Light time-of-flight sensor

Abstract

Light time-of-flight sensor (22) with modulation gates (Gam, Gbm) in light-sensitive and read fingers (Ga, Gb) in light-insensitive areas, wherein the modulation gates (Gam, Gbm, G0) and read fingers (Ga, Gb) are arranged in parallel strips which form a light time-of-flight pixel (23) in groups, wherein a light-time-of-flight pixel (23) has at least two read fingers (Ga, Gb), characterized in that that the light time-of-flight sensor (22) has at least one light time-of-flight pixel row (23z) with at least three light time-of-flight pixels (23), and that the selection fingers (Ga, Gb), which are not arranged at the edge of the (23Z) line, are adjacent on two sides to a modulation gate (Gam, Gbm), wherein the readout fingers (Ga, Gb) are surrounded by two modulation gates (Gam, Gbm) acting in the same potential, wherein the readout fingers (Ga, Gb) located at the edge of the light transit time pixel (23) share a light-sensitive area (26a) with the adjacent pixel.

Description

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

[0002] The light-time sensor particularly relates to time-of-flight camera systems, particularly time-of-flight or 3D-TOF camera systems that obtain time-of-flight information from the phase shift of emitted and received radiation. PMD cameras with photonic mixer detectors (PMD) are particularly suitable as time-of-flight or 3D-TOF cameras. These are described, among other things, in applications EP 1 777 747 B1, US 6 587 186 B2, and also DE 197 04 496 C2 and are available, for example, from ifm electronic GmbH or PMD-Technologies GmbH as the O3D frame grabber or CamCube. The PMD camera, in particular, allows for a flexible arrangement of the light source and detector, which can be arranged either in one housing or separately.

[0003] Furthermore, from DE 198 21 974 A1 and from WO 02 / 33817 A1, a device for detecting the phase and amplitude of electromagnetic waves or signal waves is known, in which light-transmitting modulation gates are arranged in the photosensitive area and charge-collecting accumulation gates are arranged in strip form.

[0004] DE 10 2004 037 137 A1 deals with a method and a device for distance measurement in which, among other things, an indirect TOF method, in particular a PMD sensor, is combined with a triangulation method.

[0005] Furthermore, WO 02 / 49339 A2 discloses a 3D image sensor that uses so-called quantum efficiency modulation for distance measurement.

[0006] DE 10 2011 053 219 A1 also discloses a PMD pixel with color-selective subpixels, each of which has a pixel area that is selectively photosensitive for a different spectral range.

[0007] EP 1 513 202 A1 describes a time-of-flight pixel for determining the phase and amplitude of modulated emitted and demodulated received light. Demodulation occurs by introducing a potential into the time-of-flight pixel, and the charge carriers are accumulated within a detection area of ​​the time-of-flight pixel. The proposed pixels are surrounded by a metallization that is at a ground or 0 V potential.

[0008] DE 10 2004 016 624 A1 describes a photonic mixer or photonic mixer pixel with a photosensitive layer, at least two modulation gates, and at least two readout electrodes. The modulation gates are connected to a modulation device that modulates the potential of the modulation gates relative to each other and to the readout electrodes. Within a pixel, two adjacent mixer elements can have a common readout electrode.

[0009] The object of the invention is to improve the fill factor on a time-of-flight sensor.

[0010] The object is advantageously achieved by the light transit time sensor according to the invention according to the preamble of the independent claim.

[0011] Advantageously, a time-of-flight sensor is provided with modulation gates in light-sensitive areas and readout fingers in light-insensitive areas. The modulation gates and readout fingers are arranged in parallel strips that, in groups, form a time-of-flight pixel, and each time-of-flight pixel has at least two readout fingers. The embodiment further provides that the time-of-flight sensor has at least one time-of-flight pixel row with at least three time-of-flight pixels, and that the readout fingers that are not arranged at the edge of the row are adjacent to a modulation gate on two sides.

[0012] This approach has the advantage that there is no need for an isolation distance separating the individual pixels and the fill factor can be increased.

[0013] In a further preferred embodiment, the readout fingers comprise a readout gate and a readout node, wherein the readout node is formed as a pn structure with metallization. In contrast to structures formed as a pn junction across the entire finger width, a structure comprising a gate and a readout node can be formed spatially smaller, thereby further improving the fill factor.

[0014] Preferably, the readout nodes are arranged in an end region of the readout finger near the readout circuit in order to shorten signal paths.

