Today, both CCD and CMOS sensors can be envisaged for nearly all visible sensors and instruments designed for space needs. Indeed, detectors built with both technologies allow excellent electro-optics (EO) performances to be reached, the selection of the most adequate device being driven by their functional and technological features and limits. The first part of the paper shortly recalls how far CMOS Image Sensors (CIS) EO performances have been improved these last years. The second part reviews the advantages of CMOS technology for space applications, illustrated by examples of CIS developments performed by EADS Astrium and Supaéro/CIMI for current and short term coming space programs.
Solid-state optical sensors are now commonly used in space applications (navigation cameras, astronomy imagers, tracking sensors...). Although the charge-coupled devices are still widely used, the CMOS image sensor (CIS), which performances are continuously improving, is a strong challenger for Guidance, Navigation and Control (GNC) systems. This paper describes a 750x750 pixels CMOS image sensor that has been specially designed and developed for star tracker and tracking sensor applications. Such detector, that is featuring smart architecture enabling very simple and powerful operations, is built using the AMIS 0.5μm CMOS technology. It contains 750x750 rectangular pixels with 20μm pitch. The geometry of the pixel sensitive zone is optimized for applications based on centroiding measurements. The main feature of this device is the on-chip control and timing function that makes the device operation easier by drastically reducing the number of clocks to be applied. This powerful function allows the user to operate the sensor with high flexibility: measurement of dark level from masked lines, direct access to the windows of interest… A temperature probe is also integrated within the CMOS chip allowing a very precise measurement through the video stream. A complete electro-optical characterization of the sensor has been performed. The major parameters have been evaluated: dark current and its uniformity, read-out noise, conversion gain, Fixed Pattern Noise, Photo Response Non Uniformity, quantum efficiency, Modulation Transfer Function, intra-pixel scanning. The characterization tests are detailed in the paper. Co60 and protons irradiation tests have been also carried out on the image sensor and the results are presented. The specific features of the 750x750 image sensor such as low power CMOS design (3.3V, power consumption<100mW), natural windowing (that allows efficient and robust tracking algorithms), simple proximity electronics (because of the on-chip control and timing function) enabling a high flexibility architecture, make this imager a good candidate for high performance tracking applications.
Today, CCD and CMOS image sensors have found many applications in general public domains. However
their use for scientific and space applications requires high electro optical performances and strong abilities to
predict them prior to the image sensors design and conception. Sensitivity and image quality are two important
electro-optical characteristics for an image sensor. The Quantum Efficiency (QE) and the Modulation Transfer
Function (MTF) are respectively the common metrics used to quantify them. Because of an important number of
parameters influencing the MTF and the QE, their analytical calculation is not an easy task. This paper describes
an analytical model of MTF and QE of CCD and CMOS image sensors. The model has been developed in order
to take into account a maximum number of parameters: pixel size, photosensitive area size and shape, EPI-layer
and substrate doping concentration, junction depth. The effect of top layer oxide stacks on the resulting optical
transmission coefficient and so on QE can also be taken into account. The study is established in the case of
CMOS photodiode pixels and buried channel CCD pixels. The MTF and QE modeling results are compared
with experimental results. MTF and QE measurements are realized on different pixels types having different
photosensitive area shapes and using different technologies. A part of these measurements are performed on a
frontside-illuminated CMOS sensor and on a thinned backside-illuminated CMOS image sensor, both of them are
manufactured using CMOS technology dedicated to image sensors. The other part of MTF and QE measurements
are performed on thinned backside-illuminated N-buried channel CCD sensor. Finally the MTF and QE models
are used to make performance predictions, and the effects of various parameters on the MTF and the QE are
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 efficiency 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.
Sensitivity and image quality are two of the most important characteristics for all image sensing systems. The
Quantum Efficiency (QE) and the Modulation Transfer Function (MTF) are respectively the common metrics
used to quantify them, but inter-pixel crosstalk analysis is also of interest. Because of an important number of
parameters influencing MTF, its analytical calculation and crosstalk predetermination are not an easy task for
an image sensor, particularly in the case of CMOS Image Sensor (CIS). Classical models used to calculate the
MTF of an image sensor generally solve the steady-state continuity equation in the case of a sinusoidal type of
illumination to determine the MTF value by a contrast calculation. One of the major drawbacks of this approach
is the difficulty to evaluate analytically the crosstalk. This paper describes a new theoretical three-dimensional
model of the diffusion and the collection of photo-carriers created by a point-source illumination. The model can
take into account lightly-doped EPI layers which are grown on highly-doped substrates. It allows us to evaluate
with accuracy the crosstalk distribution, the quantum efficiency and the MTF at every needed wavelengths. This
model is compared with QE, MTF measurements realized on different pixel types.
