JP2008258201A - Rear surface irradiation type solid-state imaging element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a rear surface irradiation type solid-state imaging element having an oxide film in which its film thickness is closely controlled while using an SOI wafer. <P>SOLUTION: This rear surface irradiation type solid-state imaging element 54 is provided with a semiconductor substrate 1 having an oxide film 2 of 1-50 nm in thickness on it rear surface; a light-receiving section 44 and a charge transfer section 46 formed on the front surface of the semiconductor substrate 1; and a color filter 19 and microlenses 22 formed on the oxide film 44. The semiconductor substrate 1 having an oxide film 2 is formed of an SOI wafer having an oxide film and a silicon layer formed on a silicon substrate by removing the silicon substrate with a solution principally containing KOH. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solid-state image sensor, and particularly relates to a back-illuminated solid-state image sensor.

  A solid-state imaging device is used in an optical apparatus such as a digital still camera or a video camera. In general, a solid-state imaging device includes a semiconductor substrate, a light receiving portion formed on the surface side of the semiconductor substrate, and a charge transfer portion that is formed on the semiconductor substrate and transfers charges generated in the light receiving portion. Further, a color filter and a microlens are formed in the opening above the light receiving unit. Incident light to the solid-state imaging device is irradiated to the light receiving unit through the macro lens and the color filter, and is photoelectrically converted by the light receiving unit. Charges generated in the light receiving unit by photoelectric conversion are transferred to the outside via the charge transfer unit.

  In recent years, due to the demand for miniaturization, the area of the opening has been reduced, and the presence of a plurality of insulating films between the microlens and the light receiving portion, there is a concern that the sensitivity of the solid-state imaging device may be reduced.

In order to solve the problem, Patent Document 1 discloses a back-illuminated solid-state imaging device that uses an SOI (Silicon on Insulator) wafer and photoelectrically converts light incident from the back side of a semiconductor substrate.
Japanese Patent Laid-Open No. 2005-268738

  In the above-mentioned patent document 1, after forming an element on a silicon layer of an SOI wafer, the silicon substrate and the oxide film are removed, the back surface is exposed, and an antireflection film is formed on the exposed surface, thereby forming a backside irradiation type. A method for manufacturing a solid-state imaging device is disclosed. This back-illuminated solid-state imaging device requires a step of removing an oxide film and a step of forming an antireflection film.

  In order to solve the above-described problems, an object of the present invention is to obtain a back-illuminated solid-state imaging device having an oxide film whose film thickness is carefully controlled while using an SOI wafer.

  In order to achieve the above object, according to the present invention, a back-illuminated solid-state imaging device according to the present invention includes a semiconductor substrate having an oxide film with a film thickness of 1 to 50 nm on the back surface, and light reception formed on the surface side of the semiconductor substrate. SOI wafer in which an oxide film and a silicon layer are formed on a silicon substrate, a semiconductor substrate including the oxide film, a color filter formed on the oxide film, and a microlens. The silicon substrate is removed with a solution containing KOH as a main component.

  An SOI wafer having an oxide film having a thickness that matches a predetermined design value is prepared, and a silicon substrate is selectively etched with a KOH solution whose concentration and temperature are adjusted to have an oxide film for a back-illuminated solid-state imaging device. A semiconductor substrate is used. After the silicon substrate is selectively etched with a KOH solution, re-etching using an HF solution or the like is not performed. The oxide film formed on the SOI wafer is not removed, and is used as the oxide film of the back-illuminated solid-state imaging device. As a result, a back-illuminated solid-state imaging device having an oxide film whose film thickness is carefully controlled can be obtained.

  In the backside illumination type solid-state imaging device of the present invention, it is preferable that the film thickness is adjusted so that the oxide film has an antireflection function. This is because it is not necessary to form a new antireflection film because the oxide film has an antireflection function.

  The back-illuminated solid-state imaging device of the present invention preferably includes a coating film that is formed on an oxide film and that exhibits an antireflection function in combination with the oxide film. This is because a desired antireflection effect can be easily obtained by combining the oxide film and the coating film.

  In the backside illumination type solid-state imaging device of the present invention, the coating film is preferably composed of a high refractive index film and a low refractive index film in order from the oxide film side. This is because the combination of an oxide film, a high refractive index film, and a low refractive index film has an excellent antireflection function.

  In the backside illumination type solid-state imaging device of the present invention, the high refractive index film is selected from the group consisting of silicon nitride, cerium oxide, zirconium oxide, yttrium oxide, hafnium oxide, tantalum oxide, titanium nitride, and titanium oxide. It is preferable that one material is included. This is because these materials are suitable as a high refractive index film.

  In the backside illumination type solid-state imaging device of the present invention, the low refractive index film includes one material selected from the group consisting of magnesium fluoride, silicon oxide, silicon nitride, silicon oxynitride, and silicon nitride. preferable. This is because these materials are suitable as a low refractive index film.

