Analyzing issues for 300mm
07/01/2001
Stephen Kay, Doug Anberg, Ultratech Stepper Inc., San Jose, California
Thomas Goodman, Peter Elenius, Flip Chip Technologies LLC, Phoenix, Arizona
 Inside view of the Ultratech Saturn Spectrum 300 wafer stepper. |
overview
With 300mm wafer processing, expect to see a shift from conventional proximity aligners to more applications of 1x steppers for backend bump processing. While passé for leading-edge frontend applications, 1x steppers provide clear advantages for semiconductor manufacturing's backend, in cost of ownership, automation, and processing. Advantages include broadband exposure, superior automatic alignment, and overlay performance. The 2µm resolution capability of these tools is more than adequate for even difficult bump applications. In addition, these tools' low 0.16 NA provides a large depth of focus (5.0µm at the resolution limit of 2.0µm), which is useful in imaging thick resist.
The move to 300mm wafers poses unique problems for backend bump lithography (see "Wafer bumping goes 300mm"). Consider the increasing requirement for stepper technology versus conventional proximity printing, a change being driven by both technical and economic necessity.
Stepper technology versus proximity printing
Yield. In the bump process, as in all semiconductor manufacturing, yield is a primary driver in the selection of a tool set. This is even more critical in the selection of tools for a 300mm fab, where a single wafer can be worth tens of thousands of dollars by the time it reaches the bump process step. Even one tenth of one percent yield loss is not acceptable for bump-processing lithography.
Conventional proximity printing onto resist to define the electroplating pattern in bump processing is fraught with potential yield loss, including
- resist damage caused by photomask-to-wafer contact;
- defects caused by residual resist left on the photomask after an exposure; and
- alignment errors introduced by the expansion of the photomask due to heat generated during typical prolonged single exposures.
Steppers eliminate these yield detractors by eliminating the need for close mask-to-wafer proximity and providing the ability to align and expose on a site-by-site basis, improving alignment capabilities, as well as reducing issues caused by prolonged high-intensity exposure [1].
The increased use of I/O redistribution, as a technique to increase the number of bumped I/Os possible on a chip, is creating tighter and smaller resolution requirements for bump and wafer-level chip-scale package (CSP) processes. With resolution for the redistribution layer routinely reaching down to 10µm, and future requirements headed toward 4µm, a typical proximity aligner resolution and alignment performance will no longer be adequate. Here again, 1x stepper technology provides a solution with better than 2µm resolution and alignment capability routinely <0.5µm.
CoO. Cost of ownership is also a critical factor in the selection of lithography tools for bump or wafer-level CSP; primary to any discussion about CoO is the issue of operational costs. Initial system cost is important, but over a tool's lifetime this can amount to only a fraction of the overall cost of owning and operating the equipment.
System throughput (in wafers/hour, wph) and the cost of photomasks are key to the operational cost of any bump-lithography tool. Unfortunately, throughput numbers quoted by most suppliers of semiconductor equipment are normally not the same values that fabs will experience. Consider, for example, a stated proximity aligner throughput specification of 61wph at 200mJ/cm2 and 39wph at 4000mJ/cm2 on 200mm wafers. While on the surface this specification is comparable to what is quoted for many stepper platforms (Fig. 1a), reality is completely different (Fig. 1b) because a typical bump process requires an extremely high dose compared to a typical frontend semiconductor process — a typical semiconductor process dose is 80-500 mJ/cm2; a typical bump process requires 1000-6000 mJ/cm2. (The throughput comparison of various exposure systems presented in Fig. 1 was done with 200mm wafers because there is little information available on 300mm-system performance.)
With a typical "bump" thick photoresist process (12-100µm), proximity aligner photomasks may require frequent cleaning due to resist outgassing and incidental contact between the photomask and the wafer. This causes issues that can have an effect on throughput due to the time lost by switching and cleaning photomasks, and can also limit the lifetime of photomasks themselves. In some cases, maintaining multiple copies of a photomask may be required to ensure optimum throughput; while one mask is on the aligner, one might be in cleaning, plus a spare may be available in case one of the other two is damaged.
Photomask size and cost are already serious issues in proximity aligners for 200mm wafers. The transition to 300mm magnifies this issue considerably, especially when one considers the estimated >$15,000 cost of the 13-in. or 14-in. photomask that will be required for proximity printing to 300mm wafers. This, coupled with the potential need for multiple photomask copies to maintain throughput, makes photomask costs a major issue at 300mm.
A stepper, on the other hand, can use a fixed reticle size no matter what wafer size is used. These range in cost from $1000-1500, depending on whether they are 6-in. or 5-in. reticles.
