Method for manufacturing silicon wafer

Abstract

A method for manufacturing a silicon wafer having a denuded zone in a surface layer by performing a heat treatment to a silicon wafer, including: a step A, performing a first rapid heat treatment of 0.01 msec or more and 100 msec or less to an upper surface layer alone of the silicon wafer to be treated at 1300 C. or more and a silicon melting point or less by using a first heat source which heats the silicon wafer to be treated from above; and a step B, holding the silicon wafer to be treated at 1100 C. or more and less than 1300 C. for one second or more and 100 seconds or less by a second rapid heat treatment using a second heat source which heats the silicon wafer to be heated, and decreasing the temperature at a falling rate of 30 C./sec or more and 150 C./sec or less.

Claims

1. A method for manufacturing a silicon wafer which has a denuded zone in a surface layer by performing a heat treatment to a silicon wafer to be treated, the method comprising: a step A of performing a first rapid heat treatment of 0.01 msec or more and 100 msec or less to an upper surface layer alone of the silicon wafer to be treated at a temperature of 1300 C. or more and a silicon melting point or less by using a first heat source which heats the silicon wafer to be treated from above; and a step B of holding the silicon wafer to be treated at a temperature which is 1100 C. or more and less than 1300 C. for one second or more and 100 seconds or less by a second rapid heat treatment using a second heat source which heats the silicon wafer to be heated, and decreasing the temperature at a temperature falling rate of 30 C./sec or more and 150 C./sec or less, wherein the step A is performed during the step B, and the silicon wafer to be treated is heated from below by the second heat source in the step B.

2. The method for manufacturing a silicon wafer according to claim 1, wherein a xenon lamp is used as the first heat source.

3. The method for manufacturing a silicon wafer according to claim 1, wherein a halogen lamp is used as the second heat source.

4. The method for manufacturing a silicon wafer according to claim 1, wherein the silicon wafer to be treated is sliced out from a silicon single crystal ingot which is grown by a Czochralski method and has oxygen concentration of 7 ppma or more and 20 ppma or less.

5. The method for manufacturing a silicon wafer according to claim 1, wherein the silicon wafer to be treated is sliced out from a silicon single crystal ingot which is grown by a Czochralski method and has nitrogen concentration of 110.sup.11 to 110.sup.15 atoms/cm.sup.3.

6. The method for manufacturing a silicon wafer according to claim 1, wherein the silicon wafer to be treated is sliced out from a silicon single crystal ingot which is grown by a Czochralski method and has carbon concentration of 110.sup.16 to 110.sup.17 atoms/cm.sup.3.

7. The method for manufacturing a silicon wafer according to claim 1, wherein the silicon wafer to be treated is sliced out from a silicon single crystal ingot which is grown by a Czochralski method and whose entire plane in a radial direction is an N region.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flowchart showing an example of a procedure of a method for manufacturing a silicon wafer according to the present invention;

(2) FIG. 2 is a schematic view showing an example of a single-crystal pulling apparatus which can be used in the method for manufacturing a silicon wafer according to the present invention;

(3) FIG. 3 is a schematic view showing an example of a heat treatment apparatus which can be used in the method for manufacturing a silicon wafer according to the present invention;

(4) FIG. 4 is an explanatory drawing for explaining how crystal defects change relative to a pulling rate; and

(5) FIG. 5 shows measurement results of TDDB in Example and Comparative Examples 1 and 2, where (a) shows a measurement result of Example, (b) shows a measurement result of Comparative Example 1, and (c) shows a measurement result of Comparative Example 2.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

(6) The present invention will now be described hereinafter in detail.

(7) As described above, in manufacture of a silicon wafer, BMDs which turn to gettering sites must be formed in a bulk region of the wafer, and dielectric breakdown strength of an oxide film must be increased.

(8) To provide a silicon wafer which has excellent dielectric breakdown strength of an oxide film and high BMD density, as a conventional method, there is, e.g., a method based on a high-temperature RTA treatment. For example, there can be considered a method for performing the high-temperature RTA treatment of 1300 C. or more based on the method disclosed in Patent Literature 4. In case of this method, since large-sized oxide precipitates which can cause degradation of the dielectric breakdown strength of the oxide film can be dissolved, the excellent dielectric breakdown strength of the oxide film can be provided. Further, implantation of Va enables highly densely forming the BMDs.

