Yttrium iron garnet disks on gadolinium gallium substrates for microwave applications
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to microwave electronic devices employing yttrium iron garnet, and, in particular, to a process for providing lanthanum-doped yttrium iron garnet disks suitable in such devices. 
     2. Description of the Prior Art 
     Yttrium iron garnet (Y 3  Fe 5  O 12  ; YIG) is an important material for microwave elecronic devices because of its high Q value at microwave frequencies. Currently, spheres of YIG are widely used as narrow band filters, microwave resonators and the like. The spheres are individually fabricated from flux grown YIG single crystals by a a crystal growth process which commonly requires a few weeks. The fabrication of crystals into small spheres is time consuming and requires sophisticated polishing techniques. 
     Liquid phase epitaxy (LPE) growth of YIG films on gadolinium gallium garnet (Gd 3  Ga 5  O 12  ; GGG) subtrates, with subsequent processing into photoetched disks, has been disclosed in Vol. MAG-9, IEEE Transactions on Magnetics, pp. 535-7 (1973). Since YIG has a smaller lattice parameter than GGG, Pb 2+  ions were incorporated in the YIG lattice to achieve a closer match of lattice parameter constants in order to reduce strains that would otherwise arise due to lattice mismatch. The lead content, however, apparently resulted in a broadening of the ferromagnetic resonance linewidth, thus rendering the YIG disks less suitable for microwave resonant applications. Further, during processing of the YIG film to photoetch YIG disks, sputtered SiO 2  was used to mask the YIG film. The sputtered SiO 2  apparently resulted in surface damage of the YIG film, in consequence of which ferromagnetic resonance linewidth was further broadened. 
     A study of dependence of lattice parameter on composition in substituted yttrium iron garnet epitaxial layers grown on GGG substrates has been published; see, e.g., Vols. 17 and 26, Journal of Crystal Growth, pp. 322-328 (1972) and 122-126 (1974), respectively. In the study, yttrium is substituted by gadolinium, samarium and lanthanum, while iron is substituted by gallium. No useful methods for fabricating microwave devices are given, however. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a mass production process is provided for fabricating arrays of thick, damage-free, yttrium iron garnet (YIG) disks. The process comprises 
     a. forming a thin film of yttrium iron garnet, doped with about 0.5 to 1.5 atom percent trivalent lanthanum ions (La 3+ ), on a gadolinium gallium garnet substrate; 
     b. forming a thin layer of SiO 2  on the lanthanum-doped yttrium iron garnet layer; 
     c. forming a photoresist mask layer on the SiO 2  layer; 
     d. removing portions of the photoresist mask layer to expose portions of the underlying SiO 2  layer; 
     e. removing the exposed portions of the SiO 2  layer to expose portions of the underlying YIG layer; and 
     f. removing the exposed portions of the YIG layer to form an array of isolated La:YIG disks supported on the GGG substrate. 
     The method, which utilizes the well-known isothermal LPE dipping process, provides narrow linewidth YIG films. Lattice mismatch between YIG and GGG is minimized by incorporating about 0.5 to 1.5 atom percent La 3+  ions on the dodecahedral sites of the YIG lattice. 
     La:YIG films are grown at a temperature of 950° to 960° C, which is approximately 100° C higher than prior art Pb,Pt:YIG films. Higher temperature melts are less viscous and thus leave smaller and fewer flux mesas on the surface of the film during flux spinoff after the film growth process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 thourgh 6 in cross-section depict the sequence of processing steps of a microwave device in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     Thick YIG films, suitable for use in microwave devices, can be grown on GGG substrates only if the film substrate lattice mismatch is small. Undoped YIG films greater than 10 micrometers in thickness commonly crack due to the strain at the film-substrate interface. The lattice constant of undoped YIG is 0.008 A smaller than congruently grown GGG. The method disclosed herein employs nonmagnetic La +3  to expand the YIG lattice to substantially match that of GGG. This is in contrast to the prior art of microwave devices, which incorporates Pb 2+  in the flux to expand the YIG lattice and Pt 4+  from the growth crucible as a charge compensating ion. 
