Germanium photoelectric detector and method for improving long wave response of germanium photoelectric detector through stress memorization

Abstract

The present invention relates to the technical field of germanium photoelectric detectors, and provides a germanium photoelectric detector and a method for improving long wave response of the germanium photoelectric detector through stress memorization. The method comprises the following steps: defining a position on a monocrystalline silicon substrate by means of silicon oxide windowing, selectively epitaxially growing a monocrystalline germanium layer, depositing a polycrystalline silicon layer, etching the polycrystalline silicon layer to form a pattern covering the germanium layer, and then depositing silicon nitride, wherein the silicon nitride completely surrounds the upper portion and the periphery of the polycrystalline silicon layer; carrying out rapid thermal annealing on the structure at a high temperature; and removing the silicon nitride after the annealing is completed, and then manufacturing a detector.

Description

Germanium photodetector and method for improving its long-wave response through stress memory Technical Field

[0001] The present invention relates to the technical field of germanium photoelectric detectors, and in particular to a germanium photoelectric detector and a method for improving the long-wave response of the germanium photoelectric detector through stress memory. Background Art

[0002] Silicon-based epitaxial germanium photodetectors overcome the long-wavelength cutoff wavelength limitation of silicon detectors at approximately 1.1 microns, extending their response wavelength into the shortwave infrared (SWIR) band (1.1 to 2.5 microns). This extends the infrared O-band (1260 nm to 1360 nm), S-band (1460 nm to 1530 nm), and C-band (1530 nm to 1565 nm), which are crucial for optical communications. Silicon-based epitaxial germanium detectors also offer the advantages of compatibility with CMOS processes and ease of integration, making them a key component in the rapidly emerging silicon-based optoelectronic integrated circuit market, widely used in optical communications, optical sensing, and optical computing. However, germanium's absorption coefficient for infrared light waves drops sharply after 1550nm, making it impossible for germanium photodetectors to meet the detection needs of longer wavelengths of short-wave infrared and even mid-infrared bands. These needs are very important for emerging optical sensing (such as lidar, biomedical testing, etc.) and expanded optical communication bands [such as L-band (1565nm~1625nm), U-band (1625-1675nm)]. Therefore, the application of germanium photodetectors in these fields is limited.

[0003] Academics and industry continue to work hard to improve the long-wave response of germanium photodetectors. They have found that by doping germanium with certain impurities and / or creating tensile strain to reduce the band gap, the absorption coefficient of germanium in the long-wave band can be effectively increased and the cutoff wavelength can be increased, allowing detection of infrared light with wavelengths of 2 microns or even longer. Existing technologies for improving the long-wave response of germanium photodetectors can be divided into several categories:

[0004] Method 1: There is a difference in thermal expansion coefficient between the silicon substrate and the germanium epitaxial material. When germanium is grown on a silicon substrate at high temperature and then returned to room temperature for use, the germanium contracts more than silicon after cooling because the thermal expansion coefficient of germanium is greater than that of silicon, thus generating tensile strain in the germanium.

[0005] Method 2: In a metal-semiconductor-metal (MSM) detector with germanium as the absorption layer, tensile strain is generated in the germanium by utilizing the attachment or proximity of a high stress source layer such as metal or silicon nitride (see patents CN103985788B and CN106653940B);

[0006] Method three is to grow germanium-tin or germanium-lead alloys by in-situ doping of impurities such as tin and lead during epitaxial germanium growth, which changes the energy band structure and reduces the width of the forbidden band, making it possible to detect light waves with longer wavelengths (see patents CN112534590A and CN110729373B).

[0007] All of the above methods effectively improve the absorption coefficient of germanium in the long-wavelength band and extend the long-wavelength detection limit, but each has its own limitations. Method 1, due to the low melting point of germanium and the high temperature exposure to which epitaxial germanium is prone to defects, results in a limited tensile strain effect, generally below 0.3%. Method 2 is only applicable to MSM detectors. Although these detectors have high bandwidth, they suffer from significant dark current and high metal absorption, resulting in a limited signal-to-noise ratio and limited application scenarios. Method 3 significantly improves the long-wavelength response, but growing high-quality germanium-tin and germanium-lead alloys is extremely difficult. The epitaxial thickness is limited, defects are easily generated, and high dark currents are likely to occur, limiting device performance. Furthermore, doping requires additional dopant sources, which can contaminate the epitaxial equipment cavity. Therefore, it is necessary to develop new tensile strained germanium manufacturing technologies to overcome the shortcomings of the above methods, or to serve as a synergistic or complementary method to achieve even better long-wavelength response enhancements.

