Method for manufacturing light-reception element

Abstract

The invention provides a method for manufacturing a light receiving element, which can selectively etch only phosphorus deposits deposited on the surface of a wafer. This method for manufacturing a light-receiving element uses an epitaxial wafer in which an InP buffer layer, an InGaAs light-absorbing layer, and an InP capping layer are grown on an InP substrate, and comprises: a step in which an InGaAs contact layer is formed on the InP capping layer; a step in which a portion of the InGaAs contact layer is removed by photolithography to form an InGaAs contact part; a step for forming a protective film so as to cover the InP capping layer and the InGaAs contact part; a step for forming an opening pattern portion in a portion of the protective film; a step for diffusing an impurity into the opening pattern portion in a phosphorus-containing gas atmosphere; a step for selectively removing phosphorus deposits adhering to the surface in the step for diffusing impurities, using a strongly alkaline aqueous solution; and a step for forming an electrode on the InGaAs contact part and the rear surface of the InP substrate.

Description

Technical Field

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

[0002] Generally, a semiconductor optical element is used, which has an n-type semiconductor, a light absorbing layer, and a p-type semiconductor layer in this order. The light absorbing layer is an intrinsic layer (also called i-layer, undoped layer, etc.) to which impurities are intentionally not added (doped). Such a semiconductor optical element generally has 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 literature Patent Literature

[0003] Patent Document 1: Japanese Patent Application Publication No. 2009-105105 Summary of the invention Technical problem to be solved by the invention

[0004] A manufacturing method is to use InGaAs / InP system materials to make an epitaxial wafer having a photodiode (PD) structure, and diffuse impurities to form a PD having a PIN structure (hereinafter sometimes simply referred to as "PIN-PD"). In this manufacturing method, phosphorus caused by the gas environment of the diffusion process may be precipitated on the surface of the PIN-PD structure after the impurity diffusion process. Phosphorus precipitation is easy to occur when the cooling rate is increased. Phosphorus precipitates will hinder the light reception of the PD and reduce the sensitivity, and will become the main cause of Ohmic hindrance when forming a contact electrode, so it must be removed before forming the electrode.

[0005] Before the impurity diffusion, for example, a protective film such as SiO2 is formed and patterned so that the semiconductor part is exposed from the opening, and the impurities are diffused from the exposed area, thereby forming a PN junction inside the epitaxial wafer. During the impurity diffusion, in order to maintain a pressure or temperature above the phosphorus dissociation pressure, the gas environment is maintained in a phosphorus-containing gas environment while the impurity diffusion process is performed. After the diffusion process, in order to prevent excessive diffusion, it is advisable to cool down quickly.

[0006] However, if the cooling rate is high, the wafer surface will cool quickly, so the phosphorus in the gas environment will easily precipitate on the wafer surface. Since the transmittance of light will be reduced by the precipitated phosphorus, the light receiving characteristics of the light receiving element will be deteriorated. In addition, the precipitated phosphorus will also become the main cause of the increase in ohmic resistance when forming electrodes on the wafer.

[0007] As described above, phosphorus precipitates existing on the surface of the wafer become the main cause of deterioration of the characteristics of the light-receiving element and should be removed. Therefore, a method is required for selectively removing only the phosphorus precipitates without changing the structure of the wafer side. However, there is no known technical invention regarding a selective etching method that removes only the phosphorus precipitates without etching the InGaAs and InP constituting the wafer.

[0008] When etching only InP, a hydrochloric acid-based etchant is more suitable, and when etching only InGaAs, a mixture of hydrogen peroxide and acid is more suitable. Phosphorus precipitates are easily etched materials, and any type of acid can be etched. However, a technical invention of an etchant that etches only phosphorus precipitates and is selective for InP and InGaAs has not yet been known.

[0009] The present invention is completed to solve the above-mentioned problems to be solved, and its purpose is to provide a method for manufacturing a light-receiving element, which can selectively remove phosphorus precipitates by selective etching in the PIN-PD manufacturing process using InGaAs / InP system materials (epitaxial wafers). The selective etching only etches the phosphorus precipitates precipitated on the wafer surface after impurity diffusion treatment and etches the InP and InGaAs very little. Technical means of solving problems

[0010] The present invention is completed to achieve the above-mentioned purpose, and provides a method for manufacturing a light-receiving element, which uses an epitaxial wafer to manufacture the light-receiving element, and the epitaxial wafer is formed by epitaxially growing an InP buffer layer, an InGaAs light absorption layer, and an InP cap layer in sequence on an InP substrate, and the manufacturing method includes: a process of forming an InGaAs contact layer on the InP cap layer; a process of removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; a process of forming a protective film in a manner of covering the InP cap layer and the InGaAs contact portion; a process of forming an opening pattern portion in a portion of the protective film in a manner of including the InGaAs contact portion; a process of diffusing impurities to the opening pattern portion in a phosphorus-containing gas environment; a process of selectively removing phosphorus precipitates attached to the surface in the process of diffusing the impurities using a strong alkaline aqueous solution; and a process of forming electrodes on the InGaAs contact portion and the back side of the InP substrate.

