Process for dissolution of a crystalline material

Details Process for dissolution of a crystalline material Process for dissolution of a crystalline material

1992-06-22

1994-01-04

Powell, William A.

Adhesive bonding and miscellaneous chemical manufacture

Delaminating processes adapted for specified product

Delaminating in preparation for post processing recycling step

156636, 156645, 156654, 252 792, B44C 122

Patent

active

052756971

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for chemical dissolution of a crystalline material. It applies especially to the chemical machining of piezoelectric materials.
The proliferation of radiocommunication devices in which the basic device is a piezoelectric resonator has led to a rapid development of piezoelectric materials. Quartz (SiO.sub.2) is the material most commonly employed at present. Many investigations are being made into other materials, especially berlinite (AlPO.sub.4) or gallium orthophosphate (GaPO.sub.4) because they have properties which differ slightly from those of quartz.
A resonator is made from a lamina of piezoelectric material. At least one electrode is deposited on each main face of the lamina. The resonator is characterised by its resonance frequency. This frequency corresponds to a well-determined thickness of the lamina, between two electrodes. The surface quality of the lamina must be as good as possible, so as not to perturb the frequency response of the resonator. To give the lamina the desired thickness, it is machined at least in its central part. Various machining processes are known. The higher the frequency of the resonator, the thinner is the lamina. At low frequency the machining may be mechanical. The lamina is lapped with an abrasive. For example, in the case of the AT section of quartz a thickness of 40 micrometers corresponds approximately to a frequency of 40 MHz. With higher frequencies it is no longer possible to perform mechanical machining. An ion process can be employed. The lamina is bombarded with accelerated argon ions. The process gives good results from the viewpoint of surface quality, but is costly.
It is also possible to employ a dissolving process which employs, in solution, a solvent for the material to be etched. For example, a berlinite lamina is immersed in a solution of hydrofluoric acid (HF) or of ammonium bifluoride (NH.sub.4 HF.sub.2). This process is less costly than ion machining but is not always wholly satisfactory from the viewpoint of surface quality.
In fact, crystalline materials do not have a perfect structure. They unavoidably contain structural defects such as dislocations which are flush with the surface of the material. Dissolving using the solvent begins preferentially at the dislocations. Etch pits are then formed at the surface of the material. They become hollow and are transformed into etch channels which run completely across the lamina. The final result is that the surface quality of the lamina does not improve. The pits and channels give rise to severe perturbations of the frequency response of the resonator. This process can be employed only with laminae of high crystalline quality and of high purity.
To overcome these disadvantages, the present invention proposes a process for controlled dissolution of a crystalline material consisting of a number of kinds of ions, making it possible to obtain a correct surface quality even with a crystalline material exhibiting structural defects. This dissolution process is low in cost. This process consists in employing a solution containing, before dissolution, a solvent for the crystalline material and at least two kinds of ions of the crystalline material. The molar concentration of ion of each kind is, on the one hand, higher than zero and, on the other hand, lower than the molar concentration of ion of each kind which the solution would have at saturation if it contained all of the constituent ions of the crystalline material, under the pressure and temperature conditions of the dissolution.
It has been found according to the invention that this process makes it possible to obtain very satisfactory results with berlinite (AlPO.sub.4) as crystalline material, the solvent being especially hydrochloric acid, sulphuric acid or phosphoric acid. The dissolution preferably takes place at a temperature between 20.degree. C. and 150.degree. C. and at atmospheric pressure.
The material is not necessarily berlinite; it can be envisaged that the crystalline material may be gallium or

REFERENCES:
patent: 4411731 (1983-10-01), Miller
patent: 4626732 (1986-12-01), Debaisieux et al.
patent: 4686324 (1987-08-01), Debaisieux et al.
patent: 4859898 (1989-08-01), Aubry et al.
patent: 4985306 (1991-01-01), Morizane et al.
Proceedings of the 39th Annual Frequency Symposium, May 29-31, 1985, Philadelphia(US) pp. 301-308; J. Dowset, et al.: "Etch Processing of Bulk and Surface Wave Devices".

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