Avalanche photodiode structure for OTDR and manufacturing method thereof

Abstract

The invention discloses an avalanche photodiode structure for an OTDR and a manufacturing method of the avalanche photodiode structure, the avalanche photodiode structure comprises an InP multiplication layer, specifically, a protection ring penetrating through the InP multiplication layer is arranged in the InP multiplication layer, Zn diffusion is adopted by the protection ring, so that the gradient of the protection ring is increased, and the thickness of the protection ring is smaller than that of the InP multiplication layer. The gain of the avalanche photodiode structure is improved; a dielectric film is arranged on the InP multiplication layer, the thickness of the dielectric film is 8000-10000, and the dielectric film is made of silicon dioxide. The protection ring is arranged in the structure, and the slope of the protection ring is increased, so that the voltage of the side multiplication voltage division region of the protection ring is reduced, the voltage of the effective multiplication region is increased, the gain of the avalanche photodiode is improved, and the OTDR has high responsivity and high sensitivity; in addition, in the machining process, the tunneling current is reduced, the signal-to-noise ratio is increased, and the sensitivity of the OTDR is improved by controlling the height of the side multiplication voltage division area.

Description

Technical Field

[0001] The present invention relates to the technical field of chip manufacturing, and particularly to an avalanche photodiode structure for OTDR and a manufacturing method thereof. Background Art

[0002] With the rapid development of optical communication, fiber optic communication has become the main transmission mode in modern communication due to its advantages such as wide bandwidth, low loss, and strong anti-interference ability. The performance of optical fibers determines the quality of network transmission, so their detection and maintenance are particularly important. As the most important detection tool in the application of fiber optic communication systems, the Optical Time-Domain Reflectometer (OTDR for short) uses Fresnel reflection and Rayleigh scattering generated when light travels in optical fibers to analyze the attenuation characteristics of optical fibers, joint losses, and the location of fiber optic fault points, and has become an essential device for current fiber optic fault diagnosis and line maintenance.

[0003] The OTDR system mainly uses classical photodetectors. Since a PIN photodiode can generate at most one pair of electron-hole pairs even at the maximum responsivity, it is a device without internal gain. In order to make the OTDR obtain higher responsivity, sensitivity, and improve detection accuracy, an avalanche photodiode (APD for short) is generally used.

[0004] The existing preparation process of avalanche photodiodes with a guard ring structure is relatively mature and has the advantage of low cost. In the existing preparation process of avalanche photodiodes with a guard ring structure, when the working voltage is reduced by 1V, the gain of the avalanche photodiode is usually less than 35, and the corresponding dark current is usually greater than 3nA. On this basis, if the existing process of the guard ring structure is not improved, it is difficult to further increase the gain of the avalanche photodiode and further reduce the dark current. This makes it impossible to improve the responsivity, sensitivity, and detection accuracy of the OTDR using avalanche photodiodes.

[0005] In view of this, overcoming the defects of the existing technology is an urgent problem to be solved in this technical field. Summary of the Invention

[0006] The technical problem to be solved by the present invention is how to overcome the bottleneck of the existing avalanche photodiode, that is, when the working voltage is reduced, the gain of the avalanche photodiode is low, and the corresponding dark current is large, resulting in low measurement accuracy of the OTDR.

[0007] The embodiments of the present invention adopt the following technical solutions: In a first aspect, the present invention provides an avalanche photodiode structure for OTDR. The avalanche photodiode structure includes an InP multiplication layer. Specifically: A protection ring is arranged in the InP multiplication layer. The protection ring is formed by Zn diffusion. During the diffusion process, the lateral diffusion rate is controlled to increase the slope of the protection ring, so as to improve the gain of the avalanche photodiode structure. A dielectric film is arranged on the InP multiplication layer. The thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

[0008] Preferably, a charge layer is arranged under the InP multiplication layer. The bottom surface of the protection ring is connected to the charge layer, and the diameter of the protection ring is 2 μm - 4 μm. Preferably, the InP multiplication layer includes an effective multiplication region. The bottom surface of the effective multiplication region abuts against the charge layer. By reducing the height of the effective multiplication region, the signal-to-noise ratio of the avalanche photodiode structure is improved.