(9) However, as a result of earnest studies conducted by the present inventors, it has been discovered that the method disclosed in Patent Literature 4 may lead to generation of slip dislocations since a temperature of the RTA treatment is high. Further, as a result of the earnest studies, it has been revealed that the RTA treatment at 1200 C. or less does not lead to generation of slip dislocations but fails to provide the high dielectric breakdown strength of the oxide film.

(10) As a result of repeatedly examining the problems, the present inventors have considered that these problems can be solved by performing two different types of rapid heat treatments, especially heating a surface layer and a bulk at different temperatures, and thereby bringing the present invention to completion.

(11) Although the method for manufacturing a silicon wafer according to the present invention will now be described in detail hereinafter with reference to the drawings, the present invention is not restricted thereto.

(12) FIG. 1 is a flowchart showing an example of a procedure to implement the method for manufacturing a silicon wafer according to the present invention.

(13) An overall flow of the implementing procedure will be first described. First, a silicon wafer to be treated is prepared. The silicon wafer prepared herein is not restricted in particular, but quality of a silicon wafer to be treated can be determined in advance so that the silicon wafer having the desired quality can be provided by the manufacturing method according to the present invention.

(14) As described above, as a method for imparting desired quality (BMDs, grown-in defects, and the like) to the silicon wafer to be treated, there can be considered, e.g., adjusting each condition at the time of pulling a silicon single crystal ingot, which can be a base of the silicon wafer to be treated, by a Czochralski method.

(15) Then, as shown in FIG. 1, a heat treatment is performed to the thus obtained silicon wafer to be treated so that a silicon wafer which has a denuded zone in a surface layer and can impart a gettering capability can be manufactured. According to the method for manufacturing a silicon wafer of the present invention, at least a step A and a step B described below are carried out. The step A is performing a first rapid heat treatment to a surface layer on an upper side alone of the silicon wafer to be treated at a temperature of 1300 C. or more and a silicon melting point or less at 0.01 msec or more and 100 msec or less by a first heat source which heats the silicon wafer to be treated from above. The step B is holding the silicon wafer to be treated at a temperature of 1100 C. or more and less than 1300 C. for one second or more and 100 seconds or less by a second rapid heat treatment using a second heat source which heats the silicon wafer to be treated, and then decreasing a temperature at a temperature falling rate of 30 C./sec or more and 150 C./sec or less. When the step A and the step B described above are carried out, the surface layer of the silicon wafer to be treated can be free from defects. Thus, according to the present invention, it is possible to manufacture a silicon wafer which has high dielectric breakdown strength of an oxide film and enables formation of BMDs at high density in a bulk.

(16) The step A can be performed during the step B. For example, the step A can be performed during holding at the above temperature in the step B. By doing so, the surface layer can be assuredly heated to 1300 C. or more at the step A. In this case, at the step B, the silicon wafer to be treated is heated from below by using the second heat source.

(17) Further, the step A and the step B can be separately performed. In this case, an order of the step A and the step B is not restricted in particular.

(18) Here, examples of an apparatus which enables manufacturing a silicon single crystal from which the silicon wafer to be treated is sliced out and an apparatus which enables performing heat treatments to the silicon wafer to be treated will now be described, respectively.

(19) FIG. 2 is a schematic view showing an example of a single-crystal pulling apparatus which can be used in the method for manufacturing a silicon wafer according to the present invention. At the time of pulling a silicon single crystal by the Czochralski method, for example, such a single-crystal pulling apparatus as shown in FIG. 2 can be used. As shown in FIG. 2, this single-crystal pulling apparatus 1 is constituted of a pulling chamber 2, a crucible 3 provided in the pulling chamber 2, a heater 4 arranged around the crucible 3, a crucible holding shaft 5 and its rotation mechanism (not shown) which rotate the crucible 3, a seed chuck 7 which holds a seed crystal 6 of silicon, a wire 8 which pulls the seed chuck 7, and a take-up mechanism (not shown) which rotates or takes up the wire 8. Furthermore, a heat insulting material 9 is arranged around a periphery of the heater 4. A silicon single crystal 10 is pulled from a silicon melt 11 which is a starting material by the wire 8.

(20) Next, the apparatus to perform heat treatments to the silicon wafer sliced out from the silicon single crystal 10 pulled by the single-crystal pulling apparatus 1 will now be described. FIG. 3 is a schematic view showing an example of a heat treatment apparatus which can be used in the method for manufacturing a silicon wafer according to the present invention. The heat treatment apparatus (which will be also referred to as an FLA apparatus hereinafter) shown in FIG. 3 has a chamber 12 made of quartz, and a silicon wafer 19 is thermally treated in this chamber 12.

