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As lithography wavelengths reduce, the depth of focus decreases rapidly as well, resulting in the need for flatter photomasks with specifications under 0.25 microns. As the industry begins to extend 193 nm technology to smaller line widths, and with the significantly tighter flatness requirements for 157 nm and EUV masks, it will become necessary to inspect finished masks for flatness. By measuring the mask flatness after the pellicle has been mounted, the deformations caused by stress relief due to exposure of the reticle and mounting the pellicle can be taken into account reducing the overall uncertainty of the lithography process. This paper describes measurement techniques applied to a tool that utilizes near normal incidence interferometry to perform concurrent flatness and thickness measurements on finished reticles, as well as other techniques to bring the reticle flatness measurements more in line with the exposure tools. The flatness of the reticle is a critical aspect of the lithography system performance. Without sufficient control over flatness, the features on the image plane become distorted, resulting in image placement errors. As the line widths become finer, and especially in the case where wavelengths are being extended beyond their original nodes, control of the reticle flatness becomes more critical. In many cases, the last time the reticle flatness is measured, is after the photoresist is applied, or even as far back as the blank state. The influence of later changes such as film deposition, exposure, and pellicle mounting may only be accounted for through tightening the flatness spec on the blank itself. By measuring the flatness later in the process, it is possible to have greater control over the individual process steps, and to meet the flatness requirements for the lithographic process without excessively tight tolerances upstream. In order to interferometrically measure the reticle flatness with the pellicle mounted, it is necessary to either measure the surface through the pellicle, or through the back surface of the photomask. In either situation there will be interference patterns generated between the reference surface, the pellicle, the reticle surface, and the back surface of the photomask, as well as interference between each of these surfaces and each other. In the case of hard pellicles for 157 nm lithography, an extra surface is introduced. The total number of potential interferograms for a reticle with a soft pellicle would be 6, and for a hard pellicle 10. Retrieving the information about the surface of interest then becomes very complicated. The Corning Tropel UtraFlat is a near normal incidence interferometer with some unique measurement advantages over traditional normal incidence interferometers. By illuminating the surface at approximately 45° angle of incidence, it is possible to eliminate extraneous fringe patterns optically by limiting the spatial coherence of the laser source. Limiting the spatial coherence optically eliminates fringe patterns from surfaces far apart due to the relative shear distances of these interfered beams, while maintaining signal from the closer surfaces. In this manner it is possible to restrict the measurement data to the back surface flatness, or the backside flatness plus the thickness variation of the mask in a controlled manner. Utilizing this same technique from the pellicle side, it is possible to measure pellicle flatness and thickness variation in its final, mounted state, a key factor in overall system performance for 157 nm lithography. Figure 1 shows an example of an interferogram from a photomask blank measured from the backside with flatness and thickness variation fringes. Figure 2 shows the isolated backside flatness as well as the front side flatness measured by combining the thickness variation and backside flatness from a reticle with a pellicle mounted. Figure 3 shows the change in form over a 130 x 130 mm quality area caused by mounting the pellicle to the reticle. An alternative technique for tracking late process changes is to store the blank front and back surface measurements before coating, exposure, and pellicle mounting, and by measuring the back surface flatness through each of these steps, it is a simple matter to track the changes in the form from each step directly in the back surface, and apply this shape change to the front surface measurement. This technique has advantages later in the process since a finished reticle may have significant variation in reflectivity due to exposure, resulting in high frequency contrast variation on the thickness variation fringes, which may complicate the previously discussed methods. By increasing the control over late process flatness changes, it should be possible to improve reticle performance more directly than simply tightening the requirements on the blanks. Most importantly, it is the finished reticle flatness that the lithography process requires, and not simply the blank flatness. By measuring the finished reticle the performance is known, not simply implied, ensuring that the real lithographic needs are met, which are becoming more and more stringent as line widths and wavelengths reduce.
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