SIMS: Ion Imaging

KJ3055-Surface Chemical Analysis- APPENDIX 3

Secondary ion Mass Spectrometry (SIMS): a short overview[1]

The bombarding primary ion beam produces monatomic and polyatomic particles of sample material and resputtered primary ions, along with electrons and photons.(Fig. 1). The secondary particles carry negative, positive, and neutral charges and they have kinetic energies that range from zero to several hundred eV.

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).

Fig. 1. Surface material sputtering by an ion beam (e.g Ar⁺).

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. Fig. 2 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.

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.

To convert the time axis into depth, the SIMS analyst uses a profilometer to measure the sputter crater depth. A profilometer is a separate instrument that determines depth by dragging a stylus across the crater and noting vertical deflections. At the end of the phosphorous depth profile , profilometry gives 0.74 mm for the crater depth. Total crater depth divided by total sputter time provides the average sputter rate.

Fig. 2. Depth profile for phosphorus in a silicon sample. Phosphorus implantation was performed in order to convert silicon into a p-type semiconductor. “Dose” stands for the primary ion beam intensity in ion/cm³; “Energy” represents primary ion kinetic energy. The horizontal axis displays the etching time. The vertical scale is a logarithm scale and displays secondary ion counts per second (s^-1).

Ion Imaging

Ion images show secondary ion intensities as a function of location on sample surfaces. Image dimensions vary from 500 mm to less than 10 mm. 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.

The example (microbeam) images (Fig. 3) 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.

Three-dimensional analyses are possible by acquiring images as a function of sputtering time (image depth profiles). Microscope sputtering rates exceed microbeam rates, often by several orders of magnitude. Thus microscope imaging produces depth scales more compatible with the scale of the lateral images. Microbeam imaging usually provides a better combination of image features, except when faster sputtering is required for three-dimensional analysis or for removing an overlayer before image acquisition.

Fig. 3. Gold (left) and sulfur (right) distribution in a pyrite (FeS₂) grain. The numerical scales and the associated colors represent different ranges of secondary ion intensities per pixel.