Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 116

where NA is the numerical aperture of the imaging lens system. Most photolithography systems use
a mercury lamp as a light source. Mercury lamp's wavelengths of I-line, H-line, G-line, and E-line are
365 nm, 404.7 nm, 435.8 nm, and 546.1 nm, respectively.
Because of its simplicity and fast prototyping process, photolithography of thick resists is a favored
technology for the fabrication of micromixers. Thick resists structured by photolithography can be
used as a template for molding polymeric devices or for electroplating of metallic parts. For the
molding application, the resist structure should have high aspect ratio, which is suitable for making
microchannels. The high aspect ratio requires special resists, such as SU-8 or high-energy beam (e.g.,
X-ray). If conventional UV source is used for the exposure, a thick-resist layer may degrade the
resolution because the best depth of focus offered by proximity printing is only on the order of 5

m.
For a simple estimation, the resolution is approximately one-third of the resist thickness. Figure 4.1
shows the typical steps for patterning a silicon oxide layer on a silicon substrate using
photolithography.

m

4.1.1.2 Chemical vapor deposition
The patterns transferred from the glass mask to the photoresist are often further copied to a functional
layer by etching. The functional layer is deposited before applying the photoresist. Chemical vapor
deposition (CVD) is one of the many techniques for creating material films on a substrate. CVD utilizes
chemical reaction between gaseous reactants to form a single solid product. The solid product is formed
as a thin film on a heated substrate surface. The other reaction products should be in the gaseous form so
that they can leave the reaction chamber. CVD processes are categorized based on reaction conditions.
The common processes are atmospheric-pressure chemical vapor deposition (APCVD), low-pressure
chemical vapor deposition (LPCVD), and plasma-enhanced chemical vapor deposition (PECVD).
APCVD and LPCVD processes require relatively high temperatures ranging from 500 Cto
800 C. The high process temperature causes metals with low eutectic temperature with silicon, such
as gold (380 C) or aluminum (577 C), to melt. Thus, metals with high eutectic temperature, such as
tungsten, are suitable for deposition before APCVD or LPCVD processes. The alternative for a low-
temperature substrate material is PECVD, which only requires temperatures typically on the order of
100 300 C. Table 4.1 lists common chemical reactions used in CVD of different material films and

their corresponding process parameters.

e

4.1.1.3 Thermal oxidation
Silicon dioxide can be deposited with CVD if the substrate is a material other than silicon. If the
substrate is silicon, thermal oxidation is the simplest technique to create a silicon dioxide layer. Based
on the type of oxidizer, thermal oxidation is categorized as dry oxidation or wet oxidation. Dry
oxidation utilizes pure oxygen to form silicon oxide at high temperatures from about 800 Cto
1,200 C:

Si þ O 2 / SiO 2 : (4.3)

The oxidant in wet oxidation is water vapor:

(4.4)
Since the thickness of the silicon oxide layer can be controlled in an oxidation process, thermal
oxidation can be used for accurately adjusting gaps in microfluidic devices with submicron precision.

Si þ 2H 2 O / SiO 2 þ H 2 [: