JP2003234467A - Method for manufacturing solid-state image pickup element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce any fixed pattern noise by reducing fluctuation of etching of each opening at the time of using a tungsten layer for a shading film. <P>SOLUTION: A tungsten layer whose shading performance is excellent is used for a shading film covering an electrode. Also, dry etching using mixed gas including isotropic etching gas made of SF<SB>6</SB>and anisotropic etching gas made of Cl<SB>2</SB>is carried out to a site above a photo-diode to form a vertically shaped opening whose lower edge has the minimum width which is 1 μm or less not following a tungsten grain. The fine etching fluctuation of each opening can be reduced, and the problem of any fixed pattern noise can be prevented. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image pickup device in which a plurality of photoelectric conversion elements are arranged on a semiconductor substrate, electrodes are laid between the photoelectric conversion elements, and the electrodes are covered with a light shielding film to shield light. In particular, the present invention relates to a method for manufacturing a solid-state image sensor of a type that reads out electric charges accumulated according to the amount of light received from an opening formed by a light shielding film.

More particularly, the present invention relates to a method of manufacturing a solid-state image pickup device using tungsten as a light-shielding film, and more particularly to a solid-state image pickup device in which fixed pattern noise is suppressed by reducing variation in etching between openings. It relates to a manufacturing method.

[0003]

2. Description of the Related Art As a device for capturing an image of a photographed subject as digital data, a CCD (Char
A solid-state imaging device such as a ge coupled device (charge coupled device) is known. This type of image sensor receives light that has entered through a lens and converts it into an electrical signal according to the amount of light.

A CCD is, for example, a MOS on the surface of a semiconductor.
(Metal Oxide Semiconductor) has a structure in which a large number of electrodes (matrix) are arranged, and charges are injected by a signal to the input part, and the injected charges are transferred by applying a drive signal to each electrode. The electric charges can be taken out as a voltage at the output part such as FGA or FDA.

FIG. 5 shows a C called an interline type.
1 schematically shows a horizontal cross-sectional structure (conventional example) of a CD solid-state imaging device.

As shown in the figure, the pixel unit is a photodiode section 1 formed of a combination of a p-layer and a thick-film n-layer,
A p-layer read-out portion 2 in which a boron layer barrier is formed;
A vertical charge transfer portion 3 composed of a combination of layers and p layers,
It is composed of a vertical transfer electrode 4 made of polycrystalline silicon for applying a voltage necessary for reading out the charges accumulated in the photodiode part 1. The reading section 2, the vertical charge transfer section 3, and the vertical transfer electrode 4 are insulated by the gate insulating film 5. The gate insulating film 5 is generally composed of an oxide film, such as SiO ₂ (silicon oxide),
MO consisting of 3 layers of SiN (silicon nitride) and SiO ₂
NOS (Metal Oxide Nitride Oxide Semiconductor)
The structure is adopted.

The light receiving surface of the photodiode section 1 is defined by the opening formed by the light shielding film 6. When light is incident, the electrons bound to the lattice are released and become free electrons, and free electrons and holes are generated. As a result of these electrons and holes moving to the depletion region, a reverse current proportional to the intensity of light is generated. The n layer of the photodiode part 1 accumulates the electric charges generated by the incidence of light for a certain period.

When reading the charges accumulated in the photodiode section 1, a pulse having a predetermined read voltage is applied to the vertical transfer electrodes 4. In response to this, the accumulated charges flow into the vertical charge transfer unit 3 over the boron layer barrier of the p-layer read unit 2 to perform the read operation.

In order to prevent the charges accumulated in the photodiode section 1 from accidentally flowing into the vertical transfer transfer section 3'of the adjacent pixel unit when a read voltage is applied, the photodiode section 1 and the adjacent pixel are prevented. A p-layer channel stop unit 7 is inserted between the unit vertical transfer transfer units 3 ′. The barrier of the channel stop unit 7 is set higher than the barrier of the reading unit 2.

As can be seen from FIG. 5, in the CCD solid-state image sensor, the electrodes arranged between the photodiode portions are covered with the light-shielding film 6, so that the accumulated charges photoelectrically converted according to the amount of received light can be surely obtained. Forward.

Conventionally, an aluminum layer is often used as the light shielding film 6. However, since aluminum has a high reflectance, in the solid-state image sensor, the pattern width and shape of the opening of the light-shielding film 6 formed by patterning are non-uniform because a large amount of exposure is reflected in the process of forming the opening. There was a problem of becoming.

