CA1168134A - Method of chemically polishing both sides of an sc- cut quartz crystal plate - Google Patents

Abstract

ABSTRACT

Both sides of an SC-cut quartz crystal plate whose theta (.theta.) angle is between about -33° and -36° and whose phi (?) angle is between about 20°
and 26° are chemically polished by lapping the quartz plate with an abrasive and etching the lapped quartz plate in a 25 to 30 percent solution of hydro-fluoric acid (HF) in water, the etching being carried out until surface rough-nesses of between 0.05 µm and 0.04 µm are obtained.

Description

3 ~

This invention relates in general to a method of chemically polishing both sides of an SC-cut quartz crystal plate and in particular, to a method of chemically polishing both sides of an SC-cut quartz crystal plate whose ~ (theta) angle is between about -33 and -36 and whose ~ (phi) angle is between about 20 and 26, where ~ and ~ are the angles of cut as defined in the "IEEE Standard on Piezoelectricity", Standard No. 176-]978. The invention described and claimed in this application represents an improvement over the method described and claimed in U.S. Patent 4,274,907 issued June 23, 1981 to John R. Vig and Ronald J. Brandmayr for "Method of Chemically Polishing a Doubly Rotated Quartz Plate".
~ oubly rotated crystals, particularly the SC-cut have the potential for providing improved crystal resonators for applications in navigation, communications and identifications systems. In *4,274,907, it was disclosed and claimed that both sides of an SC-cut crystal could be chemically polished with a solution of NH4F (40%):HF(49%)=4:1. About two hours of etching time was required at 75C to etch ~ f =15f f where f is the initial frequency in MHz, f ~ is the final frequency in MHz and ~ f=f~-f in KHz. ~4,274,907 also discloses and claims that a 5 percent to 11 percent solution of hydrofluoric acid (H~) should be used to chemically polish hoth sides of a doubly rotated SC-cut quartz crystal in about two and one half hours at 75C to the same depth of etch.
The general object of this invention is to provide a method of chemically polishing both sides of an SC-cut quartz crystal plate at a Easter etching rate than disclosed and claimed in U.S. Patent 4,274,907. A particular object of the invention is to provide a method of chemically polishing both sides of an SC-cut quartz crystal plate whose ~ (theta) angle is between about -33 and -36 and whose 0 (phi) angle is between 20 and 26 , where ~ and are the angle of cut as definecl in the "IEEE Standard on Piexoelectricity", Standard No. 176-1978 at a faster etching rate than disclosed and claimed in U.S. Patent 4,274,907.

The foregoing objects have now been obtained by a method comprlsing lapping the SC-cut quartz crystal plate with an abrasive and etching the lapped quartz plate in a 25 to 30 percent solution of hydrofluoric acid (HF) in water, the etching being carried out until a surface roughness of between 0.05,um and 0.04,um is obtained.
Etching can be considered as a five step process in which the etch-ant must diffuse to the surface, be adsorbed, react chemically and the result-ing reaction products must then be desorbed and diffuse away from the surface.
The etching rate may be limited by anyone of these steps. In chemical polish-ing, the rate controlling s-tep is generally the diffusion to or from the surface. Diffusion control means that, in particular, the rate at which a reaction takes place at the surface is higher than the rate of diffusion, that is, the etchant molecules at the surface react at the rate that is faster than the rate at which the concentration at the surface can be replen-ished by the diffusion of other etchant molecules. A depleted surface layer therefore exists, outside which the etchant concentration is uniform, but inside which the concentration decreases to near zero at the surface.
~ nder such conditions, the etching is principally determined not by the properties of the surface being etched, but by the diffusion. It is clear that if a surface initially consists of hills and valleys, the proba-bility of an etchant molecule diffusing to the top of a hill will be much greater than the probability of it diffusing to the bottom of a valley. The hills will therefore be etched faster than the valleys, and the surface will become increa~singly smooth as the etching progresses.
Eventually, the surface becomes so smooth that the depleted layer can have a uniform thickness. From that point, the surface is etched evenly everywhere, and the surface smoothness no longer improves with further etching.
Chemically polished surfaces are therefore not perfectly flat but are micro-scopically undulating.
For a given etchan-t, the time required to produce a chemically polished surface is primarily a function of: the etching bath temperature, ~168:~3~

