US 3458426 A
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Description (OCR text may contain errors)
July 29, 1969 w. v. RAUSCH E 3,458,425
SYMMETRICAL SPUTTERING APPARATUS WITH PLASMA CONFINEMENT I Filed May 25, 1966 4 Sheets-Sheet l 1N VEN TQRS VV/L L MM ERA UJCH 1 y/4R THUR CALDERONJR.
ATTQRNEYJ July 29, 1969 w. v. RAUSCH ET AL 3,458,426
' SYMMETRICAL SPUTTERI NG APPARATUS WITH PLASMA GONFINEMENT Filed May 25, 1966 4 Sheets-Sheet 2 l N VliN '1 OR) I/V/L L MM V164 UJCH ART/10R (AL oERo/VJR.
AT ToR/vEKr y 969 w. v. RAUSCH ET AL 3,458,426
SYMMETRICAL SPUTTERING APPARATUS WITH PLASMA CONFINEMENT 4 Sheets-Sheet 5 Filed May 25, 1966 IN vislvwm l/V/LL 1AM 14/64 use/4 BY ARTHUR CALDERON Jk.
A T TORNE Kr y 9, 1969 w. v. RAUSCH ET AL SYMMETRICAIJ SPUTTERING APPARATUS WITH PLASMA GONFINEMENT Filed May 25, 1566 4 Sheets-Sheet 4 R U M mxm N HAD WRI. LKQ
MR Mu 5 MR United States Patent 3 458 426 SYMMETRICAL sPUTrERiNG APPARATUS WITH PLASMA CONFINEMENT William V. Rausch, Minneapolis, and Arthur Calderon,
Jr., Minnetonka, Minn, assignors to Fabri-Tek Incorporated, Edina, Minn., a corporation of Wisconsin Filed May 25, 1966, Ser. No. 552,917
Int. Cl. 'C23c 15/00 US. Cl. 204-298 19 Claims This invention is concerned with deposition apparatus, and more particularly with improved structure for the deposition of thin films on substrate materials.
Conventional processes for the deposition of thin films use either thermal evaporation or low energy sputtering of the desired materials with subsequent condensation of the sputtered material on a glass or other material substrate.
These two methods have various advantages and disadvantages, well known to those skilled in the art. One of the primary limitations of both methods has been the inability to deposit uniform films over very large substrate areas, which would reduce the individual substrate cost. The apparatus of this invention enables low energy sputtering whereby a large number of substrates can be processed in a single cycle of operation, thus reducing the individual substrate cost while simultaneously maintaining or improving the composition control, film thickness uniformity, and deposition rate. These improvements are made possible by a cylindrical arrangement of the sputtering fixtures, by incorporating symmetry of the fixture arrangement, and by utilizing a magnetic field for control of the sputtered plasma. The magnetic field is also used as an orienting field when thin film magnetic memory elements are deposited. Prior art sputtering apparatus has generally been of a planar arrangement or with a rodshaped target with substrates held on each of four sides of the target.
Briefly described, the apparatus of this invention comprises a deposition chamber formed by a cylindrical jar. Upper and lower plates are attached to the jar, and each of the upper and lower plates carries apparatus for evacuating the chamber, filament members, apparatus for inserting the gas into the evacuated chamber, and feedthroughs for various electrical, mechanical and cooling apparatus used in the deposition process. A pair of modified Helmholtz coils are wound around the outside of the cylindrical jar, including a control coil connected between the pair. A plasma restrictor is mounted inside and arouind the longitudinal axis of the deposition chamber. An annular anode member is mounted substantially at the center of the chamber, around and transverse to the plasma restrictor. The anode member substantially divides the chamber into an upper and a lower section. Mounted in the upper and lower sections, in substantial mirror symmetry around the anode member, are a pair of cylindrical target members, a pair of cylindrical control target mem bers, a pair of right polygonal prism shaped substrate holders, and pairs of substrate heating and cooling apparatus.
