Biomedical Engineering Reference
In-Depth Information
powder blasting is the fast processing time. Because of this reason, the ink supply channels of inkjet
print head are machined by powder blasting.
The resolution of powder blasting depends on the particle size. As a rule of thumb, the smallest cut
is about three times that of the particle size [102] . High resolutions are kept by the use of a hard metal
mask, which is machined by another technique, such as laser micromachining. Alternatively, masks
made of thick-resist foils and polyimide resist can also be used [102] . With particle velocities on the
order of 80
200 m/s, the erosion rate is on the order of 1 mm/min [101] .
e
m depth were fabricated
in glass [101] . Changing the incident angle of the powder beam can lead to channels with slanted walls.
Since glass has a relatively high zeta potential, powder blasting can be the economic way for mass
fabrication of active micromixers based on electrokinetic instability. Shlautmann et al. fabricated
microchannels with 85-
Using 30-
m
m alumina particles, microchannels of 100-
m
m width and 10-
m
m alumina particles [103] . In the
fabrication of micromixers, powder blasting can be used for drilling fluidic access through a substrate.
m
m width and 22-
m
m depth in glass using 9-
m
4.3.6 Ultrasonic micromachining
Another practical technique for cutting microchannels and opening access holes is ultrasonic drilling.
This technique uses ultrasonically induced vibrations delivered to a tool to machine the substrate.
When combined with abrasive slurry, ultrasonic abrasion can handle hard, brittle materials, such as
glass and silicon.
High-frequency electrical signal is converted through a piezoelectric transducer into mechanical
oscillation. The energy is acoustically transmitted to the machining tool. A piezoelectric stack actuator
can work as the piezoelectric transducer. The actuation frequency is typically 20 kHz. Abrasive slurry
flows around the cutting tool and causes microscopic grinding between the surfaces of the tool and the
workpiece. Through this process, the machined area becomes an exact counterpart of the tool. In the
fabrication process of micromixers, ultrasonic machining can be used for drilling access holes in glass
and silicon. Hole diameters on the order of 100
e
200
m
m can be machined with this method.
4.4 PACKAGING
4.4.1 Anodic bonding
Anodic bonding is the oldest bonding technique in silicon-based micromachining. The technique is
applied to a glass wafer and a silicon wafer. The required conditions for the process are the bonding
temperature on the order of 400 C and the high electrical field with bonding voltage about 1 kV.
Figure 3.20 describes the typical setup of anodic bonding. Silicon is connected to the positive electrode
and works as an anode, which gives the name to this bonding technique.
Anodic bonding induces a large temperature change to the glass/silicon stack. If the thermal
expansion coefficients of glass and silicon do not match, the stress upon cooling will cause cracks in
either silicon or glass. Thus, the glass wafer should have matching thermal expansion coefficient.
Glasses suitable for this purpose are Corning 7740 (Pyrex), Corning 7750, Schott 8329, and Schott
8330. Bonding between two silicon layers can also be achieved with anodic bonding by coating a thin
glass layer on one of the two wafers. The glass layer can be deposited by different techniques, such as
PVD and spin-on glass. Glass material with a matching thermal expansion coefficient is used as
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