Wednesday 13 September 2017 

Speaker: Konstantin Stefanov (Open University)


Title: "Design and Performance of a Pinned Photodiode CMOS Image Sensor Using Reverse Substrate Bias”

Abstract: A new pinned photodiode (PPD) CMOS image sensor (CIS) has been developed and characterised. The sensor can be fully 
depleted by means of reverse bias applied to the substrate, and the principle of operation is applicable to very thick sensitive 
volumes. Additional n-type implants under the pixel p-wells, called Deep Depletion Extension (DDE), have been added in order 
to eliminate the large parasitic substrate current that would otherwise be present in a normal device. The first prototype has 
been manufactured on a 18 µm thick, 1000 Ω.cm epitaxial silicon wafers using 180 nm PPD image sensor process at TowerJazz 
Semiconductor. The chip contains arrays of 10 µm and 5.4 µm pixels, with variations of the shape, size and the depth of the 
DDE implant. Back-side illuminated (BSI) devices were manufactured in collaboration with Teledyne e2v, and characterised 
together with the front-side illuminated (FSI) variants. The presented results show that the devices could be reverse-biased 
without parasitic leakage currents, in good agreement with simulations. The new 10 µm pixels in both BSI and FSI variants 
exhibit nearly identical photo response to the reference non-modified pixels, as characterised with the photon transfer curve. 
Different techniques were used to measure the depletion depth in FSI and BSI chips, and the results are consistent with the 
expected full depletion. This development has the potential to greatly increase the quantum efficiency of scientific PPD CIS 
at near-infrared and soft X-ray wavelengths, due to the ability to realise sensors with sensitive thickness in excess of 100 µm. 
Likely applications are in sensors for astronomy, Earth observation, hyperspectral imaging, high speed imaging, spectroscopy, 
microscopy and surveillance, as well as for soft X-ray (<10 keV) imaging at synchrotron light sources and free electron lasers.