Positive-resist structures

Fully developed 3D microstructures are depicted in Figure 3.77. However, edges of the positive-resist structures are rounded in Figure 3.75. This is mainly caused by proton diffusion during the postexposure step [264], The prismatic cavities shown in Figure 3.75c may be a possible approach for switchable grating devices if they are filled with a low molecular weight liquid crystal. 

If, on the other hand, a vapor cloud s explosive potential is the starting point for, say, advanced design of blast-resistant structures, TNT blast may be a less than satisfactory model. In such cases, the blast wave s shape and positive-phase duration must be considered important parameters, so the use of a more realistic blast model may be required. A fuel-air charge blast model developed through the multienergy concept, as suggested by Van den Berg (1985), results in a more realistic representation of a vapor cloud explosion blast. 

It has been noted previously that radiation exposure of a positive resist causes chain scission in its structure. As a result, its molecular weight decreases. According to Ku and Scala (3), the decrease in the number average molecular weight resulting from such irradiation is given by the expression ... 

The incorporation of PDMSX into conventional novolac resins has produced potential bilevel resist materials. Adequate silicon contents necessary for O2 RIE resistance can be achieved without sacrificing aqueous TMAH solubility. Positive resist formulations using an o-cresol novolac-PDMSX (510 g/mole) copolymer with a diazonaphthoquinone dissolution inhibitor have demonstrated a resolution of coded 0.5 pm L/S patterns at a dose of 156 mJ/cm2 upon deep-UV irradiation. A 1 18 O2 etching selectivity versus hard-baked photoresist allows dry pattern transfer into the bilevel structure. 

Another interesting positive-tone polyacrylate DUV resist has been reported by Ohno and coworkers (82). This material is a copolymer of methyl methacrylate and glycidyl methacrylate. Such materials are negative e-beam resists, yet in the DUV they function as positive resists. Thermal crosslinking of the images after development provides relief structures with exceptional thermal stability. The reported sensitivity of these copolymers is surprising, since there are no obvious scission mechanisms available to the system other than those operative in PMMA homopolymer, and the glylcidy side-chain does not increase the optical density of the system. 

Fig. 20 Top left Schematic of a buried channels structure fabricated using photoacid p.2 and positive-resist p.3 (745nm/80fs). Top right Two-photon fluorescence micrograph of a vertical cross section, perpendicular to the channels (the channel-to-channel spacing is 8 xm) Bottom Two-photon micrographs of the structure at the surface and at a depth of 10 jim (the length of the channels is 50 p.m). The sections where the polymer has been removed appear dark in the images. Adapted from [214]. Reproduced with permission from AAAS...

The other major class of positive resists is based on polyfolefin sulfones) which are alternating copolymers of sulfur dioxide and the respective olefin having the general structure. 

A second olefin copolymer with a more promising structure is that with isobutylene (2-methylpropene-l). Poly isobutylene itself is a chain-scissioning polymer which has been studied often. It is not much used as a positive resist despite its G(s) of 1.5 to 5 (21) because its Tg is so low, about -60°C. 

Relationship between dry etch resistance under ion bombardment and polymer structure has recently clarified ( 1 ) As for sensitivity predictions, theoretical formulas for sensitivities of crosslinked positive resists have been proposed ( 2 ). Formulas for sensitivities of copolymer negative resists have also been derived and reported ( 3 ). Unfortunately, however, no valid schemes are yet available for predicting resolution capabilities of resist materials. 

Figure 3.3 Photoresist profiles. (A) Positive resist (a) desired resist profile for lift-off i.e. exposure-controlled profile, also called overcut (b) perfect image transfer by applying a normal exposure dose and relying moderately on the developer (c) receding photoresist structure with thinning of the resist layer i.e. developer control, also called undercut. (B) Negative resist the profile is mainly determined by the exposure. Development swells the resist a little but has otherwise no influence on the wall profile.

Hanabata, Y. Uetani, and A. Furuta, Novolak design for high resolution positive photoresists. II. stone wall model for positive photoresist development, Proc. SPIE 920, 349 482 (1990). C.G. Willson, R. Miller, D. McKean, N. Clecak, T. Tompkins, D. Hofer, J. Michl, and J. Downing, Design of a positive resist for projection lithography in the mid UV, Polym. Sci. Eng. 23, 1004 (1983) M.K. Templeton, C.R. Szamanda, and A. Zampini, Dissolution kinetics of positive photo resists the secondary structure model, Proc. SPIE 771, 136 (1987). 

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