Closed ion source

With applications requiring measurement of pressures between 10 − 4 and 10 − 3 Torr, the problem of ambient and process gases can be significantly reduced by replacing the OIS configuration with a CIS sampling system. Such an ionizer sits on top of the quadrupole mass filter and consists of a short, gas-tight tube with two openings for the entrance of electrons and exit of ions. The ions are formed close to a single extraction plate and exit the ionizer. Electrically insulative alumina rings seal the tube and the biased electrodes from the rest of the quadrupole mass assembly. The ions are produced by electron impact directly at the process pressure. Such design has been applied to gas chromatography mass spectroscopy instruments before adaption by quadrupole gas analyzers. Most commercially available CIS systems operate between 10 − 2 and 10 − 11 Torr and offer ppm level detectability over the entire mass range for process pressures between 10 − 4 and 10 − 2 Torr. The upper limit is set by reduction in mean free path for ion-neutral collisions which takes place at higher pressures, and results in the scattering of ions and reduced sensitivity.

The CIS anode may be viewed as a high conductance tube connected directly to the process chamber. The pressure in the ionization area is virtually the same as the rest of the chamber. Thus the CIS ionizer produces ions by electron impact directly at the process pressure whilst the rest of the mass analyzer is kept under high pressure. Such direct sampling provides good sensitivity and fast response times.