Gas-cluster ion-beam smoothing of chemo-mechanical-polish processed GaSb(100) substrates

Gas-cluster ion-beam (GCIB) processing of surfaces provides individual atoms within an accelerated gas cluster (∼1,500 atoms per cluster), an energy approximately equal to the individual bond energy of the target surface atoms. The gas-cluster beam is thus capable of providing smoothing and etching of the extreme surface of numerous semiconductors, metals, insulators, and magnetic materials. For semiconductor material systems, the gas-cluster processing effect on the surface and subsurface material is of critical interest for device and circuitry application integrity. In the case of III–V GaSb, chemo-mechanical or touch polishing is the final step in the semiconductor-wafer manufacturing process, often leaving scratches of various depths or damage on the polished surface. In this paper, we report the GCIB etching and smoothing of chemical-mechanical polished GaSb(100) wafers. Using a dual-energy, dual gas-cluster source process, ∼100 nm of material was removed from a GaSb(100) surface. Atomic-force microscopy (AFM) imaging and power spectral-density (PSD) analysis shows significant decrease in the post-GCIB root-mean-square (Rms) roughness and peak-to-valley measurements for the material systems. X-ray rocking-curve analysis has shown a 24-arcsec reduction in the full-width at half-maximum (FWHM) of the (111) x-ray diffraction peak of GaSb. High-resolution transmission-electron microscopy (HRTEM) shows the crystallinity of the subsurface of the pre- and post-GCIB surfaces to be consistent, following the 1 × 10¹⁶ ions/cm² total-fluence processes, with dislocation density for both pre- and post-GCIB cases below the HRTEM resolution limit. X-ray photoelectron spectroscopy (XPS) indicates a strong Ga 3p electron binding-energy intensity for gallium-oxide formation on the GaSb surface with the use of an oxygen GCIB process. Analysis of the Ga 3p electron binding-energy peaks in the XPS data in conjunction with HRTEM indicates a higher Ga or GaSb content in the near-surface layer (less stoichiometric-oxide presence) with use of a CF₄/O₂ GCIB process. The same peak analysis indicates that the surface gallium-oxide state is nearly unchanged, except in thickness, with the use of an O₂-GCIB second step. The material results suggest that GCIB provides a viable method of chemo-mechanical polish (CMP) damage removal on group III–V material for further device processing.     

Gallium arsenide surface chemistry and surface damage in a chlorine high density plasma etch process

In an effort to monitor ion-driven surface chemistry in the high density plasma etching of GaAs by Cl₂/Ar plasma chemistries, we have applied mass spectrometry and careful substrate temperature control. Etch product chlorides were mass analyzed while the substrate temperature was monitored by optical bandgap thermometry and as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. By ensuring that the substrate temperature does not deviate during process variations, the changes in product mass peak intensities are a direct measure of changes in the ionassisted surface chemistry which promotes anisotropic etching. Experimental results show that ion-assisted surface chemistry is optimum when sufficient Cl and Cl⁺ are present in the incident plasma flux. These conditions are met at low coupled microwave powers (<300 W) and low total process pressures (<1.0 mTorr) for input gas mixtures of 25% Cl₂ in Ar. Three mechanistic regions are identified for surface chemistry as a function of incident ion energy: 1) largely thermal chemistry for <50 eV; 2) ion-assisted chemistry for 50–200 eV; and 3) sputtering for >200 eV. Photoreflectance measurements of the surface Fermi level show significant damage for ion energies >75 eV. However, in situ and ex situ surface passivations can recover the surface Fermi level for up to 200 eV ion energies, in good correlation to the onset of sputtering and subsurface damage. Thus, anisotropic, low damage pattern transfer is possible for ion energies between 50 and 200 eV.     

