DE102013209161A1 - Transit Time Sensor - Google Patents

Abstract

Light transit time sensor (22) with modulation gates (Gam, Gbm) in light-sensitive areas and readout fingers (Ga, Gb) in light-insensitive areas, the modulation gates (Gam, Gbm, G0) and readout fingers (Ga, Gb) being arranged in parallel strips which are arranged in groups Form time-of-flight pixels (23), a time-of-flight pixel (23) having at least two readout fingers (Ga, Gb), characterized in that the time-of-flight sensor (23) has at least one line of light-time pixel (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) line, adjoin on two sides on a modulation gate (Gam, Gbm).

Description

The invention relates to a light transit time sensor according to the preamble of the independent claim. The light transit time sensor relates, in particular, to light transit time camera systems, in particular, time-of-flight or 3D TOF camera systems, which acquire transit time information from the phase shift of an emitted and received radiation. PMD cameras with photonic mixer detectors (PMD) are particularly suitable as the time of flight or 3D TOF cameras, as described, inter alia, in the applications EP 1 777 747 B1 . US Pat. No. 6,587,186 B2 and also DE 197 04 496 C2 described and, for example, by the company, ifm electronic GmbH 'or' PMD Technologies GmbH 'as a frame grabber O3D or as CamCube relate. In particular, the PMD camera allows a flexible arrangement of the light source and the detector, which can be arranged both in a housing and separately.

Furthermore, from the DE 198 21 974 A1 a device for detecting phase and amplitude of electromagnetic waves, in which light-transmitting modulation gates are arranged in the photosensitive region and charge accumulating accumulation gates in strip form.

The object of the invention is to improve the fill factor on a light transit time sensor.

The object is achieved in an advantageous manner by the inventive light transit time sensor according to the preamble of the independent claim.

Advantageously, a light transit time sensor is provided, with modulation gates in photosensitive and read-out fingers in light-insensitive areas, wherein the modulation gates and read-out fingers are arranged in parallel strips which form a group of light-propagation time pixels and wherein a light transit time pixel has at least two read-out fingers. The embodiment further provides that the light transit time sensor has at least one light transit time pixel line with at least three light transit time pixels, and that the read-out fingers, which are not arranged at the edge of the line, adjoin a modulation gate on two sides.

This procedure has the advantage that it dispenses with an isolation path separating the individual pixels and that the fill factor can be increased.

In a further preferred embodiment, the read-out fingers have a readout gate and a readout node, wherein the readout node is formed as a pn-structure with a metallization. In contrast to structures that are formed over the entire finger width as a pn junction, a structure of gate and read nodes can be made spatially smaller, whereby the fill factor can be further improved.

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

In a further embodiment, the read-out nodes are bordered on at least three sides by the read-out gate. This makes it advantageous to avoid crosstalk or penetration of the modulation gates.

In a further embodiment, it is provided to arrange at least three modulation gates between two read-out fingers, wherein the modulation gates, which adjoin the read-out fingers both have a transparency in an infrared as well as in a visual range. As a result, it is advantageously possible to provide the sensor with additional functionality without impairing the performance of the 3D image acquisition.

The invention will be explained in more detail by means of embodiments with reference to the drawings.

They show schematically:

1 the basic principle of a time-of-flight camera based on the PMD principle,

2 a modulated integration of the runtime-shifted generated charge carriers,

3 a cross-section of a PMD pixel,

4 a top view of a PMD pixel,

5 a top view of a PMD pixel with readout node,

6 a top view of a PMD pixel with forked feeler fingers,

7 a cross section through a PMD pixel according to the invention with two modulation gates,

8th a schematic plan view of a PMD pixel according to the invention,

9 a cross section and a plan view of a passing PMD pixel line,

10 a top view of a passing PMD pixel line with minimum pixel size,

11 an array of multiple passing PMD pixel lines,

12 a formation of a PMD pixel line with RGB filters,

13 one PMD pixel row with three modulation gates each,

14 two PMD pixel lines with different phase angles.

