Page 214 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

194 MICROSTEREOLITHOGRAPHY FOR MEMS

UV light source

Figure 7.28 Apparatus for mask-projection method of microfabrication

where the various cosine and sine coefficients are defined by

(7.11)

and the limits on the integrals are defined by,
p 1 =2 (x/a - 0.
p 2 = 2 (x/a + 0.
q 1 = 2 (y/a - 0. (7.12)
2
m = z)/a

where A is the wavelength of the UV light, a is the length of each side of the square, and
h is the distance between the mask and the surface of the resin.
The light intensity inside the resin, 7(x, y, z), can be calculated according to the
Beer-Lambert law expressed in a slightly different form from that of Equation (7.13)

I x,y,z) = I d(x, y, 0)exp(-az) (7.13)
(
where a is the absorption coefficient of the resin. The irradiation exposure E(x, y, z) =
I x,y,z)t, where t is the irradiation time period. The resin solidifies once the irra-
(
diation exposure reaches the threshold value E c. The parameters a and E 0 determine
the solidification properties of the resin and must be determined experimentally. Letting