handbook of modern sensors

554 18 Sensor Materials and Technologies
Fig. 18.16. Lift-off technique.
(A)
(B)
(C)
(D)
(E)
18.3.2.1.5 Lift-Off
Lift-off is a stenciling technique often used to pattern noble metal films. There are a
number of different techniques; the one outlined here is an assisted lift off method. A
thin film of the assisting material (e.g., oxide) is deposited. A layer of resist is put over
this and patterned, as for photolithography, to expose the oxide in the pattern desired
for the metal (Fig. 18.16A). The oxide is then wet etched so as to undercut the resist
(Fig. 18.16B). The metal is then deposited on the wafer, typically by a process known
as evaporation (Fig. 18.16C). The metal pattern is effectively stenciled through the
gaps in the resist, which is then removed, lifting off the unwanted metal with it (Fig.
18.16D). The assisting layer is then stripped off too, leaving the metal pattern alone
(Fig. 18.16E).
18.3.2.2 Wafer bonding
There are a number of different methods available for bonding micromachined silicon
wafers together, or to other substrates, to form larger more complex devices.Amethod
of bonding silicon to glass that appears to be gaining in popularity is anodic bonding
(electrostatic bonding). The silicon wafer and glass substrate are brought together and