     The process provided by the invention forms a thin epitaxial film of a lanthanum-doped yttrium iron garnet film on a substrate of gadolinium gallium garnet. Following deposition of the La:YIG film, the films are then processed into an array of disks by first applying a thin layer of SiO 2 , spinning on a photoresist mask, and exposed the photoresist through a hole pattern mask. The resulting disk pattern is maintained through an SiO 2  etch, employing an HF etchant and then a garnet etch, employing a hot H 3  PO 4  etchant. The SiO 2  layer is impervious to the high temperature phosphoric acid etch and is a necessary part of the process unless a photoresist which is also impervious to hot phosphoric acid is used. The SiO 2  layer may be removed if desired or left intact. If left intact, it can serve as a passivating layer for the La:YIG disks, since the SiO 2  layer does not adversely affect ferromagnetic resonance linewidth. 
     The process is more conveniently described with reference to FIGS. 1 through 6. In FIG. 1, a substate 10 of gadolinium gallium garnet (Gd 3  Ga 5  O 12  ; GGG) supports a thin film 11 of yttrium iron garnet (Y 3  Fe 5  O 12  ; YIG). However, in accordance with the invention, the thin film is formed by a process which incorporates about 0.5 to 1.5 atom percent trivalent lanthanum ions (La 3+ ) on the dodecahedral sites of the YIG crystal lattice. The composition of the film may thus be represented as 
     
         (Y.sub.3-x La.sub.x)Fe.sub.5 O.sub.12 
    
     where x ranges from about 0.015 to 0.045. 
     The value of x is constrained by lattice parameter mismatch considerations. The lattice parameter of undoped YIG is about 0.008 A less than that of GGG. As a consequence of this lattice mismatch, stresses in the film arise which, if sufficiently severe, lead to cracking of the YIG film. A vlue of x of about 0.015 to 0.045 reduces the lattice mismatch by at least a factor of two and permits growth of films ranging in thickness up to about 20 to 30 μm. For growth of thicker films approaching 100 μm, the value of x must range from about 0.02 to 0.03; that is, about 0.67 to 1.0 atom percent of the yttrium ions on the dodecahetral sites must be replaced by La 3+ . 
     Growth of La:YIG films is carried out employing an isothermal liquid phase epitaxy (LPE) procedure, employing growth of the film on the substrate from a molten solution of constituent oxides plus flux at a constant temperature maintained at a super-cooled condition. The substrate is conveniently fabricated by the Czochralski technique. Since those procedures are well-known and form no part of this invention, details are omitted herein. Use of the procedure permits growth of La:YIG films at about 950° C to 960° C, under conditions of about 10° C supercooling. Such a high temperature avoids viscous melts. Less viscous melts promote faster growth rate of the films, thus permitting better control over thickness uniformity. Less viscous melts also make subsequent processing more efficient, since flux removal after formation of the films is easier. Further, at such high temperatures, Pb incorporation from the flux is very small. 
     Growth of the La:YIG film is carried out under conditions such that a growth rate of at least about 1 μm/min is maintained. Such a growth rate is required to avoid haze and facet formation. 
     Following formations of the La:YIG film, a thin layer 12 of SiO 2  is formed over the film, as shown in FIG. 2. While there are many procedures available for forming SiO 2  layers, a process that avoids possible surface damage to the La:YIG film is preferred, since damaged films are usually less suitable for microwave applications. SiO 2  layers that produce substantially no damage in the La:YIG film are conventiently formed by chemical vapor deposition (CVD) of SiO 2  by decomposition of silane in oxygen at about 450° C. Such CVD procedures are well-known in art. Alternatively, SiO 2  layers are conveniently formed by using dopant-free spin-on SiO 2  source solutions which typically comprise organosilicon compositions. An example of such a source solution is sold under the trade designation Accuspin® solutions (available from Allied Chemical Corp., Morristown, N. J.). 
     The spin-on SiO 2  source solutions are conveniently applied to the La:YIG film by spinning the solution onto the film using a conventional photoresist spinner. The source solution is converted into SiO 2  by decomposition at about 200° C. 
