Lecture 5: Secondary ion mass spectroscopy (SIMS) Assoc. Prof. Dr. Zainovia Lockman, PPKBSM, USM EBB 245. Materials Characterisation

1.Introduction 2.Measurements from SIMS 3.Instrumentation 4.Limitations of SIMS (lecture presentations/notes 2011)

SIMS is a technique for surface chemical analysis It has similar use as Auger and XPS but has other added benefits SIMS examines the mass of ions, instead of energy of electrons escape from a solid surface to obtain information on surface chemistry SIMS relies on sputtering of secondary particles by primary ion The mass of the secondary ions will be measured sputtering Particles being sputtered out: secondary particles

Detection of all the chemical elements in the periodic table, including hydrogen which cannot be detected by the AES or XPS Detection of elements in concentrations as low as 10 − 6, while AES or XPS detection limits are concentration levels of 0.1 atom% Limitation of the detection to the top one or two atomic layers of a solid surface (<1 nm) Distinguish between different isotopes of elements. SIMS can be equipped with imaging, so imaging with SIMS is possible

Detector: Display Record Process SIMS uses energized primary particles, usually ions such as Ar +, Ga + and Cs +, to bombard a solid surface in order to induce sputtering of secondary particles from an area (as shown in the figure). The interactions between primary ions and the solid are however rather complicated: 1.the secondary particles include electrons, neutral species of atoms or molecules, and ions can be emitted out. 2.the majority of the secondary particles are neutral and not useful in SIMS. Only the secondary ions generated by the bombarding process carry chemical information. 3. the interactions are often more than a simply one-to-one knock-out of a surface ion by a primary ion, implantation of ions can also be induced

Only a small portion of secondary particles ( ∼ 1% of total secondary particles) are ionized and become the secondary ions that are analyzed in SIMS. A sputtered particle faces competition between ionization and neutralization processes when it escapes a sample surface. Ionization probability represents the chance of a sputtered particle being an ion. The ionization probability is strongly affected by the electronic properties of the sample matrix. Direct collision sputtering: direct impact between a primary ion and a surface atom (fast sputtering) Collision cascade: a primary ion never has chance to collide with a surface atom; instead, atoms in the solid transfer the impact energy to surface atoms after a series of collisions (slow sputtering) Thermal sputtering: density of primary ions is high, vaporisation around the surface area impacted by primary ions

I m = I p Y m α + θ m η The secondary ion current (I m ) of chemical species m is collectively determined by the primary ion flux (I p ), the sputter yield (Y m ) and the ionization probability. α + represents the probability for positive ions θ m is the fractional concentration of species m in the surface layer η is the transmission of the detection system. The transmission is defined as the ratio of the ions detected to ions emitted, and it varies from 0 to 1 depending on the analyzer. The sensitivity of SIMS to element m is controlled by the factors Y m, α + and η. Y m generally increases with beam energy and with the primary ion mass.

Sputtering is process that obviously can damage a surface. The energetic primary ions remove atoms, atom clusters, molecules or molecular fragments from the surface region of a sample. The chemical structures of the surface may be destroyed during SIMS examination. This is especially true when the high flux of primary ions bombards a surface. SIMS with high flux of primary ion bombardment is referred to as dynamic SIMS. As dynamic SIMS is able to remove many layers of atoms in the surface region and provide elemental distributions in a depth profile For surface chemical structures analysis a static SIMS is used. Static SIMS has a lower flux of primary ions to avoid the possibility that any surface area is bombarded by primary ions twice

Identifying elements from SIMS spectra is relatively simple. The elements can either be identified by their exact mz − 1 or by detecting expected isotopes. Moreover, one can identify individual ions, ion clusters and molecular fragments according to their mz − 1 values: useful in the determination of molecules of polymer for example (monomer mass of a given polymer can be detected). SIMS can be either destructive or non-destructive to the surface being analyzed as mentioned previously. The destructive type: the dynamic SIMS is particularly useful for depth profiling of chemistry. The nondestructive type: static SIMS is for surface chemical examination.

To obtain accurate mass measurement, a mass correlation with flight time should be calibrated. The calibration can be done using a known ion mass in the sample. There are general rules that are helpful for element identification, given as follows. The great majority of metals give exclusively positive ions in static SIMS conditions. The most electropositive elements, such as alkali and alkali earth metals, give intense positive ion peaks. The most electronegative elements, such as O, F, Cl, Br and I give intense negative ion peaks. Hydrogen gives a high yield of both H + and H − ions. The CH − peak is commonly more intense than the C − peak. N is not usually detectable by an elemental peak. P is usually detected from peaks of PO 2 − and PO 3 −.

