Synthesis and Patterning Methods for Nanostructures Useful for Biological Applications - Nanotechnology for Biology and Medicine - page 30

Fig. 1 Comparison of the available lithography techniques (image adapted from Brodie and
Muray 1982 )
on a surface features with dimensions on the order of nanometers. Lithography
(Madou 2002 ) can be performed using light (optical- or photolithography), elec-
trons (e-beam lithography), ions (i-beam lithography), or X-ray (X-ray lithography,
LIGA) depending on the desired minimum feature size of the outputs (Fig. 1 ).
Photolithography is the most common and widely used technique in applications
requiring micron and submicron feature sizes, like electronic, integrated circuits
manufacturing, and microfl uidic devices. It fi nds a lot of applications in bio-MEMS
devices for its applicability to patterning of organic and inorganic materials, hydro-
gels, membranes, and ion-selective electrodes (Lambrechts 1992 ) . With this tech-
nique, a design or pattern can be transferred at once on the surface of a device by
exposure with light. Similarly, e-beam, i-beam, and X-ray lithography use the expo-
sure to beams of electrons, ions, or X-ray for the design transfer.
All the lithographic techniques are multistep processes. As an example, we
describe the steps followed in the specifi c case of photolithography of a Si/SiO 2
substrate (Fig. 2 ), keeping in mind that other techniques follow a very similar path.
The fi rst step is to coat the surface of the desired substrate with a thin polymer layer
of a positive or negative photoresist (Fig. 2a ), which is then covered by a prede-
signed, light-blocking mask and exposed to UV light (Fig. 2b ). After exposure, the
cured photoresist remains on the substrate as a protective coating, and the uncured
portions are removed by the developer (Fig. 2c ). This step is then followed by the
etching of the substrate and the fi nal removing of the photoresist (Fig. 2d ). The type
of resist chosen allows having a direct duplication of the mask used (as in the case
of positive resist) or inverse of it (negative resist) exactly like in the case of fi lm
photography developing.