Biomedical Engineering Reference
In-Depth Information
as 'Organic CantiFET, can be thought of as a low cost polymer counter-
part of a similar device in silicon, a MOSFET embedded microcantilever
reported earlier [49, 50]. When SU-8 microcantilevers with embedded
OFET undergo nanomechanical motion during sensing events, the strain
sensitive organic semiconductor pentacene responds to this by changing
its mobility and hence the drain current of the transistor. h e change in
drain current can be measured and recorded using appropriate signal con-
ditioning circuitry in order to perform the sensing operation. A simple
dif erential amplii er circuit with sensing and reference transistors could
be used in a complete sensor coni guration.
Such an arrangement of organic CantiFET retains the advantages of ered
by Wheatstone bridge used for piezoresistive sensors.
9.3.1
Process Integration of Organic CantiFET
h e CantiFET device chip consisted of two CantiFET devices for dif-
ferential current measurement scheme, in which one of the devices was
considered as the measurement cantilever and other being the reference
cantilever [28]. Arrays of such CantiFET device chips were fabricated.
h e transistor covered the whole length of cantilever so that the design is
appropriate for nanomechanical cantilever sensors involving surface stress
measurements. h e thickness of the SU-8 gate dielectric has been chosen
based on the preliminary study conducted on the current voltage (I-V)
characteristics of back gate back contact (BGBC) pentacene OFETs using
SU-8 as dielectric [28].
h e process integration of pentacene OFET with solution processed
polymer materials like SU-8 is a real challenge as pentacene is known to
get degraded by organic solvents. For this,
a novel process sequence is
followed, in which, one develops all the required layers except the penta-
cene layer for OFET on the SU-8 cantilever and then deposits pentacene
layer at er the releasing the device chips from the substrate to form the
i nal organic cantiFET. h is process has inherent advantages such as (1)
this avoids the exposure of pentacene layer to organic solvents used in
SU-8 lithography (2) obviates the need for patterning of pentacene layer
which is also known to be a non-trivial problem. h e schematic of this
fabrication sequence is given in i gure and process steps are detailed as
below.
h e process started with an oxidized silicon wafer in which the silicon
dioxide layer served as the sacrii cial layer for Figure 9.11(C2)). Gate elec-
trode and contact pads of the OFET were formed using gold (Au) with a
