Masaru Takakura

The Role of Fluorine Termination in the Chemical Stability of HF-Treated Si Surfaces

The principal role of silicon-fluorine bonds in the chemical nature of HF-etched Si surfaces has been investigated by angle-resolved X-ray photoelectron spectroscopy. The native oxide growth thickness and the fluorine coverage have been systematically measured as functions of HF concentration, pure water rinse time, air or N2+O2 gas exposure time and gas phase H2O concentration. It is shown that the native oxide growth is strongly suppressed by the existence of Si-F bonds of about 0.12 monolayers on the surface. This is explained by a model in which Si-F bonds chemically stabilize the surface reactive sites such as atomic steps as supported by the result of the layer-by-layer oxidation of Si.

Layer-by-Layer Oxidation of Silicon

Growth kinetics of native oxide on HF-treated Si surfaces terminated with Si-H bonds has been studied by angle-resolved x-ray photoelectron spectroscopy. The oxide growth rate in pure water for an n + Si(100) surface is significantly high compared to that of p + , and the n or p type Si oxidation rate is in between. This is explained by the formation of ions through electron transfer from Si to adsorbed O 2 molecules and the resulting enhancement of the oxidation rate. The oxide growth on Si(100) is faster than (110) and (111) as interpreted in terms of the steric hindrance for molecular oxygen adsorption on the hydrogen terminated silicon 1×1 surface structures.

Cleaning and Oxidation of Heavily Doped Si Surfaces

HF-treated Si surfaces and the oxidation kinetics in pure water or in clean room air have systematically been studied by x-ray photoelectron spectroscopy (XPS). The oxidation of heavily-doped n-type Si appears to proceed parallel to the surface, resulting in the layer-by-layer oxidation. The oxide growth rate in pure water for heavily-doped n-type Si is significantly higher than that of heavily-doped ptype Si. This is explained by the electron tunneling from the Si conduction band to adsorbed O 2 molecules to form the O 2 state. O 2 ions easily decompose and induce a surface electric field, enhancing the oxidation rate. The growth rate of native oxide on heavily-doped n-type Si is less sensitive to the crystallographic orientations than the case of lightly doped Si where the steric hindrance against oxygen molecules significantly lowers the oxidation rate of the (110) and (111) surfaces. We suggest that the decomposed oxygen can penetrate into Si without steric hindrance. It is also found that the oxidation of heavily-doped n-type Si in pure water is effectively suppressed by adding a small amount (10 ∼ 3600 ppm) of HCI.

Early stage of silicon oxidation studied by in situ X-ray photoelectron spectroscopy

In this study, a clean silicon surface is oxidized in a UHV chamber and the surface suboxide compositions are analyzed using in situ X-ray photoelectron spectroscopy. It is found that the predominant suboxides are Si2O3 and SiO irrespective of crystallographic orientations in the early stages of oxidation. This is interpreted in terms of a significant number of atomic steps existing on the clean Si surface. The observed chemical shift of O(1s) core level signal is explained by the partial charge transfer from the first and second nearest-neighbor silicon atoms to oxygen.

Chemical Bonding Features of Fluorine and Boron in BF2+-Ion-Implanted Si

The chemical bonding configurations of fluorine and boron atoms in a BF2+-ion-implanted Si network have been studied by using X-ray photoelectron spectroscopy, infrared absorption and Raman scattering measurements. It is concluded that fluorine atoms in as-implanted Si are mainly incorporated as BF bonds. By annealing at 900°C, the BF bonds are thermally decomposed to form four fold-coordinated acceptors as well as thermodynamically stable SiFx bonds (x=2, 3) in the matrix.

BF2+ Ion Implantation into Very-Low-Temperature Si Wafer

BF2+ ions at 30 keV were implanted into silicon wafers held at -100°C or room temperature. The recrystallization process of the implanted wafers was examined by Rutherford backscattering (RBS) and Raman scattering. The value of χmin for wafers implanted at -100°C and annealed at 600°C is 0.05, close to that of crystalline Si, while it is 0.08 for the room-temperature implantation. The Raman spectrum for the case of low-temperature implantation also exhibits a smaller amount of residual defects than that for the room-temperature implantation.

Structural and Electrical Characterization of U Ltra-Thin SiO 2 Grown on Hydrogen-Terminated Silicon Surfaces

The surface microroughness of Si(100) wafers has been studied by FT-IR-ATR. The final wafer clean in an 0.1% HF + 1% H 2 O 2 aqueous solution significantly improves the hydrogenterminated surface morphology as demonstrated by a sharp SiH 2 stretching vibration peak accompanied with the weak SiH and SiH 3 peaks. The ultra-thin gate oxide grown on such surface exhibits nearly ideal tunneling current transport. The cleaning in 4.5% HF reduces the SiH 2 peak height and enhances SiH 3 , making the surface rough. Nevertheless, the tunneling characteristics are hardly influenced with such spectral change.

In-Depth Profiling of Suboxide Compositions in the SiO2/Si Interface by Angle-Resolved X-Ray Photoelectron Spectroscopy

A clean silicon surface was oxidized in a UHV chamber and in-depth profiling of suboxide compositions was carried out by using angle-resolved X-ray photoelectron spectroscopy. The existence of Si-H bonds near the oxide surface is implied because the background hydrogen in the UHV chamber reacts with the oxide surface. This hydrogenated Si bond looks like Si3+ in the Si2p spectrum. It is also shown that the SiO2/Si interface is atomically abrupt.

Native Oxide Growth and Hydrogen Bonding Features on Chemically Cleaned Silicon Surfaces

The most advanced MOS devices now utilize the gate oxide as thin as 140 A Future giga-bit memory with a minimum feature size of 0.14 µm will need 50 A thick gate oxide with sufficient reliability and precise thickness uniformity. This implies that a few-angstrom-thick native oxide formed on a silicon wafer must be completely removed or otherwise the native oxide thickness must be exactly controlled and the surface should be kept clean until the wafer is loaded to the furnace. Also, the microroughness on the wafer must be minimized to get the flat Si02/Si interface. Native oxide on the silicon surface has currently been removed by diluted HF treatment. The surface is chemically stable compared to the atomically clean surface because the Si dangling bond is terminated with hydrogen1. Also, Si-F bonds remaining on the silicon surface after the HF treatment appears to passivate chemically reactive sites2. The pH modified BHF treatment of Si(111) surfaces and further boiling or room temperature rinse in ultra pure water have led to the formation of atomically-flat hydrogen-terminated surfaces as demonstrated by surface sensitive infrared spectroscopy3~6. The atomically flat surface is hardly oxidized for more than 300min, while the rough surface with many atomic steps or microfacets is easily oxidized5. It is difficult to prepare an atomically flat Si(100) surface by employing the HF or BHF treatment. The oxidation kinetics of hydrogen terminated Si(111) and (100) surfaces and the SiO2/Si interface structure will provide further insight on the nature of chemically treated silicon surfaces.