Interconnection structure between microcircuits

Abstract

The invention proposes a set of two electronic microcircuits assembled together by implementing a set of electrical couplings between corresponding connection points of the two microcircuits. The first microcircuit is a transducer microcircuit with at least one photoelectric transduction element using charge accumulation, and the second microcircuit is a readout microcircuit with at least one element for reading the electrical charges generated by a respective photoelectric transduction element. The connection points comprise facing surface elements separated by a dielectric. The coupling between the two circuits is capacitive. This is particularly applicable to the hybridization of two matrix circuits, one for transduction and one for readout, manufactured using different technologies. Fig. 3

Description

Title of the invention: Interconnection structure between microcircuits

[0001] The present invention relates generally to hybrid circuits, and in particular to a new approach to electrical coupling between two circuits produced using different technologies and / or requiring assembly by superposition after their manufacture.

[0002] To take a practical example, a matrix sensor is composed of pixels essentially containing a transduction element and a local reading circuit. The transduction element and the local reading circuit can either be produced on the same substrate or on two different substrates.

[0003] The production of transduction elements and reading circuits on two different substrates is typically dictated by questions of compatibility and optimization of their production. Today, reading circuits are produced in C-MOS technology on a silicon substrate, while the transduction elements can be produced using much more varied techniques.

[0004] The transduction elements can for example be produced on a silicon substrate using a manufacturing process highly optimized for transduction. They can alternatively be produced with less common semiconductor materials, for example semiconductors of the IILV family for wavelengths in the infrared range.

[0005] Mention will be made here in particular of InGaAs substrates for applications in the near infrared, in particular for SWIR sensors (for "Short Wave InfraRed" in English), InSb substrates for sensors in the mid-infrared (MWIR for "Middle Wave InfraRed"), and mercury cadmium telluride (MCT) for applications of LWIR sensors ("Long Wave InfraRed" in English, or CdTe for the detection of X-rays.

[0006] Such a sensor produced on two assembled substrates is commonly called a “hybrid sensor”, the term “hybridization” designating the assembly process between the two substrates.

[0007] The present invention applies in particular to a hybrid sensor whose transduction elements consist of PN junctions, generally photodiodes which transform incident photons into electrical charges.

[0008] There are mainly two types of circuits for reading the electrical charges generated in a photodiode.

[0009] Thus [Fig.lA] shows a voltage follower readout circuit, where the photoelectric charge generated by the photodiode PD is accumulated in the capacitance of the photodiode junction and the associated parasitic capacitances. The potential variation is read by a voltage follower SF in the readout circuit, the output of which constitutes the pixel signal SP. This structure, called SFP (for "Source Follower Pixel" in English), has the advantage of being very simple and compact. A disadvantage is that the polarization of the photodiode can vary during the exposure, causing non-linearity phenomena.

[0010] [Fig.lB] proposes a more advanced solution called CTIA (for "Capacitive Trans-Impedance Amplifier"), which proves to be much better in polarization stability. An electrode of the photodiode PD, here its anode, is connected to the input of an operational amplifier AMP with a capacitor C between its other input and its output. It has several advantages: firstly the polarization is much more stable thanks to the high gain of the operational amplifier; then, because the photoelectric charge generated by the photodiode is transferred to the capacitor C the sensitivity can be modulated by acting on the gain of the amplifier.

[0011] Figs. 1A and 1B show that a direct electrical connection between the photodiode PD and the reading circuit is necessary. This interconnection involves hybridization with a direct electrical contact point at each pixel of the sensor.

[0012] Figs. 2A to 2D show different known technical solutions for such direct hybridization, the common principle being that for each pixel, an electrode is formed on the substrate of the photodiodes and on that of the reading circuit. Then the two substrates are placed face to face, aligned and mechanically and electrically connected together.

