Cutting and Slicing of Ingot (Part-3)

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)