The present disclosure generally concerns image sensors, and more particularly is directed to image sensors called polarimetric, adapted to recording information relative to the polarization of the captured light.
The measurement of the light polarization information on acquisition of an image may be advantageous for many applications. It enables in particular to implement image improvement processings, adapted according to the considered application. For example, it enables to decrease or conversely to enhance reflections on an image of a surface such as a glass pane or water. It further makes it possible to detect manufactured objects in a natural environment (camouflage detection or demining applications, for example), the latter generally having a polarization signature. Among the applications likely to take advantage of the measurement of the polarization information, one may also mention industrial control applications, biomedical applications, for example, contrast improvement applications for the capture of images in a diffusing medium (fog, submarine imaging, etc.), or also applications of distance mapping or depth image acquisition, where the polarization may provide information relative to the orientation of the surface of the manufactured objects, and thus help the 3D reconstruction as a complement to another modality such as the active illumination by structured light or by time-of-flight measurement.
To measure the polarization information, it has already been provided to successively acquire, by means of a same sensor, a plurality of images of a same scene, by placing at each acquisition a polarizer in front of the sensor, and by changing polarizer between two successive acquisitions. This results in relatively bulky acquisition systems, with the presence in front of the sensor of a mechanism, for example a motor-driven rotating wheel or platen having the different polarizers fixed thereto, enabling to change the polarizer between two acquisitions. Another limitation is linked to the need to successively acquire a plurality of images of the scene to record a plurality of polarization states. This may in particular raise an issue when the scene varies over time.
To overcome these limitations, it has been provided to place an array of polarizing filters in front of the image sensor. There however remains a limitation in that the polarizing filters block part of the light signal received by the acquisition system. Thus the general sensitivity or general quantum efficiency of the acquisition system is relatively low.
It would be desirable to at least partly overcome certain limitations of known solutions of polarimetric image acquisition.
For this purpose, an embodiment provides a polarimetric image sensor formed inside and on top of a semiconductor substrate, the sensor comprising a plurality of pixels, each comprising:
According to an embodiment, the plurality of pixels comprises at least first and second pixels adapted to measuring radiations according respectively to first and second distinct polarizations, wherein:
According to an embodiment, in each pixel, the polarization structure of the pixel comprises a plurality of parallel bars.
According to an embodiment, the parallel bars are metallic.
According to an embodiment, in each pixel, the diffraction structure of the pixel comprises a plurality of cavities or trenches extending vertically in the substrate on the side of the illumination surface of the photosensitive region.
According to an embodiment, the cavities or trenches extend down to a depth in the range from 50 to 500 nm.
According to an embodiment, the plurality of pixels comprises different pixels adapted to measuring radiations in different wavelength ranges, and, in each pixel, the polarization structure and/or the diffraction structure are adapted according to the wavelength range intended to be measured by the pixel.
According to an embodiment, in each pixel, the polarization structure and/or the diffraction structure are adapted according to the angle of incidence of the radiations received by the pixel.
According to an embodiment, the sensor comprises an interconnection stack covering a surface of the substrate opposite to the diffraction structures and to the polarization structures.
According to an embodiment, the polarization structures are polarizing filters.
According to an embodiment, the polarization structures are polarization routers.
The foregoing features and advantages, as well as others, will be described in detail in the rest of the disclosure of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:
Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties.
For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the photodetection elements and the electronic control circuits of the described image sensors have not been detailed, the described embodiment being compatible with usual embodiments of these elements. Further, the possible applications that the described image sensors have not been detailed, the described embodiments being compatible with all or most known applications of polarimetric image acquisition devices.
Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements.
In the following disclosure, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “upper”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made, unless specified otherwise, to the orientation of the figures.
Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%.
Sensor 100 is formed inside and on top of semiconductor substrate 101. Substrate 101 is for example made of a single-crystal semiconductor material. Substrate 101 is for example made of silicon, for example, of single-crystal silicon.
Sensor 100 comprises a plurality of pixels P formed inside and on top of semiconductor substrate 101. In top view, pixels P are for example arranged in an array of rows and columns.
Sensor 100 further comprises, on the side of a first surface of substrate 101, called front side, corresponding to the lower surface of substrate 101 in the orientation of the drawing, a stack 105 of insulating and conductive layers (for example, metallic), called interconnection stack, having elements of interconnection (for example, interconnection conductive tracks and vias) of the sensor pixels, formed therein.
