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5

X-Ray–Based

Fabrication

5.1 Introduction ......................................................................5-1
5.2 DXRL Fundamentals ........................................................5-4
X-Ray Mask Fabrication • Thick X-Ray Photoresist • DXRL
Exposure (The Direct LIGA Approach) • Development
• PMMA Mechanical Properties
5.3 Mold Filling ....................................................................5-14
5.4 Material Characterization and Modification ................5-20
5.5 Planarization ..................................................................5-24
5.6 Angled and Reentrant Geometry ..................................5-24
5.7 Multilayer DXRL Processing ..........................................5-26
5.8 Sacrificial Layers and Assembly ....................................5-29
Todd Christenson 5.9 Application Examples ....................................................5-30
HT MicroAnalytical, Incorporated 5.10 Conclusions ....................................................................5-46

5.1 Introduction

Originally conceived for the fabrication of smaller microelectronic features, X-ray lithography also has
attributes of great utility in micromechanical fabrication. In contrast to the many micromachining
processes that have been developed from microelectronic processing, however, X-ray based approaches may
be performed largely without a tightly controlled clean-room environment. The mode of X-ray based
microfabrication most commonly used places this type of processing in the additive category where a sac-
rificial mold is used to define the desired structural material. As a result, this technique lends itself to a very
rich and ever-expanding material base including a variety of plastics, metals, and glasses, as well as ceram-
ics and composites. The idea of using X-rays to define molds extends from the 1970s when its precedent
involved defining high-density coils for magnetic recording read/write heads and high-density magnetic
bubble memory overlays. This was where the use of X-rays for Very Large Scale Integration (VLSI) lithog-
raphy was initially investigated [Romankiw et al., 1970, 1995; Spiller et al., 1976; Spears and Smith, 1972].
The distinction from VLSI X-ray lithography is that the mold or photoresist thickness for micromachining
interests is generally much greater than 50 microns and may be well over 1 millimeter. X-ray processing at
these thicknesses has prompted the nomenclature deep X-ray lithography, or DXRL, based microfabrication.
The primary utility of DXRL processing extends from its ability to precisely and accurately define a
mold. Consequent component definition via mold filling thus is determined directly by mold acuity and
stability. Exceptional definition in this regard is possible with highly collimated X-rays that may be

5-1

© 2006 by Taylor & Francis Group, LLC