     The SiO 2  layer that is formed is adequate to protect the La:YIG film during subsequent etching of portions of the film. However, for improved adhesion and densification of the SiO 2  layer to the La:YIG film, it is preferred that the SiO 2  layer be annealed at about 400° C to 900° C. The improved adhesion results in less  pg,7 undercutting of the SiO 2  layer during etching and hence better definition of the La:YIG disks. The improved densificiation results in less possibility of the etchant penetrating the SiO 2  layer. 
     While not critical, a temperature of about 400° C is considered to be the minimum temperature at which the improved adhesion and densification are obtained. At that temperature, a time of about 30 min is sufficient in order to realize the beneficial effects and is the minimum preferred time, while a time of about 60 min produces no further improvements and, consistent with economic considerations, is the maximum preferred time. 
     While not critical, a temperature of about 900° C is considered to be the maximum temperature at which the improved adhesion and densification are obtained. At 900° C, a time of about 10 min is sufficient in order to realize the beneficial effects and is the minimum preferred time, while a time of about 20 min produces no further improvements and, consistent with economic considerations, is the maximum preferred time. 
     The atmosphere in which the annealing is permformed is not critical, other than that it be chemically unreactive with the La:YIG film. Preferably, an inert atmosphere, such as nitrogen, is employed. 
     The thickness of the SiO 2  layer is not critical, other than being thick enough to form a continuous layer and thin enough to be removed by etching in a reasonable amount of time. A thickness about 0.5 μm is generally sufficient. 
     Next, a photoresist mask layer 13 is formed over the SiO 2  layer, as shown in FIG. 3. The photoresist material is not critical and is applied by well-known techniques. 
     Portions of the photoresist layer are then exposed through a hole pattern mask, employing well-known techniques. Any wavelength of electromagnetic radiation commonly used, such as visible light, UV, soft X-ray and electron beam, may be employed for exposing the portions of the photoresist layer. While any pattern may be used, a geometrical array of holes that maximizes the number of La:YIG devices on the GGG slice is preferred. The dimensions of the holes are selected to result ultimately in the formation of La-YIG disks about 1 to 2 mm in diameter. The undesired portions of the photoresist layer are then removed by well-known techniques to expose portions of the underlying SiO 2  layer, as shown in FIG. 4. 
     The exposed portions of the SiO 2  layer are then removed by an HF etchant to expose portions of the underlying La:YIG film, as shown in FIG. 5. While many buffered oxide etchants are suitable, two etchants that have been found to be particularly useful comprise either a solution of 40% NH 4  F and 49% HF in a ratio of about 4 to 1, which is considered to be a fast etchant, or a solution of 40% NH 4  F and 49% HF in a ratio of about 10 to 1, which is considered to be a slow etchant. Other etchant compositions intermediate these two etchants are also suitable. Similar buffered oxide etchants are available under the trade designations BOE 1235 (fast etchant about 1200 A/min) and BOE 500 (slow etchant, about 500 A/min), available from Allied Chemical Corp., Morristown, N.J. These etchants are conveniently used at room temperature. 
     The exposed portions of the La:YIG film are then removed by a hot H 3  PO 4  etchant to form isolated disks of La:YIG supported on the GGG substrate. The photoresist layer is also removed, usually prior to removal of the portions of the La:YIG film. A conventional solvent is employed for removal of the photoresist layer. The resulting structure is shown in FIG. 6. The SiO 2  layer may be either removed or left in place. However, it is preferable to leave the SiO 2  layer intact, since the layer may protect the La:YIG film during subsequent processing into devices. 
     The H 3  PO 4  is conveniently employed as an aqueous solution of 85% H 3  PO 4 . An elevated temperature is used. A temperature of about 160° C results in a fast etch and may be employed in conjunction with the fast HF etchant. A temperature of about 140° C results in a slow etch and is preferably employed in conjunction with the slow HF etchant for reasons described below. 
