KR102815489B1 - Image sensor and manufacturing method thereof - Google Patents

Abstract

The present invention discloses an image sensor and a method for manufacturing the same. The image sensor is characterized in that a phase detection auto-focus micro lens is formed on a color filter, and the micro lenses are formed in multiples, one housing the other.

Description

Image sensor and manufacturing method thereof {IMAGE SENSOR AND MANUFACTURING METHOD THEREOF}

The present invention relates to an image sensor and a method for manufacturing the same, and more particularly, to an image sensor having auto-focus pixels for phase detection.

Recently, in the performance comparison of mobile devices, the comparison of camera functions, smooth operation due to installation of various apps, and maximization of storage performance are becoming more important than the superiority of communication functions. In addition, with the development of semiconductor technology, the image sensors of camera modules mounted on mobile devices are becoming more diverse in their configuration and form.

Traditionally, image sensors have usually consisted of a combination of a photodiode, a color filter, and a micro lens to form a single pixel, but recently, so-called dual-cell and quad-cell systems have been developed that combine multiple photodiodes to represent a single pixel, and are being used as a method to increase sensitivity. In addition , a technology has been developed that uses some of the multiple pixels to detect the phase difference from the subject, and this is called phase detection auto focus ( PDAF ) technology. Phase detection auto focus (PDAF) technology is a technology that enables the camera to focus more quickly by comparing the phase difference of a pair of divided lights that passed through a pair of auto focus pixels to determine whether the subject is in focus.

In some phase detection auto focus (PDAF) technologies, the size of the micro lens used for auto focus is formed to be larger than the size of other micro lenses. For example, as illustrated in FIG. 1, the micro lens (130) for auto focus is larger than other micro lenses (110). This is also called a quad cell. In the case of a quad cell, as illustrated in detail in FIG. 2, the micro lens (130) for auto focus is twice as large in diameter and four times larger in area than a general micro lens (110) for image capture.

However, even in the case of quad cells, the sizes of the photodiodes formed inside the semiconductor substrate are mostly the same and not different from each other. This is because photodiodes of the same size must be arranged in series so that adjacent cells experience the same manufacturing process environment. In other words, the degree of interference occurring during the manufacturing process such as film deposition, exposure, and etching also occurs at the same level only when the sizes are the same. Due to the extremely fine and sensitive nature of the semiconductor manufacturing process, the structural characteristics and electrical characteristics of each photodiode cell are uniform only when the degree of interference is the same.

Meanwhile, as illustrated in FIG. 3, each photodiode (190) formed inside a semiconductor substrate is electrically separated from each other by an insulating film (170). The insulating film (170) is also formed along the deep wall surface of the photodiode, and this method of insulation by a deep trench is also called DTI ( Deep Trench Isolation ). In the present invention, it is called a deep trench insulating film or DTI insulating film.

However, there is a problem that some of the light collected through the micro lens (130) for autofocus is refracted and directed toward the DTI insulating film (170) between each photodiode (190) as shown by the arrow in Fig. 3. This problem causes some of the light that should be directed to the photodiode to be dispersed, thereby reducing the light collection efficiency of the image sensor. This ultimately leads to a decrease in the detection efficiency of the camera module that adopts the phase detection focus method, and a decrease in the sensitivity and operation speed of the autofocus.

Patent Document 1: Korean Registration No. KR 10-0672706 (2007.01.16) Patent Document 2: Korean Registration No. KR 10-1439434 (2014.09.02) Patent Document 3: Korean Registration No. KR 10-0399939 (2003.05.17)

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

An image sensor according to one embodiment of the present invention includes: a photodiode formed on a semiconductor substrate; a color filter formed on an upper portion of the photodiode; an image capturing micro lens formed on an upper portion of the color filter to capture an image from a subject; a phase detection auto-focus micro lens formed on an upper portion of the color filter to detect a phase difference of incident light; and the phase detection auto-focus micro lens includes multiple micro lenses having different radii of curvature.

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

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

According to an embodiment of the present invention, incident light dispersed or scattered by an insulating film between each photodiode is collected to cause the photodiode to perform richer photoelectric conversion, thereby improving the autofocus function of the camera module.

