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Monitoring of plasma processes.

In recent years, there has been a rapid growth in the extent to which ion-beam techniques and plasmaassisted processes have been used both in the development and manufacture of new devices and materials. Many new applications rely on plasma-surface interactions (as for example in plasma-assisted etching and deposition) and plasma induced surface modifications, (eg. oxidation and nitriding). Often the effectiveness of the particular plasma-surface interaction will depend upon the energy of the positive ions incident on the surface as it is these which furnish the active species. In many cases, this energy is not known with any degree of certainty and optimization of the process is largely a hit-and-miss affair. The ability to accurately measure the energy of these ions gives a much better understanding of the process.

Similarly, the mass spectra of neutral and ionized species can yield much information about the chemical reactions occurring.

VG Quadrupoles have developed a combination of a quadrupole mass spectrometer (mass ranges up to 1000 amu) and an energy analyzer which, together with an appropriate sampling system, offers a sophisticated instrument for gaining maximum energy and mass information from plasma or ion beam processes.

This article is designed to explain some of the principles involved and the energy filter/analyzer required to handle specific applications.

The quadrupole of choice for this type of application is the VG SXP Elite series. Full technical information is available in publications obtainable from VG Instruments Canada, 2280 St. Laurent Blvd., Ottawa K1G 4K1.

Considerations

Transmission of ions by the quadrupole

It is important that ions entering a Quadrupole mass spectrometer (QMS) undergo a certain number of oscillations in the quadrupole electric field in order to be adequately resolved. The higher the ion energy relative to the quadrupole rods, the lower the number of oscillations experienced by the ions, due to their greater velocity, and consequently the lower the mass resolution.

At first sight the sampling of ions by a quadrupole mass spectrometer, either from a plasma or an ion beam experiment, would appear to be a trivial operation. One just turns off the mass spectrometer ion source. However, the type of mass spectrum obtained from such an experiment shows very poor mass resolution[1]. The energy of the ions traversing the quadrupole rods may be controlled using the Pole Bias control on the SXP Elite series of QMSs. This floats the rods relative to earth potential, hence the energy of the ions in the filter,[E.sub.o] is given by: [E.sub.o] = [E.sub.o]- [V.sub.Pole Biasl where [E.sub.o] is the energy of the ions relative to earth. Negative ions formed in a high energy process may be retarded on entering the analyzer by applying a negative pole bias potential and vice versa; these ions would otherwise experience too few oscillations and be poorly resolved. This also has the advantage that the ions spend less time in the defocussing fringing fields at the entrance and exit of the quadrupole rod assembly. Empirically we require [E.sub.o] to be in the range 1-10 eV for ions in the range 1-1000amu.

Sampling of ions

In order to obtain a well-resolved mass spectrum from externally generated ions the energy spread, (delta) [E.sub.o], of the ions must be limited. There are two cases; that of an approximately mono-energetic source of ions, ie. (delta)[E.sub.o] < 8eV. In such a case the ions can be focused into the QMS using Ion Transfer Optics, a lens assembly which fits onto the front of the SXP Elite series. The mass spectrum is optimised by adjusting the pole bias potential.

Alternatively, the ions are generated with a large spread of energy, eg. from a plasma. In order to obtain well resolved mass spectra, an energy filter is required to select an energy 'window', (delta) E < 8eV, for mass analysis.

A Cylindrical Mirror Analyzer (CMA)

In order to do true analysis of ions, from a plasma or ion beam experiment, a high-performance electrostatic analyzer is required, such as that used by Coburn et al[2] to monitor plasma processes. Here the plasma was sampled through an orifice in the grounded electrode of the discharge region. The ions were focused into a spherical energy analyzer and then into a quadrupole mass spectrometer. Using this arrangement both energy and mass analysis can be performed on the positive ions present in the discharge. However, the requirement for a 90 degrees bend in the analyzer housing to accommodate the energy analyzer is usually inconvenient. An on axis geometry is preferred. The Cylindrical Mirror Analyzer Figure 1 does have on axis geometry for the source and image. Further, if the incident beam into the radial electric field is at 42.3 degrees, the CMA exhibits second-order focusing which together with its high transmission make it superior to all other electrostatic analyzers.

However, the requirement of a point source for the analyzer is problematic from the point of view of interfacing a CMA to a quadrupole. This problem was overcome by a Schubert and Tracy[3] by angling the CMA relative to the quadrupole and the source. However, this still presents problems and further only uses a small sector of the energy analyzer. Clearly this does not represent any real improvement over the hemi-spherical analyzer. VG Quadrupoles have patented a deflector arrangement which acts as a 42.3 degrees virtual ring source for the CMA. A broadly-parallel beam of ions entering the instrument are deflected into the CMA at the optimum angle of 42.3 degrees and focused back again on exiting the analyzer. This allows full use of the CMA's 360 degrees symmetry giving high transmission. With this geometry, a number of advantages are immediately apparent:

1. no direct line of sight from the plasma/source to the detector;

2. analysis of neutral species can also be performed since the ion source is mounted in front of the CMA (see Figure 2). Neutral species enter the ion source and are ionized before passing through the energy filter. Hence we can distinguish between thermal neutrals (background gas) and nonthermal neutrals from the plasma. Further these nonthermal neutrals can also be energy analyzed;

3. high transmission.

