Growth of semiconductor single crystals

Details Growth of semiconductor single crystals Growth of semiconductor single crystals

1993-02-22

2001-09-25

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from liquid or supercritical state

Having pulling during growth

C117S014000, C117S015000, C117S024000, C117S202000, C117S208000

Reexamination Certificate

active

06294017

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for growing single crystalline material.
2. Discussion of Prior Art
Apparatus for growing crystals by the known Czochralski technique is described for example in GB 1,494,342. A seed crystal is dipped into a melt of the crystal to be grown then rotated and slowly withdrawn. By suitable adjustment of the melt temperature and the rotation and withdrawal rates a desired shape of crystal is grown. Ideally a crystal increases smoothly from the seed diameter up to the desired diameter and then continues at a uniform diameter until tapered off at growth end. Uniform growth diameter has been achieved for the difficult to grow III-V materials such as GaAs with apparatus as described in GB 1,494,342 where the growing crystal is weighed by a load cell during growth.
In this the crystal weight or some function of weight is compared with an expected value and any error which occurs is used to correct the power supplied to the melt and/or the pull speed.
One weakness of the method may be found during the grow-out phase when the system attempts to control the increase in diameter from a narrow seed to a much larger final diameter. The signals are initially small and the control of cone angle is not precise. A simple increase in system gain during this phase may help but can easily lead to instability or roughness of the crystal shape leading to crystalline faults.
Smooth control of cone angle is important in the growth of many materials including GaAs single crystals in order to achieve low densities of dislocations in the crystal.
A further and more severe problem arises when certain semiconductors (e.g. GaAs) are grown at low pull rates especially combination with isothermal melts required for better quality crystals. It is possible for the crystal to grow out across the surface of the melt and simultaneously to be growing down under the surface. In spite of this real growth of the crystal the weight sensor may detect little or no increase in weight of the crystal. This lack of weight change is due to the near horizontal surface tension forces (which therefore have little effect on the load cell) and also the fact that the density of the solid in the ‘anomalous’ materials is less than that of the liquid. This means that the solid below the melt surface is buoyant and subtracts from the measured weight.
Under these conditions control may be lost completely and the melt surface freezes, often breaking the seed and terminating the run. This can be an expensive failure in production runs where large and costly melts are involved.
SUMMARY OF THE INVENTION
The present invention overcomes the above problem by providing a method and apparatus for controlling the grow-out phase during crystal growth.
According to this invention a method of controlling the diameter of a crystal in a crystal growing system includes
the steps of providing a melt of material in a crucible heated by a heater,
bringing a seed crystal into contact with the melt,
providing relative rotation between the seed crystal and melt,
withdrawing and rotating the seed slowly so that a crystal grows from the seed crystal,
weighing the growing crystal or the melt to provide a signal indicative of the weight of the growing crystal,
providing a feedback loop using the weight signal to control the heater and hence control crystal diameter,
characterised by
the steps of applying small perturbations to the system,
providing a signal processing on measured crystal weight signals,
and using the result of the signal processing to vary the growth conditions and obtain the required diameter changes during grow out from the seed diameter to the full crystal diameter.
The perturbations may be variations in: the power signal to the heater, the pull rate, the rotation rate of crystal and/or crucible, or an applied magnetic field. These perturbations may be a pseudo random series of pulses, a rectangular wave, or a series of short pulses, or even the noise inherent in growth systems.
The signal processing may be cross or auto correlation, or signal averaging, or Fourier analysis.
The results of the signal processing may be used to vary the power applied to the heater, or the pull or rotation rates or the strength of an applied magnetic field.
The perturbations may be applied during grow out from a seed to a full diameter phase only or during the growth of the complete crystal.
The amount of perturbation is small. When growing anomalous material it is in principle quite simple to obtain an immediate indication of the crystal diameter masked by the growth under the melt. For example the pull rod is suddenly moved axially. The change in apparent weight of the crystal indicates the present diameter. Following measurement the crystal is restored to its original position. In practice there is considerable noise present in the crystal pulling system due to, e.g. vibration, convection in the melt, turbulence of the hot gases etc. This means that large movements of the crystal would be required in order to achieve accurate measurement, especially for small seed diameters. Such large perturbations would seriously degrade the crystal perfection and therefore a more refined technique is necessary.
The signal processing of the present invention allows use of small perturbations that do not degrade crystal growth, and yet provide accurate measurement of growing diameters. A typical perturbation amplitude may be less than the amplitude range of noise present.
According to this invention apparatus for growing crystals comprises:
a crucible for containing a melt of the crystal to be grown,
a heater for heating the melt,
means for providing relative rotation between a seed crystal and the melt,
means for pulling a crystal from the melt,
a load cell cell for measuring the weight of a growing crystal, and feedback means for controlling the temperature of the melt in response to changes in the load cell output to grow a controlled diameter crystal,
characterised by:
signal processor means for processing signals received from the load cell and from signals applied to the feedback means,
means for comparing the output of the signal processor with a reference signal to provide a correction signal for the feedback means.
The signals applied to the feedback means may include small amplitude test signals of a pseudo random nature, a rectangular wave, or other suitable perturbing shape. Such test signals may be applied from a test signal generator and mixed into the signals applied to the feedback means.
The signal processor may include a cross or auto correlator, a signal averager, or a Fourier analyser.
An interface may be arranged between the load cell and the signal processor whereby part of the apparatus operates on analogue signals and the remainder operates on digital signals. The signals applied to the heater, and pull rod and signals received from the load cell may be analogue whilst the signal processing and error correction is performed with digital signals. Alternatively the whole system may operate with digital signals or with analogue signals.
Low pull rates and isothermal melts are needed for a variety of crystals where high crystal perfection is required. For example the materials which could be used in integrated circuits e.g. Si, GaAs, InP, InAs, InSb, Ge. Also for detector materials a range of halides and chalcogenides are needed eg CdTe, and PbTe.


REFERENCES:
patent: 2908004 (1959-10-01), Levinson
patent: 3013721 (1961-12-01), Foster
patent: 3621213 (1971-11-01), Jen et al.
patent: 3761692 (1973-09-01), Cope
patent: 3934983 (1976-01-01), Bradsley et al.
patent: 3980438 (1976-09-01), Castonguay et al.
patent: 4668481 (1987-05-01), Watanabe et al.
patent: 4857278 (1989-08-01), Gevelber et al.
patent: 1816068 (1969-08-01), None
patent: 0115121 (1984-08-01), None
patent: 1 434 527 (1976-05-01), None
patent: 1 465 191 (1977-02-01), None
patent: 1 494 342 (1977-12-01), None
patent: 2140704A (1984-12-01), None
patent: 58-145694 (1983-08-01), None
patent: 58-14

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