Photodetector and photoelectric image sensor with high dynamic range

Abstract

The invention relates to a photodetector element which comprises a photodiode (PPD), a linked broadcasting node (FD), a controlled recovery switch (RST) able to impose a voltage on the broadcasting node, and a charge transfer switch (TX) able to transfer the charge accumulated in the photodiode to the linked broadcasting node. A control device controls the switches to generate a response of the photodetection element, which response has a linear phase (LIN) followed by a logarithmic phase (LOG). The control device generates two exposures of the photodiode with two different operating parameters, and a computing device that receives the corresponding signals generates a logarithmic unified response signal taking into account a gradient (K1; K2) of the response as well as an offset (OS1; OS2) between the signals.

Description

[0001] HIGH DYNAMIC RANGE PHOTOELECTRIC PHOTODETECTOR AND IMAGE SENSOR

[0002] Field of invention

[0003] The present invention relates generally to photoelectric image sensors with a pixel matrix.

[0004] Background of the invention

[0005] Image sensors, historically developed for shooting (photography, cinematography, etc.), are becoming essential components in intelligent systems such as autonomous cars, mobile robots, etc.

[0006] In such applications, the image captured by an image sensor is analyzed by software often based on neural networks. The reliability of these analyses depends largely on the quality and properties of the image provided by the sensor. In this case, the desired quality and properties of the image are very different from those desired for photography or cinematography, where the interpretation is done by the human vision system.

[0007] Fundamentally, for neuromimetic recognition systems, small variations in the quality and characteristics of the image are essential. Indeed, this recognition process is essentially a comparison between the input image and images "seen" by the same system previously.

[0008] It is well known that an image sensor with a logarithmic response can contribute to such small variations, as evidenced by all vision systems of living beings. But the realization of an image sensor with a logarithmic response capable of operating with a performance similar to that of vision systems of living beings is difficult. The realization of a logarithmic sensor is necessarily based on a physical phenomenon that is in itself exponential, because a simple mathematical logarithmic conversion of a conventionally taken image with a linear response does not allow to cover the desired operating dynamics for a logarithmic sensor.

[0009] Fig. 1 illustrates a logarithmic response photodetector based on a nonlinear conversion element.

[0010] The simplest implementation of such a photodetector is based on a non-linear resistor that transforms a current of photoelectric origin in a photodiode into a voltage with a logarithmic response. This non-linear resistor can be either a PN diode or a transistor connected as a diode. Despite its simplicity, this photodetector structure has many critical flaws, in particular:

[0011] - a large disparity in response from one photodetector to another, generating very strong fixed spatial noise,

[0012] - loss of sensitivity in low light due to leakage current in the non-linear resistor,

[0013] - a strong variability of the response with temperature variations, and

[0014] - strong afterglow in low light levels.

[0015] This is why such a basic approach has not had industrial success.

[0016] A second approach is based on the use of a photodiode in solar cell mode, where the open-circuit voltage on a photodiode is taken as the image signal. This solution significantly reduces the fixed spatial noise. However, an anti-blooming structure is required to maintain image resolution. Under these conditions, photocharge collection is greatly reduced. A reset operation is required to compensate for the dispersion of pixel responses and eliminate afterglow. The sensitivity of this type of photodetector is very low compared to that of conventional photodetectors. Patent EP1354360B1 describes such a solution.

[0017] A third approach is to create a photoreceptor that initially operates in charge accumulation (linear mode) and then sees its response evolve into a logarithmic mode. Operating the photodetector initially in accumulation mode allows a readout of the photodiode in charge mode. A readout in charge mode allows a correlated double readout to be performed, which eliminates switching noise during readout. All modern conventional photodetectors use this type of operation. In a charge-coupled photodetector, switching noise can be almost completely eliminated, the only residual noise being the base noise in the readout transistor.For example, the read noise in a modern charge-coupled photodetector is typically a few electrons (the best performing can go down to % electron) and the read noise in a photodetector with a logarithmic response is a few tens or even hundreds of electrons. Documents US9413991 B2, US9843750B2 and US20150281607A1 give examples of photodetectors with a mixed linear / logarithmic response. This mixed linear / logarithmic solution is very interesting in that it allows a considerable improvement in sensitivity at low light levels and a conservation of logarithmic operation in strong light.

[0018] But a major drawback is that the transition point between linear and logarithmic mode is very sensitive to certain physical parameters of components; in particular, it changes significantly depending on the temperature.

[0019] There have been many attempts to correct this non-uniformity, but none have been satisfactory.

