Manufacturing method of the structure

Abstract

To provide a technique capable of suppressing fluctuation in an etching condition in PEC etching for group III nitride.SOLUTION: A method for manufacturing a structure includes a step of preparing an etching target object having at least a first layer composed of group III nitride and a second layer which is formed on the first layer and composed of group III nitride having a composition different from that of the group III nitride constituting the first layer, and a step of irradiating the second layer with light through an etching solution containing peroxodisulfate ions while immersed in the etching solution, and generating holes in the III group nitride constituting the second layer to etch the second layer. In the step of etching the second layer, the etching solution at the start time of etching of the second layer is made acidic, thereby maintaining a state where the etching liquid is acidic during a period when the second layer is etched.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a structure. [Background technology]

[0002] Group III nitrides such as gallium nitride (GaN) are used as materials for manufacturing semiconductor devices such as light-emitting elements and transistors.

[0003] Photoelectrochemical (PEC) etching has been proposed as an etching technique for forming various structures in group III nitrides such as GaN (see, for example, Non-Patent Document 1). PEC etching is a wet etching method that causes less damage than general dry etching, and is preferable in that the equipment required is simpler than special dry etching methods that cause less damage, such as neutral beam etching (see, for example, Non-Patent Document 2) and atomic layer etching (see, for example, Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] J. Murata et al., “Photo-electrochemical etching of free-standing GaN wafer surfaces grown by hydride vapor phase epitaxy”, Electrochimica Acta 171 (2015) 89-95 [Non-patent document 2] S. Samukawa, JJAP, 45(2006)2395. [Non-patent document 3] T. Faraz, ECS J. Solid Stat. Scie.&Technol., 4, N5023 (2015). Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a technique capable of suppressing variations in etching conditions in PEC etching of group III nitrides. [Means for solving the problem]

[0006] According to one aspect of the present invention, preparing an etching object having at least a first layer made of a Group III nitride and a second layer formed on the first layer and made of a Group III nitride having a composition different from that of the Group III nitride constituting the first layer; a step of irradiating the second layer with light through the etching solution containing peroxodisulfate ions while the object to be etched is immersed in the etching solution to generate holes in the Group III nitride constituting the second layer, thereby etching the second layer; and In the step of etching the second layer, the etching solution is made acidic at the start of etching the second layer, thereby maintaining the acidic state of the etching solution during the period in which the second layer is etched. is provided. [Effects of the Invention]

[0007] A technique is provided that can suppress variations in etching conditions in PEC etching of group III nitrides. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1(a) is a schematic cross-sectional view illustrating an etching target according to a first embodiment of the present invention, and FIG. 1(b) is a schematic view of a PEC etching apparatus illustrating a PEC etching process according to the first embodiment. [Figure 2] FIG. 2(a) is a graph showing the results of the first experimental example, and FIG. 2(b) is a graph showing the results of the second experimental example. [Figure 3]3(a) and 3(b) are graphs showing the results of the third experimental example. [Figure 4] FIG. 4(a) is a laser microscope image showing the results of the fourth experimental example, and FIG. 4(b) is an SEM image showing the results of the fourth experimental example. [Figure 5] FIG. 5(a) shows various data of AFM measurements showing the results of the fourth experimental example, and FIG. 5(b) shows an AFM image showing the results of the fourth experimental example. [Figure 6] FIG. 6 is a schematic diagram of a PEC etching apparatus illustrating the pretreatment step according to a modification of the first embodiment. [Figure 7] FIG. 7(a) is a schematic cross-sectional view illustrating a HEMT according to the second embodiment, and FIG. 7(b) is a schematic cross-sectional view illustrating a wafer used as a material for the HEMT of the second embodiment. [Figure 8] FIG. 8(a) is a schematic cross-sectional view illustrating an object to be processed according to the second embodiment, and FIG. 8(b) is a schematic cross-sectional view of a PEC etching apparatus illustrating a PEC etching step. [Figure 9] FIG. 9(a) is a graph showing the relationship between etching time and etching depth in the fifth experimental example, and FIG. 9(b) is an optical microscope photograph showing a plan view of the recess formed in the fifth experimental example, and a schematic diagram of the cross-sectional shape of the recess formed in the fifth experimental example. [Figure 10] FIG. 10(a) shows various data obtained by AFM measurement of the recesses formed in the fifth experimental example, and FIG. 10(b) is an AFM image of the recesses formed in the fifth experimental example. [Figure 11] FIG. 11 is an optical microscope photograph showing a plan view of the recesses formed in the sixth experimental example, and a schematic diagram of the cross-sectional shape of the recesses formed in the sixth experimental example. [Figure 12] FIG. 12(a) shows various data obtained by AFM measurement of the recesses formed in the sixth experimental example, and FIG. 12(b) is an AFM image of the recesses formed in the sixth experimental example. DETAILED DESCRIPTION OF THE INVENTION

[0009] <First embodiment of the present invention> A method for manufacturing a structure according to a first embodiment of the present invention will be described. The manufacturing method according to this embodiment includes an etching step (hereinafter also referred to as a PEC etching step) using photoelectrochemical (PEC) etching on an etching target 10 (hereinafter also referred to as a wafer 10), which is the material of the structure. PEC etching will hereinafter also be referred to simply as etching.

[0010] Fig. 1(a) is a schematic cross-sectional view illustrating a wafer 10 to be PEC etched. First, as shown in Fig. 1(a), the wafer 10 is prepared. At least the surface 20 (the surface 20 to be etched) of the wafer 10 is made of a group III nitride.

