18-08-2012, 04:55 PM
MEMS FABRICATION
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ABSTRACT
This summary of selected microelectromechanical systems
(MEMS) processes guides the reader through a wide
variety of fabrication techniques used to make micromechanical
structures. Process flows include wet bulk etching
and wafer bonding, surface micromachining, deep trench
silicon micromachining, CMOS MEMS, and micromolding.
Introduction
Microelectromechanical systems (MEMS) technology
encompasses an enormous variety of applications, including
sensors of almost any kind, imagers, ink jets, micropositioners,
optical beam steering and filtering, microphones,
RF tunable components and switches. Microfluidics is a
specialty area that has grown out of merging MEMS technology
with the physics of fluid dynamics, chemistry and
increasingly the biological sciences. The common thread
binding these disparate application themes is the ability to
manufacture devices and systems using batch microfabrication
processes. MEMS are made using the same standard
process steps used in integrated circuit manufacturing,
including photolithography, wet and dry etching, oxidation,
diffusion, low-pressure chemical vapor deposition
(LPCVD) and sputter deposition. Some unit processes,
such as plating, molding and substrate bonding, are more
common in MEMS than in mainstream IC manufacture
MEMS Materials
Requirements of a MEMS process flow are inclusion of
one or more mechanical materials, unit processes to shape
(micromachine) these materials and, in most cases, unit
processes to release parts of the structural material from
other anchored materials. The choice of micromachining
process usually starts with a specification of device dimensions
and tolerances. Structures over 10 μm in thickness
usually dictate bulk micromachining, while structures
under 10 μm usually incorporate surface micromachining
or hybrid bulk/surface micromachining.
MEMS Process Flows
No single process flow can be used to fabricate all possible
MEMS. However, a handful of canonical process
flows cover the basic MEMS fabrication concepts and form
a basis for many other derivatives. The canonical process
flows covered in the following discussion are silicon wet
etching and bonding, surface micromachining, deep reactive-
ion etched silicon micromachining, CMOS MEMS,
and microstructural molding processes.
Silicon wet etching and bonding
Silicon is a very useful micromechanical material with
Young’s modulus and high yield strength similar to stainless
steel. For micromechanical applications, it is extremely
robust and stable material. Several anisotropic silicon
etchants exist (e.g., potassium hydroxide (KOH), ethylene
diamene pyrocatechol (EDP) and tetramethyl ammonium
hydroxide (TMAH)), which exhibit preferential etching
along the <100> and <110> crystallographic directions and
orders of magnitude smaller etch rate in the <111> direction.
Silicon etch pit orientation in a <100> wafer is shown
in Figure 3 for a timed etch. A long etch will terminate
everywhere on {111} planes. The alkaline nature of these
etchants rules out conventional photoresist masking.
Instead. silicon oxide or silicon nitride masks must be used.
Conclusion
MEMS fabrication incorporates numerous materials
within an enormous variety of different process flows. This
short tutorial is not at all comprehensive. Several good
books cover the area [21][22][23][24], though any book
becomes dated as MEMS fabrication innovations continue
to be reported.