In the example of
Each pixel P of sensor 100 comprises a photosensitive region 103 formed in substrate 101. Each photosensitive region 103 for example comprises a photodetection element 107, for example, a photodiode. In the shown example, the photosensitive regions 103 of pixels P are laterally separated from one another by insulating walls 109. Insulating walls 109 are for example made of a dielectric material, for example, silicon oxide. As a variant (not detailed in the drawings), insulating walls 109 comprise external lateral walls made of a dielectric material, for example, silicon oxide, and a central wall made of an electrically-conductive material, for example, doped polysilicon or a metal. In this example, insulating walls 109 extend vertically through the entire thickness of substrate 101. The thickness of substrate 101 is for example in the range from 1 to 20 μm, for example, from 3 to 10 μm. As a variant, insulating walls 109 may be omitted.
Each pixel P comprises a polarization structure 111, for example, a polarizing filter, arranged in front of the photosensitive region 103 of the pixel, on the side of the illumination surface of photosensitive region 103, that is, on the upper surface side of substrate 101 in the orientation of
Each polarization structure 111 is adapted to mainly transmitting light rays according to a predefined polarization.
In the example of
The polarization structures are for example metal structures comprising openings and mainly transmitting radiations according to the predefined polarization, and absorbing or reflecting radiations according to the other polarizations. The metal structures are for example made of aluminum or of copper. As a variant, other metals may be used, for example, silver, gold, tungsten, or titanium.
As an example, a filling material 115, for example, a dielectric material, for example, silicon oxide, silicon nitride, alumina (Al2O3), tantalum oxide, or hafnium oxide, fills the openings formed in the metal structures. In this example, material 115 further covers polarization structures 111, forming a planarization layer 115. As a variant, the openings of polarization structures 111 may be left vacant or filled with air.
In practice, the selection of the patterns and the sizing of polarization structures 111 may be performed by means of known electromagnetic simulation tools.
As an example, pixels P are distributed into macropixels M, each comprising at least two adjacent pixels P. In each macropixel M, the pixels P of the macropixel have different polarization structures 111. Thus, in each macropixel M, the pixels P of the macropixel measure light radiation intensities received according to different polarizations.
The polarization structures are for example metal gates, each formed of a plurality of regularly spaced apart parallel metal bars, mainly transmitting radiations according to a linear polarization perpendicular to the metal bars, and absorbing radiations according to the other polarizations.
As a variant, the number of pixels P per macropixel M may be different from four. Further, the described embodiments are not limited to linear polarization structures 111. AS a variant, each macropixel M may comprises one or a plurality of linear polarization structures 111 and/or one of a plurality of circular polarization structures 111.
As an example, the polarization structures 111 of the pixels P of same position in the different macropixels M of the sensor are adapted to mainly transmitting the same polarization orientation.
As an example, the polarization structures 111 of same polarization orientation in the different macropixels M of the sensor are all identical, to within manufacturing dispersions.
As a variant, the polarization structures 111 of same polarization orientation in the different macropixels M of the sensor have patterns adapted according to the position of macropixel M on the sensor, to take into account the main direction of incidence of the light rays received by the macropixel.
In the example of
As an example, the polarization structures 111 of same polarization orientation in the different macropixels M of the sensor have patterns adapted according to the pixel color, that is, to the wavelength range mainly transmitted by the color filter 113 of the pixel.
As a non-limiting example, for linear polarizations of the type illustrated in
In practice, for reasons of manufacturing method simplification, it is preferable for the height of the metal bars of polarization structures 111 to be the same in all the sensor pixels P. Thus, compromises may be made between the complexity of forming of polarizers 111 and their polarization filtering performance.
In the example of
In the example of
According to an aspect of an embodiment, each pixel P of sensor 100 further comprises a diffraction structure 119 formed on the side of the illumination surface of the photosensitive region 103 of the pixel, that is, on the side of its upper surface in the orientation of
In the shown example, the diffraction structure comprises structures, for example, cavities or trenches, formed in substrate 101 on the upper surface side of the photosensitive region 103 of the pixel. The structures are for example filled with a material having a refraction index different from that of the material of the substrate, for example, silicon oxide. The structures for example have a lateral dimension in the order of λ/N, λ designating the main wavelength of sensitivity of the pixel, that is, for example, the wavelength for which the quantum efficiency of the pixel is maximum, or the main wavelength intended to be measured by the pixel, and N being the refraction index of the substrate, for example, made of silicon, in order to be placed in diffraction mode.
