Low Energy Hydrogen Ion Bombardment Research Articles

Ultra-high vacuum direct bonding of a p–n junction GaAs wafer using low-energy hydrogen ion beam surface cleaning

Low-energy hydrogen ion bombardment is used to clean GaAs surfaces. The hydrogen ions produce contamination-free surfaces without changes in surface composition (stoichiometry) and surface roughness. The wafers were brought into contact at room temperature after cleaning under ultra-high vacuum (UHV), and bonded over the whole area (2 inches) without application of external mechanical pressure. After bonding, the p-GaAs/n-GaAs wafer pair was annealed at 200 °C for 30 min under UHV conditions (<5×10 −10 mbar) to improve the interface bonding strength and to achieve a full-area wafer bonding. Infrared (IR) imaging of the as-bonded wafers directly reveal the real bonding behaviour. High-resolution transmission electron microscopy images reveal that the wafers have been directly bonded without damage of the crystal lattice or intermediate layer and the interface is smooth. Current–voltage characterization shows near-ideal forward characteristics and the recombination in p–n junction space charge region.

GaAs surface cleaning by low-energy hydrogen ion bombardment at moderate temperatures

Low-energy hydrogen ion beam treatment is used to remove contaminations of carbon and oxygen from gallium arsenide (GaAs) surfaces. Real time spectroscopic ellipsometry is utilized to monitor the time evolution of the in situ ellipsometric parameters ⟨ε1⟩ and ⟨ε2⟩ during exposure of GaAs surfaces to inert H2 molecules, hydrogen plasma radicals, and hydrogen ions with energies up to Eion=400eV. Model analysis of the ellipsometric data allows the determination of both, the oxide layer degradation and the hydrogen-induced dielectric function alteration of GaAs. The degradation of the oxide layer as well as the changes of the dielectric function of GaAs depends on the surface temperature and hydrogen ion energy. Ellipsometry reveals oxygen clean surfaces for hydrogen ion bombardment with an energy of Eion=300eV at temperatures as low as 150°C, which is confirmed by in situ x-ray photoelectron spectroscopy measurements. The incorporation behavior of hydrogen and therefore the dielectric function alteration depends on the doping type of GaAs. In hydrogen-modified semi-insulating GaAs the observed energy redshift of the critical points E1 und E1+Δ1 is mainly caused by the increase of the intrinsic stress level, whereas in p-type GaAs additionally surface band bending effects are responsible for the change of the dielectric function of GaAs.

Removal of Oxygen from the Si(100) Surface in a DC Hydrogen Plasma

This paper focuses on the removal of oxygen from a Si(100) surface by hydrogen plasma etching under high-vacuum conditions, i.e., the reaction-chamber base pressure is ≈10-7 mbar. The efficiency of oxygen removal from the Si(100) surface in the employed DC hydrogen glow discharge, with the sample at room temperature, is found to depend on the ratio of H2- and H2O partial pressures, the latter of which depends on the reaction-chamber base pressure and the amount of H2O liberated from the reaction-chamber walls by the plasma. This ratio reflects the competition between reduction of the Si surface by atomic hydrogen and reoxidation of the Si surface by residual H2O. A low-energy hydrogen ion bombardment is found to facilitate the reduction of the Si surface oxides. This process is most efficient when the sample potential is kept at 25 V below the plasma potential, i.e., when the surface is struck by 25-V protons.

Reduction of the chemical erosion of doped graphite due to surface modification during low energy D + bombardment

The chemical erosion yield of pure graphite during hydrogen bombardment is strongly reduced by dopants like Ti, Si, or B. Preferential sputtering and changes of the surface composition as well as influences of the dopant-carbon bonding on methane formation consequently change the erosion yield. Surface changes may be accentuated at low energy hydrogen ion bombardment and elevated temperatures. The changes of the surface compositions of titanium doped graphites and of pure TiC during bombardment with D + ions below an ion energy of 1 keV and temperatures up to 1000 K was investigated. Ti concentrations up to 75 at% at room temperature and stoichiometric TiC concentrations at enhanced temperatures were found. Qualitative investigations of the Auger carbon signal fine structure confirms the phase changes. Due to these changes in material composition the chemical erosion yield is suppressed and the erosion dominated by physical sputtering.

