John V. Jensen

Dark-field high-transmission chromeless lithography

Dark field (i.e. hole and trench layer) lithographic capability is lagging that of bright field. The most common dark field solution utilizes a biased-up, standard 6% attenuated phase shift mask (PSM) with an under-exposure technique to eliminate side lobes. However, this method produces large optical proximity effects and fails to address the huge mask error enhancement factor (MEEF) associated with dark field layers. It also neglects to provide a dark field lithographic solution beyond the 130nm technology node, which must serve two purposes: 1) to increase resolution without reducing depth of focus, and 2) to reduce the MEEF. Previous studies have shown that by increasing the background transmission in dark field applications, a corresponding decrease in the MEEF was observed. Nevertheless, this technique creates background leakage problems not easily solved without an effective opaqueing scheme. This paper will demonstrate the advantages of high transmission lithography with various approaches. By using chromeless dark field scattering bars around contacts for image contrast and chromeless diffraction gratings in the background, high transmission dark field lithography is made possible. This novel layout strategy combined with a new, very high transmission attenuating layer provides a dark field PSM solution that extends 248nm lithography capabilities beyond what was previously anticipated. It is also more manufacturing-friendly in the mask operation due to the absence of tri-tone array features.

Achieving sub-half-micron I-line manufacturability through automated OPC

We present results of a verification study of totally automated optical proximity correction (OPC) for mask redesign to enhance process capability. OPC was performed on an aggressive 0.35 micrometer i-line LSI logic SRAM design using the automated OPC generation code Eoptimask, employing the aerial image simulation code FAIM, both from Vector Technologies, Inc. Three different tests were performed, varying in the aggressiveness and type of corrections made. The key issues addressed in this work are the predictive capability of the aerial image simulation and, particularly, the ability of automatically generated OPC to significantly improve the fidelity of the final printed resist image for different geometries. The results of our study clearly demonstrate the utility of automated OPC based on aerial image simulation. Key experimental results include: two-fold increase of depth of focus latitude; demonstration of the feasibility of full off-axis illumination on the stepper; successful resolution of different feature types (posts, lines and spaces) on the wafer to correct CD at a single common exposure and focus condition. Future research will address detailed issues in reticle manufacture and inspection which are critical for cost-effective large-scale OPC.

New methodology for evaluating and quantifying reticle line end shortening

A more precise and accurate method of quantifying line end effects on binary photomasks becomes necessary as reticle features continue to decrease in size. A new methodology for measuring and evaluating line ends was developed. By performing multiple step-wise measurements across a single line end feature using a fixed-width region of interest, a simulated representation of the line end profile could be generated. A high n-order polynomial fit was then applied to the resultant data set and a minimum line end value was extrapolated. This methodology reduced the measurement error directly caused by the region-of-interest (ROI) placement and sizing while, at the same time, it improved the accuracy and precision of the measurement. The generated line end profiles may be further used for modeling, simulation, or characterization.

Statistical analysis of poly line printability affected by sPSM manufacturing errors

LSI Logics OPC package, Molotof, integrated into several RET flows has been successfully applied for strong phase shift mask simulation and optimization. Molotof simulator was used to predict sPSM imaging performance in response to statistical errors of alternating phase shift reticle manufacturing. Mask manufacturing errors were reproduced by generating a virtual gds mask with random values of sPSM control parameters such as phase depth, phase width and phase intensity. By measuring critical dimensions and image placement errors of a simulated aerial image for each random event, the image printability performance was calculated. The approach allows for quantitative evaluation and optimization of a strong PSM manufacturing specification by analyzing the distributions of critical dimensions and image placement errors. 2D-model, metrology and simulation flow for performing statistical analysis are discussed. Sensitivity to a single parameter variation and full statistical analysis of the 90nm poly line imaging performance affected by manufacturing errors is presented. The optimum range of phase depth, phase width and phase intensity, yielded 100% of critical dimensions and image placement errors, complying with 90nm technology design rules was found in simulation. Simulation results are confirmed by empirical data.

Sensitivity of the 65-nm poly line printability to sPSM manufacturing errors

A methodology and a Monte Carlo simulation flow with integrated LSI Logics OPC package, Molotof, was applied to the 65nm poly line sensitivity analysis. Strong phase shift mask (sPSM) manufacturing specifications were optimized to obtain image critical dimensions (CD) and image placement errors (IPE) complying with technology design rules. Reticle manufacturing statistical errors of phase depth, phase width, and phase intensity imbalance were used to generate a virtual sPSM for imaging poly lines. A criterion for qualifying reticle specification is to obtain all latent image CDs and IPEs within a design rule allowed range for a given mask specification. The approach allows for computing reticle and litho budgets into CD imaging performance. We present simulation and empirical results of statistical analysis of the 65nm poly line (clear field) printability, and a method for optimizing a strong phase shift reticle specification. Sensitivity to a single parameter variation and full statistical analysis of the 65nm poly line imaging performance affected by manufacturing errors is presented. The optimum reticle specification, yielded 100% of critical dimensions and image placement errors, was found in simulation and confirmed by empirical data.

Double Step process for manufacturing reticle to reduce gate CD variation

In low k1 lithography, reticle quality decides the process capability. Therefore, we must minimize CD errors on the reticle plate. Double Step process (DS process) is a unique method to improve CD uniformity of line patterns on the active region of poly layer reticle. In DS process, poly layer design is divided into the active region and the non-active region. And then, these two regions are processed individually. By using this procedure, pattern density variation across the reticle plate is reduced when making line patterns on the active region. As a result, the loading effect of the dry etching process reduced, and CD uniformity of these patterns can be improved. Using this technique of reticle fabrication, CD uniformity could be improved. Particularly, the range of CD variation of line patterns in logic cells was drastically reduced from 29nm to 20nm.