JP2015023259A - Solid-state image pickup device and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid-state image pickup device which can inhibit color mixture between neighboring pixels while reducing increase in dark current in a back-illumination solid-state image pickup device; and provide a manufacturing method of the solid-state image pickup device.SOLUTION: A solid-state image pickup device according to an embodiment comprises: a plurality of pixels which are provided on a semiconductor layer and each of which has a light-receiving part for converting incident light from a first surface side of the semiconductor layer to signal charge and storing the signal charge; electrodes provided on the first surface of the semiconductor layer and on a second surface side opposite to the first surface, for reading the signal charge stored in the pixel; and a pixel isolation structure which is provided between the neighboring pixels and has a trench provided in a depth direction running from the first surface side to the second surface side of the semiconductor layer. A depth of the trench of the pixel isolation structure between a red color pixel and a pixel of a color other than red color is deeper than a depth of a trench of the pixel isolation structure between pixels of the color other than red color.

Description

Embodiments described herein relate generally to a solid-state imaging device and a method for manufacturing the same.

Solid-state imaging devices such as CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image sensors include a front-illuminated type and a back-illuminated type, and a semiconductor layer surface side on which a CMOS part such as a signal readout circuit is formed. A structure that receives light from the opposite surface, that is, the back surface side is called a back surface irradiation type. Compared with the front side irradiation type, the back side irradiation type has advantages in that light utilization efficiency is high and sensitivity is high because a circuit such as a metal wiring formed in the semiconductor layer is not formed on the light receiving surface.

However, in back-illuminated solid-state imaging devices as well, in recent years, pixel miniaturization has progressed with a significant increase in the number of pixels, and color mixing between adjacent pixels has become a problem. As a structure for suppressing color mixing between adjacent pixels, there is a structure in which a light shielding film is formed between pixels. The light shielding film blocks incident light from adjacent pixels and suppresses color mixing between adjacent pixels. For example, as disclosed in Patent Document 1, a light shielding film is formed by forming a trench between pixels and embedding a conductive member such as tungsten. At this time, there is a phenomenon in which dark current increases due to the formation of trenches in silicon (Si) between pixels.

JP 2009-88030 A

An object of an embodiment of the present invention is to provide a solid-state imaging device capable of suppressing color mixing between adjacent pixels while reducing an increase in dark current in a back-illuminated solid-state imaging device, and a manufacturing method thereof. Objective.

The solid-state imaging device according to the embodiment includes a plurality of pixels provided on a semiconductor layer, each of which includes a light receiving unit that converts light incident from the first surface side of the semiconductor layer into a signal charge and accumulates the signal charge. Provided on the second surface side opposite to the first surface of the semiconductor layer and between the adjacent pixels, the electrode for reading out signal charges accumulated in the pixel, and the semiconductor layer A pixel isolation structure having a trench provided in a depth direction from the first surface side toward the second surface side of the trench of the pixel isolation structure between the red pixel and the non-red pixel. The depth is deeper than the depth of the trench of the pixel isolation structure between pixels other than red.

It is a schematic diagram showing the solid-state imaging device concerning a 1st embodiment. It is sectional drawing along the A line of FIG. It is the example of arrangement | positioning of the pixel of each color and a pixel separation structure at the time of seeing the solid-state imaging device concerning the embodiment from the upper surface. FIG. 4 is a cross-sectional view taken along line C in FIG. 3. It is sectional drawing along the D line of FIG. It is the absorption efficiency of light of 530 nm which is a green region in the red pixel with respect to the depth of the trench. The amount of dark current with respect to the depth of the trench. It is sectional drawing of the solid-state imaging device which concerns on 2nd Embodiment. It is a graph of the trench depth at the time of etching with the same time length while changing the trench width.

  Hereinafter, embodiments will be described with reference to the drawings. The same parts in the drawings are denoted by the same reference numerals, detailed description thereof will be omitted as appropriate, and different parts will be described. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between the parts, and the like are not necessarily the same as actual ones. Further, even when the same part is represented, the dimensions and ratios may be represented differently depending on the drawings.