[0015] In a further embodiment, the readout nodes are enclosed by the readout gate on at least three sides. This advantageously prevents crosstalk or penetration to the modulation gates.

[0016] In a further embodiment, at least three modulation gates are arranged between two readout fingers, with the modulation gates adjacent to the readout fingers being transparent in both the infrared and the visual range. This advantageously makes it possible to equip the sensor with additional functionality without compromising the performance of 3D image capture.

[0017] The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

[0018] They show schematically: Fig. 1 the basic principle of a time-of-flight camera based on the PMD principle, Fig. 2 a modulated integration of the time-shifted generated charge carriers, Fig. 3 a cross-section of a PMD pixel, Fig. 4 a top view of a PMD pixel, Fig. 5 a top view of a PMD pixel with readout nodes, Fig. 6 a top view of a PMD pixel with forked readout fingers, Fig. 7 a cross-section through a PMD pixel according to the invention with two modulation gates, Fig. 8 a schematic plan view of a PMD pixel according to the invention, Fig. 9 a cross-section and a top view of a continuous PMD pixel row, Fig. 10 a top view of a continuous PMD pixel line with minimum pixel size, Fig. 11 an arrangement of several continuous PMD pixel lines, Fig. 12 a formation of a PMD pixel line with RGB filters, Fig. 13 a PMD pixel row with three modulation gates each, Fig. 14 two PMD pixel lines with different phase positions.

[0019] In the following description of the preferred embodiments, like reference numerals designate like or comparable components.

[0020] Fig. 1 shows a measuring situation for an optical distance measurement with a time-of-flight camera, as known, for example, from DE 197 04 496 C2.

[0021] The time-of-flight camera system 1 comprises a transmitting unit or illumination module 10 with an illumination light source 12 and associated beam-shaping optics 15, as well as a receiving unit or time-of-flight camera 20 with receiving optics 25 and a time-of-flight sensor 22. The time-of-flight sensor 22 has at least one pixel, but preferably a pixel array, and is designed in particular as a PMD sensor. The receiving optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmitting unit 10 is preferably designed as a reflector. However, diffractive elements or combinations of reflective and diffractive elements can also be used.

[0022] The measuring principle of this arrangement is essentially based on the fact that the time of flight of the emitted and reflected light can be determined based on the phase shift of the emitted and received light. For this purpose, the light source 12 and the time-of-flight sensor 22 are jointly subjected to a specific modulation frequency or modulation signal with a first phase position a via a modulator 30. Depending on the modulation frequency, the light source 12 emits an amplitude-modulated signal with phase a. In the illustrated case, this signal or the electromagnetic radiation is reflected by an object 40 and, due to the distance traveled, impinges on the time-of-flight sensor 22 with a second phase position b correspondingly phase-shifted.In the time-of-flight sensor 22, the signal of the first phase position a of the modulator 30 is mixed with the received signal, which has the time-of-flight-dependent second phase position b, wherein the phase shift or the object distance d is determined from the resulting signal.

[0023] For a more precise determination of the second phase position b and thus the object distance d, it may be provided to change the phase position a with which the time-of-flight sensor 22 is operated in order to adjust predefined phase shifts Δφ. Similarly, it may also be provided to specifically shift the phase with which the illumination is driven.

[0024] The principle of phase measurement is shown schematically in Fig. 2. The upper curve shows the temporal progression of the modulation signal used to control the illumination 12 and the time-of-flight sensor 22, here without phase shift. The light b reflected from the object 40 hits the object according to its time of flight t. L phase-shifted to the time-of-flight sensor 22. The time-of-flight sensor 22 collects the photonically generated charges q during the first half of the modulation period in a first readout finger Ga and in the second half of the period in a second readout finger Gb. The charges are typically collected or integrated over several modulation periods. The phase shift and thus the distance of the object can be determined from the ratio of the charges qa, qb collected in the first and second gates Ga, Gb.

[0025] As already known from DE 197 04 496 C2, the phase shift of the light reflected from the object, and thus the distance, can be determined, for example, using a so-called IQ (in-phase quadrature) method. To determine the distance, two measurements are preferably performed with modulation signals whose phase positions are shifted by 90°. For example, φ mod + φ0 and φ mod + φ 90 , whereby the phase shift of the reflected light can be determined from the charge difference Δq(0°), Δq(90°) determined in these phase positions using the well-known arctan or arctan2 relationship. φ=arctanΔq(90°)Δq(0°)

[0026] To improve accuracy, further measurements can be carried out with phase positions shifted by, for example, 180°. φ=arctanΔq(90°)−Δq(270°)Δ(0°)−Δq(180°)

[0027] Of course, measurements with more than four phases and their multiples and a correspondingly adapted evaluation are also conceivable.

