Color image sensor and method for fabricating the same

[0002] The present invention relates to an image sensor; and, more particularly, to a color image sensor and a method for fabricating the same.

[0003] Generally, an image sensor, e.g., a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor is employed to scan and convert an optical image into electrical signals.

[0004] Referring to FIG. 1, there is shown a pixel unit of a conventional image sensor including a photodiode (PD) and four NMOS transistors. The four NMOS transistors include a transfer transistor 12, a reset transistor 14, a drive transistor 16 and a select transistor 18. The transfer transistor 12 transfers photoelectric charges generated in the photodiode to a sensing node. The reset transistor 14 resets the sensing node in order to sense a next signal. The drive transistor 16 acts as a source follower. The select transistor 18 outputs data to an output terminal OUT in response to an address signal. A load transistor 20, which is positioned between the output terminal OUT of the pixel unit and the ground voltage level, receives a biasing signal from an external device for the sake of biasing the pixel unit. A capacitance of a floating diffusion is referred to as “Cfd”.

[0005] Referring to FIG. 2, there is shown the photodiode of the pixel unit in FIG. 1. The photodiode includes a P⁺ silicon substrate 21, a P-epi (epitaxial) layer 22, field oxide layers 23, an N⁻ diffusion region 24 and a P⁰diffusion region 25. The photodiode (PD) has a PNP junction structure in which the P-epi 22, the N⁻ diffusion region 24 and the P⁰diffusion region 25 are stacked.

[0006] Referring to FIG. 3, a conventional color image sensor includes pixel units and each pixel unit includes a photodiode. Photodiodes 102a, 102band 102care formed in the color image sensor. Non-photosensing regions 103 are formed between photodiodes 102a, 102band 102c. An interlayer insulating layer 105ais formed on the photodiodes 102a, 102band 102cand the non-photosensing regions 103. Light shield layers 104 is formed on the interlayer insulating layer 105ato shield light incident on the non-photosensing regions 103. An interlayer insulating layer 105bis formed on the light shield layers 104. Red, green and blue filters 106a, 106band 106care formed on the interlayer insulating layer 105b. A buffer layer 107 is formed on the color filters 106a, 106band 106cto improve the planarization of upper portions of the color filters 106a, 106band 106cand the transitivity of light. Micro lenses 108a, 108band 108care formed on the buffer layer 107.

[0007] The photodiode 102areceives light from an object and integrates photoelectric charges from the light via the micro lens 108aand the red filter 106a. The photodiode 102breceives the light of the object and integrates the photoelectric charges from the light via the micro lens 108band the green filter 106b. The photodiode 102creceives the light of the object and integrates the photoelectric charges from the light through the micro lens 108cand the blue filter 106c. The pixel units consist of red, green and blue pixel units to output color signals of red, green and blue components of the light from the object. Conventionally, the red, green and blue pixel units are formed by the same fabrication process, respectively, so that the photodiodes 102a, 102band 102cof the red, green and blue pixel units have substantially the same depletion region structure each other. Accordingly, there is a problem that the conventional color image sensor having photodiodes of the same depletion region structure may not improve light sensitivity since it is variable according to wavelengths of the red, green and blue components of the light.

[0008] Referring to FIG. 4, a graph depicts the absorption length of a light wavelength in Si, GaAs and Ge mediums when light is incident on a medium. The light wavelength of 770 nm is absorbed up to the absorption length of 7 μm in the Si medium while the light wavelength of 390 nm is absorbed up to the absorption length of 0.1 μm in the Si medium. Therefore, the light sensitivity of a photodiode depends on a kind of medium and wavelengths of red, green and blue components of light.

[0009] Referring to FIG. 5, a graph depicts light sensing characteristics of the conventional color image sensor in FIG. 3. The photodiode of the color image sensor receives light from an object. The light includes red, green and blue components, which have a different wavelength from each other. The red component has the longest wavelength among the red, green and blue components. The green component has a wavelength longer than the blue component. Since the blue component has a wavelength shorter than the red or green component, a part of the blue component is lost before the blue component reaches the depletion region. Accordingly, the photodiode have low light sensitivity of the blue component. Also, when a red pixel unit reaches a saturation state, a blue pixel unit does not reach the saturation state. Furthermore, when an image process is performed on the basis of color signals from the red pixel unit, color signals from the green and blue pixel units can be lost, so that colors represented by the conventional image sensor are limited and a dynamic range for the sake of processing data is reduced. In other words, color signals from the red, green and blue pixel units may display different colors from actual colors of an object because the red, green and blue pixel units have different light sensing characteristics from each other.

[0010] It is, therefore, an object of the present invention to provide a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.

[0011] It is another object of the present invention to provide a method for fabricating a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.

[0012] In accordance with an aspect of the present invention, there is provided a color image sensor for scanning and converting an optical image into electrical signals, comprising: a red photodiode including a P-type semiconductor layer and a first N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a first depletion region positioned beneath the surface of the P-type semiconductor layer; a green photodiode including the P-type semiconductor layer and a second N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a second depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the second depletion region is more adjacent to the surface of the P-type semiconductor layer than the first depletion region; and a blue photodiode including the P-type semiconductor layer and a third N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a third depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the third depletion region is more adjacent to the surface of the P-type semiconductor layer than the second depletion region.

