Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2001-07-09
2003-04-08
Hiteshew, Felisa (Department: 1765)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C117S013000, C117S020000
Reexamination Certificate
active
06544490
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a silicon wafer in which oxygen precipitation is stably obtained regardless of device production process and position in crystal and a method for producing it, as well as a method for evaluating defect regions of a silicon wafer of which pulling conditions are unknown.
BACKGROUND ART
In recent years, in connection with the use of finer devices accompanying the use of higher integration degree of semiconductor circuits such as DRAM, demand for quality of silicon single crystals produced by the Czochralski method (it may be also abbreviated as “CZ method” hereinafter) from which substrates therefor are produced is becoming higher. In particular, since there are defect called grown-in defects such as FPD, LSTD and COP and they degrade device characteristics, reduction of these defects is considered important.
Prior to explanation of those defects, there will be given first general knowledge of factors determining densities of defects introduced into silicon single crystals, a void type point defect called vacancy (also abbreviated as V hereinafter), and an interstitial type silicon point defect called interstitial silicon (interstitial-Si, also abbreviated as I hereinafter).
A V-region in a silicon single crystal means a region containing many vacancies, i.e., depressions, holes and so forth generated due to shortage of silicon atoms, and an I-region means a region containing many dislocations and aggregations of excessive silicon atoms generated due to excessive amount of silicon atoms. Between the V-region and the I-region, there should be a neutral region (also abbreviated as N-region hereinafter) with no (or little) shortage or no (or little) surplus of the atoms. Further, it has become clear that the aforementioned grown-in defects (FPD, LSTD, COP etc.) should be generated strictly only with supersaturated V or I, and they would not be present as defects even though there is little unevenness of atoms so long as V or I is not saturated.
It is known that densities of these two kinds of point defects are determined by the relationship between the crystal pulling rate (growing rate), and the temperature gradient G in the vicinity of the solid-liquid interface in the crystal in the CZ method. It has also been confirmed that defects distributed in a ring shape called OSF (Oxidation Induced Stacking Fault) are present in the N-region between the V-region and the I-region. Since OSFs are generated in a shape of concentric circle observed in a wafer surface when the wafer is sliced from a single crystal, there is used a term of OSF ring.
Those defects generated during the crystal growth are classified as follows. For example, when the growth rate is relatively high, i.e., around 0.6 mm/min or higher, grown-in defects considered to be originated from voids, i.e., aggregations of vacancy-type point defects, such as FPD, LSTD and COP, are distributed over the entire cross-section of the crystal along the radial direction at a high density, and a region containing such defects is called V-rich region (region in which supersaturated vacancies form void defects). When the growth rate is 0.6 mm/min or lower, with the decrease of the growth rate, the aforementioned OSF ring is initially generated at the circumferential part of the crystal, and L/D (large dislocations, abbreviation of interstitial dislocation loops, which include LSEPD, LFPD and so forth), which are considered to be originated from dislocation loops, are present outside the ring at a low density. A region containing such defects is called I-rich region (region in which supersaturated interstitial silicons form dislocation loop defects). When the growth rate is further lowered to around 0.4 mm/min or lower, the OSF ring shrinks and disappears at the center of wafer, and thus the entire plane becomes the I-rich region.
Recently, there has been discovered a region called N-region between the V-rich region and the I-rich region, and outside the OSF ring, in which neither of the void-originated FPD, LSTD and COP, the dislocation loop-originated LSEPD and LFPD and OSF are present. This region exists outside the OSF ring, and shows substantially no oxygen precipitation when it is subjected to a heat treatment for oxygen precipitation and examined by X-ray analysis or the like as for the precipitation contrast. This region is present at rather I-rich side, and the interstitial silicon point defects are not so rich as to form LSEPD and LFPD.
Presence of the N-region was also confirmed inside the OSF ring, in which neither of void-originated defects, dislocation loop-originated defects and OSFs were present.
Because these N-regions are formed obliquely with respect to the growing axis when the growth rate is lowered in a conventional growing method, it exists as only a part of the wafer plane.
As for this N-region, according to the Voronkov's theory (V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643), it was proposed that a parameter of F/G, which is a ratio of the pulling rate (F) and the crystal solid-liquid interface temperature gradient (G) along the growing axis, determined the total density of the point defects. In view of this, because the pulling rate should be constant in a plane, for example, a crystal having a V-rich region at the center, I-rich region at the periphery, and N-region between them is inevitably obtained at a certain pulling rate due to distribution of G in the plane.
Therefore, improvement of such distribution of G has recently been attempted, and it has become possible to produce a crystal having the N-region spreading over an entire transverse plane of the crystal, which region could previously exist only obliquely, for example, at a certain pulling rate when the crystal is pulled with a gradually decreasing pulling rate F. Further, such an N-region spreading over an entire transverse plane can be made larger to some extent along the longitudinal direction of the crystal by pulling the crystal at a pulling rate maintained at the value at which the N-region transversely spreads. Furthermore, it has also become possible to make the N-region spreading over the entire transverse plane somewhat larger along the growing direction by controlling the pulling rate considering the variation of G with the crystal growth to compensate it, so that the F/G should strictly be maintained constant.
As further classification of the N-region, it is known that there are NV-region (region in which there are many vacancies, but void defects are not detected), which is present outside the OSF ring and adjacent to it, and NI-region (region in which there are many interstitial silicons but dislocation loop defects are not detected), which is adjacent to the I-rich region.
Furthermore, in a silicon substrate produced by the CZ method, control of oxygen precipitation is becoming increasingly important in view of internal gettering effect against heavy metal impurities in addition to the importance of the reduction of such grown-in defects. However, since the oxygen precipitation strongly depends on the heat treatment conditions, it is a very difficult problem to obtain suitable oxygen precipitation in the device production process, which may be different for every user. Furthermore, wafers are subjected to a heat treatment not only in the device production step, but also a heat treatment in the crystal pulling step, in which the temperature is changed from the melting point to room temperature (thermal history of crystal). Therefore, in an as-grown crystal, there already exist oxygen precipitation nuclei formed during the thermal history of the crystal (grown-in precipitation nuclei). Such presence of grown-in precipitation nuclei makes the control of oxygen precipitation still difficult.
The oxygen precipitation process in the device production process can be classified into two kinds of processes. One is a process in which grown-in precipitation nuclei that remained after the initial heat treatment of the device production step grow, and the other one is a process in
Iida Makoto
Shigeno Hideki
Takeno Hiroshi
Hiteshew Felisa
Shin-Etsu Handotai & Co., Ltd.
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