Volume 1, Number 1, February 1998

Induction Heating and the Floating Zone Method
The need for ultra-high purity materials has led to improvements in the zone processing method of accurately controlling the level of impurities and alloying elements within an otherwise pure material. By heating a very small region of the material to be refined to its molten state and controlling the movement of this zone, impurities can be extracted from the solid material and collected by the molten or liquid zone. As the molten zone traverses the source material and re-solidifies, the solidified material will have a lower impurity level (dependent on alloying element). This process can be repeated multiple times resulting in ultra-high purity materials. Alternatively, zone processing can be used to control the level and distribution of alloying elements in a given material.

The use of induction heating in the zone processing method offers several advantages over other heating methods. By controlling the electromagnetic field, stirring can be induced in the molten zone resulting in higher process efficiency. Additionally, through coil design the heated zone length can be controlled and tailored for optimum efficiency. Coupling the induction power supply with a closed loop feedback circuit also offers improved process control. Induction heating also enables the use of controlled atmospheres, including vacuum, and non-contact heating or cold wall chamber heating, which can further reduce the level of trace impurities.

Interest in semi-conductor devices for operation at higher temperatures has stimulated the production of high purity single crystals of silicon of known orientation. Unfortunately, the high chemical reactivity of silicon and its high melting point (1412°C) offer special problems with respect to zone processing. The most practical solution to these problems has proven to be the floating zone technique, and most of the very pure silicon produced today is processed in this way. Since the floating zone method is capable of growing single crystals with a given orientation, as well as zone refining and leveling, it is ideally suited to the production of silicon crystals for semiconductor devices.

Much recent work in the technology of silicon production has been directed toward the development of single crystals of larger diameter. The fundamental principles of the floating zone method, however, pose practical limitations to the diameter of bar which can be successfully processed. These limitations indicate that the maximum diameters processed depend significantly upon the ability of the processing equipment to maintain a narrow liquid zone.

FIGURE 1 shows a power supply used to successfully process bars of silicon up to 1-1/4 inches (31.75 mm) in diameter.

This equipment consists of a Lepel 15 kW, 2.5 to 5 MHz, three-phase (T-15-3MC) induction heating generator, a remote tank assembly with matching transformer, and a single-turn induction heating coil.

The generator shown in FIGURE 1 operates at a frequency of 2.5 to 5 MHz using a single-turn coil and the matching transformer. Saturable core reactors are located in the input of the generator to minimize D.C. ripple and to provide smooth and noise-free power control. In addition, ripple filter capacitors may be placed between the R.F. filter capacitor and ground to further reduce D.C. ripple. Continuous power control is achieved manually by operation of a powerstat placed in the primary to control the saturable core reactors. Remote power control can be accomplished through a 4 - 20 mA analog input signal or digital serial port connection. The serial port connection also enables remote monitoring of critical power supply parameters.

FIGURE 2 is a close-up of the remote tank assembly with built in matching transformer. The transformer has a multiturn water-cooled primary and a single-turn skirt secondary, also water-cooled and grounded. The induction coils can be tailored for the specific application.

Using the equipment shown in FIGURES 1 AND 2, a liquid zone can be moved along the silicon bar to produce a uniform bar of high purity. FIGURE 3 shows the liquid zone in the silicon bar during processing.

In processing a raw silicon bar to produce a single crystal of desired orientation, the bar can be preheated using either a small molybdenum wire coil or carbon susceptor at one end to initiate induction heating. The molybdenum wire or carbon susceptor, which heats readily by induction and in turn heats the silicon, is placed in close proximity to the silicon rod, making the silicon sufficiently conductive to thereafter be heated directly by induction. The preheated silicon source rod and a monocrystal seed are placed in proper position relative to the coil after preheating. After heating one end of the polycrystalline bar to a high temperature, a molten zone is produced at the polycrystal-seed interface. In operation, the silicon bar is moved through the coil while rotating; the solidified material forming a smooth straight monocrystal having an orientation identical to that of the original seed. Upon reaching the top of the polycrystalline bar, the power is reduced to solidify the molten zone.

FIGURE 4 shows a schematic of a typical floating zone set-up. While the processing details for zone refining and growing single crystals of silicon vary from plant to plant, the equipment and procedure described here have proven successful for processing silicon rods up to 1-1/4 inches (31.75 mm) in diameter.