[0013] In accordance with another aspect of the present invention, there is provided a method for fabricating a color image sensor for scanning and converting an optical image into electrical signals, comprising the steps of: (a) forming a P-type semiconductor layer on a substrate; (b) forming field oxide layers on the P-type semiconductor layer to define regions for red, green and blue photodiodes; (c) providing an ion implantation mask having different mask patterns for the red, the green and the blue photodiodes; (d) implanting impurity ions into the P-type semiconductor layer through the use of said ion implantation mask to form N-type diffusion regions in the P-type semiconductor layer; and (e) applying a thermal process to the resulting structure to form different first, second and third depletion regions corresponding to the red, the green and the blue photodiodes.

[0014] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:

[0015] FIG. 1 is a circuit diagram showing a pixel unit of a conventional image sensor;

[0016] FIG. 2 is a cross-sectional view describing the photodiode in FIG. 1;

[0017] FIG. 3 is a cross-sectional view describing a conventional color image sensor;

[0018] FIG. 4 is a graph depicting absorption length corresponding to a wavelength in various mediums;

[0019] FIG. 5 is a graph depicting light sensing characteristics of the conventional color image sensor in FIG. 3;

[0020] FIG. 6 is a cross-sectional view describing photodiodes of a color image sensor in accordance with the present invention; and

[0021] FIGS. 7A and 7B are top views describing ion implantation masks to the photodiodes of FIG. 6.

[0022] Referring to FIG. 6, it will be understood that photodiodes of red, green and blue pixel units have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.

[0023] A photodiode of the red pixel unit 710 receives a red component of light and generates photoelectric charges from the red component of the light. The photodiode of the red pixel unit 710 has a depletion region 702 adjacent to the surface of a P-epi layer 701.

[0024] A photodiode of a green pixel unit 720 receives a green component of light and generates photoelectric charges from the green component of the light. Since the green component has a wavelength shorter than the red component, the photodiode of the green pixel unit 720 has a depletion region 703 more adjacent to the surface of the P-epi layer 701 than that of the red pixel unit 710.

[0025] A photodiode of a blue pixel unit 730 receives a blue component of light and generates photoelectric charges from the blue component of the light. Since the blue component has a wavelength shorter than the green component, the photodiode of the blue pixel unit 730 has a depletion region 704 more adjacent to the surface of the P-epi layer 701 than that of the green pixel unit 720.

[0026] The photodiodes of the red, green and blue pixel units 710, 720 and 730 each has the lightly doped P-epi layer 701, an N⁻ diffusion region formed in the P-epi layer 701, and a P⁰diffusion region formed between the surface of the P-epi layer 701 and the N⁻ diffusion region.

[0027] The color image sensor in accordance with the present invention controls the distance between the surface of the P-epi layer and the N⁻ diffusion region to compensate characteristic difference between pixel units. For example, in the photodiode of the blue pixel unit receiving the blue component with the shortest wavelength, the depletion region of the blue pixel unit is more adjacent to the surface of the P-epi layer than that of the red and green pixel units. Although the blue component has the shortest wavelength, since the depletion region of the photodiode in the blue pixel unit is more adjacent to the surface of the P-epi layer than that in the red and green pixel units, the blue component can sufficiently reach the depletion region and the photodiode of the blue pixel unit can improve the light sensitivity of the blue component.

[0028] One ion implantation mask is employed to control a distance between the surface of the P-epi layer and the N- diffusion region, that is, to control an impurity ion concentration implanted into the depletion region. The ion implantation mask has different mask patterns for the red, green and blue pixels. Accordingly, when impurity ions are implanted in the P-epi layer over the ion implantation mask, an impurity ion concentration depends on the mask patterns of the red, green and blue pixel units.

[0029] Referring to FIGS. 7A and 7B, top views of ion implantation masks 800 and 850 are employed to control the impurity ion concentration implanted into the depletion region. The ion implantation masks 800 and 850 include mask regions 810, 820, 830, 860, 870 and 880 for the red, green and blue pixel units. The ion implantation masks 800 and 850 include screening patterns to control an impurity ion concentration. The mask region 810 for a red pixel unit has not a screening pattern. The mask regions 820 and 830 for the green and blue pixel units have the screening patterns 840 and 850 of a square type. The mask regions 870 and 880 for the green and blue pixel units have the screening patterns 890 and 900 of a rectangular type. The number of the screening patterns 850 of the mask region 830 is more than that of the screening patterns 840 of the mask region 820. Similarly, the number of the screening patterns 900 of the mask region 880 is more than that of the screening patterns 890 of the mask region 870. Preferably, the distance “A” between two screening patterns for the green pixel unit is longer than the distance “B” between two screening patterns for the blue pixel unit. Also, the size “a” of each screening pattern for the green pixel unit is greater than the size “b” of each screening pattern for the blue pixel unit.

[0030] After the ion implantation over the ion implantation mask, a thermal process is applied to the resulting structure. At this time, an uneven portion of the depletion region is formed closely with the surface of a P-epi layer in the photodiodes of the green and blue pixel units. The present invention can be applied to a PN junction photodiode as well as a PNP junction photodiode. Also, the color image sensor in accordance with the present invention can be fabricated by using the conventional fabrication process except for the ion implantation mask different from a conventional ion implantation mask.

[0031] Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.