Voltage mode global shutter photosensitive detector and method based on composite dielectric gate capacitor

Abstract

The application provides a voltage type global shutter photosensitive detector based on a composite dielectric gate capacitor and a method. In a unit of the detector, a global shutter structure is arranged between a composite dielectric gate MOS capacitor and a reading transistor, and specifically includes a switch transistor and a switch capacitor. The structure of the switch transistor includes a bottom insulating dielectric layer, a control gate and a source-drain electrode. The source of the switch transistor is connected with the composite dielectric gate MOS capacitor. The drain of the switch transistor is connected with the switch capacitor in series. Charge signals are coupled from the composite dielectric gate MOS capacitor to the switch capacitor, so that the charge signals are converted into voltage signals. The control gate of the reading transistor is connected with the source of the switch transistor. Voltage signals are input from the control gate end of the reading transistor, and are read out by the drain of the reading transistor. The application can realize a global exposure function on the basis of a traditional photosensitive detector, and can effectively solve the analog domain noise caused by dark current at a photosensitive interface.

Description

Technical Field

[0001] This invention relates to photosensitive imaging detectors, specifically a novel imaging device, working mechanism, and signal readout applicable to the infrared, visible light, and ultraviolet light bands. It relates to a voltage-based global shutter photosensitive detector based on a composite dielectric gate capacitor and its working method. Background Technology

[0002] Currently, the main imaging devices are CCD and CMOS-APS. CCD appeared earlier, and its basic structure consists of MOS capacitors connected in series. After applying a pulsed bias voltage to the gate, a depletion region is formed to collect photoelectrons. Subsequently, the photoelectrons are read out in the form of charge packets by pulse timing. CCD has disadvantages such as complex manufacturing process, high cost, and high power consumption. In contrast, CMOS image sensors have lower cost and power consumption than CCD. CMOS-APS uses diodes for photosensitive sensing, and typically consists of one photodiode and three to six transistors for signal amplification and readout. Addressing and signal readout are achieved through external decoding circuitry. Because each detection unit contains several transistors, it results in a small fill factor for individual pixels, low sensitivity, and high noise.

[0003] The above comparison reveals that CCD and CMOS each have their own advantages and disadvantages. In light of this, patent CN201210442007 proposes a dual-transistor photodetector based on a composite dielectric gate MOSFET, which overcomes the drawbacks of complex readout methods in CCDs and the small fill factor and difficulty in miniaturizing the size of CMOS-APS. This detector contains two transistors, one for photosensitive and the other for readout. However, to ensure that the number of photoelectrons in the detector is not affected by the order of readout time, the imaging chip using this detector array must use a mechanical shutter, thus lacking global exposure functionality. This significantly limits its application range. Therefore, a new detector structure is needed to overcome this drawback and achieve global exposure functionality for the imaging chip. Summary of the Invention

[0004] To address the technical problems of existing detectors, this invention proposes a photodetector based on a composite dielectric gate structure that incorporates a new switching transistor and coupling capacitor to achieve a global shutter. Another objective of this invention is to provide a method for operating this photodetector.

[0005] The technical solution adopted by the device of the present invention is as follows:

[0006] A voltage-type global shutter photodetector based on a composite dielectric gate capacitor comprises a composite dielectric gate MOS capacitor for photosensitive sensing, a composite dielectric gate transistor for signal readout, and a global shutter structure. The global shutter structure is disposed between the composite dielectric gate MOS capacitor and the composite dielectric gate transistor. The global shutter structure includes a switching transistor and a switching capacitor. The switching transistor comprises a bottom insulating dielectric layer, a control gate, and source / drain terminals. The source of the switching transistor is connected to the composite dielectric gate MOS capacitor, and the drain of the switching transistor is connected in series with the switching capacitor to ground. A charge signal is coupled from the composite dielectric gate MOS capacitor to the switching capacitor, thereby converting the charge signal into a voltage signal. The composite dielectric gate transistor also comprises a bottom insulating dielectric layer, a control gate, and source / drain terminals, with its control gate connected to the source of the switching transistor. The voltage signal is input from the control gate terminal of the composite dielectric gate transistor and read out from the drain of the composite dielectric gate transistor.

