Abstract:
A method for manufacturing a langasite single crystal substrate comprises the steps of polishing at least one of the main surfaces of a raw material substrate; and wet-etching said polished main surface of the substrate with a solution including H 3 PO 4 , HNO 3  and CH 3 COOH.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a langasite single crystal substrate, a langasite single crystal substrate, and a piezoelectric device using the substrate, such as a piezoelectric resonator and a surface acoustic wave (SAW) device. 
     2. Description of the Related Art 
     Conventionally, a single crystal substrate, including a piezoelectric substrate, is subjected mainly to wet etching after polishing for the purpose of cleaning the surface. On the other hand, a single crystal substrate material for surface acoustic wave (SAW) use is subjected to roughening the rear surface side, in order to eliminate the effects of reflection of the bulk waves at the rear surface, the waves propagating across the substrate. Thus, the difference of the states of the front and rear surfaces will cause warps. Since these warps exert influences on the cracking in the substrate or on the accuracy of photolithography in forming an electrode, the wet etching treatment is applied as a general measure to alleviate the warps of the substrate. 
     As an etching solution (etchant) for use in manufacturing a langasite (La 3 Ga 5 SiO 14 ) substrate, an acidic aqueous solution comprising HCI, HF and H 2 O is known (for example, “Etching LANGASITE and Quartz Crystals”, Proceedings of the IEEE International Frequency Control Symposium (1994), pp. 245-250). 
     This is an aqueous solution obtained by mixing HCI, HF and H 2 O at a volume ratio of 1:50:150. It is heated to 70° C. for etching. It was reported that when polishing was performed using a colloidal silica, the surface state at the time of polishing treatment was maintained after etching for two hours when this etching solution was used, and a good surface state was obtained. 
     However, as a result of our experiments for confirming the effects of the etching method using a conventional etching solution, it was found that film was formed on the surface of a substrate, the film being supposedly made of LaF 3 . Although some improvement was observed by increasing the amount of HCI in the etching solution, it was not possible to completely eliminate the LaF 3  film. Furthermore, since the etching temperature was as relatively high as 70° C., a time-based change in the compositional ratio of the etching solution occurred due to the evaporation of H 2 O, and, therefore, it was difficult to maintain a similar etching state. Furthermore, because the etchant contains hydrofluoric acid, it was necessary to put more effort to secure safety regarding the apparatuses and the waste water treatment, resulting in more complicate handling. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a method for manufacturing a langasite single crystal substrate which furnishes a good surface state, by an etching treatment in a short time, at a relatively low temperature and without forming a film on the substrate surface. Other objects of the present invention are to provide a langasite single crystal substrate obtained by the above-described method and to produce a piezoelectric device using it. 
     To achieve the above-described objectives, the method for manufacturing a langasite single crystal substrate according to the present invention comprises the step of polishing at least one of the main surfaces of a raw material substrate and the step of wet-etching the above-described main surface of the polished substrate with a solution including H 3 PO 4 , HNO 3  and CH 3 COOH. 
     In the langasite single crystal substrate according to the present invention, at least one of the main surfaces of the single crystal substrate is wet-etched with a solution including H 3 PO 4 , HNO 3  and CH 3 COOH. 
     A piezoelectric device according to the present invention comprises a langasite single crystal substrate, wherein at least one of the main surfaces of the single crystal substrate is wet-etched with a solution including H 3 PO 4 , HNO 3  and CH 3 COOH and comprises a transducer formed on the above-described one of the main surfaces of the single crystal substrate. 
     It is noted that the langasite single crystal substrate recited in the present invention is a piezoelectric single crystal substrate having a Ca 3 Ga 2 Ge 4 O 14 -type crystal structure. 
     According to the manufacturing method of the present invention, it is possible to obtain a langasite single crystal substrate having a good surface state by an etching treatment in a short time, at a relatively low temperature and without forming a film on the surface of a substrate. Therefore, such a langasite single crystal substrate can be used to manufacture a piezoelectric device such as a surface acoustic wave device with good properties. 
     For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing one example of an SAW filter according to an embodiment of a piezoelectric device of the present invention. 
     FIG. 2 is a graph showing the relationship between the etching time and the surface warpage of a substrate in the present invention. 
