Laser-induced Reactions in a Deep UV Resist System: Studied with Picosecond Infrared Spectroscopy
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ABSTRACT One of the most technologically important uses of organic photochemistry is in the imaging industry where radiation-sensitive organic monomers and polymers are used in photoresists. A widely-used class of compounds for imaging applications are diazoketones; these compounds undergo a photoinduced Wolff rearrangement to form a ketene intermediate which subsequently hydrolyses to a base-soluble, carboxylic acid. Another use of organic molecules in polymer matrices is for dopant induced ablation of polymers. As part of a program to develop diagnostics for laser-driven reactions in polymer matrices, we have investigated the photoinduced decomposition of 5-diazo-2,2-dimethyl-1,3-dioxane-4,6-dione (5-diazo Meldrum's acid, DM) in a PMMA matrix using picosecond infrared spectroscopy. In particular, irradiation of DM with a 60 ps 266 nm laser pulse results in immediate bleaching of the diazo infrared band (v = 1 2172 cm- 1). Similarly, a new band appears within our instrument response at 2161 cm- (FWHM = 29 cm- 1) and is stable to greater than 6 ns.; we assign this band to the ketene photoproduct of the Wolff rearrangement. Using deconvolution techniques we estimate a limit for its rate of formation of 'r < 20 ps. The linear dependence of the absorbance change with the pump power (266 nm) even above the threshold of ablation suggest that material ejection take place after 6ns. INTRODUCTION One of the most technologically important uses of organic photochemistry is in the imaging industry where radiation-sensitive organic monomers and polymers are used in photoresists.1 Common examples include systems in which photochemically formed acids become soluble in aqueous base (i.e. positive photoresists) 2 and where organic dopants sensitize a polymer towards ablation. 3 The chemistry which occurs in these systems is the basis for the manufacture of high-density electronic circuits, as well as for the production of printing plates. One of the most widely-used class of compounds for imaging applications are diazoketones. 4 These compounds undergo a photoinduced Wolff5 rearrangement 6 to form a ketene intermediate which subsequently hydrolyses to a base-soluble carboxylic acid. As part of 7 a program to develop diagnostics for laser-driven reactions in polymer matrices , we have investigated the photo-induced decomposition of 5-diazo-2,2-dimethyl- 1,3-dioxane-4,6-dione (5diazo Meldrum's acid, DM) in a PMMA matrix. This particular diazoketone is sensitive to deep UV (200-260 nm) making it suitable for high resolution lithographic applications. 8 It has historically been difficult to study the chemistry of such resists as many of the intermediates are short-lived and absorb only in the UV. Recent advances in ultrafast infrared spectroscopy, however, now allow us to directly examine the initial steps occurring in UV photoresists with infrared spectroscopy. Herein, we describe the first room temperature observation of intermediates in the photochemistry of DM with ultrafast infrared spectroscopy. Shown in Fig. 1 is the generally