Plasma cleaned Si analyzed in situ by x-ray photoelectron spectroscopy, secondary ion mass spectrometry, and actinometry

Silicon surfaces are cleaned in an electron cyclotron resonance excited hydrogen plasma and characterized by in situ x-ray photoelectron spectroscopy and in situ static secondary ion-mass spectrometry. Emission spectroscopy and actinometry are used to characterize the hydrogen plasma. Exposure to the plasma for 3 to 4 minutes without applying heat or bias to the substrate completely removes the native silicon oxide resulting in a hydrogen terminated surface that is resistant to reoxidation. Adventitious hydrocarbon, when present on the surface, is also completely removed by the plasma. A shift in the isotope ratios of silicon suggests that a clean 〈100〉 silicon surface is monohydride terminated, whereas a 〈111〉 silicon surface appears largely dihydride terminated. A depth profile of the silicon isotope ratios shows a temporal instability, which with the assignment of a H 1s state in the valence-band spectra provides evidence that the hydrogen is concentrated at the surface and has not diffused deep into the silicon lattice. The oxygen removal rate has the following characteristics: two distinct microwave operating regimes separated by a discontinuity in power around 600 W; a singularity corresponding to rapid oxygen removal at 2.5 mTorr; an abrupt and near monotonic decrease in oxygen removal above 14 mTorr; and an invariance of the removal rate to ion-energy from about 10 to 100 eV. The density of hydrogen excited species and the ground state hydrogen atom density are correlated with the oxygen removal rate under all conditions except high pressure, where the density of hydrogen ions is low. This suggests an ion-induced etching mechanism whereby the native silicon oxide removal is enhanced with low-energy hydrogen ion bombardment.

Removal of a Thin SiO2 Layer by Low-Energy Hydrogen Ion Bombardment at Elevated Temperatures

Low-energy (0.5, 1.0 keV) hydrogen ion bombardment at elevated temperatures (450–500°C) was shown to be useful as a surface cleaning method to remove a thin oxide layer (presumably SiO2) on Si substrates. In situ Auger electron spectroscopy (AES) analysis indicated that the SiO2 layer dissociated under the presence of ion implanted hydrogen at elevated temperatures. Neither hydrogen ion bombardment at room temperature (RT) nor postannealing after the RT ion bombardment had any removal effect regarding the oxide layer. By comparison, He+ ion bombardment, combined with postannealing at 500°C, produced a Si-rich SiOx surface, but the surface oxide could not be removed completely. Consideration of hydrogen and helium ion bombardments indicated that a chemical sputtering effect caused by the ion implanted hydrogen atoms in SiO2 played a significant role in the removal of SiO2.

Importance of Surface Preparation in Direct Ion Beam Deposition(Ibd)

ABSTRACTFundamental aspects of direct ion beam deposition (IBD) are discussed, stressing surface preparation and contamination problems. Residual gas contamination, ion beam induced metal contamination, and presence of surface native oxide before deposition are shown to be the major factors hindering low temperature epitaxial growth in IBD. A low energy hydrogen ion bombardment is demonstrated as an effective surface preparation method to remove surface native oxide in the case of silicon deposition.

Low-energy hydrogen ion bombardment damage in silicon: An i n s i t u optical investigation

I n situ and spectroscopic ellipsometry measurements have been applied to probe damage mechanisms induced by low-energy hydrogen ion bombardment of single-crystal silicon. Ion beam voltages from 20 to 1500 V have been employed, covering the range found to passivate grain boundaries in polycrystalline Si and near-surface defects in dry etched single-crystal Si. The spectroscopic data for room-temperature samples after bombardment provide information needed to model the in situ data, obtained using a fixed photon energy of 3.4 eV at a nominal sample temperature of 200 °C. Models for the in situ ellipsometry data for ion energies &amp;lt;500 V allow determination of the evolution of damaged surface layers as a function of exposure time. Two layers dominate the time evolution of the optical data: (1) a polycrystalline Si (p-Si)-like layer, modeled as an effective medium mixture of c-Si, a-Si, and void, which develops on the scale of 1 min (dose: 6×1017 ions/cm2), and (2) a more slowly evolving transparent layer, interpreted as an oxide. The latter layer, also observed in earlier studies, may be attributed to oxidation caused by vacuum residual oxygen ions in the hydrogen ion beam. The oxidation consumes the top-most p-Si-like region of heaviest damage.

Surface blistering of metals due to low energy hydrogen bombardment.

Surface blistering of metals due to low energy hydrogen ion bombardment, determining solar absorptance change in gold-plated specimens