Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
1999-09-14
2001-01-30
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S020000
Reexamination Certificate
active
06179910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for manufacturing silicon single crystals and relates to wafers adapted for producing semiconductors. In particular, this invention relates to a method for manufacturing CZ silicon single crystals suitable for high-temperature heat treatments of CZ silicon wafers in a hydrogen atmosphere (hereinafter referred as hydrogen heat treatment). Furthermore, this invention relates to wafers that are cut out from single crystals manufactured by the above method and undergo a hydrogen heat-treatment to get rid of defects formed therein.
2. Description of the Related Art
Most substrates used for fabricating semiconductor components are silicon single crystals, which are dominantly manufactured by the CZ method. In the CZ method, polycrystalline silicon is fed into a crucible and is heated to melt into a liquid. Then, a seed crystal disposed in a seed chuck is immersed into the liquid stored in the crucible; subsequently, the seed is rotated and pulled to grow a silicon single crystal of preset diameter and length.
Silicon single crystals acquired through the above process are sliced to obtain wafers, and subsequently devices are mounted on the activated surfaces of the wafers. However, due to the tendency of miniaturization and high-integration of device structures, the voltage endurance characteristic of a gate oxidation layer has become highly important. Japanese Patent Gazette JB HEI 3-80338 disclosed a method for reducing defects contained in the gate oxidation layer during its forming process. The '338 patent suggested heat-treating a silicon wafer in a non-oxidizing atmosphere containing hydrogen gas prior to the process of forming a thermal oxidation thin layer on the surface of the silicon wafer. It has been known that grown-in defects contained in a CZ silicon wafer disappear after a hydrogen heat treatment and the oxidation layer voltage endurance characteristic of the treated CZ silicon is excellent. The above grown-in defects, such as LSTD (Laser Scattering Tomograghy Defects), FPD (Flow Pattern Defects), or COP (Crystal Originated Particle),can be detected near the outer surface layer of a wafer.
However, the distinguishable outcome brought about by hydrogen heat treatment can only be found near the most outer surface layer of the wafer. The inventors of this invention noticed that the smaller the defect size, the better the result of eliminating defects achieved through hydrogen heat treatment. Therefore, a proposal (Japanese Patent Application No. HEI 9-27213) was offered. Namely, the proposal is a method for raising the cooling rate while passing through the defect producing temperature zone in the process of pulling a crystal so as to miniaturize the defect sizes.
However, when a silicon single crystal diameter is over 200 mm, the heat capacity is larger than that of conventional small-diameter silicon single crystals; it is quite difficult to obtain a cooling rate capable of miniaturizing the defect sizes to a significant extent. In other words, due to insufficient cooling in the process of the pulling operation, it is quite difficult to eliminate defects existing in large silicon ingots with the aid of hydrogen heat treatment.
SUMMARY OF THE INVENTION
The object of this invention is to provide a method for manufacturing silicon single crystals capable of eliminating void defects existing in deep regions of a silicon single crystal despite its size.
It is another object of this invention to provide a wafer adapted for producing semiconductors, which is manufactured according to the above method and maintains a great defect-free depth.
In view of the above-mentioned problems, the inventors of this invention intended to find a method, other than adjusting the cooling rate, capable of eliminating defects during pulling a CZ silicon single crystal. They investigated the ratios (V/G) between the pulling speeds (V) of silicon single crystals and the temperature gradients (G) along the crystal axes under various conditions so as to acquire a control factor. The temperature gradients were measured at a temperature near melting point and at sites located in the central portions of the crystals being pulled.
On the whole, it is well-known that grown-in defects existing in the interior of the ring-shaped oxidation induced stacking fault (OSF ring) contained in a CZ silicon single crystal are different from those existing in the exterior of the OSF ring which is induced during a thermal oxidation treatment. Void defects existing in the interior of the OSF ring are considered as an aggregate of vacancies, which will deteriorate the reliability of the gate oxidation layer in MOS devices. Dislocation clusters existing in the exterior of the OSF ring are regarded as self-interstitilals cohesion bodies, which will also deteriorate the characteristics of semiconductor components. In addition, it has been reported that the above defect seeds relate to the pulling speeds (V) of silicon single crystals and the above temperature gradients (G) along crystal axes (M. Hourai et. al. 18
th
International Conference on Defects in Semiconductors (1995)). Usually, elevating the pulling speed of a silicon single crystal can get rid of OSF rings to avoid current leak, which is caused by the existence of dislocation clusters and happens at pn junction surfaces.
After investigating the control factor, namely the ratios (V/G) between the pulling speeds (V) of silicon single crystals and the temperature gradients (G) along the crystal axes, the inventors found that defects could be removed from the zone extending from the outer surface to a depth of at least 3 &mgr;m. In other words, when an OSF ring began to appear on the outer periphery of a wafer cut out from a silicon single crystal, specifically under a condition of V/G values being smaller than 0.25 mm
2
/° C. min and the radius of the OSF ring being larger than half the radius of the wafer, the silicon single crystal could be made free of dislocation clusters and defects therein could be removed by a hydrogen heat treatment from the zone extending from the outer surface to a depth of at least 3 &mgr;m.
Therefore, to achieve the above objects, in the method for manufacturing silicon single crystals according to this invention, silicon single crystals are pulled under a condition that the radius of a ring-shaped oxidation induced stacking fault (OSF ring) of a wafer is larger than half the radius of the wafer during the process of thermal oxidation treatment.
Furthermore, the method for manufacturing silicon single crystals according to this invention is characterized in that the ratios (V/G) between the pulling speeds (V) of silicon single crystals and the temperature gradients (G) along the crystal axes are smaller than 0.25 mm
2
/° C. min and the radius of the each OSF ring is larger than half the radius of the wafer.
Furthermore, wafers adapted for producing semiconductors according to this invention are made of silicon single crystals pulled under a condition that the radius of a ring-shaped oxidation induced stacking fault (OSF ring) of the wafer is larger than half the radius of the wafer during the process of thermal oxidation treatment, and the wafers are heat-treated by hydrogen gas or non-oxidation gas so as to eliminate void defects.
Alternatively, wafers adapted for producing semiconductors according to this invention are made of silicon single crystals pulled under a condition of V/G values being smaller than 0.25 mm
2
/° C. min and the radius of the each OSF ring being larger than half the radius of the wafer, and the wafers are heat-treated by hydrogen gas or non-oxidation gas so as to eliminate void defects.
Furthermore, this invention intends to provide silicon single-crystal wafers adapted for annealing, characterized in that void defects contained therein have sizes smaller than 120 nm. Moreover, in order to meet the above requirements, in the Czochralski method, it is appropriate to pull silicon single crystals on the following conditi
Matsukuma Shin
Nakamura Kozo
Saishoji Toshiaki
Tomioka Junsuke
Yokoyama Takashi
Hiteshew Felisa
Komatsu Electronic Metals Co., Ltd
Sheppard Mullin Richter & Hampton LLP
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