[0008] Summary of the Invention

[0009] The purpose of the present invention is to provide a germanium photodetector and a method for improving its long-wave response through stress memory, so as to solve the problems existing in the prior art of improving the absorption coefficient of germanium in the long-wave band and extending the long-wave detection limit.

[0010] The present invention discloses a method for improving the long-wave response of a germanium photodetector by using a stress memory technology, comprising the following steps:

[0011] A single crystal germanium layer is selectively grown epitaxially on a single crystal silicon substrate using silicon oxide windows to define positions, and then a polysilicon layer is deposited and etched into a pattern to cover the germanium layer, followed by deposition of silicon nitride, which completely surrounds the polysilicon layer above and around the edges.

[0012] Performing rapid thermal annealing on the structure at high temperature;

[0013] After annealing, the silicon nitride is removed and the detector is made.

[0014] During rapid thermal annealing at high temperatures, various materials undergo thermal expansion according to the linear expansion coefficients listed in Table 1. This expansion is a recoverable elastic deformation. The Young's modulus of each material in Table 1 allows comparison of their stiffness, which decreases in the order of silicon nitride > single-crystalline silicon (the Young's modulus of polycrystalline silicon can be assumed to be the same as that of single-crystalline silicon) > single-crystalline germanium > silicon oxide. The rigid silicon nitride surrounds the polycrystalline silicon from above and on all sides, forcing the polycrystalline to expand primarily downward, squeezing the softer germanium epitaxial layer. Similarly, the silicon substrate also expands upward, squeezing the softer germanium epitaxial layer. The germanium epitaxial layer is stiffer than the surrounding silicon oxide, allowing it to expand outward and squeeze the silicon oxide. Thus, during high-temperature annealing, the germanium epitaxial layer develops strain that compresses vertically and stretches horizontally.

[0015] When the temperature continues to rise to a certain level, the polysilicon grains will undergo recrystallization. At this time, the expansion of the polycrystalline includes both a part of irreversible plastic deformation and a part of recoverable elastic deformation. When the temperature drops to room temperature, the elastic deformation of the polycrystalline disappears, but this part of the plastic deformation is maintained and continues to squeeze the germanium layer downward, so the germanium presents compressive strain in the vertical direction and tensile strain in the horizontal direction. The overall effect is that rapid thermal annealing allows the plastic deformation caused by the recrystallization of the polysilicon to be maintained when it returns to room temperature, and continues to squeeze the germanium epitaxial layer downward, generating vertical compressive strain and horizontal tensile strain in it, reducing the band gap of germanium and increasing the absorption coefficient of long-wave light, thereby producing a stronger response to long-wave light and increasing the cutoff wavelength of photodetection.

[0016] Furthermore, the single crystal silicon substrate is an SOI substrate or a bulk silicon substrate.

[0017] Furthermore, when the single crystal silicon substrate is an SOI substrate, the single crystal silicon substrate comprises three parts from bottom to top: a silicon substrate, a buried silicon oxide layer and a top silicon layer.

[0018] Furthermore, the thickness of the buried silicon oxide layer is 0.1 to 3 microns. A thicker buried oxide layer can reduce the parasitic effect of the substrate in the radio frequency band and improve the bandwidth of active devices, but it will also increase the manufacturing cost of the SOI substrate.

[0019] Furthermore, the thickness of the top silicon layer is greater than 0.2 micrometers, ensuring the quality of silicon-based germanium epitaxy and the lateral conductivity of the P-type doped layer.

[0020] Furthermore, the top silicon layer includes a P layer, a P layer with gradually increasing doping concentration, and a P layer with gradually increasing doping concentration. + Layer and P ++ layer.

[0021] Furthermore, the silicon oxide is deposited twice, namely, a first silicon oxide layer and a second silicon oxide layer are deposited respectively. The first silicon oxide layer is used to define a germanium epitaxial window, and the second silicon oxide layer is used to define a contact hole.

[0022] Furthermore, the thickness of the first silicon oxide layer is 0.1-2 microns.

[0023] Furthermore, the thickness of the second silicon oxide layer is 0.3-1 micrometer.

[0024] Furthermore, the single crystal germanium layer includes an undoped epitaxial germanium layer and a doped epitaxial germanium layer.

