WO2012095582A1

WO2012095582A1 - Cmos linear image sensor with motion-blur compensation

Info

Publication number: WO2012095582A1
Application number: PCT/FR2012/000010
Authority: WO
Inventors: Yvon Cazaux
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2011-01-10
Filing date: 2012-01-09
Publication date: 2012-07-19
Also published as: EP2664132A1

Abstract

The invention relates to an image sensor with time delay and integration, including an array of light-sensitive pixels (Px) organized into rows and columns. Each pixel (Px1) of a column includes a light-sensitive element (Do), a storage node (C2o), and a first transfer transistor (TG1o) connecting the light-sensitive element to the storage node. Each pixel (Px1) of a column, except for the last, also includes a second transfer transistor (TG2o) that connects the storage node (C2o) of the pixel to the light-sensitive element (De) of the next pixel (Px2) in the column. Both transfer transistors are connected so as to be operative at the same time. With such a configuration, a sliding group of a plurality of consecutive pixels in a column is defined, the group of pixels is exposed, the information of the group of pixels is aggregated, and the process starts over after the group of pixels is shifted by one pixel.

Description

LINEAR IMAGE SENSOR IN TECHNOLOGY

CMOS A COMPENSATION OF EFFECT OF FILE

Technical field of the invention

The invention relates to a linear image sensor intended to capture a scanning image, in particular to a time delay and integration sensor, better known by the acronym TDI (of the English "Time Delay and Integration").

State of the art

The principles of a TDI image sensor are described, for example, in the article entitled "A Large Area TDI Image Sensor for Low Light Level Imaging", by Michael G. Farrier et al - IEEE Journal of Solid-State Circuits , Flight. SC-15, No. 4, August 1980.

A TDI sensor is typically used to capture the image of an object moving at high speed and observed under poor lighting conditions. It is generally performed in CCD (Charge-Coupled Device) technology, which has so far made it possible to obtain the best performance in terms of sensitivity.

Figure 1 shows schematically a TDI sensor in CCD technology, as described in the aforementioned article. It comprises an array of photosensitive sites, or photosites, the rows of which are generally, as shown, substantially longer than the columns. In the example of the above-mentioned article, a row contains 1028 photosites, while a column has only 128. For terrestrial satellite photography, a row can have around 12000 photosites, and the matrix contains some dozens of rows. The rows of the matrix are arranged perpendicular to the displacement of the object whose image is to be captured. The displacement of this image, relative to the sensor, is represented by falling arrows. These arrows also correspond to the displacement of the electric charges in the CCD registers, in synchronism with the displacement of the image. Each row grabs a corresponding slice of the object during an exposure time consistent with the speed of the image. This causes an accumulation of negative charges (electrons) in the photosites of the row. When a slice of the image inputted by a row i has moved at the level of the row i + 1, the charges accumulated in the row i are transferred in the row i + 1, which continues, during a new time of exposure, to accumulate loads for the same slice. The transfers of charges from one row to the next thus take place in synchronism with the displacement of the image.

The last row of the matrix thus contains, at each transfer cycle, the sum of the charges accumulated by all the rows for the same slice. The sensitivity of the sensor is, in theory, multiplied by the number of rows.

At the end of each charge transfer and exposure cycle, the charges of the last row of the array are transferred to a shift register 12 for reading information from the last row. The charges stored in the photosites of this register are shifted photosites by photosite to a charge-voltage converter 14 at the end of the row, where a voltage corresponding to the total charge of each photosite can be taken by a processing circuit, generally external to the sensor. .

With CCD technology being used less and less for image sensors, in favor of CMOS technology, the use of this latest technology is being considered for TDI sensors.

[Time-Delay-Integration Architectures in CMOS Image Sensors, Gérald Lepage, Jan Bogaerts, and Guy Meynants-IEEE Transactions on Electron Devices, Vol. 56, NO. 11, November 2009] discloses solutions for obtaining TDI functionality using a CMOS image sensor.

In a CMOS image sensor, light is also captured as charges at the pixel level. On the other hand, each pixel being provided with its own voltage reading circuit, it is impossible to transfer charges from one pixel to another.