(First embodiment)
FIG. 1 is a schematic diagram illustrating a solid-state imaging device according to the first embodiment. FIG. 2 is a sectional view taken along line A in FIG. In other words, FIG. 1 is a cross-sectional view taken along line B of FIG.

The solid-state imaging device according to the present embodiment includes a semiconductor layer 10, a plurality of pixels 20 provided in the semiconductor layer 10, an electrode 30, and a pixel separation structure 40. The plurality of pixels 20 include a red pixel 21 and a pixel 22 other than red. The non-red pixel 22 is, for example, a blue pixel or a green pixel.

The semiconductor layer 10 is, for example, a silicon layer, and its conductivity type is p-type. The pixel 20 detects light incident on the semiconductor layer 10. Each pixel 20 includes a light receiving unit 13 that converts light incident from the first surface side of the semiconductor layer 10 into signal charges and accumulates the signal charges. The electrode 30 is provided on the second surface side opposite to the first surface of the semiconductor layer 10. The electrode 30 is used to read out signal charges accumulated in the pixel 20.

The pixel isolation structure 40 is provided between adjacent pixels, and has a trench provided in a depth direction from the first surface side to the second surface side of the semiconductor layer 10. In the trench, for example, an insulating film and a light shielding film are formed on the bottom and side surfaces. The pixel separation structure 40 blocks incident light from adjacent pixels and suppresses color mixing between adjacent pixels. On the other hand, a dark current is generated by forming a trench in the semiconductor layer 10.

In FIG. 1, the depth of the trench is indicated by d. The inventors obtained the knowledge that the increase in dark current is larger as the depth d of the trench is deeper through experiments described later. That is, it is preferable that the depth d of the trench is shallow from the viewpoint of suppressing an increase in dark current.

The light absorption coefficient has wavelength dependence, and the absorption coefficients of silicon are 8 × 10 ⁴ (cm ⁻¹ ) and 6 × 10 ^{3 at} wavelengths of 400 nm (blue), 500 nm (green), and 700 nm (red), respectively. (Cm ⁻¹ ) and 2 × 10 ³ (cm ⁻¹ ). That is, red light having a long wavelength has a low absorption rate into silicon, and is then easily absorbed in the order of green and blue. In other words, blue light is absorbed in the shallow part of the semiconductor layer, green light is absorbed in the middle of the semiconductor layer, and red light is absorbed in the deep part of the semiconductor layer. In other words, blue light can suppress color mixing between adjacent pixels even when the trench depth d is shallow, but it is preferable to increase the trench depth d for red light.

Therefore, in the solid-state imaging device according to this embodiment, the depth of the trench of the pixel isolation structure 40 between the red pixel 21 and the non-red pixel 22 is such that the trench of the pixel isolation structure 40 between the non-red pixels 22. Deeper than the depth of. By doing so, it is possible to suppress color mixing between adjacent pixels while reducing an increase in dark current.

  Next, the solid-state imaging device according to the present embodiment will be described in detail with reference to FIGS. 1 and 2. In the following description, the conductivity type of the semiconductor layer 10 will be described as a p-type, but the present invention is not limited to this. That is, the semiconductor layer 10 may be n-type.

  As shown in FIG. 1, a plurality of light receiving portions 13 are provided inside the semiconductor layer 10. The light receiving unit 13 has an n-type conductivity different from that of the semiconductor layer 10. The light receiving unit 13 absorbs light incident on the semiconductor layer 10 and generates photocarriers.

  As shown in FIG. 2, each light receiving unit 13 is surrounded by a pixel separation structure 40, and one pixel 20 includes one light receiving unit 13. The pixel isolation structure 40 is a part of the semiconductor layer 10 and its conductivity type is p-type.

On the second surface side of the semiconductor layer 10, a p ⁺ layer 19 is selectively provided in a portion corresponding to each pixel 20. The p ⁺ layer 19 includes a higher concentration of p-type impurities than the semiconductor layer 10. An n ⁺ layer 17 is provided between the p ⁺ layer 19 and the light receiving unit 13. The n ⁺ layer 17 contains a higher concentration of n-type impurities than the light receiving unit 13.

