MATRIX SENSOR WITH LOGARITHMIC RESPONSE AND EXTENDED TEMPERATURE OPERATING RANGE

Abstract

A matrix sensor with logarithmic response and extended temperature operating range, including a plurality of active pixels each defined by a photodiode (PD) operating in solar cell mode, the photodiode being formed by a semiconductor junction in a substrate (11), a reverse-biased junction (20) being present at a distance (d), from the junction of the photodiode, that is less than the diffusion length of the charges in the substrate, the reverse-biased junction (20) being produced by a diffusion to a depth (p) greater than that (p) used in the formation of the source or drain of transistors of the sensor, adjacent to the photodiode.

Claims

1. A logarithmic-response matrix-array sensor having an extended temperature operating range, said sensor including a plurality of active pixels each defined by a photodiode operating in solar-cell mode, the photodiode being formed by a semiconductor junction in a substrate, a reverse-biased junction being present at a distance from the junction of the photodiode smaller than the diffusion length of charge carriers in the substrate, the sensor including one or more reference pixels that are used to generate a reference voltage that serves to compensate for a temperature-related shift in the response of the active pixels, this or these reference pixels being masked from incident light and placed virtually under given illumination conditions by injecting a current into the junction of the photodiode.

2. The sensor as claimed in claim 1, including a capacitance for injecting a charge into the photodiode in order to forward bias it before read-out of a voltage representative of the illumination received by the photodiode.

3. The sensor as claimed in claim 1, current being injected into the photodiode of a reference pixel through an electrical resistance that is connected to a voltage source that generates a current in the same direction as the photoelectric current generated by the photodiode.

4. The sensor as claimed in claim 1, current being injected into the photodiode of a reference pixel through a capacitance connected to a ramp voltage source that generates a current in the same direction as the photoelectric current generated by the photodiode.

5. The sensor as claimed in claim 1, the substrate being a p-type semiconductor, in particular p-type silicon, and the photodiode including an n.sup.+-type region.

6. The sensor as claimed in claim 1, the reverse-biased junction being generated by an n.sup.+-type region.

7. The sensor as claimed in claim 1, the n.sup.+-type region of the reverse-biased junction being defined by an n-doped well of a PMOS transistor for reading the voltage of the photodiode.

8. The sensor as claimed in claim 1, including for each pixel a transistor for resetting the photodiode, applying, when in the on state, a predefined voltage to the photodiode.

9. The sensor as claimed in claim 2, the capacitance for injecting charge into the photodiode, in order to forward bias it before read-out of the voltage, being a parasitic capacitance of the reset transistor.

10. The sensor as claimed in claim 2, the capacitance for injecting charge into the photodiode, in order to forward bias it before read-out of the voltage, being a capacitance produced specifically.

11. The sensor as claimed in claim 2, the initial bias voltage of the photodiode following the injection of charge by means of the capacitance being comprised between 0.1 and 0.2 V.

12. The sensor as claimed in claim 1, the reverse-biased junction extending under the junction of the photodiode.

13. The sensor as claimed in claim 1, the reverse-biased junction extending on at least two opposite sides on either side of the junction of the photodiode and better still all the way around the photodiode.

14. A method for operating the sensor of claim 1, including resetting the photodiode by closing a reset transistor and injecting a charge into the photodiode in order to forward bias it at the start of the phase for measuring the light received by the photodiode and obtaining a logarithmic response in a wide-temperature-range operating range.

15. The method as claimed in claim 14, the temperature range encompassing at least the range ?15? C. to 60? C.

16. The method as claimed in claim 14, the voltage of the photodiode of an active pixel being corrected by the voltage read from a reference pixel, in order to generate a signal representative of the illumination received by the active pixel and independent of the temperature in the operating range.

17. The method as claimed in claim 14, current being injected into the photodiode of a reference pixel through a capacitance connected to a ramp voltage source that generates a current in the same direction as the photoelectric current generated by the photodiode, the ramp voltage consisting of the falling front of a control signal of the reset transistor.

