Page 19 - Principles and Applications of NanoMEMS Physics

1. NANOELECTROMECHANICAL SYSTEMS 5

solvent-free dry layer. The resulting PR film thickness is inversely
proportional to the square root of the rotational speed, and directly
proportional to the percent of solids in it. Determining these parameters is
one of the first steps in developing a process.

1.2.1.2 Wafer Patterning

Once a uniform solid PR layer coats the wafer, this is ready for
patterning. This is accomplished by interposing a glass mask, which contains
both areas that are transparent and areas that are opaque, between a UV
source and the PR-coated wafer. As a result, selective chemical changes are
effected on the PR in accordance with the desired pattern, Figure 1-2. When
it

(a)
(a)
Photoresist (PR
Photoresist (PR) )
SiO 2 2
SiO
Si
Si
Mask
(b) Mask
(b)
SiO 2 2
SiO
Si Si
Negative PR
Negative PR
Positive PR
Positive PR
SiO
SiO 2 2
(c)
(c)
SiO
SiO
2 2
Si Si Si Si
SiO
SiO
SiO SiO 2 2
(d)
(d) 2 2
Si Si Si Si
Figure 1-2. Wafer patterning with positive and negative photoresists. (After [24]).
is desired that the created pattern be identical to that in the glass mask, a
positive PR, which hardens when exposed to UV light, is employed.
Otherwise, when it is desired that the created pattern be the negative of that
in the mask, a negative PR is employed. In the former case, UV exposure