Light receiving element and method for assembling the same

Abstract

To provide: a photosensitive element capable of increasing the sensitivity and speed of the APD also for radiation of long wavelengths; and an assembly method thereof.SOLUTION: A photosensitive element comprises: a semiconductor substrate 138 having a radiation receiving area 139 through which an incident radiation 125 can enter the photosensitive element 100; a first contact region 113 connected to a first contact 102; and a second contact region 119 connected to a second contact 104. A multiplication region for multiplying the charges generated from the incident radiation 125 is formed at the first contact region 113 when a voltage is applied between the first contact 102 and the second contact 104. The first contact 102 and the second contact 104 are arranged on a side of the semiconductor substrate 138 opposite the radiation receiving area 139. A radiation reflecting area 106 is disposed on a side of the semiconductor substrate 138 opposite the radiation receiving area 139.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a light receiving element and an assembly method for providing the light receiving element.

[0002] Light-receiving elements such as photodiodes are widely used to convert radiation into an electrical signal, whereby radiation enters the photodiode and is absorbed, generating an electrical current. [Background technology]

[0003] Such photodetectors are known in the fields of safety devices for industrial automation and automotive applications. In particular, LIDAR (Light Detection and Ranging) technology is used in advanced driver assistance systems (ADAS) and is a key technology enabling autonomous driving. In these LIDAR systems, different detector styles based on different photodetectors are used. One type of photodetector is the avalanche photodiode (APD).

[0004] APDs are silicon-based semiconductor devices that can be very compact in size, have high quantum efficiency, and have relatively high gain. The internal structure of an APD includes heavily doped p-type and n-type regions formed from an intrinsic or lightly doped p-type semiconductor and contacting the cathode and anode. By using heavily doped and lightly doped deep regions of the silicon crystal, a very high internal field is maintained across the pn junction, producing high internal gain. A high external reverse bias voltage is supplied through these metal contacts. When radiation enters an APD, the high internal field creates an avalanche effect, multiplying the generated electron-hole pairs and generating an electrical signal at the output.

[0005] A typical APD is fabricated using a complementary metal-oxide semiconductor (CMOS) process in which a p-doped epitaxial layer (epi-layer) is placed on a heavily doped p-type substrate. The anode is connected to the heavily doped p-type substrate, and the cathode is placed on the opposite side of the APD. During operation, radiation enters the APD on the cathode side (front side), and the internal structure of the APD allows a generated charge to flow between the cathode and anode. These APDs are wire-bonded to corresponding carriers.

[0006] Such a CMOS-fabricated single-photon avalanche diode (SPAD) has been proposed by G. Paternoster et al. in their contribution to the 16th (Virtual) Trento workshop on Advanced Silicon Radiation Detectors, held February 16–18, 2021 (https: / / indico.cern.ch / event / 983068 / contributions / 4223039 / attachments / 2191308 / 3703680 / TREDI21_Paternoster.pdf (retrieved March 2, 2023)). A back-illuminated SPAD is proposed, in which one metal contact (cathode) is placed on the top surface of the SPAD and another metal contact (anode) is placed on the bottom surface of the SPAD. Radiation enters the SPAD from the bottom surface. Charge generated in the silicon flows vertically through the SPAD between the anode and cathode.

[0007] One drawback of current solutions is that the sensitivity and fill factor of front-illuminated APDs are limited by the wire bonding of the APD. Furthermore, thick epitaxial layers in the near-infrared spectral region force a compromise between the sensitivity and speed requirements of the APD. Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved photodetector with reduced size, improved sensitivity and speed to overcome the drawbacks of the prior art. [Means for solving the problem]

[0009] This object is solved by the subject matter of the independent claims. Advantageous embodiments of the present disclosure are the subject matter of the dependent claims.

[0010] The present disclosure is based on the idea that by improving the structure of the APD, the sensitivity and speed of the APD can be improved, even for longer wavelength radiation.

[0011] In particular, the photodetector comprises a semiconductor substrate having a radiation-receiving region through which incident radiation can enter the photodetector. The photodetector further comprises a first contact region connected to the first contact and a second contact region connected to the second contact. A multiplication region is formed in the first contact region for multiplying charge generated from the incident radiation when a voltage is applied between the first and second contacts. The first and second contacts are arranged on opposite sides of the semiconductor substrate from the radiation-receiving region. Furthermore, a radiation-reflecting region is arranged on the semiconductor substrate opposite the radiation-receiving region.

