Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2001-06-21
2003-05-27
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S013000, C117S020000
Reexamination Certificate
active
06569237
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of pulling up a nitrogen-doped silicon single crystal, which is intended for use in the manufacture of a semiconductor device, and to a method of manufacturing an epitaxial wafer using a silicon wafer prepared from a silicon single crystal produced by that method. More particularly, it relates to a method of manufacturing a high-quality epitaxial wafer while scarcely giving rise to stacking faults, dislocations and other defects in an epitaxial layer (hereinafter collectively referred to as “epitaxial layer defects”) when it is grown on a wafer sliced from a nitrogen-doped silicon single crystal, and to a method of producing such a single crystal to serve as a raw material for the epitaxial wafer.
DESCRIPTION OF THE PRIOR ART
In the art, silicon single crystals for use in manufacturing semiconductor devices are produced by the Czochralski method (CZ method).
FIG. 1
is a sectional view schematically illustrating a producing apparatus used in the CZ method. The producing apparatus comprises a crucible
1
disposed in the middle of the apparatus, and the crucible comprises a quartz vessel
1
a
and a graphite vessel
1
b
with the quartz vessel closely fitted therein. A heater
2
is disposed in a manner surrounding the crucible
1
and a raw silicon material is contained in a molten form
3
as melted by the heater. Above the crucible
1
, there is suspended a pulling shaft
4
with a seed crystal
5
mounted thereon, and the shaft pulls up a single crystal
6
while allowing it to grow from the lower end of the seed crystal
5
. A heat shield
7
is disposed in a manner surrounding the growing single crystal
6
.
With the recent increase in the integration density of silicon semiconductor devices, quality requirements imposed on silicon wafers on which devices are formed have become more and more severe. For example, severer limitations are imposed than ever on dislocations and like crystal defects and/or metal impurities in the so-called “device active region” where devices are formed, with the increasing fineness of circuits as resulting from the increase in integration density, since such defects and impurities increase the leakage current and shorten the life of a carrier.
Wafers sliced from silicon single crystals produced by the CZ method generally contain about 10
18
atoms/cm
3
of supersaturation oxygen. Due to the thermal history in the steps of device formation, such oxygen forms oxide precipitate nuclei and thereby induces crystal defects such as dislocations and stacking faults. In the process of device manufacture, however, the so-called DZ layer (denuded zone) which is free of crystal defects and has a thickness of about tens of micrometers is formed near the wafer surface by diffusion of oxygen to the outside when the wafer is maintained at about 1100° C. for several hours in the step of field oxide film formation by LOCOS (local oxidation of silicon) or well diffusion layer formation. The DZ layer serves as a device active region, so that the occurrence of crystal defects is spontaneously prevented.
However, with the increase in the integration density of semiconductor devices, the high-energy ion implantation technique has been introduced for well formation by which the device process is carried out at a low temperature of 1000° C. or less. At such a temperature, the above-mentioned outward diffusion of oxygen does not occur to a sufficient extent, hence the DZ layer formation near the surface becomes insufficient. Therefore, attempts have been made to reduce the oxygen content in wafers, but such attempts have been unsuccessful in perfectly suppressing the formation of crystal defects.
Under such circumstances, epitaxial wafers having an epitaxial layer substantially free of crystal defects as formed therein have been developed and are now widely used in the manufacture of highly integrated devices. However, even epitaxial wafers high in crystallinity are used, the device characteristics are degraded due to contamination of the epitaxial layer with metal impurities in the subsequent device process steps.
The opportunity and influences of such contamination with impurity metal elements increase since the process becomes more complicated with the increase in integration density. The contamination may be eliminated basically by cleaning the process environment and materials used. However, it is difficult to render the device process completely free of contaminants, hence the gettering technology becomes necessary as a measure for solving that problem. This is a means for entrapping contaminant impurity elements in a region (sink) other than the device active region to thereby render the contaminants harmless.
The gettering technology includes intrinsic gettering (hereinafter referred to as “IG” for short) which comprises entrapping impurity elements by utilizing oxygen-caused oxide precipitates spontaneously induced during heat treatment in the device process steps. However, when a wafer is heat-treated at a temperature as high as 1050-1200° C. in the epitaxial step, oxide precipitate nuclei occurring within the wafer sliced from a silicon single crystal shrink and vanish, whereby it becomes difficult to sufficiently induce oxide precipitates to serve as gettering sources within the wafer in the subsequent device process steps. Thus, even if the gettering technology is applied, a problem arises that any satisfactory IG effect cannot be exerted on metal impurities throughout the whole process.
To overcome such a problem, methods of producing silicon single crystals have been proposed in the art which comprise doping the single crystals with nitrogen while they are grown by the CZ method, to thereby induce formation, within wafers, of oxide precipitate nuclei hardly vanishing even upon high temperature heat treatment in the epitaxial step (cf. e.g. Japanese Patent Application Laid-Open (Kokai) No. H11-189493 and Japanese Patent Application Laid-Open (Kokai) No. 2000-44389).
According to the methods proposed, a silicon single crystal having oxide precipitate nuclei which hardly shrink or vanish can be obtained by increasing the thermal stability of oxide precipitate nuclei in the crystal by doping it with nitrogen while growing it by the CZ method. It is alleged that oxide precipitate nuclei remaining in wafers sliced from such single crystal after the epitaxial step form oxide precipitates from the early stages of the device step and thus effectively serve as sinks for gettering, so that the effects of IG can be expected.
Later investigations, however, have revealed that thermally stable oxide precipitate nuclei which will not vanish even upon high temperature heat treatment can indeed be obtained by high concentration nitrogen doping of wafers but these oxide precipitate nuclei readily induce epitaxial layer defects. In other words, high concentration nitrogen doping results in the formation of stable oxide precipitate nuclei near the wafer surface but these nuclei induce stacking faults, dislocations and the like, namely epitaxial layer defects, in the epitaxial layer, which is the device active region. These defects cause an increase in the leakage current and degradation in the gate oxide integrity, among others.
SUMMARY OF THE INVENTION
In view of the epitaxial layer defect problem caused by conventional nitrogen doping, it is a primary object of the present invention to provide an epitaxial wafer scarcely showing epitaxial layer defect development as a result of suppressed growth of thermally stable oxide precipitate nuclei in spite of its being derived from a wafer sliced from a silicon single crystal pulled up in a nitrogen-doped form as well as a method of pulling up a silicon single crystal to serve as a raw material for such wafer.
To specify the temperature range in which thermally stable oxide precipitate nuclei are formed in nitrogen-doped single crystals, the present inventors made experiments in which a raw material silicon melt was doped with 1×10
14
atoms/cm
3
of nitrogen
Asayama Eiichi
Ono Toshiaki
Tanaka Tadami
Armstrong Westerman & Hattori, LLP
Hiteshew Felisa
Sumitomo Metal Industries Ltd.
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