To create silicon, three crucial reactions have to be completed in sequence.
The obtained polycrystalline crystal has a purity range of 99.99999%. At this stage, we have a silicon rod that has many crystals (Merill L. Minges, 1989).
Two methods can be used to obtain single-crystal silicon i.e. the Czochralski method and the float zone technique. Czochralski method is used to grow single crystals on metals while the float zone method is used when high purity crystals are desired.
I. The Czochralski method
In this method, a small single crystal seed that is attached to a rod is dipped into the molten silicon. The seed is then slowly removed from the molten silicon while the rod is rotating. The figure below illustrates the process.
Graphite is used to make the crucible while its lining is made of fused silica. The process requires an inert atmosphere gas condition such (e.g. argon) at a pressure ranging from 0.13 – 6.7-kilo pascals. Silicon concentration is adjusted by adding highly doped silicon pellets to the melt. By constantly rotating the crystal seed, the concentration of the dopant is uniformly distributed. The rotation also does provide a uniform thermal condition as crystal growth takes place. The molten silicon has to be maintained at temperatures above 425 degrees Celsius. The melt in contact with the crystal seed solidifies as a result of a reduction in temperature. The solidification occurs at the same crystal orientation as that of the starting seed. The rod pulling rate and the melt temperatures determine the ignot diameter (Merill L. Minges, 1989). Sorting of the wafers obtained by this process is based on resistivity.
II. The Float Zone Method
For this process, a chamber having argon in controlled amounts is required. First, a polycrystalline rod is placed in the chamber. Then a single seed crystal that has a specified orientation is attached to the rod at one end. Melting as a result of the action of an induction coil that is positioned around the rod occurs and it begins with the single – seed crystal. The heating coil moves along the length of the rod causing a shift in the molten zone. When the molten silicon re-solidifies, it forms a single crystal growth with a specific orientation (Merill L. Minges, 1989). The float zone process is shown in the figure below.
This process produces single crystal silicon ignots that have high resistivity and moderate diameter values. The heating coil motion determines the crystal diameter. Typical resistivity values of produced wafers by the floating zone process varies from 25 – 30000 ohm-cm.
III. Silicon-on-Sapphire (SOS) method
The Epitaxial growth of silicon occurs on a silicon wafer, silicon-on-sapphire material is formed. This discovery was made in the year 1964. By using the Czochralski method, sapphire crystals can be obtained. The obtained sapphire is divided to form wafers which have to undergo chemical and mechanical polishing. The sapphire-based wafers then undergo hydrogen etching carried out in an epitaxial reactor at a temperature of 1150 degrees Celsius. Pyrolysis process of silane at temperatures of between 900 to 1000 degrees Celsius deposits a silicon film on the wafer. Since there exists a lattice mismatch affecting sapphire and silicon, a higher defect density exists in the silicon firm particularly in extremely thin firms though the defect appears to decrease as the thickness of the firm increases. The main defects of wafers obtained by this method are twins and stacking errors. At room temperature, the silicon firm is always under compressive stress as a result of the different thermal expansion coefficients of sapphire and silicon (Colinge, 2004).
3. Ingot Processing
After the crystal growth procedure is done, the top and bottom ends of the ingot are cut off. The cut surface is then polished to form a flat surface that has the same diameter as the ingot. Afterward, a steel saw blade that has a diamond cutting edge divides the ingot into circular portions called wafers. For one to classify a silicon slice, one has to define the dopant used, the diameter of the substrate, the resistivity of the layer, the type of conductivity, the thickness of the slice, the orientation, surface finish quality and the flatness degree among other specifications. The numbering of the slice is from the crystal seed end to the top surface. Wafer slicing process introduces imperfections to the wafer thus necessitating for polishing. After polishing, a flat surface is obtained that aides in the photo-lithography step (Merill L. Minges, 1989). The process of polishing also reduces the thickness of the wafer that aids in proper handling without damage. The wafer has a substrate on which devices may be built.
4. Wafer- surface cleaning
Photo-resist imaging procedure needs a clean surface thus there is a need for the removal of the surface contaminants before the process. Organic contaminants can be removed by the wet chemistry process that entails burning the material at temperatures between 900 and 1200 degrees Celsius while in an oxidation furnace. Inorganic contaminants as a result of airborne salts, storage cans, or the processing materials can be removed by absorption. Wafer scrubbing procedure may also be used to remove airborne particles contaminating the wafer surface. Scrubbing combined with chemical action will considerably remove the surface impurities (Merill L. Minges, 1989).
5. Wafer initial Probing
This process involves carrying out tests on the wafer to ensure that the wafer is of the desired quality before further processing. The deformities of the wafer, resistivity, doping levels at different depths plus oxide quality tests are detected in this stage. The existence of pinholes is found out to study the oxide layer (Merill L. Minges, 1989).
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