Page 202 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

182 MICROSTEREOLITHOGRAPHY FOR MEMS

Scanning
mirror

Resin
surface

Figure 7.11 Spherical beam swept over a flat resin plane in classical SL. From Jacobs (1996)

It is believed that, in classical MSL, too many mobile optical elements lead to poor
focusing. As shown in Figure 7.11, the free liquid resin surface defines a horizontal plane,
whereas the motion of the focused laser spot describes a portion of the surface of a sphere.
Thus, theoretically, the spot size varies during the scanning process. In classical SL, this
defocussing may not be critical as a larger beam size (>100 um) and longer focal depth
are used and an acceptable beam size and shape can be maintained across the flat resin
surface (Jacobs 1996). In MSL, however, the depth of focus is relatively short when
striving for high-resolution fabrication; in other words, to maintain the focus point at the
resin surface is highly critical, and this is the reason classical MSL apparatus employs a
dynamic lens for focusing. Even so, focusing still remains a major concern from recent
reports (Bertsch et al 1997).
Although this classical MSL system possesses some focusing problems that prevent
high-resolution fabrications, it has a fast fabrication speed. Therefore, classical MSL
is still an attractive option, as fabrication speed is always the first consideration of
production.

7.3.2 IH Process

A series of integrated harden (IH) polymer SL processes have been developed by Ikuta
and these are based on the classical scanning MSL method. The IH processes are designed
to overcome the beam-focussing problem present in a classical MSL system.