Secondary Ion Mass Spectrometry1-10 keVStatic with low energy for surface. Dynamic with high energy for depth studyBombardment of a sample surface with a primary ion beam followed by mass spectrometry of the emitted secondary ions constitutes secondary ion mass spectrometry (SIMS).The best SIMS reference is Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends, by A. Benninghoven, F. G. Rdenauer, and H. W. Werner, Wiley, New York, 1987 (1227 pages).Ion beam-solid interactionIon Beam SputteringThe bombarding primary ion beam produces monatomic and polyatomic particles of sample material and resputtered primary ions, along with electrons and photons. The secondary particles carry negative, positive, and neutral charges and they have kinetic energies that range from zero to several hundred eV.Primary beam species useful in SIMS include Cs+, O2+, O , Ar+, and Ga+ at energies between 1 and 30 keV. Primary ions are implanted and mix with sample atoms to depths of 1 to 10 nm.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.The sputter yield is the ratio of the number of atoms sputtered to the number of impinging primary ions. Typical SIMS sputter yields fall in a range from 5 and 15.Three theoretical models for sputteringThe collision cascade model has the best success at quantitatively explaining how the primary beam interacts with the sample atoms. In this model, a fast primary ion passes energy to target atoms in a series of binary collisions. Energetic target atoms (called recoil atoms) collide with more target atoms. Target atoms that recoil back through the sample surface constitute sputtered material. Atoms from the sample's outer monolayer can be driven in about 10 nm, thus producing surface mixing. The term knock-on also applies to surface mixing.The SIMS ionization efficiency is called ion yield, defined as the fraction of sputtered atoms that become ionized.Sputtering Yield VS Ion Yield(1) Sputtering yield S:Sputtering is quite efficient, S>1 for Ar+ of few keV(2) Ion Yield = # secondary ions/# atoms sputtered(3) Sputtering Yield in SIMS (Y) = # secondary ions / # incident ions= Sputtering yield S x Ion yieldBombard surface with high energy ions and desorb ions ( 99%) (SNMS)A Schematic diagram of the energy distribution of residual gas ions, secondary ions, sputtered neutrals, electrons and other particlesEnergy of sputtered atoms is low (1 keV in, 10-20 eV out)Energy distribution of sputtered atomsSecondary Ion Energy DistributionsThe sputtering process produces secondary ions with a range of (translational) kinetic energies. The energy distributions are distinctly different for atomic and molecular ions. Molecular ions have relatively narrow translational energy distributions because they have kinetic energy in internal vibrational and rotational modes whereas atomic ions have all kinetic energy in translational modes. The following figure shows typical energy distributions for mono, di, and triatomic ionsS(E) peaks at 10-30 keV depending upon materialIncident ion Energy dependence of sputtering yield (S)Sputtering yields also vary with incident particle mass -incident ion typeSputtering yields vary considerably between elements (for fixed incident particle and energy) - depends on cohesive energy U (heat of sublimation)Sputtering yield varies for element in different matrices elements, oxidesSputtering yield varies as ~1/cos for moderate incidence angles (more energy concentrated at surface) but falls off at very grazing incidence (scattering dominates, no penetration)Ion yields vary over many orders of magnitude for the various elements. The most obvious influences on ion yield are ionization potential for positive ions and electron affinity for negative ions.Secondary Ion Yields -- Elemental EffectsThis figure shows the logarithm of positive ion yields plotted as a function of ionization potential. The ion yields are relative to silicon in a silicon matrix with oxygen sputtering.This figure shows a similar treatment for negative ions where the logarithms of relative ion yields are plotted against electron affinities. The ion yields are relative to silicon for measurements in a silicon matrix with cesium ion sputtering. The four halides are the elements that deviate furthest from the trend line.The correlations of ionization potential with secondary ion yields are not perfect. Variations depend both on the sample matrix and on the element itself. For example, the presence of oxygen in the sample enhances positive ion yields for most elements, but fluorine exhibits anomalously high positive ion yields in nearly all samples. Some elements, such as helium and neon fall outside the trend shown in the picture.Secondary Ion Yields -- Primary Beam EffectsOxygen bombardment increases the yield of positive ions and cesium bombardment increases the yield of negative ions. The increases can range up to four orders of magnitude.Oxygen enhancement occurs as a result of metal-oxygen bonds in an oxygen rich zone. When these bonds break in the ion emission process, the oxygen becomes negatively charged because its high electron affinity favors electron capture and its high ionization potential inhibits positive charging. The metal is left with the positive charge. Oxygen beam sputtering increases the concentration of oxygen in the surface layer.The enhanced negative ion yields produced with cesium bombardment can be explained by work functions that are reduced by implantation of cesium into the sample surface. More secondary electrons are excited over the surface potential barrier. Increased availability of electrons leads to increased negative ion formation.Ion yield leads to different analysis conditions for different elements as indicated on the periodic table - The choosing of ions.- Sputtering in elements best understood- Sputtering in single crystals, complex materials less well understood- Quantitation of sputtering or sputtering rate difficult because of large number of variablesSputtering very popular but- Leaves embedded incident particle in solid- Substantial damage and interlayer mixing- Preferential sputtering of one componentSummary