In order to accurately characterize a plasma or other ‘high’ pressure reaction cell, you need to obtain a representative sample. Sampling the plasma as a molecular beam through rapid pressure reduction immediately after ions, radicals and neutrals leave the plasma, allows for an unadulterated snapshot of the plasma chemistry. With molecular beam sampling, you are looking directly at the chemical soup which comprises the plasma. This includes noise sources such as photons, electrons, metastable neutrals, and even particulates in addition to the ions, neutrals and radicals to be monitored.
A cross beam ionizer allows for the separation of the analyte signal, which is comprised of plasma ions and ions created in the ionizer from plasma molecules and neutrals, from these sources of noise. The use of a cross beam ionizer will also protect the quadrupole analyzer and detector from direct bombardment with the reactive species common to plasmas used in semiconductor applications. We have developed a novel two stage molecular beam sampling system centered around a new cross beam deflector ionizer, yielding a compact vacuum system with efficient pumping in both stages.
There are a wide variety of species present in a high-pressure source such as a plasma: metastable neutrals, radicals, positive ions, negative ions, electrons, photons, clusters and particulates. A plasma monitoring system must allow selective monitoring of these various species, without interference from the others. In order to obtain a representative sample, a molecular beam can be generated through rapid pressure reduction to a collision-free vacuum level. Ions present in the sampled gas need to be separated from the bulk gas flow, preferably using an energy filter for increased selectivity.
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