Smooth Si Surfaces Research Articles

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Abstract Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp3) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼1010 cm−2 were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp3 carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp3 carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.

Limiting Si/SiO2 interface roughness resulting from thermal oxidation

The changes effected by oxidation on purposely roughened and initially smooth Si surfaces are followed via atomic force microscopy and spectroscopic ellipsometry and a technique called spectroscopic immersion ellipsometry. Initially, rough and smooth Si surfaces yield opposite roughening trends upon thermal oxidation. Rough surfaces become smoother, and smooth surfaces become rougher, ultimately yielding a limiting roughness of about 0.3 nm root-mean square. A consideration of the distribution of surface roughness features plus the thermodynamics of small features are used to explain these trends. It is also reported that the changes of interface roughness are primarily the result of the oxidation reaction and not from the high temperatures.

Preparation of smooth Si(001) surfaces by glancing angle sputtering

Abstract The (0 0 1) surface of Si is irradiated with 30 keV Ga + ions at various angles of incidence. The surface topography of the irradiated sample is observed with atomic force microscopy (AFM). While the roughness increases with fluence at angles of incidence smaller than 75°, the roughness does not change or even decreases with fluence at glancing angles. The root-mean-square (RMS) roughness decreases from 0.4 to 0.15 nm at the incident angle θ i =85°, indicating that atomically smooth surfaces can be prepared by the glancing angle sputtering. The mechanism of the surface smoothing by glancing angle sputtering is discussed in terms of specular reflection of the incident ion and step edge erosion.

H-terminated silicon: spectroscopic ellipsometry measurements correlated to the surface electronic properties

A correlation was established between the morphological structure of the Si surface caused by chemical preparation and its electronic interface properties by combined monitoring of (i) the surface roughness and hydrogen and oxide coverage by ex situ (UV-VIS-IR) and in situ (UV-VIS) spectroscopic ellipsometry (SE), and (ii) the density D it,min and energetic distribution D it( E) of interface states by surface photovoltage (SPV) measurements. With these systematic investigations a wet-chemical H-termination procedure was optimized and very smooth Si(111) surfaces without any native oxide coverage were prepared, characterized by an intrinsic surface state distribution and a very low surface state density D it,min<1.5×10 10 cm −2 eV −1. On these surfaces the resonant absorption due to the Si–H bonds was directly observed by infrared ellipsometry.

A spectroscopic immersion ellipsometry study of SiO 2-Si interface roughness for electron cyclotron resonance plasma and thermally oxidized Si surfaces

By immersing a film-covered substrate in a liquid whose refractive index matches that of the film, the sensitivity of ellipsometry to the film/substrate interface can be greatly enhanced. This spectroscopic immersion ellipsometry (SIE) technique was employed for the study of SiO 2-Si interface roughness for electron cyclotron resonance (ECR) plasma and thermally oxidized Si surfaces. The SIE results show that for an initially rough Si surface, oxidation reduces the interface roughness, while for an initially smooth Si surface, the interface roughens as the oxidation proceeds. These results were confirmed by atomic force microscopy measurements with fractal analysis. The results indicate that there are two competing mechanisms for the interface roughness evolution during oxidation. The smoothing is believed driven by the reduction of free energy dictated by the Kelvin equation, and enhanced by the external electric field during ECR oxidation. The roughening is possibly due to a chemical-reaction-enhanced roughening transition. The results also show that SIE emerges as a powerful technique to study interfaces non-destructively.

Comparison of Si/SiO2 interface roughness from electron cyclotron resonance plasma and thermal oxidation

The effects of microwave electron cyclotron resonance (ECR) plasma oxidation on smooth as well as purposely roughened Si surfaces were studied using atomic force microscopy with fractal analysis and the results were compared to thermal oxidation. In addition, spectroscopic immersion ellipsometry was employed to obtain information on the Si/SiO2 interface without the necessity of physically removing the oxide overlayer. The results are concordant and show that during both the ECR plasma and thermal oxidation processes, the Si/SiO2 interface roughness decreases for the purposely roughened Si surfaces, but increases for initially smooth Si surfaces. Besides the initial surface roughness dependence, the change of this interface roughness depends mostly on the extent of oxidation, but also on temperature. The decreasing roughness for plasma oxidation is thought to be driven by the Kelvin equation, but enhanced by an applied external electric field. Ion bombardment, localized defects, and nucleation are eliminated as causative for roughening of initially smooth Si surfaces, and the results suggest a chemically enhanced roughening transition.

