CA1284382C - Magnetic memory - Google Patents
Magnetic memoryInfo
- Publication number
- CA1284382C CA1284382C CA000538580A CA538580A CA1284382C CA 1284382 C CA1284382 C CA 1284382C CA 000538580 A CA000538580 A CA 000538580A CA 538580 A CA538580 A CA 538580A CA 1284382 C CA1284382 C CA 1284382C
- Authority
- CA
- Canada
- Prior art keywords
- bit
- intermediate layer
- structures
- major surfaces
- memory film
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
- G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
- G11C11/14—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements
- G11C11/15—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements using multiple magnetic layers
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
- G11C11/02—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements
- G11C11/14—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using magnetic elements using thin-film elements
Abstract
ABSTRACT
A digital memory based on a memory cell having two magnetoresistive, ferromagnetic film portions separated by an intermediate layer, all of limited thickness.
A digital memory based on a memory cell having two magnetoresistive, ferromagnetic film portions separated by an intermediate layer, all of limited thickness.
Description
MAGNETIC MEMORY
The present invention relates ~o ~erro~
magnetic thin film memories, and more particularly, to ferromagnetic thin film memories in which states of the memory cells based on magne~ization direction are determined through magnetoresi~ti~e properties of the thin film.
Digital memories are used very extensively in-computers and computer system component , in digital signal processing system , and in other device~ based on digital circuits. In those such devices and systems where (il the memory used must permit access to any bi~ stored therein randomly (a random access memory or RAM)~ and (ii) where such acces~ must be accomplished in times on the order of the time taken to change states in ~uch device or system, memories based on storage in electrical circuits in monolithic integrated circuit have become dominant. ~owever, such integrated circuit or emiconductor memorie~
still have shortcomings with respect to what is desired in such memories. Primarily they are a) such semiconductor memories lose information upon loss of electrical power, b) they consume electrical power ~tL~
continually during use, and c) they are subject to having the information content thereof scrambled in the presence of impinging radiationa Such shortcomings can be overcome by the use of memories where bit storage is based on alternative states of magnetization in magnetic materials used in each memory cell; typically thin film material O
~owever, such magnetic memories have shortcoming~ of their ownO Many ferromagnetic thin film memorie~ used inductive sensing to determine the magnetization state of the magnetic film materlal used in a cell for storing a bit, This sensing scheme limits the ability to reduce cell sizes sufficiently to make a dense enough memory to be cost competitive with semi-conductor memory. This limit is given effect becausethe signal levels inductively sensed in such magnetic memories declines with reduced thicknesses and widths for the thin film portions used in a cell to store a bit due to there then being less flux linkage to be inductively sensed. The maximum packing densi~y of thin film memory cells providing inductively sensed output signals is not at a density sufficiently high to be competi~ive in CQSt with semiconducta~ memories.
Further, such magnetic memorie~ have usually ~een formed on a substrate not a part of an integrated ', 3,_t~
circuit. This means there were large numbers of interconnections required between the decoding circuits provided in monolithic integrated circuits and the magnetic memory storage cells leading to S difficult technical problem~ with costly solutions.
An alternative arrangement for sensing states of magnetization in thin film magnetic material portion~ used in memory cells for storing bits is based on choosing a thin film ferromagnetic material which also exhibits a sufficient magnetoresi~tance property. Because changes in electrical re~istance of such a material with the application, removal or change in magnitude of a magnetic field do not depend to first order on the dimensions of the film portion, the film portion to store a bit can be made very small to thereby improve the`packing density of cells in a magnetic memory. Furthermore, such an array of cells containing film portions to store bits can he provided right on a monolithic integrated circuit surface to thereby considerably ease the making of electrical interconnections between the decoding circui~s and the memory cells.
However, other problems arise when such ferromagnetic thin films used for each bit are reduced to being very small and packed very closely together on such a surface so as to be very near to one L~
-~1- 64159-951 another. The magnetic situation can become much more complex with fields in one film portion serving as a bit storage site affecting neighboring storage cell ~ilm portions and vice-versa.
Furthermore, a resultank magnetization intended to occur along the easy access of an anisotropic ferromagnetic film can be unstable as to direction and magnitude because of substantial demagnetizing fields occuring in a memory cell thin film portionv SUMMARY OF THE INVENTION
The present invention provides a digital memory having a bit structure in a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory film exhibiting magnetoresistance is provided. ~his separating material is less than 100 A thick, and said memory film is le~s than 300 A thick. The easy access of magnetization in the film is provided in a selected direction.
In accordance with the present invention there is provided a magnetoresistive sensing, ferromagnetic thin film based digital memory, said memory comprising:
a first storage line structure having a first storage line pair of end terminals adapted to conduct electrical current in at least one direction, said storage line end terminals having electrically connected in series therebetween a plurality of bit structures with each said bit structure electrically connected at a bit juncture to at least one other said bit structure, each said bit structure to comprise at -4a- 64159-95 1 least a structure comprising:
an intermediate layer of a kind of separating material, said intermediate layer having two major surfaces on opposite sides thereof such that said major surfaces are separated by less than one hundred Angstroms of said separating material but sufficiently separated for said kind of said separating material to prevent any exchange interaction from coupling thereacross, and a memory film on each of said intermediate layer major surfaces with said memory film being of a thickness less than three hundred Angstroms and of a magnetoresistive, anisotropic ferromagnetic material, and a plurality of word line structures each having a pair of word line end terminals adapted to conduct electrical current in at least one direction, each said pair of said word line end terminals having an electrical conductor electrically connected therebetween which is located across an electrical insulating layer from said memory film on one of said major surfaces of said intermediate layer of a selected one of said bit structures.
BRIEF DESCRIPTION OF THE D_AWINGS
Figure 1 shows a portion of an array of memory cells of the present invention each containing an anisotropic ferromagnetic thin film portion for storing a bit, ~ .
.... ~ ~ , , , j, L~
Figure 2 shows a memory cell in more detail of the kind shown in Figure 1, and Figure 3 show~ a diagrammatic cro~fi section of the bit structure shown in ~igure 2, set out in Figure 3A, and a graph of magnetization alon~ on axis~
as set out in Figure 3B.
A metallic thin film useful in making magnetic thin film memorie~ is an alloy of nickel, cobalt and iro~. Typically, the proportions are chosen to strongly reduce or eliminate any magneto-~trictive effects in the film and to improve certaln other properties of the film for the application at hand. As a pos~ible example, the film material might comprise approximately sixty percent (603) nickel, twenty-ive percent (25~) cobalt, and fifteen percent (15~) iron. In ~ome situations other materials are added to the alloy in relatively small amounts to improve certain properties o ~he f ilm.
Such films can be fabricated by vacuum deposition or other methods and, when done in the presence of a magnetic ~ield oriented in a selected direction, the resulting magnetic thin film will exhibit uniaxial anisotropy with the easy axi8 parallel to the magnetic ~ield orientation. Because , . .
~L~
vary large demagnetizing fields would otherwise result, the magnetization vector of such a film will always lie in the plane of the film.
Furthermore, ln accord with thermodynamic~, s the magnetization in ~uch a film will arrange itself to minimize the magnetic energy. In the absence of any externally applied magnetic field~ such minimi-zation occurs when ~he magnetization ve~tor of a film portion parallels the easy axis of the film portion pointing in either direction along such axi~.
~ owever, the situation in ~uch a film portion changes in the presence of externally applied magnetic fields and the minimization of magnetic energy may then occur with the magnetization vector oriented at an angle with respect to the easy axis. A~ long as the magnetization of the film portion is in a single domain state, the magnetization vector then can be caused to rotate wi~h respect to the easy axis to angleg determined by the externally applied field~, and this can occur without substantially affecting the magnitude of ~he magnetization.
In such a state with external magnetlc fields applied to the film portion, the ~otal magne~ic energy can be approximately determined. The minimum of this energy can be calculated as a ba~i~ for determining the angle of the magnetization vector with respect to ~L ~ ~
the easy axis, u~ually as a function of the magnetic field componen~s parallel with and perpendicular to the easy axis.
In addition, the critical values for external magnetic field~ can be found governing tran~itions of the magnetization vector position rom unstable to stable statesO The equation for such critical field~
is found to be in the form of a hypocycloid, u~ually termed an a~troid, so that external field~ of values within the astroid leave the magnetization in a stable angle but those of value~ out~ide the a troid lead to potential in tability. Thi~ instability is manifested as a flipping of the magnetization vector from pointing at least to some extent along one direction of the easy axis to pointing to at least some extent in the opposite direction. Thus, the magnetization vector can be made to switch from one direction along the easy axis to the opposite which means the magnetization vector can be in one of two different state~ which provides the basis for storing a binary bit.
