CN115497969A - Backside illuminated image sensor and method of manufacture - Google Patents

Backside illuminated image sensor and method of manufacture Download PDF

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Publication number
CN115497969A
CN115497969A CN202210691110.1A CN202210691110A CN115497969A CN 115497969 A CN115497969 A CN 115497969A CN 202210691110 A CN202210691110 A CN 202210691110A CN 115497969 A CN115497969 A CN 115497969A
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sensor
region
elements
refractive index
periodic structure
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M·杜瓦
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STMicroelectronics Crolles 2 SAS
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STMicroelectronics Crolles 2 SAS
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Priority claimed from FR2106486A external-priority patent/FR3124309A1/en
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Publication of CN115497969A publication Critical patent/CN115497969A/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/1463—Pixel isolation structures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14625—Optical elements or arrangements associated with the device
    • H01L27/14627—Microlenses
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14634—Assemblies, i.e. Hybrid structures

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  • Engineering & Computer Science (AREA)
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  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
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Abstract

Embodiments of the present disclosure relate to backside illuminated image sensors and methods of manufacture. The integrated sensor includes a substrate made of a first semiconductor material having a first optical refractive index. The substrate comprises an array of pixels, wherein each pixel has a photosensitive active region formed by a refractive contrast region comprising a matrix of a first semiconductor material and a periodic structure embedded in the matrix. The periodic structure extends from the backside of the substrate and has a two-dimensional periodicity in a plane parallel to the backside. The value of the periodicity is associated with the wavelength of the optical signal and the first refractive index. The elements of the periodic structure are formed of a second optically transparent material having a second refractive index that is less than the first refractive index. These elements are located at positions defined by the periodicity, except at one position, preferably the center, of a defined area, which area is free of the corresponding one of the elements.

