A solid-state imaging device provided with microlenses

Abstract: In a solid-state imaging device having a microlens above a light receiving section formed on a substrate, the microlens comprises a first lens layer (3) formed on a smooth surface of the substrate and a second lens layer (4) formed on the first lens layer. In a transverse direction, the second lens layer is extended down on both sides of the first lens layer to be in contact with the substrate. In a longitudinal direction, the second lens layer is kept on a top surface of the substrate. The second lens layer is subjected to softening treatment.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a solid-state imaging device a method for producing the same integratedly having a microlens array formed of a transparent resin or the like for use as an image input device such as a movie camera device, a still camera, a facsimile machine and a wordprocessor. 2. Description of the Prior Art: Figure 8 shows a known solid-state imaging device. This solid-state imaging device comprises a substrate 11 , photodiodes 12,··· as pixels formed in a top portion of the substrate 11, and microlenses 13,··· formed of a transparent resin above the photodiodes 12,··· . A light is converged on the photodiode 12 through the microlens 13 , whereby a photosensitivity of the device is improved. The reference numerals 16,··· denote registers constituting CCDs (cathode coupled devices), and a shaded portion indicated by the reference numeral 17 denotes a smoothing layer for smoothing a top surface of the substrate 11 , which is ragged because of the photodiodes 12,··· , the registers 16,··· and the like formed thereon. As the above microlens 13 , a strip-shaped microlens 14 (Figure 9a ) and a rectangular microlens 15 (Figure 9b ) are well known. These microlenses 14 and 15 are each formed in the following way. First, as shown in Figures 10a and 10b , a lengthy strip pattern 14a or a rectangular pattern 15a is formed of a resin or a resist on the substrate 11 by photo-etching or the like, after the ragged top surface of the substrate 11 due to the photodiode, the register and the like is smoothed by the smoothing layer. Then, the lengthy strip pattern 14a or the rectangular pattern 15a is softened by heating or the like to make a convex curved surface by surface tension. As illustrated in Figure 11 , an arranged pitch of the microlenses 15 is determined in accordance with the positional relationship between the microlenses 15 and the corresponding photodiodes 12 . For improving the photosensitivity of the solid-state imaging device, a microlens should be made rectangular in order to have the largest possible area and the smallest possible gaps from the adjacent microlenses. Additionally, in the case of the rectangular microlens 15 having a curvature over all the directions of an upper surface thereof, the light incident from all the directions can be converged. In the case of the strip-shaped microlens 14 , however, only a portion of the light incident from all the directions can be converged due to the shape thereof, resulting in a slightly lower photosensitivity than that of the rectangular microlens 15 . As shown in Figures 11 and 12 , the rectangular microlens 15 has different widths in a longitudinal direction ("a" direction) and in a transverse direction ("b" direction) although having the same height H . Accordingly, the microlens 15 has different curvatures in the "a" and the "b" directions. In the case that the width W in the "a" direction is larger than the width W in the "b" direction, if the width W in the "a" direction tion is suitable to obtain a good light convergence (Figure 13a ), the width W in the "b" direction is short and a poor light convergence is obtained (Figure 13b ). In consequence, the light convergence and the photosensitivity cannot be enhanced to a sufficiently high level in the conventional solid-state imaging device. JP-A-60-38 989 discloses a solid-state image pickup device and its manufacture. The device has an array of microlenses having convex outer curved surfaces, formed on a planar resin coating layer over respective light-receiving elements. EP-A-0 511 404, corresponding to WO-A-92/09105 published on 29th May 1992, represents a state of the art under Article 54(3) EPC for the designated states DE, FR, GB of the present application. This document discloses a solid-state imaging device (and the method of its manufacture) in which a plurality of photodiodes arranged in a matrix array are covered by respective microlenses, each microlens having different curvatures in the row and column directions of the matrix. The microlens array comprises a plurality of lower lens strip layers extending in the column direction over a plurality of the photodiodes, and a respective upper lens layer provided over each photodiode. SUMMARY OF THE INVENTION A solid-state imaging device according to this invention is defined by independent claims 1 and 2. A method, according to this invention, for producing a solid-state imaging device is defined by independent claim 4. The dependent claims relate to preferred embodiments of the invention. The invention disclosed herein makes possible achieving the objective of providing a solid-state imaging device having improved light convergence and hence improved photosensitivity. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1a is a front view of a solid-state imaging device according to an embodiment of the present invention, and Figure 1b is a side view thereof. Figures 2a , 3a and 4a are front views illustrating a production procedure of the above solid-state imaging device, and Figures 2b , 3b and 4b are side views illustrating the same. Figure 5 is a perspective view of a first lens layer 3 formed on a substrate 1 . Figure 6 is a perspective view of a second lens layer 4 formed on the first lens layer 3 . Figure 7 is a view illustrating the light convergence of the above solid-state imaging device shown in Figure 1 . Figure 8 is a front sectional view of a conventional solid-state imaging device. Figure 9a is a perspective view of a conventional strip-shaped microlens, and Figure 9b is a perspective view of a conventional rectangular microlens. Figure 10a is a perspective view of a step of production of the microlens shown in Figure 9a , and Figure 10b is a perspective view of a step of production of the microlens shown in Figure 9b . Figure 11 is a plan view of the conventional solid-state imaging device. Figure 12 is a view illustrating a curvature of a microlens. Figure 13a is a view illustrating a good light convergence, and Figure 13b is a view illustrating a poor light convergence. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described by way of illustrating embodiments with reference to the accompanying drawings. Figure 1a is a front view of a solid-state imaging device of the present embodiment, and Figure 1b is a side view thereof. The solid-state imaging device comprises a substrate 1 , a smoothing layer 2 formed on the substrate 1 , a first lens layer 3 formed on the smoothing layer 2 , and a second lens layer 4 formed on the first lens layer 3 . A top surface of the substrate 1 is ragged due to a photodiode, a register, and the like (not shown) formed thereon, but is smoothed by the smoothing layer 2 . The first and the second lens layers 3 and 4 are both formed of the same transparent resin. A microlens, which is formed of an overlapping portion of the first and the second lens layers 3 and 4 , is located above the photodiode, thereby converging light on the photodiode. Such a solid-state imaging device is manufactured in the following way. First, the photodiode, the register and the like are formed on the substrate 1 , and then the smoothing layer 2 is formed on the ragged top surface of the substrate 1 as shown in Figures 2a and 2b . If necessary, a color filter may be formed on the smoothing layer 2 , which may further be covered with a protective film. Thereafter, the first lens layer 3 is formed on the smoothing layer 2 made of a material which has a high photosensitivity and a high transparency such as acrylic resin, novolac resin, polystyrene and epoxy resin, since the first lens layer 3 constitutes a portion of the microlens as mentioned above. The first lens layer 3 is arranged in strips as shown in Figures 3a (front view) and 3b (side view) by pasting the above material all over the smoothing layer 2 , exposing the material to light selectively by the use of a photomask, and then removing unnecessary portions. Acrylic resin, novolac resin,polystyrene and epoxy resin are more effective in improving the light quantity of the device than other materials such as polyimide resin. The second lens layer 4 is formed of a material such as acrylic resin, novolac resin, polystyrene and epoxy resin, which has a high photosensitivity and a high transparency as well as being softened when heated at a high temperature and then hardened when further heated. These materials are more effective in improving the light quantity than other materials such as polyimide resin. The second lens layer 4 is arranged in islands as shown in Figures 4a (front view) and 4b (side view) by, for example, pasting the above material all over the smoothing layer 2 , exposing the material to light selectively by the use of a photomask, and removing unnecessary portions. In this way, the first lens layer 3 formed in strips (Figure 5 ) is superimposed by the second lens layer 4 (Figure 6 ) formed in islands. The second lens layer 4 is extended down on side surfaces of the first lens layer 3 in a transverse direction thereof so as to be in contact with the smoothed surface of the substrate 1 . Thereafter, the second lens layer 4 is softened by heating so that the second lens layer 4 is in contact with the above side surfaces of the first lens layer 3 in the transverse direction (Figure 1a ) but is kept on a top surface of the first lens layer 3 in a longitudinal direction (Figure 1b ). If the second lens layer 4 is not formed so as to be extended on the side surfaces of the first lens layer 3 in the transverse direction, there is a possibility that the second lens layer 4 is not