A. M. Yarosh, O. V. Prokhorenko, S. V. Ermolenko
In the 80 th of the XX th century a new piezoelectric material langasite (La3Ga5SiO14), that has received a wide application in piezoelectronics, has appeared. Langasite is a strong piezoelectric, it has a large value of its piezoelectric modulus and sufficiently high stability of its parameters.
Filter langasite resonators have a capacitive factor of about 100 units, i.e. four times wider frequency interval as compared with crystal resonators. Today the actual problem is manufacturing langasite crystal elements with high quality polished surfaces useful for manufacturing HF higher Q resonators.
Technology equipment for quartz production can be used for manufacturing langasite piezoelectronic devices. Solutions based on orthophosphoric and hydrofluoric acids are used for chemical polishing and etching.
Langasite crystal elements after grinding on corundum M14 and etched on both sides on
a depth of 28 m maximum in HCl:HF:H2O = 1:10:30 solution at 85 C were used; the intermediate operation was mechanical polishing for the damaged layers alignment.
Subsequent etching was performed in solutions HCl:HF:H2O = 1:50:150 at temperature 85 C which was determined by experiments. It was determined that the maximum roughness is observed at 10-12 m etch with a deformed layer thickness of 10-15 m.
Long-term etching in HCl:HF:H2O = 1:50:150 solution results in forming a nonsoluble film on the surface. The film has a porous structure which does not influence the quality of etching and is easily removed by washing. After etching the langasite surface the film thickness can achieve 1 m but it provides high-speed and polishing etching the matrix with 3,5-3 nm roughness.
Manufacturing langasite elements as inverse mesa structure has shown that the etch profile has steep slopes in all directions. This is an indication of the solution isotropic properties relative to crystallographic axes. Plane-parallel plates remain within the allowable limits and cut angle deviations are not observed.
Protective coating for forming inverse mesa structure and electrodes was selected. For the protective coating thermal evaporated Cu-film was used and for the electrode coating Ag film with a vanadium underlayer. Vanadium was selected as an underlayer due to its good adhesion to langasite and its easy removal from the crystal element surface while removing the protective coating using less aggressive solutions that is essential for the work with langasite.
The proposed variant of the chemical treatment provides the high quality surface while manufacturing plate langasite resonators and forming the inverse mesa structure.