Photosensor comprising a photodiode array

Abstract

A photosensor comprising: a photodiode array comprising a first photodiode and a second photodiode each comprising a photoactive region and a peripheral region; and a metal layer comprising a plurality of metal traces disposed in the peripheral regions of the first and second photodiodes, wherein the first photodiode is connected to a first subset of the plurality of metal traces, and wherein the second photodiode is connected to a second, different subset of the plurality of metal traces.

Description

TECHNICAL FIELD

[0001] The present invention relates to a light sensor comprising an array of photodiodes. BACKGROUND

[0002] A light sensor can comprise an array of photodiodes for detecting incident light. Each photodiode has a light active region in the center surrounded by a peripheral region for wiring. Preferably, to avoid cross-talk between the photodiodes, the light sensor can comprise a light shield in the peripheral region. The closer the light shield is positioned to the photodiode, the more effective the light shield. SUMMARY

[0003] Aspects of the invention provide a light sensor and a method of using in the manufacture of a light sensor as specified in the accompanying claims.

[0004] A preferred embodiment of the invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0005] Figure 1 A schematic cross-sectional view of a photodiode in a light sensor is shown;

[0006] Figure 2 A schematic top cross-sectional view of a portion of a light sensor according to an embodiment having a row of four photodiodes is shown;

[0007] Figure 3 A schematic top cross-sectional view of a portion of a light sensor according to another embodiment is shown;

[0008] Figure 4 A schematic top cross-sectional view of a portion of a light sensor according to another embodiment is shown, wherein the photodiodes in the array are arranged along a stepped diagonal; and

[0009] Figure 5 A schematic top cross-sectional view of a portion of a light sensor according to another embodiment is shown, wherein a row of four photodiodes has four separate input / output channels. DETAILED DESCRIPTION

[0010] One problem with using light sensors is the need to shield the peripheral region of each photodiode (i.e. the peripheral region surrounding the light active region at the center of the 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 provided to the silicon, the better the shielding effect. However, in a photodiode array, metal 1 (i.e. the lowest metal layer of the back-end stack) is needed for wiring to connect to the photodiodes. Therefore, metal 2 (i.e. the second lowest metal layer of the back-end stack) is the closest metal layer to the silicon that can be used to provide the light shield. Because the light shield needs to be a light-tight part (no holes are allowed), electrical signals cannot be transmitted to or from the photodiodes above the light shield. If the light shield is in metal 2, signals from or above metal 3 cannot be transmitted to the photodiodes.

[0011] Figure 1 A schematic cross-sectional view of a photodiode 2 is shown. The photodiode 2 comprises a substrate 4 including a light active region 6 defined by a doped region, providing one or more pn junctions for converting incident light into an electrical signal. A first metal layer 8 (metal 1) is separated from the substrate 4 by an interlayer dielectric layer 10 (e.g. comprising silicon oxide). The first metal layer 8 comprises a plurality of metal wires 12 for connecting to the photodiode 2. A contact 14 is connected to the wires 12 to output the electrical signal. The photodiode 2 further comprises an ultraviolet (UV) window 16 for receiving incident light into the light active region 6. A second metal layer 18 (metal 2) comprises a light shield 20 for blocking light 22 incident on the peripheral region of the photodiode 2. There can also be a not shown metal layer (e.g. metal 3). A passivation layer 24 (e.g. a silicon nitride layer) covers the back-end stack spaced apart from the UV window 16.

[0012] Wiring of the electrical signals of a plurality of different photodiodes should be implemented in a single metal layer (crossing is not possible). Therefore, all signals of a row or column in a photodiode array have to pass through all photodiodes of that row or column. For example, in a 4x4 array, each row or column can need to carry four different signals.