[0020] Summary of the invention

[0021] The present invention aims to propose a solution that makes it possible to make a photodetector with LIN-LOG response free of fixed spatial noise while retaining the advantages in low light level and high light level, to thus obtain a photodetector, and more generally a pixel matrix sensor, which has a large dynamic range and a low noise level, this type of sensor being particularly expected nowadays, in particular for intelligent systems based on artificial vision. This solution can be declined in several implementations, with one or two photodetectors and with different control parameters.

[0022] According to a first aspect, a photodetector element is thus proposed, intended in particular to form a matrix image sensor, comprising:

[0023] - a buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity,

[0024] - a diffusion node (FD) linked to the photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage,

[0025] - a controlled recovery switch (RST) capable of imposing a voltage on the broadcast node,

[0026] - a charge transfer switch (TX) capable of transferring the charge accumulated in the photodiode to the linked diffusion node, and

[0027] - a photodiode exposure control device capable of controlling the reset switch and the charge transfer switch to generate a response signal from the photodetection element, this response signal intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, element characterized in that the control device is capable of causing two successive exposures of the photodiode with two different exposure durations, and in that it further comprises a calculation device receiving as input the two response signals corresponding to the two exposures and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals:

[0028] - by determining a ratio (R) between the values ​​of the two signals, and, depending on the value of this ratio: - by forming an output signal (FS) by converting the signal value obtained with the shortest exposure time into its logarithm, by applying a pre-established slope coefficient (K1), or

[0029] - by forming an output signal (FS) by applying a pre-established offset (OS1) to the signal value obtained with the longest exposure time.

[0030] According to a second aspect, a photodetector element is proposed intended in particular to form a matrix image sensor, characterized in that it comprises:

[0031] - a first buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity,

[0032] - a second buried photodiode (PPD) exposed to the same radiation source and having an accumulation capacity,

[0033] - at least one diffusion node (FD) linked to each photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage,

[0034] - at least one controlled restoration switch (RST) capable of imposing a voltage on the broadcast node,

[0035] - two charge transfer switches (TX) capable of transferring the charge accumulated in the two photodiodes to the diffusion node, and

[0036] - a photodiode exposure control device capable of controlling the recovery switches and the charge transfer switches with different exposure durations, to generate two different response signals from the two photodiodes, these response signals intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, the exposures of the two photodiodes having at least partial temporal overlap, the photodetector further comprising a calculation device receiving as input the two response signals obtained with the two photodiodes respectively and capable of generating a unified logarithmic response signal over the entire response range from a combination of the two response signals:

[0037] - by determining a ratio (R) between the values ​​of the two signals, and, depending on the value of this ratio:

[0038] - by forming an output signal (FS) by converting the signal value obtained with the shortest exposure time into its logarithm, by applying a pre-established slope coefficient (K1), or

[0039] - by forming an output signal (FS) by applying a pre-established offset (OS1) to the signal value obtained with the longest exposure time.

[0040] An optional aspect of the two photodetector elements presented above includes the following additional feature:

[0041] * the exposure periods defining the two exposure durations end substantially at the same time for each capture. According to a third aspect, a photodetector element is proposed, intended in particular to form a matrix image sensor, comprising:

[0042] - a buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity,

[0043] - a diffusion node (FD) linked to the photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage,

[0044] - a controlled recovery switch (RST) capable of imposing a voltage on the broadcast node,

[0045] - a charge transfer switch (TX) capable of transferring the charge accumulated in the photodiode to the linked diffusion node, and

[0046] - a photodiode exposure control device capable of controlling the reset switch and the charge transfer switch to generate a response signal from the photodetection element, this response signal intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, element characterized in that the control device is capable of causing two successive exposures of the photodiode with two different accumulation capacity values, and in that it further comprises a calculation device receiving as input the two response signals corresponding to the two exposures and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals by determining a difference (DS) between the values ​​of the two signals, and,depending on the value of this difference:,

[0047] - by forming an output signal (FS) by converting the signal value obtained with the largest accumulation capacity into its logarithm, by applying a pre-established slope coefficient (K2), or

[0048] - by forming an output signal by applying a pre-established offset (OS2) to the signal value obtained with the smallest accumulation capacity.

[0049] According to a fourth aspect, a photodetector element is proposed, intended in particular to form a matrix image sensor, characterized in that it comprises:

[0050] - a first buried photodiode (PPD) exposed to a radiation source and having a first accumulation capacity value,

[0051] - a second buried photodiode (PPD) exposed to the same radiation source and having a second accumulation capacity value different from the first,

[0052] - at least one diffusion node (FD) linked to each photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage,

[0053] - at least one controlled restoration switch (RST) capable of imposing a voltage on the broadcast node,

[0054] - two charge transfer switches (TX) capable of transferring the charge accumulated in the two photodiodes to the diffusion node, and - a photodiode exposure control device capable of controlling the recovery switches and the charge transfer switches to generate two different response signals from the two photodiodes, these response signals intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, the exposures of the two photodiodes having at least partial temporal overlap,the photodetector further comprising a calculating device receiving as input the two response signals obtained with the two photodiodes respectively and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals by determining a difference (DS) between the values ​​of the two signals, and, depending on the value of this difference:,

[0055] - by forming an output signal (FS) by converting the signal value obtained with the largest accumulation capacity into its logarithm, by applying a pre-established slope coefficient (K2), or

[0056] - by forming an output signal by applying a pre-established offset (OS2) to the signal value obtained with the smallest accumulation capacity.

