J. Irby

Cleaning and passivation of the Si(100) surface by low temperature remote hydrogen plasma treatment for Si epitaxy

This paper presents the results of a study of the hydrogen-passivated Si(100) surface prepared by a remote hydrogen plasma treatment which serves the dual purpose of cleaning and passivating the Si(100) surface prior to low temperature Si epitaxy by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD). The remote hydrogen plasma treatment was optimized for the purposes of cleaning and passivation, respectively. To achieve a clean, defect-free substrate surface, the remote hydrogen plasma process was first optimized using Transmission Electron Microscopy (TEM) and Auger Electron Spectroscopy (AES). For hydrogen passivation, the substrate temperature was varied from room temperature to 250° C in order to investigate the degree of passivation as a function of substrate temperature by examining the amount of oxygen readsorbed on the substrate surface after air exposure. Low temperature Si expitaxy was subsequently performed on the air-exposed substrates without further cleaning to evaluate the effectiveness of the hydrogen passivation. It was found that better Si surface passivation is achieved at lower substrate temperatures as evidenced by the fact that less oxygen is observed on the surface using AES and Secondary Ion Mass Spectroscopy (SIMS) analyses. The amount of readsorbed oxygen on the H-passivated Si surface after a two hour air exposure was found to be as low as 0.1 monolayer from SIMS analysis. Using Reflection High Energy Electron Diffraction (RHEED) analysis, different surface reconstructions ((3 × 1) and (1 × 1)) were observed for H-passivated Si surfaces passivated at various temperatures, which was correlated to the results of AES and SIMS analyses. Epitaxial growth of Si films at 305° C was achieved on the air-exposed Si substrates, indicating a chemically inert Si surface as a result of hydrogen passivation. A novel electron-beam-induced-oxygen-adsorptiom phenomena was observed on the Hpassivated Si surface. Scanning Auger Microscopy (SAM) analysis was performed to study the reaction kinetics as well as the nature of Si—H bonds on the H-passivated Si surface. Preliminary results show that there is a two-step mechanism involved, and oxygen adsorption on the H-passivated Si surface due to electron beam irradiation may be due to the formation of O-H groups rather than the creation of Si—O bonds.

Si atomic layer epitaxy based on Si2H6 and remote He plasma bombardment

Abstract Atomic layer epitaxy (ALE) of Si has been demonstrated by using remote He plasma low energy ion bombardment to desorb H from an H-passivated Si(100) surface at low temperaturea and subsequently chemisorbing Si 2 H 6 on the surface in a self-limiting fashion. Si substrates were prepared using an RCA clean followed by a dilute HF dip to provide a clean, dihydride-terminated (1 × 1) surface, and were loaded into a remote plasma chemical vapor deposition system in which the substrate is downstream from an r.f. noble gas (He or Ar) glow discharge in order to minimize plasma damage. An in situ remote H plasma clean at 250°C for 45 min was used to remove surface O and C and to provide an alternating monohydride and dihydride termination, as evidenced by a (3 × 1) reflection high energy electron diffraction (RHEED) pattern. It was found necessary to desorb the H from the Si surface to create adsorption sites for Si- bearing species such as Si 2 H 6 . Remote He plasma bombardment for 1–3 min was investigated over a range of temperature (250°C−410°C), pressures (50–400 mTorr) and r.f. powers (6–30 W) in order to desorb the H and to convert the (3 × 1) RHEED pattern to a (2 × 1) pattern which is characteristic of either a monohydride termination or a bare Si surface. It was found that as He pressures and r.f. powers are raised the plasma potential and mean free paths are reduced, leading to lower He bombardment energies but higher fluxes. Optimal He bombardment parameters were determined to be 30 W at 100 mTorr process pressure at 400°C for 1–3 min. He was found to be more effective than Ar bombardment because of the closer match of the He and H masses compared with that between Ar and H. Monte Carlo TRIM simulations of He and Ar bombardment of H-terminated Si surfaces were performed 3o validate this hypothesis and to predict that approximately 3 surface H atoms were displaced by the incident He atoms, with no bulk Si atom displacement for He energies in the range 15–60 eV. The He bombardment cycles were followed by Si 2 H 6 dosing over a range of partial pressures (from 10 −7 Torr to 1.67 mTorr), temperatures (250°C–400°C) and times (from 20s to 3 min) without plasma excitation, because it is believed that Si 2 H 6 can chemisorb in a self-limiting fashion on a bare Si surface as two silyl (SiH 3 ) species, presumably leading to a H-terminated surface once again. The Si 2 H 6 dosing pressures and times corresponded to saturation dosing (about 10 6 langmuirs). Alternate Si 2 H 6 dosing and He low energy ion bombardment cycles (about 100–200) were performed to confirm the ALE mode of growth. It was found that the growth per cycle saturates with long Si 2 H 6 dosing at a level which increases slightly with He bombardment time. At 400°C, for 2 min He bombardment at 100 mTorr and 30 W, the growth per cycle saturates at about 0.1 monolayers cycle −1 , while for 3 min He bombardment the Si growth saturates at about 0.15 monolayers cycle −1 . It was also confirmed that the growth is achieved only by using alternate He bombardment and Si 2 H 6 dosing. He bombardment alone for a comparable time (3 min × 100 cycles) causes a negligible change in the Si film thickness (less than 5 A). Similarly, thermal growth using Si 2 H 6 under these conditions for (3 min × 100 cycles) causes negligible deposition (less than 5 A).