[0013] [Fig.2A] shows an approach by interposing a PA polymer adhesive and applying pressure. This pressure is maintained by removing the polymer adhesive before and during curing. This approach is conceptually simple, but it requires very good uniformity in height of the electrodes on both substrates and excellent parallelism of the tools. Furthermore, it presents a risk of failure if the stresses generated by the removal of the adhesive are released when the sensor is exposed to adverse conditions such as high temperature, humidity, etc. For these reasons, this known approach is considered unreliable.

[0014] [Fig.2B] shows a metal fusion approach. Typically, Indium electrodes are very popular due to their low melting point and high ductility at very low temperatures.

[0015] [Fig.2C] shows a more recent approach, called "Copper-Pillar" in English, which consists of creating an electrode in the form of CC copper columns on each substrate, one of the columns being capped with an SM solder material. The solder volume makes it possible to compensate for the non-uniformity in the height of the copper columns.

[0016] Finally [Fig.2D] illustrates the most recent approach to molecular bonding, theo rically more reliable and the most reproducible. It is based on the fact that covalent bonds can be created between two dielectric surfaces (typically silicon dioxide) in very close contact at a fairly moderate temperature (< 300 °C). This approach requires atomic-scale flatness for both surfaces in contact and extreme cleanliness, since the initial bonding involves Van Der Walls forces. The implementation cost is therefore potentially high.

[0017] It will be recalled here that two key points for conventional hybridization are, on the one hand, the creation of a large number of direct electrical contacts, at the level of all the pixels of the sensor, and on the other hand, alignment which is all the more demanding as the pixel pitch becomes small.

[0018] These difficulties are far from being resolved today and the manufacturing yield remains relatively low due to a significant proportion of defective products, particularly for high-resolution sensors. It is observed that the greatest difficulty in hybridization lies in achieving reliable electrical contact between the photodiodes and the reading circuits, while precision alignment has become relatively easy to ensure thanks to advances in digital microscopes. Summary of the invention

[0019] The present invention aims to propose a new principle of hybridization making it possible to solve all or part of the problems set out above linked to hybridization by direct contact.

[0020] To this end, a set of two electronic microcircuits assembled together by producing a set of electrical couplings between homologous connection points of the two microcircuits is proposed, a first microcircuit being a transducer microcircuit with at least one photoelectric transduction element by charge accumulation, and the second microcircuit being a reading microcircuit with at least one element for reading the electrical charges generated by a respective photoelectric transduction element, and the connection points comprising facing surface elements separated by a dielectric, the coupling between the two circuits being a capacitive coupling.

[0021] This circuit advantageously but optionally comprises the following additional characteristics, taken individually or in any technically compatible combinations:

[0022] * the or each photoelectric capture element is a photodiode, an electrode of the photodiode being connected to a respective surface element located on the first microcircuit and its other electrode being connected to a reference potential.

[0023] * the or each surface element located on the second microcircuit is connected to the input of a read element comprising an infinite or quasi-infinite direct current input impedance circuit associated with a reset switch.

[0024] * the circuit with infinite or quasi-infinite input impedance in direct current is a voltage follower circuit.

[0025] * the or each surface element located on the second microcircuit is connected to the input of a reading element comprising a capacitive transimpedance circuit associated with a reset switch.

[0026] * the or each surface element located on the second microcircuit comprises a first part connected to the input of the capacitive transimpedance circuit and a second part belonging to a feedback connection, the first and second parts forming separate capacities with the corresponding surface element of the first microcircuit.

[0027] * the first microcircuit comprises a matrix of photo transduction elements electrical by accumulation of charges, and the second microcircuit comprises a homologous matrix of elements for reading the electrical charges generated by the respective photoelectric transduction elements.

[0028] * the assembly comprises at least one direct electrical connection between the first and second microcircuits for control signals common to the different elements of the matrices.

[0029] * the first and second microcircuits each comprise at least one element additional surface, the two facing surface elements providing additional capacitive coupling for control signals common to the different elements of the matrices.