[0025] Furthermore, the difference between the thickness of the single crystal germanium layer and the thickness of the first silicon oxide layer does not exceed 0.1 micrometers, ensuring that the surface of the germanium after epitaxy is substantially flat, which is conducive to compatibility with CMOS technology to achieve transistor integration and multi-layer technology connection.

[0026] Furthermore, the thickness of the doped epitaxial germanium layer is 0.05 to 0.2 micrometers, and the surface doping concentration is 1x10 18 ~5x10 20 cm -3 .

[0027] Furthermore, the silicon nitride has a thickness of 0.05 to 0.3 microns.

[0028] Furthermore, the polysilicon layer includes N-doped + layer and doped N ++ layer.

[0029] Furthermore, the pattern of the polysilicon layer after photolithography and etching is slightly larger than the size of the germanium epitaxial window, that is, the width of the polysilicon layer is 0.5-1 micron larger than the width of the upper surface layer of germanium.

[0030] Furthermore, the polysilicon layer has a thickness of 0.05 to 0.2 micrometers and a surface doping concentration of 1x10 18 ~5x10 20 cm -3 .

[0031] Furthermore, the silicon nitride is removed by wet etching.

[0032] Furthermore, the high-temperature rapid thermal annealing temperature ranges from 700° C. to 900° C., which is slightly lower than the melting point of germanium (937° C.), to avoid uncontrollable morphology and defect generation caused by melting recrystallization.

[0033] Furthermore, the high-temperature rapid thermal annealing is performed in an inert atmosphere, and the annealing time is 5 to 300 seconds.

[0034] High-temperature annealing is beneficial for the recrystallization of polysilicon to produce irreversible plastic deformation, thereby generating and memorizing strain in germanium. However, the upper limit of the time should be considered to avoid excessive diffusion of dopants in germanium at high temperatures and the generation of structural defects in germanium.

[0035] Furthermore, the polysilicon may also be replaced by amorphous silicon or a stack of polysilicon and amorphous silicon, which will be converted into polysilicon after high-temperature annealing.

[0036] The second object of the present invention is to protect a germanium photodetector manufactured using the above method.

[0037] Furthermore, a passivation protection layer is provided on the detector, which can improve the reliability and service life of the germanium photodetector.

[0038] Furthermore, the detector is provided with an anti-reflection film to reduce light emission and achieve better application effects.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. Utilizing the differences in stiffness (Young's modulus), thermal expansion (linear thermal expansion coefficient), and plastic deformation between different semiconductor and dielectric materials, a stress memory method is used to generate tensile strain in silicon-based germanium epitaxy, thereby improving the absorption coefficient and photoelectric response of germanium in longer wavelengths.

[0041] 2. Using stress memory technology, the stress source layer silicon nitride exerts gravity, and the polysilicon material memorizes and transmits stress, achieving a horizontal tensile strain effect in the germanium material, improving the photoelectric response in the long-wave infrared direction and increasing the cutoff wavelength;

[0042] 3. Compared with the strain effect caused by the difference in thermal expansion coefficients between germanium and silicon, this method avoids the defects and high process equipment costs of existing doping methods. It also does not have the negative effects of metal-induced strain in MSM detectors, namely higher dark current, optical loss, and the resulting limited signal-to-noise ratio.

[0043] 4. It is suitable for both vertically incident circular detectors and horizontally coupled waveguide PIN and PN detectors, and can also be used for avalanche photodiodes. It is suitable not only for SOI substrates but also for bulk silicon substrates, with high flexibility and good cost-effectiveness.

[0044] 5. The materials and processes are compatible with CMOS, with simple structure and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only represent some embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0046] FIG1 is a schematic diagram of the cross-sectional structure of a germanium photodetector according to the present invention;

[0047] FIG2 is a schematic diagram of the structure of the germanium photodetector after the silicon nitride layer is deposited during the preparation process of the present invention.

[0048] The arrows in the figure represent the direction of strain.

