Biomedical Engineering Reference
In-Depth Information
(
c
)
(
a
)
Sacrificial layer
First SU-8 layer
Bottom Cr/Au contact
(
d
)
Thick SU-8 pad
Top Cr/Au contact
SU-8/ZnO nanocomposite layer
(
b
)
SU-8 layer (1.8 m)
Cr/Au layer (90 nm)
SU-8/ZnO nanocomposite layer
(2.7
m)
Cr/Au layer (90 nm)
Thick SU-8 pad (200
m)
Removal of sacrificial layer
Released cantilever die (flip chip)
Figure 9.21
(a) Schematic illustration of the process for fabricating l exible SU-8/ZnO
cantilevers. (b) Schematic of cantilever beam showing various layers and their thicknesses
(c) SEM image of SU-8/ZnO nanocomposite cantilevers. (d) Magnii ed optical image of
SU-8/ZnO nanocomposite cantilevers showing Au electrodes [31].
it was lithographically patterned using the regular SU-8 processing steps.
h is layer acts as contact via layer at er release of the cantilevers and also to
insulate the top gold layer of the stack. A stack of Cr/Au was then deposited
for a total thickness of 90 nm and patterned for contact pads. h en a well
dispersed SU-8/ZnO nanocomposite was spin coated to get a thickness of
2.7 μm. h is composite layer is patterned by regular SU-8 processing steps.
At er this step, the deposition and patterning of second electrode layer of
Cr/Au was performed. h e frame and device die were patterned using SU-8
2100. h e frames were i nally released from the substrate by etching the
sacrii cial layer in buf ered HF acid solution. h e l oating SU-8 frames in
the HF solution were slowly transferred in to DI water and the Cr layer on
the gold contact pads is removed using Cr etchant. h e schematic of cantile-
ver beam showing various layers used with their thicknesses is represented
in i gure 9.21(b). h e SEM and optical images of fabricated cantilevers are
shown in i gure(c) and i gure (d) respectively. From SEM image, it is noted
that the width of cantilever beam is 50 μm and the length is 250 μm.
9.5.3
Characterization of SU-8/ZnO Cantilevers as
Vibration Sensors:
For an oscillating mult
i
layer cantilever, the resonant frequency is expressed
as follows [75]:
()
EI
2
1
n
f
=
(9.5)
(
)
2
2
L
h
+
h
np np
p p