[0028] Fig. Figure 3 shows a cross-section through a pixel of a photonic mixer detector as known, for example, from DE 197 04 496 C2. The modulation photogates G am , G0, G bm form the light-sensitive area of ​​a PMD pixel. According to the modulation gates G am , G0, G bm applied voltage, the photonically generated charges q are transferred either to one or the other readout finger G a , G b directed. In the example shown, the readout finger is designed as a pn junction or diode.

[0029] Fig. Figure 3b shows a potential curve where the charges q are directed towards the first readout finger G a while the potential according to Fig. 3c the charge q in the direction of the second readout finger Gb The potentials are specified according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in the range of 1 to 100 MHz. For example, a modulation frequency of 1 MHz results in a period of one microsecond, so the modulation potential changes every 500 nanoseconds.

[0030] In Fig. 3a also shows a readout unit 400, which may already be part of a PMD time-of-flight sensor designed as a CMOS. The readout finger G a , G bThe detected photonically generated charges are collected over a plurality of modulation periods. In a known manner, the voltage then present at the readout fingers Ga, Gb can be tapped at a high impedance, for example, via the readout unit 400. The integration times should preferably be selected such that the time-of-flight sensor or the readout fingers, or their capacitances and / or the light-sensitive areas, do not reach saturation for the expected amount of light.

[0031] Fig. Figure 4 shows a top view of a time-of-flight pixel 23 with typical length specifications. The distance or length between the two readout fingers Ga, Gb is referred to as the channel spacing L K and the distance between the longitudinal symmetry axes of the two readout fingers Ga, Gb as finger pitch L FPAccording to the invention, the pixels are not separated from one another by a field oxide, so that a further modulation gate is directly connected to the readout finger.

[0032] The extension of the gates or readout fingers parallel to the channel spacing L K is the gate length L G and the perpendicular extension is the gate width B G While the gates in their latitudes B G are essentially the same length, the gate lengths are typically varied depending on the application. In the illustrated case, for example, the middle modulation gate G0 is longer than the two outer modulation gates G am , G bm , for example to influence a modulation contrast.

[0033] The Auslesefingers G a , G bcan be constructed and structured in different ways. For example, it is known from DE 197 04 496 C2 to design the readout fingers as pn junctions or diodes. The diode preferably extends across the entire width of the readout fingers. The readout fingers are preferably opaque or covered with an opaque layer, preferably a metallization. The metallization can also simultaneously form the contact for the diode.

[0034] Fig. Figure 5 shows a preferred embodiment according to the invention in which the readout fingers Ga, Gb have a gate structure and an integration or readout node. In the illustrated case, a readout gate AG extends a , AG b across the entire width B G of the readout finger Ga, Gb. In one end region of the readout finger Ga, Gb there is a readout node AK a , AK b As already mentioned in Fig. 3 shown, is located at the Auslesegate AG a , AG b a voltage that forms a potential well for the charge carriers q below the gate. The charge carriers collected there flow to the respective readout node AK primarily due to diffusion processes. a , AK b and can be tapped there by the readout unit 400, for example as voltage.

[0035] Preferably, the readout fingers Ga, Gb are covered with an opaque layer, preferably a metal layer. The metal layer can also serve to contact the readout node. In a further embodiment, it is also conceivable to influence the transparency of the readout gate itself, for example, by silicide coating.

[0036] Furthermore, variations in the size and position of the readout nodes AK a , AK b For example, the readout nodes AK a , AK bthe same length as the Auslesegates AG a , AG b Multiple readout nodes can also be distributed across the gate structure. In particular, readout nodes can be arranged at both ends of the gate. A central readout node is also conceivable. The readout nodes are preferably designed as a pn junction or a diode.

[0037] Fig. Figure 6 shows another variant in which the readout fingers or gate structure are extended in the area of ​​the readout nodes. This branching allows the readout nodes AK a , AK b surrounded by a gate structure and separated by a minimum distance from the neighboring modulation gate Gam, Gbm. This approach advantageously reduces or inhibits electrical penetrations from the modulation gate to the readout nodes.

