Proximity, printing

Figure 1. Schematic of contact or proximity printing using positive and...

In spite of the limitations of contact and proximity printing, these technologies are widely used, primarily because of their relatively low cost and ease and simplicity of operation. Several companies use these techniques to produce a variety of solid state devices including VLSI memory. 

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. 

Figure 1. Linewidth versus gap for Deep UV and X-ray proximity printing. Theoretical points correspond to the Gruen range for the maximum energy photoelectrons. Experimental points were measured by Feder and Spiller (27).

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. 

The quality of pattern transfer differs greatly among the three modes of printing. As an example, a mask with parallel bundles of slits and spaces between slits with dimensions comparable with the slits can be considered. In this case, optical interference results in distorted images. The theoretical minimum dimension (for both space and slit) that allows resolvable interference peaks for contact or proximity printing is approximated by ... 

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. 

Figure 3.36. SEMs of dry-developed images generated by the process shown in Figure 3.35 a) J.0- xm images printed with a Perkin-Elmer M500, UV-2 mode (b) 0.75-p.m images printed with a Perkin-Elmer M500, UV-2 mode and (c) 0.5- xm images proximity printed with a synchrotron X-ray source. (Reproduced with permission from reference 127. Copyright 1985 Society of...

In the proximity printing, a gap space is placed between the mask and the surface layer of the substrate. 

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... 

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). 

As stated above, there are three main hthographic printing modes, namely, contact printing, proximity printing, and projection printing (see Fig. 13.24). 

Figure 13.24 Schematic of (a) contact (z 0) and proximity printing and (b) projection...

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... 

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