[0057] The two elements above optionally have the following additional characteristics, taken individually or in any technically compatible combinations between them:

[0058] * the different accumulation capacity values ​​are fixed and obtained by construction;

[0059] * different accumulation capacity values ​​are obtained by variable polarization at the charge transfer switch(es) (TX).

[0060] According to another aspect, a pixel matrix photoelectric sensor is proposed, characterized in that each pixel comprises a photodetector element as defined above, and in that it comprises a common digital processing device forming the calculation devices respectively associated with the different pixels.

[0061] Brief description of the drawings

[0062] Other aspects, aims and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof, given by way of non-limiting example and with reference to the accompanying drawings. In the drawings:

[0063] Figure 1 illustrates three versions of a photodetector with a logarithmic response of known type, Figure 2 illustrates in its left part a photodetector with a mixed linear / logarithmic response and in its right part a timing diagram of the signals used in such a photodetector,

[0064] Figure 3A illustrates in the upper part the physical structure of a conventional charge-coupled photodetector and in its lower part the structure of the energy bands in such a photodetector,

[0065] Figure 3B is a timing diagram of the signals involved in the photodetector of Figure 3A, Figure 4A illustrates the energy bands in two operating modes of such a photodetector,

[0066] Figure 4B is a timing diagram of the signals involved in the photodetector of Figure 4A,

[0067] Figure 4C shows the different responses of the photodetector of Figure 4A as a function of its operating regime,

[0068] Figure 5 shows the variation in response of this same photodetector by varying the exposure time and the height of an energy barrier under the grid,

[0069] Figure 6 shows in its upper part the response of this same photodetector with two different exposure durations and shows in its lower part the evolution of the ratio between the signals respectively obtained,

[0070] Figure 7 shows in its upper part the response of this same photodetector with two levels of maximum potential well and shows in its lower part the evolution of the difference between the signals respectively obtained,

[0071] Figure 8 shows in its upper part the responses obtained with two different exposure durations and in its lower part the shape of the corrected response obtained by processing the output signals,

[0072] Figure 9 shows in its upper part the responses obtained with two different maximum potential well levels and in its lower part the shape of the corrected response obtained by processing the output signals, and

[0073] Figure 10 schematically illustrates two modes of obtaining the two different signals in the photodetector.

[0074] Detailed Description of Preferred Embodiments

[0075] We will first recall the structure and behavior of a linear / logarithmic or LIN / LOG photodetector.

[0076] As mentioned above, the logarithmic response in the photodetector is created by controlled leakage of the photoelectric charge. For example, in a photodetector with a converter diode, the charge leakage is achieved by the conduction of the PN junction of the converter diode. If in this case, it is possible to precharge the signal node voltage to a voltage that initially blocks the conduction of the PN junction, so that there will be no charge leakage at the beginning of the exposure. The photon-derived charge accumulates and the signal node voltage decreases linearly. The evolution of the signal node voltage SIG is related to the light intensity by a linear law, before the converter diode is unblocked and the evolution of the SIG voltage gradually becomes logarithmic as a function of the light intensity. Figure.2 illustrates such a LIN / LOG photodetector where the cathode of a PD photodiode is connected on the one hand to a reference voltage VREF via a reset transistor MRST and on the other hand to a voltage VDD via an NMOS conversion transistor LOG.

[0077] It is understood that if we take the initial voltage of the output signal SIG just after the cut-off of the reset transistor MRST, and we subtract it from the voltage taken at the end of the exposure, we can eliminate the switching noise introduced by the transistor MRST.

[0078] RECTIFIED SHEET (RULE 91) ISA / EP In this basic version of a LIN-LOG photodetector, it can be seen that: a) at low light levels, the photodetector operates in the linear regime and the switching noise of the MRST transistor is compensated by determining the difference between two readings taken at the beginning and end of the exposure, as just indicated; here, very good sensitivity is obtained; b) the transition between the LIN-LOG regimes depends not only on the light intensity but also on the exposure time or exposure time: the longer the exposure time, the lower the light intensity at the transition point; c) the transition point between the two regimes is strongly dependent on both the type of conversion element (a PN diode, a MOSFET junction, etc.) and the disturbance that the MRST transistor introduces at the end of the reset.

