Residual-gas analyzers are finding their way out into the heat-treat shop, primarily for process control and contamination monitoring in vacuum systems. Any tool that can help us better understand what is happening inside our vacuum furnaces is a welcome addition. Let’s learn more.
What is an RGA?
A residual-gas analyzer, or RGA for short (Fig. 1), is a compact mass spectrometer. Originally designed for use only in the laboratory, the technology is robust enough to operate out on the shop floor. These devices are often mounted in-situ on a vacuum furnace (Fig. 2). A typical RGA analysis can reveal how much of a particular gas species is present either in the vacuum vessel or in the pump manifold (Fig. 3).
In many cases an RGA is used to monitor the quality of the vacuum and can easily detect minute traces of impurities, which possess sub-ppm detectability in the absence of background interferences in the low-pressure gas environment (down to 10-14 torr levels). Applications include finding both small and large leaks in a vacuum system, distinguishing leaks from outgassing, quantifying the process background and determining the effectiveness of gas-line purging.
An RGA can act as a helium leak detector (helium having an atomic mass unit of 4) but cannot measure the leak in a quantitative way (i.e., it is difficult to determine the speed of the leak). With a vacuum system pumped down to lower than 10-5 torr, an RGA can check the integrity of the vacuum seals and the quality of the vacuum so as to detect air leaks, virtual leaks and other contaminants at low levels before a process is initiated.
Differentiating Leaks from Outgassing
The basic RGA scans from atomic mass units 2 to 50 (Table 1) and can often identify excessive outgassing (i.e., high peaks for water at mass 17 and 18), differentiating it from a leak in which we find high peaks at 14 and 28 (both nitrogen) and sometimes 32 (oxygen). Other indicators of an air leak are argon (40) and carbon dioxide (44). The oxygen peak is often missing or very low, even if there is an air leak.
An RGA scan of a vacuum system prior to running a process can provide valuable insights into the condition of the vacuum environment. When problems occur (and they invariably do), having a baseline for comparative purposes is extremely helpful.
How an RGA Works
The internals of an RGA (Fig. 4, online only) consist of an ion source, mass spectrometer and a measurement section. The residual gas is ionized when it collides with the electrons discharged from the high-temperature filament, and the ions that result accelerate and converge at the mass spectrometer. Direct and alternating current voltages in the mass spectrometer are applied to a series of cylindrical electrodes (quadropoles), which allow the ions to be separated by mass. The separated ions are detected as electric current by a Faraday cup with the ion current being proportional to the mass (partial pressure) of the residual gas.
The operation of the RGA is quite simple, although the mathematics of the quadrupole mass analyzer section is complex. First, an “ionizer” converts many neutral gas molecules into positive ions in a well-controlled region at a specified electric potential. These ions are next accelerated by a series of electrostatic “lenses” and formed into a beam that has about 20 eV of energy. The ion beam is subsequently passed into the quadrupole mass analyzer region. This region acts as a filter. It will very nicely pass through ions with a user-chosen mass-to-charge ratio (M/e), but all other ions get pushed aside into walls, where they neutralize and become undetectable. The ions that are passed through this filter are detected as current either at a Faraday cup or using a secondary electron multiplier (also known as a “channeltron”). The channeltron gives a large amplification of the signal from ions and is consequently used to enhance the sensitivity of the RGA. By choosing a mass-to-charge ratio and making a measurement of the signal obtained, one can immediately find out the number of those molecules present in the ionizer region of the RGA. By sweeping through a whole range of M/e ratios, one can find a whole range of molecules that are present and begin to understand the full range of chemical components in the gas.
The actual analyzer is located in the vacuum and consists of the following principle components:
- An ion source ionizes neutral gas particles, which are then sorted in the mass filter on the basis of their mass-to-charge ratio (m/e ratio).
- The ion current is measured using a Faraday detector or a secondary electron multiplier (SEM) after the ions have left the separating system. The measured current is a parameter of the partial pressure of the respective gas molecules or a parameter of fractals that may possibly have been generated in the ion source.
- A data-analysis system processes the ion currents measured with the aid of the detector and presents these currents in various forms. Today, data-analysis software programs are capable of supporting the user in interpreting mass spectra.
Maintenance and Repair
Despite our best efforts, an RGA will need to be repaired. The heat-treat environment coupled with possible exposure to atmospheric pressure, pump oils or other contaminants are a fact of life. Filament replacement, ion-source cleaning and filter cleaning are examples of what can be done in the field. However, disassembly of certain components (such as the mass filter assembly) will result in costly and time-consuming factory repairs. An RGA is still a delicate instrument and should be treated as one.
Residual gas analyzers can be a very effective tool to analyze system gas loads resulting from real leaks, virtual leaks or chamber-wall outgassing. RGAs have a number of advantages over traditional, dedicated gas leak detectors, including the ability to differentiate between different gas species and comparable sensitivity levels, the ability to detect internal or “virtual” leaks, and the ability to detect and analyze outgassing problems.
- Residual Gas Analyzers, What’s Hot Newsletter, Vac Aero International, April 2014.
- RGAs - more than just leak detectors, Semiconductor European, MKS Instruments, March 2000.
- CPM Compact Process Monitor, Inficon Product Bulletins & Training Information (www.inficon.com)
- MKS Instruments (www.mks.com)
- Horiba Semiconductor (www.horiba.com)
- Pfeiffer Vacuum (www.pfeiffer-vacuum.com)
- University of Texas at Dallas