A part of the n ⁺ layer 17 extends to the second surface and faces the electrode 30 through the insulating film 33. Further, the p layer 15 is provided at a position adjacent to the extending portion 17 a of the n ⁺ layer 17. The p layer 15 includes, for example, a p-type impurity having a higher concentration than the semiconductor layer 10 and a lower concentration than the p ⁺ layer 19. The p layer 15 faces the electrode 30 through the insulating film 33.

  For example, when a positive gate bias is applied to the electrode 30, an inversion channel is formed at the interface between the p layer 15 and the insulating film 33. The photocarrier photoelectrically converted in the light receiving unit 13 is output as a photocurrent through the inversion channel and a source / drain region (not shown).

  On the other hand, an antireflection film 50 is provided on the semiconductor layer 10 on the first surface side. The antireflection film 50 is an insulating film. The antireflection film 50 can be a single layer film or a multilayer film. The refractive index (or equivalent refractive index) of the antireflection film 50 is higher than the refractive index of the film or layer directly provided thereon. For example, the refractive index of the antireflection film 50 has a value between the refractive index of the semiconductor layer 10 and the refractive index of a film or layer directly provided on the antireflection film 50 in the visible light region. And the reflection of incident light is reduced.

  As shown in FIG. 1, a color filter 53 and a microlens 55 are provided on the antireflection film 50 via a flattening film 51. The planarizing film 51 relaxes unevenness on the surface of the antireflection film 50. Further, the refractive index is smaller than the refractive index of the antireflection film 50. For the planarizing film 51, for example, a silicon oxide film can be used.

  The color filter 53 and the micro lens 55 are provided for each pixel. That is, the microlens 55 collects light incident on each pixel and efficiently enters the light receiving unit 13. The color filter sorts the wavelength of incident light for each pixel.

  The solid-state imaging device according to the present embodiment further includes a pixel separation structure 40 that demarcates the boundary of each pixel. The pixel separation structure 40 is provided between the adjacent pixels. When viewed from the top, as shown in FIG. 2, the pixel separation structure 40 is provided in a lattice shape so as to surround each pixel.

As the pixel isolation structure 40, a trench is formed in the depth direction from the first surface side to the second surface side of the semiconductor layer 10. Some material may be embedded in the trench, or it may be a cavity. Here, a case where the insulating film 41 is provided on the bottom and side surfaces of the trench and the light shielding film 42 is embedded inside the insulating film 41 is considered as the pixel isolation structure 40.

  The light shielding film 42 suppresses leakage of light incident into the pixel 20 to adjacent pixels. That is, the light shielding film 42 absorbs or reflects the incident light of the pixel 20. The light shielding film 42 may be a conductive film or an insulating film. For example, a conductive film such as a metal can be used for the light shielding film 42. For example, the light-shielding film 42 can be a conductive film containing at least one of silicon, titanium, tungsten, aluminum, and hafnium.

  The light shielding film 42 desirably has a high visible light region reflectance. Thereby, the light which injects into the light-receiving part 13 can be increased, and the photosensitivity of the pixel 20 can be improved. From this viewpoint, for example, aluminum is preferably used as the light shielding film 42. Further, as the light shielding film 42, a multilayer dielectric film that reflects visible light may be used.

  Further, a ground or negative voltage may be applied to the light shielding film 42. By doing so, holes are generated at the interface between the silicon and the trench of the semiconductor layer, and dark current can be reduced.

  For the insulating film 41, for example, silicon oxide or hafnium oxide can be used. Further, as the insulating film 41, for example, an insulating film having a negative fixed charge film such as zirconium oxide may be used. By using an insulating film having a negative fixed charge film, dark current can be reduced.

The pixel isolation structure 40 has a structure in which a P-type silicon layer is provided on the bottom and side surfaces of a trench, an insulating film 41 is provided on the inner side, and a light shielding film 42 is embedded on the inner side of the insulating film 41. There may be. With such a structure, holes are generated at the interface between the silicon and the trench in the semiconductor layer, and dark current can be reduced.

As shown in FIG. 1, the depth of the trench of the pixel isolation structure 40 is such that the trench of the pixel isolation structure 40 between the red pixel 21 and the non-red pixel 22 is the pixel between the non-red pixels 22. Deeper than the trench of the isolation structure 40.