[0011] The structure of the wafer 10 is not particularly limited. One example of the structure of the wafer 10 (used in the experimental examples described below) is a stacked structure of an n-type gallium nitride (GaN) substrate 11 and an n-type GaN layer 12 epitaxially grown on the GaN substrate 11. The n-type GaN layer 12 has an n-type impurity concentration of, for example, 5×10 16 / cm 3 and has a thickness of 3 μm.

[0012] The object to be subjected to the PEC etching process, i.e., the object to be immersed in (contact with) the etching solution 201, is referred to as the processing object 100. The processing object 100 includes at least a wafer 10 and may further include, for example, a mask 50 as a component required for the PEC etching process. The mask 50 is formed on the etching surface 20 of the wafer 10 in a pattern with openings for the regions 21 to be etched. The material of the mask 50 is not particularly limited as long as it is a material that can withstand the etching solution 201 and function as a mask. For example, the mask 50 may be made of a resist, silicon oxide, silicon nitride, or a metal such as titanium. The shape, size, and etching depth of the regions 21 to be etched may be appropriately selected as needed. The processing object 100 can be regarded as an intermediate structure (intermediate structure) for obtaining a final structure.

[0013] 1(b) is a schematic cross-sectional view of a PEC etching apparatus 200 illustrating the PEC etching process. The PEC etching apparatus 200 includes a container 210 containing an etching solution 201 and a light source 220 that emits ultraviolet (UV) light 221. The PEC etching apparatus 200 may also include a support table 240 on which a wafer 10 is placed at a predetermined height.

[0014] In the PEC etching process, while the processing object 100 (i.e., the wafer 10) is immersed in an etching solution 201, UV light 221 is irradiated onto the etching surface 20 via the etching solution 201, thereby etching the group III nitride constituting the etching surface 20. Details of the etching solution 201, the UV light 221, and the mechanism of PEC etching will be described later.

[0015] The PEC etching apparatus 200 preferably further includes a pH meter 230. The PEC etching step is preferably performed while measuring the pH of the etching solution 201 with the pH meter 230 to determine whether the pH is within an appropriate range. In this case, the probe 231 of the pH meter 230 is preferably positioned so that the shadow of the probe 231 caused by the UV light 221 is not cast on the surface 20 to be etched.

[0016] The method for manufacturing a structure according to this embodiment may include other steps such as electrode formation and protective film formation, as required.

[0017] Next, details of the etching solution 201, UV light 221, and the mechanism of PEC etching will be described. GaN will be exemplified as a group III nitride to be etched.

[0018] The etching solution 201 contains oxygen, which is used to generate oxides of Group III elements contained in the Group III nitrides that make up the surface to be etched 20, and further contains peroxodisulfate ions (SO) as an oxidizing agent that accepts electrons. 2-) is used. In this embodiment, the etching solution 201 is acidic at the start of etching the group III nitride that forms the etching target surface 20.

[0019] A first example of the etching solution 201 is a mixture of an aqueous solution of potassium hydroxide (KOH) and an aqueous solution of potassium peroxodisulfate (K2S2O8). Such an etching solution 201 is prepared by, for example, mixing a 0.001 M aqueous solution of KOH and a 0.05 M aqueous solution of K2S2O8 in a 1:1 ratio. The concentrations of the aqueous KOH solution, the aqueous K2S2O8 solution, and the mixing ratio of these aqueous solutions may be adjusted as needed.

[0020] A KOH solution alone is alkaline, while a K2S2O8 solution alone is acidic. Therefore, if the concentration of the KOH solution is excessively high (for example, 0.01 M), the etching solution 201, which is a mixture of the KOH solution and the K2S2O8 solution, becomes alkaline (such an alkaline etching solution 201 will be hereinafter also referred to as the etching solution 201 of the reference example). In the etching solution 201 of the first example, the concentration of the KOH solution is appropriately low, so that the etching solution 201, which is a mixture of the KOH solution and the K2S2O8 solution, is acidic.

[0021] The PEC etching mechanism when the etching solution 201 of the first example is used will be described. When the wafer 10 is irradiated with UV light 221 having a wavelength of 365 nm or less, pairs of holes and electrons are generated in the GaN constituting the etching surface 20. The generated holes convert the GaN into Ga 3+ and N2 (chemical formula 1), and then Ga 3+ is hydroxide ion (OH -) to generate gallium oxide (Ga2O3) (Chemical formula 2). The generated Ga2O3 is then dissolved in acid (or alkali). In this way, PEC etching of GaN is performed. The generated holes react with water, and oxygen is generated as the water is decomposed (Chemical formula 3). [ka] [ka] [ka]

[0022] In addition, when K2S2O8 dissolves in water, peroxodisulfate ions (S2O8 2- ) is produced (chemical formula 4), and S2O8 2- When UV light 221 is irradiated onto the surface, sulfate ion radicals (SO4 -* The electrons paired with the holes react with the SO4 -* When the radicals react with water, the water is decomposed and hydrogen is generated (Chemical formula 6). At this time, sulfate ions (SO4 2- ) is generated, the acidity of the etching solution 201 becomes stronger. -* By using radicals, electrons generated in pairs with holes in GaN can be consumed, which allows PEC etching to proceed smoothly. As PEC etching proceeds, the acidity of the etching solution 201 becomes stronger (the pH decreases). [ka] [ka] [ka]

[0023] A second example of the etching solution 201 is a mixture of an aqueous solution of phosphoric acid (H3PO4) and an aqueous solution of potassium peroxodisulfate (K2S2O8). Such an etching solution 201 is prepared by, for example, mixing a 0.01 M aqueous solution of H3PO4 and a 0.05 M aqueous solution of K2S2O8 in a 1:1 ratio. The concentrations of the aqueous solutions of H3PO4 and K2S2O8, and the mixing ratio of these solutions may be adjusted as needed.

