Crystal Blanks
A crystal blank is the quartz wafer exclusive of any mounting
structure or hermetic package. Blanks can be any of the typical
crystal cuts, AT, BT, SC, FC, IT and GT. Crystal blanks are commonly
supplied to OEMs to be processed in to various frequency control
products: hybrid oscillators, hybrid VCXOs, sealed crystal units and
sensors.
There are two basic types of banks: un-plated blanks and plated
blanks . Un-plated blanks have no electrodes deposited on each side
of the quartz wafer. Common to both un-plated blanks are surface
finish and blank geometry.
Surface finish is the texture and quality of the crystal blank's
surface. It is specified by the particle size, in microns, of the
final abrasive used in the processing of the blank: 5 micron, 3
micron, and 1 micron. Surface finish better than 1 micron is a
"polished" finish used in the manufacture high-precision resonators.
Blank geometry refers to the, diameter, thickness and cross section
of the crystal blank. For AT, BT and doubly rotated blanks,
frequency is a function of thickness. Diameter has a direct
relationship to thickness but also is constrained by the mechanical
limitations of the package. A good rule of thumb is the higher the
fundamental frequency, the smaller the blank and vice versa. The
cross section of the blank may have to be manipulated/shaped to
provide optimum performance of the resonator. The cross section
designs include but are not limited to plano-plano, beveled (edges),
plano-convex, convex, and ring-supported (inverted-mesa).
Common Blank Cross Sections:
Inverted-mesa/ring Supported Crystal Blank:
Since diameter and frequency are directly related, a number of blank
diameters (in inches) are available to provide products over the
broad range of frequencies from 1-250 MHz:
|
.150 |
.270 |
.390 |
|
.200 |
.295 |
.420 |
|
.220 |
.323 |
.550 |
|
.240 |
.350 |
.600 |
|
.250 |
.370 |
.650 |
2.5 mm X
5.0 mm (rectangular strip)
UN-PLATED BLANKS:
Un-plated blanks
are typically supplied to oscillator OEMs who will finish the
plating process by adding their own electrodes and inserting them
into crystal holder, hybrid oscillators or sensors. Since a raw
blank is in such an early stage of the manufacturing process and
further processing steps will have a direct effect on the electrical
performance of the blank, only very rough approximation of
frequency, stability are part of the specification. Frequency is
typically specified as the nominal plus or minus approximately +/-
0.5% or +/-500 ppm. Stability, verified only by x-ray diffraction
would typically +/- 1 minute of arc.
PLATED BLANKS:
Plated blanks are similar to un-plated blanks except that they
include the electrodes for electrical interface to the circuit.
Again, the OEM's intent is to use them as a building block for the
final product, hybrid oscillator, crystal unit or sensor. Due to a
plated blank's being further along in the resonators finishing
process, the specification can be more stringent that that of an
un-plated blank. Calibration tolerance is typically specified at +/-
0.2% or +/- 200 ppm. Stability for the blank, excluding any
exogenous effects can be verified by placing a sample group into
holders and testing over temperature to verify performance.
Several items are part of any electrode specification for plated
blanks. They include type of metal(s), electrode size, and
configuration.
Electrodes provide the electrical interface to the circuit requiring
they be conductive. Various conductive metals are used in the
manufacture of plated blanks, not all of which adhere well to
quartz. Often chromium is first evaporated onto the blank to provide
a good interface with the quartz. After the chrome is placed on the
plank, an overlay of nickel, gold or silver is added. Aluminum is
another metal that is used without the chrome underlay since it
bonds very well with the quartz. Aluminum oxidizes quickly
establishing a non-conductive layer which must be removed before
further processing. Tuning to frequency is accomplished by either
adding or removing metal to the electrode.
Electrode size is a design criteria and is selected to achieve
specific resonator performance such as pullability/deviation,
spurious suppression, spurious free band-pass, Q, phase-noise,
motional resistance, capacitance, inductance etc. within the
confines of frequency and package size. Since all are inter-related,
it is nearly impossible to change one with out effecting other.
Electrode diameters range from 0.010 through 0.500 inches and are
spaced in such a way as to provide over-lap in crystal motional
parameters.
Electrode configuration refers to the angle formed by the electrodes
(on opposing sides of the blank) to each other. The three standard
configurations are:
90 degrees
120 degrees
180 degrees
(Go to top of page) |
