Since the announcement in March 2002 of plans to develop an advanced light source to meet the future spectral power and cost requirements of photolithography, we have made significant progress in the development and productization of the core technology for an ultra line-narrowed, excimer light source based on a master oscillator-power amplifier (MOPA) approach. In this paper, we will focus on the architecture and performance of the first generation of production-ready, MOPA-based ArF light sources developed at Cymer, Inc. This first generation of MOPA-based ArF light sources is referred to as the XLA 100 product series.
In this study, process latitude, mask error enhancement factor and iso-dense bias have been experimentally measured as a function of the KrF excimer laser bandwidth. The experiment results are in agreement with photoresist simulations over a range of imaged nominal feature sizes from 120nm to 300nm at 0.6/0.75 NA/(sigma) . The mask error enhancement factor (MEEF) is shown to vary by approximately 2.3 percent for 160nm and 3.3 percent for 150nm isolated lines per 0.1pm of excimer-laser bandwidth, characterized by the full width at half maximum (FWHM). The 180nm line iso-dense bias exhibits a shift of approximately 2nm per 0.1pm FWHM. Under the given process conditions, linear empirical relationships are derived for the dependency of MEEF and iso-dense offset on FWHM excimer-laser spectral width for a range of imaged CDs. Such considerations can be used to augment the existing predictive CD-control estimation and model-based optical proximity correction.
Quantification of projection lens aberrations in lithographic exposure systems has gained significant importance due to more stringent critical dimension control and image fidelity requirements. As linewidths shrink, the impacts of wavefront aberrations on imaging become more pronounced. Therefore, minimization of the wavefront aberrations across the image field is desired and has led to the development of a number of measurement approaches. The proposed techniques have been evaluated extensively for characterization and specification of lens systems, adjustments, matching, and periodic control and monitoring of lithography systems for volume production. In this study, we discuss the contribution of excimer laser bandwidth towards lens aberrations. We carry out simulations of the effects of image contrast on conventional projection patterning, to evaluate the degree of aberation-induced linewidth changes depending on image contrast level. Also, experiments have been conducted to measure the response of wavefront error as a function of spectral bandwidth for a 0.6NA stepper and scanner. Depending on the field location, a positive relationship is observed between the measured aberration level and bandwidth. We propose a formalism to correlate the aberration measurement with aberration response to wavelength offset, presented elsewhere. The wavefront error, in this work, is measured using a commercially available in-situ interferometric technique, whose response is largely insensitive to focal plane changes and partial coherence.
Exposure tools for 248nm lithography have reached a level of maturity comparable to those based on i-line. With this increase in maturity, there is a concomitant requirement for greater flexibility from the laser by the process engineers. Usually, these requirements pertain to energy, spectral width and repetition rate. By utilizing a combination of laser parameters, the process engineers are often able to optimize throughput, reduce cost-of-operation or achieve greater process margin. Hitherto, such flexibility of laser operation was possible only via significant changes to various laser modules. During our investigation, we found that the key measure of the laser that impacts the aforementioned parameters is its F2 concentration. By monitoring and controlling its slope efficiency, the laser's F2 concentration may be precisely controlled. Thus a laser may tune to operate under specifications as diverse as 7mJ, (Delta) (lambda) FWHM < 0.3 pm and 10mJ, (Delta) (lambda) FWHM < 0.6pm and still meet the host of requirements necessary for lithography. We discus this new F2 control technique and highlight some laser performance parameters.
Now that 1000 Hz KrF excimer laser based DUV lithography tools are firmly established in production, emphasis is shifting from development towards improving the productivity and profitability of the manufacturing process, thereby reducing the cost per wafer. In this arena, laser manufacturers are competing now not only on performance but also on cost and productivity enhancements that the laser can offer to the lithography process.