The present invention relates to a method of manufacturing a silicon single crystal to be used as a semiconductor device and a silicon wafer manufactured from the grown silicon single crystal.
A silicon single crystal used as a semiconductor highly integrated circuit material is mainly manufactured by the Czochralski method (hereafter referred to as the CZ method)
When the silicon single crystal is grown by the CZ method, it is known that when the single crystal is processed to a wafer and then thermally treated at a high temperature, a ring-shaped oxidation induced stacking fault, which is called an OSF ring, is caused.
When the OSF ring is caused in the single crystal, there is about 105 to 106 cmxe2x88x923 of a grown-in defect, which is related to a point defect (vacancy) degrading the gate oxide integrity (GOI) characteristics of a MOS-type device, on the inside area of the OSF ring. This grown-in defect seems to have a hollow in it and has an octahedron base structure.
The quality of the silicon single crystal can be controlled by adjusting the positions of such areas.
For example, (1) when the OSF ring is made to disappear on the outermost periphery of the crystal, a crystal which contains no OSF ring and no grown-in defects (dislocation cluster) due to an interstitial silicon can be obtained; (2) when the OSF ring is generated between the center and the outermost periphery of the crystal, and the pulling rate and oxygen concentration of the crystal are adjusted, a crystal having extremely reduced size of OSF nuclei and a wide grown-in defect-free area formed on the ring and its outside area can be obtained; and (3) when the OSF ring is made disappear at the center, a crystal completely free from the grown-in defect on its entire surface can be obtained.
Here, the OSF ring includes a generally observable OSF ring and a latent OSF ring which has a very small OSF nuclei and cannot be seen without using special means.
It is known that the position where the OSF ring is formed is determined by the V/G value calculated from the temperature gradient G from the melting point to 1300xc2x0 C. in the silicon single crystal pulled and a pulling rate V. It is recognized that a crystal having the OSF ring at a predetermined position can be obtained by keeping the V/G value at a predetermined value while pulling the crystal.
For example, the invention described in Japanese Patent Laid-Open Publication No. Hei 8-330316 discloses a method which calculates a temperature distribution in a furnace by heat transfer calculations in view of a hot zone structure of a CZ furnace, calculates the G value from the temperature distribution in a direction of a crystal length, and determines the pulling rate V according to a pulling length according to the G distribution.
The invention described in Japanese Patent Laid-Open Publication No. Hei 8-268794 measures the temperature distribution at a predetermined position in the crystal by a radiation thermometer or measures the temperature distribution near the surface of the crystal by thermocouples, and makes heat transfer calculation of a temperature gradient G within the crystal or calculates it on-line from a regression formula obtained by experiments. And, there is described a method of controlling the pulling rate V so that the V/G value becomes constant according to the G value.
Thus, it is made possible to produce or disappear the OSF ring at a target position by controlling the V/G value.
Also, supersaturated oxygen is contained in the silicon single crystal produced by the CZ method. When heat treatment is performed in a device forming process after processing the single crystal into wafers, a heat-treatment originated fine defect (oxygen precipitate) is formed due to the supersaturated oxygen contained in the single crystal.
When the oxygen precipitates are present in the device active region of the wafer surface layer, a junction leakage current characteristic is degraded.
On the other hand, when the oxygen precipitates are present in the wafer, it is effective to remove contaminants mingled during the device forming process from the device active region. In other words, an intrinsic gettering layer (called IG layer) is formed.
In order to effectively use the oxygen precipitates formed by the supersaturated oxygen contained in the crystal grown by the CZ method, the wafer formed from the silicon single crystal is subjected to a high-temperature heat treatment at a temperature about 1150xc2x0 C. to out diffuse oxygen in the wafer surface zone, so that the oxygen concentration in the wafer surface layer is lowered, and a denuded zone (DZ) where an oxygen precipitate or a defect caused is not present, is formed on the wafer surface. Then, the heat treatment is further performed at a temperature between 500xc2x0 C. to 900xc2x0 C. to form an oxygen precipitation nucleus within the wafer in order to form an IG layer (hereafter this method is called the DZ-IG treating method).
By performing the aforesaid treatment, a high-quality wafer can be produced in which the device active region on the wafer surface layer is free from defects, while an absorbing layer for removing a contaminant from the device active region is present within the wafer.
The aforesaid method of adjusting the OSF ring forming position has the following problems.
