Method for Measuring Profiles of Photoacid Patterns in Chemically Amplified Resists
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Method for Measuring Profiles of Photoacid Patterns in Chemically Amplified Resists Gilbert D. Feke,1 Robert D. Grober,1 Gerd Pohlers,2 and James F. Cameron2 Department of Applied Physics, Yale University, New Haven, CT 06520-8284, U.S.A. 2 Microelectronic Materials Research and Development Laboratories, Shipley Company, Marlborough, MA 01752, U.S.A.
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ABSTRACT We describe a method based on single molecule probing of acid concentration to measure the profiles of photogenerated acid patterns in chemically amplified resist films. We further present preliminary data which demonstrates the viability of this method. INTRODUCTION Chemically amplified resists (CARs) [1] are widely used by the semiconductor industry and continue to be developed in response to the increasingly demanding requirements of production lithography. In this class of resists the radiation pattern incident at the wafer is recorded by a photogenerated catalyst, typically a strong Brønsted acid produced by the photolytic decomposition of a photoacid generator (PAG) [2]. The photoacid is activated by a postexposure bake (PEB) to catalyze multiple chemical reactions in the resist matrix and thereby locally alter the dissolution rate, a process called chemical amplification. The resist is then developed with the spatially dependent dissolution rate defining the ultimate pattern. Precise control of the spatial distribution of photoacid during lithographic processing is paramount for maximizing lithographic resolution and minimizing critical dimension variation. An obvious (though nontrivial) example of exercising this control is the focusing of the aerial image at the wafer. Another example is the design of resist compositions and optimization of processing conditions to minimize the diffusion of the acid from exposed to unexposed areas during PEB. Yet another example is the use of base additives to neutralize residual acid in the unexposed areas. In each of these examples, the objective is to maximize the sharpness of the acid concentration profiles. As the dimensions of lithographic features continue to decrease, the modeling, control, and monitoring of acid generation and diffusion are becoming even more crucial issues in the design and optimization of resist compositions and lithographic processes. Photoacid distribution is generally inferred from developed patterns. However, because developed patterns represent the convolution of each and every lithographic process, it is not possible to determine the photoacid distribution at each stage and hence unambiguously extract fundamental resist chemistry parameters or characterize individual processes. Furthermore, in many cases it may be desirable to inspect the outcome of a particular process before proceeding to the next. Several methods of latent image detection have been developed in response to this problem. These have included atomic force microscopy [3-8], thermal probe microscopy [4], photon tunneling microscopy [9-11], infrared microscopy [7,12], and fluorescence microscopy of resist doped with a
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