     FIG. 3 is a top view of a single crystal substrate according to the present Examples, showing the scanning direction of a surface roughness meter at the time of measuring the warpage of a substrate. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention are explained in detail with reference to the drawings. 
     FIG. 1 is a perspective view showing one example of an SAW filter according to an embodiment of a piezoelectric device of the present invention. The SAW filter  1  in FIG. 1 includes a langasite single crystal substrate  2  such as La 3 Ga 5 SiO 4 . Interdigital transducers  6   a  and  6   b  are formed with an A 1  thin film on the surface of this langasite single crystal substrate  2 . Furthermore, one interdigital transducer  6   a  is connected to one input/output terminal  8   a  and is grounded, while the other interdigital transducer  6   b  is connected to the other input/output terminal  8   b  and is also grounded. A langasite single crystal substrate according to the present invention can be used as the langasite single crystal substrate  2  of this type in an SAW filter  1 . 
     The composition of the etching solution for use in manufacturing a langasite single crystal according to the present invention is a composition known as a wet-type etching solution for etching an A 1  film to form an A 1  electrode on the surface of a piezoelectric substrate (for example, Japanese Unexamined Patent Publications Nos. 7-50537, 10-93369, 10-135759, etc). 
     In our practice of designing a piezoelectricity-application device by employing a langasite single crystal, generally formed is an Al thin film in a thickness of 50-300 nm on a substrate surface and an electrode by photolithography. In the course of the manufacturing, a solution mixture consisting of H 3 PO 4 , HNO 3  and CH 3 COOH is, used as an etching solution. It was found that when a langasite substrate was immersed in this acidic solution, etching of the substrate surface occurred while keeping its mirror surface state which had been obtained by a polishing process intact. Furthermore, when a wafer with a roughened rear surface was immersed in the same way, it was observed that warps which had occurred to the wafer were alleviated in a short time. 
     Each of H 3 PO 4 , HNO 3  and CH 3 COOH constituting the etching solution according to the present invention can etch a langasite substrate. While with H 3 PO 4  and HNO 3 , etching proceeds very rapidly at a relatively low temperature, conspicuous unevenness will appear on the surface, preventing one from obtaining a smooth surface state. With CH 3 COOH, etching can proceed with a relatively low speed, and yet fails to provide a smooth surface state. However, the etching solution according to the present invention can provide a good surface state by controlling the etching speed at an adequate value through mixing HNO 3  and CH 3 COOH in a base solution of H 3 PO 4  having a very large etching speed. In general, it is preferable that the etching solution comprises about 50 to 90 vol. % of H 3 PO 4 , about 70 to 80 vol. % of HNO 3  and about 6 10 vol. % of CH 3 COOH. 
     Working Examples 
     First, an La 3 Ga 5 SiO 14  single crystal was selected as a langasite single crystal. One such single crystal grown along the &lt;001&gt;axis direction and having a diameter of 84 mm was vertically cut along the crystal growth axis with an inner peripheral blade slicing machine into slices having a thickness of 0.6 mm, followed by an edge surface processing to prepare a disc substrate with a diameter of 76.2 mm. This substrate was subjected to a dual side polishing processing using a dual surface lapping apparatus and using abrasive grains GC #1,000 to obtain a thickness of 0.4 mm. Only one of the surfaces of this substrate was subjected to mechanochemical polishing using a colloidal silica to realize a finishing thickness of 0.35 mm. At this point, the warpage of the La 3 Ga 5 SiO 14  single crystal substrate was about 0.15 mm at its maximum, caused by the convex profile toward the polished surface side. 
     Next, a solution obtained by mixing H 3 PO 4 , HNO 3  and CH 3 COOH, each of special grade (99% purity or more), at a volume ratio shown for Sample 1 or Sample 2 in Table 1 was put in a vessel and heated to 40° C. in a thermostat. When the solution temperature became constant, the above-described La 3 Ga 5 SiO 14  single crystal substrate which had been preheated at 40° C. was immersed in the solution for minutes for etching. Then, it was subjected to cleansing with a 5% HCI aqueous solution at room temperature, followed by washing with running water. 
     As a result, the warpage was 0.02 mm or less throughout the substrate surface after the 10 minute etching, resulting in an improved warpage, a level which does not pose any problem in practice. Furthermore, regarding the surface state, there was no film formed on the substrate surface and the surface state obtained at the polishing was maintained, which was different from the case in which the conventional HF—HCI—H 2 O type etching solution was used. The results are shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Working Example 
                   
