Note: Descriptions are shown in the official language in which they were submitted.
This invention relates in general to a method of treating a quartz
plate and in particular to such a method that will produce a chemically
polished quartz crystal surface.
High precision and high frequency quartz resonators, particularly
those for high shock applications, require quartz plates whose surfaces are
free of imperfections, such as scratches and pits. The most common method
of achieving such surfaces has been mechanical polishing. The difficulty
with mechanical polishing, however, has been its inability to produce defect-
free surfaces, at the correct frequency, with a high yield. Moreover, as
has been known since the last century, even when the polished surfaces appear
to be free of defects when examined at high magnification, the surfaces
contain hidden defects. These defects can be revealed by etching subsequent
to polishing.
The general object of this invention is to provide a method of
overcoming the difficulties associated with the mechanical polishing of
quartz plates. A further object of the invention is to provide such a method
that will produce a polished quartz crystal surface that is free of defects.
A particular object of the invention is to provide a method of polishing quarts
chemically. Another object of the invention is to provide a method of making
quart~ plates of great strength suitable for high shock resonator applica-
tions.
The foregoing objects have now been obtained by a method involving
lapping the quartz plate with an abrasive and then etching the quartz plate
in a fluoride type etchant at least until the damage produced by lapping is
removed.
Etching can be considered as a five step process in which the
etchant must diffuse to the surface, be adsorbed, react chemically, and the
resulting reaction products must then be desorbed and diffuse away from the
surface. The etching rate may be limited by any one of these steps. In
chemical polishing, the rate controlling step 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
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diffusion; that is, the etchant molecules at the surface react at a rate
which is faster than the rate at which the concentration at the surface
can be replenished 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~
Under such conditions, the etching is principally determined not
by the properties of the surface be~ing etched, but by the diffusion. It is
clear that if a surface initially consists of hills and valleys, the probabil-
ity of an etchant molecule diffusing to the top of a hill will be muchgreater 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 increasingly 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
eve~ywhere, and the surface smoothness no longer improves with further etching.
Chemically polished surfaces are therefore not perfectly flat but are micro-
scopically undulating.
Of the known etchants for quartz, particularly desirable are fluor-d
ide type etchants such as aqueous solutions of ammonium bifluoride, and of
mixtures of hydrogen fluoride and ammonium fluoride.
For a given etchant, the time required to produce a chemically
polished surface is primarily a function of the etching bath temperature and
of the particle size distribution of the lapping abrasive used for final
lapping the quartz plates~ The higher the etching bath temperature, the more
rapidly 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
other parameters is discussed in "Chemically Polished Quartz" by John R. Vig,
John W. LeBus and Raymond L. Filler, published in the Proceedings of the 31st
Annual Symposium on Frequency Control, 1-3 June 1977, and in Research and
Development Technical Report ECoM-4548, November 1977. 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 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 quartz or swept (i.e., "electrolyzed")
cultured quartz are used, the incidence of etch pits 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 Electrolysis
; of Quartz," has the lowest incidence o etch channelsO
~-~ AT-cut plano-plano natural quartz plates are final lapped with a 3
micrometer aluminum oxide abrasive and then etched in a saturated solution of
ammonium bifluoride at an etch bath temperature of 75C. Chemically polished
surfaces are produced in less than 30 minutes.
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 cuto The process will also produce
plates of great strength, which is particularly important for high shock
applications.
The particular apparatus used to carry out the etching method is
not critical. One particular apparatus that can be conveniently used includes
a 1000 ml glass outer beaker containing water and a floating 400 ml Teflo ~
beaker, which in turn contains the saturated ammonium bifluoride solution. A
temperature controlled stirring hot plate with a thermistor sensor can be
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38g~;2
used to control the temperature of the water surrounding the Teflon~beaker.
The temperature of the ammonium bifluoride solution can thus be controlled to
about + 105C. A thick Teflon disc with a diameter slightly larger than the
outer beaker is used as a cover to minimize evaporation from the beakers.
The weight of this disc also serves to push the inner beaker down to assure
that the fluid level in the inner beaker is always about 3 cm below the
water level in the outer 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~jig which is
designed to assure that only point contacts exist between the jig and the
plates. The plates are agitated slowly in both directions by means of a
constant speed electric motor. The motor is set to rotate the etching jig
through an angle of approximately 360 before reversing direction. The rate
of agitation is about 5 cycles per minute.
A convenient etching procedure involves first the preparation of
a saturated solution of ammonium bifluoride in a Teflon container. The
ammonium bifluoride (NH4F~HF) flakes are mixed with distilled water, and the
solution heated to the desired tempera-ture. The amount of NH4FHF used is
such that after the solution reaches the equilibrium temperature, some undis-
solved flakes remain in the bottom of the container throughout the etchingprocess. (The solubility of NH4F-HF in water increases from 61 gms per 100
ml of solution at 60 C to 86 gms per 100 ml at 100 C.) The solution prepara-
tion 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.