  FIG. 3 shows an arrangement example of the pixels 20 of each color and the state of the pixel separation structure 40 when the solid-state imaging device according to the present embodiment is viewed from above. FIG. 4 is a cross-sectional view taken along line C in FIG. FIG. 5 is a cross-sectional view taken along line D in FIG. 3 to 5, there are a green pixel 23 and a blue pixel 24 as the pixels 22 other than red. As shown in FIGS. 3 to 5, the pixel separation structure 40 around the red pixel 21 is deeper than the pixel separation structure 40 around the non-red pixel 22.

Next, a method for manufacturing the solid-state imaging device according to this embodiment will be described. A step of forming a trench of the pixel isolation structure 40 between the non-red pixels 22 to a first depth in order to change the peripheral trench depth between the red pixel 21 and the non-red pixel 22; A step of forming a trench of the pixel isolation structure 40 between the pixel 22 other than red and a second depth deeper than the first depth is performed. First, the periphery of the red pixel 21 is patterned by lithography, and the semiconductor layer 10 is etched to the first depth to form a trench. Next, the periphery of the pixel 22 other than red is patterned by lithography, and the semiconductor layer 10 is etched to the second depth to form a trench. By changing the etching time of the semiconductor layer 10, trenches having different depths can be formed. In the above description, the trench around the red pixel 21 is etched first, but this order may be reversed.

Thereafter, the pixel isolation structure 40 is formed by performing the step of forming the insulating film 41 and the step of forming the light shielding film 42 in the trench.

Next, a specific example of this embodiment will be described. The following specific numerical values and materials are examples, and are not limited to these numerical values and materials.

Here, in the element in which the red pixel 21 and the green pixel 23 are adjacent to each other, the pixel separation structure 40 is formed between the red pixel 21 and the green pixel 23. Specifically, a trench is formed in the semiconductor layer 10, silicon oxide is formed as the insulating film 41 inside the trench, and then tungsten is embedded as the light shielding film 42, thereby forming the pixel isolation structure 40. Then, two-dimensional color mixing spectral calculation was performed when the trench depth was changed. The pixel pitch is 1.1 μm, and the photodiode depth is 2.5 μm.

FIG. 6 shows the absorption efficiency of light of 530 nm, which is the green region in the red pixel 21, with respect to the depth of the trench. That is, it is possible to see the color mixture with respect to the depth of the trench. From FIG. 6, it can be seen that when the trench depth is 0 μm, that is, when there is no light shielding film 42, there is much color mixing. It can be seen that the color mixture decreases as the trench becomes deeper.

FIG. 7 shows the amount of dark current with respect to the depth of the trench. FIG. 7 shows that the dark current increases as the trench becomes deeper.

6 and 7, it can be seen that when the trench is deepened, the color mixture between the pixels is reduced, but the dark current is increased. That is, the dark current and the color mixture are traded off with respect to the depth of the trench.

Here, considering the case of the pixel layouts as shown in FIGS. 3, 4 and 5, the light incident on the red pixel 21 has a long wavelength, so the absorption efficiency into the silicon layer is poor, and the adjacent green color There is much color mixture to the pixel 23. On the other hand, since the light incident on the blue pixel 24 has a short wavelength, the absorption efficiency into the silicon layer is high, and there is little color mixing in the adjacent green pixel 23. Thus, the light absorption efficiency to the silicon layer has wavelength dependency. Therefore, even if the trench depth between the blue pixel 24 and the green pixel 23 is shallow, the dark current can be reduced without deteriorating the color mixture.

Further, when the depth of the trench between all the pixels is 1 um, the dark current increases by 100 (arbitrary multiplier) as compared with the case where there is no trench. On the other hand, when the trench depth around the red pixel 21 is 1 μm and the trench depth around the non-red pixel 22 is 0.5 μm, the increase in dark current is 74. Thus, by forming the trench in the semiconductor layer 10 by reducing the trench depth around the non-red pixels 22 to 0.5 μm, compared to the case where the trench depth around all the pixels is 1 μm. The increase in dark current that occurs can be reduced.