Characterization of chemically prepared Si-surfaces by uv-vis and IR spectroscopic ellipsometry and surface photovoltage

Roughness and oxide coverage of silicon (Si) surfaces after various chemical treatments were investigated using ex situ spectroscopic ellipsometry (SE) both in the ultraviolet-visible (uv-vis) and in the infrared (IR) spectroscopic region. For control and optimization of electronic interface properties the large-signal field-modulated surface photovoltage technique was used for contactless measurement of the surface band-bending and the energetic distribution of interface states D it( E). Applying a special hydrogen (H)-termination procedure, very smooth Si(111) surfaces without any native oxide coverage were prepared, characterized by a U-shaped intrinsic surface state distribution and very low surface state density D it.min < 3 × 10 10 cm −2 eV −1. On these surfaces the resonant absorbance due to the SiH bonds was observed directly by IR ellipsometry. By comparing the SE data and interface state distributions of differently treated silicon surfaces the density of intrinsic and extrinsic surface states was correlated to the surface roughness and oxide coverage.

Hydrogen desorption from ion-roughened Si(100)

Low energy Kr+ ions were used to create rough Si(100) surfaces. Hydrogen adsorption and desorption on these surfaces were studied using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A comparison of hydrogen TPD data from smooth and ion-roughened Si(100) surfaces showed no significant shift for the dihydride and monohydride related desorption features, revealing that ion impact roughening does not change the activation barriers for hydrogen desorption. In contrast to smooth Si(100), HREELS spectra from rough Si(100) surfaces with different hydrogen coverages showed a dihydride scissor mode at 0.8 ML and dihydride desorption below monolayer coverage in the corresponding TPD experiment. The vibrational spectra of hydrogen covered rough surfaces generally showed a smaller ratio of the scissor mode/bending mode intensity than the smooth Si(100) surfaces, suggesting a lower dihydride/monohydride coverage ratio. The corresponding TPD results on the other hand showed increased dihydride desorption and higher total coverages up to 2 ML. Although different dihydride units on terrace sites and at edge sites could not be unambiguously identified in the vibrational spectra, they provide a reasonable explanation for this inconsistency.

New approach to preparing smooth Si(100) surfaces: Characterization by spectroellipsometry and validation of Si/SiO2 interfaces properties in metal-oxide-semiconductor devices

Wet chemical removal of thermally grown SiO2 layers on Si(100) substrates has been studied as a function of the pH of the etching solutions in the range of −0.32–1.6 by adding controlled amounts of H2SO4 to a 1:30 HF:H2O mixture. Characterization of the stripped Si(100) surfaces by spectroellipsometry showed that the smoothest surfaces were obtained at a 1:0.50:30 HF (49 wt %):H2SO4(98 wt %):H2O etch with a pH of approximately 0.5. Electrical characterization of metal-oxide-semiconductor (MOS) capacitors fabricated on these surfaces with oxide layers prepared by remote plasma enhanced chemical vapor deposition showed (i) the lowest density of interface traps, Dit, (ii) the lowest tunneling currents, J0, and that (iii) the highest breakdown fields, EBD, occurred at the same pH value that produced the smoothest surfaces. In contrast, MOS capacitors fabricated with high-temperature thermally grown oxides were not significantly affected.