18. The method as claimed in claim 14, the temperature range encompassing at least the range ?50? C. to 100? C.

Description

[0045] The invention will possibly be better understood on reading the following detailed description of nonlimiting example embodiments thereof and on examining the appended drawing, in which:

[0046] FIG. 1 is an equivalent circuit diagram of a pixel of a sensor according to the invention;

[0047] FIG. 2 schematically and partially shows the CMOS structure of a pixel;

[0048] FIG. 3 illustrates the charge profile in the substrate of the photodiode;

[0049] FIG. 4 is a diagram analogous to that of FIG. 1, of a variant embodiment of the pixel;

[0050] FIG. 5 illustrates the generation of a current simulating exposure in a reference pixel;

[0051] FIG. 6 is a view analogous to FIG. 5 of a variant embodiment;

[0052] FIG. 7 is a timing diagram illustrating a way of generating the charge to be injected by the ramp;

[0053] FIG. 8 shows the variation of the voltage generated by a pixel as a function of the illumination level, in the absence of injection of charge prior to the read-out of the voltage of the photodiode in the anode-cathode direction;

[0054] FIG. 9 shows the voltage generated by a pixel according to the invention in the anode-cathode direction, after injection of charge prior to the start of the exposure of the photodiode, as a function of the illumination level; and

[0055] FIG. 10 is a schematic representation of a simplified equivalent circuit of the photodiode; and

[0056] FIGS. 11 and 12 schematically show variant embodiments of the sensor.

[0057] FIG. 1 schematically and partially shows the electronic circuit of a pixel of an optical sensor according to the invention. This pixel forms part of a detector matrix-array including rows and columns of pixels. Each pixel includes a photodiode PD associated with electronics for reading its voltage, which have not been described in detail; examples of circuits for reading photodiodes in solar-sell mode are for example described in patent FR 2 943 178.

[0058] The open-circuit voltage of the photodiode PD in solar-cell mode is sampled at 14 by way of output signal. The photodiode is reset (reset operation) after each read cycle by closing a reset transistor 10 that is controlled by a signal RST, as illustrated in FIG. 1, this transistor applying, when closed, a predefined potential to the terminals of the photodiode.

[0059] The photodiode PD may be formed by diffusing n dopants into a p-type substrate 11, as illustrated in FIG. 2, using a conventional CMOS integration technology.

[0060] The reset operation allows the photoelectric charge stored on the cathode of the photodiode PD to be emptied, but that stored in the substrate must also be empty.

[0061] If the density of pixels in the matrix array of the sensor is low, the charge carriers that make up this charge recombine naturally in the substrate. In contrast, if the pixel matrix-array is dense, this being the case in a sensor according to the invention, these charge carriers must be absorbed expressly.

[0062] According to the invention, a reverse-biased junction 20 is created in the substrate 11 at a distance d from the photodiode PD smaller than the diffusion length.

[0063] The reset transistor 40 includes junctions 41, 42 the n.sup.++ regions of which extend to a depth p smaller than the depth p of the region 20.

[0064] Diffusion length characterizes the distance that minority carriers travel in the substrate before recombining. This distance is commonly called Lp in a p-type substrate, as in the considered example. Lp is for example determined as described in Physics of Semiconductor Devices, which book was written by S.M. Sze and published by John Wilet & Sons in 1981, ISBN 0-471-05661-8. Lp is for example comprised between 50 ?m and 200 ?m in a standard substrate for fabrication of CMOS circuits.

[0065] The junction 20 may be created with an n.sup.+-type region, for example the well N.sub.well of at least one PMOS transistor used for read-out of the voltage of the photodiode PD, this transistor not being shown in the drawing for the sake of clarity. Examples of read circuits using PMOS transistors are described in FR 2 943 178.

[0066] As a variant, in particular when the read circuit used includes, as described in FR 2 920 590, only NMOS transistors, an n-diffusion may be formed in proximity to the photodiode PD. This n-diffusion may form part of the diffusions forming the active or passive components of the pixel, for example the sources or drains of the NMOS transistors.

[0067] In the case of a p-type substrate 11 made of silicon, the n.sup.+ region of the photodiode is for example formed either by diffusion, or by ion implantation, with arsenic or phosphorus, and the same process is used for the reverse-biased n.sup.+ region.

[0068] For a single photodiode in a substrate, equation (1) below governs the relationship between the current I.sub.D and the voltage V.sub.D of said photodiode.

[00001]$\begin{array}{cc}{I}_D=I_S.Math.e^{\frac{V_D}{V_t}}-I_S& \left(1\right)\end{array}$

[0069] V.sub.t is the voltage of thermal origin, typically about 26 mV at 20? C. and I.sub.S is the saturation current of the junction of the photodiode.

[0070] The static open-circuit voltage of the illuminated diode, in solar-cell mode, is given by equation (2) below. I.sub.? is the photoelectric current.

[00002]$\begin{array}{cc}{V}_D=V_t.Math.\log \left(\frac{I_{?}+I_S}{I_S}\right)& \left(2\right)\end{array}$

[0071] It may be seen that the voltage across the photodiode no longer varies logarithmically when Is becomes large. It will be noted that Is doubles about every 7? C. in silicon.