The energy of the ions transmitted by the CMA is determined by a combination of the pass energy of the CMA and a retard potential applied to be CMA. The analyzer transmission is determined by the pass energy as is the FWHM. Thus, at a constant pass energy and a variable retard potential, the energy spectrum can be scanned at constant FWHM. This is the usual mode of operation for dispersive electrostatic analyzers.

Mass spectrometry is not necessarily an intrusive technique and the sampling probe of the SXP-CMA instrument can be configured in a variety of ways depending on the purpose of the investigation. VG Quadrupoles has designs for the following configurations:

1. sampling through driven electrode to monitor plasma-sample interactions;

2. between the electrodes monitoring the bulk plasma;

3. sampling through the ground electrode to monitor plasma-sample interactions;

4. flush with the chamber walls to monitor plasma-wall interaction.

Differential pumping As many plasma applications occur at pressures above the working range of a quadrupole mass spectrometer, it is often necessary to sample both ions and neutrals via an inlet system into a differentially pumped quadrupole system. Figure 3 shows such a sample system, where ions are sampled via an orifice and are then energy and mass analyzed. A bellows assembly is included to give a degree of movement to the sampling point. The system would typically be pumped by a turbo-molecular pump backed by a rotary pump.

Specialized Options VG Quadrupoles has built a reputation of supplying customized systems to researchers at the frontiers of science which meet their specific requirements. Some specialized features that have been supplied to various customers to-date include:

- two stages of differential pumping via twin orifices to

give molecular beams;

- cryopumped assemblies to give minimum backgrounds;

-dual plate orifice assembly for reducing plasma

perturbations;

-chopper modulated beam inlet;

-magnetic shielding;

-high-voltage floating analyzer (to 3KV);

-high-temperature sampling;

-feedback-control voltage signals;

-high-vacuum RGA slide valve;

-custom-designed optics;

-coincidence software for laser-induced plasmas.

Detection of ions

Positive ion detection may be achieved using either a Faraday plate detector or an analogue Channel Electron Multiplier (CEM) if better detection limits are required. For negative ions, the situation is more complex.

Negative ions are normally present in low levels, thus although they can be detected by a Faraday detector, it is rare that a sufficient concentration is present to make this useful. An electron multiplier is needed. In order to produce the correct potential gradient for electron amplification along the CEM, the collector must be at a positive potential relative to the mouth of the CEM. For positive ions the mouth is at up to -3kV and the collector is normally at earth potential. However, for negative ions the mouth of the CEM must be at a high-positive potential, in order to attract the ions, and consequently the collector is maintained at an even higher positive potential. This poses problems in signal handling if an analogue CEM is used, ie. a high voltage isolation amplifier is required. These problems can be avoided by using a pulse counting CEM. Here the signal is the AC coupled via a blocking capacitor and passed on to pulse handling electronics such as the VG Quadrupoles SPC10 unit which combines a pre-amp, discriminator and ratemeter in one unit. The output is TTL compatible and can be processed by a data system.

Examples of Application

The VG CMA/Quadrupole combination finds applications in the following areas:

1. energy and mass analysis of charged particles from plasma;

2. energy and mass analysis of neutral particles from plasma;

3. energy tunable SIMS detector;

4. secondary neutral mass spectrometer (SNMS);

5. ion-scattering spectroscopy (ISS) detector;

6. residual-gas analysis (RGA).

Electrostatic Plasma Probe

To complement the mass spectrometry diagnostic instruments previously described, VG Quadrupoles also supplies a unique PC-controlled Electrostatic Plasma Probe for plasma diagnostics.

The instrument incorporates a single cylindrical Langmuir probe, driven by software which employs the definitive theory of Laframboise[4] to give rapid measurements of the plasma parameters in medium density DC plasmas.

The following plasma parameters are continuously displayed:

- positive ion density

- bulk electron density

- fast electron density

- bulk electron temperature

- fast electron temperature

- plasma potential

- floating potential References: 1. Satake et al., Jap. Jr. Appl. Phys., 15, 1976, 1359 2. H.F. Winters, J.W. Coburn, T.J. Chang, J. Vac. Sci. Technol., 81, 1983, 469 3. R. Schubert, J.C. Tracy, Rev. Sci. Inst., 44, 1973, 489 4. J.G. Lafamboise, University of Toronto, Institute of Aerospace Studies Report No. 100, 1966
COPYRIGHT 1991 Chemical Institute of Canada
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Golding, Martin
Publication:Canadian Chemical News
Date:Apr 1, 1991
Words:1784
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