Abstract:
A method of etching a surface of a substrate comprising lithium tantalate (LiTaO 3 ) and chemically similar materials is disclosed. The method includes contacting the surface with a mixture comprising hydrofluoric acid and sulfuric acid. In addition to its etching action, the mixture is also capable of polishing the thus treated surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of etching a surface of a substrate comprising LiTaO 3  and chemically similar materials and more particularly, to a method of etching LiTaO 3  with a mixture comprising hydrofluoric acid and sulfuric acid. 
     2. Description of the Prior Art 
     Recently, lithium tantalate (LiTaO 3 ) has become of great interest in the electrical device industry. An inherent problem in the use of LiTaO 3  has been the etching and polishing of this material. Lithium tantalate (LiTaO 3 ) can be etched with an aqueous mixture consisting of HNO 3  and HF. However, the etching can only be carried out at a reasonable rate at the boiling temperature of the mixture. With respect to the polishing of LiTaO 3 , the HNO 3  -HF mixture does not polish a surface of LiTAO 3  so that optical inspection thereof for surface and interior defects is possible. An etchant which both etches the LiTaO 3  and polishes a surface thereof at a reasonable rate at a moderate temperature is therefore desirable and a method of employing such an etchant is an object of this invention. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of etching a surface of a substrate comprising LiTaO 3  and chemically similar materials and more particularly, to a method of etching LiTaO 3  with a mixture comprising hydrofluoric acid and sulfuric acid. 
     Briefly, the method comprises contacting a surface of a substrate, comprising lithium tantalate (LiTaO 3 ) with a mixture comprising hydrofluoric acid and sulfuric acid whereby etching thereof occurs. Additionally, the mixture has a polishing effect on the etched surface. 
    
    
     DETAILED DESCRIPTION 
     The present invention is based upon the discovery of a unique chemical system for etching lithium tantalate and chemically similar compounds such as lithium niobate (LiNbO 3 ) and barium titanate. 
     The etching system comprises a mixture comprising hydrofluoric acid, HF and sulfuric acid, H 2  SO 4 , contained in at least one suitable liquid solvent carrier. The solvent carrier may be either an aqueous or nonaqueous liquid. A preferred carrier solvent is water. 
     The concentrations of the respective components, HF and H 2  SO 4 , employed depend on the rate of etch desired at a particular temperature. In this regard, it is to be noted that a preferred mixture concentration for 49 weight percent aqueous HF and 97 weight percent aqueous H 2  SO 4  typically ranges from about 0.40 to about 0.90 volume fraction of 49 weight percent aqueous HF (about 42.172 moles of HF/1000 gms. H 2  O, 40.173 moles of H 2  SO 4  /1000 gms. H 2  O to about 47.535 moles/1000 gms. H 2  O and 3.354 moles/1000 gms. H 2  O, respectively). 
     A substrate to be treated, i.e., comprising LiTaO 3 , is immersed in or exposed in some conventional manner to the etching solution mixture wherein the LiTaO 3  is etched. The temperature of the etching solution is maintained at a temperature between room temperature to the boiling point of the particular mixture employed. In this regard, the temperature selected depends upon the parameters such as the etching mixture employed and the etch rate desired. After the substrate has been immersed in the etching solution mixture for the requisite period of time, it is removed therefrom and rinsed with a suitable solvent, e.g., water, and then dried. It is to be noted that exposure to the HF-H 2  SO 4  mixture leads to polishing of the LiTaO 3  substrate surfaces. It is also to be noted that exposure of LiTaO 3  to 100 percent of 97 weight percent aqueous H 2  SO 4  does not lead to etching thereof (within experimental error) at 90° C. even after 40 hours of exposure thereto. 
     Specific examples of the etching of a LiTaO 3  substrate are as follows: 
     149 slices (AT cut) comprising LiTaO 3  were ground to a 12μ finish. The samples were then immersed at 90° C., in an etching solution comprising 65 volume percent of an aqueous 49 weight percent HF solution and 35 volume percent of an aqueous 97 weight percent H 2  SO 4  solution. The samples were maintained in the etching solution for two hours to etch the surfaces thereof, i.e., to remove surface blemishes, damages and irregularities. After about two hours and a removal of 24 microns from the surfaces thereof, the samples were removed from the etchant solution and water rinsed. 
     Different samples from among the 149 etched slices were then subjected to varying volume fractions of aqueous 49 weight percent HF and aqueous 97 weight percent H 2  SO 4  solutions at varying temperatures and time intervals. An etching rate for each sample was then determined by measuring the weight change thereof and the area of each sample, using standard techniques and equipment. The rate was then calculated from this data and the density of LiTaO 3  [ 7.454 g/cm 3  as reported by R. T. Smith, Applied Physics Letters, 11, 1461 (1967)] using a conventional procedure. The various experimental rates are given in Tables I to VI. 
     
