Online proceedings - EDA Publishing Association

imorph. Bimorph array of single beam was also prepared
for direct comparisons. The fabrication is mainly involved of
commonly-used surface micromachining and deep reactive
ion etching technology. The top and bottom layer of the
bimorph was Mo (500 nm-thick) and Au (500 nm-thick),
respectively. The former has the thermal expansion
coefficient of about 4.9 ppm/ o C. The latter has the thermal
expansion coefficient of about 14.4 ppm/ o C. Figure 5 is
fabrication process of the 3D bimorph, in which SOI wafer
was used and its device layer has high resistivity (> 1000
Ω⋅cm). Spray coating method was used for the resist coating
in order to get good coverage on non-planar surface and the
fabrication of isolation gap. Other key processes include the
conformal deposition of thin metal film and the formation of
the isolation structure between the adjacent bimorph.
Because the beam height was 35 μm, the sputtering method
was utilized for the deposition of thin metal film.
Fig 4 Fabrication sequence of the triple-beam bimorph
Fig. 5 Fabrication sequence of the 3D bimorph.
The displacement measurement of the triple-beam
bimorph was carried out in a home-made quartz stage by
11-13
May 2011, Aix-en-Provence, France
using a confocal scanning laser microscopy (OPTELICS
S130, LASERTEC). The tip displacements were in-situ
measured. The 3D bimorph was examined on a Cascade
9000 analytical probe station with a heater (Temptronic Co.)
by using Agilent 4284A LCR meter.
III. RESULTS AND DISCUSSION
Figure 6 are the SEM images of the prepared triple-beam
bimorphs. Traditional single-beam bimorph was also
prepared for direct comparisons of thermal response
behavior. The bi-metal layer structure is visible in Fig. 6. It
is noteworthy that all the prepared bimorphs have large
initial deflection because of the residual stress that resulted
from the deposition and micromachining of the thin films.
The initial deflection was within 10 ~ 30 μm. The
triple-beam bimorph had almost same initial bends as the
single-beam or traditional bimorph. It was also found that
the initial bends of the triple-beam bimorph was not the
average value of those of the side and middle beam. The side
beam was dominantly determined the initial bends of the
triple-beam bimorph. Figure 7 representative Z-images for
the displacement measurement of the triple-beam bimorph.
The triple-beam bimorph had tilted during the upward
deflection upon temperature increasing, which possibly
resulted from alignment error during the fabrication. The
initial bends and tilts of the triple-beam bimorph would be
barriers to the practical application. As well, the package of
the triple-beam bimorph would be difficult.
Figure 8 are the SEM images of the prepared 3D bimorphs,
respectively. It consisted of ten 5 μm-thick Si/0.4 μm-thick
Au bimorph in five pairs. Good leading connections were
formed across the cavity edge. The isolation gaps were well
formed, too. Figure 9 was optical photo of the chip after the
dicing. The chip was 1.2 mm square. The 3-D bimorph was
robust and can stand for conventional dicing process with
simple protection by polymer sheet. We could draw the
conclusion that the new 3D layout could simplify the
microfabrication process and thus the cost.
Figure 10 is the measured displacements of the prepared
bimorph upon the increasing of temperature from 25 o C to
38 o C, 41 o C and 44 o C. The triple-beam bimorph exhibited
different behaviors from the traditional single-beam
bimorph upon the increasing of temperature. Firstly, the
former had larger tip displacements than the latter. For
example, the tip displacement was about 2 μm for the
triple-beam bimorph with the temperature increasing from
41 to 44 o C while the other was only about 0.5 μm. The larger
tip displacement makes bigger increment of dimension and
therefore higher sensitivity possible. For example, the
triple-beam thermometer could be consisted of 20 bimorphs
at the displacement increment of 100 nm for temperature
sensing between 41 and 44 o C. Larger displacement and
beam-length increment also suggested that the fabrication
process was more feasible. We could draw the conclusion
that the triple-beam bimorph is prior to the single-beam one
in the views of better temperature sensing and easier
fabrication.
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