[0007] Furthermore, the composite dielectric gate MOS capacitor comprises, from bottom to top, a bottom insulating dielectric layer, a floating gate, a top insulating dielectric layer, and a control gate, wherein the floating gate is connected to the control gate of the composite dielectric gate transistor and the switching capacitor through a metal layer.

[0008] Furthermore, the floating gate is made of polycrystalline silicon, metal, or semiconductor material; the control gate is made of polycrystalline silicon, metal, or transparent conductive electrode; the bottom insulating dielectric layer is made of silicon oxide or SiON; and the top insulating dielectric layer is made of a silicon nitride / silicon oxide / silicon nitride sandwich structure, a silicon oxide / alumina oxide / silicon oxide sandwich structure, silicon oxide, or alumina oxide.

[0009] Furthermore, the composite dielectric gate transistor and the global shutter structure are integrated on the same wafer, while the composite dielectric gate MOS capacitor is integrated on another wafer.

[0010] The present invention also provides a method for operating a voltage-based global shutter photodetector based on a composite dielectric gate capacitor, wherein during exposure, the photocharge generated by the composite dielectric gate MOS capacitor is not directly coupled to the composite dielectric gate transistor for readout, but is temporarily stored by coupling the charge signal to the switching capacitor through the switching transistor; after exposure, all switching transistors are turned off simultaneously, and then the voltage signal is read sequentially through the composite dielectric gate transistor, thereby realizing a charge-to-voltage global shutter function.

[0011] Furthermore, during exposure, the control gate of the switching transistor is positively biased and in the on state. At the same time, the composite dielectric gate MOS capacitor vertically transfers the generated photocharge from the substrate to the floating gate, and then temporarily stores it inside the switching capacitor in the global shutter structure through coupling. After the device exposure is completed, a negative bias is applied to the control gate of the switching transistor to make it in the off state. At the same time, a ramp voltage is applied to the control gate terminal of the composite dielectric gate transistor to convert the acquired charge signal into a voltage signal and read it out from the drain of the composite dielectric gate transistor.

[0012] Furthermore, the method includes the following steps:

[0013] Step 1, generation of photogenerated charge: A positive bias voltage is applied to the control gate terminal of the composite dielectric gate MOS capacitor, and a negative bias voltage is applied to the substrate. The vertical electric field in the vertical direction forms a depletion region on the substrate. The photogenerated charge generated by the light is collected below the composite dielectric gate MOS capacitor and then coupled into the floating gate of the composite dielectric gate MOS capacitor.

[0014] Step 2, Transfer and storage of photogenerated charge: All switching transistors are driven by a forward conduction voltage. During global exposure, a forward conduction voltage is applied to the control gate of the switching transistors, and the collected charge is coupled from the floating gate of the composite dielectric gate MOS capacitor to the switching capacitor for temporary storage. After the photoelectron transfer is completed, a zero bias voltage of the same potential as ground is applied to the control gate of the composite dielectric gate MOS capacitor. At this time, there is no longer a depletion region in the composite dielectric gate MOS capacitor, and no photoelectrons are collected.

[0015] Step 3, reading of photogenerated charge: Apply a zero bias voltage to the control gate of the switching transistor to turn off all switches. Simultaneously, ground the source of the composite dielectric gate transistor and apply a suitable positive bias voltage to the drain. At this time, the composite dielectric gate transistor couples the photoelectrons collected by the switching capacitor to the gate of the readout transistor. At the same time, apply a ramp voltage to the control gate of the composite dielectric gate transistor, and the charge signal is converted into a voltage signal, which is sequentially read by the drain of the composite dielectric gate transistor. This realizes the function of charge-voltage global shutter. By inputting the voltage signal from the control gate of the composite dielectric gate transistor and reading it from its drain, the number of photoelectrons collected by the composite dielectric gate MOS transistor can be calculated.

