This short note contains a simple introduction to microlithographic masks for use with visible or near-ultraviolet exposure sources, such as a laser-beam pattern generator or a mask aligner. Flexible masks are briefly described first, for low resolution and critical applications. Then rigid masks are discussed, for precision and high quality applications.
 

Flexible masks

The mechanical support of flexible mask is made of a polymeric film with a thickness between 0.1 and 0.5 mm. The film is coated with a layer of photographic emulsion which, after exposure and development, becomes the contrast medium of the mask.

Due to the nature of their support, flexible masks are not able to maintain high mechanical and thermal stability. Moreover, the grain size of the photographic emulsion prevents them from being used for resolutions below a few microns. Due to such limitations, flexible masks are commonly used for low resolution patterning, that is for a linewidth higher then 20 micron. For such applications, the poor planarity control of the polymeric support does not limit the focusing possibility of the light beam used for the exposure, since low numerical aperture (NA) lenses can be used (low resolution = small NA = high depth-of-field).

The allocation of a flexible mask in a pattern generator requires specific care for keeping it in place and flat. Mechanical holders or magnetic strips are usually employed, while the use of a vacuum chuck is not recommended, since the mask surface can be bent in correspondence of the suction points.

The use of flexible mask is limited to the fabrication of printed circuits, low resolution thin film circuits and in general in low resolution applications where a single process layer is often used or non-critical overlapping of more layer can be tolerated.
 

Rigid masks

The mechanical support in rigid masks is usually 1 to 2 mm thick and is made of soda lime glass or pure silica (amorphous silicon dioxide). The first material is used when the exposure of the patterned masks is made by visible or near-ultraviolet light (such as a low-medium resolution mark-aligner or stepper). In such case the mask is referred to simply as a "glass" mask. The second material is transparent down to deep-UV wavelengths, and is adopted for advanced mask-aligners and steppers. The mask, in this case, is referred to as a "quartz" mask, even if it is not quartz crystal but amorphous silica.

The contrast medium for glass masks is normally a thin film of iron oxide or chromium, or a layer of fine-grain photographic emulsion. Oxide masks are cheaper but exhibit less contrast, resolution and durability than chromium masks. Hence, they are used in less demanding applications.

Chromium masks offer maximum image quality, as well as less sensitivity to surface damage than oxide masks. Both oxide and chromium masks must be coated with a photosensitive layer - positive or negative resist - for being patterned. Instead, emulsion-coated masks are ready for use. In other terms, in oxide and chromium masks the contrast medium and the photosensitive medium are different, while in emulsion masks a single layer acts both as sensitive material and as contrast medium.

As a consequence, when using chromium or oxide masks the patterning process can be made positive or negative by choosing the type of resist layer. Instead, in emulsion masks the pattern is intrinsically negative, due to the nature of the sensitive layer. Nevertheless, a positive pattern can be obtained even with emulsion masks, through a special "inverting" development process. When a silica substrate is used ("quartz" mask) the contrast medium is normally chromium, being this the perfect match material for such a high quality substrate.

A fundamental parameter for the choice between oxide or chromium on one side and photographic emulsion on the other is the achievable resolution. When a photographic plate - i.e. an emulsion based mask - is used, the minimum linewidth cannot be smaller than the grain size of the emulsion. For the best commercially available plates, this results in a resolution around 2 micron. If oxide or chromium masks are used, the resolution is that achievable with the polymeric resist layer. No grain is then involved, resulting in a maximum resolution intrinsically better than that of emulsion masks.