Solid-state image pickup device

Abstract: In a solid-state image pickup device, a high melting point metal film is used in a cell portion region (photoelectric conversion portion) as a light shielding film which defines the opening area of a photoelectric conversion element and an aluminum film which is covered with a high melting point metal is used in a peripheral region as a wiring.

Description: The present invention relates to a solid-state image pickup device and in particular to a structure of a metal film which forms wirings and light shielding films of the solid-state image pickup device. Referring to Figs. 1(a) and 1(b), there are shown sectional views of a cell portion region (photoelectric conversion portion) and a peripheral region. In the drawing, reference numeral 1 denotes an n-type semiconductor substrate; 2 a p-type well formed thereon; 3 an n-conductivity type photoelectric conversion region formed in the p-type well 2; 4 an n-conductivity type charge transfer region also formed in the p-type well 2; 5 an element isolation region which is formed between elements by doping a p-type dopant at a high concentration; 6 an insulating film formed on the p-type well 2; 7 an electric charge transfer electrode formed of polysilicon serving also as an electric charge read out electrode; 8 an aluminum wiring formed on the peripheral region; and 8b an aluminum light shielding film having an opening above the photoelectric conversion region 3. A color filter film 10 and a microlens 11, etc. are formed above the solid-state image pickup device as shown in Fig. 2. An aluminum thin film is used as the light shielding film in the above mentioned prior art solid-state image pickup device. The aluminum thin film should have a thickness of about 0.8 µm in order to obtain an enough light shielding ability. The width of the opening of the photoelectric conversion element has been reduced as high density integration of the solid-state image pickup devices has recently been remarkably advanced. A device having 380 thousand pixels and compatible with a 1/3 inch (8.5 mm) tube has an opening width not higher than 2 µm. It is essential to condense incident lights by forming the microlens 11 on the photoelectric conversion element as shown in Fig. 2 in order to prevent lowering of the sensitivity of the device if the opening area is reduced. However, if a conventional aluminum film is used as the light shielding film, the area of the opening formed in the upper side 8ba of the light shielding film 8b would become equal to that in the underside 8bb of the film 8b since the thickness of the aluminum thin film is larger than the dimension of the opening of the light shielding film. That is, the opening would be formed in narrow width along the total height (usually not less than 1 µm) of the aluminum film including the thickness of the electric charge transfer electrode 7. Accordingly, the incident light from the periphery of the microlens 11 will be reflected in the vicinity of the edge of the opening of the aluminum light shielding film 8b, resulting in a lowering of the sensitivity of the device. Regions 14 other than the region 13 to be exposed are exposed due to random reflections of the light on the surface of the aluminum light shielding film 8b as shown in Fig. 3 when a negative photo-resist is used which will become a dye matrix 12 of the color filter in the solid-state image pickup device using a conventional aluminum light shielding film. As a result of this, patterning accuracy is lowered. There is the possibility that projections which are referred to as "hillock" may be formed on the aluminum wiring or aluminum light shielding film. This may cause the reliability of the products to be lowered. WO-A-9 002 418 describes a solid-state image pickup device comprising photoelectric conversion elements and a thin film made of a refractory material which serves as a light-shielding film defining the opening area of the photoelectric conversion elements. Furthermore, it is disclosed that the light-shielding film may have a double layer structure. It is an object of the present invention to obviate the above mentioned defects of the prior art and to provide a solid state image pickup device having a high patterning accuracy. This object is achieved by a solid state image pickup device as defined in the claim. Figs. 1(a) and 1(b) are sectional views showing a prior art image pick-up device; Figs. 2 and 3 are sectional views explaining the problems of the prior art; Figs. 4(a) and 4(b) are sectional views useful for understanding the present invention; Fig. 5 is a sectional view explaining the effect of the device of Figs. 4(a) and 4(b); and Figs. 6(a) and 6(b) are sectional views showing an embodiment of the present invention. An image pick-up device useful for understanding the invention as claimed will be described with reference to the drawings. Referring now to Figs. 4(a) and 4(b), there are shown a cell portion region and a peripheral region of the cell portion of a