Bombardment-induced Gibbsian Segregation Research Articles

Rapid fabrication technique for aluminum optics by inducing a MRF contamination layer modification with Ar+ ion beam sputtering.

Aluminum optics are widely used in modern optical systems because of high specific stiffness and high reflectance. Magnetorheological finishing (MRF) provides a highly deterministic technology for high precision aluminum optics fabrication. However, the contamination layer will generate on the surface and bring difficulties for the subsequent processes, which highly limit the fabrication efficiency and precision. In this study, characteristics of the contamination layer and its formation process are firstly revealed through experimental and theoretical methods. Impurities such as abrasives are embedded into the aluminum substrate causing increasing surface hardness. The influence of the contaminant layer on machining accuracy and machining efficiency is analyzed in this study. Based on the analysis, ion beam sputtering (IBS) is induced as a contamination layer modification method. Impurities will be preferential sputtered during the process. Surface hardness and brightness will restore to the state before MRF. Moreover, the thickness of the contamination layer reduces dynamically during IBS because of the bombardment-induced Gibbsian segregation and sputter yield amplification mechanism. Consequently, we proposed a combined technique that includes MRF, IBS and smoothing polishing. Comparative experiments are performed on an elliptical shape plane surface. The results indicate that the efficiency has been increased sevenfold and surface precision is also highly improved. Our research will promote the application of aluminum optics to the visible and even ultraviolet band.

Surface composition changes in Au-Cu alloys: interplay of mass-related preferential sputtering and bombardment-induced segregation

The surface composition changes caused by heating and ion sputtering were studied in five Au–Cu alloys using the AES calibration curve method. A new phenomenon was found in ion-sputtered alloys: component enrichment at the near-surface region (beneath the topmost layer) is strongly related to the bulk composition. For copper-riched alloys, gold will be enriched at the near-surface region; for gold-riched alloys, copper will be enriched at the near-surface region. This phenomenon does not occur in heated alloys, so the factors governing thermo-activated segregation are excluded. Concerning the factors relating to ion sputtering, we suggest a combination of bombardment-induced Gibbsian segregation (BIGS) and mass-related preferential sputtering, both of which are related to bulk composition. Copyright © 1999 John Wiley & Sons, Ltd.

Calculation of sputtering yields under high-fluence bombardment of a Cu xNi 1-x alloy

Under 2 keV Ar ion bombardment of a Cu xNi 1−x alloy, at high fluence and at low temperature, Cu and Ni sputtering yields, the total sputtering yield (Cu+Ni yields), and the sputtering yields from the first surface layer and from beneath this layer for Cu and Ni, have been separately calculated by an improved kinetic BIGS (bombardment-induced Gibbsian segregation) model. Calculations indicate that these sputtering yields are strongly dependant on the Cu (segregating species) bulk composition. A cause of the possible correlation between sputtering angular distribution and bulk composition of the segregating species is also analysed for the segregating system.

Dependence of surface-compositional and angular distributions on the bulk composition in the sputtering of CuPt and CuNi alloys

Under 1 keV Ar ion bombardment of CuPt and CuNi alloys at high fluence at low temperature, the depletion degree of the segregating species at the second surface layer, and the normalized sputtering yield ratios (non-segregating species/segregating species) in the near-normal direction (15°) and at oblique angles (75°), have been estimated by an improved kinetic BIGS (Bombardment-Induced Gibbsian Segregation) model. For both the bombarded CuPt and the bombarded CuNi alloys, calculations show that the depletion degree of the segregating species at the second surface layer is small when its bulk composition equals the extreme value, but this depletion degree becomes large when its bulk composition equals the intermediate value. Furthermore, the difference of the normalized sputtering yield ratio between 15° and 75° is small when the bulk composition of the segregating species equals the extreme value, but this difference becomes large when that of the segregating species equals the intermediate value.

