Contact and proximity printing

In contact printing [see Fig. 13.24(a)] the mask and the photoresist-coated substrate are in direct contact during exposure, with the mask held chrome-side down on the resist film. Patterns on the mask are transferred to the entire wafer hy broadband radiation. The theoretical resolution hmit of contact printing is given by  

The early 1960s saw the development of the contact aligners, the first practical exposure systems that were sold commercially by companies such as Microtech, Electroglas, Preco, and Kulicke Soffa. In these contact aligners, patterns were imaged and exposed in a photoresist via a mask that was in intimate physical contact with the upper surface of the photoresist. Such exposures created yield 

Optics for photolithography, in Microlithography Science and Technology, J.R. Sheats and B.W. Smith, Eds., p. 172, Marcel Dekker, New York (1998). 

Bowden, The lithographic process the physics, in Introduction to Microlithography, L.F. Thompson, C.G. Willson, and M.J. Bowden, Eds., pp. 22 23, American Chemical Society, Washington, DC (1994). 

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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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. 

This is the scanned-source equivalent of contact and proximity printing. 

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

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. 

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 ... 

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

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. 

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... 

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

In applying the process depicted in Fig. 9.1, the mask may either be placed directly onto the wafer (contact printing) or may be positioned a short distance in front of the wafer (proximity printing). In either case, the minimum feature size amounts to a couple of micrometers, and thus does not satisfy toda/s industrial demands. However, fine-line features down to the sub-micrometer range can be obtained with projection techniques, as described in the next subsection. 

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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