The mirror symmetry around the annular anode, and the substantially cylindrical shape of the substrate holder, that is the right polygonal prism shape, enable the deposition of films on a high plurality of substrates during a single evacuation cycle. Further, the use of the plasma restrictor and the magnetic field set up by the Helmholtz coils promotes etficient control of the plasma available for deposition.
In the drawings:
FIG. 1 is a planar external view of an embodiment of the deposition apparatus of this invention;
3,458,426 Patented July 29, 1969 ice FIG. 2 is a sectional view taken along the section line 2--2 of FIG. 1;
FIG. 3 is an enlarged view of a portion of the drawing of FIG. 2;
FIG. 4 is an orthographic view of a substrate holder forming a portion of the apparatus of this invention;
FIG. 5 is a schematic representing the electrical connections of the sputtering apparatus of the embodiment of this invention; and
FIG. 6 is a schematic representing the electrical interconnection of magnetic coils used in the apparatus of this invention.
In FIG. 1 there is shown a cylindrical jar 10, covered at one end by a plate 11, and covered at the other end by a plate 12. Plate 12 is connected through a throat 17 to a flange connection 14. Flange connection 14 is also connected to a member 15 containing baffies and valves. Member 15 is connected to a diffusion pump 16. Plate 11 is connected through a throat 18 to a flange connection 19. Flange connection 19 is also connected to a member 21 containing battles and valves. Member 21 is connected to a diffusion pump 22. Three magnetic coils 24, 25 and 26 are shown wound around jar 10.
In FIG. 2 there is shown a sectional view of jar 10, including coils 25 and 26, plate 12, and a portion of throat 17. It can be seen that the connection of jar 10 and plate 12 is sealed by an L-seal gasket 13. Within jar 10, there are shown a pair of plates 28 and 29, mounted in parallel spaced relation on plate 12 by a plurality of bolts such as 31 and 32. Mounted on plate 29' is a plasma restrictor 30, here shown in the form of a sealed glass cylinder. Mounted on plate 28 is an annular radiation shield 34, and within shield 34 is mounted a circular filament 35. A set of cooling coils 38 is mounted below plate 28.
There is also shown mounted on plate 28 a shutter 37, here shown as a movable or telescoping shutter. A tube 39 is shown passing through throat 17 and having one end opening into jar 10. The other end of tube 39 is connected to a source of gas, such as a noble gas (not shown). Immediately surrounding plasma restrictor 30, there is shown a symmetrical primary target 41. Around target 41 there is shown a helical control target 42. Around target 42 there is shown a substrate holder 45. Around substrate holder 45 there is shown a cylindrical substrate heater 46, and helically wound around 46 there are shown cooling coils 48.
Also shown, substantially at the center of the chamber formed by jar 10, is an annular anode 50 around plasma restrictor 30. Above anode 50, there are shown mounted a portion of further deposition apparatus, substantially in mirror symmetry to the above described apparatus which is mounted below anode 50. The further apparatus is represented by a primary target 51, a helical control target 52, a substrate holder 55, a substrate heater 56 and cooling coils 58.
It is to be understood that plate 11, though not shown in the drawings of FIGS. 2 and 3 carries substantially the same equipment as that shown carried by plate 12, including an upper portion of plasma restrictor 30. Thus plate 11 would carry another filament, another filament radiation shield, another telescopic shutter, and another tube from a source of gas. In throat 17 there is shown mounted a valve 59 for varying the amount of vacuum pressure from diffusion pump 16 shown in FIG. 1.
Referring now to FIG. 3, there is shown an enlarged sectional view of a portion of the apparatus of FIG. 2. Again there is shown jar 10 having wound thereon coils 25 and 26, mounted on plate 12, and sealed by gasket 13. This enlarged view more clearly shows the spaced relation of the various deposition apparatus including filament 35 in shield 34, shutter 37, primary target 41,
control target 42, substrate holder 45, substrate heater 46, and cooling coils 48. There is also shown a feedthrough 61 in plate 12 through which various electrical, mechanical and cooling inputs are fed to the proper apparatus.
