Biomedical Engineering Reference
In-Depth Information
of different, especially also very high voltages and very high frequencies, respectively,
have to be applied to the different subsystems, isolated electric wiring with high isola-
tion and shielding in combination with low capacitive loading and low loss is manda-
tory. Otherwise cross talk and dark currents will limit the SNR and RF-losses would
require high power drivers and—especially critical for microsystems, where cooling
is limited—generate high losses and cause intolerable heating.
As will be shown, the chosen glass-silicon-glass sandwich construction with the
electrical conductors on glass, the areas exposed to the analyte made of silicon or
glass, and the use of functional elements simultaneously for different tasks fulfills
all the cited demands and, due to their special design, all the necessary subsystems
can be generated in one patterning process.
Before the design, simulation, fabrication, pressures, potentials, electronics, and
software are discussed in detail in the sections to follow, an overview on the basic
functional principles of the system is given. According to Fig.
1
starting from left to
right the system comprises electron generation and acceleration including an elec-
tron optic, analyte ionization and its supply, ion extractor, optic, and accelerator,
mass separator, energy filter, and the detector with an optional amplifier.
Electrons for the ionization of the sample gas are generated in a microwave
(2.45 GHz) argon plasma burning in a chamber with a diameter of 150 mm at a pres-
sure of about 100 Pa. Argon is supplied from a small bottle at a pressure of 6 bar via
a capillary which, in conjunction with the electron extraction gap in the chamber,
reduces the pressure to the desired 100 Pa. The plasma is ignited by a spark gener-
ated by a high voltage pulse between two integrated platinum electrodes on the
bottom glass.
The electrons are extracted through a slit in the chamber wall, the anode, that
simultaneously acts as electron optic and a pressure stage between the 100 Pa in the
plasma chamber and <1 Pa outside, to which the mass spectrometer chip is evacu-
ated in an appropriate housing.
The electrons are accelerated across this low pressure region between the plasma
chamber and the adjoining ionization chamber to typically 100 eV, the optimum for
the ionization of most gases. They are focused to the input slit into ionization cham-
ber, into which the sample gas is introduced via another capillary, alternatively via
a microchannel chip, which reduces the pressure from atmospheric again to about
15 Pa, the pressure for maximum ionization efficiency for the chamber diameter of
also 150 mm. Analyte ions are extracted from the ionization chamber through a
second slit on the opposite side into an ion optic and are then accelerated into the
mass separator to an energy of typically 100 eV.
This mass separator is a new type, especially accommodated to be compatible
with the 2 ½-D geometries available in this fabrication process. The “synchronous
ion shield (SIS)” analyzer selects the mass, for which the filter is transparent, by a
traveling high frequency rectangular pulse stream (rise and fall time <1 ns) of a
swept frequency (0-270 MHz), which is supplied to a comb shaped electrode
arrangement.
According to Fig.
2
, which depicts the electric field between the electrodes (top)
perpendicular to the ion trajectory, ions which have the same speed as the pulse