Description

Backside illuminated image sensor and method of manufacture
Cross Reference to Related Applications
The present application claims priority to french patent application No. 2106486, filed on 18/6/2021, which is incorporated herein by reference in its entirety to the maximum extent allowed by law.
Technical Field
Implementations and embodiments relate to Back Side Illumination (BSI), and more particularly to enhancement of quantum efficiency thereof, particularly in the infrared range.
Background
The integrated image sensor comprises an array of pixels in the substrate, which array of pixels is associated with control and processing electronics with respect to signals provided by the pixels.
When the image sensor is to be irradiated with an optical signal through the back surface of the substrate, it is called a back surface irradiation type.
The quantum efficiency of a pixel is defined by the ratio of the number of electrons generated to the number of photons received when the pixel absorbs a light signal.
The higher the efficiency, the better the sensitivity of the sensor and the better the signal-to-noise ratio.
Therefore, there is a need to improve the quantum efficiency of the back-illuminated integrated image sensor.
There is also a need to provide gain in terms of crosstalk (interference light signals reflected or diffracted from one pixel to its neighboring pixels) caused by optical coupling between pixels and in terms of modulation transfer function in order to obtain excellent contrast on an image.
Disclosure of Invention
In one embodiment, each pixel is formed with a photonic periodic structure having a two-dimensional periodicity in a plane parallel to the back surface, the photonic structure having a defect, for example at the level of its central region.
Therefore, according to an aspect, a back-illuminated integrated image sensor is proposed.
The sensor is intended to be illuminated by an optical signal, such as (but not limited to) a signal in the infrared range having a wavelength of 940 nm.
The sensor includes a substrate having a backside and comprising a first semiconductor material (e.g., silicon) having a first optical index of refraction.
The sensor includes an array of pixels in a substrate.
Each pixel has a photosensitive active area.
Each photosensitive active region is a refractive contrast region comprising a matrix formed of a first semiconductor material (e.g. silicon) and a periodic structure embedded in the matrix formed of a second material (e.g. silicon dioxide) having a lower refractive index than the first material.
The periodic structure extends from the back surface into the substrate and has a two-dimensional periodicity in a plane parallel to the back surface.
The two-dimensional periodicity is not necessarily the same along two orthogonal directions of the plane.
The periodic structure is referred to as a photonic crystal because the value of the periodicity is associated with the wavelength of the optical signal and the first index of refraction, the first material guiding the optical signal to propagate through the photosensitive region.
Thus, conventionally, the periodicity value is at least substantially equal to a ratio between a wavelength of the optical signal and a refractive index of the first material. That is, to further enhance performance, the periodicity value may be taken to be equal to a few tens of percent increase in the above ratio, e.g., 30%.
The periodic structure has a plurality of elements, such as rods, formed of a second optically transparent (i.e., non-absorptive) material and having a second index of refraction that is less than the first index of refraction.
Furthermore, the periodic structure has a region, such as a central region, which is free of at least one of the elements.
Thus, due to the presence of the defect in this central region (absence of at least one low refractive index and optically transparent element), the light radiation will be confined to a certain volume of the pixel, for example in the centre of the pixel, and the periodic structure will slow down its propagation in the first material (e.g. silicon).
This will therefore lead to a greater absorption in this first material and hence to a greater amount of electrons being generated during the pixel integration time, resulting in an enhancement of the quantum efficiency.
The periodic structure has a honeycomb structure such as an element in its periodic plane.
Furthermore, it is particularly advantageous if, in the periodic plane, the crystal density of all the optically transparent elements occupies at least 50% of the surface area of the photosensitive region.
This makes it possible to further improve the quantum efficiency.
As mentioned above, the elements may be rods having cross-sections of various shapes in a periodic plane (circular, rectangular, star-shaped, etc.).
When the first material is silicon and the wavelength of the optical signal is equal to 940nm, the spacing between the optically transparent elements in the periodic plane may be on the order of 400nm (i.e., equal to within +/-30% of 400 nm).
The second optically transparent material may be, for example, silicon dioxide.
Each rod may have another cross-section with a diameter of 200nm (i.e., within 200nm +/-25%).
In order to ensure a correct collection of the charges generated to the transistors of the processing electronics, it is preferred that the structure extends in the above-mentioned matrix to a position located at a distance in front of the substrate.
Furthermore, in order to further increase the absorption efficiency in the first material, it is advantageous that the sensor further comprises an optical mirror positioned facing each photosensitive area, the photosensitive area being opposite the back side.
It is advantageous to further enhance the operation of the sensor when the pixel array has a periodic arrangement with a pitch N which is an integer ratio to the period of the periodic structure (i.e. equal to k times the period of the periodic structure, k being an integer).
According to another aspect, a method for manufacturing a backside illuminated sensor, the backside illuminated sensor being intended to be illuminated by an optical signal, the method comprising forming a pixel array in a substrate having a backside and comprising a first semiconductor material having a first optical refractive index, the pixel array comprising for each pixel a light sensitive active area according to one embodiment.
According to this aspect of the method, the forming of each photosensitive active region comprises the formation of periodic structures embedded in a matrix formed of a first semiconductor material, said periodic structures extending in said matrix from said back surface, the periodic structures having a two-dimensional periodicity in a plane parallel to the back surface, the value of the periodicity being associated with the wavelength of the optical signal and a first refractive index, the periodic structures having a plurality of elements formed of a second optically transparent material having a second refractive index smaller than the first refractive index and having an area, preferably a central area, devoid of at least one of the elements.
According to one implementation, the forming of the periodic structure includes forming a mask on the backside defining the locations of the elements, etching the substrate through the mask, and filling the holes resulting from the etching with a second material.
According to one implementation, the etching of the matrix stops at a distance from the front of the substrate.
According to one implementation, the first material is silicon and the second material is silicon dioxide.
According to one implementation, the method further includes forming an optical mirror facing the pixel, the photosensitive region being opposite the back surface.
Drawings
Further advantages and features of the invention will emerge from a study of the detailed description of implementation and embodiments, in no way limiting, and the accompanying drawings, in which:
FIG. 1 schematically illustrates an integrated image sensor;
fig. 2 is a cross-sectional view of a pixel;
FIG. 3 is a sectional view taken along line III-III of FIG. 2;