Further, 1x steppers eliminate the mask's proximity to a wafer, thereby reducing any potential for contact. In addition, they include the availability of pellicle protection for photomasks and automated handling of reticles. They also provide superior resolution and alignment performance. In use for backend applications, 1x steppers are able to maintain as close to ideal throughput as possible. Overall, they provide all of the technical performance necessary for current and future bump process requirements [1] at a favorable CoO (Fig. 2).
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Figure 2. Estimated CoO for various lithography methods on a typical bump lithography process requiring a 4000mJ/cm2 exposure energy for a variety of monthly wafer outputs.
Solder ball pitch. As with 200mm wafers, the need to reduce solder ball pitch will also continue with 300mm wafers. The 1999 SIA International Technology Roadmap for Semiconductors predicts that 0.30mm solder ball pitch will not be required until the 50nm technology node, expected to be in production in 2011. The small size and weight requirements of new handheld, portable Internet access devices, however, are driving packaging densities at an accelerated rate. If this trend continues, packaging density and solder ball pitch are likely to exceed the Roadmap's expectations, and 0.30mm pitches may be needed on 300mm wafers as early as the year 2005. Contact-proximity aligners may very well find these pitches difficult or impossible to deal with in anything other than full contact mode, further increasing the chances of yield loss due to the lithography step.
 Figure 3. The first bumped 300mm wafer processed at Flip Chip Technologies. (Insert shows SEM of bumps.) |
Flexibility. Another key concern when purchasing equipment for 300mm bump processing is flexibility. While many 300mm fabs coming on line in 2001 will be foundries, and with frontend fabs supporting only 300mm wafers, the bump fab line may be required to support multiple fabs and multiple wafer sizes. This is primarily due to the desire to keep wafer-bumping cost at a minimum and to use the available equipment and fab space fully. Typically, wafer bumping is done separately from the frontend fab and the ability to handle multiple wafer sizes and thickness, while not a necessity, is a distinct cost advantage if available. The inherent design of a contact-proximity aligner makes this change difficult, cumbersome, and time consuming. 1x steppers, on the other hand, have been able to automate this process and make it transparent to users, providing a truly flexible and robust bump lithography tool.
300mm bump-specific stepper results
The wafer-bumping industry has shown widespread acceptance and interest in stepper technology for 300mm wafers, with the first 300mm bump-specific 1x stepper shipping in June of 2000. While stepper technology for 300mm bump processing and wafer-level CSP is just beginning to ramp, the number of 300mm wafer fabs coming on line in 2001, and the current level of interest in 1x stepper technology, make it clear that support will be required for 300mm wafer bump lithography in 2001. We proved the concept and capability for 300mm bumping by producing the first bumped 300mm wafers almost a year ago (July 2000, see Fig. 3). Following this achievement, we have seen demand for 300mm bump tools increase dramatically in both the foundry and IDM communities.
Specifically, our work with the Ultratech Saturn Spectrum series of bump-specific lithography tools has shown that these tools eliminate many process issues associated with proximity aligners, such as incidental contact between a photomask and wafer, alignment run-out errors, and the inability to provide automation in world-class production facilities.
These steppers provide an illumination bandwidth from 350nm to 450nm, which includes the g-, h- and i-line output spectra. This means that it is possible to expose either g-line or i-line photosensitive films using one stepper, greatly improving flexibility. In addition, the broadband exposure provides a higher wafer plain exposure-energy intensity (i.e., 1900mW/cm2), enabling more effective processing of thicker resist films required in bump and wafer-level CSP processing [2, 3]. Photosensitive films that are sensitive to all three wavelengths will see exposure times drop, increasing throughput. This high wafer-plane intensity and increased throughput translate directly into fewer tools required.
In addition, auto-alignment capability allows superior alignment without special alignment targets [1]. Using a proprietary machine-vision alignment system, the stepper can align to any unique feature on a wafer, eliminating any need for tool-specific, custom stepper targets. With auto-alignment to existing wafer features, this stepper can be inserted into a process line with minimum impact, requiring only that a new bump reticle be ordered. No changes to prior masking levels are required. Moreover, this can be done for any wafer manufactured on any stepper.
A stepper with this kind of flexible auto-alignment capability would need to be able to capture targets and align through very thick films, such as polyimide or resist films more than 100µm thick. This is not an issue, since the system provides total overlay of 0.5µm or better, compared with the typical 1-2µm overlay capability of proximity aligners [3].
The stepper provides fully automatic changes in wafer size, from 150-300mm, and orientation, as well as automatic support for differing wafer thickness, from 0.4-4.0mm, with no hardware or software modifications. This makes it uniquely suited for foundries or bump lines supporting multiple fab locations. The ability to handle varying wafer thickness is critical when lithography on thinned or pre-background wafers is required. These thinned wafers also have issues with warpage due to the stress levels placed on them by thick films applied during the bumping process.
