SOLID-STATE IMAGE PICKUP DEVICE
A solid-state image pickup device 1 is back surface incident type and includes a semiconductor substrate 10, a semiconductor layer 20 and a light receiving unit 30. The solid-state image pickup device 1 photoelectrically converts light incident on the back surface S2 of the semiconductor substrate 10 into signal electrical charges to image an object. The semiconductor substrate 10 has a resistivity ρ1. A semiconductor layer 20 is provided on the surface S1 of the semiconductor substrate 10. The semiconductor layer 20 has a resistivity ρ2. Where, ρ2>ρ1. A light receiving unit 30 is formed in the semiconductor layer 20. The light receiving unit 30 receives signal charges produced by the photoelectric conversion.
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This application is a division of application Ser. No. 11/774,726 filed on Jul. 9, 2007, which claims foreign priority to Japanese patent application No. 2006-188904 filed on Jul. 10, 2006. The entire contents of each of these applications are hereby expressly incorporated by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a solid-state image pickup device.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2002-33469 describes a back surface incident type of solid-state image pickup device. In the solid-state image pickup device, light incident on the back surface of a semiconductor substrate from an object is photoelectrically converted inside the semiconductor substrate. Electric charges produced by the conversion are received by a light receiving unit to image the object.
Japanese Patent Laid-Open No. 2000-217803 in addition to Japanese Patent Laid-Open No. 2002-33469 is known as a prior art document related to the present invention.
The present inventor has recognized as follows. The solid-state image pickup device described in Japanese Patent Laid-Open No. 2002-33469 leaves room for improvement in terms of easy release of excess charges. In this respect, the resistivity of the whole substrate needs to be reduced to easily release excess charges. This, however, makes it difficult to directly use a platform in an existing device process without modification when a light receiving unit is formed in the substrate.
SUMMARYAccording to one aspect of the present invention, there is provided a solid-state image pickup device photoelectrically converting light incident on a back surface of a semiconductor substrate into signal electric charges to image an object, the solid-state image pickup device comprising: a first semiconductor layer forming a part or whole of the semiconductor substrate and having a first resistivity; a second semiconductor layer provided on a front surface of the semiconductor substrate and having a second resistivity higher than the first resistivity; and a light receiving unit provided in the second semiconductor layer and receiving the signal electric charges produced by the photoelectric conversion.
The solid-state image pickup device is provided with a first semiconductor layer with a relatively lower resistivity (a first resistivity). This enables excess charges to be easily released from the surface of the semiconductor substrate. On the other hand, a second semiconductor layer on which the light receiving unit is provided has a higher resistivity (a second resistivity) than the first semiconductor layer. Unlike the case where both the first and the second semiconductor layer have the first resistivity, a platform in an existing device process can be directly used when a light receiving unit is formed on the second semiconductor substrate.
According to another aspect of the present invention, there is provided a solid-state image pickup device comprising: a first semiconductor layer of first conductivity type having first and second main surface portions; a plurality of diffusion regions of second conductivity type formed in the first main surface portion of the first semiconductor layer; and a second semiconductor layer of said first conductivity type formed over the second main surface portion of the first semiconductor layer and receiving light incident thereon, the second semiconductor layer being lower in resistivity than the first semiconductor layer.
The present invention realizes a solid-state image pickup device which can be easily produced and has a structure capable of easily releasing excess charges.
The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which:
Preferred embodiments of a solid-state image pickup device according to the present invention are described in detail below with reference to the drawings. In the description of the drawings the same constituent elements are given the same reference numerals to omit duplicated description.
First EmbodimentThe semiconductor substrate 10 in the present invention is a p+-type silicon substrate. The semiconductor substrate 10 has a resistivity ρ1 (a first resistivity). The resistivity ρ1 is, for example, 0.01 Ωcm and is preferably ρ1≦0.1 Ωcm. The thickness of the semiconductor substrate 10 is preferably not greater than the absorption length of the above light, that is, of light used in image pickup. If the wavelength of the above light is 1 μm, for example, the absorption length is approximately 100 μm.
The semiconductor layer 20 is provided on a front surface S1 of the semiconductor substrate 10. The semiconductor layer 20 in the present invention is a p-type silicon layer. The semiconductor layer 20 has a resistivity ρ2 (a second resistivity). Where, ρ2>ρ1. The resistivity ρ2 is, for example, 10 Ωcm. It is preferably that 5 Ωcm≦ρ2≦100 Ωcm. The semiconductor layer 20 is formed by an epitaxial growth method, for example.