[0012] The use of a radiation-reflecting region to reflect incident radiation doubles the effective distance the radiation must travel in the semiconductor substrate. This advantageously increases the likelihood that the radiation, also known as light, will be absorbed by the substrate. This is particularly noteworthy because electromagnetic energy has different absorption depths depending on wavelength. Longer wavelengths in the red and infrared regions are absorbed deep within the substrate. Reflecting incident light allows for the absorption of longer wavelengths while advantageously reducing the thickness of the substrate. Reducing the thickness of the substrate advantageously increases the speed and sensitivity of the photodetector.

[0013] The radiation reflecting regions according to the present disclosure may be regions of different sizes that can be used to reflect radiation of various wavelengths, thereby the present invention is not limited to a particular reflecting material but may be any reflecting material, and the material used can be selected depending on the radiation wavelength to be reflected, making the present invention applicable to a wide number of applications.

[0014] The radiation receiving regions of the present invention are preferably formed from metal-free materials.

[0015] According to an advantageous further development of the present disclosure, the first contact region of the light receiving element comprises a region of different conductivity of a first conductivity type, and the second contact region comprises a region of different conductivity of a second conductivity type.

[0016] According to an advantageous further development of the present disclosure, the first contact region comprises a heavily doped first region of the first conductivity type arranged above a first well of the first conductivity type, and a second region of the first conductivity type arranged radially outward of the first region and the first well.

[0017] According to an advantageous further development of the present disclosure, the second contact region comprises a highly doped third region of the second conductivity type arranged above a second well of the second conductivity type.

[0018] Advantageously, the semiconductor substrate is a p-type substrate, the first conductivity type is n-type and the second conductivity type is p-type.

[0019] According to an advantageous further development of the present disclosure, the second contact region is at least partially surrounded by a trench, the trench including doped sidewalls of the second conductivity type. The trench advantageously provides a low path resistance and reduces the possibility of background noise. The trench including doped sidewalls can improve charge transfer efficiency. Preferably, the depth of the shallow doping of the second conductivity type on the trench sidewalls increases with the depth of the trench. Alternatively, the doping concentration on the trench sidewalls is substantially uniform at different depths of the trench.

[0020] According to an advantageous further development of the disclosure, the radiation reflecting area has a diameter D of 230 μm.

[0021] According to an advantageous further development of the present disclosure, an anti-reflection coating (ARC) or Bragg filter is arranged below the semiconductor substrate in the radiation-receiving region. The anti-reflection coating advantageously improves the radiation transparency of the radiation-receiving region. Furthermore, the ARC can be used to improve or control the absorption properties of the semiconductor substrate at a desired wavelength. Advantageously, the properties of the ARC can be selected depending on the wavelength of interest.

[0022] According to an advantageous further development of the disclosure, a heavily doped fourth region of the second conductivity type is arranged between the semiconductor substrate and the antireflection coating or Bragg filter, which advantageously improves the charge transfer efficiency to the multiplication region.

[0023] According to an advantageous further development of the present disclosure, the semiconductor substrate comprises silicon. The use of a thin layer of silicon has the favorable effect of making it possible to reduce the breakdown voltage.

[0024] According to an advantageous further development of the present disclosure, the photodetector is a linear-mode avalanche photodiode fabricated on the basis of bipolar technology. Fabrication of the photodetector on the basis of bipolar technology is advantageous because it uses a silicon substrate and does not require the use of a thick epitaxial (EPI) layer. This advantageously overcomes the drawbacks of conventional solutions using EPI layers, which require a compromise between the sensitivity and speed of the photodetector.

[0025] According to an advantageous further development of the disclosure, the radiation reflecting region comprises a metal and / or a metal alloy.

[0026] According to an advantageous further development of the present disclosure, the semiconductor substrate has a depth that is smaller than the total absorption depth of wavelengths in the red-infrared spectrum. The particularly small depth of the substrate allows for a small package size of the photodetector. At the same time, the idea of ​​using radiation reflecting areas allows these wavelengths to still be absorbed in a thin substrate.

[0027] According to an advantageous further development of the disclosure, a third well of the second conductivity type is arranged below the first contact region and is completely surrounded by the semiconductor substrate.

[0028] According to an advantageous further development of the present disclosure, the photodetector has a sensitivity of more than 70% (A / W) in the spectral range from 800 nm to 930 nm. In particular, the photodetector advantageously has a peak sensitivity of 73% (A / W) at a wavelength of 905 nm. The term sensitivity in this context relates to the amount by which an object reacts when receiving a photon. Exemplarily, a sensitivity value can be reached with a semiconductor substrate thickness of 30 μm.

[0029] The present invention further relates to an assembly method for providing a light receiving element according to an advantageous development of the present disclosure, further comprising the step of assembling the light receiving element on a carrier, the carrier being arranged on the side of the light receiving element opposite the radiation receiving area.