[0024] Both H3PO4 aqueous solution and K2S2O8 aqueous solution are acidic. Therefore, an etching solution obtained by mixing H3PO4 aqueous solution and K2S2O8 aqueous solution, which will be given as a second example, is acidic at any mixing ratio.

[0025] The PEC etching mechanism when the etching solution 201 of the second example is used is presumed to be the same as that when the etching solution 201 of the first example is used, (Chemical Formula 1) to (Chemical Formula 3) are replaced with (Chemical Formula 7). In other words, the GaN, holes generated by irradiation with UV light 221, and water react with each other to form Ga2O3 and hydrogen ions (H + ) and N2 are generated (Chemical formula 7). The generated Ga2O3 is dissolved in acid. In this way, PEC etching of GaN is performed. Note that electrons generated in pairs with holes as shown in (Chemical formula 4) to (Chemical formula 6) are converted into SO8 2- The mechanism by which the etching liquid is consumed is the same as when the etching liquid 201 in the first example is used. [ka]

[0026] As shown in (chemical formula 5), ​​S2O8 2- From SO4 -* The radicals can be generated by at least one of irradiation with UV light 221 and heating. When irradiation with UV light 221 is used, S2O8 2- Increased light absorption by SO4 -*In order to efficiently generate radicals, it is preferable that the wavelength of the UV light 221 is 200 nm or more and less than 310 nm. In other words, irradiation of the UV light 221 generates holes in the group III nitride in the wafer 10 and also generates SO in the etching solution 201. 2- From SO4 -* From the viewpoint of efficiently generating radicals, it is preferable that the wavelength of the UV light 221 is 200 nm or more and less than 310 nm. 2- From SO4 -* When generating radicals by heating, the wavelength of the UV light 221 may be 310 nm or more (365 nm or less).

[0027] Irradiation with UV light 221 produces S2O8 2- From SO4 -* When generating radicals, the distance L from the surface 20 to be etched of the wafer 10 to the upper surface 202 of the etching solution 201 is preferably, for example, 1 mm or more and 100 mm or less, and more preferably 5 mm or more and 100 mm or less. If the distance L is too short, for example, less than 1 mm, the SO4 generated in the etching solution 201 above the wafer 10 may be generated. -* The amount of radicals may become unstable due to fluctuations in the distance L. Also, if the distance L is excessively long, for example, exceeding 100 mm, an excessive amount of SO4 may be generated in the etching solution 201 above the wafer 10, which does not contribute to PEC etching. -* The generation of radicals reduces the utilization efficiency of the etching solution 201.

[0028] In addition, S2O8 2- The etching solution 201, which contains KSO and is acidic at the start of etching, is not limited to those in the first and second examples described above. For example, it is also possible to use only a KSO solution as the etching solution 201. In this case, the concentration of the KSO solution may be, for example, 0.025M.

[0029] PEC etching can also be performed on Group III nitrides other than the exemplified GaN. The Group III element contained in the Group III nitride may be at least one of aluminum (Al), gallium (Ga), and indium (In). The concept of PEC etching of Al or In components in Group III nitrides is similar to that explained for Ga components with reference to (Chemical Formula 1), (Chemical Formula 2), or (Chemical Formula 7). That is, PEC etching can be performed by generating holes by irradiating UV light 221, thereby generating Al oxides or In oxides, and dissolving these oxides in acid (or alkali). The peak wavelength of the UV light 221 (light 221) may be appropriately changed depending on the composition of the Group III nitride to be etched. Based on the PEC etching of GaN, if Al is contained, light with a shorter wavelength can be used, and if In is contained, light with a longer wavelength can also be used. Impurities such as conductivity-determining impurities may be added to the Group III nitride to be PEC etched, as necessary.

[0030] In the PEC etching according to this embodiment, the etching solution contains S2O8 as an oxidizing agent. 2- (more specifically, S2O8 2- SO4 produced from -* The PEC etching can be progressed by consuming electrons generated together with holes by light irradiation of the group III nitride (by radicals). In other words, the PEC etching can be performed in a manner in which electrons are emitted directly (without going through external wiring) from the processing object 100 into the etching solution 201.

[0031] In contrast, a PEC etching technique that does not use such an oxidizing agent is one in which electrons generated in the group III nitride are released into the etching solution from a cathode electrode immersed in the etching solution via wiring extending outside the etching solution. In contrast to such electroded PEC etching that uses a cathode electrode, the PEC etching according to this embodiment is electrodeless (contactless) PEC etching that does not require the provision of such a cathode electrode.

[0032] Next, first to fourth experimental examples relating to PEC etching of this embodiment will be described. Figures 2(a) and 2(b) are graphs showing the results of the first and second experimental examples, respectively. Figures 2(a) and 2(b) also show the results of a reference experimental example.

[0033] In the first experimental example, an etching solution 201 was used, which was a 1:1 mixture of a 0.001 M KOH aqueous solution and a 0.05 M K2S2O8 aqueous solution, as described above as the etching solution 201 of the first example. In the second experimental example, an etching solution 201 was used, which was a 1:1 mixture of a 0.01 M H3PO4 aqueous solution and a 0.05 M K2S2O8 aqueous solution, as described above as the etching solution 201 of the second example. In the reference experimental example, an etching solution 201 was used, which was a 1:1 mixture of a 0.01 M KOH aqueous solution and a 0.05 M K2S2O8 aqueous solution, as described above as the etching solution 201 of the reference example.

[0034] In this experimental example, the etching solution 201 was irradiated with UV light 221, and the change in pH of the etching solution over time was measured. UV light 221 with a peak wavelength of 260 nm was irradiated. Note that in this experimental example, PEC etching was not performed by immersing the wafer 10 in the etching solution, and only the change in pH of the etching solution was measured. However, it is believed that the change in pH of the etching solution would be approximately the same even if PEC etching were performed.