The light receiving unit 30 is formed in the semiconductor layer 20. Specifically, the light receiving unit 30 is provided in the surface layer on the front surface (or, the surface on the opposite side of the semiconductor substrate 10) side of the semiconductor layer 20. The light receiving unit 30 receives signal electric charges produced by the above photoelectric conversion. The light receiving unit 30 in the present invention is an n-type impurity diffusion layer. The light receiving unit 30 and the semiconductor layer 20 adjacent thereto form a photodiode.
A MOSFET 50 is also formed in the semiconductor layer 20. That is to say, the solid-state image pickup device 1 includes a MOS image sensor formed of the light receiving unit 30 and a logic circuit unit formed of the MOSFET 50. The MOSFET 50 includes an n-type impurity diffusion layer 52 functioning as source and drain area and a gate electrode 54.
An interconnect layer 60 is provided on the surface of the semiconductor layer 20. An interconnect (not shown) is formed in the interconnect layer 60.
One example of operation of the solid-state image pickup device 1 is described with reference to
The effects of the present invention are described below. The semiconductor substrate 10 with a small resistivity ρ1 is provided on the solid-state image pickup device 1. This causes excess charges introduced by electro-static discharge (ESD) when the finger is brought into contact with the back surface S2 to be readily released from the semiconductor substrate 10 connected to the fixed electric potential (for example, ground potential), precluding circuit elements (MOSFET 50 or the like) in the solid-state image pickup device 1 from being electrostatically broken down. If ρ1≦0.1 Ωcm, a marked effect can be obtained.
The semiconductor layer 20 on which the light receiving unit 30 is provided has a higher resistivity ρ2 than that of the semiconductor substrate 10. This enables the platform in an existing device process to be used without modification when the light receiving unit 30 and MOSFET 50 are formed on the semiconductor layer 20, unlike the case where both the semiconductor substrate 10 and the semiconductor layer 20 have the resistivity ρ1. This is because the resistivity ρ2 can be set to a value within an available range of the existing device process because the value of the resistivity ρ2 does not need following the resistivity ρ1 to be reduced. If 5 Ωcm≦p2≦100 Ωcm, it is particularly preferable to use the existing device process without modification. If the existing device process cannot be used, a problem is caused in that a standard logic process is prevented from being used and circuit components such as a standard macro from being used. However, according to the present embodiment, this problem can be avoided.
Part of signal electric charges produced by photoelectric conversion disappears due to recombination before reaching the light receiving unit 30. In this respect, if the thickness of the semiconductor substrate 10 is not greater than the absorption length of the light used for image pickup, the ratio of recombining signal charges can be sufficiently lowered, providing the solid-state image pickup device 1 having extra-high sensitivity.
A fixed electric potential is applied to the semiconductor substrate 10 to allow excess charges to be more easily released. This further suppresses the occurrence of electrostatic breakdown.
When the semiconductor layer 20 is formed by the epitaxial growth method, or when the semiconductor layer 20 is formed of an epitaxial layer, the semiconductor layer 20 having a higher resistivity than the semiconductor substrate 10 is easily formed. The resistivity ρ1 of the semiconductor substrate 10 can be sharply changed to the resistivity ρ2 of the semiconductor layer 20.
Second EmbodimentWhen the whole semiconductor substrate 10 corresponds to the first semiconductor layer as in the solid-state image pickup device 1, the thickness of the whole semiconductor substrate 10 needs to be reduced from the viewpoint that the ratio of recombining signal charges is lowered as described above. On the other hand, the thickness of the semiconductor substrate 10 needs to be increased from the viewpoint that the strength of the semiconductor substrate 10 or the strength of the solid-state image pickup device is sufficiently secured.
The present embodiment can satisfy these opposing requirements. That is to say, a configuration in which the first semiconductor layer is provided only at part of the semiconductor substrate 10 enables the ratio of recombining signal electric charges to be lowered even if the thickness of the whole semiconductor substrate 10 is increased, as long as the first semiconductor layer is positioned in the vicinity of the back surface S2 of the semiconductor substrate 10. Specifically, it is preferable that a distance d1 between the face of the first semiconductor layer on the side of the semiconductor layer 20 (refer to
The solid-state image pickup device according to the present invention is not limited to the above embodiment, but may be modified in various forms. Various configurations in addition to those illustrated in the above embodiments are possible as the configuration of the semiconductor substrate 10 as long as at least part of the semiconductor substrate 10 is formed by the first semiconductor layer with the resistivity ρ1. For example, as illustrated in
Alternatively, as illustrated in
Although the above embodiments show examples where the N-channel MOSFET 50 is provided, a P-channel MOSFET may be provided instead of the MOSFET 50 or in addition to the MOSFET 50.
