Page 421 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

FABRICATION OF A MEMS-IDT ACCELEROMETER 401

3. The sacrificial layer is then etched with hydrofluoric acid (HF) to release the
seismic mass.
4. The seismic mass is now ready to be flip-chip bonded to the SAW substrate.

These steps are only one of the many possible methods of realising these structures.
After the sacrificial oxide is removed in HF, the wafers are rinsed in deionised (DI)
water and dried. The surface tension of the water under the structures tends to pull them
down to the surface of the wafer, and in some cases, causes them to stick permanently.
This problem can be avoided by using thick structural and sacrificial layers and short
structures. There are several other methods of adhesion prevention that rely on avoiding
the problem of surface tension. These include the following:
• Freeze drying (sublimation) of the final rinsing solution (DI water, t-butyl alcohol)
• Using an integrated polymer support structure during release etching and then ashing
in oxygen
• Gradually replacing acetone with photoresist, then spinning and ashing the resist
• Electrically fusing (vaporising) micromechanical support structures

14.3.1 Fabrication of the SAW Device

The SAW device consists of a piezoelectric substrate on which IDTs are deposited. The
IDTs can be sputtered through a mask or they can be etched using the lift-off technique as
described in Chapter 12. The choice of the method is dependent on the minimum feature
size to be fabricated and adhesion of the electrode metal to the piezoelectric substrate.
The fabrication of the IDTs is essentially a single mask process.
For example, in the lift-off technique, fabrication begins with standard photolitho-
graphy using standard equipment such as a wet bench, resist spinner, hot plates, and
an evaporator. The lithium niobate wafers are cleaned using acetone, isopropanol, and
trichloroethylene (in turn) at about 60°C for about 10 minutes. The wafers are then
thoroughly rinsed in DI water for about 5 minutes and subsequently heated at 125°C
(on a hot plate) for about 10 minutes to remove surface moisture. Upon cooling the
wafer on a heat sinking plate, Shipley 1813 photoresist is spin-coated (at 4000 rpm
for 55 seconds) on the wafer after soaking the top face with an adhesion agent called
hexamethyl disilazane (HMDS). The wafer is then heated at 125°C for 2 minutes in
what is commonly referred to as the soft-bake. The wafer is then exposed to ultravio-
2
let (UV) light (of 15 mW/cm ) for 1.2 seconds such that the regions of the resist that
are exposed become soluble to the developer (DI water and MF3 12 in a 1:1 ratio). A
negative mask, whereby the patterns are glass set against a background of chrome, is
used for this purpose. The wafer is developed until the sections that have been exposed to
UV light and therefore soluble are etched away. The wafer is then hard-baked at 125 °C
for 1 minute 30 seconds. The patterned wafer is coated with 20 nm of chromium using
electron gun evaporation, which is deposited to improve the adhesion of the subsequent
thermally evaporated 120 nm gold layer. The wafer is then submerged into acetone to
facilitate the lift-off process.