1. Introduction

When a solid sample is sputtered by primary ions of few keV energy, a fraction of the particles emitted from the target is ionized. Secondary Ion Mass Spectrometry consists of analyzing these secondary ions with a mass spectrometer. Secondary ion emission by a solid surface under ion bombardment supplies information about the elemental, isotopic and molecular composition of its uppermost atomic layers.
The secondary ion yields will vary greatly according to the chemical environment and the sputtering conditions (ion, energy, angle). This can add complexity to the quantitative aspect of the technique.

The SIMS technique provides a unique combination of extremely high sensitivity for all elements from Hydrogen to Uranium (detection limit down to ppb level for many elements), high lateral resolution imaging (down to 40 nm), and a very low background that allows high dynamic range (more than 5 decades). This technique is "destructive" by its nature (sputtering of material). It can be applied to any type of material (insulator, semiconductor, metal) that can stay under vacuum.
It allows molecular as well as elemental characterization of the first top monolayer in the static SIMS( see TOF-SIMS) mode. It allows also the investigation of bulk composition or depth distribution of trace elements in the dynamic SIMS mode, with a depth resolution ranging from one to 20-30 nanometers. 

In static SIMS, dedicated to the analysis of the top monolayer, the primary ion dose is kept below 1E12 ions/cm2 and the mass spectrum reveals MOLECULAR information. In dynamic SIMS mode, the primary ion dose is not limited and exceeds 1E12 ions/cm2. In this case, only ELEMENTAL information can be obtained from the mass spectrum. This mode allows "bulk" and in-depth analyses.
The ionization yield of most elements varies by decades, depending on the chemical environment. This property is used in SIMS instruments to increase the sensitivity of the technique: a dynamic SIMS instrument will be generally equipped with Oxygen and Cesium primary ion beams, in order to enhance, respectively, positive and negative secondary ion intensity by 2 to 3 orders of magnitude compared to the use of noble gas ions. Oxygen gas can also be flooded onto the surface to oxidize it and increase positive ion emission.
When sending keV ions onto a solid surface, (at least !) three phenomena occur simultaneously: 
- the sputtering of (mainly) the top monolayer atoms, induced by the collision cascade,
- the ionization of a small fraction of the secondary particles,
- the primary ion implantation in the solid. 
Starting from the surface (or going through an interface), the concentration of the implanted primary species (oxygen or cesium) will vary, then reach an equilibrium (after a few nm, depending on the conditions). As soon as this is achieved, reliable quantification is possible with reference standard samples, using Relative Sensitivity Factors.

One of the main application of dynamic SIMS is the analysis of trace element depth distribution (for example, dopants in semiconductors). Impact ion energy is adjusted depending on the applications. Low energy (down to 200-300 eV) is used to reduce atomic mixing due to the collision cascade and improve depth resolution to the nanometer level. High energy (up to 20-30 keV) is chosen to go deeper (10-20 microns), faster (Ám per min), and improve detection limits and image resolution.


2. Sample Preparation

The measurement is performed under ultra-high vacuum (UHV), the samples should have a flat surface and can stay in UHV system. For insulator samples, 10-20nm gold film should be coated on the surface. Please cut your samples into about 10mmx10mm pieces for easy handling.


3. Instrument at MCPF

MCPF is equipped with Cameca IMS 4f Dynamic SIMS system. The typical specification is shown as below,

Ion Sources High brightness duoplasmatron O2+, O- and Ar+
Surface ionization, Cs microbeam source
Accelerating Voltages Cs+: 5 to 13 kV
O2+: 5 to 17.5 kV
O-: 5 to 12.5 kV
Sample Max. Size 1 inch in diameter, 0.5 inch in thichness
Extraction Voltage 4.5 kV
Detection Limit B in Si: 5x1013 at./cm3 at 300 MRP with O2+ primary ions
P in Si: 5x1014 at./cm3 at 5000 MRP with Cs+ primary ions
As in Si: 5x1014 at./cm3 at 5000 MRP with Cs+ primary ions
H in Si: 1x1017 at./cm3 at 300 MRP with Cs+ primary ions

4. Application Examples

 


Last updated on 23/08/2007