Conventional positive-resist

Some sensitivities with 364 nm irradiation are as follows DMA/PEMA, 0.0059 DPA/PEMA, 0.0057 DPA/PPSQ, 0.0092 cm2/mJ. (This value is the initial rate dA/dt divided by the intensity see ref. 15.) A comparable datum for DMA/PPSQ is not available, but by comparing the doses required to reach a specified absorbance, one finds that DMA/PPSQ is slightly faster than DMA/PEMA the ratio is about 1.5 (+/-20%). These numbers are a little smaller than those for conventional positive resist (49). With deep UV irradiation, however, they will be 7-20 times larger due to the larger extinction coefficient, which helps fulfill one of the key prerequisites of a CEL (or PIE) material for the deep UV. 

Several groups have investigated three-component systems encompassing both chemical amplification and dissolution inhibition. As stated earlier, Smith and Bonham (63) reported resist materials composed of a binder resin (novolac), a nonpolymeric compound containing acid-labile functional groups such as acetals, and a trihalomethyl-substituted 5-triazine acid photogenerator. The acid-labile compound acts as a novolac dissolution inhibitor in a manner analogous to the action of DNQ in conventional positive resists. However, in this case, the inhibitor is not photochemically active. Instead,... 

The absorption characteristics of resist films at the exposure wavelength of an excimer laser are important. For example, an application of conventional positive resists to 248-nm KrF excimer laser lithography is limited by ex-... 

It is assumed that the solution development of a conventional positive resist occurs in three steps, namely, diffusion of developer from the bulk solution to the surface of the resist, reaction of the developer with the resist, and diffusion of the product back into the solution. It is also assumed that the last step, diffusion of the dissolved resist into solution, occurs very rapidly, and therefore can be ignored. ... 

Though conventional positive resists have performed well at 436,405, and 365 nm wavelength (common outputs of commercial exposure sources), the potential resolution gain has been small. This has given a push toward shorter wavelengths for achieving higher resolution. 

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). 

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. 

Interest in solution inhibition resist systems is not limited to photoresist technology. Systems that are sensitive to electron-beam irradiation have also been of active interest. While conventional positive photoresists may be used for e-beam applications (31,32), they exhibit poor sensitivity and alternatives are desirable. Bowden, et al, at AT T Bell Laboratories, developed a novel, novolac-poly(2-methyl-l-pentene sulfone) (PMPS) composite resist, NPR (Figure 9) (33,34). PMPS, which acts as a dissolution inhibitor for the novolac resin, undergoes spontaneous depolymerization upon irradiation (35). Subsequent vaporization facilitates aqueous base removal of the exposed regions. Resist systems based on this chemistry have also been reported by other workers (36,37). 

Conventional, positive photoresists can be used for wavelengths down to = 300 nm. However, most of these resists have optical densities > 1 at wavelengths below 300 nm and as a consequence, are not useful at such low wavelengths since most of the light is absorbed near the surface with very little light reaching the resist/substrate interface. If we are to operate below 300 nm, new resist systems and processes must be designed. 

The earliest work in DUV lithography utilized PMMA resist (77) and required exposure times of tens of minutes. Researchers were forced to tolerate the excessive exposure times because conventional, positive NUV resists are not useful for DUV exposure. These NUV resist materials (the... 

Figure 5. Typical sensitivity or response curve for a positive electron resist. Positive resist response may also be plotted using other conventions (see...

Conventional positive photoresists only have limited utility in the mid-UV and deep-UV range. While it appears feasible to modify the structure of the o-quinonediazides to optimize their absorption characteristics in the mid-UV range, (8,9) or even to use appropriate sensitizers for conventional resists,... 

Several attempts have been made to redesign the traditional two-component near-UV positive resist systems to make them compatible with the deep-UV. Recall that the major problems associated with deep-UV exposure of conventional resists are related to non-bleaching of the o-quinonediazide sensitizer on exposure because of photoproduct absorbance, and strong absorption of the novolac resin. Willson and coworkers34 attempted to solve this problem using dissolution inhibitors based on 5-diazo Meldrums acid, which undergoes photochemical decomposition as follows ... 

Apart from multi-level layer resist systems, conventional positive-tone resists can be classified into two categories one-component and two-component systems. Classical examples of the former systems are polyfmethyl methacrylate), and poly (butene-1-sulfone) (2,3). Typical examples of the latter system are AZ-type photoresists, which are mixtures of cresol-formaldehyde-Novolak resins and a photoactive compound acting as a dissolution inhibitor... 

We have developed a silicon containing positive photoresist in the same manner that we developed an iodine containing resist previously. In other words, silicon compounds are blended into a conventional positive photoresist. 

We have indicated that intensity dependent phenomena may be useful in at least two distinct ways. One is to obtain something approaching a "threshold detector" resist response. To obtain a threshold development response in typical positive resists is difficult, since the development rate is in general a smoothly varying function of the photochemical reaction progress. The application of a layer of polymer with the bleaching characteristics shown in Figure 5 provides a way to obtain such threshold response with conventional resists, provided an excimer laser is used in the illumination system. 

Many kinds of silicon-containing photoresists have been reported(6-11) to have both O2 RIE resistance and high resolution. Among them a positive photoresist called ASTRO( ), a mixture of conventional positive photoresist and a silicon resin developed by Hitachi for the imaging layer of bi-level resist system, was selected as an etching mask in this experiment for the following reasons. First its... 

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. 

The bottom PMMA layer can be replaced with more sensitive DUV positive resists such as P(MMA-OM) and PMIPK (c Section 3.1.2) to reduce blanket-exposure time. Conventional positive photoresists are opaque enough below 250 nm to act as an excellent mask for the image transfer exposure of PMMA, which is sensitive to the DUV radiation ranging from 200 to 230 nm. On the other hand, PMIPK is most sensitive in the range... 

However conventional positive electron beam resists like PMMA(4) or PBS(5.) do not have excellent dry etching resistance. The electron beam sensitivities of these positive resists primarily result from radiation-induced degradation of polymer main chains. If the main chain bonding force of these polymers is weakened in order to improve sensitivities, the dry etching resistances of these polymers will decrease. In such cases, sensitivity to electron beam exposure and dry etching resistance are in a trade-off relationship. 

Bowden and his coworkers(j).) proposed a new type of positive electron beam resist which consists of an alkali-soluble novolac and polymeric dissolution inhibitor. The positive working mechanism of this new type positive resist( NPR ) is similar to that for the conventional positive photoresist 10). It was also found that poly(2-methylpentene-l sulfone)( PMPS ) is good as a polymeric dissolution inhibitor for NPR(lil). In addition, it was clarified that one of the difficulties with NPR is phase separation in the resist films(10)(n). 

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