[0009] Preferably, the concentration of the InP multiplication layer is 1e16 cm -3 -1.5e16 cm -3 , and the thickness is 2.5 μm - 3.0 μm. Preferably, the avalanche photodiode structure further includes an InP substrate, an InP buffer layer, an InGaAs absorption layer, an InGaAsP graded layer, an InP charge layer, and an InGaAs contact layer. The InP buffer layer is arranged on the InP substrate. The doping concentration of the InP buffer layer is not less than 5e17 cm -3 , and the thickness is 1.0 - 2.0 μm. The InGaAs absorption layer is arranged on the InP buffer layer. The InGaAs absorption layer is of type I. The doping concentration of the InGaAs absorption layer is not greater than 1e15 cm -3 , and the thickness is 1.5 - 2.0 μm. The InGaAsP graded layer is arranged on the InGaAs absorption layer. The InGaAsP graded layer is of type N. The doping concentration of the InGaAsP graded layer is 1e16 cm -3 -1.5e16 cm -3 , and the thickness is 0.15 - 0.20 μm. The InP charge layer is arranged on the InGaAsP graded layer. The InP charge layer is of type N. The doping concentration of the InP charge layer is 1.5e17 cm -3 -2.0e17 cm -3 , and the thickness is 0.2 - 0.25 μm. The InP multiplication layer is disposed on the InP charge layer; The InGaAs contact layer is disposed on the InP multiplication layer, and the doping concentration of the InGaAs contact layer is 1e19 cm -3 -1.5e19 cm -3 , and the thickness is 0.2 - 0.3 um.

[0010] Preferably, the avalanche photodiode structure further includes a P electrode and an N electrode; The P electrode is disposed on the InGaAs contact layer, and the P electrode is made of one or more of Ti, Pt, and Au; The N electrode is disposed on the lower surface of the InP substrate, and the N electrode is made of one or more of Ti, Pt, and Au.

[0011] Preferably, the avalanche photodiode structure further includes a SiNx antireflection film, the SiNx antireflection film is disposed on the photosensitive surface of the InP multiplication layer, the thickness of the SiNx antireflection film is 1600 Å - 1800 Å, and the SiNx antireflection film has an antireflection effect on wavelengths of 1.55 um and 1.31 um.

[0012] In a second aspect, the present invention further provides a method for manufacturing an avalanche photodiode structure for OTDR, the method being applicable to the avalanche photodiode structure for OTDR in the first aspect, and including: A protection ring is disposed in the InP multiplication layer, wherein the protection ring uses Zn diffusion to increase the slope of the protection ring and improve the gain of the avalanche photodiode structure; A dielectric film is disposed on the InP multiplication layer, wherein the thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

[0013] Preferably, disposing a protection ring in the InP multiplication layer includes: An InGaAs contact layer is fabricated on the InP multiplication layer; A dielectric film is deposited on the InP multiplication layer and the InGaAs contact layer, and part of the dielectric film is removed to form a protection ring diffusion region; Zn diffusion is performed through the protection ring diffusion region, and the lateral diffusion rate is controlled to dispose a protection ring in the InP multiplication layer.

[0014] Preferably, disposing a dielectric film on the InP multiplication layer includes: After the fabrication of the protection ring is completed, the remaining dielectric film is removed; Deposit a dielectric film on the InP multiplication layer and the InGaAs contact layer, and remove the dielectric film on the InGaAs contact layer and the dielectric film on the InP multiplication layer between the InGaAs contact layers, so as to retain the dielectric films distributed on both sides of the InP multiplication layer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: A protection ring is provided in the avalanche photodiode structure of the embodiment of the present invention. By increasing the slope of the protection ring, the voltage of the side multiplication voltage-dividing region of the protection ring is reduced, and the voltage of the effective multiplication region is increased, thereby improving the gain of the avalanche photodiode and enabling the OTDR to have high responsivity and high sensitivity; during the processing, by controlling the height of the side multiplication voltage-dividing region, the tunneling current of the avalanche photodiode is reduced, thereby improving the signal-to-noise ratio and the sensitivity of the OTDR.

