Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 347

basic fabrication steps are employed by the microchip manufacturers: (i) thin fi lm dep-
osition (ii) photolithography, (iii) etching, and (iv) substrate bonding. Thin fi lms are
deposited using various chemical or physical methods for a variety of different pur-
poses in microstructures: masking materials, structural materials, sacrifi cial materials,
and electrical devices. After thin fi lm deposition, the photolithography step is per-
formed in order to transfer a designed pattern onto a substrate. The patterned substrate
is then etched using various chemicals in the liquid or gas phase. Finally, substrate
bonding is conducted either to integrate multiple functionalities or for packing use.
These steps can be repeated numerous times in random sequences depending on the
complexity of the design and process.

Thin fi lm deposition
A wide variety of techniques including chemical vapor deposition (CVD), thermal oxi-
dation, physical deposition (sputtering, spin coating or E-beam), and electroplating are
utilized to deposit thin fi lms of different materials such as silicon, silicon nitride, sili-
con oxide, etc. onto a substrate. The properties of the deposited fi lm will be strongly
dependent on the process designed for a specifi c application and materials used as
well. CVD involves the reaction of chemicals in a gas phase to form the deposited fi lm
of inorganic materials such as silicon oxide, silicon nitride, and polysilicon. The most
popular CVD methods are low pressure CVD (LPCVD) and plasma enhanced CVD
(PECVD). LPCVD, usually at

100-300 mTorr and evaluated temperature, generally
results in high quality, uniform, and conformal fi lms. The main drawback of LPCVD
is high temperature and slow growth. PECVD uses plasma to dissociate the reactive
molecules under a fairly low temperature, which is often the reason for this choice.
However, the obtained fi lms are often less conformal and of poor quality despite
high deposition rate and high fl exibility in operating conditions. Thermal oxidation is
typically performed on semiconductor substrates especially Si wafers by heat treat-
ment from 800 to 1200ºC in an atmospheric O 2 (dry oxidation) or H 2 O vapor (wet oxi-
dation). PVD is done by evaporation or sputtering. In evaporation systems, materials to
be deposited are heated in a vacuum chamber and generate a cloud of the vapor of the
material following thin fi lm deposition on the top of the substrate. In sputtering, posi-
tive ions from plasma bombard a target of the fi lm material, which remove atoms or
clusters from the target into the plasma as neutral species. Due to the directionality of
the arriving material, sputtered fi lms are non-conformal. One reason for the widespread
use of sputtering is the ability to deposit alloys and materials with a high melting point
like tungsten. If liquids are used as a coating material, spin coating is often preferred.
Electroplating can be used to form very thick metal layers, for instance in creating
thick mechanical structures out of metal or in creating tools for injection molding.

Photolithography
This technique is used to transfer a computer-generated pattern onto a substrate. Here, a
fi lm of photoresist is spin-coated onto the substrate and exposed to UV light through a
photolithographic mask; the light exposure transfers the desired pattern to the photore-
sist. Depending on whether the resist material is “positive” or “negative”, the photoresist