Some Remarks about Lithography

Three main types of optical lithography are used as illustrated in Fig. 4.6. The major differences between these techniques are the position of the mask. The first step in hthography is always the alignment of mask and substrate. 

In proximity exposure, the mask and substrate are not directly in contact. Instead the mask is aligned with the substrate in close proximity at small distance, which is usually in the order of 5-40 pm. On the one hand, this reduces the maximum possible resolution of structural details, i.e. the larger the distance between the mask and the substrate the lower the optical resolution. 

On the other hand, the wear of the mask is experiencing during handling significant lower. 

Nowadays, wafer steppers are used for the lithographic structuring of large diameter wafers at high resolution. This equipments use the step and repeat principle. Only small area of the substrate of about 1 cm is exposed to the mask in a single step. Afterwards, the substrate is moved to the next area followed by a subsequent exposure step and so on, which will be repeated until the entire wafer is exposed to the mask. 

Sotomayor-Torres [479] gives an overview of alternative lithography methods. 

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