22-11-2012, 05:23 PM
Silicon Wafer Processing
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Crystal Growth and Wafer Slicing Process
• The sand used to grow the wafers has to be a very
clean
• The sand is heated to about 1600 degrees C – just
above its melting point
• A pure silicon seed crystal is placed into the
molten sand bath
• The seed is pulled out slowly as it is rotated
• The result is a pure silicon cylinder: an ingot
• The ingot is sliced into very thin wafers
• Wafers are polished until they are very smooth
and just the right thickness
Photolithography
• When the design is ready glass photomasks are made - one mask for
each layer of the circuit.
•These glass photomasks are used in a process called photolithography.
• The wafers are exposed to a multiple-step photolithography process
that is repeated once for each mask
Photolithography –Coating of photoresist
• The wafer is uniformly coated with a thick light-sensitive liquid
called photoresist. The coating is applied while the wafer is
spinning (1500-8000 rpm)
• The photoresist thickness is in the range: 0.5 - 2μm. Thickness
uniformity of ~5nm is required.
• The wafer is heated in order to cure the photoresist (soft baking).
Photolithography - Exposure to UV
• Parts of the wafer are selected for exposure by carefully
aligning a mask between an ultraviolet light source and
the wafer.
• In the transparent areas of the mask, light passes
through and exposes the photoresist.
•A mask is a square glass plate with a patterned emulsion of
metal film on one side
•The mask is aligned with the wafer, so that the pattern can be
transferred onto the wafer surface
•Each mask must be aligned to the previous one
•The photoresist is exposed through the pattern on the mask
with a high intensity ultraviolet light