Resist Requirements for Electron Projection and Direct Write Nanolithography
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Resist Requirements for Electron Projection and Direct Write Nanolithography Leonidas E. Ocola Agere Systems, Murray Hill NJ 07974, USA ABSTRACT This paper will discuss aspects of resist properties required for nanolithography using high-energy electrons. A comprehensive image formation model is presented for positive and negative chemically amplified resists where most aspects have been considered (from exposure through development) for e-beam nanolithography (EPL, DWL). A series of optimization studies are performed using the modulation of soluble site density after PEB as a metric to characterize resist imaging performance. Base loading, blocking fraction and acid diffusion properties are investigated. Slower acid diffusion near regions with deprotected sites dominates during PEB and contributes to a sharp image modulation. Electron beam exposed nanostructures exhibit spatial fluctuations of soluble sites in the material left after development, which may alter etch resistance and pattern collapse. Soluble site fluctuations arise from the low statistics involved in the image formation and are significant contributors to the sidewall roughness. INTRODUCTION As critical dimensions of IC devices rapidly march towards sub-100 nm dimensions, there is much effort in developing the technologies to enable their fabrication. Advanced resist technology has made significant progress towards providing resist materials every time the semiconductor industry has moved to a lithographic technology that uses shorter wavelength photons. But sub-100 nm dimensions are below most wavelengths available for photon-based lithography. This created the need for the development of alternative lithographic technologies, of which electron projection lithography (EPL) [1] is a viable candidate. Direct write e-beam lithography (DWL) is another lithographic technique that is commonly used in the semiconductor industry. The main feature of DWL is its high resolution. For this reason DWL is used to pattern the masks used in optical lithography and EPL, as well as for highly specialized device patterning applications. Although EPL and DWL are able to produce high-resolution images, and the use of high-energy electrons enables the transfer of this image into the resist without any significant amount of scattering, the resist exposure is just the beginning of the image formation process. The resist must be treated as a system, where different components interact and participate in different stages of the image recording process until the final topography is obtained. This paper will discuss the material requirements needed for chemically amplified resists to be capable of high-resolution nanostructures when exposed with highenergy electrons. Chemically amplified resists are widely used in the semiconductor industry because of the high sensitivity and chemical etch resistance properties they posses and needed for device fabrication. The image formation process in resists exposed using optical lithography differs from that in resists exposed with high-
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