Page 136 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 115

and 450ºC, thus reducing the deleterious effects of humidity. The sense resistor and
the heater reside over a 2-µm-thick silicon membrane to minimize heat loss through
the substrate. Consequently, a mere 47 mW is sufficient to maintain the membrane
at 400ºC. There are a total of four electrical contacts: two connect to the tin-oxide
resistor, and the other two connect to the polysilicon heater. The simplest method to
measure resistance is to flow a constant current through the sense element and
record the output voltage (see Figure 4.29).
The particulars of the fabrication process for the MiCS carbon monoxide sensor
and its predecessor by Motorola are not publicly disclosed, but demonstrations of
similar devices exist in the literature. A simple process would begin with the forming
of a heavily doped, p-type, 2-µm-thick layer of silicon either by epitaxial growth or,
alternatively, by ion implantation and annealing. The deposition of a silicon nitride
layer follows. A chemical vapor deposition (CVD) step provides a polysilicon film
that is later patterned and etched in the shape of the heater. The polysilicon film is
doped either in situ during the CVD process or by ion implantation and subsequent
annealing. An oxide layer is then deposited and contact holes etched in it. The pur-
pose of this layer is to electrically isolate the polysilicon heater from the tin-oxide
sense element. The tin-oxide layer is deposited by sputtering tin and oxidizing it at
approximately 400ºC. An alternative deposition process is sol-gel, starting with a
tin-based organic precursor and curing by firing at an elevated temperature. The
tin-oxide layer is patterned using standard lithography and etched in the shape of
the sense element. Sputtered and patterned aluminum provides contact metalliza-
tion. Finally, an etch from the back side in potassium hydroxide or EDP forms a thin
membrane by stopping on the heavily doped p-type surface silicon layer. Naturally,
a masking layer (e.g., silicon nitride) on the back side of the substrate and protection
of the front side are necessary. It is also possible to etch all of the silicon and stop at

Package opening
Mesh Tin oxide
Charcoal filter Metal Polysilicon heater
contact
Silicon dioxide
Insulating
layer
P-type silicon
Silicon
Anisotropically
Mesh
etched silicon
membrane

Surface tin oxide

Bulk tin oxide

Figure 4.29 Illustration of a carbon monoxide sensor, its equivalent circuit model, and the final
packaged part. The surface resistance of tin-oxide changes in response to carbon monoxide. A
polysilicon heater maintains the sensor at a temperature between 100° and 450ºC in order to
reduce the adverse effects of humidity. (After: [34].)