New Stepper Tools Needed New application-specific bump-bond stepper tools are needed that are versatile, cost-effective, reliable and capable of producing very high production yields to keep packaged device prices competitive. Such a tool would provide all the yield and automation advantages of a stepper, in combination with other bump application-specific features, to meet all of the bump- bond lithography processing needs in a single machine. This tool would also have the ability to provide broadband exposures, thus allowing the use of any g-line or i-line resist process on the same lithography machine. In addition, it would be able to auto-align without special alignment targets, process both thin and thick films, automatically switch between all wafer sizes, shapes and orientations, and still provide superior cost of ownership. These features would increase total productivity dramatically. Broadband wavelength exposure capability would provide an illumination bandwidth from 350 nm to 450 nm, which includes the g-, h- and i-line output spectra, making it possible to expose either g-line or i-line photosensitive films on the same stepper, greatly improving flexibility. In addition, if a fab uses films that are sensitive to all three wavelengths, exposure time will drop, increasing throughput (Figure 4).
The ability to auto-align without special alignment targets means that any unique feature on the wafer can be trained and used as an alignment target, eliminating the need for tool-specific, custom stepper targets. With auto-alignment to existing wafer features, the stepper could be inserted in the line, and a reticle for only the new bump layer could be imaged without any changes to the prior masking levels. Moreover, this could be done for any wafer manufactured on any stepper. A stepper with this kind of flexible auto-alignment capability should be able to capture targets and align through very thick films, such as polyimide or resist films more than 100 microns thick (Figure 5). It would also be able to align to existing structures with a total overlay of 0.5 microns or better, as compared to the typical overlay capability of contact aligners of 1 to 2 microns.
Global Alignment Global alignment capability would eliminate the need for fine-align target searching due to large, wafer-to-wafer array offsets that might be present on wafers processed on older aligners. This, in turn, would eliminate any throughput impacts from those offsets. This is especially important for bump foundry applications where array placement repeatability can vary from lot to lot and is not controlled by the foundry. A stepper capable of processing both thin, 1- to 10-micron, and thick, 10- to 100-micron, films could be designed with very large focus offsets that would allow imaging of thin and thick films over this entire range. To achieve the high throughputs required on thick photosensitive films with very high dose requirements, as well as superior cost of ownership, the bump-specific stepper should be capable of producing at least three times the wafer-plane intensity of a typical stepper. This equates to an intensity of 1500 to 2000 mWatts/cm2 at the wafer plane. Fully automatic changes in wafer size and orientation could be achieved via reticle data control. Software that allows stepper job file control of wafer sizes from 100 mm-200 mm (4 to 8 inches), of any orientation, notched or flat, would allow full control from the job file, without the need for operator intervention. To complete this ideal bump-application-specific stepper, a method of compensating for differences in wafer thickness (background vs. non-background wafers) would automatically sense wafer thickness and allow processing of different thickness variations, ranging from as thin as 200 microns for some pre-background wafers up to the thickness of standard 200 mm wafers, with no mechanical adjustments needed. Since the thicker films that are required for the bump process sometimes warp wafers considerably, the ideal bump-specific stepper should be able to handle wafers with severe warpage of up to 100 microns.
Support Requirements Many of the foundries that are beginning to offer flip-chip bump processing are experienced in back-end processes, such as packaging, but not in the use of steppers or other lithography tools. These users will need training in how to integrate these new tools into their production flow, how to set up the lithography process and how to operate the machines. But new tools, concepts or strategies are not going to work without fab service, support and training provided by the tool's manufacturer. Global Resources Since the majority of the world's bump foundries are in Asia, equipment suppliers must be able to deal effectively with the location, logistics and cultural issues of the area, regardless where a supplier's home base might be located. Therefore, global resources, global offices and the ability to provide extensive applications and engineering support, in any fab around the world, are becoming key parts of a stepper tool manufacturer's strategy. Summary With the explosive growth of flip-chip packaging expected to occur over the next few years, new application-specific lithography tools must be found for the unique needs of bump-bond processing. Although steppers have added automation to bump bonding, and contribute zero yield impact from the lithography step, they have had severe limitations. New stepper tools serving the bump processing market must integrate several features in a single machine in order to be more versatile and productive.
These include broadband (g-, h- and i-line) exposure capability, the ability to auto-align without special alignment targets, high reliability, high wafer plane intensity for high through-put, and the ability to process both thin and thick films. These steppers must also automatically switch between all wafer sizes, shapes and orientations and still provide superior cost of ownership.
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