[0079] In such a photodetector, good image quality is obtained in the linear regime thanks to double reading and differentiation. But in the logarithmic regime, the image quality is rather poor because of the dispersion and instability of the transition point between the two regimes. In addition, an improvement in sensitivity is obtained, but the suppression of the reset noise requires a reading at the beginning of the exposure and another at the end of the exposure. This not only requires an image memory, but also the relatively long time between the two readings leads to signal degradation due to low-frequency noise in the transistors. Finally, the dark current is very high because of the necessary contact on the photodiode. These considerations indicate that it would be preferable to use a charge-coupled photodetector.

[0080] Figure 3A illustrates the microelectronic structure of a conventional charge-coupled photodetector and, in the lower part, the representation of the energy levels. The photoelectric charge accumulates in a buried PPD photodiode whose surface of the N-doped region denoted NPD is protected by a P+ doping connected to the substrate SUB. The transfer transistor TX and the reset transistor RST are schematically illustrated. The PW zones designate P-doped wells.

[0081] This configuration ensures a very low dark current and thus creates the possibility of a complete desertion of the N zone. A transfer transistor TX connects this buried photodiode PPD to a diffusion node FD. A reset transistor RST is connected between a voltage source VDD and the FD node. To read the photoelectric charge of the buried photodiode PPD, the FD node is first reset via the transistor RST. Then the gate of the transistor TX is biased to a high voltage and a conduction channel is thus formed under this transistor TX to transfer the charge from the photodiode PPD to the FD node.

[0082] The physical parameters of the PPD photodiode and the TX transfer transistor are adjusted so that the entire photonic charge is transferred to the FD node at the end of the TX transistor action. It is understood that this reading operation also empties the PPD photodiode, which is ready for another acquisition cycle.

[0083] With particular reference to the timing diagram in Figure 3B, two voltage signals are taken, one after the action of transistor RST and the other after the action of transistor TX. The difference between these two voltages gives the output signal. This operation is commonly called "Correlated Double Sampling (CDS)". Thanks to this differentiation and the very short time interval between the two readings, the reset noise can be almost completely eliminated.

[0084] If we now introduce a charge leakage path between the PPD photodiode and a charge drain during exposure, a hybrid LIN-LOG response can be obtained for this charge-coupled photodetector. It is known that charge can naturally overflow from the photodiode during exposure in a charge-coupled photodetector. Therefore, special attention must be paid to the presence of a charge drain to prevent excess charge from propagating to neighboring photodetectors in the case of a matrix sensor, otherwise the image quality will be greatly degraded. For this reason, a charge-coupled photodetector with a LIN-LOG response has a special arrangement for evacuating excess charge in the PPD diode.

[0085] A simple implementation to drain excess charge of photonic origin is to lower the energy level barrier under the gate of the TX transistor so that it is lower than that of the substrate. In this case, the excess charge takes this path to the FD node instead of spilling into the substrate and contaminating the neighboring pixels. The RST transistor must then remain on.

[0086] As illustrated in Figure 4A, when the accumulated charge level reaches the energy barrier under the TX gate, the charge spills over to the FD node and is efficiently drained to a voltage source via the RST transistor.

[0087] The operating timing diagram, shown in Figure 4B, is very similar to that of a conventional charge-coupled photodetector. The essential difference is that the reset signal RST must be maintained for the exposure time of the photodetector as illustrated in the timing diagram.

[0088] It can be physically demonstrated that the amount of residual charge in the buried PPD photodiode during this overflow is proportional to the logarithm of the light intensity reaching it. The article "QLog Solar-Cell Mode Photodiode Logarithmic CMOS Pixel Using Charge Compression and Readout", Yang Ni, Sensors 2018, 18, 584, gives a detailed explanation of this operation.

[0089] Figure 4C shows the shape of the LIN-LOG response of the photodetector of Figure 4A as a function of photon intensity.

[0090] The LIN-LOG transition point can be adjusted either by the voltage applied to the TX transfer transistor during exposure or by the exposure time. Figure 5 shows in its left part the evolution of the LIN-LOG response as a function of the barrier height and the exposure time.

[0091] It is well known that the threshold voltage of a MOS transistor strongly depends on the interface state between the gate and the channel in the substrate. This interface state is unstable and has a very large dispersion. Therefore, the transition point of a LIN-LOG charge-coupled photodetector is very dispersed from one photodetector to another. An image sensor composed of this type of photodetector for each pixel therefore suffers from a very strong fixed spatial noise, it being noted that many efforts have been devoted to correcting this fixed spatial noise, without however obtaining a simple and stable solution.