[0030] The advantageous structure of the photodetector allows the device to be assembled onto a carrier via flip-chip bonding. This assembly method increases reliability and reduces the package size of the device. Furthermore, the assembled photodetector can be easily integrated into existing solutions for further processing. Additionally, using flip-chip bonding instead of the standard wire-bonding assembly method has an advantageous effect on the signal-to-noise ratio (SNR) and fill factor of the photodetector.

[0031] For a better understanding of the present disclosure, the present disclosure will be described in more detail using examples shown in the following drawings, in which identical parts are designated by identical reference numerals and part names. Furthermore, some features or combinations of features of the various examples shown and described may also represent independent solutions, solutions of the present invention, or solutions according to the present disclosure. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 illustrates a photodetector according to the present disclosure. [Figure 2] FIG. 2 is a diagram showing the arrangement of light receiving elements according to the present disclosure. [Figure 3] FIG. 1 illustrates an assembled photodetector according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0033] Further, a preferred embodiment of the present invention will be described with reference to FIG.

[0034] 1 illustrates a photodetector 100 according to the present disclosure. The photodetector 100 includes a semiconductor substrate 138 including a first contact region 113 and a second contact region 119. The first contact region 113 includes a region of different conductivity of a first conductivity type, and the second contact region 119 includes a region of different conductivity of a second conductivity type.

[0035] The first contact region 113 is connected to the first contact 102, and the second contact region 119 is connected to the second contact 104. The first contact 102 is preferably referred to as the cathode of the photodetector 100, and the second contact 104 is preferably referred to as the anode. The second contact 104 is preferably disposed radially outward of the first contact 102. Furthermore, the first contact 102 and the second contact 104 preferably comprise a metal and / or a metal alloy.

[0036] Furthermore, first contact region 113 includes a heavily doped first region 112 of the first conductivity type disposed above a first well 114 of the first conductivity type. Furthermore, a second region 116 of the first conductivity type is disposed radially outward of first region 112 and first well 114. Second contact region 119 includes a heavily doped third region 118 of the second conductivity type disposed above a second well 120 of the second conductivity type.

[0037] Preferably, the first conductivity type is n-type and the second conductivity type is p-type. Furthermore, the semiconductor substrate 138 is illustratively selected as a p-type substrate comprising silicon. Therefore, in a preferred example, the first contact region 113 includes differently doped regions of n-type, and the second contact region 119 includes differently doped regions of p-type.

[0038] Additionally, second contact region 119 is at least partially surrounded by trench 136. Trench 136 advantageously includes doped sidewalls of the second conductivity type to improve charge transfer efficiency. The depth of the doped sidewalls of the second conductivity type can thereby increase with the depth of the trench. Alternatively, the doping concentration at the trench sidewalls can be substantially uniform at different depths of the trench.

[0039] The trenches 136 provide a low path resistance, improving charge collection efficiency and reducing potential sources of background noise.

[0040] Additionally, a third well 122 of the second conductivity type is preferably disposed below the first contact region 113 and is completely surrounded by the semiconductor substrate 138 .

[0041] In an advantageous example of the present disclosure, the photodetector 100 is a linear-mode avalanche photodiode (APD) fabricated based on bipolar technology. Fabricating an APD based on bipolar technology overcomes the drawbacks of standard CMOS technology, which limits the flexibility of developing such detectors. This limitation arises from the inability to modify the process flow, which must be optimized for high-performance transistors as wells. A further advantage of fabricating an APD based on bipolar technology is that design requirements such as the size of the active area, the distance between the cathode and anode, and the thickness of the semiconductor substrate are no longer limiting factors. Furthermore, the contact openings in the semiconductor substrate for metal deposition can be freely selected. Furthermore, the use of bipolar technology advantageously provides an APD with low input impedance. At the same time, high drive current, speed, and gain can be achieved.

[0042] Incident radiation 125 enters the photodetector 100 through a radiation-receiving region 139, which is preferably formed from a metal-free material. When a voltage is applied between the first contact 102 and the second contact 104, a multiplication region is formed in the first contact region 113. The applied voltage strengthens the electric field around the multiplication region. As the incident radiation enters the photodetector, it is absorbed, generating electron-hole pairs. These charge carriers flow to the multiplication region, where a high electric field exists, and are multiplied. At their highest velocity, the charge carriers collide with other atoms, generating new electron-hole pairs. This results in a high photocurrent.

[0043] The first contact 102 and the second contact 104 are located on one side of the photodetector 100 so that charge generated by incident radiation 125 flows laterally through the photodetector 100 .