  According to the present invention, it is possible to obtain a back-illuminated solid-state imaging device having an oxide film whose thickness is carefully controlled while using an SOI wafer.

  A preferred embodiment of a backside illuminated solid-state imaging device according to the present invention will be described below with reference to the accompanying drawings. However, the present invention is not limited to these examples.

[Solid-state imaging device]
FIG. 1 is a perspective view showing an external shape of a solid-state imaging device according to an embodiment of the present invention. The solid-state imaging device 50 includes a solid-state imaging element chip 52 on which a back-illuminated solid-state imaging element 54 is formed, a frame-shaped spacer 56 surrounding the back-illuminated solid-state imaging element 54 provided on the solid-state imaging element chip 52, and spacers A cover glass 58 that is attached and seals the back-illuminated solid-state imaging device 54 is configured. In order to perform wiring with the outside, the solid-state imaging device chip 52 includes a plurality of pads 60.

[Solid-state image sensor]
In the present invention, as shown in FIG. 2, an SOI wafer 30 is used to form a back-illuminated solid-state imaging device. The SOI wafer 30 includes a silicon substrate 31, a silicon oxide film 32, and a silicon layer 33. There are several types of methods for manufacturing the SOI wafer 30. SIMOX (Separation by IM planted Oxygen) is a method of forming a buried silicon oxide film between a silicon substrate and a silicon layer by injecting oxygen ions into the silicon substrate and then performing heat treatment at a high temperature. ELTRAN (Epitaxial Layer TRANsfer) (registered trademark of Canon Inc.) forms a porous film on a seed silicon substrate as a seed, forms an epitaxial layer and a silicon oxide film layer on the porous film, and forms a silicon oxide film layer. In this method, a supporting silicon substrate is pasted through the substrate, and the seed silicon substrate is separated by a porous film. Smart-Cut (registered trademark of SOITEC Co., Ltd.) is a method in which hydrogen ions are implanted into a silicon substrate on which an oxide film is formed, and then another silicon substrate is bonded, and the silicon substrate is peeled off from the hydrogen ion implanted portion to form an SOI wafer. How to get. SOI wafers manufactured by other methods can be suitably used in the present invention.

  In order to form the functional unit 34 including the charge transfer unit and the light receiving unit on the SOI wafer 30 manufactured by the above method, a plurality of semiconductor processes are performed. As a result, as shown in FIG. A functional unit 34 including a charge transfer unit and a light receiving unit is formed on the surface layer of the silicon layer 33.

  Next, in order to remove the silicon substrate 31 from the SOI wafer 30, the SOI wafer 30 is etched with a KOH solution. The KOH solution is a mixed solution of KOH (potassium hydroxide), IPA (isopropyl alcohol) and water, and the KOH concentration is adjusted to 10 to 50 vol%. Further, the temperature of the KOH solution is controlled at 40 ° C to 75 ° C.

  The reason why the KOH solution is used as the etching solution is that the silicon substrate 31 and the silicon oxide film 32 have a large selection ratio (Si: SiO2 = 100: 1 or more), so that the etching can be performed at 0.2 to 10 μm / min. This is because 32 can be an etching stop layer.

  The silicon substrate 31 is removed from the SOI wafer 30, and the silicon layer 33 including the silicon oxide film 32 becomes the semiconductor substrate 1 including the silicon oxide film 2 of the back-illuminated solid-state imaging device. The present invention is characterized in that the silicon oxide film 32 of the SOI wafer 30 becomes the silicon oxide film 2 of the back-illuminated solid-state imaging device.

  However, when the silicon oxide film 32 of the SOI wafer 30 is different from the desired film thickness, the film thickness of the silicon oxide film 32 may be adjusted by etching. Since the silicon oxide film 32 is formed to have a predetermined film thickness, even if a deviation occurs in the film thickness, the difference is small. Since the etching amount is small, it is easy to accurately adjust the silicon oxide film 32 of the SOI wafer 30 to the film thickness required as the silicon oxide film 2 of the solid-state imaging device. The silicon oxide film 32 is adjusted with, for example, a KOH solution. After removing the silicon substrate 31, the silicon oxide film 32 can be etched by using a KOH solution. Since the etching rate of the KOH solution with respect to the silicon oxide film 32 is slow, it is suitable for adjusting the film thickness.

  FIG. 3 shows a cross-sectional structure of a back-illuminated CCD type solid-state imaging device 54 to which the present invention is applied. The solid-state imaging device 54 includes a p-type semiconductor substrate 1 having a p-type silicon layer 1a and a p ++ type silicon layer 1b having a higher impurity than the p-type silicon layer 1a. The p-type semiconductor substrate 1 includes a first surface 40 and a second surface 42 opposite to the first surface 40. In this specification, the 2nd surface 42 of the p-type semiconductor substrate 1 is called a back surface, and the solid-state image sensor which irradiates light from the 2nd surface 42 side is called the solid-state image sensor of back irradiation.

