Sand is the base material for making the
wafers. It is melted, purified and then melted
again in a radio frequency (RF) furnace. Figure 13-4 shows the molten silicon in a quartz
crucible. A seed crystal is lowered into the
furnace until it touches the melt. After a little
of the molten silicon freezes around the seed
crystal, the seed begins to rotate and is slowly
retracted from the furnace. A large, single crystal of silicon forms as the silicon moves away
from the melt and cools.
Pulled crystals are also called ingots. Ingots
are ground to a cylindrical shape and then sliced
into thin wafers with a diamond saw. The wafers
are then ground flat and polished to a mirror finish. The polished wafers are sent on to the wafer
fabrication area, or clean room where temperature, humidity, and dust are all tightly controlled.
After a thorough cleaning, the wafers are exposed to ultra pure oxygen to form a layer of
silicon dioxide (SiO2). Next, the wafers are
coated with photoresist, which is a material that
hardens when exposed to light. The exposure is made through a photomask. Each mask has a
pattern that will be transferred to the wafer. The
unhardened areas of the photo resist, caused by
the opaque areas of the photo mask, wash away
during the developing step. The wafer is then
etched to remove the silicon dioxide and expose the patterned areas of the substrate. The
exposed areas act as windows to allow penetration by impurity atoms. The remains of the
photoresist are removed with chemicals or
plasma gas. Figure 13-5 shows the major steps
in this mostly photolithographic process. The
wafer is reoxidized and the photolithographic
sequence is repeated from 8 to 20 times, depending on the complexity of the IC being
manufactured. Thus, photolithography is considered the core process in IC fabrication.
When the basic circuit has finally been completed, the surface is passivated using a silicon
nitride coating. This coating acts as an insulator
and also serves to protect the surface from
damage and contamination.
Sand is the base material for making thewafers. It is melted, purified and then meltedagain in a radio frequency (RF) furnace. Figure 13-4 shows the molten silicon in a quartzcrucible. A seed crystal is lowered into thefurnace until it touches the melt. After a littleof the molten silicon freezes around the seedcrystal, the seed begins to rotate and is slowlyretracted from the furnace. A large, single crystal of silicon forms as the silicon moves awayfrom the melt and cools.Pulled crystals are also called ingots. Ingotsare ground to a cylindrical shape and then slicedinto thin wafers with a diamond saw. The wafersare then ground flat and polished to a mirror finish. The polished wafers are sent on to the waferfabrication area, or clean room where temperature, humidity, and dust are all tightly controlled.After a thorough cleaning, the wafers are exposed to ultra pure oxygen to form a layer ofsilicon dioxide (SiO2). Next, the wafers arecoated with photoresist, which is a material thathardens when exposed to light. The exposure is made through a photomask. Each mask has apattern that will be transferred to the wafer. Theunhardened areas of the photo resist, caused bythe opaque areas of the photo mask, wash awayduring the developing step. The wafer is thenetched to remove the silicon dioxide and expose the patterned areas of the substrate. Theexposed areas act as windows to allow penetration by impurity atoms. The remains of thephotoresist are removed with chemicals orplasma gas. Figure 13-5 shows the major stepsin this mostly photolithographic process. Thewafer is reoxidized and the photolithographicsequence is repeated from 8 to 20 times, depending on the complexity of the IC beingmanufactured. Thus, photolithography is considered the core process in IC fabrication.When the basic circuit has finally been completed, the surface is passivated using a siliconnitride coating. This coating acts as an insulatorand also serves to protect the surface fromdamage and contamination.
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