Stepper tech overcomes packaging problems - Comment - wafer-level, chip-scale packaging for DRAMs - Editorial

Electronic News, May 20, 2002 by Manish Ranjan, Stephen Kay

THE TREND OF MINIATURIZing devices has pushed assembly and packaging toward the adoption of wafer-level, chip-scale packaging (WLCSP).

With advances in process maturity and cost reduction, WLCSP will invade the DRAM sector, a move that will lead to its adoption in mainstream volume production. With the widespread growth of flip-chip and WLCSP packages, however, traditional methods of lithography such as contact and proximity aligners are no longer adequate to address the technology demands in a cost-effective manner. As a result, 1x stepper technology has been introduced as a de-facto standard for meeting the flipchip and WLCSP lithography dynamics created by larger wafer sizes and shrinking pad pitches.

Because of the advantages in yield and imaging performance, semiconductor manufacturers have long abandoned the use of contact and proximity aligners for front-end applications. To address the technology needs and increase process yields, device manufacturers are now applying traditional front-end lithography equipment to leading edge, back-end lithography. This adoption has also been accelerated because the imaging requirements of wafer bumping are subject to the same production necessities as front-end semiconductor fabrication.

The use of projection optics, as provided on a 1x stepper, offers superior imaging performance. The resolution obtained using a contact or proximity aligner is a direct function of the gap between the wafer and the photomask. The best resolution is obtained when the wafer and the photomask are in direct contact. This leads to several issues with respect to imaging performance and yield. Stepper or projection optics resolution, however, is a function of the wavelength of the exposure light and the numerical aperture of the system and not the proximity of the photomask to the wafer. Furthermore, because lx steppers produce a projected image, the ability to focus this image at various depths within a thick resist film enables superior side wall angle and overall image performance regardless of the resist film in use. This is especially critical in the advanced packaging market where the use of thick resist films is quite common.

On the other hand, contact and proximity aligners have virtually no focus capabilities and have much poorer performance in these types of films. Contact and proximity aligners also have difficulty maintaining exposure uniformity over large wafer sizes because they have to illuminate the entire wafer for each exposure. Steppers illuminate a much smaller area for each step and therefore are able to control the illumination uniformity to a much tighter degree. This results in improved CD uniformity and a more consistent resist sidewall profile.

The use of enhanced global alignment during exposure using stepper technology provides significantly better alignment capability. Because the image is stepped over the surface of the wafer, aix stepper can compensate for mask run out, isotropic wafer scaling, rotation errors and orthogonality errors by adjusting the exposure locations on the wafer. Contact and proximity aligners conduct alignment on a global basis using two alignment targets and cannot compensate for mask run out or grid errors. In addition, unlike the quartz reticles used for imaging using stepper technology, contact aligners use sodalime photomasks, which further degrade the alignment performance.

The use of stepper technology also eliminates the incidental wafer-to-photomask contact witnessed during imaging using contact aligners. This contact during imaging by contact aligners reduces the life of the photomask and introduces residual resist on the photomask, which leads to repeating damages during imaging. The use of enhanced global alignment also contributes to significantly better process yields by reducing the alignment errors and improving the quality of the image being exposed. Steppers provide a superior imaging performance and significantly better CD control, resulting in higher overall product yields. In addition, the high level of process automation provided by stepper technology reduces operator intervention and provides a more robust process.

Manish Ranjan is senior product marketing manager for packaging applications, and Stephen Kay is director of product marketing for advanced technology at Ultratech Stepper Inc. in San Jose.