Hisashi Furuya

Effects of Thermal History on Microdefect Formation in Czochralski Silicon Crystals

The mechanism of formation of microdefects in Czochralski silicon crystals is investigated, with particular attention to the effects of the thermal history in the growth process on the nucleation and growth of oxygen precipitates. The density and size of the nuclei of oxygen precipitates in the asgrown state of a crystal subjected to a special thermal history are analyzed in detail. The behaviors of the nuclei at various temperature ranges of the thermal history are then discussed on the basis of these analyses.

Effects of Thermal History on the Formation of Oxidation-induced Stacking Fault Nuclei in Czochralski Silicon during Crystal Growth

The formation of oxidation induced stacking fault (OSF) nuclei during Czochralski silicon crystal growth was investigated using crystals subjected to in situ annealing for either 1 or 4 hours by halt of pulling during crystal growth. The effects of in situ annealing at temperatures from about 1160° C to 1030° C can be summarized as follows. In positions held at about 1090° C and 1030° C, the radial distributions of OSFs, LSTDs (light scattering tomography defects), and OPs (oxygen precipitates after annealing) changed greatly. In the position held at about 1090° C, large LSTDs were formed but the OSF-ring disappeared in the region where it should have been present if pulling was not halted. In the position held at about 1030° C, the defects in the OSF-ring grew, its width increased, and the OP density both inside and outside the OSF-ring decreased with holding time. However, the radial distribution of defects at the position held at about 1160° C was similar to that for a reference crystal which was not subjected to halt of pulling. These results indicate that these defects were not formed until 1160° C, the large LSTDs were formed at 1160–1090° C, the OSF nuclei were formed at 1090–1030° C, and the OP nuclei inside and outside the OSF-ring were formed below 1030° C. We discussed the influence of point defects upon the formation of OSF nuclei.

Detection of Bulk Microdefects underneath the Surface of Si Wafer Using Infrared Light Scattering Tomography

A new method has been developed to detect bulk microdefects (BMD) underneath the surface of a Si wafer using infrared light scattering tomography. In this method, infrared light enters from the cleaved surface obliquely and is totally reflected at the surface of the wafer, in order to eliminate the effect of scattering at the surface. BMD not only underneath the surface of the polished silicon wafer, but also underneath the wafer surface on which devices are fabricated, are easily detected by this method.

Dependence of crystal nature on the gettering efficiency of iron and nickel in a Czochralski silicon wafer

The gettering efficiency of both iron and nickel in a Czochralski silicon wafer strongly depends on the crystal nature, i.e. an interstitial silicon dominant crystal or a vacancy dominant crystal. After heat treatment of dynamic random access memory, iron is mainly precipitated in the vacancy dominant crystal region rather than the interstitial silicon dominant crystal region, since the density of silicon oxide precipitate in the vacancy dominant crystal region is approximately two orders higher than that in the interstitial silicon dominant crystal region. This indicates that the mechanism of iron gettering is associated with relaxation gettering by silicon oxide precipitates. Otherwise, nickel silicides at the wafer surface are dominantly produced in the interstitial silicon crystal region, while nickel is chiefly precipitated at silicon oxide precipitates in the vacancy dominant crystal region. Also, this result demonstrates that the mechanism of nickel gettering is associated with relaxation gettering by silicon oxide precipitates, since silicon oxide precipitates are virtually not produced in the interstitial silicon dominant crystal region. A super silicon wafer designed for proximity gettering by silicon oxide precipitates shows a superior gettering efficiency for iron and nickel via the elimination of the memorized crystal nature.

Simulation of degradation of dielectric breakdown field of thermal SiO/sub 2/ films due to voids in Si wafers

Degradation of the dielectric breakdown field of thermal SiO/sub 2/ film caused by voids that are formed during growth of silicon single crystal has been a serious problem with reliability of MOS devices. To understand the degradation of breakdown field, local thinning of oxide film grown on pits (i.e., voids exposed at the wafer surface) is simulated using a simple model, and the degradation of breakdown field expected from the thinning is compared with experimental reports. In the model, oxide film grown on the inner surface of a sphere is calculated by assuming that deformation of oxide film is visco-elastic and that oxidation reaction rate is reduced by compressional normal stress acting on the Si/SiO/sub 2/ interface. The calculated results show appreciable thinning of oxide film, which explains the low breakdown field observed experimentally. It also helps to understand the unique degradation characteristics reported for pits and voids: lower breakdown field for thicker oxide film and recovery of breakdown field by chemical etching. No clear pit size dependence observed in the experiments suggests that the oxide thinning is localized at corners of voids.