[0011] By using such a method for manufacturing a light-receiving element, in a PIN-PD manufacturing process using an InGaAs / InP system material (epitaxial wafer), etching of InP and InGaAs can be suppressed, and phosphorus precipitates precipitated on the wafer surface after an impurity diffusion process can be selectively removed.

[0012] At this time, the impurity can be zinc (Zn) and the protective film can be SiO 2 .

[0013] According to this configuration, p-type conductivity can be easily formed, and a protective film can be easily formed.

[0014] At this time, an aqueous solution prepared by dissolving one or more alkalis selected from the following in water can be used as the strong alkaline aqueous solution: tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH and Ca(OH)2.

[0015] This makes it possible to more reliably and stably remove only the phosphorus precipitates by etching.

[0016] In this case, an aqueous solution having a pH exceeding 12 can be used as the strongly alkaline aqueous solution.

[0017] This makes it possible to more reliably and stably remove only the phosphorus precipitates by etching. Effects of the Invention

[0018] As described above, the method for manufacturing a light-receiving element of the present invention can suppress the etching of InP and InGaAs in the PIN-PD manufacturing process using InGaAs / InP-based materials (epitaxial wafers), and selectively remove phosphorus precipitates precipitated on the wafer surface after impurity diffusion treatment. Therefore, high-quality PIN-PDs can be manufactured with a high yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a diagram for explaining the manufacturing process of the light receiving element. Figure 2 Show Figure 1 The subsequent manufacturing process of the light receiving element. Figure 3 Show Figure 2 The subsequent manufacturing process of the light receiving element. Figure 4 Show Figure 3 The subsequent manufacturing process of the light receiving element. Figure 5 The following is a result of evaluating the thickness of the phosphorus precipitate and the thickness of the InGaAs contact portion with respect to the etching time when etching is performed using a strong alkaline aqueous solution on a PIN-PD having phosphorus precipitates after impurity diffusion. Figure 6 The resistivity of the comparative example and the resistivity of the example are compared to show the results of investigating the influence of the etching process on the formation of the ohmic contact. DETAILED DESCRIPTION

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

[0021] As described above, a method for manufacturing a light-receiving element has long been sought, which can selectively remove phosphorus precipitates by selective etching in the PIN-PD manufacturing process using InGaAs / InP system materials (epitaxial wafers), and this selective etching only etches the phosphorus precipitates precipitated on the wafer surface after impurity diffusion treatment and etches the InP and InGaAs very little.

[0022] After repeated research on the above-mentioned problem to be solved, the inventors found the following facts and completed the present invention: a method for manufacturing a light-receiving element, which uses an epitaxial wafer to manufacture the light-receiving element, and the epitaxial wafer is formed by epitaxially growing an InP buffer layer, an InGaAs light-absorbing layer, and an InP cap layer in sequence on an InP substrate, and the manufacturing method includes: a step of forming an InGaAs contact layer on the InP cap layer; a step of removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; a step of forming a protective film in a manner of covering the InP cap layer and the InGaAs contact portion; and a step of forming a protective film on the protective film. A process for forming an opening pattern portion in a portion of the protective film in a manner that includes the InGaAs contact portion; a process for diffusing impurities to the opening pattern portion in a phosphorus-containing gas environment; a process for selectively removing phosphorus precipitates attached to the surface in the process of diffusing impurities using a strong alkaline aqueous solution; and a process for forming electrodes on the InGaAs contact portion and the back side of the InP substrate; through the manufacturing method of the light-receiving element, in the PIN-PD manufacturing process using InGaAs / InP system materials (epitaxial wafers), the etching of InP and InGaAs can be suppressed, and the phosphorus precipitates precipitated on the wafer surface after the impurity diffusion treatment can be selectively removed.

[0023] The present invention will be described below with reference to the accompanying drawings.

