Method for fabricating solid-state imaging device

Disclosed is a method for fabricating a solid-state imaging device including a semiconductor substrate of a first conductivity type, a plurality of light-receiving sections provided at a distance in the surface region of the semiconductor substrate, and channel stop regions of a second conductivity type provided between the adjacent light-receiving sections in the surface region and in the internal region of the semiconductor substrate. The method includes the steps of forming a first photoresist layer having openings corresponding to positions at which the channel stop regions are formed; ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a first energy through the first photoresist layer as a mask; forming a second photoresist layer having openings; and ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a second energy through the second photoresist layer as a mask.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating a solid-state imaging device. More particularly, the invention relates to a method for fabricating a solid-state imaging device in which channel stop regions are formed in the surface region and in internal regions by injecting impurities, the channel stop regions being located between light-receiving sections.

2. Description of the Related Art

In order to improve the sensitivity of a solid-state imaging device, in a known technique, an overflow barrier region is formed deep in a semiconductor substrate, and high-resistivity depletion regions under light-receiving sections (photosensors) are extended in the depth direction.

FIG. 1is a partial sectional view which shows a solid-state imaging device having extended depletion regions, andFIG. 2is a partial plan view of the solid-state imaging device shown in FIG.1.FIG. 1is the sectional view taken along the line I—I of FIG.2.

As shown inFIG. 2, in a solid-state imaging device12, a plurality of light-receiving sections14are arranged in a matrix on a semiconductor substrate22. A vertical charge transfer register16is provided for each column of the light-receiving sections14. Signal charges generated by each light-receiving section14in response to light are fetched by the adjacent vertical charge transfer register and are transferred vertically (in the V direction shown in FIG.2). At the ends of the vertical charge transfer registers16, a horizontal charge transfer register (not shown in the drawing) extends perpendicular to the direction in which the vertical charge transfer registers16extend. The signal charges from each column of the light-receiving sections14, which are transferred through the corresponding vertical charge transfer register16, are supplied to the horizontal charge transfer register, are transferred horizontally (in the H direction shown in FIG.2), and are finally output as image signals.

Each vertical charge transfer register16includes first transfer electrodes18and second transfer electrodes20, and as shown inFIG. 2, the first transfer electrodes18and the second transfer electrodes20are placed alternately in the V direction so as to be partially overlapped with each other. The corresponding first transfer electrodes18and second transfer electrodes20of the individual vertical charge transfer registers16are connected to each other and they are driven in phase.

As shown inFIG. 1, in the solid-state imaging device12, at a deep position, for example, at a depth of 3 μm or more, of a semiconductor substrate22of a first conductivity type, for example, n-type, an overflow barrier region24which is a semiconductor well region of a second conductivity type, for example, p type, is formed. A high-resistivity semiconductor region, i.e., a so-called high-resistivity epitaxial layer (depletion region)26, having a higher resistivity than that of the overflow barrier region24, is formed by epitaxial growth on the overflow barrier region24. The high-resistivity epitaxial layer26has a thickness of 2 μm or more, and preferably, 5 μm or more, and is formed as a p-type region or n-type region having a lower concentration than that of the overflow barrier region24, or as a non-doped (intrinsic semiconductor) region.

The light-receiving sections14, each including a high-concentration p-type region28and an n-type region30, are formed at a distance in the surface region of the semiconductor substrate22. The transfer electrodes18and20are deposited with an insulating layer32therebetween on the surface of the substrate between two adjacent light-receiving sections14. The surface of the substrate except the light-receiving sections14is covered with a shading film34.

Between two adjacent light-receiving sections14, a p-type region36is formed in the surface region, and a low-concentration p-type region38is formed in the internal region above the overflow barrier region24, the p-type region36and the p-type region38being vertically aligned. The p-type regions36and38constitute a channel stop region40. The p-type region38is formed, for example, at a depth of 1 μm or more from the surface of the substrate. By forming such a channel stop region40, holes generated by photoelectric conversion in the deep region of the light-receiving section14can also be transferred to the channel stop region, and the light-receiving sections14can be isolated reliably so that color mixing between pixels can be prevented.

FIGS. 3Ato3C show the process for forming the channel stop regions40, each including the p-type region36located in the surface region and the p-type region38located in the internal region in the semiconductor substrate22.

First, as shown inFIG. 3A, a photoresist layer44having openings42corresponding to channel stop region-forming positions is formed on the surface of the semiconductor substrate22. Next, as shown inFIG. 3B, using the photoresist layer44as a mask, a p-type impurity is ion-implanted at a relatively high energy to form the p-type region38. Then, as shown inFIG. 3C, using the photoresist layer44as a mask, a p-type impurity is ion-implanted at a relatively low energy to form the p-type region36.

However, when the channel stop region40is formed by the method described above, when the p-type region38is formed in the internal region, a high ion-implantation energy is required, and in particular, when the region is the deep region of the substrate, the ion-implantation energy is 1 MeV or more and the photoresist layer44must have a thickness of 3 μm or more so as to withstand such a high energy. As a result, it is difficult to form a fine mask pattern of the photoresist layer44, and it is not possible to scale down the channel stop regions40. Therefore, the array pitch of the light-receiving sections14is limited, which is an obstacle to the miniaturization of the solid-state imaging device12and an increase in density in the solid-state imaging device12.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for fabricating a solid-state imaging device in which the channel stop regions can be scaled down.

In one aspect, the present invention provides a method for fabricating a solid-state imaging device including a semiconductor substrate of a first conductivity type, a plurality of light-receiving sections provided at a distance in the surface region of the semiconductor substrate, and channel stop regions of a second conductivity type provided between the adjacent light-receiving sections in the surface region and in an internal region of the semiconductor substrate. The method includes the steps of forming a first photoresist layer having openings corresponding positions at which the channel stop regions are formed; ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a first energy through the first photoresist layer as a mask; forming a second photoresist layer having openings corresponding to positions at which the channel stop regions are formed; and ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a second energy through the second photoresist layer as a mask.