When the trench depth around the red pixel 21 is 1 μm and the trench depth around the non-red pixel 22 is 0.2 μm, the increase in dark current is 63. Next, when the trench depth between all the pixels is set to 0.7 μm, the dark current increases by 90. On the other hand, when the trench depth around the red pixel 21 is 0.7 μm and the trench depth around the non-red pixel 22 is 0.5 μm, the dark current increases by 71. As described above, by reducing the trench depth around the non-red pixel 22 by about 10 to 90% with respect to the trench depth around the red pixel 21, color mixing is suppressed without significantly increasing the dark current. be able to.

According to the present embodiment, the solid-state imaging device according to the present embodiment is such that the depth of the trench of the pixel isolation structure between the red pixel and the non-red pixel is the trench of the pixel isolation structure between the non-red pixels. Deeper than the depth of. By doing in this way, color mixing between adjacent pixels can be suppressed by the pixel isolation structure having a trench while reducing an increase in dark current.

(Second Embodiment)
A solid-state imaging device according to the second embodiment will be described below. FIG. 8 is a cross-sectional view of the solid-state imaging device according to the present embodiment. As shown in FIG. 8, the solid-state imaging device according to the second embodiment is substantially the same as the solid-state imaging device according to the first embodiment. In the solid-state imaging device according to the second embodiment in FIG. 8, the same components as those in the solid-state imaging device according to the first embodiment shown in FIG. .

In the solid-state imaging device according to the second embodiment, the pixel separation structure 45 is different from the pixel separation structure 40 of the solid-state imaging device according to the first embodiment. In the solid-state imaging device according to the second embodiment, the width of the trench of the pixel isolation structure 45 between the red pixel 21 and the non-red pixel 22 is equal to the width of the trench of the pixel isolation structure 45 between the non-red pixel 22. Wider than. Regarding the point that the depth of the trench of the pixel isolation structure 45 between the red pixel 21 and the non-red pixel 22 is deeper than the depth of the trench of the pixel isolation structure 45 between the non-red pixel 22, This is the same as the solid-state imaging device according to the embodiment.

Next, a method for manufacturing the solid-state imaging device according to this embodiment will be described. In the first embodiment, the trench around the red pixel 21 and the trench around the non-red pixel 22 are formed in different steps. However, in this embodiment, the trench around the red pixel 21 and the non-red pixel are formed. The trenches around 22 are formed in the same process. That is, in the first embodiment, lithography is performed twice, but in this embodiment, in one lithography, a trench around the pixel 22 other than red and a red pixel having a deeper depth than that. 21 around the trench.

The manufacturing method according to this embodiment includes a first width trench in the pixel isolation structure 45 between the non-red pixels 22 and a first trench in the pixel isolation structure 45 between the red pixel 21 and the non-red pixel 22. A step of patterning a trench having a second width larger than the width of 1 by lithography, and a step of etching silicon in accordance with a pattern formed in the patterning step. Thereafter, the pixel isolation structure 45 is formed by performing the step of forming the insulating film 41 and the step of forming the light shielding film 42 in the trench.

In the solid-state imaging device according to this embodiment, the width of the trench of the pixel isolation structure 45 between the red pixel 21 and the non-red pixel 22 is greater than the width of the trench of the pixel isolation structure 45 between the non-red pixel 22. Therefore, the microloading effect when etching silicon can be used.

FIG. 9 shows a graph of the trench depth when etching is performed for the same time length while changing the trench width. FIG. 9 shows that the etching depth increases as the trench width increases. In this embodiment, the microloading effect is used to make the trench width around the red pixel 21 wider than the trench width of the non-red pixel 22, so that lithography is performed once and silicon is etched once. The trench around the red pixel 21 can be deepened.