Such ferromagnetic thin film as tho~e just de~cribed further exhibit magnetoresistance.
Differences in direction between that o the magnetization vector in the thin film and that of current passed through the thin film leads to ,cL~ r~
differences in the effective electrical resistance in ~he direction of the current. The maximum resistance occurs when the magnetization vector in the film and the current direction are parallel, while the minimum occurs while they are perpendicular. The resistance of a magnetoresis~ive resistor can be shown to be given by a constant value representing the minimum plus an additional value depending on the angle between the current direction in the film and the magnetization ve~tor therein. This additional resistance follows the square o the co~ine of that angle.
Thus, external magnetic field~ can be used to vary the angle of the magnetization vector in ~ùch a film portion with respect to the easy axis of the film, and can vary it to such an extent as to cause switching of the magnetization vector between two stable states which occur as magnetizations in opposite directions along the easy axis. Further, the state of the magnetization vector in such a film portion can be measured or sensed by the change in resi~tance encountered by a current directed through this film portion. Thi~ provides a ba~is for a film portion to serve as a bit storage means in a memory cell, the state of which i6 subject to being determined by effects occurring in current~ applied to this portion.
_.iJrd~
A part of an array of such ~ilm portions, 10, is shown in Figure 1 together forming a digital memory. For now, the film portion~ 10 can be thought of as being a single layer of film but because of problems in such a memory as indic~ted above, the structures 10 in practice will be more complex as ~ill be described below. Each of bit structure~ 10 in a horizontal row is electrically joined to another in that row by interconnection juncture3, 11. At opposite ends of each row there is provided end terminating regions, 12. Each row then represents a number of bit structures 10 electrically inter-connected in serie3 by junction interconnection~ 11 between end terminations 12 to thereby form a storage line structure.
Figure 1 is s~own with a vertical break to indicate there may be a very large number of such bit structures in each row, a number much larger than shown. Similarly, Figure 1 has a horizontal break to show there may be a much larger number of storage lines than has been shown in forming a digital memory.
Al~o shown in Figure 1 are a number of word lines, 13, one for each corresponding bit structure 10 in each storage line structure. Each word line 13 is an electrical conductor occurring in series between end terminations, 14. Each word line 13 i8 formed , . .. . .. ..
, over portion~ of a corresponding bit structure 10 in each storage line ~tructure and, although not specifically indicated in Figure 1, is ~ormed with an insulating layer between word line 13 and adjacent portions of bit ~tructure 10, If such an insulating layer was shown, all line8 but lines representing word lines 13 and end terminations 14 would be in dashed form to show them being below the insulating layer.
Also not shown is a further protective and lo insulating layer formed over th~ entire structure shown in Figure 1. Showing ~uch a layer would require all lines in Figure 1 to be in dashed form to indicate all the structures shown there as being below such a protective layer.
Storage line end terminations 12 permit providing current through each ætorage line from one o end terminations 12 connected to that line to the other connected at the opposite end thereof. Thus, end ~erminations 12 are also electrically connected to 20 other circuits such as sensing circuits, write control circuits, decoding circuit~, or the likeO
5imilarly, word line end termination~ 1~
permit current to be passed through word line~ 13 f rom an end termination 14 at one end thereof to that end 25 termination 14 provided at the other end. Word line end terminations 14 are also connected to other circuits such as current supply circuits, write control circuits, decoding circuits, or the like.
Junction interconnections ll~ storage line end terminations 12, word lines 13, and word line end terminations 14 can all be formed of a convenient conductor material. Since the digital memory portion of Figure l is intended to be provided on a surface portion of a monolithic in~egrated circuit 80 that circuits in the other portions of the integrated circuit can be conveniently connected to end terminations 12 and 14, a typical conductor material used in integrated circuits would be appropriate. An aluminum layer~ perhaps containing an additional alloying metal such as copper, on a titanium-tungsten base layer is one e~ample.
Bit structures 10, however, must be considered in more de~ail because the structure ~hereof must overcome those problems indicated above associated with ferromagnetic thin film memorie~. The first of those problems is the effect of one bit ~tructure on neighboring bit structures, and vice vèrsa, if a single thin film portion i~ used to provide each ~uch bit structure. As bit Rtructures 10 become more and more compact and located closer and closer to one another, ~o thereby impxove packing density, the interaction of the magnetic fields occurring in one upon its neighbors becomes quite (r~
significant. The effect is usually deleterious in that such fields will often act to increase the demagne~izing field experienced in it~ neighborsr An arrangement to more closely confine the magnetic fields occurring in a bit structure to ju~t that bit structure is shown in Figure 2. This i~ a bit structure which would be satisfactory for u e a~ a bit structure 10 in Figure 1, and accordingly, the designations of struc~ures in Figure 1 which appear in Figure 2 are carried over to Figure 2.
The further structural detail shown in Figure 2 includes that bit ~tructure 10 is formed over a semiconductor ma~erial body, 20, as used in a monolithic integrated circuit, and directly on an insulating layer, 21, supported on a major surface of body 20 in the integrated circuit. Only a ~mall portion of the integrated circuit is shown, and then only a small portion of semiconductor material body 20 is shown of that integrated circuit portion~ Juncture interconnect~ons 11 are shown comprising aluminum alloyed with four percent ~4%) copper approximately 5000 A thick, and disposed on the exposed major surface of insulating layer 21.
Also disposed on thi~ exposed major surface of insulating layer 21 is bit structure 10 shown comprised of a lower ferromagnetic thin film, 22, and ...l ~ ~ ` ,t ~,o ~ ~
an upper ferromagnetic thin film, 23. Ferromagnetic thin film layers 22 and 23 arQ each as de cribed above in that they exhibit uniaxial anisotropy, magneto-resistance, little magneto~striction, and are of an alloy compositionO Xn between ferromagnetic thin film layers 22 and 23 i3 a further thin film layer, 24, which usually would not exhibit ferromagnetism but may be either an electrical conductor or an electrical insulator. hayer 24 mu~t, however, prevent the exchange interactlon between electron ~pins on neighboring atom~ from coupling acro~ between layers 22 and 23 to lock toge~her the magnetization vectors in each. A typical choice for layer 24 would be silicon nitride. An insulating layer, 25, covers bit structure 10 although only a part of it is shown in Figure 2.
The "sandwich" structure of Figure 2 is effective in reducing magnetic fields outside bit structure 10 because the magnetic fields occurring in either of erromagnetic thin film layer~ 22 and 23 are, to a considerable extent, confined to the magnetic path provided by the other. Thu~, the effect of magnetic fields occurring in either of layers 22 and 23 on neighboring bit tructures is much reduced.
A further coninement of magnetic field~
occurring in bit structure 10 of Figure 2 can be ~'L~ d 1~-achieved by providing magnetic material on the ides of bit structure 10, as more or less indicated by the dashed lines, ~6~ These lines are to sugqest such magnetic material being used as part of a 3ingle anular ferromagnetic thin film arrangement comprising also layers 22 and 23, or which may be provided separately from layers 22 and 23. Such an addition of magnetic side material to bit structure 10 would improve ~he confinement of magnetic field~ in bit lQ struc~ure 10. on the other hand, the addition of such side magnetic structures 26 means additional fabri~
cation process effort, complexity, and c03t. No indication has been made in Figure 2 for the greater room required under in~ulating layer 25 to accommoda~e the side magnetic material 26 although, obviou~ly, layer 25 would have to cover such side magnetic material also.
Finally, word line 13 i8 shown in Figure 2 disposed on the major ~urface of in~ulating layer 25.
Word line 13 is typically compri~ed of an aluminum layer alloyed with approximately four percent (4~) copper on a titanium-tungsten base layer in a total thickness of 5000 A. A protective and in~ulating layer over the entire structure of Figure 2 would be us~d in practice but is not shown.
The orthogonality of word line 13 and bit structure 10, the magne~oresistive properties of - , .
. . '. :
.
.;
~L~
layers 22 and 23, and the desire to have as large an output signal as possible leads to a choice of providing the easy axis in layers 22 and 23 in one o~
two principle direction~. Bit structure 10 can be operated in a longitudinal mode with the ea~y axis for layers 22 and 23 directed parallel to bit struc~ure 10 between juncture interconnections 11. Alternatively, the easy axis in layer~ 22 and 23 can be formed perpendicular to this first choice and parallel to word line 13 leading to operation in the transver~e mode. A possible operating scheme w1th non-destructive readout of the bit ~tate in a bit structure 10 can be sketched for each operating mode.
