Wednesday, January 15, 2014

World's Smallest Quadrupole RGA

This is an analytical technique used for identifying gases present in a vacuum chamber. The equipment used in performing the analysis is a mass filter of quadrupole type. A typical RGA has three major parts, namely, an ionizer, a mass analyzer and an ion detector.
 
Residual gas analyzers operate by creating a beam of ions from samples of the gas being analyzed. The resulting blend of ions is separated into individual species through their charge-to-mass ratios. The output of an RGA is a spectrum that shows the relative intensities of the various species present in the gas. This output is referred to as a mass scan or also mass spectrum. 
 

The molecules of the gas analyzed are converted to ions by an ionizer through electron impact ionization. An ionizing electron beam is generated by a hot emission filament and extracted by means of an electric field then used to strike the gas atoms to ionize them. This hot filament is easily destroyed by reactive gases like oxygen, which is why conventional RGAs are required to operate at pressures of less than 10-4 mbar. When required to operate at higher pressures, normally differential pumping must be used, there-by adding significant cost to the assembly. The exception is with the revolutionary Micropole™, a miniature-size RGA designed to overcome such limitations by working at 0.9 Pascal (9 x 10-3 mbar).
Ions from the gas are distinguished from each other in terms of their masses by the mass analyzer of the RGA. There exist various techniques for mass separation, but mass analyzers used in RGA's usually employ the RF quadrupole which has four cylindrical rods that are provided with combinations of AC and DC voltages of varying frequency. Only ions that possess the right mass-to-charge ratio can reach the ion collector for a given applied frequency. A Faraday cup may be used for detecting ions at less sensitive ranges, while ion detection at higher sensitivity would require electron multipliers. 


World’s most miniature quadrupole sensor on 16mm CF or KF!
 
The amount of current at specific masses (unit charge is usually assumed), depends on several factors:
  • The probability that an electron will cause ionization which is derived from the cross section for ionization
  • The probability that a particular molecular fragment will produce an ion
  • The probability that an ion so produced will make it into the quadrupole
  • The probability that the ion will pass through the quadrupole rods and be detected
Example: What will be the relative peak height of peaks due to nitrogen and H2S? First, look up their cross sections for ionization at http://physics.nist.gov/PhysRefData/Ionization.

At 70 eV hydrogen sulfide has a cross section of 4.145 square Ã…ngstroms and N2 has a cross section of 2.508 square Ã…ngstroms. So H2S gives about twice as much ion current as does N2. This is not the end of the story as one would need to know the cracking pattern of N2 and H2S.

H2S produces peaks at 32, 33, and 34 with 32 and 33 each about 40% of the 34 peak. Nitrogen produces peaks at 28 and 14 (this latter at 7% of 28). Putting both of these bits of information together gives the 34 peak at about the same size as the 28 peak if both gasses are present in equal amounts.

The mass spectra are normally shown as a chart with the mass-to-charge ratio on the x-axis and the relative intensity on the y-axis. The various peaks exhibited by a mass spectrum need to be interpreted correctly since these can be ambiguous in certain cases, such as when two different molecules exhibit the same mass. Knowledge of how two different molecules with the same mass would dissociate into smaller fragments of different mass-to-charge ratios (referred to as cracking patterns) allows absolute identification of the gas.

In the semiconductor industry, RGAs are used in identifying gases, vapors, and residues for the purpose of fixing leaks in a vacuum system and eliminating contaminants that cause process problems or product failures. For instance, it is widely used to analyze residual gases inside a hermetically sealed package to pinpoint the cause of problems such as die corrosion.

In the electron microscopy (EM) field, an RGA is quite useful to understand what contaminants might be in the sample chamber from various sources such as vacuum pumps or from manipulations.

Generally solvents as acetylene, acetone, methyl or isopropyl alcohols, among many more, are used to clean pump oil and other contamination in vacuum chambers. Often these too are far from innocent and can leave their fingerprints in a vacuum chamber, prompting the operator to take a mass spectrum to help determine the contaminant and try to pre-empt or solve a latent problem that can strongly impair the quality of the experiment or production in process.



 

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