contacted with the substrate 1 by softening. In the construction of this embodiment, each strip of the first lens layer 3 and the second lens layer 4 on the top and the side surfaces of the above strip form a microlens. In such a microlens, each curved portion of the second lens layer 4 in a first cross section taken in a first direction is in contact with the smoothed surface of the substrate 1 , and each curved portion thereof in a second cross section taken in a second direction is in contact only with the smoothed first lens layer 3 . If the first lens layer 3 is softened by heating together with the second lens layer 4 , it is preferred that the first lens layer 3 is slightly softened by heating or the like before the second lens layer 4 is formed. In a solid-state imaging device having the above construction, the light converging state in the longitudinal and the transverse directions can be aligned as will be explained hereinafter. In Figure 7 , the height of the second lens layer 4 in the first cross section is H1 . The height of the second lens layer 4 in the second cross section, namely, a portion which virtually functions as a lens in the second cross section is H2 . The height H2 is adjusted by changing the thickness t of the first lens layer 3. By determining the thickness t based on the height H1 , the width W1 of the first cross section and the width W2 of the second cross section, it is possible to align curves of the first and the second cross sections. In other words, it is possible to uniformize curvatures in the longitudinal and the transverse directions. Consequently, the light converging state in the two directions can be aligned, thereby providing the microlens with a sufficiently high photosensitivity. Practically, the photosensitivity can be improved by several tens of percent. Although the second lens layer 4 is formed in islands in the above embodiment, the second lens layer 4 may be formed in strips which are perpendicular to the strips of the first lens layer 3 . The second lens layer 4 may be softened by irradiating microwave, infrared rays, ultraviolet rays, far-ultraviolet rays or electronic beams instead of heating. In such a case, the second lens layer 4 is formed of a material which is softened by the above-mentioned irradiation, and once softened, the second lens layer 4 should be hardened again.

Claims: A solid-state imaging device having a plurality of microlenses which respectively overlie a plurality of light-sensitive regions (12) arranged on a substrate (1), wherein each of said microlenses (13) comprises a first lens layer (3) formed over the corresponding light-sensitive region as a strip, and a second lens layer (4) formed above said corresponding light-sensitive region as an island extending transversely over said first lens layer between portions of said substrate on either side of said strip, said second lens layer having a convex outer curved surface providing uniform curvature in the longitudinal and transverse directions of said strip of said first lens layer (3). A solid-state imaging device having a plurality of microlenses which respectively overlie a plurality of light-sensitive regions arranged on a substrate, wherein each of said microlenses comprises a first lens layer formed over the corresponding light-sensitive region as a strip, and a second lens layer formed above said corresponding light-sensitive region as a strip extending transversely over said first lens layer, said second lens layer having a convex outer curved surface providing uniform curvature in the longitudinal and transverse directions of said strip of said first lens layer. A device according to claim 1 or claim 2, wherein at least one of the first lens layer (3) and the second lens layer (4) is made of a material selected from the group consisting of acrylic resin, novolac resin, polystyrene, and epoxy resin. A method for producing a solid-state imaging device having a plurality of microlenses (13) which respectively overlie a plurality of light-sensitive regions (12) arranged on a substrate (1), the method comprising: forming a plurality of first lens layers over said plurality of light-sensitive regions, each of said first lens layers being formed as a strip (3); forming a plurality of second lens layers above said light-sensitive regions, each of said second lens layers being formed as an island (4) extending transversely over said first lens layer between portions of said substrate on either side of said strip; and softening each of said second lens layers (4) so that each of said second lens layers has a convex outer curved surface providing uniform curvature in the longitudinal and transverse directions of said strip (3) of said first lens layer. A method according to claim 4, wherein at least one of the first lens layer (3) and the second lens layer (4) is made of a material selected from the group consisting of acrylic resin, novolac resin, polystyrene, and epoxy resin.

Inventors: NAKAI JUNICHI, NAKAI, JUNICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L31
IPC codes: H01L31, G02B3, H04N5, H01L27