[0016] Step 4, Reset of photogenerated charge: Turn on the switch, and at the same time apply a negative bias voltage of the same magnitude as the bias voltage applied to the substrate to the control gate of the composite dielectric gate MOS capacitor and the control gate of the composite dielectric gate transistor. The source and drain of the composite dielectric gate transistor are grounded, and photoelectrons disappear by recombination with holes in the substrate.

[0017] The mechanism of this invention for transferring charge signals to achieve a global shutter is as follows: When the switching transistor is forward-biased, the same negative bias voltage as the substrate is applied to the composite dielectric gate MOS capacitor. This causes photoelectrons generated by illumination to couple from the floating gate of the photosensitive MOS capacitor into the switching capacitor under a longitudinal electric field for temporary storage. After exposure, all photogenerated charges have been completely stored in the capacitor of the global shutter structure. At this time, a zero bias voltage is applied to the gate of the switching transistor to turn it off. By applying a positive voltage to the control gate of the readout transistor, the charge signal is transferred from the capacitor to the control gate of the readout transistor. Photoelectrons couple into the control gate of the composite dielectric gate transistor and change the surface potential. The source of the readout transistor is grounded, and the voltage signal is read from the drain, thereby calculating the number of generated photocharges. This invention achieves a global exposure function by applying different voltages to the source, drain, and gate of the photosensitive composite dielectric gate MOS capacitor, the readout transistor, and the switching transistor, controlling the opening and closing of the photoelectron collection channel during exposure, as well as the coupling and transfer in various structures of the device.

[0018] The photodetector proposed in this invention, based on the composite dielectric gate device structure, adds a global switching transistor and a switching capacitor between the photosensitive composite dielectric gate MOS capacitor and the readout composite dielectric gate transistor. This achieves a global shutter without increasing the lateral area of ​​the original detector and is compatible with advanced stacking processes, showing great application potential. The method of converting charge signals into voltage to achieve the global shutter function in this invention has the following specific features and advantages:

[0019] (1) It can realize the global exposure function: the working mode of using capacitor coupling to generate photocharge and then transmitting voltage signal through source follower replaces the original working mode. The flexible pressure method can add global shutter function to the photodetector, which is beneficial to expanding the application of photodetector in dynamic scenes compared with rolling shutter exposure.

[0020] (2) Low noise: By directly coupling the charge through an external capacitor, the noise in the analog domain caused by the direct conversion of charge into a voltage signal is avoided. This effectively avoids the influence of dark current noise at the interface and improves imaging quality.

[0021] (3) Existing stacking technology can be used to stack two wafers, in which the photosensitive transistor is placed on the lower wafer, and the global shutter and readout transistor are placed on the upper wafer. Metal vias and interconnects are added between the two to facilitate the transmission of charge signals. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the device of the present invention;

[0023] Figure 2This is the AA' cross-sectional view of the device structure diagram of the present invention;

[0024] Figure 3 This is a top view of the device of the present invention;

[0025] Figure 4 This is the equivalent circuit diagram of the device of the present invention. Detailed Implementation

