Patent Application
20080053370
In feature of the silicon single crystal production method of the invention, when the crystal raw materials in the crucible are melted to dip the seed crystal into the melt in the crucible by the CZ method, after the melt temperature is set to the optimum temperature at which the seed crystal is brought into contact with the melt surface, the melt temperature is lowered, the seed crystal is pulled up while the pulling rate is increased, and the neck portion is formed by setting the pulling rate to a constant rate at the time that the pulling diameter reaches the target neck diameter.
In the silicon single crystal production method of the invention, because the transition is made to the dip process after the crystal raw materials in the crucible are melted, in dipping the seed crystal into the melt of the crucible, it is necessary that the melt temperature be set to the optimum temperature at which the seed crystal is brought into contact with the melt surface.
As used herein, “the optimum temperature at which the seed crystal is brought into contact with the melt surface” shall mean the melt temperature stabilized by the so-called “seed crystal fitting” operation, and “the optimum temperature at which the seed crystal is brought into contact with the melt surface” means the temperature of the melt surface in which the meniscus shape at the contact interface, e.g., the hang-over of a crystal-laiden line is observed to estimate the melt surface temperature, the heater power (electric power) is controlled based on the estimation, and the input heat to the melt are adjusted to stabilize the melt surface. In the specific operation, in the state in which the growth rate of the seed crystal 1 is zero, the heater power is adjusted to stabilize the melt surface temperature such that the meniscus 3 having the predetermined shape is formed around the leading end portion of the seed crystal 1.
The temperature at which the melt temperature is stabilized depends on the pulling apparatus used, and the temperature at which the melt temperature is stabilized varies when the seed crystal diameter or target neck diameter varies even in the same pulling apparatus. Therefore, “the optimum temperature at which the seed crystal is brought into contact with the melt surface” defined in the invention cannot uniquely be determined.
Accordingly, in the silicon single crystal production method of the invention, it is necessary that the “optimum temperature” be correctly acquired to control the melt temperature. Usually, a method for controlling the melt temperature is adapted such that the heater power (electric power) is adjusted to control the heater temperature based on data obtained by optically measuring the melt temperature.
However, in a method for measuring the melt surface temperature using means for optically measuring the temperature such as a single-color thermometer and a two-color thermometer, the temperature measurement is easily affected by a disturbance factor such as SiO evaporation generated in growing the crystal, and the measured temperature varies depending on a measurement point of the melt surface due to the influence of melt heat convection. Therefore, the measured temperature has poor reliability, and the measured temperature cannot be applied to the Dash method in which the precise temperature control is required.
In the silicon single crystal production method of the invention, desirably the heater temperature at which the crystal raw materials in the crucible are melted is measured, and the melt temperature is adjusted by adjusting heater power (electric power) to control the heater temperature based on the temperature measurement result. The heater temperature corresponds to the melt temperature one-on-one, and the heater temperature is not affected by the disturbance factor such as the SiO evaporation or the melt heat convection even if the heater temperature is measured with the radiation thermometer or the two-color thermometer, so that the melt temperature can correctly be measured as the optimum temperature at which the seed crystal is brought into contact with the melt surface.
In the silicon single crystal production method of the invention, it is mandatory that after the melt temperature is set to the optimum temperature at which the seed crystal is brought into contact with melt surface, the melt temperature is lowered, and the pull-up is performed while the pulling rate of the seed crystal is enhanced.
Even in the state in which a temperature distribution of the melt is stabilized, when the melt temperature is kept at the optimum temperature at which the seed crystal is brought into contact with melt surface, the neck portion having the target diameter cannot be formed because the melt temperature is in the higher state, or the neck portion cannot be grown because the severance is generated between the seed crystal and the melt. Therefore, it is necessary that the melt temperature be lowered to the temperature suitable to the formation of the neck portion, and the melt temperature is lowered by decreasing the heater power (electric power) to control the heater temperature. At this point, desirably the melt temperature is lowered by an extent in the range of 4 to 5° C.
As described above, even if the heater power for heating the crystal raw materials in the crucible is lowered to start the heater temperature control, the time lag is generated until the melt temperature reaches the temperature suitable to the formation of the neck portion, and the liquid covering is generated in the outer peripheral portion of the seed crystal before the melt temperature reaches the temperature suitable to the formation of the neck portion, which sometimes causes the generation of the dislocation. In this regard, in the silicon single crystal production method of the invention, the liquid covering is not generated in the outer peripheral portion of the seed crystal by pulling the seed crystal while the pulling rate of the seed crystal is increased.
Because the pulling rate which can be applied to the formation of the neck portion also depends on a hot zone structure (temperature distribution inside a crucible) of the growing apparatus, the pulling rate cannot quantitatively be determined. According to studies of the inventors, in pulling up the silicon single crystal having a diameter of 300 mm, the pull-up is effectively performed while the pulling rate is gradually increased in the range of 0 to 5 mm/min. Although desirably the pulling rate is linearly increased, the pulling rate may be increased in a step manner of a short time duration.
