How Chips are Manufactured

Photolitography (Cont'd)

The first thing that is done to the raw wafer is to grow silicon dioxide (SiO2) on it, by exposing the wafer to extreme heat and gas. This growth is similar to the way rust grows on metal when exposed to water, but it happens faster.

Next the wafer is coated with a substance called photoresist, which becomes soluble when exposed to ultraviolet light. The first mask is applied and the wafer is exposed to ultraviolet light. The “soft” part of photoresist is removed using solvent and then the parts of the silicon dioxide layer that were revealed are removed in a process called etching. The rest of the photoresist is removed, so now we have the wafer with a silicon dioxide layer with the same shape as the first mask.

Another silicon dioxide layer is applied on the wafer, a polysilicon layer is applied on top of it and then another photoresist layer is applied on top of them. The second mask is applied and the wafer is exposed to ultraviolet light again. The “soft” part of photoresist is removed using solvent and then the parts of the polysilicon and silicon dioxide layers that were revealed are etched away. The rest of the photoresist is removed, so now we have the wafer with a silicon dioxide layer with the same shape as the first mask and on top of it a polysilicon and silicon dioxide layers with the same shape as the second mask.

After these two steps, a process called doping (or ionization) takes place. Here the exposed areas of the wafer are bombed with various ions, to alter the way the exposed areas conduct electricity. The exposed areas will be transformed either into a P-type semiconductor (i.e., positively charged) or into a N-type semiconductor (i.e., negatively charged), depending on the chemicals used: phosphorus, antimony and arsenic are typically used to create a N-type semiconductor layer, while boron, indium and gallium are typically used to create a P-type semiconductor layer. The stacking of semiconductor layers will create the transistors.

The layering and masking are repeated, following the layout of the next mask. A metal is then dropped on the wafer, filling eventual holes that were created to make electrical connections between the layers. Another masking and etching processes are done to add the electrical connections.

This process is repeated all over again until the chip is done, i.e., all masks were used. The exact manufacturing process and number of layers depends on the component being manufactured. For a Pentium 4 processor, it uses 26 masks and 7 metal layers.

Figure 3: Transistors built inside the chip and the metal connection between them.

Figure 4: Wafer with Pentium 4 processor chips after being manufactured.

The chips on the wafer are then tested and the wafer is sent to the next step on the chip manufacturing process, where the chips are cut from the wafer, have their terminals attached and are packed. After that they are tested, labeled and sold.