碲镉汞红外探测器电极接触研究北大核心CSCD

金属/碲镉汞电极接触是红外焦平面探测器的重要组成部分,对器件的性能与稳定性影响较大。然而在碲镉汞金属化过程中,金属离子能量较高将有可能对碲镉汞表面造成损伤。本文采用了离子束沉积系统生长金属电极,探究了束流、束压以及热处理等条件对碲镉汞红外探测器接触性能的影响。研究表明,碲镉汞在生长电极后表面会受到一定程度的损伤;随着离子能量的升高,对材料表面损伤加剧。在I-V曲线中,电极沉积损伤较大的器件表现出软击穿现象;在热处理后,在一定程度上可以修复电极沉积时能量过大造成的损伤,提高了电极接触性能。     

The influence of nitrogen ion energy on the quality of GaN films grown with molecular beam epitaxy

Since the growth of GaN using molecular beam epitaxy (MBE) occurs under metastable growth conditions, activated nitrogen is required to drive the forward synthesis reaction. In the process of exciting the nitrogen using a plasma or ion-beam source, species with large kinetic energies are generated. Impingement on the growth surface by these species can result in subsurface damage to the growing film, as well as an enhancement of the reverse decomposition reaction rate. In this study, we investigate the effect of the kinetic energy of the impinging nitrogen ions during growth on the resulting optical and structural properties of GaN films. Strong band-edge photoluminescence and cathodoluminescence are found when a kinetic energy of ∼10 eV are used, while luminescence is not detectable when the kinetic energies exceeds 18 eV. Also, we find that the use of conductive SiC substrates results in more homogeneous luminescence than the use of insulating sapphire substrates. This is attributed to sample surface charging in the case of sapphire substrates and subsequent variation in the incident ion flux and kinetic energy across the growth surface. This study clearly shows that the quality of GaN films grown by MBE are presently limited by damage from the impingement of high energy species on the growth surface.     

The effect of boron,oxygen, and fluorine in ion implanted GaAs

Effects of boron, fluorine, and oxygen in GaAs have been investigated by electrical characterization using current-voltage, capacitance-voltage and deep level transient spectroscopy techniques. Ion implantation at 100 keV energy was conducted with doses of 10¹¹ and 10¹²/cm². Carrier compensation was observed in each implanted sample. The compensation effect strongly depended on ion implantation condition and ion species. More free carriers were compensated for higher dose and heavier species; however, severe surface damage would also be induced that degrade electrical performance. Rapid thermal annealing treatment showed the heavier ion implanted samples to be more thermally stable. Defect levels for each implanted species were compared and identified. A native shallow defect E4 was easily removed by ion implantation. In higher dose and heavier ion implantation, both electron and hole traps were induced. However, in some cases, heavier ion implantation also removed native defects. Acceptor-type surface states were created by implantation that degrade material electrical characteristics and also reduce the effect of compensation. The damage induced traps were mostly point-defects or point-defect/impurity complexes as evidenced by sensitivity to heat treatment.     

Pt and W ohmic contacts to p-6H-SiC by focused ion beam direct-write deposition

Low resistance Pt and W ohmic metallizations to p-type 6H-SiC, using focused ion beam (FIB) surface-modification and in-situ direct-write metal deposition without annealing, are reported. FIB (Ga) surface-modification and in-situ deposition of Pt, and W showed minimum contact resistance values of 2.8 × 10⁻⁴ ohm-cm² to 2.5 × 10⁻⁴ ohm-cm², respectively. A comparison with ex-situ pulse laser deposited Pt on surface-modified areas showed comparable contact resistance values and similar behavior. Auger and secondary ion mass spectroscopy analysis showed a significant (∼4% a.c.) incorporation of Ga within a 15 nm distance from the SiC surface with surface-modification. Atomic force micros-copy studies showed that surface-modification process smooths out the SiC surface significantly.     

Glancing-angle ion bombardment for modification and monitoring of semiconductor surfaces

Using glancing-angle ion bombardment for surface modification rather than conventional near-normal incidence ions has the advantages of reducing damage and implantation projected ranges, reducing channeling, reducing sputtering, and preferentially removing surface asperities leading to flat surfaces. The effects of bombardment conditions on the surface morphology and perfection of GaAs (001), InP (001), and Si (001) surfaces are reported. Air-exposed surfaces were cleaned and smoothened to near atomic flatness without damage under optimal conditions. Sputtering yield, measured using film thicknesses and changes in reflection high-energy electron diffraction oscillations, decreased with decreasing incidence angle. The low sputtering yield and minimal damage make a glancing-angle geometry ideal for real-time characterization by ion scattering spectroscopy. Surface composition measurements on single monolayers of InAs on GaAs showed that the glancing-angle Ar beam did not measurably change the In coverage over relatively long times. A new ion beam monitoring technique was also developed that utilizes the advantages of glancing-angle ions. Specular scattering of 3 keV He ions was observed for incidence angles of 2–6° from GaAs (001). Oscillation in the specularly scattered ion current during GaAs growth were observed with periods corresponding to monolayer growth times. The oscillations allow a simple quantitative interpretation based on scattering by adatoms and step edges.     