[0072] With an n-doped region formed by the junction 20 in proximity to the photodiode, i.e. at a distance smaller than the diffusion length Lp, the variation in the voltage across the photodiode is influenced by the bias of this n-doped region.

[0073] A model based on the diffusion of minority charge carriers between the photodiode and the nearby n-doped region allows the equation relating the voltage and current of the photodiode to be derived.

[0074] As FIG. 3 shows, an n-diffusion reverse biased by a voltage VA (called the over-exposure protection voltage) in proximity to a photodiode in solar-cell mode changes the profile of the minority carriers (namely electrons in this precise case) in the p-type zone. Thus, the charge injected into the substrate 11 by the photodiode diffuses toward the n-doped region, which is reverse biased, according to a diffusion law.

[0075] In a substrate intended for the fabrication of an image sensor, crystal quality is excellent. Therefore, for a small distance between the n-type regions with respect to the diffusion length, the distribution of minority carriers is substantially triangular.

[0076] It is possible to derive a current-voltage relationship for this photodiode:

[00003]$\begin{array}{cc}{I}_D=I_S.Math.e^{\frac{V_D}{V_t}}-I_S.Math.e^{-\frac{V_{\mathrm{AB}}}{\mathrm{Vt}}}& \left(3\right)\end{array}$

[0077] From this relationship (3) it may be seen that the current/voltage curve of the photodiode does not pass through the point (0, 0). This deviation from the point (0, 0) is the reason the temperature drift effect is seen, because when the current is zero in the photodiode, corresponding to darkness, the voltage across the photodiode is not zero.

[0078] The voltage across the photodiode in solar-cell mode may be described by relationship (4) below.

[00004]$\begin{array}{cc}{V}_D=V_t.Math.\log \left(\frac{I_{?}+I_S.Math.e^{-\frac{V_{\mathrm{AB}}}{V_t}}}{I_S}\right)& \left(4\right)\end{array}$

[0079] It may be seen that reverse biasing the n-doped junction 20 allows a substantially logarithmic variation to be maintained even when the current I.sub.S is high, because the exponential term dependent on ?V.sub.AB/V.sub.t is negligible.

[0080] The variation of the current Is with temperature nevertheless leads in this case to a voltage drift when the photodiode is in the dark, which it is possible to correct as described below.

[0081] When the photodiode PD is associated with a reset transistor 10, the photodiode varies from an initial voltage, denoted V.sub.D0.

[0082] The simple equivalent circuit illustrated in FIG. 10 may be used to model the dynamic behavior of the photodiode after the reset operation. Analysis of this equivalent circuit allows a differential equation (5) to be derived, the solution of which gives the voltage V.sub.D of the photodiode after an exposure time t.

[00005]$\begin{array}{cc}{V}_D=V_T.Math.\ln .Math..Math.\frac{I_{?}+I_{\mathrm{AB}}}{\left[\left(I_{?}+I_{\mathrm{AB}}\right).Math.e^{-\frac{V_{D.Math..Math.0}}{V_T}}-I_s\right].Math.e^{-\frac{\left(I_{?}+I_{\mathrm{AB}}\right).Math.t}{V_T.Math.C_D}}+I_s}& \left(5\right)\end{array}$

[0083] In this equation. I.sub.AB=I.sub.s exp (?V.sub.AB/V.sub.t), V.sub.DO is the initial voltage across the photodiode and C.sub.D the capacitance of the photodiode, the other terms having the same meaning as in equation (4).

[0084] In an image sensor, exposure time is often set to a value lower than or equal to the capture period, which is constant.

[0085] If the voltage across the photodiode at the end of exposure is plotted as a function of the level of illumination falling on the photodiode, a complex response is obtained, as FIG. 8 shows, for various temperature levels and currents I.sub.S.

[0086] At high light flux, the response is strictly logarithmic, but at low flux, the response may be linear at low temperature, because the parasitic capacitance of the photodiode must be recharged after the reset operation.

[0087] In comparison, with a photodiode in solar-cell mode without the nearby reverse-biased junction 20, the photoelectric response rapidly collapses with temperature. It is impossible in this case to restore the loss of sensitivity via subsequent processing.

[0088] From equation (5) it may be seen that if the initial voltage V.sub.D0 of the photodiode in solar-cell mode is set to a positive value, i.e. if the photodiode is forward biased during the reset phase instead of being short-circuited, the response becomes logarithmic over the entire temperature range. The effect of temperature on the response may then be summed up as a simple shift, as illustrated in FIG. 9, in which the variation of voltage as a function of illumination has been shown for a temperature range extending from ?20? C. to 90? C.

