Getting the masks produced

Firstly you will need to tell them which is the top structure of the design: i.e. which one to work with when writing the mask. In the example given previously, this would be the WAFER structure. You will also need to tell them which layers to write as masks (each layer being produced as a different mask) - i.e. the GDSII "stream" numbers and the names, to be on the safe side - and obviously the minimum feature size.

Blanks. The mask blank that you use will be larger than the wafer that you will wish to fabricate the design on, by about 1 inch (25mm): so for a 100mm diameter wafer, one would use a 125mm square mask. Mask and wafer dimensions may be specified in either inches or mm, and the people writing your mask should have a set of standard sized blanks, corresponding with the common wafer sizes, in stock.

For UV photolithography, masks will be chrome on either low expansion glass, or quartz (fused silica) glass. The quartz glass is a more expensive option, but transmits UV light better and is more stable. For micromachining work, the low expansion glass will probably be acceptable; however your foundry, where the photolithography is performed, may have a preference. For X-ray lithography (for LIGA), more exotic mask materials are required. These are often specific to a particular organisation, but are generally formed using a gold film (to absorb the X-rays) which has been patterned on a thin membrane (e.g. an etched silicon membrane).

Dark field / light field. When masks are produced, each layer must be specified as either dark field (positive resist) or light field (negative resist). With light field, a polygon on your design will appear as a polygon of chrome (which will block UV) on the mask. The converse is true for dark field (Figure 21).

Figure 21.

Mirroring. The last option that you may specify is that a layer is to be mirrored. This is useful when machining both sides of the wafer, where the image on the back will need to be the mirror image of itself if it were to be fabricated on the front. By specifying the mirroring at the mask fabrication stage, rather than in the design software, one can easily check the alignment of the structures on both sides.