Steps of photolithography process

Photolithography (optical lithography) is the process of transferring a circuit pattern from a mask onto a light-sensitive layer (photoresist) on the silicon wafer, so that selected regions can then be etched, doped or coated. It is repeated for each layer of the IC.

Steps

 1 Clean + oxide    2 Spin resist     3 Expose (UV)
 [ SiO2   ]         [= resist =]      UV  UV  UV
 [ Si     ]         [  SiO2    ]      [##] [##] mask
                    [  Si      ]      [= resist =]
 4 Develop          5 Etch oxide      6 Strip resist
 [=]  [=] resist    [=]  [=] resist   [oxide] [oxide]
 [oxide gap oxide]  [ Si windows  ]   [ Si wafer   ]

1. Wafer cleaning and preparation: remove dust, organics and moisture; grow or deposit the layer to be patterned (SiO2, poly-Si or metal). A dehydration bake and an adhesion promoter (HMDS) are applied.
2. Photoresist coating: a few drops of liquid resist are placed on the wafer, which is spun at 3000-6000 rpm to give a uniform film about 1 µm thick.
3. Soft bake (pre-bake): about 90-100 °C removes solvent and hardens the resist.
4. Alignment and exposure: the mask (reticle) is aligned to marks already on the wafer and the resist is exposed to UV light through it.
5. Development: a developer dissolves the exposed resist (positive resist) or the unexposed resist (negative resist), leaving the pattern.
6. Hard bake (post-bake): about 120 °C increases the resist's resistance to etching.
7. Etching (pattern transfer): wet (HF) or dry (plasma) etching removes the layer where it is not protected by resist.
8. Resist stripping and inspection: the remaining resist is removed with solvents or oxygen plasma and the pattern is inspected; then doping or the next layer follows.