[0026] This example provides a device structure for a voltage-type global shutter photodetector based on a composite dielectric gate capacitor. The detector units are all fabricated on a P-type substrate, including a composite dielectric gate transistor (readout transistor) and a composite dielectric gate MOS capacitor for photosensitive functions. Between them is a switching transistor for implementing the global shutter function and a switching capacitor for temporarily storing charge. Existing stacking technology is used for dual-wafer stacking, with the photosensitive composite dielectric gate MOS capacitor placed on the lower wafer, and the global shutter structure and readout transistor on the upper wafer, connected by metal vias and interconnects. The specific structure is described as follows: A photosensitive composite dielectric gate MOS capacitor is used to realize the photosensitive function of the photodetector; a global shutter structure is used to realize the transfer of photogenerated charge between the composite dielectric gate MOS capacitor and the readout transistor, including a switching transistor and a switching capacitor. The source terminal of the switching transistor is connected to the floating gate of the composite dielectric gate MOS capacitor and the control gate of the readout transistor. The drain of the switching transistor is connected in series with the switching capacitor to ground. The charge signal is coupled from the composite dielectric gate MOS capacitor to the switching capacitor, thereby converting the charge signal into a voltage signal. The composite dielectric gate transistor operates in a source follower mode, that is, the acquired voltage signal is input from the gate terminal of the composite dielectric gate transistor and read out from the drain terminal of the composite dielectric gate transistor. The readout transistor has a particularly high input impedance, a low output impedance, and a voltage amplification factor of approximately 1, which can reconstruct the acquired charge signal. The global shutter structure works as follows: by controlling the conduction of the switching transistor, the photogenerated charge generated by the composite dielectric gate MOS capacitor is transferred to the switching capacitor in the global shutter structure. By controlling the cutoff of the switching transistor, the photogenerated charge stored on the switching capacitor is converted into a voltage signal by the readout transistor and read out, thus realizing the charge-to-voltage type global shutter operation.

[0027] This embodiment describes the fabrication process of a voltage-type global shutter photodetector array based on a composite dielectric gate capacitor: First, the composite dielectric gate MOS capacitor is constructed, mainly forming a bottom insulating dielectric layer, a charge coupling layer, a top insulating dielectric layer, and a control gate. Next, the switching transistor and readout transistor are constructed, mainly forming a bottom insulating dielectric layer, a control gate, and the source and drain regions of the transistors. Finally, the processes include via fabrication, metal interconnect fabrication, stacking, passivation layer fabrication, and wafer surface planarization.

[0028] like Figure 1 As shown, the photosensitive composite dielectric gate MOS capacitor of the detector unit includes, from bottom to top, a bottom insulating dielectric layer, a floating gate, a top insulating dielectric layer, and a control gate. The dielectric layer is formed by deposition. In this embodiment, the bottom insulating dielectric layer is made of silicon oxide, and the top insulating dielectric layer is a three-layer structure of silicon oxide-silicon nitride-silicon oxide. The dielectric layer thickness is about ten nanometers, the floating gate thickness is about tens of nanometers, and the gate length and width are both submicron to several micrometers. Metal vias are drilled on the floating gate, and the read transistor and the photosensitive composite dielectric gate MOS capacitor are fabricated on two wafers through the metal layer, using a stacking process for dual-wafer stacking. Simultaneously, the read transistor and global shutter structure are fabricated on the p-type substrate of the upper wafer using the same process. The read transistor includes, from bottom to top, an insulating dielectric layer, a control gate, and source / drain regions. The global shutter structure includes a switching transistor and a switching capacitor. The switching transistor includes, from bottom to top, an insulating dielectric layer, a control gate, and source / drain regions, while the switching capacitor is a layer of polysilicon deposited on the p-type substrate. For the photosensitive composite dielectric gate MOS capacitor, as... Figure 4This can be represented as two capacitors, C1 and C2. A switching capacitor is externally connected to the floating node between C1 and C2, and an external switching transistor controls the transfer and storage of charge signals within it. During exposure, a negative voltage is applied to the substrate to form a depletion region as a photoelectron collection region. The depletion region extends from the substrate surface to a depth of several micrometers. Photoelectrons entering the depletion region are eventually collected to the substrate surface under the influence of a vertical electric field. Then, a forward conduction voltage is applied to the switching transistor to drive it, keeping the switching transistor on during global exposure. The generated photosensitive charge is then coupled to the switching capacitor C3 for temporary storage. When all the photogenerated charge has been transferred, a zero bias voltage is applied to the control gate of the switching transistor to turn it off, and a ramp voltage is applied to the control gate of the readout transistor. At this time, the photogenerated charge will couple to the surface of the readout transistor and change the threshold voltage. The number of photogenerated charges is calculated by reading the offset of the threshold voltage at the drain of the readout transistor. This invention introduces a global shutter structure for flexible transfer, storage, and readout of photogenerated charge. This global shutter structure enables multiple detectors on the entire wafer to simultaneously sense light and then read it sequentially. This global shutter structure increases the sensitivity of charge signals and effectively avoids the influence of dark current noise at the interface, thereby improving image quality.