[0024] like Figure 1 As shown, a first conductive type InP substrate 1 is used as a starting substrate. For example, a 1 μm thick undoped InP buffer layer 2 is stacked, and then a 3 μm thick undoped InP buffer layer 2 is stacked. x Ga 1－x An InGaAs light absorbing layer 3 of As (0.4≤x≤0.6) or the like and an undoped InP cap layer 4 of, for example, 1 μm thick are epitaxially grown in sequence, thereby preparing an epitaxial wafer 10 .

[0025] Furthermore, a 0.15 μm thick undoped In x Ga 1－xAn InGaAs contact layer 5A of As (0.4≤x≤0.6) or the like is epitaxially grown on the InP cap layer 4 to form an epitaxial wafer having a light-receiving element structure as an epitaxial functional layer (a step of forming an InGaAs contact layer).

[0026] Then, if Figure 2 As shown, a portion 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). When forming the pattern, a sulfuric acid hydrogen peroxide solution can be used, which can rapidly and selectively etch InGaAs and very slowly etch the InP layer.

[0027] Then, if Figure 3 As shown, after forming the InGaAs contact portion 5B, a protective film 6 composed of SiO2 or the like is formed on the surface. An opening pattern 7 is formed on a portion of the formed protective film, and an impurity for imparting the second conductivity type is diffused into the opening pattern 7 portion in a phosphorus-containing gas environment (a process of diffusing impurities). SiO2 is preferably used as the protective film because it can be formed relatively easily.

[0028] In the process of diffusing impurities, for example, when diffusing Zn, which imparts p-type conductivity, a Zn source gas such as dimethyl zinc (DMZn) is mixed in a phosphorus-containing gas atmosphere at normal pressure while maintaining a temperature of 400°C or higher. Specifically, phosphine (PH3) gas is flowed into a gas atmosphere using hydrogen (H2) as a carrier gas to form a phosphorus-containing gas atmosphere, and the partial pressure is maintained at 1×10 －6 The phosphorus-containing gas atmosphere is maintained during the temperature increase and during the temperature decrease until 400°C.

[0029] After the diffusion process, the wafer is taken out and immersed in a strong alkaline aqueous solution (aqueous solution containing a strong alkaline material) to remove the phosphorus precipitates by etching (a process of selectively removing the phosphorus precipitates). By using a strong alkaline aqueous solution to remove the phosphorus precipitates attached to the surface of the wafer, the etching of InP and InGaAs is extremely small, so that only the phosphorus precipitates precipitated on the surface of the wafer after the impurity diffusion process can be selectively etched and removed.

[0030] When the strong alkaline material forms an aqueous solution, it is preferably pH>12. It should be noted that the upper limit of the pH can be, for example, below 14 (pH≤14). In addition, such a strong alkaline material can be selected, for example: tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH, Ca(OH)2, etc., and an aqueous solution formed by dissolving one or more alkalis selected from these materials in water can be used. If it is such a strong alkaline aqueous solution, it is possible to more reliably and stably etch and remove only the phosphorus precipitate.

[0031] Table 1 shows the results of investigating the etching properties of phosphorus precipitates and the selectivity of etching properties between InGaAs and InP when the wafer is immersed in an acidic aqueous solution as an etchant, and Table 2 shows the results of investigating the etching properties of phosphorus precipitates and the selectivity of etching properties between InGaAs and InP when the wafer is immersed in a strong alkaline aqueous solution as an etchant. The etching selectivity and phosphorus precipitate etching properties were confirmed by the following method.

[0032] Prepare a substrate having an InP / InGaAs structure by selectively etching InP to form an InP pattern, and a substrate having an InGaAs / InP structure by selectively etching InGaAs to form an InGaAs pattern, and perform diffusion treatment to form phosphorus precipitates on the substrate surface. Immerse these substrates in various etchants. When there is selective etching of phosphorus precipitates, the thickness and size of the patterned InP layer or InGaAs layer will not change, and only the phosphorus precipitates will disappear. At this time, it is judged as "having etching selectivity" (etching selectivity: ○). On the other hand, when the thickness and size of the InP layer or InGaAs layer changes, it is judged as "having no etching selectivity" (etching selectivity: ×). It should be noted that when there is etching selectivity for InGaAs and no etching selectivity for InP, it is also regarded as "having no etching selectivity" (etching selectivity: ×). The same is true when it is the opposite. In addition, when the patterned InP portion or InGaAs layer portion has been etched, although there is no etching selectivity, the phosphorus precipitates have been removed together with the underlying layer (InP layer portion or InGaAs layer portion). Since the phosphorus precipitates themselves have disappeared, they are defined as "removed" (phosphorus precipitates: ○).

