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1993 | Buch

Introduction and Historical Perspective Kapitel lesen Erstes Kapitel lesen

Crystal Pulling from the Melt

verfasst von: Dr. Donald T. J. Hurle

Verlag: Springer Berlin Heidelberg

Enthalten in: Professional Book Archive

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Über dieses Buch

Crystal pulling is an industrial process and provides the bulk of semiconductor crystals for the semiconductor industry. Initially a purely empirical process, the increase in importance and size of the industry has led to basic research into the fundamentals of the process - particularly the modelling of heat and mass transfer. The book has been written by the recognized authority on Czochralski crystal-growth techniques. It is an attempt to strengthen the interface between the practical crystal grower and the applied mathematician involved in analytical and computer modelling. Its focus is on the physics, chemistry and metallurgy of the process. From reviews: "... There is a need for a modern, non-trivial text on Czochralski growth ... and Dr. Hurle is eminently suited to write such a text."; "Dr. Hurle is probably uniquely qualified to write a book on ... (the Czochralski) growth process. ... He has published a great deal of very substantial as well as innovative work in this area."

Inhaltsverzeichnis

Frontmatter
1. Introduction and Historical Perspective
Abstract
The elegant, but deceptively complex, technique of pulling single crystals from a melt by first dipping a seed crystal into that melt has been developed over the last forty years to become the dominant technique for the production of bulk single crystals of a wide range of materials for the electronics and electro-optic industries.
Donald T. J. Hurle
2. Elements of the Process
Abstract
At its most basic (Fig. 2.1) the technique consists of a crucible which contains the charge material to be crystallised and a heater arrangement (shown as RF induction heating in the figure but various other forms are also in common use) to heat the crucible and charge to above the melting point of the latter. A pull rod with a chuck containing a seed crystal at its lower end is positioned axially above the crucible. This seed crystal is dipped into the melt and the melt temperature adjusted until a meniscus is supported. The pull rod is then slowly rotated and lifted and, by careful adjustment of the heater power, a crystal of the desired diameter can be grown. The whole assembly is maintained in an envelope which permits control of the ambient gas and enables visual observation of the crystal to be made. Figure 2.2 shows a small silicon single crystal being pulled from a melt contained in a silica crucible heated by RF induction.
Donald T. J. Hurle
3. Techniques and Technology
Abstract
A detailed description of the process can only be made with respect to specific materials. The wide applicability of the technique — from materials like bismuth which melts at only 271 °C [76] up to refractory oxides melting at temperatures approaching 2500 °C [77] necessitates a diverse technology. There is no such thing as the universal crystal pulling machine. However, research pullers are usually modular in construction to permit fitment of different heaters, chambers etc. Commercial pullers, on the other hand, are designed for one specific material; indeed often for just one specific size and specification of single crystal of that material.
Donald T. J. Hurle
4. Convection and Flow in the Melt
Abstract
The importance of melt flow in controlling dopant incorporation and distribution on both macroscopic and microscopic scales was made evident in the previous chapter for the cases of silicon, gallium arsenide and oxide crystal growth.
Donald T. J. Hurle
5. Heat Transport
Abstract
In a later chapter we progress to asking questions about the stability of the growth process and to this end we need to understand the heat transfer processes throughout the crystal pulling assembly. We have seen that numerical simulation provides physical insight into the nature of the flows in the melt and their coupling to the thermal boundary conditions but that to deal with questions of stability one must properly take account of the crystal-melt interface as a thermodynamic phase boundary whose shape represents the solution of the problem and not a boundary condition. Such problems are known as Stefan problems (see for example Ockendon and Tayler [79]). First, though, we determine the temperature field in the cooling crystal.
Donald T. J. Hurle
6. Mass Transport and Solute Segregation
Abstract
Most crystals required for commercial application need to be doped with a deliberately added solute in order to acquire the needed properties. (One significant exception to this is undoped semi-insulating gallium arsenide). The device specification will require that the doping level is precisely controlled and that it be uniform throughout the crystal boule both on a macroscopic and on a microscopic scale. Since the pulling process is essentially a batch process, segregation of solute at the interface results in a progressive increase in solute concentration in the crystal as the melt is consumed for solutes having a segregation coefficient (k) less than unity. The reverse is true for solutes with k greater than unity.