In the following description of the preferred embodiments, like reference characters designate like or similar components.

1 shows a measurement situation for an optical distance measurement with a light transit time camera, as for example from the DE 197 04 496 C2 is known.

The light transit time camera system 1 comprises a transmitting unit or a lighting module 10 with an illumination light source 12 and associated beam shaping optics 15 as well as a receiving unit or light runtime camera 20 with a receiving optics 25 and a light transit time sensor 22 , The light transit time 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 consists of improving the imaging characteristics of multiple optical elements. The beam shaping optics 15 the transmitting unit 10 is preferably formed as a reflector. However, it is also possible to use diffractive elements or combinations of reflective and diffractive elements.

The measurement principle of this arrangement is essentially based on the fact that, based on the phase shift of the emitted and received light, the transit time of the emitted and reflected light can be determined. For this purpose, the light source 12 and the light transit time sensor 22 via a modulator 30 acted upon together with a specific modulation frequency or modulation signal with a first phase position a. The light source sends according to the modulation frequency 12 an amplitude modulated signal with the phase a. This signal or the electromagnetic radiation is in the illustrated case of an object 40 reflects and hits due to the distance traveled correspondingly phase-shifted with a second phase position b on the light transit time sensor 22 , In the time of flight sensor 22 becomes the signal of the first phase a of the modulator 30 mixed with the received signal having the second time phase condition b, mixed, wherein the phase shift or the object distance d is determined from the resulting signal.

For a more accurate determination of the second phase position b and thus of the object distance d, it may be provided that the phase position a coincides with that of the light transit time sensor 22 is operated to change pre-tuned phase shifts Δφ. Equally effective, it can also be provided to selectively shift the phase with which the lighting is driven.

The principle of phase measurement is schematically in 2 shown. The upper curve shows the time course of the modulation signal with the illumination 12 and the light transit time sensor 22 , here without phase shift, be controlled. The object 40 Reflected light b strikes the light transit time sensor out of phase in accordance with its light transit time t _L 22 , The light transit time sensor 22 collects the photonically generated charges q in a first read-out finger Ga during the first half of the modulation period and in a second read-out finger Gb in the second half-cycle. The charges are typically collected or integrated over several modulation periods. From the ratio of the charges qa, qb collected in the first and second gate Ga, Gb, the phase shift and thus a distance of the object can be determined.

Like from the DE 197 04 496 C2 already known, the phase shift of the light reflected from the object and thus the distance, for example, by a so-called IQ (in-phase quadrature) method can be determined. To determine the distance, two measurements are preferably carried out with modulation signals whose phase positions are shifted by 90 °. Thus, for example, φ _mod + φ ₀ and φ _mod + φ ₉₀ , wherein from the charge difference Δq (0 °), Δq (90 °) determined in these phase positions, the phase shift of the reflected light via the known arctan or arctan2 relationship can be determined.

To improve the accuracy of further measurements can be performed with shifted by 180 °, for example, phase positions.

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

3 shows a cross section through a pixel of a photonic mixer as it is for example from the DE 197 04 496 C2 is known. The modulation photogates G _am , G ₀ , G _bm form the light-sensitive area of a PMD pixel. In accordance with the voltage applied to the modulation gates G _on , G ₀ , G _bm , the photonically generated charges q are directed either to one or to the other read-out finger G _a , G _b . The read-out finger is formed in the illustrated example as a pn junction or diode.

3b shows a potential curve in which the charges q flow in the direction of the first read-out finger G _a , while the potential according to 3c the charge q flows in the direction of the second read-out finger G _b . The potentials are specified according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in a range of 1 to 100 MHz. At a modulation frequency of, for example, 1 MHz results in a period of one microsecond, so that the modulation potential changes accordingly every 500 nanoseconds.