[0029] The operating method of the photodetector in this embodiment includes the following steps:

[0030] (1) Generation of photogenerated charge: A positive bias voltage is applied to the control gate of the photosensitive composite dielectric gate MOS capacitor, and a negative bias voltage is applied to the substrate. The vertical electric field in the vertical direction forms a depletion region on the P-type bulk silicon. The photogenerated charge generated by the light is collected below the composite dielectric gate MOS capacitor.

[0031] (2) Transfer of photogenerated charge and shutdown of collection: Apply a forward conduction voltage to the switching transistor to keep the switching transistor on during global exposure. The collected charge is then coupled to the switching capacitor for temporary storage. After the photoelectron transfer is completed, apply a zero bias voltage of the same as ground potential to the control gate of the composite dielectric gate MOS capacitor. At this time, there is no longer a depletion region in the composite dielectric gate MOS capacitor, and no photoelectrons are collected.

[0032] (3) Photoelectron readout: Apply zero bias voltage to the control gate of the switching transistor to turn off all switches, and at the same time, ground the source of the read transistor and apply a suitable positive bias voltage to the drain. At this time, the photoelectrons collected by the switching capacitor will couple to the read transistor, changing the threshold voltage of the composite dielectric gate transistor. By applying a ramp voltage to the control gate of the read transistor and reading the change value of the ramp voltage at its drain, the number of photoelectrons collected by the read transistor can be calculated. In this way, the charge signal can be converted into a voltage signal and read sequentially by the read transistor, thus realizing the function of charge-voltage global shutter.

[0033] (4) Reset of photoelectrons: Apply a forward conduction voltage to the switching transistor, and at the same time apply a negative bias voltage of the same magnitude as the bias voltage applied to the substrate to the control gate of the composite dielectric gate MOS capacitor and the control gate of the read transistor. The source and drain of the read transistor are grounded, and the photoelectrons disappear by recombination with the holes in the substrate.

Claims

1. A voltage-mode global shutter photosensitive detector based on composite gate capacitor, the unit of the detector comprising a composite gate MOS capacitor for light sensing, a composite gate transistor for reading signal and a global shutter structure, characterized in that, A global shutter structure is positioned between a composite dielectric gate MOS capacitor and a composite dielectric gate transistor. The global shutter structure includes a switching transistor and a switching capacitor. The switching transistor comprises a bottom insulating dielectric layer, a control gate, and source / drain terminals. The source of the switching transistor is connected to the composite dielectric gate MOS capacitor, and the drain of the switching transistor is connected in series with the switching capacitor to ground. A charge signal is coupled from the composite dielectric gate MOS capacitor to the switching capacitor, thereby converting the charge signal into a voltage signal. The composite dielectric gate transistor also comprises a bottom insulating dielectric layer, a control gate, and source / drain terminals, with its control gate connected to the source of the switching transistor. The voltage signal is input from the control gate terminal of the composite dielectric gate transistor and read out from its drain.

2. The composite dielectric gate capacitance based voltage mode global shutter photosensitive detector of claim 1, wherein, The composite dielectric gate MOS capacitor comprises, from bottom to top, a bottom insulating dielectric layer, a floating gate, a top insulating dielectric layer, and a control gate. Its floating gate is connected to the control gate of the composite dielectric gate transistor and the switching capacitor through a metal layer.