[0092] If we analyze the response of a photodetector with LIN-LOG response, whatever its concrete implementation, this response is composed of 3 zones, as illustrated in figure 4C where the abscissa scale is a logarithmic scale: a first linear zone (LIN), an intermediate zone (X) then a logarithmic zone (LOG).

[0093] As explained, the initial LIN part of the response is very accurate because the voltage conversion is based on a count of the number of electrons of photonic origin accumulated in the photodiode, and is carried out on the intrinsic capacitance of the FD node. This process is very stable and relatively accurate. The variation of the capacitance of the floating diffusion node FD is easily corrected by a gain, and the dark thermal load can be corrected by a simple offset voltage. In general, a photodetector with LIN-LOG response realized in CMOS technology requires almost no correction.

[0094] In the following discussion, it is assumed that the linear response of the LIN-LOG photodetector is accurate, either intrinsically or after correction.

[0095] It can be seen that the LOG part of the response does indeed follow a logarithmic distribution because this process is governed by thermodynamic laws that electrons follow. The dispersion is found in the offset value and in the logarithmic slope value. The slope of a LOG response is directly related to the absolute temperature, and it can also be modulated by the readout gain in a particular design. It can be measured and its correction is easy, for example by measuring the response at two different light levels to determine this slope.

[0096] In the following discussion, we will assume that the slope of the LOG response of a LIN-LOG responding photodetector is accurate either intrinsically or after correction.

[0097] The problem lies in the intermediate region X between the LIN and LOG regions, because this transition region is difficult to model and varies with the physical parameters of the components and the operating temperature. The signal from a LIN-LOG photodetector can be located in one of these regions with a completely different fixed spatial noise: very weak fixed spatial noise in the LIN region, and very strong in the X or LOG region. Therefore, this region cannot be identified via a simple response from this photodetector.

[0098] We will now describe a solution according to the present invention for:

[0099] - determine the intermediate zone X of a photodetector with LIN-LOG response,

[0100] - correct the offset of the LOG response and obtain a unified logarithmic response over the entire operating range of a photodetector with LIN-LOG response.

[0101] It should be noted that these operations are performed by a digital processing circuit associated with the photodetector and receiving its output signals, or in the case of an implementation in the form of a pixel matrix image sensor, associated with all the pixels.

[0102] We will first describe the process of determining the intermediate zone X. We know that the transition point between LIN and LOG responses can be influenced by different physical or functional parameters. For example, a long exposure time advances the transition point, while a short exposure time pushes the transition point back. A larger accumulation capacity pushes the transition point back, and a smaller accumulation capacity pushes the transition point forward.

[0103] We recall here that the LIN response is precise and stable in absolute value, that the LOG response is precise and stable in slope, and that between the LIN response and the LOG response, there is an unstable and non-uniform intermediate response, which is difficult to calibrate.

[0104] In order to correct the dispersion of the LIN-LOG response, it is necessary to know the exact position of the LIN-LOG transition zone, however if we only look at the value of the signal from a photodetector with LIN-LOG response, we cannot know this position.

[0105] A first approach to determine the transition zone is to operate the same LIN-LOG response photodetector with two different exposure times T1 and T2. We denote by R the ratio between the two exposure times:

[0106] R = T1 / T2 and T1 and T2 are chosen so that

[0107] R > 1

[0108] We define SIG1 and SIG2 as the signals obtained with exposure times T1 and T2, respectively.

[0109] As illustrated in Figure 6:

[0110] - when the report:

[0111] RS = SIG1 / SIG2 is equal to R, this means that SIG1 and SIG2 are both in the LIN area,

[0112] - when this same ratio is equal to 1, this means that SIG1 and SIG2 are all in the LOG zone,

[0113] - when it is determined that RS is between 1 and R, this means that SIG1 or SIG2 is in the intermediate zone X.

[0114] A second approach to determine the transition zone is to modulate the maximum accumulation capacity FW (for "Full Well" in English) of the LIN-LOG response photodetector. A first accumulation capacity value FW1 is used to obtain a first response signal SIG1 and a second accumulation capacity value FW2 is used to obtain a second response signal SIG2, and it is assumed that the light intensity is stationary during the two exposures.

[0115] When the SIG1 and SiG2 signals are in the LIN zone, the response is not impacted by the variation of the accumulation capacitance, since the voltage is proportional to the number of photons arriving at the photodiode.

[0116] On the other hand, if the light intensity is such that the SIG1 and SIG2 signals are in the LOG zone, we observe a relatively constant difference D between SIG1 and SIG2. By analyzing the difference:

[0117] DS = SIG1 - SIG2 it is possible to determine the operating area of ​​SIG1 and SIG2 as shown in Figure 7.

