Residual Gas Analysis (RGA)

Users in many applications are faced with the question of how clean the vacuum apparatus actually is. It is also very important for process monitoring to know the composition of the residual gas. Vacuum pressure measurement gives us information about how many particles still remain in the chamber but not about what particles they are. Quadrupole mass spectrometers (QMS) can be used to perform a residual gas analysis that answers this question. Once it is known which species are left in the chamber, additional purification steps can be taken or the application itself can be started.

How does it work?

Like all mass spectrometric instruments, quadrupole mass spectrometers (QMS), are made up of three parts: an ion source, an analyzer and a detector. All components in a quadrupole mass spectrometer require high vacuum to function. The neutral gas particles are ionized in the ion source. A common ionization method that is also used in Pfeiffer Vacuum's QMS is electron impact ionization.

Ions generated in this way pass through extraction lenses before entering the analyzer with its electric quadrupole field.

Using voltage comprising both an alternating and direct voltage component, an electric field is generated within 4 high-precision metal rods. This quadrupole field is where ion discrimination takes place on the basis of their mass-to-charge ratio.

Accordingly, only ions of one species are able to pass through the rod system and reach the detector. All other ions are located on unstable trajectories and are discriminated in the apparatus. The detector in its simplest form is an electrically conductive hollow body known as a Faraday cup. The ions are neutralized by the release of charges and the resulting current is detected.

A Faraday cup is sufficient in terms of signal strength to perform a residual gas analysis in high vacuum. Where very small ion currents are present, as in the ultra-high vacuum range, or where very fast measurement is required, a secondary electron multiplier (SEM) is normally used. Utilizing a mass spectrometric setup such as this, the user can detect which substances are still present in the vacuum system.

Requirements

Pressures of <10-4 hPa are required for ions to pass through the analyzer with its electrical quadrupole. These pressures are necessary to ensure the mean free path is long enough for the cations to reach the detector on their trajectory without colliding on the way. To achieve the necessary vacuum pressure, it is ideal to use a combination of a backing pump and a turbopump, either as a modular or a ready-to-use pumping station.

Gas inlet systems developed for different pressure ranges enable the user to adjust the pressure precisely to the QMS. Because the QMS can be damaged if used at pressures that are too high for it, it is recommended that a total pressure measurement is integrated.