In 3a is also a readout unit 400 which, if appropriate, may already be part of a CMOS PMD light transit time sensor. The photonically generated charges detected in the read-out finger G _a , G _{b are} collected over a plurality of modulation periods. In a known manner, then the voltage applied to the read-out fingers Ga, Gb voltage, for example via the readout unit 400 be tapped high impedance. The integration times are preferably to be selected such that the light transit time sensor or the read-out fingers or their capacitances and / or the light-sensitive areas do not saturate for the expected amount of light.

4 shows a plan view of a light transit time pixel 23 with typical length specifications. The distance or the length extension between the two read-out fingers Ga, Gb is referred to as the channel spacing L _K and the distance between the longitudinal axes of symmetry of the two read-out fingers Ga, Gb as a finger pitch L _FP . According to the invention, it is provided that the pixels are not separated from one another by a field oxide, so that a further modulation gate is connected directly to the read-out finger.

The extent of the gates or the read-out fingers parallel to the channel spacing L _K is the gate length L _G and the extent perpendicular thereto the gate width B _G. While the gates are essentially the same length in their widths B _G , the gate lengths are typically varied depending on the application. In the illustrated case, for example, the middle modulation gate G ₀ longer than the two outer modulation gates G _am , G _bm , for example, to influence a modulation contrast.

The read-out fingers G _a , G _b can be constructed and structured in different ways. From the DE 197 04 496 C2 For example, it is known to form the read-out fingers as pn junctions or diodes. Preferably, the diode extends over the entire width of the read-out fingers. Preferably, the read-out fingers are formed opaque or covered with an opaque layer, preferably a metallization. If necessary, the metallization can simultaneously form the contacting of the diode.

5 shows a preferred embodiment of the invention, in which the read-out fingers Ga, Gb have a gate structure and an integration or readout node. In the illustrated case, a readout gate AG _a , AG _b extends over the entire width B _{G of} the readout finger Ga, Gb. In an end region of the read-out finger Ga, Gb is a read-out node AK _a , AK _b . As already in 3 shown, is applied to the readout gate AG _a , AG _b a voltage which forms a potential well for the charge carrier q below the gate. The charge carriers collected there flow primarily through diffusion processes to the respective read-out node AK _a , AK _b , where they can be read by the readout unit 400 for example, be tapped as a voltage.

The read-out fingers Ga, Gb are preferably covered with an opaque layer, preferably a metal layer. If necessary, the metal layer can also serve for contacting the readout node. In a further embodiment, it is also conceivable to influence the readout gate itself in terms of transparency, for example by siliciding.

Furthermore, variations of the size and position of the readout nodes AK _a , AK _{b are} conceivable. For example, the readout nodes AK _a , AK _{b have} the same length as the readout gates AG _a , AG _b . Also, multiple readout nodes may be distributed over the structure of the gate. In particular, readout nodes can be arranged at both ends of the gate. Also, a central readout node is conceivable. The readout nodes are preferably designed as a pn junction or as a diode.

6 shows a further variant in which the read-out finger or the gate structure is extended in the region of the readout nodes. By this Aufgabelung the readout nodes AK _a , AK _{b are surrounded} by a gate structure and separated with a minimum distance from the adjacent modulation gate Gam, Gbm. By such a procedure, electrical passages from the modulation gate to the read-out nodes are advantageously reduced or inhibited.

As already mentioned, a PMD pixel is typically separated from the neighboring matrix pixel by means of a field oxide and an optical cover. As a result, crosstalk of the individual matrix pixels with one another is typically minimized. For very small pixels, however, the fill factor plays an increasingly important role. In particular, it is disadvantageous if the separating field oxide matrix has a larger dimension than the read-out fingers. If the pixel pitch is on the same order of magnitude as the fingerpitch, the edge area or separation area plays an increasingly important role in terms of fill factor. The typical finger pitch L _FP , ie read-out finger plus modulation gates, is on the order of about 7-10 μm. The height and width of the fingers and the channel length are scalable in a very wide range for PMD structures, similar to transistors.

