Adsorption Of SiH4 Research Articles

Impact of hydrogen coverage on silane adsorption during Si epitaxy from ab initio simulations

Epitaxy by Chemical Vapor Deposition (CVD) is a commonly used technique for the growth of Si alloys in microelectronic devices. In this work, we perform Density Functional Theory (DFT) simulations to study the impact of H coverage on the dissociative adsorption of silane on a Si(001) surface. Silane adsorption is systematically found thermodynamically favorable but its kinetics is limited by H2 desorption which exhibits an activation energy of 2.4 eV. In addition, our calculations suggest that hydrogenated surfaces tend to reduce the adsorption activation energies compared to uncovered surfaces. This work provides an atomistic description of the SiH4 adsorption mechanisms and associated energies for the modeling of the epitaxial deposition process using large scale simulation methods.

Theoretical Analysis of Si2H6 Adsorption on Hydrogenated Silicon Surfaces for Fast Deposition Using Intermediate Pressure SiH4 Capacitively Coupled Plasma

The rapid and uniform growth of hydrogenated silicon (Si:H) films is essential for the manufacturing of future semiconductor devices; therefore, Si:H films are mainly deposited using SiH4-based plasmas. An increase in the pressure of the mixture gas has been demonstrated to increase the deposition rate in the SiH4-based plasmas. The fact that SiH4 more efficiently generates Si2H6 at higher gas pressures requires a theoretical investigation of the reactivity of Si2H6 on various surfaces. Therefore, we conducted first-principles density functional theory (DFT) calculations to understand the surface reactivity of Si2H6 on both hydrogenated (H-covered) Si(001) and Si(111) surfaces. The reactivity of Si2H6 molecules on hydrogenated Si surfaces was more energetically favorable than on clean Si surfaces. We also found that the hydrogenated Si(111) surface is the most efficient surface because the dissociation of Si2H6 on the hydrogenated Si(111) surface are thermodynamically and kinetically more favorable than those on the hydrogenated Si(001) surface. Finally, we simulated the SiH4/He capacitively coupled plasma (CCP) discharges for Si:H films deposition.

Electron-enhanced atomic layer deposition of silicon thin films at room temperature

Silicon thin films were deposited at room temperature with electron-enhanced atomic layer deposition (EE-ALD) using sequential exposures of disilane (Si2H6) and electrons. EE-ALD promotes silicon film growth through hydrogen electron stimulated desorption (ESD) that creates reactive dangling bonds and facilitates Si2H6 adsorption at low temperatures. Without hydrogen ESD, silicon growth relies on thermal pathways for H2 desorption and dangling bond formation at much higher temperatures. An electron flood gun was utilized to deposit Si films over areas of ∼1 cm2 on oxide-capped Si(111) substrates. The silicon film thickness was monitored in situ with a multiwavelength ellipsometer. A threshold electron energy of ∼25 eV was observed for the Si film growth. A maximum growth rate of ∼0.3 Å/cycle was measured at electron energies of 100–150 eV. This growth rate is close to the anticipated growth rate assuming dissociative Si2H6 adsorption on dangling bonds on representative single-crystal silicon surfaces. The Si growth rate also displayed self-limiting behavior as expected for an ALD process. The silicon growth rate was self-limiting at larger Si2H6 pressures for a fixed exposure time and at longer electron exposure times. The silicon growth rate versus electron exposure time yielded a hydrogen ESD cross section of σ = 5.8 × 10−17 cm2. Ex situ spectroscopic ellipsometry showed good conformality in thickness across the ∼1 cm2 area of the Si film. Si EE-ALD should be useful for a variety of applications.

Understanding inherent substrate selectivity during atomic layer deposition: Effect of surface preparation, hydroxyl density, and metal oxide composition on nucleation mechanisms during tungsten ALD.

