Secondary Ion Mass Spectrometry

Sputtering Process

High-energy primary ions are generated and accelerated toward the sample surface to initiate the physical sputter process. When the primary ions collide with the sample, they transfer their kinetic energy to the target atoms, causing a cascade of atomic collisions that results in the ejection of surface atoms and molecular fragments. Most of these sputtered particles are neutral, but a small fraction, typically one to ten percent, are ionized during the emission process.

These secondary ions are then accelerated by an electrostatic field into the mass spectrometer for analysis. The continuous removal of surface material by the primary ion beam allows the system to analyze progressively deeper layers of the specimen over time, creating a continuous record of the chemical composition as a function of depth.

Ionization Yield

The efficiency of the ionization process is highly dependent on the chemical environment of the sample and the choice of primary ion species. Primary ion beams of oxygen are used to enhance the yield of positive secondary ions, which is necessary for analyzing electropositive elements like boron and sodium. Conversely, primary ion beams of cesium are used to enhance the yield of negative secondary ions, making them ideal for analyzing electronegative elements like phosphorus and arsenic.

This chemical sensitivity, known as the matrix effect, means that the raw secondary ion signal does not directly correspond to the absolute concentration of the element. To obtain quantitative data, analysts must use calibrated reference standards of the same material matrix to convert the raw ion counts into precise concentration values.

Depth Profile

The collection of secondary ion intensity as a function of sputtering time is converted into a depth profile that shows the concentration distribution of specific elements through the thickness of the sample. This profile provides an accurate measurement of the thickness of thin films, the sharpness of material interfaces, and the diffusion behavior of dopants during thermal processing. In failure analysis, depth profiling is used to identify trace contaminant layers that can cause adhesive failure or electrical leakage in microelectronic assemblies.

The spatial resolution of the depth profile can reach the nanometer scale, allowing for the characterization of ultra-thin barrier layers and quantum well structures. However, the technique is destructive because it physically sputters away the material being analyzed, which requires that samples be carefully selected and positioned before starting the measurement run.