[0035] 2(a) and 2(b), the horizontal axis represents the time from the start of irradiation with UV light 221, and the vertical axis represents the pH of the etching solution 201. The time on the horizontal axis can be considered as the time from the start of PEC etching.

[0036] As shown in FIG. 2(a), in the reference experiment, the etching solution 201 of the reference example has a pH of about 12 at the start of etching, making it alkaline. As described above, the irradiation of UV light 221 causes the SO4 2- Therefore, the longer the etching time, the lower the pH of the etching solution 201. The etching solution 201 of the reference example becomes neutral with a pH of 7 when the etching time is 40 minutes, and thereafter becomes acidic.

[0037] PEC etching progresses when the generated Ga2O3 is dissolved in an alkali or acid. Therefore, in the reference experimental example, etching stops while the etching solution 201 is near neutral. Also, in the reference experimental example, the pH of the etching solution 201 fluctuates greatly over time, which causes large temporal fluctuations in the dissolution conditions of Ga2O3. Therefore, it is difficult to maintain temporal uniformity in PEC etching.

[0038] As shown in FIG. 2(a), in the first experimental example, the etching solution 201 of the first example has a pH of 4.4 at the start of etching, making it acidic. Therefore, the etching solution 201 of the first example remains acidic from the start of etching. Therefore, by using the etching solution 201 of the first example, it is possible to prevent the PEC etching from stalling due to the etching solution 201 becoming neutral. Furthermore, compared to when the etching solution 201 of the reference example is used, it is possible to reduce the pH fluctuation range during the etching period.

[0039] As shown in FIG. 2(b), in the second experimental example, the etching solution 201 of the second example has a pH of 2.3 at the start of etching, making it acidic. Therefore, like the etching solution 201 of the first example, the etching solution 201 of the second example remains acidic from the start of etching. Therefore, by using the etching solution 201 of the second example, it is possible to prevent the PEC etching from stopping due to the etching solution 201 becoming neutral, as in the etching solution 201 of the first example, and it is possible to reduce the pH fluctuation range during the etching period compared to when the etching solution 201 of the reference example is used.

[0040] In this embodiment, by using an acidic etching solution 201 at the start of etching, as in the first and second examples, the etching solution 201 remains acidic throughout the entire etching period. This prevents the PEC etching from stalling due to the etching solution 201 becoming neutral. Furthermore, compared to the case where the etching solution 201 of the reference example is used, in which the etching solution 201 changes from alkaline to acidic during the etching period, the pH fluctuation range during the etching period can be reduced.

[0041] Furthermore, maintaining the etching solution 201 in an acidic state is preferable from the viewpoint of facilitating the use of a resist mask as the mask 50. In contrast, if the etching solution 201 is alkaline, the resist is easily peeled off, making it difficult to use a resist mask as the mask 50. When an acidic etching solution 201 is used, a mask material other than resist (silicon oxide, silicon nitride, etc.) may be used as needed.

[0042] In addition, it is also possible to prevent PEC etching from stalling by continuously replacing the etching solution 201 of the Reference Example before the etching solution 201 reaches neutrality so as to maintain the alkaline state. However, in this embodiment, the labor required to replace the etching solution 201 is cumbersome. By using the etching solution 201 of the First and Second Examples, it becomes easy to perform PEC etching for a long period of time (for example, preferably 30 minutes, more preferably 60 minutes, even more preferably 90 minutes or more, and even more preferably 120 minutes or more) without replacing the etching solution 201, i.e., using the same etching solution 201.

[0043] Furthermore, by not replacing the etching solution 201, it is possible to prevent the etching solution 201 from being agitated. 2- , SO4 -* Since the supply state of radicals and the like can be made more uniform over time, it becomes easier to improve the flatness of the etched surface (to improve in-plane uniformity). This is also an advantage when performing etching for a long period of time. In this embodiment, it is not necessary to keep the etching solution 201 stationary, and PEC etching may be performed while flowing the (acidic) etching solution 201, as necessary. When flowing the etching solution 201, the same etching solution 201 may be circulated (the etching solution 201 is not replaced), or new etching solution 201 may be continuously supplied (the etching solution 201 is replaced).

[0044] The use of the etching solution 201 of the second example also provides the following effects: The etching solution 201 of the second example has an acid dissociation constant pK of more than 0 and less than 7. aAn example of a weak acid having a first acid dissociation constant pK of H3PO4 is shown. By using the etching solution 201 of the second example, the buffering effect of the weak acid can be utilized in the acidic region where the pH is greater than 0 and less than 7, which makes it easier to reduce the pH drop during the etching period, in other words, to keep the pH close to a constant value during the etching period. a1 is 2.12.

[0045] Therefore, by using the etching solution 201 of the second example, the temporal fluctuation of the pH of the etching solution 201 can be further reduced compared to the embodiment using the etching solution 201 of the first example, and therefore the temporal fluctuation of the dissolution conditions of Ga2O3 can be reduced. Therefore, it becomes easier to maintain the temporal uniformity of the PEC etching.

[0046] From the viewpoint of minimizing the temporal variation in the pH of the etching solution 201, the following can be said as a guideline: The decrease in the pH of the etching solution 201 at the end of etching relative to the pH of the etching solution 201 at the start of etching is, for example, preferably 4 or less, more preferably 3 or less, even more preferably 2 or less, even more preferably 1.5 or less, and even more preferably 1 or less.

[0047] From this viewpoint, the following can be said as another guideline: 2- All are SO4 2- The decrease in pH of the etching solution 201 when the pH is changed to 0.05 with respect to the pH of the etching solution 201 at the start of etching is, for example, preferably 4 or less, more preferably 3 or less, even more preferably 2 or less, even more preferably 1.5 or less, and even more preferably 1 or less.