Area-selective thin film deposition is expected to be important for advanced sub-10 nanometer semiconductor devices, enabling feature patterning, alignment to underlying structures, and edge definition. Several atomic layer deposition (ALD) processes show inherent propensity for substrate-dependent nucleation. This includes tungsten ALD (W-ALD) which is more energetically favorable on Si than on SiO2. However, the selectivity is often lost after several ALD cycles. We investigated the causes of tungsten nucleation on SiO2 and other "non-growth" surfaces during the WF6/SiH4 W-ALD process to determine how to expand the "selectivity window." We propose that hydroxyls, generated during the piranha clean, act as nucleation sites for non-selective deposition and show that by excluding the piranha clean or heating the samples, following the piranha clean, extends the tungsten selectivity window. We also assessed how the W-ALD precursors interact with different oxide substrates though individual WF6 and SiH4 pre-exposures prior to W-ALD deposition. We conclude that repeated SiH4 pre-exposures reduce the tungsten nucleation delay, which is attributed to SiH4 adsorption on hydroxyl sites. In addition, oxide surfaces were repeatedly exposed to WF6, which appears to form metal fluoride species. We attribute the different tungsten nucleation delay on Al2O3 and TiO2 to the formation of nonvolatile and volatile metal fluoride species, respectively. Through this study, we have increased the understanding of ALD nucleation and substrate selectivity, which are pivotal to improving the selectivity window for W-ALD and other ALD processes.

Carrier gas effects on aluminum-catalyzed nanowire growth

Aluminum-catalyzed silicon nanowire growth under low-pressure chemical vapor deposition conditions requires higher reactor pressures than gold-catalyzed growth, but the reasons for this difference are not well understood. In this study, the effects of reactor pressure and hydrogen partial pressure on silicon nanowire growth using an aluminum catalyst were studied by growing nanowires in hydrogen and hydrogen/nitrogen carrier gas mixtures at different total reactor pressures. Nanowires grown in the nitrogen/hydrogen mixture have faceted catalyst droplet tips, minimal evidence of aluminum diffusion from the tip down the nanowire sidewalls, and significant vapor–solid deposition of silicon on the sidewalls. In comparison, wires grown in pure hydrogen show less well-defined tips, evidence of aluminum diffusion down the nanowire sidewalls at increasing reactor pressures and reduced vapor–solid deposition of silicon on the sidewalls. The results are explained in terms of a model wherein the hydrogen partial pressure plays a critical role in aluminum-catalyzed nanowire growth by controlling hydrogen termination of the silicon nanowire sidewalls. For a given reactor pressure, increased hydrogen partial pressures increase the extent of hydrogen termination of the sidewalls which suppresses SiH4 adsorption thereby reducing vapor–solid deposition of silicon but increases the surface diffusion length of aluminum. Conversely, lower hydrogen partial pressures reduce the hydrogen termination and also increase the extent of SiH4 gas phase decomposition, shifting the nanowire growth window to lower growth temperatures and silane partial pressures.

Radical Production Processes on Heated Metal Catalyst Surfaces

Free radical species, such as H, O, OH, CH3, and SiH3, can be ejected efficiently to the gas phase by catalytic decomposition of material gases on heated metal surfaces. The detailed reaction mechanisms for the production of these radicals are still hard to be specified, but the mechanisms can be presumed by measuring the radical densities in the gas phase under various conditions. In this review, the results on the detection of these radical species are summarized, and the possible mechanisms taking place on the catalyst surfaces are discussed. The experimental procedures to identify the radical species are also reviewed shortly. In the production of Si atoms from SiH4, the rate determining step changes from Si desorption to SiH4 adsorption with the increase in the catalyst temperature. In the decomposition of H2O, the OH desorption energy depends on the surface coverage. In O2 systems, decomposition takes place not only on the metal surfaces, but also on the second adsorbed layers.

Energetics and Rate Constants of Si2H6 and Ge2H6 Dissociative Adsorption on Dimers of SiGe(100)-2 × 1

Dissociative adsorption of Si2H6 and Ge2H6 on various buckled dimers of SiGe(100)-2 × 1 surface has been studied using density functional theory at the B3LYP level. Two paths, individually leading to (I) two chemisorbed SiH3 or GeH3 and (II) surface hydrides plus a gas-phase SiH4 or GeH4, are considered in the energetics calculation. Contrary to the intuitive mechanism involving a four-center transition state (TS) that yields two SiH3(a) or GeH3(a) in one step, the intermolecular bond scission on a dimer is accomplished in two consecutive steps of three-center TS, in which the first step is rate-limiting. The calculated energy barrier of the first step in Si−Si bond scission is higher than that of the H−Si bond scission on Si*−Si dimer by 9 kcal/mol. Therefore, in contrast to the conventional wisdom of favoring Si−Si bond cleavage over H−Si as the first step in chemisorption on Si(100)-2 × 1, the calculation result supports Niwano and co-workers' experiments that concluded that Si2H6 adsorption without breaking the Si−Si bond was preferred. On the other hand, the Ge alloying on the surface reduces the barrier height difference between these two paths. Hence, the energy blockade between dehydrogenation and intermolecular bond scission decreases when Ge2H6 is added in Si2H6 chemical vapor deposition. Rate constants are also calculated and the results are qualitatively in line with the Ge2H6 reactivity on Si(100) measured by Lam et al.

