is an aps-c sensor really cheaper to produce? You would think the 70d sensor with its smaller photo sites and dual pixel tech would be more expensive than the 6d sensor which is about the same mp count?
Pixel size doesn't matter significantly (as long as the density is achievable with the process scale being used (500 nm, 180 nm, etc.). Area matters - you get a lot more APS-C sensors than FF from a wafer; the larger sensor also means a higher QC failure rate, raising the cost further.
I suppose that's true, I just don't think that it is the huge cost difference its made out to be and if its the sensors area and not pixel count is what makes it expensive then they should hurry up with the high mp ff sensor
Neuro's got it right here.
The silicon wafers used for IC and sensor fabrication have a fixed manufacturing cost per wafer*. So if you can get ten times the number of good parts out of a wafer, the part cost will be - everything else equal - 1/10th the original cost.
There are a couple of things that affect the actual yield of a wafer:
1. How many (rectangular) dies that can be put on the (round) wafer. The "square peg in round hole" problem. See Wikipedia
. Plugging in some numbers for a 300mm wafer size, I get 1610/610 = 2.64 times more APS-C sensors than FF sensors.
2. Intrinsic die yield. This is usually modeled as falling exponentially with the die area, so an APS-C sensor will intrinsically have exp(1.62
)~=13 times higher yield than a FF sensor. The yield is based on how likely fabrication defects will not impact a die of size X, with an inherent defect density of A0
. The defect density is related to the fabrication line itself (it usually starts out high, and then improves over the lifetime of the fab line, as the engineers learn how to control and improve the process line).
3. Feature density. This reflects how dense the wires/transistors etc are, and will somewhat counteract the die yield above. I'll ignore this below, since I don't have data and this has never been significant enough for me to bother about when considering the yield of a particular product.
So for APS-C vs FF sensors, fabricated on the same line, a single 300mm wafer will yield ~2.64 * exp(1.62
) ~ 34 times more good dies
of APS-C size than of FF size.
I probably forgot a number of details in relation to the yield - apologies in advance for that - but hopefully this shows why an APS-C sensor will always
have a cost advantage over a FF sensor.
As a reference, the cost of a processed 180nm wafer for mixed analog/digital designs can be on the order of USD2000 (ex fab cost). Lower geometry wafers are generally more expensive, higher geometry less expensive.
*) This assumes a number of technicalities, such as using the same fab options, metal layers, equipment utilization rates etc etc etc. To the first order and for this discussion, this is a good assumption.