[0048] In the etching solution 201 of the first and second examples, S2O8 2- All are SO4 2- The pH of the etching solution 201 when the pH changes to 1.5 (that is, the limiting pH when the etching time is increased) is about 1.5.

[0049] When a weak acid is contained in the etching solution 201 of the second example, it is more preferable to select the following conditions: pH of the etching solution 201 at the start of etching, acid dissociation constant pK of the weak acid a The difference between the acid dissociation constant pK of the weak acid is preferably ±1 or less, more preferably ±0.5 or less. a Therefore, the pH of the etching solution 201 at the start of etching is a By reducing the difference with respect to the pH, it becomes easier to utilize the buffering effect of the weak acid.

[0050] The pH of the etching solution 201 at the start of etching is determined by the acid dissociation constant pK a As a result, as the PEC etching progresses, the pH of the etching solution 201 decreases, and the acid dissociation constant pK a , it becomes easier to utilize the buffering effect of the weak acid for a longer period of time.

[0051] In addition, the acid dissociation constant pK of the weak acid contained in the etching solution 201 a is the S2O8 contained in the etching solution 201. 2- All are SO4 2- It is preferable that the pH of the etching solution 201 is higher than the pH of the etching solution 201 when the pH changes to the limit (hereinafter also referred to as the limit pH). This makes it possible to utilize the buffering effect of the weak acid while the pH of the etching solution 201 approaches the limit pH.

[0052] 3(a) and 3(b) are graphs showing the results of Experimental Example 3, showing the relationship between etching time and etching depth in PEC etching under various conditions. The slope of the graph represents the etching rate.

[0053] 3(a) shows the results of etching the processing object 100 on which a mask 50 made of silicon oxide (SiO2) was formed using the etching solution 201 of the reference example (square plots), the results of etching the processing object 100 on which a mask 50 made of silicon oxide was formed using the etching solution 201 of the second example (triangle plots), and the results of etching the processing object 100 on which a mask 50 made of resist was formed using the etching solution 201 of the second example (circular plots). UVC ) was set to 260 nm, and the irradiation intensity (I) was set to 4 mW / cm 2 The distance L (d electrolyte ) was set to 5 mm.

[0054] It can be seen that PEC etching can be performed at roughly the same etching rate when using the etching solution 201 of the reference example and when using the etching solution 201 of the second example. It can also be seen that when using the etching solution 201 of the second example, PEC etching can be performed at roughly the same etching rate whether the mask 50 is made of silicon oxide or resist. Since the etching solution 201 of the reference example is alkaline, it is difficult to use a resist mask 50.

[0055] 3(b) shows another result of etching the processing object 100 on which a resist mask 50 is formed, using the etching solution 201 of the second example. As with the result shown in FIG. 3(a), it can be seen that by using the etching solution 201 of the second example, PEC etching using a resist mask can be performed well.

[0056] Next, a fourth experimental example will be described. Fig. 4(a) is a laser microscope image showing the results of forming a resist mask with a line-and-space pattern of 10 μm width and performing PEC etching with the etching solution 201 (using phosphoric acid) of the second example. The dark areas in the laser microscope image indicate the etched areas.

[0057] Figure 4(b) is a scanning electron microscope (SEM) image of the etched area of ​​the sample shown in Figure 4(a). The root mean square (RMS) surface roughness of the etched area over a 5 μm square area was 2.76 nm.

[0058] Fig. 5(a) shows various data obtained by atomic force microscope (AFM) measurement of the sample shown in Fig. 4(a). Fig. 5(b) shows an AFM image of another sample in which a resist mask was formed and etched with the second example etching solution 201 (using phosphoric acid).

[0059] From these experimental examples, the inventors of the present invention have found that S2O8 2- The inventors have found that PEC etching is possible using an etching solution 201 that contains the above-mentioned compound and is acidic from the start of etching. The inventors have also found that, for example, phosphoric acid, a weak acid, can be preferably used to prepare such an etching solution 201. Furthermore, the inventors have also found that PEC etching using a resist mask can be performed well by using such an etching solution 201. Performing PEC etching using such an etching solution 201 and using a resist mask in the PEC etching have not been proposed before, and the inventors are the first to propose them.

[0060] As described above, according to this embodiment, by performing PEC etching using etching solution 201 that is acidic from the start of etching, it is possible to perform PEC etching with reduced temporal fluctuations in etching conditions compared to when PEC etching is performed using etching solution 201 that is alkaline from the start of etching. Furthermore, using such an acidic etching solution 201 makes it easier to use a resist mask as mask 50.

[0061] <Modification of the first embodiment> Next, a modified example of the first embodiment will be described. In the above embodiment, the use of the etching solution 201 of the first example (FIG. 2(a)) exemplified a mode in which the pH at the start of etching is higher than when the etching solution 201 of the second example (FIG. 2(b)) is used, i.e., the pH fluctuation range during the etching period is larger. In this modified example, the use of the etching solution 201 of the first example also exemplified a mode in which the pH at the start of etching can be lowered, i.e., the pH fluctuation range during the etching period can be reduced.

[0062] 6 is a schematic diagram of a PEC etching apparatus 200 illustrating a pre-treatment process added in this modification. The pre-treatment process is carried out before the process of etching the wafer 10 (FIG. 1(b)). The pre-treatment process is carried out by irradiating the etching solution 201 with UV light 221 while the wafer 10 is not immersed in the etching solution 201, thereby oxidizing SO8. 2- Part of the SO4 2- This is a process of lowering the pH of the etching solution 201 by changing the pH of the etching solution 201 to

[0063] 2(a), when the etching solution 201 of the first example is used, UV light 221 is irradiated for 20 minutes before the start of etching to lower the pH of the etching solution 201 to 2.5. Thereafter, the wafer 10 is immersed in the etching solution 201 to start etching, and the pH at the start of etching becomes, for example, 2.5.