Accordingly, in the silicon single crystal production method of the invention, desirably the melt temperature is lowered by an extent in the range of 3 to 4° C., and the seed crystal is pulled up while the pulling rate of the seed crystal is gradually increased in the range of 0 to 5 mm/min. Thus, the melt temperature is lowered by an extent in the range of 3 to 4° C., and the seed crystal is pulled up while the pulling rate of the seed crystal is gradually increased. Therefore, the reduced portion 4a whose diameter is successively shrunk is formed in the lower end portion of the seed crystal shown in
In the silicon single crystal production method of the invention, the pulling rate is set to a constant rate at the time that pulling diameter reaches the target neck diameter, and the neck portion is formed as shown in
In the seed crystal used in the silicon single crystal production method of the invention, a crystal diameter of the lower end portion thereof which is in contact with the melt surface is set to 8 mm or less. Usually, the seed crystal diameter ranges from 20 to 10 mm, and a highly sophisticated technique is required to form the reduced portion to narrow down the reduced portion to the neck portion target diameter ranging from 3 to 6 mm. In dipping the seed crystal in the melt, the fluctuation in melt temperature becomes large. Therefore, in order to stabilize the melt temperature distribution while the strength of the neck portion is secured, the crystal diameter of the lower end portion which is in contact with the melt surface can be set to 8 mm or less.
The silicon single crystal production method of the invention is suitable to the case in which the seed crystal having the crystal orientation <110> is used to pull up the silicon single crystal having the crystal orientation <110>. As described above, when compared with the seed crystal having the crystal orientation <100>, from the standpoint of the crystal structure, the seed crystal having the crystal orientation <110> includes a crystal plane {111} which is a slip plane parallel to the pulling axial direction. Therefore, the dislocation generated in the seed crystal resulting from the contact with the melt surface hardly escapes out of the seed crystal, and the dislocation is taken over to the grown crystal through the neck portion. However, in the present invention, when the seed crystal is dipped in the melt, the melt temperature is set to the optimum temperature at which the seed crystal is brought into contact with the melt surface, whereby the generation of a thermal strain caused by the dip of the seed crystal can be eliminated as much as possible to suppress the generation of the slip dislocation.
The advantages of the silicon single crystal production method according to the present invention will be described based on Examples in which the specific process is applied. In Examples, the 8-inch silicon single crystals having the crystal orientations <100> and <110> respectively are grown, and a test of a dislocation-free ratio is performed.
First the 140-kg polycrystalline silicon materials which are the crystal raw materials are loaded in the 24-inch quartz crucible, and the crystal raw materials in the crucible are melted. At the stage in which the transition is made to the dip process using the seed crystal having the crystal orientation <100>, the downward movement of the seed crystal is tentatively halted before the seed crystal is dipped in the melt, and the seed crystal is pre-heated to increase the temperature of the seed crystal, which releases the thermal shock (heat shock) caused by the contact with the melt surface. Then, the seed crystal moves downward while rotated, the downward movement of the seed crystal is halted, and the fitting operation is performed to achieve the stabilization such that the melt temperature becomes the optimum temperature at which the seed crystal is brought into contact with the melt surface.
After observing that the predetermined meniscus shape is formed at the lower end of the seed crystal, the melt temperature is lowered by 4 to 5° C. to adjust the heater power such that the melt temperature becomes the temperature suitable to the formation of the neck portion. The operation for pulling up the seed crystal is started at the same time of starting the heater power control, and the target diameter of 5 mm is obtained in the neck portion while the pulling rate is gradually increased from 0 mm/min. After the target diameter is obtained in the neck portion, the neck portion is formed at the constant pulling rate of 5 mm/min, and the shoulder portion, the body portion, and the tail portion are successively pulled up.
The test of the dislocation-free ratio is performed for the 20 silicon single crystals thus pulled up, and the dislocation-free ratio of 90% (18 are good out of 20 silicon single crystals) is obtained.
Then, the silicon single crystal having the crystal orientation <110> is pulled up using the seed crystal having the crystal orientation <110>, and the test of the dislocation-free ratio is performed. Similarly, the 140-kg polycrystalline silicon materials which are the crystal raw materials are loaded in the 24-inch quartz crucible, and the crystal raw material in the crucible is melted.
In the dip process, after the seed crystal is pre-heated, the seed crystal moves downward while rotated, and the seed crystal is dipped in the melt. Then, the downward movement of the seed crystal is halted to perform the fitting operation, and the fitting operation is performed to achieve the stabilization such that the melt temperature becomes the optimum temperature at which the seed crystal is brought into contact with the melt surface.
After observing that the predetermined meniscus shape is formed at the lower end of the seed crystal, while the melt temperature is lowered by 4 to 5° C. to adjust the heater power such that the melt temperature becomes the temperature suitable to the formation of the neck portion, the operation for pulling up the seed crystal is started. Then, the target diameter of 5 mm is obtained in the neck portion while the pulling rate is gradually increased from 0 mm/min to 5 mm/min. After the target diameter is obtained in the neck portion, the neck portion is formed at the constant pulling rate of 5 mm/min, and the shoulder portion, the body portion, and the tail portion are successively pulled up.
The test of the dislocation-free ratio is performed for the 18 silicon single crystals thus pulled up, and the dislocation-free ratio of 83% (15 are good out of 18 silicon single crystals) is obtained.
According to the silicon single crystal production method of the invention, in pulling up the silicon single crystals having the crystal orientations <100> or <111> and <110>, the generation of the slip dislocation is suppressed, and the dislocation-free can easily be achieved to enhance the crystal quality irrespective of the crystal orientation by improving the dip process of dipping the seed crystal in the melt and the necking process of forming the neck portion in the Dash method. Therefore, the production cost is largely reduced, and the invention can widely be applied as the efficient Dash method.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-240458 | Sep 2006 | JP | national |
