D. E. Sykes

Surface morphology of Si(100), GaAs(100) and InP(100) following O2+ and Cs+ ion bombardment

Abstract Low energy ion bombardment is an important technique in the processing and analysis of electronic materials. Inert gas and reactive ions are used as the primary excitation agent in Secondary Ion Mass Spectrometry (SIMS) to obtain the depth distribution of dopants and contaminants in semiconductors. Ion etching is also used for pattern delineation in the fabriatiion of devices in the electronics and telecommunications industries. Although the technique is becoming increasingly important, ion erosion of surfaces does not always proceed uniformly and artefacts such as pits, pyramids, cones, facets and other ion-induced topographies are frequently observed. In this paper a systematic study of the effects of 5.5 keV ion bombardment on clean well-characterized surfaces of some important electronic materials (Si(100), Cr-doped semi-insulating GaAs(100) and Fe doped InP(100)) is reported using O22 and Cs+ primary ion species. Relatively high doses of1019 ions cm−2 and 1020 ions cm−2 have been used to ensure that the observed features are properly developed. Detailed studies of the morphologies have been made using scanning electron microscopy. The implications of the work for SIMS and the integrity of concentration-depth profiles are discussed.

Improved sputter-depth profiles using two ion guns

Abstract A method is described which leads to a substantial improvement in the depth resolution of sputter-depth profiles obtained using Auger electron spectroscopy. Two ion guns are used to develop the profile, each symmetrically inclined to the surface normal, whose effect is to suppress the formation of ion induced topography. The effectiveness of the method is illustrated by comparing profiles obtained using one and two ion guns through the same Ag/Cu multi-layer thin film sandwich structure.

SIMS with sample rotation : An experimental novelty or a practical necessity?

The application of sample rotation during SIMS analysis of practical samples is described. It is shown that sample rotation gives improved depth resolution in profiles of metal layers on flat semiconductor substrates, allowing thin interfacial layers to be identified. For thick metal layers on non-ideal substrates, the benefits of sample rotation are less clear. Sample rotation is also shown to be useful as a simple in situ sample preparation tool for the removal of metal overlayers on semiconductor substrates where the objective of the analysis is the semiconductor material rather than the metal itself.

Surface Characterisation of Laser Modified Human Tooth Enamel Using Laser Microprobe Mass Spectrometry and Scanning Electron Microscopy

A technique in preventative dentistry currently under development is the modification of the tooth surface by the fusion of glassy metal oxides into the enamel surface using a CO2 laser. This could provide a protective coating to the tooth which would be applied in the mouth. Such a radical treatment is intended to produce major changes in chemistry at the tooth surface and characterisation of these changes is a necessary part of the development of the technology. LMMS and SEM have been used, as part of a feasibility study, to characterise coatings deposited with laser assisted fusion on the surface of human tooth enamel, in particular the chemical and topographical changes taking place. The results demonstrate that the analytical approach adopted provides useful qualitative information about the physical and chemical changes taking place. They highlight the fact that although deposition of the coating is patchy, areas are present where the coating has apparently fused to the tooth enamel. Changes in the chemistry of the tooth enamel subjected to laser irradiation were identified, both in the absence of coating precursors and with precursor materials present.

SIMS analysis of GaAs/GaAIAs superlattice including sub-monolayer grown by MBE

Abstract A GaAs/GaAIAs superlattice structure containing layers of sub-monolayer thicknesses was analysed using two SIMS instruments with markedly different analytical conditions. The results from the two instruments are consistent. Layers between 25 A and 50 in A in thickness can be resolved whilst layers as thin as 1.6 A can be detected. Secondary ion images indicate that the resolution is limited by morphology which is present through the structure, rather than by atomic mixing.

SIMS analysis using molecular ions for the study of boron precipitation in silicon

Abstract A method of determining, by SIMS analysis, whether or not an impurity species is present in the form of a dilute solution or as discrete precipitates is described and illustrated for the case of boron in silicon. For a sample containing randomly distributed boron atoms (ion implanted boron in silicon) the molecular cluster signal B n shows a power law relationship to the B monomer signal ( B n = k B n ) for both positive and negative secondary ions, n

The effect of oxygen flooding on the secondary ion yield of Cs in the cameca IMS 3f

Abstract It is well known that the positive secondary ion yields of surfaces exposed to ion bombardment in a SIMS experiment are critically dependent upon the oxygen concentration in the analysed surface. Oxygen flooding of the surface is a technique that can be used to increase the secondary ion yield of a surface bombarded by inert gas ions. Alternatively, when an oxygen primary beam is used at low angles of incidence (as in the Cameca geometry), oxygen flooding enhances the ion yield by creating a surface oxide layer. In this paper we report observations of the effect of oxygen flooding on the useful ion yield of caesium implanted into silicon analysed using O2+ primaries in a Cameca IMS 3f. The results from caesium are compared with those from boron and arsenic doped silicon. Contrary to expectation the useful yield of caesium is found to be reduced by oxygen flooding. Possible mechanisms to account for this observation are discussed.

SIMS analysis of isotopic impurities in ion implants

Abstract The n-type dopant species Si and Se used for ion implantation in GaAs are multi-isotopic with the most abundant isotope not being chosen because of potential interferences with residual gases. SIMS analysis of a range of 29 Si implants produced by several designs of ion implanter all showed significant 28 Si impurity with a different depth distribution from that of the deliberately implanted 29 Si isotope. This effect was observed to varying degrees with all fifteen implanters examined and in every 29 Si implant analysed to date 29 Si + , 29 Si ++ and 30 Si implants all show the same effect. In the case of Se implantation, poor mass resolution results in the implantation of all isotopes with the same implant distribution (i.e. energy), whilst implants carried out with good mass resolution show the implantation of all isotopes with the characteristic lower depth distribution of the impurity isotopes as found in the Si implants. This effect has also been observed in p-type implants into GaAs (Mg) and for Ga implanted in Si. A tentative explanation of the effect is proposed.

Surface Chemical Analysis

The physical bases of surface chemical analysis techniques are described in the context of semiconductor analysis. Particular emphasis is placed on the SIMS (secondary ion mass spectrometry) technique, as this is one of the more useful tools for routine semiconductor characterization. The practical application of these methods is addressed in preference to describing the frontiers of current research.

SIMS without sums

Abstract Secondary Ion Mass Spectrometry (SIMS) is probably the most powerful analytical technique for the characterization of semiconductor materials. Its strength lies in its ability to perform high sensitivity chemical analysis directly on semiconductor materials with high spatial and depth resolution. As part of the continuing series on assessment techniques, III–Vs Review presents a mathematics-free overview of the process.