Diazo Meldrums acid

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 ... 

IR spectroscopy has been applied to study the pyrolysis of isonitroso Meldrum s acid 126 <1997PCA3936> and the photodecomposition of diazo-Meldrum s acid 127 <1996JA1551, 1997MI43> in the latter case, the application of a new method, ultrafast IR spectroscopy, was necessitated because the reaction was complete within only 20 ps. 

Many modifications of this basic chemistry have been explored to tailor these resists to deep-UV radiation. For example, changes have been made in the sensitizer so that it bleaches in this wavelength region. Early work in this area was performed on diazo-Meldrum s acid (54) (see structure). This compound functioned as a deep-UV-bleachable dissolution inhibitor however, it was somewhat volatile and, consequently, could be depleted via evaporation during soft bake. More-recent studies have therefore focused on less-volatile sensitizers incorporating heteroatom substitution (55) and on increases in molecular weight (56). 

Diazo-Meldrum s acid, a deep-UV bleachable sensitizer... 

Two interesting attempts to redesign the dissolution-inhibitor-matrix-resin systems have been reported. Grant et al. (37) proposed an alternative inhibitor for novolac resins. They found that 5-diazo-Meldrum s acid (structure 3.5) exhibits an intense, bleachable absorbance at 254 nm with a sharp... 

Scheme 3.2. Photochemical decomposition of 5 diazo-Meldrum s acid.

Therefore we have developed an experimental setup which provides mid-IR spectroscopy on timescales from pico- to milliseconds. For the proof-of-princi-ple experiments an azo compound, i.e., diazo Meldrum s acid, was selected. This compound was used as dopant in a polymer matrix, allowing variation of the absorption strength of selected mid-infrared bands over of a wide range. The IR absorption bands of azo functional groups are additionally in a range where nearly no other IR bands are located (-2200 cm-1). Azo compounds are also structurally closely related to the triazene groups in our designed polymers and are classical representatives of photoactive compounds. 

Scheme 10 Mechanism of the photodecomposition of diazo Meldrum s acid. REPRINTED WITH PERMISSION OF [Ref. 188], COPYRIGHT (1996) American Chemical Society...

Acid 20 has been transformed to the acid chloride 21 which has been condensed with Meldrums acid to give, after solvolysis with methanol, the p-keto ester 22. Subsequent treatment with / -tosylazide gave the diazo compound 23 which has been stereoselectively cyclized to 2-carbomethoxy-tricyclo[3.3.0.0]octan-3-one 24 with copper acetylacetonate as catalyst. 

Resists with diazo-Meldrum s acid, like their DNQ counterparts, are based on the principle of transformation (or removal) of a dissolution inhibitor by a photoreaction. The photolabile inhibitor itself, diazo-Meldrum s acid, behaves like diazoqui-none in that when formulated with novolac, it can inhibit the base solubility of the latter. On irradiation, it decomposes according to Scheme 7.10, forming an acid, which accelerates the dissolution of novolac in aqueous base. 

Diazo-Meldrum s acid does have one serious drawback it sublimates out of the coating during the prebake step. Enormous efforts were made with a view to finding related chromophores that do not sublimate. Successful candidates include diazo-pyrazolidine dione (DPD) (XIV), diazotetramic acid (XV), and diazopiperidine dione (XVI). ... 

It has also been reported that the replacement of the lactone link (in diazo-Meldrum s acid) by a lactam improves hydrogen bonding in the acidic resin and lowers its vapor pressure.Furthermore, Schwarzkopf has reported the synthetic route to 2-diazodimedones (XVII), which are highly photoreactive and which do not sublime at bake temperatures below 95°C. ... 

The photochemistry of diazo Meldrum s acid (33) (Scheme 7) has been investigated experimentally and theoretically. Irradiation of (33) in MeOH with 254 nm light results in efficient loss of N2 and Wolff rearrangement, affording the ester (36) via the intermediate ketene (35). There was no product derived from trapping of a singlet carbene, suggesting that the Wolff... 

Several attempts have been made to design two-component deep-UV positive systems with increased sensitivity. An example is a system based on a novolac polymer and 5-diazo Meldrum s acid as a dissolution inhibitor [20]. The Meldrum s acid derivative provides a bleachable chromophore at 250 nm, which is converted to volatile compounds on irradiation according to the scheme shown in Figure 6.12,... 

Bogdanova, A. and Popik, V.V. (2003) experimental and theoretical investigation of reversible interconversion, thermal reactions, and wavelength-dependent photochemistry of diazo Meldrum s acid and its diazirine isomer, 6,6-dimethyl-5,... 

Diazabicyclo[2.2.2]octane, 92 Ethyl (Z)-3-bromoacrylate, 130 Ethyl diazo(lithio)acetate, 230 Meldrum s acid, 172 Trimethylsilyl trifluoromethanesulfo-nate, 329... 

Isopropylidene diazomalonate (1). Mol. wt. 170.13, m.p. 93-95°. This compound is prepared by diazo transfer from tosyl azide to Meldrum s acid. ... 

Cooperative catalysis in multi-component reactions highly enantioselective synthesis of y-hydroxyketones with a quaternary carbon stereocenter, (b) X. Han, M. Gan, H. Qin, J. Ji, X. Zhang, L. Jiang, W. Hu, Synlett 2011, 1717-1722. Trapping of oxonium ylides with Michael acceptors highly diastereoselective three-component reactions of diazo compounds with alcohols and ben-zylidene Meldrum s acids/4-oxo-enoates. 

In order to get sensitivity in the deep UV region (254 nm), 5-diazo-2,2-dimethyl-l,3-dioxane-4,6-dione (9 the 5-diazo derivative of Meldrum s acid) was used as the photoactive compound. This compound has been shown to decompose via an analogous mechanism to the NDS photoactive compounds at 436 nm (equation 7). Resists formulated with these compounds in novolac resins have been reported to have high sensitivity and maintain good etch resistance. However, (9) appears to volatilize readily from the resist films. Several derivatives of these compounds that are stable in polymer films have been recently reported. 

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