Michael Arnz

Toward a comprehensive control of full-field image quality in optical photolithography

This paper shows, that as resolution is pushed into regions below 0.6 (lambda) /NA, understanding the effects of wavefront aberrations is crucial to producing stepper systems that can meet end-user requirements. We show how aberrations can affect the choice of optimum NA and partial coherence for a given reticle object when considering critical dimension uniformity and depth of focus. The ability to measure the complete wavefront and extract meaningful full-field aberration data is shown using an advanced through-the-lens interferometer that operates at the wavelength and bandwidth of the lithographic radiation. The impact of aberrations an image quality criteria is shown through a sensitivity analysis using an imaging approximation model that represents various image criteria as a weighted sum of aberration coefficients. The validity and use of such a model is shown by correlation to full- field experimental measurements.

High-resolution and high-precision pattern placement metrology for the 45 nm node and beyond

In order for the lithography world to continue on its path to ever smaller features, process solutions such as OPC assist features and double patterning / exposure strategies put more and more focus on the quality of photomasks. The community roadmap requires for the 45nm/32nm node nominal mask features of 120 nm and 85 nm, respectively. Small feature sizes in combination with tight overlay budgets of only 4.8 nm or even 3.4 nm for the 32 nm node illustrate the need for a registration metrology tool with high resolving power and yet unprecedented specifications on reproducibility and accuracy. Carl Zeiss reports on the concept and the project timeline of its new registration tool currently under development. Novel concepts such as the high resolution at-wavelength imaging optics, an integrated full-field autofocus system, and a correlation algorithm for image analysis are presented as examples for the innovative approach to achieve the very demanding goal.

Photomask Registration and Overlay Metrology by means of 193 nm Optics

This paper reports on the current status of PROVETM - a new photomask registration and overlay metrology system currently under development at Carl Zeiss. The scope of the project is to design and build a photomask pattern placement metrology tool which is serving the 32 nm node. Performance specifications of the tool are actually driven by double exposure/ double patterning approaches which will help to extend the 193 nm lithography platforms, while keeping the semiconductor industry conform to ITRS roadmap requirements. A secondary requirement of pattern placement metrology tools is the CD measurement option for design features of interest. Combining both registration and CD measurement reduces the number of process operations a photomask has to encounter during manufacture. Optical design considerations are discussed, which led to the tool being designed for 193 nm illumination corresponding to at-wavelength metrology for most current and future photomask applications. The concept enables registration and CD metrology by transmitted or reflected light. The short wavelength together with a NA of 0.6 also provides sufficient resolution even at working distances compatible with the use of pellicles, hence enabling the tool for qualification of final, production ready masks. Imaging simulations with a rigorous Maxwell solver prove our chosen optical concept to be adequate for the various mask types (e.g. COG, MoSi, EUV) commonly used today and presumably in the future. The open concept does enable a higher NA for future, pellicle free applications.

Monte Carlo simulations of image analysis for flexible and high-resolution registration metrology

The continuous progress of PROVE, the new photomask registration and overlay measurement tool currently under development at Carl Zeiss has been reported at mask related conferences since its first publication at EMLC 2008. The project has moved in the past year from a final design on paper to functional hardware in the lab. Major tool components such as the climate control unit, the automated mask handling system and the metrology stage have been assembled and successfully tested. The scope of this paper is to report on the current status of PROVE and furthermore present results from simulations utilizing the image analysis routines of the tool. Monte-Carlo simulations were used to analyze the impact of several realistic tool limitations (camera noise, stage and focus noise and imaging telecentricity) on the image analysis process. The evaluation itself was based on a conventional threshold approach to perform both registration and CD measurement simultaneously. The results show, that the routines can deal with the tool imperfections and limit the contribution to the reproducibility error for standard registration markers to a negligible part. Even single contact holes suffer only from small errors, when camera noise is low and image averaging is increased. Employing a generally used test pattern the CD test results also confirm a sufficiently small error contribution to the CD non-uniformity reproducibility.

PROVE: a photomask registration and overlay metrology system for the 45 nm node and beyond

The continuous progress in semiconductor technology has caused mask feature sizes shrinking to 120 nm for the 45nm node and down to 85 nm for the 32nm node. Along with the smaller features, mask image placement accuracy has to improve to 3.4 nm by 2013. Applying double patterning in particular requires rigorous manufacturing control over level to level registration in order to achieve the specified yield and device speed. There is currently no registration tool that ensures image placement performance at the minimum feature size of current and future technology nodes. This work describes fundamental concepts and working principals of a new metrology tool currently under development at Carl Zeiss for measuring image placement and critical dimension in photomask manufacturing. The design of the instrument will be discussed with special emphasis on its optical components. Benefits and advantages using 193nm illumination as well as contrast simulations on different types of masks are presented.