Impact of Chemomechanical Polishing on the Chemical Composition and Morphology of the Silicon Surface

ABSTRACTThe polishing technology used for manufacturing ultraflat and smooth Si surfaces on a large scale is the chemomechanical polishing (CMP) technique. This technique combines the chemical corrosive removal of silicon atoms and the mechanical transport of the agents. The removal rates strongly depend on the interaction of mechanical parameters and the chemistry involved in the polishing process like the pH of the alkaline polishing slurry used. Removal of Si during CMP is explained by a nucleophilic attack of OH− to silicon atoms catalyzing the corrosive reaction of H2O resulting in cleavage of silicon backbonds. Characterization of the surface chemistry of the silicon wafer after polishing by X-Ray Photoelectron Spectroscopy and High-Resolution Electron Energy Loss Spectroscopy reveals an oxide free, predominantly hydride covered silicon surface displaying hydrophobic properties. Morphological features like microroughness as well as localized surface irregularities on the silicon surface, also referred to as Light Point Defects, depend on different strongly interacting process parameters. Microroughness is reduced by CMP by several orders of magnitude as characterized by lightscattering techniques and Atomic Force Microscopy.

In situ investigation of silicon surface cleaning and damage by argon electron cyclotron resonance plasmas

An argon electron cyclotron resonance (ECR) plasma process has been optimized to successfully remove oxide films from a silicon surface at elevated temperatures leaving smooth Si surfaces devoid of an amorphized silicon damage layer. Etch rates of over 10 nm/min have been achieved at ion energies below 100 eV. The low ion energy (−50 V dc bias) and high ion fluxes (1×1016 ions/cm2 s) represent a significant improvement from conventional Ar ion sputter cleaning processes. In situ spectroscopic ellipsometry and ex situ atomic force microscopy were used to characterize the surface condition during and after cleaning to establish a 700 °C argon plasma cleaning process for silicon. Real-time single wavelength ellipsometry was used to study the cleaning kinetics, determine the optimal end point, and elucidate a controversy about the level of damage in the argon ECR plasma cleaning process.

Early stages of H 2O adsorption on clean Si(100)

Clean and smooth Si(100) surfaces have been exposed to low doses of water vapor, up to 100 L at most, under ultra-high vacuum. Structural and electronic property changes of the surface have been studied by LEED, AES and photoemission yield spectroscopy. Special attention has been payed in order to get a quantitative indication of the oxygen coverage, by a careful calibration of the Auger lines. It is found that the first stage of adsorption saturates when all the dangling bonds are removed, at an oxygen coverage of 0.25 monolayer. This result is in contradiction with the model where H 2O dissociates into H and OH, each one removing one dangling bond, which would give 0.5 monolayer at saturation. In agreement with similar observations made in the dissociative interaction of NH 3 with Si(100), we propose that either H or OH is able to remove the two dangling bonds of a surface Si dimer.

In Situ Low Temperature Cleaning and Passivation of Silicon by Remote Hydrogen Plasma for Silicon-Based Epitaxy

ABSTRACTRemote Plasma-enhanced Chemical Vapor Deposition (RPCVD), which involves nonthermal, remote plasma excitation of precursors, has been demonstrated to be a novel and attractive technique for low temperature (150-450C) Si and Sil-xGex epitaxy for applications in Si ULSI and novel Si heterostructure devices which require compact doping profiles and/or heterointerfaces. An in situ low temperature remote hydrogen plasma clean in the Ultra-High Vacuum (UHV) deposition chamber in order to achieve a chemically passive, hydrogenated Si surface with minimal O, C and N contamination, is a critical component of the process. The ex situ wet chemical cleaning consists of ultrasonic degreasing and a modified RCA clean, followed by a final dilute HF dip. The in situ clean is achieved by remote plasma excited H, where H introduced through the plasma column is r-f excited such that the plasma glow does not engulf the wafer. In situ AES analysis shows that the remote H plasma clean results in very substantial reduction of the C, O and N contamination on the Si surface. We believe that the H plasma produces atomic H which, in turn, produces a reducing environment and has a slight etching effect on Si and SiO2 by converting them to volatile byproducts. TEM analysis of the wafers subjected to this clean indicate that defect-free surfaces with dislocation loop densities below TEM detection limits of 105 /cm2 are achievable. Corroborating evidence of achieving an atomically clean, smooth Si surface by remote H plasma clean as obtained from in situ RHEED analysis will also be presented. After in situ H cleaning at low pressures (45 mTorr), typically for 30 min. at a substrate temperature of 310 C, we observe both stronger integral order streaks compared to the as-loaded sample and the appearance of less intense half-order lines indicative of a (2 × 1) reconstruction pattern, indicating a monohydride termination. A (3 × 1) reconstruction pattern is observed upon H plasma clean at lower temperatures (250 C), which can be attributed to an alternating monohydride and dihydride termination. Results of air exposure of hydrogenated Si surfaces by AES analysis indicate that the (3 × l) termination is chemically more inert towards readsorption of C and 0. Successful Si homoepitaxy and Si/Sil-xGex heteroepitaxy under a variety of surface cleaning conditions prove that by a combination of these cleaning techniques, and by exploiting the inertness of the H-passivated Si surface, very low defect density films with 0 and C levels as low as 1X1018 cm−3 and 5×1017 cm−3, respectively, can be achieved.

