Dark-field mask

The etching procedure using a negative photoresist is identical except that the lithographic mask must be opaque in the regions where metal is to be removed — this is known as a dark-field mask. Exposure of negative resist to UV light makes it insoluble in its developer. 

Fig. 1 The light field and dark field polarities of a typical mask.

Note masks are either comprised of soda-lime glass (coated with either a photographic emulsion, Pe203, or Cr films), or quartz (with a Cr film). Due to the absorption of UV light by glass, the latter is required for deep UV (DUV) photolithography. Masks may be classified as either light-field or dark-field whereas the former is mostly clear with opaque patterns, the latter is an opaque mask, with transparent features. 

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