Positive-lithographic resists

As a general rule, the sensitivity of conventional electron beam resists is not sufficient for economic throughput in an x-ray lithographic system. This is particularly true of positive electron resists such as PMMA, the most widely used x-ray resist for experimental purposes, whose sensitivity of >500 mJ/cm2 is some 100 times too slow for practical application. Even PBS only shows a sensitivity of 94 mJ/cm2 to PdLa x-rays. Consequently, the major research effort has concentrated on negative resists because of their higher inherent sensitivity. 

Microlithography, Xerography. Because of their photosensitivity, polysilanes are under intense investigation for use as positive photoresist materials (94) (see Lithographic resists). They are particularly attractive because both wet and dry development techniques can be used for imaging (131,132). The use of polysilanes for xeroprinting has been reported (133). Thermal and optical sensors based on the photodegradation of polysilanes have been developed (134). 

The ultimate test of usefulness is, of course, the lithographic performance of a resist. The often-quoted sensitivity of PMMA of 50 pC/cm2 assumes conventional developing conditions and is measured by competitive dissolution rates of exposed and unexposed films. A secondary measure of sensitivity for positive-working resists is the G(s) value, the yield of chain scissions per 100 e.v. of absorbed energy. In this case, mea-... 

Figure 12.4 Contrast curves for (a) positive-tone and (b) negative-tone resists. The intercept of the curve and abscissa in positive-tone resists is called the dose to clear" and is designated as Do, while in negative resists, it marks the onset of cross-linking, and is designated as Dq. This should not be confused with the lithographic dose to print, which tends to be approximately 1.6-2.2 times higher. The absolute value of the slope of the tangent to the contrast curve at its intercept with the abscissa is defined as the resist contrast. It is usually defined in terms of an auxiliary dose value Di, which is obtained by continuing the above tangent line to the full resist film thickness (normalized to 1.0).

Photoacid diffusion behavior in t-BOC-blocked chemically amplified positive DUV resists under various conditions was studied. Based on the experimental results, it was confirmed that only one mechanism dominated the acid diffusion in the resist film, and two diffusion paths, i.e., the remaining solvent in the resist film and hydrophilic OH sites of base phenolic resin, existed. Moreover, the effects of molecular weight dispersion, acid structure, and additional base component on both acid-diffusion behavior and lithographic performance were revealed. Finally, the acid diffusion behavior in the resist film was clarified and the acid diffusion length that affected the resist performance could be controlled. 

Fig. 3. Lithographic process using positive- and negative-resist systems. The element (a) is (b), exposed to uv radiation (c), developed and (d), the metal is...

Figure 2 Typical lithographic response (contrast) curves for (a) positive and (b) negative resists.

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