In FIG. 4 there is shown an orthographic view of substrate holder 45. It can be seen in FIG. 4 that holder 45 is in substantially the form of a right polygonal prism, which is in itself substantially cylindrical, and is especially suitable for holding a plurality of substrates. The number of sides of the polygonal prism of FIG. 4 is merely exemplary of the substrate holder of this invention, and it is not intended that the apparatus of this invention be limited to the particular polygonal prism shown.
In FIG. 5 there is shown an electrical wiring diagram disclosing the interconnections of the filaments, anode, and targets of this invention. A source of AC power 65 is connected across filament 35 which is mounted on plate 12. One end of filament 35 is connected to a common ground. Another source of AC power 66 is connected across a filament 36 which is mounted on plate 11. One end: of filament 36 is also connected to the common ground.
A variable source of DC power 70 is shown having a positive terminal connected to anode 50'. A negative terminal of source 70 is connected through a variable impedance 71 to cylindrical primary target 41. The negative terminal of source 70 is connected through a variable impedence 72 to cylindrical primary target 51. A variable source of DC power 75 is shown having a positive terminal connected to anode 50. Source 75 has a negative terminal connected through a variable impedance 76 to helical control target 42. The negative terminal of source 75 is connected through a variable impedance 77 to helical control target 52. The positive terminal of source 75 is connected through a resistor 81 and a variable source of DC power 80 to the common ground. The dotted lines 63 of FIG. 5 represent the plasma field set up in the chamber formed by jar 10.
In FIG. 6 magnetic coils 24 and 26 are shown serially connected across a source of DC power 84. Control coil 25 is shown connected across another source of DC power 83.
From the above discussion of FIGS. 1-6 and the physical relations of the apparatus shown therein, it will be apparent that the apparatus of this invention possesses double or mirror image symmetry with respect to an imaginary plane passed through annular anode 50, as seen best in FIG. 2. This symmetry is particularly advantageous because it allows deposition on twice as many substrates, while simultaneously, when used in conjunction with the variable magnetic field set up by coils 24, 25 and 26, and with plasma restrictor 30, it maintains uniformity of film thickness and deposition rate.
For a better understanding of the operation of the apparatus of this invention, it should be understood that in normal engineering terminology the process called low energy sputtering specifically means the degradation of a surface by ionic bombardment. Deposition by sputtering refers to the removal of target or source material by impinging gas ions, generally noble gas, and subsequent condensation of this material upon substrates.
Primary targets 41 and 51 may be composed of a standard alloy. Control targets 42 and 52, which are here shown as helical but may be a mesh or the like, surrounding the respective primary targets 41 and 51, can be of a similar composition with a variance in the ratio or percent of the alloy materials. With reference to FIG. 5, cylindrical primary targets 41 and 51 are maintained at a negative voltage by variable source 70. Variations of the voltage determine, to some extent, the sputtering rates of the alloy of targets 41 and 51, which are the primary source of the deposited alloy material. Helical control targets 42 and 52 are maintained at a negative voltage somewhat less than the potential of primary targets 41 and 51, by source 75. This voltage determines the sputtering rate of the alloys of the control targets. Proper adjustment of sources 70 and will allow deposition of films of compositions within a range determined by the alloys of the primary and control targets. A particular composition of film may be determined experimentally. This control over the resultant film composition is highly desirable, as commercially available high purity alloys of precise composition are prohibitively expensive for most practical thin film devices. Most any desirable alloy film can be deposited using the apparatus of this invention.
In particular, magnetic thin films used for high speed computer memories must exhibit a property called uniaxial anisotropy. that is, the films must have an easy and a hard axis of magnetization. This anisotropic property is induced in magnetic films by the magnetic field supplied by the modified Helmholtz coils 24 and 26. These coils, in conjunction with control coil 25, and plasma restrictor 30, are also used to control the density and density-variation of the plasma.
