Sputtering of compound semiconductor surfaces. I. Ion-solid interactions and sputtering yields

Several phenomena occur on the surface of a solid when being bombarded by energetic ions. A short general review is given of the major ion-solid interactions on compound semiconductor surfaces. An in-depth discussion is presented of the total sputtering yields of component semiconductors. For this discussion, GaAs is assumed to be the prototype compound semiconductor because most experimental measurements exist for GaAs. To exclude any chemical effects in the sputter yields, only the total sputtering yield data for argon ion bombardment of GaAs are compared with the predictions of the major sputtering theories, with particular attention to the Sigmund theory for linear cascade sputtering. Different proposals of each of the parameters in this theory are presented and compared with the GaAs data. These parameters are the surface binding energy, the nuclear stopping power, and the factor α, which represents the fraction of energy available for sputtering. Use of the different parameters results in a large variation in the predictions. Topics also considered are the angle dependence of the sputtering yields, sputter threshold energy, and channeling effects in the sputter yields of compound semiconductors. Spike sputtering effects are evident in the sputtering yields of GaAs by krypton and xenon ions.     

Quantitative auger electron analysis of homogeneous binary alloys: Chromium in gold

Quantitative Auger electron analysis of Cr/Au alloys with up to 20% Cr has been accomplished. The surface composition of scribed areas were compared to bulk compositions and it was shown that corrections for variation of density, escape depth, and electron backscattering must be included; these corrections change the measured surface Cr concentrations by approximately 15%. Alloy sputter yield ratios have been calculated from surface concentrations after sputtering with Ar or Ne (0.5, 1.0, 1.5, and 2.0 keV). The sputter yield ratio of Cr to Au was 0.5 at 1% Cr (significant preferred sputtering) but was near unity at 20% Cr (no preferred sputtering). The sputter yield ratio was nearly independent of ion species and ion energy. The 2 keV argon ion sputter yields for pure Cr and Au were determined to be 2.0 and 7.9 atoms/ion, respectively. However, the 2 keV argon ion sputter yield for Au in the alloys drops rapidly from 7.9 atoms/ion for pure Au, to 5 atoms/ion at 10–20% Cr. The sputter yield for Cr in alloys (5 atoms/ion) is relatively independent of composition and is 2.5 times higher than the yield of pure Cr. No simple model is known by which pure elements sputter yields could be used to predict alloy sputtering behavior.     

Real-time in situ spectroscopic ellipsometry investigation of the amorphous to crystalline phase transition in Mo single layers

Molybdenum single layers were grown by ion beam sputter deposition onto [001] Si substrates. Argon or xenon was used as sputter gas. The layer growth was monitored by real-time in situ spectroscopic ellipsometry in the visible spectral region. A volume phase transition from amorphous to polycrystalline molybdenum layer growth was observed during the deposition process. The time regime of the phase transition as well as the layer thickness at which the phase transition occurs, depends on the sputter regime, especially on sputter species and deposition-pressure range. The thermodynamic approach of energy minimisation is discussed as the driving force for the Mo phase transition. A moderate backscattered particle bombardment of the growing molybdenum film provides the activation energy for the recrystallisation process. A self-diffusion-like process is made responsible for atomic rearrangement of the entire as-deposited thin-film volume. The molybdenum phase transition is connected to thin-film densification and therefore volume contraction. PACS 68.65.Ac; 81.15.Cd; 78.67.Pt     

Sputter yields in diamond bombarded by ultra low energy ions

Artificial diamond is an ideal material for high power, high voltage electronic devices, and for engineering use in extreme environments. Diamond process development requires parallel development in characterization techniques such as ultra low energy SIMS (uleSIMS), especially in the ability to depth profile for impurities and dopants at high depth resolution.As a contribution to the background knowledge required, we have measured the sputter yields of single crystal high pressure high temperature (HPHT) diamond using O₂⁺, Cs⁺ and Ar⁺ primary ions in the energy range 300 eV to 2 keV. We compare these with yields for silicon and GaAs. We show that the erosion rates with oxygen are ∼10 times what would be expected from ballistic processes and essentially energy independent in the measured range. This result agrees with the anomalously high sputter yield observed in the ion etching context. Conversely, positive ion yields for elements such as boron are very low in comparison with silicon. This points to a reactive ion etching process liberating CO or CO₂ rather than sputtering as the principal erosion process.This is both problematic and beneficial for SIMS analysis. Oxygen can be used to reach buried structures in diamond efficiently, and the effects of the near-normal incidence beam are planarizing as they are in silicon. Conversely, since positive ion yields are low, alternative probes or strategies must be found for high sensitivity profiling of electropositive elements.     