FIG. 4 schematically illustrates the values of periodicity associated with a wavelength; and
fig. 5 to 8 illustrate an implementation of a method for manufacturing a pixel.
Detailed description of the invention
In fig. 1, reference numeral CPT denotes an integrated image sensor, which is very schematically shown to comprise a substrate 1, for example made of silicon, the substrate 1 having a back side FAR and a front side FAV.
The back side FAR is intended to be illuminated by an optical signal SL, for example an infrared signal, typically by means of optical devices such as lenses, optionally colour filters, polarizing means, these examples being non-limiting.
The sensor includes a pixel array PXAR in a substrate 1.
Here, the pixel array has a periodic arrangement with a pitch N between pixels PX.
The image sensor CPT comprises on the front side of the substrate 1 an interconnect portion 2, which is generally denoted by the person skilled in the art using the abbreviation BEOL (back end of line).
This portion 2 generally comprises metal tracks and vias for providing interconnections between the different elements of the sensor, in particular the pixels, and transistors with respect to the control and processing electronics of the signals emitted by the pixels, which are not shown here for the sake of simplicity.
That is to say that these control electronics have a conventional structure known per se.
Fig. 2 schematically shows a cross-sectional view of the pixels PX of the pixel array PXAR.
The pixel PX comprises a refraction-contrast photosensitive region forming a matrix 50 formed of a first semiconductor material, typically silicon of a substrate.
Furthermore, the refraction contrast photosensitive region comprises a periodic structure 5 embedded in a matrix 50 and comprises a plurality of elements 51 formed of a second optically transparent (non-absorbing) material, for example silicon dioxide.
When the optical refractive index of the first material (silicon) is different from the refractive index of the second material (silicon dioxide), the photosensitive region is said to be index-contrast.
The refractive index of the first material forming the matrix 50 has a refractive index greater than the refractive index of the second optically transparent material.
For example, the refractive index of silicon is close to 3.4, and the refractive index of silicon dioxide is close to 1.46.
As shown in fig. 2, the elements 51 of the periodic structure 5 extend from the back surface FAR of the substrate to a depth less than the depth of the substrate.
In other words, the element 51 (typically a rod) stops at a distance d from the front surface FAV of the substrate. The distance d is for example of the order of 1 to 2 micrometers.
On the back surface FAR of the photosensitive area, an optical device 4, such as a lens here, is arranged.
In addition to the metal tracks 20, the interconnect 2 further comprises an optical metal mirror 3, which optical metal mirror 3 is arranged to face the photosensitive area of the pixel, more specifically the front face FAV.
The metal mirror is advantageously formed from the metal of the metal layer of the interconnect.
As shown in fig. 3, fig. 3 is a sectional view along line III-III of fig. 2, the periodic structure 5 has two-dimensional periodicity in a plane, which is the plane of fig. 3, and which is a plane parallel to the back surface FAR.
The periodic structure has a honeycomb arrangement of elements 51 in its periodic plane at locations defined by the periodicity/spacing and the matrix, and here has a central region 500, the central region 500 having defects (i.e., regions without at least one of the elements 51).
In other words, the central region 500 comprises a first material, here silicon.
As schematically shown in fig. 4, the value of the periodicity is associated with the wavelength of the optical signal SL and with the first refractive index of the first material (here silicon).
In order to increase the density of the second material, as in the case of a honeycomb structure, the periodicity in the planar direction may optionally be different from the periodicity in the other orthogonal directions.
More specifically, for a wavelength of the optical signal SL equal to 940nm corresponding to infrared rays, the pitch between the elements 51 in the periodic plane is of the order of 400nm ± 30% (the sides of an equilateral triangle connecting the centers of the three rods 51).
To further enhance the operation of the sensor, the pitch N of the pixels PX is advantageously an integer ratio with the pitch between the elements 51.
Furthermore, the diameter of the rods 51 is of the order of 200nm ± 25%, and the crystal density of the cross-section of all the rods occupies at least 50% and for example 50% of the surface area of the photosensitive region.
A defect of the periodic structure, preferably the center of the periodic structure (i.e. the absence of at least one rod at a position in the preferably central region 500 that would otherwise have rods according to a defined pitch or periodicity) forms a waveguide for the optical radiation SL, and the periodic structure surrounding this waveguide will slow down the propagation rate of the optical signal in silicon and thus increase the absorption time and quantum efficiency of the pixel.
Therefore, a 25% quantum energy increase can be obtained compared to a pixel without a periodic structure having defects in the photosensitive region.
In operation, the optical signal illuminates the back of the sensor, enters the central region 500 of the photosensitive area after passing through the lens 4, propagates in the waveguide described above, reflects on the metallic mirror 3 and is distributed in the waveguide. This return in the waveguide further increases the absorption time in the silicon and thus further increases the quantum energy of the pixel. The quality of the reflection for the same pixel also provides gain in terms of crosstalk and modulation transfer function due to the presence of the waveguide (and not diffracting into neighboring pixels).
An implementation of the method for manufacturing such a pixel will now be described with more specific reference to fig. 5 to 8.
Obviously, all pixels are implemented simultaneously, but fig. 5 to 8 only show steps for one pixel.
In fig. 5, a substrate is provided that includes an epitaxial region 50 (host) of silicon having a height of, for example, 6 to 9 microns.
The FAV in front of the photosensitive area (in front of the substrate 50) supports the interconnect 2 comprising the metal mirror 3.
On the back side of the substrate 50, a mask 7, for example a resin mask, is formed, the holes of which define the positions of the future silica rods (see fig. 6).
Then, as shown in fig. 7, the silicon of the substrate 50 is subjected to conventional etching known per se through the holes of the mask 7, and the holes thus formed are filled with silicon dioxide to form the rods 51.
It should be noted here that, as mentioned above, it is particularly advantageous that the end of the rod 51, and therefore the end of the hole resulting from the etching, is located at a non-zero distance d (for example of the order of 1 to 2 microns) from the front face of the substrate 50, in order to promote a good collection of electrons resulting from the absorption of the optical signal in the silicon.
Then, as shown in fig. 8, the mask 7 is removed, the back surface is polished, and the lens 4 is fixed to the back surface of the pixel, thereby forming the pixel shown in fig. 2.
The invention is not limited to the described implementations and embodiments, but encompasses all variants.
Thus, even if the region of the periodic structure having defects is preferably the central region, it may not be central, particularly for manufacturing reasons.
Further, other first materials are also possible, such as germanium or a silicon/germanium alloy. Additional second materials are possible, such as silicon nitride or high dielectric constant dielectrics, known to those skilled in the art as "high K".
Furthermore, other wavelengths of the optical signal SL (optionally in the visible range) are possible in exchange for the adjustment of the periodic structure size.

