US 3887451 A
A method is disclosed for sputtering epitaxially a layer of stoichiometric garnet composition from a single target wherein the target is composed of a mixture of the separate components of the sputtered layer.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Cuomo et al.
. 5] June 3, 1975 Sadagopan, Scarborough, all of N.Y.
 Assignee: International Business Machines Corporation, Armonk, NY.
221 Filed: Dec. 29, 1972 21 Appl. No.: 319,589
 U.S. Cl 204/192; 204/298  Int. Cl. C23c 15/00  Field of Search 204/192  References Cited UNITED STATES PATENTS 3,437,577 4/1969 Kay et a1 204/192 3,573,099 3/1971 Moore et a1. 204/192 3,607,698 9/1971 Kay et a1 204/192 OTHER PUBLICATIONS Primary Examiner-Oscar R. Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, or FirmBernard N. Wiener [57-] ABSTRACT A method is disclosed for sputtering epitaxially a layer of stoichiometric garnet composition from a single target wherein the target is composed of a mixture of the separate components of the sputtered layer.
lllustratively, both at a substrate temperature of approximately 450C and at another substrate temperature between 800-850C, there was obtained formation of a film of gallium substituted yttrium iron garnet (GazYlG). A target was made up of the desired stoichiometry with a mixture of the individual oxides pressed to 85% of the compounds theoretical density. Generally, the steps of the method are: (l) applying a radiofrequency bias to the substrate during sputtering to prevent the deposition of an easily resputtered component of the target; and (2) changing the power density to the target during deposition.
Specifically, a target was made up of a mixture of individual oxides Y O Ga O Fe O which was pressed to 8.5% of its theoretical density. Exemplary films of stoichiometric composition were obtained with a radio-frequency bias on the substrate in the range approximately from ground to 100 volts and with power density to the target in the range of approximately 5 to 65 wattslin The stoichiometric ratio for the composition was Y Fe Ga,O, where 0 X 3, whereas the ideal stoichiometric ratio of [Fe ,Ga /Y is 1.667. This procedure obtained stoichiometric ratios approximately in the range 1.4 to 1.6.
11 Claims, 7 Drawing Figures l Lsi 1 as I es T ATETFTFRJTTTQ I975 38873151 I 45 45 i 55 51 l T T\\\\'\\ T\\\FW 1. VACUUM 29 TARGET 2e SUBSTRATE 150 0 SUBSTRATE 15b M150 0. THERMALLY FLOATING GQEIIT 900C l I J FIG. 2
ANODE 25 5 4 l I H 5 .1 4 M 2 0 .m 4 7 m M 8 G 8 a 8 0% w 5 I e 1| 0 5 TO F 5 I M 5 E Q 5 d 8 VI G M n T A I 0 X F R 5 T E M T E /0 D l S m E 0 A I 4 T 2 W S 7 T 7 0 S 5 w P ML .1 N G c a M O 0 G E 2 R U I N O CL U 5 E S B U 1 5 ll A DI. F M EL Du M .1. C N m I 0 w TI. TAI S G D M M U 0 H M O M H N H F P as U W 8 DH PHI 0 N 0 R 0 A M l G M R A Ran L A T F. I ZJ TH. 2 I1 0 0 0 0 0 o 5 5 .71 9 n0 4 mEfimmzw 20 3 NEE 03 TIME (Hr$.)
