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
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.
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