6.3.6 Application notes
Mass spectrometer analysis is every bit as varied as vacuum applications. The above-described gas inlet systems with heated capillaries are used for gas analysis in the pressure range of up to atmospheric pressure. Gas flows can be channeled directly to gas-tight ion sources in order to reduce the background noise of the vacuum environment. Gas beams are passed through crossbeam ion sources, with the beam exiting into a vacuum pump or trap.
Figure 6.18: QMS with gas inlet system and crossbeam ion source
Figure 6.19: Differentially pumped QMS with various gas inlets
In the pressure range $p$ < 10 hPa (etching, sputtering or other coating processes), the gas is admitted into the mass spectrometer via an orifice or a valve. A turbopump is attached to the measuring system for pressure reduction. There are special versions for corrosive gases.
At extremely low pressures, particularly in the UHV range, open ion sources are used which have a particularly small surface area and therefore low outgassing rates (grid ion source). Due to the low gas densities, secondary electron multipliers (SEM) that are arranged perpendicular to the axis of the quadrupole must be used as detectors. To improve the signal-to-noise ratio, a turbopump that pumps down the inflowing neutral particles is attached opposite the SEM.
Secondary ion mass spectrometry (SIMS) represents a special case. In this process, ions are shot onto surfaces that in turn release positively or negatively charged secondary ions. These are detected directly by a QMS without an ion source. The measuring arrangement described in the preceding section is used in this case as well.