An Introduction to MEMS
Prime Faraday Technology Watch – January 2002
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Figure 21. Surface micromachining of a cantilever beam using a sacrificial layer [2,3].
These layers (or thin films) are deposited and subsequently dry etched in sequence, with the
sacrificial material being finally wet etched away to release the final structure. Each
additional layer is accompanied by an increasing level of complexity and a resulting difficulty
in fabrication. A typical surface micromachined cantilever beam is shown in Figure 21.
Here, a sacrificial layer of oxide is deposited on the silicon substrate surface using a pattern
and photolithography. A polysilicon layer is then deposited and patterned using RIE
processes to form a cantilever beam with an anchor pad. The wafer is then wet etched to
remove the oxide (sacrificial) layer releasing and leaving the beam on the substrate. More
complex MEMS structures can be achieved using structural polysilicon and sacrificial silicon
dioxide, including sliding structures, actuators and free moving mechanical gears. Figures 22
shows the process flow for the fabrication of a micromotor by the commercially available
Multi-User MEMS Process (MUMPS).
The levels of complexity achievable with MEMS has already been shown in Figure 16. In
this case, five mechanical levels of micromachined polysilicon can be achieved using Sandia
Ultra-Planar Multi-Level Technology (SUMMiT).
The success of the surface micromachining process depends on the ability to successfully
remove all of the sacrificial layers to free the structural elements so that they can be actuated.
This step is responsible for curtailing the yield (percentage of the devices on a wafer that
function properly) and reliability of fabricated MEMS due to the phenomenon known as
Figure 22. Surface micromachining of a MEMS micromotor using the Multi-
User MEMS Process (MUMPS) [30].
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