the particle size distribution of the lapping abrasive used for final lapping the quartz plates, the angles of cut of the quartz plate and the etchant composition. The higher the etching bath temperature, the more rapid]y the etching takes place, and the coarser the abrasive~ the greater the amount of material which must be removed by etching in order to produce chemically polished surfaces. The details of how the chemical polishing is influenced by the etching bath temperature, abrasive particle size, and some other para-meters is discussed in "Chemically Polished Quartz" by John R. Vig, John W.
LeBus and Raymond L. Filler, published in the Proceedings of -the 31st ~nnual Symposium on Frequency Control, 1-3 June 1977, in Research and Development Technical Report ECoM-45~8, November 1977, and in "Etching Studies on Singly and Doubly Rotated Quartz Plates" by John R. Vig, Ronald J~Brandmayr and Raymond L. Filler, published in the Proceedings of the 33rd Annual Symposium on Frequency Control, 30 ~ay - 1 June 1979. For the aluminum oxide abrasives evaluated, the etching should remove from the thickness of the plates an amount which is at least twice the average particle diameter in the final abrasive.
For example, if a 3 ~m abrasive is used for final lapping the quartz plates, the chemical polishing process should remove at least 6,um from the plate thicknesses in order to produce surfaces which can be described as chemically polished.
The angles of cut are also important variables since the inherent etching rates vary greatly with crystallographic direction. For example, in concentrated HF, the etching rate of quartz a]ong the Z-direction is over 100 times higher than along the -X direction.
The quality of quartz used is another important consideration.
When chemical polishing is attempted on groups of plates made of several different cultured quartz varieties, it is found that for many of the plates the etching produces large numbers of undesirable etch pits and etch channels.
This is particularly true for plates made of relatively low Q, fast grown materials. When plates made of natural quart7 or swept (i.e., "electrolyzed") -' ~ 1 6~3~

cultured quartz are used, the incidence of etch pit and etch channels is far fewer. Of the cultured quartz varieties, the one variety which has been vacuum swept in accordance with the method described in U.S. Patent No.
3,932,777 issued January 13, 1976 to James Claude King for "Vacuum Electro-lysis of Quartz", has the lowest incidence of etch channels.
The SC-cut crystals to he chemically polished are made of nat~lral quartz and have nominal angles of ~=Zl561-20~ and ~=-34 12l-5'. The diameters are 14 mm and the blanks have an initial frequency of 4.660 MHz.
The crystals are plano-plano and have 1 ,um or 3 ~m lapped surfaces. Before 0 chemical polishing, the crystals are cleaned thoroughly. The lapped plates are then etched in a 25 to 30 percent solution of hydrofluoric acid (HF) in water, the etching being carried out at an etching bath temperature of about 75 C for about 30 minutes until surface roughnesses of between 0.05 ,um and 0.04~m are obtained.
The chemical polishing process can remove large amounts of material from lapped plates while simultaneously producing an improved surface finish, without producing shifts in the angles of cut. The process will also produce plates of great strength, which is particularly important for high sbock appli-cat;ons.
'0 The particular apparatus used to carry out the etching method is not ~ critical. One particular apparatus that can be conveniently used includes a k~ ~
Teflon beaker that is externally heated by means of a temperature controlled heating jacket. The temperature of the etching solution can thus be controlled to about +2 C. A thick Teflon disc with a diameter slightly larger than the outer beaker is used as a cover to minimize evaporation from the beaker. A
hole through the center of the disc permits the agitation of crystals during etching.
The quartz plates are loosely held in a Teflon fixture which`-;s designed to assure that only point contacts exist between the crystals and the fixture i0 and the fixture is rotated in both directions by means of a constant speed o/~f~f ~e- f/~ o/oe~y/e~e ~ ~ 6~34 electric motor (or in vertical motion) The motor is set to rotate the etch-ing fixture through an angle of approximately 360 before reverfiing direction.
The rate of agitation is about 5 cycles per minu-te (l2 cycles per minute for vertical motion)~
A convenient etching procedure involves first the preparation of a 25 to 30 percent solution of hydrofluoric acid in distilled water. (The solution preparation and the etching are performed under a vented hood to prevent inhalation of the vapors from the etching bath). Then, the plates are cleaned thoroughly. It is particularly important to remove all contaminants such as finger prints waxes and greases, which may be impervious to the etchant~ Any number of cleaning techniques may be used, as long as contaminants that are impervious to the etchant are removed. One me-thod which has consis-tently produced good results involves placing the crystal plates into slots in the etching fixture, agitating ultrasonically in a detergent solution, then rinsing thoroughly in distilled water, then degreasing the plates with a sol-vent, followed by plasma cieaning.
The plates are rinsed in hot water, then are transferred, while wet, into the etching bath, and are shaken vigorously to make sure that there are no trapped air bubbles in the etching fixture.
After the plates reach the desired frequency, the etching fixture is removed rapidly from the etching bath and is immersed immediately into a con-tainer of hot water, given a thorough rinse under running hot water, then agitated ultrasonically in hot water, then given another rinse in running water, then dried by spin drying~ A thorough rinse is important in order to remove all residues of the etchant.
The plates are usually etched to the desired frequency by first measuring the etching rates as a function of temperature, selecting a suitable temperature between 20C and 90C, calculating the etching time required to reach the desired frequency, and etching the plates for a time slightly less than the time calculated. The reason for etching for less than the time cal-culated is that experience has shown that there are slight variations from 136~34 plate to plate in the rates at which quartz plates etch at a given temperature.
In practice, therefore, in order to etch a group of quartz plates to a narrow range about a target frequency, an iterative procedure is often necessary.
That is, the plates are etchecl for a time slightly less than the time calcu-lated, then the plates are rinsed, dried and the -frequencies are measured.
Those plates whose frequency is within the target range are removed from the etching fixture, a new etching time is calculated for the rernaining plates, the plates are etched again, rinsed, dried, measured, and the process is repeated until all the plates in the group have been etched to the proper frequency range.
The plates are then inspected under a microscope for uniformity of etch, and for defects such as scratch marks, etch pits and etch channels.
The inspection of etched plates is performed under a microscope at about ~OX magnification, with the light incidence perpendicular to the axis of the microscope and with using a black background. First, the plate is inspected for surface irregularities such as scratch marks, pits and twinned areas by tilting the plate so as to reflect light into the microscope. The crystal plate is then inspected for etch channels by holding it so that the light incidence is in the plane of the plate (i.e., edge illumination). The etch channels are most visible when the edge illumination is incident along a direction perpendicular to the direction of the channels. For example, in many types of cultured quartz, the etch channels tend to be along directions near the Z direction. These channels are most easily visible therefore with the light incident from the X direction. To help make the etch channels more visible without rotating the crystals, it is helpful to use for the edge illum ination two lights incident at a right angle to each other~ or a ring light.
The etch channels appear as small, bright streaks which extend through the plate from one side to the other. The thicker the plate, the longer the streaks,and the deeper the plate has been etched, the brighter the streaks.
Many abrasives are suitable for use in the method of the invention ~ ~ 68~3~

