Page 369 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

SAW-IDT MICROSENSOR FABRICATION 349

Figure 12.2 Basic layout of a photolithographic mask plate showing an IDT structure: (a) positive
and (b) negative fields

the wafer surface is essential. The cleaning of the wafers should be performed in a fume
cupboard (in a clean room) to allow the safe and fast removal of any possible harmful
fumes produced during the cleaning process (Campbell 1998; Atashbar 1999).
The wafers are initially cleaned of any surface contaminants, such as dust, grease,
or any other soluble organic particles, by immersion in trichloroethylene 2 at 60 °C for
10 minutes, followed by an acetone bath at 60 °C for 10 minutes. The wafers are then
rinsed with methanol and finally with deionised water. It is best to avoid the use of
nitrogen gas for drying the sample during the aforementioned procedure so as to minimise
further surface contaminants. Instead, a slow evaporation in a protected fume cupboard
is employed. Further cleaning is then undertaken for the removal of the more obstinate
contaminants. The wafers are immersed in a mixture of three parts of deionised water
(3H 2O), one part ammonium hydroxide (NH 4OH), and one part of 30 percent unsta-
bilised hydrogen peroxide (H 2O 2) at 75 °C for 10 minutes. Caution is required because
the mixture is harmful, and it is recommended that the hydrogen peroxide is added last
so as to minimise any reaction side effects. Next, the wafers are placed in a solution of
industrial grade detergent and subjected to ultrasonic agitation at 60 °C for ten minutes.
Following a rinse in deionised water, the wafers are placed in a circulating deionised
water bath for 30 minutes. The wafers are then dried using compressed filtered nitrogen
and stored in an appropriate container and environment.

12.2.3 Metallisation

A metal layer now needs to be deposited, from which IDT structures are to be formed. In
general, aluminum is evaporated using, for example, a Kurt Lesker™ E-beam evaporator.
Aluminum is employed because it is commonly used in IC foundries and exhibits chemical
3
resistance to many different liquids .
Typically, a 100 to 150 nm layer of aluminum is deposited on the clean surface of a
quartz wafer. For example, the beam voltage of an E-beam evaporator is set to 6 keV
during the deposition of 150 nm of aluminum, the pre-evaporation pressure is set at
10 -6 torr, and the beam current is set to almost 100 mA. This gives an evaporation rate
of 0.2 nm/s. It is to be noted that aluminum could have also been evaporated onto the

' Caution needs to be exercised since trichloroethylene fumes are toxic.
3
Clearly, strong acids attack aluminum and should be avoided.