Referring to FIG. 6 it will be apparent that modified Helmholtz coils 24 and 26 are independently energized by source 84. The distance between coils 24 and 26 is mechanically variable, and this distance, in conjunction with the current in control coil 25, which is separately energized by source 83, and the currents in coils 24 and 26, will determine the shape of the resultant magnetic field. In general, if the distance between coils 24 and 26 is equal to their radius, the magnitude of the field at the anode end of primary targets 41 and 51 is approximately one-half of the magnitude of the field at the filament end of the primary targets. This variation of magnetic field magnitude will tend to pinch or reflect the plasma 63 toward the anode end of targets 41 and 51, thus reducing the ion density gradient. It is necessary that the density of the plasma in the area of targets 41 and 51 be uniform if uniformity of thickness of the resultant films is to be maintained. Control coil 25 is energized for additional control over the shape of the magnetic field. This controls the plasma density distribution, because the magnetic field from control coil 25 will tend to increase the field magnitude at the anode end of targets 41 and 51 to a greater extent than at the filament end of targets 41 and 51. The spacing between coils 24 and 26, the current in coils 24 and 26, and the current in coil 25 can be established for optimum film thickness uniformity by experiment. Once these factors are established, the average density of plasma 63 in the target area can be controlled.
While maintaining the ratio of the currents in coils 24 and 26, and coil 25, to assure film thickness uniformity, the magnitude of the magnetic field in the area of targets 41, and 51 is varied by varying the currents in coils 24, 25 and 26. Note that the optimum ratio of these currents is maintained a constant regardless of current magnitudes. Because the cylindrical plasma restrictor 30 inhibits gas ionization at the center of the chamber formed by jar 19, increases in currents in Helmholtz coils 24 and 26, and control coil 25, will tend to compress the plasma 63 in the target area, thus increasing the average density of plasma 63 while maintaining uniformity of the density. The sputtering rate is a function of the plasma density, therefore, the current settings for optimum deposition rates are determined experimentally for each material to be deposited. The sputtering rate is also a function of target voltage, which must be considered simultaneously.
Assuming that optimum primary and control target voltages, and optimum plasma density variation, have been established as described above, the deposition procedure can be commenced. First, substrates are firmly mounted on substrate holders 45 and 55. The vacuum chamber in jar 10 is then sealed, pumps 16 and 22 are turned on, and the chamber pumped down to a high degree of vacuum (for example, 10 mm. Hg or less). Substrate heaters 46 and 56, and filaments 35 and 36, are energized sufficiently to out-gas impurities. Filament power, by means of sources 65 and 66, is then increased for optimum thermionic emission from filaments 35 and 36, and the filament temperature is stabilized. A positive potential is then applied to anode 50. Note source 80 of FIG. 5, and also note that filaments 35 and 36 are connected to the common ground.
With the positive potential on anode 50, valve 59 in throat 17, and a similar valve in throat '18, are moved to throttle down the vacuum pump inlet. A gas, usually a noble gas such as argon, is bled in through tube 39 and a similar tube in plate 11, at a flow rate substantially equal to the throttled pumping rate. These rates are adjusted such that the gas pressure in the chamber formed by jar is, for example, 1 to 10 microns. Under these conditions, thermionically emitted electrons ionize the noble gas atom by collision in transit to anode 50 from filaments 35 and 36, and a dense plasma 63 is initiated. Large potentials, negative with respect to anode 50, are then applied to primary targets 41 and 51 and control targets 42 and 52. The targets then act like large negative Langmuir probes, immersed in the plasma, and space charge sheaths are formed adjacent to the various target surfaces. The positive ions of the plasma are accelerated through the space charge sheaths and as they impinge on the target surfaces, sputtering occurs. During the entire above described operation, shutter 37, and a similar shutter attached to plate 11, are extended to cover substrates on substrate holders 45 and 55. Thus, as the ions impinge on the target surfaces, the targets are sputtered clean, with the shutters such as 37 shielding the substrates.
When adequate time has been allowed for removal of the contaminating surface layer on the targets, shutter 37 and the corresponding shutter in plate 11 are removed to allow the sputtered particles to be deposited on substrates held by substrate holders 45 and 55. If desired, a higher potential may be used during cleaning to speed up the decontamination process.
The number of sputtered atoms per impinging incident ion is proportional to the energy of the impinging ion for the normal operating target potentials. This energy is very nearly determined by the potential between targets 41, 51, 42, and 52 and anode 50. Thus, the composition of the resulting deposited film can be controlled by merely adjusting target voltages.
