Geoscience Reference
In-Depth Information
The structural defects in quartz are basically the dislocations. Their origin can
be due, generally, to foreign particles, thermodynamical parameters governing the
crystal growth, or hydrodynamic conditions inside the reaction vessel.
The asymmetry in the dissolution behavior of quartz under hydrothermal conditions
is an interesting topic in order to enhance growth rate on certain directions
[51,52]
.
5.2.6 Processing of
α
-Quartz for High-Frequency Devices
The study of the type of defects present on silica surfaces would be of great impor-
tance for a better understanding of electrical or piezoelectric properties. The devel-
opment of high-frequency devices (24 MHz
!
100 MHz or more) induces some
strong constraints concerning the shaping of the
-SiO
2
material. The low-defect,
high-purity synthetic quartz should have the following characteristics
[53]
:
α
Parameter
Desirable
So far Achieved
,
10/cm
2
Etch channel density
,
86
Inclusion density
,
10/bar
Impurity concentrations (ppb)
Al
,
200
700
Li
300
300
,
Na
,
500
1640
K
,
40
300
Fe
,
100
1800
Q (3500/3800 cm
2
1
)
,
2.5
3
10
6
.
2.5
3
l0
6
Strain
None
Variable
Fringe distortion
,
0.05 RMS Variable
The existence of defects, either physicochemical or structural, in synthetic
quartz crystals leads to the critical modifications of devices. There are three differ-
ent parameters on which the quality of quartz depends
[54]
:
i. Chemical impurities, e.g., (OH)
2
distribution, induce a large decrease of the acoustic Q.
The substitution O
2
2
!
(OH)
2
being coupled with the cationic one (Si
4
1
!
M
3
1
), the
M
3
1
impurities can play an important role concerning the chemical aging of resonators
and its behavior versus ionizing radiations.
ii. Crystal defects like dislocations, etching channels, and fractures, which influence the
acoustic distribution.
iii. Inclusions (iron, sodium, acmite, aluminum, and so on).
As the application of quartz goes to higher and higher frequency devices, the
thickness of the quartz plate drops. For example:
24 MHz range applications:
70
μ
m thickness
B
100 MHz range applications:
6
7
μ
m thickness
:
B
