Yuichi Hirofuji

Si-Beam Radiation Cleaning in Molecular-Beam Epitaxy

A new surface-cleaning process for Si-MBE, termed Si-beam radiation cleaning (Rad clean), and a model of the cleaning process have been examined. Epitaxial Si layers of high quality have been obtained as a result of the formation of an inactive, very clean natual oxide, removal of this oxide together with contaminants slightly adsorbed on it, and dispersion of remaining contaminants without their developing into defect nuclei. Epitaxial Si layers with etch pit densities of less than 103/cm2 have reproducibly been obtained by carrying out growth at 500°C following the Rad clean process at 800°C for 2 min.

New Experimental Findings on Stress Induced Leakage Current of Ultra Thin Silicon Dioxides

A new degradation mode (named B mode) of Stress Induced Leakage Current (SILC) of 4nm-thick ultra thin silicon dioxides is proposed. It is shown that B mode SILC is a local area and temperature dependent phenomenon, which is different from the previous reported mode (named A mode). There is a close relationship between dielectric breakdown (time to railure) and B mode SILC (time to B mode shift). It is also shown that both dielectric breakdown and B mode shift concern with the SiO2 /Si interface roughness. Also the essential difference on the reliability evaluation between constant current and constant voltage stressing due to the existence of B mode SILC is proposed.

Effect of Bottom Oxide on the Integrity of Interpolysilicon Ultrathin ONO (Oxide/Nitride/Oxide) Films

The bottom oxide was found to play an important role on the integrity of interpolysilicon ultrathin oxide/nitride/oxide (ONO) films. Natural oxides grown on phosphorus‐doped polysilicon by diffusion from source contain defects and weak spots, which degrade the integrity of ONO films grown on these. On in situ phosphorus‐doped LPCVD polysilicon, defect‐free oxides can be obtained and introduce much better yield in ONO films. Dielectric strength measurement by voltage ramping method cannot separate this difference of integrity. We were able to discriminate this difference only by time dependent dielectric breakdown measurement in which negative voltage was applied to the upper polysilicon. The intrinsic wearout phenomena of ONO films, which have been observed on single‐crystal silicon surfaces, and depend only on the thicknesses of bottom oxide, nitride, and top oxide, were observed also on polysilicon if the bottom oxide has grown on in situ phosphorus‐doped LPCVD polysilicon.

MOLECULAR BEAM EPITAXIAL GROWTH OF InP.

InP thin crystalline films were grown on (100)-oriented GaAs by molecular beam epitaxy (MBE) and evaluated by RHEED, SEM and IMA. Thin films with high crystalline quality were obtained when the substrate temperature was about 240°C and temperatures of In and P cells were 840~880°C and 370~400°C, respectively. Furthermore, Sn atoms were easily doped into InP during MBE growth, and the surface morphology of InP was greatly improved by Sn doping.

PIG-Type Compact Microwave Metal Ion Source

A compact microwave metal ion source using PIG (Penning lonization Gauge) geometry has been newly developed. This modified PIG ion source introduces a microwave discharge and can generate stable and high density plasma. The size of the source is 60 mm in diameter and 80 mm long. The microwave discharge power is 20~60 W at a frequency of 2.45 GHz. A tantulum ion beam current of 3.2 µA has been measured with the extraction aperture (2 mm diameter) at an extraction voltage of 13 kV.

Low-Energy Double-Ion -Beam Deposition System

A low-energy double-ion-beam deposition system for high-quality thin-dielectric-film formation has been developed. The system consists of two beam lines (a mental ion beam line and a gas ion beam line) and has a new type of ion-beam deceleration electrodes which can be moved like a folding screen. In this system, decelerated Ta+ and oxygen ion beams of about 80 and 180 µA/cm2, respectively, were obtained in a final-energy range of 100~200 eV. In a deposition test, by simultaneously irradiating with a mass-separated Ta+ ion beam and an oxygen ion beam, pure and soichiometric Ta2O5 films of 50 nm thickness were obtained in one hour at room temperature.

Orientation dependence of crystal defects formation in Si molecular beam epitaxy

The origin of crystal defect in epitaxial Si film grown by the molecular beam epitaxial (MBE) method has been studied. Examination went down to the detail of crystal defect density, unintentionally doped carrier concentration, and carbon contaminations of the epitaxial film with both substrate orientations of (100) and (111). The crystal defect originated from the epitaxial interface has been affected by the excessive carbon content formed at the interface during the substrate cleaning process, because both the defect and the amount of carbon with (100) are statistically much lower than those with (111). Both unintentionally doped hole concentration which has a run number dependence and the crystal defect formed during the epitaxial growth have been much smaller with (100) than with (111). A similar trend in the run number has also been observed in terms of the residual gases of CO and CO2 in the vacuum chamber. From these facts, it has been concluded that characteristics of the MBE film grown on the (111...

Doping of Trench Side-Walls Using an Arsenic Planar-Type Solid-Diffusion Source (S-D Source) and Analysis of Doping Uniformity by Secondary Ion Mass Spectroscopy (SIMS)

We developed a new method of doping side-walls of sub-micron-width trenches using an arsenic planar-type solid-diffusion source at a low pressure. The sheet resistivity, which was measured on the wafer surfaces, is found to be controlled by the oxygen concentration in the tube. Arsenic was doped uniformly in the 6-inch diameter wafer with a standard deviation of 2.7%. The doping was reproducible; the standard deviation of the doping for many batches was 3.0%. A new method of evaluating uniformity of trench side-wall doping using SIMS was also developed. The doping uniformity is confirmed to be within the accuracy of the SIMS measurement.

Characteristics of Penning ionization gauge type compact microwave metal ion source

A sputtering ion source utilizing both microwave and Penning ionization gauge (PIG) ionizations for the primary ion beam deposition is described. The size of the source is 60 mm in diameter and 80 mm long. The continuous ion beams of refractory metals (Ta, W, Mo) with the ion currents of 100 to 150  μA are obtained for gas pressure on the order of 10−3 Torr in the plasma production chamber, microwave power under 40 W, and PIG discharge power under 80 W. This source has been operated with two different discharge modes, either glow discharge or arc discharge. Operation of both modes becomes quite stable by introducing microwave power. Metal ion production by glow discharge is more efficient in power consumption than that by arc discharge.

Sputtering ion source with simultaneous use of microwave and Penning‐ionization‐gauge discharge

Characteristics of Penning‐ionization‐gauge‐type compact microwave metal‐ion source are described. This ion source can operate with two different discharge modes: the positive‐resistance region mode and the negative‐resistance region mode. In the positive‐resistance region, a normalized emittance of a Ta+‐ion beam is 1.8×10−7 m rad, the plasma density is 4×1012 cm−3, and the electron temperature is about 6 eV. In the negative‐resistance region, the emittance is 2.7×10−7 m rad. This ion source is suitable for operation in the positive‐resistance region where the microwave discharge is dominant.