Pattern transfer process consists of taking a suitable patterned mask and imagining it on surface of wafer which is precoated with resist.

Four types of exposure techniques are used:

1. Contact printing –

   Contact exposure is the oldest used method. The photomask is in direct contact to the resist layer, the structures are transferred in a scale of 1:1.

   Thus disruptive scattering or diffraction effects only appear at the edges of the structures.

   This method allows only moderate feature sizes. Because all chips are exposed simultaneously the wafer throughput is very high, the construction of the lithographic unit is simple.

   However, the disadvantages are obvious: the mask is contaminated because of its contact to the resist and can be scratched as well as the resist layer can be damaged. If there are particles between the mask and the resist, the optical imaging is degraded.

2. Proximity printing –

   In proximity exposure there is no direct contact of the photomask and the resist.

   Thus only a shadow image is projected onto the wafer which results in a much worse resolution of the structures, contact issues are prevented therefore.

   Proximity printing avoids defect generation because a small gap ranging from $10^{-50}μm$ is introduced between mask and wafer.

   The separation is usually controlled by a flow of nitrogen gas. The gas flow keeps the mask away from the wafer surface.

   The main disadvantage of proximity printing is a severe reduction in resolution due to diffraction spreading.

3. Projection printing –

   The exposure via projection uses the so-called step-and-repeat technique. Thereby only one or a few dies were projected onto the wafer at a time.

   The entire wafer is exposed step by step - die by die.

   The advantage of this method is that the structures on the reticle are enlarged 4-fold or 10-fold.

   If the structures are projected onto the wafer in reduced scale, also defects, like particles, are reduced.

   In contrast to other exposure methods the resolution is improved.

   Besides the projection with lenses also a projection with a complex system of mirrors can be used (scale 1:1).

   Compared to lenses there are no color aberrations and thermal expansion of the photomask can be adjusted. However, mirror images can be distorted or warped. The resolution is limited due to the scale of 1:1.