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In current digital camera design, a partially stacked sensor sits conceptually between a conventional backside-illuminated (BSI) CMOS sensor and a fully stacked CMOS sensor.
Baseline: conventional BSI CMOS
In a standard BSI sensor, the light-sensitive photodiodes are on one layer, and the read-out circuitry sits on the same silicon substrate. This limits how much circuitry can be devoted to fast read-out, because everything must coexist on a single die.
Fully stacked sensor
A fully stacked sensor (as used in cameras such as Sony’s A1 or A9 series) separates the photodiodes and the read-out/logic circuitry into distinct layers, often with an additional dedicated memory (DRAM) layer. This architecture allows:
Very high read-out speeds
Greatly reduced rolling-shutter distortion
Extremely fast continuous shooting and video read-out
The trade-offs are higher cost, increased complexity, and greater power and heat management demands.
Partially stacked sensor
A partially stacked sensor uses stacking selectively rather than across the entire sensor. In practical terms:
Some read-out or processing circuitry is moved to a separate layer beneath the photodiodes.
Other circuitry remains on the main sensor layer.
There is typically no full-width, dedicated DRAM layer as found in flagship stacked sensors.
What this achieves
Compared with a conventional BSI sensor, a partially stacked design provides:
Faster read-out than non-stacked sensors
Improved rolling-shutter performance
Better support for higher frame rates and more advanced AF processing
However, compared with a fully stacked sensor, it delivers:
Slower read-out
Less rolling-shutter suppression
Lower manufacturing cost and power consumption
In the context of the Sony A7 V
For the A7 V, Sony appears to be using a partially stacked architecture to balance performance and price:
Noticeably faster read-out than the A7 IV’s non-stacked sensor
Improved video and electronic-shutter usability
Without the expense and thermal complexity of a true stacked sensor like those in the A9 III or A1
In short, a partially stacked sensor is a hybrid design: some of the benefits of stacking are realised, but without going “all in” on a fully stacked, DRAM-equipped sensor.