DE102015218484A1 - Reference pixel array for an image sensor - Google Patents

Abstract

Reference pixel array (26) for an image sensor, comprising an array of pixels (21) at least partially having a transmissive region and means (Ga, Gb) for integrating carriers (q), at least one pixel (21) of the reference pixel array (26 ) is formed as a covered pixel (22) in that the covered pixel (22) is completely covered by an opaque layer (28).

Description

The invention relates to a reference pixel array for an image sensor, an image sensor and in particular also a light transit time sensor according to the preamble of the independent claims.

As a light transit time sensor should be understood in particular a sensor that wins a runtime information from the phase shift of an emitted and received radiation. PMT cameras with photonic mixer detectors (PMD) are particularly suitable as the time of flight or 3D TOF cameras, as they are used in the DE 197 04 496 C2 described and obtained from the company, ifm electronic GmbH 'or' PMD Technologies GmbH 'as O3D sensor or as Cam Board.

The object of the invention is to improve the reliability of an image sensor or the reliability of the distance measurements of a light transit time camera or a light transit time sensor.

The object is achieved in an advantageous manner by the reference pixel array according to the invention and the image sensor according to the invention as generically defined by the independent claims.

Advantageously, a reference pixel array is provided for an image sensor, comprising an array of pixels having at least partially a transparent region and means for integrating charge carriers, wherein at least one pixel of the reference pixel array is formed as a covered pixel by completely covering the pixel with an opaque layer is.

By doing so, it is possible to easily increase the dynamic range of a reference pixel array. The actively operated but covered pixels in the reference pixel array receive their light signal only indirectly, for example via reflections on the metallization, and thus obtain significantly less light than directly irradiated pixels. Thus, it is possible that even with strong irradiation, a pixel can always be found whose integration nodes are not in saturation and / or have a suitable signal-to-noise ratio.

It is particularly useful if the reference pixel array has a plurality of covered pixels and at least two covered pixels are immediately adjacent.

By this procedure, the dynamic range can be increased further, since on the one hand the adjacent edge regions can enter through the light and, on the other hand, the travel distance of the indirectly incident light is increased and, as a result, the quantity of light detectable at the pixel is reduced.

Furthermore, it is advantageous if an arrangement of the covered pixels within the reference pixel array repeats itself at least once.

This repetitive arrangement provides a plurality of indirectly irradiated reference pixels that should receive substantially the same amount of light. It is thus possible, for example, to determine an average amount of light via a plurality of reference pixels which are assigned to the same amount of light.

In a further embodiment, it is provided to combine a plurality of covered pixels into a common pixel in an evaluation and / or electrically. By this procedure, for example, statistical fluctuations of the pixels can be compensated.

It is particularly advantageous to provide an image sensor for detecting light, having an array of pixels which at least partially have a light-transmissive region and means for integrating charge carriers, wherein a part of the pixels are used as reference pixels, and wherein these reference pixels are used as reference pixel array according to US Pat the aforementioned variants are formed.

This procedure ensures in a simple manner that the pixels operated as reference pixels have the same electrical properties as the pixels of the remaining sensor.

In a further embodiment, it is provided that the image sensor is designed as a light transit time sensor with light transit time pixels and reference pixels for phase measurement of a modulated light, wherein at least one pixel of the reference pixels is completely covered with an opaque layer.

Furthermore, a light cycle camera with a reference pixel array or image sensor according to one of the aforementioned embodiments is provided.

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

They show schematically:

1 a time of flight camera system,

2 a sensor with reference pixels

3 a cross section of a light transit time pixel,

4 a cross-section of a light transit time sensor with open and covered light transit time pixels,

5 a distribution of covered pixels,

6 a cascading distribution of covered pixels,

7 a light incidence to be considered for the next but one pixel neighbor,

8th a combination of different distributions of covered pixels,

9 a consideration of the light incidence of a covered second-order pixel.

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 23 , The light transit time sensor 23 has a pixel array and is designed in particular as a PMD sensor.

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 23 via a modulator 220 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 23 , In the time of flight sensor 23 becomes the signal of the first phase a of the modulator 220 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 23 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.

