Single Crystal Growing for Wafer Production

Integrated circuits are built on single-crystal silicon substrates that possess a high level of purity and perfection. Single-crystal silicon is used in VLSI fabrication instead of polycrystalline silicon since the former does not have defects associated with grain boundaries found in polysilicon. Such defects have been known to limit the lifetimes of minority carriers.

Aside from the need to be single-crystalline in nature, silicon substrates must also have a high degree of chemical purity, a high degree of crystalline perfection, and high structure uniformity. The acquisition of such high-grade starting silicon material involves two major steps: 1) refinement of raw material (such as quartzite, a type of sand) into electronic grade polycrystalline silicon (EGS) using a complex multi-stage process; and 2) growing of single-crystal silicon from this EGS either by Czochralski or Float Zone process.

Czochralski Crystal Growth

Czochralski (CZ) crystal growth, so named in honor of its inventor, involves the crystalline solidification of atoms from a liquid phase at an interface.  The basic CZ crystal growing process is more or less still the same as what has been developed in the 1950's.

CZ crystal growing consists of the following steps.

1) A fused silica crucible is loaded with a charge of undoped EGS together with a precise amount of diluted silicon alloy.

2)  The gases inside the growth chamber are then evacuated.

3) The growth chamber is then back-filled with an inert gas to inhibit the entrance of atmospheric gases into the melt during crystal growing.

4)  The silicon charge inside the chamber is then melted (Si melting point = 1421 deg C).

5)  A slim seed of crystal silicon (5 mm dia. and 100-300 mm long) with precise orientation tolerances is introduced into the molten silicon.

6)  The seed crystal is then withdrawn at a very controlled rate. The seed crystal and the crucible are rotated in opposite directions while this withdrawal process occurs.   

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