Positive photoresists, effects

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Two component, positive photoresists (see Section 3.5.b) represent systems with unusual exposure characteristics caused by the standing wave effect (see Section 2.1.f) and "bleaching" or change in optical density during exposure (see Sections 3.5 and 3.9). Both of these phenomena result in nonlinear exposure throughout the thickness of the resist film, and result in uneven developing rates as a function of film thickness, making evaluation of these systems difficult. 

Ester derivatives of 4-hydroxyl polystyrene, (III), prepared by Suetsugu [3] were effective as chemical amplification type positive photoresists. 

Positive photoresist amplifier perfluoro polymers, (IV), (V), (VI), and (VII), prepared by Kanna [4], Hohle [5], DiPietro, [6], and AUen, [7], respectively, were effective with an exposed light source of 160 nm and thus usable with an F2 excimer laser beam. ... 

DALY ET AL. Effects of Additives on Positive Photoresist Development... 

Knowledge that silyl substituents may be incorporated into standard resist chemistry to effect etching resistance has prompted several workers to evaluate silylated novolacs as matrix resins for conventional positive-photoresist formulations. Typically, these resists operate via a dissolution inhibition mechanism whereby the matrix material is rendered insoluble in aqueous base through addition of a diazonaphthoquinone. Irradiation of the composite induces a Wolff rearrangement to yield an indenecarboxylic acid (Figure 4), which allows dissolution of the exposed areas in an aqueous-base developer (35). 

The curve is accompanied by one in which only B has been changed such that it represents the absorbance of the resist at 254 nm (B = 1.96) rather than at 436 nm (B = 0.058), with all other parameters kept constant. The effect is very large and sufiScient to render the image useless for subsequent processes. Thus, conventional positive photoresists do not function adequately in DUV lithography. 

Shaw and M Hatzakis, Developer temperature effects on e beam and optically exposed positive photoresist, J. Electrochem. Soc. 126(11), 2026 2031 (1979). 

Effects of Developer Concentration on Linewidth Control in Positive Photoresists... 

The positive resist materials evolved from discoveries made by the Kalle Corporation in Germany who developed the first positive-acting photoresist based on the use of a novolac matrix resin and a diazoquinone photoactive compound or sensitizer. The original materials were designed to produce photoplates used in the printing industry. These same materials have been adopted by semi-conductor fabrication engineers and continue to function effectively in that more demanding application. 

Organosilanes are particularly effective as positive ultraviolet photoresists (Section 3.5.1). The oxide layer exposed where the resist has been removed may then be etched with an appropriate solution (buffered aqueous HF in the case of a silica layer) to expose the silicon underneath. Application of the desired metal over the whole surface is then followed by removal of the remaining photoresist, taking the unwanted parts of the metal layer with it and leaving the metal contact on the Si that had been exposed by etching. Variations on this approach can be used to attach shaped deposits of other electronically active materials. 

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