The technique of Secondary Ion Mass Spectrometry (SIMS) is the most sensitive of all the commonly-employed surface analytical techniques - this is because of the inherent sensitivity associated with mass spectrometric-based techniques.
There are various types of SIMS equipment, each having similar characteristics and each possessing their own specific specialties and disadvantages.
This device uses an ion gun to sputter materials under high vacuum. The sputtered materials/particles are charged and accelerated through a magnetic field at a 90 degree bend. The materials/particles separate base on their individual atomic weights since heavier particles will be diverted at a lesser degree than lighter particles under the same magnetic field. The particles are then detected visually by a channel plate or quantitatively by an electron multiplier.
Quadrupole SIMS instruments use low extraction fields to extract secondary ions from the sample, allowing low energy ion bombardment - giving high depth resolution and analysis of ultra-shallow and very thin films. Also because of the low extraction fields of the Quad-SIMS instrument, it is easier to do charge compensation for the analysis of insulating material like SiO2 and Si3N4. We have developed protocols for H analysis in Si-R Nitrides, B & P concentrations in BPTEOS, and N concentration in Thin Oxynitrides, Ge concentration in SiGe samples with OCE (Optical Conductivity Enhancment). The full wafer Quad-SIMS instrument allows us to map wafers - looking for dose or profile shape variations across a wafer.
Similar to the other SIMS, ToF SIMS is also used to obtain elemental and molecular chemical information about surfaces.
In ToF-SIMS analysis, the sample is placed in an ultrahigh vacuum environment where primary ions bombard and sputter atoms, molecules, and molecular fragments from the sample surface. The mass of the ejected particles (i.e. secondary ions) are analyzed via time-of-flight mass spectrometry.
In the ToF analyzer, ejected ions are accelerated into the analyzer with a common energy (but different velocities depending on the particle mass). Due to the differences in velocities, smaller ions move through the analyzer more rapidly than the larger ions. The mass of the secondary ions are determined by their travel time through the analyzer. SIMS is a surface sensitive analysis method since only the secondary ions generated in the outermost 10 to 20 Å region of a sample have sufficient energy to overcome the surface binding energy and escape the sample surface for detection and analysis.
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.
This is mass analysis to determine the elemental and/or molecular composition of the top monolayer of a material's surface. Can distinguish elements and molecules whose masses range from 1 amu to >10,000 amu. Can be quantitative with standards.
Low doses of primary ions ensure that every secondary ion comes from an undisturbed region of the surface, allowing true surface analysis with SIMS sensitivities.
Using a raster probe, static SIMS generates images to determine the lateral distribution of secondary ions, with a lateral resolution <0.2µm.