Light travel time pixels

Abstract

Light travel time pixels with a photoactive area (PAB) in which photoelectrons are generated when radiation penetrates, wherein a discard node (DD) is arranged on a first side of the photoactive area (PAB) and memory areas (MA, MB) are arranged on a second and third side of the photoactive area (PAB), wherein each memory area (MA, MB) is followed by a transfer gate (TXA, TXB) and a readout area (DA, DB), wherein either on the second or third side of the photoactive area a single translucent control gate (SG) is arranged and the spatial distance of the control gate (SG) to one of the memory areas (MA, MB) is closer than to the opposite memory area (MA, MB).

Description

[0001] The invention relates to a light-time-of-flight pixel according to the preamble of the independent claim.

[0002] Time-of-flight pixels, as used here, specifically refer to pixels that determine distances from the phase shift of emitted and received radiation. PMD pixels with photomixing detectors (PMDs), as described, among others, in DE 197 04 496 A1, are particularly suitable as time-of-flight pixels, or 3D pixels. These pixels are used especially in 3D cameras, such as those available from companies like 'ifm electronic GmbH' or 'pmdtechnologies ag' as O3D cameras.

[0003] Furthermore, from DE 10 2019 100 460 A1, time-of-flight pixels with a so-called rejection node are known. The rejection node consists of an additional diode, which is separated from the rest of the pixel, for example, by a transfer gate. This arrangement serves to collect photogenerated electrons that are generated during the global readout of the pixel matrix and thus preferably prevent an unwanted change in the charges collected below the integration node.

[0004] DE 10 2020 132 868 A1 shows a time-of-flight pixel in a semiconductor substrate, wherein two centrally arranged photogates form a photoactive region, storage regions are arranged in the semiconductor substrate below the photogates and delimiting the photoactive region, and wherein the photogates only partially cover the storage regions. Transfer gates and readout diodes are arranged adjacent to the storage regions.

[0005] The object of the invention is to reduce the area requirement and power consumption of a time-of-flight pixel.

[0006] The problem is solved by the light-time-of-flight pixel according to the invention.

[0007] Advantageously, a light-time-of-flight pixel is provided, with a photoactive area in which photoelectrons are generated when radiation penetrates, wherein a discard node is arranged on a first side of the photoactive area and storage areas are arranged on a second and third side of the photoactive area, where each storage area is followed by a transfer gate and a read area, wherein either on the second or third side of the photoactive area a single translucent control gate is arranged and the spatial distance of the control gate to one of the memory areas is closer than to the opposite memory area.

[0008] Furthermore, a first doping area is located below the control gate in front of the storage area and a second doping area is located in front of the reject node.

[0009] This approach has the advantage that the photogenerated charges can be distributed between a first and second storage node as well as a discard node using a single control gate.

[0010] Preferably, the first and second doping regions are designed as threshold voltage implants such that the threshold voltage of the first doping region is lower than the threshold voltage of the second doping region.

[0011] The light-time-of-flight pixel is advantageously designed in such a way that that the photoelectrons generated in the photoactive range by applying different control voltages to the control gate, the light is directed in different directions within the time-of-flight pixel. - wherein a first control voltage, which is lower than the first threshold voltage, directs the photoelectrons towards the remote storage node, - wherein a second control voltage, which lies between the first and the second threshold voltage, directs the photoelectrons towards the nearest storage node, - and wherein a third control voltage, which is greater than the second threshold voltage, directs the photoelectrons towards the fault node.

[0012] They show schematically: Fig. 1 a pixel structure according to the invention in top view, Fig. 2 a cross-section of the structure according to Fig. 1 with potential curves, Fig. 3 a timing diagram for the operation of the pixel according to the invention.

[0013] The core idea of ​​the invention is that a pixel is electrostatically designed such that the photogenerated charge carriers can be directed to different pixel areas using three different potential levels of a control gate. For example, at low control voltage, the charge carriers are directed to channel B, at medium control voltage to channel A, and at high control voltage to a discard node. By using a single control gate, the area requirement and power consumption of the pixel can be reduced. The advantage over single-channel pixels with a modulation gate is that the entire signal can be utilized despite the use of only one gate.

[0014] Fig. Figure 1 shows a light-time-of-flight pixel according to the invention, consisting of a photoactive region or mixer region PAB in the center of the structure with two adjacent storage regions, MA and MB, and a reject node DD. Above this lies a control gate SG on one side, which is close to one of the two storage regions MA, MB.

[0015] Below the control gate SG, there are additional first and second doping areas VA and VD (shown in gray), which are used to set the threshold voltages of the individual areas. These areas are implemented, for example, in the form of implants. The respective transfer gates TXA and TXB are adjacent to the memory areas MA and MB, which in turn are adjacent to the readout diodes DA and DB.

