14-11-2012, 04:30 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.
Surface micromachining
Advantages of surface micromachining are a) structural
and spacer features, especially thicknesses, can be smaller
than 10 μm in size, b) the micromachined device footprint
can often be much smaller than bulk wet-etched devices, c)
it is easier to integrate electronics below surface microstructures,
and d) surface microstructures generally have
superior tolerance compared to bulk wet-etched devices.
The primary disadvantage is the fragility of surface microstructures
to handling, particulates and condensation during
manufacturing.
Polysilicon (polycrystalline silicon) micromachining is
arguably the most common form of surface micromachining.
Polysilicon has mechanical properties similar to singlecrystal
silicon, which explains its popularity for MEMS. A
basic process flow, shown in Figure 5, starts with low-pressure
chemical vapor deposition (LPCVD) of a thin silicon
nitride layer on top of a silicon substrate (a).
DRIE silicon micromachining
In 1996, an Advanced Silicon Etch (ASE™) process
was introduced that can make trenches in silicon with depth
to width aspect ratios over 20:1 and with nearly vertical
sidewalls [9][10][11]. The deep trench etch sequence is
illustrated in Figure 7, and is also known as deep reactiveion
etching (DRIE). The mask is usually either photoresist
or silicon oxide, however other mask materials can be used.
In step (a), a high density inductively coupled SF6 plasma
etch achieves selectivity to the mask of around 100:1. The
etch is normally run for about 8 to 12 s, which corresponds
to around a 0.2 to 0.5 μm etch depth. The gas in the plasma
chamber is then switched to C4F8 for around 8 s, which
deposits a thin fluorocarbon polymer onto the wafer surface.
The following etch step © uses physical ion assist to
etch the polymer at the bottom of the trench, leaving some
sidewall polymer. The polymer masks lateral etching and
thereby maintains the vertical sidewall profile. The desired
trench depth is obtained by cycling etch steps (a) and deposition
step (b), with an effective etch rate of around 1 μm/
min. The effective etch rate is a function of the trench
aspect ratio, an effect known as aspect-ratio dependent
etching (ARDE).
CMOS MEMS
The term “CMOS MEMS” most often describes processes
that create microstructures directly out of the metal/
dielectric interconnect stack in foundry CMOS. The metallization
and dielectric layers, normally used for electrical
interconnect, now serve a dual function as structural layers.
For example, the suspended n-well of Figure 3(d) is considered
CMOS MEMS, since its beam suspension is made
from the CMOS interconnect stack.
There is significant motivation for making MEMS out
of CMOS. Leveraging foundry CMOS for MEMS is fast,
reliable, repeatable, and economical. Electronics can be
placed directly next to microstructures, enabling arrayed
systems on chip. In CMOS MEMS, multiple conductors
can be placed inside of the microstructures, which enables
placement of multiple electrically isolated capacitive sensors
and electrostatic actuators. The gate polysilicon can be
embedded in the microstructures as heater resistors,
piezoresistors, or thermocouples.
Microstructural molding processes
Polymer micromolding is a common way to make
microchannels, because of the ease of processing, low cost,
and bio-compatibility. Molds with micron-scale features
may be made from photosensitive polymers (e.g., SU-8) or
by DRIE silicon micromachining. Poly(dimethylsiloxane)
(PDMS) is often chosen as the microstructural material
[15]. PDMS is spin cast onto the mold, cured and peeled
off. A short exposure in an oxygen plasma activates the
PDMS surface and results in instant bonding to other
PDMS or glass surfaces. Other polymers may be molded
onto silicon masters through either hot embossing, casting,
or injection molding.
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. The interested reader is encouraged to
browse through the major journals in the MEMS field,
where virtually all of the fabrication concepts are published
[25][26][27][28].