Fast MTF measurement of CMOS imagers at the chip level using ISO 12233 slanted-edge methodology

MTF measurement methods for imaging devices usually require the use of an optical system to project the image of the object onto the detector. So, MTF results quality strongly depends on the accuracy of the optical adjustments (alignments, focusing…). Dedicated edge patterns have been implemented at the chip level on a CMOS imager. One of them emulates the target used in the ISO 12233 slanted-edge technique and the others one are inspired by the knife-edge method. This allows to get the MTF data without optical focusing. In order to validate the results, comparisons have been made between MTF measurements using these patterns and results obtained through direct measurements with the transmissive slanted-edge target and sine target

CMOS pixels crosstalk mapping and its influence on measurements accuracy for space applications

Due to different local intra-pixel sensitivity and crosstalk between neighboring pixels, the Pixel Response Function of detectors (PRF - signal of the pixel as a function of a point source position) is generally non-uniform. This may causes problems in space application such as aperture photometry and astrometry (centroiding). For imaging applications, an important crosstalk yields to a loss of resolution, i.e. a poor image quality, commonly quantified by the Modulation Transfer Function (MTF). So, crosstalk study is of primary importance for our applications. A dedicated test chip (using a technology optimized for imaging applications) has been developed in order to get both MTF data and influence of the various areas of the pixel to its own response and the one of its neighbors. The results obtained with pixel kernels and direct MTF measurements, performed on the same chip at different wavelengths, are analyzed and compared in order to correlate them. So it is possible to draw conclusions -that can be applied at the design level - allowing to get a better MTF and to minimize errors on aperture photometry and centroiding computation

Fast MTF measurement of CMOS imagers using ISO 12233 slanted-edge methodology

The ISO 12233 standard provides a fast and efficient way of measuring Modulation Transfer Function (MTF) of digital input devices (such a digital still camera) using a normalized reflective target based on a slanted-edge method. A similar methodology has been applied for measuring MTF of CMOS image sensors, using 12233 slanted-edge technique associated with a prototype transmissive target. In order to validate the results, comparisons have been made between MTF measurements of image sensor implemented using a 0.25µm process, using this method and sine target direct measurements

Optoelectrical performance evolution of CMOS image sensors exposed to gamma radiation

In this paper we present a study of ionizing radiation effects, up to 5 kGy, in several CMOS image sensors manufactured using a commercial 0.18 μm technology dedicated to imaging

Overview of ionizing radiation effects in image sensors fabricated in a deep-submicrometer CMOS imaging technology

An overview of ionizing radiation effects in imagers manufactured in a 0.18-μm CMOS image sensor technology is presented. Fourteen types of image sensors are characterized and irradiated by a 60Co source up to 5 kGy. The differences between these 14 designs allow us to separately estimate the effect of ionizing radiation on microlenses, on low- and zero-threshold-voltage MOSFETs and on several pixel layouts using P+ guard-rings and edgeless transistors. After irradiation, wavelength dependent responsivity drops are observed. All the sensors exhibit a large dark current increase attributed to the shallow trench isolation that surrounds the photodiodes. Saturation voltage rises and readout chain gain variations are also reported. Finally, the radiation hardening perspectives resulting from this paper are discussed

Estimation and Modeling of the Full Well Capacity in Pinned Photodiode CMOS Image Sensors

This letter presents a simple analytical model for the evaluation of the full well capacity (FWC) of pinned photodiode (PPD) CMOS image sensors depending on the operating conditions and on the pixel parameters. While in the literature and technical documentations FWC values are generally presented as fixed values independent of the operating conditions, this letter demonstrates that the PPD charge handling capability is strongly dependent on the photon flu

Radiation damages in CMOS image sensors: testing and hardening challenges brought by deep sub-micrometer CIS processes

This paper presents a summary of the main results we observed after several years of study on irradiated custom imagers manufactured using 0,18 µm CMOS processes dedicated to imaging. These results are compared to irradiated commercial sensor test results provided by the Jet Propulsion Laboratory to enlighten the differences between standard and pinned photodiode behaviors. Several types of energetic particles have been used (gamma rays, X-rays, protons and neutrons) to irradiate the studied devices. Both total ionizing dose (TID) and displacement damage effects are reported. The most sensitive parameter is still the dark current but some quantum eficiency and MOSFET characteristics changes were also observed at higher dose than those of interest for space applications. In all these degradations, the trench isolations play an important role. The consequences on radiation testing for space applications and radiation-hardening-by-design techniques are also discussed

Theoretical evaluation of MTF and charge collection efficiency in CCD and CMOS image sensor

Classical models used to calculate the Modulation Transfer function (MTF) of a solid-state image sensor generally use a sinusoidal type of illumination. The approach, described in this paper, consists in considering a point-source illumination to built a theoretical three dimensional model of the diffusion and the collection of photo-carriers created within the image sensor array. Fourier transform formalism is used for this type of illumination. Solutions allow to evaluate the spatial repartition of the charge density collected in the space charge region, i.e. to get the Pixel Response Function (PRF) formulation. PRF enables to calculate analytically both MTF and crosstalk at every needed wavelengths. The model can take into account a uniformly doped substrate and an epitaxial layer grown on a highly doped substrate. The built-in electric field induced by the EPI/Substrate doping gradient is also taken into account. For these configurations, MTF, charge collection efficiency and crosstalk proportion are calculated. The study is established in the case of photodiode pixel but it can be easily extended to pinned photodiode pixels and photogate pixels

Generic radiation hardened photodiode layouts for deep submicron CMOS image sensor processes

Selected radiation hardened photodiode layouts, manufactured in a deep submicron CMOS Image Sensor technology, are irradiated by 60Co gamma-rays up to 2.2 Mrad(SiO2) and studied in order to identify the most efficient structures and the guidelines (recess distance, bias voltage) to follow to make them work efficiently in such technology. To do so, both photodiode arrays and active pixel sensors are used. After 2.2 Mrad(SiO2), the studied sensors are fully functional and most of the radiation hardened photodiodes exhibit radiation induced dark current values more than one order of magnitude lower than the standard photodiode