Optical sensors comprising an array of photodiodes

Abstract

Optical sensor comprising: an arrangement of photodiodes (2) comprising a first photodiode (2a) with a first optically active region (6) surrounded by a first edge region (26) and a second photodiode (2b) with a second optically active region (6) surrounded by a second edge region (26), wherein the first edge region (26) and the second edge region (26) do not overlap, and a metal layer (8) comprising a plurality of metal wires (12) arranged in the first edge region (26) of the first photodiode (2a) and in the second edge region (26) of the second photodiode (2b), wherein the first photodiode (2a) is directly electrically connected to each metal wire of a first subset of metal wires (12a, 12b) of the plurality of metal wires (12) and the second photodiode (2b) is directly electrically connected to each metal wire of a second, distinct subset of metal wires (12c, 12d) of the plurality of is connected to metal wires (12).

Description

TECHNICAL AREA The invention relates to optical sensors comprising an arrangement of photodiodes. GENERAL STATE OF THE ART An optical sensor can comprise an array of photodiodes for detecting incident light. Each photodiode has an optically active area in the center, surrounded by a rim for wiring. Preferably, to prevent crosstalk between photodiodes, the optical sensor can include a light shield in the rim. The closer the light shield is to the photodiode, the more effective it is. US 7,683,407 B1 discloses a pixel cell and an imaging device, as well as a method for manufacturing them, wherein the pixel cell has several metallization and through-layers formed over a photosensitive area. The metallization and through-layers form a step-like light tunnel structure that increases the ability of the photosensitive area to capture incident light. Gäbler, D., Henkel Ch., Thiele S.: “CMOS integrated UV-photodiodes” In: 30th Eurosensors Conference EUROSENSORS 2016, 4-7 September 2016, Budapest, Hungary. Amsterdam: Elsevier, 2-16 (Procedia Engineering; 168) pp. 1208-1213. ISSN 187-7058 reveals highly sensitive UV photodiodes integrated into a versatile analog 0.18-micrometer high-voltage CMOS technology. SUMMARY Aspects of the present invention relate to an optical sensor and a method for manufacturing an optical sensor, as set out in the attached claims. Preferred embodiments of the invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic cross-section of a photodiode in an optical sensor; Fig. 2 shows a schematic top-view cross-section of a portion of an optical sensor according to one embodiment, comprising a row of four photodiodes; Fig. 3 shows a schematic top-view cross-section of a portion of an optical sensor according to another embodiment; Fig. 4 shows a schematic top-view cross-section of a portion of an optical sensor according to one embodiment, wherein the photodiodes are arranged in stepped diagonal lines; and Fig. 5 shows a schematic top-view cross-section of a portion of an optical sensor according to another embodiment, wherein a row of four photodiodes comprises four separate input / output channels. DETAILED DESCRIPTION One problem with optical sensors is the need to shield the edge (i.e., the border region surrounding the optically active area at the center of the photodiode) of each photodiode from incident light. A light shield (i.e., a light-tight structure) can be provided in one of the metal layers. The closer the light shield is to the silicon, the better the shielding effect. However, in an array of photodiodes, a metal layer (i.e., the bottommost metal layer of the backend stack) must be used for wiring to connect to the photodiodes. Consequently, metal layer 2 (i.e., the second-bottom metal layer of the backend stack) is the metal layer closest to the silicon that can be used to provide a light shield. Because the light shield must be light-tight (no holes are permitted), electrical signals cannot be routed to or from the photodiodes above the light shield.If the light shielding is arranged in metal 2, no signals from metal 3 or above can be directed to the photodiodes. Fig. 1 shows a schematic cross-section of a photodiode 2. The photodiode 2 comprises a substrate 4, which includes an optically active region 6 defined by doped areas providing one or more pn junctions for converting incident light into electrical signals. The first metal layer 8 (metal 1) is separated from the substrate 4 by an interdielectric layer 10 (comprising, for example, silicon oxide). The first metal layer 8 includes a plurality of metal wires 12 for connecting to the photodiode 2. Contacts 14 connect to the wires 12 to output the electrical signals. The photodiode 2 further includes an ultraviolet (UV) window 16 for receiving incident light in the optically active region 6. The second metal layer 18 (metal 2) includes a light shield 20 for blocking light 22 incident on the edge region of the photodiode 2. There may be additional metal layers (e.g., metal 3) that are not shown.A passivation layer 24 (e.g. a silicon nitride layer) covers the backend stack, except in the UV window 16. Wiring the electrical signals from several different photodiodes should be achieved within a single metal layer (no crossing is possible). Consequently, all signals in a row or column of photodiodes must pass through all photodiodes in that row or column. For example, in a 4x4 array, each row or column might need to carry four different signals. Fig. 2 shows a top-view cross-section of part of an optical sensor according to one embodiment. The optical sensor comprises a row of four photodiodes 2a, 2b, 2c, and 2d connected