mm? 2: 3 on 2 v 52:05;
' SHEET :25 2o EE; "E22 $5: a mass $2 I Z g wgws d v e 2231; H :2: 25:2: L 2
1 METHOD FOR SPUT'IZERING- GARNET COMPOUND LAYER BACKGROUND or THE .lNvliNTlON chiometric and the prior art accumulation rate was relatively poor. Target enrichment in the deficientmaterial has been described at: i
a. Applied Physics Letters, 15, 256 ('196 9 Tak'ei, e t al. The deficient material is added to the desiredcornpounded material and mixed with it. They used mixed oxide targets rich in bismuth oxide for'sp uttering single crystal bismuth titanate film/The yield differe r ice between the compound and the added deficient component is not predictablefFurther, the added presence of the excess material shifts the target composition so that many phases are present. I g ,7
b. IBM J. of Resand Dev. 13., 696 (T969), by Sawatzky, et al., discloses cation deficiencies in garnet films, principally iron whereinthe iron deficiencies are supplemented in the filmswithithe use of an auxiliary independenttarget of iron.,,ln greater detail .films of gadolinium iron garnet Gd Fe O were prepared by radio-frequency sputtering butwere found to have cation deficiencies, e.g., film depositedfroma stoichiometric garnet target atsubstrate temperatures of 500C had about a.25% iron.defi ciency,,while film deposited at 40C had about a 7% iron deficiency. The loss in iron was compensated by the auxiliary iron target so that stoichiometric composition garnet films were prepared. However, a composition gradient was naturally introduced alongthe film surface. Thelatter method has problems of n'onuniformity. Since the addition of the deficient iron is across the film surface, vi.tis somewhat difficult bythis latter technique=to produce stoichiometric film with large area Y c. U.S. Pat. No. 3,607,698 by Kay, et al., discloses and claims the method of making singlephase rare earth garnet thin films in a sputtering apparatus comprising the steps of:. maintaining a garnet substrate-hav ing a (1 l l orientation directedtoward'the sputtering flux at a temperature lessthan 500C; maintainingan atmosphere in said apparatus of at least IO percent pressure oxygen; sputtering material from a source containing said rare earth iron garnet in bulk form to said substrate; crystallizing the depositedfilm which si-- Sci. and Tech. 8, 512 1971') by J. L. vdssen:
thev film composition by radio frequency bias sputt'erin'gh'as been discussed at J. Vac.
" Although both mixed oxide targets and biasing of a growing filmduringsputtering have been discussed in the priorart literature, illustratively, the use of a target of an enriched mixture of oxides with power density on the-target,radio-frequencybias on the substrate, and substrate temperature being controlled interdependently for a given'accumulation rate has not been previously accomplished.
OBJECTS OF THE INVENTION vlt is an object of this invention to produce garnet compositions by sputtering; 7 It is another object of this invention to produce gar- 'net compositions in an amorphous form by radiofrequency sputtering on Gadolinium Gallium Garnet -('GGG) substrates in a variety-of orientations.
' It is yet another object to convert the amorphous garnet composition of the preceding object to epitaxial single crystal films-by heating.
It is anobject of this invention to produce amorphous garnet films at substrate temperatures less than 675C.
'lt is yet another object to prepare garnet compositions in amorphous form and in crystalline form from targets with compositions with excess of the resputtered species by adjusting the substrate temperature and radio-frequency bias on the substrate to produce stoichiometric garnet composition.
It is another object of" this invention to practice the preceding objectby a' method which includes starting with a target compound of a mixture of materials,
transporting these materials to an appropriate substrate where they are reacted to produce a garnet structure rapidly.
"It is an object ofthis invention to provide a method for growing an epitaxial layer of garnet by sputtering of the components thereof from a target to a substrate c'o'ntrollably bysetting interdependently the operasired layer according to the stoichiometry thereof varying' interdependently-the recited operational parameters'of the preceding'object.
It is another object of this invention to practice the preceding object by establishingthe' levels of 't he recited operational parameters for a given accumulation rate of thelayer at the substrate.
It is another object of this invention to produce garnet films with suitable properties for bubble domain applications.
It is another object of-thisinvention to provide the crystalline garnet film of the preceding object epitaxiallyin single crystalline form.
It is another object of this invention to' produce garnet film for magnetic bubble device by sputtering from a single target'where the film is produced initially in amorphous form-and is then converted to crystalline form. v 4 It isanother object of this invention to provide a method for p'roducing a layer with a given composition by "radio-frequency sputtering said layer onto a substrate wherein-there is included varying interdepenature; radib frequency power to said target, and radiofrequency bias on said substrate, to establish said given composition of said layer on said substrate.
SUMMARY OF THE INVENTION It has been discovered for the practice of this invention that yield, stoichiometry and accumulation rate of a sputtered garnet film can be controlled by interdependently varying magnitudes of radio-frequency power at the target, and both temperature and radiofrequency bias on the substrate. In general, it has been determined for practice of this invention that garnet structure so produced is suitable for bubble domain device.