(21) Moreover, in the FLA apparatus shown in FIG. 3, the first rapid heat treatment is performed by using Xe flash lamps (xenon lamps) 13 arranged on a chamber upper portion. The second rapid heat treatment is performed by using halogen lamps 14 arranged on a chamber lower portion. In this case, the first rapid heat treatment is flash lamp annealing (FLA: a rapid temperature raising/falling heat treatment) performed in a very short time. Thus, the FLA apparatus shown in FIG. 3 can perform the FLA which is the first rapid heat treatment during the second rapid heat treatment (during high-temperature holding of the RTA in particular). It is to be noted that the first heat source, the second heat source, the first rapid heat treatment, and the second rapid heat treatment are not restricted thereto, and they can be separately performed.

(22) A non-illustrated wafer insertion opening formed to be openable/closable by a gate valve is provided in an auto shutter 15. Further, the silicon wafer 19 is arranged on a support section 17 formed on a quartz tray 16. Furthermore, a non-illustrated temperature measurement special window is provided to the chamber 12, and a temperature of the silicon wafer 19 can be measured by a pyrometer 18 installed outside the chamber 12 through the special window. As described above, the single-crystal pulling apparatus and the heat treatment apparatus equal to counterparts in the conventional examples can be used, and their structures are not restricted in particular.

(23) Each step in the flowchart in FIG. 1 will now be described in detail hereinafter. As described above, first, the silicon wafer to be treated is prepared. In the first rapid heat treatment in the manufacturing method according to the present invention, as will be described later, since the upper surface layer alone of this silicon wafer to be treated is heated to annihilate the grown-in defects, quality of the silicon wafer to be treated could be determined on a preparation stage so that the defects can be annihilated at low temperatures in a short time.

(24) For example, at the time of pulling the silicon single crystal by using the single-crystal pulling apparatus 1 shown in FIG. 2, V/G can be adjusted by changing a pulling rate V (V/G may be adjusted by changing a hot zone to vary G) so that the single crystal to be pulled can be a single crystal whose entire plane in the radial direction is an N region. As a matter of course, an entire plane in the radial direction of a silicon wafer sliced out from this crystal is the N region, and no grown-in defect is present in this wafer.

(25) Since oxide precipitate nuclei alone are present in this silicon wafer of the N region and they can be annihilated by a heat treatment at a relatively low temperature, a cost or a treatment time required for formation of the DZ layer can be further reduced. This is also effective for a decrease in contamination or suppression of generation of slip dislocations.

(26) Here, it is preferable to set oxygen concentration in this silicon single crystal to 7 ppma or more and 20 ppma or less. When the oxygen concentration is 7 ppma or more in this manner, since the grown-in oxide precipitate nuclei are appropriately present in the single crystal or the silicon wafer sliced out therefrom, the oxide precipitate nuclei are grown by the heat treatment and the BMDs are formed in the device process, and the gettering function can be provided.

(27) Further, since the oxygen concentration is 20 ppma or less, sizes of the grown-in defects or the oxide precipitate nuclei formed during crystal growth do not increase beyond necessity, and the oxide precipitate nuclei in the surface layer can be more assuredly annihilated. Furthermore, since an original degree of supersaturation of oxygen is not too high, annihilating the oxide precipitate nuclei in the surface layer by the first rapid heat treatment enables effectively preventing the oxygen from being again precipitated even if the heat treatment is performed in the device process, thereby avoiding appearance of BMDs on the surface.

(28) As described above, it is preferable for the oxygen concentration to fall within the above range so that the new oxide precipitate nuclei are not formed by the regular device heat treatment. The oxygen concentration is more preferably set to 15 ppma or less.

(29) Further, it is preferable for the silicon single crystal to have nitrogen concentration of 110.sup.11 to 110.sup.15 atoms/cm.sup.3. When the nitrogen is contained at the concentration, the size of the grown-in defects can be reduced, and hence the defects in the surface layer can be assuredly annihilated as compared with a case where nitrogen doping is not performed, which is effective. Moreover, it is also known that formation of the BMDs is promoted or mechanical strength of a wafer is increased when the nitrogen is contained, there are advantages that generation of slip dislocations at the time of the heat treatment can be suppressed and a control range over the BMDs in the bulk can be enlarged.

