KR20220041563A - Image sensor and manufacturing method thereof - Google Patents

Abstract

The present invention discloses an image sensor and a manufacturing method thereof. The image sensor has phase detection autofocus micro lenses formed on the upper part of a color filter, and the micro lenses are formed in multiples, wherein one accommodates the other. The light sensitivity may be improved by condensing incident light, which is dispersed or scattered by an insulating film between each photodiode.

Description

Image sensor and manufacturing method thereof

The present invention relates to an image sensor and a manufacturing method thereof, and more particularly, to an image sensor having autofocus pixels for phase detection.

In the recent performance comparison of mobile devices, rather than superiority or inferiority of communication functions, comparison of camera functions, smooth operation due to installation of various apps, maximization of storage performance, etc. are becoming more important. Also, with the development of semiconductor technology, the configuration and shape of the image sensor of the camera module mounted on the mobile device are gradually diversifying.

Traditionally, image sensors usually consist of a combination of photodiodes, color filters, and microlenses to form one pixel. is used as a method for Also, a technology that uses some of several pixels to detect a phase difference from a subject has been developed, which is called a phase detection auto focus ( PDAF ) technology. Phase-detection autofocus (PDAF) technology determines whether a subject is in focus by comparing the phase difference between a pair of divided light passing through a pair of autofocus pixels, allowing the camera to focus more quickly. It is a skill that makes

In some of the phase detection autofocus (PDAF) technologies, the size of a microlens used for autofocus is formed larger than that of other microlenses. For example, as shown in FIG. 1 , the micro lens 130 for auto focus is larger than the other micro lenses 110 . This is also called a so-called quad cell, and in the case of a quad cell, as shown in detail in FIG. The area is four times

However, even in the case of a quad cell, the size of the photodiodes formed inside the semiconductor substrate is not different from each other and most of them are formed the same. This is because, although photodiodes having the same size must be sequentially arranged, adjacent cells undergo the same manufacturing process environment. That is, it is because the degree of interference occurring in the manufacturing process such as film formation, exposure, and etching, etc., also occurs to the same degree only when the sizes are the same. Due to the extremely fine and sensitive characteristics of the semiconductor manufacturing process, the structural and electrical characteristics of each cell of the photodiode become uniform only when the degree of interference is the same.

Meanwhile, as shown in FIG. 3 , each photodiode 190 formed inside the semiconductor substrate is electrically separated from each other by the insulating layer 170 . The insulating layer 170 is also formed along the deep wall of the photodiode, and the insulating method by such a deep trench is also called deep trench isolation ( DTI ) . In the present invention, it is referred to as a deep trench insulating film or a DTI insulating film.

However, there is a problem in that some of the light focused through the microlens 130 for autofocus is refracted and directed toward the DTI insulating film 170 between the photodiodes 190 as shown by the arrow in FIG. 3 . This problem causes a portion of light to be directed to the photodiode to be dispersed, thereby lowering the light collecting efficiency of the image sensor. This in turn leads to a reduction in detection efficiency of a camera module adopting a phase detection focus method, and a reduction in autofocus sensitivity and operation speed.

Patent Document 1: Korean Registration No. KR 10-0672706 (Jan. 16, 2007) Patent Document 2: Korean Registration No. KR 10-1439434 (2014.09.02) Patent Document 3: Korean Registration No. KR 10-0399939 (2003.05.17)

SUMMARY The technical problem to be solved by the present invention is to provide an image sensor capable of reducing light loss and reliably detecting a phase to increase autofocus sensitivity and operational efficiency, and a method for manufacturing the same.

An image sensor according to an embodiment of the present invention includes a photodiode formed on a semiconductor substrate; a color filter formed on the photodiode; a micro lens for image capturing that is formed on the color filter and captures an image from a subject; a micro lens for auto-focusing for phase detection that is formed on the color filter and detects a phase difference of incident light; The microlens for phase detection autofocus includes a plurality of microlenses having different radii of curvature.

A method of manufacturing an image sensor according to an embodiment of the present invention includes forming a photodiode; forming a deep trench insulating layer; forming a color filter layer; inner lens forming step; and forming an outer lens having a greater curvature than the inner lens.

According to an embodiment of the present invention, light sensitivity can be improved by condensing incident light that is dispersed or scattered by the insulating film between each photodiode.

