As photomask pattern complexity continues to increase, it becomes more challenging to control write times of shaped e-beam tools. This raises the related concerns of increased mask costs and extended mask cycle times. A strategy for sub-100 nm technology nodes is to use high-speed DUV laser pattern generators for as many layers as possible, reserving e-beam tools for only the most critical layers. With 248 nm optics and high-NA partially coherent imaging, the Sigma7500 increases the application space available to laser pattern generators. Image profiles are steepened with phase shifting methods, and pattern fidelity is improved with on-line corner enhancement. In the Sigma architecture, mask patterns are imaged with full fidelity and addressability in each writing pass. Because of this, the Sigma7500 provides additional means to improve write time by reducing the number of exposure passes. Platform improvements have resulted in a 2-pass writing accuracy that meets the 4-pass specification of the previous system. Write time is typically under two hours in 2-pass mode, compared to approximately three hours for 4-pass. The Sigma7500 can generally be used for all binary mask layers at the 90 nm technology node, and for about half the layers at 45 nm. The ProcessEqualizerTM function addresses long range CD errors arising from mask process effects. Mask data is sized in real time to compensate for process errors related to local pattern density, and also to correct for static process CD signatures. With a through-the-lens alignment system and both grid matching and pattern matching capabilities, the tool is also suited to 2nd layer patterning for advanced phase shifting mask (PSM) applications down to 45 nm, with extendibility to 32 nm. Process integration is facilitated by the use of standard FEP-171 chemically amplified resist (CAR).
Phase shifting mask (PSM) development activity is increasing as 193nm optical lithography is extended beyond the 90nm technology node. The requirements on second level mask patterning of advanced PSM have for many applications exceeded the capability of i-line pattern generators, and it is natural for deep-ultraviolet (DUV) pattern generators to be employed for this task. The Sigma7300 DUV mask writer has the attributes required for advanced PSM applications: high resolution, tight CD uniformity and pattern placement, an accurate alignment system, and grid matching to first level exposures. The paper reports system performance for the parameters that constitute the error budget for second level patterning and shows that the mask writer is suitable for second level patterning of advanced PSMs. The alignment system employs the same laser and optics that are used for system calibration and exposure, including a CCD camera that links system calibration to alignment. As a result overlay performance is stable, and is shown to be independent of alignment mark image contrast changes due to different mask materials or resist thicknesses. The mask process uses a conventional chemically amplified resist, and does not require the use of charge dissipating top layers. Combined with throughput that is essentially independent of pattern complexity, DUV pattern generation provides a high yield solution for second level patterning of advanced PSMs for the 65nm and 45nm nodes.
This paper presents the properties of the Sigma7300 which is a commercial DUV laser pattern generator based on spatial light modulator (SLM) technology designed to meet the requirements of the 65-nm technology node and below. The introduction of spatial light modulators provides a possibility for optical mask writers to combine high resolution and accuracy with short write time making it possible to write most of the high end mask layers in a cost effective way. The Sigma7300 mask writer is developed by Micronic Laser Systems whereas the SLM, which is a combined MEMS and CMOS component with individually controllable movable micromirrors, is developed by the Fraunhofer-IPMS institute in Dresden. The SLM allows parallel writing of one million pixels with a frame rate of up to 2 kHz. The technology offers resolution enhancement advantages from stepper technology not available in other mask patterning tools.
With each new technology generation, photomask manufacturing faces increasing complexity due to shrinking designs and accelerating use of reticle enhancement techniques. Denser and more complex patterns on the mask result in lower yields and long write and turn-around times, important factors for the rapidly increasing mask related costs in IC manufacturing. Laser pattern generators operating at DUV wavelengths were recently introduced to provide cost effective alternatives to electron-beam systems for printing of high-end photomasks. DUV wavelengths provide the required resolution and pattern fidelity. Optical tools that use raster writing principles and massively parallel printing ensure short and predictable write times for photomasks almost independent of pattern complexity.
One such high-volume production system, the Sigma7300, uses spatial light modulator (SLM) technology and a 248 nm excimer laser for printing. Partially coherent imaging and multi-pass printing as in a lithography scanner further increases resolution and pattern accuracy. With four-pass printing the system provides resolution and pattern accuracy meeting mask requirements for critical layers at the 90-nm node and sub-critical layers at the 65-nm node and beyond.
The paper discusses how mask layout can be optimized to take full advantage of the speed potential provided by the SLM-based writer. It shows how flexible use of the writing principle can provide cost effective writing solutions for many layers in high-end mask sets. Resolution and pattern accuracy results from the Sigma7300 will be presented together with write times for different types of designs.
This paper presents the properties of a second-generation DUV laser pattern generator based on spatial light modulator technology and designed to meet the requirements of the 90-nm to 65-nm technology nodes. The system, named Sigma7300, is described and major changes compared to its predecessor are pointed out. These changes result in improved pattern accuracy and fidelity as well as system reliability and maintenance. This improved performance is accompanied with greatly reduced writing times of typically 3 Hrs. per mask. Performance data is presented that shows the system meets the resolution requirement of 260 nm with CD linearity of 10 nm and assist line resolution of 140 nm. CD uniformity data and registration data are also presented that indicates that the system meets the requirements for most layers at the 90-nm and 65-nm nodes.