Through the use of smaller manufacturing processes and the miniaturization of the readout electronics, it is possible to bring 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 and small dimensions.

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

7 and 8th show such a continuous pixel structure.

7a shows a cross section of such a structure with four read-out fingers Ga, Gb and two modulation gates Gam, Gbm between two read-out fingers Ga, Gb. The modulation gates Gam, Gbm are as already in 3 represented translucent and form a photosensitive area 26 of the time of flight pixel 23 , The read-out fingers Ga, Gb are covered with a mask and form a light-insensitive area 27 , Below the read-out fingers Ga, Gb doping regions are indicated, which may be n-doped or p-doped in accordance with the base material of the semiconductor. As already shown in the above examples, this can also be a gate structure with read-out nodes.

In 7b shows a possible potential course for the in 7a shown structure, analogous to the representation according to 3c ,

8th shows a plan view of the pixel structure according to 7a , The read-out fingers Ga, Gb are from a readout unit 400 read out, with the same read-out fingers together on an evaluation 420 are performed, for example, the charge differences Δq or the charge sums Σq the read-out finger Ga, Gb determined. That from the modulator 30 originating modulation signal is via a potential lead 35 fed to the modulation gates Gam, Gbm in antiphase. The modulation gates Gam, Gbm in the immediate vicinity of a selection finger Ga, Gb are each at the same potential. The elite unit 400 and the potential supply 35 are as well as the Auslesefinger Ga, Gb covered with an opaque mask.

This in 7 and 8th shown time of flight pixels is according to the invention, as in 9 represented, continuous and arranged without separation in a pixel row. In the 9 displayed pixel line in four single pixels 23.1 - 23.4 divided up. The pixel boundaries are each marked with dashed dotted lines. The read-out fingers which lie at the edge of the light-time pixel have a photosensitive area 26a shared with the neighboring pixel. Depending on the potential position, the charge carriers generated there thus flow once to the neighboring pixel and the other time to the own read-out finger. However, since this process is repeated on both outer read-out fingers with opposite signs, this charge transfer compensates on average again, so that effectively, as in 9 shown, only half the common photosensitive area 26a is active for the respective pixel.

10 shows a smallest possible pixel structure with only two read-out fingers Ga, Gb and four modulation gates Gam, Gbm, where, as in the pixels shown so far, the read-out fingers are surrounded by two equal-potential modulation gates. The pixel pitch L _PP is in this case only slightly larger than the finger pitch L _FP .

11 shows an arrangement with multiple PMD pixel rows 23z in which, for a space-saving arrangement, two pixel rows each have a surface area for the potential supply lines 35 and elite units 400 share.

12 respectively. 12a shows a variant with three modulation gates Gam, Gbm, G0 have different optical filters, so as to generate, for example, a visual color information.

For detecting a color information, for example, a potential according to 12b be applied to the pixel structure. By applying color filters Red R, Green G and Blue B over the modulation gates in combination with a suitable control of the gates, it is possible to extract color information from the sensor, whose resolution is higher than the physical resolution with respect to the 3D Values.

The modulation gates on the sides of the read-out fingers are provided with the same filters on both sides which, in addition to transparency in one of the three primary colors, are also transparent in the infrared. The middle gate is permeable only in the infrared.

In the color or RGB mode, the middle gates G0 are preferably driven at 0 volts, while the outer modulation gates Ga, Gb are supplied with a voltage between the readout or separation gates. The charge carriers generated below the color filters are collected at the nearest read-out finger Ga, Gb. The potential barrier under the middle modulation gate G0 can not be overcome by the charge carriers.

For a photon mixture is preferably a potential according to 12c applied to the modulation gates, wherein the modulated illumination takes place in the infrared IR.