Quantitative study of oxygen enhancement of sputtered ion yields. I. Argon ion bombardment of a silicon surface with O2 flood

A novel quantification approach is applied to determine in situ the amount of surface oxygen within the sputtered particle escape depth during steady-state sputter depth profiling of silicon under simultaneous oxygenation with an oxygen flood gas or with an oxygen primary ion beam. Quantification is achieved by comparing the secondary ion intensities of ¹⁶O that is adsorbed or implanted at the Si surface with the measured peak intensities of a calibrated ¹⁸O ion implant used as a reference standard. Sputtered ion yields can thereby be related to surface oxygen levels. In the present work the dependences of the partial silicon sputter yield Y and of the positive and negative secondary ion useful yields UY(X^±) (X = B, O, Al, Si, P) on the oxygen/silicon ratio, O/Si, in the sputtered flux are studied for ⁴⁰Ar⁺ bombardment of Si with simultaneous O₂ flooding. The silicon sputter yield is found to decrease with increasing flood pressure and O/Si ratio by up to a factor of 3. Both positive and negative secondary ion yields are enhanced by the presence of oxygen at the silicon surface. The useful ion yield of Si⁺ scales non-linearly with the atom fraction of surface oxygen; this behavior is shown to invalidate models that suggest that Si⁺ ion yield enhancement is dominated either by isolated oxygen atoms or by formation of SiO₂ precipitates. In contrast a microscopic statistical model that assumes that local Si⁺ ion formation depends only on the number of oxygen atoms coordinated to the Si atom to be ejected fits the ion yield data quantitatively.     

Diagnostics of Low-Pressure Oxygen RF Plasmas and the Mechanism for Polymer Etching: A Comparison of Reactive Sputter Etching and Magnetron Sputter Etching

We have compared low-pressure oxygen RF plasmas and the etching of photoresist in a reactive sputter etch reactor and in a magnetron etch reactor using Langmuir probe, optical emission actinometry, and mass spectrometry measurements. The Langmuir probe data allow the determination of the plasma ion density and electron temperature, and thus the ion flux onto the substrate. The optical data yield information on the presence of O atoms and O2+ ions. Stable reactant and product species are monitored with a mass spectrometer. The main difference between the two reactors is that in magnetron sputter etching (MSE), the ion flux to the substrate is about an order of magnitude higher, under comparable plasma conditions, than in reactive sputter etching (RSE). This accounts for the higher etch rate in MSE. However, the etch yield per ion is higher in RSE because of the higher ion energy. Etch rates correlate neither with the ion flux to the substrate nor with the density of O atoms in the plasma, but change in parallel with the consumption of reactant gas. We conclude that in etching a polymer in a low-pressure oxygen plasma, the main neutral reactant species are O2 molecules, and an important role of the ions is to remove reaction products from the substrate surface.     

The influence of implanted xenon on the sputtering yield of silicon

Following implantation labeling with either 200 or 270 keV Xe⁺ the sputtering yield of silicon bombarded with 20 keV Xe⁺ has been determined in situ by means of the backscattering technique (Y = 3.0 ± 0.3 (atoms/ion)). Yield enhancement by up to 60% was observed in cases where the implantation-induced xenon concentrations exceeded the saturation concentration during sputtering. The effect is attributed to (i) an increase in energy deposition at the surface introduced by pronounced xenon loading of the target and (ii) lowering of the surface binding energy. As a consequence the energy dependence of the xenon sputtering yield of silicon is expected to be strongly affected by the energy dependence of the xenon saturation concentration in silicon. Available experimental data support this idea.     

Energy spectra and temperature distributions of clusters produced during ion-beam sputtering of metals

A method for evaluating the energy spectra and temperature dependences of the yield of neutral and charged clusters that consist of N≥5 atoms and are produced by ion bombardment of metals is proposed. The results are presented in the form of simple formulas. Theoretical energy spectra of clusters emitted as a result of bombarding niobium, tantalum, and iron targets by atomic ions of gold or xenon and temperature dependences of the yield of silver clusters produced by bombarding the targets with xenon ions are compared with experimental data.

