Biomedical Engineering Reference
In-Depth Information
FIGURE 4.20
Setup for anodic bonding.
a source or target for the evaporation or sputtering process. The bonding processes work as usual with
the glass-covered silicon wafer replacing the glass wafer in
Fig. 4.20
. Because of the much thinner
glass layer on the order of 0.5
m, much lower bonding voltages are needed for the same field
strength of bonding glass to silicon. The bonding voltage in this case is on the order of several tens of
volts. A thin metal layer on glass or silicon does not affect the bonding quality. Electric interconnects
can therefore survive the bonding process. Because of the optically transparent glass, this bonding
technique is suitable for making micromixers for biochemical applications, where optical access for
manipulation and evaluation of the fluid are required.
4
m
e
4.4.2
Direct bonding
Direct bonding refers to the bonding process between two substrates of the same material. Direct
bonding applies to a variety of materials such as silicon, glasses, polymers, ceramics, and metals.
Silicon direct bonding
, also called
silicon fusion bonding
, seals two silicon wafers directly under
high temperature. An intermediate layer makes the fabrication process more complex and more
complicated. The advantage of this technique is the lack of thermal stress because of the perfectly
matching thermal expansion coefficient of the two silicon wafers. Silicon direct bonding utilizes the
reaction between hydroxyl (OH) groups at the surface of the oxide layers of the two silicon wafers.
These oxide layers can be either native or deposited. Hydration of the silicon wafers is achieved by
immersing them in an H
2
O
2
/H
2
SO
4
mixture, boiling nitric acid, or diluted H
2
SO
4
. The bonding
process is carried out at temperatures between 300
C and 1000
C. Annealing the bonded stack at
high temperatures (800
C to 1100
C) further improves the bond quality.
Because of the optical transparency, glasses are relevant for many micromixers for life-science
applications. Bonding between glasses is called
glass
glass bonding.
Many applications use soda-
lime glass, which consists of SiO
2
,Na
2
O, CaO, MgO, and a small amount of Al
2
O
3
. The following
bonding process is used for two soda-lime glass slides. First, the glass wafers are cleaned in an
ultrasonic bath and subsequently 10 minutes in a solution of [5 H
2
O: 1 NH
3
(25%): 1 H
2
O
2
(20%)] or
[6H
2
O: 1 HCl (37%): 1 H
2
O
2
(20%)]. After removing moisture by annealing at 130
C, the two wafers
are thermally bonded together at 600
C for 6
e
8h
[104]
.
Many polymers are thermally bonded at temperatures above their glass transition temperatures. In
cases of polymers with low surface energy, such as PDMS, a surface treatment with oxygen plasma
seals the two polymer parts at room temperature. Polymer
e
polymer bonding can be achieved with the
e
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