For the longitudinal operating mode, the easy axis and ~erromagnetic thin film layers 22 and 23 extends between the juncture interconnections 11 with the anisotropy magnetic field, HR, and the magneti-zationy M, both directed therealong in the absence of any externally applied magnetic fields. Information, or the state of the digital bit stored in bit structure 10, i~ stored in layers 22 and 23 in the absence of external magnetic ~ields by having the vector for magnetization M polnted in one direction between interconnections 11 or in the other direction.
If the magnetization direction is caused to rotate from a direction along the ,ea~y axis by .
, ~L~3 external magnetic fields~ the electrical resistance of layers 22 and 23 changes with this magnetization direction rotation because of the magnetoxesistive properties thereof. For the kinds of materials in layers 22 and 23, the maximum change in resistance is on the order of a few percent of the minimum resistance value.
To read the state of a bit structure 10, currents are passed through ~-he storage line of Figure 1 in which the bit structure occurs and through that word line 13 passing over such bit structure. The sense line current magnitude is set so that the magnetic field generated by such current rotates the magnetization direction in layers 22 and 23 to a significant angle from the easy axis. If current through the word line is now provided, the magnetic field associated with that current will for one state of the magnetization of layers 22 and 23 increase the angle of rotation, and for the other state decrease the angle of rotation.
Such changes in the angle of magnetization direction by these rotations cause different changes in ~he electrical resistance of layers 23 and 24 because of the magnetoresistive propertles of these layers. The bit state which leads to an increase in the angle of rotation of the magnetization vector with .
~17-the application of the word current will lead to a lower resistance if that is the state taken in bit struc~ure 10, while the opposite state will lead ~o an increase resistance. Such changes in resistance will s affect the sense current on the sense line which effects can be detected to determine the state of the corresponding bit structure. Both the sense current and the word current must be kept small enough ~o that the magne~ic field~ generated ~hereby do not exceed the critical fields described by the astroid plot for the bit structure under consideraSion. Otherwise~
switching the magnetization vector from one state to the other may occur.
Just the opposite requirement for magni~udes of the bit and sense currents occur when a de~ired state of the bit structure is to be written into that structure. The sense and word currents are set to be insufficient individually to cause switching of the magnetization vector, but cumulatively to be enough for such switching. The state set is determined by the direc~ion of current flow through the word line.
A similar scheme can be used for operating in the transverse mode with the easy axis in film~ 22 and 23 perpendicular to the easy axis direction in the longitudinal mode, i.e. parallel to word line 13. The magnetization vector then points in one direction or ' - : . ' ` :
: ' ' ,: ' ~ . ' ~ - ' .
the other along this easy axis to determine the state of ~it structure 10. Current is supplied along the word line sufficient to cause a magnetic field which rotates the magnetization vector to an angle from the easy axis. Current supplied along the sense line will then cause the rotation to increase when the bit structure is in one state and decrease when it is in the opposite state leading to a detectable difference in the electrical resistance in layers 23 and 24 because of the magnetoresistive effect. Again for non-destructive readout, these currents must be ~mall enough that the magnetic fields generated thereby cannot exceed the critical field level determined by the astroid for bit structure 10. Again, for setting the state for the bit structure, larger currents are applied along the sense and bit lines with the direction of the current on the sense line determining the state occurring in the bit structure.
To achieve either of the foregoing operations, however, bit ~tructures 10 must be carefully constructed. 5mall ferromagnetic ~hin film portions are subject to very high demagnetizing fields because the effective "free poles'l are closer together along the edges of the film leading to larger demagnetizing fields. This can be seen since the widths of bit structure 10 in the direction of word , --lg--line 13 will be on the order of 0.1 mic~on to, at most, a ~ew microns. Such dimensions are necessary to meet the high density requlrement for such bit structures to keep cost~ low on a cost per sell basi~, and because small curren~s in the sen~e or word lines allow faster switching. Such currents can be kept small only if the thin film portions are also small.
A ferromagnetic thin film portion typical of the kind being considered here, tha~ is, 1000 A thick and in the form of a square 2 microns on a side, can be approximated by an inscribed elipsoid for purposes of calculating its demagnetization field. Assuming that the thin film ~quare has a saturation magneti-zation value of M~, a thicknes~ of T, and a major axis of length 2r, the following equation applles for the demagnetization fi~ld ~D in the corresponding inscribed elip~oid:
TM
HD ~ 4 r If the satura~ion magnetization is around 10,000 Gau~, the uniform demagnetizing field will be on the order of 785 Oersteds, a field ~trength which i~ two orders of magnltude larger ~han ~ypical ani~otropy field strengths HK in such ferromagnetic thin films.
These demagnetizing fields would undoub~edly dominate the behavior of sush a film portion and result in ', ' ' ' '' ' ~ , ' .
instability in the magnetization of such a film in the sense that the magnetization would be forced from lying entirely along the easy axis to lying at least in part in some other direction even in the absence of external fields.
Aqain, the "sandwich" arrangement for the bit structure in Figure 2 provides aid in this situation because the demagnetizing field~ in each of films 22 and 23 act to cancel one ano~herr Nevertheless, very larqe uncancelled field~ will stlll occur because of the drop in field strength with di~tance given the separation between film. 22 and 23. Even if a large fraction of the magnetizing fields are cancelled, the remaining uncancelled portions of two rather large demagnetizing fields can ~till be on the order of anisotropy field strengths ~R leading to the kinds o~
instabilities in the magnetization of the devices as indicated above.
Con~ider in Figure 3 a diagrammatic cross ~ection of bit structure 10 o~ Figure 2 p~rallel to word line 13 where the cross section view in Figure 3A
shows ferromagnetic thin films 22 and 23 and separating film 24. This cross section is taken relatively far from either of juncture interconnec-tions 11. Insulating layers~ protective layers,semiconductor material body substrate and the like are ,: ~
.. .. .
.. ~ ' ,. . .
, ' . ~ ' '~L,s~
ignored and therefore omitted from the cros~ section of Figure 3. Each of ferromagnetic thin films 22 and 23 are shown in Figure 3 to have a thickness designated TF while the separating film 24 which is free of any ferromagnetic properties is shown to have a thickness Ts. The transverse operating mode has been chosen here, so tha~ the easy axis in each film is parallel to word line 13. The magnetization shown for each film, M22(x) and M23(x), are both shown as a function of x for rea~ons to be described, and are shown in oppocite directions along the x axis which is an arrangement that minimizes the magne ic energy.
Near the edges of films 22 and 23, anisotropy fields are dominated by the demagnetizing field~ due to the "free poles" at the edge~. If the magneti-zations of films 22 and 23 were saturated, the demagnetizing ields would approach MS/2 in the films or about 5,000 Oersteds for films with tha alloys described here. Typical films of these alloys will have a coercitivity and an anisotropy field in the order of only 20 Oersteds, leading to instabil- ities in the magnetization at the edges of these films.
In such large demagnetizing fields, electron spins at the edge of the ~trip are constrained to lie nearly parallel to the long dimension of these films, i.e. along the z axis. The direction of these electron spin~ only gradually turn to pointing across the films further inward toward the center of the film~ where the demagnetizing fields are no longer overcoming the anisotropy field. The rate, shape and distance of occurrence are all a complex function depending on magnetostatic~ of the situation, the ~uantum exchange interaction between adjacen~ atom electron ~pin I and anisotropy considera~ions not unlike tho3e leading to ~eel walls.
lo This is reflected in the simplified graph in ~igure 3B below the film~ shown in Figure 3A where the magnetiza~ion along the x axi~ is shown to be at zero at the outer edges of the films and gradually increasing towards the interior of the films to the saturation value Ms occurring in the central portion~
of the film~ for interior film width distance S. In the region~ of width D between the exterior edgeR of the film and the point where magnetic saturation begins, the magnetizations are in transition from pointing along the z axis to along the x axis.
Detailed analysis has shown that for films of the kind being considered here having a film thickness of 150 A (TF) separated by 50 ~ (~S) leading to an anisotropy field ~R f 25 Oersteds, that distance D is about 0.4 microns in a 2 micron wide film, A film only 1 micron wide, would saturate for only about 0.2 microns in the central regions of the film, and therefore the x directions magnetization would be only marginally stable.
Therefore, films with easy direc~ions on the transverse axis do not truly saturate across the films. Furthert films that are thicker or having greater separation therebetween, or both, are found to have even less of the central interior regions o~ the strip in magnetic saturation along the x axis leading to even less ~tability.
The uncancelled field in one film, that is, the magnetic field in one of films ~2 and 23 due to the demagnetization fields in Pach of these films can be found from again using inscribed elipsoids in the x-y plane of the cross sections of films 22 and 23 in Figure 3 as a ba~is for such a determination. The uncancelled field in the chosen film can be found by f inding the field at the midpoint of such a film because the average effect of field in the film can be approximated as the field occurring at such midpoint.
This estimate of the average effect of field through the film is reasonable because the exchange in~eractions between the electron spins of adjacent atoms in the film are so strong through the thickness of a very thin ferromagnetic film that the spins of -2~-electrons on such adjacent atoms throughout this thickness are constrained to align within a few degrees of one another.
These in~cribed elipRoid~ have their major axes along the wid~h of films 22 and 23 of a length equal to w or 2r. The magnetization~ of the films are saturated inside th se elipsoids. The center of one film is separated from the other by the distance TF
Ts.
The uncancelled field in one film, ~unca~ -can be written:
uncan HDg ( (Tf + Ts) /r), where ~D is the demagnetizing field occurring within one of ~he elipsoids in one of the films due to just the surface poles of that film, and g, as a function of the argument (Tf + Ts)/r, is a cancellation factor arlsing becau~e of the action of the demagnetizing ield of the other film opposing the demagnetizing field of the fir~t film. In the situation where Tf +
Ts is much less than r, the uncancelled field has been found to satisfy the approxi~ation:
HUncan ~ HDg((Tf + Ts)/r Tf ~ T~3 ~ HD[2.4 ( r ) ~ .
Experiment has confirmed the accuracy of this approximationO
The value of the anisotropy magnetic field HK
in ferromagnetic thin films of the nature being considered here is typically 10 to 30 Oersteds determined primarily by tbe cbemical composition of the film but al~o depending on various other parameters such as angle and deposition of the film on the substrate, the substrate temperature, ani~otropic strainq resulting in the film, and the like. In any event, to keep the demagnetization fields in a film portion from dominating the anisotropy field too great an extent, the uncancelled portions of he demagnetizing field~ in the film should be kept in the range from 2 to 6 Oer~teds. That i8, the ratio of HUncan to ~R is a measure of the stability of the magnetization. The ratio is unacceptable with a value o~ 1 and one can be quite confiden~ of a value of 10 so an intermediate ratio value such as 5 is a reasonable choice.
To determine then the permitted thickneqses for the films in the bit structure 10 of Figure 2, the first equation above and the last equation above can be combined and a choice for the uncancelled magnetic fields of 3 Oersteds or less leads to the following in equality:
(- f 9) ~2 4( f s)] < 3 '~L~
If w in Figure 3 i9 taken to be 2 microns, and the saturation magnetization in films 22 and 23 is again taken to be 10,000 Gauss~ thi~ inequality can be rewritten as follows:
Tf(Tf + Ts) < 16, 0000 This last inequality is sufficient for ju~t the conditions assumed in~ofar as a desirable width for bit structure 10 t and the material composition leading to the magnetixation saturation used in reaching thi~
inequality. Further, the 3 Oersted limit on acceptable uncancelled demagnetization fields is somewhat arbitrary. In other design situations, then, another inequality would be used.
Nevertheless, values of permitted thin film thicknesses allowing stable magnetizations in films 22 and 23 in ~his design situation are of significance in illustra~ing the acceptable ranges of values. They can be obtained from thi~ la-~t inequality on assuming one o~ the thickness value~. If ~he thickness of 20 separating film 24 is chosen to have a thickness of 50 A, then the ferromagnetic films 22 and 23 mu~t be less than abou~ 105 A in thickness.
This thickne~s choice or separa~ing film 24 is not an unreasonable choice in that the film needs 25 only to be thick enough to break the exchange interaction coupling between electron spins in atoms at the edges of each of ferromagnetic film layers 22 and 23. Typically, a separation on the order of 10 angstroms is sufficient to eliminate such exchange coupling between layer~ 22 and 23. The material for layer 24 has been chosen to be an in~ulating film, silicon nitride, which give~ good fabrica~ion process results. The choice of a conductor, although partially shorting the magnetoresi~tive respon~e signal, still has the advantage of shorting layers 22 and 23 together CO that the sense current flowing in ~hese layers i8 distributed more uniformly there-between, particularly if there is a defe~t in one of the other layer along the current paths. A further alternative, the material in layer 24 could in some situations be either a ferromagnetic material or a ferrimagnetic ma~erial if there is ~ufficient exchange interaction mismatch with layers 22 and 23 to prevent the exchange interaction from coupling therebetween.
With thi3 latter choice of materials, ~he possible use of an outer magnetic material covering on the sides 26 could be eliminated as flu~ closure could be provided by this chosen material for layer ~4.
Thus, the thickne~s chosen for intermediate layer 24 is a reasonable one and leads to rather thin ferromagnetic films for layers 22 and 23 in this design example. In practice over the range of ~cceptable designs to give sufficient bit structure density and operating rapidity for a digital memory, the thickness of films 22 and 23 should be less than 300 A, and preferably less than 200 A. The thickness in these situations of intermediate layer 24 should be less than 100 ~. -Restricting the thickness of ferromagnetic films 22 and 23 ~ufficiently to achieve a relatively low uncancelled demagnetizing field in each is a good design practice because there may be further demagnetization fields arising in a practical design not accounted for in the foregoing analysi For instance, there will be some demagnetizing fields occuring along the z axis in films 22 and 23 which would also be reduced by a restricted film thickness.
Further, while film~ 22 and 23 have been shown to be of comparable thickness and width, this may not necessarily be the best design in each situation~
Further, a different alloy material for each of films 22 and ~3 may be desirable in some design situations.
These sorts of differences may lead to additional demagnetizing field strengths which also would be reduced by limiting the thickness of such ~ilms.
Bit structure 10 can very readily be provided in a monolithic integrated circuit chip. Because bit structure 10 is formed on insulating layer 21 and "
.
;'~ .
--~29 -would have only insulating protective layers thereover, and because none of these layers are magnetically permeable, bit structure 10 can be designed without reference to the integrated circuit structure environment. Further, the interconnections between the digital memory and the remaining portions of the integrated circuits can be provided by the normal integrated circuit fabrication process steps for providing interconnections.
Only the memory cell array construction steps need to be added to the normal steps used for abricating monolithic integrated circuits. In some circumstances, the additional steps needed to construct ths memory array can be integrated with already existing monolithic integrated circuit fabrication process steps to minimize or possibly eliminate addi~ional fabrioation steps in providinq the digital memory on a monolithic integra~ed circuit chip.
The present invention relates ~o ~erro~
magnetic thin film memories, and more particularly, to ferromagnetic thin film memories in which states of the memory cells based on magne~ization direction are determined through magnetoresi~ti~e properties of the thin film.
Digital memories are used very extensively in-computers and computer system component , in digital signal processing system , and in other device~ based on digital circuits. In those such devices and systems where (il the memory used must permit access to any bi~ stored therein randomly (a random access memory or RAM)~ and (ii) where such acces~ must be accomplished in times on the order of the time taken to change states in ~uch device or system, memories based on storage in electrical circuits in monolithic integrated circuit have become dominant. ~owever, such integrated circuit or emiconductor memorie~
still have shortcomings with respect to what is desired in such memories. Primarily they are a) such semiconductor memories lose information upon loss of electrical power, b) they consume electrical power ~tL~
continually during use, and c) they are subject to having the information content thereof scrambled in the presence of impinging radiationa Such shortcomings can be overcome by the use of memories where bit storage is based on alternative states of magnetization in magnetic materials used in each memory cell; typically thin film material O
~owever, such magnetic memories have shortcoming~ of their ownO Many ferromagnetic thin film memorie~ used inductive sensing to determine the magnetization state of the magnetic film materlal used in a cell for storing a bit, This sensing scheme limits the ability to reduce cell sizes sufficiently to make a dense enough memory to be cost competitive with semi-conductor memory. This limit is given effect becausethe signal levels inductively sensed in such magnetic memories declines with reduced thicknesses and widths for the thin film portions used in a cell to store a bit due to there then being less flux linkage to be inductively sensed. The maximum packing densi~y of thin film memory cells providing inductively sensed output signals is not at a density sufficiently high to be competi~ive in CQSt with semiconducta~ memories.
Further, such magnetic memorie~ have usually ~een formed on a substrate not a part of an integrated ', 3,_t~
circuit. This means there were large numbers of interconnections required between the decoding circuits provided in monolithic integrated circuits and the magnetic memory storage cells leading to S difficult technical problem~ with costly solutions.