In the practice of one aspect of this invention, a mixed oxide target is utilized. In the practice of another aspect of this invention, a stoichiometric compound garnet target may be utilized, but the accumulation rate is less than for comparable growth conditions for an unreacted target. Practice of another aspect of this invention utilizes an enriched mixed oxide target wherein the component normally deficient in the produced film is an enriched species. The enriched target is readily fabricated and the yield of the enriched species is predictable. The radio-frequency bias on the substrate removes the easily resputtered species from the growing film in a predictable way thereby producing the resultant stoichiometric garnet film.
Through the practice of this invention with mixed oxide component targets, desirable accumulation rate and stoichiometry of garnet film are obtained. Therefore, use of an unreacted mixture of the oxides as a target in radio-frequency sputtering obtains a substantial improvement in the rate of accumulation of the film compared to use of a reacted target of stoichiometric composition with time to grow garnet film thicknesses less by approximately onethird.
By a practice of this invention there is a method of producing stoichiometric garnet structure film where A is a rare earth element of the lanthanide series, e.g.,,Gd, Eu, Tb, Er; B is Y, La or Sc; C is Al or Ga; and D is Fe, Ni or C0, through the steps of: establishing a single source target; establishing radio-frequency power at target in the range of approximately 5 watts/- sq.in. to I watts/sq.in.; and establishing substrate temperature in the range approximately 0C to 1,000C. In greater detail, it is beneficial to utilize a target for practice of this invention which is a mixture of the components of said film and radio-frequency power at the target in the range of 2 watts/sq.in. to 23 watts/- sq.in. with a substrate temperature in the range of approximately 500C to 650C.
Specifically, for a grown garnet film of composition Y Ga Fe O where O y 3, the target is preferably a composite mixture of the oxides Y O Ga O and Fe O and is grown with radio-frequency bias on the substrate of less than approximately 200 volts d.c. level peak-to-peak, e.g., in the range of watts/sq. in. to I00 watts/sq. in. However, for some operational circumstance, the target may be of garnet structure.
In an illustrative example of the practice of this invention for obtaining a stoichiometric composition of A,B ,C,,D ,,O where O x 3 and O y 3, in a garnet film, the following are exemplary operational parameters: temperature of the substrate is approximately in the range 500C to I,OOOC; radio-frequency power to the target is approximately in the range 35 watts/sq. in to 60 watts/sq.in; and the radio-frequency bias on the substrate is desirably in the range of 200 volts d.c. level to 25 volts d.c. level, e.g., volts d.c. level, where the lower voltage may suitably be below 25 volts. I
It has been demonstrated that D may be Fe in excess up to 27% of the target by weight composition. For an example of an excess of Fe in said target of approximately 27% Fe, the power density in the target is approximately 51 watts/sq. in. and the radio-frequency bias on the substrate is approximately in the range of volts d.c. level to 25 volts d.c. level. For another example, the film may be deposited in crystalline form when the temperature on the substrate is greater than approximately 650C. For another example, the film may be deposited in amorphous form when the excess Fe is up to approximately 40% Fe and the temperature is less than approximately 650C. It has also been demonstrated that a variable substrate bias may be utilized while substrate temperature is kept fixed.
Generally, in accordance with the objects of this invention, there is provided a method for producing a layer with a given composition comprising the steps of: establishing a target which includes the components of said layer and a substrate; radio-frequency sputtering said layer onto said substrate; establishing operational parameters such that said substrate is at a given temperature, radio-frequency power to said target is at a given power density thereat, and radio-frequency bias on said substrate is at a given d.c. level; and varying interdependently said operational parameters to establish said given composition of said layer on said substrate. The target may be an unreaeted mixture of different compounds of at least two components of said layer, e.g., oxides of said components. Additionally, by varying interdependently said operational parameters, there may be obtained a given accumulation rate of thickness of said layer on said substrate.
More particularly, in accordance with the objects of this invention, there is provided a method for producing a layer with garnet stoichiometry comprising the steps of: establishing a target which includes the components of said layer and a substrate with garnet composition; radio-frequency sputtering said film onto said substrate including, establishing operational parameters such that saidjsubstrate is at a given temperature, radio-frequency power to said target is at a given power density thereat, and radio-frequency bias on said substrate is at a givendc. level; and varying interdependently said operational parameters to establish a given accumulation rate of thickness of said layer on said substrate. Illustratively, when said temperature is established approximately in the range of 650C to ,l,000C, said power density on said target is approximately in the range of 35 watts/sq. in. to 60 watts/sq. in., and said radio-frequency bias on said substrate is approximately in the range of 200 volts d.c. level to 25 volts d.c. level; and said accumulation rate of thickness of said layer is approximately in the range of 1 Angstrom/sec. to 3 Angstroms/sec.