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.
Nowadays, CMOS image sensors are widely considered for space applications. Their performances have been
significantly enhanced with the use of CIS (CMOS Image Sensor) processes in term of dark current, quantum efficiency
and conversion gain. Dynamic Range (DR) remains an important parameter for a lot of applications. Most of the
dynamic range limitation of CMOS image sensors comes from the pixel. During work performed in collaboration with
EADS Astrium, SUPAERO/CIMI laboratory has studied different ways to improve dynamic range and test structures
have been developed to perform analysis and characterisation. A first way to improve dynamic range will be described,
consisting in improving the voltage swing at the pixel output. Test vehicles and process modifications made to improve
voltage swing will be depicted. We have demonstrated a voltage swing improvement more than 30%. A second way to
improve dynamic range is to reduce readout noise A new readout architecture has been developed to perform a
correlated double sampling readout. Strong readout noise reduction will be demonstrated by measurements performed on
our test vehicle. A third way to improve dynamic range is to control conversion gain value. Indeed, in 3 TMOS pixel
structure, dynamic range is related to conversion gain through reset noise which is dependant of photodiode capacitance.
Decrease and increase of conversion gain have been performed with different design techniques. A good control of the
conversion gain will be demonstrated with variation in the range of 0.05 to 3 of initial conversion gain.
Today, both CCD and CMOS sensors can be envisaged for nearly all visible sensors and instruments designed for space needs. Indeed, detectors built with both technologies allow excellent electro-optics performances to be reached, the selection of the most adequate device being driven by their functional and technological features and limits. The first part of the paper presents electro-optics characterisation results of CMOS Image Sensors (CIS) built with an optimised CMOS process, demonstrating the large improvements of CIS electro-optics performances. The second part reviews the advantages of CMOS technology for space applications, illustrated by examples of CIS developments performed by EADS Astrium and Supaero/CIMI for current and short term coming space programs.
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.
The Modulation Transfer Function is a common metric used to quantify image quality but inter-pixel crosstalk analysis is also of interest. Because of an important number of parameters influencing MTF, its analytical calculation and crosstalk predetermination are not an easy task for a CMOS image sensor, due to the use of several metal line and transistor in a close proximity of the photodetector.
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. In order to evaluate the contribution of pixel elementary patterns (particularly the in-pixel readout circuitry), several kernels of shielded pixels have been implemented with the central pixel locally unmasked. Analyze of the kernel responses provides a good insight on both Quantum Efficiency and crosstalk contributors. Additionally, the top metal layer has been used to implement metal edge pattern allowing the on-chip measurement of Edge Spread Function so the MTF.
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 and draw conclusions that can be applied at the design level.
Imaging detectors are key elements for optical instruments and sensors on board space missions dedicated to Earth observation (high resolution imaging, atmosphere spectroscopy...), Solar System exploration (micro cameras, guidance for autonomous vehicle...) and Universe observation (space telescope focal planes, guiding sensors...). This market has been dominated by CCD technology for long. Since the mid-90s, CMOS Image Sensors (CIS) have been competing with CCDs for consumer domains (webcams, cell phones, digital cameras...). Featuring significant advantages over CCD sensors for space applications (lower power consumption, smaller system size, better radiations behaviour...), CMOS technology is also expanding in this field, justifying specific R&D and development programs funded by national and European space agencies (mainly CNES, DGA and ESA). All along the 90s and thanks to their increasingly improving performances, CIS have started to be successfully used for more and more demanding space applications, from vision and control functions requiring low-level performances to guidance applications requiring medium-level performances. Recent technology improvements have made possible the manufacturing of research-grade CIS that are able to compete with CCDs in the high-performances arena. After an introduction outlining the growing interest of optical instruments designers for CMOS image sensors, this paper will present the existing and foreseen ways to reach high-level electro-optics performances for CIS. The developments and performances of CIS prototypes built using an imaging CMOS process will be presented in the corresponding section.
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.
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 0.25 μm process, using this method and sine target direct measurements.