FIG. 2 schematically represents an architecture envisaged in this article by Lepage et al. A matrix 10 'of NxM pixels Px is associated with a matrix 16 of memory cells, of the same size and configuration (here NxM = 5x5). In principle, the matrix of pixels 10 'takes pictures at a rate corresponding to the time (called "line time" TL) that an image slice sweeps the pitch of the rows of pixels. Thus, after N line time, the same image slice has been entered by each of the N rows of the matrix of pixels. Each row of memory 16 is temporarily associated with the same slice of the image. It accumulates the brightness levels recorded for this slice by all rows of pixels. Once all levels have been accumulated for the slice, the memory array is read, reset, and circularly associated with a new slice of image.

We realize that we must proceed to the accumulation of all the rows of the matrix of pixels at each line time.

While in CCD technology the operations of accumulation of brightness levels correspond to simple transfers of charges, in CMOS technology these operations are notably more complex. They involve multiplexing on pixel read buses, analog-to-digital conversions, addition operations, and memory access operations. As a result, there are difficulties in CMOS technology to maintain the same frame rate (or line time T _L ) as in CCD technology. Thus, the resolution in number of rows of the pixel matrix must be adapted to the minimum expected line time and the desired pixel pitch.

In some applications, as described in particular in the aforementioned article by Lepage et al., It is desired to spatially oversample the image in the direction of displacement in order to improve the so-called modulation transfer function (MTF). of yarn, representative of the sharpness of the reproduced image. This amounts to increasing the number of rows of pixels while decreasing the pitch to maintain the dimensions of the sensor. The temporal constraints thus increase with the square of the subdivision factor.

For certain applications, it would furthermore be desirable for the pixels to have an anti-glare function, making it possible, in very bright conditions, to evacuate excess charges.

Summary of the invention

It is noted that there is a need to provide a TDI type image sensor CMOS technology to improve the spindle MTF, without significantly increasing the time constraints, and to achieve an anti-glare function.

This need is satisfied by providing a temporal delay and integration image sensor comprising a matrix of photosensitive pixels organized in accordance with FIG. rows and columns, each pixel of a column comprising a photosensitive member, a storage node, and a first transfer transistor connecting the photosensitive member to the storage node. Each pixel of a column, except the last one, further comprises a second transfer transistor which connects the storage node of the pixel to the photosensitive member of the next pixel of the column. Both transfer transistors are connected to be active at the same time. Each pixel further comprises a reset transistor connecting the pixel storage node to a supply potential.

The anti-glare function is obtained by controlling this sensor according to the following steps: a) periodically activating the first and second transfer transistors of each pixel to cause charge transfer to the storage node of the pixel; b) reading the potential on the storage node of the pixel after each activation of the first and second transfer transistors; c) in an interval between two successive activations, bring the gate of the transfer transistors to a polarization potential defining an anti-glare threshold of the photosensitive elements; d) conducting the pixel reset transistor during said interval; e) blocking the reset transistor outside said gap.

The preceding steps are implemented in offset of half a period between the pixels of even rank and the pixels of odd rank of a column.

According to one embodiment, a first reading of the potential on the storage node is made between the start of the blocking of the reset transistor and the activation of the pixel transfer transistors; and performing a second reading of the potential on the storage node between the activation of the transfer transistors and the end of the blocking of the reset transistor.

The duration of said interval is preferably between one half-period and one period. Brief description of the drawings

Other advantages and features will emerge more clearly from the following description of particular embodiments given as non-limiting examples and illustrated with the aid of the accompanying drawings, in which: FIG. 1, previously described, represents schematically a conventional TDI image sensor, made in CCD technology; Figure 2, previously described, schematically shows a conventional TDI sensor made in CMOS technology; FIG. 3 diagrammatically represents a TDI sensor embodiment in CMOS technology making it possible to improve the FTM of yarn; Figures 4a and 4b symbolize a sensor of the type of Figure 3 in two measurement phases; Figure 5 is a timing diagram illustrating an operation of a sensor of the type of Figure 3; FIG. 6 represents an embodiment of a TDI image sensor making it possible to relax the temporal constraints further; and FIG. 7 is a timing diagram illustrating an alternative embodiment of a sensor of the type of FIG. 3, providing an anti-glare function.

