Proximity theoretical resolution

Practical and technical difficulties associated with contact printing led to a modification of the latter technique, such that the mask and the wafer were separated by a small, accurately controlled gap of 10-25 p,m during exposure. Introduced in the early 1970s, this technique, known as proximity printing, minimizes mask and wafer damage caused by contact, but at the expense of resolution. This gap, however, limited resolution because of diffraction. The theoretical resolution of proximity printing is given by... 

However, due to the defectivity problem associated with contact printing, as discussed above, the first noncontact aligners, called proximity aligners, were introduced in 1973 by Kasper Instruments. In proximity printing mode, the mask does not come into direct physical contact with the wafer [see Fig. 13.24(a)]. Rather, it is held 10-25 p.m above the upper surface of the resist-coated wafer. This gap, however, limited resolution on account of diffraction. The theoretical resolution limit of proximity printing is given by the expression... 

A careful examination of the database is always necessary if the flux rates were calculated from pore water profiles of variant resolution. A commonly used method to minimize such discrepancies consists of the application of mathematical fit-functions (e.g. Sayles et al. 1996). As long as the sampling procedure includes at least all essential features of an anticipated concentration curve (zones of release and fixation), the application of non-linear functions will permit a theoretical depth resolution of any desired precision and thns a theoretical flux calculation in close proximity to the boundary. 

These shortcomings led to the use of proximity printing where the mask was deliberately kept out of contact with the resist, accepting the loss in resolution caused by the increase in diffraction effects (Fig. 7.14). The theoretical limit for resolution is a simple extension of the equation for contact printing... 

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