Page 422 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

402 MEMS-IDT MICROSENSORS

14.3.2 Integration of the SAW Device and Seismic Mass

14.3.2.1 Wafer bonding

If two wafers with surface oxide layers are immersed in sulfuric acid or nitric acid and
then brought into contact, they will stick immediately. If these wafers are subsequently
annealed at high temperature, the resulting bond can be essentially perfect. The quality of
the bond depends on the types of oxides used, the temperature, and the cleanliness of the
process. If the wafers can be annealed at 1000°C, any common (thermal, phosphosilicate
(PSG), etc.) oxide will do. Because the oxide, rather than the silicon, is involved in
the bonding, it is also possible to bond silicon to quartz or other glass substrates. This
technique is often used in what is known as a 'dissolved wafer process.' Further details
of silicon-bonding techniques may be found in Sections 5.6 and 5.7.

14.3.2.2 Incorporation of electronics into the accelerometer

When the electromagnetic signal is converted to an acoustic signal on the surface of a
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piezoelectric, the initial wavelength is reduced by a factor of about 10 . This results in
the dimensions of acoustic wave devices becoming compatible with IC technology. Use
of silicon substrates sputtered with piezoelectric zinc oxide instead of other piezoelectric
materials, such as lithium niobate and stable temperature (ST)-quartz, makes it feasible to
integrate electronics on the sensor substrate. This combination has the following distinct
advantages:

• Onboard signal processing and data manipulation is possible
• The need for signal amplification may be eliminated in certain cases
• Information loss by transmission is eliminated
• The overall system is miniaturised and a smart sensor is created.

However, there are some drawbacks to this approach and these are as follows:

• Production processes must be compatible with each other
• The number of production steps (mask levels) may increase, which may make the
yield low
• The contrasting demand that the electronics must be protected from the environment,
while the sensor must be exposed to the environment. This complicates the packaging
requirements
• Optimisation of the sensor and the electronics may involve conflicting requirements

14.4 TESTING OF A MEMS-IDT ACCELEROMETER

The MEMS-IDT accelerometer needs to be tested so that its design can be optimised. We
will first describe, discuss, and evaluate the measurement setup and calibration procedure