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

Figure 1 illustrates an array of pixels of an autofocus image sensor employing a quad cell.
Figure 2 is a top view and a partial detailed view of Figure 1.
Figure 3 is a drawing showing the refraction of incident light by a micro lens.
Figure 4 is a top view and a partial detailed view showing one embodiment of the present invention.
FIG. 5 is a drawing showing multiple refraction of incident light according to one embodiment of the present invention.
Figure 6 is a graph comparing the change in photocurrent according to the incident angle of light.
Figure 7 is a three-dimensional drawing for showing a cross-sectional cut line of the present invention.
Fig. 8 illustrates the upper surface of the micro lens portion among the patterns for each layer. Fig. 9 illustrates the upper surface of the color filter array among the patterns for each layer.
Figure 10 illustrates the upper surface of the metal grid and deep trench insulating film of the insulating layer among the patterns for each layer.
Figure 11 illustrates the upper surface of a pattern for forming a photodiode among the patterns of each layer.

Hereinafter, with reference to the attached drawings, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. Among the reference numerals presented in each drawing, the same reference numerals represent the same components.

In explaining 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 is omitted.

Although the terms first, second, etc. may be used to describe various components, said components are not limited by said terms, and said terms are used only to distinguish one component from another.

In an embodiment of the present invention, an image sensor is disclosed having multiple micro lenses configured for phase detection autofocus.

FIG. 4 illustrates an array of pixels of a phase detection auto-focus image sensor (400) using a quad cell structure as an embodiment of the present invention.

Referring to FIG. 4, the image sensor (400) includes 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 o n C hip). The auto-focus pixel also includes a micro lens (430, 440) as a component. The image capture pixel is a pixel for capturing an image of a subject, and may include red (R), green (G), and blue (B) pixels, and in the case of a quad cell, one color is expressed by four adjacent image capture pixels. The micro lens for auto-focus is composed of an outer lens (430), an inner lens (440), and a multiple.

Among the components of each pixel, the photodiodes are separated from each other by a deep trench insulation film (DTI, Deep Trench Isolation , 470). Therefore, light received by each photodiode is converted into current without mutual interference with adjacent photodiodes. In the embodiment of FIG. 4, the size of the photodiode (490) formed at the bottom of the multi-structure auto-focus micro lens (430, 440) is also formed to be the same size as the photodiode (450) of the image capture pixel, so that the multi-structure auto-focus micro lens (430, 440) occupies four photodiode areas.

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

The photodiodes (490) are formed inside a semiconductor substrate, and a DTI insulating film (470) surrounds each photodiode, so that they are electrically insulated from each other and interference between them is also minimized. A color filter (460) and a thin insulating film (420) are formed in sequence on the upper portion of the photodiode (490).

The inner lens (440) and the outer lens (430) may be made of an insulating material used in semiconductor manufacturing. For example, the material may be an insulating film containing silicon oxide or nitride (N). In addition, the inner lens (440) and the outer lens (430) may be formed using a passivation layer used as an insulating layer at a near-final stage of the semiconductor preprocess. The same applies to the thin insulating film (420). The thin insulating film (420) may be formed in advance using a material such as a passivation layer to ensure the stability of a process that may occur when forming a micro lens. The thin insulating film (420) acts as an insulator and buffer between the color filter (460) layer and the micro lens layer to be formed later. The buffering function refers to mitigating process damage caused by thermal expansion, exposure, etching, etc. during the subsequent manufacturing process when the materials forming each layer are different from each other. Therefore, depending on the need, 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. In addition, even if they are the same material, the density and refractive index may be different depending on the conditions of the manufacturing process, and furthermore, the thin insulating film (420) may be omitted.

The micro lens for autofocus is composed of multiple lenses including an outer lens (430) and an inner lens (440). The inner lens (440) is small enough to be sufficiently accommodated inside the outer lens (430), and its radius of curvature is also smaller than that of the outer lens.

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

Although not illustrated, as an example of another embodiment of the present invention, the thin insulating film (420) may be omitted or 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 portions of the two lenses contact each other to form a point of contact.

Light incident on 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. As in the embodiment of the present invention illustrated in FIG. 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, light directed toward the DTI insulating film (470) between the photodiodes changes direction and is directed toward the photodiode (490) and absorbed therein. If there were no secondary refraction, the primarily refracted light would reach the DTI insulating film (470) as before and be dispersed or scattered, resulting in loss, but due to the secondary refraction, it can be stored inside the photodiode, thereby increasing the amount of light, which has the effect of increasing the sensitivity of the image sensor. In some cases, the term 'sensitivity' in the present invention may be used synonymously with 'photocurrent amount'.