Description of a preferred embodiment of the invention

In order to increase the FTM of yarn, it is proposed to subdivide each pixel into two (or more), as has already been proposed in the prior art, but there is also provided a particular architecture of pixel matrix limiting the increase of the pixels. temporal constraints to the value of the subdivision factor, instead of increasing them with the square of the subdivision factor. Thus, by subdividing each pixel into two, the time constraints increase by a factor of only 2, instead of 4.

This is achieved by increasing the temporal resolution of the sensor without increasing its spatial resolution. "Temporal resolution" refers to the number of measurements taken per unit of time, while "spatial resolution" refers to the number of measurements taken per unit of distance. The fact increasing the temporal resolution makes it possible to improve the FTM of yarn, while the fact of conserving the spatial resolution makes it possible to limit the resources necessary to process the image. The temporal resolution is increased by subdividing each pixel, in the direction of displacement, into several sub-pixels covering the same area as the pixel. The spatial resolution is maintained by aggregating into a single value the values of a sliding group of consecutive subpixels corresponding to the size of a pixel. This group "slides" at the speed of movement of the image, that is to say it shifts from one pixel to one line time. In order to effectively increase the temporal resolution, a new aggregation occurs each time the group has shifted by one sub-pixel.

FIG. 3 schematically represents the first pixels of a column of a TDI sensor embodiment in CMOS technology making it possible to implement this principle. This sensor is of type "global shutter" ("snapshot" in English), that is to say that all the pixels of the sensor perform integration at the same time.

In the sensor embodiments described hereinafter by way of example, each pixel is subdivided in two in the direction of movement to go from a FTM of yarn from 0.64 to 0.9.

Each pixel Px comprises a photodiode D, whose intrinsic capacitance C1, or integration capacitance, makes it possible to accumulate the charges generated by the light striking the pixel. A transfer transistor TG1 connects the photodiode D1 to the gate of a follower transistor M2. The gate capacitance of the transistor M2 as well as the capacitances of the other components connected to the gate of the transistor M2 form a buffer capacitance C2, or storage node. A read transistor RD connects the source of the follower transistor M2 to a column bus Bc. The read transistors RD of the pixels of a row are controlled by a selection line common to the row. A reset transistor RST connects the capacitor C2 to a positive supply line Vdd.

For the sake of convenience, the control signals of the transistors hereinafter have the same name as the transistors. In addition, the references of certain elements of the odd pixels have a suffix "o", while the references of the same elements of the even pixels have a suffix "e".

This type of pixel, as described up to now, is a conventional "4T" type pixel making it possible to produce a global shutter sensor, that is to say a sensor. sensor to expose all its pixels at the same time and read the pixel levels successively after exposure. The operation is in short the following.

Initially, the transistors TG1, RST and RD are blocked. The capacitor C1 integrates the charges generated by the light striking the photodiode D. Before the end of the exposure, the transistor RST is actuated briefly to reinitialize the buffer capacitance C2. At the end of exposure, the transistor TG is actuated briefly to transfer the charges of the capacitor C1 to the buffer capacitor C2. This transfer is total in the case where the photodiode D is of type "locked" (or "pinned" in English), which results in the resetting of the capacitor C1 for a new phase of exposure.

During each exposure phase, the voltage level corresponding to the previous exposure is stored on the buffer capacity C2. This voltage level can be transferred at any time on the bus Bc by operating the read transistor RD, this before a new reset by the transistor RST.

Each pixel of FIG. 3, with respect to a conventional "4T" pixel, comprises an additional transfer transistor TG2, connecting the capacitance C2 of the pixel to the photodiode D of the next pixel of the column. (The last pixel of the column will be devoid of such an additional transfer transistor.) The transfer transistors TG1 and TG2 of the same pixel are synchronously controlled - their gates receive the same control signal for this purpose. Preferably, the two transfer transistors TG1 o and TG2o of all the odd pixels are synchronously controlled by a single line TGo, while the two transfer transistors TG1e and TG2e of all the even pixels are synchronously controlled by a single line. TGe.

It may be desired that the first pixel Px1 of the column behaves exactly like the other pixels. For this purpose, for example, a reset transistor Trst connected between the photodiode of the pixel Px1 and the line Vdd is provided. This transistor Trst is activated by the control line TGe of the transfer transistors of even pixels. Thus, the capacitance C1 of the pixel Px1, the charge of which is not transferable in a capacitor C2 of a preceding pixel, is reinitialized at the moment when the transfer transistors of the even pixels are controlled. Furthermore, the last pixel of the column is a conventional pixel - it does not include a second transfer transistor TG2, since there is no next pixel to connect this transistor.