An alternative arrangement for sensing states of magnetization in thin film magnetic material portion~ used in memory cells for storing bits is based on choosing a thin film ferromagnetic material which also exhibits a sufficient magnetoresi~tance property. Because changes in electrical re~istance of such a material with the application, removal or change in magnitude of a magnetic field do not depend to first order on the dimensions of the film portion, the film portion to store a bit can be made very small to thereby improve the`packing density of cells in a magnetic memory. Furthermore, such an array of cells containing film portions to store bits can he provided right on a monolithic integrated circuit surface to thereby considerably ease the making of electrical interconnections between the decoding circui~s and the memory cells.
However, other problems arise when such ferromagnetic thin films used for each bit are reduced to being very small and packed very closely together on such a surface so as to be very near to one L~
-~1- 64159-951 another. The magnetic situation can become much more complex with fields in one film portion serving as a bit storage site affecting neighboring storage cell ~ilm portions and vice-versa.
Furthermore, a resultank magnetization intended to occur along the easy access of an anisotropic ferromagnetic film can be unstable as to direction and magnitude because of substantial demagnetizing fields occuring in a memory cell thin film portionv SUMMARY OF THE INVENTION
The present invention provides a digital memory having a bit structure in a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory film exhibiting magnetoresistance is provided. ~his separating material is less than 100 A thick, and said memory film is le~s than 300 A thick. The easy access of magnetization in the film is provided in a selected direction.
In accordance with the present invention there is provided a magnetoresistive sensing, ferromagnetic thin film based digital memory, said memory comprising:
a first storage line structure having a first storage line pair of end terminals adapted to conduct electrical current in at least one direction, said storage line end terminals having electrically connected in series therebetween a plurality of bit structures with each said bit structure electrically connected at a bit juncture to at least one other said bit structure, each said bit structure to comprise at -4a- 64159-95 1 least a structure comprising:
an intermediate layer of a kind of separating material, said intermediate layer having two major surfaces on opposite sides thereof such that said major surfaces are separated by less than one hundred Angstroms of said separating material but sufficiently separated for said kind of said separating material to prevent any exchange interaction from coupling thereacross, and a memory film on each of said intermediate layer major surfaces with said memory film being of a thickness less than three hundred Angstroms and of a magnetoresistive, anisotropic ferromagnetic material, and a plurality of word line structures each having a pair of word line end terminals adapted to conduct electrical current in at least one direction, each said pair of said word line end terminals having an electrical conductor electrically connected therebetween which is located across an electrical insulating layer from said memory film on one of said major surfaces of said intermediate layer of a selected one of said bit structures.
BRIEF DESCRIPTION OF THE D_AWINGS
Figure 1 shows a portion of an array of memory cells of the present invention each containing an anisotropic ferromagnetic thin film portion for storing a bit, ~ .
.... ~ ~ , , , j, L~
Figure 2 shows a memory cell in more detail of the kind shown in Figure 1, and Figure 3 show~ a diagrammatic cro~fi section of the bit structure shown in ~igure 2, set out in Figure 3A, and a graph of magnetization alon~ on axis~
as set out in Figure 3B.
A metallic thin film useful in making magnetic thin film memorie~ is an alloy of nickel, cobalt and iro~. Typically, the proportions are chosen to strongly reduce or eliminate any magneto-~trictive effects in the film and to improve certaln other properties of the film for the application at hand. As a pos~ible example, the film material might comprise approximately sixty percent (603) nickel, twenty-ive percent (25~) cobalt, and fifteen percent (15~) iron. In ~ome situations other materials are added to the alloy in relatively small amounts to improve certain properties o ~he f ilm.
Such films can be fabricated by vacuum deposition or other methods and, when done in the presence of a magnetic ~ield oriented in a selected direction, the resulting magnetic thin film will exhibit uniaxial anisotropy with the easy axi8 parallel to the magnetic ~ield orientation. Because , . .
~L~
vary large demagnetizing fields would otherwise result, the magnetization vector of such a film will always lie in the plane of the film.
Furthermore, ln accord with thermodynamic~, s the magnetization in ~uch a film will arrange itself to minimize the magnetic energy. In the absence of any externally applied magnetic field~ such minimi-zation occurs when ~he magnetization ve~tor of a film portion parallels the easy axis of the film portion pointing in either direction along such axi~.
~ owever, the situation in ~uch a film portion changes in the presence of externally applied magnetic fields and the minimization of magnetic energy may then occur with the magnetization vector oriented at an angle with respect to the easy axis. A~ long as the magnetization of the film portion is in a single domain state, the magnetization vector then can be caused to rotate wi~h respect to the easy axis to angleg determined by the externally applied field~, and this can occur without substantially affecting the magnitude of ~he magnetization.
In such a state with external magnetlc fields applied to the film portion, the ~otal magne~ic energy can be approximately determined. The minimum of this energy can be calculated as a ba~i~ for determining the angle of the magnetization vector with respect to ~L ~ ~
the easy axis, u~ually as a function of the magnetic field componen~s parallel with and perpendicular to the easy axis.
In addition, the critical values for external magnetic field~ can be found governing tran~itions of the magnetization vector position rom unstable to stable statesO The equation for such critical field~
is found to be in the form of a hypocycloid, u~ually termed an a~troid, so that external field~ of values within the astroid leave the magnetization in a stable angle but those of value~ out~ide the a troid lead to potential in tability. Thi~ instability is manifested as a flipping of the magnetization vector from pointing at least to some extent along one direction of the easy axis to pointing to at least some extent in the opposite direction. Thus, the magnetization vector can be made to switch from one direction along the easy axis to the opposite which means the magnetization vector can be in one of two different state~ which provides the basis for storing a binary bit.
Such ferromagnetic thin film as tho~e just de~cribed further exhibit magnetoresistance.
Differences in direction between that o the magnetization vector in the thin film and that of current passed through the thin film leads to ,cL~ r~
differences in the effective electrical resistance in ~he direction of the current. The maximum resistance occurs when the magnetization vector in the film and the current direction are parallel, while the minimum occurs while they are perpendicular. The resistance of a magnetoresis~ive resistor can be shown to be given by a constant value representing the minimum plus an additional value depending on the angle between the current direction in the film and the magnetization ve~tor therein. This additional resistance follows the square o the co~ine of that angle.
Thus, external magnetic field~ can be used to vary the angle of the magnetization vector in ~ùch a film portion with respect to the easy axis of the film, and can vary it to such an extent as to cause switching of the magnetization vector between two stable states which occur as magnetizations in opposite directions along the easy axis. Further, the state of the magnetization vector in such a film portion can be measured or sensed by the change in resi~tance encountered by a current directed through this film portion. Thi~ provides a ba~is for a film portion to serve as a bit storage means in a memory cell, the state of which i6 subject to being determined by effects occurring in current~ applied to this portion.
_.iJrd~
A part of an array of such ~ilm portions, 10, is shown in Figure 1 together forming a digital memory. For now, the film portion~ 10 can be thought of as being a single layer of film but because of problems in such a memory as indic~ted above, the structures 10 in practice will be more complex as ~ill be described below. Each of bit structure~ 10 in a horizontal row is electrically joined to another in that row by interconnection juncture3, 11. At opposite ends of each row there is provided end terminating regions, 12. Each row then represents a number of bit structures 10 electrically inter-connected in serie3 by junction interconnection~ 11 between end terminations 12 to thereby form a storage line structure.
Figure 1 is s~own with a vertical break to indicate there may be a very large number of such bit structures in each row, a number much larger than shown. Similarly, Figure 1 has a horizontal break to show there may be a much larger number of storage lines than has been shown in forming a digital memory.
Al~o shown in Figure 1 are a number of word lines, 13, one for each corresponding bit structure 10 in each storage line structure. Each word line 13 is an electrical conductor occurring in series between end terminations, 14. Each word line 13 i8 formed , . .. . .. ..
, over portion~ of a corresponding bit structure 10 in each storage line ~tructure and, although not specifically indicated in Figure 1, is ~ormed with an insulating layer between word line 13 and adjacent portions of bit ~tructure 10, If such an insulating layer was shown, all line8 but lines representing word lines 13 and end terminations 14 would be in dashed form to show them being below the insulating layer.
Also not shown is a further protective and lo insulating layer formed over th~ entire structure shown in Figure 1. Showing ~uch a layer would require all lines in Figure 1 to be in dashed form to indicate all the structures shown there as being below such a protective layer.
Storage line end terminations 12 permit providing current through each ætorage line from one o end terminations 12 connected to that line to the other connected at the opposite end thereof. Thus, end ~erminations 12 are also electrically connected to 20 other circuits such as sensing circuits, write control circuits, decoding circuit~, or the likeO
5imilarly, word line end termination~ 1~
permit current to be passed through word line~ 13 f rom an end termination 14 at one end thereof to that end 25 termination 14 provided at the other end. Word line end terminations 14 are also connected to other circuits such as current supply circuits, write control circuits, decoding circuits, or the like.
