Method for manufacturing a light receiving element

Abstract

To provide a manufacturing method of a light receiving element, capable of only performing a selection etching of a phosphate deposit on a wafer front surface.SOLUTION: A manufacturing method of a light receiving element, contains: a step of forming an InGaAs contact layer onto an InP cap layer by using an epitaxial wafer in which an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer are grown on an InP substrate; a step of forming an InGaAs contact part by removing one part of the InGaAs contact layer by a photo-lithography method; a step of forming a protection film so as to cover the InP cap layer and the InGaAs contact part; a step of forming an open pattern part to one part of the protection film; a step of diffusing an impurity in an atmosphere containing phosphate in the open pattern part; a step of selectively removing a phosphate deposit adhered onto a front surface in the step of diffusing the impurity by using a strongly basic solution; and a step of forming an electrode to the InGaAs contact part and the InP substrate back surface.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a light-receiving element. [Background technology]

[0002] Semiconductor optical elements that have an n-type semiconductor layer, a light absorbing layer, and a p-type semiconductor layer in that order are commonly used. The light absorbing layer is an intrinsic layer (also called an i-layer or an undoped layer) to which no impurities are intentionally added (doped). Such semiconductor optical elements generally have a so-called PIN structure in which the light absorbing layer is sandwiched between a p-type semiconductor layer and an n-type semiconductor layer (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-105105 A Summary of the Invention [Problem to be solved by the invention]

[0004] In a manufacturing method in which an epitaxial wafer having a photodiode (PD) structure is made using InGaAs / InP-based materials and impurities are diffused to produce a PD having a PIN structure (hereinafter, sometimes simply referred to as "PIN-PD"), phosphorus may precipitate on the surface of the PIN-PD structure due to the atmosphere in the diffusion process after the impurity diffusion process. Phosphorus precipitation is likely to occur when the temperature drop rate is high. Phosphorus precipitates inhibit the light reception of the PD, reducing its sensitivity, and also become an ohmic obstruction factor when forming the contact electrode, so they must be removed before forming the electrode.

[0005] Before impurity diffusion, a protective film such as SiO2 is formed and patterned to expose the semiconductor portion from the opening, and impurities are diffused from the exposed region to form a P / N junction inside the epitaxial wafer. During impurity diffusion, the pressure or temperature must be maintained at or above the phosphorus dissociation pressure, so the impurity diffusion process is performed while maintaining the atmosphere as a phosphorus-containing atmosphere. After diffusion process, it is desirable to quickly lower the temperature to prevent excessive diffusion.

[0006] However, if the cooling rate is high, the wafer surface cools quickly, which makes it easier for phosphorus in the atmosphere to precipitate on the wafer surface. The precipitated phosphorus reduces the light transmittance, degrading the light receiving characteristics of the photodiode. The precipitated phosphorus also increases the ohmic resistance when an electrode is formed on the wafer.

[0007] Thus, phosphorus precipitates present on the wafer surface are a cause of deterioration of the light receiving element characteristics and should be removed. Therefore, a method is required to selectively remove only the phosphorus precipitates without changing the structure of the wafer. However, there is no known disclosure of a technique related to selective etching that removes only the phosphorus precipitates and does not etch the InGaAs and InP that constitute the wafer.

[0008] It is known that a hydrochloric acid-based etchant is suitable for etching only InP, and a mixture of hydrogen peroxide and acid is suitable for etching only InGaAs. Phosphorus precipitates are an easy-to-etch material, and can be etched with any type of acid. However, no technical disclosure of an etchant that etches only phosphorus precipitates and has selectivity for InP and InGaAs has been known until now.