It should be noted that cylindrical plasma restrictor 30 is sealed, thus reducing the volume of gas that must be pumped into jar 10 during each deposition cycle and thus improving the over-all economic efiiciency of the apparatus of this invention. Further, the only major limitation on the size of the apparatus of this invention, and thus the number of substrates available for deposition during each cycle, would be the mechanical construction and electrical isolation problems. Cooling coils 48 and 58, which surround, respectively, substrate heater assemblies 46 and 56, serve the double function of minimizing the amount of radiant energy reaching jar 10, and rapidly cooling the deposition apparatus after completion of a deposition cycle.
From the above description of the structure and operation of the apparatus of this invention, it is apparent that the use of an annular anode, around which deposition apparatus is placed in substantial mirror symmetry, and the use of a centrally located plasma restrictor to decrease the volume of the deposition chamber, along with the use of magnetic fields, results in efiicient and high quality deposition apparatus which allows the deposition of films on substrates with greater quality control and significantly lower cost.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. Apparatus for the deposition of films on a substrate comprising:
a vacuum chamber;
evacuating means connected to said vacuum chamber;
anode means mounted within said vacuum chamber and substantially dividing said vacuum chamber into an upper chamber and a lower chamber;
first and second filament means mounted, respectively, in said upper and lower chambers and in substantial mirror symmetry;
first and second target means mounted, respectively, in said upper and lower chambers and in substantial mirror symmetry;
first and second substrate holding means mounted, re-
spectively, in said upper and lower chambers and in substanital mirror symmetry;
first and second movable shutter means mounted, re-
spectively in said upper and lower chambers and in substantial mirror symmetry;
filament power supply means connected to each of said filament means;
target power supply means;
means connecting said target power supply means and said anode means in electrical circuit with each of said target means;
means mounted adjacent said upper and lower chambers for creating a magnetic field generally parallel to the axis of the discharge therein;
a source of gas particles and means for directing said gas particles into said vacuum chamber; and
said gas particles becoming ionized in said vacuum chamber to impinge upon said target means for sputtering metal particles ofi said target means for deposition on substrates mounted on said substrate holding means, said shutter means shielding the substrates until said target means are sputtered clean and then moving to allow deposition on the substrates.
2. The apparatus of claim 1 including:
first and second control target means mounted, respectively, in said upper and lower chambers and in substantial mirrer symmetry.
3. The apparatus of claim 1 in which:
said vacuum chamber has cylindrical walls;
said filament means are circular;
said target means are cylindrical and mounted with the longitudinal axis substantially on the vacuum chamber longitudinal axis; and
said substrate holding means are substantially right polygonal prisms and mounted with the longitudinal axis substantially on the vacuum chamber longitudinal axis, surrounding and in spaced relation to the respective target means.
4. The apparatus of claim 3 including:
first and second cylindrical control target means;
control target power supply means;
means connecting said control target power supply means and said anode means in electrical circuit with each of said control target means; and
said first control target means mounted in said upper chamber between and in spaced relation to said first target means and said first substrate holder means;
said second control target means mounted in said lower chamber between and in spaced relation to said second target means and said second substrate holder means.
5. The apparatus of claim 3 including:
first and second annular shield means mounted, respectively, around said first and second filament means.
6. The apparatus of claim 3 in which said movable shutter means are telescopic and are mounted to shield the substrates on the respective substrate holder means from the respective target means in the extended position.
7. The apparatus of claim 3 in which said means for creating a magnetic field in said vacuum chamber compr1se:
a pair of modified Helmholtz coils wound around the cylindrical walls of said vacuum chamber, in spaced relation to one another;
a first source of energy;
said pair of coils and said first source of energy connected in electrical circuit;
a control coil wound around the cylindrical walls of said vacuum chamber between said pair of coils; and
a second source of energy connected in electrical circuit with said control coil.
8. The apparatus of claim 3 including:
first and second substrate heater means mounted, respectively, adjacent said first and second substrate holder means; and
cooling means mounted adjacent each of said substrate heater means and said filament means.