[0118] First, the first approach based on exposure times and the ratio between signals is used to determine the operating area of ​​SIG1 and SIG2.

[0119] Thus, when the RS ratio is between R-TH1 and 1 +TH2 (TH1 and TH2 being tolerance coefficients determined by experiment), this means that SIG1 (which corresponds to the longest exposure time) has entered the LOG regime while SIG2 is still in the LIN regime. Recall that SIG2 then presents a precise and stable response while SIG1, located in the LOG zone, suffers from a very dispersed shift. Typically, the TH1 and TH2 coefficients are approximately equal to Rx0.2 and Rx0.8, respectively.

[0120] Referring now to Figure 8, according to the invention, the signal SIG2 is transformed into its logarithmic value LSIG2 weighted with the slope of the logarithmic response of SIG1, i.e.: LSIG2 = K1*log(SIG2)

[0121] Always when the RS ratio is between R-TH1 and 1 +TH2, the value of SIG1 must be equal to LSIG2. In this case, the photodetector has two equal measurements, SIG1 and LSIG2, for the same light intensity (which is assumed here to be constant between the two exposures).

[0122] LSIG2 being an accurate and stable response, because it is obtained when the photodetector is still operating in the LIN area, it is therefore possible to obtain an OS1 correction offset value which allows SIG1 to be aligned with LSIG2:

[0123] OS1 = LSIG2 - SIG1

[0124] This OS1 parameter can be calculated either during a calibration phase or each time that, for a given exposure, the RS value falls in the interval between R-TH1 and 1 +TH2.

[0125] It is thus possible to generate a unified logarithmic response FS of the photodetector which is determined as follows:

[0126] FS = LSIG2 if RS > 1 +TH3

[0127] FS = SIG1 + OS1 if RS <= 1 +TH3

[0128] The threshold TH3 is preferably substantially equal to R / 2. Figure 8 graphically illustrates this correction and the obtaining of the unified output signal FS.

[0129] Now referring to Figure 9, when using the second approach based on modulation of the FW accumulation well depth, two exposures of the photodetector are made with two different energy levels of the TX barrier.

[0130] The output processing circuit then calculates the difference DS between the two response signals obtained SIG1 and SIG2.

[0131] In case the value of DS is zero or less than a tolerance value TH4 (chosen to cope with possible measurement noise), this means that the values ​​of the SIG1 and SIG2 signals are both in the LIN zone. When this difference DS is greater than TH4, then the values ​​of the SIG1 and SIG2 signals are in the LOG zone.

[0132] It is possible to set two tolerance thresholds TH5 and TH6, and when DS is determined to be within this interval, it means that the value of the SIG1 signal is in the LIN zone while the value of the SIG2 signal is in the LOG zone. The values ​​of TH5 and TH6 are chosen to determine with certainty that SIG1 is indeed in the LIN zone and that SIG2 is indeed in the LOG zone. These values ​​can be determined by simulations and / or by actual measurements. Typically, TH5 = Dx0.2, and TH6 = Dx0.8 can be set.

[0133] The digital processing device then mathematically transforms SIG1 into its logarithm weighted by the logarithmic slope of SIG2 (K2):

[0134] LSIG1 = K2*LOG(SIG1)

[0135] Recall that the LSIG1 value is an accurate and stable response, because it is obtained when the photodetector is still operating in the LIN regime. It constitutes a reference value. Recall also that the value of SIG2 must be corrected so that it coincides with LSIG1 when DS is in the interval [TH5, TH6], the offset value being:

[0136] OS2 = LSIG1 - SIG2

[0137] It therefore becomes possible to generate a unified logarithmic response of the FS pixel, which is calculated as follows:

[0138] FS = LSIG1 if DS < TH7

[0139] FS = SIG2 + OS2 if DS >= TH7

[0140] For the threshold TH7, a value approximately equal to D / 2 is preferably chosen. Figure 9 graphically illustrates this correction and the obtaining of the unified signal FS.

[0141] We will now describe a possible approach for dynamically updating correction parameters.

[0142] As seen above, the main correction parameters are the logarithmic slope K1 or K2 and the offset value OS1 or OS2. The parameter K1 or K2, essentially linked to the temperature and the gain of the reading chain, can either be measured during a calibration phase or extracted from a control circuit integrated into the sensor.

[0143] The offset value OS1 or OS2 is determined from the responses with two different exposure times or from the responses with two different integration capacity values, respectively. The initial offset value can be obtained in a calibration procedure with stable light intensity. Since its value changes in a rather complex manner with temperature and operating conditions, it is advantageous if this value is updated frequently during use of the LIN-LOG response photodetector.