13 FIG. 12 shows a construction in which a higher virtual pixel resolution than the physical pixel resolution can be realized. Typically, the pixels used are square, but this is not mandatory. A pixel consists of at least two read-out fingers Ga, Gb and the modulation gates Gam, Gbm, G0 or MOD. In the example shown here, the actual pixel cell has a format of 2: 1 in the ratio of width to height. By realizing the pixels as passing matrix pixels, it is possible to achieve a virtually higher pixel resolution by calculating distance values from the charge differences of all read-out fingers Ga, Gb with the respectively adjacent read-out finger Ga, Gb. The resulting virtual pixel pitch L _VPP is then 1: 1 in the illustrated example while the physical pixel pitch L _{PP is} 2: 1.

Typically, in a PMD sensor, four frames are recorded in temporal succession with four different phase angles. Although this allows a high spatial resolution but a limited temporal resolution.

14 shows a structure in which can be realized by sharing the read-out fingers four different phases in a narrow spatial area. About the modulator 30 The upper and lower pixel line are subjected to a modulation signal, wherein the lower pixel line is applied with a shifted by 90 ° modulation signal. By suitable interconnection adjacent pixels in a row of pixels can be acted upon by a shifted by 180 ° phase position. By this procedure, a distance value can be determined in a frame by offsetting two times two neighboring pixels. This makes it possible to significantly minimize the motion artifacts without greatly affecting the spatial resolution.

LIST OF REFERENCE NUMBERS

10

 transmission unit

12

 Illumination light source

15

 Beam shaping optics

20

 Receiving unit, TOF camera

22

 Transit Time Sensor

23

 Transit Time pixels

23Z

 Time of flight pixel row

25

 receiving optics

26

 photosensitive area

26a

 common photosensitive area

27

 light-insensitive area

28a

 active range for readout finger Ga

28b

 active range for read finger Gb

30

 modulator

35

 potential supply

40

 object

400

 readout unit

420

 evaluation

Gam, G0, Gbm, MOD

 modulation gates

Ga, Gb

elite finger

AG _a , AG _b

elite Gates

AK _a , AK _b

readout node

q

 charges

qa, qb

 Charges on the reading finger Ga, Gb

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1777747 B1 [0001]
• US 6587186 B2 [0001]
• DE 19704496 C2 [0001, 0028, 0033, 0036, 0041]
• DE 19821974 A1 [0002]

Claims (5)

Light transit time sensor ( 22 ) with modulation gates (Gam, Gbm) in light-sensitive and read-out fingers (Ga, Gb) in light-insensitive regions, wherein the modulation gates (Gam, Gbm, G0) and read-out fingers (Ga, Gb) are arranged in parallel strips which in groups have a light-propagation time pixel ( 23 ), wherein a light transit time pixel ( 23 ) has at least two read-out fingers (Ga, Gb), characterized in that the light transit time sensor ( 23 ) at least one light-time-pixel row ( 23z ) with at least three light-time pixels ( 23 ), and that the read-out fingers (Ga, Gb), which are not at the edge of the ( 23Z ) Are arranged on two sides on a modulation gate (Gam, Gbm). Light transit time sensor ( 22 ) according to claim 1, wherein the read-out fingers (Ga, Gb) have a readout gate (AG _a , AGB _b ) and a readout node (AK _a , AK _b ), wherein the readout node is formed as a pn-structure with a metallization. Light transit time sensor ( 22 ) according to claim 2, wherein the read-out node (AK _a , AK _b ) in an end region of the read-out finger (Ga, Gb) in the vicinity of the read-out circuit ( 400 ) is arranged. Light transit time sensor ( 22 ) according to claim 3, wherein the read-out node (AK _a , AK _b ) on at least three sides of the read-out gate (AG _a , AGB _b ) is enclosed. Light transit time sensor ( 22 ) according to claim 1, wherein between two read-out fingers (Ga, Gb) at least three modulation gates (Gam, Gbm, G0) are arranged, wherein the modulation gates (Gam, Gbm) adjacent to the read-out fingers (Ga, Gb) both a transparency in an infrared (IR) as well as in a visual range (R, G, B).

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