Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 147

such as absorption and reflection characteristics. Theoretically, the minimum achievable focal spot
diameter and, consequently, the smallest size are about twice that of the laser wavelength.
The choice of power depends on the desired structure size and the ablation rate. When excimer or
Nd:YAG lasers with a pulse duration of a few tens of nanoseconds are utilized, a single laser pulse will
typically vaporize the surface material to a depth of 0.1

m (see Table 4.9 ). Since each pulse
removes such a thin layer of material, the depth of the machined trench can be controlled accurately by
the number of laser pulses. Furthermore, laser pulses of very short duration eliminate heat flow to
surrounding materials. Consequently, clean and accurate structures can be achieved with shortly
pulsed lasers. There are two modes of laser micromachining: direct writing and using a mask [99] .In
the direct writing mode, the laser beam is focused on the substrate surface. The pattern is scanned
using a precision x

1 m

e

y stage or galvano scanning mirrors. In this mode, the smallest structure depends
on the accuracy of the scanning system, and is on the order of 25

e

m. In the masking mode, the
mask determines the detailed shape of the structure. Therefore, the minimum structure size can be
brought down to twice that of the laser wavelength.
Laser micromachining is suitable for fabrication of microchannels and fluidic access holes. An
LIGA-like technique uses laser machining instead of X-ray lithography to machine PMMA [100] .
Furthermore, the laser beam can be used for sealing polymeric devices fabricated with other tech-
niques or making shadow masks.

50 m

e

4.2.3 Polymeric surface micromachining
Polymeric surface micromachining technique is similar to its silicon-based counterpart. A functional
layer is structured on top of a sacrificial layer. Removing the sacrificial layer results in a freely movable
structure. Polymers can work as both sacrificial and functional layers. With SU-8 as the functional
layer, polymers, such as polystyrene, or metals, such as chromium, were used as sacrificial layers.
Silicon was used directly as sacrificial material as well as the handling substrate for the fabrication of
polymeric valves, polymeric micropumps, and polymeric microgrippers.

4.2.3.1 SU-8
SU-8 is a thick-film resist, which can be structured using UV lithography. With a Young's modulus of
4 5 GPa and a Poisson's ratio of 0.22, hardbacked SU-8 poses excellent mechanical properties and

can be used for movable parts. The sacrificial material for the release of the SU-8 part can be the silicon
substrate, a metal layer, or a polymer layer. Figure 4.18 (a) shows a micro-check valve made with this
technology [71] . The valve was first structured on silicon substrate with a two-layer process.

e

Table 4.9 Typical Ablation Depths Per Pulse of Different

Material (Nanosecond Laser)
Material

Depth Per Pulse ( m m)

Polymers 0.3 e 0.7

Ceramics and glass 0.1 e 0.2

Diamond 0.05 e 0.1

Metals 0.1 1.0

e