[0016] The first and second doping regions VA and VD are designed as threshold voltage implants such that the threshold voltage of the first doping region VA is lower than the threshold voltage of the second doping region VD. The threshold voltage of the first doping region VA is lower than the threshold voltage of the second doping region VD. For example, the threshold voltage of the first doping region could be 1 V and the second threshold voltage 2.7 V.

[0017] Fig. 2 shows that in Fig. The pixel shown is a cross-sectional view, and various potential profiles are observed when applying application-typical control voltages to the control gate SG. In addition to the structural section, the potential sections for the following modes are shown: active channel B (low potential U1), active channel A (medium potential U2) and hold (high potential U3). Active Channel B:

[0018] If the control gate is set to a low potential U1, for example 0 volts, a potential minimum forms under the control gate SG. This results in a potential increase in the photoactive region PAB towards channel B. Photogenerated electrons are thus directed to channel B and stored there in the storage node MB. Active Channel A:

[0019] If the control gate SG is set to a medium potential U2, for example 1 volt, a potential similar to the storage area MA or A-channel is created beneath it. This results in a potential increase in the photoactive area PAB towards channel A. Photogenerated electrons are thus directed to channel A and stored there in the storage area MA. Hold:

[0020] If the control gate SG is set to a high potential U3, for example 2.7 volts, a potential maximum forms beneath it. Photogenerated electrons collect beneath the control gate SG and can be discharged towards the rejection node DD. The access point of the rejection node VD to the area beneath the control gate SG is only conductive in this mode. The storage areas MA and MB are designed such that a backflow of the already collected electrons to the control gate SG is prevented.

[0021] Fig. Figure 3 shows a possible timing diagram for the time-of-flight pixel according to the invention. As is typical for an iTOF method based on the phase measurement principle, the received modulated light is first demodulated by splitting it between the A and B channels or storage nodes. In the example shown, the pixel is designed for global shutter operation, so that after the modulated integration, the pixel is set to hold and the photoelectrons still being generated are directed to the discard node to prevent, for example, saturation or overflow of the photoactive area PAB. After the storage nodes MA and MB have been read, the discard node can be switched off, as shown in Figure 3. Fig. As shown in section 2, the modulated integration can be restarted.

[0022] Naturally, the pixel according to the invention also allows for other timing possibilities.

[0023] In a further embodiment, it is also conceivable to design the pixel for a rolling shutter operation, so that a holding phase and a derivation of the photoelectrons to a rejection node can be dispensed with, so that no rejection node and second doping region VD is necessary.

[0024] In a further embodiment, it is possible to provide two control gates for the demodulation of the photoelectrons, whereby one or possibly both control gates can have a first and / or a second doping region. In this case, one or possibly both control gates could be connected to a discard node.

Claims

[1] Light-time-of-flight pixel with a photoactive area (PAB) in which photoelectrons are generated when radiation penetrates, wherein a discard node (DD) is arranged on a first side of the photoactive area (PAB) and memory areas (MA, MB) are arranged on a second and third side of the photoactive area (PAB), wherein each memory area (MA, MB) is followed by a transfer gate (TXA, TXB) and a readout area (DA, DB), wherein either on the second or third side of the photoactive area a single translucent control gate (SG) is arranged and the spatial distance of the control gate (SG) to one of the memory areas (MA, MB) is closer than to the opposite memory area (MA, MB). [2] Light time-of-flight pixel according to claim 1, wherein a first doping area (VA) is arranged below the control gate (SG) in front of one of the memory areas (MA, MB) and a second doping area (VD) is arranged in front of the discard node (DD). [3] Light time-of-flight pixel according to claim 2, wherein the first and second doping regions are configured as threshold voltage implants such that a first threshold voltage (UA) of the first doping region (VA) is lower than a second threshold voltage (UD) of the second doping region (VD). [4] Light time-of-flight pixel according to claim 3, where the light travel time pixel is designed in such a way, that the photoelectrons generated in the photoactive area (PAB) are directed in different directions in the light-time-of-flight pixel by applying different control voltages to the control gate (SG), - wherein a first control voltage (U1), which is lower than the first threshold voltage (UA), directs the photoelectrons towards a storage node (MB) of the two storage areas (MA, MB) that is further away from the control gate (SG), - wherein a second control voltage (U2), which lies between the first and the second threshold voltage (UA, UD), directs the photoelectrons towards a storage node (MA) of the two storage areas (MA, MB) that is spatially closer to the control gate (SG), - and wherein a third control voltage (U3) which is greater than the second threshold voltage (UD) directs the photoelectrons towards the rejection node (DD). [5] Light time-of-flight pixel according to claim 4, where the first control voltage is smaller than the second control voltage, and the second control voltage is lower than the third control voltage. [6] Time-of-flight sensor with a matrix of time-of-flight pixels according to the preceding claims. [7] Time-of-flight camera with a time-of-flight pixel according to one of claims 1 to 5 or a time-of-flight sensor according to claim 6.

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