JP3960067B2 - Manufacturing method of solid-state imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a solid-state imaging device in which a plurality of photoelectric conversion elements are arranged on a semiconductor substrate and electrodes are laid between the photoelectric conversion elements and the electrodes are covered with a light shielding film to shield the light. The present invention relates to a method for manufacturing a solid-state imaging device of a type that reads out charges accumulated according to the amount of light received from an opening formed by a film.
[0002]
More particularly, the present invention relates to a method for manufacturing a solid-state imaging device using tungsten as a light-shielding film, and more particularly, to a method for manufacturing a solid-state imaging device that suppresses fixed pattern noise by reducing variations in etching for each opening. .
[0003]
[Prior art]
A solid-state image sensor such as a CCD (Charge Coupled Device) is known as a device for capturing a photographed subject image as digital data. This type of image sensor receives light incident through a lens and converts it into an electrical signal corresponding to the amount of light.
[0004]
A CCD has a structure in which a large number of electrodes (such as a metal oxide semiconductor (MOS) structure) are arranged on a semiconductor surface (in a matrix), for example, and a charge is injected by a signal to an input unit to apply a drive signal to each electrode. Thus, the injected charge can be transferred along the transfer electrode, and the charge can be taken out as a voltage at an output section such as FGA or FDA.
[0005]
FIG. 5 schematically shows a cross-sectional structure (conventional example) in the horizontal direction of a CCD solid-state imaging device called an interline type.
[0006]
As shown in the figure, the pixel unit is composed of a photodiode unit 1 composed of a combination of a p-layer and a thick n-layer, a p-layer readout unit 2 formed with a boron layer barrier, and a combination of the n-layer and the p-layer. The vertical charge transfer unit 3 and the vertical transfer electrode 4 made of polycrystalline silicon to which a voltage necessary for reading out the charges accumulated in the photodiode unit 1 is applied. The read unit 2, the vertical charge transfer unit 3, and the vertical transfer electrode 4 are insulated by a gate insulating film 5. The gate insulating film 5 is generally composed of an oxide film, and employs, for example, a MONOS (Metal Oxide Nitride Oxide Semiconductor) structure composed of three layers of SiO ₂ (silicon oxide), SiN (silicon nitride), and SiO _2. .
[0007]
The light receiving surface of the photodiode portion 1 is defined by an opening formed by the light shielding film 6. When light is incident, electrons bonded to the lattice are released to 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 unit 1 accumulates the charge generated by the incident light for a certain period.
[0008]
When reading out the electric charge accumulated in the photodiode unit 1, a pulse having a predetermined read voltage is applied to the vertical transfer electrode 4. In response to this, the stored charge flows over the boron layer barrier of the p-layer read unit 2 and flows into the vertical charge transfer unit 3 to perform a read operation.
[0009]
When applying a read voltage, to the accumulated charge of the photodiode portion 1 is prevented from flowing accidentally to the vertical charge transfer portion 3 'of the adjacent pixel, the photodiode portion 1 and the adjacent pixels A p-layer channel stop 7 is inserted between the vertical charge transfer units 3 '. The barrier of the channel stop unit 7 is set higher than the barrier of the reading unit 2.
[0010]
As can be seen from FIG. 5, in the CCD solid-state imaging device, 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 are transferred reliably.
[0011]
Conventionally, an aluminum layer is often used for the light shielding film 6. However, since aluminum has a high reflectivity, in the solid-state imaging device, since the exposure is reflected during the opening formation process, the pattern width and shape of the opening of the light shielding film 6 formed by patterning are not uniform. There was a problem of becoming.
[0012]
If the pattern of the light shielding film 6 becomes inaccurate, variations in sensitivity occur between the photodiodes, and the characteristics of the solid-state imaging device as a whole deteriorate. In addition, when the solid-state imaging device is highly integrated, the problem of fixed pattern noise (image unevenness) due to minute variations in the opening area becomes obvious.
[0013]
In order to provide a solid-state imaging device that can reduce the problems associated with the reflectance of such a light-shielding film and can be surely patterned in Japanese Patent Application Laid-Open No. 09-232552 already assigned to the present applicant. It is disclosed that the light shielding film is formed of a tungsten layer.
[0014]
When tungsten is used for the light shielding film, the light shielding property is good, but there is a problem that etching becomes difficult.
