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 produce ripples with O+2 produce no observable topography. These results are discussed using several existing theories for ripple formation and growth. Ripple growth and the variations in secondary ion yield are modeled by accounting for the change in local angles of incidence as the ripples grow. This model describes well the variation in secondary ion yield assuming an exponential growth rate. Ripple formation is discussed in terms of a balance between roughening (by sputtering-induced surface stress and by the dependence of the sputtering yield on surface curvature) and smoothing (by both diffusion and ion mixing). Variation in ripple wavelength with energy is not simply explained by these theories. Surface smoothing by cascade ion mixing can, however, make the wavelength, as observed, independent of ion flux. Finally, the possibility of formation of ripples by phase separation within the SiOx surface layer is discussed.

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.

Quantitative depth profiling resonance ionization mass spectrometry of GaAs/AlGaAs heterojunction bipolar transistors J. Vac. Sci. Technol. B 10, 385 (1992); 10.1116/1.586363High resolution secondary ion mass spectrometry depth profiling using continuous sample rotation and its application to superlattice and deltadoped sample analysis 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)/ Alo 3 GIlo. 7 As(5 nm) superiattice. Profiles without sample rotation with 1.0-7.0 keY 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 keY at 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 sample rotation during SIMS depth profiling prevents and can reverse the development of surface topography that both degrades depth resolution and changes secondary ion yield. Thus, interpretation and quantitation of SIMS analysis is facilitated.

In order to provide a consistent judgment on the capabilities and limitations of the sample rotation approach to depth profiling, a round robin (interlaboratory comparison) was organized between four laboratories on identical samples using AES, XPS and SIMS. The sample consisted of an Ni/Cr multilayer with a total of 16 alternating Ni and Cr layers with a single layer thickness of 30 om. Sputter profiling was performed with a rastered beam of 3 keV Ar' ions at an incidence angle of 45" to the surface normal, with and without sample rotation. Test runs were additionally performed with BCR standard samples of 30 om thick Ta,O, layers on Ta. For sample rotation, depth profiles of the Ni/Cr multilayer by AES and SIMS show a marked improvement in depth resolution of about a factor of two for lower sputter depth (30 nm) and four to five for greater sputter depth (450 om). The depth resolution deteriorates with depth for stationary samples, but is found to be independent of depth when using sample rotation. For XPS, the depth resolution improvement for sample rotation is less pronounced. Agreement between the different laboratories and techniques is excellent and clearly demonstrates the capabilities of sample rotation in depth profiling studies.

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 depth corresponding to a depth resolution ΔZ=3.5 nm. Use of AFM enabled determination of the atomic mixing ΔZm and sputtering statistics ΔZss components of depth resolution to be identified directly for the first time. These components were 3.1 and 1.5 nm, respectively.