According to an embodiment of the present invention, the autofocus function of the camera module can be improved by condensing incident light dispersed or scattered by the insulating film between each photodiode to cause the photodiode to generate more abundant photoelectric conversion.

In addition, according to the embodiment of the present invention, since light loss during phase detection can be reduced, even when used in a high-resolution camera module having a small light-receiving area, the auto-focus success probability can be improved.

1 shows an array of pixels of an auto-focus image sensor to which a quad cell is applied.
Fig. 2 is a top view of Fig. 1 and a partial detail view thereof;
3 is a diagram illustrating refraction of incident light by a micro lens.
4 is a top view showing an embodiment of the present invention and a partial detailed view thereof;
5 is a diagram illustrating multiple refraction of incident light according to an embodiment of the present invention.
6 is a diagram comparing the change of photocurrent according to the incident angle of light.
7 is a three-dimensional view for indicating a cross-sectional cut line of the present invention.
8 is a view showing a top surface of a micro lens part among patterns for each layer. 9 illustrates a top surface of a color filter array among patterns for each layer.
FIG. 10 shows the upper surface of the metal grid of the insulating layer and the deep trench insulating layer among the patterns for each layer.
11 illustrates a top surface of a pattern for forming a photodiode among patterns for each layer.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily carry out the present invention. The same reference numerals among the reference numerals presented in the respective drawings indicate the same members.

In describing the present invention, if it is determined that a detailed description of a related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted.

Terms such as first and second may be used to describe various elements, but the elements are not limited by the terms, and the terms are used only for the purpose of distinguishing one element from other elements. used

In an embodiment of the present invention, an image sensor in which a micro lens configured for phase detection autofocus is configured in multiples is disclosed.

4 illustrates an array of pixels of a phase detection autofocus image sensor 400 using a quad cell structure as an embodiment of the present invention.

Referring to FIG. 4 , the image sensor 400 may include an array of pixels, and the array of pixels may include an image capture pixel and an auto-focus pixel. The image capture pixel includes a micro lens 410 as a component. Since the micro lens is formed on a semiconductor substrate, it is also called MLOC ( Micro Lens on C hip ) . The auto focus pixel also includes micro lenses 430 and 440 as components. An image capture pixel is a pixel for capturing an image of a subject, and may include red (R), green (G), and blue (B) pixels. In the case of a quad cell, four adjacent image capture pixels represent one color. The micro lens for autofocus is composed of an outer lens 430, an inner lens 440, and multiple.

Among the components of each pixel, photodiodes are separated from each other by an insulating layer ( DTI , Deep Trench Isolation, 470 ) having a deep trench structure. Therefore, the light received by each photodiode is converted into a current without mutual interference with the adjacent photodiode. In the embodiment of FIG. 4 , since the size of the photodiode 490 formed under the microlenses 430 and 440 for autofocus of the multi-structure is also the same size as the photodiode 450 of the pixel for image capture, the multi-structure The micro lenses 430 and 440 for autofocus of occupies four photodiode areas.

5 is a cross-sectional view taken along the cut line S in FIG. 4 , and an embodiment of the present invention will be described in more detail with reference to this.

The photodiodes 490 are formed inside the semiconductor substrate, and the DTI insulating film 470 is surrounded between the photodiodes, and thus, since they are electrically insulated from each other, interference with each other is also minimized. A color filter 460 and a thin insulating layer 420 are sequentially formed on the photodiode 490 .

The material of the inner lens 440 and the outer lens 430 may be an insulating material used in semiconductor manufacturing. For example, it may be a silicon oxide film or an insulating film containing nitride (N). In addition, the inner lens 440 and the outer lens 430 may be formed by using a passivation layer used as an insulating layer in an almost final stage of the semiconductor preprocess. This is also the same for the thin insulating film 420 . The thin insulating film 420 may be formed of the same material as a passivation layer in advance for stability of a process that may occur when forming the microlens. The thin insulating film 420 functions as an insulating and buffering layer between the color filter 460 layer and a micro lens layer to be formed later. When the materials forming each layer are different from each other, the buffering action refers to reducing process damage caused by various kinds of thermal expansion, exposure, etching, etc. caused by the continuous manufacturing process. Therefore, if necessary, the thin insulating film 420 , the inner lens 440 , and the outer lens 430 may be made of the same material or may be made of insulating materials of different materials. Also, the same material may have different densities and different refractive indexes depending on the conditions of the manufacturing process, and furthermore, the thin insulating film 420 may be omitted.