In Fig. 5, in order to show the effect of secondary refraction by the inner lens (440), light reaching the photodiode (490) is indicated by a solid arrow, and light proceeding only through primary refraction without the inner lens (440) is indicated by a dashed arrow.

The degree of refraction, or refraction angle, of the inner lens (440) is affected not only by the geometric shape of the inner lens (440) but also by its material. Originally, the inherent 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 components used to form the inner lens (440), the composition ratio of the compounds, etc. also affect the refractive index. For example, even if 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 found the appropriate conditions for a greater amount of light to reach the photodiode through repeated experiments and simulations in various environments.

The results of an experiment comparing an embodiment of the present invention having a pair of phase detection automatic detection pixels (AF_R, AF_L) and having only one inner lens (440) and a case without an inner lens are shown in Fig. 6. The upper figure shows a case without an inner lens, and the lower figure shows a case with an inner lens. The vertical axis represents the amount of photocurrent, i.e., the sensitivity of the pixel, and the horizontal axis represents the incident angle of light.

The photocurrent was measured within the range of the incident angle of light from -20 ^o to +20 ^o , and the measurement points that serve as the standards for comparison were selected as +10 ^o and -10 ^o , respectively. A pair of photocurrents detected by a pair of phase detection automatic detection pixels increase (AF_L) or decrease (AF_R) as the incident angle increases, and the directions of the increase and decrease are opposite to each other. This tendency is due to the structural aspect of screening a part of a pair of auto-focus micro lenses for phase detection, and a detailed description thereof is omitted. In Fig. 6, points C1 and C2 represent the lowest values of the photocurrent. Since this lowest value corresponds to crosstalk or an unwanted baseline component, it is desirable that it is smaller. In addition, points P1 and P2 are the maximum values of the photocurrent, so it is desirable that it is larger.

The difference between the maximum and minimum values, i.e. (P1-C1, P2-C2), is larger when the inner lens is present. For an objective comparison, the difference values when +10 ^o and -10 ^o are averaged for each case with and without the inner lens, and the results of comparing them are shown in Table 1. In other words, if this is expressed as a formula, it is a comparison of the values of .

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

Meanwhile, in order to determine the effect of the present invention, the ratio of the sensitivity of the image capture pixels representing green and the auto focus pixels was used as an index (hereinafter, 'SENSE') and this value was measured. This value is preferable when it is closer to 1 because the two pixels to be compared have no difference in sensitivity. In one embodiment of the present invention, this value was measured as 1.2 when there was an inner lens, and as 1.3 when there was no inner lens. Through this comparison, it can be seen that one embodiment of the present invention has a further improvement in the value representing the sensitivity. The above explanation is summarized in Table 1 below.

[Table 1]

In order to determine the effectiveness of the present invention, various other indices may be utilized. One of the other indices may include crosstalk, which indicates the degree of interference between each pixel. In addition, in the present invention, the sensitivity difference 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 at all. In addition, the sensitivity difference of a red image capture pixel may also be compared in the same manner. In addition, as mentioned above, the refractive index of the inner lens (440) may be varied to measure the changes in the various indices.

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

The results in Table 2 are obtained by closely measuring the contrast of auto focus pixels (AF contrast), sensitivity index (SENSE), crosstalk of green pixels, and crosstalk of red pixels, respectively, while fixing the refractive index of the outer lens (430) to 1.58 and the reference wavelength of measurement to 540 nm, and then changing the refractive index of the inner lens (440) between 1.7 and 2.0. The figures in the leftmost column in Table 2 are values measured when there is no inner lens (440) 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.

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

In order to obtain useful experimental results as described above, a simulation experiment can be performed using a well-written commercial computer program with an image sensor formed with a microlens array for image capture and a microlens for phase detection autofocus. For the simulation experiment, the image sensor is cut in cross section (A-A line) as shown in Fig. 7, and the so-called photocurrent component converted into current by a photodiode is detected at an appropriate depth inside the semiconductor substrate.

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

FIG. 8 is a top view showing a pattern for forming a top-layer micro lens, as shown and described in one embodiment of the present invention. Micro lenses (410) for image capture are arrayed, and an outer lens (430) pattern is formed among the micro lenses for autofocus.

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

Fig. 10 shows a grid-shaped metal grid and a square-shaped deep trench insulating film as a pattern for forming an insulating layer between a photodiode and a color filter. The area of the part where the micro lens for autofocus is formed is larger and can be easily recognized.

Figure 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 pixels for image capture and pixels for auto focus.