3. The composite dielectric gate capacitance based voltage mode global shutter photosensitive detector of claim 2, wherein, The floating gate is made of polycrystalline silicon, metal, or semiconductor material; the control gate is made of polycrystalline silicon, metal, or transparent conductive electrode; the bottom insulating dielectric layer is made of silicon oxide or SiON; the top insulating dielectric layer is made of silicon nitride / silicon oxide / silicon nitride sandwich structure, silicon oxide / alumina / silicon oxide sandwich structure, silicon oxide, or alumina.

4. The composite dielectric gate capacitance based voltage mode global shutter photosensitive detector of claim 1, wherein, The composite dielectric gate transistor and the global shutter structure are integrated on the same wafer, while the composite dielectric gate MOS capacitor is integrated on another wafer.

5. A method of operating a voltage-mode global shutter photosensitive detector based on a composite dielectric gate capacitor as claimed in claim 1, wherein, During exposure, the photocharge generated by the composite dielectric gate MOS capacitor is not directly coupled to the composite dielectric gate transistor for readout. Instead, the charge signal is coupled to the switched capacitor for temporary storage through the switching transistor. After exposure, all the switching transistors are turned off simultaneously, and then the voltage signal is read sequentially through the composite dielectric gate transistor, thereby realizing a charge-to-voltage global shutter function.

6. The method of working according to claim 5, characterized in that, During exposure, the control gate of the switching transistor is positively biased and in the conducting state. At the same time, the composite dielectric gate MOS capacitor vertically transfers the generated photocharge from the substrate to the floating gate, and then temporarily stores it inside the switching capacitor in the global shutter structure through coupling. After the device exposure is complete, a negative bias voltage is applied to the control gate of the switching transistor to put it in the off state; at the same time, a ramp voltage is applied to the control gate terminal of the composite dielectric gate transistor to convert the acquired charge signal into a voltage signal and read it out from the drain of the composite dielectric gate transistor.

7. The method of working according to claim 6, characterized in that, The method includes the following steps: Step 1, generation of photogenerated charge: A positive bias voltage is applied to the control gate terminal of the composite dielectric gate MOS capacitor, and a negative bias voltage is applied to the substrate. The vertical electric field in the vertical direction forms a depletion region on the substrate. The photogenerated charge generated by the light is collected below the composite dielectric gate MOS capacitor and then coupled into the floating gate of the composite dielectric gate MOS capacitor. Step 2, Transfer and storage of photogenerated charge: All switching transistors are driven by a forward conduction voltage. During global exposure, a forward conduction voltage is applied to the control gate of the switching transistors, and the collected charge is coupled from the floating gate of the composite dielectric gate MOS capacitor to the switching capacitor for temporary storage. After the photoelectron transfer is completed, a zero bias voltage of the same potential as ground is applied to the control gate of the composite dielectric gate MOS capacitor. At this time, there is no longer a depletion region in the composite dielectric gate MOS capacitor, and no photoelectrons are collected. Step 3, reading of photogenerated charge: Apply a zero bias voltage to the control gate of the switching transistor to turn off all switches. Simultaneously, ground the source of the composite dielectric gate transistor and apply a suitable positive bias voltage to the drain. At this time, the composite dielectric gate transistor couples the photoelectrons collected by the switching capacitor to the gate of the readout transistor. At the same time, apply a ramp voltage to the control gate of the composite dielectric gate transistor, and the charge signal is converted into a voltage signal, which is sequentially read by the drain of the composite dielectric gate transistor. This realizes the function of charge-voltage global shutter. By inputting the voltage signal from the control gate of the composite dielectric gate transistor and reading it from its drain, the number of photoelectrons collected by the composite dielectric gate MOS transistor can be calculated. Step 4, Reset of photogenerated charge: Turn on the switch, and at the same time apply a negative bias voltage of the same magnitude as the bias voltage applied to the substrate to the control gate of the composite dielectric gate MOS capacitor and the control gate of the composite dielectric gate transistor. The source and drain of the composite dielectric gate transistor are grounded, and photoelectrons disappear by recombination with holes in the substrate.

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