H. A. M. de Grefte

Concentration profiles of boron implantations in amorphous and polycrystalline silicon

Abstract Boron was implanted in amorphous silicon at energies in the range 30–200 keV and in polycrystalline silicon at energies in the range 70–800 keV. The boron distributions were measured with secondary ion mass spectrometry. By comparing the boron distributions in amorphous silicon and in polycrystalline silicon it was found that the used polycrystalline silicon behaves similarly to amorphous silicon for the boron stopping process. It was found that with the first four moments of the experimental distributions, an analytic description of the experimental distributions can be given by distributions of the Pearson system. The moments are determined by curve fitting of Pearson distributions to experimental distributions. In this way a systematic extrapolation of the low energy distributions outside the surface is automatically obtained. The moments are compared with the moments calculated by Winterbon and others. A satisfactory correspondence between the experimental and theoretical average ranges and s...

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.

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.

The measurement of small ion currents with the aid of photomultipliers

A short survey of the various methods for measuring small ion currents (10−9 to 10−20 A) is given: 1. Ion current measurement (with d.c. amplifier, vibrating-reed electrometer amplifier or multiplier). 2. Ion counting (with open multiplier or scintillation detector). In particular, technical problems encountered in lowering the background in a Daly-type scintillation counter are discussed. In the arrangement used, the ions separated in a mass spectrometer are accelerated on to an auxiliary electrode which is at a potential of −25 kV. Opposite this auxiliary electrode (converter) is situated a scintillator. The secondary electrons released from the converter by the ions are accelerated on to the scintillator and give rise to photons which generate photoelectrons in the cathode of a photomultiplier. These photoelectrons are finally detected as a voltage pulse at the anode of the photomultiplier with the of well-known pulse counting methods. The influence of the following parameters on the background of the arrangement is discussed: field emission from the converter, the absorption of cosmic radiation and radioactive radiation in the scintillator, the choice of the scintillator material, the optimum dimensions of the scintillator, thermal emission from the photocathode, and spurious effects of the electronic equipment. Methods for lowering these unwanted contributions are discussed. The lowest background obtained was 0.02 counts/sec corresponding to approximately 3 × 10−21 A. A method is described to determine the average number γ′ of electrons released at the converter per impinging active ion i.e. an ion actually releasing electrons.

Interpretation of secondary ion mass spectra by means of fingerprint spectra and secondary ion imaging

A passive layer, of several thousand Å thickness, formed on a polycrystalline nickel electrode, has been examined using secondary ion mass spectrometry (SIMS) by spottering with a 5.5 keV, 13μA·cm−2,40Ar+ primary beam. Concentration profiles were detived by monitoring the intensities of atomic and molecular mass peaks as a function of sputtering time (i.e. depth). Nickel was present throughout the layer but not as the element since the relative intensities of the Nin+ (n=1, 2, 3, 4) peaks, constituting part of its fingerprint spectrum, differed from those in the fingerprint spectrum of elemental nickel. These values were eventually reached, signifying piercing of the layer and thus providing a means of estimating its thickness. Imaging of58Ni+ showed the presence of nickel in at least two different modifications in the layer, both with higher Ni+ yields than the bulk nickel. Their fractional coverages were estimated from the images taken at various depths. The resulting profile of the Ni+ originating from one of these modifications was found to be proportional to the16O− profile, indicating that these ions originate from the same molecule.This example demonstrates the advantage of combining different SIMS modes (viz. depth profiles, fingerprint spectra and imaging) in tackling certain analytical problems.

Experimental Analysis of Concentration Profiles of Boron Implanted in Silicon

The concentration profiles of boron implantations in silicon are measured using secondary ion mass spectrometry. In this method the implanted silicon is sputtered by bombardment with 5.5 keV oxygen ions. The resulting secondary B+-current is measured continuously as a function of time. The time scale is transformed into a depth scale by measuring the erosion rate. The reliability of this method is checked and discussed. This method is used in the study of the specific shape of the profiles of boron implanted in a dense crystal direction. This was done by varying the implantation conditions such as temperature, crystal direction, crystal perfection. The boron profiles in amorphous silicon were compared with theory. Another aspect studied is the profile distortion due to heat treatments. By comparison of the boron profiles with corresponding electrical profiles valuable additional information was obtained.