In order to form a structure of an image sensor like one embodiment of the present invention, the following manufacturing process steps must be performed.

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

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

Afterwards , a deep trench insulation film ( DTI ) is formed to insulate the top and sidewall of each photodiode to form an insulating layer on top of each photodiode. The mask pattern used at this time is as shown in Fig. 10. ( Deep trench insulation film formation step)

Red, green, and blue color filter layers that display three primary colors are formed on the upper part of the deep trench insulating film. At this time, a plurality of photodiodes correspond to one color filter area. In other words, the size of each unit color filter that is arrayed and repeatedly formed is larger than the size of each unit photodiode that is also arrayed and repeatedly formed. For example, in the case of a quad cell, one unit color filter includes and corresponds to four photodiode areas. (Color filter layer formation step)

An interlayer insulating material, such as a silicon oxide film or silicon nitride film, is selected and formed on top of the color filter layer. (Thin insulating film formation step) This step may be omitted to obtain manufacturing advantages.

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

An outer lens (430) layer is formed on top of the inner lens (440) layer. The material selected at this time may include a silicon oxide film or a silicon nitride film. The size of the outer lens is preferably such that all or part of the inner lens can be accommodated therein, so the curvature of the outer lens is preferably larger than the curvature of the inner lens. When the inner lens is accommodated, the crest of the inner lens may touch the crest of the outer lens or may maintain a certain distance therefrom. The shape of the outer lens is formed so that the upper part is convex so that it can perform the function of a 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, by configuring the micro lenses of the phase detection auto focus (PDAF) pixels in multiple layers, the loss of incident light can be reduced and the reliability of phase difference detection can be increased. In addition, the auto focus contrast (AF Contrast) performance and the sensitivity of the image sensor can be improved while minimizing crosstalk between each pixel.

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

100: Quad cell PDAF array 110: Standard micro lens
130: Micro lens for autofocus 150, 450: Color filter
170, 470: Deep trench insulation 190, 490: Photo diode
400: Quad cell PDAF array 420: Thin insulating film
430: Outer Lens 440: Inner Lens

Claims (13)

Photodiodes formed on a semiconductor substrate;
A deep trench insulating film electrically insulating between the photodiodes;
A color filter formed on top of the above photodiodes;
An image capturing micro lens formed on top of the photo diodes and corresponding to the photo diodes one-to-one to capture an image from a subject;
A micro lens for phase detection auto-focus, corresponding to four adjacent photodiodes among the above photodiodes and formed on the upper part of the four adjacent photodiodes to detect the phase difference of incident light;
An image sensor having a structure in which the above-described phase detection autofocus microlens comprises multiple microlenses having different radii of curvature, wherein one of the multiple microlenses is housed inside the other.
In paragraph 1,
An image sensor in which the above multiple micro lenses are plane-convex lenses having different heights.
In paragraph 1,
An image sensor wherein one of the multiple micro lenses has a height higher than the image capturing micro lens.
delete In paragraph 1,
An image sensor in which the micro lens to be accommodated is an inner lens and the micro lens that accommodates it is an outer lens.
In paragraph 5,
An image sensor in which the radius of curvature of the inner lens is smaller than the radius of curvature of the outer lens.
In paragraph 1,
An image sensor wherein any one color among the above color filters has a size corresponding to a plurality of the above photodiodes.
A step of forming photodiodes on a semiconductor substrate;
A step of forming a deep trench insulating film for electrically insulating between the above photodiodes;
A step of forming a color filter layer on top of the above photodiodes;
A step of forming an image capturing micro lens corresponding to the above photodiodes one-to-one and formed on each upper portion of the photodiodes to capture an image from a subject;
A step of forming an inner lens corresponding to four adjacent photodiodes among the above photodiodes and formed on top of the four photodiodes;
A step of forming an outer lens having a larger radius of curvature than the inner lens;
A method for manufacturing an image sensor including:
In Article 8,
A method for manufacturing an image sensor, wherein the area of any one unit color filter of the color filter layer has a size corresponding to the area of several photodiodes.
In Article 8,
A method for manufacturing an image sensor in which the top portion of the outer lens is in contact with the top portion of the inner lens.
In Article 8,
A method for manufacturing an image sensor, wherein the height of the outer lens is equal to or greater than the height of the inner lens.
delete In Article 8,
A method for manufacturing an image sensor, comprising a step of additionally forming a thin insulating film between the color filter layer forming step and the inner lens forming step.

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