               
             
          
           
               
                   
                 Sample 
                 Sample 
                 Comparative Example 
               
             
          
           
               
                 Component 
                 1 
                 2 
                 Sample 3 
                 Sample 4 
                 Sample 5 
               
               
                   
               
             
          
           
               
                 H 3 PO 4   
                 11 
                 16 
                 0 
                 0 
                 0 
               
               
                 HNO 3   
                 3 
                 1 
                 0 
                 0 
                 0 
               
               
                 CH 3 COOH 
                 1 
                 2 
                 0 
                 0 
                 0 
               
               
                 HF 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                 HCl 
                 0 
                 0 
                 50 
                 5 
                 1 
               
               
                 H 2 O 
                 0 
                 1 
                 150 
                 15 
                 3 
               
               
                 Surface 
                 0.02 
                 0.02 
                 0.05 
                 0.07 
                 0.07 
               
               
                 Warpage 
                 mm 
                 mm 
                 mm 
                 mm 
                 mm 
               
               
                 Surface State 
                 Excellent 
                 Good 
                 Poor 
                 Poor 
                 Poor 
               
               
                 Remarks 
                   
                   
                 Film 
                 Film 
                 Film 
               
               
                   
                   
                   
                 formed 
                 formed 
                 formed 
               
               
                   
               
               
                 Note:  
               
               
                 Each value in the cells indicates a volume rate of each component.  
               
             
          
         
       
     
     It is noted that the surface warpage in Table 1 was a result of scanning with a surface roughness meter along the direction of the diameter on the surface of a single crystal substrate which had been subjected to an etching treatment. FIG. 2 shows the relationship between the etching time and the surface warpage of the substrate in Sample 1. The legends location A, location B, location C, and location D in FIG. 2 indicate warpage toward lines A, B, C and D respectively in a top view of a single crystal substrate shown in FIG.  3 . 
     Also, the surface state in Table 1 was determined depending on the surface flatness and whether there was a film or deposits, from the observation of a single crystal substrate surface obtained by the etching treatment, with a differential interference microscope. The term, “Excellent” represents the state that neither surface unevenness nor film nor deposits were observed, the term “Good” represents the state that there were neither film nor deposits observed while there was slight surface unevenness observed, and the “Poor” represents the state that there were both surface unevenness and deposits observed. 
     Comparative Examples 
     The dual side polishing treatment and the mechanochemical polishing treatment were performed in the same way as in the Working Example. An La 3 Ga 5 SiOl 4  single crystal with a diameter of 76.2 mm thus obtained was subjected to etching with etching solutions obtained by mixing ffF, HCI and H 2 O at the HF:HCI:H 2 O volume ratios shown in Table 1 for Samples 3 to 5. It is noted that the etching was performed as follows: an etching solution was put into a vessel and was heated to 70° C. in a thermostat; when the solution temperature became constant, the above-described La 3 Ga 5 SiO 14  single crystal substrate which had been preheated at 70° C. was immersed in the solution for one hour for etching; and then it was subjected to cleansing with a 5% HCI aqueous solution at room temperature followed by washing with running water. 
     Next, the warpage and the surface state of single crystal substrates obtained by the etching treatment were observed as in the case of the Working Example 1. The results are shown in Table 1. 
     As shown in Table 1, a film was formed on the surface of an La 3 Ga 5 SiO 14  single crystal substrate in each Comparative Example, and a phenomenon that cracks appeared as the substrates were dried was observed. Furthermore, the surface state of the substrates which had been obtained by the polishing processing could not be maintained. 
     Although in the above-described Examples, explanations were made based on the case in which the langasite single crystal was an La 3 Ga 5 SiO 14  single crystal, a piezoelectric single crystal having a Ca 3 Ga 2 Ge 4 O 14 -type crystal structure, such as one composed of at least three elements selected from the group consisting of La, Ba, Sr, Ga, Ge, Si, Al, Ti, Nb, and Ta, provides the same effect as a langasite single crystal. 
     While preferred embodiments of the invention have been disclosed, various modes of carrying out the principles disclosed herein are contemplated as being within the scope of the following claims. Therefore, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.