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 in-pixel p-wells, called Deep Depletion Extension (DDE), have been added to the manufacturing process in order to eliminate the large parasitic substrate current that would otherwise be present in a normal device.
The first device has been manufactured on a 18 µm thick, 1000 Ω.cm epitaxial silicon wafers using the well-established 180 nm PPD image sensor process from 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, as predicted by TCAD simulations. The new pixels in both BSI and FSI variants exhibit nearly identical photon transfer curve, image lag and linearity to the reference, non-modified pixels. Special attention was paid to the verification that full depletion of the epitaxial layer can be achieved, using different complementing methods.
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.