A new pinned photodiode (PPD) CMOS image sensor with reverse biased p-type substrate has been developed and characterized. The sensor uses traditional PPDs with one additional deep implantation step to suppress the parasitic reverse currents, and can be fully depleted. The first prototypes have been manufactured on an 18 µm thick, 1000 Ω·cm epitaxial silicon wafers using 180 nm PPD image sensor process. Both front-side illuminated (FSI) and back-side illuminated (BSI) devices were manufactured in collaboration with Teledyne e2v. The characterization results from a number of arrays of 10 µm and 5.4 µm PPD pixels, with different shape, the size and the depth of the new implant are in good agreement with device simulations. The new pixels could be reverse-biased without parasitic leakage currents well beyond full depletion, and demonstrate nearly identical optical response to the reference non-modified pixels. The observed excessive charge sharing in some pixel variants is shown to not be a limiting factor in operation. This development promises to realize monolithic PPD CIS with large depleted thickness and correspondingly high quantum efficiency at near-infrared and soft X-ray wavelengths.
A prototype CMOS Image Sensor optimised for soft X-ray applications has been designed by the Centre for Electronic Imaging in partnership with Teledyne-e2v. The device features four different pixel variants (three variants of 40 μm pitch pixels, and one variant of 10 μm pixels) each covering a quarter of the 2 × 2 cm2 image area. The pixel designs feature fully depleted pinned photodiodes using reverse substrate bias and have been optimised for low noise operation, high responsivity and low image lag. The fabricated front-illuminated devices have been tested in a custom-built vacuum test setup at operating temperatures between -30°C and -40°C. The sensors feature less than 5 e- RMS readout noise and energy resolution of 142 eV at Mn-Kα (5.9 keV). The response to soft X-ray with different sensor parameters (e.g. pixel pitch, deep-depletion extension implant depth, and back-bias voltage) is also studied.
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.
Lateral charge diffusion is one of the main contributors to the Point Spread Function (PSF) in CMOS image sensors, due to the small depth to which they can be depleted. This can have an adverse effect on the spatial resolution of the sensor and the measured shape of the observed object. In this paper, PSF measurements are made on a novel CMOS detector capable of reverse bias and full depletion. The PSF is measured with the Virtual Knife Edge (VKE) technique at five wavelengths, from 470 nm to 940 nm, to ascertain wavelength dependence. The inter-and intra-pixel non-uniformity is examined to determine the difference between pixels as well as within the pixels themselves. Finally, the pixel structure is also evaluated using a 1 µm spot of light to examine the effect of the internal layout of a pixel on the sensitivity to light. These factors all impact precision astronomical measurements and so need to be understood before use in science missions.
2020). Mitigating charge spill-back induced image lag with a multi-level transfer gate pulse in PPD image sensors. In: X-Ray, Optical, and Infrared Detectors for Astronomy IX (Holland, Andrew D. and Beletic, James eds.), Proceedings of SPIE,
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