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5-12 MEMS: Design and Fabrication

80

70

60

PMMA stress (MPa) 50

40

30

20

10

0
0 0.01 0.02 0.03 0.04 0.05
Axial strain (laser based)

FIGURE 5.12 Tensile curve of PMMA (ICI CQ grade) measured with load frame and laser extensometer system for
a DXRL patterned tensile coupon geometry with 750µm gauge width and 150 µm thickness.
aresulting defect “foot” in the electroform. Electroplating a few microns of sacrificial metal before the
structural material can resolve this situation in some circumstances. Further study of G–G developer
behavior has shown that this alkaline solvent mixture induces a chemical reaction with exposed PMMA
prior to dissolution and that development depends strongly on PMMA stereochemistry [Schmalz et al.,
1996]. Another related issue concerns the origins and tailoring of PMMA sidewall roughness that has
direct implications for optical components and tribological behavior. Results from exposures at the
CAMD facility in Baton Rouge, Louisiana, have shown PMMA sidewall roughnesses as high as 25 nm
and as low as 5 nm RMS with typical results near 12 nm. This result seems to concur with the typical
resolution quoted for PMMA of roughly 50 to 100 Å [Van der Gaag and Sherer, 1990].
PMMA development is substantially improved via high-frequency acoustic agitation or megasonic agi-
tation [Ehrfeld,1991]. The development rate is increased, and redeposition of dissolved yet marginally high-
molecular-weight photoresist is alleviated, thereby decreasing sidewall particulates and roughness.Analysis
indicates an acoustic streaming effect likely is responsible for enhanced development rates in photoresist
with structure sizes over a few micrometers [Nilson and Griffiths, 2002]. Studies of transport processes for
PMMA trench development indicate that forced convective transport is effective in increasing development
rate only for trench structures with an aspect ratio of less than about five [Griffiths and Nilson, 2002].

5.2.5 PMMA Mechanical Properties

The origin of design rules in the DXRL-based molding process rests on the mechanical limitations of the mold
material, PMMA. Atensile curve for a high-molecular-weight linear PMMA material is shown in Figure 5.12;
it was measured with a milliscale tensile pulling technique. Table 5.4 provides a summary of properties

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