[0038] As already mentioned, a PMD pixel is typically separated from the neighboring matrix pixel using a field oxide and an optical cover. This typically minimizes crosstalk between the individual matrix pixels. However, for very small pixels, the fill factor plays an increasingly important role. It is particularly disadvantageous if the separating field oxide matrix has a larger dimension than the readout fingers. If the pixel pitch is on the same order of magnitude as the finger pitch, the edge region or separation region plays an increasingly important role with regard to the fill factor.

[0039] The typical fingerpitch L FP , i.e., readout fingers plus modulation gates, is on the order of approximately 7 - 10 µm. The height and width of the fingers and the channel length of PMD structures are scalable over a very wide range, similar to transistors.

[0040] By using smaller manufacturing processes and miniaturizing the readout electronics, it is possible to increase the pixel pitch L PP or the size of a pixel in the order of the finger pitch L FP Furthermore, there is a desire for sensors with the highest possible resolution while being small in size.

[0041] To solve these technical requirements, a continuous PMD pixel structure without separating field oxides is proposed.

[0042] Fig. 7 and Fig. 8 show such a continuous pixel structure.

[0043] Fig. Figure 7a shows a cross-section of such a structure with four readout fingers Ga, Gb and two modulation gates Gam, Gbm between each of the readout fingers Ga, Gb. The modulation gates Gam, Gbm are as already shown in Fig. 3, they are translucent and form a light-sensitive region 26 of the time-of-flight pixel 23. The readout fingers Ga, Gb are covered with a mask and form a light-insensitive region 27. Below the readout fingers Ga, Gb, doping regions are indicated, which can be n- or p-doped depending on the base material of the semiconductor. As already shown in the examples above, this can also be a gate structure with readout nodes.

[0044] In Fig. Figure 7b shows a possible potential curve for the Fig. 7a shown structure, analogous to the representation according to Fig. 3c.

[0045] Fig. Figure 8 shows a top view of the pixel structure according to Fig. 7a. The readout fingers Ga, Gb are read out by a readout unit 400, with readout fingers of the same name being fed together to an evaluation unit 420, which determines, for example, the charge differences Δq or the charge sums Σq of the readout fingers Ga, Gb. The modulation signal originating from the modulator 30 is fed in phase opposition to the modulation gates Gam, Gbm via a potential supply line 35. The modulation gates Gam, Gbm in the immediate vicinity of a readout finger Ga, Gb are each at the same potential. The readout unit 400 and the potential supply line 35, as well as the readout fingers Ga, Gb, are covered with an opaque mask.

[0046] The Fig. 7 and Fig. 8 shown light-time pixel is, according to the invention, as in Fig. 9, arranged continuously and without separation in a pixel line. Fig. The pixel row shown in Figure 9 is divided into four individual pixels 23.1 - 23.4. The pixel boundaries are marked with dot-dash lines. The readout fingers located at the edge of the time-of-flight pixel share a light-sensitive area 26a with the neighboring pixel. The charge carriers generated there thus flow, depending on the potential level, once to the neighboring pixel and once to their own readout finger. However, since this process is repeated at both outer readout fingers with opposite signs, this charge transfer balances out on average, so that effectively, as in Fig. 9, only half of the common light-sensitive area 26a is active for the respective pixel.

[0047] Fig. Figure 10 shows a smallest possible pixel structure with only two readout fingers Ga, Gb and four modulation gates Gam, Gbm. As in the pixels shown so far, the readout fingers are surrounded by two modulation gates with the same potential. The pixel pitch L PP is in this case only slightly larger than the finger pitch L FP .

[0048] Fig. 11 shows an arrangement with several PMD pixel rows 23z, in which two pixel rows each share a surface area for the potential supply lines 35 and readout units 400 for a space-saving arrangement.

[0049] Fig. 12 or Fig. Figure 12a shows a variant with three modulation gates Gam, Gbm, G0 which have different optical filters to generate, for example, visual color information.

[0050] To capture color information, for example, a potential according to Fig. 12b are applied to the pixel structure. By applying color filters Red R, Green G, and Blue B over the modulation gates in combination with appropriate gate control, it is possible to extract color information from the sensor with a resolution higher than the physical resolution in terms of 3D values.

[0051] The modulation gates on the sides of the readout fingers are equipped with the same filters on both sides, which, in addition to being transparent in one of the three primary colors, are also transparent in the infrared. The middle gate is only transparent in the infrared.

