4. Chemical Impurities in Quartz Crystal
Both cultured and natural quartz contain chemical impurities that can affect
resonator performance. Chemical impurities are those that form chemical bonds
with silicon and oxygen in quartz. Aluminum, iron, hydrogen and fluorine are
typical chemical impurities. They are held to a much lower level in cultured
quartz than that often found in natural quartz. However, chemical impurities are
not evenly distributed in cultured quartz. The +x , -
x, z regions, and so-called s regions that occasionally form, contain different
levels of chemical impurities. The two z regions contain the least amount of
impurities. The +x region contains more impurities that the z region, and the -
x region has yet more impurities. Density of impurities in the s regions, which
are generally small, is between that in the z regions and that in the +x region.
When wide seeds are used for culturing, the z regions of a lumbered bar are
large and the +x and - x regions are small. When
narrow, less expensive seeds are used, the z regions are smaller and the +x and -
x regions larger. In general, the chemical impurities can result in degrade in
the resonator performance such as radiation hardness, susceptibility to twining,
oscillator short term and long term stability, and filter loss.
5. Resonator Q and Crystal Q
The Q value of a crystal resonator is the ratio of energy stored to energy
lost during a cycle:
Q º 2p Energy
stored during a cycle / Energy lost during a cycle
The value is important because it is a measure of the power required to drive
the resonator. The Q is primarily a function of the atmosphere in which a
resonator operates, surface imperfection, mechanical attachments and other
factors resulting from processing and mounting the resonators.
Quartz lumbered bars also are assigned a Q value, but the Q for a quartz bar
is not based on a direct measurement of energy stored and energy lost. Instead,
the Q of a quartz bar is a figure of merit based on impurities in the bar.
Chemical impurities in cultured quartz are measured by directing an infrared
light through the z regions in a cross-section slice of a lumbered bar. The
difference in transmittance at two specific wavelengths (3,500 nm and 3,800 nm)
is measured, and Q value is calculated from these data. Quartz having a high Q
contains less impurity than those with low Q, and "Infrared Q"
measurements, per EIA Standard 477-1, are routinely used by quartz growers and
users as an indicator of quartz quality.
The value of Q for a resonator generally is not identical to that for the
quartz bar from which the resonator was cut. However, the Q of a resonator can
be affected when Q of the quartz bar is below a critical level. A Q value of 1.8
million or higher for cultured quartz is an indication that chemical impurities
will not be a factor in the final Q of a resonator for most applications. Quartz
having such values for Q is generally called electronic grade (Grade C). Premium
grade quartz has a Q of 2.2 million (Grade B), and special premium has a Q of
3.0 million (Grade A). It is important to be aware of that the Q value for
cultured quartz is based on impurities in the z region only. Therefore, even
where crystal Q is adequate for an application, resonator Q and frequency vs.
temperature behavior can be adversely affected where the active portion (between
the electrodes) of a resonator includes +x, - x, or s
region material.
Quartz crystal wafers containing only z-region material can be successfully
cut only from bars grown from wide seeds, which are relatively expensive.
Fortunately, electrodes rarely cover the entire surface area of a resonator
wafer, and impurities contained in +x, - x, or s
region does not adversely affect resonator operation when these impurity
material lie outside the active portion. Thus, resonators for most applications
can use quartz grown from a relatively cheap narrow seed.
6. Summary
Piezoelectric quartz crystal, discovered in 1880 by the famous Curie couple
and once obtained at high cost from rough-hewn natural crystal, is now grown
artificially by a process that produces crystals of specified size and purity.
This cultured quartz has lowered the cost and reduced the size of resonators
critical to the timing of today’s digital circuits.
|
