Page 39 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

PATTERN TRANSFER 21

Photoresist
SiO 2
Si substrate

Photomask

(b)

Positive resist ^-^\ Negative resist
\
Develop

Final image
(e)

Figure 2.10 Formation of images after developing positive and negative resists (Sze 1985)

2.3.4 Lift-off Technique

The pattern-transfer technique, referred to as lift-off, uses a positive resist to form the resist
pattern on a substrate. The steps of the technique are shown in Figure 2.11. The resist is
first exposed to radiation via the pattern-carrying mask (Figure 2.11 (a)) and the exposed
areas of the resist are developed as shown in Figure 2.1 l(b). A film is then deposited over
the resist and substrate, as shown in Figure 2.11(c). The film thickness must be smaller
than that of the resist. Using an appropriate solvent, the remaining parts of the resist and
the deposited film atop these parts of the resist are lifted off, as shown in Figure 2.1 l(d).
The lift-off technique is capable of high resolution and is often used for the fabrication
of discrete devices.