[0009] The present invention has been made to solve the above-mentioned problems, and has an object to provide a method for manufacturing a photodetector that can selectively remove phosphorus precipitates that have been deposited on the wafer surface after an impurity diffusion process in a PIN-PD manufacturing process using an InGaAs / InP-based material (epitaxial wafer), by etching only the phosphorus precipitates that have been deposited on the wafer surface after an impurity diffusion process, and by selective etching that minimizes etching of InP and InGaAs. [Means for solving the problem]

[0010] The present invention has been made to achieve the above-mentioned object, and provides a method for manufacturing a photodetector by using an epitaxial wafer in which an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer are epitaxially grown in this order on an InP substrate, the method including the steps of forming an InGaAs contact layer on the InP cap layer, removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion, forming a protective film so as to cover the InP cap layer and the InGaAs contact portion, forming an opening pattern portion in a portion of the protective film so as to include the InGaAs contact portion, diffusing an impurity into the opening pattern portion in a phosphorus-containing atmosphere, selectively removing phosphorus precipitates attached to the surface in the impurity diffusing step by using a strongly basic aqueous solution, and forming electrodes on the InGaAs contact portion and the back surface of the InP substrate.

[0011] According to this method for manufacturing a photodetector, in the PIN-PD manufacturing process using an InGaAs / InP-based material (epitaxial wafer), etching of InP and InGaAs can be suppressed while selectively removing phosphorus precipitates that have formed on the wafer surface after the impurity diffusion process.

[0012] In this case, the impurity may be zinc (Zn) and the protective film may be SiO2.

[0013] These materials can be easily made to have a p-type conductivity and can also be easily provided with a protective film.

[0014] In this case, the strongly basic aqueous solution may be one in which one or more bases selected from tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH, and Ca(OH)2 are dissolved in water.

[0015] This makes it possible to more reliably and stably etch away only the phosphorus precipitates.

[0016] In this case, the strongly basic aqueous solution may have a pH of more than 12.

[0017] This makes it possible to more reliably and stably etch away only the phosphorus precipitates. Effect of the Invention

[0018] As described above, the method for manufacturing a photodiode according to the present invention makes it possible to selectively remove phosphorus precipitates that have been deposited on the wafer surface after the impurity diffusion process while suppressing etching of InP and InGaAs in the PIN-PD manufacturing process using InGaAs / InP-based materials (epitaxial wafers). Therefore, high-quality PIN-PDs can be manufactured with high yields. [Brief description of the drawings]

[0019] [Figure 1] 1 is a diagram illustrating a manufacturing process of a light receiving element. [Diagram 2] The manufacturing process of the photodetector is shown next to FIG. [Diagram 3] The manufacturing process of the photodetector is shown next to FIG. [Figure 4] The manufacturing process of the light-receiving element continues from FIG. [Diagram 5] The graph shows the results of evaluating the thickness of the phosphorus precipitate and the thickness of the InGaAs contact part versus etching time when a PIN-PD having phosphorus precipitates after impurity diffusion is etched using a strong basic aqueous solution. [Figure 6] The effect of the etching process on the formation of ohmic contacts was investigated, and the results of comparing the resistivity of the comparative example and the example are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention will be described in detail below, but the present invention is not limited thereto.

[0021] As described above, in the PIN-PD manufacturing process using InGaAs / InP-based materials (epitaxial wafers), there has been a demand for a method for manufacturing a photodetector that can selectively remove phosphorus precipitates by etching only the phosphorus precipitates that have formed on the wafer surface after the impurity diffusion process and by using selective etching that minimizes etching of InP and InGaAs.

[0022] As a result of intensive research into the above-mentioned problems, the present inventors have discovered a method for manufacturing a photodetector using an epitaxial wafer in which an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer are epitaxially grown in this order on an InP substrate, the method comprising the steps of: forming an InGaAs contact layer on the InP cap layer; removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; forming a protective film so as to cover the InP cap layer and the InGaAs contact portion; The inventors have found that a method for manufacturing a light-receiving element, which includes the steps of: forming an aperture pattern portion so as to include an impurity; diffusing an impurity into the aperture pattern portion in a phosphorus-containing atmosphere; selectively removing phosphorus precipitates attached to the surface in the impurity diffusing step using a strongly basic aqueous solution; and forming electrodes on the InGaAs contact portion and the back surface of the InP substrate, can selectively remove phosphorus precipitates that have been deposited on the wafer surface after an impurity diffusion process while suppressing etching of InP and InGaAs in a PIN-PD manufacturing process using an InGaAs / InP material (epitaxial wafer), and have completed the present invention.