Modeling and Simulation of Arsenic-Doped-Silicon Low-Pressure Chemical Vapor Deposition

In this paper, we present the modeling and simulation of As-doped-Si low-pressure chemical vapor deposition (LPCVD), in which a Si film growth step using SiH4 and an AsH3 adsorption step are alternately performed (sequential doping is carried out). We focus on the Si film growth in the second step of sequential doping, which involves Si film growth (first step) → AsH3 adsorption → Si film growth (second step). The growth thickness and As concentration in the Si film are evaluated with respect to deposition time. As atoms adsorbed on the Si surface after the AsH3 adsorption step are taken up by the Si layer grown in the second step, and some of these atoms precipitate on the Si film surface. The adsorption of SiH4 is suppressed owing to the effect of the As atoms precipitated on the Si film surface, resulting in a decrease in the growth rate of the Si film. On the basis of this finding, we construct a surface reaction model by determining the sticking coefficient of SiH4 from the growth plots in the second step. The Si film growth profiles on submicron holes are analyzed by topography simulation using the surface reaction model, and simulation results are compared with experimentally obtained results.

High-Speed Rotating-Disk Chemical Vapor Deposition Process for In-Situ Arsenic-Doped Polycrystalline Silicon Films

We have developed high-speed rotating-disk chemical vapor deposition (CVD) equipment for polycrystalline silicon (poly-Si) films. This CVD equipment has an enhanced ability to reduce the boundary layer thickness at a given temperature above a wafer surface, and to suppress vapor-phase reactions. We investigated in-situ arsenic-doped poly-Si film deposition using silane (SiH4), arsine (AsH3) and nitrogen (N2) in a high-speed rotating-disk CVD as functions of AsH3 flow rate and deposition temperature. Both the deposition rate and resistivity decreased with increasing AsH3 flow rate. A deposition rate of 120 nm/min, a resistivity of 16 mΩ·cm, a film thickness nonuniformity of ±5%, and a number of particles of less than 20 (over 200 nm in diameter) were achieved at a deposition temperature of 680°C for in-situ arsenic-doped poly-Si deposition on a 200-mm-diameter silicon (Si) wafer. Moreover, it was confirmed that the concentration of As in the poly-Si film was low at the initial stage of deposition, and that this process has a high gap filling capability in a hole of 0.18 µm width and 7 µm depth. It was also confirmed that there were conditions for a high step coverage of more than 1. These properties are inferred to be due to the adsorbed AsH3 preventing the adsorption of SiH4.

Temperature dependence of the surface reactivity of SiH 3 radicals and the surface silicon hydride composition during amorphous silicon growth

For hydrogenated amorphous silicon (a-Si:H) film growth governed by SiH 3 plasma radicals, the surface reaction probability β of SiH 3 and the silicon hydride (–SiH x ) composition of the a-Si:H surface have been investigated by time-resolved cavity ringdown and attenuated total reflection infrared spectroscopy, respectively. The surface hydride composition is found to change with substrate temperature from –SiH 3-rich at low temperatures to SiH-rich at higher temperatures. The surface reaction probability β, ranging from 0.20 to over 0.40 and with a mean value of β=0.30±0.03, does not show any indication of temperature dependence and is therefore not affected by the change in surface hydride composition. It is discussed that these observations can be explained by a-Si:H film growth that is governed by H abstraction from the surface by SiH 3 in an Eley-Rideal mechanism followed by the adsorption of SiH 3 at the dangling bond created.