[0013] Figure 2A top sectional view of a portion of a photosensor according to an embodiment is shown. The photosensor comprises a row of four photodiodes 2a, 2b, 2c and 2d connected by four metal wires 12a, 12b, 12c and 12d for providing input and output signals to the photodiodes 2a, 2b, 2c and 2d. Each photodiode 2a, 2b, 2c and 2d comprises a photoactive region 6 for detecting incident light, and a peripheral region 26 around the photoactive region 6. The metal wires 12a, 12b, 12c and 12d pass through the peripheral region 26 of each photodiode 2a, 2b, 2c and 2d in the row. The first (leftmost) photodiode 2a comprises a contact 28a in the peripheral region 26 for connecting the photodiode 2a to the first metal wire 12a and to the second metal wire 12b. The first metal wire 12a and the second metal wire 12b are similarly provided in the peripheral region 26 of the second (adjacent) photodiode 2b. However, the second photodiode 2b does not comprise any contacts provided to connect the first and second wires 12a, 12b to the second photodiode 2b, so any signals from or to the first photodiode 2a bypass the second photodiode 2b. The second photodiode 2b comprises a contact 28b connecting the second photodiode 2b to the third metal wire 12c and the fourth metal wire 12d, by which input and output signals can be provided to the second photodiode 2b. The first photodiode 2a does not comprise a contact in the lower half of the peripheral region 26, so the first photodiode 2a is not connected to the third and fourth metal wires 12c, 12d. Thus, signals to and / or from the second photodiode 2b bypass the first photodiode 2a. Advantageously, the four metal wires are provided in the same metal layer (typically metal 1). The layout of the metal wires 12a, 12b, 12c and 12d in combination with the arrangement of the contacts 28a and 28b of the adjacent photodiodes enables separate input and output to the photodiodes 2a and 2b in the row without the need to transmit signals in different metal layers. Each photodiode 2a, 2b, 2c and 2d preferably comprises a light shield (not shown) as a light-tight structure covering the peripheral region 26 while leaving the photoactive region 6 exposed. Preferably, the light shield is provided in metal 2, which is possible when all the wiring of the photodiodes 2a, 2b, 2c and 2d is provided in metal 1.

[0014] The third photodiode 2c comprises contacts in the upper half of the peripheral region 26, thereby connecting to the first and second metal wires 12a and 12b. The first and third diodes 2a and 2c are thereby connected together, and the output signal 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 comprises contacts 28d in the lower half of the peripheral 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.

[0015] Figure 3 A top sectional view of two photodiodes 2a and 2b arranged next to each other in a light sensor according to another embodiment is shown. Each photodiode comprises a photoactive region 6, and a peripheral region 26 surrounding the photoactive region 6. Four metal wires 12a, 12b, 12c and 12d are arranged in the peripheral region 6 of the two photodiodes 2a and 2b. The photodiodes 2a and 2b have essentially a square shape and can be divided into four quadrant regions Ql, Q2, Q3 and Q4. The first photodiode 2a comprises contacts 28 in the three quadrant regions Ql, Q2 and Q4, while the second photodiode 2b comprises contacts in the quadrant regions Q2, Q3 and Q4. Thus, 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 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 where the contacts 28a and 28b can be placed. Each photodiode 2a and 2b has three quadrant regions where contacts can be arranged to be connected to the appropriate metal wires 12a, 12b, 12c and 12d. In a straight row or column, only two quadrant regions per diode can be used for connecting to the wires, while separating the signals from the adjacent diodes.

[0016] Figure 4 An array 30 of photodiodes comprising two types (type 1 and type 2) of photodiodes is shown. The array comprises a first photodiode 2a of a first type (type 1) and a second photodiode 2b of a second type (type 2), which can be Figure 3The first and second photodiodes 2a, 2b are shown. The first type (type 1) of photodiode comprises contact portions 28a connected to the first and second metal wires 12a, 12b. The second type (type 2) of photodiode comprises contact portions 28b connected to the third and fourth metal wires 12c, 12d. The metal wires 12a, 12b, 12c and 12d run diagonally through the array and are provided in the same metal layer (typically metal 1). The diagonal "step" of each photodiode in the array can carry two signals. Each photodiode in the array preferably comprises a light shield (not shown) for shielding the peripheral region of each diode from light.

[0017] Figure 5 A schematic diagram of a row of four photodiodes 2a, 2b, 2c and 2d in a light sensor according to one embodiment is shown. The row comprises four metal wires 12a, 12b, 12c and 12d representing four separate channels (one channel per photodiode 2a, 2b, 2c and 2d in the row). The metal wires 12a, 12b, 12c and 12d are provided in the same metal layer in the peripheral region 26 of each photodiode 2a, 2b, 2c and 2d and do not cover the light active region 6. The first (leftmost) photodiode 2a comprises two contact portions 28a connecting the first photodiode 2a to the first metal wire 12a. The first photodiode 2a does not comprise any contact portions provided to connect to the other metal wires 12b, 12c and 12d. Thus, the input and output signals of the first photodiode 2a are transmitted only on the first metal wire 12a. Similarly, the second photodiode 2b in the row comprises contact portions 28b connecting to the second metal wire 12b, the third photodiode 2c in the row comprises contact portions 28c connecting to the third metal wire 12c and the fourth (rightmost) photodiode 2d comprises contact portions 28d connecting to the fourth metal wire 12d. Thus, each photodiode 2a, 2b, 2c and 2d in the row has a dedicated metal wire 12a, 12b, 12c and 12d determined by the arrangement of the contact portions 28a, 28b, 28c and 28d.

