Method for manufacturing single crystal

A method for manufacturing a single crystal, comprising:

• setting in advance, for each of a plurality of pull-up lengths, a target value of a pull-up speed of the single crystal, wherein the setting is performed before starting pulling-up of the single crystal by a Czochralski method;
• calculating a pull-up speed moving average value from a plurality of actual values of pull-up speeds measured from a starting time point that pulling-up of a predetermined length of the single crystal has been started until a current time point while the single crystal is being pulled up;
• calculating, at the current time point, a corrected value of the target value of the pull-up speed based on the target value of the pull-up speed at the current time for a corresponding pull-up length and the pull-up speed moving average value as of the current time, such that the pull-up speed moving average value matches the target value of the pull-up speed when there is a difference between the pull-up speed moving average value and the target value of the pull-up speed;
• achieving the target value of the pull-up speed at the current time point by continuously and repeatedly conducting a correction of a target value of a pull-up speed from the starting time point until the current time point;
• pulling up the single crystal based on the calculated corrected value of the target value of the pull-up speed at the current time point,
• wherein
• when α is a past pull-up length for calculating an actual value of the pull-up speed moving average value and β is a future pull-up length,
• a corrective pull-up length (α+β) used for calculating the corrected value of the target value of the pull-up speed is changed in accordance with an actual value of the diameter of the single crystal,
• wherein the past pull-up length is a pull-up length relating to the single crystal at a past time point that is earlier than the current time point, and the future pull-up length is a pull-up length relating to the same single crystal at a future time point that is later than the current time point, and
• wherein
• a state of pulling up the single crystal is monitored, and when |D1−D0| is not more than a threshold value t1 where D0 is a target value of the diameter of the single crystal and D1 is the actual value of the diameter of the single crystal, the corrective pull-up length (α+β) used for calculating the corrected value of the target value of the pull-up speed is set to be shorter than a reference pull-up length γ used for evaluating the pull-up speed moving average value,
• when |D1−D0| is greater than the threshold value t1 and is not more than a threshold value t2, the corrective pull-up length (α+β) used for calculating the corrected value of the target value of the pull-up speed is set to be maintained at the reference pull-up length γ used for evaluating the pull-up speed moving average value, and
• when |D1−D0| is greater than the threshold value t2, the corrective pull-up length (α+β) used for calculating the corrected value of the target value of the pull-up speed is set to be longer than the reference pull-up length γ used for evaluating the pull-up speed moving average value.