by four metal wires 12a, 12b, 12c, and 12d to provide input and output signals for the photodiodes 2a, 2b, 2c, and 2d. Each photodiode 2a, 2b, 2c, and 2d comprises an optically active area 6 for detecting incident light and a border area 26 surrounding the optically active area 6. The metal wires 12a, 12b, 12c, and 12d run through the border area 26 of each photodiode 2a, 2b, 2c, and 2d in a row. The first (leftmost) photodiode 2a includes contacts 28a in the edge region 26, which connect the photodiode 2a to the first and second metal wires 12a and 12b. The first and second metal wires 12a and 12b are similarly arranged in the edge region of the second (adjacent) photodiode 2b.However, the second photodiode 2b has no contacts arranged to connect the first and second metal wires 12a and 12b to the second photodiode 2b, and thus any signals to or from the first photodiode 2a bypass the second photodiode 2b. The second photodiode 2b has contacts 28b that connect it to the third and fourth metal wires 12c and 12d, which can thereby provide input and output signals for the second photodiode 2b. The first photodiode 2a has no contacts in the lower half of the edge region 26 and is therefore not connected to the third and fourth metal wires 12c and 12d. Consequently, signals to and / or from the second photodiode 2b bypass the first photodiode 2a. Advantageously, the four metal wires are arranged in the same metal layer (typically metal 1).The arrangement of the metal wires 12a, 12b, 12c, and 12d, in combination with the placement of the contacts 28a and 18b of adjacent photodiodes, allows for individual inputs and outputs for the photodiodes 2a and 2b in a single line, without requiring signals to be routed through different metal layers. Each photodiode 2a, 2b, 2c, and 2d preferably includes a light shield (not shown), which is a light-tight structure that covers the edge region 26 while leaving the optically active region 6 exposed. Preferably, the light shield is arranged in metal 2, which is possible if all the wiring of the photodiodes 2a, 2b, 2c, and 2d is arranged in metal 1. The third photodiode 2c includes contacts in the upper half of the edge region 26, thus connecting it to the first and second metal wires 12a and 12b. The first and third photodiodes 2a and 2c are therefore connected to each other, and the output signals on the first and second metal wires 12a and 12b will be the sum of the signals from the first and third photodiodes 2a and 2c. Similarly, the fourth photodiode 2d includes contacts 28d in the lower half of the edge region 26 and is connected to the third and fourth metal wires 12c and 12d, so that the output on these wires is the sum of the signals from the second and fourth photodiodes 2b and 2d. Fig. 3 shows a top-view cross-section of two photodiodes 2a and 2b arranged side by side in an optical sensor according to another embodiment. Each photodiode comprises an optically active area 6 and a border area 26 surrounding the optically active area 6. Four metal wires 12a, 12b, 12c, and 12d are arranged in the border area 26 of the two photodiodes 2a and 2b. The photodiodes 2a and 2b have a substantially square shape and can be divided into four quadrants Q1, Q2, Q3, and Q4. The first photodiode 2a comprises contacts in three quadrants, which are Q1, Q2, and Q4, while the second photodiode 2b comprises contacts in quadrants Q2, Q3, and Q4. This means that the first photodiode 2a is connected to the first and second wires 12a and 12b, while the second photodiode is connected to the third and fourth metal wires 12c and 12d, so that the two diodes are not connected to the same wires.An advantage of this embodiment is the larger area over which contacts 28a and 28b can be placed. Each photodiode 2a and 2b has three quadrants in which contacts can be arranged to connect to the appropriate metal wires 12a, 12b, 12c, and 12d. In an even row or column, only two quadrants per diode can be used for connection to the wires, while the signals from adjacent diodes are kept separate. Fig. 4 shows an array 30 of photodiodes comprising photodiodes of two types (Type 1 and Type 2). The array includes a first photodiode 2a of the first type (Type 1) and a second photodiode 2b of the second type (Type 2), which can be the first and second photodiodes 2a and 2b shown in Fig. 3. Photodiodes of the first type (Type 1) include contacts 28a connecting to a first and second metal wire 12a and 12b. Photodiodes of the second type (Type 2) include contacts 28b connecting to a third and a fourth metal wire 12c and 12d. The metal wires 12a, 12b, 12c, and 12d run diagonally through the array and are located in the same metal layer (typically Metal 1). Each diagonal "staircase" of photodiodes in the array can carry two signals. Each photodiode in the arrangement preferably includes a light shield (not shown) for shielding the edge region of each diode from light. Fig. 5 shows a schematic representation of a row of four photodiodes 2a, 2b, 2c, and 2d in an optical sensor according to one embodiment. The row comprises four metal wires 12a, 12b, 12c, and 12d, which represent four separate channels (one for each photodiode 2a, 2b, 2c, and 2d in the row). The metal wires 12a, 12b, 12c, and 12d are arranged in the same metal layer in the edge region 26 of each photodiode 2a, 2b, 2c, and 2d and do not cover the optically active area 6. The first (leftmost) photodiode 2a comprises two contacts 28a, which connect the photodiode 2a to the first metal wire 12a. The first photodiode 2a has no contacts arranged to connect to the other metal wires 12b, 12c, and 12d. Consequently, input and output signals for