[0016] In addition, the embodiment of the present invention uses silicon dioxide as the passivation dielectric film. The silicon dioxide dielectric film not only has good blocking and masking effects on diffusion, but is also less affected by temperature, which can further reduce the dark current on the surface of the avalanche photodiode structure, thereby improving the sensitivity and measurement accuracy of the OTDR of the present invention. Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a schematic diagram of the principle of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 2 It is a schematic diagram of the overall structure of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 3 It is a flowchart of a method for obtaining an avalanche photodiode epitaxial wafer in the manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 4 It is a schematic diagram of the structure of an InGaAs contact layer fabricated in the manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 5 It is a schematic diagram of the structure of depositing a dielectric film by vapor deposition and forming a protection ring diffusion region by photolithography etching in the manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 6 It is a schematic structural diagram of forming a P-type diffusion region in a manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 7 It is a schematic structural diagram of forming an antireflection film in a manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Figure 8 It is a schematic structural diagram of forming a P electrode in a manufacturing method of an avalanche photodiode structure for OTDR provided by an embodiment of the present invention; Among them, the reference numerals are: 1 - InP substrate; 2 - InP buffer layer; 3 - InGaAs absorption layer; 4 - InGaAsP graded layer; 5 - InP charge layer; 6 - InP multiplication layer; 7 - InGaAs contact layer; 8 - dielectric film; 9 - antireflection film; 10 - P electrode; 11 - N electrode; 12 - effective multiplication region; 13 - side multiplication voltage division region. Detailed implementation manners

[0019] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] In the description of the present invention, the orientation or positional relationships indicated by terms such as "inside", "outside", "longitudinal", "transverse", "upper", "lower", "top", "bottom", etc. are based on the orientation or positional relationships shown in the accompanying drawings. These are only for the convenience of describing the present invention and do not require the present invention to be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0021] In the present invention, terms such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of this application, unless otherwise stated, the meaning of "a plurality" is two or more.

[0022] In this application, unless otherwise clearly defined and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral one; it can be directly connected or indirectly connected through an intermediate medium. In addition, the term "coupling" can be a way of electrical connection for signal transmission.

[0023] In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0024] Embodiment 1: Embodiment 1 of the present invention provides an avalanche photodiode structure for OTDR, as Figure 1 - Figure 2 shown. The avalanche photodiode structure includes an InP multiplication layer 6. Specifically: A guard ring is provided in the InP multiplication layer. The guard ring uses Zn diffusion. During the diffusion process, the lateral diffusion rate is controlled to increase the slope of the guard ring, so as to improve the gain of the avalanche photodiode structure. A dielectric film is provided on the InP multiplication layer. The thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

[0025] As Figure 1 shown, the guard ring in the embodiment of the present invention is the upper region corresponding to a, and the slope of the guard ring is the radian corresponding to the side multiplication voltage division region 13.

[0026] In the existing avalanche diode structure with a guard ring structure, the research on the guard ring is not in-depth. Therefore, when using open-tube Zn diffusion, the problem of the slope of the guard ring is not considered. Compared with the existing avalanche photodiode structure with a guard ring structure, in the embodiment of the present invention, when manufacturing the guard ring structure, the method of open-tube Zn diffusion is adopted to control the slope of the guard ring in the avalanche photodiode, so that the slope of the guard ring is as large as possible. When actually manufacturing the avalanche photodiode structure for OTDR in the embodiment of the present invention, a guard ring diffusion region is formed through photolithography and etching processes. Specifically, open-tube Zn diffusion is adopted, and the doping concentration is 1e17 cm -3 -1e18 cm -3 is used for processing. During the diffusion process, by controlling the lateral diffusion rate, the slope of the guard ring is increased to reduce the voltage division of the side multiplication region of the guard ring, so that the voltage of the effective multiplication region 12 increases, thereby improving the gain and making the OTDR of the present invention have high responsivity and high sensitivity.

