Page 306 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

286 MICROSENSORS

We now provide, via a worked example, the process sequence for a resistive gas
microsensor.

Worked Example E8.2: Silicon-Resistive Gas Sensor Based on a Microhotplate

Objective:
To fabricate a resistive planar gas microsensor from bulk silicon micromachining tech-
niques based on a solid diaphragm microhotplate. The small thin diaphragm (less than
1 urn thick) should result in low power consumption.

Process Flow:
A five-mask process has been used to fabricate a resistive gas microsensor with the
main steps shown in Figure 8.55 (Pike 1996). The initial substrate was a 3", 280 nm
thick, single-sided polished, (100) oriented, SCS wafer. Before processing, the wafers
were given an identity mark with a diamond scribe and then subjected to a standard
multistage cleaning process, which removed any organic contaminants, adsorbed layers,
and particulates. A standard cleaning process was used before each thin film deposition
20
to ensure adequate adhesion .

1. An 80 nm dry SiO 2 film was thermally grown on the wafer at 1100 °C. Note that the
intrinsic stress in the thermally grown SiO 2 films must be negligible at 1100 °C - an
essential requirement for mechanical stability of the membrane structure.
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2. A 250 nm thick layer of low-stress SiN x was then deposited by LPCVD.
3. The microheaters were defined by patterning a thin platinum film with Mask 1
using a lift-off technique. Specifically, a photoresist was first spin-coated onto the
wafer and exposed to UV light using Mask 1. Before the photoresist was developed,
it was exposed to chlorobenzene to harden the photoresist surface. Hence, during
developing, the photoresist was undercut slightly because of the surface modifi-
cation. This profile ensures that no side coverage occurs during the metallisation
deposition, so that when the photoresist was removed, it did not interfere with
the metallisation that had bonded to the substrate. The photoresist layer acts as a
sacrificial layer, which was removed later with acetone, revealing the mask image
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patterned into the metallisation . To improve metal adhesion to the substrate, it is
common to use a thin adhesion layer of a more reactive metal. Therefore, before
sputtering down 200 nm of Pt, a 10 nm tantalum (Ta) adhesion layer was first
deposited.
4. A standard cleaning process prepared the substrate for a second 250-nm layer of
LPCVD low-stress SiNx, which insulates the microheater electrically from the elec-
trodes deposited in a later stage. Mask 2 was used to open up the contact windows
in the SiN x. This required the plasma etching of the SiN x to reveal the Pt heater
contact pads. The patterned photoresist layer used was then stripped off before
another cleaning stage.

20
See Chapter 4 for details of wafer cleaning.
21
The nitride is slightly silicon-rich and so nonstoichiometric.
22
This is a lift-off process such as that used to make SAW IDT microsensors (Chapter 12).