[0049] In the figure: 1-SOI substrate, 2-silicon substrate, 3-buried oxide layer, 4-top silicon layer, 5-first silicon oxide layer, 6-germanium layer, 7-polysilicon layer, 8-silicon nitride layer, 9-second silicon oxide layer. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0051] Example 1

[0052] A vertical-incidence germanium PIN detector that uses a stress memory method to enhance long-wave response, as shown in FIG1-FIG2, comprises the following steps:

[0053] 1. Select an 8-inch P(100)SOI substrate 1, a silicon substrate 2 with a thickness of 725 μm and a resistivity of about 10 ohm·cm, a buried oxide layer 3 with a thickness of 3 μm, and a top silicon layer 4 with a thickness of 0.22 μm;

[0054] 2. Use photoresist to mask other areas and perform medium dose P on the SOI top silicon layer 4 in the germanium photodetector area. + Type ion implantation, which is subsequently used to transport carriers laterally and collect photocurrent;

[0055] 3. Mask other areas with photoresist and perform ring-shaped ion implantation on the top silicon layer 4 of the SOI in the germanium photodetector area to form heavily doped P ++ Type ion implantation, and high temperature rapid thermal annealing activation, and then used to form ohmic contact with the metal electrode;

[0056] 4. Deposit a 0.6 micron first silicon oxide layer 5, and define a window for selective silicon-based germanium epitaxy by photolithography and dry etching the first silicon oxide layer 5;

[0057] 5. Silicon-based selective germanium epitaxy of 0.6 microns, using a two-step germanium epitaxy process of low temperature followed by high temperature to reduce defect density;

[0058] 6. Low-pressure chemical vapor deposition (LPCVD) polysilicon 0.15 micron, removing the polysilicon on the back of the substrate and leaving only the polysilicon on the front;

[0059] 7. Unmasked N-type polysilicon + Ion implantation doping, and then photolithography of the heavily doped polysilicon contact area for N ++ Ion implantation and activation by annealing;

[0060] 8. Photolithography and dry etching of the polysilicon, leaving only the polysilicon above the germanium epitaxial layer, leaving the retained polysilicon layer 7 1 micron wider on both sides than the germanium layer 6 in the cross-sectional view to ensure that the polysilicon can completely cover the germanium;

[0061] 9. Implementing a stress generation and memory process in the germanium layer 6: Depositing a silicon nitride layer 8 (the silicon nitride can deviate from the standard Si3N4 ratio and can be nitrogen-rich or silicon-rich silicon nitride, or oxygen-doped silicon oxynitride) using low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD) equipment. The silicon nitride layer 8 has a thickness of 0.05 to 0.3 microns, and the silicon nitride deposited on the back side is removed. Performing a high-temperature rapid thermal annealing in a nitrogen inert atmosphere: The temperature range is 700 to 900°C, which should be slightly lower than the melting point of germanium, 937°C, to avoid uncontrollable morphology and defect generation caused by melt recrystallization; The annealing time is 5 to 300 seconds in an inert atmosphere. High-temperature annealing is conducive to polycrystalline recrystallization deformation, thereby generating and memorizing strain in the germanium. However, the upper limit of the time should be considered to avoid excessive diffusion of dopants in the germanium and the generation of structural defects in the germanium. After the annealing is completed, the silicon nitride is removed using a hot phosphoric acid wet method. Taking all the above into consideration, this embodiment uses LPCVD to deposit 0.15 micron Si3N4 and remove the silicon nitride on the back of the substrate, performs rapid thermal annealing under nitrogen / 900°C / 10 seconds, and then boils in hot phosphoric acid at 155°C for 60 minutes to remove the silicon nitride.

[0062] 10. Deposit a second silicon oxide layer 9 with a thickness of 0.5 microns as a pre-metal dielectric (PMD)

[0063] 11. Photolithographic Contact Holes: Above the detector's cathode lies a second silicon oxide layer 9, 0.5 microns thick. Above the positive region lies the first silicon oxide layer 5 and the second silicon oxide layer 9, whose combined thickness is 0.6 + 0.5 microns, or 1.1 microns. Given such a significant difference in oxide layer thickness, two photolithography steps should be performed to open the positive and negative contact holes, respectively. This avoids severe overetching of the negative contact hole when the positive contact hole is opened in a single photolithography and etching process, as overetching would cause significant etching of the highly doped polysilicon, reducing the quality of the ohmic contact. This embodiment employs a method of photolithography and etching the positive contact hole first, followed by photolithography and etching of the negative contact hole, to minimize exposure of the negative contact hole on the more complex and fragile germanium layer 6 / polysilicon layer 7.

[0064] 12. Use the conventional two-step annealing method to form low-resistance metal silicide NiSi: 1) Sputter 10 nanometers of metal nickel, and the nickel in the contact hole is in contact with the positive electrode P ++ N in silicon and negative electrode contact area ++ The nickel contacts the polysilicon, and the remaining nickel adheres to the second silicon oxide layer 9 or the silicon oxide sidewall of the contact hole; 2) 280℃ / 300 seconds / nitrogen atmosphere annealing, the nickel in contact with silicon reacts to form high-resistance, nickel-rich silicide Ni x Si phase (x is greater than 1, mainly Ni2Si), the nickel in contact with silicon oxide does not react and remains in the metallic nickel state; 3) Etching in heated concentrated sulfuric acid for 180 seconds to remove the metallic nickel that has not reacted to form silicide; 4) Annealing at 400℃ / 30 seconds / nitrogen atmosphere to transform the nickel-rich silicide into a stable, low-resistance NiSi phase silicide.

