Deep-UV positive resists

The polymers employed to formulate today s deep UV positive resists are predominantly 4-hydroxystyrene copolymers (Fig. 31), which has resulted from realization that only a small amount of a protecting group can make the resin insoluble in aqueous base. For example, as mentioned earlier, more than 90% deprotection is needed to render the fBOC resist soluble in aqueous base, meaning that 10% protection sufficiently reduces the dissolution rate of PHOST. Thus, the partial protection approach has overcome the problem associated with the much faster dissolution rate of PHOST than novolac resins in aqueous base and allowed one to make chemical amplification positive resists compatible with the industry standard 0.263 N TMAH developer which had been selected for the... 

A DNS-based positive photoresist can be used for deep-UV exposures if the binder resin in the photoresist itself does not absorb. Since novolacs made fi-om the condensation of formaldehyde with pure p-cresol (instead of commercial cresol mixtures) are found to have very transparent windows at about 250 nm, a DNS/p-cresol novolac-based photoresist gives a positive image after deep-UV exposure and development with aqueous base. A copolymer of styrene and maleimide is also used in place of novolac as a binder for DNS-based deep-UV positive 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 ... 

Deep-W Lithography. The important issues for deep-UV lithography (200-250 nm) are aligner optics and resist materials. Problems in aligner optics stem from the decreased transparency of standard lens materials in this frequency range, which necessitates the use of more-expensive construction materials such as quartz. Typical near-UV positive resists are not useful for deep-UV lithography because of unacceptable absorption at... 

Table 9.2 Sensitivities of deep UV positive-tone resists [15].

Resists used to define circuit patterns are radiation-sensitive and may be either positive- or negative-working. As a result of the fine lines, there has been movement away from optical Hthography and iato the mid- or deep-uv regioas. Developmeatal work has also beea focused oa electroa beam, x-ray, and ion-beam exposure devices and resists (9,10). 

Sci. in press). In these studies, the PIQ (2.0 ym thick) was used as an underlayer. Thus, the film consisting of the polymer 11 and PIQ prepared on a silicon wafer was exposed to deep UV-light with the use of Canon contact aligner PLA-521 through a photomask for 5 to 6 s (UV intensity 72 mV/cm2 at 254 nm). The resulting film was then developed with a 1 5 mixture of toluene and isopropyl alcohol for 15 s and rinsed with isopropyl alcohol for 15 s. A positive resist pattern was obtained after treatment of the film pattern with 02 RIE under the condition of 0.64 W/cm2 (RF power 7 MHz, 02 pressure 3 mtorr). 

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. 

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. 

The photoresponsive properties of molecular glasses also have been applied in the design of resists for semiconductor lithography. In a resist, irradiation changes the solubility of the materials, making it more or less soluble (positive or negative resist, respectively). The search for new resist materials follows the development of lithographic techniques toward deep-UV and electron beam... 

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,... 

Positive Resists for Deep UV. Resists based on poly(methyl methacrylate) (PMMA) have been widely used (20,21). The spectral absorption of a variety of methacrylates is similar, showing absorption below 260 nm with a peak at... 

A modification of these systems involving replacement of poly (4-vinylphenol) with poly(methyl isopropenyl ketone) (PMIPK) was reported by Nakane and co-workers (41). Although PMIPK is a positive-acting singlecomponent resist in the deep UV, it functions as a negative resist when mixed with an aromatic bisazide such as 2,6 di(4-azidobenzylidene)-4-... 

Poly(p-formyloxystyrene) was also subjected to quantitative sensitivity analysis using a calibrated multidensity resolution mask. A plot of the normalized thickness remaining as a function of dose is provided in Figure 11. The resist exhibits a sensitivity of approximately 70 mJ/cm2 in the deep UV and has a contrast ( ) comparable to that of the classical diazonaphtoquinone-novolac positive resists that are commonly employed in semiconductor manufacturing. 

These results confirm the potential of poly (p-formyloxystyrene) as a useful resist material combining such interesting properties as ability to be imaged in both positive or negative tone, ease of preparation, and activity in the deep UV, with a moderate sensitivity and good contrast. 

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