BSI: No Panacea
May 21, 2010
In a few earlier posts I have mentioned the new generation of Sony sensors boasting “back-side illumination,” and marketed as Exmor-R (as distinct from Sony’s conventional sensors, just branded Exmor).
Back-side illumination (BSI in the industry jargon) is a tricky and costly chip-fabrication technique, where after depositing all the wiring traces on a silicon wafer, the substrate is flipped over and almost entirely thinned away. This leaves the wiring on the underside of the light-sensitive photodiodes (as Sony describes here), so these unobstructed pixels will theoretically collect more light.
BSI is promoted as one of the technological breakthroughs which might help save image quality, even as manufacturers race to cram more megapixels into tiny sensor areas. In fact, the IMX050CQK actually scaled back its pixel count to 10 Mp, compared to the 12 and 14 that have been becoming increasingly common in the point & shoot market.
Sony introduced the chip in its own first models in the fall of 2009, for example in the WX1. But clearly Sony found it advantageous to spread the sensor development costs over a larger production run, and apparently they’ve aggressively marketed the chip to other camera makers as well. Pretty much any 10 Mp camera sold this year advertising a backside-illuminated sensor uses it. It seems particularly popular in today’s nutty “Ultra Zoom” market segment.
As reviews of these new BSI-based cameras filter out, the word seems to be that they do offer decent image quality—but hardly anything revolutionary. If their high-ISO images look smooth, it seems to be partly thanks to noise reduction processing, which can destroy detail and add unnatural, crayon-like artifacts.
The issue is simply that the IMX050CQK remains a very tiny chip: The image area used is about 6 x 4.5mm. That image circle, just 7.5mm, is about the same diameter as a pencil. Photon noise and diffraction blur using such a tiny sensor will always limit image quality.
It’s interesting that Sony must resort to some special hocus-pocus to overcome this, such as a low-light mode that quickly shoots 6 frames and then “stacks” them to smooth out noise.
In truth, it seems that the real development breakthrough for Sony’s new sensor was in readout speed. Not only is HD video a must-have bullet point for every camera today, the fast readout allows Sony to offer several gee-whiz features such as fast shooting bursts, “sweep panorama,” and the image-stacking functions I mentioned.
I had all this on my mind when looking at an interesting bit of chip-maker PR that Image Sensors World posted the other day. Aptina (who I’ve mentioned before) is a leading CMOS sensor producer—although most are sold in the exploding phone-cam market.
Their perspective is that BSI’s advantages must be weighed against its problems (especially see page 5 of this PDF).
“Crosstalk” means that the signal recorded by one pixel (which ought to correspond to just one color, e.g. red) might bleed into its neighbors (which ought to represent green and blue). So if BSI has a problem with increased crosstalk, its raw data will require some extra massaging by the camera’s processor to create the final image. Potentially, this could blur fine detail more, compared to a conventional front-side sensor.
Aptina claims that it is only when pixel sizes drop to 1.1 microns that BSI’s advantages become worthwhile. For reference, that would mean a 22-megapixel point & shoot, or an 8 MP iPhone camera.
On camera discussion boards, you’ll sometimes hear the wish expressed that back-side illumination could come to larger sensors (like APS-C), and somehow allow a great revolution in their sensitivity.
But this seems to be a vain hope. Aside from the high cost (and unproven technology) of thinning such large chip areas, there is simply much less advantage when pixel dimensions grow to a reasonable 5 or 6 microns across. The wiring traces obstruct a much smaller fraction of the light-sensitive area then.
This was noted by Sony representative Toru Katsumoto during the European NEX camera launch. (Attention Toru: No, Sony did not invent BSI, nor create the name.)
Sensor technology has indeed evolved rapidly over the past decade. But we’re reaching a point where technology tricks can no longer overcome physics. Tiny sensors have inherent limitations; ones we may choose to tolerate only when portability is the overriding design criteria.
But when it comes to image quality, larger sensors still win.