Low energy ion beam machining of ULE® substrates: Evaluation of surface roughness

In order to apply ion beam figuring (IBF) to final shape correction of the substrates of projection optics for EUVL, ion beam machining characteristics such as high-spatial frequency roughness (HSFR) and mid-spatial frequency surface roughness (MSFR) of ULE^® substrate were investigated. Our previous research confirmed that the surface roughness of the ULE^® machined by Ar⁺ ion beam with energy of 3–10 keV decreases with decreasing the ion beam energy. Therefore, we have conducted our research on ion beam machining of ULE^® substrate by Ar⁺ ion beam with energy from 0.2 to 2 keV. The HSFR and MSFR of the mechanically pre-finished ULE^® substrate were 0.06 and 0.07 nm rms, respectively; whereas, the HSFR and MSFR of the substrate irradiated by Ar⁺ ion beam at energy of 0.3 keV were less than 0.10 and 0.08 nm rms, respectively. The HSFR is the best result among our previous and other current research.     

Processing-texture relationships in dual-unbalanced magnetron deposition of TiN films

There has been sustained interest in using TiN and other sputter deposited thin film materials in electronics applications, such as barrier coatings. However, it is difficult to produce “pin-hole free” coatings using conventional magnetron sputtering, since the high bias potentials required to produce dense films often result in substrate damage. “Unbalanced” magnetron sputtering may offer a low energy alternative since the ion-to-deposited-atom ratio can be greatly increased, permitting the ion-bombardment energy to be reduced to <200 eV, without sacrificing film density, hardness, or adhesion. As has been demonstrated previously, ion energy can have a profound effect on film texture, but what affect the “substitution” of ion flux for ion energy will have on film texture has not been determined. In this work, TiN films were deposited onto M2 steel via “unbalanced” magnetron sputtering in an attempt to correlate changes in film texture and film stress, with ion energy and flux.     

Fabrication of carbon nanofibers using only ion beam irradiation to glassy carbon

Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) are new carbon-based materials. However, the production of CNFs and CNTs is very difficult due to the complicated processes and high temperature involved. Therefore, a method of fabrication is required that enables high throughput at a low cost.Our previous study reported that oxygen ion beam energy of 500 eV applied to glassy carbon (GC) forms the finest pitch conical anti-reflection (AR) structures, and that an irradiation time of more than 24 min fabricates conical AR structures with heights of more than 250 nm. After the fabrication of the AR structures, irradiation by an argon ion (Ar⁺ beam changes the surface morphology, and oblique angle irradiation can form CNFs. Thus, we carried out oblique Ar⁺ beam irradiation on conical carbon protrusions on GC fabricated by oxygen ion beam irradiation. As a result, CNFs have been formed using oxygen and argon ion beam irradiation at room temperature. In addition, multi-wall CNT can be obtained by two-step ion beam irradiation.     

Rutherford backscattering studies on high-energy Si-implanted InP

High-energy Si-implantations into InP:Fe were examined using Rutherford backscattering (RBS) via channeling measurements. Variable-fluence implantations at 3 MeV and variable-energy implantations for a fluence of 3 x 10¹⁴ cm⁻² were done. A damagestudy on the 3 MeV Si-implanted samples by RBS indicated formation of a continuous, buried amorphous layer for a fluence of ≈5 x 10¹⁴ cm⁻². The quality of the crystal in the region of the amorphous layer was poor after annealing at any temperature (≤900° C), indicating that the crystallization during annealing resulted in either a highly defective material or a polycrystal. For samples with damage below the continuous amorphous level, damage recovery is essentially independent of damage concentration. In the variable-energy-implanted samples, the region of damage moved deeper below the sample surface with increasing energy.     