Junction interconnections ll~ storage line end terminations 12, word lines 13, and word line end terminations 14 can all be formed of a convenient conductor material. Since the digital memory portion of Figure l is intended to be provided on a surface portion of a monolithic in~egrated circuit 80 that circuits in the other portions of the integrated circuit can be conveniently connected to end terminations 12 and 14, a typical conductor material used in integrated circuits would be appropriate. An aluminum layer~ perhaps containing an additional alloying metal such as copper, on a titanium-tungsten base layer is one e~ample.
Bit structures 10, however, must be considered in more de~ail because the structure ~hereof must overcome those problems indicated above associated with ferromagnetic thin film memorie~. The first of those problems is the effect of one bit ~tructure on neighboring bit structures, and vice vèrsa, if a single thin film portion i~ used to provide each ~uch bit structure. As bit Rtructures 10 become more and more compact and located closer and closer to one another, ~o thereby impxove packing density, the interaction of the magnetic fields occurring in one upon its neighbors becomes quite (r~
significant. The effect is usually deleterious in that such fields will often act to increase the demagne~izing field experienced in it~ neighborsr An arrangement to more closely confine the magnetic fields occurring in a bit structure to ju~t that bit structure is shown in Figure 2. This i~ a bit structure which would be satisfactory for u e a~ a bit structure 10 in Figure 1, and accordingly, the designations of struc~ures in Figure 1 which appear in Figure 2 are carried over to Figure 2.
The further structural detail shown in Figure 2 includes that bit ~tructure 10 is formed over a semiconductor ma~erial body, 20, as used in a monolithic integrated circuit, and directly on an insulating layer, 21, supported on a major surface of body 20 in the integrated circuit. Only a ~mall portion of the integrated circuit is shown, and then only a small portion of semiconductor material body 20 is shown of that integrated circuit portion~ Juncture interconnect~ons 11 are shown comprising aluminum alloyed with four percent ~4%) copper approximately 5000 A thick, and disposed on the exposed major surface of insulating layer 21.
Also disposed on thi~ exposed major surface of insulating layer 21 is bit structure 10 shown comprised of a lower ferromagnetic thin film, 22, and ...l ~ ~ ` ,t ~,o ~ ~
an upper ferromagnetic thin film, 23. Ferromagnetic thin film layers 22 and 23 arQ each as de cribed above in that they exhibit uniaxial anisotropy, magneto-resistance, little magneto~striction, and are of an alloy compositionO Xn between ferromagnetic thin film layers 22 and 23 i3 a further thin film layer, 24, which usually would not exhibit ferromagnetism but may be either an electrical conductor or an electrical insulator. hayer 24 mu~t, however, prevent the exchange interactlon between electron ~pins on neighboring atom~ from coupling acro~ between layers 22 and 23 to lock toge~her the magnetization vectors in each. A typical choice for layer 24 would be silicon nitride. An insulating layer, 25, covers bit structure 10 although only a part of it is shown in Figure 2.
The "sandwich" structure of Figure 2 is effective in reducing magnetic fields outside bit structure 10 because the magnetic fields occurring in either of erromagnetic thin film layer~ 22 and 23 are, to a considerable extent, confined to the magnetic path provided by the other. Thu~, the effect of magnetic fields occurring in either of layers 22 and 23 on neighboring bit tructures is much reduced.
A further coninement of magnetic field~
occurring in bit structure 10 of Figure 2 can be ~'L~ d 1~-achieved by providing magnetic material on the ides of bit structure 10, as more or less indicated by the dashed lines, ~6~ These lines are to sugqest such magnetic material being used as part of a 3ingle anular ferromagnetic thin film arrangement comprising also layers 22 and 23, or which may be provided separately from layers 22 and 23. Such an addition of magnetic side material to bit structure 10 would improve ~he confinement of magnetic field~ in bit lQ struc~ure 10. on the other hand, the addition of such side magnetic structures 26 means additional fabri~
cation process effort, complexity, and c03t. No indication has been made in Figure 2 for the greater room required under in~ulating layer 25 to accommoda~e the side magnetic material 26 although, obviou~ly, layer 25 would have to cover such side magnetic material also.
Finally, word line 13 i8 shown in Figure 2 disposed on the major ~urface of in~ulating layer 25.
Word line 13 is typically compri~ed of an aluminum layer alloyed with approximately four percent (4~) copper on a titanium-tungsten base layer in a total thickness of 5000 A. A protective and in~ulating layer over the entire structure of Figure 2 would be us~d in practice but is not shown.
The orthogonality of word line 13 and bit structure 10, the magne~oresistive properties of - , .
. . '. :
.
.;
~L~
layers 22 and 23, and the desire to have as large an output signal as possible leads to a choice of providing the easy axis in layers 22 and 23 in one o~
two principle direction~. Bit structure 10 can be operated in a longitudinal mode with the ea~y axis for layers 22 and 23 directed parallel to bit struc~ure 10 between juncture interconnections 11. Alternatively, the easy axis in layer~ 22 and 23 can be formed perpendicular to this first choice and parallel to word line 13 leading to operation in the transver~e mode. A possible operating scheme w1th non-destructive readout of the bit ~tate in a bit structure 10 can be sketched for each operating mode.
For the longitudinal operating mode, the easy axis and ~erromagnetic thin film layers 22 and 23 extends between the juncture interconnections 11 with the anisotropy magnetic field, HR, and the magneti-zationy M, both directed therealong in the absence of any externally applied magnetic fields. Information, or the state of the digital bit stored in bit structure 10, i~ stored in layers 22 and 23 in the absence of external magnetic ~ields by having the vector for magnetization M polnted in one direction between interconnections 11 or in the other direction.
If the magnetization direction is caused to rotate from a direction along the ,ea~y axis by .
, ~L~3 external magnetic fields~ the electrical resistance of layers 22 and 23 changes with this magnetization direction rotation because of the magnetoxesistive properties thereof. For the kinds of materials in layers 22 and 23, the maximum change in resistance is on the order of a few percent of the minimum resistance value.
To read the state of a bit structure 10, currents are passed through ~-he storage line of Figure 1 in which the bit structure occurs and through that word line 13 passing over such bit structure. The sense line current magnitude is set so that the magnetic field generated by such current rotates the magnetization direction in layers 22 and 23 to a significant angle from the easy axis. If current through the word line is now provided, the magnetic field associated with that current will for one state of the magnetization of layers 22 and 23 increase the angle of rotation, and for the other state decrease the angle of rotation.
Such changes in the angle of magnetization direction by these rotations cause different changes in ~he electrical resistance of layers 23 and 24 because of the magnetoresistive propertles of these layers. The bit state which leads to an increase in the angle of rotation of the magnetization vector with .
~17-the application of the word current will lead to a lower resistance if that is the state taken in bit struc~ure 10, while the opposite state will lead ~o an increase resistance. Such changes in resistance will s affect the sense current on the sense line which effects can be detected to determine the state of the corresponding bit structure. Both the sense current and the word current must be kept small enough ~o that the magne~ic field~ generated ~hereby do not exceed the critical fields described by the astroid plot for the bit structure under consideraSion. Otherwise~
switching the magnetization vector from one state to the other may occur.
Just the opposite requirement for magni~udes of the bit and sense currents occur when a de~ired state of the bit structure is to be written into that structure. The sense and word currents are set to be insufficient individually to cause switching of the magnetization vector, but cumulatively to be enough for such switching. The state set is determined by the direc~ion of current flow through the word line.
A similar scheme can be used for operating in the transverse mode with the easy axis in film~ 22 and 23 perpendicular to the easy axis direction in the longitudinal mode, i.e. parallel to word line 13. The magnetization vector then points in one direction or ' - : . ' ` :
: ' ' ,: ' ~ . ' ~ - ' .
the other along this easy axis to determine the state of ~it structure 10. Current is supplied along the word line sufficient to cause a magnetic field which rotates the magnetization vector to an angle from the easy axis. Current supplied along the sense line will then cause the rotation to increase when the bit structure is in one state and decrease when it is in the opposite state leading to a detectable difference in the electrical resistance in layers 23 and 24 because of the magnetoresistive effect. Again for non-destructive readout, these currents must be ~mall enough that the magnetic fields generated thereby cannot exceed the critical field level determined by the astroid for bit structure 10. Again, for setting the state for the bit structure, larger currents are applied along the sense and bit lines with the direction of the current on the sense line determining the state occurring in the bit structure.
