John Lewellen

Interactions of double patterning technology with wafer processing, OPC and design flows

Double patterning technology (DPT) is one of the main options for printing logic devices with half-pitch less than 45nm; and flash and DRAM memory devices with half-pitch less than 40nm. DPT methods decompose the original design intent into two individual masking layers which are each patterned using single exposures and existing 193nm lithography tools. The results of the individual patterning layers combine to re-create the design intent pattern on the wafer. In this paper we study interactions of DPT with lithography, masks synthesis and physical design flows. Double exposure and etch patterning steps create complexity for both process and design flows. DPT decomposition is a critical software step which will be performed in physical design and also in mask synthesis. Decomposition includes cutting (splitting) of original design intent polygons into multiple polygons where required; and coloring of the resulting polygons. We evaluate the ability to meet key physical design goals such as: reduce circuit area; minimize rework; ensure DPT compliance; guarantee patterning robustness on individual layer targets; ensure symmetric wafer results; and create uniform wafer density for the individual patterning layers.

Double-patterning interactions with wafer processing, optical proximity correction, and physical design flows

In this paper we study interactions of double patterning technology (DPT) with lithography, optical proximity correction (OPC) and physical design flows for the 22-nm device node. DPT methods decompose the original design intent into two individual masking layers, which are each patterned using single exposures and existing 193-nm lithography tools. Double exposure and etch patterning steps create complexity for both process and design flows. DPT decomposition is a critical software step that will be performed in physical design and also in mask synthesis. Decomposition includes cutting (splitting) of original design intent polygons into multiple polygons, where required, and coloring of the resulting polygons. We evaluate the ability to meet key physical design goals, such as reduce circuit area, minimize relayout effort, ensure DPT compliance, guarantee patterning robustness on individual layer targets, ensure symmetric wafer results, and create uniform wafer density for the individual patterning layers.

Polarization-dependent proximity effects

To meet the imaging resolution requirements, driven by the evolution of IC design rules, leading-edge scanners incorporate projection lenses with hyper-NAs. Moreover, immersion scanners are being introduced into IC manufacture. Both dry and immersion tools explore the lens design regimes of unprecedented complexity. The need to predict, to analyze and to control the IC pattern CDs is met by various photolithography simulators. The continuing demand for simulation accuracy is reflected by the requirement to quantify the scanner projection lens fingerprints, i.e. projection lens infinitesimal excursions from the ideal performance. The scanner engineering community has been relying on photolithography simulators to analyze the impact of the projection lens fingerprints on the imaging characteristics. However small, these excursions are always present in the projection tools and they control important imaging characteristics such as overlay, CD uniformity, across-field exposure latitude, to name but a few. Customarily, phase front aberrations and lens pupil apodization signatures have been used to predict the scanners imaging responses. Of course, the need to design, to manufacture and to deploy scanners of ever improving quality resulted in dramatic reductions of these non-ideal imaging excursions. The evolution of IC designs and imaging tools complexity escalate the requirements for imaging simulation accuracy. Simultaneously, predicting scanner imaging response has become a key mission in the Deign For Manufacture arena. In view of these developments, it necessary to pose a question if the conventional equipment engineering and imaging simulation methodologies predict scanner imaging responses with the accuracy required by the IC design rules. Differently put, the question is: what is necessary to provide simulation accuracy required by the current IC design rules? This report attempts to address these questions.

The impact of mask design on EUV imaging

EUV exposure tools are the leading contenders for patterning critical layers at the 22nm technology node. Operating at the wavelength of 13.5nm, with modest projection optics numerical aperture (NA), EUV projectors allow less stringent image formation conditions. On the other hand, the imaging performance requirements will place high demands on the mechanical and optical properties of these imaging systems. A key characteristic of EUV projection optics is the application of a reflective mask, which consists of a reflective multilayer stack on which the IC layout is represented by the reflectivity discontinuities1. Several mask concepts can provide such characteristics, such as thick absorbers on top of a reflective multi-layer stack, masks with embedded absorbers, or absorber-free masks with patterns etched in a reflective multilayer. This report analyzes imaging performance and tradeoffs of such new mask designs. Various mask types and geometries are evaluated through imaging simulations. The applied mask models take into account the topographic nature of the mask structures, as well as the fundamental, vectorial characteristics of the EUV imaging process. Resulting EUV images are compared in terms of their process stability as well as their sensitivities to the EUV-specific effects, such as pattern shift and image tilt, driven by the reflective design of the exposure system and the mask topography. The simulations of images formed in EUV exposure tools are analyzed from the point of view of the EUV mask users. The fundamental requirements of EUV mask technologies are discussed. These investigations spotlight the tradeoffs of each mask concept and could serve as guidelines for EUV mask engineering.