DRAWINGS FOR THE INVENTION FIG. 1 is a schematic drawing partially in section and partially in circuit diagram of apparatus suitable for radio-frequency sputtering of material compositions according to the principles of this invention.
FIG. 2 is a schematic drawing illustrating the use of the apparatus of FIG. 1 wherein alternatively a sub strate is in thermal contact with the anode electrode and wherein a substrate is thermally floating relative to the anode electrode. FIG. 3 is a chart showing data in graphical form of the stoichiometric or molar ratio (Fe Ga,/Y plotted versus power density on the target for several growth conditions in the practice of this invention.
FIG. 4 is a chart showing data in graphical form of accumulation rate on the substrate versus power density on the target for radio-frequency sputtering according to the principles of this invention for targets of (a) powders of mixed oxides, Y 05 Ga Fe; 0 and (b) stoichiometric compound Y Fe Ga 0, illustrating that the target of mixed oxides yields a faster accumulation rate on the substrate according to the principles of this invention.
FIG. 5 is a graph showing data on accumulation rate.
versus radio-frequency bias on the substrate for an unreacted garnet target with excess Fe and a particular power density on the target illustrating that the accu-. mulation rate at the substrate decreases with increase of the radio-frequency bias.
FIG. 6 presents data in graphical form of molar ratio of the grown stoichiometric garnet film according to the principles of this invention illustrating that the molar ratio is higher for a gallium-backed substrate at z 450C than for a substrate thermally floating at z 900C.
FIG. 7 is a line diagram showing an idealized plot of terrnperature versus time illustrating the amorphous to crystalline transformation of a garnet film according to the principles of this invention.
GENERAL DESCRIPTION OF APPARATUS FOR THE PRACTICE OF THE INVENTION A schematic diagram of apparatus suitable for the radio-frequency sputtering of garnet materials according to the principles of this invention is shown in FIG. 1.
Generally, the cathode holder 3 and the anode 25 are arranged in a diode configuration. A shutter 47 is placed between the cathode and anode and axial and horizontal motion are provided thereto to cover the substrate during presputtering of the target and also to protect the target during sputter cleaning of the substrate prior to deposition. Heating of the substrate is obtained by resistance heaters 43 embedded in the anode assembly. The structure 41 is generally constructed of stainless steel. The molybdenum substrate holder is precoated with garnet'of approximately the target composition to prevent resputtering of molybdenum on the substrate 15. The radio-frequency power is split between the target 26 and the substrate 15 by the network 5 Y which provides independent radiofrequency bias control on the substrate.
The target is made of oxides pre-reacted to form the garnet compound which are ground, mixed and fired several times and then formed and fired into a disc. The preferable type of target is unreacted and comprised of the metal oxides which are mixed and pressed at high pressure and temperature and are only slightly'sintered.
Generally, both reacted and unreacted targets have stoichiometric composition Y Fe;, Ga, O, Additionally, a suitable target may also be unreacted GdzGazYlG' with excess Fe of up to approximately 40% compensation, e.g., 27% Fe. lllustratively, the substrate showed single crystal electron diffraction pat terns with Kikuchi lines, indicating a surface nearly free from lattice distortion. Surfaces with the l I l], 1 l0], l00]orientations were. found suitable for growth of epitaxial single crystal films of garnet stoichiometric composition.
SPECIAL DESCRIPTION OF APPARATUS FOR PRACTICE OF THE INVENTION FIG. 1 depicts an exemplary radio-frequency sputtering system, suitable for producing garnet film in accordance with the principles of this invention. As shown in the figure, an RF source 1 is coupled to target holder assembly 3, via the impedance matching network 5 and coupling capacitor 7. Impedance matching network 5 comprises variable capacitors 5a and 5b and inductor 5c and is employed to match the impedance of the sputtering system 2 to the impedance of RF source 1. During RF sputtering, it is possible to control to some extent the properties of the deposited films by applying a radio-frequency bias voltage to the substrate 15. For that purpose, there is provided a network comprising AC impedance elements 9, 11, and 13 which ensure that an appropriate bias is on substrate 15.