[0144] The processing circuit can be provided to calculate the value of RS (respectively of DS) during use, and each time the calculated value is within a given interval, the value of OS1 (respectively OS2) is recalculated. This continuous update makes it possible to follow changes in the operating conditions of the photodetector and to maintain the effectiveness of the correction. Alternatively, this update can be carried out unconditionally, at regular time intervals.

[0145] It should be noted that in real use, the light intensity varies, and this variation can induce errors between two shots. A continuous and adaptive correction can then be adopted to follow the evolution of the conditions. It can also be planned that the digital processing circuit of the photodetector applies a more intelligent update procedure to:

[0146] - apply a smoothing function on updates,

[0147] - rule out abnormal correction values, which can occur when the light varies greatly between the two exposures.

[0148] An example of an architecture for implementing the invention in the context of a pixel matrix image sensor comprises a substrate where the photoelectric conversion elements and the control transistors are made. A specialized companion processor is attached to this substrate and is programmed to apply the corrections described above to the output signals of each pixel and to generate an image with a unified logarithmic response FS. This image can be directly used by a conventional downstream processing unit.

[0149] We will now describe an approach to optimizing the signal-to-noise ratio.

[0150] It is recalled here that when using the exposure time modulation approach, the unified response at low light level uses the SIG2 response obtained with the shortest exposure time. However, under low light conditions, this solution is not optimal. Under low light conditions, the SIG1 and SIG2 signals are both in the LIN mode, but the SIG2 signal has a lower signal-to-noise ratio than the SIG1 signal, which is generated with a longer exposure time. In order to optimize performance, one can then choose to generate the unified FS response using the response given by the SIG1 signal (a division by the value of R will be applied) instead of SIG2. This allows for better image quality at low light levels. The decision threshold between generating the FS response as described above and as described in this paragraph can be determined by experiment on a real sensor.

[0151] And when the integration capacity modulation approach is implemented, in low light conditions, an average can be performed between the SIG1 and SIG2 signals to obtain a better signal-to-noise ratio. Indeed, in low light conditions, the SIG1 and SIG2 signals theoretically have the same value, and calculating an average of the two values ​​allows a reduction of random noise. As in the previous case, the decision threshold can be determined experimentally.

[0152] We will now describe an implementation of the invention making it possible to ensure a superposition in time of the two exposure phases leading to the generation of the signals SIG1, SIG2.

[0153] We first recall that the approaches for developing a unified logarithmic response as described above are based on two sequential readings of the same photodetector with the different operating parameters, namely:

[0154] - either the variation in exposure time,

[0155] - or the variation in accumulation capacity.

[0156] It should be noted here that it would also be possible to combine these two approaches. This implies an extension of the overall duration of an acquisition, which can sometimes pose problems for applications of matrix sensors equipped with such photodetectors and which must process rapid movements, for example in automobile vision.

[0157] In this case, with reference to Figure 10, a double photodetector is produced according to the invention, i.e. with two photodiodes and two associated control circuits whose shooting parameters are different, either by programming or by construction. Each double photodetector simultaneously or almost simultaneously provides the two signals SIG1 and SIG2 from which the same correction as that described previously is applied to obtain a unified logarithmic response.

[0158] Thus, the left part of Figure 10 illustrates the two sequential approaches based on a single photodetector, one with modulation of the exposure duration (designated by T1 / T2), and the other with modulation of the accumulation capacity (designated by FW1 / FW2).

[0159] The right part of the figure shows the same approaches with two photodetectors for the same pixel, which has the advantage of ensuring at least partial simultaneity of captures and thus shortening the overall acquisition and processing time for an image.

[0160] For example, if the approach based on two different exposure durations is used, the control signals are programmed so that the two exposure phases end at the same time or substantially at the same time. In this case, the SIG1 and SIG2 signals are delivered at the same time, and it is not necessary to provide a buffer memory to store the signal which is delivered first in the case of the embodiment with a single photodetector. If the approach based on modulation of the accumulation capacity is used, this modulation can be implemented in hardware, by providing different physical characteristics for the two photodiodes, for example by using different doping levels.

[0161] Of course, the present invention is in no way limited to the embodiments described and shown in the drawings, but those skilled in the art will be able to make numerous variations or modifications thereto.

Claims

Claims 1. Photodetector element intended in particular to form a matrix image sensor, comprising: - a buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity, - a diffusion node (FD) linked to the photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage, - a controlled recovery switch (RST) capable of imposing a voltage on the broadcast node, - a charge transfer switch (TX) capable of transferring the charge accumulated in the photodiode to the linked diffusion node, and - a photodiode exposure control device capable of controlling the reset switch and the charge transfer switch to generate a response signal from the photodetection element, this response signal intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two element modes, characterized in that the control device is capable of causing two successive exposures of the photodiode with two different exposure durations, and in that it further comprises a calculation device receiving as input the two response signals corresponding to the two exposures and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals: - by determining a ratio (R) between the values ​​of the two signals, and, depending on the value of this ratio: o by forming an output signal (FS) by converting the signal value obtained with the shortest exposure time into its logarithm, by applying a pre-established slope coefficient (K1), or o by forming an output signal (FS) by applying a pre-established offset (OS1) to the signal value obtained with the longest exposure time.

