Image sensor color filters

Abstract: The invention provides an image sensor and a method of manufacture thereof. The invention is based on the remarkable mitigation of problems associated with conventional arrangements simply by using a negative polyimide rather than a positive polyimide to form the inter-layers (56, 60) between adjacent color filters (54, 58, 62).

Description: The present invention includes a method of manufacturing a color filter for use in a solid-state image sensor or a contact type image sensor such as charge coupled device(CCD) or bucket brigade device(BBD). For a solid-state image sensor, a particular wave of light to be incident on a surface of the image sensor like CCD or BBD is selectively radiated in order to obtain a color, for which there has been made a color filter for color separation divided by red, green, blue or yellow, cyan, magenta. In this case, for a display device such as liquid crystal display(LCD), the receiving of light is just changed. Currently, as a color filter, there is being widely adapted an organic dye type(or on-chip type) which obtains a desired spectrum characteristic in a manner that an easily dyed photosensitive resin is patternized by a photo-etching technique and the resultant pattern is dyed by a dyeing material. The method of manufacturing such a conventional color filter which is formed on the charge coupled devices is now explained. By way of an example therefor, it is preferably taken cross-sectional view of the image sensor of which the color filter is formed on an interline transfer charge coupled device(referred to as IT-CCD hereinbelow), as shown in Fig.1. The conventional technique of Fig.1 is described in Matsushidat technique report(90.6 vol.36. No.3) and Nichei Micro Device(1989 December version). Referring to Fig.1, IT-CCD comprises photodiodes 32, 34, 36 and 38 for optical-to-electrical converting the light to be incident on the surface of p-type semiconductor substrate, an active area having a charge-transfer part repeatedly arranged for transferring charges, the active area being adjacent to the photodiodes 32, 34, 36 and 38, and a pad area having pad part 52 for externally transferring a signal, the pad area being adjacent to the active area. The photodiodes 32, 34, 36 and 38 may have PNP structure by being formed on p-type semiconductor substrate 30 or PNPN structure in order to remove dark current. The charge transfer part 50 comprises an insulating layer 44 of SiO 2 for insulating a first conductive layer 42 of polycrystal silicon transferring charges, a second conductive layer 46 of polycrystal silicon formed on the insulating layer 44 and a light blocking layer 48 of dual layer of metal aluminium Al and SiW x formed on the second conductive layer 46. On the IT-CCD, color filter is formed. First, on the entire surface of IT-CCD, a flat layer 40 is formed in order to form a uniformed color filter layer and to prevent the light blocking layer 48 and the metal pad part 52 as metal layers from being corroded due to the acid radical of a photosensitive material when a color pattern array(referred to as a color filter layer hereinafter) is dyed over the entire surface of IT-CCD. The flat layer 40 is formed of a positive polyimide having an excellent light transmissivity and a photo-sensitivity at a hardening temperature. Thereafter, according to the conventional method for manufacturing color filter layer, a photo-sensitive layer having photo-sensitivity by using ammonium dichromate on hydrophile macromolecule material such as polyvinyl alcohol, casein or gelatine is evenly deposited. Then, after drying it, only the first color pattern portion is exposed and developed by mask exposing method, thereby forming a desired pattern. By dyeing the pattern with a dyeing material having a predetermined spectrum characteristic, magenta layer of the first color filter layer 54 is formed at the position corresponding to the first photodiode 32. Then, the second and the third color filter layers are formed in the same manner. A first inter-layer 56 is formed as a dye-proof layer for protecting the first color filter 54 from being dyed by yellow dyeing material on the entire surface of the flat layer 40 and the first color filter layer 54. The first inter-layer 56 is formed of a positive polyimide. After forming the first inter-layer 56, the second filter layer 58 of yellow layer is formed on the first inter-layer 56 corresponding to the second and the third photodiodes 34 and 36. In order to prevent the second color filter layer 58 from being dyed by cyan material, a second inter-layer 60 of positive polyimide is formed on the entire surface of the first inter-layer 56 and the second color filter layer 58. A third color filter layer 62 of cyan layer is formed at the position corresponding to the third photodiode 36 disposed on the second inter-layer 60. The overlapped portion 66 of the second color filter layer 58 and