Semiconductor Crystal Pulling

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Heating A Graphite Susceptor For MOCVD Crystal Growth

Overview

Advances in electronic technology require complex materials and production methods. In the semiconductor manufacturing process, the refining and subsequent doping of silicon and germanium crystals is of utmost importance. To operate properly, the semiconductor material must contain single crystals with strictly controlled orientation, with the correct amounts of electrically active impurities.

The process known as crystal pulling is one method used to produce semiconductor crystals which meet these requirements.

Using Induction

Modern induction heating provides many advantages over other heating methods. Heating with induction provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time. Solid state induction power supplies are capable of heating very small areas with repeatability and precision.

High frequency RF induction heating has been used extensively within the field of semiconductor crystal pulling. Advantages include the strict control over the highly critical temperature of the molten mass, a clean non-contact method of heating the crucible, and the ability to perform within varying atmospheres. Most crystal pulling operations utilizing high frequency RF induction heating involve multi turn coils, large crucibles, and high impedance levels.

Typical RF power supplies for these systems range from 1 to 10kW, depending on material and application requirements.

How the Process Works

For the production of quality semiconductor crystals, the following basic method is followed:

The semiconductor material is refined through a process known as Zone Refining. This process is based on the principle that the effect of impurities on the melting point of a material will control the impurity concentration of impurities between the molten and just-frozen material. By causing a molten zone of the material to travel along a rod of the material, impurities can be left at one end. Zone refining produces impurity levels based on the number of passages made along the rod by the molten zone. Electrically active impurity levels of one part per billion in silicon have been quoted.

After refining, the semiconductor material is melted within a crucible, usually made of carbon graphite. By maintaining the melt at a specific temperature, a single crystal of the material (with the required orientation) can be inserted into the molten mass on the end of a rod. After which, it is possible to withdraw the single crystal having the molten material adhere to the crystal and solidify maintaining the same orientation.

The process of crystal pulling is usually facilitated by rotating the rod and/or the crucible. Often, the temperature of the molten semiconductor is controlled through the use of a thermocouple and a radiation pyrometer.