The use of Langmuir probe measurements to investigate the reaction mechanisms of remote plasma-enhanced chemical vapor deposition

Langmuir probe measurements of plasma density and electron temperature have been used to investigate the reaction kinetics in remote plasma-enhanced chemical vapor deposition (RPCVD) of Si on Si (100) substrates. The increased growth rate for negative substrate bias indicates that positively charged ions are involved in the deposition reaction. A comparison of growth rate and plasma density data indicates that the growth rate is proportional to the ion flux. It is concluded that the rate limiting reaction in RPCVD is H desorption from the hydrogenated Si surface by ion bombardment.

Advances in remote plasma-enhanced chemical vapor deposition for low temperature in situ hydrogen plasma clean and Si and Si 1- x Ge x epitaxy

Remote plasma-enhanced chemical vapor deposition (RPCVD) is a low temperature growth technique which has been successfully employed inin situ remote hydrogen plasma clean of Si(100) surfaces, silicon homoepitaxy and Si1- xGex heteroepitaxy in the temperature range of 150–450° C. The epitaxial process employs anex situ wet chemical clean, anin situ remote hydrogen plasma clean, followed by a remote argon plasma dissociation of silane and germane to generate the precursors for epitaxial growth. Boron doping concentrations as high as 1021 cm−3 have been achieved in the low temperature epitaxial films by introducing B2H6/He during the growth. The growth rate of epitaxial Si can be varied from 0.4Å/min to 50Å/min by controlling therf power. The wide range of controllable growth rates makes RPCVD an excellent tool for applications ranging from superlattice structures to more conventional Si epitaxy. Auger electron spectroscopy analysis has been employed to confirm the efficacy of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection high energy electron diffraction and transmission electron microscopy have been utilized to investigate the surface structure in terms of crystallinity and defect generation. Epitaxial Si and Si1-xGex films have been grown by RPCVD with defect densities below the detection limits of TEM (~105 cm-2 or less). The RPCVD process also exploits the hydrogen passivation effect at temperatures below 500° C to minimize the adsorption of C and 0 during growth. Epitaxial Si and Si1-xGex films with low oxygen content (~3 × 1018 cm-3) have been achieved by RPCVD. Silicon and Si/Si1-xGex mesa diodes with boron concentrations ranging from 1017 to 1019 cm-3 in the epitaxial films grown by RPCVD show reasonably good current-voltage characteristics with ideality factors of 1.2-1.3. A Si/Si1-xGex superlattice structure with sharp Ge transitions has been demonstrated by exploiting the low temperature capability of RPCVD.In situ plasma diagnostics using single and double Langmuir probes has been performed to reveal the nature of the RPCVD process.

Low‐temperature growth of GexSi1−x/Si heterostructures on Si(100) by remote plasma‐enhanced chemical vapor deposition

Low‐temperature growth processes are needed in order to fully exploit the potential of GexSi1−x/Si heterostructures. Remote plasma‐enhanced chemical vapor deposition has been successful for silicon homoepitaxy at substrate temperatures as low as 150 °C. We report the growth of GexSi1−x/Si heterostructures with values of x between 0.07 and 0.73, and at substrate temperatures of 305 and 450 °C. The films grown at 450 °C have excellent crystallinity, low defect densities, and very abrupt interfaces, while films grown at 305 °C have degraded crystallinity.