FIGS. 4a and 4b show a column of pixels of the type of FIG. 3 with two operating phases of the sensor. The column is parallel to the displacement of the image. The circles represent the capacitances C2 and the triangles the reading circuits (transistors M2 and RD).

The case represented corresponds to the one where the temporal resolution is divided by two. Thus, each pixel represented is in fact a subpixel, and two consecutive subpixels of the column, which will be designated "pixel pair", form a single pixel at the original spatial resolution. As shown, a pair of pixels preferably occupies a square surface, and each pixel is twice as wide as high, so that the original form factor is retained.

Opposite the column, in the form of a vertical bar, there is shown a reference image, or pattern, scrolling from top to bottom. The pattern includes alternating light slices and dark slices at the pitch of pixel pairs, that is, the Nyquist boundary for the original spatial resolution. Each slice is thus the height of a pair of pixels and sweeps the pair of pixels in a line time T _L. In FIG. 4a, the first dark slice of Ja mire has passed in front of the first two pixels P x 1 and P x 2 of the column, while the first clear slice has passed in front of the two following pixels P x 3 and P x 4 of the column. This configuration is repeated along the column.

As represented by arrows, the two transfer transistors of each of the odd pixels are activated, which results in the charges integrated by the photodiodes of the pixels Px1 and Px2, both of which have seen the first dark slice of the test pattern. , are summed in the first capacitance C2, while the charges integrated by the photodiodes of the pixels Px3 and Px4, both having seen the first clear slice of the pattern, are summed in the third capacitor C2. This configuration is repeated along the column, so that each capacitor C2 of odd rank, that is to say each capacitor C2, receives the sum of the charges of the pair of neighboring pixels. The voltage levels of the capacitors C2o will be read in turn on the column bus Bc during the next line half-time, during which a new integration on the photodiodes begins.

In Figure 4b, corresponding to half a line time later, the first dark slice of the pattern shifted one pixel down and scanned the pixels Px2 and Px3, while the first clear slice swept the two following pixels Px4 and Px5. This configuration is repeated along the column.

As represented by arrows, the two transfer transistors of each of the even pixels are activated, whereby the charges integrated by the photodiodes of the pixels Px2 and Px3, both having seen the first dark slice of the target , are summed in the second capacitor C2, while the charges integrated by the photodiodes of the pixels Px4 and Px5, both having seen the first clear slice of the pattern, are summed in the fourth capacitor C2. This pattern is repeated along the column, so that each C2 of even rank, i.e., each C2e, receives the sum of the charges of the pair of neighboring pixels.

The voltage levels of the capacitors C2e will be read in turn on the column bus Bc during the next line half-time. The voltage levels of the odd C2o capacitors will have been read during the current line half-time. We realize that we can thus sample the same slice of image every half-time line, that is to say that we reach a temporal resolution twice as good, using at each sampling a pair of pixels. whose information is aggregated, that is to say that we do not increase the spatial resolution. As a result, as can be seen from FIGS. 4a and 4b, the number of levels to be read on the bus during a line time is proportional to the subdivision factor, instead of being proportional to the square of the factor. subdivision. If the pixel levels were to be read individually on the bus Bc, there would be twice as many readings to be done for each of FIGS. 4a and 4b, ie at each half-time line.

FIG. 5 is a timing diagram illustrating an example of evolution of the main signals relating to the pixels Px1 and Px2 during the phases of FIGS. 4a and 4b, and of subsequent phases. The chronogram is subdivided by vertical mixed lines into periods equal to half a line time. The first signal is representative of the average of the charge states of the capacitances C1 of the pixels Px1 and Px2. It will be noted that a zero charge level of the capacitors C1 and C2 corresponds to a high potential (for example Vdd), whereas a rising charge level, corresponding to a growing number of electrons, evolves decreasing from the potential high.

During the first half-line, corresponding to FIG. 4a, the pixel Px1 sees a transition from the dark slice to a clear slice and the pixel Px2 sees the dark slice during the whole half-time line. The C1 capabilities of the Px1 and Px2 pixels load at low levels. Before the end of the first half-line, the RST signal is activated to reset the C2 capabilities. Such a reset is repeated with a period of one line time or, preferably, half a line time, as shown.