  A plurality of n-type impurity diffusion layers 4 are formed on the first surface 40 side of the p-type semiconductor substrate 1 in order to accumulate charges generated in the p-type semiconductor substrate 1 in response to incident light. The n-type impurity diffusion layer 4 has a two-layer structure including an n-type impurity diffusion layer 4 a and an n − -type impurity diffusion layer 4 b formed in this order from the first surface 40 side. Charges generated in the n-type impurity diffusion layer 4 and charges generated when incident light from the second surface 42 side passes through the p-type semiconductor substrate 1 are accumulated in the n-type impurity diffusion layer 4. The n-type impurity diffusion layer 4 and the p-type semiconductor substrate 1 that generate an electric charge in response to incident light constitute the light receiving unit 44.

  A high concentration p + -type impurity diffusion layer 5 is formed on each n-type impurity diffusion layer 4. This is to prevent dark charges generated on the first surface 40 of the p-type semiconductor substrate 1 from being accumulated in each n-type impurity diffusion layer 4. An n + -type impurity diffusion layer 6 having a higher concentration than the n-type impurity diffusion layer 4 is formed in each p + -type impurity diffusion layer 5 from the first surface 40 side toward the inside of the p-type semiconductor substrate 1. The n + -type impurity diffusion layer 6 functions as an overflow drain for discharging unnecessary charges accumulated in the n-type impurity diffusion layer 4. The p + type impurity diffusion layer 5 functions as an overflow barrier for the overflow drain.

  The charge transfer channel 12 is formed on the first surface 40 side of the p-type semiconductor substrate 1 at a position away from the n-type impurity diffusion layer 4 and the p + -type impurity diffusion layer 5. A p-type impurity diffusion layer 11 having a lower concentration than the p + -type impurity diffusion layer 5 is formed so as to surround the charge transfer channel 12.

  A gate insulating film 20 made of a silicon oxide film, an ONO film or the like is formed on the first surface 40 of the p-type semiconductor substrate 1, and a transfer electrode 13 made of polysilicon or the like is further formed on the gate insulating film 20. ing. Charges generated in the light receiving portion are transferred by the charge transfer channel 12 and the transfer electrode 13 to constitute a CCD (charge transfer portion 46). The surface of the transfer electrode 13 is covered with an oxide film 14.

  An element isolation layer 15 is formed between the adjacent n-type impurity diffusion layers 4 in order to prevent charge from leaking to the adjacent n-type impurity diffusion layers 4.

  An insulating film 9 such as silicon oxide is formed on the first surface 40 so as to cover the transfer electrode 13 and the oxide film 14. The electrode 7 is formed through a contact hole formed in the insulating film 9 so as to be electrically connected to the n + -type impurity diffusion layer 6. An electrode 8 is provided on the insulating film 9 so as to be electrically connected to the electrode 7. A protective layer 10 is formed on the electrode 8. Further, a support substrate 48 made of silicon, glass or the like is provided on the protective layer 10 by a bonding method such as a surface activation technique.

A p ++ type silicon layer 1b is provided from the back side to the inside of the p type semiconductor substrate 1 in order to prevent dark charges generated on the back side of the p type semiconductor substrate 1 from moving to the n type impurity diffusion layer 4. . A terminal is connected to the p ++ type silicon layer 1b, and a predetermined voltage can be applied to the terminal. The impurity concentration of the p ++ type silicon layer 1b is, for example, 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ .

  An aluminum pad 16 is formed on the first surface 40 and covered with an insulating film 9. A through hole 17 is formed in the p-type semiconductor substrate 1 to expose the aluminum pad 16. A metal wire (not shown) is wire bonded to the exposed surface of the aluminum pad 16 through the through hole 17. An insulating layer 18 is formed on the side wall of the through hole 17 in order to prevent the metal wire from contacting the p-type semiconductor substrate 1 in the through hole 17.

  A silicon oxide film 2 that is transparent to incident light is formed on the second surface 42 of the p-type semiconductor substrate 1. In the present invention, the silicon oxide film formed when the SOI wafer is manufactured is used as the silicon oxide film 2.

  A coating film 3 is formed on the silicon oxide film 2. The coating film 3 is composed of a single film or a plurality of films whose refractive index and film thickness are appropriately selected in consideration of the refractive index and film thickness of the silicon oxide film 2 so as to function as an antireflection film. The combination of the silicon oxide film 2 and the coating film 3 substantially constitutes an antireflection film. By adjusting the film thickness of the silicon oxide film 2, an antireflection film may be formed only by the silicon oxide film 2.

