Page 308 - Sami Franssila Introduction to Microfabrication
P. 308
28
MEMS Process Integration
MEMS devices come in a bewildering variety, with question and unconventional patterning approaches are
regard to structures, materials and functions. Whereas all needed. Through-wafer structures require double-sided
CMOS technologies are close relatives, MEMS devices processing of the wafer, and even without through-holes,
are made with a multitude of related, distantly related there is often a need to align structures on the two sides
and unrelated technologies. Pressure sensor operation of the wafer. Double-side alignment is also mandatory
can be based on piezoresistive, capacitive, thermal for structured wafer bonding.
conductance or resonance mechanisms; and the first MEMS devices are not ‘solid-state devices’ in the
three share some structural features and fabrication sense that they are not solid throughout but have free-
steps whereas the fourth bears more resemblance to standing, moving, rotating, vibrating and sliding parts
gyroscopes and RF oscillators. with air gaps or vacuum cavities. These create addi-
Identical DRIE fabrication steps are utilized in tional topology challenges for the following process and
making microfluidic valves, variable optical attenuators, packaging steps. Capillary forces in drying, silicon dust
accelerometers and enzyme microreactors. Anisotropic and vibrations during dicing or stresses and tempera-
wet etching is similarly used for a plethora of applications ture in encapsulation may damage delicate mechanical
that have nothing in common at the device level, even structures. Cavities can sometimes be handled without
though they share some of the crucial fabrication steps. problems, but high temperatures and changing pressures
MEMS technologies require new materials: nickel as during fabrication can cause some design limitations,
mechanical material, copper as thick electroplated metal, especially when the cavity roof is a thin diaphragm.
platinum as chemically inert electrode in microfluidics,
palladium as catalyst, gold as low-resistivity metalliza- 28.1 DOUBLE-SIDE PROCESSING
tion, SnO 2 as gas sensitive film, zinc oxide as piezo-
electric material, PZT as ferroelectric material, VO 2 Although intricate three-dimensional topography can
as strong temperature coefficient of resistivity mate- build up on the wafer surface by etching and deposition
rial, and the list goes on. Some of these are known processes, utilization of both sides of the wafer leads
materials from other applications: gold is routinely used large-scale 3D structures that pose special problems of
in GaAs microwave circuits, polyimide films are well- their own. Processing must be tailored so that both sides
known materials in chip packaging and the printed cir- of the wafer are under controlled conditions at all times.
cuit board industry, and Teflon coating is widely used Double-side processing is intricately intertwined with
in frying pans, but many are new in microdevices or in process equipment, which has historically been designed
thin-film form. for top surface processing only, and therefore processes
MEMS structures have high aspect ratios and highly on wafer backside have been neglected and they depend
complex 3D shapes resulting from DRIE or from heavily on particular equipment designs.
anisotropic wet etching and wafer bonding. These put Three kinds of processes take place on the wafer
new requirements for subsequent lithography, dop- backside:
ing and thin-film steps, and introduce novel metrol-
ogy requirements. The fact that MEMS devices have • patterning;
through-wafer holes limits some process steps: for • blanket processing (doping, growth and deposition);
instance, spinning of resist over holes is out of the • unintentional processes.
Introduction to Microfabrication Sami Franssila
2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)