In each pixel P, structure 119 enables to diffract the light at the input in the semiconductor material of the substrate, to increase the length of the optical path of the rays in the photosensitive region 103 of the pixel, and thus favor the absorption, and accordingly the photoconversion, of the incident rays by the photosensitive region 103 of the pixel.
As an example, the diffraction structures 119 of the different pixels P of the sensor are adapted according to the main wavelength intended to be measured by the pixel and/or to the main angle of incidence of the rays reaching the pixel (that is, according to the position of the pixel in the sensor).
According to an aspect of the embodiment of
Thus, in the embodiment of
The presence of diffraction structures 119 enables, in each pixel, to improve the sensitivity to the pixel polarization, that is, the quantum efficiency of the pixel illuminated according to the pixel polarization. Diffraction structures 119 further enable to increase the extinction coefficient or contrast of the pixels, that is, in each pixel, the ratio of the quantum efficiency of the pixel illuminated according to the pixel polarization to the quantum efficiency of the pixel illuminated according to the polarization orthogonal to the pixel polarization.
Diffraction structures 119 are for example strongly asymmetrical to favor the diffraction of light according to the pixel polarization over the other polarizations.
The selection of the patterns and the sizing of diffraction structures 119 may be performed by means of electromagnetic simulation tools.
It should however be noted that polarization structures 111 and diffraction structures 119 may be different in terms of design and of materials, and do not use the same physical principles. In particular, the role of polarization structures 111 is to mainly transmit the main pixel polarization and to absorb or reflect the other polarizations, while the role of diffraction structure 119 is to more strongly diffract the main pixel polarization with respect to the other polarizations.
As a result, the structures 111 and 119 of a same pixel P do not necessarily have the same main axis and their respective orientations may depend on parameters such as the wavelength, the aperture ratio, and/or the size of the pixel.
Thus, as a variant, not shown, each diffraction structure 119 may be formed of trenches orthogonal to the metal bars of the polarization structure 111 of the pixel.
It should further be noted that the lateral dimensions and the repetition period of the trenches of diffraction structures 119 are not necessarily identical to the lateral dimensions and to the repetition period of the metal bars of polarization structures 111. Further, the depth of the trenches of the diffraction structure may be different from the height of the metal bars of the polarization structures.
In the example considered hereabove where each macropixel M comprises a plurality of pixels P adapted to respectively measuring different linear polarizations, the polarization structures 111 of the different pixels for example have the same pattern, rotated by an angle θ equal to the angle formed between the polarization direction to be measured and a reference direction. Similarly, the diffraction structures 119 of the different pixels for example have the same pattern, also rotated by angle θ.
In the example of
Before the thinning step, a substrate 401, used as a support handle, is bonded, by its lower surface, to the lower surface of interconnection stack 105.
It should be noted that in the cross-section views of
Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants may be combined, and other variants will occur to those skilled in the art.
In particular, although examples of forming of back-side illumination (BSI) sensors have been described hereabove, the described embodiments may be adapted to front-side illumination (FSI) sensors. In this case, the photosensitive region of each pixel is illuminated through interconnection stack 105. Diffraction structures 119 are then formed on the side of the front surface (lower surface in the orientation of
Further, in the case of a front-side illumination sensor, polarization structures 111 may be formed in one or a plurality of metal levels of interconnection stack 105. Here again, this enables to introduce no additional step for the manufacturing of polarization structures 111.
Further, the described embodiments are not limited to the examples disclosed hereabove of forming of polarization structures 111 formed of opaque metal patterns laterally surrounded with a transparent dielectric material. As a variant, polarization structures 111 may be formed based on transparent or semi-transparent materials having an index contrast, to improve the transmission of polarizers. For example, for polarization structures intended to operate in near infrared, for example at a wavelength in the order of 940 nm, silicon patterns, for example, made of amorphous silicon, surrounded with a dielectric material of smaller refraction index, for example, silicon oxide, may be used. For pixels intended to measure visible radiations, silicon nitride or titanium oxide patterns, surrounded with a dielectric material of smaller refraction index, for example, silicon oxide, may be used.
Further, examples of polarizing structures 111 formed on parallel bars having an orientation selected according to the linear polarization direction to be transmitted have been described hereabove. These are sub-wavelength structures which may be called one-dimensional (1D) metasurfaces, since they are structured along a single axis. As a variant, polarization structures 111 may be formed based on two-dimensional metasurfaces (2D) formed of pillars or pads arranged in the form of a two-dimensional array. To obtain the desired polarizing effect, pillars having, in top view, an elongated shape, for example, in a rectangle or an ellipse, according to a direction selected according to the linear polarization direction to be transmitted, will be selected.