  Since the coating film 3 is formed of an insulating film, the insulating layer 18 on the side wall of the through hole can be formed in the same process as the coating film 3. This has the effect of omitting the process.

  A plurality of color filters 19 are formed on the coating film 3. Each color filter 19 is configured to transmit light in different wavelength ranges. In order to prevent color mixing, a light shielding member 21 is formed between the color filters 19. A material that does not transmit light, such as W, Mo, Al (aluminum), or a black filter, is used as the light shielding member 21. A micro lens 22 is formed on the color filter 19 in order to efficiently guide incident light from the back surface to the n-type impurity diffusion layer 4 which is a charge generation region.

  The solid-state image sensor may be the back-illuminated CCD solid-state image sensor described in this embodiment or the back-illuminated CMOS solid-state image sensor.

  FIG. 4 shows the configuration of the silicon oxide film 2 and the coating film 3 formed in the backside illumination type solid-state imaging device. FIG. 4A shows a solid-state imaging device provided with a microlens, and FIG. 4B shows a solid-state imaging device provided with a lid glass instead of the microlens.

  As shown in FIG. 4A, the semiconductor substrate 1 includes a silicon oxide film 2. The silicon oxide film 2 has a thickness of 25 nm and a refractive index of 1.45. On the silicon oxide film 2, a coating film 3 composed of two layers is formed. The coating film 3 includes a first film 3 a made of titanium oxide having a high refractive index and a second film 3 b made of silicon oxide having a low refractive index in order from the semiconductor substrate 1. The film thickness of the first film 3a is 40 nm and its refractive index is 2.0. The second film 3b has a thickness of 80 nm and a refractive index of 1.46. On the second film 3b, a color filter 19 and a microlens 22 are formed in order from the semiconductor substrate 1.

  Next, as shown in FIG. 4B, the semiconductor substrate 1 includes a silicon oxide film 2. The silicon oxide film 2 has a thickness of 25 nm and a refractive index of 1.45. A coating film 3 composed of two layers is formed on the silicon oxide film 2. The covering film 3 includes a first film 3 a made of titanium oxide having a high refractive index and a second film 3 b made of silicon oxide having a low refractive index in order from the semiconductor substrate 1. The film thickness of the first film 3a is 40 nm and its refractive index is 2.0. The second film 3b has a thickness of 130 nm and a refractive index of 1.46. On the second film 3b, a color filter 19 and a lid glass 24 having a thickness of 1 mm or more are provided in order from the semiconductor substrate 1.

  In FIG. 4, the structure of the two-layer coating film is shown. However, in order to obtain a predetermined antireflection rate, the film material, refractive index, film thickness, and number of films are appropriately selected and formed on the semiconductor substrate. .

  As described above, according to the present invention, it is possible to obtain a solid-state imaging device having an oxide film whose thickness is precisely controlled while using an SOI wafer.

The perspective view which shows the external appearance shape of the solid-state imaging device concerning this invention Schematic sectional view for explaining an embodiment of the present invention Schematic sectional view of the solid-state imaging device of the present invention Schematic sectional view for explaining an embodiment of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate, 2 ... Silicon oxide film, 3 ... Cover film, 19 ... Color filter, 22 ... Micro lens, 44 ... Light-receiving part, 46 ... Charge transfer 50, solid-state imaging device, 54 ... solid-state imaging device

Claims (6)

A back-illuminated solid-state imaging device,
A semiconductor substrate having an oxide film with a thickness of 1 to 50 nm on the back surface, a light receiving portion and a charge transfer portion formed on the front surface side of the semiconductor substrate, a color filter and a microlens formed on the oxide film, With
The back surface is characterized in that the semiconductor substrate provided with the oxide film is obtained by removing the silicon substrate from a SOI wafer in which an oxide film and a silicon layer are formed on a silicon substrate with a solution containing KOH as a main component. Irradiation type solid-state image sensor.   The backside illumination type solid-state imaging device according to claim 1, wherein the oxide film has a film thickness adjusted so as to have an antireflection function.   The back-illuminated solid-state imaging device according to claim 1, further comprising a coating film formed on the oxide film and exhibiting an antireflection function in combination with the oxide film.   The back-illuminated solid-state imaging device according to claim 3, wherein the coating film includes a high refractive index film and a low refractive index film in order from the oxide film side.   5. The backside irradiation according to claim 4, wherein the high refractive index film includes one material selected from the group consisting of silicon nitride, cerium oxide, zirconium oxide, yttrium oxide, hafnium oxide, tantalum oxide, titanium nitride, and titanium oxide. Type solid-state imaging device.   6. The back-illuminated solid-state imaging device according to claim 4, wherein the low refractive index film includes one material selected from the group consisting of magnesium fluoride, silicon oxide, silicon nitride, silicon oxynitride, and silicon nitride.

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