Cutting and Squaring of Ingot:
In
the next process the ingot is cut from head to tail and squaring from side to
side.
Slicing of the Ingot:
In
the next step the Ingots are sliced into wafers using a Wire Saw machine or
other type of saw. De-ionized water is used to cool the blade on this ID
(inside diameter) saw. The saw is slicing 120mm thick (minimum) wafers.
Lapping: Lapping is next; in
this step the Ingots have now become rough cut Silicon wafers with saw marks
and other defects on both sides of the wafer. Also at this point the wafer is
much thicker than it will be when it is finished. Lapping the wafers
accomplishes several things, it removes saw marks and surface defects from the
front and backside of the wafers and it thins the wafer and relieves a lot of
the stress accumulated in the wafer from the sawing process. Both before and
after the lapping process many in-process checks will be done on the Silicon
wafers and more fall-outs will occur. After lapping the wafers go thru several
cleaning /etching steps using sodium hydroxide or acetic and nitric acids to
remove microscopic cracks and surface damage caused by the lapping process,
this is followed by followed by de-ionized water rinses.
Edge grinding or
rounding is an important part of the wafer manufacturing process, it is
normally done before or after lapping, this rounding of the edge of the wafer
is very important! If it is not done the wafers will be more susceptible to
breakage in the remaining steps of the wafer manufacturing process and the
device manufacturing processes to come. If you look at the edge of a finished
wafer you will see the edge rounding even in the notch area of 120mm and 300mm
wafers. On the best Prime wafers the edges are also highly polished,
this can improve cleaning results on wafers and reduce breakage up to 400%. Process
Specialties has seen a notable yield differential between poorly
and perfectly edge rounded material.
Polishing: Polishing is the next step in the wafer manufacturing
process. Most Prime wafers go through 2-3 polishing steps using progressively
finer slurry (slurry is the polishing compound). The polishing is
normally done on the front side of the wafer, but sometimes it is done on both
sides. Polishing is done on huge precision machines that are capable of
extraordinary tolerances. Prior to final polishing some wafers may
receive that is called backside damage; two examples would be bead blast
and brush damage. The wafers may also receive a backside coating of Poly silicon,
all these treatments are done to the backsides of the wafer for the purpose of Gettering defects
(later in the device manufacturing process these backside treatments will draw
defects in the Silicon towards the backside of the wafer and away from the
front side where the devices are being built, this is called Gettering). After
polishing the wafers are rinsed in DI water and scrubbed to remove any residual
slurry compounds from the wafer.
Final Cleaning: The next step in the
process after polishing is a rather intense regimen of cleans and scrubs to
remove trace metals, residues and particles from the surface(s) of the finished
Silicon wafers. Normally most wafer manufacturers use a final cleaning method
developed by RCA in the 1970's the first part of this clean is called SC1 and
consists of Ammonium Hydroxide followed by a dilute Hydrofluoric acid clean
followed by a DI water Rinse. Next the SC2 clean which consists of Hydrochloric
acid and Hydrogen peroxide followed by a DI water rinse. Many companies modify
these cleans to make them even more effective. After all this cleaning and
rinsing the finished wafers will now go through a front and backside scrub to
remove even the smallest particles.
Final sort and inspection: This is one of the
last steps in the long wafer manufacturing process. It is here that the wafers
either meet or fail the specifications the customers (IC manufacturers) have
asked for. There are many specifications the final prime wafers must meet according
to agreements made between the customers and the Silicon manufacturer. We will
talk about these specifications in a generalized form here, some specifications
are tighter, and some more relaxed depending on the end user and their
requirements.
(To be Continue)