[0023] The following description will be given with reference to the drawings.

[0024] As shown in FIG. 1, a first conductive type InP substrate 1 is used as a starting substrate. After a non-doped InP buffer layer 2 having a thickness of, for example, 1 μm is laminated, a non-doped InP buffer layer having a thickness of, for example, 3 μm is laminated.x Ga 1-x An epitaxial wafer 10 is prepared by epitaxially growing an InGaAs light absorption layer 3 such as As (0.4≦x≦0.6) and an undoped InP cap layer 4 having a thickness of, for example, 1 μm in this order.

[0025] Further, on the InP cap layer 4, for example, a non-doped In x Ga 1-x An InGaAs contact layer 5A such as As (0.4≦x≦0.6) is epitaxially grown to form an epitaxial wafer having a light receiving element structure as an epitaxial functional layer (step of forming an InGaAs contact layer).

[0026] 2, a part of the InGaAs contact layer 5A is removed by photolithography to form an InGaAs contact portion 5B (a step of forming an InGaAs contact portion). For pattern formation, a sulfuric acid / hydrogen peroxide solution can be used, which selectively etches InGaAs quickly and has an extremely slow etching rate for the InP layer.

[0027] Next, as shown in Fig. 3, after the InGaAs contact portion 5B is formed, a protective film 6 made of SiO2 or the like is formed on the surface. An opening pattern 7 is formed in a part of the formed protective film, and impurities for imparting a second conductivity type to the opening pattern 7 are diffused in a phosphorus-containing atmosphere (a step of diffusing impurities). SiO2 is preferable as the protective film because it can be formed relatively easily.

[0028] In the process of diffusing impurities, for example, when diffusing Zn to impart p-type conductivity, the diffusion process is carried out by maintaining a temperature of 400°C or higher in a phosphorus-containing atmosphere at normal pressure and mixing in a Zn source gas such as dimethyl zinc (DMZn). Specifically, a phosphorus-containing atmosphere is created by flowing phosphine (PH3) gas into an atmosphere with hydrogen (H2) as a carrier gas, and the partial pressure is 1×10 -6 The phosphorus-containing atmosphere is maintained during the temperature increase, and the phosphorus-containing atmosphere is maintained up to 400°C during the temperature decrease.

[0029] After the diffusion process, the wafer is taken out and immersed in a strong basic solution (a solution containing a strong basic material) to etch and remove the phosphorus precipitates (a process for selectively removing phosphorus precipitates). By using a strong basic solution to remove the phosphorus precipitates attached to the wafer surface in this way, etching of InP and InGaAs is minimized, and only the phosphorus precipitates that have precipitated on the wafer surface after the impurity diffusion process can be selectively etched away.

[0030] The strong base material is preferably one that has a pH>12 when dissolved in an aqueous solution. Examples of such strong base materials include tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH, and Ca(OH)2, and an aqueous solution in which one or more bases selected from these materials are dissolved in water can be used. Such a strong base aqueous solution can more reliably and stably etch away only the phosphorus precipitate.

[0031] Table 1 shows the results of an investigation into the selectivity of etching for phosphorus precipitates and etching for both InGaAs and InP when a wafer is immersed in an acidic aqueous solution as an etchant, and Table 2 ...

[0032] A substrate having an InP / InGaAs structure is used to selectively etch InP to form an InP pattern, and a substrate having an InGaAs / InP structure is used to selectively etch InGaAs to form an InGaAs pattern. A diffusion process is performed to form phosphorus precipitates on the substrate surface. These substrates are immersed in various etchants. If there is selective etching of phosphorus precipitates, the thickness and size of the patterned InP layer or InGaAs layer do not change, and only the phosphorus precipitates disappear. In this case, it is judged that there is "etching selectivity". On the other hand, if the thickness and size of the InP layer or InGaAs layer change, it is judged that there is "no etching selectivity". In addition, if there is etching selectivity for InGaAs and no etching selectivity for InP, it is also judged as "no etching selectivity". The same applies to the reverse case. In addition, when the patterned InP or InGaAs layer portion is etched, there is no etching selectivity, but the phosphorus precipitates are removed together with the underlayer (InP or InGaAs layer portion) and the phosphorus precipitates themselves no longer exist, so this is defined as "removed."

