Analysis of Atmospheric Plasmas

By AZoNano

Table of Contents

Introduction
Equipment
Gas Analysis
Plasma Characterization
Conclusions
About Hiden Analytical

Introduction

A Hiden HPR-60 inlet system with an EQP 1000 series mass analyzer gauge was utilised for atmospheric plasma analysis. This specific sampling inlet was designed in order to penetrate the plasma chamber and enable sensitive measurement of the plasma at up to atmospheric pressure. To minimize the pressure from atmospheric to gauge compatible pressures, a system of three cones with small orifices was used to divide the inlet into three stages. Each stage was pumped separately. MASsoft was used to gather data about neutral, positive and negative species through the modes available through the MASsoft controlling software.

Equipment

The Hiden HPR-60 inlet system is a molecular beam sampling mass spectrometer. This is developed for precise process sampling for applications such as plasma etch and deposition, chemical vapour deposition, atmospheric or cluster research, combustion and nanoparticle studies and flow reactors. In a number of applications, the process pressure must be reduced to be compatible to the operating pressure of the quadrupole mass spectrometer. This can be achieved by a design, such as in Figure 1.

Figure 1. Schematic of inlet sampling system with orifice sizes and pumping locations.

The sampling part comprises several chambers separated by orifices. Each chamber is pumped separately to obtain a sequential pressure drop from the sampling area through the chambers to the gauge. While using an optimum combination of orifice sizes and pumping rates, sampling can be performed at atmospheric pressures without exposing the gauge to overpressure. The pressure drop enables the sample to remain as a coherent beam, reducing collisions between sample and background gases and eliminating interactions with surfaces, improving accuracy considerably. The orifices can be interchanged and will impact the modulation and beam focus. The pumping efficiency is maximized by this design.

In this specific application, the mass spectrometer selected for this inlet system was the Hiden EQP-1000. This is a combined 45° sector field energy analyser and mass spectrometer with 9mm quadrupole rods. The mass range options can be chosen from 300, 510, 1000, 2500 amu, providing the best performance to the relevant research application. The triple filter is fitted as standard, enhancing accuracy, sensitivity and mass discrimination.

Gas Analysis

Argon gas was used to evaluate the residual gas analysis (RGA) mode of the software. As the gas passes through the three stages, it cools down quickly and dimerises. Therefore the signal analyzed is the Ar2 80 amu peak, which can be seen in Figure 2. Even for this low concentration species, it can be clearly observed that the count rate was consistently over 50,000 counts per second.

Figure 2. Scan for Ar gas, RGA mode.

The molecular beam alignment was evaluated as good since the background level, equivalent to the beam being “off” was at a level of counts per second. Usually, this will be compared with the strongest Ar 40 peak to examine the amount of dimerisation. However, this peak was too intense for measurement. Hence the secondary Ar 36 peak was measured at a level of approx. 0.3% of that of the Ar 40 peak. The calculated ratios are summarized in Table 1. The scans are seen in Figures 3a and 3b.

Table 1. Calculated ratios for Ar species.

Pump Location

SEM / Counts s-1 Ratio to Ar 80
Ar 80
53,000
1 : 1
Ar 36 (0.3%)
2.1 x 106
40 : 1
Ar 40 (100%)
7.0 x 108 (extrapolated)
13,207 : 1

Figure 3a. Mass scan for Ar36, RGA mode. 
Figure 3b. Mass scan for Ar80, RGA mode.

Hence the amount of dimerisation and the beam sensitivity to these small levels of species was calculated. Mostly, only smaller species are measured using the gauge. However, higher mass species are also investigated. To test the high mass sensitivity of the gauge, the signal from a sample of heptacosafluorotributylamine, PFBTA (C12F27N), was measured. This is a large compound with an intense peak of mass 219, which can be seen in the mass scan in RGA mode seen in Figure 4. The peak is clearly seen, demonstrating that the mass calibration of the gauge extends up to the higher masses.

Figure 4. Mass scan for PFBTA, RGA mode.

To test the detection limits of the gauge, air was sampled. The levels of krypton, Kr, and Xenon, Xe, were extracted by looking for their main peaks, at mass 84 and 132 respectively. The two scan results can be seen in Figures 5a and 5b.

Figure 5a. Mass scan for Xe, RGA mode.

Figure 5b. Mass scan for Kr, RGA mode.

Again the counts rates offer an indication of the concentration in air. These are 700 and 2300, approximately corresponding with the rarities in normal atmospheric air of 1 ppm and 87 ppm respectively, after the relative sensitivity of the mass gauge has been taken into account.

The electron attachment energy was also measured for N2O in negative ion RGA mode. The scan is seen in Figure 6 with the strongest signal for negative oxygen ions, mass 16, which is a source of this ion.

Figure 6. Electron attachment scan for RGA mode.

Plasma Characterization

To use the ion analysis ability of the gauge, the sample was changed from neutral gas to plasma. For this experiment the “atmospheric plasma” was produced in the form of a propane flame. The tip of the flame was positioned close to the first cone orifice for optimal sampling. The bias voltages were set on the cones and pressure readings taken. These set conditions for the instrument are summarized in Table 2.

Table 2. Conditions for the Ar gas experiment.

Stage Diameter / mm Bias / V Pressure Reading / Torr
Sample
--
--
9.0 x 10-1 (atmos.)
Orifice 1
0.1
+30
7.5 x 10-1
Orifice 2
0.4
-6
4.9 x 10-4
Orifice 3
0.6
-30
2.0 x 10-7

The voltages were adjusted to obtain the required beam through the cone inlet, allowed detailed analysis. Optimization of the voltages enhances the acceleration effect of the pressure drop and ensures good focus on the source.

Figures 7a and 7b shows the mass data and the energy data respectively, obtained from the gauge in positive ion SIMS mode. Hence only ions are produced in the plasma and are transported down the molecular beam path will be analysed.

Figure 6a. Mass Scan for atmospheric plasma.  
Figure 6b. Ion Energy Scan of atmospheric plasma.

Figure 7a clearly shows the mass 19 peak. The count rate of 18,000 suggests good focus along the molecular beam path. Other peaks are at a smaller level, but resolved from the noise. Figure 6b shows the energy is centred at zero volts with a peak of 15,000 counts per second. The characteristics of the plasma are therefore defined.

Conclusions

The gauge functions in all the needed modes of RGA detecting positive, neutral and negative species. Furthermore, the inlet system permits the secondary ions from plasma to be analyzed at up to atmospheric level. Also appearance potential, ion energies and other plasma characteristic can be studied. As well as reducing the pressure through the stages of the inlet system, the three cones can have individual potentials applied to them. This feature is useful for beam studies.

About Hiden Analytical

Hiden Analytical is a leading manufacturer of quadrupole mass spectrometers for both research and for process engineering. Their products Our products address a diverse range of applications including:

  • Precision gas analysis
  • Plasma diagnostics by direct measurement of plasma ions and ion energies
  • SIMS probes for UHV surface science
  • Catalysis performance quantification
  • Thermo-gravimetric studies

These analytical instruments are designed to work over a pressure range extending from 30 bar processes down to UHV/XHV.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

For more information on this source, please visit Hiden Analytical.

Date Added: Jan 21, 2012 | Updated: Jun 11, 2013
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