If the pattern of the light-shielding film 6 becomes inaccurate, variations in sensitivity will occur between the photodiodes, and the characteristics of the solid-state image pickup device as a whole will deteriorate. Further, when the solid-state imaging device is highly integrated with a large number of pixels, the problem of fixed pattern noise (image unevenness) due to minute variations in the opening area becomes apparent.

[0013] Japanese Patent Laid-Open No. Hei 09 09, which has already been assigned to the present applicant
Japanese Patent Publication No. 232552 discloses a light-shielding film formed of a tungsten layer in order to provide a solid-state imaging device capable of reliably patterning while reducing the problems associated with the reflectance of the light-shielding film. ing.

When tungsten is used for the light-shielding film, the light-shielding property is good, but etching is difficult.

For example, when a tungsten layer is dry-etched with a mixed gas containing an isotropic etching gas SF ₆ and an anisotropic etching gas CL ₂ , etching progresses along the grains of tungsten, resulting in etching surface. May become uneven. Such uneven surface causes fixed pattern noise (image unevenness).
Therefore, the solid-state image sensor cannot endure high density of multiple pixels.

FIG. 6 shows a cross-sectional view of a substrate in which etching has progressed along the grain of tungsten. As shown in the figure, the etching surface is uneven.

Further, FIG. 7 shows a state in which the vicinity of the opening of the light shielding film at this time is viewed from above. As can be seen from the figure, the shape of the opening is not stable, and the pattern width and shape are not uniform.

Since the shape of the opening is not uniform, variations in sensitivity occur among the photodiodes, and the characteristics of the solid-state image pickup device as a whole deteriorate. Further, when the solid-state imaging device is highly integrated with a large number of pixels, the problem of fixed pattern noise (image unevenness) due to minute variations in the opening area becomes apparent. In addition, since the aperture length of the light shielding film varies, the distance from the transfer portion is not constant and the smear characteristic is unstable.

[0019]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an excellent manufacturing method for a solid-state image pickup device capable of reading out accumulated charges according to the amount of light received from an opening formed by a light shielding film. To do.

A further object of the present invention is to provide an excellent method for manufacturing a solid-state image sensor using tungsten as a light shielding film.

A further object of the present invention is to provide an excellent method for manufacturing a solid-state image pickup device capable of suppressing fixed pattern noise by reducing variations in etching between openings.

[0022]

The present invention has been made in consideration of the above problems, and a first aspect thereof is that a plurality of photoelectric conversion elements are arranged on a semiconductor substrate. A method for manufacturing a solid-state imaging device in which electrodes are laid between and covered with a light-shielding film to shield light, and a tungsten layer is formed on a semiconductor substrate on which a plurality of photoelectric conversion elements and electrodes running between the photoelectric conversion elements are formed. A step of forming a light-shielding film made of, a step of forming a mask pattern in which a portion of the light-shielding film corresponding to the light receiving region of the photoelectric conversion element is opened, isotropic etching gas and anisotropic etching A step of forming an opening for the light-receiving region in the light-shielding film by dry etching using a mixed gas containing a gas. It is a method.

A second aspect of the present invention is a solid-state imaging device in which a plurality of photoelectric conversion elements are arranged on a semiconductor substrate, electrodes are laid between the photoelectric conversion elements, and the electrodes are covered with a light shielding film to shield light. A method of manufacturing a plurality of photoelectric conversion elements and a step of forming a light-shielding film on a semiconductor substrate on which electrodes running between the photoelectric conversion elements are formed, and a portion corresponding to the photoelectric conversion element of the light-shielding film. , The minimum width of the lower end is 1 μm
A method of manufacturing a solid-state image sensor, comprising the following step: forming an opening formed so as to be perpendicular to a substrate.

Here, a tungsten layer having an excellent light shielding property is used as the light shielding film. In addition, an isotropic etching gas composed of SF ₆ and N ₂ are formed on the photodiode.
And dry etching using a mixed gas containing an anisotropic etching gas of Cl ₂ is performed. More specifically, etching is performed using a mixed gas having a ratio of SF ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1.

Thus, isotropic etching gas and N ₂
And dry etching using a mixed gas containing an anisotropic etching gas containing Cl ₂
It is possible to realize vertical etching on the tungsten light-shielding film that does not follow the tungsten grains.