Inductor 9 is coupled at one end point to the node be tween impedance matching network 5 and coupling capacitor 7, and at another point to movable wiper arm 17 with contact 18. The center of inductor 9 is grounded and lower end 19 is floating, with the wiper arm 17 contact 18 on inductor 9 acting to provide a complete path. The wiper arm 17 is coupled to variable capacitor 11 and variable inductor 13 which is coupled to capacitor 21. Meter 23 is coupled to the junction between capacitor 21 and anodic substrate holder 25 via inductor 27 and is employed to measure the peak-topeak radio-frequency voltage bias on the anodic substrate holder. Inductor 27 and capacitor 29 are employed to isolate the meter 23 from the radio-frequency bias on the substrate.
The grounded center tap inductor 9 inverts the RF voltage so that when the RF signal from source 1 is positive, the voltage between the grounded center tap and the floating end of inductor 9 is negative and vice versa. When wiper arm 17 is moved on inductor 9, the amplitude of the inverted voltage is varied. Variable capacitor l1 and variable inductor 13 are employed to tune the phase shift network, which may be varied over 360C. Thus, both the amplitude of the RF substrate bias and the phase thereof may be adjusted. Therefore, the RF load, i.e., the impedance of the sputtering system, varies in accordance with the sputtering system parameters. Thus, where it is desirable to obtain a maximum peak-to-peak voltage on the substrate 15, for a particular wiper arm 17 setting, the entire radiofrequency circuit is adjusted to resonance.
Substrate 15 is mounted on substrate holder 25. Target 26 is mounted in conductive relationship with target holder assembly 3. Cathode target holder assembly 3 is water cooled, with water entering and exiting via pipes 4 in accordance with the arrows shown at the top of the assembly. Respective Helmholtz coils 31 which are energized by a direct voltage source, not shown, surround the cathode target holder assembly 3 and pedestal portion 51 of anode substrate holder 25. Coils 31 provide a magnetic field of intensity approximately 30 to 80 gauss perpendicular to the plane oftarget 26 and substrate 15. This magnetic field increases the concentration of electrons in the sputtering environment to increase sputtering efficiency. Additionally. the magnetic field acts to increase the bias on the substrate 15. Ground shield 33 around target 26 limits and focuses the sputtering to the control portion of target 26. Shield 33 is removably affixed to mount 35 to permit changing of targets 26.
Ceramic sleeves 37 and 39 insulate the. cathode target holder assembly 3 from the metal sputtering chamber 41 and the housing portion 35a of mount 35, respectively. Heating assembly 43 maintains'the substrate 15 at the desired temperature. Cooling coils 45 cool the anode structure and therefor the substrate 15. Shutter arrangement 47 is movably positionable between substrate 15 and target 26. Turning knob assembly 49 which is external to chamber 41 permits movement of the shutter 47 from the region between substrate 15 and target 26. Pedestal portion 51 of the anode substrate holder is mounted upon insulation 53 to isolate electrically the substrate holder assembly from metal chamber 41.
The sputtering chamber incorporates a titanium sublimation pump 55 surrounded by liquid nitrogen container 57. Before the sputtering is initiated, the sublimation pump getters active species, such as carbon and carbon-bearing compounds, from within the chamber onto the surface of the cryogenically cooled drum-59 of the pump. Titanium filament 61 is energized viaan electrical source, not shown. High purity oxygen enters -at port 63 and is passed through the"titanium pu'mp'in to the sputtering chamber. The high purity oxygen becomes further purified in the titanium pump and is used to sputter-clean the surface of substrate 15.
"Sputtering chamber 41 is pre-pu mped down-'. to a pressure'of from 2.0 to 8.O l" Torr while substrate is maintained at the desired temperature for sputtering. This pre-pumping is achieved via the port at lower right of chamber 41. lllustratively, a freon-cooled diffusion pump may be used to achieve'the desired vacuum in the sy'steml The system is then backfilled-with high purity oxygen until a pressure of approximately 2X10 Torr is reached. The oxygen is scrubbed in the titanium sublimation pump and the sputtering system is further readied for sputtering.