[0064] In this modification, the pH of the etching solution 201 is lowered to a predetermined pH in advance by the pretreatment process, and then etching is started. This reduces the amount of pH decrease from the start to the end of etching compared to when the pretreatment process is not performed, thereby suppressing fluctuations in etching conditions.

[0065] Second Embodiment Next, a method for manufacturing a structure 150 according to a second embodiment will be described. An example of the structure 150 is a high electron mobility transistor (HEMT). Hereinafter, the structure 150 will also be referred to as a HEMT 150. Here, the structure refers to a wafer 10 having a structure formed by PEC etching, or a member including such a wafer 10. Here, the high electron mobility transistor (HEMT) refers not only to a HEMT element on which electrodes and the like are formed, but also to a semiconductor laminate member used in a HEMT element (for a HEMT).

[0066] In this embodiment, as will be described in detail later, a recess (gate recess) 110 in which a gate electrode 152 of a HEMT 150 is disposed is formed by PEC etching using an acidic etching solution 201, as described in the first embodiment.

[0067] In this embodiment, the wafer 10 to be subjected to PEC etching has a lower layer made of a group III nitride and an upper layer formed on the lower layer and made of a group III nitride having a different composition from the group III nitride constituting the lower layer (preferably a group III nitride having a larger band gap than the group III nitride constituting the lower layer), i.e., a heterojunction structure made up of a stack of group III nitrides having different compositions. As an example of such a heterojunction structure, this embodiment illustrates the stacked structure of the channel layer 12b and the barrier layer 12c in the HEMT 150.

[0068] As will be described in the fifth and sixth experimental examples below, the inventors of the present invention have found that when PEC etching is performed on such a heterojunction structure, using an acidic etching solution 201 can suppress fluctuations in etching conditions compared to using an alkaline etching solution 201, and more specifically, can allow etching to proceed uniformly within the surface. This embodiment is based on this finding.

[0069] First, a description will be given of the structure of the HEMT 150 and the wafer 10 used as the material for the HEMT 150. Fig. 7(a) is a schematic cross-sectional view illustrating the HEMT 150, and Fig. 7(b) is a schematic cross-sectional view illustrating the wafer 10.

[0070] The wafer 10 has a substrate 11 and a group III nitride layer 12 (hereinafter also referred to as epitaxial layer 12) formed (epitaxially grown) on the substrate 11. A semi-insulating substrate is used as the substrate 11. Here, "semi-insulating" means, for example, a substrate having a resistivity of 10 5 This refers to a state in which the resistivity is Ωcm or more. Examples of semi-insulating substrates include semi-insulating silicon carbide (SiC) substrates and semi-insulating GaN substrates. Examples of semi-insulating GaN substrates include Fe-doped and manganese-doped GaN substrates.

[0071] When a SiC substrate is used as the substrate 11, the epitaxial layer 12 has a stacked structure including, for example, a nucleation layer 12a made of aluminum nitride (AlN), a channel layer 12b made of gallium nitride (GaN), a barrier layer 12c made of aluminum gallium nitride (AlGaN), and a cap layer 12d made of GaN. The cap layer 12d may be omitted. When a GaN substrate is used as the substrate 11, the nucleation layer 12a may be omitted from the epitaxial layer 12.

[0072] In the stacked structure of the channel layer 12b and the barrier layer 12c, a two-dimensional electron gas (2DEG) is generated near the upper surface of the channel layer 12b, which serves as the channel of the HEMT 150. Note that, instead of GaN, AlGaN may be used as the material for the channel layer 12b. The AlGaN used for the channel layer 12b has a lower Al composition (narrower band gap) than the AlGaN used for the barrier layer 12c.

[0073] The substrate 11 is not limited to a SiC substrate or a GaN substrate, and other substrates (such as a sapphire substrate or a silicon (Si) substrate) may be used. The layered structure of the epitaxial layer 12 may be selected appropriately depending on the type of substrate 11, the desired characteristics of the HEMT 150, and the like.

[0074] The surface 20 of the epitaxial layer 12 is composed of, for example, the c-plane of the Group III nitride that constitutes the epitaxial layer 12. Here, "composed of the c-plane" means that the crystal plane with the lowest index closest to the surface 20 is the c-plane of the Group III nitride crystal that constitutes the epitaxial layer 12.

[0075] In the HEMT 150 of this embodiment, the gate electrode 152 is formed on the bottom 120 of a recess (gate recess) 110 formed on the surface (upper surface) 20 of the epitaxial layer 12. The bottom 120 of the recess 110 is located within the thickness range of the barrier layer 12c, and the thickness of the barrier layer 12c below the recess 110 (the thickness from the upper surface of the channel layer 12b to the bottom 120 of the recess 110) may be set to a predetermined thickness so that the threshold gate voltage of the HEMT 150 becomes a predetermined value. The source electrode 151 and the drain electrode 153 are formed on the surface 20 of the epitaxial layer 12. A protective film 154 is formed so as to have openings on the upper surfaces of the source electrode 151, the gate electrode 152, and the drain electrode 153.

[0076] The gate electrode 152 is formed of, for example, a Ni / Au layer in which a gold (Au) layer is stacked on a nickel (Ni) layer. The source electrode 151 and the drain electrode 153 are each formed of, for example, a Ti / Al / Au layer in which an Al layer is stacked on a titanium (Ti) layer and an Au layer is further stacked on the Al layer.