Further, it should be noted that, in the above-described examples, polarization structures 111 have a polarization filtering function. In other words, the polarization which is desired to be measured is transmitted by structure 111, while the orthogonal polarization is absorbed or reflected. This results in a loss of light flux in the order of 50% in the case of a depolarized light. As a variant, polarization structures 111 may be sorting structures, adapted to deviating the light differently according to its polarization. Structures 111 adapted to deviating light towards respectively two neighboring photosensitive regions 103 for two orthogonal polarizations may particularly be provided. As an example, in the case of a macropixel M of the type described in relation with
Such polarization sorting structures, also called polarization routers, may be formed based on two-dimensional (2D) or three-dimensional (3D) metasurfaces. The shape and the sizing of the patterns of the metasurfaces may be determined by means of electromagnetic simulation tools, for example, by using reverse design methods, for example, of the type described in the article entitled “Multifunctional volumetric meta-optics for color and polarization image sensors” of Philip Camayd-Munoz et al. (Vol. 7, No. 4/April 2020/Optica) or in the article entitled “Empowering Metasurfaces with Inverse Design: Principles and Applications” of Zhaoyi Li et al. (https://doi.org/10.1021/acsphotonics.1c01850).
It should further be noted that, in a back-side illumination sensor of the type described in relation with
The reflection on the metal tracks of the interconnection stack may result in at least partially polarizing the light according to a direction depending on the orientation of said metal tracks. Preferably, for each pixel P of the sensor, the metal tracks of interconnection stack 105 located in front of the pixel are oriented according to a direction selected according to the pixel polarization, for example, to favor the polarization of the light reflected according to the polarization orientation intended to be measured by the pixel. Thus, preferably, the metal tracks of interconnection stack 105 located in front of pixels intended to measure different polarizations, have different orientations.
Further, the described embodiments are not limited to the above-described examples of application to visible or near infrared sensors. Other wavelength ranges may take advantage of polarizing pixels. For example, the described embodiments may be adapted to infrared sensors intended to measure radiations of wavelength in the range from 1 to 2 μm, for example, based on InGaAs or on germanium.
Finally, the practical implementation of the described embodiments and variations is within the abilities of those skilled in the art based on the functional indications given hereabove. In particular, those skilled in the art will be capable, based on the indications of the present disclosure, of designing, sizing, and forming the polarization structures and the diffraction structures of the sensor pixels, so that these structure cooperate to obtain the desired effect of improvement of the tradeoff between the sensitivity and the pixel polarization extinction coefficient.
Polarimetric image sensor (100) formed inside and on top of a semiconductor substrate (101), the sensor including a plurality of pixels (P), each may be summarized as including: a photosensitive region (103) formed in the semiconductor substrate (101); a diffraction structure (119) formed on the side of an illumination surface of the photosensitive region (103); and a polarization structure (111) formed on the side of the diffraction structure (119) opposite to the photosensitive region (103).
Said plurality of pixels (P) may include at least first and second pixels adapted to measuring radiations according respectively to first and second distinct polarizations, wherein:
In each pixel (P), the polarization structure (111) of the pixel may include a plurality of parallel bars.
Said parallel bars are metallic.
In each pixel (P), the diffraction structure (119) of the pixel may include a plurality of cavities or trenches extending vertically in the substrate on the side of the illumination surface of the photosensitive region (103).
Said cavities or trenches may extend down to a depth in the range from 50 to 500 nm.
Said plurality of pixels (P) may include different pixels adapted to measuring radiations in different wavelength ranges, and wherein, in each pixel, the polarization structure (111) and/or the diffraction structure (119) are adapted according to the wavelength range intended to be measured by the pixel.
In each pixel (P), the polarization structure (111) and/or the diffraction structure (119) may be adapted according to the angle of incidence of the radiations received by the pixel.
Sensor (100) may include an interconnection stack covering a surface of the substrate (101) opposite to the diffraction structures (119) and to the polarization structures (111).
The polarization structures (111) may be polarizing filters.
The polarization structures (111) may be polarization routers.
The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
Number | Date | Country | Kind |
---|---|---|---|
2208214 | Aug 2022 | FR | national |