'Next, theshutter 47 is positioned between the target 26 and substrate 15 by knob 49 and the circuitry-power is turned on. Oxygen is established in chamber 4l,via
port 63. Oxygen plasma is generated and components of the target are sputtered upon shutter 47. Usually a pre-sputtering period of from 5 to minutes is-adequate, and it has been found that the average time for effective pre-sputtering is about 15 minutes.
Then, the substrate 15 is sputter cleaned in an oxygen environment. The system is first pumped down againto the base pressure of 2Xl0' Torr and is back-filled with oxygen. Simultaneously, radio frequency voltage 18 is applied to produce a dclevel bias on the substrateof approximately 150 volts. With shutter=47 in conductive contact with shield 33, which is grounded via the walls of mount and chamber 41, there is sputter cleaning Torr may be employed.
PRACTICE OF THE INVENTION FIG. 2 is a schematic diagram illustrating functionally a portion of the apparatus to indicate two ways that a substrate 15 may be mounted on anode 25 for the practice of this invention. Substrate 15a is shown supported via a layer of gallium, i.e., it is gallium-backed, by modybdenum block 25; and substrate 15b is shown as thermally floating, i.e., it is placed on block 25 and is somewhat insulated therefrom by the lack of effective thermal control thereto. Consequently, substrate is at the same temperature z 450C as the block 25, whereas substrate 15b reaches a higher temperature z 900C, i.e., it is thermally floating. Thus, the Gd Ga O substrate wafers 15 were either gallium-backed to the molybdenum substrate holder or allowed to float thermally as illustrated in FIG. 2. The gallium backed substrate 15a is within a few degrees of the temperature of the molybdenum holder. The temperature of a substrate 15b as measured by optical pyrometry reached 900C and greater depending on the power input to the target; Identical atom flux from the target impinged on the higher temperature floating substrate 15b and on the lower temperature gallium backed substrate 15a.
For an exemplary film growth procedure, with reference to FIGS. 1 and 2, the system is pumped to below 10 Torr, and pure oxygen is admitted into the system to about 2.5X10 Torr, which is maintained by pumping. With target 26 and substrate 15 shielded and the anode 25 grounded, the system is pre-sputtered for about 30 minutes. Thengthe shield 47 is moved from target region to allow sputter cleaning of the substrates for about 10 minutes. The shutter shield 47 is then opened thereby permitting atom flux from the target to reach the substrates 15a and 15b. The radio-frequency bias on the substrate 15 is adjusted to a predetermined level and deposition occurs over a period, e.g., up to approximately 38 hours.
After deposition, the grown films are annealed in flowing oxygen in an open tube furnace. Desirable results were obtained when the grown films were maintained from 900C to l,l00C for 24 hours, and then slowly cooled, e.g., 50C/hr. Circular erruptions occurred in thefilm when rapidly heated to annealing temperature, whereasfilm slowly heated to annealing temperature showed tensile cracking. However, by selecting an intermediate rate of heating to annealing temperature, large nearly perfect films were obtained as regards such circular erruptions and tensile cracks. Proper lattice matching between the substrate and garnetfilm. reduces the tendency for cracking to take place. Illustrative thicknesses of grown films, as measured. by. a Taylor-Hobson Tally vSurf instrument, ranged from 0.01 to 15 microns, of which I to 5 micron films were typical. Optical absorption methods were alsoemployed in determining film thickness and the results compared closely. to the mechanical measure I EXEMPLARY DATA FOR Tin; INVENTION Exemplary garnet films according tothe principles of this inventionwere examined from various interdependent operational parameters of the sputtering 'of radiofrequency .bias on the substrate; power density on. the target, substrate temperature and accumulationrate of the film at the substrate. The film composition as determined by electron microprobe analysis; was normalized to 3 moles of Yttrium. The ratio of the sum of the normally occupied Fe sites to therare earth sites in a material with garnet composition 1 was considered as the stoichiometric or molar ratio.
For comparison, the molar ratio for the ideal stoichiometric composition is 1.667. FIG. 3 is a plotof the molar ratio as a function; of power density in watts/iii on the target showing results of reacted and unreacted targets either at --450C or z 900C.
