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MEMS Fabrication 57

Of the three MEMS fabrication technologies previously discussed, surface
micromachining is the most amenable to integration with electronics to form an
integration of electonics and MEMS technology (IMEMS) process. There are
several challenges to the development of an IMEMS process:

. Large vertical topologies: Microelectronic fabrication requires planar sub-
strates due to the use of precision photolithographic processes. Surface
micromachine topologies can exceed 10 mm due to the thickness of the
various layers.
. High-temperature anneals: The mitigation of the residual stress of the sur-
face micromachine structural layers can require extended period time at high
temperatures (such as several hours at 11008C for polysilicon). This would
have adverse effects due to the thermal budget of microelectronics that is
limited due to dopant diffusion and metallization.
There are three strategies for the development of an IMEMS process. 34

. Microelectronics first: This approach overcomes the planarity constraint
imposed by the photolithographic processes by building the microelectronics
before the nonplanar micromechanical devices. The need for extended
high temperature anneals is mitigated by the selection of MEMS materials
35
(e.g., aluminum, amorphous diamond ), and selection of the microelectronic
metallization (e.g., tungsten instead of aluminum), which make the MEMS
and microelectronic processing compatible. Examples of this IMEMS ap-
proach include an all-tungsten CMOS process that was developed by
36
researchers at the Berkeley Sensor and Actuator Center seen in Figure
33
3.18. The TI DMD (Texas Instruments Incorporated, Dallas, TX) uses the
microelectronics first approach and utilizes an aluminum structural layer
MEMS and photoresist sacrificial layer MEMS, which enables low-tempera-
ture processing.
. Interleave the microelectronics and MEMS fabrication: This approach
may be the most economical for large-scale manufacturing since it optimizes
and combines the manufacturing processes for MEMS and microelectronics.
However, this requires extensive changes to the overall manufacturing flow
in order to accommodate the changes in the microelectronic device or the
MEMS device. Analog devices has developed and marketed an accelerometer
and gyroscope that illustrates the viability and commercial potential of the
interleaving integration approach. 32
. MEMS fabrication first: This approach fabricates, anneals, and planarizes
the micromechanical device area before the microelectronic devices are
fabricated, which eliminates the topology and thermal processing constraints.
The MEMS devices are built in a trench, which is then refilled with oxide,
planarized, and sealed to form the starting wafer for the CMOS processing
as seen in Figure 3.19. This technology was targeted for inertial sensor

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