Eun-Hee Cirlin

Influence of O+2 energy, flux, and fluence on the formation and growth of sputtering‐induced ripple topography on silicon

The formation of ripples on Si(100) by O+2 sputtering at an angle of incidence of 40° and energies from 1 to 9 keV has been studied using secondary ion mass spectrometry and scanning electron microscopy. At 1 keV no ripples are observed. Between 1.5 and 9 keV ripples are observed oriented perpendicular to the ion direction with average wavelengths that increase, from ∼100 to 400 nm, approximately linearly with O+2 energy. Two‐dimensional fast Fourier transforms of secondary electron images are used to investigate the frequency distribution of the ripples. For the conditions studied, the distributions of frequencies appear approximately Gaussian. At 1.5 keV, the wavelength and growth rate with sputtered depth are independent of flux for fluxes from 15 to 150 μA/cm2. Accompanying ripple formation are changes in secondary ion yields. The changes occur abruptly at depths that increase, from ∼0.2 to 5.6 μm, with O+2 energy. In contrast, sputtering with Ar+ at 1.5 and 7 keV to depths 5–10 times those that produ...

Ion‐induced topography, depth resolution, and ion yield during secondary ion mass spectrometry depth profiling of a GaAs/AlGaAs superlattice: Effects of sample rotation

Effects of sample rotation and sputtering conditions on the depth resolution and ion yield during secondary ion mass spectrometry (SIMS) sputter depth profiles have been studied on bulk GaAs and a GaAs(5 nm)/Al0.3Ga0.7As (5 nm) superlattice. Profiles without sample rotation with 1.0–7.0 keV O+2 show a rapid degradation of the depth resolution with increasing sputter depth. Profiles with Ar+ show only slight degradation. Scanning electron microscope (SEM) studies indicate that degradation is associated with development of periodic surface ripples. The wavelength of the ripples is energy dependent and increases with increasing ion impact energy. With sample rotation, no degradation of the depth resolution is observed and SEM micrographs indicate that surfaces sputtered with rotation are smooth. In addition, with 3.0 keV O+2 significant changes in the secondary ion yield of AsO+ from bulk GaAs are observed at a depth of ∼200 nm. No changes are observed with sample rotation. Our results demonstrate that sampl...

Auger electron spectroscopy and secondary ion mass spectrometry depth profiling with sample rotation

Abstract Recently, there has been a rapid increase in the application of multilayered structured materials, as opposed to bulk materials, in many areas of technological development. Accurate characterization of the structure and composition of advanced multilayers such as superlattices, quantum wells, contacts, and coatings is important for materials and device fabrication technology. Surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, and secondary ion mass spectrometry (SIMS) in conjunction with ion beam sputtering (sputter depth profiling) are at present the most widely used methods for characterization of modern multilayer thin film materials and devices. Ion-beam-induced surface topography, however, can limit depth resolution, and with SIMS, can also cause changes in the secondary ion yield. These changes are due to the high sensitivity of secondary ion yield to the local angle of incidence on sputter-roughened surfaces. Degradation of depth resolution and changes in secondary ion yields during sputter depth profiling have often limited studies of thin film interdiffusion, segregation, oxidation at interfaces, and impurity effects. Much theoretical and experimental work has been carried out to try to improve depth resolution including the use of low ion beam energy, high angle of incidence, and two ion guns. Recent studies of AES and SIMS with sample rotation have shown that depth resolution can be improved substantially and that constant secondary ion yields in SIMS can be achieved. We will first provide an overview of the studies made by various groups to improve depth resolution of metal multilayers using AES with rotation. Next we will review recent investigations of SIMS using sample rotation including studies of the effects of sample rotation on O2+ ion-beam-induced topography, secondary ion yield, and the depth resolution of electronic, metallurgical and dielectric materials. The results presented demonstrate that SIMS with sample rotation provides constant secondary ion yield, and depth-independent depth resolution because sample rotation prevents ion-beam-induced roughness and reduces the effect of the inhomogeneity of low energy ion beams.

THEORY OF IMPROVED RESOLUTION IN DEPTH PROFILING WITH SAMPLE ROTATION

We advance a theory that explains why sample rotation during depth profiling leads to a dramatic improvement in depth resolution. When the sample is rotated, the smoothing effects of viscous flow and surface self‐diffusion can prevail over the roughening effect of the curvature‐dependent sputter yield and generate a smooth surface. If the sample is not rotated initially and the depth resolution declines, we predict that subsequent rotation leads to improved resolution. This phenomenon has already been observed experimentally.

High resolution secondary ion mass spectrometry depth profiling using continuous sample rotation and its application to superlattice and delta‐doped sample analysis

We report the development of secondary ion mass spectrometry (SIMS) with continuous sample rotation to achieve high resolution that is independent of depth during compositional and dopant sputter depth profiling. Sample rotation reduces the ion beam‐induced surface roughness that often limits the depth resolution in SIMS. To illustrate the effects of sample rotation on SIMS analysis, measurements both with and without rotation were performed on a molecular beam epitaxy grown AlGaAs/GaAs superlattice with 100 periods and 5‐nm‐thick layers, and a boron delta‐doped (atomically planar) Si sample, with dopant concentrations from 1016–1021 atoms/cm3 at depths of 0.5–2.0 μm.

