Biomedical Engineering Reference
In-Depth Information
Fig. 20
REM picture of the silicon structures
2 .
Silicon wafer
Photolithography and DRIE etching of the 500 nm step
Photolithography and DRIE etching of the 5
m m step
3 .
Assembly of the sandwich
Anodic bonding of the bottom glass wafer and the silicon wafer
Photolithography and DRIE etching through the silicon wafer
Removing of released structures (Puzzle-Process)
Anodic bonding of the top glass wafer
All mechanical structures of the PIMMS are fabricated in a one mask anisotropic
deep reactive-ion etching process (DRIE, Bosch- or ASE(Advanced Silicon-Etch)-
Process. This process allows the production of silicon structures of high aspect ratio,
because it etches highly anisotropically with approximately vertical side walls. The
characteristic aspect ratio is achieved by combining isotropic plasma-etching and
passivation in alternating process steps. In order to achieve separated highly con-
ductive structures, such as plasma chamber walls and the finger electrodes of the
separator, the DRIE process is applied to a low resistive n-doped silicon wafer
(300 mm), which is anodically bonded to a glass wafer (500 m m).
In order to guarantee a constant etch rate across the wafer independent of feature
size the etching is restricted to narrow gaps around the desired patterns. This allows
to remove narrow as well as extended areas of the silicon. The areas to be removed
are not bonded to the bottom glass substrate. The released areas are removed after-
wards just by turning the sandwich upside down.
Figure
20
shows a REM picture of the silicon structures.
Metal conductors are necessary for applying electrical potentials to the silicon
structures. Due to the small size and the material a direct welding or wire bonding
