US RE29336 E
A method of etching a surface of a substrate comprising lithium tantalate (LiTaO.sub.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.
1. A method of etching a surface of a substrate comprising LiTaO.sub.3, which comprises contacting the surface with a mixture comprising HF present in an amount ranging from about 42.172 to about 47.535 molal and H.sub.2 SO.sub.4 present in an amount ranging from about 40.173 to about 3.354 molal.
2. A method of treating a body comprising lithium tantalate to obtain a polished surface thereof, which comprises contacting the surface with a mixture comprising HF present in an amount ranging from about 42.172 to about 47.535 molal and H.sub.2 SO.sub.4 present in an amount ranging from about 40.173 to about 3.354 molal.
3. A method of optically examining a substrate comprising LiTaO.sub.3 which comprises treating the substrate with a mixture comprising HF present in an amount ranging from about 42.172 molal to about 47.535 molal and H.sub.2 SO.sub.4 present in an amount ranging from about 40.173 to about 3.354 molal.
4. The method as defined in claim 3 which further comprises directly examining said treated substrate with optical means. .Iadd. 5. A method of etching a surface of a substrate comprising lithium tantalate, which comprises contacting the surface with a mixture comprising HF and H.sub.2 SO.sub.4. .Iaddend. .Iadd. 6. A method of treating a body comprising lithium tantalate to obtain a polished surface thereof, which comprises contacting the surface with a mixture comprising HF and H.sub.2 SO.sub.4. .Iaddend. .Iadd. 7. A method of etching a surface of a substrate comprising lithium niobate, which comprises contacting the surface with a mixture comprising HF and H.sub.2 SO.sub.4..Iaddend.
1. Field of the Invention
This invention relates to a method of etching a surface of a substrate comprising LiTaO.sub.3 and chemically similar materials and more particularly, to a method of etching LiTaO.sub.3 with a mixture comprising hydrofluoric acid and sulfuric acid.
2. Description of the Prior Art
Recently, lithium tantalate (LiTaO.sub.3) has become of great interest in the electrical device industry. An inherent problem in the use of LiTaO.sub.3 has been the etching and polishing of this material. Lithium tantalate (LiTaO.sub.3) can be etched with an aqueous mixture consisting of HNO.sub.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.sub.3, the HNO.sub.3 -HF mixture does not polish a surface of LiTAO.sub.3 so that optical inspection thereof for surface and interior defects is possible. An etchant which both etches the LiTaO.sub.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.
The present invention is directed to a method of etching a surface of a substrate comprising LiTaO.sub.3 and chemically similar materials and more particularly, to a method of etching LiTaO.sub.3 with a mixture comprising hydrofluoric acid and sulfuric acid.
Briefly, the method comprises contacting a surface of a substrate, comprising lithium tantalate (LiTaO.sub.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.
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.sub.3) and barium titanate.
The etching system comprises a mixture comprising hydrofluoric acid, HF and sulfuric acid, H.sub.2 SO.sub.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.sub.2 SO.sub.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.sub.2 SO.sub.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.sub.2 O, 40.173 moles of H.sub.2 SO.sub.4 /1000 gms. H.sub.2 O to about 47.535 moles/1000 gms. H.sub.2 O and 3.354 moles/1000 gms. H.sub.2 O, respectively).
A substrate to be treated, i.e., comprising LiTaO.sub.3, is immersed in or exposed in some conventional manner to the etching solution mixture wherein the LiTaO.sub.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.sub.2 SO.sub.4 mixture leads to polishing of the LiTaO.sub.3 substrate surfaces. It is also to be noted that exposure of LiTaO.sub.3 to 100 percent of 97 weight percent aqueous H.sub.2 SO.sub.4 does not lead to etching thereof (within experimental error) at 90
Specific examples of the etching of a LiTaO.sub.3 substrate are as follows:
149 slices (AT cut) comprising LiTaO.sub.3 were ground to a 12μ finish. The samples were then immersed at 90 comprising 65 volume percent of an aqueous 49 weight percent HF solution and 35 volume percent of an aqueous 97 weight percent H.sub.2 SO.sub.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.sub.2 SO.sub.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.sub.3 [ 7.454 g/cm.sup.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 = 25Volume 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 = 38Volume 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 = 51Volume 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 = 64Volume 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 = 77Volume 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 = 90Volume 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.sub.3 substrate are as follows:
1. A lithium tantalate (LiTaO.sub.3) slice (AT cut) was ground to a 12μ finish. The slice was then immersed at 90 aqueous mixture comprising 65 volume percent HF (49 weight percent) and 35 volume percent H.sub.2 SO.sub.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.