For example, if the trench depth around the red pixel 21 is 0.7 um and a 0.5 um trench is formed around the non-red pixel 22, the trench width around the red pixel 21 is 220 nm, and the non-red pixel 22 periphery. This can be realized by processing under the etching conditions used in FIG. 9 using a mask having a trench width of 150 nm. This time, the trench depth is controlled by setting the difference between the periphery of the red pixel 21 and the other trench width to 70 nm, but the difference of 70 nm in the trench width can be changed by changing the conditions for etching silicon. . When it is desired to reduce the difference in the trench width, it is sufficient to use an etching condition that has a large loading effect, that is, a dependency of the depth on the trench width that is greater than the etching of the experiment obtained in FIG. On the other hand, when it is desired to increase the difference in trench width, an etching condition with a small loading effect may be used. The loading effect in dry etching of silicon varies depending on the influence of ion energy, pressure, total gas flow rate, additive gas, and the like. For example, if the pressure is reduced or the total gas flow rate is increased, the loading effect can be reduced. Thus, by making the trench dimensions around the red pixel 21 wider than the trenches around the non-red pixel 22, the trench depth around the red pixel 21 can be obtained by one dry etching due to the loading effect of dry etching of silicon. Can be made deeper than the trenches around the pixels 22 other than red.

According to the present embodiment, the solid-state imaging device according to the present embodiment is such that the depth of the trench of the pixel isolation structure between the red pixel and the non-red pixel is the trench of the pixel isolation structure between the non-red pixels. Deeper than the depth of. By doing in this way, color mixing between adjacent pixels can be suppressed by the pixel isolation structure having a trench while reducing an increase in dark current.

In the solid-state imaging device according to this embodiment, the width of the trench of the pixel isolation structure between the red pixel and the non-red pixel is wider than the width of the trench of the pixel isolation structure between the pixels other than red. Thereby, the trench around the pixel other than red and the trench around the red pixel having a deeper depth can be formed in the same process.

Although embodiments of the present invention have been described, the embodiments have been presented by way of example and are not intended to limit the scope of the invention. The embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

10: Semiconductor layer, 13: Light receiving portion, 15: p layer, 17: n + layer, 17a: Extension portion of n + layer 17, 19: p + layer, 20: pixel, 21: red pixel, 22: pixel other than red 23: green pixel, 24: blue pixel, 30: electrode, 33: insulating film, 40: pixel separation structure, 41: insulating film, 42: light shielding film, 45: pixel separation structure, 50: antireflection film, 51: Flattening film, 53: color filter, 55: microlens

Claims (10)

A plurality of pixels provided on the semiconductor layer, each of which has a light receiving portion for converting and storing light incident from the first surface side of the semiconductor layer into a signal charge;
An electrode provided on a second surface side opposite to the first surface of the semiconductor layer, for reading out signal charges accumulated in the pixels;
A pixel isolation structure having a trench provided between adjacent pixels and provided in a depth direction from the first surface side to the second surface side of the semiconductor layer;
A solid-state imaging device, wherein a depth of a trench of the pixel isolation structure between a red pixel and a pixel other than red is deeper than a depth of a trench of the pixel isolation structure between pixels other than red. The solid-state imaging device according to claim 1, wherein the pixel isolation structure includes a light shielding film and an insulating film embedded in the trench. The solid-state imaging device according to claim 2, wherein the light shielding film is formed of a metal including any one of silicon, titanium, tungsten, aluminum, and hafnium. The solid-state imaging device according to claim 2, wherein a negative voltage is applied to the light shielding film. The solid-state imaging device according to claim 1, wherein the pixel isolation structure includes a P-type silicon layer. The solid-state imaging device according to claim 1, wherein the pixel separation structure includes an insulating film including a negative fixed charge film. The width of the trench of the pixel isolation structure between a red pixel and a pixel other than red is wider than the width of the trench of the pixel isolation structure between pixels other than red. The solid-state imaging device according to any one of 6. A method for manufacturing a solid-state imaging device having a pixel separation structure between a plurality of pixels and adjacent pixels on a semiconductor layer,
Forming a trench of the pixel isolation structure between pixels other than red to a first depth;
Forming a trench having the pixel isolation structure between a red pixel and a non-red pixel to a second depth deeper than the first depth. Forming an insulating film in the trench;
The method for manufacturing a solid-state imaging device according to claim 8, further comprising a step of forming a light shielding film in the trench. A method for manufacturing a solid-state imaging device having a pixel separation structure between a plurality of pixels and adjacent pixels on a semiconductor layer,
A first width trench in the pixel separation structure between pixels other than red, and a second width trench wider than the first width in the pixel separation structure between red pixels and non-red pixels. Patterning by lithography, and
And a step of etching silicon according to the pattern formed in the patterning step.

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