Thursday, May 27, 2010

BSI Manufacturing and Equipment

ElectroIQ published Yole Développement article on BSI and WLC as the next generation drivers of image sensor technology. A quote below describes BSI porcess and manufacturing equipment:

"The BSI process flow starts with making the photo diodes. Then the device wafer is bonded at low temperature to a silicon or glass carrier, using either adhesive polymers or molecular oxide-to-oxide bonding. US-based 3D-IC company Ziptronix offers one low-temperature bonding solution, which presses together ultraflat wafers, well prepared through specific surface preparation treatments. Next, the 1mm thick photo diode wafer is thinned down to 40-50μm with Disco or Accretech grinding tools, then thinned further with CMP, and finally etched down to an etch-stop layer at 5μm. This radical wafer thinning typically requires precise control of wet etching after initial grinding and CMP. One option, used by Sony and others, is to use SOI wafers from Soitec, using the buried oxide layer as an inner etch stop layer at the oxide interface -- though the high cost of SOI wafers may limit the process to high-end imaging applications only. Others, including OmniVision, working with TSMC and Xintec, claim to have developed a lower-cost alternative process using bulk silicon wafers with graded implant layers. The trick is to find a highly selective etch chemistry that will stop precisely at the required 5μm thin silicon interface, just before reaching the photodiode structures.

Also critical is the annealing process, since this 5μm thin silicon film needs to include a very narrow implant gradient, to prevent recombination in the epi silicon and to push the photons down to the photo diodes. Since typical annealing ovens can only be controlled to about 30μm layer precision, the finer implant gradients require annealing with a nano-second, local heating laser process. The French equipment company Excico supplies a tool that uses a UV excimer type of laser source with a large spot for tight precision with better image quality and higher throughput.

The article also overviews the wafer level camera AF approaches:

"Companies are also working on integrating autofocus functions at the wafer level. A number of players, including SEMCO, are working on electro active polymers, whose thickness can be controlled and driven electrically by applying a defined voltage. Others like Siimpel (recently acquired by Tessera) have a MEMS- based solution, using a spring-like silicon structure.

But the front-runner currently appears to be a potentially breakthrough technology using liquid crystal polymers. The startup LensVector plans to start production this year of a four-layer stack of 8-in. glass wafers, encapsulating liquid crystal polymers that change shape when voltage is applied to the driving electrodes -- all in less than 500μm total thickness. This technology has the potential to significantly bring down the size and cost of camera modules in the future.

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