the first photodiode 2a are transmitted only on the first metal wire 12a.Similarly, the second photodiode 2b in the row has contacts 28b that connect to the second metal wire 12b, the third photodiode 2c in the row has contacts 28c that connect to the third metal wire 12c, and the fourth (farthest right) photodiode 2d has contacts 28d that connect to the fourth metal wire 12d. Therefore, each photodiode 2a, 2b, 2c, and 2d in the row has an associated metal wire 12a, 12b, 12c, and 12d, respectively, which is determined by the placement of contacts 28a, 28b, 28c, and 28d. In general, embodiments described herein provide an optical sensor comprising an arrangement of photodiodes, the first and second photodiodes each comprising an optically active region surrounded by a boundary region, and a metal layer comprising a plurality of metal wires, each metal wire being arranged in the boundary region of the first photodiode and in the boundary region of the second photodiode, the first photodiode being connected to a first subset of metal wires of the plurality of metal wires, and the second photodiode being connected to a second, distinct subset of metal wires of the plurality of metal wires. A subset of metal wires comprises one or more metal wires. The embodiments can thereby provide a configuration of metal wires in the same metal layer that allows signals from each photodiode to be transmitted to other photodiodes in, for example,The photodiodes are passed by a row or column of the arrangement. In preferred embodiments, the photodiodes are brought directly into contact with the metal wires without any transistors or other intervening components. The optical sensor can include a third photodiode next to the second photodiode, electrically connected to the first subset of metal wires but not to the second. The optical sensor can also include a fourth photodiode next to the third photodiode, electrically connected to the second subset of metal wires but not to the first. In this case, every second photodiode in a row or column is interconnected and provides summed output signals. For example, an 8x8 array can have four channels in each row, with each channel connected to two photodiodes in that row. Alternatively, each photodiode in a row or column can have a separate channel (i.e., a separate subset of metal wires for inputs and outputs). The optical sensor can include a third photodiode next to the second photodiode and electrically connected to a third subset of metal wires from the plurality of metal wires, different from the first and second subsets of metal wires, with the third subset of metal wires being located in the boundary regions of the first, second, and third photodiodes.The optical sensor can further comprise a fourth photodiode next to the third photodiode and electrically connected to a fourth subset of metal wires from the plurality of metal wires, different from the first, second and third subset of metal wires, wherein the fourth subset of metal wires is arranged in the boundary regions of the first, second, third and fourth photodiode. The first, second, and third photodiodes can be arranged in a row, a column, or a stepped diagonal. In each case, metal wires connecting to one of the photodiodes must pass by the other two photodiodes, as the metal wires are confined to a single metal layer. When the photodiodes are arranged in a stepped diagonal line, each photodiode can have contacts in three quadrants of the photodiode. The metal layer is typically the first metal layer (Metal 1) of a complementary metal-oxide-semiconductor (CMOS) backend stack. The optical sensor typically includes a second metal layer that contains a light shield. The light shield is preferably located in Metal 2 (above Metal 1, which contains the wiring). The light shield can be a continuous metal layer in the edge region of the first photodiode, but it does not cover the optically active area.The sensor may further comprise an interdielectric layer beneath the metal layer (which insulates the metal layer from the underlying silicon) and contacts in the interdielectric layer which connect the multitude of metal wires to the arrangement of photodiodes. The embodiments described herein also provide a method for use in the manufacture of an optical sensor, wherein the method comprises providing an arrangement of photodiodes comprising a first and a second photodiode, each comprising an optically active region and a boundary region, and providing a metal layer comprising a plurality of metal wires arranged in the boundary regions of the first and second photodiodes, wherein the first photodiode is connected to a first subset of metal wires of the plurality of metal wires, and wherein the second photodiode is connected to a second, different subset of metal wires of the plurality of metal wires. The step of providing an array of photodiodes can include providing a substrate that contains the array of photodiodes. The step of providing the metal layer can include depositing an interdielectric layer, forming contacts in the interdielectric layer to provide contacts for the underlying array of photodiodes, depositing the metal layer onto the interdielectric layer, and patterning the metal layer to form the multitude of metal wires. The process can involve depositing a second metal layer over the first and patterning the second metal layer to form a light shield at the edge of the first photodiode and at the edge of the second photodiode. The deposition and patterning steps of the second metal layer are typically performed in a complementary metal-oxide-semiconductor (CMOS) backend-of-line (BEOL) process.