(30) Additionally, it is preferable for the silicon single crystal to have carbon concentration of 110.sup.16 to 110.sup.17 atoms/cm.sup.3. It is known that the BMDs can be easily formed by the heat treatment in the device process when the carbon is contained at the concentration in this manner, which is advantageous. Further, the carbon functions as a catalyst when the oxygen fixes a dislocation such as a slip, thereby suppressing the slip dislocations.

(31) It is to be noted that these concentrations can be adjusted by using a method equal to those in the conventional examples. For example, in case of the nitrogen concentration, a silicon wafer or the like doped with nitrogen by the Czochralski method can be put into a starting material in a crucible to adjust its concentration.

(32) Furthermore, as described above, it is preferable for an entire plane in the radial direction of the silicon single crystal to be the N region. The grown-in defects, e.g., COPs or OSF nuclei are not present in a silicon wafer sliced out from such an N-region single crystal ingot, and the oxide precipitate nuclei alone, which can be annihilated at a lower temperature than the grown-in defects can be, are present. Thus, the present invention which makes the surface layer free from defects is effective for reducing costs of the heat treatment. Moreover, the treatment can be performed at lower temperatures, which is advantageous to a decrease in contamination or slip dislocations.

(33) Slicing is carried out to the silicon single crystal pulled while adjusting the grown-in defects, the oxygen concentration, and others, and this sliced piece can be used as the silicon wafer to be treated.

(34) Then, the heat treatments including the step A and the step B are performed to the thus provided silicon wafer to be treated. The steps A and B are rapid heat treatments. Although the rapid heat treatment conditions in the step A and the step B are as described above, they will be described in more detail hereinafter. When the step A is performed, the large-sized oxide precipitates which can cause degradation of the TDDB characteristics can be dissolved. At this time, since a heating time to reach 1300 C. or more is very short, an increase in temperature of a back surface side (a lower side of the wafer) is small, and generation of slip dislocations can be suppressed. When the step B is performed under the described conditions, vacancies can be frozen in a bulk of the wafer, thus providing high BMD density.

(35) At this time, when an apparatus having the first heat source and an apparatus having the second heat source are prepared respectively, the step A and the step B can be separately performed. In this case, a heating atmosphere can be changed depending on the step A and the step B. Moreover, when an apparatus having the first heat source and the second heat source, e.g., an FLA apparatus shown in FIG. 3 is prepared, the step A can be carried out during the step B. In this case, the number of the apparatus used for manufacture of wafers can be one.

(36) As the first heat source in this case, a laser annealing apparatus or the like can be used, but using a flash lamp having a rare gas such as a xenon sealed therein, especially an Xe flash lamp is preferable. In this case, heating can be readily carried out at a temperature of 1300 C. or more described later, and the entire plane of each wafer in the radial direction can be uniformly heated.

(37) Additionally, as the second heat source, a halogen lamp can be used. Consequently, the second rapid heat treatment can be easily performed.

(38) Here, a heating temperature (a maximum temperature in heating in particular) provided by the first heat source is set to 1300 C. or more and a melting point of silicon (1412 C.) or less. When the heating temperature of the first heat source is less than 1300 C., the grown-in defects and the oxide precipitate nuclei in the surface layer cannot be sufficiently dissolved. When the heating temperature of the first heat source exceeds the melting point of silicon, the silicon wafer to be treated may be possibly deformed.

(39) Further, a heating time provided by the first heating source (a total irradiation time in the step A of the flash lamp annealing) is set to 0.01 msec or more and 100 msec or less. When the heating time of the first heat source is less than 0.01 msec, the grown-in defects and the oxide precipitate nuclei in the surface layer on a wafer upper side (a front side) cannot be sufficiently dissolved. Furthermore, when the heating time of the first heat source exceeds 100 msec, slip dislocations may be possibly generated. It is preferable to set the heating time of the first heat source to 20 msec or less in particular to avoid an increase in temperature on the wafer back surface.

(40) Moreover, a heating temperature provided by the second heat source is set to 1100 C. or more and less than 1300 C. When the heating temperature provided by the second heat source is less than 1100 C., Va cannot be implanted, and the BMD density cannot be higher than that before the heat treatment. Additionally, when the heating temperature provided by the second heat source is 1300 C. or more, slip dislocations are produced in a wafer. It is to be noted that setting the heating temperature of the second heat source to 1150 C. or more is particularly preferable to highly densely form the BMDs.