including alumin~n oxide, silicon carbide, diamond, cerium oxide, and zircon-ium oxide. Abrasives having different particle size distributions and/or shapes, produce different equilibrium surface topographies upon chernical polishing. The more uniformly disturbed the surface is prior to etching, the smoother wi]l be the chemically polished surface. Accordingly, one should lap the plates with progressively finer abrasives prior to etching. The final abrasive should be as fine as possible. It is highly desirable -to have the average particle size in the final abrasive 5 micrometers or less.
The temperature at which the etching is performed can vary from about 20 C to about 90C. The higher the temperature, the faster the etch rate. A convenient etching temperature has been found to be 75 C.
During etching, proper agitation, preEerably in -two directions, clocl~wise and counterclockwise or in an up and down vertical motion, is important to assure that the crystals are etched uniformly on both sides.
Agitation also serves to minimize temperature gradients in the etch bath, which in turn minimizes plate to plate etching rate variations.
After lapped plates are polished chemically, the surfaces are micro-scopically undulating, i~e., the topographies consist of hills and valleys.
In some high frequency applications, the undulations can scatter the acoustic waves and thereby degra~e the resonators' Q. The undulations, however, can be removed by polishing the plates chemomechanically; that is, by*combined chemical and mechanical action, for example, with cerium oxide and water, or with a colloidal silicia polishing agent such a Syton as manufactured by Monsanto Company or Ludox as manufactured by DuPont Gompany. The chemomechan-ical polishing can produce a smooth, undamaged surface which remains smooth upon further etching.
The crystals are then surface profiled using a ~encor Alpha-step profilometer~ Surface roughnesses between 0.05 ,um and 0.04 Jum are obtained.
The surface roughness values are estimated by calculating the root mean square deviation from an imaginary center line through the Alpha-step profile which was chosen so that the areas under the profile above and be]ow the line are approximately equal, as estimated visually.

e r~l a/~h s -3 !1 The reason that the 25 to 30 percent solutions of hyclrofluoric acid provide faster etching rates than those previously reported, while at the same time chemically polishing both sides of the plate, is not exactly known. This is because the etching process is extremely complex, that is, the various by products are unknown and the various diffusion, adsorption~ reaction, and desorption rates are not known. When concentration greater than 30% are used for etching, the etching results in one side of the qclartz plate being substan-tially rougher than the other side~ Such heavier concentration solutions are therefore not suitable for chemically polishing both sides of SC-cut quartz crystal plates.
~ e wish it to be understood that we do not desire to be limited to the exact details as described for obvious modifications will occur to a person skilled in the art.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Method of chemically polishing both sides of an SC-cut quartz plate whose theta (.theta.) angle is between about -33° and -36°
and whose phi (?) angle is between about 20° and 26°, comprising lapping the quartz plate with an abrasive, thoroughly cleaning the plate and etching the lapped quartz plate in about a 25 to 30 percent solution of hydrofluoric acid (HF) in water, such etching being carried out at a temperature of about 20-90°C for about 0.5 hour until surface roughnesses of between 0.05 µm and 0.04 µm are obtained.

2. Method according to claim 1 wherein the abrasive is selected from the group consisting of aluminum oxide, silicon carbide, cerium oxide, and zirconium oxide.

3. Method according to claim 2 wherein the abrasive is aluminum oxide.

4. Method according to claim 2 wherein the abrasive is silicon carbide.

5. Method according to claim 1 wherein the quartz plate is agitated during etching.

6. Method according to claim 1 wherein the quartz plate is thoroughly rinsed after etching to remove all residues of the etchant.

7. Method according to claim 1 wherein the quartz plate is made of a material selected from the group consisting of natural quartz and swept cultured quartz.