[0027] In addition, the SiNx thin film used in the prior art as a dielectric film needs to be processed at high temperature during the open-tube Zn diffusion process, resulting in relatively large residual stress. When the temperature increases and decreases rapidly, the internal defects of the material increase, and the dark current will increase. In the embodiments of the present invention, silicon dioxide is used as the passivation dielectric film. The silicon dioxide dielectric film not only has good blocking and masking effects on diffusion, but is also less affected by temperature, which can effectively reduce the dark current on the surface of the avalanche photodiode structure, thereby improving the sensitivity and measurement accuracy of the OTDR of the present invention. Next, in combination with the corresponding drawings and formulas, the slope of the protection ring in the embodiments of the present invention is increased to improve the gain of the avalanche diode structure, so as to achieve the purpose of high sensitivity of the OTDR. As Figure 1 shown, when the slope of the protection ring of the avalanche photodiode is small, there will be a side multiplication voltage division area 13 on the side of the protection ring, which reduces the effective avalanche area voltage and the gain will decrease; according to the gain formula: M = 1 / [1 - (V / V B ) n , where V is the reverse bias voltage applied to the APD, V B is the avalanche breakdown voltage of the APD, and n is determined by the incident light band and the APD structure. By increasing the slope (such as Figure 1 ), the voltage of the side multiplication voltage division area 13 can be reduced, so that the voltage V of the effective multiplication area 12 increases, thereby increasing the gain and making the OTDR have high responsivity and high sensitivity.

[0028] Next, the specific structural composition of the avalanche photodiode structure for OTDR in the embodiments of the present invention will be specifically described. A charge layer is grown under the InP multiplication layer 6 in the embodiments of the present invention, and the bottom surface of the protection ring is connected to the charge layer, and the diameter of the protection ring is 2 um - 4 um. The avalanche photodiode structure in the embodiments of the present invention reduces the PIN active area corresponding to the protection ring, and sets the diameter of the protection ring (refer to the a value in Figure 5 , and the a value is the diameter of the protection ring) within the range of 2 um - 4 um. For example, it can be, but is not limited to, set to 3 um, so that the protection ring can effectively reduce the dark current in the avalanche photodiode structure.

[0029] In order to improve the sensitivity of the OTDR, the InP multiplication layer 6 in the embodiments of the present invention includes an effective multiplication area 12, and the bottom surface of the effective multiplication area 12 abuts against the charge layer. Among them, reducing the height of the effective multiplication area 12 (refer to Figure 1 and Figure 2As shown, use "d" to represent the height of the effective multiplication region 12), so as to reduce the height of the effective multiplication region 12 and improve the signal-to-noise ratio of the avalanche photodiode structure. In the embodiment of the present invention, by controlling the height of the effective multiplication region 12, the tunneling current of the APD is reduced, thereby improving the signal-to-noise ratio and the sensitivity of the OTDR. Specifically: By appropriately reducing the thickness of the effective multiplication region 12, the gain-bandwidth product can be increased, the noise can be reduced, and the signal-to-noise ratio can be improved. When the externally applied reverse bias voltage remains unchanged, the electric field distribution intensity of the thinner effective multiplication layer 12 is larger. The high electric field distribution is beneficial to increasing the impact ionization rate of electrons and holes, reducing the carrier transport time and transport distance, reducing the avalanche time, and reducing the dark current noise. However, due to the existence of the dead zone effect, the thickness of the effective multiplication region 12 cannot be infinitely reduced.

[0030] As Figure 2 shown, the avalanche photodiode structure in the embodiment of the present invention further includes an InP substrate 1, an InP buffer layer 2, an InGaAs absorption layer 3, an InGaAsP graded layer 4, an InP charge layer 5, and an InGaAs contact layer 7; the InP buffer layer 2 is disposed on the InP substrate 1, and the doping concentration of the InP buffer layer 2 is not less than 5e17 cm -3 , and the thickness is 1.0 - 2.0 um; the InGaAs absorption layer 3 is disposed on the InP buffer layer 2, the InGaAs absorption layer 3 is of type I, and the doping concentration of the InGaAs absorption layer 3 is not greater than 1e15 cm -3 , and the thickness is 1.5 - 2.0 um; the InGaAsP graded layer 4 is disposed on the InGaAs absorption layer 3, the InGaAsP graded layer 4 is of N type, and the doping concentration of the InGaAsP graded layer 4 is 1e16 cm -3 - 1.5e16 cm -3 , and the thickness is 0.15 - 0.20 um; the InP charge layer 5 is disposed on the InGaAsP graded layer 4, the InP charge layer 5 is of N type, and the doping concentration of the InP charge layer 5 is 1.5e17 cm -3 - 2.0e17 cm -3 , and the thickness is 0.2 - 0.25 um; the InP multiplication layer 6 is disposed on the InP charge layer 5, the InGaAs contact layer 7 is disposed on the InP multiplication layer 6, and the doping concentration of the InGaAs contact layer 7 is 1e19 cm-3 - 1.5e19 cm-3, and the thickness is 0.2 - 0.3 um. Among them, the InGaAs contact layer 7 can be a P+ type InGaAs contact layer.

