On Figure 5 you can see a chase (pay attention on the floor). These are the machines that process the wafers and where the technician is putting the lot on Figure 4.

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Figure 5: A chase.

The number of clean rooms and machines is impressive, because of the number of steps necessary to manufacture a CPU wafer. A Pentium 4 CPU uses 26 photolithographic masks. For each mask it may be necessary several steps to process it, plus the doping and metal layer stages. Thus the CPU manufacturing process can have hundreds of steps.

Just to give an example to clarify, for processing the first chip layer using the example we posted on our How Chips are Manufactured tutorial, the following steps would be necessary:

• Grow silicon dioxide on the wafer.
• Apply a layer of photoresist.
• Apply the first mask.
• Expose the wafer to ultraviolet light.
• Remove the “soft” part of the photoresist layer using solvent.
• Remove the exposed parts of silicon dioxide (etching process).
• Remove the rest of the photoresist material.

As you can see, in this example for processing the first layer it would be necessary seven steps, each one occurring in a different place and using different machinery. On real chips more steps may be necessary. So imagine a CPU like Pentium 4 that has 26 masks.

On the steps where ultraviolet light is being applied, the clean room uses orange light, not white light like the other clean rooms, as you can see on Figure 6, because the photoresist layer is sensitive to white light.

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Figure 6: Clean room running a photolithographic process (the light bulbs are orange).