The micro lens for auto focus is composed of multiple including an outer lens 430 and an inner lens 440 . The inner lens 440 is small enough to be sufficiently accommodated in the outer lens 430 , and the radius of curvature is also smaller than that of the outer lens.

Due to the characteristics of the semiconductor manufacturing process, starting from the photodiode 490 at the bottom, the outer lens at the top is sequentially formed.

Although not shown, as an example of another embodiment of the present invention, the thin insulating film 420 may be omitted and not formed during the manufacturing process.

Also, although not shown, as an example of another embodiment of the present invention, the inner lens 440 and the outer lens 430 may have the same height, so that the uppermost layer portions of the two lenses contact each other to form a contact point.

Light incident to the micro lens for autofocus is primarily refracted by the outer lens 430 . The degree of refraction can be easily obtained according to Snell's law. 5 , when the inner lens 440 is provided, light refracted through the outer lens 430 is secondarily refracted again by the inner lens 440 . Due to the secondary refraction, the light directed to the DTI insulating film 470 between the photodiodes is changed to be directed toward the photodiode 490 and is absorbed therein. If there is no secondary refraction, the primary refracted light reaches the DTI insulating film 470 as in the prior art and is dispersed or scattered, resulting in loss. It has the effect of increasing the sensitivity of the sensor. In some cases, in the present invention, the term 'sensitivity' may be used synonymously with 'photocurrent'.

In FIG. 5 , light reaching the photodiode 490 is a solid arrow in order to show the effect of secondary refraction by the inner lens 440 , and the light that proceeds only through primary refraction without the inner lens 440 is Indicated by a dashed arrow.

The degree of refraction, that is, the refraction angle, of the inner lens 440 is affected not only by the geometric shape of the inner lens 440 but also by the material thereof. Originally, the intrinsic refractive index of a material is a function of the dielectric constant and permeability of the material. Therefore, the type or content ratio of the component used to form the inner lens 440, the composition ratio of the compound, etc. also affect the refractive index. For example, even when the material of the inner lens 440 is a silicon oxide film or a silicon nitride film, they have different refractive indices. Therefore, the researchers of the present invention have found an appropriate condition for a greater amount of light to reach the photodiode through repeated experiments and simulations in various environments.

6 shows the results of an experiment by comparing an embodiment of the present invention having only one inner lens 440 and without an inner lens having a pair of automatically detecting phase detection pixels AF_R and AF_L. The upper figure shows the case without an inner lens, and the lower figure shows the case with the inner lens. The vertical axis represents the amount of photocurrent, that is, the sensitivity of the pixel, and the horizontal axis represents the incident angle of light.

The amount of photocurrent was measured in the range of the incident angle of light from -20 ^o to +20 ^o , but the measurement points used as the basis for comparison were selected when they were +10 ^o and -10 ^o , respectively. A pair of photocurrent amounts detected by a pair of phase detection automatic detection pixels increase (AF_L) or decrease (AF_R) as the incident angle increases, but the directions of increase and decrease are opposite to each other. This tendency is due to a structural aspect of screening some areas of a pair of autofocus microlenses for phase detection, and detailed description thereof will be omitted. In FIG. 6, points C1 and C2 indicate the lowest value of the photocurrent. Since this minimum value corresponds to crosstalk, or an undesired base value component, the smaller the value, the better. In addition, since the points P1 and P2 are the maximum values of the photocurrent, it is preferable that the points are larger.

The difference between the maximum value and the minimum value, that is, (P1-C1, P2-C2), appears larger when there is an inner lens. For objective comparison, with and without the inner lens, the two difference values at +10 ^o and -10 ^o were averaged, and then the comparison results are shown in Table 1. That is, when expressed as a formula, the values of are compared.

When there is no inner lens, this average value is 5.2, and when there is one inner lens as in the embodiment of the present invention, this average value reaches 8.0. Since this average value indicates the degree of autofocus contrast (AF contrast), the larger the value, the more desirable the result. Other conditions used in this experiment were that the outer lens height ( H eight ) and radius of curvature ( R adius of Curvature ) were 0.9㎛ and 0.7㎛ (H/R=0.9/1.0), respectively, and the inner lens was 0.89 respectively. μm and 0.7 μm (H/R=0.89/0.7). The refractive index (n) of the inner lens was 1.8.

