Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 126

FIGURE 4.5
Buried channel with highly boron-doped silicon layer as cover: (a) boron doping; (b) opening etch access,
anisotropic wet etching; and (c) deposition of silicon oxide and silicon nitride.

Glass is a familiar material in chemistry and life sciences. Microchannels in glass have been used
widely for applications in these fields [20,24] . Glass consists mainly of silicon dioxide and therefore
can be etched with oxide etchants listed in Table 4.2 . The microchannels in glass are sealed by thermal
bonding to another glass plate. Most glass types can be etched in fluoride-based solutions [21,22] .
Photolithography and subsequent etching can be used for photosensitive glasses, such as Foturan, to
make microchannels [23] .
Isotropic etching in silicon results in semicircular channel shapes similar to those of glass etching.
Microchannels with trapezoidal cross-sections are formed by anisotropic etching of {100}- [23] or
{110}-wafers [24] . Microchannels etched in silicon are sealed either by anodic bonding to a glass
wafer or by thermal direct bonding to another silicon wafer. A glass cover is ideal for micromixers that
need optical access to the flow in the microchannel.
Sealing microchannels with anodic bonding or direct bonding has a drawback of wafer-to-wafer
misalignment. Misalignments and voids trapped during bonding processes can change the desired
cross-sectional shapes and, consequently, the function of the intended micromixer. Fabrication of
covered channels in a single wafer can overcome the problems associated with misalignment and
wafer bonding. In general, these covered microchannels are fabricated and buried in a single substrate.
Sealing is achieved by covering the etch access with a subsequent deposition process.
Figure 4.5 illustrates the main steps of making a buried channel in {100}-wafer. The process starts
with a highly boron-doped silicon layer with a doping concentration higher than 7

10 19 cm e 3
( Fig. 4.5 (a)). This layer works as etch stop and masking layer for the subsequent wet-etching process.
Etch accesses are opened by RIE through the highly boron-doped layer. The buried channel is formed
by anisotropic etching. The boron-doped layer remains intact during the etch process ( Fig. 4.5 (b)).
After anisotropic etching, the access gaps are sealed by thermal oxidation ( Fig. 4.5 (c)). The final
deposition of silicon nitride covers the entire structure [25] . The burying depth of the above-described
channel depends on the thickness of the highly boron-doped layer, which is a maximum 5

m due to
limits of diffusion processes [25,26] . The process shown in Fig. 3.6 overcomes this problem by using
deep trenches etched by DRIE [27,28] . Figure 4.6 shows the basic steps of this technique. The process
starts with DRIE of a narrow trench ( Fig. 4.6 (a)). The depth of this trench defines the burying depth of
the channel. In the next step, the trench wall is protected by deposition of silicon nitride or by thermal
oxidation ( Fig. 4.6 (b)). The layer at trench bottom is then removed by RIE to create the etch access.
Anisotropic or isotropic etching can be used to form the channel ( Fig. 4.6 (c)). After stripping the

m