Defects in the Oxidation-Induced Stacking Fault Ring Region in Czochralski Silicon Crystal

We have examined the defects in the oxidation-induced stacking fault (OSF) ring region in the as-grown Czochralski silicon crystals subjected to in situ annealing by the halt of pulling during crystal growth using secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) in order to investigate the nature of OSF nuclei. Oxygen aggregates were observed in the ring region in the crystals held for 4 h and 16 h using SIMS. The aggregate density in the crystal held for 16 h corresponded to the density of the grown-in defects without a flow pattern revealed by non-agitated Secco etching. TEM observation indicated that one of the grown-in defects in the ring region in the crystal held for 4 h was oxygen precipitate with a dislocation loop. OSF density decreased with holding time and density of another defect which was not an OSF increased with holding time in the ring region held at temperatures below about 1060°C. We believe that the OSF nuclei were oxygen precipitates and the dislocation loop was appeared around the precipitate during the halt of pulling process, so that OSF nuclear density was decreased.

Stress-Enhanced Diffusion of Boron at the Interface of a Directly Bonded Silicon Wafer

We have investigated the interface of Si/Si directly bonded wafers using X-ray topography (XRT), spreading resistance (SR), and secondary ion mass spectroscopy (SIMS). The residual stress induced at the interface of the bonded wafer was observed as ring patterns by XRT when the two wafers, flat and convex, were bonded. It appears that the residual stress enhanced the diffusion of boron atoms and aggregated them near the bonded interface. However, the resistivity increased in the region under the compressive stress in spite of the aggregation of boron.

Detection of very small defects and tiny inclusions just under mirror polished surfaces of silicon wafers by inside total reflection

Defects just beneath the mirror surface of a silicon wafer grown by the Czochralski method is critical for epitaxial growth on the surface. Inside total reflection of an IR laser beam allowed only the light scattered by defects and tiny inclusions in the denuded zone of the wafer to be observed through the surface, because part of the scattered light makes an incident angle smaller than the critical one while the unscattered part of the beam is totally reflected and thus does not emerge from the wafer. Background brightness was caused by light scattering from tiny inclusions such as interstitial oxygen atoms in the zone, and inhomogeneity of this background indicated distribution fluctuation of the defects with gettering function.

Effect of Interstitial Oxygen on Formation of Amorphous SiOx Layerin Directly Bonded Czochralski Silicon Wafers

We have investigated the effect of interstitial oxygen in CZ silicon wafers on the formation of the amorphous SiOx layer (x=0.2-0.3 after annealing at 1100°C for 2 h) at the interface in directly bonded silicon wafers using high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray (EDX). As the annealing time became longer, the thickness and the oxygen concentration in this layer increased, depending on the concentration of interstitial oxygen in wafers. These changes are caused by the diffusion of oxygen atoms from the bulk toward the bonded interface. However, they must not play such an important role in the initial stage of the formation of this layer because the layer existed discontinuously and was not affected by rotational misorientation or the microroughness.

Radial Distribution of Oxygen Precipitates in Czochralski Silicon Single Crystals

The effect of annealing conditions on the radial distribution of oxygen precipitates in Czochralski (CZ) silicon crystals was examined through two-step annealing. When the preannealing temperature was lower than 800°C, the oxygen precipitation depended on the initial oxygen concentration, and precipitation near the periphery of the crystal was often suppressed. When the preannealing temperature was higher than 900°C, the oxygen precipitation was enhanced near the periphery. This result indicates that extremely stable nuclei, whose density is low compared with the total amount of nuclei, exist near the periphery of the CZ crystals.