[0044] 1 also shows that an anti-reflective coating (ARC) or Bragg filter 132 is advantageously located below the semiconductor substrate 138. The term "below" is a relative term meaning that the ARC or Bragg filter 132 is located in a radiation-receiving area 139 of the photodetector 100, and the incident radiation 125 strikes the ARC or Bragg filter 132 before entering the semiconductor substrate 138. A heavily doped fourth region 128 of the second conductivity type is preferably located between the ARC / Bragg filter 132 and the semiconductor substrate 138. The fourth region 128 preferably reduces the contribution of trapped charge or defect points on the side of the photodetector where the radiation enters.

[0045] Additionally, a glass wafer 134 is positioned below the ARC or Bragg filter 132. The glass wafer 134 is used as a handle to facilitate processing of the photodetector 100. The glass handle wafer is removed after the bonding process. Therefore, it is advantageous for the incident radiation 125 to first enter the ARC or Bragg filter, then the fourth region 128, and then the semiconductor substrate 138.

[0046] The ARC and Bragg filter preferably enhance the absorption characteristics at the desired wavelengths to allow incident radiation to enter the substrate.

[0047] The incident radiation 125 that is not absorbed on its way thereto is reflected by a radiation reflecting region 106 disposed on the photodetector 100. This radiation reflecting region 106 is disposed on the opposite side of the photodetector 100 from the radiation receiving region 139. The radiation reflecting region 106 is preferably disposed on the same side of the photodetector 100 as the first contact 102 and the second contact 104. Furthermore, the radiation reflecting region 106 is advantageously disposed approximately in the center of the photodetector. The first contact 102 advantageously radially surrounds the radiation reflecting region 106. The second contact 104, in turn, radially surrounds the first contact 102.

[0048] Incident radiation 125 is reflected by radiation reflecting region 106, producing reflected light ray 126. Thus, incident radiation 125 is substantially reflected back to the multiplication region and semiconductor substrate 138, where it can be absorbed. Reflection of incident radiation 125 doubles the effective distance the radiation must travel in the photodetector. Thus, the present disclosure enables absorption of long wavelength radiation.

[0049] In particular, the reflection of the incident radiation 125 allows the depth of the semiconductor substrate 138 to be effectively reduced without affecting the radiation that can be absorbed. The depth of the semiconductor substrate 138 is advantageously selected to be less than the total absorption depth for wavelengths in the red-infrared spectrum. Thus, a particularly compact and small photodetector is obtained that still absorbs long wavelengths.

[0050] Providing a photodetector with a comparable thin substrate and radiation-reflecting region improves both the speed and sensitivity of the photodetector. Additionally, the thin silicon substrate enables a low temperature coefficient and low breakdown voltage of the photodetector, especially in the near-infrared spectral region.

[0051] Furthermore, it is preferred that the radiation reflecting region 106 has a diameter D of 230 μm and comprises a metal and / or a metal alloy. Illustratively, the radiation reflecting region 106 is formed from aluminum.

[0052] The proposed photodetector 100 advantageously achieves a peak sensitivity of 70% to 80% (A / W) for wavelengths in the spectral range of 800 nm to 930 nm. Advantageously, a peak sensitivity of 73% (A / W) can be achieved at a wavelength of 905 nm. Exemplarily, this sensitivity value can be reached with a semiconductor substrate thickness of 30 μm.

[0053] 2 shows the layout of the photodetector 100. As can be seen, the radiation-reflecting region 106 is located approximately centrally and preferably has a diameter of 230 μm. The first contact 102 is positioned radially outward of the radiation-reflecting region 106. The second contact 104 is located radially outward of the first contact 102 and is electrically isolated from the first contact 102. A channel stop 146 is preferably located radially outward of the second contact 104.

[0054] 1, the first contact 102 and the second contact 104 are illustratively electrically isolated by an oxide-containing layer 108. Illustratively, the layers 108 are separated by nitride-containing layers 110.

[0055] FIG. 3 shows the photodetector 100 assembled on a carrier 144. This illustration highlights certain advantageous features of the photodetector 100 of the present invention. By locating the first contact 102 and the second contact 104 on the same side of the photodetector 100, it is possible to couple these contacts via flip-chip bonding, which is a highly reliable and easy-to-integrate bonding method. Via a bonding element 140, each contact 102, 104 is bonded to a respective bonding element 142 of the carrier 144. Thus, the carrier 144 is specifically positioned on the side of the photodetector 100 opposite the radiation-receiving area 139. [Explanation of symbols]