More specifically, due to the effect of Cl ₂ gas and N ₂ gas, a depositable product is deposited on the etching sidewall of tungsten, so that the etching proceeds in the vertical direction without following the tungsten grain.

As a result, the minimum width of the lower end is 1 μm or less,
Vertical openings can be formed in the tungsten shading film that do not follow the tungsten grains.

Therefore, minute etching / variation in each light-shielding film opening is reduced, and the problem of fixed pattern noise (image unevenness) can be solved. Further, since the opening length of the light shielding film does not vary, the distance from the transfer portion is stabilized and the smear characteristic is also stabilized.

Further objects, features and advantages of the present invention are as follows.
It will be apparent from the embodiments of the present invention described later and the more detailed description based on the accompanying drawings.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below in detail with reference to the drawings.

The processing procedure for forming a light-shielding film of a tungsten layer on a semiconductor substrate according to the present invention will be described with reference to FIGS.
Will be described with reference to.

After forming the transfer electrode 4 and the photodiode portion 1 on the semiconductor substrate, a silicon oxide film 9 as an interlayer electrode layer is formed by a CVD (Chemical Vapor Deposition) method (see FIG. 1). .

Next, after forming the tungsten light shielding film 6 on the substrate, the resist pattern 11 for forming the opening is masked by the photoresist method (see FIG. 2).

Next, the microwave ECR (Electron Cyc
lotron Resonance: Electron cyclotron resonance) Etching is performed with an etcher. The etching conditions at this time are, for example, SF ₆ : Cl ₂ : Ar:
It is a mixed gas having a ratio of N ₂ = 2: 2: 14: 1, the etching pressure is 2.0 Pa, the microwave excitation power is 1100 W, the RF excitation power is 45 W, and the lower electrode temperature is 0 ° C.

Here, SF₆Isotropic etching gas
Used as N₂And Cl ₂Is anisotropic etch
Used as a gas.

When the tungsten layer is etched under such conditions, a depositable product is deposited on the etching sidewall of tungsten due to the effect of Cl ₂ gas and N ₂ gas,
As a result, it is possible to realize vertical etching that does not follow the tungsten grains.

FIG. 3 shows a cross-sectional view of the substrate in the vicinity of the opening of the light-shielding film formed under the above-described etching conditions without extending along the grain of tungsten. As shown in the figure, the etching surface does not include irregularities, and has vertical side walls. Further, the minimum width of the lower end can be controlled to 1 μm or less.

Further, FIG. 4 shows a state in which the vicinity of the opening of the tungsten light shielding film at this time is viewed from above. As can be seen from the figure, the shape of the opening is stable, and its pattern width and shape are uniform.

Therefore, according to the present embodiment, since it is possible to realize vertical etching that does not follow the tungsten grains, minute etching variations between the openings of the light shielding film are reduced, and fixed pattern noise ( The problem of image unevenness) can be solved. Further, since the opening length of the light shielding film does not vary, the distance from the transfer portion is stabilized and the smear characteristic is also stabilized.

Although the microwave ECR etcher is used as the etching apparatus in the above-described embodiment, the gist of the present invention is not limited to this. For example,
Parallel plate type RIE (Reactive Ion Etching), high pressure narrow gap type plasma etching device, inductively coupled plasma etching device, magnetron RIE, transformer coupled plasma etching device,
Other etching equipment may be used, such as a helicon wave plasma etching equipment.

[Supplement] The present invention has been described in detail with reference to the specific embodiments. However, it is obvious that those skilled in the art can modify or substitute the embodiments without departing from the scope of the present invention. That is, the present invention has been disclosed in the form of exemplification, and the contents of this specification should not be construed in a limited manner. In order to determine the gist of the present invention, the section of the claims described at the beginning should be taken into consideration.

[0042]

As described above in detail, according to the present invention,
It is possible to provide an excellent manufacturing method for a solid-state imaging device that can read out the accumulated charge according to the amount of light received from the opening formed by the light shielding film.

Further, according to the present invention, it is possible to provide an excellent method for manufacturing a solid-state image pickup device using tungsten as a light shielding film.

Further, according to the present invention, it is possible to provide an excellent method for manufacturing a solid-state image pickup device capable of suppressing fixed pattern noise by reducing the variation in etching for each opening.