     

Electron production by metastable rare gas atom impact on a molybdenum (110) surface

Electron production by impact of thermal energy metastable helium, argon, and xenon atoms on a molybdenum (110) surface was studied. The electron yield was always highest for a clean surface and decreased considerably when the surface was exposed to either oxygen or carbon monoxide. The change in electronic yield with gas coverage of the surface was most drastic for xenon and smallest for helium metastables. The similarity of the present results to earlier measurements of electron production by low energy noble gas ion impact led us to believe that metastable deexcitation proceeded via resonance ionization followed by Auger neutralization in the present cases.     

The State of Arts of Hall Thruster

Electric propulsion is broadly defined as the acceleration of a working fluid for propulsion by electrical heating and/or by electric and magnetic body forces. Compared with chemical propulsion, electric propulsion has the characteristic of higher specific impulse, lower thrust, lighter weight and longer lifetime. So electric propulsion is generally suitable for satellite attitude control, the orbit transfer and raising, orbit correction, resistance compensate, position keeping, reposi- tion, space exploration and interplanetary flight.     

Simultaneous measurement of ionization and scintillation from nuclear recoils in liquid xenon for a dark matter experiment

We report the first measurements of the absolute ionization yield of nuclear recoils in liquid xenon, as a function of energy and electric field. Independent experiments were carried out with two dual-phase time-projection chamber prototypes, developed for the XENON dark matter project. We find that the charge yield increases with decreasing recoil energy, and exhibits only a weak field dependence. These results are the first unambiguous demonstration of the capability of dual-phase xenon detectors to discriminate between electron and nuclear recoils down to 20 keV, a key requirement for a sensitive dark matter search.     

Angle of incidence effects in a molecular solid

We investigate the influence of the angle of incidence on the sputter yield when bombarding molecular solid, benzene, with C₆₀. Our simulations show that at normal incidence, essentially all of the projectile energy is deposited into the substrate within ∼2.5 nm of the surface. However, at 75° incident angle, only 35% of the projectile energy is deposited within a depth of less than 1.5 nm of the surface while 65% of the projectile energy is reflected. Therefore, important aspects of the collision process which are dependent upon energy deposition, such as sputter yield, ejection depth, and molecule dissociation, may change as the incident angle changes.     

Sputtering and secondary ion yields of metals subjected to oxygen ion bombardment

The yields of sputtering and secondary ion emission of metals under oxygen ion bombardment were simultaneously measured for multi-layer targets with known layer thickness. The sputtering yield of each layered metal was determined by the time to sputter away the thin film. The experimental results revealed that the sputtering yields of metals were reversely proportional to the energy transfer factor in the classical head-on collision model and a linear relationship was found between the ionization potential and a modified degree of ionization which can be expressed by the ionization potential and the secondary O⁺₂ current.     

ToF-SIMS analysis of a fluorocarbon-grafted PET with a gold cluster ion source

Cluster ions have been recognized as a superb primary species in time of flight secondary ion mass spectroscopy (ToF-SIMS) compared with monatomic primary ions, as they significantly enhance the secondary ion yields from bulk samples. Self-assembled monolayers provide an important system for studying the fundamental mechanism involved in the yield enhancement.We used a gold cluster ion source to analyze a new type of self-assembled monolayer: a fluorocarbon-grafted polyethylene terephthalate. In addition to the structure details, which helped to understand the grafting mechanism, ToF-SIMS analysis revealed that fluorocarbon secondary ion yield enhancements by cluster ions were due to the enhanced sputter efficiency. A larger information depth may also be expected from the enhancement. Both mathematical definitions of damage cross-section and disappearance cross-section were revisited under a new context. Another cross-section parameter, sputter cross-section, was introduced to differentiate the beam induced sputter process from damage process.     

