Page 372 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

352 IDT MICROSENSOR FABRICATION

UV light for 6 seconds. To improve the lift-off capability, the wafer is then immersed in
a chlorobenzene bath at room temperature for 3 to 3.5 minutes. It is important to note
that this time varies depending on the intensity of the UV exposure lamp; typically, for
2
an intensity of 21 W/cm , the characteristic time in chlorobenzene ranges from about 220
to 280 seconds. This is an extremely critical step, and the procedure should be validated
before it is applied to the set of SAW wafers.
Chlorobenzene modifies the surface of the photoresist by developing a characteristic
'lip' in the developed pattern. This creates a discontinuity at the edges of the patterned
photoresist when a metal is evaporated on the surface of the wafer; thus, unwanted metal
is subsequently removed more easily. The wafer is then baked in an oven at 75 °C for
30 minutes to remove any excess solvents from the photoresist and allowed to cool to
room temperature for approximately 15 minutes. The wafer is then developed in a mixture
(ratio 1:4) of AZ-450® developer (Hoechst) and deionised water for 40 seconds.
Again, great care should be taken at this stage, as under or overdeveloping the photore-
sist layer will degrade the fabrication success. As in the etching process, an immersion
method is strongly recommended, whereby the wafer is slowly agitated during the devel-
oping process at 10 second intervals, followed by a deionised water rinse and then
close inspection with a microscope. It is important to prevent damage to the photoresist-
patterned structures at this stage, so extremely gentle agitation is required in the immersion
step, and the use of compressed filtered nitrogen for drying the wafer should be avoided.
Close inspection of the wafer surface using an optical microscope is then performed to
examine the transferred SAW-IDT pattern. The edges of the photoresist patterns should
be well defined and sharp to facilitate the lift-off process. Again, if found unacceptable,
the photoresist can be removed using acetone and the entire procedure can be repeated
before the metallisation of the wafer.
After the metallisation of the photoresist-patterned wafer using the metal evaporation
technique (Section 12.2.4), the photoresist is removed by immersing the wafers in an
acetone bath at room temperature for 30 minutes. Ultrasonic agitation may be used to
assist in the removal process but caution is advised as damage to small patterned structures
(feature sizes <2 nm) may occur.
Once the IDT designs have been successfully transferred to the wafers via the lift-off
process, the wafers are then ready for dicing (Section 12.2.5).
A summary of the photolithography process for both the etching and lift-off procedures
is shown in Figure 12.4.

12.2.5 Wafer Dicing

The wafers are finally cut into small, individual chips using, for example, a Deckel™ wire
saw, together with a diamond impregnated wire and slurry. The slurry is made from a
mixture of silicon, glycerol, and deionised water (3:5:1) and has a particle size of 25 um.
Before cutting the wafer, a thick layer of AZ-4562® positive photoresist (Hoechst) is
spun at 2000 rpm for 30 seconds, following the deposition of a thin HMDS layer spun on
to improve the photoresist adherence on the wafer. The wafer is then baked in an oven at
75 °C for 30 minutes and then allowed to cool to room temperature. The resulting thick
layer protects the delicately patterned IDT structures during the debris cutting.