Tessera Incorporates TSVs Into Optoelectronics Packaging

Sally Cole Johnson, Contributing Editor -- Semiconductor International, 3/18/2008

In a déjà vu manner, Tessera Technologies Inc. (San Jose) recently acquired a few more smart optics and imaging companies — namely EyeSquad and FotoNation — and quickly turned out another advanced wafer-level chip-scale packaging (WLCSP) technology for image sensors and other optical devices destined for electronic products with a camera.

“Tessera’s strategy is to offer miniaturization and they’re going after the optoelectronics packaging arena,” said Jim Walker, research vice president at Gartner (Stamford, Conn.). “They’ve acquired several optics companies during the past couple of years and created a bit of a niche. Now Tessera has taken Shellcase’s process and minimized it again — with through-silicon vias this time — and it gives them a good handle on the technology and the market. From an optoelectronics packaging perspective, they don’t have many serious competitors.”

To back up a little bit, in case you haven’t been following Tessera closely, they entered the consumer optoelectronics packaging market a little more than two years ago by first acquiring Shellcase (and its patented family of WLCSP technologies Shellcase OC, Shellcase OP, as well as chip-on-board Shellcase CF), followed by Digital Optics Corp. Soon after, Tessera rolled out a WLCSP technology known as “RT” for its razor-thin profile.

Shellcase MVP

Tessera’s latest WLCSP intellectual property (IP) offering is the Shellcase MVP (short for micro via packaging). It’s an ultrathin (~500 µm) high-performance through-silicon via (TSV) solution that enables 3-D stacking with digital signal processing (DSP) or flash memory, and is compatible with reflowable wafer-level cameras.

While Tessera declined to give away its process step secrets for MVP, the unique development here is the way in which they create the via through the pad without damaging the silicon. “We managed to get away from the problems and reliability issues incorporated into the silicon die when using standard TSV processes,” divulged Schlomo Oren, general manager of Tessera Israel-Jerusalem.

MVP’s basic process steps, which are very similar to those of the RT, are shown in the Figure. The design rules feature a scribe line width of ≤100 µm, bond pad size of ≤50 µm and bond pad pitch of ≤110 µm. As far as metallization and routing go, the MVP has direct contact to CIS, through pad T-contact via, no process in the scribe line, and no special design mask is required by the image sensor fabrication facility.

MVP's general process steps. (Source: Tessera)

A JEDEC moisture sensitivity level (MSL) reliability rating of 1 means that MVP can be used in harsh environments, such as automotive ones. “We’ve run extensive tests on this device, in excess of 4000 cycles [requirement is only 2000 cycles: -40 to 125°C] without failures,” Oren noted. “We’re very confident about the reliability of this package, which should be attractive to the automotive industry because its reliability requirements are much higher than standard consumer optics.

“Another benefit of MVP is that it uses existing manufacturing infrastructures in the market,” Oren said. “This will significantly shorten the shift from technology introduction to high-volume manufacturing. In such a dynamic market, it’s extremely important to minimize the time between technology introduction and new product availability.”

MVP is offered in both cavity and non-cavity formats, as well as leaded or lead-free bump formats, and is currently available for licensing from Tessera.

WLCSP market

Zooming in on the WLCSP market, analysts have been predicting a bit of a slow migration from wire bonding and flip-chip to WLCSP. “We’re seeing an increase in the adoption of wafer-level packaging in general,” Walker said. Gartner, for example, predicted last year that the total WLCSP market would progress from 8905 millions of units in 2007 to 11,974 millions of units in 2011. The image sensor segment of the market is also expected to be one of the faster growing segments, with an estimated 15.3% compound annual growth rate between 2006 and 2011.

Prismark (Cold Spring Harbor, N.Y.), an electronics consulting firm, projected last year that by 2011 WLCSP will encompass 50% of the market, while wire bonding and chip-on-board will slip to 40% and flip-chip cavity and others will hang on at 10%.