(41) A heating time provided by the second heat source is set to one second or more and 100 seconds or less. When the heating time of the second heat source is less than one second, the oxide precipitate nuclei formed at an ingot pulling stage cannot be grown. When the heating time of the second heat source exceeds 100 seconds, productivity is lowered.

(42) A temperature falling rate in the second rapid heat treatment is set to 30 C./sec or more and 150 C./sec or less. When the temperature falling rate at the time of rapidly decreasing a temperature is less than 30 C./sec, vacancies cannot be frozen in a wafer, and the BMDs cannot be highly densely formed. Further, when the temperature falling rate is higher than 150 C./sec, the slip dislocations may be produced by rapid cooling in some cases.

(43) It is to be noted that a temperature raising rate in the second rapid heat treatment can be set to, e.g., 30 C./sec or more and 70 C./sec or less.

(44) When the heat treatments are carried out under the conditions, it is possible to provide a wafer which has a TDDB good chip yield of, e.g., 90% or more and in which the BMDs can be highly densely formed by a device heat treatment or the like.

(45) Additionally, the first rapid heat treatment and the second rapid heat treatment can be carried out in a non-oxidizing atmosphere of argon, hydrogen, helium, or a mixed gas of these materials. In case of performing the first rapid heat treatment (a heat treatment of a surface layer region) and the second rapid heat treatment in the non-oxidizing atmosphere in this manner, since equilibrium concentration of oxygen on the surface is lower than that of the oxidizing atmosphere, the oxygen can be efficiently outwardly diffused. Consequently, the oxygen concentration near the surface can be lowered and rapidly reach solid solubility limit or less, and hence the oxide precipitate nuclei or the grown-in defects can be more readily annihilated, thereby improving quality in the outermost surface layer in particular. Further, when the heat treatment atmosphere in the first rapid heat treatment and the second rapid heat treatment is hydrogen, its reducing action facilitates dissolving defects caused due to the oxide precipitates, and hence the quality of the surface can be further improved.

(46) Furthermore, the first rapid heat treatment and the second rapid heat treatment can be performed in a nitride film forming atmosphere containing nitrogen and ammonia. In case of performing the first rapid heat treatment and the second rapid heat treatment in the nitride film forming atmosphere in this manner, as described in Patent Literature 2, it is known that vacancies can be efficiently implanted into a wafer and the implanted vacancies promote oxygen precipitation. The vacancy implantation promotes the oxygen precipitation in this manner and, at the same time, annihilation of the grown-in oxide precipitate nuclei during heating can be suppressed. That is, as compared with a case where heating is performed in an atmosphere which does not require the vacancy implantation rather than the nitride film forming atmosphere, a width of the DZ layer can be prevented from increasing beyond necessity and can be decreased. When the DZ layer is narrow and a bulk having the BMDs is close to a device region, a diffusion distance of metal impurities mixed during a device process to reach the BMDs as gettering sites is reduced, and the metal impurities can be efficiently gettered.

(47) On the other hand, the first rapid heat treatment and the second rapid heat treatment can be carried out in an oxidizing atmosphere containing oxygen. In case of performing the first rapid heat treatment and the second rapid heat treatment in the oxidizing atmosphere in this manner, interstitial Si (I) is implanted, the oxide precipitate nuclei are apt to be easily dissolved, and hence the width of the DZ layer can be increased. Alternatively, the DZ layer can be formed by a heat treatment at a lower temperature/in a shorter time.

(48) As described above, the method for manufacturing a silicon wafer according to the present invention enables providing a silicon wafer which cannot be provided by conventional methods, i.e., a silicon wafer which has the excellent TDDB characteristic and high BMD density in a bulk.

EXAMPLES

(49) The present invention will now be more specifically described hereinafter with reference to an example and comparative examples, but the present invention is not restricted these examples.

(50) A silicon wafer to be treated in which an Nv region and an Ni region are mixed was prepared, and Example and Comparative Examples 1 and 2 described below were carried out.

(51) A conductivity type, a resistivity, oxygen concentration, a diameter, and a crystal axis orientation of the silicon wafer are follows. Conductivity type: P type Resistivity: 17 to 20 .Math.cm Oxygen concentration: 13 to 14 ppma (JEITA) Diameter: 300 mm Crystal axis orientation: <100>
Doping of nitrogen and carbon was not performed.