To compensate for measurement uncertainties can be on the sensor 23 further light time-of-flight pixels 21 be used for a reference measurement. Preferably, for the referencing, light transit time pixels are suitable at the edge of the sensor. These pixels can also be arranged offset from the image-capturing pixel matrix for referencing purposes. In particular, it is also conceivable to arrange these reference pixels on a separate chip.

In order to be able to recognize and compensate for changes in the electrical properties during operation, the reference pixels can be illuminated, for example, with a reference signal / light. In particular, it may be provided to decouple a part of the emitted light and to illuminate the reference pixels with this light. Depending on the differences between the expected and measured results, the measurements of the image-capturing pixel matrix can be corrected.

2 shows such a sensor 23 , in which besides the picture-taking pixels 21 spatially offset reference pixels 26 are arranged, via a light pipe 260 Light from the light source 12 to the reference pixels 26 is introduced. Ideally, the reference pixels 26 the same electrical properties as the image-capturing pixels 21 on.

3 schematically shows a light time-of-flight pixel 21 with modulation gates Gam, Gbm in a translucent area and integration nodes in an opaque area. The semiconductor structure is typically provided with a transparent, electrically insulating cover layer 27 Consisting for example of SiO ₂ or Si ₃ N _4, covered. Areas where no light is wanted are metallized 28 Mistake.

The incident radiation in the translucent area hν generated in the semiconductor carriers q, which are supplied in phase synchronism depending on the applied modulation voltage ModA, ModB either the first or second integration node Ga, Gb.

In the case shown, the integration nodes are formed as diodes in a p-doped base material. Of course, the doping can also be complementary. The charges accumulated at the integration nodes Ga, Gb are then passed on via read-out circuits to, for example, column lines.

4 shows a light transit time sensor according to the invention 23 with a reference pixel area 26 the open and covered time of flight pixel 21 . 22 having. The light transit time pixels 21 . 22 correspond to the in 3 represented pixel. The covered time of flight pixels 22 On the other hand, the original light-transmissive areas with an opaque layer differ from each other 28 are covered.

The core idea of the invention is to advantageously use the typically disadvantageous effect of signal crosstalk between neighboring pixels by light scattering or forwarding for the provision of different useful signal amplitudes, in order thus to reduce the dynamic range of the light transit time pixels or reference pixels 26 to expand.

By the under-illumination of the covered pixels 22 Thus, it is possible to get the light hν through the open light-time pixels 21 penetrates and with decreasing amplitude on the adjacent covered pixels 22 forward. The intensity of the under-illumination depends on the distance of the covered pixel 22 to an uncovered pixel 21 and second, the number of pixels covered 22 surrounding uncovered pixels 21 from.

The illumination is mainly driven by the light pipe in the semiconductor itself. Semiconductors, in particular silicon, have a wavelength-dependent penetration depth which, starting from the blue light, increases in the direction of red and infrared radiation. The penetration depth or the range of the penetrating light is essentially predetermined by the semiconductor material itself.

Furthermore, an under illumination of the covered pixels takes place 22 through a light pipe in the transparent cover layer 27 of the sensor 23 , Due to the metallization 28 the topcoat 27 the light can be forwarded in particular by multiple reflections. It is also conceivable, by using different cover layer materials having different refractive indices, for example SiO ₂ with n = 1.41 and Si ₃ N ₄ with n = 2.0, to form the cover layer composite as light guide. The range of the penetrating light can thus, for example, by the structure of the cover layer 27 as well as the shape, position, arrangement, size and / or material of the metallization 28 to be influenced. By way of example, the light range can also be influenced by the different reflection properties of the materials which can be used for the metallization.

The covered time of flight pixels 22 are operated in the same way as the open time of flight pixels 21 only with the difference that the charge carriers q generated in these areas are not generated by direct, but by indirect light incidence.

5 shows top views of various reference pixel arrangements 26 with covered pixels 22 , In 5a) is the covered pixel of eight open pixels 22 surrounded and receives from the entire environment an underlight. 5b) shows a covered pixel 22 with five open pixel neighbors 21 and 5c) a variant with three open pixel neighbors 21 , By the number of surrounding open pixel neighbors 21 In this way, the intensity of the sublight of the covered pixels can be easily determined 22 influence. Basically, such structures can be realized in a standard process without the need for additional masks.