Specifically, the invention described in Japanese Patent Laid-Open Publication No. Hei 8-330316 is the method which calculates G from the hot zone structure of the CZ furnace and determines V in order to have a predetermined V/G value and has disadvantages (a) the accuracy of the heat transfer calculation is poor, and (b) it takes time to calculate. Though the hot zone is worn and degraded with the number of pulling times, a change of G involved in such wearing and degradation is hardly taken into account. It was difficult to accurately control the V/G value because of these problems.
The invention described in Japanese Patent Laid-Open Publication No. Hei 8-268794 is the method which measures a temperature gradient of the crystal surface through online by a radiation thermometer or thermocouples and controls V according to the G value calculated from the measured value to keep the V/G value at a constant level. This method has problems, such as; (a) when the radiation thermometer is used to measure the light reflected from the surface of a melt, the inner wall of a chamber and the surface of the crystal is penetrated with stray light, and the crystal surface temperature cannot be measured accurately; (b) when a thermocouple is used to measure the crystal surface temperature, the thermocouple is influenced by heat radiation from the melt or a heater even if it is mounted near the surface of the crystal, therefore the crystal surface temperature cannot be measured accurately. Besides this, even if the temperature is measured by the aforesaid methods, the measured temperature is a temperature of the crystal surface and not a measured value of the actual temperature within the crystal having the OSF ring formed therein. The temperature inside the crystal is merely estimated from the surface temperature, therefore it is difficult to control the OSF ring diameter with high precision.
Thus, it is not easy to correctly know the G value which is the temperature gradient within the single crystal at the time of being pulled.
The DZ treating method which is used as a treating method for forming the aforesaid high-quality wafer has the following disadvantages.
The DZ treating method out-diffuses the supersaturated oxygen present on the wafer surface zone by the high-temperature heat treatment and decreases undetected oxygen precipitates present on the wafer surface layer or secondary defects induced by the oxygen precipitates by selective etching used for evaluation.
However, the aforesaid DZ-IG treating method cannot eliminate the grown-in defects formed at crystal pulling.
Specifically, the oxygen precipitates are eliminated from the wafer surface layer by the treatment of out diffusion of oxygen, but the grown-in defects formed during the crystal growth are still present, and a real defect-free layer is not formed.
Therefore, the present invention provides a method for manufacturing a silicon single crystal with an OSF ring located at a desired position by designing a predetermined pulling rate pattern in view of a relation between an OSF ring diameter and a crystal pulling rate for each crystal pulling length in order to control the OSF ring diameter without measuring the actual temperature gradient G within the single crystal. However, it is an object of the invention to provide a high-quality single crystalline silicon wafer, which is free from defects such as grown-in defects in a device active region and has an intrinsic gettering effect, by using a silicon single crystal having an OSF ring formed at a desired position.
The invention described in claim 1 relates to a method of manufacturing a silicon single crystal grown by the Czochralski method, the method is characterized in that a crystal is pulled up in a CZ furnace by changing an average pulling rate for the crystal having a predetermined length more than two times; a relation between a crystal pulling length, an average pulling rate and an OSF ring diameter is measured; an average pulling rate pattern at each pulling rate of the crystal for generation or disappearance of an OSF ring at a predetermined position is designed based on the measured results; and the crystal is pulled according to the average pulling rate pattern.
The present invention relates to a method of manufacturing a silicon single crystal, to obtain the crystal of pre-determined OSF ring diameter, according to the relation of OSF ring diameter and average pulling rate at each pulling length, without measuring a G value.
A pulling rate V does not indicate an instantaneous pulling rate but means an average value of pulling rate measured for 10 minutes or more. Changes of the instantaneous pulling rate do not affect the OSF ring diameter in the crystal, but changes of the average pulling rate over 10 minutes or more affect the OSF ring diameter in the crystal. Therefore, the term xe2x80x9caverage pulling ratexe2x80x9d is used in the present invention.
A temperature gradient at any part of the crystal is determined from (1) a hot zone structure, (2) a crystal pulling length X at any portion (a pulling length after the OSF ring has a predetermined diameter) and (3) a distance in a direction of the crystal diameter from the center of the crystal at any part.
We can know the rotation of OSF ring diameters and average pulling rate in each pulling length by measurement of the OSF ring diameters of all crystal length, for several average pulling rate crystals. In view of the regression from the experimental values, a relation between the OSF ring diameter, the crystal pulling length and the average pulling rate can be measured.