Limiting factors for secondary ion mass spectrometry profiling

Understanding the limitations of depth profiling with ion sputtering is essential for accurate measurements of atomically abrupt interfaces and ultra‐shallow doping profiles. The effects of cascade mixing, sputtering statistics, ion‐induced roughness, the inhomogeneity of ion beams, and sample rotation on the depth resolution of Si δ‐doped, AlAs, and InAs monolayers in GaAs and an AlGaAs(5 nm)/GaAs(5 nm) superlattice were investigated. Atomic force microscopy (AFM) investigation of the ion‐induced surface ripple formation on a GaAs substrate sputtered with 3 keV O+2 at angle of incidence θ=40° showed that ripples form rapidly below 200 nm depth. AFM measured root mean square roughness of Si δ‐doped GaAs sputtered with 2 keV O+2 was 0.8 and 2.6 nm with and without sample rotation showing that ripples play a dominant role in depth resolution degradation at shallow depth under these conditions of bombardment. Sample rotation yielded the lowest full width at half‐maximum, 4.1 nm for a Si δ layer at 120 nm dep...

Interdiffusion study of HgTe–HgCdTe superlattice. I. Low‐temperature Auger sputter depth profiling

It has been suggested that some discrepancies between theoretical and experimentally measured band‐gap values may arise from interdiffusion which occurs during superlattice growth. Previously, no analytical technique had been sufficiently developed to provide compositional depth profiling with high enough depth resolution to characterize superlattice structures. We report the development of Auger sputter depth profiling (SDP) of HgCdTe superlattices at cryogenic temperatures and the subsequent investigation of superlattice interdiffusion using this technique. First, we studied Hg desorption, sputter rates, and preferential sputtering on molecular‐beam epitaxy (MBE) and liquid phase epitaxy grown HgTe and CdTe standards as well as HgCdTe samples at room temperature and at −126 °C to understand the effect of electron and ion beams on these materials and subsequent analysis. Next, we were successful in obtaining a high‐resolution Auger SDP of an MBE grown ‘‘square‐wave’’ HgTe–Hg0.15Cd0.85Te (52 A/80 A) and a...

Influence of ion mixing on the depth resolution of sputter depth profiling

During sputter depth profiling, the bombarding ions for sputtering also produce ion mixing at interfaces. As a result, ion bombardment changes the original composition profile of the specimen, setting a limit on the depth resolution of the sputter depth profiling technique. The origin and magnitude of the effect of ion mixing on the depth resolution of sputter depth profiling have been studied using x‐ray photoelectron spectroscopy, Auger electron spectroscopy, and Ar+ sputter depth profiling of Pt/Zr, Pt/Mo, Pt/Ti, and Pt/Ni interfaces. It is shown that the effect of ion mixing is an intrinsic limitation on the depth resolution; it depends on the thermodynamic properties, such as the heat of mixing and the cohesive energy, of the solids under examination. The magnitude of the effect of ion mixing is compared with that of the instrument‐dependent limitations on the depth resolution of sputter depth profiling.

Low‐temperature Auger depth profile study of HgCdTe double‐layer heterojunction

Low‐temperature Auger depth profiling has been investigated and developed for characterizing double‐layer heterojunction device structures of Hg1−xCdxTe/Hg1−yCdyTe/CdZnTe . We report compositional measurements and high‐resolution structural analysis of HgCdTe heterojunction devices using Auger sputter depth profiling at cryogenic temperatures. A liquid phase epitaxy grown graded double‐layer heterojunction (DLHJ) sample of Hg1−xCdxTe/Hg1−yCdyTe/CdZnTe was studied for the development of this technique. One‐half of the sample was Auger depth profiled at low temperature, and the other half was taper etched for stepwise C–V and low‐temperature Auger surface composition measurements for comparison. Using optimized electron and ion beam parameters, the DLHJ gradient composition was measured with depth resolution <225 A to a depth of 1 μm.

Depth profiling and ion‐induced mixing of AlAs monolayers in GaAs

Monolayers of AlAs, in a matrix of GaAs grown by molecular beam epitaxy, were characterized using 1.0 keV O2+ secondary ion mass spectrometry (SIMS) employing sample rotation to reduce uneven sputtering and improve depth resolution. Under optimal conditions, a full width at half maximum resolution of 2.6 nm was obtained. This resolution is discussed in terms of surface roughening, the cascade mixing model, and preferential sputtering. Cascade mixing predicts well the mixing estimated from experimental measurements. In addition, using this SIMS characterization procedure, mixing from 280 keV Ar+ bombardment was studied as a function of depth. The mixing with depth varied as dictated by cascade mixing. However, quantitative estimates of the mixing were only ∼0.2 of the observed values.