The stepper has specially designed hardware and vacuum setups to handle up to 100µm of wafer warpage. All wafer size, thickness, and illumination spectra selections are controlled via a reticle job control file, and no hardware modifications are necessary.
 Figure 4. Comparison of measured CD uniformities for a) a proximity aligner and b) a 1x stepper (10?m line in 10?m-thick PMER P-LA900PM resist). |
The Saturn Spectrum 300 is fully SECS/GEM compliant and interfaces to all major coat-develop tracks. As with all 300mm tools, full compliance with I300I standards and support for a FOUP interface is required. Support for these requirements is under development.
The system is capable of processing both thin and thick films, automatically switching between a variety of wafer sizes, shapes, and orientations.
The 2µm resolution capability of these tools is more than adequate for even the most difficult bump applications. In addition, this tool's low 0.16 NA provides an extremely large depth of focus (5.0µm at the resolution limit of 2.0µm), which is useful in imaging thick resist. This increased depth of focus also enables superior resist sidewall angle performance that translates into more consistent and reliable plating performance, critical for maintaining the required bump shape. It can also deliver the alignment control and critical dimension (CD) uniformity needed for successful imaging in a bump and wafer-level CSP process. Again, this is critical in controlling the size of the bump in the case of solder bumping and the shape of the bump in the case of gold bumping for LCD drivers [1] (Fig. 4).
Conclusion
With 300mm wafer fabs finally coming on line, there are clear indications that flip chip or wafer-level packaging will play a significant role for an increasing variety of ICs. The ability to fabricate bumps or process wafer-level CSPs on 300mm wafers has the same inherent difficulties as other aspects of 300mm wafer processing. New equipment must be developed to meet the challenges associated with this increased wafer size. The economics of 300mm wafer bumping and the types of devices being produced on 300mm wafers call for a change in requirements for bump lithography and wafer-level CSP.
References
- D. Anberg et al., "1x Broadband Wafer Stepper for Bump and Wafer-level CSP Applications," Pan Pacific Microelectronics Symposium Proceedings, 2000.
- W. Flack, W. Fan, S. White, "The Optimization and Characterization of Ultra-thick Photo-resist Films," in Advances in Resist Technology and Processing XV Proceedings, SPIE 3333, pp. 1288-1303, 1998.
- B. Todd, W. Flack, S. White, "Thick Resist Photo Imaging Using a Three-Wavelength Exposure Stepper," Micromachining and Microfabrication Process Technology Proceedings, SPIE 3874, 1999.
Stephen Kay received his BS in business information systems from the University of Phoenix. Kay is director of product marketing for advanced packaging technologies at Ultratech Stepper Inc., 3050 Zanker Rd., San Jose, CA 95134; ph 408/321-8835, fax 408/325-6444, [email protected].
Doug Anberg received his BS in chemistry and electronic engineering from California Polytechnic University and his MS in engineering management from Santa Clara University. Anberg is senior director of marketing at Ultratech Stepper Inc.
Thomas Goodman received his BS in polymer science from Penn State University and his MS in macromolecular science and engineering from Case Western Reserve University. Goodman is the manager of strategic applications and business development for the flip chip division of Kulicke & Soffa Industries (formerly Flip Chip Technologies LLC) in Phoenix, AZ.
Peter Elenius received his BS in mechanical engineering and his MS in manufacturing systems from the University of Wisconsin, Madison. Elenius is VP of technology and CTO for the flip chip division of Kulicke & Soffa Industries.
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Wafer bumping goes 300mm
There are four trends that drive bumping and wafer-level CSP capability to address 300mm process technology:
- the industry's "general" ramp to 300mm wafer fabrication;
- the growing trend toward flip chip and wafer-level CSP as primary packaging choices;
- the economies of scale for wafer bumping at 300mm; and
- the types of devices driving the initial ramp toward 300mm.
The technology of packaging or bumping die in wafer form is applicable to a wide range of devices. Low I/O devices, such as integrated passive devices or EEPROMs, rely on wafer-level CSP technology for a small form factor, low-cost packaging solution. On the other end of the spectrum, high-performance, high-I/O-count devices, such as microprocessors and peripheral logic, use flip chip mounting, not only for its superior electrical performance and low parasitics, but also for its ability to enable pad-limited designs that cannot be wire bonded.
Fabrication facilities are being built around the world to produce a number of different devices on 300mm wafers, including DRAM, logic, and processors. These high-performance devices will be the first products to be bumped on 300mm wafers. As the technology and availability of 300mm processing broadens, more cost-sensitive devices for consumer products will begin to be produced on the larger-size wafers.
Semiconductor manufacturers are now applying traditional frontend lithography equipment and methodologies to backend packaging applications, including 1x wafer steppers for bump lithography. With 300mm wafers, this shift will become even more evident given the inability of conventional proximity aligners or 5x reduction steppers to meet requirements.