Both types of targets for the data of FIG. 3 havethe stoichiometric composition Y Fe Ga ,O The substrates were gadolinium gallium garnet Gd Ga O Substrates that are gallium-backed and at z 450C are compared'to thermally floating examples which are at temperatures from 850C to 980C. For comparison purpose, films were deposited on both lowertemperature substrate 15a and higher temperature substrate 15b of FIG. 2 simultaneously. The samples at z 450C are amorphous and reddish while films deposited at z 900C arecrystalline and range in color from amber to greenish-yellow.
The difference in composition between films depos- 10 within 2% of the stoichiometric composition of the target, whereas for the reacted garnet target there was an accumulation rate of about 0.4A/sec with greater than I I 1% deviation from the target composition.
With reference to FIGS. 5 and 6, unreacted targets "with: 27% excess Fe -(Gd Y ,Fe Ga 0, were also 'mally'fl'o'ating 'at temperature 900C. FIG. 5 shows the accumulation rate on the substrate as a function of The films on the Ga-backed z 450C substrates'are amorphous and nonmagnetic and upon crystallization ited on the high temperature z 900C substrate films deposited on and the lower temperatures= 450C substrate is considerably smaller when'produced from the unreacted target than when produced from the reacted target. The unreacted target produces an approximately constant differential' in composition between the higher temperature substrate and the lower temperature substrate whereasv the reacted compounded garnet target resulted in films that vary widely in composition. As the power density on the target is decreased, the composition of the grown filrriapproaches more closely that of the target. Theoretically, the tendency for the convergence of compositions of the higher and lower temperature substrates at lower'powers results because the heating of the'thermally isolated. substrates is decreased to a'point where the films are deposited in amorphous form at approximately "the temperature of the substrate-holder. Sputtering atrelatively low power density on the target produced more nearly ideal film stoichiometry. Films prepared from the unreacted, i.e., mixed component, target approach the target stoichi ometry before the reacted, i.e.,.com-pounded garnet, target. Therefore, 'there is an improvement-in the ratio of the species sputtered fromtheunreacted-targetcompared to the species sputter from the reacted target.
FIG. 4 shows the accumulation rate on the substrate as a function of power density for boththe unreacted target and the reacted target. Illustratively, the accumulation'rate from the unreacted target exceeds'the accumulation rate for the reacted target by a factor of about 2 to 3. Illustratively, at watts/in on the target with substrates at 450C, sputtered films were prepared from an unreacted target with an accumulation rate on the substrate of 1.5A/sec. Such films were become single crystalline garnet structures; occasionally small amounts of a second phase were found and phase appearing occasionally.
' The data of FIGS. 5 and 6 show the trends of' composition of thefilrnproduce'd and accumulation rate on the substrate with radio-frequency bias on the substrate and indicate a large influence of the bias on film composition, particularly at relatively high temperature. Therefore, it has been determined for practice of this invention that radio frequency bias control of the substrate is advantageous for the fabrication of stoichiometric films of multi-component systems.
FIG. 6 presents plots of molar ratio versus radio- .frequency bias on the substrate for both galliumbacked substrates 450C and thermally floating substrates 900C. Generally, it is shown by FIG. 6 that unreacted targets withexce ss Fe can be used to prepare stoichiometric garnet compositions and structures by adjusting the power density to the target which can be generally greater than to a stoichiometric target and also by adjusting the temperature range at which stoichiometric films can be prepared as well as the bias that -is applied to the substrate. It will therefore be understood from the foregoing discussion that the interrelationship among target composition, power density, substrate temperature and substrate bias can be selectively ,controlled to obtain stoichiometric garnet compositions and structures.
CONSIDERATIONS FOR THE INVENTION tering. It has been demonstrated that composition control may be successfully achieved for single crystal gar- .net film at substrate temperatureshigher than 500C,
which is the upper'temperature limit given in the noted prior art US. Pat. No. 3,607,698 for the preparation of single crystal GdzGazYIGfilms.