Meanwhile, in order to examine the effect of the present invention, the ratio of the sensitivity of the pixel for image capture and the pixel for autofocus representing green was used as an index (hereinafter, 'SENSE') and this value was measured. Since there is no difference in sensitivity between the two pixels to be compared, the closer this value is to 1, the more preferable. In an embodiment of the present invention, when the inner lens is present, this value is 1.2, and when there is no inner lens, this value is measured to be 1.3. By this comparison, it can be seen that the numerical value indicating the sensitivity is further improved in one embodiment of the present invention. The above description is summarized in Table 1 below.

[Table 1]

Several other indicators may be used to examine the effectiveness of the present invention. One of the other indicators may include crosstalk indicating the degree of interference between pixels. Also, in the present invention, a difference in sensitivity between a green image capture pixel located far away from the auto focus pixel and a green image capture pixel located adjacent to the auto focus pixel may be compared so as not to be affected by the auto focus pixel at all. Also, in the same way, a difference in sensitivity of a red image capture pixel may be compared. In addition, as mentioned above, it is possible to measure changes in the various indicators while variously changing the refractive index of the inner lens 440 .

As described above, the results of measuring various indicators through various experiments are shown in Table 2. Among them, the crosstalk for each of green and red may be expressed by the following equation, and has a percentage (%) unit.

The results in Table 2 show that the refractive index of the outer lens 430 is fixed at 1.58 and the reference wavelength for measurement is fixed at 540 nm, and then the refractive index of the inner lens 440 is changed between 1.7 and 2.0 while changing the contrast (AF) of the autofocus pixel. contrast), sensitivity index (SENSE), crosstalk of green pixels, and crosstalk of red pixels were carefully measured, respectively. In Table 2, the values in the leftmost column are values measured when the inner lens 440 is not present for comparison.

[Table 2]

These various experimental results in the present invention show that the presence of the inner lens 440 presented as an embodiment of the present invention is more effective in applying the phase detection auto focus ( PDAF ) technology. tells what

As can be expected from various improved values in Tables 1 and 2, it can be implemented by variously changing the material, refractive index, height, radius of curvature, distance from the outer lens, etc. of the inner lens from one embodiment of the present invention. It is natural to have In addition, it is natural that examples in which the number of inner lenses is not just one but multiple are also included in the scope of the core idea of the present invention.

In order to obtain the useful experimental results described above, it is possible to simulate using a well-designed commercial computer program with an image sensor having a micro lens array for image capture and a micro lens for automatic focusing for phase detection. For the simulation, as shown in FIG. 7 , the so-called photocurrent component converted into a current by a photodiode is detected at an appropriate depth inside the semiconductor substrate by cutting the image sensor in a cross section (line A-A).

In order to fabricate a multi-micro lens as in an embodiment of the present invention using a semiconductor manufacturing technology, it is preferable to use the mask patterns of FIGS. 8 to 11 .

8 is a top view showing a pattern for forming a microlens of the uppermost layer, shown and described in an embodiment of the present invention. Micro lenses 410 for image capture are arrayed, and an outer lens 430 pattern is formed among micro lenses for auto focus.

9 shows a pattern for forming a color filter described in an embodiment of the present invention. The colors of red, green, and blue were divided by differences in patterns for convenience. In addition, when the outer lens 430 and the inner lens 440 of the present invention are selected to be formed on the green color filter, they are illustrated to overlap the green color filter as shown in FIG. 9 .

10 shows a grid-shaped metal grid and a square-shaped deep trench insulating film as a pattern for forming an insulating layer present between the photodiode and the color filter. The area where the microlens for auto focus is to be formed is larger and can be easily recognized.

11 shows a pattern for forming a photodiode. The photodiodes formed inside the semiconductor substrate are repeatedly arrayed with the same size without distinguishing between the image capturing pixel and the autofocusing pixel.

In order to form the structure of the image sensor as in the embodiment of the present invention, the following manufacturing process steps should be performed.

In order to form a photodiode inside a semiconductor substrate, a PN junction diode is manufactured using a technique of ion implantation or diffusion of impurities. The exposure pattern used at this time uses a shape as shown in FIG. 11. (Photodiode formation step)

Hereinafter, well-known photolithography techniques such as exposure and etching using a mask of a corresponding pattern are commonly used in each step.