To assure that the surfaces are etched evenly, it is particularly important
to remove all contaminants such as 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 method
which has consistently produced good results involves the immersion of the
blanks in ethyl alcohol in a Petri dish the bottom of which is lined with
open cell urethane foam, and then scrubbing both sides of each blank with a
-foam swab. The crystal plates are then placed into the slots in the etching
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~1%~3842
fixture and agitated ultrasonically in a detergent solution, then rinsed
thoroughly in distilled water. A second satisfactory cleaning technique
involves plasma cleaning in an oxygen plasma followed by a thorough rinse in
distilled water.
From the final rinse the plates 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. During etching, the plates are
agitated in both directions to assure even etching of both sides.
After the plates reach the desired frequency, the etching fixture
is removed rapidly from the etch bath and is immersed immediately into a
container of hot water, given a thorough rinse under running hot water, then
agitated ultrasonically in hot water, then given another rinse in running
distilled 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 etch 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
calculated is that experience has shown that there are slight variations from
plate to plate in the rates at which quartz plates etch at a gi~en 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 necessaryO
That is, the plates are etched for a time slightly less than the time calcula-
ted, 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 remaining 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.
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389L~
The inspection of etched plates is performed under a microscope
at about 40 X 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 micro-
scope. 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 illumin-
ation). 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 quartæ, the etch channels tend to be
along directions near the Z directionO 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 illumination 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. To facilitate inspection for etch channels, it is
desirable to etch the plates until a minimum of 16 ~m is removed from the
plate thicknesses.
In the method of the invention, the etching is believed to be
diffusion controlled. That is, for a lapped initial surface, the surface
becomes increasingly smooth as the etching progresses. The rougher the
initial surface, the rougher the final equilibrium surface, where the depleted
surface layer of etching solution has uniform thickness everywhere, and the
hills therefore, are no longer etched faster than the valleys. A smooth,
undisturbed initial surface remains smooth ever after a large amount of
material is removed by the etching. No signs of preferential etching along
the different crystallographic axes appear on blanks which have been suitably
polished mechanically prior to etching.
Many abrasives are suitable for use in the method of the invention
including aluminum oxide, silicon carbide, diamond, cerium oxide, and
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zirconium oxide. Abrasives having different particle siæe distributions
and/or shapes, produce different equilibrium surface topographies upon
chemical polishing. The more uniformly disturbed the surface is prior to
etching, the smoother will be the chemically polished surface. Accordingly,
one should lap the plates with progressively finer abrasives prior to etch-
ing. The final abrasive should be as fine as possible. It is highly desir-
able 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 90 C. The higher the temperature, the faster the etch
rate. A convenient etching temperature has been found to be 75C.
During etching, proper agitation, preferably in both directions,
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 etch r~te variations.
After lapped plates are polished chemically, the surfaces are
microscopically undulating, i.e9, the topographies consist of hills and
valleys. In some high frequency applications, the undulations can scatter
the acoustic waves and thereby degrade 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 silica polishing agent such as Syton~as manu-
factured by Monsanto Company or Ludox as manufactured by DuPont Company.
The chemomechanical polishing can produce a smooth, undamaged surface which
remains smooth upon further etching.
Example 2
AT-cut plano-plano quartz plates are final lapped with 1 um
aluminum oxide abrasive to a frequency of 18.0 MHz. The plates are then
etched in a saturated solution of ammonium bifluoride at 75C, to a frequency
of 25.0 MHz. The surfaces at this point are microscopically undulating,
with a surface roughness of about 0.12 ,um. The plates without etch channels
are selected, and are polished chemomechanically with Ludox~until the
~ ~rq ~ f k
38~Z
undulations are removedO At the completion of chemomechanical polishing,
the frequencies are about 26.0 MHz. The plates are then etched again in the
saturated solution of ammonium bifluoride, at 75C, until a frequency of
50.3 MHz is reached. The surfaces have remained smoothO The fundamental
mode 50 MHz resonators which are fabricated of these plates show no Q degra-
dation.
Example 3
Resonators are prepared using chemically polished plates. All of
the resonators are fundamental mode, in the range of 18 MHz to 22 MHæ. The
plates are plano-plano, AT-cut, 6.4 mm diameter, and are made of vacuum
swept cultured quartz. The depths of etch range from ~ f=-2-f ff to ~ f=22f ff
where ~ f is the change in frequency in kHz, and f and ff are the initial
and final frequencies, respectively, in MH . The surfaces prior to etching
are final lapped with a 1 micrometer aluminum oxide abrasive. Subsequent
to etching, the plates are free of etch channels. In the case of the highest
Q resonators, the motional capacitances range from 12 fF to 13.5 fF, the
resistances range from 3 ohms to 5 ohms, and the Q's range from 140,000 to
210,000 with no apparent Q degradation with depth of etching.
The method of the invention may allow manufacturers to stock lapped
plates at only a few frequencies at each commonly used angle of cut and then
etch the plates to the required frequencies as the need arises~ The method
may also permit the manufacturing of miniature contoured high frequency
resonators, since such small diamter resonators can now be contoured at con-
ventional frequencies and then etched up to high frequencies. If a masking
material which is resistant to the etch solution can be found, the method
may also permit the fabrication of high frequency resonators and filters
with the inverted mess structure.
In addition to the AT-cut, the etching technique described has
also been shown to be capable of chemically polishing BT-cut quartz plates,
and ST-cut quartz plates which are frequently used in surface acoustic wave
devices.
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z
We 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 artO
