K. Makihara

Very thin oxide film on a silicon surface by ultraclean oxidation

Very thin oxide films with a high electrical insulating performance have been grown by controlling preoxide growth using the ultraclean oxidation method. The current level through the ultraclean oxide is lower than that through the conventional dry oxide including thicker preoxide. The barrier height at the silicon‐oxide interface for electrons emission from silicon to oxide for the ultraclean oxide is little decreased as the thickness is thinner, while the barrier height for conventional dry oxide is drastically decreased. The growth rate of ultraclean oxide at 900 °C is governed by a simple parabolic law even in the range of 5–20 nm.

Preoxide-Controlled Oxidation for Very Thin Oxide Films

Very thin oxide films with high insulating performance and high reliability are formed by controlling preoxide growth with an ultraclean oxidation method during heating of the wafer to thermal oxidation temperature. The current level through the preoxide-controlled ultraclean oxide is lower than that through the oxide incorporating a thicker preoxide, and the preoxide-controlled ultraclean oxide has high reliability with respect to hot electron injection.

Chemical Oxide Passivation for Very Thin Oxide Formation

Electrical characteristics of the very thin oxides including oxide films formed in various chemicals were evaluated by means of the ultraclean oxidation system. The very thin oxides including the oxide films formed in H2SO4/H2O2, O3/H2O, and Hot-H2O22 have been found superior terms of reliability to those including the oxide films formed in NH4OH/H2O2/H2O and HCI/H2O2/H2O. The oxide films formed in H2SO4/H2O2, O3/H2O, and Hot–H2O2 have been found to function as the controlled passivation films which suppress increase of surface microroughness when heating of the wafer to thermal oxidation temperature in the inert gas ambience.

Influence of silicon wafer surface orientation on very thin oxide quality

Effects of silicon wafer surface orientation on very thin oxide quality were studied, testing Si(100) and (111) wafers. It has been found that the very thin oxide quality is determined by the silicon wafer surface orientation, and that when Si(111) is oxidized, SiO2/Si(111) interface microroughness increases as oxide becomes thicker than 10 nm, resulting in a degradation of oxide films quality on Si(111). When oxide thickness is decreased less than 10 nm, Si/SiO2 interface smoothness is maintained similar for Si(100) and (111) but SiO2/Si interface for Si(111) exhibits larger interface charges and larger threshold‐voltage shift due to hot‐electron injection than that for Si(100).

Effect of Silicon Wafer In Situ Cleaning on the Chemical Structure of Ultrathin Silicon Oxide Film

The effect of silicon wafer in situ cleaning on the chemical structures of thermally grown silicon oxide films was studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy. After the silicon wafer in situ cleaning was performed by the decomposition of native oxides in high vacuum, the nearly 1.6-nm-thick thermal oxides were formed in dry oxygen at 800°C. If the heating time for the decomposition of native oxides was too short, intermediate states transformed from native oxides were found to remain on the surface of the oxide films. On the other hand, if the heating time was too long, the amount of intermediate states at the interface was found to increase as a result of the increase in interface roughness. The optimum condition for in situ cleaning is heating at 900°C for 30 minutes in high vacuum.

Highly-reliable ultra-thin oxide formation using hydrogen-radical-balanced steam oxidation technology

A new oxide formation technology featuring oxidation in a strongly reductive ambient followed by an Ar gas annealing has been developed. We call this oxidation method H/sup */-H/sub 2/O oxidation. In this oxidation process, only the strong Si-O bond which is perfectly resistant even to the strongly reductive ambient survives and a high integrity oxide film can be grown. From the results of the breakdown tests by stepped voltage (E/sub BD/) and constant current stress (Q/sub BD/), the gate voltage shift (Vg shift) measurement under constant current stress, and the stress induced leakage current test, it has been revealed that the thin oxide film featuring high-integrity in breakdown and strong immunity from electrical stress is obtained by H/sup */-H/sub 2/O oxidation with post-oxidation-annealing in Ar ambient. This oxidation method is effective to form the thin oxide used under high electric field condition such as a tunnel oxide for flash memories.<<ETX>>

Silicon Wafer Orientation Dependence of Metal Oxide Semiconductor Device Reliability

The reliability of very thin gate oxides is studied using Si(100) and Si(111) wafers. When Si(111) wafer is oxidized to form thick oxide such as field oxide, the Si–SiO2 interface microroughness increases. The increased microroughness degrades the dielectric breakdown characteristics. Moreover, the oxide films on Si(111) are inferior to those on Si(100) in reliability even when the Si–SiO2 interface microroughness is of the same level. Oxide quality is determined by silicon wafer orientation.

Wafer quality specification for future sub-half-micron ULSI devices

It is pointed out that the surface microhardness dominating the electrical properties of very thin oxide films is strictly influenced by the wafer quality. The increase of the surface microhardness in some processes is shown to depend strongly on the silicon vacancy cluster concentration in the wafer. An epitaxial wafer having low silicon vacancy concentration is superior for sub-half-micron ULSI devices.<<ETX>>

Effect of Preoxide on the Structure of Thermal Oxide

It is found from X-ray photoelectron spectroscopy study on the chemical structure of silicon dioxide, whose thickness is in the range of 1.2 nm to 8.6 nm, that the oxidation-induced chemical shift depends mainly on the distance, and that 0.4- and 0.6-nm-thick preoxide, which is formed in dry oxygen at 300° C, modifies the structure of oxide near the surface.