In-die photomask registration and overlay metrology with PROVE using 2D correlation methods

According to the ITRS roadmap, semiconductor industry drives the 193nm lithography to its limits, using techniques like double exposure, double patterning, mask-source optimization and inverse lithography. For photomask metrology this translates to full in-die measurement capability for registration and critical dimension together with challenging specifications for repeatability and accuracy. Especially, overlay becomes more and more critical and must be ensured on every die. For this, Carl Zeiss SMS has developed the next generation photomask registration and overlay metrology tool PROVE® which serves the 32nm node and below and which is already well established in the market. PROVE® features highly stable hardware components for the stage and environmental control. To ensure in-die measurement capability, sophisticated image analysis methods based on 2D correlations have been developed. In this paper we demonstrate the in-die capability of PROVE® and present corresponding measurement results for shortterm and long-term measurements as well as the attainable accuracy for feature sizes down to 85nm using different illumination modes and mask types. Standard measurement methods based on threshold criteria are compared with the new 2D correlation methods to demonstrate the performance gain of the latter. In addition, mask-to-mask overlay results of typical box-in-frame structures down to 200nm feature size are presented. It is shown, that from overlay measurements a reproducibility budget can be derived that takes into account stage, image analysis and global effects like mask loading and environmental control. The parts of the budget are quantified from measurement results to identify critical error contributions and to focus on the corresponding improvement strategies.

Improving registration metrology by correlation methods based on alias-free image simulation

The increased industry requirements for pattern registration tools in terms of resolution and in-die measurement capability lead to the development of the new photomask registration and overlay metrology system PROVETM at Carl Zeiss. Performance measures of the tool are actually driven by double exposure/ double patterning approaches which will help to extend the 193nm lithography platforms while keeping the semiconductor industry conform to ITRS roadmap requirements. To achieve the challenging specifications, PROVETM features beside a highly stable hardware system new image analysis methods which are designed to meet the requirements both for standard markers as for in-die features. For that, in addition to conventional threshold-based image analysis, PROVETM will provide a more accurate correlation analysis to measure pattern placement errors with respect to design images. This correlation is based on an aerial image simulation of the corresponding reference design patterns. Since reproducibility and accuracy specifications at camera level are far below the pixel size of the CCD, sophisticated algorithms have to be used to avoid super-pixelling effects. It will be shown that super-pixelling effects of discretized design images will either lead to placement errors or to unrealistic small design pixel dimensions, connected with huge image sizes. The solution is an alias-free forward transform that performs the discretization in Fourier space and will not disturb the pattern placement. It is indicated by simulations that this allows the detection of an arbitrary sub-pixel placement error with high accuracy. Furthermore, it is demonstrated that correlation methods reduce the impact of camera noise compared to threshold methods, in particular for small in-die features as contact holes.

Correlation method based mask to mask overlay metrology for 32nm node and beyond

The new photomask registration and overlay metrology system PROVETM at Carl Zeiss has been developed and already delivered to customers to meet the increased industry requirements for pattern registration tools in terms of resolution and in-die measurement capability. Main drivers of the tool performance specifications are double exposure and double patterning approaches which will help to extend the 193nm lithography platforms while keeping the semiconductor industry conform to ITRS roadmap requirements. To guarantee the demanding tool performance, PROVETM features highly stable hardware components for the stage and environmental control. Moreover, sophisticated image analysis algorithms as for instance correlation methods have been developed to overcome limitations of standard approaches. In this paper we focus on mask-to-mask overlay metrology as one of the critical components of modern lithography. To achieve the challenging requirements with the PROVETM tool, an overlay reproducibility budget is prepared that takes into account stage, image analysis and global effects like mask loading and environmental control. The corresponding parts of the budget are quantified by means of PROVETM overlay measurements, performed on typical box-in-frame structures. This allows the identification of critical error contributions and the corresponding improvement strategies. In particular, it will be shown, that the new developed correlation methods of PROVETM will significantly reduce image analysis and camera noise contributions.

Lithographic process simulation for scanners

In scanner systems wafer and reticle move continuously with respect to the projection optics. This movement across the image field results in varying lateral shift and focus positions and in an averaging of aberrations from different field positions of the projection system. Several approaches for the effective simulation of these effects are discussed. Based on simulated and experimental data, scanner effect are quantified and compared to results of static stepper exposure.

Flare metrology used for PSD reconstruction

During the last years due to low-k1 imaging , mid- and longrange flare as well as the necessity to quantify it , became more and more important for lithography optical systems. The so-called power spectral density (PSD) was proven to be an adequate means for describing rangedependent flare. At Carl Zeiss SMT AG a PSD metrology was concepted and sucessfully tested as one method for newest generation systems. We will give an outline into this aerial image based metrology and especially pay attention onto: 1. signal calibration as well as accurate focussing and lateral positioning 2. vignetting-free detection in face of large numerical apertures (near 1) 3. correlation with alternative flare metrics used in our house Both the necessity and the potential for extending the technique towards 193 nm immersion lithographic systems will be basically shown , too. The application of well-known models like the ABC and (as special case) the fractal PSD-model will be discussed for typical measurements. Limit cases and their physical meaning will be deduced for the fractal model.