Page 207 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

SCANNING METHOD 187

Figure 7.18 Some microparts fabricated using the mass-IH process of Ikuta: (a) five 3-D micro-
structures on a table of 4.5 mm diameter and (b) a micropipe with a lateral window

Figure 7.19 (a) Comparison of the solidification processes of conventional MSL (I) and the new
super-IH process (II) and (b) processes needed to make movable gear and shaft (I). Conventional
MSL needs support structure, whereas the new super-IH process (II) does not need a support

It has been shown by Ikuta et al. (1998) that a significant surface tension of the
liquid monomer decreases the precision of the fabrication process. The super-IH process
has been developed to address this problem and can be used to solidify the monomer
at a specific point in 3-D space by focusing a laser beam into a liquid UV-curable
monomer. The 3-D microstructure is now fabricated by scanning the focused spot in all
three dimensions inside the liquid, thus obviating the need for any supports or sacrificial
layers. Figure 7.19 illustrates the difference between the conventional MSL processes and
the super-IH process (Ikuta et al. 1998) in that it does not need a support (a) and therefore
can be used to make, for example, a movable gear and shaft in one step. Because the
beam is writing directly into the resist, the effects of the monomer viscosity and surface
tension are greatly diminished.
A schematic diagram of the experimental setup of the super-IH process is shown in
Figure 7.20 and consists of a He–Cd laser of 442 nm wavelength, an optical shutter,
a galvano-scanner set, an X-Y-Z stage, an objective lens, and a computer (Ikuta et al.
1998). The laser beam is focused inside the monomer volume by coordinating the beam
scanning and Z-stage movements; thus, the 3-D structures are formed inside the liquid.
The properties of this system must be precisely tuned to ensure that polymerisation
only takes place at the point of focus. The UV monomer system used in the super-IH
process is a mixture of urethane acrylate oligomers, monomers, and photoinitiators.
Some interesting polymer microparts with free-moving elements have been fabricated
using the super-IH process (Ikuta et al. 1998). Figure 7.21 shows a scanning electron