                       TABLE I______________________________________(Temperature = 25° C)Volume  Molality of  Molality of  EtchingFraction   HF (moles/   H.sub.2 SO.sub.4 (moles/                             Rateof HF   1000 g. H.sub.2 O)                1000 g. H.sub.2 O)                             (μm/hr.)______________________________________0.00     0.000       329.684      --0.10    26.206       149.783      --0.20    35.053       89.045       0.609390.30    39.498       58.530       0.046950.30    39.498       58.530       0.046950.35    40.983       48.336       0.097930.35    40.983       48.336       0.088540.40    42.172       40.173       0.120740.40    42.172       40.173       0.116720.45    43.146       33.490       0.174400.45    43.146       33.490       0.160990.47    43.487       31.143       0.187820.47    43.487       31.143       0.178430.50    43.957       27.916       0.210630.50    43.957       27.916       0.237460.60    45.234       19.151       0.214650.60    45.234       19.151       0.238800.65    45.745       15.643       0.233430.65    45.745       15.643       0.228070.70    46.192       12.572       0.224040.70    46.192       12.572       0.226720.75    46.587       9.862        0.201230.75    46.587       9.862        0.190500.80    46.938       7.452        0.174400.80    46.938       7.452        0.169040.90    47.535       3.354        0.107320.90    47.535       3.354        0.105981.00    48.024       0.000        0.050981.00    48.024       0.000        0.03354______________________________________ 
    
     
                       TABLE II______________________________________(Temperature = 38° C)Volume Fraction  Etching Rateof HF            (μm/hr.)______________________________________0.00             0.006710.00             0.009390.10             0.006710.10             0.004020.20             0.028170.20             0.033540.30             0.079150.30             0.136840.35             0.201230.35             0.193180.40             0.268310.40             0.295140.45             0.308560.45             0.370270.47             0.453450.47             0.407830.50             0.461500.60             0.599680.65             0.563460.70             0.590290.75             0.550040.75             0.513820.80             0.469550.80             0.344780.90             0.233431.00             0.187821.00             0.13818______________________________________ 
    
     
                       TABLE III______________________________________(Temperature = 51° C)Volume Fraction  Etching Rateof HF            (μm/hr.)______________________________________0.00             0.053660.10             0.026830.20             0.036220.30             0.368930.30             0.434670.35             0.402470.40             0.804940.40             0.810300.45             0.931040.47             1.015560.60             1.341560.60             1.517310.65             1.328150.65             1.515960.70             1.220820.75             1.377780.75             1.301310.80             1.033000.80             1.282530.90             0.630530.90             0.727131.00             0.429301.00             0.39710______________________________________ 
    
     
                       TABLE IV______________________________________(Temperature = 64° C)Volume Fraction  Etching Rateof HF            (μm/hr.)______________________________________0.00             0.073790.00             0.075130.10             0.024150.10             0.021460.20             0.134160.40             2.146500.45             2.167960.47             2.548970.47             2.622750.47             2.194790.50             2.039170.60             2.951440.65             3.488060.70             3.219750.75             2.951440.80             2.683120.90             1.878190.90             1.412661.00             0.83311______________________________________ 
    
     
                       TABLE V______________________________________(Temperature = 77° C)Volume Fraction  Etching Rateof HF            (μm/hr.)______________________________________0.00             0.228070.10             0.040250.10             0.022810.20             0.187820.20             0.245510.30             1.517310.35             1.914410.40             3.085590.40             3.331100.45             3.780520.47             4.315800.50             4.657900.60             6.439500.60             6.304000.65             6.037030.65             6.176550.70             5.983360.70             6.285220.75             5.768710.75             4.779980.80             5.902870.90             4.427150.90             3.749661.00             1.744031.00             1.72122______________________________________ 
    
     
                       TABLE VI______________________________________(Temperature = 90° C)Volume Fraction  Etching Rateof HF            (μm/hr.)______________________________________0.0              0.096590.20             0.603700.20             0.351490.30             1.744030.30             1.809770.35             2.951440.35             3.288170.40             3.890530.40             5.575530.45             7.781060.45             6.896970.47             7.070030.50             8.988460.50             8.655760.60             11.403270.60             9.456670.65             12.208210.65             10.825060.70             11.134960.70             10.055000.75             11.403270.75             10.194530.80             10.464180.90             7.512740.90             7.347731.00             4.427151.00             5.084521.00             4.25007______________________________________ 
    
     Specific examples of polishing a LiTaO 3  substrate are as follows: 
     1. A lithium tantalate (LiTaO 3 ) slice (AT cut) was ground to a 12μ finish. The slice was then immersed at 90° C. for 30 minutes, in an aqueous mixture comprising 65 volume percent HF (49 weight percent) and 35 volume percent H 2  SO 4  (97 weight percent). The thus treated slice was rendered semi-opaque thereby enabling optical examination by conventional apparatus and means for gross defects therein. 
     2. The procedure of (1) above was repeated except that the slice was immersed in the mixture for four hours. The slice was rendered almost transparent as well as having the surfaces thereof polished.