5.2 Experimental
5.2.1 Freestanding Mesh Electronics Fabrication............
Uniform single-crystalline nanowires were synthesized using the Au
nanocluster-catalyzed vapor–liquid–solid growth mechanism in a home-built CVD
system described previously [ 22 ]. Key steps in the fabrication of freestanding mesh
electronics were discussed in previous chapters, in brief [ 14 ]: (1) Thermal depo-
sition were used to deposit a 100-nm nickel metal layer over the whole silicon
wafer, where the nickel served as thefinal relief layer for freestanding electronics.
(2) A 300- to 400-nm layer of SU-8 photoresist was spin cast over the entire
chip. (3) Photolithography and developing process was used to pattern the bottom
SU-8 layer for passivating and supporting the whole device structure. (4) A 300- to
400-nm layer of SU-8 photoresist was spin cast over the entire chip, then (5) the
synthesized nanowires were directly printed from growth wafer over the SU-8 layer
by the contact printing. Photolithography and developing process was used to
define regular SU-8 patterns for immobilizing nanowires. (6) To fabricate metal
electrode electrophysiological sensor, photolithography and electron beam depo-
sition were used to define and deposit 20 20 μm^2 Pt pad. (7) Photolithography
and thermal deposition were used to define and deposit the metal contact to address
each nanowire device and form interconnections to the input/output pads for the
array. Symmetrical Cr/Pd/Cr (1.5/50–80/1.5 nm) metal was sequentially deposited
followed by metal liftoff to define metal contact for semiconductor. Symmetrical
Cr/Au/Cr (1.5/50–80/1.5 nm) metal was sequentially deposited followed by metal
liftoff to define global interconnects. Nonsymmetrical Cr/Pd/Cr (1.5/50–80/50–
80 nm) metal was sequentially deposited followed by metal liftoff to define
mechanics-driven self-organized region as described in previous chapters. (8) A
300- to 400-nm layer of SU-8 photoresist was spin cast over the entire chip and
photopatterned as the top passivation layer (9) A 300- and 500-nm thick layers of
LOR 3A and S1805 photoresist can be deposited by spin-coating and defined by
photolithography to further protect the device region if necessary [ 23 ]. (10) The 2D
syringe injectable electronics were released from the substrate by etching of the
nickel layer. (11) If the device region was protected by photoresist protection layer,
electronics need to be transferred into deionized (DI) water for rinsing and then
dried on substrate, exposed in ultraviolet light to sensitize the photoresist protection
with subsequently immersed in developer solution to dissolve the protection on
device region. The structure of the mesh electronics is in Fig.5.1.
5.2.2 Mesh Structure Design...........................
General mesh electronics: Design of mesh structure is illustrated in Fig.5.2.We
call the ribbon along the injection direction as the longitudinal ribbon and call the
66 5 Syringe Injectable Electronics