Effects of evolving surface morphology on yield during focused ion beam milling of carbon

We investigate evolving surface morphology during focused ion beam bombardment of C and determine its effects on sputter yield over a large range of ion dose (10¹⁷-10¹⁹ ions/cm²) and incidence angles (Θ = 0-80°). Carbon bombarded by 20 keV Ga⁺ either retains a smooth sputtered surface or develops one of two rough surface morphologies (sinusoidal ripples or steps/terraces) depending on the angle of ion incidence. For conditions that lead to smooth sputter-eroded surfaces there is no change in yield with ion dose after erosion of the solid commences. However, for all conditions that lead to surface roughening we observe coarsening of morphology with increased ion dose and a concomitant decrease in yield. A decrease in yield occurs as surface ripples increase wavelength and, for large Θ, as step/terrace morphologies evolve. The yield also decreases with dose as rippled surfaces transition to have steps and terraces at Θ = 75°. Similar trends of decreasing yield are found for H₂O-assisted focused ion beam milling. The effects of changing surface morphology on yield are explained by the varying incidence angles exposed to the high-energy beam.     

Correlation of process parameters and properties of TiO<Subscript>2</Subscript> films grown by ion beam sputter deposition from a ceramic target

The correlation between process parameters and properties of TiO₂ films grown by ion beam sputter deposition from a ceramic target was investigated. TiO₂ films were grown under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. Systematic variations of film properties with the scattering geometry, namely the scattering angle, have been revealed. There are also considerable differences in film properties when changing the process gas from Ar to Xe. Similar systematics were reported for TiO₂ films grown by reactive ion beam sputter deposition from a metal target [C. Bundesmann et al., Appl. Surf. Sci. 421, 331 (2017)]. However, there are some deviations from the previously reported data, for instance, in growth rate, mass density and optical properties.     

电容损耗对脉冲氙灯放电特性的影响

研究了储能电容自身损耗对氙灯放电特性的影响,对脉冲氙灯放电回路进行了分析与改进。在实际电容自身损耗不为零的情况下,将电容等效为理想电容与损耗阻抗的串联,给出了计算氙灯放电特性的改进公式,并详细计算和分析了电容自身损耗增大情况下的氙灯放电电流和放电功率变化情况,计算结果表明:损耗增大会导致氙灯放电电流、功率峰值下降,闪光时间缩短,显著影响激光泵浦效率;电容损耗增大还会导致LC放电回路出现电容反充电现象,影响氙灯正常工作并缩短灯的寿命。实际的储能电容充放电实验证明,电容损耗增大会导致氙灯闪光的波形峰值下降、闪光时间缩短和电极溅射加剧,验证了理论分析的合理性。另外,实验证明环境因素对储能电容自身损耗的增大具有非常显著的影响。     

Temperature-dependent sputtering of metals and insulators

The temperature dependence of the sputter yield and the energy spectrum of sputtered atoms have been investigated on the basis of a standard model for thermal spikes. A high-temperature and a low-temperature regime have been identified in the temperature spectrum making up the evaporation yield. The high-temperature component of the yield as well as the associated energy spectrum are only very weakly dependent on ambient target temperature. The relative variation is the less pronounced the higher the spike temperature. The low-temperature component is associated with the long-time behavior of the spike, and measurable evaporation takes place over time intervals where spikes overlap. The importance of time constants for macroscopic heat transport is pointed out. The results are shown to provide a framework within which experimental results on the temperature dependence of the sputter yield of metals can be explained. The results are also consistent with measured temperature dependences in the sputter yield of insulators.On leave from Instytut Fizyki, Uniwersytet Jagiellonski, PL-30-059 Krakow, Poland     

Defect mediated desorption of the KBr(001) surface induced by single highly charged ion impact

The individual impacts of slow (300 eV/amu) highly charged Xe ions induce nanometer sized pitlike structures on the KBr (001) surface. The volume of these structures shows a strong dependence on the ions potential energy. Total potential sputter yields from atomically flat (001) terraces are determined by imaging single ion impact sites. The dependence of the sputter yield on the ions initial charge state combined with structure formation at low and high-fluence irradiations indicates that agglomeration of defects into complex centers plays a major role in the desorption process induced by the potential energy.     

Electronic excitation of the matrix during drift of excess electrons through solid xenon

It is shown experimentally that the exciton luminescence λ=172 nm) quantum yield excited by excess electrons drifting through solid xenon at 77 K in fields of 10 kV/cm amounts to 20±5 per electron and that luminescence takes place during the entire drift process. A CW bulky discharge through solid xenon (with a current up to 20 A/cm²) is realized, and intense visible luminescence due to excitation of impurities by electron impacts is observed. The prospects for using solid rare gases as matrices for studying processes in low-temperature plasmas and for creating effective electric energy converters in the vacuum ultraviolet range are discussed.