Lithographic resist materials, design

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... 

The basic concepts employed in early photolithography, both in materials and processing, have been extrapolated to modern lithographic technologies including x-ray and electron beam. In the remainder of this book we will discuss in detail the fundamental principles of chemistry and physics as they apply to the design and use of resist materials. 

Three extensive introductory chapters by Everhart, Broers, and Bowden provide a solid foundation in the physics and chemistry of the lithographic process together with an overview of current resist systems. These 3 chapters, coupled with 20 chapters from outstanding radiation polymer chemists throughout the world, provide a firm basis for understanding the important fundamental concepts in radiation chemistry as applied to design, development, and application of resist materials. 

There are two aspects of resist sensitometry 1) the measuiement of resist response to radiation which involves measurements designed to determine the intrinsic radiation sensitivity of the materials from which the resist is constituted and 2) lithographic sensitivity which is a measure of the efficiency with which these radiochemical conversions can be expressed in relief image formation. 

To accommodate the diverse needs of lithographic processes and device design specifications, resist properties vary. However, a few primary characteristics common to all resists can be used to gauge their performance. These characteristics include sensitivity, contrast, resolution, and etching resistance. Because resist performance is strongly operation dependent, comparison between materials must be made under identical conditions. 

Combining the lithographic and etch mask functions into a single polymer can be a major challenge, especially for deep-UV lithography. The latitude in resist design is limited, because at least 10 and preferably 15 wt % of the polymer structure must be reserved for silicon. A few materials, like silicon-substituted poly(methyl methacrylates) (6) and polysilanes (7, 8), have been used as positive two-layer resists for deep-UV lithography, but these materials suffer from either poor to moderate sensitivities to deep-UV radiation or an excessive absorption in the UV that limits exposure depth in the resist layer. 

Resolution has become a crucial requirement for microlithography. It depends not only on material itself, but also on lithographic apparatus limitations, development processes and so many other complex factors. In designing a high resolution resist, therefore, starting material, or basic structure has to be chosen by an evidence of resolution capability. Low molecular weight (Mw = 3 x 10 0 poly(2-vinylnaphthalene) showed high resolution less than 0.1 p. m line on Si substrate by ordinary procedure. 

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