Activation of the RST signal can occur at any time within half a line time. Preferably, it is activated, as shown, towards the middle of each half line time, which will make it possible to carry out a correlated double sampling to compensate for the noise of the black level. Thus, between the activation of the RST signal and the end of the half-line time, the black levels of the capacitors C2 are transferred to the bus Bc, so that these levels can be subtracted from the useful levels transferred to the next phase. At the end of the first half-line, the pairs of transfer transistors TGo of the odd pixels are activated. The capacitance charges C1 of the pixels Px1 and Px2 are transferred and summed in the capacitance C2o of the pixel Px1, the voltage level of which has a corresponding amplitude step (low here).

The transistors TGo are thereafter periodically activated with a period of one line time.

During the second half-line, corresponding to FIG. 4b, the capacitances C1 of the pixels Px2 and Px3 see what the pixels Px1 and Px2 had seen at the half-time preceding line. The C1 capabilities of these pixels (level not shown for the pixel Px3) load at low levels. The pixel Px1 seeing during all this a clear slice, its C1 capacity is loaded at a high level.

At the end of the second half-line, the pairs of transfer transistors TGe are activated with even pixels. The C1 capacitance loads of the Px2 pixels and Px3 are summed in the capacitance C2e of the pixel Px2, whose voltage level has a corresponding amplitude step (again low).

The transistors TGe are thereafter periodically activated with a period of one line time. During the third half-time line, the capacitances C1 of the pixels Px1 and Px2 which see scrolling a clear slice, load at high levels.

At the end of the third line half-time, the pairs of transfer transistors TGo of the odd pixels are activated again. The capacitance charges C1 of the pixels Px1 and Px2 are summed in the capacitance C2o of the pixel Px1, the voltage level of which has a corresponding amplitude step (high this time).

The half-times line follow one another in a similar way. It can be seen that the levels on C2 capacitances show a significant periodic variation with a period of one line time, corresponding to the pitch of the pattern of the pattern.

By thus increasing the temporal resolution by a factor of 2, without affecting the spatial resolution, the FTM of yarn was passed from 0.64 to 0.9.

The principle described is valid for any factor N, although factors greater than 2 do not significantly improve the FTM of yarn (0.955 is obtained for a factor of 3, and 0.975 for a factor of 4).

To increase the temporal resolution by a factor N, which will increase the time constraints by a factor N also (instead of N ² ), each pixel of FIG. 3 comprises a transfer transistor TG1 connecting the photodiode D to the capacitance of C2 storage of the pixel, and N-1 additional transfer transistors (TG2, TG3 ... TGN) connecting the storage capacitor C2 to the respective photodiodes of the following N-1 pixels. These N pixel transfer transistors are activatable at the same time to summon in the storage capacitor C2 of the pixel the charges of the N photodiodes of the N-tuple of pixels thus formed. There is provided a bus of N control lines of the transfer transistors, the line of rank i being activated at a time iT _L / N of each line time, and controlling the transfer transistors of the pixels of ranks i + kN, where k = 0, 1, 2, ... Figure 6 shows a TDI sensor architecture in CMOS technology, offering an optional solution to relieve time constraints by a factor of 2, in the context of a global shutter sensor. In combining this solution with that which has just been described (with N = 2), it is possible to offer a TDI sensor in CMOS technology having an excellent FTM of yarn without having time constraints more difficult to satisfy than for a conventional sensor. With this architecture, it is proposed to divide by two the time required for the operations of accumulation of the brightness levels in memory. Indeed, an important factor in time constraints is the time required to accumulate a current brightness level with a value stored in memory in an architecture of the type of FIG. 2. For each column of pixels, two channels are provided. independent accumulation, each associated with a separate memory, which is used simultaneously or almost simultaneously.

In general, an accumulation memory 16a is associated with a first half of the rows, and a separate accumulation memory 16b is associated with the second half of the rows. It is thus possible to write in the memory 16a a value corresponding to a pixel of the first half at the same time as a value corresponding to a pixel of the second half is written in the memory 16b.