[0018] In summary, embodiments described herein provide a photosensor comprising: a photodiode array comprising a first photodiode and a second photodiode each comprising a photoactive region surrounded by a peripheral region; and a metal layer comprising a plurality of metal wires, each metal wire disposed in the peripheral region of the first photodiode and disposed in the peripheral region of the second photodiode, 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 subset of metal wires comprises one or more metal wires. Embodiments can thereby provide a layout of metal wires in the same metal layer that allows signals from each photodiode to be transmitted past other photodiodes in, for example, a row or column in the array. In preferred embodiments, the photodiodes directly contact the metal wires without any transistors or other intermediate devices.

[0019] The photosensor can comprise: a third photodiode immediately adjacent to the second photodiode and electrically connected to the first subset of metal wires but not to the second subset of metal wires. The photosensor can further comprise: a fourth photodiode immediately adjacent to the third photodiode and electrically connected to the second subset of metal wires but not to the first subset of metal wires. In this case, each of the other photodiodes in the row or column are connected together and provide a summed output signal. For example, an 8x8 array can comprise: four channels in each row, wherein each channel is associated with two photodiodes in that row.

[0020] Alternatively, each photodiode in the row or column can have a separate channel (i.e. a separate subset of metal wires for input and output). The photosensor can comprise: a third photodiode immediately adjacent to the second photodiode and electrically connected to a third subset of metal wires of the plurality of metal wires that is different from the first subset of metal wires and the second subset of metal wires, wherein the third subset of metal wires is disposed in the peripheral regions of the first photodiode, the second photodiode, and the third photodiode. The photosensor can further comprise: a fourth photodiode immediately adjacent to the third photodiode and electrically connected to a fourth subset of metal wires of the plurality of metal wires that is different from the first subset of metal wires, the second subset of metal wires, and the third subset of metal wires, wherein the fourth subset of metal wires is disposed in the peripheral regions of the first photodiode, the second photodiode, the third photodiode, and the fourth photodiode.

[0021] The first, second, third and fourth photodiodes can be arranged along a row or column or along a staircase diagonal. In each case, a metal wire connected to one of the photodiodes has to pass the other two photodiodes, since the metal wire is bound to one metal layer. When the photodiodes are arranged along a staircase diagonal, each photodiode can comprise a contact in three quadrant regions of the photodiode. The metal layer is typically the first metal layer (metal 1) of a complementary metal-oxide-semiconductor (CMOS) back-end stack. The photosensor typically comprises a second metal layer comprising a light shield. The light shield is preferably in metal 2 (above metal 1 comprising wiring). The light shield can comprise a continuous metal layer in a peripheral region of the first photodiode that does not cover the light active region. The sensor can further comprise an interlayer dielectric layer below the metal layer (insulating the metal layer from the underlying silicon) and a contact in the interlayer dielectric layer connecting a plurality of metal wires to the array of photodiodes.

[0022] The embodiments described herein also provide a method for use in fabricating a photosensor, the method comprising: providing an array of photodiodes comprising a first photodiode and a second photodiode, the first and second photodiodes each comprising a light active region and a peripheral region; and providing a metal layer comprising a plurality of metal wires, the plurality of metal wires being provided in the peripheral regions of the first and second photodiodes, wherein the first photodiode is connected to a first subset of the plurality of metal wires, and wherein the second photodiode is connected to a second, different subset of the plurality of metal wires.

[0023] The step of providing the array of photodiodes can comprise providing a substrate comprising the array of photodiodes. The step of providing the metal layer can comprise: depositing an interlayer dielectric layer; forming a contact in the interlayer dielectric layer for providing contact to the underlying array of photodiodes; depositing the metal layer on the interlayer dielectric layer; and patterning the metal layer to form the plurality of metal wires.

[0024] The method can comprise depositing a second metal layer above the first metal layer and patterning the second metal layer to form a light shield in the peripheral region of the first photodiode and in the peripheral region of the second photodiode. The steps of depositing and patterning the metal layer are typically performed in a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL) process.

