handbook of modern sensors

546 18 Sensor Materials and Technologies
gas, and the gas ions bombard the target. The kinetic energy of the bombarding ions
is sufficiently high to free some atoms from the target surface. Hence, the escaped
sputtered atoms deposit on the surface of the sample.
The sputtered techniques yields better uniformity, especially if a magnetic field
is introduced into the chamber, allowing for better directing of the atoms toward
the anode. Because this method does not require a high temperature of the target,
virtually any material, including organic, can be sputtered. Moreover, materials from
more than one target can be deposited at the same time (cosputtering), permitting a
controlled ratio of materials. For example, this can be useful for sputtering nichrome
(Ni and Cr) electrodes on the surface of the pyroelectric sensors.
18.2.5 Chemical Vapor Deposition
A chemical vapor phase deposition (CVD) process is an important technique for the
production of optical, optoelectronic, and electronic devices. For sensor technologies,
it is useful for forming optical windows and the fabrication of semiconductor sensors
where thin and thick crystalline layers have to be deposited on the surface.
The CVD process takes place in a deposition (reaction) chamber, one of the versions
of which is shown in a simplified form in Fig. 18.7. The substrates or wafers are
positioned on a stationary or rotating table (the substrate holder) whose temperature
is elevated up to the required level by the heating elements. The top cover of the
chamber has an inlet for the carrier H 2 gas, which can be added by various precursors
and dopants. These additives, while being carried over the heated surface of the
substrate, form a film layer. The gas mixture flows from the distribution cone over
the top surface of the wafers and exits through the exhaust gas outlets. The average
gas pressure in the chamber may by near 1 atm, or somewhat lower. For example, a
Fig. 18.7. Simplified structure of a CVD reactor chamber.