Method of producing epitaxial wafers

Details Method of producing epitaxial wafers Method of producing epitaxial wafers

2002-06-18

2004-03-23

Le, Dung (Department: 2818)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

Fluid growth from gaseous state combined with subsequent...

C438S200000, C438S487000, C438S765000

Reexamination Certificate

active

06709957

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of producing epitaxial wafers exhibiting high gettering effect by doping a silicon single crystal with nitrogen in the process of growing thereof, slicing silicon wafers from the thus-grown single crystal ingot, subjecting the wafers obtained to predetermined heat treatment and then treating them for epitaxial growth.
DESCRIPTION OF THE PRIOR ART
In recent years, the tendency toward higher degree of integration of silicon W semiconductor integrated circuit devices has been rapidly increasing and, accordingly, silicon wafers on which devices are formed have been subjected to increasingly severe quality standards. Thus, since circuits become finer or thinner with the increase in integration density, crystal defects, such as dislocations, and metal element impurities other than a dopant in the so-called device active region where devices are formed on a wafer are subjected to more rigorous limitations than ever before, for such defects and impurities will cause increases in leakage current and/or shorten the life of a carrier.
In the prior art, a wafer sliced from a silicon single crystal obtained by the Czochralski method (hereinafter referred to as “CZ method” and such wafer as “CZ wafer”) is used in producing a semiconductor device. This wafer generally contains interstitial oxygen at a supersaturated concentration of about 10
18
atoms/cm
3
. While oxygen is effective in preventing the occurrence of dislocations and thereby enhancing the silicon wafer strength and in providing a gettering effect, it is well known that oxygen deposits in an oxide form and thus induce crystal defects such as dislocations and/or stacking faults upon heating during device production.
Meanwhile, in the process of device production, the wafer is maintained at a temperature as high as 1,100 to 1,200° C. for several hours for the formation of a field oxide film by LOCOS (local oxidation of silicon) or the formation of a well diffusion layer and, as a result, a defect-free denuded zone with a thickness of about several tens of micrometers is formed near the wafer surface owing to diffusion of interstitial oxygen to the outside. This denuded zone serves as a device active region and a condition with reduced crystal defects is thus provided.
However, the employment of the high-energy ion implantation method for well formation and a temperature of not higher than 1,000° C. for device production in conjunction with device miniaturization corresponding to the increasing density of integration makes it difficult to allow the above-mentioned oxygen diffusion to a sufficient extent and thereby form a denuded zone in the vicinity of the surface. Thus, attempts have been made to reduce the oxygen content in the wafer. However, the formation of crystal defects cannot be suppressed to a satisfactory extent but oxygen reduction rather causes deterioration in wafer performance characteristics; no satisfactory results have thus been obtained. Therefore, an epitaxial wafer produced by allowing an epitaxial Si layer substantially free of crystal defects to grow on a silicon wafer has been developed and is widely used in producing a high integration density device.
However, even when an epitaxial wafer highly perfect as a crystal is used, the chance of contamination of the epitaxial layer by metal element impurities in the process of device production increases and the influence thereof also increases since the device process becomes complicated with the increasing degree of integration.
To cope with this contamination problem, there is the technique of gettering. This is the technique comprising collecting or capturing contaminant impurity elements at sites (sinks) outside the device active region to thereby avoid their adverse effects. One gettering technology is the so-called intrinsic gettering (IG) which uses oxygen-induced crystal defects (bulk micro defects; BMDs) induced during heat treatment in the device process as sinks.
In the case of an epitaxial wafer, however, the temperature reaches a level as high as 1,050 to 1,200° C. in the step of epitaxial layer formation, so that oxygen precipitates to serve as nuclei of micro defects in the wafer reduce in size or disappear. It is thus difficult to induce a sufficient number of BMDs to serve as sources of gettering in the wafer in the subsequent device process. In particular when the device process is carried out at a lower temperature, as mentioned above, the growth of oxygen precipitates becomes slow and a problem arises in that no sufficient gettering effect upon metal impurities can be expected not only in the initial stage of the device process but also throughout the device process.
Therefore, extrinsic gettering (EG) has been used combinedly as an alternative gettering method. This method comprises introducing crystal defects by causing distortion by means of extrinsic factors such as sandblasting, grinding, laser irradiation, ion implantation, and growth of a Si
3
N
4
or polycrystalline Si film on that side of the wafer which is reverse to the side on which devices are formed. The combined use of the EG method causes not only the increased cost problem due to the increase in the number of steps but also such problems as particle generation resulting from detachment of silicon fragments from the distorted layer and deterioration in flatness due to the growth of a polycrystalline silicon film.
To overcome the above problems, a method has been proposed which does not depend on the combined use of EG but improves the IG capacity itself by doping with an impurity capable of promoting oxygen precipitation in the step of single crystal growth. For example, Japanese Patent Application Laid-Open No. 11-189493 proposes a method which selects nitrogen as the element to promote oxygen precipitation and thus provide gettering effect and according to which the doping is carried out to a nitrogen concentration of not less than 1×10
13
atoms/cm
3
to thereby form, within the wafer, stable precipitates which will hardly disappear even in the epitaxial step involving high temperature treatment.
However, when nitrogen doping is carried out to a specified concentration of not less than 1×10
13
atoms/cm
3
according to the proposed method, nitrogen segregation occurs from the top to the tail of the single crystal ingot grown and thus the nitrogen concentration varies all over the whole length of the single crystal ingot. The BMD density, which influences the gettering effect, varies accordingly, hence no uniform gettering effect can be expected from the top to the tail of the single crystal ingot.
Further, Japanese Patent Application Laid-Open No. 2000-044389 proposes a method of increasing the gettering effect which comprises using a CZ wafer doped with 1×10
10
to 5×10
15
atoms/cm
3
of nitrogen and subjecting the same, before epitaxial growth treatment, to heat treatment at a temperature not lower than 900° C. but not higher than the melting point of silicon, desirably 1,100° C. to 1,250° C. Allegedly, this method can suppress the formation of defects within the epitaxial layer and can bring about a gettering effect. However, even this proposed method does not take into consideration the nitrogen segregation occurring from the top to the tail of the single crystal ingot grown, and it is difficult, by this method, to obtain a uniform gettering effect all over the single crystal ingot.
SUMMARY OF THE INVENTION
The present invention has been made in view of the problem that epitaxial wafers differ in gettering effect depending on the site of slicing from a single crystal ingot owing to nitrogen segregation on the occasion of growth by the CZ method. Thus, it is an object of the present invention to provide a method of producing epitaxial wafers having an equal and high level of gettering effect irrespective of the site of slicing from a single crystal ingot while suppressing the formation of defects in the epitaxial layer.
Among micro defects of a crystal which are caused by a conten

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