[0077] The HEMT 150 has an isolation trench 160 that separates adjacent elements. The isolation trench 160 is provided so that its bottom is located deeper than the upper surface of the channel layer 12b, that is, so that the 2DEG between adjacent elements is separated by the isolation trench 160.

[0078] Next, we will explain the method for manufacturing the HEMT 150. The method for manufacturing the HEMT 150 according to this embodiment includes the steps of preparing a processing object 100 for PEC etching (i.e., a wafer 10) and etching the barrier layer 12c by PEC etching.

[0079] 8(a) is a schematic cross-sectional view illustrating an example of a processing object 100. The processing object 100 has a structure in which a mask 50 and a cathode pad 30 are provided on the epitaxial layer 12 of a wafer 10. The processing object 100 in this example is an embodiment in which the cathode pad 30 is used as (at least one of) the source electrode 151 and the drain electrode 153 of a HEMT, and specifically, for example, has a structure in which a mask 50 for PEC etching is formed on a member at the stage in which the source electrode 151 and the drain electrode 153 are formed on the surface 20 of the wafer 10.

[0080] The mask 50 is formed on the surface 20 of the epitaxial layer 12, and has openings in regions 21 where the recesses 110 are to be formed (hereinafter also referred to as the etched regions 21), as well as openings that expose the top surfaces of the cathode pad 30 (the source electrode 151 and the drain electrode 153). The mask 50 is made of a non-conductive material, such as resist or silicon oxide.

[0081] The cathode pad 30 is a conductive member formed of a conductive material and is arranged to contact at least a portion of the surface of the conductive region of the wafer 10 (epilayer 12) that is electrically connected to the etched region 21.

[0082] When the substrate 11 of the wafer 10 is semi-insulating and the mask 50 is made of a non-conductive material, the processing object 100 preferably further includes a cathode pad 30. The cathode pad 30 is a conductive member made of a conductive material such as metal, and is provided so as to contact at least a portion of the surface of the conductive region of the processing object 10 that is electrically connected to the etching region 20. The cathode pad 30 is provided so that at least a portion of the cathode pad 30, for example, the upper surface, comes into contact with the etching solution 310 during PEC etching.

[0083] The etching region 20 where PEC etching occurs is considered to function as an anode region where holes are consumed. In contrast, the surface of the cathode pad 30, a conductive member electrically connected to the etching region 20, that comes into contact with the etching solution 310 is considered to function as a cathode region where electrons are consumed (emitted). If the back side of the etching target 10 is made of a non-conductive member and the mask 50 is made of a non-conductive material, it is difficult to ensure a cathode region where electrons are consumed unless the cathode pad 30 is formed. Even in such a case, forming the cathode pad 30 makes it easier to ensure a cathode region, thereby facilitating PEC etching.

[0084] The inventors of the present application have found that if the edge of the mask used for PEC etching is made of a conductive material, the edge shape of the recess formed by PEC etching tends to be irregular and not follow the edge of the mask, whereas if the edge of the mask is made of a non-conductive material, it is easier to control the edge shape of the recess formed by PEC etching to follow the edge of the mask. Therefore, it is preferable that the mask edge defining the etched region 21 (i.e., the edge of the recess 110) is defined by the mask 50 made of a non-conductive material. It is preferable that the cathode pad 30 is positioned away from the edge of the recess 110 (in a plan view) (at a position that does not define the edge of the recess 110). From the viewpoint of properly controlling the edge shape of the recess 110, the distance D between the edge of the mask 50 (in a plan view) and the edge of the cathode pad 30 is set to 0.05 mm. OFF is preferably 5 μm or more, and more preferably 10 μm or more.

[0085] FIG. 8(b) is a schematic cross-sectional view of a PEC etching apparatus 200 showing a step of etching the barrier layer 12c by PEC etching (hereinafter also simply referred to as a PEC etching step).

[0086] In the PEC etching step, the processing object 100 is immersed in an etching solution 201, and the region to be etched 21 and the cathode pad 30 (at least a portion of the cathode pad 30, for example, the upper surface) are in contact with the etching solution 201. In this state, UV light 221 is irradiated onto the surface 20 of the epitaxial layer 12 via the etching solution 201. In this manner, the group III nitride constituting the region to be etched 21 is etched, thereby forming the recess 110. More specifically, the recess 110 is formed by etching the (cap layer 12d and) barrier layer 12c of the epitaxial layer 12.

[0087] In the processing object 100 of this embodiment, the etched region 21 (bottom 120 of the recess 110), which is the anode, and the cathode pad 30, which is the cathode, are electrically connected via the 2DEG. Therefore, as the PEC etching progresses, the barrier layer 12c thins and the 2DEG below the recess 110 decreases, making it difficult for the PEC etching to proceed. Eventually, the PEC etching can be automatically stopped when a predetermined, uniform thickness of the barrier layer 12c remains below the recess 110. This predetermined thickness can be adjusted, for example, by the intensity of the UV light 221. In this way, the formation of the recess 110 can be completed by automatically stopping the PEC etching.

[0088] In the PEC etching step of this embodiment, the etching solution 201 is preferably acidic (i.e., pH less than 7). That is, by making the etching solution 201 acidic at the start of etching the barrier layer 12c, the etching solution 201 is maintained in an acidic state throughout the period in which the barrier layer 12c is being etched. By making the etching solution 201 acidic, it is possible to more appropriately automatically stop the PEC etching when a predetermined uniform thickness of the barrier layer 12c remains below the recess 110, as described above.

[0089] Although an example has been given of forming the recess 110 by performing PEC etching on the processing object 100 in a state where the element isolation groove 160 is not formed, the recess 110 may also be formed by performing PEC etching on the processing object 100 in a state where the element isolation groove 160 is formed.