Ion beam induced reduction of metallic cations in yttria-zirconia

Samples consisting in polycrystalline films of 8 wt.% Y 2O 3ZrO 2, 0.8 mm thick, deposited by plasma spray technique, were irradiated with 1–4 keV Ar ions in the fluence range between 1 × 10 16 and 7 × 10 17 ions/cm 2. The formation of radiation-induced Zr(III) and Zr(II) as well as Y(II) suboxides is demonstrated by using X-ray photoelectron spectroscopy (XPS). In particular, reduced species start to be observed when the energy of the irradiating ions is higher than 1 keV, while the relative amount of the low valence cations produced for a given fluence in the altered layer has been found to depend on the primary ion energy. The results obtained by angular dependent XPS (ADXPS) analysis suggest that the in-depth distribution of the Zr and Y reduced species also depends on the primary ion energy. In particular, for samples irradiated with 2 keV Ar ions the Zr (and Y) reduced species are detected at the surface, while for samples irradiated with 4 keV ions the reduced species are found to be mostly localized in depth. The observed irradiation effects are explained in terms of bombardment-induced Gibbsian segregation (BIS) mechanism.

XPS and LEISS studies of ion bombarded GaSb, InSb and CdSe surfaces

Ion bombardment effects in GaSb, InSb and CdSe crystal surfaces were studied with a combination of X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS). Both XPS and LEISS showed that neglecting surface contamination, the composition of the crystal surfaces prior to ion bombardment were close to their stoichiometric values of 1:1. During 3 keV Ar + beam bombardment, XPS analysis showed that the atomic ratios of Sb Ga , Sb In and Se Cd in the bombarded surfaces decreased from 1 to 0.71, 0.92 and 0.87, respectively. LEISS results, indicated that these ratios increased to 3.33, 1.63 and 1.32, respectively. The variances between XPS and LEISS measurements are attributed to a difference in the sampling depth between the two techniques, and give a clear indication of how ion bombardment changes the surface composition of these compound semiconductors. These changes may be described in terms of bombardment-induced Gibbsian segregation.

Matrix effect of bombardment-induced Gibbsian segregation on Cu depletion at a Cu x Ni100?x subsurface

A dynamic Monte-Carlo program, including an improved BIGS (Bombardment-Induced Gibbsian Segregation) model was employed to study the matrix effect of BIGS and its influence on Cu depletion at the subsurface under 1 keV Ar-ion bombardment of a Cu−Ni alloy. The calculation results show that not only at high fluences (the steady state) but also at low fluences, the Cu subsurface depletion depends on its bulk composition. This is because the atomic jump rateW21 of Cu from the second layer to the first layer is correlated to Cu bulk composition at any fluence. We also found that the product of the non-segregating species concentration at the first layer with the segregating species concentration at the second layer plays a more important role than other parameters in determiningW21 under low current conditions.

Matrix effect of bombardment-induced Gibbsian segregation on Cu depletion at a CuxNi100?xsubsurface

Matrix effect of bombardment-induced Gibbsian segregation on Cu depletion at a CuxNi100?xsubsurface

Matrix effect of bombardment‐induced Gibbsian segregation on Cu depletion at a CuXNi100−X subsurface

A dynamic Monte Carlo program, including an improved bombardment‐induced Gibbsian segregation (BIGS) model was employed to study the matrix effect of BIGS and its influence on Cu depletion at the subsurface under 1 keV Ar ion bombardment of a Cu–Ni alloy. The calculated results show that not only at high fluence (the steady‐state) but also at low fluence the Cu subsurface depletion depends on its bulk composition. This is because the atomic jump rate W21 of Cu from the second layer to the first layer is correlated to Cu bulk composition at any fluence. We also found that the product of the nonsegregating species concentration in the first layer with the segregating species concentration in the second layer plays a more important role than other parameters in determining W21 under low current conditions.

Influence of bombardment-induced Gibbsian segregation on alloy sputtering

This is a summary of the topic concerning the influence of bombardment-induced (or enhanced) Gibbsian segregation (BIGS or BEGS) on alloy sputtering, to which we have contributed ourselves experimentally over the past ten years. The following topics are treated: (1) What is BIGS? (2) Why was BIGS missed by some experimentalists? (3) Some new controversial points concerning BIGS. (4) Detection of BIGS by using different energy Auger line combinations (DEALC). (5) Detection of BIGS by using angle-resolved Auger electron spectroscopy (ARAES), and (6) the significance of BIGS.

