Kunihiro Yagi

Germanium and Silicon Film Growth by Low-Energy Ion Beam Deposition

The design and characteristics of a low-energy ion beam deposition system are discussed. In the system, metal ions with an energy of 100 eV are deposited onto the substrate at a current density of 4–5 µA/cm2. Germanium single crystalline films are deposited on germanium (111) and silicon (111) substrate at substrate temperatures above 300°C. In the case of deposition below 200°C, films are found to be amorphous and re-crystallized by annealing above 300°C. When the ion energy over 500 eV is used, sputtering of the substrate is dominant and deposition is not observed for Ge+ ions and the silicon substrate combination. The results demonstrated the feasibility of growing thin film by low-energy ion beam deposition.

Observation of p-n Junctions with a Flying-Spot Scanner Using a Chopped Photon Beam

A flying-spot scanner that employs a chopped photon beam emitted from a cathode ray tube is reported. The photon beam scanns a planar p-n junction put on a metal electrode through a In2O3-coated transparent electrode and a 15 µm-thick mylar spacer. The ac photovoltage is picked up with electrodes through a condensor formed by the spacer. The photovoltage signal modulates brightness of another cathode ray tube to form a scanning image. Mean wavelength and chopping frequency of the photon beam are 507 nm and 2 kHz. Scanning image analysis is done using photocurrent density equations based on a step-like junction model. Three kinds of junctions, a solar cell, a partly-deep junction and a non-uniformly ion implanted one, have been evaluated to show the validity of the present method.

Photoemission study of the SiO2 /Si interface structure of thin oxide films on Si(100), (111), and (110) surfaces

The SiO2/Si interface structure of thin oxide films thermally grown on Si(100), (111), and (110) surfaces under device processing conditions has been investigated using high‐resolution photoemission spectroscopy with synchrotron radiation. The intensity distribution of the so‐called suboxides, Si1+, Si2+, Si3+, displays a strong dependence on the crystallographic orientation of the substrate over the oxidation temperature range from 600 to 900 °C; Si1+ is enhanced in intensity on Si(111) and (110), while the Si2+ intensity is larger than the Si1+ one on Si(100). This orientation dependence is explained in terms of the bond topology of the substrate surface. A Si(110) surface exhibits a rather large Si3+ intensity as compared to Si(100), (111) surfaces, suggesting that Si–Si bonds on the outermost layer of a Si(110) surface are easily broken by oxygen atoms to generate the bridge bond Si–O–Si. The presence of an interfacial Si atom to which hydrogen is bonded is clearly observed. The total suboxide intensity, i.e., the sum of the suboxide intensities depends on both oxidation temperature and substrate orientation, which is interpreted by means of the interfacial roughness and the surface Si atom density of the substrate. It is shown that an ordered crystalline phase of SiO2 is present at the interfacial region. The generation of this phase has a Si(111)‐preferred orientation.

Ac Surface Photovoltages in Strongly-Inverted Oxidized p-Type Silicon Wafers*

Surface photovoltages in Si wafers excited with a chopped 559 nm-wavelength photon beam are analysed using a new half-sided junction model. In this model, the wafer surface with the depletion layer is considered to be one half of the p-n junction. Chopping frequency ranges from 2 Hz through 20 kHz. Four 76 mm-diameter p-type Si wafers having resistivities of 260, 92, 17 and 1.0 mΩ m are used after forming 360 nm-thick wet-oxide layer on their front surfaces. In these wafers, photovoltage increases with resistivity. In three high-resistivity wafers with strongly-inverted surfaces, the inversion capacitances and conductances limit the photovoltages at low frequencies. The obtained inversion time-constant is 7 s for the 17 mΩ m wafer.

Low-energy mass-separated ion beam deposition of materials

Abstract A low energy mass-separated ion beam deposition system was developed. Ge+ ions of 100 μA were obtained at an acceleration energy of 100 eV with a beam spot size of 5 mm o at 5 × 10−8 Torr. Material buildup has been observed at the range of ion energy where the self-sputtering yield is below unity. The incident ion energy dependence of the reactions of the F+ and CFx+ ions with a silicon substrate has been measured, as well as the physical phenomena conventionally observed between impinging ions and a substrate material. Epitaxial growth of Ge films on Ge and Si single crystal substrates has been observed at 300°C with a Ge+ ion energy of 100 eV. Deposited Ge crystal properties were evaluated.