[0033] As shown in Table 1, although the acid etchant has an etching ability for phosphorus precipitates, the etching rate for InGaAs and InP is not slower than the etching rate for phosphorus precipitates, and it was found that the etching selectivity is almost nonexistent.

[0034] [Table 1]

[0035] As shown in Table 2, when a strong basic etchant was used, etching of phosphorus precipitates proceeded, but the etching rate of InGaAs and InP was slow, indicating high etching selectivity.

[0036] [Table 2]

[0037] In addition, Fig. 5 shows the plot of the thickness of phosphorus precipitates and the thickness of the InGaAs contact part against the etching time when a 2.38% TMAH solution was used as an etchant for removing phosphorus precipitates for PIN-PD after impurity diffusion. The thickness of the phosphorus precipitates was measured by processing the cross-section after processing and observing it with a SEM. In addition, for thin films, TEM samples were prepared and observed. The phosphorus precipitate thickness monotonically decreased, whereas the thickness of the InGaAs part was almost constant. Although not shown in the figure, even in the etching evaluation of the InP layer alone, the etching of the InP layer was extremely small, being equal to or less than that of InGaAs. It can be seen that by using a strong basic solution such as TMAH, the phosphorus precipitates are selectively etched, and InGaAs and InP are hardly etched. The concentration of TMAH is preferably 0.5% or more. It is preferable that the aqueous solution containing other strong basic materials can maintain a pH>12, and any concentration can be selected.

[0038] After the process of selectively removing the phosphorus precipitate, an electrode 8 is formed on the InGaAs contact portion of the opening pattern 7 as shown in Fig. 4. An electrode 9 is also formed on the back surface of the InP substrate 1 to manufacture a light receiving element (a process of forming an electrode). EXAMPLES

[0039] The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

[0040] (Example) Starting from an n-type InP substrate, a 1 μm thick non-doped InP buffer layer was laminated, and then a 3 μm thick non-doped In 0.5 Ga 0.5 An epitaxial wafer was prepared by successively growing an As light absorption layer and a 1 μm-thick non-doped InP cap layer. A 0.15 μm-thick non-doped In was grown on the InP cap layer. 0.5 Ga 0.5 An As contact layer was formed and an epitaxial wafer having a light-receiving element structure as an epitaxial functional layer was prepared (Figure 1).

[0041] In 0.5 Ga 0.5 Part of the As contact layer was removed by photolithography to form the InGaAs contact area. A sulfuric acid / hydrogen peroxide solution was used for pattern formation (Figure 2).

[0042] In 0.5 Ga 0.5 After forming the As contact area, a SiO2 film was formed on the surface as a protective film. An opening pattern was formed in part of the formed SiO2 film, and Zn was diffused into the opening pattern area (Figure 3). Zn diffusion was performed by maintaining a temperature of 400°C or higher in a phosphorus-containing atmosphere at normal pressure, flowing phosphine (PH3) gas with hydrogen (H2) as the carrier gas to create a phosphorus-containing atmosphere, and the partial pressure of the PH3 gas was 1×10 -6 The atmosphere was maintained at a pressure of atm or higher. Dimethyl zinc (DMZn) gas was mixed into the atmosphere to carry out the diffusion process. The phosphorus-containing atmosphere was maintained during the temperature rise, and the phosphorus-containing atmosphere was maintained during the temperature fall up to 400°C.

[0043] After the diffusion process, the wafer was removed and immersed in a 2.38% tetramethylammonium hydroxide (TMAH) aqueous solution with a pH of 13 for 190 minutes to etch the phosphorus precipitates. After that, electrodes were formed on the InGaAs contact parts of the opening pattern, and an electrode was also formed on the back side of the n-type InP substrate to produce a photodetector (Figure 4).