[0031] In addition, the avalanche photodiode structure described in the embodiments of the present invention further includes a P electrode 10 and an N electrode 11. The P electrode 10 is disposed on the InGaAs contact layer 7, and the P electrode 10 is 800 Å Ti, 900 Å Pt, or 6000 Å Au. The N electrode 11 is disposed on the lower surface of the InP substrate 1, and the N electrode 11 is 800 Å Ti, 900 Å Pt, or 1000 Å Au. It should be noted that in the embodiments of the present invention, the P electrode is disposed on the InGaAs contact layer, and the P electrode is made of one or more of Ti, Pt, and Au. The N electrode is disposed on the lower surface of the InP substrate, and the N electrode is made of one or more of Ti, Pt, and Au.

[0032] As Figure 2 shown, the avalanche photodiode structure of the embodiments of the present invention includes a multi-layer structure. An InP buffer layer 2, an InGaAs absorption layer 3, an InGaAsP graded layer 4, an InP charge layer 5, an InP multiplication layer 6, and an InGaAs contact layer 7 are sequentially grown on the InP substrate 1 to obtain an avalanche photodiode epitaxial wafer. After obtaining the avalanche diode epitaxial wafer, a P+-type InGaAs contact layer 7 is fabricated through photolithography and chemical etching processes. A dielectric film 8 is deposited by plasma enhanced chemical vapor deposition (PEVCD), and a guard ring diffusion region is protected through photolithography and etching processes. The dielectric film 8 is removed by etching and etching processes, and the dielectric film 8 is continuously deposited by PEVCD, and a P-type diffusion region is formed through photolithography and etching processes. An antireflection film 9 is deposited by PECVD, and the antireflection film 9 outside the photosensitive surface is removed through photolithography and etching processes. A P metal ring is formed on the P+-type InGaAs contact layer 7 through photolithography, electron beam evaporation, and stripping processes. Then, through chemical mechanical polishing (CMP) processes, the epitaxial wafer is thinned and polished to 150 ± 20 μm, and an N electrode 11 is sputtered on the back surface by magnetron sputtering. Finally, by cleaving the epitaxial wafer along the crystal orientation, a single avalanche photodiode die is obtained.

[0033] During the fabrication process of the avalanche photodiode in the embodiments of the present invention, the concentration of the InP multiplication layer 6 is 1e16 cm -3 -1.5e16 cm -3 , and the thickness is 2.5 μm - 3.0 μm. Since the heterojunction dark current mainly includes generation-recombination current and tunneling current, the G-R current plays a major role at a lower bias voltage. The dark current I d = I PIN + I APD = A j ·q·X m ·n i / 2τ, where A j is the photosensitive area, q is the electron charge, and Xm is the depletion region width, n i is the intrinsic concentration of the material, and τ is the minority carrier lifetime. As Figure 2 , the dark current can be reduced by reducing the PIN active area corresponding to the guard ring and making its diameter a be 2 μm - 4 μm; at the same time, due to the design of the guard ring, there are electric fields both laterally and longitudinally in the guard ring, so the substrate InP multiplication layer 61e16 cm -3 -1.5e16 cm -3 and the InGaAs absorption layer 3 < 1e15 cm -3 doping to weaken the electric field around the guard ring, thereby reducing the dark current; by controlling d to reduce the APD tunneling current, thereby improving the signal-to-noise ratio and the sensitivity of the OTDR.