2. Photodetector element intended in particular to form a matrix image sensor, characterized in that it comprises: - a first buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity, - a second buried photodiode (PPD) exposed to the same radiation source and having an accumulation capacity, - at least one diffusion node (FD) linked to each photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage, - at least one controlled restoration switch (RST) capable of imposing a voltage on the broadcast node, - two charge transfer switches (TX) capable of transferring the charge accumulated in the two photodiodes to the diffusion node, and - a photodiode exposure control device capable of controlling the recovery switches and the charge transfer switches with different exposure durations, to generate two different response signals from the two photodiodes, these response signals intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, the exposures of the two photodiodes having at least partial temporal overlap, - the photodetector further comprising a calculation device receiving as input the two response signals obtained with the two photodiodes respectively and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals: - by determining a ratio (R) between the values ​​of the two signals, and, depending on the value of this ratio: o by forming an output signal (FS) by converting the signal value obtained with the shortest exposure time into its logarithm, by applying a pre-established slope coefficient (K1), or o by forming an output signal (FS) by applying a pre-established offset (OS1) to the signal value obtained with the longest exposure time.

3. Element according to claim 2, characterized in that the exposure periods defining the two exposure durations end substantially at the same time for each capture.

4. Photodetector element intended in particular to form a matrix image sensor, comprising: - a buried photodiode (PPD) exposed to a radiation source and having an accumulation capacity, - a diffusion node (FD) linked to the photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage, - a controlled recovery switch (RST) capable of imposing a voltage on the broadcast node, - a charge transfer switch (TX) capable of transferring the charge accumulated in the photodiode to the linked diffusion node, and - a photodiode exposure control device capable of controlling the reset switch and the charge transfer switch to generate a response signal from the photodetection element, this response signal intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, element characterized in that the control device is capable of causing two successive exposures of the photodiode with two different accumulation capacity values, and in that it further comprises a calculation device receiving as input the two response signals corresponding to the two exposures and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals by determining a difference (DS) between the values ​​of the two signals, and, depending on the value of this difference: - by forming an output signal (FS) by converting the signal value obtained with the largest accumulation capacity into its logarithm, by applying a pre-established slope coefficient (K2), or - by forming an output signal by applying a pre-established offset (OS2) to the signal value obtained with the smallest accumulation capacity.

5. Photodetector element intended in particular to form a matrix image sensor, characterized in that it comprises: - a first buried photodiode (PPD) exposed to a radiation source and having a first accumulation capacity value, - a second buried photodiode (PPD) exposed to the same radiation source and having a second accumulation capacity value different from the first, - at least one diffusion node (FD) linked to each photodiode and capable of transforming a charge variation of photoelectric origin in the photodiode into voltage, - at least one controlled restoration switch (RST) capable of imposing a voltage on the broadcast node, - two charge transfer switches (TX) capable of transferring the charge accumulated in the two photodiodes to the diffusion node, and - a photodiode exposure control device capable of controlling the recovery switches and the charge transfer switches to generate two different response signals from the two photodiodes, these response signals intrinsically having an initially linear phase (LIN) followed by a logarithmic phase (LOG), with a potentially indeterminate transition (X) between the two modes, the exposures of the two photodiodes having at least partial temporal overlap, the photodetector further comprising a calculation device receiving as input the two response signals obtained with the two photodiodes respectively and capable of generating a unified logarithmic response signal over the entire range of the response from a combination of the two response signals by determining a difference (DS) between the values ​​of the two signals, and, depending on the value of this difference: - by forming an output signal (FS) by converting the signal value obtained with the largest accumulation capacity into its logarithm, by applying a coefficient of pre-established slope (K2), or - by forming an output signal by applying a pre-established offset (OS2) to the signal value obtained with the smallest accumulation capacity.

6. Element according to claim 5, characterized in that the different accumulation capacity values ​​are fixed and obtained by construction.

7. Element according to claim 4 or 5, characterized in that the different accumulation capacity values ​​are obtained by variable polarization at the charge transfer switch(es) (TX).

8. Photoelectric sensor with pixel matrix, characterized in that each pixel comprises a photodetector element according to one of claims 1 to 7, and in that it comprises a common digital processing device forming the calculation devices respectively associated with the different pixels.

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