Thereafter, in order to form an insulating layer on each of the photodiodes, a deep trench insulating layer ( DTI ) is formed to insulate an upper part and a sidewall of each photodiode by partitioning the insulating layer. The mask pattern used at this time is as shown in FIG. 10 . (Deep trench insulating film formation step)

A red, green, and blue color filter layer displaying three primary colors is formed on the deep trench insulating layer. In this case, a plurality of photodiodes correspond to one color filter area. That is, the size of each unit color filter repeatedly formed in an array is larger than the size of each unit photodiode repeatedly formed in an array. For example, in the case of a quad cell, one unit color filter includes four photodiode areas to correspond to it. (Color filter layer formation step)

An interlayer insulating material that may include a silicon oxide layer or a silicon nitride layer is selected and formed on the color filter layer. (Thin insulating layer forming step) This step may be omitted to take advantage of manufacturing.

An inner lens 440 layer is formed on the color filter layer or the thin insulating layer. In this case, the material selected may include a silicon oxide film or a silicon nitride film. The shape of the inner lens is to make the lower part flat and the upper part convex to perform the function of the plano-convex lens. (Inner lens formation step)

An outer lens 430 layer is formed on the inner lens 440 layer. In this case, the material selected may include a silicon oxide film or a silicon nitride film. Since the size of the outer lens is preferably large enough to accommodate all or part of the inner lens therein, it is preferable that the curvature of the outer lens is greater than the curvature of the inner lens. In accommodating the inner lens, the bottom of the inner lens may be in contact with the bottom of the outer lens or may maintain a certain distance. The shape of the outer lens is formed so that the upper part is convex to perform the function of the convex lens. (Outer lens formation step)

As described above, according to the embodiment of the present invention or the core idea of the present invention, the loss of incident light can be reduced and the reliability of the phase difference detection is increased by configuring the microlenses of the phase detection autofocus (PDAF) pixels in multiples. . In addition, it is possible to improve autofocus contrast performance and image sensor sensitivity while minimizing crosstalk between pixels.

Although the present invention has been described with reference to the embodiments shown in the drawings, these are merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible therefrom. Accordingly, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

100: quad cell PDAF array 110: normal micro lens
130: micro lens for autofocus 150, 450: color filter
170, 470: deep trench insulating film 190, 490: photodiode
400: quad cell PDAF array 420: thin insulating film
430: outer lens 440: inner lens

Claims (13)

a photodiode formed on a semiconductor substrate;
a color filter formed on the photodiode;
a micro lens for image capturing that is formed on the color filter and captures an image from a subject;
a micro lens for auto-focusing for phase detection that is formed on the color filter and detects a phase difference between incident light;
and a plurality of micro lenses having different radii of curvature in the phase detection autofocus micro lens.
The method of claim 1,
The multiple micro lenses are plane-convex lenses having different heights.
The method of claim 1,
One of the plurality of micro lenses is an image sensor having a higher height than the micro lenses for image capturing.
The method of claim 1,
An image sensor having a structure in which one of the plurality of micro lenses is accommodated inside the other.
5. The method of claim 4,
The micro lens to be accommodated is an inner lens, and the micro lens to be accommodated is an outer lens.
6. The method of claim 5,
An image sensor in which a radius of curvature of the inner lens is smaller than a radius of curvature of the outer lens.
The method of claim 1,
An image sensor having a size corresponding to a plurality of the photodiodes in any one color among the color filters.
photodiode formation step;
forming a deep trench insulating film;
forming a color filter layer;
inner lens forming step;
forming an outer lens having a greater curvature than the inner lens;
A method of manufacturing an image sensor comprising a.
9. The method of claim 8,
An area of an arbitrary unit color filter of the color filter layer has a size corresponding to an area of a plurality of photodiodes.
9. The method of claim 8,
A method of manufacturing an image sensor such that the crest of the outer lens is in contact with the crest of the inner lens.
9. The method of claim 8,
A method of manufacturing an image sensor such that a height of the outer lens is equal to or greater than a height of the inner lens.
9. The method of claim 8,
The method of manufacturing an image sensor such that the outer lens completely accommodates the inner lens therein.
9. The method of claim 8,
and further forming a thin insulating film between the color filter layer forming step and the inner lens forming step.

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