Quantitative Auger electron spectroscopy analysis of the Au-Ni system

Quantitative AES analysis was carried out for the Au-Ni system. The aim is to discuss whether we can use different-energy Auger transitions to determine the changes of composition in depth and how we can optimize the results. Using nine in-situ-scraped standard samples with compositions from 10 to 90 at% Au, eleven calibration curves, corresponding to eleven combinations of Au and Ni Auger transitions with energies from 43 to 2111 eV, were obtained. Particularly discussed was the method by which reliable calibration curves were obtained. This was especially true for the Auger transitions of Au(95 eV) and Ni(102 eV): one of which may have a very weak intensity at corresponding extremes of composition. These calibration curves were then used to detect surface composition changes in five alloys (Au90Ni10, Au70Ni30, Au50Ni50, Au30Ni70 and Au10Ni90) after bombarding them with 1 keV Ar ions. The results suggest that a bombardment-induced Gibbsian segregation (BIGS) existed in the bombarded Au-Ni alloys and the BIGS ratio is strongly dependent on the bulk composition. This article demonstrates how to resolve complicated near-surface profiles.

Enrichment transformation between individual elements in the near-normal direction during binary alloy sputtering

Evolutions of angular distributions and surface composition profiles of a sputtered Pt 0.5 Cu 0.5 alloy were simulated under 1 keV Ar ion bombardment at normal incidence, with the help of a dynamic Monte Carlo program including preferential sputtering and BIGS (bombardment-induced Gibbsian segregation) processes etc. It is found during the PtCu alloy sputtering that the Cu enrichment in the near-normal direction is regularly transformed into the Pt one as the Ar ion fluence increases. Below a low fluence of about 1.7 ions/cm 2 , Cu is preferentially ejected in the near-normal direction; above 1.7 ions/cm 2 , on the other hand, Pt is preferentially ejected in the near-normal direction instead of Cu. Effects of preferential sputtering and BIGS processes on enrichment transformation between Cu and Pt elements in the near-normal direction are also discussed.

Surface composition changes caused by Ar ion bombardment of Au-Ni alloys

Surface composition changes caused by 1 keV Ar ion bombardment of a series of Au-Ni alloys at room temperature were studied. Using 11 combinations of different energy Auger spectra, the variation of the composition with depth from ∼ 0.4 to 1.5 nm in a bombarded Au-Ni alloy was obtained. We found that a drastic variation in concentration took place within the first few layers. This is evidently indicating that a bombardment-induced Gibbsian segregation exists in the Au-Ni system. We also found that the degree of gold depletion at the second layer depended strongly on the bulk composition, thus this depletion became slight as the gold content in alloy was high. In addition, we have roughly estimated the concentration of Au at the topmost layer of a bombarded Au-Ni alloy and found that this value tentatively might be smaller than the bulk one. This result differs from two known examples: C(Au, top) > C(Au, bulk) in a bombarded Au-Cu alloy and C(Cu, top) = C(Cu, bulk) in a bombarded Cu-Ni alloy. Comparing our experimental results with a recent theory presented by Kelly, we suggest that a more sophisticated theory should take this new phenomenon into account.

Diffusion and Phase Formation During Combined Heating and Sputter Etching

ABSTRACTSputter etching normally removes material with an overall composition equal to the target composition. At room temperature, this steady-state removal process is usually quickly established by the formation of an altered surface layer which compensates for preferential sputtering, together with an altered subsurface layer which reflects the existence of bombardment-induced Gibbsian segregation (GS). At elevated temperature, ordinary GS, diffusion and the presence of compound phases may strongly affect the development of the steady-state condition. In this paper, we describe two cases in which diffusion and GS become significant processes during sputter etching of a compound material AxB1−x on an underlying substrate of B atoms. In the first case, diffusion is dominated by the substrate species. This case is demonstrated by the sputtering of TiSi2 on (100) Si by 300 eV Ar+. At room temperature, normal sputtering of the TiSi2 layer occurs. At elevated temperature (500–700°C), however, Si diffuses from the substrate to the surface, where it undergoes GS. This allows the development of a steady-state highly selective sputtering of Si atoms, accompanied by movement of the TiSi2 layer into the substrate. In the second case, diffusion is dominated by the non-substrate species. This case is demonstrated by CoSi2, for which the development of steady-state etching occurs in two distinct stages which are strongly temperature-dependent. In this material, Co is the dominant diffusing species, and it is unclear whether or not Si GS occurs. What is clear, however, is that Ar+ bombardment at 500–600°C leads to partial decomposition of the CoSi2 layer and diffusion of Co towards the substrate Si, where new CoSi2 is formed. The combination of sputter etching and elevated temperature is shown to be a controllable environment for compound phase formation at a buried interface.

