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Chapter 3 – Theory of Operation
Introduction
Backside illuminated scientific CMOS (BSI Scientific CMOS) sensors are a recent development
in image sensor technology. BSI CMOS sensors are able to provide the highest levels of
sensitivity with a near perfect 95% quantum efficiency (QE). This QE coupled with optimized
pixels enable CMOS sensors which have high frame rates, high pixel counts, and low
electronic noise provide the most complete low-light scientific imaging solution.
CMOS Image
Sensor
Structure
A major difference between traditional CCD sensors and CMOS sensors is the location where
charge-to-voltage conversion of accumulated photoelectrons takes place. CCD sensors
transfer the pixels accumulated signal in charge packets in “bucket brigade” fashi on across
the sensor to a common output node where charge is converted to a voltage. The voltage is
then sampled using off-chip Analog-to-Digital Converters (ADC) and transferred to the PC as
digital grey values.
While providing excellent quantitative photometry and very high image quality, the large
number of transfers and sequential digitization of pixels results in low frame rates. This
speed penalty increases with the number of pixels to be digitized.
CMOS sensors leverage many of the same analog signal concepts used in CCD
’ s, but
places the output node circuitry inside each pixel. This eliminates the charge transfer
process. To read the signal from a given row, the accumulated charge is converted to a
voltage inside the pixel, then each pixel in the row is connected to the appropriate
column voltage bus, where the on-chip ADCs covert the voltages to an 8-bit, 11-bit or 12-
bit grey value.
The parallel digitization of all pixels in a row provides CMOS devices with a tremendous
speed advantage. Imagine a CCD with 2048 x 2048 pixels
– and each pixel’s voltage is
measured in 1µsec. To read a single row, 2048 voltage measurements are performed in
serial fashion taking slightly longer than 2ms, and when repeated for 2048 rows, the entire
image takes over 4 seconds to be digitized.
On a CMOS device
– the entire 2048 voltage conversions needed to digitize a row happen
in parallel.
If the time to digitize a pixel remains at 1µsec
– the time to read the entire frame is now
2ms. In practice, the time saving is split between faster frame rates and slowing the rate of
pixel measurement to reduce electronic noise. For example, if the time to measure a pixel