Negative deep-UV resists

New Negative Deep-UV Resist for KrF Excimer Laser Lithography... 

A photosensitive composition, consisting of an aromatic azide compound (4,4 -diazidodi-phenyl methane) and a resin matrix (poly (styrene-co-maleic acid half ester)), has been developed and evaluated as a negative deep UV resist for high resolution KrF excimer laser lithography. Solubility of this resist in aqueous alkaline developer decreases upon exposure to KrF excimer laser irradiation. The alkaline developer removes the unexposed areas of this resist. 

As for negative deep UV resist, O Toole et al. have exhibited half-micron pattern resolution in 0.5 micron film thickness using the new resist and PIE process (10). The pattern profiles, however, were re-entrant, due to the large photo absorption and the applications to single-layer-resist system have not been presented (11). 

Figure 2 shows the exposure characteristics for azide-styrene resin resist film with an azide concentration from 10 to 40 wt% (based on the styrene resin weight) and Figure 3 shows the contrast of the resist films as a function of the azide concentration. Development was done with a 60s immersion in 0.83% TMAH solution. The styrene resin matrix alone has been found to be a negative deep UV resist. However, rather low contrast (1.48) and low sensitivity (2.5 J/cm2) are observed. The contrast and the sensitivity of the styrene resin is remarkedly increased by adding the azide, as shown in Figures 2 and 3. 

A negative deep UV resist, consisting of a photosensitive 4,4 -diazidodiphenyl methane and a poly-(styrene-co-maleic acid half ester) resin, has been found to meet the requirements needed for KrF excimer laser lithography. 

Novolac- or phenolic resin-based resists usually show no pattern deformation induced by swelling during development in aqueous alkaline solution. Examples of such resists are naphtho-quinonediazide/novolac positive photoresists, novolac-based positive electron-beam resist (NPR) (1), and azide/phenolic negative deep-UV resist (MRS) (2). Iwayanagi et al.(2) reported that the development of MRS proceeds in the same manner as the etching process. This resist, consisting of a deep-UV sensitive azide and phenolic resis matrix, is also sensitive to electron-beams. This paper deals with the development mechanism of non-swelling MRS and its electron-beam exposure characteristics. 

H. Tomioka, Submicron optical lithography utilizing a negative deep UV resist MRS, Proc. SPIE 539, 151 (1985). 

A negative deep UV resist can be produced from a composition of 3-octylPT with the cross-linker ethylene [l,2-bis(4-azido-2,3,5,6-tetrafluorobenzoate)] [150]. Grid patterns can be reproduced without distortion on an electrically conductive electron beam resist material containing substituted PT like 3-dodecyloxyPT [151,152]. 

Figure 4. SEM micrograph depicting nominal 0.3 /jum line/space images obtained in the negative, deep-UV resist, XP8843 available from the Shipley Co.

A negative deep-UV resist composed of poly( p-vinylphenol) and a bisazide has been developed (5). A limitation has been that a high percentage of bisazide (20 wt%) to resin is required such that a resist film of 1 jam thickness is virtually opaque in the 200-300 nm region. Consequently, undercut profiles are typically observed after development and the processing conditions have to be carefully controlled to maintain line-width and reproducibility (2). Other negative resists such as novolac resin with a bisazide (15-20 wt%) (9), an acidic resin with a bisazide (30 wt%) 10) and poly(methyl methacrylate) with a bisazide (20-25 wt%) 11) also suffer because of the poor cross-linking efficiency of the bisazide. 

Table 1. Bisazides in Polystyrene as Negative Deep-UV Resists...

H. Iwasaki, T. Itani, M. Fujimoto, and K. Kasama, Acid size effects of chemically amplified negative resist on lithographic performance, Proc. SPIE 2195, 164 172 (1994) U. Schedeli, N. Miinzel, H. Holzwarth, S.G. Slater, and O. Nalamasu, Relationship between physical properties and lithographic behavior in a high resolution positive tone deep UV resist, Proc. SPIE 2195, 98 110 (1994). 

A comparative study of polystyrene (PS) with bis(perfluorophenyl) azides 1-2 and the corresponding non-fluorinated bisazides 3-4 as deep-UV resists is reported. Inclusion of as low as 1.2 wt-% of 1 in PS led to 70% retention of thickness after photolysis and development. PS containing 2.4 wt-% of 1 is > 100 times more sensitive as a deep-UV negative resist than PS itself. The presence of 1 in PS also increased the contrast of the resist. On a molar basis, 1 was about 10 times as effective as non-fluorinated bisazide 3 in cross-linking PS while 2 was about 6 times as effective as 4. PS containing 2.4 wt-% of 1 was found to have a deep-UV sensitivity of 5-10 mJ cm and resolution of about 0.5 /tm. 

Pol ene (PS) is a negative deep UV (25) and electron beam resist showing high resolution (24) but low sensitivity (25). Apparentty no attempt has been made to improve the sensitivity of PS by adcUtion of a bisazide. We expected that addition of bis-PFPA to PS should result in a deep-UV resist with increased sensitivity. Thus, various amounts of bis-PFPA 1 or 2 and the corresponding nonfluorinated bisazide 3 or 4 were separately added to PS. The resist solutions were then spin-coated on NaQ discs, baked, photolyzed and developed. The intensity of the IR CH stretching absorption at 2924 cm" before and after development was used to estimate the retention of film thidcness. 

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). 

From these observations, a photooxidative scheme has been developed in which a hydrophobic resist becomes hydrophilic upon oxidation induced by deep UV (248 and 193 nm) radiation. Subsequent treatment with TiCLi followed by oxygen reactive ion etching then affords high-resolution, negative-tone patterns. Studies are currently underway to minimize the line edge roughness and background residue present in such patterns. 

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... 

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