[0030] * the additional surface elements surround the connection points.

[0031] * the additional capacitive coupling is capable of conveying a signal for resetting the photoelectric transduction elements of the first microcircuit.

[0032] * the dielectric includes an adhesive for securing the two microcircuits.

[0033] * the adhesive contains a permittivity increasing additive.

[0034] * the dielectric comprises a layer of dielectric material assembled to the two microcircuits by molecular bonding.

[0035] * the or each photoelectric capture element comprises a photodiode with avalanche, the capacitive coupling ensuring isolation from the bias voltage of the or each photodiode. Brief description of the drawings

[0036] Other aspects, aims and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof, given by way of non-limiting example and with reference to the accompanying drawings. the drawings:

[0037] [Fig. 1 A] and [Fig. 1B] are schematic diagrams of a hybrid sensor pixel with two conventional reading technologies,

[0038] [Fig.2A], [Fig.2B], [Fig.2C] and [Fig.2D] schematically illustrate four techniques known hybridization between a substrate of transduction elements and a substrate of reading elements for such a hybrid sensor,

[0039] [Fig.3] is a circuit diagram of a pixel of a hybrid sensor according to a first embodiment of the invention,

[0040] [Fig.4] is a more detailed diagram of the circuit of [Fig.3],

[0041] [Fig.5] is a timing diagram illustrating the different signals involved in the circuit of [Fig.4],

[0042] [Fig.6A] illustrates the hybridization of the two substrates in a hybrid sensor according to the invention,

[0043] [Fig.6B] illustrates a variant of the hybridization of [Fig.6A],

[0044] [Fig.6C] illustrates another embodiment of the hybridization of [Fig.6A],

[0045] [Fig.6D] illustrates another embodiment of the hybridization of [Fig.6B], and

[0046] [Fig.7] is a circuit diagram of a pixel of a hybrid sensor according to a second embodiment of the invention.

[0047] Detailed Description of Preferred Embodiments

[0048] In the following, two possible implementations of a hybrid image sensor according to the present invention will be described, one with the combination for each pixel of a photodiode and a reading circuit of the SFP source follower type, and the other with the combination for each pixel of a photodiode and a reading circuit of the CTIA amplifier type.

[0049] In both cases, the photodiode is reset to zero at the start of each exposure either by a switching transistor or by a virtual ground generated for example by an operational amplifier.

[0050] According to the invention, a capacitive hybridization is provided between the substrate of the photodiodes and the substrate of the reading circuits making it possible to read the potential variations on these photodiodes, while ensuring the resetting of the photodiodes between two exposures. Thus, each photodiode is connected to its reading circuit without direct contact, but via a capacitance formed between two electrodes brought face to face and at a short distance during the assembly of the two substrates.

[0051] It is understood that thanks to the passage of the signal via a capacitor, there is no risk of the signal disappearing as is the case in a direct contact mode when a contact is defective, as recalled in the introduction.

[0052] Indeed, a certain capacitance value will always exist between a photodiode and its reading circuit, and only the variation of the capacitance value at the level of a pixel can impact the reading performed on the pixel in question. And such uniformity defects, when they exist, are generally easy to correct by digital post-processing.

[0053] The capacitors are formed by flat electrodes of appropriate dimensions and shapes made facing each other on the two substrates which will then be assembled, typically using standard techniques for producing conductive contact plates.

[0054] We will first describe the implementation and operation of the invention with a source follower SFP reading circuit.

[0055] Considering a pixel comprising a photodiode and an SFP reading circuit, it is understood that if a capacitor is inserted between the photodiode and the reading circuit, it is indeed possible to capture the variation in potential generated by the accumulation of charges during the exposure. But it remains necessary to ensure the repolarization of the photodiode in reverse between two exposures (resetting the pixel to zero), and solutions for doing this will be described later.

