Contact printing limitations

On the basis of equation (3.2), for a gap of 10 iim, using 400 nm exposing radiation, the resolution limit is about 3 /u,m which is poorer than that of contact printing. 

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... 

Contact printing The resolution is the limited by the optical skin depth of the mask material. 

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... 

Nanoimprinting A master stamp is used directly for hot embossing of a substrate, usually made of a polymer. This allows the stamp to produce many patterned substrates. Although there is no diflraction limit here, hot embossing has been applied mainly for low-resolution applications, in various fields such as miniaturized total-analysis Systems (p,TAS), microfluidics, and microoptics. UV-nanoimprint lithography corresponds to contact printing with UV exposure. 

Transfer printing employs the intermediate step of printing dye dispersions or dye solutions onto a temporary substrate, usually paper. From the paper, the dye is transferred to the textile by heat and steam, while printed paper and textile are in close contact. The advantages and limitations of the process have been described (33). 

Silver is often preferred as an undercoat for rhodium by reason of its high electrical conductivity. A further advantage of silver in the case of the thicker rhodium deposits (0-0025 mm) applied to electrical contacts for wear resistance is that the use of a relatively soft undercoat permits some stress relief of the rhodium deposit by plastic deformation of the under-layer, and hence reduces the tendency to cracking , with a corresponding improvement in protective value. Nickel, on the other hand, may be employed to provide a measure of mechanical support, and hence enhanced wear resistance, for a thin rhodium deposit. A nickel undercoating is so used on copper printed connectors, where the thickness of rhodium that may be applied from conventional electrolytes is limited by the tendency of the plating solution to attack the copper/laminate adhesive, and by the lifting effect of internal stress in the rhodium deposit. 

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. 

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