the third color filter layer 62 has green color. In order to protect the third color filter 62 from an external impact, a third inter-layer 64 of positive polyimide is formed on the entire surface of the second inter-layer 60 and the third color filter layer 62. Thereafter, for the purpose of wire bonding, the flat layer, the first, the second and the third inter-layer 56, 60 and 64 formed as above-described are removed at a time by photo-etching process so as for the pad area 52 to be exposed. In the manufacturing method color filter on the IT-CCD as described above, as for the complementary color type with an improved color sensitivity, it is noted that cyan layer and yellow layer are mutually overlapped in order to form green layer. That is, the first, the second and the third color filter layers 54, 58 and 62 are formed on the IT-CCD, and inter-layer 56, 60 and 64 are formed as a transparent dye-proof layer for prevent the colors from mixing in the color filter layer. Recently, as the light receiving part is being decreased in accordance with the high-efficiency of the IT-CCD, the area per unit pixel is in a tendency toward reduction. Accordingly, the allowance margin against a defect in the inter-layer and the align accuracy for the color filter layer is also reduced. Such a need causes defects for the inter-layer in thermal hardening process. That is, in the thermal hardening process carried out each time after forming each color filter layer, the inter-layer is deformed due to a thermal shrinkage, resulting in a possibility that the color filter layer would be moved or the inter-layer would be cracked. For the purpose of avoiding it, there has been used a process in which inter-layer is made by stacking different types of positive polyimides on each other. Nevertheless, a problem still remains as far as concerns the thermal shrinkage according to the hardening process for the color filter layer. In particular, the inter-layer is always formed of a positive polyimide because of the excellent light transmissivity of the material. However, the material has severe thermal shrinkage in its characteristic, so that a stress is acted on the interface between the color filter layer and the inter-layer, causing the movement of the color filter layer and even the image defect such as a wave trace or white blot due to a crack of the inter-layer, resulting in degrading the characteristic of the image sensor. US Patent No. 4357415 describes a method of making a solid state imaging device having a color filter comprising a plurality of color filter layers separated by dye impermeable inter-layers of a photocrosslinkable polymer. However, the problem of degradation caused by thermal shrinkage of the inter-layer material is not addressed. It is an object of the present invention to provide a manufacturing method of a color filters having a superior spectrum characteristic and high sensitivity which mitigates the described thermal shrinkage problem while retaining the characteristic of acting as a barrier between the different dye materials. According to a first aspect of the present invention, there is provided a method of manufacturing a color filter having a plurality of color filter layers comprising the step of forming a respective inter-layer between adjacent color filter layers characterised by forming at least one inter-layer from a layer made of a negative polyimide. According to a second aspect of the present invention there is provided an image sensor having plurality of color filter layers each separated from the next by an inter-layer characterised by at least one inter-layer comprising a layer of a negative polyimide having relatively low thermal shrinkage. An embodiment of the invention will now be described in detail, by way of example only and with reference to the accompanying drawings, in which; Fig. 1 is a vertical cross-sectional view of an image sensor of CCDs type for explaining a conventional color filter manufacturing method. Fig.2 is a manufacturing processing figure of the color filter according to the preferred embodiment of the present invention. With particular reference to Fig.2 of the drawings;- A color filter is generally formed in a shape of stripes or mosaic on an image sensor including CCDs or BBD. In this embodiment, it is described a color filter formed on the IT-CCD image sensor having the same structure as the conventional image sensor including photodiodes and light blocking layer of Fig.1. First, as shown in Fig. 2A, polyimide having an excellent light transmissivity and an unchanged photo-sensitivity at a hardening temperature on the IT-CCD is deposited with thickness of 3.5µm, which is then heated for 5 minutes at 170°C in a hot plate to form a flat layer 40. As a preferable positive polyimide, a CFGR-2010(Tokyo Ohka Kogyo Co. LTD) may be used, and as a developing liquid and a rinse, CRFT-Developer, Rinse(Tokyo Ohka Kogyo Co., LTD) may be used. Referring