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 BSI Sensor

A Whizzy Small Sensor is Still A Small Sensor

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.

So I was interested to read the review just posted by Jeff Keller of Nikon’s P100 ultrazoom camera, which uses this chip. See his conclusions here.

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.

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If you’re interested in a behind-the-scenes peek into the imaging-chip industry, check out the blog “Image Sensors World.”

Much of this revolves around cell-phone cameras, which today are by far the largest consumer of imaging chips. And that’s a market where the drive for miniaturization is even more extreme than with point & shoot cameras. For a phone-cam to boast 2 megapixels, 4 megapixels, or more, each pixel must be tiny.

And it’s finally happened: A company called OmniVision has introduced the industry’s first 1.1 micron pixel. That’s about 60% of the area of today’s typical point & shoot pixels.

At that scale, the light-gathering area of each pixel is so minuscule that back-side illumination practically becomes mandatory. The reasons are well explained in OmniVision’s “technology backgrounder” PDF.

Back Side Illumination

OmniVision Explains Back Side Illumination

This document’s introduction says,

“Evidently, pixels are getting close to some fundamental physical size limits. With the development of smaller pixels, engineers are asked to pack in as many pixels as possible, often sacrificing image quality.”

Which is an amusingly candid thing to say—considering that they are selling the aforementioned chips packed with “as many pixels as possible.”

What are these “fundamental limits”? Strangely, OmniVision’s document never once mentions the word “diffraction.” But as I’ve sputtered about before, with pixels the size of bacteria, diffraction becomes a serious limitation.

Because of light’s wavelike nature, even an ideal, flawless lens cannot focus light to a perfect point. Instead, you get a microscopic fuzzy blob called the Airy disk.

Now, calling it a “disk” is slightly deceptive: It is significantly brighter in the center than at the edge. Thus, there is still some information to extract by having pixels smaller than the Airy disk. But by the time the Airy disk covers many pixels, no further detail is gained by “packing in” additional ones.

Our eyes are most sensitive to light in the color green. For this wavelength, the Airy disk diameter in microns is the f/ratio times 1.35. (In practice, lens aberrations will make the blur spot larger than this diffraction limit.)

But even using a perfect lens that is diffraction-limited at f/2.3, the Airy disk would cover four 1.1 micron pixels.

Airy Disk versus Pixels

Pixels much smaller than the Airy Disk add no detail

A perfect lens working at f/3.5 (which is more realistic for most zooms) will have an Airy disk covering nine pixels of 1.1 micron width. This is one of the “fundamental physical size limits” mentioned in OmniVision’s document.

Manufacturing a back-illuminated chip is quite complex. And for OmniVision to be able to crank them out in quantity is a technological tour de force. As I wrote earlier, there are still a few tweaks left to make imaging chips more sensitive per unit area; this is one of them.

Perhaps this helps explain another curiously candid statement I saw recently.  Sony executive Masashi “Tiger” Imamura was discussing the “megapixel race” in a PMA interview with Imaging Resource. And he said,

” …making the pixel smaller on the imager, requires a lot of new technology development. […] So, as somebody said, the race was not good for the customers, but on the other hand, good for us to develop the technologies. Do you understand?”