[0015]
For example, when a tungsten layer is dry-etched with a mixed gas containing isotropic etching gas SF ₆ and anisotropic etching gas CL ₂ , the etching surface becomes uneven as a result of etching progressing along the tungsten grains. It may become. Such an uneven surface causes fixed pattern noise (image unevenness) and cannot withstand the increase in the density of multiple pixels of the solid-state imaging device.
[0016]
FIG. 6 shows a cross-sectional view of a substrate on which etching has progressed along the tungsten grains. As shown in the figure, the etched surface is uneven.
[0017]
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.
[0018]
Since the shape of the opening is not uniform, sensitivity variation occurs between the photodiodes, and the characteristics of the solid-state imaging device as a whole deteriorate. In addition, when the solid-state imaging device is highly integrated, the problem of fixed pattern noise (image unevenness) due to minute variations in the opening area becomes obvious. Further, since the opening length of the light shielding film varies, the distance from the transfer unit is not constant, and the smear characteristic is also unstable.
[0019]
[Problems to be solved by the invention]
An object of the present invention is to provide an excellent manufacturing method for a solid-state imaging device capable of reading out electric charges accumulated according to the amount of light received from an opening formed by a light shielding film.
[0020]
A further object of the present invention is to provide an excellent manufacturing method of a solid-state imaging device using tungsten as a light shielding film.
[0021]
It is a further object of the present invention to provide an excellent method for manufacturing a solid-state imaging device capable of suppressing fixed pattern noise by reducing variations in etching for each opening.
[0022]
[Means and Actions for Solving the Problems]
The present invention has been made in consideration of the above-described problems. A solid substrate 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 block light. A method for manufacturing an image sensor,
Forming a light shielding film made of a tungsten layer on a semiconductor substrate on which a plurality of photoelectric conversion elements and electrodes running between the photoelectric conversion elements are formed;
Forming a mask pattern in which a portion corresponding to a light receiving region of the photoelectric conversion element of the light shielding film is opened;
An opening for the light receiving region is formed in the light shielding film by dry etching using a mixed gas containing an isotropic etching gas using SF ₆ and an anisotropic etching gas using N ₂ and Cl _2. Drilling step;
A method of manufacturing a solid-state imaging device.
[0023]
In the method of manufacturing a solid-state imaging device according to the present invention, in the step of forming the opening, the opening is formed so that the minimum width of the lower end is 1 μm or less and is perpendicular to the substrate.
[0024]
Here, a tungsten layer having excellent light shielding properties is used as the light shielding film. Further, dry etching using a mixed gas containing an isotropic etching gas composed of SF ₆ and an anisotropic etching gas composed of N ₂ and Cl ₂ is performed on the portion on the photodiode. More specifically, etching is performed using a mixed gas having a ratio of SF ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1.
[0025]
In this way, by performing dry etching using an isotropic etching gas and a mixed gas containing an anisotropic etching gas containing N ₂ and Cl ₂ , the tungsten light-shielding film is made into tungsten grains. It is possible to achieve vertical etching that does not follow.
[0026]
More specifically, depositable products are deposited on the etching sidewalls of tungsten due to the effects of Cl ₂ gas and N ₂ gas, so that etching proceeds in the vertical direction without following the tungsten grains.
[0027]
As a result, a vertical opening having a minimum width of 1 μm or less at the lower end and not along the tungsten grain can be formed in the tungsten light-shielding film.
[0028]
Therefore, minute etching / variation for each opening of the light shielding film 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 unit is stabilized, and the smear characteristic is also stabilized.
[0029]
Other objects, features, and advantages of the present invention will become apparent from more detailed description based on embodiments of the present invention described later and the accompanying drawings.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0031]
A 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.
[0032]
After the transfer electrode 4 and the photodiode portion 1 are formed 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).
[0033]
Next, after forming a tungsten light-shielding film 6 on the substrate, the resist pattern 11 for forming an opening is masked by a photoresist method (see FIG. 2).
[0034]
Next, etching is performed with a microwave ECR (Electron Cyclotron Resonance) etcher. The etching conditions at this time are, for example, that the etching gas is a mixed gas having a ratio of SF ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1, the etching pressure is 2.0 Pa, and microwave excitation The power is 1100 W, the RF excitation power is 45 W, and the lower electrode temperature is 0 ° C.
[0035]
Here, SF ₆ is used as an isotropic etching gas, and N ₂ and Cl ₂ are used as an anisotropic etching gas.