Limited thickness epitaxy of semiconductors and Si MBE down to room temperature

Si MBE on smooth Si(100) surfaces is shown to occur at temperatures down to room temperature. We demonstrate that Si deposition at constant temperature becomes amorphous after growth of a limiting epitaxial thickness h epi. At room temperature, h epi is ≈ 10–30 Å and increases rapidly at higher temperatures with a rate-dependent activation energy in the range 0.4–1.5 eV. Experiments suggest that neither impurity segregation nor defect build-up cause the ultimate nucleation of the amorphous phase, and the effect may be linked to surface roughening during growth at low temperatures. The possibility that some step arrangements may be sufficient to trigger nucleation of the amorphous phase is discussed. We show that dopants are active even at very low growth temperatures, so that limited-thickness epitaxy provides a solution to the “doping problem” of thermally-activated surface segregation of dopants in Si MBE. Our observations of Si/Si(111), Ge/Si(100), and GaAs/GaAs(100) suggest that limited-thickness epitaxy may occur in MBE deposition of almost any semiconductor.

Surface and Near-Surface Atom Dynamics During Low Energy Xe Ion Bombardment of Si and Fcc Surfaces

ABSTRACTSurface and near-surface processes have been studied during low energy Xe ion bombardment of Si (001) and fcc surfaces using molecular dynamics simulations. Defect production is enhanced near the surface of smooth Si (001) surfaces with respect to the bulk in the energy range 20–150 eV, but is not confined exclusively to the surface layer. The extent and qualitative nature of bombardment-induced dissociation of small fcc islands on an otherwise smooth fcc (001) surface is found to depend strongly on island cohesive energy.

The Si(001)/SiO2 interface

The atomic structure of Si(001)/SiO2 interfaces, produced by the oxidation of initially smooth Si surfaces, is discussed. Soft X-ray spectroscopy shows the detailed interfacial atomic configuration to depend sensitively on the preparation conditions.

Formation of low-temperature Al/n-Si Schottky contacts using a partially ionized beam deposition technique

High quality Al/n-Si Schottky contacts have been fabricated using the partially ionized beam (PIB) deposition technique in a conventional vacuum condition without post heat treatment. The electrical characteristics of the diodes were extremely uniform across a 3 in. wafer and were stable with respect to a furnance annealing performed at 450 °C for 30 min. Scanning electron microscope examination revealed much shallower pit formation on the Si surface, as compared to that deposited by conventional means, following the heat treatment. A completely smooth Si surface was observed after a 10 s rapid thermal annealing at 450 °C. The observed behavior could be attributed to the creation of ‘‘contact openings’’ in the native oxide during deposition by ion bombardment, allowing Al to make an intimate contact to Si. This PIB deposition of metal-semiconductor contacts may be an important metallization scheme for the future low-temperature processing of shallow junctions for very high speed integrated circuits.