The mass can be compared to the mass of standard ions and with that the ions can be identified. Measure (mz − 1 ) =>All elements and isotopes (ppb) From SIMS, a mass spectrum can be gathered

Analysis of surface chemical structure requires use of the static SIMS technique to ensure that the major portion of the surface should not be affected by secondary ion emission. Time-of flight SIMS (ToF SIMS) is the most widely used static SIMS technique. As its name indicates, ToF SIMS uses the ToF mass analyzer to measure mz − 1 of secondary ions.

The time-of-flight (ToF) analyzer is the most widely used analyzer in static SIMS. The analyzer analyzes the flight time of an ion. When ions are obtained with a constant kinetic energy from an acceleration potential (V) of 3–8 kV, the flight time of ions through a distance of flight tube to reach a detector can be detected and measured. Heavier ions will have longer flight times in the tube hence differentiation can be made for sample characterisation

Interpreting SIMS spectra, particularly those of polymers, requires experience and knowledge of materials. The interpretation of polymer spectra has been the subject of extensive study. Published sources are available to help with interpretation and databases have been established in which more than a thousand spectra for surface analysis TOF SIMS can be used to identify polymeric material. As can be seen, these spectra (belonging to polystyrene) can be either that of positive ions or negative ions. +ve ions -ve ions

For depth profiling, focused ion beam rasters over a rectangular area The primary ion beam etches a well defined crater in a sample by a beam deflector Secondary ions emitted from the crater bottom should be collected for depth profiling. Beam sputtering the sample, removing layer by layer Measuring the concentration of materials from the surface to a certain depth Secondary ions ejected out from few micron down the sample the, mass measured More sputtering until a crater is being formed Mass of ions being ejected out can be mapped out as a function of thickness

Concentration of dopant atoms from the surface of the material until 1.5 µm in depth. The concentration is 1ppm near the surface (~ 0.1µm) and 1ppb 0.4µm deeper into the sample Silicon doped with dopants with varying concentration. SIMS can be used to sputter layer by layer of Si (with the dopants) and the concentration can be mapped out as shown here Sputtering of primary ions Secondary ions coming out

SIMS depth profile of the GaAs–Si–Al 2 O 3 system SIMS depth profiling shown here reveals GaAs and Si layers coating an Al 2 O 3 substrate. In a SIMS depth profile, the atomic density or secondary ion intensity of elements being analyzed is plotted against either depth (z) or sputtering time (t), and can be easily converted to depth at a constant sputtering rate.

In order to analyze surface chemical structures, static SIMS has been developed in which a low flux of primary ions is used in order to avoid the possibility that any surface area is bombarded by primary ions twice The low dose of primary ions used in static SIMS means that less than 10% of the surface atoms or molecules are bombarded A damaged surface area is defined as a region that includes the impact point of a primary ion and escape point of a secondary ion. Thus, static SIMS is considered as a non- destructive method for surface analysis

Sputtering process by primary ions Mass of secondary ions ejected out can be detected and analysed Therefore all elements and all isotopes can be analysed SIMS has high Sensitivity: parts per million/billion Lateral resolution: beam width 5nm-50 μ m, controllable area to be measured Depth resolution: beam energy can determine the depth that primary ions can penetrate (depth profiling or surface analysis: static or dynamic SIMS) Controllable information depth (a few atomic layers to several micron thick for depth profiling giving composition of film for example on a substrate. Can be used to identify metals, ceramics, polymer and semiconductors to perform various tasks and applications.

SIMS is a useful technique in materials science and engineering TASKS: Failure Analysis Quality Control Development Reverse Engineering Research ** most characterisation methods have similar tasks TO CHECK: Composition Contamination Adhesion Friction Wettability Corrosion Diffusion Residue on a surface of a material Segregation Dopants concentration Interaction between layers

SIMS requires an ultra-high vacuum environment, similar to AES and XPS. The ultra-high vacuum environment ensures that trajectories of ions remain undisturbed during SIMS surface analysis. The SIMS vacuum chamber and pumping system is not much different from that for AES and XPS. A system generating a primary ion beam consists of three main parts: ion source, ion filter and deflector as schematically A primary ion beam for SIMS analysis should ideally contain ions with identical mass. Under the same electric field for accelerating the primary ions, the time for two ions to reach the sample will be different if their mass is different. Wien filter is used to filter ions according to their mass therefore all the ions will travel together. The mass analysis system collects and analyzes the ion masses to produce mass spectra with the assistance of a computer

Not all sputtered particles are of ionised secondary ions, but molecular species that in places can dominate the mass spectrum, making analysis of some elements impossible. SIMS spectra appear more complicated than those of AES and XPS because there are more peaks than the basic ion components The sensitivity of an element is strongly dependent on the composition of the matrix and the type of primary beam used. Standards should, therefore, be close to the composition of the unknown. This is particularity true for isotopic analysis. Need of ultra high vacuum system. Roughness or topographic features of a sample surface can cause a significant reduction in mass resolution, surface must be smooth