[0056] 100 Photodetector 102 First Contact 104 Second Contact 106 Radiation reflection area 108 Oxides 110 Nitrides 112 First Area 113 First Contact Region 114 First Well 116 Second Area 118 The Third Realm 119 Second Contact Area 120 Second Well 122 Third Well 125 Incident radiation 126 Reflected Light 128 The Fourth Realm 132 Anti-reflection coating / Bragg filter 134 Glass wafer 136 Trench 138 Semiconductor Substrate 139 Radiation Receptor Area 140 Connecting Parts 142 Carrier connecting parts 144 Career 146 Channel Stopper

Claims

1. A light receiving element (100), a semiconductor substrate (138) having a radiation receiving area (139) through which incident radiation (125) can enter the photodetector (100); a first contact (102) across which a voltage is applied to a second contact (104); the second contact (104) across which a voltage is applied to the first contact (102); a first contact region (113) connected to said first contact (102); a second contact area (119) connected to said second contact (104); a radiation reflecting area (106) for reflecting the incident radiation (125); Equipped with a multiplication region is formed in the first contact region (113) for multiplying charge generated from the incident radiation (125) when a voltage is applied between the first contact (102) and the second contact (104); the first contact (102) and the second contact (104) are disposed on an opposite side of the semiconductor substrate (138) from the radiation-receiving region (139); the radiation reflecting region (106) is disposed on an opposite side of the semiconductor substrate (138) from the radiation receiving region (139); the first contact region (113) includes a region of a different conductivity type of a first conductivity; The first contact region (113) is a first region (112) of the first conductivity type connected to the first contact (102); a first well (114) of the first conductivity type disposed on a surface of the first region (112) opposite the first contact (102) and doped to a lower concentration than the first region (112); a second region (116) of the first conductivity type that is lightly doped relative to the first well (114); The second region (116) comprises: Adjacent to the first region (112) and the first well (114), and disposed radially outward of said first well (114). Light receiving element (100).

2. the first contact region (113) includes a region of a different conductivity type of the first conductivity type; 2. The photodetector (100) of claim 1, wherein the second contact region (119) comprises a region of a different conductivity type of a second conductivity type.

3. the second contact region (119) includes a region of a different conductivity type of a second conductivity; 2. The photodetector of claim 1, wherein the second contact region includes a second well of the second conductivity type and a third region of the second conductivity type that is disposed above the second well and is doped more heavily than the second well.

4. the semiconductor substrate (138) is a p-type substrate; the first conductivity type is n-type, The photodetector (100) of claim 2, wherein the second conductivity type is p-type.

5. the second contact region (119) includes a region of a different conductivity type of a second conductivity; the second contact region (119) is at least partially surrounded by a trench (136); 2. The photodetector (100) of claim 1, wherein the trench (136) includes sidewalls that are doped with the second conductivity type.

6. 2. The light receiving element (100) of claim 1, wherein the radiation reflecting area (106) has a diameter D of 230 μm.

7. an anti-reflection coating (ARC) or Bragg filter (132); 2. The photodetector (100) of claim 1, wherein the anti-reflective coating (ARC) or Bragg filter (132) is disposed below the semiconductor substrate (138) in the radiation-receiving region (139).

8. the second contact region (119) includes a region of a different conductivity type of a second conductivity; the second contact region (119) includes a second well (120) of the second conductivity type and a third region (118) of the second conductivity type disposed above the second well (120) and doped to a higher concentration than the second well (120); a fourth region (128) of the second conductivity type and more highly doped than the second well (120); 8. The photodetector (100) of claim 7, wherein the fourth region (128) is disposed between the semiconductor substrate (138) and the anti-reflection coating or the Bragg filter (132).

9. The photodetector (100) of claim 1, wherein the semiconductor substrate (138) comprises silicon.

10. The photodetector (100) of claim 1, wherein the radiation reflecting region (106) comprises a metal and / or a metal alloy.

11. The photodetector (100) of any preceding claim, wherein the semiconductor substrate (138) has a depth that is less than the total absorption depth for wavelengths in the red-infrared spectrum.

12. the second contact region (119) includes a region of a different conductivity type of a second conductivity; a third well (122) of the second conductivity type; 2. The photodetector (100) of claim 1, wherein the third well (122) is disposed below the first contact region (113) and is completely surrounded by the semiconductor substrate (138).

13. 10. The photodetector (100) of claim 1, wherein the photodetector (100) has a peak sensitivity of 73% (A / W) at a wavelength of 905 nm.

14. 14. An assembly method for providing a light receiving element (100) according to any one of claims 1 to 13, comprising the steps of: The assembly method further includes the step of assembling the light receiving element (100) on a carrier (144), the carrier (144) being positioned on an opposite side of the light receiving element (100) from the radiation receiving area (139).

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