[0065] 0.6 micron aluminum was sputtered, metal wiring patterns were photolithographically patterned, contact holes and interconnects were formed by dry etching, and the alloy was annealed at 350°C / 30min in a forming gas (a mixture of 90% nitrogen and 10% hydrogen) environment to complete the metallization step.

[0066] The present invention employs polysilicon and silicon nitride materials to double-wrap the germanium epitaxial layer, and then generates and memorizes horizontal tensile strain in the germanium layer 6 through high-temperature rapid thermal annealing, thereby improving the response in the long-wave direction of infrared light and increasing the cutoff wavelength. Although the technical solutions and embodiments of the present invention are only directed to vertically incident silicon-based germanium PIN detectors, their technical principles and methods are equally applicable to waveguide-type silicon-based germanium PIN or PN detectors with transversely coupled input light, as well as to silicon-based germanium avalanche photodiodes with vertically incident or waveguide-coupled transverse input light. Aside from the composite structure and process in which the germanium epitaxial layer is wrapped with polysilicon and silicon nitride and annealed to generate and memorize strain, the remaining structures and processes are conventional in the structural design and manufacture of silicon-based germanium photodetectors and will not be elaborated upon here.

[0067] Table 1 Young's modulus and linear thermal expansion coefficient of relevant materials (room temperature)

[0068] The above are the implementation methods listed in this embodiment, but this embodiment is not limited to the above optional implementation methods. Those skilled in the art can arbitrarily combine the above methods to obtain other various implementation methods. Anyone can derive other various forms of implementation methods based on the inspiration of this embodiment. The above specific implementation methods should not be understood as limiting the scope of protection of this embodiment. The scope of protection of this embodiment shall be based on the definition in the claims, and the description can be used to interpret the claims.

Claims

1. A method for improving the long-wave response of a germanium photodetector by using stress memory technology, characterized in that: The following steps are involved: A single crystal germanium layer is selectively grown by epitaxially opening a window with silicon oxide on a single crystal silicon substrate to define a position, and then a polysilicon layer is deposited and etched into a pattern covering the germanium layer, and then silicon nitride is deposited, and the silicon nitride completely surrounds the polysilicon layer above and around; Performing rapid thermal annealing on the structure at high temperature; After annealing, the silicon nitride is removed and the detector is made.

2. The method of improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 1, characterized in that: The single crystal silicon substrate is an SOI substrate or a bulk silicon substrate.

3. The method for improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 2, characterized in that: When the single crystal silicon substrate is an SOI substrate, the single crystal silicon substrate comprises three parts from bottom to top: a silicon substrate, a buried silicon oxide layer and a top silicon layer.

4. The method of improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 1, characterized in that: The thickness of the polysilicon layer is 0.05 to 0.2 micrometers, and the surface doping concentration is 1x10 18 ~5x10 20 cm -3 .

5. The method of improving the long-wave response of germanium photodetector by stress memory technology according to claim 1, characterized in that: The thickness of the silicon nitride is 0.05 to 0.3 microns.

6. The method of improving the long-wave response of germanium photodetector by stress memory technology according to claim 1, characterized in that: The silicon oxide is deposited twice to deposit a first silicon oxide layer and a second silicon oxide layer respectively. The first silicon oxide layer is used to define a germanium epitaxial window, and the second silicon oxide layer is used to define a contact hole.

7. The method of improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 1, characterized in that: The high temperature rapid thermal annealing temperature ranges from 700 to 900°C.

8. The method of improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 1, characterized in that: The high-temperature rapid thermal annealing is performed in an inert atmosphere, and the annealing time is 5 to 300 seconds.

9. The method of improving the long-wave response of a germanium photodetector by using stress memory technology according to claim 1, characterized in that: The single crystal germanium layer includes a non-doped epitaxial germanium layer and a doped epitaxial germanium layer.

10. A germanium photodetector, characterized in that: The method for improving the long-wave response of a germanium photodetector by using stress memory technology is used as described in any one of claims 1 to 9.

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