Wolfgang Henke

Comparison of simulation approaches for chemically amplified resists

Lithography simulators have become a standard tool in industrial and governmental research and development departments. IN contrast to the modeling approaches for the optical system and for the lithographic performance of i- line resists, there is still no consensus on the modeling of chemically amplified resist (CAR). Existing models differ in the description of the kinetics and the diffusion phenomena during post exposure bake and in the specification of the development rate. A modeling approach was established, that combines the light induced generation of photoacid, in- and out-diffusion of acid or base components, a generalized deprotection kinetics, Fickian and non-Fickian diffusion of resist components and an arbitrary development rate model. Existing models such as the effective acid model and a standard deprotection model for CAR can be considered as special cases of the implemented model. To evaluate the importance of certain options of the model and of the model parameters we have evaluated the performance of the model by comparing simulated CD data and resists profiles with experimental data.

Simulation and Process Design of Gray-Tone Lithography for the Fabrication of Arbitrarily Shaped Surfaces

This paper reports on a study of a methodology for fabrication of arbitrarily shaped silicon structures using technologies common to standard IC manufacturing processes. Particular emphasis is placed on the design and use of halftone transmission masks for the lithography step required in the fabrication process of mechanical, optical or electronics components. The design and experimental investigation of gray-tone masks were supported by lithography simulation. Results are presented for both simulated gray-tone patterns and experimental trials.

New stochastic post-exposure bake simulation method

A new method for simulating the post-exposure bake (PEB) of optical lithography is presented and applied to modeling the reaction-diffusion processes in a chemically amplified resist (CAR). The new approach is based on a mesoscopic description of the photoresist, taking into account the discrete nature of resist molecules and inhibitor groups that are attached to the resist polymers, but neglecting molecular details on an atomistic (microscopic) level. As a result, the time- and space-dependent statistical fluctuations of resist particle numbers, the correlations among them, and their effect on the printing result can be accounted for. The less molecules that are present in the volume of interest, the more important these fluctuations and correlations will become. This is the case for more and more shrinking critical dimensions (CD) of the lithographic structures but unchanged molecular sizes of the relevant resist species. In particular, the new PEB simulation method allows us to predict the behavior of statistical defects of the printed lithographic structures, which may strongly contribute to printing features like line edge roughness (LER).

Influence of optical nonlinearities of the photoresist on the photolithographic process: applications

Using a new simulation method, the influence of refractive index changes during the bleaching of the photoresist on process parameters is investigated. For standard applications using thin resists, refractive index changes above 0.1 result in considerably modified dose latitudes, swing curves and iso- dense bias. In special applications with thick resists, the same effect occurs for much smaller refractive index changes. Optimized refractive index changes can be used for the fabrication of structures with high aspect ratios.

The Impact of Lens Distortion on Dynamic Imaging in Photolithography

In dynamic imaging static lens distortion has a detrimental impact on the image contrast of dense mask patterns and leads to blurring of images of isolated features, thus decreasing process window in lithographic applications. A new method is proposed here for combining transversal vibrations caused by the stage synchronization error of reticle and wafer stages and the lens distortion in lithographic step-and-scan exposure systems. Merging the descriptions of both effects into a unique characteristic function, the combined probability density function, allows us to quantify the individual contributions to the total result, to assess the mutual interaction of both effects and finally to calculate the impact of the combined effects on the projected dynamic image. The characteristic function characterizes the resulting image blur and is used here to study the impact of specific lens distortion fingerprints. The method presented here is also applicable to other types of transversal vibrations and can be extended to combine several vibration effects into one characteristic function.

How lens aberrations influence lithographic imaging and how to reduce their effects

A method is described to assess the influence of lens aberrations on the image by analyzing the interaction of specific aberrations and diffraction patterns resulting from corresponding mask structures. In order to establish a correlation between the amplitude of individual diffraction orders and specific aberrations, the sensitivity of each diffraction order is investigated separately. The resulting information is used in order to find means to reduce the influence of the aberrations on the diffraction pattern. Several possibilities such as various mask biases, serifs and non-printable assist features can either enhance or decrease the sensitivity to specific aberrations. This method of diffraction order sensitivity study is described and experimentally tested.