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5-10 MEMS: Design and Fabrication

140 angstroms/minute may be achieved, and films as thick as 10 microns have been deposited. Increased
power levels result in further decomposition of MMA and films that are less like PMMA.
Another X-ray resist is a negative-working epoxy-based photoresist formulated with SU-8 resin, which is
also used for e-beam or deep near UV lithography and yields very high resolution [Lee et al., 1995; Khan-
®
Malek, 1998b]. The EPON resin SU-8 (SU-3 and SU-2.5 are also available), from Shell Chemical, pos-
sesses excellent high temperature stability with a glass transition temperature of more than 200°C when
cross-linked [Shaw et al., 1997]. This is due to a high epoxide group functionality (with an average near
eight) that also leads to an exceptionally high sensitivity as a photoresist. Deep X-ray lithography results
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indicate a sensitivity near 52 J/cm [Bogdanov and Peredkov, 2000], or roughly 40 times greater than
PMMA, to as low as 20 J/cm [Jian et al., 2001]. Critical exposure dose has been found to be highly depend-
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ent on the residual solvent content [Singleton et al., 2001] and concentration of photo acid generator
(PAG) [Ling et al., 2003]. A number of practical issues regarding the X-ray process use of SU-8 are critical
for its success. Adhesion, for example, depends on substrate material, plating base material, and the use of
adhesion promoters [Barber et al., 2004].Additional issues concern SU-8 X-ray contrast, which dictates min-
imum absorber thickness [Shew et al., 2003a] and oxygen relaxation [Shew et al., 2003b]. Following expo-
sure and post-exposure baking, the resulting SU-8 cross-linked epoxy also possesses a high optical refractive
index (near 1.65) and thus has advantages as an optical lens material. What remains to be found is an easy
means to chemically dissolve the SU-8 photoresist after cross-linking. Several techniques have had various
degrees of success; these include solvent cracking, downstream etching, and molten salt baths [Dentinger
et al., 2002]. In addition, sacrificial polymers may be prepared before SU-8 application to aid in removal.
Another negative X-ray photoresist that is amenable to dissolution after postexposure bake has been used.
From Japan Synthetic Rubber Co. (JSR), it is the line of NFR photoresists that may be spun cast to
several tens of microns. It employs standard TMAH-based developers; examples of structures are shown
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in Figure 5.10. Minimum X-ray dose levels of 100–300J/cm have been measured, and the resist may
readily be dissolved in n-methyl pyrolidinone (NMP) at 60–80°C. New formulations of negative chemi-
cally amplified X-ray sensitive photoresists with marked photosensitivity improvements of 40 times
greater than PMMA have also been presented [Sakai, 2003].

5.2.3 DXRL Exposure (The Direct LIGA Approach)
With its ability to expose and apply PMMA layers over 1 cm in thickness while most applications being pur-
sued require 150 to 500µm (10cm exposures have been demonstrated [Siddons et al., 1994; Guckel, 1996b]),
the question arises of how to use deep X-ray lithography most beneficially. What becomes immediately
apparent is that multiple sheets of PMMA may be stacked and exposed simultaneously [Guckel,1998a,1999a;
Siddons et al., 1997]. Accounting for the issues associated with the bonding or attachment of exposed
PMMA sheets to a substrate accommodating development and mold filling, this procedure has signifi-
cant throughput advantages. Exposures obtained with the 7.5 Tesla wiggler insertion device [Manohara,
1998] at the Center for Advanced Microstructure Devices (CAMD), at Louisana State University, reveal
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these advantages. For a 1 cm thick PMMA stack with a limited delivered top dose of 10 kJ/cm and
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bottom dose of 3 kJ/cm , an exposure time slightly greater than 4 hours results for an exposure area of

FIGURE 5.10 (a) 6µm and (b) 8µm diameter cylindrical NFR-015 photoresist pillars, 40µm tall, created with
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125J/cm X-ray dose.

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