[0056] [Fig.3] thus illustrates the diagram of a pixel with an SFP type reading circuit with the two substrates assembled by capacitive hybridization, namely a substrate of SPD photodiodes (also called a transduction substrate) and a substrate of SCL reading circuits.

[0057] An electrode of the photodiode PD, here its anode, is connected to an electrode ET intended to form with an electrode EL on the reading substrate side, a capacitance called hybridization capacitance CH, which is formed when the two substrates are assembled together. The cathode of the diode receives a RAZ signal allowing it to be reset between two exposures, as will be seen below.

[0058] On the reading circuit side, the corresponding electrode EL of the hybridization capacitor is connected to the input of a so-called source follower amplifier SF, a recovery switch MRST (typically a simple transistor) being able to selectively connect the input of the amplifier SF to a reference voltage VREF. The output of the amplifier delivers the pixel signal SP.

[0059] When the recovery switch MRST is on, the electrode EL of the hybridization capacitor CH is connected to the voltage VREF (ground or a fixed voltage source), and the photodiode PD and the hybridization capacitor CH together form a rectification circuit.

[0060] In this configuration, if an alternating or pulsed reset signal is applied during this time to the cathode of the photodiode PD, a quantity of charges accumulates in the hybridization capacitor CH thanks to the asymmetric conduction of the photodiode junction. This quantity of charge makes it possible to reverse bias the photodiode to reset it to zero.

[0061] In more detail and with reference to Figs. 4 and 5, if an electrical pulse RAZ is applied to the photodiode during the application of the control pulse of the MRST transistor (see [Fig.5]), it becomes forward biased and becomes reverse biased after the suppression of this pulse.

[0062] If the reset transistor MRST is blocked shortly after the removal of the RAZ pulse on the substrate, the variation in potential of the photodiode (voltage Vpd) under the effect of the incident light is then transmitted to the source follower SF (voltage Vin) through the hybridization capacitor CH, the voltage Vin gradually increasing (from its starting point VREF) as electrical charges of photoelectric origin are created in the photodiode PD.

[0063] We thus find at the output of the SF follower a voltage Vs which reproduces this evolution, except that the starting point is equal to VREF increased by the offset voltage Vos of the SF follower. This voltage Vs constitutes the output signal of pixel SP.

[0064] It is understood that the capacitive transmission gain depends on the ratio between the hybridization capacitance and the input capacitance of the voltage follower. If the input capacitance of the follower is low, the gain will be very close to 1, and this makes it possible to largely compensate for the variation in the value of the hybridization capacitance CH.

[0065] Referring now to [Fig.6A], the application of the reset pulse RAZ to the SPD substrate can be carried out, in a first embodiment, by a direct connection pad CD with the SPD substrate. This direct connection can be made for example with a connecting wire on the face of the SPD transduction substrate which is exposed to light, as illustrated. Alternatively, one or more direct electrical contacts can be provided at the level of the facing faces of the SPD and SCL substrates. It is advantageous in this case to provide several contacts to limit the risk of defective contact.

[0066] Referring now to [Fig.6B], it is alternatively possible to route the RAZ signal via a CRAZ coupling capacitor between the substrate of the SPD photodiodes and the substrate of the SCL reading circuits. This capacitor is advantageously produced with a large coupling surface compared to the surface of the CH capacitors existing at each pixel, so as to provide all the pixels with a sufficient quantity of charge to carry out the reset reliably.

[0067] An advantage of this embodiment of [Fig.6B] is that no direct electrical connection between the two substrates is necessary. The assembly of the hybrid sensor then becomes a simple gluing operation, ensuring a reasonably uniform thickness of dielectric (the adhesive itself) between the pairs of electrodes of the respective capacitors.

[0068] Furthermore, the absence of direct electrical contact between the photodiodes and their respective reading circuit reduces the noise linked to direct contacts and therefore improves the quality of image.

[0069] We will now describe an alternative embodiment of the follower embodiment described above, aimed at increasing the operating dynamics.

