Biomedical Engineering Reference
In-Depth Information
Fig. 1
The planar integrated micro-mass spectrometer (PIMMS)
300 mm. The final chip size presently is 7 × 10 × 1.3 mm³ (
w
×
l
×
h
), i.e., about
0.1 cm³. Typically about 100 complete mass spectrometers are fabricated in paral-
lel on a 100 mm diameter wafer with a high yield even in a laboratory fabrication
environment, which would be 250 on a 150 mm wafer, the present standard of the
“microsystem” state of the art.
The silicon wafer is attached to two Pyrex glass plates on both sides via anodic
bonding. This encapsulation allows sealing silicon to glass hermetically without an
additional interface layer. The bottom glass wafer also carries the electrical conduc-
tors to apply the potentials and electric signals to the silicon electrodes and together
with the top glass wafer forms vacuum tight seals to install channels and chambers
of different pressure regimes as well as guiding structures for the gas capillaries to
supply the sample and process gases.
As already mentioned the planar integration forces all the subsystems needed for
the spectrometer to be in one plane and the available structures are restricted to a
2 ½-D geometry, i.e., only subsystem designs are allowed, which can be realized in
such a geometry. Furthermore, the devices are to be fabricated by standard micro-
fabrication processes on wafer level, such as masking by photolithography, deep
reactive ion etching, wet chemical etching, thin film deposition, and full wafer
anodic bonding. In contrast structures typically used in common mass spectrometer
designs are mostly cylindrical in shape.
The basic functional blocks, which a mass spectrometer has to comprise, are an
ionizer for the sample gas, a source for the ionization, an ion extractor, optic, and
accelerator, a mass separator and ion detector mostly following a multiplier to
amplify the ion current and improve the signal to noise ratio (SNR). All surfaces
and components exposed to the sample gas should be inert and chemically stable,
not to limit the type of chemistry to be analyzed. As these subsystems partly func-
tion in very different pressure regimes “pressure shields,” i.e., pressure stages have
to be incorporated.
Then such a system can be operated at one base pressure, which due to the small
volume of the device can be generated by a small vacuum pump. Since a wide range
