Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 139

Table 4.7 Properties of Common Polymers for Bulk Micromachining

r(kg/m 3 ) 10 e 6 K e 1 Materials T g ( C) k(W/K m) g 3

e

Parylene-N 410 1,100 0.13 69

Parylene-C 290 1,290 0.08 35

Parylene-D 380 1,418 — 30 80

e

Polyamide 6 (PA 6) 60 1,130 0.29 80

Polyamide 66 (PA 66) 70 1,140 0.23 80

Polycarbonate (PC) 150 1,200 0.21 65

Polymethylmethacrylate
(PMMA)

106 1,180 1,190 0.186 70 90

e e

Polyimide — 1,420 0.10 0.35 30 60

e e

Polystyrene (PS) 80 100 1,050 0.18 70

e

( T g : Glass Transition Temperature, r : Density, k : Thermal Conductivity, g : Thermal Expansion Coefficient)

There are two key approaches in polymeric bulk micromachining: dry etching with oxygen plasma
and replication. Similar to silicon-based bulk micromachining, polymeric bulk micromachining uses
photolithography and etching to transfer a pattern into the bulk substrate. Oxygen plasma can be used
for etching polymers and other organic materials. Because photoresists are organic, a hard mask made
of metals such as aluminum, nickel, or titanium should be used here. Figure 4.11 shows an example of
the fabrication of a microchannel with two access holes in a bulk polymeric material. To start with,
a metal layer is deposited on the polymer surface. Sputtering is preferred because a low temperature is
required for processing polymers. The metal layer is patterned and structured using conventional
lithography and etching techniques ( Fig. 4.11 (a)). The two access holes are first etched with oxygen
plasma ( Fig. 4.11 (b)). The metal layers are then etched to form the mask for the microchannel. Access
holes and microchannel etched until the areas of the access holes are opened on the backside of the
substrate ( Fig. 4.11 (c)). Finally, the mask is etched away and the microchannel can be sealed by
thermal bonding to another polymer sheet ( Fig. 4.11 (d)).

FIGURE 4.11

Fabrication of a microchannel with fluidic access holes: (a) Patterning the metal masks for the access holes;
(b) etching of access holes; (c) patterning the mask for the microchannel and etching until the access holes are
opened; (d) etching away the mask.