handbook of modern sensors

548 18 Sensor Materials and Technologies
Fig. 18.8. Positive and negative photolithography.
18.3.1 Photolithography
Photolithography is the basic technique used to define the shape of micromachined
structures in the three techniques outlined below. The technique is essentially the
same as that used in the microelectronics industry.
Figure 18.8A shows a thin film of some material (e.g., silicon dioxide) on a
substrate of some other material (e.g., a silicon wafer). The goal of the process is to
selectively remove some silicon dioxide (oxide) so that it only remains in particular
areas on the silicon wafer (Fig. 18.8F).
The process beings with producing a mask. This will typically be a chromium
pattern on a glass plate. The wafer is then coated with a polymer which is sensitive
to UV light (Fig. 18.8B), called a photoresist. Ultraviolet light is then shone through
the mask onto the photoresist (Fig. 18.8C). The photoresist is then developed which
transfers the pattern on the mask to the photoresist layer (Fig. 18.8D).
There are two types of photoresist, termed positive (left side of Fig. 18.8) and
negative (right side of Fig. 18.8). Where the ultraviolet light strikes the positive resist,
it weakens the polymer, so that when the image is developed, the resist is washed
away where the light struck it—transferring a positive image of the mask to the resist
layer. The opposite occurs with negative resist. Where the ultraviolet light strikes
negative resist it strengthens the polymer, so when developed, the resist that was not