Solid state image pick-up element

Abstract: A solid state image pick-up element has a pixel region (13), an output region (12) located near the pixel region (13), and a field portion (11) located remote from the pixel region (13), all of which are formed at un upper layer of a semiconductor substrate (1). The pixel region (13) is provided with plural rows of light receiving portions (4) for performing photoelectric conversion with respect to light received thereby. Between the plural rows of the light receiving portions (4) are interposed transfer portions (3) for transferring signals obtained through the photoelectric conversion. The output region (12) is provided with an output portion (2) for outputting the signals transferred from the transfer portions (3). The semiconductor substrate (1) is covered with three dielectric films (5, 6, 8), and an opening (15) is formed through two dielectric films (6, 8) at a location above the output portion (2).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a solid state image pick-up element employing, for example, CCDs (charge coupled devices). The solid state image pick-up element is preferably used in an area sensor, a line sensor or the like. Description of the Prior Art Fig. 1 depicts a conventional solid state image pick-up element having a pixel region 113, an output region 112 located in the proximity of the pixel region 113, and a field portion 111 located remote from the pixel region 113. All the pixel region 113, the output region 112, and the field portion 111 are formed at an upper layer of a semiconductor substrate 101. The pixel region 113 is provided with a number of light receiving portions 104 having a photoelectric conversion function and a number of transfer portions 103 for transferring electric signals obtained through photoelectric conversion. The light receiving portions 104 and the transfer portions 103 are disposed alternately. The output region 112 is provided with an output portion 102 for outputting signals transferred from the transfer portions 103. The field portion 111 is provided with a pad portion 107. The semiconductor substrate 101 having the above-described structure is entirely covered with a BPSG (boron phosphorus silicate glass) film 105 in which B (boron) and P (phosphorus) are added to SiO 2 . The entire surface of the BPSG film 105 except an upper portion of the pad portion 107 is covered with a PSG (phosphorus silicate glass) film 106 in which P (phosphorus) is added to SiO 2 . The pad portion 107 is formed directly on the BPSG film 105. The recent trend in design of solid state image pick-up elements is to reduce the size thereof and to increase the number of pixels. With this trend, there is an increasing demand toward high sensitivity. The following two methods are conventionally known to enhance the sensitivity: (1) a method in which the area of light receiving portions is enlarged to increase the amount of incident light, thereby increasing the quantity of signals (the quantity of electric charges) which are subjected to photoelectric conversion in the light receiving portions; and (2) another method in which the capacity of a detection portion provided in an output portion is reduced to enhance the electric charge-to-voltage conversion efficiency, thereby effectively converting signals (electric charges) obtained through the photoelectric conversion in the light receiving portions to an output voltage. Fig. 2 depicts a solid state image pick-up element employing the method (1). The solid state image pick-up element shown in Fig. 2 has the same structure as that shown in Fig. 1 and further has a polymeric resin film 108 formed on the PSG film 106, a color filter 109 formed on the polymeric resin film 108, and a number of very tiny lenses 110 formed on the color filter 109. The lenses 110 act to increase the amount of incident light entering the light receiving portions 104, thereby increasing the quantity of signals which are subjected to photoelectric conversion in the light receiving portions 104. On the other hand, the method (2) is carried out, for example, by reducing the area of a detection diode provided in the output portion 102 or by shortening the wiring length between the detection diode and a driver gate provided in the output portion 102 at the first stage thereof. By doing so, the capacity of the detection portion of the output portion 102 is reduced, thereby enhancing the electric charge-to-voltage conversion efficiency. These methods, however, cannot enhance the sensitivity of solid state image pick-up elements to a sufficiently high level. Further enhancement of the sensitivity requires considerable changes in production processes or requires new materials or devices. As a result, the production cost of the solid state image pick-up elements increases. JP-A-2-312 274 discloses a color solid state image sensing device in which a photodetection element and a transfer electrode are formed on a semiconductor substrate, and a PSG film is formed thereover. A flattening layer of SiO 2 is formed on the PSG film. Three color filter layers are successively provided over the flattening layer, each separated from the adjacent color filter layer by a separating layer of SiO 2 . Further prior art solid state image sensing devices are known from "Charge Transfer Devices" by C.H. Séquin and M.F. Tompsett, Academic Press, 1975. Fig. 3.15(a) of this citation shows a device according to the preamble of claim 1. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved solid state image pick-up element having a sufficiently high sensitivity which can be readily manufactured without increasing the production cost. The present invention, as defined by claim 1, provides a solid state image pick-up element comprising a semiconductor substrate on which there are formed an array of light-receiving portions, a plurality of transfer portions for transferring electric charge obtained through photoelectric conversion of light received by said light-receiving portions, and an output region in which electric charge transferred from said light-receiving portions by the transfer portions is output from an output portion and converted to output voltage, wherein first, second and third dielectric films are formed as consecutive layers on said semiconductor substrate, the second and third layers each having an opening above said output region so that neither said second nor said third dielectric film exists above the output portion, thereby to enhance the electric charge-to-voltage efficiency. The sub-claims 2 to 6 are directed to embodiments of the invention. According to the invention, the provision of the opening in the output region reduces the capacitance of a detection element in the output region, thereby enhancing the electric charge-to-voltage conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a conventional solid state image pick-up element; Fig. 2 is a view similar to Fig. 1, but indicating another conventional solid state image pick-up element; and Fig. 3 is a view similar to Fig. 1, but indicating a solid state image pick-up element according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 3 depicts a solid state image pick-up element embodying the present invention. As shown in Fig. 3, the solid state image pick-up element has a pixel region 13, an output region 12 located in the proximity of the pixel region 13, and a field portion 11 located remote from the pixel region 13. All the pixel region 13, the output region 12, and the field portion 11 are formed at an upper layer of a semiconductor substrate 1. The pixel region 13 is provided with plural rows of light receiving portions 4, which comprise a number of photodiodes having a photoelectric conversion function. Between the plural rows of the light receiving portions 4 are interposed transfer portions 3 comprising CCDs for transferring electric signals obtained through photoelectric conversion in the light receiving portions 4. The output region 12 is provided with an output portion 2 for outputting signals transferred from the transfer portions 3. The field portion 11 is provided with a pad portion 7. The semiconductor substrate 1 having the above-described structure is entirely covered with a first dielectric film 5 comprising BPSG in which B (boron) and P (phosphorus) are added to, for example, SiO 2 . It is to be noted that the material of the first dielectric film 5 is not limited to BPSG, and Si compound such as, for example, SiO 2 or Si 3 N 4 may be used as the material of the first dielectric film 5. The pad portion 7 is formed directly on the first dielectric film 5. The entire surface of the first dielectric film 5, except an upper portion of the pad portion 7 and a specific location above the output portion 2, is covered with a second dielectric film 6 comprising PSG in which P (phosphorus) is added to, for example, SiO 2 . The second dielectric film 6 is entirely covered with a third dielectric film 8 made of polymeric resin such as acrylic resin, polyimide resin or the like. Two openings 14 and 15 are formed through the second and third dielectric films 6 and 8. The opening 14 is positioned above the pad portion 7, whereas the opening 15 is positioned above the output portion 2. These openings 14 and 15 are formed, for example, by generally available plasma-etching after the second and third dielectric films 6 and 8 have been formed on the entire surface of the semiconductor substrate 1. Particularly, the size and configuration of the opening 15 formed above the output portion 2 are rendered to be substantially the same as those of the upper surface of the output portion 2. It is to be noted that the material of the second dielectric film 6 is not limited to PSG, and Si compound such as, for example, SiO 2 or Si 3 N 4 may be used as the material of the first dielectric film 5. Also, the material of the third dielectric film 8 is not limited to acrylic resin or polyimide resin, and any other suitable polymeric resin may be used. At an upper portion of the pixel region 13, a color filter 9 is formed on the third dielectric film 8, and a number of very tiny lenses 10 made of transparent resin or the like are formed on the color filter 9. In the solid state image pick-up element having the above-described structure, because neither the second dielectric film 6 nor the third dielectric film 8 exists above the output portion 2, the capacity of the detection portion of the output portion 2 reduces, thereby enhancing the electric charge-to-voltage conversion efficiency. Furthermore, the provision of the lenses 10 above the light receiving portions 4 increases the quantity of signals which are subjected to photoelectric conversion. As a result, the solid state image pick-up element according to the present invention is rendered to have a sensitivity 20% higher than that of the conventional solid image pick-up element shown in Fig. 2. In addition, because the solid state image pick-up element according to the present invention causes no considerable changes in production processes or requires no new materials or devices, the production cost thereof does not increase. In view of reliability or the production cost, Si compound such as, for example, SiO 2 or Si 3 N 4 is preferably used as the material of the first and second dielectric films 5 and 6 whereas polymeric resin is preferably used as the material of the third dielectric film 8. In the above-described embodiment, although an opening 15 is formed through the second and third dielectric films 6 and 8 and above the output portion 2, this opening 15 may be formed through all the three dielectric films 5, 6, and 8. By doing so, the sensitivity of the solid state image pick-up element can be further enhanced. The present invention is applicable to the case where the area of a detection diode provided in an output portion is reduced or the wiring length between the detection diode and a driver gate provided in the output portion at the first stage thereof is shortened to reduce the capacity of the detection portion of the output portion for the increase in electric charge-to-voltage conversion efficiency. In this case also, an opening is preferably formed through dielectric films and above the output portion. The present invention is also applicable to an interline transfer type CCD solid state image pick-up element or a frame transfer type solid state image pick-up element. Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art.

