Until Hideo Yamanaka published his 2009 “Method and apparatus for producing ultra-thin semiconductor chip and method and apparatus for producing ultra-thin back-illuminated solid state image pick up device”. The technology of back illuminating sensors was costly, complex and required further refinement to become a widely used sensor. Yamanaka found that by rearrangement of the imaging elements, increased light could be captured and thereby improve low-light performance. However, back thinning lead to a host of other problems such as cross-talk which causes noise, dark current and color missing between adjacent pixels. Thinning also made the silicon wafer more fragile.
A traditional front illuminated camera is constructed to mimic the human eye; a lens at the front and photodetectors at the back, a back illuminated sensor arranges the wiring behind the photodiode substrate layer by flipping the silicon wafer and then thinning its reverse side so that light can strike the photodiode layer without passing through the wiring layer.
Front illuminated CMOS sensors vs. Back Illuminated CMOS.
Today back illumination technology has made some significant progress and BI chips are now available from several manufactures of Silicon Chip technology. With higher sensitivity over a broader spectral region (deep UV to near IR) several industrial camera manufacturers are introducing back illuminated cameras.
These cameras are ideal for ultra-low light applications like Astronomy, Spectroscopy and biological imaging. With back illumination, low light applications get increased Quantum Efficiency up to 95% and lower read noise at <2 e-rms. When every photon counts a camera with a back illuminated sensor should be your one and only choice.
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