[0089] It is difficult to reset a forward-biased photodiode with a MOS transistor, because in this case the source and drain of the reset transistor are also forward biased. These forward-bias junctions inject charge into the substrate that is of the same nature as the photoelectric charge, thereby preventing correct operation of the photodiode in the image sensor.

[0090] A capacitance 40 may be used to inject a charge into the photodiode, in order that it be forward biased after the reset transistor has been opened.

[0091] The capacitance 40 may be a parasitic capacitance of the reset transistor 10 as illustrated in FIG. 1, or a specific capacitance as illustrated in FIG. 4. The initial forward bias voltage V.sub.D0 applied to the photodiode via this capacitance is for example 0.15 V.

[0092] The value of the capacitance is high enough to obtain the sought-after logarithmic response at low temperatures.

[0093] For example, it is sought to obtain at T=?15? C. less than 1% dispersion in the response with respect to that at 25? C. Here, dispersion is defined as the relative deviation between the response curves.

[0094] Moreover, the optical sensor according to the invention advantageously includes one or more reference pixels that are protected from the incident light and that serve to generate a reference voltage allowing the temperature shift to be compensated for, and thus a signal that is both logarithmic and independent of temperature over a wide range may be obtained.

[0095] The one or more reference pixels are masked by a metal layer forming a screen with respect to the incident light; however, in contrast to known solutions, predefined reference illumination conditions are simulated therein.

[0096] If this reference illumination level is set high enough, it is easily possible to attenuate, or even suppress, the effect of leakage of light through the optical mask using a CMOS production process.

[0097] For example, if a metal layer allows an attenuation factor of 2000 to be achieved and if the sensitivity threshold of a logarithmic pixel is 0.01 lux, the maximum tolerable illumination of a reference pixel placed in the dark is 20 lux, this being very low.

[0098] With the proposed solution, if the reference illumination is set to 10,000 lux, even if the reference pixel receives 200,000 lux, there is only a 1% variation in the reference level.

[0099] If necessary, it is possible to apply a plurality of metal layers to the one or more reference pixels for even greater precision. Generally, precision increases with the electronically simulated level of illumination.

[0100] To simulate this level of illumination, it is possible to generate a current simulating equivalent illumination conditions, and therefore a current that flows in the same direction as that generated by the operation, in solar-cell mode, of the photodiode, using a voltage source 30 connected by an electrical resistance 31 to the photodiode PD, as illustrated in FIG. 5. In the example considered, this voltage source is negative and the choice of the voltage of the voltage source 30 and the choice of the value of the resistance 31 allows the desired current to be obtained.

[0101] Another more advantageous solution is to use a voltage ramp connected by way of a capacitance 33 to the cathode of the photodiode, as illustrated in FIG. 6. The current simulating the illumination conditions may then be adjusted via the choice of the value of the capacitance and of the slope of the ramp.

[0102] The ramp voltage source may be specifically intended to generate the sought-after current. However, it may be advantageous to exploit the falling front of a control signal of a transistor of the sensor, in particular the control signal RST of the reset transistor, as illustrated in FIG. 7. This signal RST is triggered before each exposure cycle of the photodiode PD.

[0103] FIGS. 11 and 12 show two examples of sensors according to the invention, in which the junction 20 is formed by a diffusion in the substrate 11 that extends under the junctions of the photodiodes PD.

[0104] In the example of FIG. 11, the junction 20 extends, for certain photodiodes PD, only thereunder, at a distance d smaller than the diffusion length L, thereby allowing a dense implantation to be preserved.

[0105] In the example in FIG. 12, the diffusion serving to form the junction extends both laterally on either side of each photodiode PD, and preferably also thereunder.

[0106] The diffusion for example forms cups within each of which one photodiode PD is placed.

[0107] The substrate 11 may be p-type and the junctions of the photodiodes PD and the reverse-bias junctions n-type.

[0108] The depth p of diffusion to form the reverse-biased junction is relatively large.

[0109] It is for example, in the case of a 0.18 ?m technology, at least 0.5 ?m. A larger depth allows more of the photoelectric charge created by long wavelength photons (>650 nm) to be absorbed.

[0110] The invention is not limited to the described examples. In particular, the n and p carrier types may be inverted.

[0111] The depth p and the distance d may vary within the sensor, being local values. The depths p and distance d may be easily determined, by scanning electron microscope.

[0112] The expression including a or including an must be understood as being synonymous with comprising at least one, unless otherwise specified.