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Fil-Tech, Inc.
Technical Bulletin No. 5
Effect of Electrode Metal on Quartz
Crystal Sensor Performance
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The type of metal used for the contact
electrode on a quartz crystal sensor has a
pronounced influence on the crystal’s ability to measure film thickness. As a result, Fil-Tech
provides four standard crystal: Gold, Longer Life
Gold, Alloy, and Silver.
Gold
is the mostly widely known crystal electrode material. It offers low contact
resistance, very high chemical stability, and is easy to deposit. Typically, gold crystals are used
for low-stress metal depositions such as gold, silver, and/or copper. With gold it is possible to get
frequency shifts of up to 1 Megahertz without adverse effects. However, gold electrodes are
relatively inflexible, transmitting stresses from deposited films to the underlying quartz.
Transmitted stresses may result in frequency jumps and crystal instability.
Longer
Life Gold
crystals are exlcusive to Fil-Tech and offer longer life than standard
gold crystals. Fil-Tech's proprietary process for longer life gold combines the low contact
resistance and high chemical stability of gold crystals with the plastic yielding qualities of alloy
crystals to produce a superior, longer life gold crystal. Fil-Tech recommends our longer life
gold
for anti-reflective coatings and semiconductor processes to dissipate the stresses caused by
dielectric and high stress material depositions. Laboratory experiments have shown over 200%
increase in crystal life with deposited magnesium fluroride on gold longer life.
Alloy,
an aluminum-silver composition is the best electrode for high-stress material
depositions including; silicon monoxide, silicon dioxide, magnesium fluroride, and titatium dioxide.
Deposited high-stress materials often cause erratic crystal performance produced by high tensile
or compressive stresses. These stresses cause bending of the quartz and subsequent frequency
shifts.
Alloy dissipates the stress of the deposited
film by plastic yielding or flowing. Long
before the compressive or tensile forces cause the crystal to bend the electrode will "give,"
dissipating the stress. This results in a much more stable crystal with a longer period of steady,
jump-free oscillation. Laboratory experiments have shown as much as a 400% increase in
crystal life with deposited silicon dioxide on alloy.
Silver
is an excellent all-around electrode material. Silver has a low contact
resistance
and exhibits some degree of plastic yielding. However, silver tends to tarnish in the presence
of
atmospheric sulfides. The tarnish will increase contact resistance and decrease the adherence
of film deposits on the crystal.
Tel 800-743-1743 FIL-TECH INC. 617-742-0686
Fax
COPYRIGHT FIL-TECH INC. 1998
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