Method of controlling growth of a semiconductor crystal to automatically transition from taper growth to target diameter growth

A control method for use with a crystal puller for growing a monocrystalline semiconductor crystal according to the Czochralski process, said crystal puller having a heated crucible containing a semiconductor melt from which the crystal is grown, said crystal being grown on a seed crystal pulled from the melt, said method comprising the steps of:

• pulling the growing crystal from the melt at a first target pull rate, said first target pull rate substantially following an initial velocity profile for growing a taper portion of the crystal, said taper portion of the crystal having a generally increasing diameter;
• measuring the crystal diameter during the taper portion of growth;
• estimating a slope of the crystal diameter, said estimated slope being a function of a change in crystal diameter relative to time during the taper portion of growth and the first target pull rate;
• predefining a crystal diameter measurement Di at which to initiate a transition to a body portion of the crystal from the taper portion of the crystal, said body portion of the crystal having a substantially uniform diameter greater than the predefined diameter measurement Di;
• determining a second target pull rate as a function of the estimated slope and one or more hotzone parameters, said second target pull rate corresponding to growth of the crystal to achieve the transition to the body portion of the crystal at a target diameter with zero slope;
• and
• increasing the pull rate to the second target pull rate when the measured crystal diameter reaches the predefined crystal diameter measurement Di for controlling a transition from the taper portion of the crystal to the body portion of the crystal.