Postexposure baking resist

Acid generation in photoresist films add photogeneration vs. dose, 3233/ acid present after irradiation, 32,34r add present before irradiation, 32 quantum yield, 3234 Acid hardening resin resists cross-linking adivation energy determination, 87,89 cross-linking chemistry, 87 determination of acid generated, 87-88 effect of postexposure bake temperature and time, 87... 

As discussed previously, an optional postexposure, predevelopment bake can reduce problems with the standing-wave effect in DNQ-novolac positive resists. However, such a postexposure bake step is indispensable in the image reversal of positive resists (37-41) and certain resists based on chemical amplification of a photogenerated catalyst (64-67, 77, 78). For both types of resists, the chemistry that differentiates between exposed and unexposed areas does not occur solely during irradiation. Instead, differentiation occurs predominantly during a subsequent bake. Therefore, to obtain acceptable CD control in these systems, the bake conditions must be carefully optimized and monitored. 

Brominated poly(l-trimethylsilylpropyne) is an example of a substituted polyacetylene that is suitable for bilevel-resist processes (34). Requiring both exposure and postexposure bake (PEB) steps, samples of the polypropyne having a mole fraction of bromine from 0.1 to 0.2 per monomer unit exhibit sensitivities in the order of 25 mj/cm. Submicrometer resolution has been demonstrated, and etching-rate ratios relative to hard-baked photoresist planarizing layers are —1 25. 

Postexposure bake of the wafer. A postexposure bake (PEB) improves contrast of the photoresist before its development. The PEB process causes three effects 1) diffusion of the PAC 2) solvent evaporation and 3) thermally induced chemical reactions. In general, the dissolution rate of a resist decreases as a function of a PEB temperature. PEB becomes more important for the photoresists with a chemical amplification (CA) feature. The photoresists need the PEB to complete chemical reactions initiated by exposure. 

In general, the chemical transformations associated with the chemical amplification mechanism in resists is effected through heating the exposed resist film, in a process called postexposure bake (PEB). Although, in principle, the active catalytic species (ions or radicals) could be generated from either photochemical (or radiochemical) acid or base generators, the acid generators are now used almost exclusively in advanced resist systems. ... 

Figure 11.13 SEM images of line-and-space patterns printed with 60-nm-thick Shipley XP-98248 resist on bare silicon and exposed at 157 nm. Process conditions postapplied bake 130°C/60 seconds, postexposure bake 130°C/90 seconds, developer 0.26N tetramethylammonium hydroxide (without surfactant) for 20 seconds. Unexposed resist loss 6nm. Exposure energy 1.35 mJ/cm. Note the significant surface inhibition layer, showing poisoning effects. ...

Lithographic modeling simulates several key steps in the lithographic process comprising image formation, resist exposure, postexposure bake diffusion,... 

The effects of thermally driven diffusion of active chemical species within the resist during the postexposure bake are modeled with appropriate diffusion... 

Topcoats also provide a protective function against airborne molecular contaminants such as amines. Figure 13.53 shows PEB delay stabilty of up to 15 minutes obtained on a resist film protected with a topcoat. Following the indicated PEB delay times, the resist film was postexposure baked and developed. Even after 15 minutes, the CD values of the 80-nm and 90-nm lines/space features remained remarkably stable. This is considerably better performance than that achieved when topcoat is not used. 

The rate of acid generation from BTf was larger than that of diphenyliodonium triflate(ITf). Deprotection of poly(tert-butyloxycarbonyloxystyrene) (tBOCHS) with BTf was 3 times faster than that with ITf after postexposure bake at same temperature. BTf with higher sensitivity and thermal stability may be expected to be PAG of diazo compounds applicable to microlithography resists. 

When the photoacid generator, triphenylsulfonium hexafluoroantimonate, is exposed to radiation, it decomposes to release the super acid, hexafluoroantimonic acid, in the resist film. While this photochemical reaction can occur at room temperature, the acid-catalyzed deprotection of the pendant r-butyl group of the resist polymer occurs at reasonable rates only at elevated temperature. It is therefore necessary to heat the resist film to an appropriate temperature (postexposure bake) to provide the energy that is required for the acid-catalyzed deprotection of the r-butyl group of the ester, which in turn, affords the base-soluble norbomene carboxylic acid unit isobutylene volatilizes. The extent of deprotection at constant temperature is... 

Figure 9 Contrast curves of epoxy novolac resists exposed to DUV and postexposure-baked at 40 °C for the times shown.

The resist process of chemical amplification resists is shown in Figure 2.2. The resist solution is coated on a silicone wafer. The coated resist is then prebaked. The prebaked resist is exposed by UV light of an excimer laser. The time between prebaking and exposure is interval I. The exposed resist is baked by postexposure baking (PEB). The time between exposure and PEB is interval II. The silicone wafer is developed by alkali solution (tetramethylam-monium hydroxide). The time between PEB and development is interval III. 

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