Claims

Optical sensor comprising: an arrangement of photodiodes (2) comprising a first photodiode (2a) with a first optically active region (6) surrounded by a first edge region (26) and a second photodiode (2b) with a second optically active region (6) surrounded by a second edge region (26), wherein the first edge region (26) and the second edge region (26) do not overlap, and a metal layer (8) comprising a plurality of metal wires (12) arranged in the first edge region (26) of the first photodiode (2a) and in the second edge region (26) of the second photodiode (2b), wherein the first photodiode (2a) is directly electrically connected to each metal wire of a first subset of metal wires (12a, 12b) of the plurality of metal wires (12) and the second photodiode (2b) is directly electrically connected to each metal wire of a second, distinct subset of metal wires (12c, 12d) of the plurality of is connected to metal wires (12). Optical sensor according to claim 1, wherein the first and second photodiodes (2a, 2b) are nearest neighbors in the arrangement of photodiodes. Optical sensor according to claim 1 or 2, further comprising a third photodiode (2c) next to the second photodiode (2b) and electrically connected to the first subset of metal wires (12a, 12b) but not to the second subset of metal wires (12c, 12d). Optical sensor according to claim 3, further comprising a fourth photodiode (2d) next to the third photodiode (12c) and electrically connected to the second subset of metal wires (12c, 12d) but not to the first subset of metal wires (12a, 12b). Optical sensor according to claim 1 or 2, further comprising a third photodiode (2c) located next to the second photodiode (2b) and electrically connected to a third subset of metal wires (12c) of the plurality of metal wires, different from the first and second subset of metal wires (12a, 12b), wherein the third subset of metal wires (12c) is arranged in the boundary regions (26) of the first, second and third photodiode (2a, 2b, 2c). Optical sensor according to claim 5, further comprising a fourth photodiode (2d) located next to the third photodiode (2c) and electrically connected to a fourth subset of metal wires (12d) of the plurality of metal wires (12), different from the first, second and third subset (12a, 12b, 12c) of metal wires (12), wherein the fourth subset of metal wires (12d) is arranged in the boundary regions (26) of the first, second, third and fourth photodiode (2a, 2b, 2c, 2d). Optical sensor according to one of claims 3 to 6, wherein the first, second and third photodiode (2a, 2b, 2c) are arranged in a row or a column. Optical sensor according to one of claims 3 to 6, wherein the first, second and third photodiode (2a, 2b, 2c) are arranged in a stepped diagonal line. Optical sensor according to one of the preceding claims, wherein the metal layer (8) is the first metal layer, metal 1, of a complementary metal oxide semiconductor, CMOS, backend stack. Optical sensor according to one of the preceding claims, further comprising a second metal layer (18) comprising a light shield (20). Optical sensor according to claim 10, wherein the light shielding (20) comprises a continuous metal layer in the edge regions (26) of the first and second photodiode (2a, 2b). Optical sensor according to one of the preceding claims, further comprising an interdielectric layer (10) and contacts (14) in the interdielectric layer, which connect the plurality of metal wires (12) to the arrangement of photodiodes (2). A method for manufacturing an optical sensor, the method comprising: providing an arrangement of photodiodes comprising a first photodiode (2a) with a first optically active region (6) surrounded by a first edge region (26) and a second photodiode (2b) with a second optically active region (6) surrounded by a second edge region (26), wherein the first edge region (26) and the second edge region (26) do not overlap; and providing a metal layer (8) comprising a plurality of metal wires (12) arranged in the first edge region (26) of the first photodiode (2a) and in the second edge region (26) of the second photodiode (2b), wherein the first photodiode (2a) is directly electrically connected to each metal wire of a first subset of metal wires (12a, 12b) of the plurality of metal wires (12); and wherein the second photodiode (2b) is directly electrically connected to each metal wire of a second subset of metal wires (12a, 12b).different subset of metal wires (12c, 12d) of the plurality of metal wires (12) is connected. Method according to claim 13, wherein the step of providing an arrangement of photodiodes comprises providing a substrate (4) comprising the arrangement of photodiodes. The method of claim 14, wherein the step of providing the metal layer (8) comprises: depositing an interdielectric layer (10) on the substrate (4), forming contacts (14) in the interdielectric layer (10) to provide contacts (14) for the underlying arrangement of photodiodes, depositing the metal layer (8) on the interdielectric layer (10) and patterning the metal layer (8) to form the plurality of metal wires (12). The method of claim 15, further comprising depositing a second metal layer (18) over the first metal layer (8) and patterning the second metal layer (18) to form a light shield (20) in the first edge region (26) and in the second edge region (26). Method according to claim 14 or 15, wherein the steps of depositing and patterning the second metal layer (18) are carried out in a complementary metal oxide semiconductor, CMOS, backend-of-line, BEOL process.

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