solid-state image pickup device. As shown in Figs. 4(a) and 4(b), a p-type well 2 is formed on an n-type semiconductor substrate 1. An n-conductivity type photoelectric conversion region 3, an n-conductivity type electric charge transfer area 4, and an element isolation region 5 which is a p-conductivity type highly doped concentration region are formed in the surface region of the p-type well 2. An electric charge transfer electrode 7 made of polysilicon is formed via and within an insulating film 6 in the cell portion region above the semiconductor substrate. An aluminum wiring 8 is formed on the insulating layer 6 in the peripheral region of the cell region above the semiconductor substrate. A tungsten thin film 9 which covers the aluminum wiring 8 and a tungsten light shielding film 9a which defines the opening of a photoelectric conversion element are formed in the peripheral and cell regions, respectively. If the tungsten thin film 9 is used as a light shielding film, the thin film 9 having a thickness of about 0.4 µm is capable of providing an enough light shielding ability. Accordingly, in this structure, the height of the upper surface of the light shielding film can be made lower than that of the prior art and the opening area on the upper surface can be made wider than that of the prior art. Hence, the light from the periphery of a microlens 11 is prevented from being reflected on the light shielding film as shown in Fig. 5 when a color filter layer 10 and a microlens 11 are formed on the solid-state image pickup device of the present embodiment. An unwanted region is not exposed to light on patterning of a color filter dye matrix if a tungsten thin film is used as a light shielding film since a high melting point metal like tungsten has a light reflection factor which is about a quarter of that of aluminum. This enables a precise patterning to be carried out. Since the wiring in the peripheral region is fabricated by covering the aluminum film with the high melting point metal, the aluminum film can be prevented from being etched in an etching step for patterning the high melting point metal. The tungsten thin film remains on the aluminum wiring and functions as a protective film after the patterning step to prevent a hillock from forming on the aluminum wiring. Referring now to Figs. 6(a) and 6(b), there are shown the cell portion and the peripheral regions of an embodiment of a solid-state image pickup device of the present invention. In this embodiment, when the aluminum wiring 8 is formed in the peripheral region of the cell portion by patterning the aluminum thin film, a portion corresponding to the electric charge transfer region 4 is formed with an aluminum light shielding film 8a in the cell portion region simultaneously with this. The embodiment is substantially identical with the device of fig. 4 except the foregoing. In the present embodiment, the tungsten thin film can be made thinner by providing the aluminum light shielding film 8a. As mentioned above, in the solid-state image pickup device of the present invention, the high melting point metal film is used in the cell portion region as the light shielding film which defines the opening of the photoelectric conversion element and the aluminum wiring is covered with the high melting point metal in the peripheral region. Therefore, incident light from the periphery of the microlens will not be reflected on the light shielding film so that decrease in sensitivity can be prevented. Since the high melting point metal such as tungsten has a light reflection factor of about 20% while aluminum has a factor not less than 90%, pattern distortion due to reflection of light on patterning of a color filter can be prevented from occuring in accordance with the present invention. Since the aluminum wiring is protected by the high melting point metal film, it is hard for hillocks to grow so that the reliability of the device can be enhanced.

Claims: A solid-state image pickup device, comprising: a semiconductor substrate (1); a plurality of photoelectric conversion elements formed in a photoelectric conversion portion (3) for converting incident light into electric signals; a film (9a) made of a tungsten covering an electric charge transfer electrode (7) and a part of the photoelectric conversion region (3) and serving as a light shielding film which defines openings of the photoelectric conversion elements in the photoelectric conversion portion (3), characterized by aluminum wirings (8) formed on said semiconductor substrate (1) outside the photoelectric conversion portion, wherein said film covers the aluminum wirings (8) and serves as a protective film (8) for the aluminum wirings (8), is thinner than said aluminum wirings (8) and also covers an aluminum light shielding film (8a) formed over a charge transfer region (4) in the photoelectric conversion portion.

Inventors: TORIYAMA KEIJI, TORIYAMA, KEIJI, Toriyama, Keiji, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H01L31
IPC codes: H01L27, H01L31