to Fig. 2B, magenta layer of a first color filter layer 54 is formed at the position corresponding to a first photodiode 32 disposed on the flat layer 40 in accordance with the manufacturing method of the conventional color filter layer. Thereafter, a negative polyimide having a low thermal shrinkage expansion ratio, even though its photo-sensitivity is changed at a hardening temperature, is formed with thickness of 0.8µm, and is softly baked in the hot plate for 70 seconds at 90°C. Then, after the polyimide is exposed and developed at deep UV(240∼270 nm), it is again heated in the hot plate at 105°C for 5 minutes, thereby forming a first inter-layer 56 with only the pad area 52 exposed. As a preferable polyimide, there are FVR-7, FVR-10(Fuji Chemical Co., LTD), while as a preferable developing liquid and rinse liquid, there are FVR-Developer and Rinse(Fuji Chemical Co., LTD). After forming the first inter-layer 56, as shown in Fig. 2C, yellow layer of the second color filter layer 58 is formed at the position corresponding to the second and the third photodiodes 34 and 36 disposed on the first inter-layer 56. Thereafter, on the final structure, a negative polyimide is deposited for a second color filter layer 58 not to be dyed by cyan dyeing material, and the second inter-layer 60 of dye-proof layer is formed in the same manner as previously described. Subsequently, as shown in Fig. 2D, cyan layer of a third color filter layer 62 is formed at the position corresponding to the fourth photodiode 38. The cyan layer is overlapped with the second color filter layer 58 formed on the upper part of the third photodiode 36 disposed on the second inter-layer 60. Then, a third inter-layer 64 is formed on the surface of the third color filter layer 62 in order to protect the third color filter layer 62 from an external impact. At this time, the third inter-layer 64 is formed with only the pad area 52 exposed. Thus, the first, the second and the third inter-layer are formed on the pad area 52. These inter-layer may be formed by a negative polyimide with thickness of 4.1µm, or by a successive mixture of negative polyimide with thickness of 0.6∼1.0µm and positive polyimide with thickness of 2.0∼4.0µm. Preferably, in the thickness of the mixture, the negative polyimide is 0.8µm and the positive polyimide is 3.3µm. As mentioned above, positive polyimides have conventionally been used for forming the inter-layers between the color filters. Particularly as the size of the image sensors has reduced, problems arise as a result of the thermal hardening process such that stresses are produced between the inter-layers and the color filter. This can result in misalignment of the color filters and even cracking of the inter-layers. The present invention is based upon the remarkable mitigation of these problems simply by using a negative polyimide rather than a positive polyimide to form the inter-layers.

Claims: A method of manufacturing a color filter having a plurality of color filter layers (54, 58, 62) comprising the step of forming a respective inter-layer (56, 60, 64) between adjacent color filter layers characterised by forming at least one inter-layer from a layer made of a negative polyimide. A method of manufacturing a color filter as claimed in claim 1, comprising the steps of: forming a flat layer (40) for smoothing over an image sensor (30, 32-38) having charge coupled devices or bucket brigade device, forming a first color filter layer (54) and a first inter-layer (56) of dye-proof layer on said flat layer, alternately forming color filter layers and inter-layers in the same manner as above described steps in order to form a second color filter layer (58), a second inter-layer (60), a third color filter layer (62) and a third inter-layer (64). A method according to claim 2, wherein said first, second and third inter-layers are each formed by a layer of a negative polyimide. A method according to claim 2 or claim 3, wherein the first, the second and the third inter-layers are each formed by stacked layers of negative polyimide and positive polyimide. A method according to claim 2, wherein said flat layer is formed of a positive polyimide. A method according to claim 2, wherein the method further includes a step of etching the inter-layer covering the pad area (52) each time after forming the first, the second and the third inter-layer, respectively. A method of manufacturing an image sensor having a color filter as claimed in any one of claims 2 to 8. An image sensor having plurality of color filter layers (54, 58, 62) each separated from the next by an inter-layer (56, 60, 64) characterised by at least one inter-layer comprising a layer of a negative polyimide having relatively low thermal shrinkage.

Inventors: LEE HYUNG SEOK, LEE, HYUNG SEOK
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO. LTD.
Application year: 1993
Main classification: H01L31
IPC codes: G02B5, H01L31, H01L27