Donald T. J. Hurle
7. The Use of a Magnetic Field
Abstract
The idea for the use of a magnetic field to damp melt turbulence and thereby improve microscopic homogeneity of the crystal was introduced in 1966 independently by Utech and Flemings [112] and by Chedzey and Hurle [113]. However its application to crystal pulling did not come until much later. In the late 1970s, workers with large silicon pullers observed systematic anomalies in oxygen content in crystals grown on apparently similar pullers. Specifically it was found that differing oxygen contents could be obtained depending on the sense of the crucible rotation, always with the seed rotating counter to the crucible [114]. Eventually it was discovered that this was due to the rotating magnetic field generated by the three-phase heater elements.
Donald T. J. Hurle
8. System Dynamics and Automatic Diameter Control
Abstract
In order to control the process effectively, the stability of crystal pulling against fluctuations in environmental conditions such as heater power, pull-rate etc. must be evaluated, i.e. we need to measure the dynamical response of the system to such perturbations. In the language of the electronic control engineer, the transfer function of the process must be determined. Specifically we need to know whether or not the process is stable, i.e. that, if we perturb it slightly, whether it will decay back to its original state. If the process is not stable then a servo-control system will be needed in order to maintain the desired growth conditions. In particular we will wish to grow a cylindrical crystal i.e. to ensure that the crystal radius remains constant. The response of the overall system has to be investigated but the crucial part of the process is the dynamics of the meniscus region and this is considered in the next section.
Donald T. J. Hurle
9. Morphological Stability of a Planar Rotating Interface
Abstract
Inhomogeneity of chemical composition on a macroscopic scale resulting from the segregation of solute at the crystal-melt interface during solidification of a finite melt has already been considered. Additionally it has been shown that a macroscopic radial non-uniformity can result from flow conditions under which the solute boundary layer does not remain embedded within a momentum boundary layer flow characterised by the rotating crystal. Such a situation can occur for example in the presence of an axial magnetic field. Further, we saw in Chap. 6 how the response of the interface to oscillations of zero wave number, (i.e. plane wave oscillations) gives rise to striations.
Donald T. J. Hurle
10. The Cooling Crystal
Abstract
The properties of a Czochralski-grown crystal depend not only on the conditions prevailing at the point of growth (e.g. the solute distribution resulting from segregation at a planar or cellular interface fed from a melt whose solute distribution is determined by the complex flow patterns in that melt) — but also on the processes which occur as that crystal cools from the melting point down to room temperature. Some of these changes result from attempts by the crystal to remain in or near to local thermodynamic equilibrium whilst others are non-equilibrium effects — such as the introduction of dislocations into the crystal.
Donald T. J. Hurle
11. Future Developments
Abstract
What future extensions of the technology can be envisaged? To attempt to answer this, consideration has to be given material by material.
In the case of silicon, size and uniformity are paramount. To this end it is likely that the process will be modified from a strictly batch process to a semi-continuous one where the melt is continuously replenished. This is not as straightforward as it may sound, one serious problem being the build up of impurity in the melt as a result of continuous segregation of residual solutes with segregation coefficient less than unity at the crystallising interface. Additionally, the process has to be stopped periodically to remove finite lengths of grown crystal. The limited life of a silica crucible when in contact with molten silicon sets a practical limit to any pseudo-continuous process with this material. Further developments in the control of oxygen and carbon distribution in silicon are required. Achievments of these goals may involve the use of a cusped magnetic field but that is, at present, not clearly resolved.
Donald T. J. Hurle
12. References
Donald T. J. Hurle
Backmatter
Metadaten
Titel
Crystal Pulling from the Melt
verfasst von
Dr. Donald T. J. Hurle
Copyright-Jahr
1993
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-78208-4
Print ISBN
978-3-642-78210-7
DOI
https://doi.org/10.1007/978-3-642-78208-4