Proximity printing resolution

W.M. Moreau and R.R. Schmidt, Photoresist for high resolution proximity printing, Extended Abstract 187, presented at 138th Electrochemical Society Meeting, Electrochemical Society, Pennington, NY (1970). 

Contact or proximity printing is no longer widely used because of defects caused by mask/wafer contact. It is still used in non-critical situations when such defects can be tolerated, or when the resolution required is low enough to allow an adequate gap to be left between mask and wafer. 

Proximity printing, a variation of contact printing, preserves a minimum gap of approximately 10-30 xm between the silicon wafer and the mask. Although the problem of particulate contamination is avoided, light distortion is enhanced, and a loss in resolution results. 

Moreau and Schmidt (7) demonstrated the sensitivity of poly(methyl methacrylate) (PMMA) to DUV radiation in 1970 and described most of the features of DUV lithography. The main objective of their work was to employ DUV radiation to allow larger mask-to-wafer separation at constant resolution in proximity printing. Their work demonstrated the utility of PMMA as a DUV resist, and they predicted that DUV radiation would be employed with an acrylate polymer of some type in future projection printing systems that should allow reliable printing of submicrometer features. 

In proximity printing spacing the mask away from the substrate minimizes defects that result from contact, but increases the diffraction of the transmitted light which reduces the resolution. The degree of reduction in resolution and image distortion depends on the mask to substrate distance which may vary across the wafer. For proximity printing, equation (3.1) can be rewritten as... 

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... 

Proximity printing The resolution is limited by diffraction, with a minimum line spacing of /gX, where g is the mask-surface distance. Proximity printing techniques have heen used mainly in conjunction with X-ray radiation sources. 

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... 

Figure 13.36 Resolution capability of AZ HiR 1075 i-line photoresist used in printing line/space features with 1 1.5 pitch. Processing conditions Film thickness 0.66 iJim on 1300A AZ BARLi II BARC, soft bake (proximity) 90°C/60 seconds (proximity). Exposure tool ASML/400 Scanner. Exposure conditions Dose 170 mJ/cm, annular illumination, NA 0.65, partial coherence a (outer/inner) 0.85/0.55. Postexposure bake (proximity) 110°C/90 seconds. Development 2.38% tetramethyl ammonium hydroxide developer/single puddle for 60 seconds at 21.0°C. (Courtesy of R. Dammel. °)...

Exposure systems can be classified as contact or proximity printer. In the first case the system puts a photomask in direct contact with the substrate covered by the photoresist and exposes it to a uniform light. In the proximity printer a small gap between the photomask and wafer is present. Contact printing can damage both the mask and the wafer due to the contact but in this case the optical resolution is better than in the proximity method since the gap distance is approximately zero (the resolution is approximately the square root of the product of the wavelength and the gap distance). 

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