[0052] In color or RGB mode, the middle gates G0 are preferably driven with 0 volts, while the outer modulation gates Ga, Gb are applied with a voltage between the readout or separation gates. The charge carriers generated beneath the color filters are collected at the nearest readout finger Ga, Gb. The potential barrier beneath the middle modulation gate G0 cannot be overcome by the charge carriers.

[0053] For photon mixing, a potential according to Fig. 12c is applied to the modulation gates, whereby the modulated illumination occurs in the infrared IR.

[0054] Fig. Figure 13 shows a structure or circuitry in which a higher virtual pixel resolution can be realized compared to the physical pixel resolution. Typically, the pixels used are square, but this is not mandatory. A pixel consists of at least two readout fingers Ga, Gb and the modulation gates Gam, Gbm, G0, or MOD. In the example shown here, the actual pixel cell has a 2:1 width-to-height ratio. By implementing the pixels as continuous matrix pixels, it is possible to achieve a virtually higher pixel resolution by calculating distance values ​​from the charge differences between all readout fingers Ga, Gb and the respective neighboring readout fingers Ga, Gb. The resulting virtual pixel pitch L VPP In the example shown, this is 1:1 while the physical pixel pitch L PP 2:1.

[0055] Typically, a PMD sensor captures four frames sequentially with four different phase angles. While this allows for high spatial resolution, it results in low temporal resolution.

[0056] Fig.Figure 14 shows a setup in which four different phases can be realized in a narrow spatial area by jointly using the readout fingers. The upper and lower pixel rows are supplied with a modulation signal via modulator 30, with the lower pixel row being supplied with a modulation signal shifted by 90°. By suitable interconnection, neighboring pixels in a pixel row can each be supplied with a phase shifted by 180°. This approach allows a distance value to be determined in a frame by calculating two times two neighboring pixels. This makes it possible to significantly minimize motion artifacts without significantly impairing spatial resolution. List of reference symbols 10 transmitter unit 12 Illumination light source 15 Beam shaping optics 20 Receiver unit, TOF camera 22 Light transit time sensor 23 time-of-flight pixels 23Z time-of-flight pixel line 25 Receiving optics 26 light-sensitive area 26a common light-sensitive area 27 light-insensitive area 28a active area for readout finger Ga 28b active area for read finger Gb 30 Modulator 35 Potential supply line 40 objects 400 reading unit 420 evaluation unit Gam, G0, Gbm, MOD modulation gates Ga, Gb selection finger AG a , AG b Readout gates AK a , AK b Readout node q charges qa, qb Charges at the readout finger Ga, Gb

Claims

[1] Time-of-flight sensor (22) with modulation gates (Gam, Gbm) in light-sensitive areas and readout fingers (Ga, Gb) in light-insensitive areas, wherein the modulation gates (Gam, Gbm, G0) and readout fingers (Ga, Gb) are arranged in parallel strips which, in groups, form a time-of-flight pixel (23), wherein a time-of-flight pixel (23) has at least two readout fingers (Ga, Gb), characterized by , that the time-of-flight sensor (22) has at least one time-of-flight pixel row (23z) with at least three time-of-flight pixels (23), and that the readout fingers (Ga, Gb) which are not arranged at the edge of the (23Z) row are adjacent to a modulation gate (Gam, Gbm) on two sides, wherein the readout fingers (Ga, Gb) are surrounded by two modulation gates (Gam, Gbm) with the same potential, wherein the readout fingers (Ga, Gb) located at the edge of the time-of-flight pixel (23) share a light-sensitive area (26a) with the adjacent pixel. [2] Time-of-flight sensor (22) according to claim 1, wherein the readout fingers (Ga, Gb) comprise a readout gate (AG a , Terms and Conditions b ) and a readout node (AK a , AK b ), wherein the readout node is formed as a pn structure with a metallization. [3] Time-of-flight sensor (22) according to claim 2, wherein the readout node (AK a , AK b ) is arranged in an end region of the readout finger (Ga, Gb) near the readout circuit (400). [4] Time-of-flight sensor (22) according to claim 3, wherein the readout node (AK a , AK b ) on at least three sides of the readout gate (AG a , Terms and Conditions b ) is framed. [5] Time-of-flight sensor (22) according to claim 1, wherein at least three modulation gates (Gam, Gbm, G0) are arranged between two readout fingers (Ga, Gb), wherein the modulation gates (Gam, Gbm) adjacent to the readout fingers (Ga, Gb) have transparency both in an infrared (IR) and in a visual range (R, G, B).

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