OmniBSI
This week presented interesting news as two image sensor technologies I thought might one day displace traditional types appear to have made breakthroughs on the march to commercialization. NHK showcased an active layer over IC – or AIC -type of sensor while OmniVision announced they would be sampling a backside illuminated - BSI - detector next month. Junko Yoshida’s detailed article on EETimes put me onto the backside scent.
For the traditional CMOS image sensor (front-side illuminated or FSI to use the OmniVision convention), the ratio of stack height or optical path length to pixel pitch is an important limitation. There are many drivers – both market and technical – for decreasing the pixel pitch of the image sensor. Leaving those aside for now, shrinking active pixel sensor dimensions is a given. Since the active silicon region (or photodiode) where incoming light is converted into electrical energy sits at the bottom of the metal interconnect, it’s easy to see why this dimension receives so much attention. As a result image sensor manufacturers have raced to reduce the height of the interconnect over the photodiode. This is most important with CMOS image sensors (CIS) since CCD’s do not include any processing circuits on the sensing chip. CIS devices often incorporate a great deal of signal processing on the same silicon as the sensor. Increasing integration drives more metal wiring levels if you hope to keep the total chip size under control.
About the time SoC integration on CIS was reaching a peak, a few manufacturers transitioned from aluminum to copper for the interconnect. Copper’s lower resistance allows thinner wires for a given line pitch for the same design target resistance as aluminum. Thinner metals means a shorter optical path to the photodiode. But copper integration is not ideal for image sensors since the barrier levels create unwanted interfaces resulting in reflections and optical system losses. There were also rumours of lower camera module yields for devices built with copper. Whatever the reasons, many manufacturers returned to aluminum and longer optical paths. But a few percent improvement will not eliminate the issues with optical path length, something these two new technologies avoid for the most part.
Both AIC and BSI approaches treat interconnect where it should be – literally the “back end” from the optical point of view. An AIC detector adds active silicon with photodiodes on top of the chip wiring, so light reaches it first. BSI flips the chip allowing light to hit the sensor from the back. Viewing a BSI detector in cross-section, it would like very similar to the traditional FSI device. The only differences is that light enters through the backside of the substrate opposite to the active silicon surface.
Several companies have published work on the AIC approach. The sensor in these cases is most commonly amorphous silicon patterned into detectors after traditional BEOL metal IC processing. These devices employ similar structures and materials to those used in LCD display panels. ST Micro and Samsung have been brewing this type of technology for a while. (ST and Samsung both had multiple presentations at IISW 2007. See my earlier post.)
But the NHK sensor uses an organic light sensitive layer rather than silicon. I have not been able to confirm a connection with FujiFilm, but this sounds a lot like the technology they presented at last year’s IISW. It doesn’t take too much imagination to visualize the organic sensor approach as spreading old school analog film emulsions over a state-of-the-art readout IC.
I guess it’s obvious that substrate processing must be adapted for the BSI device. That is almost assuredly based on SOI technology. The leading SOI substrate provider is Soitec. CEA-LETI spawned another company in the SOI space in 2003. Tracit Technologies offers layer transfer technology to IC’s onto SOI substrates. Last year, they helped e2V bring backside detectors to the “medium volume professional image sensor market.” I think TraciT will be an important factor in getting BSI into the high volume consumer market.
There has been a lot of news this year about the wafer-level camera (WLC) and through-silicon vias (TSV) to facilitate it. Even some reverse engineering blogs have something to say about TSV! BSI offers an interesting packaging option in addition to the many performance advantages as well. It lends itself well to flip-chip packaging. Perhaps the cost adders that have maligned the BSI approach in the press will be more than compensated by the cheaper flip chip packaging it will allow.
OVT now calls this technolgoy “BSI” for back-side imager, but it seems to have been previously called “BID” as I learned at IISW 2007. Dr. Bedaprata Pain should now be very happy that his attempts to jump-start the transition to back-side illumination may have sparked commercialization of the technology.
OmniVision’s PR states:
OmniBSI architecture delivers a number of performance improvements over FSI, including increased sensitivity per unit area, improved quantum efficiency and reduced cross talk and photo response non-uniformity, which all lead to significant improvements in image quality. Since light directly strikes the silicon, the fill factor of the image sensor is significantly improved so as to deliver best-in-class low-light sensitivity. A much higher chief ray angle enables shorter lens heights which in turn allows for thinner camera modules, which are ideal for use in the next generation of ultra-thin mobile phones. Finally, BSI technology affords a much larger aperture size, which allows for lower f stops facilitating the development of better performing camera modules with superior camera performance.
Even though this is written by the spin doctors, their statements are absolutely correct. The improved low-light sensitivity will be especially important for cameraphones as users, and therefore designers, are demanding improvements to images acquired in pubs, restaurants and parties. Nokia made this very clear at Image Sensors Europe, and those sentiments were echoed by a number of sensor manufacturers including Aptina and OmniVision.
As for the suggestion that there is little novelty to the BSI detector approach, I can’t argue that the idea was around for a while (check Stern, Proc. SPIE, vol. 1071, 1989). For example, MIT Lincoln Labs has a long history in this field. But then again, how long did it take the laser to make its way into a useful consumer product?
[...] Last week, I posted a long, wordy (your own description might not be so flattering) article about OmniVision’s OmniBSI announcement. To sum up, OmniVision appears to be first out of the gate with a backside illuminated CMOS image sensor for the mass market. Despite the length of last week’s post, I left out an important detail that every other blog or news source – regardless of credibility – mentioned. As OmniVision’s foundry partner, TSMC is obviously a key player in bringing this new technology to production. [...]