Page 54 - An Introduction to Microelectromechanical Systems Engineering

CHAPTER 3
Processes for Micromachining

“You will have to brace yourselves for this—not because it is difficult to under-
stand, but because it is absolutely ridiculous: All we do is draw arrows on a piece of
paper—that’s all!”
—Richard Feynman, explaining the Theory of Quantum Electrodynamics
at the Alix G. Mautner Memorial Lectures, UCLA, 1983.

This chapter presents methods used in the fabrication of MEMS. Many are
borrowed from the integrated-circuit industry, in addition to others developed spe-
cifically for silicon micromachining. There is no doubt that the use of process equip-
ment and the corresponding portfolio of fabrication processes initially developed
for the semiconductor industry has given the burgeoning MEMS industry the impe-
tus it needs to overcome the massive infrastructure requirements. For example,
lithographic tools used in micromachining are oftentimes from previous generations
of equipment designed for the fabrication of electronic integrated circuits. The
equipment’s performance is sufficient to meet the requirements of micromachining,
but its price is substantially discounted. A few specialized processes, such as ani-
sotropic chemical wet etching, wafer bonding, deep reactive ion etching, sacrificial
etching, and critical-point drying, emerged over the years within the MEMS com-
munity and remain limited to micromachining in their application.
From a simplistic perspective, micromachining bears a similarity to conven-
tional machining in the sense that the objective is to precisely define arbitrary
features in or on a block of material. There are, however, distinct differences.
Micromachining is a parallel (batch) process in which dozens to tens of thousands
of identical elements are fabricated simultaneously on the same wafer. Furthermore,
in some processes, dozens of wafers are processed at the same time. Another key dif-
ference is the minimum feature dimension—on the order of one micrometer—which
is an order of magnitude smaller than what can be achieved using conventional
machining.
Silicon micromachining combines adding layers of material over a silicon wafer
with etching (selectively removing material) precise patterns in these layers or in the
underlying substrate. The implementation is based on a broad portfolio of fabrica-
tion processes, including material deposition, patterning, and etching techniques.
Lithography plays a significant role in the delineation of accurate and precise pat-
terns. These are the tools of MEMS (see Figure 3.1).
We divide the toolbox into three major categories: basic, advanced, and non-
lithographic processes. The basic process tools are well-established methods and are
usually available at major foundry facilities. The advanced process tools are unique
in their nature and are normally limited to a few specialized facilities. For example,

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