[0034] In a specific application scenario, the avalanche photodiode structure described in the embodiment of the present invention further includes a SiNx antireflection film 9, the SiNx antireflection film 9 is disposed on the photosensitive surface of the InP multiplication layer 6, the thickness of the SiNx antireflection film 9 is 1600 Å - 1800 Å, and the SiNx antireflection film 9 has an antireflection effect on wavelengths of 1.55 μm and 1.31 μm.

[0035] In the avalanche photodiode structure of the embodiment of the present invention, a guard ring is provided. By increasing the slope of the guard ring, the voltage of the side multiplication voltage division region 13 of the guard ring is reduced, and the voltage of the effective multiplication region 12 is increased, thereby improving the gain of the avalanche photodiode and enabling the OTDR to have high responsiveness and high sensitivity; in the process of processing the embodiment of the present invention, by controlling the height of the side multiplication voltage division region 13, the tunneling current of the avalanche photodiode is reduced, thereby improving the signal-to-noise ratio and the sensitivity of the OTDR. In addition, the embodiment of the present invention also reduces the PIN active area corresponding to the guard ring and sets the diameter of the guard ring to 2 μm - 4 μm to reduce the dark current; since there are electric fields both laterally and longitudinally in the guard ring, the substrate InP multiplication layer 61e16 cm -3 -1.5e16 cm -3 and the InGaAs absorption layer 3 is doped not greater than 1e15 cm - 3 to weaken the electric field around the guard ring and further reduce the dark current to improve the sensitivity of the OTDR. The SiO2 thin film with a thickness of 8000 Å - 10000 Å is selected as the passivation dielectric film 8. Silicon dioxide not only has good blocking and masking effects on Zn diffusion, but is also less affected by temperature and can significantly reduce the surface dark current.

[0036] Embodiment 2: The embodiment 2 of the present invention further provides a manufacturing method for an avalanche photodiode structure for OTDR. The method is applicable to the avalanche photodiode structure for OTDR in Embodiment 1 and includes: Step 201: A guard ring is provided in the InP multiplication layer 6. The guard ring uses Zn diffusion to increase the slope of the guard ring and improve the gain of the avalanche photodiode structure.

[0037] In the embodiment of the present invention, a guard ring is provided in the InP multiplication layer 6, and Zn diffusion is adopted to increase the slope of the guard ring, reduce the voltage division on the side of the guard ring in the multiplication region, increase the voltage of the effective multiplication region 12, thereby improving the gain, and enabling the OTDR of the present invention to have high responsivity and high sensitivity.

[0038] Step 202: A dielectric film is provided on the InP multiplication layer. The thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

[0039] In the embodiment of the present invention, a dielectric film is provided on the InP multiplication layer. During the process of open-tube Zn diffusion, it needs to be carried out at high temperature, and there are large residual stresses. When the temperature increases and decreases suddenly, the internal defects of the material increase, and the dark current will increase. However, in the embodiment of the present invention, silicon dioxide is used as the passivation dielectric film. The silicon dioxide dielectric film not only has good blocking and masking effects on diffusion, but is also less affected by temperature, can effectively reduce the dark current on the surface of the avalanche photodiode structure, and further improve the sensitivity and measurement accuracy of the OTDR of the present invention.

[0040] As Figure 4 - Figure 5 shown, providing a guard ring in the InP multiplication layer includes: fabricating an InGaAs contact layer on the InP multiplication layer; depositing a dielectric film on the InP multiplication layer and the InGaAs contact layer, removing part of the dielectric film to form a guard ring diffusion region; performing Zn diffusion through the guard ring diffusion region and controlling the lateral diffusion rate to provide a guard ring in the InP multiplication layer.

[0041] The process of providing a guard ring in the InP multiplication layer in the embodiment of the present invention has been described above and will not be elaborated here.

[0042] In addition, as Figure 6 shown, providing a dielectric film on the InP multiplication layer in the embodiment of the present invention includes: after completing the fabrication of the guard ring, removing the remaining dielectric film; depositing a dielectric film on the InP multiplication layer and the InGaAs contact layer, and removing the dielectric film on the InGaAs contact layer and the dielectric film on the InP multiplication layer between the InGaAs contact layers to retain the dielectric films distributed on both sides of the InP multiplication layer. In the embodiment of the present invention, an antireflection film is deposited on the InP multiplication layer between the InGaAs contact layers (refer to Figure 8As shown above, the process of setting the dielectric film in the embodiments of the present invention has been described above and will not be elaborated here.