[0025] While specific embodiments of the application have been described above, it will be appreciated that the application can be practiced otherwise than as described. The description is not intended to limit the application. Modifications can be made by those skilled in the art with the benefit of the description without departing from the scope of the claims as described below.

[0026] Each feature disclosed or shown in the present specification, whether standing alone or in any and every combination, can be included in the present application.

Claims

1. A photosensor, comprising: a photodiode array including a first photodiode and a second photodiode, the first photodiode including a first photoactive region surrounded by a first peripheral region, the second photodiode including a second photoactive region surrounded by a second peripheral region, wherein the first peripheral region and the second peripheral region do not overlap; and a metal layer including a plurality of metal wires, wherein each metal wire of the plurality of metal wires is disposed in the first peripheral region and the second peripheral region, wherein the first photodiode is directly electrically connected to a first subset of metal wires of the plurality of metal wires, and wherein the second photodiode is directly electrically connected to a second subset of metal wires of the plurality of metal wires different from the first subset of metal wires.

2. The optical sensor of claim 1, wherein, The first photodiode and the second photodiode are nearest neighboring photodiodes in the photodiode array.

3. The optical sensor of claim 1, further comprising: A third photodiode immediately adjacent to the second photodiode and directly electrically connected to the first subset of metal wires and not the second subset of metal wires.

4. The optical sensor of claim 3, further comprising: A fourth photodiode immediately adjacent to the third photodiode and directly electrically connected to the second subset of metal wires and not the first subset of metal wires.

5. The optical sensor of claim 1, further comprising: A third photodiode immediately adjacent to the second photodiode and directly electrically connected to a third subset of metal wires of the plurality of metal wires different from the first subset of metal wires and the second subset of metal wires, wherein the third subset of metal wires is disposed in peripheral regions of the first photodiode, the second photodiode, and the third photodiode.

6. The optical sensor of claim 5, further comprising: A fourth photodiode immediately adjacent to the third photodiode and directly electrically connected to a fourth subset of metal wires of the plurality of metal wires different from the first subset of metal wires, the second subset of metal wires, and the third subset of metal wires, wherein the fourth subset of metal wires is disposed in peripheral regions of the first photodiode, the second photodiode, the third photodiode, and the fourth photodiode.

7. The optical sensor of claim 3, wherein, The first photodiode, the second photodiode, and the third photodiode are arranged along a row or a column.

8. The optical sensor of claim 3, wherein, The first photodiode, the second photodiode, and the third photodiode are arranged along a stair diagonal.

9. The optical sensor of claim 1, wherein, The metal layer is a first metal layer of a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL) stack.

10. The optical sensor of claim 1, further comprising: A second metal layer including a light shield.

11. The optical sensor of claim 10, wherein, The light shield includes a continuous metal layer in the first peripheral region and the second peripheral region.

12. The optical sensor of claim 1, further comprising: An interlayer dielectric layer, and a contact in the interlayer dielectric layer connecting the plurality of metal wires to the photodiode array.

13. A method for use in fabricating a photosensor, the method comprising: providing a photodiode array including a first photodiode and a second photodiode, the first photodiode including a first photoactive region surrounded by a first peripheral region, the second photodiode including a second photoactive region surrounded by a second peripheral region, wherein the first peripheral region and the second peripheral region do not overlap; and The arrangement includes a metal layer comprising a plurality of metal wires, wherein each metal wire of the plurality of metal wires is arranged in the first and second peripheral regions, wherein the first photodiode is directly electrically connected to a first subset of metal wires of the plurality of metal wires, and wherein the second photodiode is directly electrically connected to a second subset of metal wires of the plurality of metal wires different from the first subset of metal wires.

14. The method of claim 13, wherein, The step of arranging the array of photodiodes including a first photodiode and a second photodiode comprises arranging a substrate comprising the array of photodiodes.

15. The method of claim 14, wherein, The step of arranging the metal layer comprising a plurality of metal wires comprises: depositing an interlayer dielectric layer on the substrate; forming contacts in the interlayer dielectric layer for providing contact to the underlying array of photodiodes; depositing the metal layer on the interlayer dielectric layer; and patterning the metal layer to form the plurality of metal wires.

16. The method of claim 15, further comprising: depositing a second metal layer above the first metal layer and patterning the second metal layer to form a light shield in a peripheral region of the first photodiode and in a peripheral region of the second photodiode.

17. The method of claim 14, wherein, The steps of depositing and patterning the metal layer are performed in a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL) process. The steps of depositing and patterning the metal layer are performed in a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL) process.

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