Identification of a Deep Acceptor Level in ZnO Due to Silver Doping

There remains considerable interest in the behavior of acceptors in ZnO, the ultimate goal being the realization of device grade p-type material. Silver is a candidate acceptor, and, in this study, in situ doping of silver was performed during plasma-assisted molecular beam epitaxy. Silver concentrations, as determined by ion beam analysis, ranged between 10¹⁸ cm⁻³and 10²⁰ cm⁻³, with as much as 94% incorporated substitutionally on Zn lattice sites. Variable magnetic field Hall effect measurements detected no evidence of holes, and 4 K photoluminescence was dominated by donor bound excitons. Transient capacitance measurements, however, suggested that incorporated silver had led to the formation of an acceptor, located approximately 320 meV above the valence band edge, indicating that compensation remains a significant issue in determining the conductivity of ZnO.     

Low-Temperature Preparation of Undoped ZnO Films with High Transparency and Conductivity by Ion Beam Deposition

Low-temperature growth of undoped ZnO films with high transparency and low electrical resistance was performed by ion beam sputtering. After systematic testing, resistivity as low as 2.95 × 10⁻³ Ω cm was obtained at a substrate temperature of 150°C, ion source voltage of 850 V, and ion beam current of 30 mA. The transmittance of the ZnO films was in the range of 85% to 90%. Hall measurements showed that a high mobility of 21.41 cm²/Vs was obtained for films less than 200 nm thick. The related microstructures and physical properties were measured and are discussed.     

Strain relaxation in silicon ion molecular beam epitaxy on silicon (001) substrates during aging

Silicon strained epitaxial films were grown on Si (001) substrates by low energy ion beam assisted molecular beam epitaxy. Films grown in the range of 450– 550°C with concurrent Ar⁺ ion bombardment (100 eV) were characterized using x-ray diffraction and transmission electron microscopy and found to be disloca-tion free and ununiformly strained. During aging, the strained layers stay stable until 500°C. Relaxation of most of the strain occurred at temperatures of 500-650°C. At higher aging temperatures, the strained layers relaxed by the formation of dense dislocation structures.     

Effect of annealing on the morphology and optoelectrical characteristics of ZnO thin films grown by plasma-assisted molecular beam epitaxy

The dependence of characteristics of plasma-assisted molecular beam epitaxy-grown ZnO thin films on different postgrowth annealing conditions was investigated. It was found that, under oxygen atmosphere, annealing temperature can profoundly affect the morphological, electrical, and optical properties of ZnO thin films. In particular, the surface morphology changed from a relatively smooth surface before annealing to various island morphologies after annealing above 800°C for samples grown directly on sapphire without a buffer layer. It is speculated that intrinsic stress due to lattice mismatch drives the island formation and the high temperature provides the energy needed for this surface rearrangement. Single-field Hall-effect measurement showed that the carrier concentration improved by an order of magnitude and the mobility increased from about 30 cm²/Vs to ∼70 cm²/Vs by annealing at 750°C. Variable-field Hall effect shows that a model with two carriers, one a degenerate low-mobility electron and the other a higher mobility non-degenerate electron, is needed to explain the transport properties of the thin film. Analysis indicates that annealing at 750°C decreased the carrier concentration and increased the mobility for the high-mobility carrier. Annealing also led to a significant improvement in photoluminescence, with temperatures of ∼750–850°C yielding the best results.     

Ion-assisted molecular beam epitaxy of GaAs on Si(100)

GaAs was grown by molecular beam epitaxy (MBE) and ion-assisted MBE on Si(100) substrates. Three-dimensional (3D) island nucleation, observed during MBE growth, was eliminated during ion-assisted MBE when the ion energyE was >25 eV and the product ofE and the current densityJ was ≈6-12 eV mA/cm². IncreasingEJ to ≈15 eV mA/ cm² resulted in excessive ion damage. Decreasing the substrate temperature from 280 to 580° C during ion-assisted MBE yielded a slight decrease in surface roughness, and flatter surfaces were obtained for lower As⁴/Ga flux ratios. The suppression of 3D island nucleation led to an improvement in the crystalline perfection of thicker GaAs films. For example, the x-ray diffraction rocking-curve full-width-at-half-maximum values for 0.5 μm thick films grown at 380° C decreased from 1700 arcsec to 1350 arcsec when ion irradiation was used during nucleation. IAMBE allowed nucleation of thin, relatively flat-surfaced GaAs films even at 580° C, resulting in FWHM values of 1850 arcsec for 0.14 /μm thick films.     