[0043] Embodiment 3: Based on Embodiment 1 and Embodiment 2, Embodiment 3 of the present invention explains in detail the manufacturing method of an avalanche photodiode structure for OTDR, including: Step 1, refer to Figure 3 As shown, an InP buffer layer 2, an InGaAs absorption layer 3, an InGaAsP graded layer 4, an InP charge layer 5, an InP multiplication layer 6, and an InGaAs contact layer 7 are sequentially grown on an InP substrate 1 to obtain an avalanche photodiode epitaxial wafer. Step 2, refer to Figure 4 As shown, a P+-type InGaAs contact layer 7 is fabricated through photolithography and chemical etching processes. Step 3, refer to Figure 5 As shown, a dielectric film 8 is deposited by plasma-enhanced chemical vapor deposition (PECVD), and a protection ring with a preset slope is formed through photolithography and chemical etching processes.

[0044] Step 4, refer to Figure 6 As shown, the dielectric film 8 is removed using etching and etching processes, the dielectric film 8 is continuously deposited by PECVD, and a P-type diffusion region is formed through photolithography and etching processes.

[0045] Step 5, refer to Figure 7 As shown, an antireflection film 9 is deposited by PECVD, and the antireflection film 9 outside the photosensitive surface is removed through photolithography and etching processes.

[0046] Step 6, refer to Figure 8 As shown, a P metal ring is formed on the P+-type InGaAs contact layer 7 through photolithography, electron beam evaporation, and lift-off processes; and through chemical mechanical polishing (CMP) process, the epitaxial wafer is thinned and polished to 150 ± 20 μm, and an N electrode 11 is sputtered on the back using a magnetron sputtering process (as Figure 2 shown).

[0047] Step 7, refer to Figure 2 As shown, the epitaxial wafer is cleaved along the crystal orientation to obtain an avalanche photodiode structure.

[0048] In the avalanche photodiode structure of the embodiment of the present invention, a guard ring is provided. By increasing the slope of the guard ring, the voltage of the side multiplication voltage division region 13 of the guard ring is reduced, and the voltage of the effective multiplication region 12 is increased, thereby improving the gain of the avalanche photodiode and enabling the OTDR to have high responsivity and high sensitivity. During the processing of the embodiment of the present invention, by controlling the height of the side multiplication voltage division region 13, the tunneling current of the avalanche photodiode is reduced, thereby improving the signal-to-noise ratio and the sensitivity of the OTDR. In addition, the embodiment of the present invention also reduces the dark current by reducing the PIN active area corresponding to the guard ring and setting the diameter of the guard ring to 2um - 4um. Since there are electric fields in both the lateral and longitudinal directions of the guard ring, the doping concentration of the substrate InP multiplication layer 6 is 1e16cm -3 -1.5e16cm -3 and the InGaAs absorption layer 3 is not greater than 1e15cm -3 doping to weaken the electric field around the guard ring and further reduce the dark current to improve the sensitivity of the OTDR. Select a SiO2 thin film with a thickness of 8000Å - 10000Å as the dielectric film 8. Silicon dioxide not only has good blocking and masking effects on Zn diffusion but is also less affected by temperature, which can significantly reduce the surface dark current.

[0049] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. An avalanche photodiode structure for an OTDR, characterized in that, The avalanche photodiode structure includes an InP multiplication layer. Specifically: A protection ring is provided in the InP multiplication layer. The protection ring uses Zn diffusion. During the diffusion process, the lateral diffusion rate is controlled to increase the slope of the protection ring, thereby improving the gain of the avalanche photodiode structure. A dielectric film is provided on the InP multiplication layer. The thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

2. The avalanche photodiode structure for OTDR according to claim 1, characterized in that, A charge layer is grown under the InP multiplication layer. The bottom surface of the protection ring is connected to the charge layer, and the diameter of the protection ring is 2 um - 4 um.