6 shows another reference pixel arrangement 26 , where both the number of surrounding open pixels neighbor 21 as well as the distance to the open pixels 21 is varied. The numbers in the covered pixels indicate the number of directly adjacent, open pixels. The illustrated finger-like structure makes it possible to cover covered pixels 22 starting with eight open neighbor pixels 21 down to a covered pixel 22 that has only one open neighbor pixel form. In the fully covered structure, the pixels do not have an open neighbor 21 more on, leaving the light only depending on the penetration depth and the distance to the next open pixel 21 to the covered pixels 22 arrives.

By repeatedly repeating this finger structure, the data of individual pixels can also be statistically evaluated.

7 corresponds to the illustration according to 5 , where indicated by arrows, which has next-neighbor (neighbors 2nd degree) of the cross-labeled, covered pixels. The light intensity detected at these pixels can accordingly also be influenced by the number of neighbors after the next.

8th shows an arrangement accordingly 6 where the width and spacing of the "fingers" with the covered pixels 22 was enlarged. In addition, besides the single covered pixel 21 with eight neighbors a pixel area of 6x6 pixels covered. The entered numbers indicate the direct neighbor.

9 shows an arrangement accordingly 8th in which the next but one neighbors are marked.

The aforementioned embodiments were explained primarily with reference to a light transit time sensor, but in principle this principle can be generalized to image sensors for detecting light.

It should also be noted that due to the wavelength-dependent penetration depth, the described arrangements can also be used to determine the incident wavelength.

LIST OF REFERENCE NUMBERS

1

 Time of flight camera system

10

 transmission unit

12

 Illumination light source

15

 Beam shaping optics

20

 Receiving unit, light time camera

21

 Transit Time pixels

22

 covered light transit time pixel

23

 Transit Time Sensor

25

 receiving optics

26

 Reference pixels, array

27

 insulation layer

28

 metallization

40

 object

260

 light line

Ga, Gb

 integration node

ModA, ModB

 Modulation lead, signal

Gam, Gbm

 modulation gates

n, p

 allocations

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

• DE 19704496 C2 [0002, 0028]

Claims (7)

Reference pixel array ( 26 ) for an image sensor, with an array of pixels ( 21 ) which at least partially have a light-transmissive region and means (Ga, Gb) for the integration of charge carriers (q), characterized in that at least one pixel ( 21 ) of the reference pixel array ( 26 ) as a covered pixel ( 22 ) is formed by the covered pixel ( 22 ) completely with an opaque layer ( 28 ) is covered. Reference pixel array ( 26 ) according to claim 1, wherein the reference pixel array ( 26 ) several covered pixels ( 22 ) and at least two covered pixels ( 22 ) are immediately adjacent. Reference pixel array ( 26 ) according to claim 2, wherein an arrangement of the covered pixels ( 22 ) within the reference pixel array ( 26 ) repeated at least once. Reference pixel array ( 26 ) according to one of the preceding claims, in which a plurality of covered pixels ( 22 ) are combined into a common pixel in an evaluation and / or electrically. Image sensor ( 23 ) for detecting light, with an array of pixels ( 21 ), which at least partially have a transparent area and means (Ga, Gb) for the integration of charge carriers (q), wherein a part of the pixels ( 21 ) as a reference pixel ( 26 ) and these reference pixels ( 26 ) as a reference pixel array ( 26 ) are formed according to one of the preceding claims. Image sensor ( 23 ) according to the preceding claim, in which the image sensor is used as a light transit time sensor ( 23 ) with light-time pixels ( 21 ) and reference pixels ( 26 ) is designed for phase measurement of a modulated light, wherein at least one pixel ( 22 ) of the reference pixels ( 26 ) completely with an opaque layer ( 28 ) is covered. Photocell camera with a reference pixel array ( 26 ) according to claims 1 to 4 or an image sensor ( 23 ) according to claim 5.

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