Claims: A solid state image pick-up element comprising a semiconductor substrate (1) with a first dielectric film (5) thereon on which there are formed an array of light-receiving portions (4) , a plurality of transfer portions (3) for transferring electric charge obtained through photoelectric conversion of light received by said light-receiving portions (4), and an output region (12) in which electric charge transferred from said light-receiving portions by the transfer portions (3) is output from an output portion (2) and converted to output voltage, characterised by second and third dielectric films (6,8) which together with the lower first dielectric film are consecutive layers on said semiconductor substrate (1), the second and third layers (6,8) each having an opening (15) above said output region (12) so that neither said second nor said third dielectric film exists above the output portion (2), thereby to enhance the electric charge-to-voltage efficiency. A solid state image pick-up element according to claim 1, in which said second dielectric film (6) comprises phosphorus silicate glass or SiO 2 or Si 3 N 4 . A solid state image pick-up element according to claim 1 or claim 2, in which said third dielectric film (8) comprises polymeric resin. A solid state image pick-up element according to claim 3, wherein said polymeric resin is acrylic resin or polyimide resin. A solid state image pick-up element according to any of claims 1 to 3, in which said first dielectric film (5) also has a said opening (15) in said output region (12). A solid state image pick-up element according to any preceding claim, said pick-up element being of interline transfer type or frame transfer type.

Inventors: AOKI TETSURO, IMANAKA YASUHIRO, ISHIBE SHOUICHI, NAKAI JUNICHI, WATANABE TOORU, AOKI, TETSURO, IMANAKA, YASUHIRO, ISHIBE, SHOUICHI, NAKAI, JUNICHI, WATANABE, TOORU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L29
IPC codes: H01L27, H01L29, H04N5