Geoscience Reference
In-Depth Information
Sweeping is influenced by the electric field, strength, electrode type, and atmo-
sphere. It takes less time in air due to the availability of water in the atmosphere
[58]
.
Electrodes, usually platinum, or gold or aquadog are deposited on each of the Z
surfaces of a lumbered bar. The crystal is then subjected to a field of 1000
2000 V/cm
550
C. The current is continuously measured and
the sweeping is considered completed when the current has dropped to a constant
value. It takes usually 5
(thickness) at a temperature of 500
7 days to remove alkalies. In some quartz, the sweeping may
take 7 days or even more. Sweeping has also been used to dope quartz crystals in order
to determine the effect of the dopant on the properties of the crystallization of quartz.
A vast amount of literature data shows that sweeping cannot become a routine process
even if all the same experimental conditions are ever applied. The quality of the swept
crystal can change in such a way that the modifications are sometimes unexpected. So,
each industrially swept crystal must be controlled before using. Some of the controlling
techniques are the X-ray topography and the IR spectroscopy. These techniques help to
characterize the physicochemical defects and the
α
3500
value (Q factor). The reader
can refer to the works of Refs
[12,59,60]
.
There are several other techniques employed for the fabrication of piezoelectric
high- to ultrahigh-frequency devices based on quartz resonators. The important ones
are chemical polishing and ion beam etching (IBE). The reverse thermodynamic
relations are employed for the refinement of chemical polishing. Several solvents
like HF and NH
4
HF
2
, NaOH, KOH
xH
2
O, etc. can be used
[61]
.
Similarly, the industrial chemical etching process is specially dedicated to large
thickness removals without damaging the blank surface texture. This is most useful
for frequency applications of quartz, because the mechanical grinding and lapping
introduce surface stresses. Fluoride media is most popularly used for this type of
chemical polishing
[62]
. Cambon et al. (1994)
[63]
have tried industrial chemical
etching successfully in the temperature range of 150
xH
2
O, NaOH
180
C using concentrated
NaOH solvents. During the chemical etching process, several factors influence the
process: kinetics, etching temperature, etching time, plate orientation, SiO
2
concen-
tration, solvent concentration, wafer carrier geometry, and so on. Using this process,
about 3200 quartz plates can be processed. The resonators manufactured by this
process have demonstrated a high level of performance, even higher than those
obtained by mechanical means.
Future avenues in this direction are improvements on theoretical knowledge,
improvements in the quality of solvents, nutrients, and seeds, autoclaves' technol-
ogy, material used for autoclaves, dimensional ratio (diameter/length), crystalliza-
tion zone/dissolution zone, basic questioning of the validity of presently used
growth parameters (P, T ), and better understanding of complementary treatments
like sweeping and chemical etching.
In recent years, there have been some efforts to grow crystalline quartz films with
AT-cut plane by means of catalyst-enhanced vapor-phase epitaxy under atmospheric
pressure. It has been proved that quartz films grown by vapor-phase epitaxy show
good oscillation characteristics at room temperature
[64]
. Using electro-diffusion,
several types of structures were obtained inside and on the cathodic face of the
quartz. The study of structures growth by electro-diffusion in crystal lattices with