Claims (18)

1. A back-illuminated integrated image sensor, comprising:
a substrate having a backside configured to be illuminated by an optical signal and comprising a first semiconductor material having a first optical refractive index; and
an array of pixels in the substrate;
wherein each pixel has a photosensitive active region;
wherein each photosensitive active region is a refractive contrast region comprising a matrix formed of the first semiconductor material, and periodic structures embedded in the matrix, the periodic structures extending from the back surface in the matrix;
wherein the periodic structure has a two-dimensional periodicity in a plane parallel to the back surface;
wherein a value of the two-dimensional periodicity is associated with a wavelength of the optical signal and with the first optical refractive index; and
the periodic structure is formed of a plurality of elements made of a second optically transparent material having a second refractive index less than the first refractive index, the plurality of elements being located at positions defined by the two-dimensional periodicity; and
wherein the photosensitive active region comprises a region at one of the locations defined by the two-dimensional periodicity, the region being free of a corresponding one of the elements.
2. The sensor of claim 1, wherein the region is a central region.
3. The sensor of claim 1, wherein the periodic structure has a honeycomb arrangement of the elements in the plane of the two-dimensional periodicity.
4. The sensor of claim 1, wherein in the plane of the two-dimensional periodicity, a crystal density of all elements occupies at least 50% of a surface area of the photosensitive region.
5. The sensor of claim 1, wherein the element is a rod.
6. The sensor of claim 1, wherein the first material is silicon.
7. The sensor of claim 5, wherein the wavelength of the optical signal is equal to 940nm and a spacing between the elements in the plane of the two-dimensional periodicity is on the order of 400 nm.
8. The sensor of claim 7, wherein the second material is silicon dioxide.
9. The sensor of claim 8, wherein each rod has a cross section with a diameter on the order of 200 nm.
10. The sensor of claim 1, wherein the structures extend in the matrix to a position located at a distance from a front face of the substrate.
11. The sensor of claim 1, further comprising an optical mirror positioned to face each photosensitive region opposite the back surface.
12. The sensor of claim 1, wherein the array of pixels has a periodic arrangement with a pitch that is an integer ratio of the period of the periodic structure.
13. A method for manufacturing a backside illuminated integrated image sensor, comprising: in a substrate having a back side intended to be illuminated by an optical signal and comprising a first semiconductor material having a first optical refractive index:
forming a pixel array including a photosensitive active area for each pixel;
forming a periodic structure in each photosensitive active region, the periodic structure being embedded in a matrix formed of the first semiconductor material;
wherein the periodic structure extends in the substrate from the back side and has a two-dimensional periodicity in a plane parallel to the back side;
wherein a value of the two-dimensional periodicity is associated with a wavelength of the optical signal and with the first refractive index;
wherein forming the periodic structure comprises:
generating a plurality of elements formed of a second optically transparent material having a second refractive index less than the first refractive index, the plurality of elements being located at locations defined by the two-dimensional periodicity; and
defining a region at one of the locations defined by the two-dimensional periodicity, the region being free of a corresponding one of the elements.
14. The method of claim 13, wherein the region is a central region.
15. The method of claim 13, wherein forming the periodic structure comprises:
forming a mask on the back surface defining the positions of the elements;
etching the substrate through the mask; and
filling the holes resulting from the etching with the second material.
16. The method of claim 15, wherein the etching of the matrix stops at a distance from a front side of the substrate.
17. The method of claim 13, wherein the first material is silicon and the second material is silicon dioxide.
18. The method of claim 13, further comprising: an optical mirror is provided facing a photosensitive area of the pixel, the photosensitive area being opposite the back surface.
CN202210691110.1A 2021-06-18 2022-06-17 Backside illuminated image sensor and method of manufacture Pending CN115497969A (en)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
FR2106486A FR3124309A1 (en) 2021-06-18 2021-06-18 Backside illumination image sensor and manufacturing method
FR2106486 2021-06-18
US17/840,437 US20220406829A1 (en) 2021-06-18 2022-06-14 Backside illumination image sensor and manufacturing method
US17/840,437 2022-06-14

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