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6)  The seed crystal is then withdrawn at a very controlled rate.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.


Fig. 1.  Examples of Czochralski Pullers


Float Zone Crystal Growth


The float zone (FZ) process is another method for growing single-crystal silicon. It involves the passing of a molten zone through a polysilicon rod that approximately has the same dimensions as the final ingot.  The purity of an ingot produced by the FZ process is higher than that of an ingot produced by the CZ process. As such, devices that require ultrapure starting silicon substrates should use wafers produced using the FZ method.


The FZ process consists of the following steps.

1.  A polysilicon rod is mounted vertically inside a chamber, which may be under vacuum or filled with an inert gas.

2.  A needle-eye coil that can run through the rod is activated to provide RF power to the rod, melting a 2-cm long zone in the rod. This molten zone can be maintained in stable liquid form by the coil.

3. The coil is then moved through the rod, and the molten zone moves along with it.

4. The movement of the molten zone through the entire length of the rod purifies the rod and forms the near-perfect single crystal. 


FZ growing equipment can also use a stationary coil, coupled with a mechanism that can move the silicon rod through it.


Fig. 2.  Examples of Float Zone Crystal Growing Equipment


After the single-crystal silicon ingot has been manufactured, it undergoes a routine evaluation of its resistivity, impurity content, crystal perfection, size and weight.  It is then ground using diamond wheels to make it a perfect cylinder that has the right diameter. It then undergoes an etching process to remove the mechanical imperfections left by the grinding process. 


Fig. 3.  A Single-Crystal Silicon Rod


The cylindrical ingot is then given one or more 'flats' by another round of grinding. The largest flat, called the primary flat, is used by automated wafer handling systems for alignment.  Flats (primary and secondary) are also used to identify the crystallographic orientation and conductivity of the wafer.


The ingot is then sawn into thin wafer slices, each of which will be subjected to further etching and polishing until it is ready for use as substrates for VLSI fabrication.  The above process of silicon growing, grinding, shaping, sawing, etching, and polishing to produce input wafers is known as wafering.


Fig. 4.  An ingot slicer (left) and a wafer grinder/polisher (right)


See also:   Specifications for Si Wafers Crystal Defects Semiconductor Wafers

Semiconductor Manufacturing;   What is a semiconductor?




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