Page 64 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c003 Final Proof page 54 1.9.2005 8:59pm

54 MEMS and Microstructures in Aerospace Applications

is conformable and will deposit on the inside wall of the hub structure. The
thickness of SACOX2 defines the clearance of the hub structure. SACOX2 can
also be used as a hard mask to define MMPOLY1 using the subsequent etch that
also defines MMPOLY2.
Upon completion of the SACOX2 deposition, pattern, and etch, a 1.5-mm thick
layer of doped polysilicon, MMPOLY2 is deposited. Any MMPOLY2 layer ma-
terial that is deposited directly upon MMPOLY1 (i.e., not separated by SACOX2)
will be bonded together. Following the MMPOLY2 deposition, an anisotropic
reactive ion etch is performed to etch MMPoly2 and composite layers of
MMPOLY1 and MMPOLY2 (laminated together to form a single layer 2.5-mm
thick). The MMPOLY2 etch will stop on silicon dioxide, hence MMPOLY1 will be
protected by any SACOX2 on top of MMPOLY1 and the SACOX2 layer can be
used as a hard mask to define a pattern in MMPOLY1.
At this point in the SUMMiT V process all the layers have been conformable
(i.e., assume the shape of the underlying patterned layers). To enable the addition
of subsequent structural and sacrificial levels without the fabrication and design
constraints of the conformable layers, CMP is used to planarize the sacrificial
oxide layers. With the MMPOLY2 etch complete, approximately 6 mm of TEOS
(tetraethoxysilane) silicon dioxide (SACOX3) is deposited. CMP is used to planar-
ize the oxide to a thickness of about 2 mm above the highest point of MMPOLY2.
Following planarization, SacOx3 is patterned and etched to provide dimples
and anchors to the MMPOLY2 layer using the DIMPLE3_CUT and SACOX3_
CUT masks, respectively. The DIMPLE3_CUT etch is performed by etching all
the way through the SACOX3 layer, stopping on MMPOLY2. Then 0.4 mmof
silicon is deposited to backfill the dimple hole to provide the 0.4 mm standoff
distance. The processing of the DIMPLE3 feature will provide a repeatable standoff
distance.
A2-mm thick layer of doped poly (MMPoly3) is deposited on the CMP
planarized SACOX3 layer. The MMPOLY3 layer will be flat and not have the
topography due to the patterning of the underlying layers. This will ease design
constraint on the higher levels and enhance the use of MMPOLY3 and MMPOLY4
layers as mirror surfaces in optical applications. The MMPOLY3 layer is patterned
and etched using the MMPOLY3 mask.
The processing for the SACOX4 and MMPOLY4 layers proceeds using the
SACOX4_CUT, DIMPLE4_CUT, and MMPOLY4 mask in an analogous fashion to
the SACOX3 and MMPOLY3 layers, except that the DIMPLE4 standoff distance is
0.2 mm.
Release and drying of the SUMMiT V die are the final fabrication steps. The
device is released by etching all the exposed silicon dioxide away with a 100:1
HF:HCl wet etch. Following the wet release etch, a drying process can be employed
using either simple air evaporation, supercritical CO 2 drying, 27 or CO 2 freeze
sublimation. 28 The choice of the drying process will depend upon the design of
the particular devices. Structures that are very stiff will be less sensitive to the
surface tension forces, and they can be processed by simple air drying. Supercritical
CO 2 drying processing for large devices is a better option.

© 2006 by Taylor & Francis Group, LLC