In another aspect, the present invention provides a method for fabricating a solid-state imaging device including a semiconductor substrate of a first conductivity type and a plurality of light-receiving sections provided at a distance in the surface region of the semiconductor substrate. The method includes a first step of forming a first photoresist layer having openings at predetermined positions between adjacent light-receiving sections; a second step of ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a first energy through the first photoresist layer as a mask; a third step of forming a second photoresist layer having openings at the predetermined positions; and a fourth step of ion-implanting an impurity of a second conductivity type into the semiconductor substrate at a second energy through the second photoresist layer as a mask.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4Ato4E are sectional views which show the process steps for fabricating a solid-state imaging device according to a fabrication method of the present invention, in which the same reference numerals are used for the same elements as those shown in FIG.1. The sectional views shown inFIGS. 4Ato4E correspond to sectional views taken along the line I—I of FIG.2.

In this embodiment, as shown inFIG. 4A, a first photoresist layer2is formed on the surface of the semiconductor substrate22and openings4are formed by patterning at positions at which channel stop regions are formed between light-receiving sections14. Then, as shown inFIG. 4B, using the first photoresist layer2as a mask, p-type regions38A are formed by ion-implanting a low-concentration p-type impurity into the internal region of the semiconductor substrate22at a high energy, for example, at an energy of 1 MeV or more.

As shown inFIG. 4C, after the first photoresist layer2is removed, a second photoresist layer6is formed on the surface of the semiconductor substrate22and openings8are formed by patterning at positions at which channel stop regions are formed between the light-receiving sections14. Then, as shown inFIG. 4D, using the second photoresist layer6as a mask, p-type regions36A are formed by ion-implanting a p-type impurity into the surface region of the semiconductor substrate22at a relatively low energy, for example, at an energy that is lower than the energy for the ion-implantation shown in FIG.4B.

Consequently, channel stop regions40A, each including the p-type region36A in the surface region and the p-type region38A in the internal region, are formed at the positions for isolating the adjacent light-receiving sections14in the semiconductor substrate22.

The subsequent process steps are the same as those used in the conventional method, and as shown inFIG. 4E, the light-receiving sections14, transfer electrodes18and20, a shading film34, etc., are formed. A solid-state imaging device12A is thereby completed.

As described above, in this embodiment, different photoresist layers, i.e., the photoresist layers2and6, are formed for ion-implanting the impurity into the internal region of the semiconductor substrate22at a high energy and for ion-implanting the impurity into the surface region of the semiconductor substrate22at a low energy, respectively. Accordingly, the second photoresist layer6used for the ion-implantation into the surface region of the semiconductor substrate22can be made thin because of the low ion-implantation energy, and therefore a fine pattern can be formed easily. As a result, in the surface region of the semiconductor substrate22, the p-type regions36A constituting the channel stop regions40A can be scaled down.

On the other hand, the first photoresist layer2used for the ion-implantation into the internal region of the semiconductor substrate22must be made thick, in the same manner as that in the conventional case, because of the high ion-implantation energy, and it is difficult to form fine openings in the photoresist layer2. Therefore, the p-type regions38A constituting the channel stop regions40A in the internal region of the semiconductor substrate22have the same size as that in the conventional case. However, since the p-type regions38A in the internal region are not required to have high impurity concentrations and since the regions at this depth correspond to depletion regions, even if the p-type regions38A extend widthwise to a certain extent, the sensitivity of the light-receiving sections14is not degraded.

That is, in the method for fabricating the solid-state imaging device in this embodiment, since the p-type regions36A constituting the channel stop regions40A in the surface region of the semiconductor substrate22can be scaled down, it is possible to miniaturize the solid-state imaging device12A and to increase the density by scaling down the array pitch of the light-receiving sections14. Although the size of the p-type regions38A constituting the channel stop regions40A in the internal region of the semiconductor substrate22is the same as that in the conventional case, the sensitivity is not degraded in the light-receiving sections14.

Additionally, since two photoresist layers, i.e., the first and second photoresist layers2and6, are used in order to form the channel stop regions40A, there may be a case in which the positions of the openings4and the openings8do not completely agree with each other due to misalignment of the masks, and the p-type region38A is not formed beneath the p-type region36A. However, even if the positions of the p-type region36A and the p-type region38A are misaligned, the entire channel stop region40A is just slightly inclined, and the light-receiving section14is formed just slightly obliquely with respect to the vertical direction. Therefore, the characteristics of the light-receiving sections14, etc., are not substantially affected.

The depth D for the p-type region38A (refer toFIG. 4B) is 1 μm or more in the conventional case. However, in the present invention, since the p-type region38A is formed using a dedicated photoresist, i.e., the first photoresist layer2, the p-type region38A can be formed at a deep position easily. For example, the depth D can be set at approximately 3 μm. In such a case, the thickness of the first photoresist layer2may be set at approximately 5 μm so as to withstand high ion-implantation energy. The width W1of the opening4is, for example, 0.8 μm.

On the other hand, when the p-type region36A is formed, since the ion-implantation energy required is low, the thickness of the second photoresist layer6is 1 μm or less, for example, approximately, 0.5 μm. When the thickness of the second photoresist layer6is 1 μm or less, the width W2of the opening8can be set at, for example, 0.8 μm or less, and when the thickness of the second photoresist layer6is 0.5 μm, the width W2of the opening8can be scaled down to approximately 0.35 μm.

The configuration of the solid-state image pickup device of the present invention is not limited to that described above, and various modifications are possible without departing from the scope of the invention.