. In thepracticeof this-invention, a garnet structure is -;deposited as an amorphous layer on a GGG surface and then transformed to crystalline form by heating as illustrated by FIG. 7. The growth takes place outwardly from the interface between the film and the substrate toward the surface of the film with resultant epitaxial single crystal garnet layer on the substrate. The epitaxial growth process via the amorphous to crystalline transformation is a relatively low temperature process for epitaxial growth of garnet films, when compared to the prior art practice. The typical thermal history of garnet films undergoing amorphous to crystalline transformation is shown in FIG. 7. For Y Fe Ga O film on Gd Ga O substrate, the amorphous to crystalline transformation temperature is between 650C and 675C for a film prepared at z 450C. Amorphous garnet films deposited on sapphire substrates showed an amorphous to crystalline transformation temperature higher than for GGG substrate, and the transformation temperature varied from 50C to 100C greater than the transformation temperature of film on GGG. Theoretically, these differences in crystallization temperatures are due to differences in the interfacial surface energy between the substrate and the film. The amorphous garnet films are a deep burgundy red in transmitted light and after crystallization range from a yellowgreen to an amber-yellow depending on composition and thickness of the film.
Amorphous garnet films deposited in accordance with the principles of this invention have been found to be readily etched in dilute l-lCl, whereas the crystallized garnet deposited in accordance with the principles of this invention is only slightly soluble in dilute HCl. Patterns may readily be etched in the amorphous garnet, then recrystallized to form epitaxial garnet patterns.
Exemplary technology for providing special amorphous to crystalline transformation to obtain single crystal film is disclosed in copending application Ser. No. 319,125 filed Dec. 29, 1972 by P. Chaudari, et al., and commonly assigned.
1. Method of producing film with garnet stoichiometric composition A B C D a o where O x 3 and O y 3, wherein A is a rare earth element of the lanthanide series including Gd, Eu, Tb and Er;
B is Y, La or Sc;
C is Al or Ga; and
D is Fe, Ni or C; comprising the steps of:
a. establishing a target with substantially non-garnet structure and with said stoichiometric composition and establishing a substrate with garnet composition;
b. sputtering said film onto said substrate from said target including establishing operational parameters by:
1. establishing said substrate at a temperature approximately in the range of greater than 500C to 1,000C,
2. establishing radio-frequency power to said target approximately in the range of 2 watts/sq. in. to 100 watt/sq. in;
3. establishing radio-frequency bias on said substrate at a given d.c. level; and
c. varying interdependently said operational parameters to deposit onto said substrate a film having substantially said garnet stoichiometic composition and to establish a given accumulation rate of thick- LII ness of said film on said substrate.
2. Method as set forth in claim 1 wherein 1. said temperature of said substrate is established approximately in the range of greater than 500C to 650 C, and
2. said radio-frequency power to said target is established in the range of 2 watts/sq. in. to 23 watts/sq.
3. Method as set forth in claim 2 including the step of establishing said radio-frequency bias on said substrate less than approximately 200 volts d.c. level peakto-peak.
4. Method as set forth in claim 1 and wherein said radio-frequency power to said substrate is established approximately in the range of 35 watts/sq. in. to 60 watts/sq. in. i
5. Method as set forth in claim 4 wherein said radiofrequency bias on said substrate is varied during production of said film thereon while said temperature of said substrate is maintained at an approximately fixed value.
6. Method as set forth in claim 1 wherein power density on said target is approximately in the range 20 watts/sq. in. to 100 watts/sq. in.
7. Method as set forth in claim 1 wherein said temperature of said substrate is greater than approximately 650C,
said radio-frequency bias on said substrate is approximately volts d.c. level.
8. Method as set forth in claim 1 wherein said temperature of said substrate is less than approximately 650C.
9. Method for producing a layer with garnet stoichiometry comprising the steps of establishing a target which has substantially a garnet stoichiometric composition which is substantially nongarnet structure and establishing a substrate with garnet composition,
radio-frequency sputtering said film onto said substrate from said target including,
establishing operational parameters such that said substrate is at a given temperature approximately in the range of 650C to 1,000C, radio-frequency power to said target is at a given power density thereat approximately in the range of 35 watts/sq. in. to 60 watts/sq. in., and radio-frequency bias on said substrate is at a given d.c. level approximately in the range of 200 volts d.c. level to 25 volts d.c. level, and varying interdependently said operational parameters to deposit onto said substrate a layer having substantially said garnet stoichiometric composition and to establish a given accumulation rate of thickness of said layer on said substrate approximately in the range of 1 Angstrom/sec. to 3 Angstroms/sec.
10. Method as set forth in claim 9 wherein said target is comprised of a mixture of oxides Y 0 Ga O and Fe O and said film is produced with stoichiometric composition of 11. Method of claim 9 wherein said target is an unreacted mixture of different oxides of at least two components of said layer.