Heat Transfer Control During Crystal Growth

This invention relates to a heat transfer control system for controlling temperature and temperature gradient related defects generated in a crystal during growth by regulating thermal stress in the growing crystal.

Recent advances in high performance integrated circuit technologies have resulted in increased demand for high quality, large diameter semi-insulating material such as gallium arsenide, GaAs. Gallium arsenide is a III-V semiconductor material referring to the groups to which gallium and arsenic belong in the Periodic Table.

The primary crystalline defect in III-V semiconductor crystals grown by the LEC (liquid encapsulated Czochralski) technique is the presence of massive dislocation networks resulting from excessive thermal stresses experienced by the crystal solid during growth. Significant reduction in the density of dislocation is required for improved performance of opto-electronic as well as digital and monolithic IC devices utilizing these crystals. It has been recognized for several years that reduction of thermal stress levels below the plasticity limit requires, in principle, a reduction of temperature gradients in the solid to below critical values. Known attempts to control temperature gradients in the solid involve control of the environmental temperature distribution surrounding the crystal during growth. The known techniques have not, however, resulted in the production of dislocation-free GaAs crystals having a diameter on the order of three inches.

The apparatus for controlling heat flux from a crystal surface during crystal growth from a melt includes plural coaxial heat transfer modules surrounding the crystal and stacked along the crystal axis. The modules may be separated from one another by an insulating material. Each module includes temperature sensing devices for monitoring a temperature drop across a radially extending gradient region of the module. The module also includes a heating element responsive to the temperature drop to control heat flux through the module. In preferred embodiments, the coaxial heat transfer system penetrates into the melt for interface morphology control and extends beyond the crystal top to control heat transfer from the crystal top. The heat flux modules are mounted in a housing which forms a heat exchange cavity.

In a preferred embodiment, each heat transfer module includes a first high temperature material (ceramic or nickel) element facing the crystal and including a first temperature monitoring thermocouple. A second high temperature material element is radially spaced from the first high temperature material element and includes a second temperature monitoring thermocouple. An insulating member is disposed between the first and second high temperature material elements to create the gradient region across which the temperature drop is measured. It is preferred that this insulating member have low thermal conductivity to increase the temperature drop across the gradient region to provide high sensitivity.

FIG. 1 is a cross-sectional, elevation view of the invention disclosed herein; and

FIG. 2 is a cross-sectional, elevation view of a heat transfer control module.

The invention disclosed herein reduces the thermal stress levels in crystals to below the critical resolveld shear stress (CRSS) values of the matrix. This is achieved by controlling the heat flux from the crystal surface by surrounding the growing crystal with a coaxial heat transfer control system. The design of the heat transfer control system to be described in detail below is based on analytical results obtained from the thermoelastic study of III-V crystals grown by the LEC technique. This study indicates that since thermal stresses in the crystal are controlled by temperature gradients in the solid, the appropriate parameter to be controlled is the heat transfer from the crystal surface, not the environmental temperature distribution. The maximum rate of heat flux from the crystal (required for practical growth rates) consistent with maintaining the thermal stress levels below the plasticity limit are dynamically infuenced by crystal length.

The heat transfer control apparatus for regulating thermal stresses in a growing crystal is shown in FIG. 1. A crystal 10 such as GaAs or InP is being grown from a melt 12. A heat transfer control system 14 surrounds the crystal 10 and is coaxial with it. The heat control system 14 includes a plurality of modules such as the circled module 16 stacked one on top of the other. The modules extend beyond the top of the crystal 10 to control heat flux from the top surface. The modules are separated from one another by a low thermal conductivity insulation material 18. Because of the thermal isolation of the individual modules 16, heat flux from the crystal at any axial location is equal to the heat transfer through the corresponding module. The heat transfer control modules 16 are mounted within a housing 20 preferably made of INCONEL 9 nickel-base alloy of steel. A lower portion 22 of the housing 20 is designed to penetrate into the melt 12 so as to provide the additional capapbility of interface morphology control by controllilng the radial temperature gradient at the solidification interface. The lower portion 22 which penetrates into the melt 12 is made of boron nitride to reduce potential contamination of the melt 12. A thick piece of insulating material 24 thermally isolates the system 14 from the melt 12 to provide the accuracy needed for the control of heat flux through a lowermost module 26. Heat from the modules is extracted from a cavity 27 in the housing 20 by natural and/or forced convection of a coolant which may be the ambient gas or a fluid coolant pumped through a wound coaxial piping system 29 made of a high temperature material.

FIG. 2 shows one of the heat transfer control modules 16 in more detail. The module 16 includes a first high temperature ceramic element 30 which faces the crystal 10. Mounted within the first ceramic element 30 is a first thermocouple 32. A second ceramic element 34 includes a second thermocouple 36 and a heating element 38. The second ceramic element 34 is separated from the first ceramic element 30 by a low thermal conductivity insulating material 40. An insulating element 42 along with the insulating elements 18 completes the structure of the heat transfer control module 16. The region between the heat transfer control module 16 and the housing 20, preferably of INCONEL, a nickel-base alloy of steel, forms the heat exchange cavity 27.

Heat flux through each of the modules 16, and therefore heat flux from the crystal surface, is controlled independently by monitoring the temperature drop across the gradient region between the thermocouples 32 and 36 and adjusting the power into the heating element 38 to achieve the desired magnitude of heat flux. The insulating material 40 between the ceramic elements 30 and 34 is a low thermal conductivity material to provide high sensitivity in measuring the temperature drop across the insulator 40. The heat flux across each module 16 is direcly proportional to the heat flux at the crystal periphery so that the need for accurate quantification of the heat atransfer modes in the frowth furnace is eliminated. By controlling the heating element 38 in response to the outputs of the thermocouples 32 and 36, the heat flux from the surface of the crystal 10 is controlled so that the thermal stress level in the growing crystal is maintained below the critical resolved shear stress value. In this way, large diameter crystals are grown which are substantially dislocation free .