Hydrogen desorption on various H-terminated Si(100) surfaces due to electron beam irradiation: Experiments and modeling

Hydrogen desorption from (2×1) and (3×1) H‐terminated Si(100) surfaces due to irradiation by electron beams with 2–5 keV beam energies has been investigated both experimentally and theoretically. Auger electron spectroscopy (AES) has been employed to monitor Si, O, and C signals periodically with continuous irradiation of an electron beam on H‐terminated Si(100) surfaces. An incubation phenomenon is observed in the time evolution profiles of the Si, O, and C AES signals for all H‐terminated Si(100) surfaces. The incubation period is believed to be associated with the time required for desorption of hydrogen from the H‐terminated Si surface as a result of electron beam irradiation. Among (2×1) and (3×1) H‐terminated Si(100) surfaces, the (3×1) surface is found to have greater hydrogen coverage than (2×1) surface. The hydrogen desorption cross section is found to range from 4×10−19 to 8×10−18 cm2 and decrease with increasing beam energy in the 2–5 keV range.

In situ B-doped Si epitaxial films grown at 450° C by remote plasma enhanced chemical vapor deposition: physical and electrical characterization

In situ boron doping of Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition (RPCVD) has been studied using secondary ion mass spectroscopy (SIMS), Hall effect measurements, defect etching in conjunction with Nomarski microscopy, cross-sectional transmission electron microscopy (XTEM), and current-voltage measurements. Boron incorporation is shown to be controllable and electrically active from 7 × 1017 to over 1020 cm-3, with no dependence on process parameters (temperature, rf power, and substrate bias) in the ranges studied, other than the B2H6/SiH4 gas-phase ratio. No change in deposition rate upon introduction of B2H6 dopant gas is seen, contrary to what has been observed in several higher-temperature CVD processes. No defects such as stacking faults are seen under Nomarski microscopy, but a visible haze covers some areas ofin situ B-doped wafers. This haze appears to consist of amorphous cone-shaped structures with their apexes at the substrate-epilayer interface. The origin of the conical defects is believed to be related to some phenomenon at the initiation of growth. In order to evaluate the electrical quality ofin situ B-doped epilayers,P+/N mesa diodes have been fabricated using both homoepitaxial and heteroepitaxial (GexSi1-x)p-type epitaxial films. The electrical junction in these diodes coincides with the (epi-substrate)—interface in the grown films. To avoid interdiffusion or annealing effects during diode fabrication, all processing temperatures were kept at or below 450‡ C. Ideality factors are 1.2-1.3 for all diodes, indicating diffusion-limited transport rather than recombination in the depletion region.

Structural analysis of Ge x Si 1- x /Si layers by remote plasma-enhanced chemical vapor deposition on Si (100)

In this work, remote plasma-enhanced chemical vapor deposition (RPCVD) has been used to grow GexSi1−x/Si layers on Si(100) substrates at 450° C. The RPCVD technique, unlike conventional plasma CVD, uses an Ar (or He) plasma remote from the substrate to indirectly excite the reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions. In situ reflection high energy electron diffraction, selected area diffraction, and plan-view and cross-sectional transmission electron microscopy (XTEM) were used to confirm the single crystallinity of these heterostructures, and secondary ion mass spectroscopy was used to verify abrupt transitions in the Ge profile. XTEM shows very uniform layer thicknesses in the quantum well structures, suggesting a Frank/ van der Merwe 2-D growth mechanism. The layers were found to be devoid of extended crystal defects such as misfit dislocations, dislocation loops, and stacking faults, within the TEM detection limits (∼105 dislocations/cm2). GexSi1−x/Si epitaxial films with various Ge mole fractions were grown, where the Ge contentx is linearly dependent on the GeH4 partial pressure in the gas phase for at leastx = 0 − 0.3. The incorporation rate of Ge from the gas phase was observed to be slightly higher than that of Si (1.3:1).

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

Remote 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 Si l-x Ge x 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 SiO 2 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 10 5 /cm 2 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/Si l-x Ge x 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 1X10 18 cm −3 and 5×10 17 cm −3 , respectively, can be achieved.

Growth of GexSi1-x/Si heteroepitaxial films by remote plasma chemical vapor deposition

GexSi1−x/Si heteroepitaxial thin films have been grown using the low‐temperature remote plasma‐enhanced chemical vapor deposition (RPCVD) approach, in which the substrate is kept remote from the glow discharge, and an Ar plasma is employed to indirectly activate the reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions. Secondary ion mass spectroscopy (SIMS), plan‐view and cross‐sectional transmission electron microscopy (TEM), and in situ reflection high‐energy electron diffraction (RHEED) have been employed to analyze the films with different Ge mole fractions and thicknesses. Abrupt Si/GexSi1−x heterointerfaces with the Ge concentration changing by 10× in about 30 A (SIMS resolution limit) have been achieved. Commensurate growth has been observed for layers whose thicknesses are below the critical layer thicknesses (CLTs). Crystalline GexSi1−x/Si films with high mole fractions of Ge (up to 60%), which are thicker than the CLTs, show relaxation of misfit strain. This results in more...