Recently, in the solid-state image pickup device, the aperture of the light-shielding film tends to be downsized due to the demand for downsizing and high integration of multiple pixels. As the opening of the light-shielding film is reduced, the problem of fixed pattern noise (image unevenness) becomes apparent due to a slight variation in the opening area due to variations in etching. According to the present invention, since it is possible to realize vertical etching that does not follow tungsten grains, minute etching variations between light-shielding film openings are reduced, and fixed pattern noise (image unevenness) is reduced. It can be resolved. Further, since the opening length of the light shielding film does not vary, the distance from the transfer portion is stabilized and the smear characteristic is also stabilized.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a processing procedure for forming a light shielding film of a tungsten layer on a semiconductor substrate according to the present invention.

FIG. 2 is a diagram for explaining a processing procedure for forming a light shielding film of a tungsten layer on a semiconductor substrate according to the present invention.

FIG. 3 is a diagram for explaining a processing procedure for forming a light shielding film of a tungsten layer on a semiconductor substrate according to the present invention.

FIG. 4 is a diagram for explaining a processing procedure for forming a light shielding film of a tungsten layer on a semiconductor substrate according to the present invention.

FIG. 5 is a diagram schematically showing a horizontal sectional structure (conventional example) of a CCD called an interline type.

FIG. 6 is a diagram showing a cross-sectional view of a substrate on which etching has progressed along a grain of tungsten.

FIG. 7 is a diagram showing a state in which the vicinity of the opening of the light shielding film is viewed from above when etching progresses along the grain of tungsten.

[Explanation of symbols]

1 ... Photodiode section 2 ... Read-out section 3 ... Vertical charge transfer unit 4 ... Vertical transfer electrode 5 ... Gate insulating film 6 ... Shading film 7 ... Channel stop section 9 ... Interlayer insulating film 11 ... Resist

Claims (8)

[Claims] 1. A method for manufacturing a solid-state imaging device, wherein a plurality of photoelectric conversion elements are arranged on a semiconductor substrate, electrodes are laid between the photoelectric conversion elements, and the electrodes are covered with a light shielding film to shield light. A step of forming a light shielding film made of a tungsten layer on a semiconductor substrate on which a photoelectric conversion element and electrodes running between the photoelectric conversion elements are formed; and a portion of the light shielding film corresponding to a light receiving region of the photoelectric conversion element is opened. Forming a mask pattern, and forming an opening for the light receiving region in the light shielding film by dry etching using a mixed gas containing an isotropic etching gas and an anisotropic etching gas. A method for manufacturing a solid-state imaging device, comprising: 2. In the step of forming the opening, SF ₆ is used as an isotropic etching gas, and
_{2. The} method for manufacturing a solid-state image sensor according to claim 1, wherein N ₂ and Cl ₂ are used as the anisotropic etching gas. 3. In the step of forming the opening, SF
_{2. The} method for manufacturing a solid-state imaging device according to claim 1, wherein etching is performed using a mixed gas having a ratio of ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1. 4. A method for manufacturing a solid-state imaging device, wherein a plurality of photoelectric conversion elements are arranged on a semiconductor substrate, electrodes are laid between the photoelectric conversion elements, and the electrodes are covered with a light shielding film to shield light. A step of forming a light-shielding film on a semiconductor substrate on which a photoelectric conversion element and an electrode running between the photoelectric conversion elements are formed, and a minimum width of a lower end is 1 μm or less at a portion of the light-shielding film corresponding to the photoelectric conversion element. A step of forming an opening formed so as to be perpendicular to the substrate, and a method for manufacturing a solid-state image sensor. 5. The method for manufacturing a solid-state image sensor according to claim 4, wherein the light-shielding film is formed of a tungsten layer. 6. The step of forming an opening in the light shielding film is performed by dry etching using a mixed gas containing an isotropic etching gas and an anisotropic etching gas.
The method for manufacturing a solid-state imaging device according to claim 4, wherein. 7. The solid-state image pickup device according to claim 6, wherein SF ₆ is used as the isotropic etching gas, and N ₂ and Cl ₂ are used as the anisotropic etching gas. Production method. 8. In the step of forming an opening in the light shielding film, etching is performed using a mixed gas having a ratio of SF ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1. The method for manufacturing a solid-state imaging device according to claim 4, wherein.