Sputtering Angular Distributions of Individual Elements for Low-Energy Ar Ion Irradiation of PtCu Alloy

The dynamic Monte Carlo program including a bombardment-induced Gibbsian segregation process was used to calculate angular distributions of individual elements from sputtered Cu0.5Pt0.5 alloy in the Ar ion energy range between 0.2 and 1.5ke V. Calculated results show that for each investigated incident energy, the angular distribution of the Pt element is more forward-pointed than that of the Cu one; the reason may be that different percentage of sputtered atoms (37 similar 45% of Pt and 12 similar 19% of Cu) come from beneath the topmost layer.

Bombardment-induced segregation of Cu in Pt-Cu alloy observed using different energy Auger line combinations

Pt0.19Cu0.81 alloy scraped in situ was bombarded by energetic (0.2–2 keV) argon ions and the surface compositions under steady-state bombardment were determined with different energy Auger line combinations. It is found that the outermost surface is enriched in Cu, but that its concentration rapidly decreases to a minimum at the second layer then increases slowly to the bulk value. This fact strongly suggests the existence of a bombardment-induced Gibbsian segregation even at room temperature.

Bombardment-induced Gibbsian segregation and its role in secondary ion formation

There is good evidence that a phenomenon resembling Gibbsian segregation, i.e. involving a surface composition spike, occurs when alloys are bombarded either in the absence or presence of oxygen. The existence of such segregation means that the non-segregating species will be present in, and be emitted from, atom layer two to an untypical extent. The present work explores the consequences of this unusual situation for secondary ion formation. In particular, emission from atom layer two is shown to be numerically significant for a dilute non-segregating species but the sense of the effect, i.e. which species is non-segregating, is sometimes reported incorrectly. The non-segregating species will show characteristic differences in emission, including preferential emission near the target normal and a slightly narrower energy distribution. There is a possibility that the matrix effect associated with O 2 + bombardment or O 2 flooding in part arises because a non-segregating species passes through a greater thickness of target in escaping.

Energy dependence of the surface composition changes in AuCu alloys under Ar + ion bombardment

The energy dependence of the surface composition changes in AuCu alloys under Ar + ion bombardment has been studied. When the energy of the Ar + ions is higherthan 1 keV, the surface concentration of gold decreases with bombardment time gradually approaching the steady state value, which is somewhat lower than the bulk concentration of gold. Depth profiling indicates that gold is enriched at the outmost layer with a gold-depleted zone beneath it. When the energy of the Ar + ions is lower than 1 keV, however, the surface concentration of gold increases with time reaching a maximum, then decreases toward a steady state value, which is higher than the value of the bulk. Depth profiling indicates that gold is enriched not only at the outmost layer but also in the subsurface region. Bombardment with Ar + ions of 1 keV energy gives steady state concentration of gold, which is close to the bulk value. From these results, some possible explanation of the difference between the present observation and an early result obtained by Gillam are discussed. These results can be regarded as new evidences to support a new point of view on preferential sputtering of binary alloys. This suggests that the so-called preferential sputtering in most binary alloys may be identical to the bombardment-induced Gibbsian segregation, but when ion energy decreases toward a near-threshold condition, the mass-correlating preferential sputtering also play a role.

A summary of the theory of the preferential sputtering of alloys

Preferential sputtering of alloys arises from mass differences, chemical binding differences and bombardment-induced gibbsian segregation. The relation underlying the mass effect with a binary alloy A–B is that the ratio of the sputtered fluxes scales as ( M B/ M A) 2 m where m is a number equal to 1 4 or less. A simple derivation of this scaling is given. The relation underlying the chemical binding effects is that the ratio of the sputtered fluxes scales as U B/ U A where U i is the surface binding energy. A simple derivation which relates U B/ U A to nearest-neighbor binding energies is given. The relations underlying bombardment-induced gibbsian segregation take into account the fact that the steady state composition of the outermost atom layer is governed by mass and chemical binding effects but that of the second atom layer is governed by segregation such that the composition is depressed (for the component subject to segregation) by a factor K. There is no adequate theory of K but it is shown empirically that K is similar to the composition ratio observed in thermally activated gibbsian segregation.