Maximizing Throughput Multiple coat modules maximize the overall throughput of the system or allow different resist chemistries in the cluster. In the above case, one coating module is used to apply a thick Novolak-based photoresist, while the other is used to coat layers of BCB or polyimide. The two materials should not be processed on the same coating module since their solvents are not compatible; difficulties in process control may arise when both are present in the same coat module. In addition, both materials form compounds that are difficult to dissolve, resulting in maintenance issues. Resist layers have to be cured on hot plates to drive the solvent out of the resist. Curing wafers in parallel is important, since this process step can be quite time consuming when thick photoresist is used. Therefore up to six stacked proximity hot plates are added as one module to the cluster system. Proximity hot plates are typically better suited for thick resist application than simple contact hot plates since they allow optimizing the temperature over time for optimum solvent removal. In the above cluster configuration, a spray development module is used to process the thick Novolak-based resist. In the center of the four coat, bake and develop modules is a robot that transfers wafers between the individual process chambers, enabling multiple coating and curing of resist-important in achieving very thick layers for solder bumping. Plating Technology Liquid photoresists are often used with plating technology. The trend with narrower pitches is to employ straight wall rather than mushroom plating (Figure 4), although the latter is still frequently used. In the case of straight wall plating, however, a resist thickness of 80-100 microns is needed.
A thick resist is often difficult to achieve with only one coating step, because the solvent cannot be removed sufficiently and because the edge bead becomes too wide and too thick. With the cluster configuration, wafers can be transferred between coat and bake modules without adapting the number of coat modules to the number of coating steps. While the coat, bake and develop modules for 300 mm wafers are similar to modules used in current 200 mm systems, the 300 mm mask aligner is fundamentally different from current 200 mm equipment. Extending mask aligner technology from 200 mm to 300 mm requires a discussion of several aspects related to photomasks. The full-field proximity technology, upon which mask aligners are based, requires photomasks that are larger than the wafer. For 300 mm wafers, we have defined the standard mask to be a 14-inch square plate while 9-inch plates are employed for 200 mm wafers. Moving from 9 to 14-inch Masks Several questions come to mind when moving from 9-inch to 14-inch masks. For example, how accurately can 14-inch masks be written, and what impact do temperature variations produce on the overlay accuracy between mask and wafer? How does mask sagging in 14-inch masks compare with 9-inch masks, and how does it influence the CD uniformity? How can these large area masks be handled? Finally, relative to the large exposure area, how uniform can the exposure intensity be on the full 300 mm exposure area? Intensity Uniformity Let's start with the uniformity of intensity. In Figure 5 the intensity (5 kW lamp source) measured at several locations on the 300 mm exposure area is shown. Average intensity was 92.6 mW/cm2 with a relative variation of ±3.9%. This value is well within that usually specified for a 200 mm mask aligner. The intensity itself was, as mentioned earlier, surprisingly high because it is relatively close to the intensity available on a 200 mm system when using the same light source.
CD uniformity has been fully satisfactory so far on all materials tested. Ten micron features were easily opened using 5-micron-thick liquid Novolak-type photoresists with comfortable exposure gaps of 30 to 40 microns. Furthermore, 70-micron-thick dry film resist has been exposed with a CD control better than 1 micron on a 176-micron- wide bump opening. Initial Results These initial results suggest that the resolution capability of the 300 mm system is absolutely comparable or even slightly better than that of a 200 mm system. To minimize mask sagging, a CD control-limiting factor, 14-inch masks are thicker than 9-inch masks. Mask sagging was measured to be less than 5 microns on a 14-inch mask, a value that is far less than typical exposure gaps. This value nearly guarantees a CD control similar to 200 mm aligners. Besides controlling the mask writing temperature, it is also necessary to control the wafer and mask temperature in the mask aligner. The LithoPack employs a temperature-controlled chuck to keep the wafer at a constant temperature. In a proximity printing system, mask and wafer are in close proximity to each other, assuring that the mask temperature is indirectly controlled by the chuck. What about tool automation? In the past, the mask aligner certainly has not been a forerunner in tool automation because of its limited use in front-end technology. This, however, will change with the transition to 300 mm. Conclusions Initial results indicate that full-field proximity printing is not only well suited for 200 mm wafer-level packaging but is also applicable to 300 mm wafers as well. The LithoPack offers a new lithography tool generation for 200 and 300 mm wafers with more advanced automation compared to what is currently available. References 1. In this article, the term "wafer-level packaging" will be used for wafer-level CSPs as well as for wafer-bumping technology. 2. M. Töpper, H. Reichl, et al., "Is Wafer-Level Packaging Ready for 300 mm?" Advanced Packaging, July 2001, pp. 39-46. 3. Some recent studies arrived at a different conclusion about the throughput of mask aligner and stepper. In our studies, however, steppers were compared to mask aligners with 1 kW lamp houses. For 300 mm mask aligners, however, 5 kW lamps will be used. 4. S. Kay, D. Anberg, et al., "Analyzing Issues for 300 mm Backend Lithography," Solid State Technology, July 2001, pp. 138-142.
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