Stereolithographic System

All SL systems share the same basic elements or subsystems: the CAD design, layer-preparation functions, and a laser-scanning or imaging system. A typical SL system is schematically shown in Figure 7.7.

The CAD model is a 3-D representation of the object and is exported, most commonly, into a file with a format called STL. Slice cross sections are produced from this file and converging parts placed into a 'build' file. Process control software is then used to operate the SL writing process according to the build file. Detailed information on CAD design, STL file formats, part preparation convergence, contour slice, and so on can be found in the references (Jacobs 1992, 1996).

A uniform, flat resin layer of the desired thickness is prepared for curing through standard layer preparation. The liquid resin surface will be the foundation of each layer of the SL model; therefore, the quality of the resin surface has a direct impact on the accuracy of the final structure. Recoating blades, fluidic pumps, surface-finding lasers, and robotic systems have all been considered to achieve a highly uniform resin surface. The general requirements of the SL process are simple in concept, but their implementation is quite challenging. The SL system should satisfy all the following requirements (Jacobs

1996):

• The resin surface must be maintained precisely at the focal plane of the imaging system

• The resin surface must be uniformly flat, leveled, and free of extraneous features

• The resin surface must be a controlled distance above the previously built cross section of the part

The recoating and leveling systems work together and aim to achieve the desired goal of providing a flat layer of liquid resin of proper thickness. A leveling system is used to sense the resin height and permit adjustments. A level-finding system can vary from a

x-v scanner

2*0*

Photopolymer resin

Figure 7.7 Block diagram showing the basic elements of an SL system. From Jacobs (1992)

MICROSTEREOLITHOGRAPHY 179 simple mechanical float mechanism through to an optical proximity sensor system, which monitors the current resin height and allows this to be fed back into a computer-based closed-loop control system.

A number of other steps are required in the coating process, such as the lowering of the part in the vat full of resin, the wiping away of any excess materials, and the smoothing of the remaining materials to provide the desired thickness of liquid resin above the previously solidified layer.

The thickness of the layer typically ranges from 100 to 500 um. The recoater must be controlled very precisely to achieve thinner layers, because a recoating error of 25 um becomes very significant when building with thickness below 100 um (Jacobs 1996).

When a smooth, uniform, and accurate coating of the liquid resin has been achieved over the previously solidified polymer layer, the process is not finished until sufficient resin remains within the vat to compensate for any shrinkage that can occur during curing.

An imaging system for SL includes a light source (laser or lamp), beam delivery, and focusing elements (Figure 7.7). The laser (or lamp) chosen for the system must be appropriate for the resin to be used. Wavelength, output beam shape, and power available are all important characteristics (see equations in Section 7.1.1). Beam delivery elements are employed to fold the path of the laser beam and therefore make the SL system as compact as possible.

A typical SL system employs two orthogonally mounted, servo-controlled, galvanometer-driven mirrors to direct the laser beams onto the surface of the vat. The beam is passed though a focusing objective and then hits the resin surface. The beam exposure is controlled by a shutter according to an on-off command generated by the build file. A mechanical shutter requires typically about 1 ms to actuate, and so has now been replaced by an acoustic optical modulator that has a much lower actuation time of about 1 us, thus allowing a much faster fabrication process. The time needed to write a layer is always a critical parameter of any beam writer because it relates both to throughput and cost.

Applications of SL vary considerably from the quick-cast tooling through to struc-tural analysis. Consequently, SL can speed up product development and improve product quality through superior design and prototyping.

7.2 MICROSTEREOLITHOGRAPHY

The principle of MSL is basically the same as that of SL (Section 7.1), except that the resolution of the process is lower. In MSL, a UV laser beam is focused down to a 1 to 2 um-diameter spot that solidifies a resin layer of 1 to 10 um in thickness, whereas in conventional SL, the laser beam spot size and layer thickness are both on the order of 100 to 1000 um. Submicron control of both the x-y-z translation stages and the UV beam spot enables the precise fabrication of complex 3-D microstructures.

MSL is also called microphotoforming and was first introduced to fabricate high aspect ratio and complex 3-D microstructure in 1993 (Ikuta and Hirowatari 1993). In contrast to conventional subtractive micromachining, MSL is an additive process, and therefore, it enables the fabrication of high aspect ratio microstructures with novel smart materials. The MSL process is, in principle, compatible with silicon microtechnology

180 MICROSTEREOLITHOGRAPHY FOR MEMS

and therefore post-CMOS batch fabrication is also feasible (Ikuta et al. 1996; Zissi 1996).

Different MSL systems have been developed in recent years to improve upon their precision and speed. Basically, scanning MSL (Ikuta and Hirowatari 1993; Zissi el al.

1996; Katagi and Nakajima 1993; Zhang et al. 1999; Maruo and Kawata 1997) and projection MSL (Bertsch et al. 1997; Nakamoto and Yamaguchi 1996; Monneret et al.

1999) are the two major approaches that have been taken. Scanning MSL builds the solid microparts in a point-by-point and line-by-line fashion (Figure 7.8), whereas projection MSL builds one layer with each exposure (Figure 7.9), thus speeding up the building process by a significant factor (Beluze et al. 1999). The details of the two approaches are presented in Sections 7.3 and 7.4.

Another research effort in MSL is the incorporation of a broad spectrum of mate-rials to create MEMS with new special functions. The MSL fabrication of polymer

Z-stage

Y- stage

Figure 7.8 Principle of scanning MSL, that is, a vector-by-vector approach. From Beluze et al.

(1999)

Focusing optics -*

Photoreactor

Figure 7.9 Principle of projection MSL, that is, a plane-by-plane approach. From Beluze et al.

(1999)

SCANNING METHOD 181 microparts and their subsequent electroplating to form metallic microparts have been reported (Ikuta and Hirowatari 1993; Ikuta et al. 1996; Zissi et al. 1996; Katagi and Naka-jima 1993; Maruo and Kawata 1997; Bertsch et al. 1997; Nakamoto and Yamaguchi 1996;

Monneret et al. 1999). Functional polymer (e.g. conducting polymer) microparts possess the unusual characteristics of high flexibility, low density, and high electric conductivity (Ikuta and Hirowatari 1993). Ceramic microstructures have also been fabricated by MSL using both structural and functional ceramic materials (Zhang et al. 1999; Jiang et al.

1999). The use of MSL to make both ceramic and metallic microparts is discussed in Section 7.7.