In FIG. 6, the matrix of photosensitive pixels Px is represented with six rows and five columns, by way of example. The accumulation memory 16a is here associated with the rows of the upper half of the pixel matrix, and the accumulation memory 16b is associated with the rows of the lower half of the pixel array. The pixels of the upper half and the pixels of the lower half of each column are connected by separate read buses to their respective memories 16a and 16b. In each of these buses, an ADC analog-to-digital converter is provided. Thus, the analog levels provided by the pixels are converted to digital before being accumulated in memories 16a and 16b.

The accumulation takes place, for example, as is schematically in the memories, using an adder that replaces the contents of a memory cell by the sum of this content and the value provided by the ADC converter corresponding.

The accumulation memories 16a and 16b and the pixels Px are managed by a control circuit 18, the operation of which will be described in more detail later. The contents of the memories 16a and 16b are accessible by respective buses arriving at the two inputs of an adder 20. An adder 20 is provided per column, serving to complete the partial accumulations made in each of the memories 16a and 16b. Each of the storage memories 16a and 16b preferably has the same configuration as the pixel array, i.e., it is in the form of an array of memory cells of six rows by five columns in the array. example shown. In addition, as shown, the memories 16a and 16b are preferably arranged physically on either side of the matrix of pixels, in the direction of the columns. This facilitates the routing of connections.

Compared with the conventional configuration of FIG. 2, the memory size is doubled. This does not have a significant influence on the size of the sensor, since the latter generally has much fewer rows than columns. Thus, a relatively small number of rows of memory are added which have little effect on the width of the sensor relative to the other components of the sensor, in particular the input / output pads.

In some applications, it is desired that the pixels have an anti-glare function, allowing, under conditions of high brightness, to evacuate excess charges towards the supply lines rather than to let them overflow towards neighboring pixels. This function is usually available in conventional five-transistor or "5T" pixels. In such a pixel, the fifth N-type transistor is connected between the photodiode and the supply potential Vdd, and its gate is biased to a fixed potential set so that the transistor goes into conduction as soon as the potential of the photodiode (connected to the transistor source) goes below a glare threshold. This polarization potential, with the current technologies, is slightly positive, of the order of 200 mV.

A simple way of providing an anti-glare function in the sensor of FIG. 3 is to add in each pixel a transistor between the photodiode and the potential Vdd. This nevertheless increases the complexity of the pixels and reduces the size of the photodiode.

It can be seen that the structure of FIG. 3 makes it possible to dispense with the anti-glare transistor by judiciously controlling the transfer transistors TG and the reset transistors RST. Indeed, the anti-glare function is obtained by rendering the transistor RST of the pixel conductive, and by applying simultaneously to the gate of the transistor TG1 of the pixel the bias voltage regulating the glare threshold. The charges in excess of the photodiode can then be discharged to the potential Vdd by the transistor TG1 and the transistor RST.

However, this control of the transistors TG1 and RST of the pixel, at least during the entire period of integration of the photodiode where the anti-glare function is desirable, is not compatible with the control method of the photodiode. the sensor assembly, illustrated in FIG. 5. It can be seen that it is nevertheless possible to provide the anti-glare function permanently by exploiting the fact that the pixels of even rank and the pixels of odd rank are managed offset.

Figure 7 illustrates how.

At the beginning of a first integration phase on the photodiodes of the sensor, the RST transistors of the odd pixels, denoted RSTo, are on. This causes the storage nodes C2o of the odd pixels to remain in the unloaded state. The RST transistors of even pixels, denoted RSTe, are blocked, allowing the transfer of charges from the photodiodes to the even storage nodes C2e, and the reading thereof. At the same time, the gates of the transistors TG1o and TG2o of the odd pixels are at their level of anti-glare, of the order of 200 mV with the current technologies. The transistor TG1o thus provides the anti-glare function for the odd pixel, via the transistor RSTo of the pixel.

It turns out that the transistor TG2o is polarized as the transistor TG1o, since the gates of these two transistors are connected to one another. Thus, this transistor TG2o provides the anti-glare function for the next even pixel, also via the transistor RSTo. Transistors TG1e and TG2e of this next even pixel are blocked, and can not in any case serve as anti-glare transistors, because the transistor RSTe of this pixel is blocked. On the other hand, the storage node C2e can receive the charges of the photodiode De / C1e and be read via the read transistor RD of the pixel.

