SIMS: Secondary Ion Mass Spectrometry

1. 1 SIMS: Secondary Ion Mass Spectrometry Paolo Ghigna, Dipartimento di Chimica Fisica “M. Rolla”, Università di Pavia

2. 2 Introduction Bombardment of a sample surface with a primary ion beam followed by mass spectrometry of the emitted secondary ions constitutes secondary ion mass spectrometry (SIMS). The best SIMS reference is Secondary Ion Mass Spectrometry:

3. 3 Uses for SIMS Today, SIMS is widely used for analysis of trace elements in solid materials, especially semiconductors and thin films. The SIMS ion source is one of only a few to produce ions from solid samples without prior vaporization. The SIMS primary ion beam can be focused to less than 1 um in diameter. Controlling where the primary ion beam strikes the sample surface provides for microanalysis, the measurement of the lateral distribution of elements on a microscopic scale. During SIMS analysis, the sample surface is slowly sputtered away. Continuous analysis while sputtering produces information as a function of depth, called a depth profile. When the sputtering rate is extremely slow, the entire analysis can be performed while consuming less than a tenth of an atomic monolayer. This slow sputtering mode is called static SIMS in contrast to dynamic SIMS used for depth profiles. Shallow sputtering minimizes the damage done to organic substances present on the sample surface. The resulting ion fragmentation patterns contain information useful for identifying molecular species. Only dynamic SIMS will be treated in this surface analysis computer aided instruction package because only dynamic SIMS yields quantitative information.

4. 4 Ion Beam Sputtering The bombarding primary ion beam produces monatomic and polyatomic particles of sample material and resputtered primary ions, along with electrons and photons. The secondary particles carry negative, positive, and neutral charges and they have kinetic energies that range from zero to several hundred eV. Primary beam species useful in SIMS include Cs+, O2+, O , Ar+, and Ga+ at energies between 1 and 30 keV. Primary ions are implanted and mix with sample atoms to depths of 1 to 10 nm. Sputter rates in typical SIMS experiments vary between 0.5 and 5 nm/s. Sputter rates depend on primary beam intensity, sample material, and crystal orientation. The sputter yield is the ratio of the number of atoms sputtered to the number of impinging primary ions. Typical SIMS sputter yields fall in a range from 5 and 15.

5. 5 Sputtering Effects The collision cascade model has the best success at quantitatively explaining how the primary beam interacts with the sample atoms. In this model, a fast primary ion passes energy to target atoms in a series of binary collisions. Energetic target atoms (called recoil atoms) collide with more target atoms. Target atoms that recoil back through the sample surface constitute sputtered material. Atoms from the sample's outer monolayer can be driven in about 10 nm, thus producing surface mixing. The term knock-on also applies to surface mixing.

6. 6 Secondary Ion Energy Distributions The sputtering process produces secondary ions with a range of (translational) kinetic energies. The energy distributions are distinctly different for atomic and molecular ions. Molecular ions have relatively narrow translational energy distributions because they have kinetic energy in internal vibrational and rotational modes whereas atomic ions have all kinetic energy in translational modes. The figure shows typical energy distributions for mono, di, and triatomic ions.

7. 7 Secondary Ion Yields The SIMS ionization efficiency is called ion yield, defined as the fraction of sputtered atoms that become ionized. Ion yields vary over many orders of magnitude for the various elements. The most obvious influences on ion yield are ionization potential for positive ions and electron affinity for negative ions. 

8. 8 Secondary Ion Yields -- Primary Beam Effects Other factors affect the secondary ionization efficiencies in SIMS measurements. Oxygen bombardment increases the yield of positive ions and cesium bombardment increases the yield of negative ions. Oxygen enhancement occurs as a result of metal-oxygen bonds in an oxygen rich zone. When these bonds break in the ion emission process, the oxygen becomes negatively charged because its high electron affinity favors electron capture and its high ionization potential inhibits positive charging. The metal is left with the positive charge. Oxygen beam sputtering increases the concentration of oxygen in the surface layer. The enhanced negative ion yields produced with cesium bombardment can be explained by work functions that are reduced by implantation of cesium into the sample surface. More secondary electrons are excited over the surface potential barrier. Increased availability of electrons leads to increased negative ion formation.

9. 9 Sensitivity and Detection Limits The SIMS detection limits for most trace elements are between 1012 and 1016 atoms/cc. In addition to ionization efficiencies (RSF's), two other factors can limit sensitivity. The output of an electron multiplier is called dark counts or dark current if no secondary ions are striking it. This dark current arises from stray ions and electrons in instrument vacuum systems, and from cosmic rays. Count rate limited sensitivity occurs when sputtering produces less secondary ion signal than the detector dark current. If the SIMS instrument introduces the analyte element, then the introduced level constitutes background limited sensitivity. Oxygen, present as residual gas in vacuum systems, is an example of an element with background limited sensitivity. Analyte atoms sputtered from mass spectrometer parts back onto the sample by secondary ions constitute another source of background. Mass interferences also cause background limited sensitivity.  

10. 10 Depth Profiling Monitoring the secondary ion count rate of selected elements as a function of time leads to depth profiles. The following figure shows the raw data for a measurement of phosphorous in a silicon matrix. The sample was prepared by ion implantation of phosphorous into a silicon wafer. The analysis uses Cs+ primary ions and negative secondary ions.

11. 11 Bulk Analysis For samples with homogeneously dispersed analyte, bulk analysis provides better detection limits than depth profiling, usually by more than an order of magnitude. Faster sputter rates increase the secondary ion signal in bulk analysis. The fastest possible sputtering requires intense primary ion beams which sacrifice depth resolution because they cannot be focused as required for flat bottom (rastered) craters. Otherwise, bulk analyses are similar to depth profiles. Ion intensity data are displayed as a function of time. This provides a means for verifying that the sample is indeed homogenous. In a typical heterogeneous sample, the analyte is concentrated in small inclusions that produce spikes in the data stream.

12. 12 Ion Imaging  Ion images show secondary ion intensities as a function of location on sample surfaces. Image dimensions vary from 500 um to less than 10 um. Ion images can be acquired in two operating modes, called ion microscope or stigmatic imaging, and ion microbeam imaging or raster scanning. Ion microscopy requires a combination ion microscope/mass spectrometer capable of transmitting a mass selected ion beam from the sample to the detector without loss of lateral position information. Image detectors indicate the position of the arriving ions. Ion microscope images are usually round because the ion detectors are round. Lateral resolutions of 1 um are possible. A SIMS analyst selects images with higher lateral resolution at the expense of signal intensity and higher mass resolution at the expense of image field diameter. For ion microbeam imaging, a finely focused primary ion beam sweeps the sample in a raster pattern and software saves secondary ion intensities as a function of beam position. Microbeam imaging uses standard electron multipliers and image shape follows raster pattern shape, usually square. Lateral resolution depends on microbeam diameter and extends down to 20 nm for liquid metal ion guns. Some instruments simultaneously produce high mass resolution and high lateral resolution. However, the SIMS analyst must trade high sensitivity for high lateral resolution because focusing the primary beam to smaller diameters also reduces beam intensity.

13. 13 Ion Imaging The example (microbeam) images show a pyrite (FeS2) grain from a sample of gold ore with gold located in the rims of the pyrite grains. The image on the right is 34S and the left is 197Au. The numerical scales and the associated colors represent different ranges of secondary ion intensities per pixel.