Christoph M. Friedrich

Process proximity correction using an automated software tool

The pattern transfer process from the chip layout data to the structures on the finished wafer consists of many process steps. Although desired, none of these steps is linear in all aspects of the pattern transfer. Approaching the process limits due to the ever-shrinking linewidth, the non- linearities of the pattern transfer clearly show up. This means, that one cannot continue the practice to summarize all process influences into one bias between the data used for mask making and the final chip structure. The correction of process non-linearities is a necessity. This correction is usually called optical proximity correction (OPC), although not all effects intended for correction are of optical origin and/or not all these are effects of the neighborhood. We therefore propose to use the term PPC (process proximity correction). This paper reports our experiences with the application of OPTISSIMO, a software tool developed to perform automatically OPC/PPC for full chip designs. First, we provide a definition of PPC, which in our view has to correct all non- linearities of the pattern transfer process from layout data to the final electrically measured structures. Then, the strategy of the OPC/PPC tool OPTISSIMO, a software package to perform PPC based on process simulation, is discussed. We focus on the data handling strategy and on the process modeling of the tool under evaluation. It is shown, that full chip OPC/PPC is practicable using a well-designed hierarchy management system combined with a pattern library. Finally, it is demonstrated, that a model-based OPC/PPC tool is by definition a process simulation tool, that is able to perform all simulation tasks (like defect printability) at reasonable accuracy.

Benchmarking of software tools for optical proximity correction

The point when optical proximity correction (OPC) will become a routine procedure for every design is not far away. For such a daily use the requirements for an OPC tool go far beyond the principal functionality of OPC that was proven by a number of approaches and is documented well in literature. In this paper we first discuss the requirements for a productive OPC tool. Against these requirements a benchmarking was performed with three different OPC tools available on market (OPRX from TVT, OPTISSIMO from aiss and PROTEUS from TMA). Each of these tools uses a different approach to perform the correction (rules, simulation or model). To assess the accuracy of the correction, a test chip was fabricated, which contains corrections done by each software tool. The advantages and weakness of the several solutions are discussed.

Mask Specifications and OPC

The paper evaluates the implications of optical proximity correction (OPC) to mask specifications. This is done by simulation of 200 nm design rule structures using a conventional simulation tool for long lines and using the simulation module of an OPC-tool for more complicated design situations. Both tools were parameterized for a state-of-the- art lithography process using a 0.6 NA stepper and a chemically amplified resist. As long as OPC stays within reasonable limits (minimum feature size greater than 60% of the input pattern), the printability of a defect is about the same as in a dense line/space array. A defect of 200 nm size on a 4X mask produces change in linewidth of about 20 nm. Even for masks with subresolution structures, the increase of printability of mask defects is within 10% compared to un- corrected masks. However, simulation using resist parameters cannot reproduce the aerial image result, that masks with outriggers are less sensitive to mask linewidth variation than conventional masks.

Integration of alternating phase-shift mask technology into optical proximity correction

The paper describes the extension of optical proximity correction (OPC), which is well established for conventional chromium-on-glass mask printing, to alternating phase shift masks (altPSM). Aerial image simulation of various situations of light-field and dark-field altPSM shows that the size of the phase shifter has a great impact on the printed critical dimension (CD). Especially layouts containing non-symmetric phase shifters or shifter sizes comparable to the nominal CD do not print on target. The application of optical proximity correction to the chromium structures between the phase shifters is capable to compensate for such effects. We demonstrate the added value of OPC using a simulation-based software tool for altPSM.

High-NA illumination: a simulation study

Lithography simulation was used to calculate the influence of high-NA illumination on resolution, depth-of-focus, and exposure latitude evaluating the aerial image. Contour plots of exposure latitude versus NA & (sigma) at constant depth-of- focus values were calculated for dense lines, single contacts and isolated lines. All features were investigated using standard illumination and enhancement techniques (PSMs and/or annular illumination). For standard illumination the maximum exposure latitude is achieved for the highest NA possible if only small depth-of-focus is required e.g. thin resist layers over nearly plain or planarized substrates. In a production environment higher depth-of-focus values are necessary. In this case the optimum NA moves to lower values even for feature sizes near the resolution limit. However, in combination with PSMs (and/or annular illumination) the best conditions move to higher NA with decreasing feature sizes.

Evaluating the potential of alternating phase-shift masks using lithography simulation

This paper quantifies the expected gain in the process window of 150nm structures printed with DUV for alt PSM vs. COG masks and HT PSM. Most of the analysis was performed for dense lines and isolated lines using lithography simulation. Alt PSM show an increase of dose latitude by 9 percent and an improved DOF by 0.2 micrometers for dense liens. For isolated lines the real advantage is seen in the increase of DOF by 0.7 micrometers . Furthermore it will be demonstrated, that alternating PSM can improve the imagin performance of contacts significantly over competitive techniques. Chromeless PSM may push the ultimate resolution limit. However to vary the linewidth three adjacent quartz edges must be used, since two phase edges are instable in defocus. A phase shifting region needs to exceed a minimum width in order to enhance the contrast of the aerial image of the whole feature. Experimental data and simulations show that the required minimum phase-shifter width for an isolated line is in the region of 400nm. Simulation and experiment show, that 90 degrees edges are very sensitive to defocus and neighboring patterns. Using a 3D mask simulator, correction values for etch depth and parameters for a lateral underetch were determined in order to achieve intensity balancing for alt PSM for various feature sizes.