9. The apparatus of claim 3 in which said anode means is annular.
10. The apparatus of claim 9 including:
a cylindrical plasma restrictor mounted with the longitudinal axis along the longitudinal axis of said vacuum chamber and mounted within said annular anode means and said first and second cylindrical target means.
11. Apparatus for deposition of films by sputtering comprising:
a cylindrical vacuum chamber;
an annular anode mounted in said vacuum chamber, the plane of said anode substantially dividing said vacuum chamber into a first chamber and a second chamber;
a cylindrical plasma restrictor mounted within said vacuum chamber and extending through said annular anode, the longitudinal axis of said restrictor being the longitudinal axis of said vacuum chamber;
first and second cylindrical targets mounted, respectively, in said first and second chambers, the radius of said targets being greater than the radius of said restrictor, and the longitudinal axes of said targets being the longitudinal axis of said vacuum chamber;
first and second substrate holders in the form of right polygonal prisms mounted, repectively, in said first and second chambers, the radius of said holders being greater than the radius of said targets, and the longitudinal axes of said holders being the longi' tudinal axis of said vacuum chamber;
first and second cylindrical movable shutters mounted, respectively, in said first and second chambers, the radius of said shutters being greater than the radius of said targets and less than the radius of said holders, and the longitudinal axes of said shutters being the longitudinal axis of said vacuum chamber;
first and second circular filaments mounted, respectively, in said first and second chambers, the axes of said filaments being the longitudinal axis of said vacuum chamber;
means for providing a magnetic field in said vacuum chamber;
means connecting each of said targets and said filaments in electrical circuit with said anode;
evacuation means connected to said vacuum chamber;
a source of gas particles; and
means for providing said gas particles into said first and second chambers for ionization thereof to provide a plasma of ionized particles for sputtering of said targets.
12. The apparatus of claim 11 including:
first and second cylindrical control targets mounted, respectively, in said first and second chambers, the radius of said control targets being greater than the radius of said targets and less than the radius of said shutters, and the longitudinal axes of said control targets being the longitudinal axis of said vacuum chamber.
13. The apparatus of claim 12 in which said means for providing a magnetic field comprises:
first and second magnetic coils mounted in spaced relation around the external periphery of said cylindrical vacuum chamber, said coils connected in electrical circuit; and
a control coil connected in electrical circuit and mounted between said first and second coils around the external periphery of said cylindrical vacuum chamber.
14. Deposition apparatus comprising:
means defining a vacuum chamber;
anode means mounted in the chamber;
first and second filaments means mounted in the chamber in spaced relation and one on each side of said anode means;
first deposition means including target and substrate holder means mounted in the chamber between said first filament means and said anode means;
second deposition means including target and substrate holder means mounted in the chamber between said second filament means and said anode means;
means for providing gas particles into the chamber;
means connected to said anode, filament and target means for ionizing said gas particles in the chamber to sputter material off said target means for depositon on substrates held by said substrate holder means.
15. The apparatus of claim 14 including:
plasma restrictor means mounted in the chamber for decreasing the volume thereof, to increase the density of ionized gas particles.
16. The apparatus of claim 15 in which:
said anode means is annular in shape and mounted around said plasma restrictor.
17. The apparatus of claim 14 in which said first and second means include, respectively:
first and second movable shutter means for selectively shielding substrates on said substrate holder means from the sputtered material.
18. The apparatus of claim 17 in which:
said first and second deposition means are in substantial mirror symmetry about the plane of said anode means.
19. The apparatus of claim 14 including:
means for providing a magnetic field generally parallel to the axis of the discharge in the vacuum chamber, said means mounted on said means defining the vacuum chamber.
References Cited UNITED STATES PATENTS 3,293,168 12/1966 Schulz 204-192 OTHER REFERENCES Bertelsen et al.: IBM Technical Disclosure Bulletin, vol. 6, No. 11, April 1964, p. 45.
HOWARD S. WILLIAMS, Primary Examiner SIDNEY S. KANTER, Assistant Examiner US. Cl. X.R.