Page 371 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

SAW-IDT MICROSENSOR FABRICATION 351

remove any excess solvents from the photoresist. Then, it is cooled to room temperature
for approximately 15 minutes before exposing it to UV light in the mask aligner.
A contact mask aligner (Karl Suss MRK-3) is used to align the positive chrome mask
plate with the quartz that is wafer-coated with the photoresist. A UV light exposure of
6 seconds is subsequently required. The exposed wafer is then developed in a mixture of
(ratio 1:4) AZ–450® developer (Hoechst) and deionised water for 40 seconds. Great care
should be taken at this stage because under or overdeveloping the photoresist layer will
degrade the fabrication success. It is strongly recommended that an immersion style is
adopted, so that the wafer is slowly agitated during the developing process at 10 second
intervals, followed by a deionised water rinse and a close inspection using a microscope.
This will provide for greater control in the important developing stage of fabrication. A
'soft' post bake is then performed at 75 °C for 10 minutes, which assists in the hardening
and formation of sharp features of the photoresist. The wafer is then allowed to cool to
room temperature for approximately 15 minutes. At this stage, the IDT pattern should
have been successfully transferred to the wafer; if not, the photoresist can be stripped off
in acetone and the entire procedure repeated before the etching of the wafer.
Chemical wet-etching of the unwanted aluminum is then performed. The aluminum
layer is first etched in a solution of a commercial etchant and deionised water (3.25 g of
etchant in 50 ml of deionised water) at room temperature for approximately 60 seconds.
The etching time is extremely critical because undercutting of the structure walls may
occur if prolonged times are employed. It is strongly recommended that etching is
performed at 10 second intervals, followed by a deionised water rinse and close inspection
with a microscope.
The temperatures of the etchant solutions, together with the thickness of the metal
layers, are important factors that have a significant influence on the etching times. It is
recommended that the etching procedure is inspected for assurance before the processing
of valuable quartz wafers.
Once the IDT design has been successfully transferred to the metallised wafer via the
etching process, the wafer is ready for dicing (Campbell 1996, 1998). The dicing process
is described briefly in Section 12.2.5.

12.2.4.2 Lift-off process

The lift-off process begins with an initial cleaning of the wafers, followed by the deposition
of a positive photoresist. A similar cleaning procedure to that used for the etching process
is used to remove any possible loose surface contaminants that may have appeared during
storage since the initial wafer-cleaning procedure. Similarly, HMDS is spun on to the
wafer using, for example, a Headway Research Inc.® spinner at 3000 rpm for 60 seconds
to improve the adhesion of the resist to the wafer. After allowing the HMDS thin film
to sit for 2 minutes, AZ-1512® positive photoresist (Hoechst) is then spun at 3000 rpm
for 30 seconds. A photoresist layer of approximately 1.2 um is formed. The wafers are
then baked in an oven at 75 °C for 30 minutes to remove any excess solvents from the
photoresist. The wafers are cooled to room temperature for approximately 15 minutes
before UV light exposure.
After aligning the negative IDT chrome mask plate with the photoresist-coated wafer
having a similar orientation to that used in the etching process, the wafer is exposed to