Page 193 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

Microstereolithography for MEMS

7.1 INTRODUCTION

Stereolithography (SL) was introduced in 1981 by different teams in the USA (Hull 1984),
Europe (Andre et al. 1984), and Japan (Kodama 1981). SL is a rapid prototyping, and
manufacturing technology that enables the generation of physical objects directly from
computer-aided design (CAD) data files.
The stereolithographic process begins with the definition of a CAD model of the
desired object, followed by a slicing of the three-dimensional (3-D) model into a series
of closely spaced horizontal planes that represent the x-y cross sections of the 3-D object,
each with a slightly different z coordinate value. All the 3-D models are next translated
into numerical control code and merged together into a final build file to control the
ultraviolet (UV) light scanner and z-axis translator. The desired polymer object is then
'written' into the UV-curable resist, layer by layer, until the entire structure has been
defined (Figure 7.1).
The first commercially available SL system was produced by 3D Systems in 1987. SL
is now widely used in both the automotive and aerospace industries to fabricate industrial
products from the basic design to 'show and tell' parts at low cost - before the parts are
machined in the conventional manner.
This chapter describes in detail the different stereolithographic techniques and how they
can be used to make miniature parts (or microparts). When SL is used to make microparts,
it is usually referred to as microstereolithography (MSL). Here, we show the importance of
MSL as an enabling technology to make parts for microelectromechanical system (MEMS)
devices in materials other than silicon. It is thus a complementary technology to the
bulk- and surface-micromachining techniques described in Chapters 5 and 6, respectively.
Furthermore, MSL permits the fabrication of true 3-D devices, on the micron-to-millimetre
scale, including curvilinear and re-entrant microstructures that are difficult to make using
conventional silicon micromachining. It is sometimes referred to as the 'poor man's LIGA
process'!
The next section describes the fundamental concepts of photopolymerisation and SL
to produce an MSL system. The concept of photopolymerisation will already be familiar
to those who have read Chapters 2, 4, and 6, because it is also used in the processing of
conventional electronic materials and silicon microtechnology (e.g. see Sections 2.3, 4.3,
and 6.2).