H. W. Werner

The use of secondary ion mass spectrometry in surface analysis

Abstract Secondary ion mass spectrometry (SIMS) is based on the bombardment of solids by ions and subsequent mass analysis of the sputtered ions or of the post-ionized neutrals. Various models have been proposed for the emission of secondary ions from the target. These models can roughly be grouped into two categories: (a) lonization takes place outside the target; the sputtered particles are assumed to leave the target in an excited or super-excited state; ionization can then result from such processes as Auger de-excitation or resonance ionization. (b) Ions are generated inside the target by collision cascades initiated by a primary impinging ion. Experimental data of the element- and matrix-dependent ion yield and estimates of the minimum detectable concentrations are shown as well as the rate of consumption of the target. Methods for efficient use of the sputtered material will be discussed. Examples of the wide range of applications of SIMS for determination of the chemical composition and structure of the surface layer and for imaging of the distribution of elements in the surface are given. The SIMS results are compared with those of Auger and Ion Scattering Spectrometry. The essentially non-destructive method of static SIMS is shown to be a powerful tool for the investigation of the outermost atomic layers of a solid.

The influence of bombardment conditions upon the sputtering and secondary ion yields of silicon

Abstract The steady state sputtering yield S and the useful degree of ionisation β + η of silicon have been measured as functions of primary ion energy, atomic number, incident angle, species (rare gas or oxygen), and oxygen partial pressure. The optimum bombardment conditions for SIMS analyses are discussed, and a comparison of the instrumental factor η for two SIMS instruments is made. Variation of both S and β + with oxygen surface coverage, as determined by using Auger electron spectroscopy, is given. A simple model is used to describe the mutual intensity changes occuring in the Auger spectrum during the bombardment-induced oxidation of silicon.

Boron implantations in silicon: A comparison of charge carrier and boron concentration profiles

The concentration profiles of boron implanted in silicon were measured using secondary ion mass spectrometry. The accompanying charge carrier profiles were determined by Hall-effect sheet-resistivity measurements combined with layer removal by anodic oxidation and etching.From a mutual comparison of these profiles an electrically inactive boron fraction was found to exist in the region of maximum boron concentration. This fraction can be correlated with boron precipitates. In high dose implantations the precipitates still exist after annealing at 1000°C.In the tail of the profile a small electrically inactive boron fraction was observed. This fraction was correlated with fast diffusing non-substitutional boron. Near the surface a charge carrier peak was found that can be correlated with the damage caused by implantation.The interpretation of the observed electrical effects was facilitated by investigations on boron concentration profiles of layers implanted with different doses and annealed in accordance with different time-temperature schedules.

On the use of the saha-eggert equation for quantitative sims analysis using argon primary ions

Abstract Samples of known composition have been bombarded by Ar + ions. The resulting secondary ion mass spectra, obtained with two different types of SIMS instruments, were used to check the validity of the Saha-Eggert equation for the case of Ar + ion bombardment.

Quantitative SIMS studies with a uranium matrix

Abstract The relative degrees of positive ionisation of various elements from 6 keV O + 2 bombardment of a uranium sample have been measured. Using only one fitting parameter, most results comply to within a factor of 2 with an empirical expression. The value of this parameter depends upon the sample oxygen content and upon the range of secondary ion energies selected for the measurements. Poor results found for certain elements can be traced to oxide ion formation in lieu of the atomic ion. Ways are found to circumvent this deficiency so that a complete semiquantitative SIMS analysis is feasible.

The influence of selective sputtering on surface composition

Abstract The time dependence of the surface composition of a one-phase, two-component homogeneous system is calculated assuming different sputter yields for the two components. It is shown for a special case that the amount of material which has to be removed before a steady state situation is obtained, is approximately one monolayer. Signals obtainable with different modern surface analytical methods under steady state conditions are discussed.

Application of characteristic secondary ion mass spectra to a depth analysis of copper oxide on copper

Abstract The study of molecular and cluster ions in secondary ion mass spectrometry has led to the concept of characteristic (fingerprint) spectra for this mode of ionization. Thin films of copper oxide with decreasing oxygen concentration as a function of depth have been investigated. The dependence of all mass peaks as a function of depth can be calculated from the measured values of just one peak by means of a linear superposition of the characteristic mass spectra of copper oxide and copper. Finally it is shown that the concentration of copper and copper oxide as a function of depth can be calculated from any mass peak when using the appropriate fingerprint spectra.

Investigation of surface layers by SIMS and SIIMS

Abstract In the Secondary Ion Mass Spectrometry (SIMS) the sample to be analysed is bombarded with a beam of primary ions. The secondary ions sputtered away from the sample, characteristic for its composition near the surface at any time, are mass selected and detected in a mass spectrometer. The yields of several elements in a Fe-matrix and in technically pure samples bombarded with positive oxygen and argon ions have been determined to study the influence of the matrix and the primary ions on the ion yields. The properties of SIMS and of two of its special modes viz. Static Secondary Ion Mass Spectrometry (SSIMS) and Secondary Ion Imaging Mass Spectrometry (SIIMS) with respect to the analysis of surface layers are discussed.

Profiles of boron implantations in silicon measured by secondary ion mass spectrometry

Abstract The concentration of boron implanted in silicon as a function of depth has been measured by using secondary ion mass spectrometry. In this method the silicon substrate is sputtered continuously by ion bombardment and the boron secondary ion current is measured as a function of time. The requirements that must be fulfilfed to ensure that this time-dependent current represents the true concentration profile are formulated and have been checked by a number of experiments. It has been found, that under certain precautions, this is a reliable and fast method for the measurement of boron profiles. A number of boron profiles in the energy range 30–75 keV have been measured and are discussed. Attention is paid to the range and range straggling as well as to the existence of a small tail on the profile.

On the abundance of molecular ions in secondary ion mass spectrometry

Changes in molecular secondary ion intensities brought about by working in an environment of oxygen can be rationalised in simple statistical terms.