Surface science investigations of atomic layer deposition half-reactions using TaF5 and Si2H6

Fundamental adsorption and surface chemistry studies of TaF5 and Si2H6 on a polycrystalline Ta surface were performed in ultra-high vacuum (UHV) in the range 303–523 K to understand if these precursors may be used in applications to grow Ta-based films by atomic layer deposition. TaF5 uptake saturated in UHV conditions for less than 100 L exposure for adsorption on both clean Ta and on 144 L Si2H6-treated Ta. TaF5 dissociatively adsorbed on clean Ta, and F ligands were determined to govern the self-limiting adsorption behavior. The extent of each “half-reaction”, the reaction of TaF5 with a Si2H6-treated surface and the reaction of Si2H6 with a TaF5-treated surface, increased with surface temperature and was dependent on SiHxFy byproduct desorption. Neither Si2H6 adsorption nor Si2H6 half-reaction reached saturation in UHV conditions, as measured by the surface Si concentration. F ligands were removed during the Si2H6 half-reaction, and residual F asymtotically approached a constant value.

Quantum Chemical Study of Silane Decomposition on Hydrogen-Terminated Si(001) Surfaces

The mechanism of SiH4 adsorption to the bare Si defects in a hydrogen-terminated Si(001) surface is analyzed using ab initio molecular orbital calculations based on a cluster model. The energy barriers for the dissociative adsorption of SiH4 are estimated to be 0.24 and 0 eV, depending on the environment around the bare Si defects. This result shows that the bare Si defects become the chemisorption sites of SiH4 in chemical vapor deposition.

Arsenic-doped Si(001) gas-source molecular-beam epitaxy: Growth kinetics and transport properties

Arsenic-doped Si(001) layers with concentrations CAs up to 5×1018 cm−3 were grown on Si(001)2×1 at temperatures Ts=575–900 °C by gas-source molecular-beam epitaxy (GS-MBE) using Si2H6 and AsH3. This is almost an order of magnitude higher than the initially reported “maximum attainable” saturated CAs value for GS-MBE from hydride precursors. At constant JAsH3/JSi2H6, CAs decreases, while the film growth rate RSi increases, with Ts. Temperature programmed desorption measurements show that As segregates strongly to the growth surface and that the observed decrease in CAs at high film growth temperatures is primarily due to increasingly rapid arsenic desorption from the segregated layer. Decreasing Ts enhances As incorporation. However, it also results in lower film growth rates due to higher steady-state As surface coverages which, because of the lone-pair electrons associated with each As adatom, decrease the total dangling bond coverage and, hence, the Si2H6 adsorption rate. At constant Ts, CAs increases, while RSi decreases, with increasing JAsH3/JSi2H6. All incorporated As resides at substitutional electrically active sites for concentrations up to 3.8×1018 cm−3, the highest value yet reported for Si(001):As growth from hydride source gases, and temperature-dependent electron mobilities are equal to those of the best bulk Si:As.

Adsorption of SiH 4 or Si 2H 6 on P/Si(100) at room temperatures

Effects of predeposition of phosphorus on room temperature adsorption of SiH 4 or Si 2H 6 on Si(100) surfaces have been investigated by H 2-temperature-programmed-desorption measurements. As a result, it was clarified that the amount of adsorbed SiH 4 or Si 2H 6 molecules after saturation, as measured from the hydrogen coverage θ H(sat), decreases from that on a bare-Si surface, satisfying the relation θ H(sat)+ θ P=1, with θ P the predeposited phosphorus coverage. This relation indicates that neither SiH 4 nor Si 2H 6 chemisorbs at phosphorus sites at room temperatures. For initial adsorptions, by contrast, the adsorption of SiH 4 or Si 2H 6 were less affected, for which migration of physisorbed molecules from P-sites toward bare-Si sites is suggested.

Migration-assisted Si subatomic-layer epitaxy from Si2H6

Submonolayer by submonolayer Si epitaxy (subatomic-layer epitaxy, SALE) from Si2H6 on Si(001) has been successfully realized independent of the adsorption coverage by repeating self-limited Si2H6 adsorption and surface adatom migration induced by surface thermal excitation with Ar+ laser irradiation and self-resistive heating. With the self-limited Si2H6 adsorption and the migration assist, a substrate temperature window and a laser power window with a constant growth rate and an atomically flat surface have been obtained. The fact conversely indicates that the surface temperature control within the limited temperature range is important during the thermal excitation to obtain the atomical surface flattening. On the basis of the results of the reflection high-energy electron diffraction study on a Si2H6/Si(001) system together with the SALE growth experiments, models for the SALE growth mechanisms and the growth modes are proposed.