We propose a brief overview of different techniques to implement complex-valued filters on binary spatial light modulators (SLMs), including encoding techniques at both pixel- and cell- (i.e. group of pixels) levels. We show that group-oriented methods offer extended coding domains, at the expense of a reduced space bandwidth product and spurious noise in the correlation plane. We propose the concept of a time multiplexing technique that combines at the pixel level the pseudo-random encoding method with the minimum Euclidean distance approach. This pixel-oriented method offers the advantage to keep the full space bandwidth of SLMs, and the possibility to very efficiently encode ternary filters (-1, 0, +1) on binary phase (-1, +1) SLMs. Tested on a sequence of images acquired with an IR sensor in a tracking scenario (1 target), encoded ternary filters have offered superior performance in terms of peak-to-clutter ratio than classical BPOF (+45% in the simulation, +35% in the first optical experiments).
In this paper we compare the performance of linear and nonlinear filters on binary images. The maximum-likelihood ratio test (MLRT) processor is optimal for detection. We generalize it and use it for location. It is thus well suited as an upper reference with which we compare the performance of the other filters. We present the maximum likelihood ratio approximation (MLRA) and we compare the MLRA and the nonlinear joint transform correlator (NLJTC) with the MLRT. By having a look at the impulse responses of these processors we can explain the similarities in performance. We also compare the MLRT with classical linear filters, such as the optimal trade- off (OT) filter and the classical matched filter (CMF). We show that the automatic regularization given by the binary noise makes the CMF perform almost as good as the MLRT. The OT filter can be regularized through the choice of the tuning parameter and it also shows almost as good performance as the MLRT.
We present a compact optical correlator with Internet access. Users can remotely download images and get the optically computed correlation results back on their monitor.
A new encoding technique for Joint Transform correlators is presented, with no loss of information for correlation filters displayed on non-ideal SLMs, and narrower autocorrelation peaks.
We present in this paper a study of the influence of the target resolution and contrast on correlation filter performance. Classical filters and the Optimum filter designed by Javidi et al. for non-overlapping noise were considered and tested on a set of realistic images based on real-world non- overlapping noise realizations. We show that for the images that we have tested, there is a limit of the resolution after which classical linear filters fail to detect properly targets, while the Optimum filter still performs well. Furthermore, it was observed that for some background images and fairly low target resolutions, there are values for the target illumination that prevent the classical filter to detect the target, while again it is possible to tune the Optimum filter to make it tolerant to varying target illumination. Eventually, we propose a new composite filter for in-plane rotation tolerance based on individual optimum filters and built according to the equal correlation principle perform best for our test images.
A compact optical correlator based on binary liquid crystal spatial light modulators has been built. The correlator has a 21 X 28 cm2 footprint and can process 256 X 256 pixel images at a maximum frame rate of 220 Hz. The system is insensitive to transportation and can be used both in VLC and JTC configuration. It can process live images from an external camera as well as images from computer memory. Variations of illumination conditions can change the graylevels and perceptible details of a target considerably. When this is the case, classical filters like phase only and optimal trade-off fail to detect the target. An improvement of the results can be achieved by using the optimum generalized filter. In most cases, the contour of the target remains more or less the same even if the graylevels are fluctuating. We show that by edge-enhancing and binarizing the input images used together with the optimum generalized filter we can achieve comparable results as compared to using grayscale images.
This paper reviews some results of an ongoing research program at the Institute of Optical Research that addresses optical correlators dedicated to pattern recognition. A two- class discrimination problem with in-plane distortion is considered. To cope with the loss of information that may occur when input images are directly displayed on experimental binary SLMs in a Vanderlugt correlator, we have implemented several types of preprocessing operators on the original gray scale images. Performance criteria are presented to assess and grade preprocessors that can be implemented in real-time on an electronic frame grabber board. Optical implementation in a VanderLugt correlator is described. Experimental results are provided and emphasize the benefits of preprocessing prior to binarization. Significant improvements are found, both digitally and optically, with classical derivative and rank operators.
This paper presents experimental and simulation results from ongoing research on the influence of clutter on OTSDF filter performance. The degrading influence of overlapping clutter can be reduced by including colored noise in the filter construction or by tuning a white noise filter. Simulation results together with experimental results from a van derLugt correlator indicate that tuning of a white noise filter is the most effective for low levels of clutter in the input images whereas inclusion of colored noise is more effective for high levels of clutter. The experimental results are well in line with the simulations.
An optical system for analysis of diesel spray penetration and atomization has been developed. The basic system is based on a microscope with a long working distance (230 mm) and a modest resolution.
The magnification is variable between 1:1 and 50:1 on the camera plane. This enables the study of individual droplets with a diameter of 5 mm. A number of different techniques have been used together with the basic
system to study different parts of the spray. They include the darkground technique, double exposure, and ordinary magnification. The size, speed, and direction of propagation of the individual droplets have been calculated using the elongation of the droplet image caused by the duration time of the flash.
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