Although the p-type semiconductor substrate, p-type semiconductor layer and n-type light receiving unit are exemplified in the above embodiments, the n-type semiconductor substrate, n-type semiconductor layer and p-type light receiving unit may be used.
A portion excluding the first semiconductor layer in the semiconductor substrate may be a high-resistance semiconductor layer with a resistivity of 1000 Ωcm or more. The portion corresponds to the semiconductor substrate 10 (except the portion where the p+-type impurity diffusion layer 12 is formed) in
The present invention can be suitably applied to a charge coupled device (CCD) type of solid-state image pickup device.
It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.
Claims
1. A solid-state image pickup device photoelectrically converting light incident on a back surface of a semiconductor substrate into signal electric charges to image an object, the solid-state image pickup device comprising:
- a first semiconductor layer forming a part of the semiconductor substrate and having a first resistivity;
- a second semiconductor layer provided on a front surface of the semiconductor substrate and having a second resistivity higher than the first resistivity;
- a light receiving unit provided in the second semiconductor layer and receiving the signal electric charges produced by the photoelectric conversion; and
- wherein the first resistivity is 0.1 ohm cm or less, wherein the second resistivity is from 5 ohm cm or more to 100 ohm cm or less,
- wherein the first semiconductor layer is an impurity diffusion layer formed on the surface layer on side of the back surface of the semiconductor substrate and having an impurity concentration higher than that of the other part of the semiconductor substrate.
2. A solid-state image pickup device photoelectrically converting light incident on a back surface of a semiconductor substrate into signal electric charges to image an object, the solid-state image pickup device comprising:
- a first semiconductor layer forming a part of the semiconductor substrate and having a first resistivity;
- a second semiconductor layer provided on a front surface of the semiconductor substrate and having a second resistivity higher than the first resistivity;
- a light receiving unit provided in the second semiconductor layer and receiving the signal electric charges produced by the photoelectric conversion; and
- wherein the first resistivity is 0.1 ohm cm or less,
- wherein the second resistivity is from 5 ohm cm or more to 100 ohm cm or less,
- wherein the semiconductor substrate includes a silicon substrate and an epitaxial layer being provided on the silicon substrate and corresponding to the first semiconductor layer,
- wherein the silicon substrate is p type or p− type,
- wherein the epitaxial layer is p+ type.
3. The solid-state image pickup device according to claim 1, wherein a distance between a face of the first semiconductor layer on side of the semiconductor layer and the back surface of the semiconductor substrate is not greater than an absorption length of the light.
4. The solid-state image pickup device according to claim 1, wherein a predetermined fixed electric potential is applied to the semiconductor substrate.
5. The solid-state image pickup device according to claim 4, wherein the predetermined fixed electric potential is ground potential.
6. The solid-state image pickup device according to claim 1, wherein the second semiconductor layer is an epitaxial layer.
7. The solid-state image pickup device according to claim 2, wherein a distance between a face of the first semiconductor layer on side of the semiconductor layer and the back surface of the semiconductor substrate is not greater than an absorption length of the light.
8. The solid-state image pickup device according to claim 2, wherein a predetermined fixed electric potential is applied to the semiconductor substrate.
9. The solid-state image pickup device according to claim 3, wherein a predetermined fixed electric potential is applied to the semiconductor substrate.
10. The solid-state image pickup device according to claim 2, wherein the second semiconductor layer is an epitaxial layer.
11. The solid-state image pickup device according to claim 3, wherein the second semiconductor layer is an epitaxial layer.
12. The solid-state image pickup device according to claim 4, wherein the second semiconductor layer is an epitaxial layer.
13. The solid-state image pickup device according to claim 5, wherein the second semiconductor layer is an epitaxial layer.
14. The solid-state image pickup device according to claim 7, wherein a predetermined fixed electric potential is applied to the semiconductor substrate.
15. The solid-state image pickup device according to claim 8, wherein the predetermined fixed electric potential is ground potential.
16. The solid-state image pickup device according to claim 9, wherein the predetermined fixed electric potential is ground potential.
17. The solid-state image pickup device according to claim 14, wherein the predetermined fixed electric potential is ground potential.
18. The solid-state image pickup device according to claim 7, wherein the second semiconductor layer is an epitaxial layer.
19. The solid-state image pickup device according to claim 8, wherein the second semiconductor layer is an epitaxial layer.
20. The solid-state image pickup device according to claim 10, wherein the second semiconductor layer is an epitaxial layer.
Type: Application
Filed: May 3, 2013
Publication Date: Sep 19, 2013
Applicant: RENESAS ELECTRONICS CORPORATION (Kanagawa)
Inventor: Yasutaka NAKASHIBA (Kanagawa)
Application Number: 13/886,693