To achieve either of the foregoing operations, however, bit ~tructures 10 must be carefully constructed. 5mall ferromagnetic ~hin film portions are subject to very high demagnetizing fields because the effective "free poles'l are closer together along the edges of the film leading to larger demagnetizing fields. This can be seen since the widths of bit structure 10 in the direction of word , --lg--line 13 will be on the order of 0.1 mic~on to, at most, a ~ew microns. Such dimensions are necessary to meet the high density requlrement for such bit structures to keep cost~ low on a cost per sell basi~, and because small curren~s in the sen~e or word lines allow faster switching. Such currents can be kept small only if the thin film portions are also small.
A ferromagnetic thin film portion typical of the kind being considered here, tha~ is, 1000 A thick and in the form of a square 2 microns on a side, can be approximated by an inscribed elipsoid for purposes of calculating its demagnetization field. Assuming that the thin film ~quare has a saturation magneti-zation value of M~, a thicknes~ of T, and a major axis of length 2r, the following equation applles for the demagnetization fi~ld ~D in the corresponding inscribed elip~oid:
TM
HD ~ 4 r If the satura~ion magnetization is around 10,000 Gau~, the uniform demagnetizing field will be on the order of 785 Oersteds, a field ~trength which i~ two orders of magnltude larger ~han ~ypical ani~otropy field strengths HK in such ferromagnetic thin films.
These demagnetizing fields would undoub~edly dominate the behavior of sush a film portion and result in ', ' ' ' '' ' ~ , ' .
instability in the magnetization of such a film in the sense that the magnetization would be forced from lying entirely along the easy axis to lying at least in part in some other direction even in the absence of external fields.
Aqain, the "sandwich" arrangement for the bit structure in Figure 2 provides aid in this situation because the demagnetizing field~ in each of films 22 and 23 act to cancel one ano~herr Nevertheless, very larqe uncancelled field~ will stlll occur because of the drop in field strength with di~tance given the separation between film. 22 and 23. Even if a large fraction of the magnetizing fields are cancelled, the remaining uncancelled portions of two rather large demagnetizing fields can ~till be on the order of anisotropy field strengths ~R leading to the kinds o~
instabilities in the magnetization of the devices as indicated above.
Con~ider in Figure 3 a diagrammatic cross ~ection of bit structure 10 o~ Figure 2 p~rallel to word line 13 where the cross section view in Figure 3A
shows ferromagnetic thin films 22 and 23 and separating film 24. This cross section is taken relatively far from either of juncture interconnec-tions 11. Insulating layers~ protective layers,semiconductor material body substrate and the like are ,: ~
.. .. .
.. ~ ' ,. . .
, ' . ~ ' '~L,s~
ignored and therefore omitted from the cros~ section of Figure 3. Each of ferromagnetic thin films 22 and 23 are shown in Figure 3 to have a thickness designated TF while the separating film 24 which is free of any ferromagnetic properties is shown to have a thickness Ts. The transverse operating mode has been chosen here, so tha~ the easy axis in each film is parallel to word line 13. The magnetization shown for each film, M22(x) and M23(x), are both shown as a function of x for rea~ons to be described, and are shown in oppocite directions along the x axis which is an arrangement that minimizes the magne ic energy.
Near the edges of films 22 and 23, anisotropy fields are dominated by the demagnetizing field~ due to the "free poles" at the edge~. If the magneti-zations of films 22 and 23 were saturated, the demagnetizing ields would approach MS/2 in the films or about 5,000 Oersteds for films with tha alloys described here. Typical films of these alloys will have a coercitivity and an anisotropy field in the order of only 20 Oersteds, leading to instabil- ities in the magnetization at the edges of these films.
In such large demagnetizing fields, electron spins at the edge of the ~trip are constrained to lie nearly parallel to the long dimension of these films, i.e. along the z axis. The direction of these electron spin~ only gradually turn to pointing across the films further inward toward the center of the film~ where the demagnetizing fields are no longer overcoming the anisotropy field. The rate, shape and distance of occurrence are all a complex function depending on magnetostatic~ of the situation, the ~uantum exchange interaction between adjacen~ atom electron ~pin I and anisotropy considera~ions not unlike tho3e leading to ~eel walls.
lo This is reflected in the simplified graph in ~igure 3B below the film~ shown in Figure 3A where the magnetiza~ion along the x axi~ is shown to be at zero at the outer edges of the films and gradually increasing towards the interior of the films to the saturation value Ms occurring in the central portion~
of the film~ for interior film width distance S. In the region~ of width D between the exterior edgeR of the film and the point where magnetic saturation begins, the magnetizations are in transition from pointing along the z axis to along the x axis.
Detailed analysis has shown that for films of the kind being considered here having a film thickness of 150 A (TF) separated by 50 ~ (~S) leading to an anisotropy field ~R f 25 Oersteds, that distance D is about 0.4 microns in a 2 micron wide film, A film only 1 micron wide, would saturate for only about 0.2 microns in the central regions of the film, and therefore the x directions magnetization would be only marginally stable.
Therefore, films with easy direc~ions on the transverse axis do not truly saturate across the films. Furthert films that are thicker or having greater separation therebetween, or both, are found to have even less of the central interior regions o~ the strip in magnetic saturation along the x axis leading to even less ~tability.
The uncancelled field in one film, that is, the magnetic field in one of films ~2 and 23 due to the demagnetization fields in Pach of these films can be found from again using inscribed elipsoids in the x-y plane of the cross sections of films 22 and 23 in Figure 3 as a ba~is for such a determination. The uncancelled field in the chosen film can be found by f inding the field at the midpoint of such a film because the average effect of field in the film can be approximated as the field occurring at such midpoint.
This estimate of the average effect of field through the film is reasonable because the exchange in~eractions between the electron spins of adjacent atoms in the film are so strong through the thickness of a very thin ferromagnetic film that the spins of -2~-electrons on such adjacent atoms throughout this thickness are constrained to align within a few degrees of one another.
These in~cribed elipRoid~ have their major axes along the wid~h of films 22 and 23 of a length equal to w or 2r. The magnetization~ of the films are saturated inside th se elipsoids. The center of one film is separated from the other by the distance TF
Ts.
The uncancelled field in one film, ~unca~ -can be written:
uncan HDg ( (Tf + Ts) /r), where ~D is the demagnetizing field occurring within one of ~he elipsoids in one of the films due to just the surface poles of that film, and g, as a function of the argument (Tf + Ts)/r, is a cancellation factor arlsing becau~e of the action of the demagnetizing ield of the other film opposing the demagnetizing field of the fir~t film. In the situation where Tf +
Ts is much less than r, the uncancelled field has been found to satisfy the approxi~ation:
HUncan ~ HDg((Tf + Ts)/r Tf ~ T~3 ~ HD[2.4 ( r ) ~ .
Experiment has confirmed the accuracy of this approximationO
The value of the anisotropy magnetic field HK
in ferromagnetic thin films of the nature being considered here is typically 10 to 30 Oersteds determined primarily by tbe cbemical composition of the film but al~o depending on various other parameters such as angle and deposition of the film on the substrate, the substrate temperature, ani~otropic strainq resulting in the film, and the like. In any event, to keep the demagnetization fields in a film portion from dominating the anisotropy field too great an extent, the uncancelled portions of he demagnetizing field~ in the film should be kept in the range from 2 to 6 Oer~teds. That i8, the ratio of HUncan to ~R is a measure of the stability of the magnetization. The ratio is unacceptable with a value o~ 1 and one can be quite confiden~ of a value of 10 so an intermediate ratio value such as 5 is a reasonable choice.
To determine then the permitted thickneqses for the films in the bit structure 10 of Figure 2, the first equation above and the last equation above can be combined and a choice for the uncancelled magnetic fields of 3 Oersteds or less leads to the following in equality:
(- f 9) ~2 4( f s)] < 3 '~L~
If w in Figure 3 i9 taken to be 2 microns, and the saturation magnetization in films 22 and 23 is again taken to be 10,000 Gauss~ thi~ inequality can be rewritten as follows:
Tf(Tf + Ts) < 16, 0000 This last inequality is sufficient for ju~t the conditions assumed in~ofar as a desirable width for bit structure 10 t and the material composition leading to the magnetixation saturation used in reaching thi~
inequality. Further, the 3 Oersted limit on acceptable uncancelled demagnetization fields is somewhat arbitrary. In other design situations, then, another inequality would be used.
Nevertheless, values of permitted thin film thicknesses allowing stable magnetizations in films 22 and 23 in ~his design situation are of significance in illustra~ing the acceptable ranges of values. They can be obtained from thi~ la-~t inequality on assuming one o~ the thickness value~. If ~he thickness of 20 separating film 24 is chosen to have a thickness of 50 A, then the ferromagnetic films 22 and 23 mu~t be less than abou~ 105 A in thickness.
