William B. Howard

Modeling and Performance Metrics for Longitudinal Chromatic Aberrations, Focus- drilling, and Z-noise; Exploring excimer laser pulse-spectra

The combined impact of longitudinal chromatic aberrations, focus-drilling, and Z-noise on several lithographic performance metrics is described. After review, we investigate an improved method for simulating the lithographic behavior of longitudinal chromatic aberrations stemming from the finite bandwidth of excimer laser pulse-spectra using PROLITHTM v. 9.3.3. Additionally, we explore two methods for modeling the lithographic improvements related to focus-drilling and new PROLITH functionality for modeling the effects of Z-noise. Our case studies involve reinvestigating the RELAX process and providing a framework for accurate lithographic simulation using machine specific pulse-spectral data, modified Lorentzian, and Gaussian models. After presentation and analysis, we discuss potential applications including methods for improved focus budgets and improved mask design.

Assessment of OPC effectiveness using two-dimensional metrics

A complete evaluation of the optical proximity effects (OPE) and of their corrections (OPC) requires a quantitative description of two-dimensional (2D) parameters, both at resist- and at reticle-level. Because the 2D behaviour at line-ends and at line-corners can become a limiting factor for the yield, it should be taken into account when characterising a process, just as the CD- and pitch-linearity are already kept under control. This implies the measurement of 2D-metrics in a precise way. We used an SEM Image Analysis tool (ProDATA SIAM) to define and measure various OPC-relevant metrics for a C013 process. For the METAL (M1) process, we show that the overlap between line-ends of M1-trenches and underlying nominal contacts is a relevant metric to describe the effectiveness of hammerheads. Moreover, it is an interesting metric to combine with the CD process window. For the GATE process, we demonstrate that for a given set of metrics there is a degree of OPC aggressiveness beyond which it is not worth to go. We considered both line-end shortening (LES) and corner rounding affecting the poly linewidth close to a contact pad, and this on various logic circuits having received different degrees of fragmentation. Finally the knowledge of the actual line-end contour on the reticle allows one to simulate separately the printing effect of that area loss at reticle line-ends. The area loss measured by comparing the extracted contour to the target one is regarded as a combination of pull-back and area loss at corners. For our C013 gate process, and for the 130nm lines at a 1:1.25 duty cycle, those two parameters contribute together to approximetely 40% of the measured LES in the resist. This fact raises the question of specifications on 2D reticle parameters. We also find a linear correlation between the area loss at reticle line-end corners and the corresponding increase of LES on the wafer, which suggests a way towards putting specifications on the reticle line-ends.

Tuning and simulating a 193-nm resist for 2D applications

For some applications, the usefulness of lithography simulation results depends strongly on the matching between experimental conditions and the simulation input parameters. If this matching is optimized and other sources of error are minimized, then the lithography model can be used to explain printed wafer experimental results. Further, simulation can be useful in predicting the results or in choosing the correct set of experiments. In this paper, PROLITH and ProDATA AutoTune were used to systematically vary simulation input parameters to match measured results on printed wafers used in a 193 nm process. The validity of the simulation parameters was then checked using 3D simulation compared to 2D top-down SEM images. The quality of matching was evaluated using the 1D metrics of average gate CD and Line End Shortening (LES). To ensure the most accurate simulation, a new approach was taken to create a compound mask from GDSII contextual information surrounding an accurate SEM image of the reticle region of interest. Corrections were made to account for all metrology offsets.

Inspection of Integrated Circuit Databases through Reticle and Wafer Simulation: An Integrated Approach to Design for Manufacturing (DFM)

The present approach to Optical Proximity Correction (OPC) verification has evolved from a number of separate inspection strategies. OPC decoration is verified by a design rule or optical rule checker, the reticle is verified by a reticle inspection system, and the final wafers are verified by wafer inspection and metrology tools. Each verification step looks at a different representation of the desired device pattern with little or no data flowing between them. In this paper, we will report on a new inspection system called DesignScan that connects the data between the various abstraction layers. DesignScan inspects the OPC decorated design by simulating how the design will be transferred to the reticle layer and how that reticle will be imaged into resist across the full focus-exposure process window. The simulated images are compared to the desired pattern and defect detection algorithms are applied to determine if any unacceptable variations in the pattern occurs within the nominal process window. The end result is a new paradigm in design verification, moving beyond OPC verification at the design plane to process window verification at the wafer plane where it really matters. We will demonstrate the application of DesignScan to inspect full chip designs that utilized different Resolution Enhancement Technique (RET) and OPC methods. In doing so, we’ll demonstrate that DesignScan can identify the relative strengths and weaknesses of each methodology by highlighting areas of weak process window for each approach. We will present experimental wafer level results to verify the accuracy of the defect predictions.