The effect of dose rate and implant temperature on transient enhanced diffusion in boron implanted silicon

The effect of ion implantation dose rate and implant temperature on the transient enhanced diffusion (TED) of low energy boron implants into silicon was investigated. The implant temperature was varied between 5 and 40°C. The beam current was varied from 0.035 to 0.35 mA/cm². Three different defect regimes were investigated. The first regime was below the formation of any extended defects (5 keV B⁺ 2 × 10¹⁴/cm²) visible in the transmission electron microscope. The second regime was above the {311} formation threshold (2×10¹⁴/cm²) but below the subthreshold (type I) dislocation loop formation threshold. The final regime was above both the {311} and dislocation loop formation threshold (10 keV 5×10¹⁴/cm²). TED for these conditions is shown to be over after annealing at 750°C for 15–30 min. Secondary ion mass spectroscopy results for the three different damage regimes indicate that there is no measurable effect of dose rate or implant temperature on TED of boron implanted silicon for any of the damage regimes. It should be emphasized that the dose and energy of the boron implants is such that none of these implants approached the amorphization threshold. Above amorphization dose rate and implant temperature have dramatic effects on TED, but it appears that below the amorphization threshold there is little effect. These results suggest that for a given energy it is the ion dose not the extent of the implant damage that determines the extent of TED in boron implanted silicon.     

Molecular beam epitaxial growth of BGaAs ternary compounds

B_xGa_1−xAs ternary compounds with boron compositions varying up to x=1% have been grown by molecular beam epitaxy. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements show that grown layers are single crystal with boron composition up to 0.25% and exhibit specular surface morphology. Photoluminescence measurements indicated a monotonic increase in energy bandgap with boron composition up to 0.25%. The layers showed p-type conductivity with hole concentration reaching the low 10¹⁹ cm⁻³ range. Increasing boron concentrations leads to rough surface morphology and reduction in photoluminescence intensity. Initial results indicate that lower growth temperature may be useful for increasing boron incorporation in BGaAs compounds.     

Low energy Xe+ ion beam machining of ULE® substrates for EUVL projection optics – Evaluation of high-spatial frequency roughness

Ion beam figuring (IBF) is a suitable technology for the final shape correction of substrates used in the projection optics of EUVL tools. In order to achieve HSFR below 0.10 nm rms, we have conducted our research on ion beam machining of the ULE® substrate by Xe⁺ ion beam with energy less than 2 keV. The HSFR of the unprocessed ULE® surface was 0.06 nm rms, whereas the HSFR of the ULE® substrate machined by under 0.7 keV Xe⁺ ion beam was less than 0.08 nm rms. This HSFR (0.08 nm rms) is lower than that (0.10 nm rms) of the ULE® substrate machined with Ar⁺ ion beam. Therefore, Xe⁺ ion beam with energy under 0.7 keV can be used for figure error correction of the ULE® substrates for projection optics used in commercially available EUVL exposure tools.     

Observed correlation of Sn oxide film to Sn whisker growth in Sn-Cu electrodeposit for Pb-free solders

Localized cracking of surface oxide has been proposed as a necessary step in the nucleation of Sn whiskers in Sn electrodeposited films. To evaluate the effects of the oxide film on Sn whisker growth, a bright Sn-Cu electrodeposited film was inserted into an ultrahigh vacuum Auger system, cleaned using an Ar⁻ ion beam to remove the oxide film, and aged in the 2×10⁻⁹ Pa Auger system chamber. Whiskers and other features present during Ar⁺ ion cleaning left visible “shadows” on the surface. During aging in the ultrahigh vacuum system, new whiskers, identified by the absence of the telltale shadows, nucleated and grew. Based on these observations, the presence or absence of an oxide film has a minimal effect on Sn whisker nucleation and growth.