[0033] As shown in Table 1, it is known that the acidic etchant has etching properties for phosphorus precipitates (phosphorus precipitates: ○), but the etching rate for InGaAs and InP is not slow relative to the etching rate for phosphorus precipitates, and there is almost no etching selectivity (etching selectivity: ×).

[0034] [Table 1] hydrochloric acid sulfuric acid Phosphoric acid Hydrofluoric acid Acetic acid Citric Acid Malonic acid <![CDATA[NH3]]> Phosphorus precipitate ○ ○ ○ ○ ○ ○ ○ ○ Etch selectivity × × × × × × × ×

[0035] As shown in Table 2, it can be seen that when a strong alkaline etchant is used, although phosphorus precipitates are etched (phosphorus precipitates: ○), the etching rate for InGaAs and InP is slow and the etching selectivity is high (etching selectivity: ○).

[0036] [Table 2] TMAH TEAH TBAH KOH NaOH <![CDATA[Ca(OH)2]]> Phosphorus precipitate ○ ○ ○ ○ ○ ○ Etch selectivity ○ ○ ○ ○ ○ ○

[0037] in addition, Figure 5 The graph in Figure 2 shows the thickness of the phosphorus precipitate and the thickness of the InGaAs contact portion plotted against the etching time when a 2.38% TMAH aqueous solution is used as an etchant for removing phosphorus precipitates from a PIN-PD after impurity diffusion. The thickness of the phosphorus precipitate is measured by performing cross-sectional shape processing after treatment and performing SEM observation. In addition, in the case of a thin film, a TEM (transmission electron microscope) sample is made and observed. The thickness of the phosphorus precipitate decreases monotonically, and the thickness of the InGaAs portion is relatively almost constant. Although not shown in the figure, in the etching evaluation of the InP layer alone, the etching of the InP layer is also the same as or less than that of InGaAs and is very small. It can be seen that by using a strong alkaline aqueous solution such as TMAH, the phosphorus precipitate has been selectively etched, and InGa As and InP are almost not etched. It should be noted that the concentration of TMAH is preferably above 0.5%. Other aqueous solutions containing strong alkaline materials are preferably capable of maintaining a pH of pH>12, and any concentration can be selected.

[0038] After the selective removal of phosphorus precipitates, Figure 4 As shown, an electrode 8 is formed on the InGaAs contact portion of the opening pattern 7. An electrode 9 is also formed on the back surface of the InP substrate 1 to manufacture a light receiving element (electrode forming step). [Example]

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

[0040] (Example) Using an n-type InP substrate as the starting substrate, a 1 μm thick undoped InP buffer layer was stacked, followed by a 3 μm thick undoped In 0.5 Ga 0.5 As light absorption layer and 1 μm thick undoped InP cap layer are grown in sequence to prepare epitaxial wafers, and 0.15 μm thick undoped InP cap layer is formed on the InP cap layer. 0.5 Ga 0.5 As contact layer, thereby preparing an epitaxial wafer having a light receiving element structure as an epitaxial functional layer ( Figure 1 ).

[0041] In 0.5 Ga 0.5 A portion of the As contact layer is removed to form an InGaAs contact. The pattern is formed using a sulfuric acid hydrogen peroxide solution ( Figure 2 ).

[0042] Formation 0.5 Ga 0.5 After the As contact part, a SiO2 film is formed on the surface as a protective film. An opening pattern is formed on a part of the formed SiO2 film, and Zn is diffused into the opening pattern part ( Figure 3 The diffusion of Zn is carried out in a phosphorus-containing gas environment at atmospheric pressure, with a temperature of more than 400°C maintained, hydrogen (H2) used as a carrier gas, and phosphine (PH3) gas flowing into the phosphorus-containing gas environment. The partial pressure of PH3 gas is maintained at 1×10 －6 [atm] or more. Then, dimethyl zinc (DMZn) gas was mixed and diffusion treatment was performed. The phosphorus-containing gas atmosphere was maintained during the temperature increase and the phosphorus-containing gas atmosphere was maintained until 400°C during the temperature decrease.

[0043] After the diffusion treatment, the wafer was taken out and immersed in a tetramethylammonium hydroxide (TMAH) aqueous solution with a concentration of 2.38% and a pH of 13 for 190 minutes to etch the phosphorus precipitates. Then, an electrode was formed on the InGaAs contact portion of the opening pattern portion, and an electrode was also formed on the back side of the n-type InP substrate to manufacture a light receiving element ( Figure 4 ).

[0044] (Comparative Example) A light receiving element was manufactured under the same conditions as in the example except that the step of immersing in a tetramethylammonium hydroxide (TMAH) aqueous solution to etch phosphorus precipitates was not performed.

