Crystal pulling process

The process of growing a pure crystal is sensitive to a host of process parameters that impact the iacorporation of impurities ia the crystal, the quality of the crystal stmcture, and the mechanical properties of the crystal rod. For example, the crystal-pulling mechanism controls the pull rate of the crystallisa tion, which affects the iacorporation of impurities ia the crystal, and the crystal rotation, which affects the crystal stmcture. 

The system is usually evacuated to a suitable characteristic pressure before the actual working process begins. This happens, for example, in plants tor evaporative coating, electron-beam welding, and crystal pulling in particle accelerators, mass spectrometers, electron microscopes and others. 

Figure 19 shows sample isotherms and interface shapes predicted by the QSSM for calculations with decreasing melt volume in the crucible, as occurs in the batchwise process. Because the crystal pull rate and the heater temperature are maintained at constant values for this sequence, the crystal radius varies with the varying heat transfer in the system. Two effects are noticeable. First, decreasing the volume exposes the hot crucible wall to the crystal. The crucible wall heats the crystal and causes the decrease in... 

Process Stability and Control. Operationally, automatic control of the crystal radius by varying either the input power to the heater or the crystal pull rate has been necessary for the reproducible growth of crystals with constant radius. Techniques for automatic diameter control have been used since the establishment of Czochralski growth. Optical imaging of the crystal or direct measurement of the crystal weight has been used to determine the instantaneous radius. Hurle (156) reviewed the techniques currently used for sensing the radius. Bardsley et al. (157,158) described control based on the measurement of the crystal weight. 

In the crucible-pulling process [8], this high-purity Si, in a light vacuum or in an atmosphere of Ar or He, is melted in an Si02 crucible supported by a carbon crucible. A seed Si crystal, with prominent (100) or (101) faces, on the tip of a rod, is made to touch the melt, then the rod is pulled slowly... 

Montforte, F.R., Swanekamp, F.W., and Van Uitert, L.G. (1961) Radio-frequency technique for puUing oxide crystals without employing a crucible susceptor, J. Appl. Phys. 32, 959. First application of the skull melting process combined with crystal pulling to produce single crystals. 

Producing Junction Transistors. Junction transistors can be produced during the original crystal growing process for the silicon (or germanium) crystal by adding known n-type and petype impurities to the molten semiconductor as the solid crystal is slowly pulled from the melt. 

Silicon is produced by a well-controlled Czochralski crystal growth process in a very clean environment, that is, class of 1 or 10. In this process, a small seed crystal is dipped into a highly purified silicon melt. This seed is slowly pulled while the crucible containing the melt is rotated. The silicon crystal grows along the selected orientation of the seed to the rod. A cylindrical crystal is obtained from which slices are cut. This is followed by the atomic polishing phase. The side of one cubic face is 5.43A. The mechanical, electrical, and thermal properties of silicon have been presented in Table 10.1. 

Within the framework of this chapter, both the description of the principles and merits of the continuous crystal pulling techniques are given and the notions are provided about the hardware for industrial-scale technologies. Separately, we shall dwell on the state-of-the-art technologies, i.e. we shall try both to demonstrate the possibilities of the universal approach to the continuous crystal-growth process and prove that these techniques enable us to vary and modify the level of perfection of the grown single crystal. 

In this way, the growth process control comes down to the control exercised over three parameters temperatures of the bottom (Tbot) or lateral (Ti t) heaters, crystal pulling, and melt feeding rates. Since ds is determinable mostly by the value of WflVy, programming of this ratio leads to automatic radial outgrowth of the single crystal from the seed to the final diameter. 

The informative parameter Ax makes for a smooth crystal pulling at a constant rate that also simplifies the structure of the facility and the growth process itself Employment of an intermediate heater with the melt enabled the crucible rotation unit to be omitted and the structure of the facility to be made more rugged. 

The presence of defects and impurities is unavoidable. They are created during tire growtli or penetrate into tlie material during tlie processing. For example, in a crystal grown from tire melt, impurities come from tire cmcible and tire ambient, and are present in tire source material. Depending on factors such as tire pressure, tire pull rate and temperature gradients, tire crystal may be rich in vacancies or self-interstitials (and tlieir precipitates). 

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