A study of defect measurement techniques and corresponding effects on the lithographic process window for a 193-nm EPSM photomask

Photomasks with small dense features and high mask error enhancement factor (MEEF) lithography processes require stringent reticle quality control. The ability to quickly and accurately measure reticle defects on a high-resolution inspection system and to simulate their impact on wafer printing are key components in ensuring photomask quality. This paper discusses the correlation of measurements made with UV and DUV-based inspection systems; simulation performed with a 193nm aerial image review tool and aerial image simulation software. Ease-of-use is discussed for each technique. Data accuracy is compared to measurements performed by a Scanning Electron Microscope (SEM) on mask and wafer. Tests show that the inspection system can quickly and accurately determine sizes of most defects. The study also indicates that the simulation techniques can accurately tract the lithographic results, and can be used to reduce or eliminate the use of test wafers and expensive lithography and wafer metrology time. The outcome of this study leads to better defect dispositioning by providing techniques to determine the size and printability of reticle defects.

Improved method for measuring and assessing reticle pinhole defects

With the increased resolution of todays lithography processes, reticle pinhole defects are much more printable. Measuring the size of small pinholes using the current SEM method often produces erroneous results when compared to pinhole energy transmission. This is mainly due to the fact that SEMs do not accurately account for edge wall angle and partial filling which can dramatically reduce the pinhole transmission and subsequent printability. Since reticle inspection tools, like wafer steppers and scanners, use transmitted illumination, pinhole detection performance based upon top surface SEM defect sizing is often erroneous for small pinhole diameters. This study first uses simulation to predict printability. Then, a pinhole test reticle is developed with a variety of sub-200nm pinholes. The reticle pinholes are measured with an improved method incorporating transmission and imaged to wafer in order to assess printability.

OPC aware mask and wafer metrology

Lithography at its limit of resolution is a highly non- linear pattern transfer process. Typically the shapes of printed features deviate considerably from their corresponding features in the layout. This deviation is known as Optical Proximity Effect, and its correction Optical Proximity effect Correction or OPC. Although many other so-called optical enhancement technologies are applied to cope with the issues of lithography at its limit of resolution, almost none of these can re-store the linearity of the pattern transfer. Hence fully functional OPC has become a very basic requirement for current and future lithography processes. In general, proximity effects are two-dimensional (2d) effects. Thus any measurement of proximity effects or any characterization of the effectiveness of OPC has to be two- dimensional. As OPC modifies shapes in the data for mask writing in a way to compensate for the expected proximity effects of the following processing steps, parameters describing the particular OPC-mask quality is a major concern. One-dimensional mask specifications, such as linewidth mean-to-target and uniformity, pattern placement, and maximum size of a tolerable defect, are not sufficient anymore to completely describe the functionality of a given mask for OPC. Two-dimensional mask specifications need to be evaluated. We present in this paper a basic concept for 2d metrology. Examples for 2d measurements to assess the effectiveness of OPC are given by the application of an SEM Image Analysis tool to an advanced 130nm process.

Reticle blank inspection and its role in zero-defect manufacturing

Stringent specifications require that reticle makers carefully examine the role blanks play in reticle quality. Photronics and KLA-Tencor are jointly examining several aspects of this issue. As part of this investigation, PBS blank quality was tested in a production environment using the KLA-Tencor STARlight inspection system. PBS blanks were inspected using a 500-nm pixel with the highest sensitivity settings. We completed a comprehensive study using an effective blank defect test pattern. The test pattern was chosen to maximize the probability that a blank defect will fall on a chrome-to- quartz transition. Several test reticles were inspected and reviewed before writing, and reviewed a second time after processing. 452 defects were classified using three variables: blank defect size, blank defect type and reticle defect type. Some blank defect sizes and types transferred to the test reticles with probabilities exceeding 80%. False defect rates were less than 0.5%. Defect statistics for two blank suppliers are presented. We outline the phases of the research, present the results and discuss the implications for production reticles. We demonstrate techniques that can be used before writing and processing to assess the probability of defect transfer. Plans for a trial protocol for blank inspection are presented.

Accurate aerial image simulation using high-resolution reticle inspection images

The use of hardware-based and software-based reticle defect printability simulation systems is expanding as the cost and complexity of reticles increases. Without such systems it has become increasingly difficult to predict the lithographic significance of a defect found on a reticle. The viability of such systems can be judged using several criteria including accuracy, ease of use, level of automation, and the degree to which they can be applied to a wide range of reticle types. Simulation systems have improved in each of these areas. Automated and semi-automated systems have now been developed and integrated into reticle manufacturing. We report on advances made in a software-based simulation system which uses high-resolution reticle inspection images as the basis for the description of the reticle. We show that the simulated aerial images can be compared quantitatively to results from a hardware-based simulation system (the Zeiss AIMSTM tool) for both 193 and 248 nm EPSM reticles. The development of a new set of metrics to judge lithographic significance will be explained. Common procedural mistakes in evaluating the impact of a defect will be discussed.

Application-specific methods for creating simulation masks

Lithography simulation is being used in a wide range of applications to help lithographers solve an equally wide range of problems. A necessary input to optical lithography simulation is the specification of the mask transmittance function, m(x,y), which forms the basis for the aerial image calculation. Various methods are used to specify m(x,y). The choice of method depends, in part, on the capabilities of the simulation software package and the available information. To maximize effectiveness, efficiency and accuracy, users should choose a method of specifying m(x,y) which considers the requirements of their application. In many cases, a simple expression for m(x,y) is all that is needed. In other cases, finer detail is desirable or even necessary. This paper reviews many techniques to generate m(x,y) for the PROLITH family of lithography simulators and presents current research for the defect printability application.