     For 5 μm thick La:YIG disks stimulated by 9.45 Ghz, a linewidth of about 0.85 Oe is obtained for the fast SiO 2  etch-fast garnet etch combination, while a linewidth of about 0.45 Oe is obtained for the slow SiO 2  etch-slow garnet etch combination. Consequently, the slow etch combination is preferred, since narrower linewidths are obtained. Further, the slow etch combination results in better definition of the La:YIG disks. 
     Following the processing steps outlined above, the GGG wafer containing the disks of La:YIG is sectioned and sawn by well-known techniques to produce individual sections of GGG substrate, each supporting at least one disk of La:YIG thereon. The disks are then further processed as necessary to fabricate microwave devices therefrom, such as microwave filters, oscillators, multipliers and the like. 
     The process of the invention thus permits mass production of La:YIG disks for microwave device applications. Assuming the central 80% of the La:YIG film is usable and 0.25 mm spacings are made between disks, then approximately 800 1 mm diameter or 30 5 mm diameter disks may be diced from a 2.0 inch diameter wafer. 
     EXAMPLES 
     Johnson Matthey Grade I PbO, B 2  O 3 , Fe 2  O 3 , together with Molycorp 99.999% Y 2  O 3  and Johnson Matthey 99.999% La 2  O 3 , were used for film growth of La:YIG. One inch polished &lt;111&gt; GGG substrates were used, on which epitaxial La:YIG films were grown at rates of at least about 1 μm/min at a temperature of about 950° C under conditions of about 10° C supercooling. X-ray diffractometer scans indicated that the film substrate mismatch was less than 0.002 A. The composition for a melt used to grow the near-zero lattice mismatch La:YIG films on GGG is presented below: 
     
         ______________________________________                                    
       Oxide         Moles                                                
______________________________________                                    
       PbO           2.24                                                 
       B.sub.2 O.sub.3                                                    
                     0.152                                                
       Fe.sub.2 O.sub.3                                                   
                     0.199                                                
       Y.sub.2 O.sub.3                                                    
                     0.0211                                               
       La.sub.2 O.sub.3                                                   
                     0.0015                                               
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     Unbroken La:YIG films were grown up to 18 μm in thickness. Substantially thicker near-zero mismatch films can also be grown. 
     The La:YIG films were then processed into disk arrays by first applying a 0.5 μm layer of SiO 2 , employing a dopant-free spin-on SiO 2  source solution at room temperature. The layer was then annealed at 900° C for 15 min in N 2  to form and densify the SiO 2 . A layer of a negative photoresist was then formed over the SiO 2  layer and exposed through a 1 mm hole pattern mask. The resulting disk pattern was maintained through an SiO 2  etch and then a garnet etch. The SiO 2  layer was impervious to the high temperature phosphoric acid etch. Two etchants were employed, a fast etch and a slow etch. The fast etch comprised a solution of 40% NH 4  F and 49% HF in a ratio of 4 to 1 at 25° C for the SiO 2  layer and an 85% H 3  PO 4  at 160° C for the La:YIG layer. The slow etch comprised a solution of 40% NH 4  F and 49% HF in a ratio of 10 to 1 for the SiO 2  layer and 85% H 3  PO 4  at 140° C for the La:YIG layer. The fast SiO 2  etch rate was about 0.1 μm/min; the slow SiO 2  etch rate was about 0.25 μm/min. The fast garnet etch rate was about 0.5 μm/min; the slow garnet etch rate was about 0.3 μm/min. 
     Following the etching of the La:YIG layer, an array of La:YIG disks distributed on the GGG substrate was obtained. The GGG substrate was then diced with a wire saw to form individual squares of GGG supporting individual La:YIG disks. Linewidths of the La:YIG disks were measured on a Varian E-12 X-Band spectrometer, employing scans of 200 Oe and 20 Oe, of (a) a rough cut 5 μm thick, 1 mm by 2 mm, fast-etched La:YIG slab (200 Oe), (b) a 5 μm thick, 1 mm diameter slow-etched La:YIG disk (200 Oe) and (c) a 5 μm thick 5 mm diameter, slow-etched La:YIG disk (20 Oe). The LA:YIG disk of (c) gave the narrowest linewidth, 0.45 Oe.