J.M. Walls

An XPS study of ion-induced compositional changes with group II and group IV compounds

Abstract Inert-gas ion bombardment and plasma etching are important processes in the fabrication and micromachining of semiconductor devices. Sequential ion etching and surface analysis is also widely employed as a means of device characterisation and investigation. In both applications, the incident ions produce topograhical and compositional changes in the surface region of the material. Changes in stoichiometry and oxidation state following ion-bombardment of a range of group II and group IV compounds have been studied by X-ray photoelectron spectroscopy (XPS). In every case, stoichiometry changes caused by ion-induced decomposition of the surface is observed, generally resulting in a preferential loss of the anionic species. Such behaviour is observed for the carbonates, sulphates and nitrates of group II and IV metals. The extent of cation reduction following ion-bombardment is found to be critically dependent on the composition and nature of the original surface. For example, Pb 2+ is stable to ion bombardment in all of the compounds studied, but is reduced to Pb 0 in the monoxide. The results are discussed in relation to the known thermodynamic properties of the compounds, and the experimental data are shown to correlate well with the free energies of formation in every case.

The development of a general three‐dimensional surface under ion bombardment

Abstract A general three‐dimensional theory of the development of surface topography during ion etching is presented together with the computational methods used for its visualization. Use of the techniques is illustrated using the example of an elliptical hummock under erosion using ion sources with both uniform and non‐uniform ion‐current distributions. The analysis is based on the method of characteristics, as in the theory of non‐linear waves, and this has several advantages over existing two‐dimensional methods including its suitability for numerical manipulation, its extension into three dimensions, and its ease of use with non‐uniform beams. The use of the approach is also advantageous in two dimensions and this is considered separately in the Appendix. Areas of application of the methods include the use of ion etching in surface analysis and the microfabrication and micromachining of surface structures on certain electronic devices. The methods may also find application in explaining the effects o...

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.

Studies of the composition, ion-induced reduction and preferential sputtering of anodic oxide films on Hg0.8Cd0.2Te by XPS

Abstract Angle-resolved studies of anodic oxide films on Hg 0.8 Cd 0.2 Te, and on cadmium and tellurium bulk oxides, by X-ray photoelectron spectroscopy (XPS) have revealed the formation of a discrete altered overlayer, containing reduced tellurium (Te 0 ) species, following inert gas ion bombardment. In the presence of Cd 2+ species, recombination is shown to occur, to yield a Te 2− (telluride) species, in the form of CdTe. Only Te 2− is observed on ion-induced reduction of CdTeO 3 , whereas there is no evidence for significant Te 2− formation on either TeO 2 or anodic oxides on CMT, reflecting the low cadmium content of these latter two oxides. In addition, preferential cation sputtering leads to a homogeneous (within the XPS sampling depth) Te 4+ -depleted layer in Cd-Te mixed oxides. The phenomenon is characterised by an effective sputter yield ratio ( S Te 4+ / S Cd 2+ ) of 1.83 (±0.06). The anodic oxide on CMT, when corrected for preferential Te 4+ sputtering, yields a (Te 4+ ):(Cd 2+ ) ratio of 4.6 (±0.5), in excellent agreement with the bulk Te:Cd ratio in Hg 0.8 Cd 0.2 Te. An anodic oxide film composition of approximately 2 O 5 and > 75 mol% TeO 2 and/or mixed Hg-Te oxides is proposed.

An XPS study of ion-induced dissociation on metal carbonate surfaces

Ion-induced chemical decomposition during ion bombardment of surfaces is well known but the causes and mechanisms of this damage are still poorly understood. In general, it has been found that molecules with low free energies of formation will undergo ion-induced dissociation, but attempted correlation with thermochemical data has met with only limited success. Most experimental studies of ion-beam damage have been carried out on metal oxide substrates, which often undergo decomposition or reduction via a number of intermediate species, and result in the formation of a variety of complex reaction products. For this reason, thermodynamic models of the sputtering process have tended to be oversimplified. Better correlation with experiment would be expected within a selected range of chemically similar compounds, which undergo ion-induced decomposition via simple, one-step, irreversible reaction mechanisms to yield unique products. One such class of compounds is the alkaline earth carbonates, which undergo thermal dissociation in the solid state: MCO3(s)→MO(s)+CO2(g) ΔG = 15.6 to 51.6 kcal mol−1whereM=Mg, Ca, Sr, Ba. Photoelectron spectra from clean alkaline earth carbonate surfaces show a single, discrete, oxygen 1s feature, at a binding energy of about 531eV, and an oxygen to carbonate-type carbon atom ratio of 3:1. Following exposure to the ion beam, however, both the carbon 1s and the 531eV oxygen 1s peaks are observed to be significantly reduced in intensity, and a second oxygen 1s feature, at a binding energy of about 528eV, is produced. This new feature may be assigned to the oxide dissociation product, since further ion etching simply results in a further loss of intensity from the carbonate features, and a complementary increase in intensity from the new oxygen feature. The extent of decomposition at any given ion dose may be related to the thermochemical properties of the parent carbonate. Implications of the work in the interpretation of sputter-depth profiles of such systems are discussed.

The erosion of amorphous and crystalline surfaces by ion bombardment

Ion etching is a commercially important technique used in the micromachining of solid surfaces. In this paper the three-dimensional theory of the development of surface topography during ion bombardment is extended to include the effects of surface crystallinity. The method of characteristics is used to trace the development of edges and facets that develop on three-dimensional surfaces due to multiple stationary points present in the angular dependence of the sputtering yield. The analytical approach used facilitates subsequent numerical calculations and examples are given to illustrate the applications of the theory to both amorphous and crystalline structures.

Multilayer Broadband Antireflective Coatings for More Efficient Thin Film CdTe Solar Cells

Reflection losses limit the efficiency of all types of photovoltaic devices. The first reflection loss occurs at the glass-air interface of the photovoltaic module. If no light trapping mechanism is used about 4% of the solar energy is lost at this surface. Currently, most commercial thin-film CdTe solar modules are manufactured using NSG TEC10 glass, with no light trapping mechanism addressing the reflection at the interface of the glass with the atmosphere. To minimize the losses, a broadband multilayer thin-film coating has been designed and deposited onto the glass surface of a thin-film CdTe solar cell. The coating consisted of four dielectric layers of alternating thin films of ZrO2 and SiO2 . The layers were deposited by using high-rate-pulsed dc magnetron sputtering. Spectrophotometer measurements confirm that the transmission increased by between 2% and 5% over the spectrum utilized by the thin-film CdTe solar cell. The weighted average reflection reduced from 4.22% to 1.24%. Standard test conditions (STC) solar simulator measurements confirmed a 0.38% increase in absolute efficiency and a 3.6% relative increase in efficiency.

The development of surface topography during depth profiling in auger electron spectroscopy

Abstract There are several ways in which sputter-etching can cause interpretational difficulties when depth profiling in Auger electron spectroscopy. This paper discusses the effects on surface topography of a non-uniform ion beam current and the dependence of sputtering yield on ion incidence angle and surface composition. A three-dimensional theory is given which can be used to account for these effects and which leads to some conclusions concerning the optimum conditions for composition-depth profiling.

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.

Ion trajectories in the field-ion microscope

The trajectories of the imaging ions in the field-ion microscope have been determined for hyperboloidal and paraboloidal emitter shapes. The shape of the trajectories has been shown to be independent of the potential difference between the emitter and the microscope screen and the charge-to-mass ratio of the ion. It has also been found to vary little with tip-to-screen distance and nose curvature. The stereographic projection is found to be in good agreement with the calculated results.