Mass Analysis of Ions Using the Secondary Ion Mass Spectrometry (SIMS)

By AZoM

Introduction
Conclusion
About Hiden Analytical

Introduction

A 5 KeV Argon primary ion beam (10 nA beam current) was utilized to excite secondary ions from an aluminum surface. The Hiden EQS and Hiden EPIC were positioned as shown in Figure 1. Both instruments can mass analyze positive and negative ions from the surface but the EQS also has a 45° sector field ion energy analyzer incorporated in it. This can be used to energy analyze (energy filter) the ions.

Figure 1. Experimental setup incorporating Hiden EQS and Hiden EPIC.

Figure 2 displays the ion energy distribution of the secondary Al+ ions measured with the Hiden EQS. The maximum in the distribution is at 2 or 3 eV. It must be noted that most of the secondary ions appear to have energies below ~30 or 40 eV though there are a small number of higher energy ions with a wide spread of energies.

Figure 2. Ion energy distribution of the secondary Al+ ions measured with the Hiden EQS.

Figure 3. Energy filtered mass spectrum of ions of energies ~2.5 ± 0.5 eV.

Figure 3 shows the energy filtered mass spectrum of ions of energies ~2.5 ± 0.5 eV. The mass peaks are well resolved. The intensity of the Al+ (27 amu) peak is ~60,000 cps. The Hiden EPIC does not have an energy filter, but the mid axis potential (pole bias) facility can be used to bias the mass filter to progressively more positive potentials thereby rejecting more and more of the low energy ions. This is illustrated in Figure 4.

Figure 4. The effect of biasing the mass filter on the Hiden EPIC.

With the mid axis potential set to zero, the mass spectrum resolution of the mass peaks is very poor, because of the wide spread of ion energies entering the quadrupole field. However the signal intensity at 27 amu is large, which is 200,000 cps since no energy filtering is taking place and no transmission losses are incurred. The same spectra is shown in Figure 5 but with the signal axis plotted on a log scale. It is clearly shown that it will not be possible to measure some of the minor peaks in the spectra which are swamped by the under-resolved Al+ peak.

Figure 5. Data with the signal axis plotted on a log scale.

Figures 6, 7, 8 show the same spectra but with the mid axis potential of the Hiden EPIC set to 20, 60, 100 V. Progressively, low energy ions are rejected and the effective spread of ion energies is reduced. Thus the mass resolution is improved. The mass resolution in Figure 9 is approaching that shown in Figure 2 (the EQS spectra). However, the signal intensity considerably reduces as mid axis potential is increased. It has decreased from 200,000 cps in Figure 5 (3 times greater than the intensity of the EQS signal) to 4,000 cps in Figure 9 (15 times less than the intensity of the EQS signal).

Figure 6. The same spectra but with the mid axis potential of the Hiden EPIC set to 0V.

Figure 7. The same spectra but with the mid axis potential of the Hiden EPIC set to 60V.

Figure 8. The same spectra but with the mid axis potential of the Hiden EPIC set to 100V.

Conclusion

The mass analysis of ions with a broad ion energy distribution must be done with an instrument like the Hiden EQS with its integral mass and energy filters. The mass analysis of mono-energetic ions can be done with the Hiden EPIC. The transmission losses associated with the 45° sector field energy analyzer are minimal.

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: