Satoru Kawazu

Computer analysis of punch-through in MOSFETs

Abstract A two-dimensional numerical analysis is made for MOSFETs having short channel lengths. The short channel MOSFET is especially characterized by the existence of the punch-through current which cannot be explained by the one-dimensional MOSFET models. The two-dimensional analysis makes clear the following facts relating about the punch-through mechanism. The punch-through is a condition in which the depletion layers of the source and the drain connect mutually at the deep region in the substrate even in equilibrium. The punch-through current is injected through the saddle point of the intrinsic potential into the drain region by the electric field from the drain, at the low gate voltages.

Lateral spread of boron ions implanted in silicon

The lateral spread of implanted ions is measured in boron‐implanted silicon by junction delineation. The maximum lateral spread of a p‐type region from a mask edge is about half the junction depth. Typical values are 0.4–0.6 μ in n‐type substrates of 0.1 Ω cm at an implantation energy in the range 75–250 keV with the dose level of 1×1015/cm2, or at 150 keV with the dose in the range 1×1014–5×1015/cm2. These experimental values are well interpreted by the theory in which the lateral spread is expressed as a complementary error function with the standard deviation calculated by the LSS theory.

A numerical analysis of avalanche breakdown in short-channel MOSFETs

Abstract Recently, the electrical characteristics for the short channel MOSFETs (Metal-Oxide-Semiconductor field effect transistor) have become important because of the increasing density of LSIs (Large Scale Integrated Circuits). One of the methods to understand the characteristics of the short channel MOSFETs is the two-dimensional analysis of the MOSFETs, and many studies about threshold voltage and other items have been made by using the two-dimensional method. In this paper, the drain breakdown characteristics for the short channel MOSFETs are calculated by the two-dimensional analysis method. Consequently, one of the phenomena for the short channel MOSFETs, that the breakdown voltage decreases with increase in gate voltage, is reduced to the difference of the electric field strength distribution from that of the long channel MOSFETs. This variation of the electric field distribution is caused by the strong influence of the electric field from the drain upon the considerable region in the substrate of the short channel MOSFETs.

Stable Solution Method for Viscoelastic Oxidation Including Stress-Dependent Viscosity

A new computer program that simulates viscoelastic oxidation of silicon has been developed. Since in this program a tangential procedure is used for time stepping, numerical stability has been improved, and the instability problem that arises from the incorporation of stress dependence into oxide viscosity has been resolved. Thus, oxidation-induced stresses calculated by our program using stress-dependent viscosity have reasonable magnitude over the entire device area. Moreover, in our program, volume expansion due to oxidation of silicon was treated as a dilational strain, as opposed to its treatment as a forced displacement of oxide/silicon interface or a uniaxial strain perpendicular to the interface in most previous programs. Due to this difference, our program can simulate the generation of intrinsic stress below viscous flow temperature (ca. 960°C), and as a result, stress distribution calculated by our program changes drastically at the viscous flow temperature.

Stress compensation in laser diodes

Stress compensation in GaAs‐(Ga,Al)As laser diodes is discussed. The stress distributions in the laser diodes are theoretically simulated by the finite element method. The photoelastic effect is used to observe the actual strain fields. It is found by simulation that the stress can be minimized by optimizing the Si submount thickness. Lasers fabricated with the optimized submount successfully exhibit low strain fields.

Effects of N distribution on charge trapping and TDDB characteristics of N/sub 2/O annealed wet oxide

Wet pyrogenic oxide of different thicknesses was annealed in N/sub 2/O ambient and the N concentration in the films was studied by using SIMS (secondary ion mass spectroscopy). It was found that for a certain annealing time and temperature, the N concentration (at %) increases with decreasing wet oxide thickness and the location of the peak of N is observed near the interface of nitrided oxide and Si substrate. On the contrary, after nitridation the concentration of H is higher in the thicker wet oxide of thickness 100 /spl Aring/ and also does not change much from the surface to the interface. For the thinner wet oxide of thickness 40 /spl Aring/, the concentration of H is less and decreases toward the interface. Gate dielectrics were characterized using high-frequency and quasi-static measurements. After a constant current stress, a large distortion was observed for the N/sub 2/O annealed wet oxide of 98 /spl Aring/ whereas for the N/sub 2/O annealed wet oxide of 51 /spl Aring/ the distortion was small. With increasing stressing time, hole trap is followed by electron trapping for the wet oxide of 98 /spl Aring/ whereas for the N/sub 2/O annealed wet oxide of 51 /spl Aring/, hole trapping increases a little at the beginning and then saturates. From the TDDB characteristics, a longer t/sub BD/ was observed for N/sub 2/O annealed wet oxide of 51 /spl Aring/ compared to 98 /spl Aring/. From the experimental results, it can be suggested that the improved reliability of thin gate oxide is due to the large amount of N concentration near the interface only. Hence for the device fabrication process, if the wet oxide is nitrided in N/sub 2/O ambient, the reliability of gate oxide will be improved in the ultrathin region.

Kikuchi-Band Analysis of X-Ray Photoelectron Diffraction Fine Structure of Si(100) by Precise Angle-Resolved X-Ray Photoelectron Spectroscopy

X-ray photoelectron diffraction fine structure of Si(100) is studied with precise angle-resolved X-ray photoelectron spectroscopy. Polar-angle intensity distributions of the Si 2p photoelectron emission excited by Al Kα X-rays are measured from H-terminated Si(100) along some azimuthal angles, and are qualitatively interpreted using both forward-focusing peaks along zone axes and Kikuchi bands associated with planes of low indices ({220}, {400}, {111} and {311}). To confirm the interpretation experimentally, we prepare ultrathin Si(100) layers on SiO2 from silicon-on-insulator (SOI) wafers. The thicknesses of the ultrathin Si(100) layers are adjusted so that the SOI layer is thick enough to exhibit the forward-focusing peak along [100] but slightly thinner than the thickness in which the contribution of the Kikuchi pattern weakens. It is observed experimentally, for the first time, that the intensities of X-ray photoelectron diffraction fine structure due to Kikuchi lines decreases for such ultrathin Si(100), while the forward-focusing peak remains unchanged, as expected from our interpretation.

Effects of Oxygen Concentration and Annealing Sequence on Microstructure of Separation by Implanted Oxygen Wafer with High-Temperature Annealing

We investigate the influence of oxygen concentration and annealing sequence on the microstructure of the SIMOX (separation by implanted oxygen) wafer. Samples with different oxygen contents are compared. Higher oxygen concentration causes higher density of dislocations in the top silicon layer by the growth and dissolution of precipitates. The two-step annealing, consisting of annealing at 600°C for 8 h and 1350°C for 30 min, increases the density of dislocations because the nuclei of precipitates may effectively be formed before the high-temperature annealing. Another procedure consisting of laser annealing and 1350°C annealing causes no significant effect on the dislocation density.

Enhanced Oxidation of Silicon by Ion Implantation and its Novel Applications

The enhancement of the oxidation of silicon due to ion implantation is observed to be significant (about 2.1 times at maximum) in the case of Sb+, P+, Sn+ and Ar+ implant with doses more than 5 × 1014/cm2. The annealing before oxidation does not affect the oxidation rate. The oxidation rate constant B, so called the parabolic constant, is not affected by ion implantation with any dose examined, but the constant A, which refers to linear rate constant, decreases at the dose about 5 × 1014/cm2. It is concluded that the residual damage is less effective, but the chemical effect caused by the implanted ions to the reaction between silicon and oxygen at the interface of Si-SiO2 plays an important role in the enhancement of the oxidation rate. A novel application method of this effect to the fabrication of E/D MOS IC is also shown.

Simulation and analysis of anomalous plasma etching

This paper deals with a SIMS analysis and a physical simulation of an anomalous reactive ion etching of poly silicon for understanding the physical environments of the plasma processing. The symmetric perturbation of the plasma cylinder affected the surface migration of iron compounds resulting in symmetrically localized anomalolus etching of poly silicon.