Method of producing high-quality silicon single crystals

Details Method of producing high-quality silicon single crystals Method of producing high-quality silicon single crystals

2000-11-22

2002-10-01

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from liquid or supercritical state

Having pulling during growth

Reexamination Certificate

active

06458204

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method of producing silicon single crystals for use as semiconductor materials, namely silicon wafers. More particularly, it relates to a method of producing high-quality silicon single crystals for use as wafers, which are grown by the Czochralski method and free of grown-in defects.
DESCRIPTION OF THE PRIOR ART
It is Czochralski's method of growing single crystals that is most widely employed in the production of silicon single crystals for use as semiconductor silicon wafers.
The Czochralski method comprises dipping a silicon seed crystal in molten silicon in a quartz crucible and then pulling up the seed from the melt while allowing single crystal growth. Advances in this silicon single crystal growing technology have made it possible to produce dislocation-free large single crystals with few defects. Semiconductor devices are produced by hundreds of process steps starting with wafers which are obtained from such single crystals and serve as substrates. In that process, the substrates are exposed to a number of physical, chemical and thermal treatments, including such treatment in a severe thermal environment as high-temperature treatment at 1,000° C. or above. Therefore, there arises the problem of grown-in defects; micro-defects having latent origins in the process of single crystal growth manifest in such process of device production, possibly causing devices to underperform.
Typical examples of such micro-defects are distributed, for example, as shown in FIG.
1
. The figure is a schematic representation of the results of micro-defect distribution observation by X-ray topography of a wafer which was sliced from a silicon single crystal just after growth, immersed in an aqueous solution of copper nitrate for Cu deposition thereon and then heat-treated. As seen, this wafer shows oxidation-induced stacking faults (hereinafter referred to as “OSF”), distributed in a ring-shaped zone at about two thirds of the outer diameter. Inside the ring-shaped zone, there are found laser scattering tomography defects, also called as COP or FPD, which are equally in a Si-deficient state. The ring of OSF (ring OSF) is directly surrounded by an oxygen precipitation promoted region where there is a tendency toward formation of deposits of oxygen precipitation. The latter is further surrounded by a denuded region where no defects appear. In the peripheral or outermost zone of the wafer, which surrounds the denuded region, defects called dislocation clusters readily manifest.
Generally, the sites of development of the above defects are strongly influenced by the pulling rate in the step of single crystal growth. A single crystal grown while varying the pulling rate within limits of rate of growth for obtaining dislocation-free sound single crystals, when examined for the distribution of each kind of defect on a longitudinal section along the central axis of the crystal, namely the pulling axis, gives such results as shown in FIG.
2
.
When wafer planes resulting from perpendicular slicing to the pulling axis of single crystal are observed, a ring OSF manifests from the periphery of the crystal as the rate of growth is decreased after arriving at the desired diameter following shoulder formation. The diameter of this ring OSF, which is found initially on the peripheral region, gradually decreases with the decrease in growth rate, and the ring OSF finally disappears, whereupon the whole wafer surface is covered by the region outside the ring OSF. Thus,
FIG. 1
is the sectional view, along the line A in
FIG. 2
perpendicular to the pulling axis of the single crystal wafer grown at the pulling rate corresponding to A. When the position of ring OSF development is taken as a standard, a higher growth rate gives a faster grown single crystal wholly showing the region inside the ring OSF, while a slower growth rate gives a slower grown single crystal wholly showing the region outside the ring OSF.
It is well known that dislocations in a silicon single crystal cause deterioration in characteristics of a device formed therefrom. OSF deteriorate electric characteristics, for example increase the leakage current and these defects exist at a high density in the ring QSF. Therefore, single crystals for ordinary LSI are currently grown at a relatively high pulling rate so that the ring OSF may be distributed in the outermost region of each single crystal. In this way, the wafer is caused to be mostly composed of the region inside the ring OSF, namely of a faster grown single crystal, to thereby avoid dislocation clusters. This is also because the region inside the ring OSF is higher in gettering activity against heavy metal contamination possibly occurring in the process of device production than the region outside the ring OSF.
In recent years, the thickness of the gate oxide layer has been reduced for increasing the density of LSI and the temperatures in device production process have been lowered. Therefore, the number or density of OSF, which are apt to occur in high-temperature treatment, has become reduced and, owing also to the success in reducing the oxygen concentration in crystals, OSF, such as the ring OSF, have become less troublesome when evaluated as factors deteriorating device characteristics. The laser scattering tomography defects occurring as major defects in faster grown single crystals, however, have been shown to markedly deteriorate the dielectric strength of the gate oxide films, which are now fairly thin. With the increase in device pattern fineness, in particular, the influence of such defects becomes significant, rendering it difficult to cope with the task of increasing the scale of integration.
If it becomes possible to enlarge the oxygen precipitation promoted region just outside the ring OSF region as well as the denuded region in the defect distribution pattern shown in
FIG. 1
, the possibility will arise that wafers or single crystals having very few grown-in defects may be obtained. Thus, in Japanese Patent Application Laid-Open (JP Kokai) No. H08-330316, for instance, there is disclosed an invention relating to a method of enlarging the denuded region outside the ring OSF exclusively to the whole wafer surface or to the whole single crystal, without allowing formation of dislocation clusters, which method comprises controlling the temperature gradient within the crystal in a manner such that, in the temperature range of from the melting point to 1,300° C., the ratio V/G (where V is the pulling rate in single crystal growing (mm/min) and G is the temperature gradient (°C./mm) within the crystal in the direction of pulling axis) is 0.20 to 0.22 in the inner region ranging from the center of the crystal to 30 mm from the perimeter but gradually increases therefrom toward the boundary of the crystal. In that case, various conditions, such as the positions of the crucible and heater, the semiconical thermal radiator comprising carbon disposed around the growing single crystal and the insulator structure around the heater, are studied and selected for growing through overall heat transfer calculations so that the above temperature conditions may be satisfied.
Further, JP Kokai No. H11-79889 discloses an invention directed to a production method which comprises carrying out the pulling so that the shape of the solid-melt interface during single crystal growth may be ±5 mm relative to the average position of the solid-melt interface except for the 5-mm-wide zone around the single crystal, and controlling the inside temperatures of the furnace so that the temperature gradient difference &Dgr;G=Ge-Gc (where Gc is the temperature gradient inside the crystal in the direction of pulling axis in the central region of the crystal in the range of 1,420° C. to 1,350° C., or from the melting point to 1,400° C. and Ge is that in the peripheral region of the crystal) may be not more than 5°C./cm. In other words, it is a production method according to which the solid-melt interface during growth is kept as flat as possi

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