[0036]
When the tungsten layer is etched under such conditions, a depositable product is deposited on the etched sidewalls of the tungsten due to the effects of Cl ₂ gas and N ₂ gas, and as a result, the vertical shape does not follow the tungsten grain. Etching can be realized.
[0037]
FIG. 3 shows a cross-sectional view of the substrate in the vicinity of the opening of the light-shielding film that is drilled not along the tungsten grains using the above etching conditions. As shown in the figure, the etching surface does not include irregularities and has a vertical side wall. Moreover, the minimum width of the lower end can be controlled to 1 μm or less.
[0038]
FIG. 4 shows a state in which the vicinity of the opening of the tungsten light-shielding film is viewed from above. As can be seen from the figure, the shape of the opening is stable and the pattern width and shape are uniform.
[0039]
Therefore, according to the present embodiment, it is possible to realize vertical etching that does not follow the tungsten grain, so that minute etching variations in each light shielding film opening are reduced, and fixed pattern noise (image unevenness) is reduced. The problem can be solved. Further, since the opening length of the light shielding film does not vary, the distance from the transfer unit is stabilized, and the smear characteristic is also stabilized.
[0040]
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 RIE (Reactive Ion Etching), high-pressure narrow gap plasma etching equipment, inductively coupled plasma etching equipment, magnetron RIE, transformer coupled plasma etching equipment, helicon wave plasma, Other etching apparatuses such as an etching apparatus can be used.
[0041]
[Supplement]
The present invention has been described in detail above with reference to specific embodiments. However, it is obvious that those skilled in the art can make modifications and substitutions of the embodiment without departing from the gist of the present invention. That is, the present invention has been disclosed in the form of exemplification, and the contents described in the present specification should not be interpreted in a limited manner. In order to determine the gist of the present invention, the claims section described at the beginning should be considered.
[0042]
【The invention's effect】
As described above in detail, according to the present invention, there is provided an excellent manufacturing method for a solid-state imaging device capable of reading out the electric charge accumulated according to the amount of light received from the opening formed by the light shielding film. Can do.
[0043]
In addition, according to the present invention, it is possible to provide an excellent method for manufacturing a solid-state imaging device using tungsten as a light shielding film.
[0044]
Further, according to the present invention, it is possible to provide an excellent method for manufacturing a solid-state imaging device that can suppress variation in etching for each opening and suppress fixed pattern noise.
[0045]
In recent years, in the solid-state imaging device, the opening 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) due to minute variations in the opening area due to variations in etching becomes obvious. According to the present invention, vertical etching that does not follow the tungsten grain can be realized, so that minute etching variation in each light shielding film opening is reduced, and there is a problem of fixed pattern noise (image unevenness). Can be resolved. Further, since the opening length of the light shielding film does not vary, the distance from the transfer unit is stabilized, and the smear characteristic is also stabilized.
[Brief description of the drawings]
FIG. 1 is a view 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 view 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 view 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 cross-sectional view of a substrate on which etching has progressed along tungsten grains.
FIG. 7 is a view showing a state in which the vicinity of the opening of the light shielding film is viewed from above when etching progresses along the grains of tungsten.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Photodiode part 2 ... Read-out part 3 ... Vertical charge transfer part 4 ... Vertical transfer electrode 5 ... Gate insulating film 6 ... Light-shielding film 7 ... Channel stop part 9 ... Interlayer insulating film 11 ... Resist

Claims (3)

A method of manufacturing 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 the light.
Forming a light shielding film made of a tungsten layer on a semiconductor substrate on which a plurality of photoelectric conversion elements and electrodes running between the photoelectric conversion elements are formed;
Forming a mask pattern in which a portion corresponding to a light receiving region of the photoelectric conversion element of the light shielding film is opened;
An opening for the light receiving region is formed in the light shielding film by dry etching using a mixed gas containing an isotropic etching gas using SF ₆ and an anisotropic etching gas using N ₂ and Cl _2. Drilling step;
A method for manufacturing a solid-state imaging device, comprising: In the step of forming the opening, the opening is formed so that the minimum width of the lower end is 1 μm or less and is perpendicular to the substrate.
The manufacturing method of the solid-state image sensor of Claim 1 characterized by the above-mentioned. In the step of forming the opening, etching is performed using a mixed gas having a ratio of SF ₆ : Cl ₂ : Ar: N ₂ = 2: 2: 14: 1.
The manufacturing method of the solid-state image sensor of Claim 1 characterized by the above-mentioned.

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