Modeling growth in Si gas-source molecular beam epitaxy using Si2H6

Substrate temperature dependencies of growth rate and surface hydrogen coverage for Si GSMBE using Si2H6 have been obtained experimentally. Validity of a growth model has been confirmed by comparing the model prediction with the experimental data. The Si growth can be described as a combination of the Si2H6 adsorption and the surface-hydrogen desorption. A Si2H6 molecule adsorbs on a Si(1 0 0) surface consuming two available dangling bonds, while the desorption of surface hydrogen is characterized as a first-order reaction.

A kinetic study on tungsten deposition from SiH4 and WF6

The reaction kinetics of tungsten deposition from a SiH4/WF6 gas mixture was investigated for SiH4/WF6 feed-gas flow ratios lower than unity. By performing selective depositions, care was taken to ensure that the kinetic data was obtained in the surface-kinetics controlled regime. It was found that the reaction kinetics of the deposition process depends on the SiH4/WF6 ratio. For low SiH4/WF6 ratios (smaller than ≊0.7) the growth rate is slightly negative in [WF6] and almost quadratic in [SiH4]. The growth rate is almost independent of temperature between 200 and 350 °C. On the basis of our measurements it is argued that in this regime the surface is covered mainly with W, WFx, and F, and that the rate-limiting step is the dissociative adsorption of SiH4 on this surface. For higher SiH4/WF6 ratios (0.7≤SiH4/WF6≤1) the growth rate increases sharply as the SiH4/WF6 ratio increases. In this regime the growth rate cannot be expressed in the simple form: rate=A⋅[WF6]α⋅[SiH4]β since the growth rate is more sensitive to the SiH4/WF6 partial pressure ratio than to the absolute values of the SiH4 and WF6 partial pressure. For SiH4/WF6 ratios as high as 1, the growth rate is limited only by the flux of the species hitting the surface.

State-specific study of hydrogen desorption from Si(100)-(2×1): Comparison of disilane and hydrogen adsorption

The desorption of hydrogen from the monohydride species on Si(100) has been studied state specifically using (2+1) resonance-enhanced multiphoton ionization. The monohydride phase was prepared by dosing the surface with either disilane (Si2H6) or atomic hydrogen. Adsorption of disilane with subsequent desorption of H2 leads to the growth of an epitaxial silicon film, based on evidence obtained with scanning electron microscopy and low energy electron diffraction. We report that the rovibrational-state distribution for hydrogen desorbed from Si(100) is the same after both disilane and atomic-H adsorption. Hydrogen desorbs with low average rotational energy but with a population in the v=1 state enhanced by roughly 20 times over a thermal distribution at the temperature of the surface. The agreement between internal-state distributions for both adsorption schemes indicates that the desorption of hydrogen during epitaxial growth of Si after Si2H6 adsorption proceeds in the same manner as that for a hydrogen-prepared Si(100)-(2×1):H surface.

A reflection high-energy electron diffraction study of the Si(111) surface during gas source molecular beam epitaxy

Reflection high-energy electron diffraction was used to monitor Si(111) growth during gas source molecular beam epitaxy using disilane (Si2H6). Depending on the substrate temperature, the growing Si(111) surface was found to exhibit hydrogenated δ(7×7), (1×1), (5×5), and (7×7) reconstructions. Within the substrate temperature range of 490 to 560 °C where growth proceeded two-dimensionally, clear and well-defined intensity oscillations could be observed in the [21̄0] azimuths. Since consecutive atomic layers on the Si(111) surface are nonequivalent, the oscillation periods were found to correspond to bilayer growth. In the two-dimensional (2D) growth regime, the oscillation frequencies, and hence the growth rates deduced, were found to increase with increasing substrate temperature and flow rate of Si2H6. At higher temperatures, there was a change from 2D layer-by-layer growth to step propagation and consequently, the intensity oscillations were weak or absent. At low temperatures (<400 °C) where no dissociative adsorption of Si2H6 occurred, intensity oscillations were not observed.

Self-Limiting Adsorption of SiH<sub>2</sub>X<sub>2</sub> Molecules and Its Application to Si Atomic Layer Epitaxy by Photochemical Processes

Self-Limiting Adsorption of SiH<sub>2</sub>X<sub>2</sub> Molecules and Its Application to Si Atomic Layer Epitaxy by Photochemical Processes