[0045] Figure 6 The data obtained by comparing the resistivity when forming an electrode on an untreated wafer (comparative example) with the resistivity when forming an electrode on a wafer after etching with TMAH for 190 minutes (embodiment) are shown in FIG. It can be seen that the resistance of the comparative example (untreated object) is high, and the resistance of the embodiment (etched object) is about two orders of magnitude lower. It shows that since the phosphorus precipitates have been removed by etching, the phosphorus precipitates that would become an obstacle when the ohmic contact is formed have been removed, and at the same time, it is shown that the shape of the thickness of the contact layer will hardly change even if the selective removal of the phosphorus precipitates is treated, and the etching treatment will not have an adverse effect on the formation of the ohmic contact.

[0046] As can be seen from the above, through the embodiments of the present invention, phosphorus precipitates can be selectively removed by treatment with a strong alkaline aqueous solution, and during the removal, no adverse effects are caused on the underlying epitaxial layer.

[0047] This manual includes the following methods. [1]: A method for manufacturing a light-receiving element, wherein the light-receiving element is manufactured using an epitaxial wafer, wherein an InP buffer layer, an InGaAs light-absorbing layer, and an InP cap layer are sequentially epitaxially grown on an InP substrate, and the manufacturing method comprises: A step of forming an InGaAs contact layer on the InP cap layer; A step of removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; A step of forming a protective film so as to cover the InP cap layer and the InGaAs contact portion; A step of forming an opening pattern portion in a portion of the protective film so as to include the InGaAs contact portion; A step of diffusing impurities into the opening pattern portion in a phosphorus-containing gas environment; a step of selectively removing phosphorus precipitates attached to the surface in the step of diffusing impurities using a strong alkaline aqueous solution; and The step of forming electrodes on the InGaAs contact portion and the back surface of the InP substrate. [2]: The method for manufacturing a light-receiving element as described in [1] above, wherein the impurity is zinc (Zn) and the protective film is SiO2. [3]: A method for manufacturing a light-receiving element as described in [1] or [2] above, which uses an aqueous solution formed by dissolving one or more alkalis selected from the following in water as the strong alkaline aqueous solution: tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), KOH, NaOH and Ca(OH)2. [4]: The method for producing a light-receiving element as described in [1], [2] or [3] above, wherein an aqueous solution having a pH exceeding 12 is used as the strongly alkaline aqueous solution.

[0048] It should be noted that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely illustrative, and any embodiments having substantially the same structure and producing the same effects as the technical ideas described in the scope of the present invention are included in the technical scope of the present invention.

Claims

1. A method for manufacturing a light-receiving element, characterized in that: The light receiving element is manufactured using an epitaxial wafer, wherein the epitaxial wafer is formed by epitaxially growing an InP buffer layer, an InGaAs light absorbing layer, and an InP cap layer in sequence on an InP substrate, and the manufacturing method comprises: A step of forming an InGaAs contact layer on the InP cap layer; A step of removing a portion of the InGaAs contact layer by photolithography to form an InGaAs contact portion; A step of forming a protective film so as to cover the InP cap layer and the InGaAs contact portion; A step of forming an opening pattern portion in a portion of the protective film so as to include the InGaAs contact portion; A step of diffusing impurities into the opening pattern portion in a phosphorus-containing gas environment; A step of selectively removing phosphorus precipitates attached to the surface in the step of diffusing the impurities using a strong alkaline aqueous solution; and The step of forming electrodes on the InGaAs contact portion and the back surface of the InP substrate.

2. The method for manufacturing a light-receiving element according to claim 1, wherein: The impurity is zinc Zn, and the protective film is SiO2.

3. The method for manufacturing a light-receiving element according to claim 1, wherein: As the strong alkaline aqueous solution, an aqueous solution formed by dissolving one or more alkalis selected from the following in water is used: tetramethylammonium hydroxide TMAH, tetraethylammonium hydroxide TEAH, tetrabutylammonium hydroxide TBAH, KOH, NaOH and Ca(OH)2.

4. The method for manufacturing a light-receiving element according to claim 2, wherein: As the strong alkaline aqueous solution, an aqueous solution formed by dissolving one or more alkalis selected from the following in water is used: tetramethylammonium hydroxide TMAH, tetraethylammonium hydroxide TEAH, tetrabutylammonium hydroxide TBAH, KOH, NaOH and Ca(OH)2.

5. The method for manufacturing a light-receiving element according to any one of claims 1 to 4, characterized in that: As the strongly alkaline aqueous solution, an aqueous solution having a pH exceeding 12 is used.

Images