 and dry sandpaper. The sharp edges of the wafers are then smoothed to prevent them from chipping.
Next up: lapping, where the wafers’ surfaces are polished using an abrasive slurry until the wafers are flat to a tolerance of two thousandths of a millimetre. After that, the wafer is etched with a mixture of nitric, hydrofluoric and acetic acids to create an even smoother surface.
Step 4: Make patterns
The super-smooth wafers will now be given an oxide layer, which is used to create the required features of the circuit. This is done selectively to specific areas, and may involve the use of ion beams, hot gases and/or chemicals.
Once such chemical is known as “photoresist”, and it’s broadly similar to the chemicals used to make photographic film. Unfortunately that doesn’t cope very well with hot gas treatments, so the wafer needs to be masked. This is done by applying a patterned oxide layer that ensures the gases don’t reach the photoresisted bits that the chip designer wants to keep.
There can be up to six steps in this process:
- The wafer is heated to a high temperature in a furnace, creating a layer of silicon dioxide as the silicon reacts with oxygen
- A layer of photoresist is applied, with the wafer spun in a vacuum to ensure even coverage and then baked dry
- The wafer is exposed to UV light through a photographic mask or film, once for each chip or cluster of chips on the wafer. The wafer is moved between each exposure using a machine called a ‘stepper’
- An alkaline solution is applied, dissolving the sections of photoresist that were exposed to the UV light. Those sections are washed away
- Hydroflouric acid is used to dissolve the parts of the oxide layer where the photoresist has been washed away
- A solvent is applied to remove the remaining photoresist, leaving a patterned oxide layer in the shape of the required circuit features