[0070] It is recalled that the reset pulse RAZ on the photodiode substrate during the action of the MRST recovery pulse at the reading circuit makes it possible to initialize the photodiodes at the start of the exposure. However, if the light is too intense, photodiodes can be completely discharged, in which case the photoelectric charges of the saturated photodiodes will overflow towards the neighboring photodiodes, causing a blooming phenomenon.

[0071] A known solution to this problem is to implement reverse-biased PN junctions (AB junctions, for "anti-glare") around the photodiodes. These reverse-biased junctions absorb the photoelectric charges that overflow from the saturated photodiodes to limit the propagation of glare. These AB junctions must, however, be kept reverse-biased, including during the action of the reset pulse.

[0072] This solution has a defect of reducing the efficiency of the photoelectric conversion, because these AB junctions absorb a part of photoelectric charge generated by the photons. But an advantage of this solution is that it provides a logarithmic response after the saturation of photodiodes. This logarithmic response provides the sensor with a dynamic range that can exceed 120dB.

[0073] We will now describe with reference to [Fig.7] the embodiment and operation of the invention with a CTIA type reading circuit.

[0074] In this embodiment, the direct electrical contact between the photodiode and the CTIA amplifier of [Fig.lB] is replaced not by a single capacitor but by two capacitors CH1 and CH2.

[0075] The first capacitor CH1 plays the same role as the capacitor CH of Figs. 3 and 4. The second capacitor is added to ensure the continuity of the feedback necessary for the operation of the amplifier.

[0076] During the recovery of the amplifier AMP via the switch PRST, a negative reset pulse RAZ is applied to the substrate of the photodiodes, as in the case of a pixel with SFP readout circuit. Here again, this RAZ pulse installs a reverse bias on the photodiodes, and then the switch MRST of the amplifier is opened.

[0077] The feedback makes it possible to maintain the constant bias voltage on the photodiode and transfer the accumulated charge to the integration capacitor consisting of the capacitance CH2 during the exposure.

[0078] Advantageously, the capacitors CH1 and CH2 consist of a single electrode AND substrate side of the photodiodes, and two neighboring electrodes ELI and EL2 located opposite the electrode AND reading substrate side.

[0079] The respective surfaces of the CH1 capacitance and the CH2 capacitance are determined in particular according to the required amplification level.

[0080] We will now describe an embodiment of a bonding between the substrate of the SPD photodiodes and the substrate of the SCL reading circuits, so as to secure them together. This securing can be carried out in different ways, for example using a polymer adhesive, a molecular bond, a solvent and etc.

[0081] If a polymeric adhesive is used, as schematically illustrated in Figs. 6A and 6B, additives can be incorporated into the adhesive to increase its electrical permittivity. A high electrical permittivity increases the capacitance value between the photodiodes and the readout circuits and therefore improves the signal transmission quality between the photodiodes and the readout circuits. For example, one can use the lessons learned from the article “Improved dielectric permittivity of NBCTO / epoxy composite films with low dielectric loss”, Yanli Su, Chengdong Ba, Fei Wang, SN Applied Sciences (2019), Springer Nature Switzerland AG.

[0082] In the case of a bonding without the addition of adhesive, and as illustrated in Figs. 6C and 6D, the electrodes on the substrate side of the photodiodes and / or on the substrate side of the reading circuits are covered with a thin layer of a material, typically an oxide OX, intended to form the dielectric of the capacitances created by the hybridization, the bonding then being carried out by molecular bonding.

[0083] We will now describe a particular case of a hybrid sensor according to the invention, where the transduction photodiodes are of the avalanche effect type.

[0084] Avalanche photodiodes are very useful for increasing the sensitivity and response speed in optoelectronic systems such as single-photon detection sensors or distance measurement sensors (LIDAR). It is known that to operate these avalanche photodiodes, a very high bias voltage is required, for example more than 100V for silicon-based avalanche photodiodes. This high bias voltage is potentially dangerous for readout circuits that only support a few volts of voltage.