Example

(52) The manufacturing method according to the present invention was carried out by using the FLA apparatus shown in FIG. 3. It is to be noted that the step A was performed during the step B. Specifically, a temperature of a prepared wafer was rapidly raised (rapid temperature raising in the step B) from a room temperature to 1175 C. at a temperature raising rate of 50 C./sec provided by a halogen lamp in a mixed atmosphere containing 3% of NH.sub.3 and 97% of Ar by using the FLA apparatus shown in FIG. 3 and held for 10 seconds to preliminary heat the wafer (holding in the step B), irradiation using an Xe flash lamp was performed for 2 msec in this state to solely heat a surface layer on an upper side of the silicon wafer to be treated to 1350 C. (the step A), and a temperature falling rate from the preliminary heating temperature to 700 C. or less was set to 50 C./sec to carry out rapid cooling (rapid temperature falling in the step B).

Comparative Example 1

(53) Like Example, a wafer was prepared, its temperature was rapidly raised from a room temperature to 1175 C. at a temperature raising rate of 50 C./sec in a mixed atmosphere containing 3% of NH.sub.3 and 97% of Ar by using a commercially available rapid heating/rapid cooling apparatus (an RTA apparatus) and held for 10 seconds, and then the wafer was rapidly cooled at a temperature falling rate of 50 C./sec.

Comparative Example 2

(54) Like Example, a wafer was prepared, its temperature was rapidly raised from a room temperature to 1000 C. at a temperature raising rate of 30 C./sec provided by a halogen lamp in an atmosphere containing 100% of Ar by using the FLA apparatus shown in FIG. 3 and held for 20 seconds to preliminary heat the wafer, and irradiation using the Xe flash lamp was performed for 40 msec in this state to solely heat a surface layer on an upper side of the silicon wafer to be treated to 1300 C., and the wafer was rapidly cooled at a temperature falling rate of 30 C./sec from the preliminary heating temperature to 700 C. or less.

(55) A gate oxide film having a thickness of 25 nm was formed on the wafer of each of Example and Comparative Examples 1 and 2, and then the TDDB characteristic was evaluated.

(56) FIG. 5 shows TDDB measurement results of Example and Comparative Examples 1 and 2. In FIG. 5, (a) shows a measurement result of Example, (b) shows that of Comparative Example 1, and (c) shows that of Comparative Example 2. Although a -mode good chip yield of 99% was acquired in each of Example and Comparative Example 2, a good chip yield of Comparative Example 1 was 69%. That is because a heating temperature of the wafer surface layer was low and hence large-sized oxide precipitates which degrade the TDDB characteristic were not successfully dissolved in Comparative Example 1, but the wafer surface layer was sufficiently heated to a high temperature in each of Example and Comparative Example 2, which resulted in successful dissolution of the oxide precipitates and no defect in the surface layer.

(57) Slip dislocations in Example and Comparative Examples 1 and 2 were evaluated by an X-ray topography, temperatures on back surfaces were low in all examples, and hence no slip dislocation was confirmed.

(58) Further, the wafers of Example and Comparative Examples 1 and 2 were thermally treated in an N.sub.2 atmosphere at 800 C. for four hours. Then, temperatures of the wafers were raised to 1000 C. at a temperature raising rate of 10 C./min in the same heat treatment furnace, a heat treatment was performed at 1000 C. for 16 hours, thereafter the temperatures were decreased to 700 C., and the wafers were taken out. Subsequently, each wafer was attached to a jig having an angle of approximately 22, and oblique polishing was performed. Then, preferential etching was performed to measure BMD density by using a microscope. Consequently, the BMDs were sufficiently formed at the density of 510.sup.9 pieces/cm.sup.3 in Example and Comparative Example 1. However, the same were formed at the density of 510.sup.8 pieces/cm.sup.3 in Comparative Example 2, and this BMD density was smaller than that of Example and Comparative Example 1. As a reason for this, it can be considered that the preliminary heating temperature in Comparative Example 2 was 1000 C. which was low.

(59) As described above, Example enables fabrication of the wafer which has no slip dislocation, the BMDs highly densely formed therein, and the excellent TDDB characteristic.

(60) It is to be noted that the present invention is not restricted to the foregoing embodiment. The foregoing embodiment is an illustrative example, and any example which has substantially the same configuration and exerts the same functions and effects as the technical concept described in claims of the present invention is included in the technical scope of the present invention.