Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 232 / 362

10-4 Circuit Description
SRS Residual Gas Analyzer
Circuit Description
General Description
The specifications and features of the many circuits that drive the RGA are determined by
characteristics of the quadrupole mass spectrometer such as:
x
the ionizer settings available to the user,
x
the characteristics of the quadrupole mass filter,
x
the magnitude of the ion current levels detected during measurements,
x
an optional electron multiplier.
The ionizer consists of a dual thoriated iridium filament heated by a current of about 3A
at 1.7V. The filament and the associated repeller shield are held at a negative potential
between -13 and -97V, determined by the electron and ion energy settings (Vrepeller =
ion energy - electron energy). Electrons from the filament are accelerated toward the
anode grid cage, which is held at a positive potential of +8 (low) or +12V (high). The 25-
105 eV electrons ionize residual gas molecules in the grid cage. The ions are then
injected into the mass filter, aided by the focus plate, which is held at a negative potential
of about -90VDC (adjustable from 0 to -150V) to optimize the throughput of the mass
filter and to deflect electrons.
The mass filter requires two potentials be applied to two pairs of rods. The potentials are
given by: Vx/y(t) = +/- [ U + V
0
cos(
Z
t) ], where U is a DC potential amplitude and V
0
is
an RF potential amplitude. The physics of the quadrupole mass filter show that the
resolution of the instrument improves with the frequency
Z S
f ( the ions interact with
the RF field for more cycles ), however the required RF amplitude increases with the
square of the frequency. An operating frequency f=2.7648 MHz requires an RF
amplitude, V
0
, of 1284 V (2568 Vpp) to measure ions with mass of 300 amu (8.56
Vpp/amu.). The ratio of DC to RF amplitude, U/V
0
, controls the fractional mass
resolution, M/
'
M. For f= 2.7648 MHz, the DC potential, U, will +215VDC on one rod
pair, and -215 VDC on the other, for masses of 300 amu.
The rod structures are primarily capacitive, so to reduce the circuit drive requirements,
the rod reactance is resonated with an inductance. The advantages of this resonant drive
circuit is the primary reason for operation at a fixed frequency.
Ions with the selected mass are collected in a Faraday cup. The ion currents can be very
low, a few femtoamps, and have a very large dynamic range, up to about 100 nA. A log
I/V converter capable of simultaneously measuring ion signals over several orders of
magnitude is a natural choice. Low Pass filtering of the signal is essential in the presence
of the large rod potentials.
To improve the detection limit, by overcoming the noise floor imposed by the I/V
converter, an electron multiplier may be used. The electron multiplier needs to be biased
with as much as -2500VDC to provide gains as large as 10
7
.