[0085] With a design that takes this into account, the compatibility between the avalanche photodiodes and the reading circuits is correct. But a problem arises when one or more photodiodes is / are defective either during manufacture or during use. The high bias voltage can then reach the reading circuits through uncontrolled and unpredictable paths, causing the destruction of the component. The capacitive hybridization of the present invention, by avoiding direct electrical contact between the photodiodes and their reading circuits, ensures electrical isolation that effectively protects the reading circuits from these voltages. high.

[0086] Thus the two embodiments above, with SFP type and CTIA type reading circuit respectively, can operate with avalanche effect photodiodes without any particular adaptation.

[0087] Of course, the present invention is in no way limited to the embodiments described and shown. In particular, the capacitive hybridization of the invention can be implemented with different types of reading circuits, as well as with photoelectric transduction elements other than simple photodiodes.

Claims

Claims

1. Set of two electronic microcircuits assembled together by producing a set of electrical couplings between homologous connection points of the two microcircuits, a first microcircuit being a transducer microcircuit with at least one photoelectric transduction element by charge accumulation, and the second microcircuit being a reading microcircuit with at least one element for reading the electrical charges generated by a respective photoelectric transduction element, and the connection points comprising facing surface elements separated by a dielectric, the coupling between the two circuits being a capacitive coupling.

2. Assembly according to claim 1, characterized in that the or each photoelectric capture element is a photodiode, one electrode of the photodiode being connected to a respective surface element located on the first microcircuit and its other electrode being connected to a reference potential.

3. Assembly according to claim 1 or 2, characterized in that the or each surface element located on the second microcircuit is connected to the input of a reading element comprising a circuit with infinite or quasi-infinite input impedance in direct current, associated with a reset switch.

4. Assembly according to claim 3, characterized in that the circuit with infinite or quasi-infinite input impedance in direct current is a voltage follower circuit.

5. Assembly according to claim 1 or 2, characterized in that the or each surface element located on the second microcircuit is connected to the input of a reading element comprising a capacitive transimpedance circuit associated with a reset switch.

6. Assembly according to claim 5, characterized in that the or each surface element located on the second microcircuit comprises a first part connected to the input of the capacitive transimpedance circuit and a second part belonging to a feedback connection, the first and second parts forming separate capacities with the corresponding surface element of the first microcircuit.

7. Assembly according to one of claims 1 to 6, characterized in that the first microcircuit comprises a matrix of transduction elements photoelectric by accumulation of charges, and in that the second microcircuit comprises a homologous matrix of elements for reading the electric charges generated by the respective photoelectric transduction elements.

8. Assembly according to claim 7, characterized in that it comprises at least one direct electrical connection between the first and second microcircuits for control signals common to the different elements of the matrices.

9. Assembly according to one of claims 7 and 8, characterized in that the first and second microcircuits each comprise at least one additional surface element, the two facing surface elements providing additional capacitive coupling for control signals common to the different elements of the matrices.

10. Assembly according to claim 9, characterized in that the additional surface elements surround the connection points.

11. Assembly according to one of claims 9 and 10, characterized in that the additional capacitive coupling is capable of conveying a signal for resetting the photoelectric transduction elements of the first microcircuit.

12. Assembly according to one of claims 1 to 11, characterized in that the dielectric comprises an adhesive for securing the two microcircuits.

13. An assembly according to claim 12, characterized in that the adhesive contains a permittivity increasing additive.

14. Assembly according to one of claims 1 to 10, characterized in that the dielectric comprises a layer of dielectric material assembled to the two microcircuits by molecular bonding.

15. Assembly according to one of claims 1 to 14, characterized in that the or each photoelectric capture element comprises an avalanche photodiode, the capacitive coupling ensuring isolation from the bias voltage of the or each photodiode.

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