Thus, in this configuration where only the odd pixels are monopolized by their active reset transistors, the anti-glare function is nevertheless provided for all the pixels without adding an additional component. In the vicinity of the middle of the integration period, the even RSTe transistors are turned on while the odd RSTo transistors are off. The odd transistors TGo (TG1o and TG2o) are off, while the transistors TGe (TG1e and TG2e) are in turn polarized to provide the anti-glare function.

Preferably, as shown on an expanded scale, the transistors RST0 and RSTe have a short common conduction phase (between 0.1 and 2 με), inside which the transistors TGo and TGe have a phase polarization at the level of anti-glare. This makes it possible to maintain the active anti-glare function only during the transition between the integrations of the odd and even pixels.

As a result of this transition, the sensor is in a configuration where the roles of odd and even pixels are exchanged to provide anti-glare. The even pixels are monopolized by the active RSTe transistors and provide the anti-glare function for all the pixels, by the transistors TG1e and TG2e, while the storage nodes C2o of the odd pixels can receive charges and be read.

At the end of the first charge integration period in the photodiodes, the transistors TGo are briefly activated, causing charge transfer from the pairs of adjacent photodiodes to the storage nodes C2o of the odd pixels, as has been described in relation with Figure 5.

The transistors RST and TG then remain in the same state until half of the second integration period, where they switch back to the described configuration of the beginning of the first integration period. At the end of the second integration period, transistors TGe are briefly activated, causing charges to be transferred from pairs of adjacent photodiodes to C2e storage nodes of even pixels, as has been described in connection with FIG. 5.

The transistors TG and RST are thus controlled periodically, with a period of one line time and a half-period offset between the even and odd pixels. In other words, a period is the time interval between two successive activations of the transfer transistors (and thus two successive readings of the pixel level) and half a period corresponds to half of this period. With the operation just described, the transfer of charges from the photodiode to the storage node C2 of each pixel occurs substantially in the middle of an interval, corresponding substantially to half a period, where the RST transistor of the pixel is blocked. This allows for a correlated double-sampling. During the interval between the blocking of the transistor RST and the activation of the transfer transistors TG, the black levels stored on the capacitors C2 of the pixels of the same parity are read. During the interval between the activation of the transistors TG and the subsequent conduction of the transistors RST, the signal levels stored on the capacitors C2 are read. By subtracting the black levels off from the corresponding signal levels off pixel, levels having a better signal-to-noise ratio are obtained by suppressing the precharging noise of the reading nodes.

Claims

claims

A method of controlling a time delay image sensor and integrating comprising a matrix of photosensitive pixels (Px) organized in rows and columns, each pixel (Px1) of a column comprising:

A photosensitive element (C);

A storage node (C2o);

^• a first transfer transistor (TG1o) connecting the photosensitive member to the storage node;

^• a reset transistor (RST) connecting the storage node (C2o) of the pixel to a supply potential (Vdd); each pixel (Px1) of a column, except the last one, further comprising:

^• a second transfer transistor (TG2o) which connects the storage node (C2o) of the pixel to the photosensitive member (De) of the next pixel (Px2) of the column and which is connected to be active at the same time as the first transfer transistor; method comprising the steps of: a) periodically activating the first and second transfer transistors (TGo) of each pixel to cause charge transfer to the storage node (C2o) of the pixel; b) reading the potential on the storage node (C2o) of the pixel after each activation of the first and second transfer transistors (TG1 o, TG2o); c) in an interval between two successive activations, bring the gate of the transfer transistors (TGo) to a polarization potential defining an anti-glare threshold of the photosensitive elements; d) conducting the reset transistor (RST) of the pixel during said interval; e) blocking the reset transistor (RST) outside said gap; and wherein the preceding steps are implemented half-period off between the even-numbered pixels and the odd-row pixels of a column.

The method of claim 1, comprising the steps of:

Performing a first reading of the potential on the storage node (C2o) between the start of the blocking of the reset transistor (RST) and the activation of the pixel transfer transistors; and

Performing a second reading of the potential on the storage node between the activation of the transfer transistors and the end of the blocking of the reset transistor.

3. Method according to one of claims 1 and 2, wherein the duration of said interval is between half a period and a period.