Based on the relational expression obtained, the average pulling rate V required for generating the OSF ring at any position in a predetermined portion in the crystal pulling direction can be determined. Specifically, when the crystal is pulled, the average pulling rate pattern is previously determined according to the crystal pulling length, and the crystal is pulled up according to the average pulling rate pattern. Thus, the crystal having the OSF ring positioned at a target position can be obtained.
The invention described in claim 2 relates to a method of manufacturing a silicon single crystal to be grown by the Czochralski method according to claim 1, wherein in comprising; using a prediction formula, based on previous crystals pulling data, to predict the average pulling velocity pattern; measuring the actual value of the average pulling velocity, in view of the diameter of the OSF ring; calculating the prediction formula to compensate for the next crystal grown.
The CZ furnace comprises a carbon heater, a heat insulating material, a carbon crucible and other members. These members are continuously used for tens to hundreds of pulling times. And, these members are degraded and worn with time due to their reaction with vapor or drops of a silicon melt, reaction with gas produced from the silicon melt and carbon, reaction with the quartz crucible, and a thermal characteristic in the hot zone of the CZ furnace is also changed with time.
Therefore, it is presumed that the OFS ring diameter deviates from the initial set value as the number of pulling times increases.
According to the invention, the change of the OSF ring diameter is examined, a prediction formula of the average pulling rate pattern is corrected by the results of the previous batch.
Therefore, even when the thermal characteristics of the hot zone structure in the furnace change with time, a crystal having the OSF ring positioned at the target position can be obtained according to the corrected pulling rate pattern.
The invention can use the result of the initial batch or one batch other than the initial batch to correct the prediction value of the average pulling rate.
When measuring the OSF ring position in order to correct the prediction value, it is not necessary to measure the OSF ring positions over the entire crystal, it is sufficient by measuring the OSF ring generation position at representative main positions of several crystals. Also, the measuring may be satisfactorily made by an inferring method such as an interpolation method and an extrapolation method.
The invention described in claim 3 relates to a single crystalline silicon wafer, which is formed of only an oxygen precipitation area and/or an oxygen precipitation suppressing area grown by the Czochralski method.
Thus, when the pulling rate is adjusted by the CZ method to grow silicon single crystal consisting of only the OSF ring, the oxygen precipitation area and/or the oxygen precipitation suppressing area, a silicon single crystal having stable quality can be obtained by pulling the crystal so that the OSF ring area is formed at the center of the crystal, and predetermined areas are formed on the outside of the OSF ring area in the crystal. In other words, the outer diameter area of the OSF ring area is an index for crystal quality.
A silicon wafer obtained from this single crystal is a high-quality silicon single crystal free from the formation of grown-in defects.
The invention described in claim 4 relates to a single crystalline silicon wafer, which is characterized in that a single crystalline silicon wafer, which is formed of only an oxygen precipitation area and/or an oxygen precipitation suppressing area grown by a Czochralski method, undergoes an oxygen out diffusion treatment.
When the single crystalline silicon wafer is formed from this single crystal and subjected to the oxygen out diffusion treatment, supersaturated oxygen present on the wafer surface layer is out diffused, and a high-quality single crystalline silicon wafer, which does not have oxygen precipitates or any secondary defects induced thereby on the wafer surface layer serving as the device active region, can be formed.
The invention described in claim 5 relates to a single crystalline silicon wafer, which is characterized in that a single crystalline silicon wafer, which is formed of only an oxygen precipitation area and/or an oxygen precipitation suppressing area grown by a Czochralski method, undergoes an oxygen out diffusion treatment and further an oxygen precipitation nucleus forming treatment.
The single crystalline silicon wafer formed of the crystal consisting of only the OSF ring area, the oxygen precipitation area and/or the oxygen precipitation suppressing area grown by the Czochralski method becomes a high-quality silicon wafer free from grown-in defects. When this silicon wafer is subjected to the oxygen precipitation dispersing treatment, the wafer surface layer to be the device active region is formed as a denuded zone (DZ), free from defects such as oxygen precipitates. Besides, when the silicon wafer is subjected to the oxygen precipitation nucleus forming treatment, a high-quality single crystalline silicon wafer, which has an IG layer having an intrinsic gettering effect formed in the wafer, is formed.