As I said. Technology has been marching on.
But even with light-guides (to guide the light onto the photodiode), there are still limits as to much you can shrink pixels.
These are physical entities and you cannot shrink them indefinitely with a given technology.
That's the only point I'm making.
You make it sound like smaller pixels are always better - and that's not unconditionally true.
There's a physical limit that cannot be crossed.
That's why manufacturers are using finer and finer CMOS processes (Panasonic is at 65nm now).
And also looking for alternative solutions - like BSI, Sony's stacked technology, etc..
So, smaller pixels are generally better - but only when newer, more advanced technologies are used.
There's also the issue of the full-well capacity of a photodiode.
Smaller full-well capacity automatically lowers SNR. You should know that.
So, it's a balancing act, really, for pixel engineers.
A blanket statement like 'smaller pixels are always better' is just that - a blanket statement.
Some necessary small print needs to be added to discussion .
Sure, there is no question there are limits to how small you can shrink pixels with an FSI design. I already mentioned that ALL small form factor sensors that use 1.2µm pixels and smaller use BSI designs now.
But we are primarily talking about larger sensors. In larger sensors, we don't have the kinds of problems with maintaining incident light ratio on the photodiodes. We don't even need lightpipes...a single layer microlens works sufficiently to control over 90% of the light. A second layer would again focus any dispersal from the color filter back into the "well", again minimizing any remainder losses.
There is also a limit to how far use of finer processes will improve things for larger sensors. For smaller sensors they are essential, even with BSI, as your packing so much into increasingly small spaces. I mean, when the 0.9µm generation hits, the pixels will be smaller than the majority of the infrared bandwidth! But, large sensors still have huge pixels. It will be many generations before we drop below 3 micron pixels, assuming we ever do. It's a lot harder to make large optics perform well outside of the central FoV, and I think lens design will ultimately become the bottleneck for keeping the megapixel race alive with larger sensors.
Assuming we do reach 3 micron pixels at some point, on either a 180nm or 90nm process...that would be a 96 MEGAPIXEL sensor in full frame, and a 37 megapixel sensor in APS-C. That's WAY up there. The highest resolution sensors will probably sit around 4.5µm to 3.7µm pixel sizes for a while still, a couple DSLR generations, which puts us out another eight years approximately?
Assuming everything is manufactured on a 180nm or smaller process soon, I don't think that fill factor will be the primary or even a significant issue for APS-C and FF sensors for so long that it simply doesn't matter. In that light, I still assert that you can always do more with smaller pixels. As far as I am concerned, BRING ON THE 96mp MEGAPIXEL MONSTROSITIES!! MUHAHAHA!!