Slimmer Packaging for Image Sensors

Tessera (San Jose), known primarily for its flash and memory packaging, acquired Shellcase and Digital Optics Corp. relatively recently, then quickly turned around and developed one of the world's slimmest wafer-level chip-scale packaging (WLCSP) technologies that features a polymeric encapsulated glass-silicon structure to enable image-sensing capabilities through the actual packaging structure.

“We've taken Shellcase's existing technologies and evolved it into a thinner package, almost halving it from about 900 µm to 500 µm,” said Mitch Reifel, Tessera's manager of business development. “This is a key concern in the image module industry, because smaller is better — just like everything else in the electronics industry.”

Camera module vendors rely primarily on one of two types of technologies (Figure ). “The first is known as 'chip-on-board,' where a bare die is mounted onto a laminate and wire bonded, then you put on a housing and a lens,” Reifel said. “The second technology, chip-scale packaging, puts the imager in a reflowable package and reflows it onto a PCB, then you put on the housing and lens.”

Chip-on-board (COB) technology leads the industry with 65% market share, followed by chip-scale packaging (CSP) with 25-30% of the market. (Source: Tessera)

The company's latest technology falls into the CSP category, and its manufacturing process consists of four main steps. First, a 200 mm wafer and imager is joined with a 200 mm glass cover. This is a critical step, because once encapsulation is complete, contamination of the imager is no longer a serious concern.

During the second step, a “V” notch is made by grinding from the semiconductor to the bond pads. “The bond pad is exposed to the backside, then we lay down metal for the sputtering process to create a contact from the backside of the bond pad to the back of the die, along with routing,” Reifel explained. “This creates a lead wrapped around the image sensor from top to bottom.”

Steps three and four involve forming solder bumps on the wafer's backside and then singulating it, according to Reifel. “The result is a CSP with a ball grid array,” he added.

The technology is available in both cavity and non-cavity formats. In the cavity format, there is a space between the image area and cover glass that allows for the use of microlenses. In the non-cavity format, the glass is applied directly onto the image sensor.

And with this latest generation, the material set and design rules are all done at the wafer level. “We've narrowed the streets [width between the dies] from about 230 µm to 100 µm to yield more net die per wafer,” Reifel said. “The material set was also changed, which should lower costs by roughly 10%.”

Another significant change made was the improvement of the moisture sensitivity reliability from JEDEC MSL 2 to MSL 1. This allows the technology to be used in other applications, such as harsh automotive ones, which tend to have much higher reliability standards than the consumer imaging market.

“Essentially, we've made the package smaller, increased its reliability and made it less expensive, because these are all driving factors within the semiconductor packaging industry,” Reifel said.

For more perspective on the image sensor market and where all these ultrathin packages are headed, market research firm Techno Systems Research (Tokyo) said the market for image sensors within camera phones alone is expected to more than double from 413 million units in 2005 to 940 million units in 2009.

Demand for image sensors is coming primarily from camera phones, but also from digital cameras, automotive electronics, fax machines, digital scanners, machine vision systems, and CD/DVD portable players.

Looking to 2010, market research firm iSuppli (El Segundo, Calif.) is forecasting that revenue from image sensors for cell phones will reach $5.9B, while unit shipments of image sensors to cell phones are expected to jump to 1.2 billion. Also on the horizon, iSuppli predicts a dramatic improvement in average camera phone resolution from 4 to 5 megapixels by 2010, which should only help fuel the demand for ultrathin image sensors.

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