This thickne~s choice or separa~ing film 24 is not an unreasonable choice in that the film needs 25 only to be thick enough to break the exchange interaction coupling between electron spins in atoms at the edges of each of ferromagnetic film layers 22 and 23. Typically, a separation on the order of 10 angstroms is sufficient to eliminate such exchange coupling between layer~ 22 and 23. The material for layer 24 has been chosen to be an in~ulating film, silicon nitride, which give~ good fabrica~ion process results. The choice of a conductor, although partially shorting the magnetoresi~tive respon~e signal, still has the advantage of shorting layers 22 and 23 together CO that the sense current flowing in ~hese layers i8 distributed more uniformly there-between, particularly if there is a defe~t in one of the other layer along the current paths. A further alternative, the material in layer 24 could in some situations be either a ferromagnetic material or a ferrimagnetic ma~erial if there is ~ufficient exchange interaction mismatch with layers 22 and 23 to prevent the exchange interaction from coupling therebetween.
With thi3 latter choice of materials, ~he possible use of an outer magnetic material covering on the sides 26 could be eliminated as flu~ closure could be provided by this chosen material for layer ~4.
Thus, the thickne~s chosen for intermediate layer 24 is a reasonable one and leads to rather thin ferromagnetic films for layers 22 and 23 in this design example. In practice over the range of ~cceptable designs to give sufficient bit structure density and operating rapidity for a digital memory, the thickness of films 22 and 23 should be less than 300 A, and preferably less than 200 A. The thickness in these situations of intermediate layer 24 should be less than 100 ~. -Restricting the thickness of ferromagnetic films 22 and 23 ~ufficiently to achieve a relatively low uncancelled demagnetizing field in each is a good design practice because there may be further demagnetization fields arising in a practical design not accounted for in the foregoing analysi For instance, there will be some demagnetizing fields occuring along the z axis in films 22 and 23 which would also be reduced by a restricted film thickness.
Further, while film~ 22 and 23 have been shown to be of comparable thickness and width, this may not necessarily be the best design in each situation~
Further, a different alloy material for each of films 22 and ~3 may be desirable in some design situations.
These sorts of differences may lead to additional demagnetizing field strengths which also would be reduced by limiting the thickness of such ~ilms.
Bit structure 10 can very readily be provided in a monolithic integrated circuit chip. Because bit structure 10 is formed on insulating layer 21 and "
.
;'~ .
--~29 -would have only insulating protective layers thereover, and because none of these layers are magnetically permeable, bit structure 10 can be designed without reference to the integrated circuit structure environment. Further, the interconnections between the digital memory and the remaining portions of the integrated circuits can be provided by the normal integrated circuit fabrication process steps for providing interconnections.
Only the memory cell array construction steps need to be added to the normal steps used for abricating monolithic integrated circuits. In some circumstances, the additional steps needed to construct ths memory array can be integrated with already existing monolithic integrated circuit fabrication process steps to minimize or possibly eliminate addi~ional fabrioation steps in providinq the digital memory on a monolithic integra~ed circuit chip.
Claims (22)
1. A magnetoresistive sensing, ferromagnetic thin film based digital memory, said memory comprising:
a first storage line structure having a first storage line pair of end terminals adapted to conduct electrical current in at least one direction, said storage line end terminals having electrically connected in series therebetween a plurality of bit structures with each said bit structure electrically connected at a bit juncture to at least one other said bit structure, each said bit structure to comprise at least a structure comprising:
an intermediate layer of a kind of separating material, said intermediate layer having two major surfaces on opposite sides thereof such that said major surfaces are separated by less than one hundred Angstroms of said separating material but sufficiently separated for said kind of said separating material to prevent any exchange interaction from coupling thereacross, and a memory film on each of said intermediate layer major surfaces with said memory film being of a thickness less than three hundred Angstroms and of a magnetoresistive, anisotropic ferromagnetic material, and a plurality of word line structures each having a pair of word line end terminals adapted to conduct electrical current in at least one direction, each said pair of said word line end terminals having an electrical conductor electrically connected therebetween which is located across an electrical insulating layer from said memory film on one of said major surface of said intermediate layer of a selected one of said bit structures.
a first storage line structure having a first storage line pair of end terminals adapted to conduct electrical current in at least one direction, said storage line end terminals having electrically connected in series therebetween a plurality of bit structures with each said bit structure electrically connected at a bit juncture to at least one other said bit structure, each said bit structure to comprise at least a structure comprising:
an intermediate layer of a kind of separating material, said intermediate layer having two major surfaces on opposite sides thereof such that said major surfaces are separated by less than one hundred Angstroms of said separating material but sufficiently separated for said kind of said separating material to prevent any exchange interaction from coupling thereacross, and a memory film on each of said intermediate layer major surfaces with said memory film being of a thickness less than three hundred Angstroms and of a magnetoresistive, anisotropic ferromagnetic material, and a plurality of word line structures each having a pair of word line end terminals adapted to conduct electrical current in at least one direction, each said pair of said word line end terminals having an electrical conductor electrically connected therebetween which is located across an electrical insulating layer from said memory film on one of said major surface of said intermediate layer of a selected one of said bit structures.
2. The apparatus of claim 1 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
3. The apparatus of claim 1 wherein an easy axis of magnetization of said memory film in at least one of said bit structures substantially parallels a center line of said intermediate layer where said center line has an end point at each said bit juncture.
4. The apparatus of claim 1 wherein an easy access of magnetization of said memory film in at least one of said bit structures is substantially perpendicular to a center line of said intermediate layer where said center line has an end point at each said bit juncture.
5. The apparatus of claim 1 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that there are two separate films with one of said separate films on each of said major surfaces.
6. The apparatus of claim 1 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that ferromagnetic material substantially surrounds said intermediate layer at least where said bit structure is free of a said bit juncture.
7. The apparatus of claim 3 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that there are two separate films with one of said separate films on each of said major surfaces.
8. The apparatus of claim 3 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that ferromagnetic material substantially surrounds said intermediate layer at least where said bit structure is free of a said bit juncture.
9. The apparatus of claim 4 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that there are two separate films with one of said separate films on each of said major surfaces.
10. The apparatus of claim 4 wherein said memory film on each of said major surfaces of said intermediate layer of at least one of said bit structures is arranged such that ferromagnetic material substantially surrounds said intermediate layer at least where said bit structure is free of a said bit juncture.
11. The apparatus of claim 5 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
12. The apparatus of claim 5 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
13. The apparatus of claim 6 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
14. The apparatus of claim 6 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
15. The apparatus of claim 7 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
16. The apparatus of claim 7 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
17. The apparatus of claim 8 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
18. The apparatus of claim 8 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
19. The apparatus of claim 9 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
20. The apparatus of claim 9 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
21. The apparatus of claim 10 wherein said memory film on said major surfaces of said intermediate layer of each of said bit structures is of a thickness less than two hundred Angstroms.
22. The apparatus of claim 10 wherein a second storage line structure is provided substantially matching said first bit line structure including that structural relationship with said plurality of word line structures.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/870,068 US4780848A (en) | 1986-06-03 | 1986-06-03 | Magnetoresistive memory with multi-layer storage cells having layers of limited thickness |
| US870,068 | 1986-06-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1284382C true CA1284382C (en) | 1991-05-21 |
Family
ID=25354735
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000538580A Expired - Lifetime CA1284382C (en) | 1986-06-03 | 1987-06-02 | Magnetic memory |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4780848A (en) |
| EP (1) | EP0248355B1 (en) |
| JP (1) | JPS6342089A (en) |
| CA (1) | CA1284382C (en) |
| DE (1) | DE3778065D1 (en) |
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-
1986
- 1986-06-03 US US06/870,068 patent/US4780848A/en not_active Expired - Lifetime
-
1987
- 1987-05-29 EP EP87107788A patent/EP0248355B1/en not_active Expired
- 1987-05-29 DE DE8787107788T patent/DE3778065D1/en not_active Expired - Lifetime
- 1987-05-29 JP JP62134734A patent/JPS6342089A/en active Granted
- 1987-06-02 CA CA000538580A patent/CA1284382C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| US4780848A (en) | 1988-10-25 |
| EP0248355B1 (en) | 1992-04-08 |
| JPH0444352B2 (en) | 1992-07-21 |
| JPS6342089A (en) | 1988-02-23 |
| DE3778065D1 (en) | 1992-05-14 |
| EP0248355A3 (en) | 1990-03-28 |
| EP0248355A2 (en) | 1987-12-09 |
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