Page 163 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

REFERENCES 143

silicon in comparison with KOH. The p+ layer is subsequently removed using a
8:3:1 CH 3COOH:HNO3:HF liquid etch. A brown stain, indicating porous silicon,
resulting from the etch is removed using a mixture of 97:3 HNO3:HF solution for
about 15 seconds leaving a 5 um-thick silicon epilayer bonded to the patterned oxide
(Figure 5.2 l(b)).
3. A masked implant (as at 80 keV and 7 x 10 15 cm –2 dose) followed by a one-hour
anneal at 850 °C in dry O 2 ambient is performed for better ohmic contact. Sensing
–3
tethers are not implanted and their concentration remains at approximately 10 15 cm .
Following the implant anneal, 500 nm of Al is deposited using electron-beam deposi-
tion, patterned and sintered in N2 ambient at 375 °C for 30 minutes (Figure 5.21(c)).
4. A thin layer of plasma-enhanced chemical vapour deposition (PECVD) amorphous
silicon (500 nm) is deposited over the wafer to protect the aluminum in the subsequent
processing steps. A 1 um-thick PECVD oxide is then deposited and patterned using a
CF 4/CHF 3/He plasma etch. This oxide will act as an etch mask for the silicon trench
etch that is used to define the floating element. The oxide is plasma-etched and the
resist is subsequently removed (Figure 5.21(d)).
5. A CCl 4-based plasma etch is used to trench-etch the silicon. A BOE (7:1 H 2O:HF
is used to etch the PECVD oxide that remains after the silicon trench etch, thus
completing the fabrication of the floating element. Electrical contacts are made
through the amorphous silicon layer to the Al bonding pads either by mechan-
ical probing or by ultrasonic wire bonding. The completed sensor is shown in
Figure 5.2 l(e).

5.8 CONCLUDING REMARKS

In this chapter, we have described the process involved in bulk silicon micromachining
that is used to fashion microstructures out of silicon wafers, including a discussion of
etching and etch-stops, followed by a discussion of the bonding together of two wafers
to make the complete microdevices. Several worked examples are considered to illustrate
the role of the various processes.
The alternative approach is that of surface rather than bulk silicon micromachining.
This topic is covered in the next chapter, again with worked examples of microstruc-
tures. Together, bulk and surface micromachining represent the two key technologies that
are essential for the manufacture of many different types of silicon microsensors and
microactuators today.

REFERENCES

Apel, U. et al. (1991). "A 100-V lateral DMOS transistor with a 0.3 micron channel in a 1 micron
silicon-film-on-insulator-on silicon," IEEE Trans. Electron Devices, 38, 1655–1659.
Bong, A. (1971). "Ethylene diamine-pyrocatechol-water mixture shows etching anomaly in boron-
doped silicon," J. Electrochem. Soc., 118, 401–402.
Choi, W-S. and Smits, J. G. (1993). "A method to etch undoped silicon cantilever beam," J. Micro-
electromech. Syst., 2, 82-86.