Chemical sputtering yields of silicon resulting from F+, CFn+ (n = 1,2,3) ion bombardment

Chemical sputtering yields of crystalline silicon resulting from mass‐separated, reactive ion bombardments are measured as a function of ion kinetic energy at room temperature. Ions of F+ and CFn+ (n = 1,2,3) are bombarded independently onto a silicon surface in an ultrahigh vacuum (UHV) environment. Evolution rate of SiF4 molecules resulting from surface chemical reaction: Si+4F→SiF4↑, is measured using a quadrupole mass filter. For F+/Si ion bombardment, yield increases monotonically with ion kinetic energy and saturates at 1 keV giving a value of 0.18. For CFn+/Si ion bombardment, yields show maxima at 1200 eV (CF+), 800 eV (CF2+) and 700 eV (CF3+). At ion energy ranges above 1.5 keV, yields for CFn+/Si are about half that for F+/Si. Carbon deposition and scavenging effects are discussed in detail by relating with fluorocarbon ion bombardment.

Anomalous Diffusion of Phosphorus into Silicon

Lattice strains induced by diffusion of phosphorus, arsenic, antimony and tin into silicon are studied through an X-ray double crystal spectrometer. This quantitative experiment gives that the amount of lattice strain induced by diffusion of impurity depends both on the size of impurity atoms diffused into silicon and on the impurity concentration, which agrees with the equation by S. Prussin et al. It also leads to the result as follows; the lattice strain in the diffused layer is compensated in the case of the phosphorus content less than 4×1020 atoms/cm3 by diffusing both tin, whose atomic radius is larger than that of silicon, and phosphorus with smaller radius. Anomaly in the diffraction pattern is also observed in the case of anomalous diffusion of phosphorus under high vapor pressure of phosphorus.

Novel characterization of implant damage in SiO2 by nuclear‐deposited energy

Etching enhancement in through‐implanted SiO2 has been characterized by nuclear‐deposited energy independently of implant conditions. An empirical expression has been proposed to describe the etching rate for any implantation. The enhanced etching has been related to the Si‐O vibrational frequency shift. Etching enhancement has been found to reflect the structural change in SiO2, and to be a good measure of degradation. The structural change of SiO2 stops and the etching rate reaches a maximum for an ion dose corresponding to nuclear‐deposited energy larger than 3.4×1023 eV/cm3. This energy is equal to the total SiO bonding energy (3.8 eV) in a unit volume of SiO2.

Reliability of Nano-Meter Thick Multi-Layer Dielectric Films on Poly-Crystalline Silicon

A guiding principle of designing double layer dielectric film (SiO2/Si3N4) on poly-Si was established in order to scale down VLSIs. The oxidation of Si3N4 reduces leakage current and defect density. The double layer dielectric film with thinner top oxide layer is harder to breakdown. Hence, the thickness of the top oxide layer must be reduced in order to increase the reliability of double layer dielectrics. The perimeter edge of patterned poly-Si electrode has no affect on the reliability of double-layer dielectric films. As a result. the SiO2/Si3N4 was confirmed to be a high reliable nano-meter thick dielectrics on poly-Si.

Synchrotron‐radiation‐stimulated desorption of O+ ions from an oxidized silicon surface

The photon‐stimulated desorption of ions from a naturally oxidized Si(100) surface has been studied using synchrotron radiation (SR). For mass analysis of the PSD ions, the time‐of‐flight method was utilized. Desorption of O+ ions is clearly observed on the surface during exposure to unmonochromatized SR in the vacuum ultraviolet (VUV) region. Si 2p core level photoemission measurements show that the photoemission peak corresponding to silicon oxide is reduced in intensity after exposure to the radiation. The present experimental results indicate the possibility of removing a thin SiO2 layer on a Si(100) surface at low temperatures by exposing the surface to SR in the VUV region.