3. The avalanche photodiode structure for OTDR according to claim 2, characterized in that, The InP multiplication layer includes an effective multiplication region. The bottom surface of the effective multiplication region abuts against the charge layer. By reducing the height of the effective multiplication region, the signal-to-noise ratio of the avalanche photodiode structure is improved.

4. The avalanche photodiode structure for OTDR according to claim 1, wherein The concentration of the InP multiplication layer is 1e16 cm -3 -1.5e16 cm -3 , and the thickness is 2.5 μm - 3.0 μm.

5. The avalanche photodiode structure for OTDR according to claim 1, characterized in that, The avalanche photodiode structure further includes an InP substrate, an InP buffer layer, an InGaAs absorption layer, an InGaAsP graded layer, an InP charge layer, and an InGaAs contact layer. The InP buffer layer is disposed on the InP substrate, and the doping concentration of the InP buffer layer is not less than 5e17 cm -3 , and the thickness is 1.0 - 2.0 um; The InGaAs absorption layer is disposed on the InP buffer layer. The InGaAs absorption layer is of type I, and the doping concentration of the InGaAs absorption layer is not greater than 1e15 cm -3 , and the thickness is 1.5 - 2.0 um; The InGaAsP graded layer is disposed on the InGaAs absorption layer. The InGaAsP graded layer is of N type, and the doping concentration of the InGaAsP graded layer is 1e16 cm -3 -1.5e16 cm -3 , and the thickness is 0.15 - 0.20 um; The InP charge layer is disposed on the InGaAsP graded layer. The InP charge layer is N-type, and the doping concentration of the InP charge layer is 1.5e17 cm -3 -2.0e17 cm -3 , and the thickness is 0.2 - 0.25 um; The InP multiplication layer is provided on the InP charge layer. The InGaAs contact layer is disposed on the InP multiplication layer, and the doping concentration of the InGaAs contact layer is 1e19 cm -3 -1.5e19 cm -3 , and the thickness is 0.2 - 0.3 um.

6. The avalanche photodiode structure for OTDR according to claim 5, characterized in that, The avalanche photodiode structure further includes a P electrode and an N electrode. The P electrode is provided on the InGaAs contact layer, and the P electrode is made of one or more of Ti, Pt, and Au. The N electrode is provided on the lower surface of the InP substrate, and the N electrode is made of one or more of Ti, Pt, and Au.

7. The avalanche photodiode structure for OTDR according to claim 1, wherein The avalanche photodiode structure further includes a SiNx antireflection film. The SiNx antireflection film is provided on the photosensitive surface of the InP multiplication layer. The thickness of the SiNx antireflection film is 1600 Å - 1800 Å, and the SiNx antireflection film has an antireflection effect on wavelengths of 1.55 um and 1.31 um.

8. A manufacturing method for an avalanche photodiode structure for OTDR, the manufacturing method being used to manufacture the avalanche photodiode structure for OTDR according to any one of claims 1-7, characterized in that, Including: A protection ring is provided in the InP multiplication layer. The protection ring uses Zn diffusion to increase the slope of the protection ring, thereby improving the gain of the avalanche photodiode structure. A dielectric film is provided on the InP multiplication layer. The thickness of the dielectric film is 8000 Å - 10000 Å, and the material of the dielectric film is silicon dioxide.

9. The manufacturing method of the avalanche photodiode structure for OTDR according to claim 8, characterized in that, The step of providing a protection ring in the InP multiplication layer includes: Fabricating an InGaAs contact layer on the InP multiplication layer. Depositing a dielectric film on the InP multiplication layer and the InGaAs contact layer, and removing a part of the dielectric film to form a protection ring diffusion region. Performing Zn diffusion through the protection ring diffusion region and controlling the lateral diffusion rate to provide a protection ring in the InP multiplication layer.

10. The manufacturing method of the avalanche photodiode structure for OTDR according to claim 8, characterized in that, The step of providing a dielectric film on the InP multiplication layer includes: After completing the fabrication of the protection ring, removing the remaining dielectric film. Depositing a dielectric film on the InP multiplication layer and the InGaAs contact layer, and removing the dielectric film on the InGaAs contact layer and the dielectric film on the InP multiplication layer between the InGaAs contact layers to retain the dielectric film distributed on both sides of the InP multiplication layer.

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