Fine calibration of physical resist models: the importance of Jones pupil, laser bandwidth, mask error and CD metrology for accurate modeling at advanced lithographic nodes

In this paper, we discuss the accuracy of resist model calibration under various aspects. The study is done based on an extensive OPC dataset including hundreds of CD values obtained with immersion lithography for the sub-30 nm node. We address imaging aspects such as the role of Jones matrices, laser bandwidth and mask bias. Besides we focus on the investigation on metrology effects arising from SEM charging and uncertainty between SEM image and feature topography. For theses individual contributions we perform a series of resist model calibrations to determine their importance in terms of relative RMSE (Root Mean Square Error) and it is found that for the sub-30 nm node they all are not negligible for accurate resist model calibration.

Application of photolithographic simulation and a mask repair system in a production environment

This work represents one in a series of ongoing papers demonstrating the potential utility of integrating advanced photolithographic simulation software into a mask repair tool to provide immediate defect or repair printability feedback. The equipment used here is an AFM-technology based nanomachining photomask repair tool where the high-accuracy AFM surface topography data is fed directly into software applying rigorous solutions to Maxwells equations. The nature of these systems allows for process endpoint printability evaluation, not restricted by the optical limitations of any given apparatus, of any micro to nano-scale region of the mask in-situ with the defect repair process. In prior work, the capability of this approach was shown in good correlations to AIMSTM at 248 and 193 nm wavelengths, for binary mask repairs of varying dimensions, with no applied optical aberrations to the simulation. In this examination, the development of this system is taken to its next step by introducing it to a real photomask production environment, using production masks, for performance substantiation. Methodologies are shown for the best use of this system in streamlining the mask production process.

Impact of AFM scan artifacts on photolithographic simulation

This work represents one in a series of ongoing papers demonstrating the potential utility of integrating advanced photolithographic simulation software into a mask repair tool to provide immediate defect or repair printability feedback. The equipment used here is an AFM-technology based nanomachining photomask repair tool where the high-accuracy AFM surface topography data is fed directly into software applying rigorous solutions to Maxwells equations. The nature of these systems allows for process endpoint printability evaluation, not restricted by the optical limitations of any given apparatus, of any micro to nano-scale region of the mask concurrent with the normal defect repair process. However, known AFM scan artifacts can impact the accuracy and stability of the photolithographic simulation results, especially for mask or pattern types which have not been previously studied by the user. The relevant sources of these artifacts are identified and improvements in the AFM operation are discussed which could minimize them. The quantitative relationships between the various artifact measures and their corresponding effects on various simulation results (including relative transmission and CD) are examined for both AIMSTM aerial imaging and wafer print. From this examination, error baselines are established and software, as well as model setup, optimizations are proposed.

Validation of Nu-Flare e-beam emulation software in a simulation environment

In order to reduce mask making costs and improve wafer printability it is advantageous to determine machine parameters that will create highest probability of successful mask yield and mask image at CD and inspection. Proper simulation of actual product database helps to define the optimum e-beam machine settings for maximum probable yield and best mask pattern including OPC structures. In this paper we study the basic capability of the Nu-Flare E-beam mask writer emulation taking into account mask processing effects such as PEB. Analysis of how well software emulates the actual PEC corrections applied in the mask writer is necessary in predicting proper initial and subsequent machine settings for optimum yield and OPC structure fidelity. Comparisons of the Nu-Flare PEC emulation against actual mask PEC patterns on chrome masks are presented. Excellent agreement is found to experimental data when the PEC algorithm is modified to keep dose to the dense line pattern constant for any given setting of the eta PEC parameter.

Improved cycle time of mask repair by optimizing nanomachining with photolithographic imaging simulation

Current generation photomasks use optical enhancements such as phase shifting and aggressive OPC in an effort to maintain image contrast as CDs shrink. The result is non-intuitive complex shapes with jogs and multiple levels with different materials. The mask repair engineer is challenged to work with defects that occur in ever tightening spaces on these complex masks. Prior established nanomachining technology allows nanometer level control of material removal. To date, the challenge in developing repair strategies that will meet transmission specifications as well as maintaining aerial image contrast through focus has been mainly an empirical exercise where the mask repair is attempted and aerial image measurement among other tests are used to verify the result. This approach can be streamlined by the use of lithography simulation which rigorously models the effects of mask defects on the aerial image at the wafer. Once the topography of the defect is measured by the nanomachining mask repair tool, lithography simulation can be proactively used to develop a repair strategy for the nanomachining process. Following this repair, the simulation software can then provide immediate feedback to confirm the post repair 3-D topology from AFM surface measurements for either approval or immediate rework. This integration is initially validated using a significant set of repairs with subsequent aerial image measurements compared to some of the more common evaluative analyses.

Wavefront-based tool selection for critical level imaging

IC manufacture often has to meet stringent requirements pushing the imaging tools beyond their limits. Selection and optimization of steppers used to image patterns with critical dimensions at a fraction of wavelength has to consider tool’s aberration residue and the imaging tradeoffs of the patterned features. This report presents methodology to select tool-specific, multi-feature optima for imaging tools performing beyond their design points.