[0044] (Comparative Example) A light-receiving element was manufactured under the same conditions as in the example, except that the step of etching the phosphorus precipitate by immersing it in an aqueous solution of tetramethylammonium hydroxide (TMAH) was not performed.

[0045] Figure 6 shows data comparing the resistivity of an electrode formed on an untreated wafer (Comparative Example) with the resistivity of an electrode formed on a wafer etched with TMAH for 190 minutes (Example). It can be seen that the resistance of the Comparative Example (untreated product) is high, while the resistance of the Example (etched product) is about two orders of magnitude lower. This shows that phosphorus precipitates, which are an obstacle when forming ohmic contacts, were removed by etching, and that the shape, including the thickness, of the contact layer was hardly changed by the selective removal of phosphorus precipitates, indicating that the etching process does not adversely affect the formation of ohmic contacts.

[0046] As described above, according to the examples of the present invention, it is possible to selectively remove phosphorus precipitates by treatment with a strongly basic aqueous solution, and it has been found that the removal does not adversely affect the underlying epitaxial layer.

[0047] The present specification includes the following aspects. [1]: A method for manufacturing a photodiode using an epitaxial wafer in which an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer are epitaxially grown in that order on an InP substrate, comprising: forming an InGaAs contact layer on the InP cap layer; removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; forming a protective film to cover the InP cap layer and the InGaAs contact portion; forming an opening pattern portion in a portion of the protective film so as to include the InGaAs contact portion; diffusing impurities into the opening pattern portion in a phosphorus-containing atmosphere; selectively removing phosphorus precipitates that have adhered to the surface during the impurity diffusing step using a strongly basic aqueous solution; forming electrodes on the InGaAs contact portion and on a rear surface of the InP substrate; [2]: The method for manufacturing the light-receiving element according to [1] above, wherein the impurity is zinc (Zn) and the protective film is SiO2. [3]: The method for producing a light-receiving element according to [1] or [2] above, wherein the strongly basic aqueous solution is prepared by dissolving one or more bases selected from tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH and Ca(OH)2 in water. [4]: The method for producing a light-receiving element according to the above [1], [2] or [3], wherein the strongly basic aqueous solution has a pH of more than 12.

[0048] The present invention is not limited to the above-described embodiment. The above-described embodiment is merely an example, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included in the technical scope of the present invention. [Explanation of symbols]

[0049] 1...InP substrate; 2...InP buffer layer; 3...InGaAs light absorption layer; 4...InP cap layer, 5A...InGaAs contact layer, 5B...InGaAs contact portion, 6...protective film, 7...opening pattern, 8, 9...electrodes, 10...epitaxial wafer.

Claims

1. A method for manufacturing a light-receiving element, comprising the steps of: manufacturing a light-receiving element using an epitaxial wafer in which an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer are epitaxially grown in this order on an InP substrate, the method comprising the steps of: forming an InGaAs contact layer on the InP cap layer; removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; forming a protective film to cover the InP cap layer and the InGaAs contact portion; forming an opening pattern portion in a part of the protective film so as to include the InGaAs contact portion; diffusing impurities into the opening pattern portion in a phosphorus-containing atmosphere; selectively removing phosphorus precipitates that have adhered to the surface during the impurity diffusing step using a strongly basic aqueous solution; and forming electrodes on the InGaAs contact portion and on a rear surface of the InP substrate.

2. The impurity is zinc (Zn), and the protective film is SiO ２ 2. The method for manufacturing a light-receiving element according to claim 1,

3. The strong basic aqueous solution may include tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH, and Ca(OH). ２ 2. The method for producing a light-receiving element according to claim 1, wherein one or more bases selected from the following are dissolved in water.

4. The strong basic aqueous solution may include tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH, and Ca(OH). ２ 3. The method for producing a light-receiving element according to claim 2, wherein one or more bases selected from the following are dissolved in water.

5. 5. The method for manufacturing a light-receiving element according to claim 1, wherein the strongly basic aqueous solution has a pH of more than 12.

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