[0090] Next, fifth and sixth experimental examples relating to PEC etching of this embodiment will be described. In the fifth and sixth experimental examples, a wafer in which a GaN channel layer 12b and an AlGaN barrier layer 12c were stacked on a GaN substrate 11 was used, and recesses were formed in the barrier layer 12c by PEC etching. An acidic etching solution was used in the PEC etching of the fifth experimental example, and an alkaline etching solution was used in the PEC etching of the sixth experimental example.

[0091] In the fifth experimental example, a 0.025 M K2S2O8 aqueous solution was used as the acidic etching solution. Figure 9(a) is a graph (graph labeled "HEMT") showing the relationship between the etching time and the etching depth of the barrier layer 12c in the fifth experimental example. It can be seen that the etching depth becomes almost constant after about 10 minutes of etching, meaning that etching stops automatically. For reference, Figure 9(a) also shows a graph (graph labeled "n-GaN") showing the relationship between etching time and etching depth when an n-type GaN layer is similarly etched. When the n-type GaN layer is etched, the etching depth increases without limit in proportion to the etching time, indicating that etching does not stop automatically.

[0092] 9(b) is an optical microscope photograph showing a plan view of the recess 110 formed in the fifth experimental example, and a schematic diagram of the cross-sectional shape of the recess 110 formed in the fifth experimental example. In the fifth experimental example, the barrier layer 12c is etched uniformly in-plane to a depth halfway, and the recess 110 having a flat bottom 120 is formed.

[0093] FIG. 10(a) shows various data from AFM measurements of the recesses 110 formed in the fifth experimental example, and FIG. 10(b) shows an AFM image of the recesses 110 formed in the fifth experimental example. The etching time for the optical microscope measurement in FIG. 9(b) and the AFM measurements in FIGS. 10(a) and 10(b) was 30 minutes. The AFM measurements confirm that the recesses 110 formed in the fifth experimental example were approximately 50 nm deep and had flat bottoms 120. The arithmetic mean roughness Ra of the bottoms 120 of the recesses 110 in the fifth experimental example was 0.4 nm, which is less than 1 nm.

[0094] In the sixth experimental example, an alkaline etching solution was used, which was a 1:1 mixture of a 0.1 M KOH aqueous solution and a 0.05 M KSO aqueous solution. FIG. 11 shows a planar optical microscope photograph of the recess 110 formed in the sixth experimental example and a schematic diagram of the cross-sectional shape of the recess 110 formed in the sixth experimental example. In the sixth experimental example, etching stopped halfway through the barrier layer 12c at the center of the recess 110 in the width direction. However, at both ends of the recess 110 in the width direction, the entire thickness of the barrier layer 12c was etched and the etching even progressed to the top of the channel layer 12b. Thus, in the sixth experimental example, a recess 110 having a flat bottom 120 was not formed.

[0095] FIG. 12(a) shows various data from AFM measurements of the recesses 110 formed in the sixth experimental example, and FIG. 12(b) shows an AFM image of the recesses 110 formed in the sixth experimental example. The etching time for the optical microscope measurement in FIG. 11 and the AFM measurements in FIGS. 12(a) and 12(b) was 30 minutes. The AFM measurements confirm that the sixth experimental example formed recesses 110 with bottoms 120 that were abnormally deeper at both ends in the width direction than at the center, resulting in poor flatness. The center in the width direction was generally flat and approximately 50 nm deep, while the both ends were approximately 150 nm deeper than at the center. The arithmetic mean roughness Ra of the bottoms 120 of the recesses 110 in the sixth experimental example was 51 nm, which was greater than 50 nm.

[0096] In this way, the inventors of the present application have discovered that when etching the barrier layer 12c using electrodeless PEC etching, by using an acidic etching solution (from the start of etching the barrier layer 12c) as in the fifth experimental example, etching can proceed more uniformly across the surface than when an alkaline etching solution is used as in the sixth experimental example, and a recess 110 with a flatter bottom 120 can be formed.

[0097] When an alkaline etching solution is used, the arithmetic mean roughness Ra of the bottom 120 is 50 nm or more, but by using an acidic etching solution, the barrier layer 12c can be etched to form the recess 110 so as to achieve higher flatness, specifically, so that the arithmetic mean roughness Ra is preferably 10 nm or less, more preferably 5 nm or less, and even more preferably 1 nm or less.

[0098] Although the mechanism behind the difference in the progress of electrodeless PEC etching depending on whether the etching solution is acidic or alkaline is unclear, as in the fifth and sixth experimental examples, considering that the automatic etching stop that occurs in acidic solutions is less likely to occur in alkaline solutions, it is believed that performing PEC etching on a heterojunction structure composed of stacked III-nitride layers with different compositions (more specifically, performing PEC etching on the upper layer of a heterojunction structure in which the upper layer has a larger band gap than the lower layer) contributes to the occurrence of this phenomenon. Furthermore, it is possible that this phenomenon is caused or accelerated by the fact that the layer to be PEC-etched (barrier layer 12c in this example) is composed of AlGaN.

[0099] Therefore, when performing electrodeless PEC etching on such a heterojunction structure, preferably when performing electrodeless PEC etching on the AlGaN layer of such a heterojunction structure, it is preferable to use an acidic etching solution in order to ensure uniform etching within the surface.

[0100] <Other embodiments> Although the embodiments and modifications of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and modifications, and various changes, modifications, combinations, etc. are possible without departing from the spirit and scope of the present invention.

[0101] For example, in the above description, S2O8 2- Although the embodiment in which potassium peroxodisulfate (K2S2O8) is used as the catalyst is shown, S2O8 2-Alternatively, the catalyst may be supplied from, for example, sodium peroxodisulfate (Na2S2O8), ammonium peroxodisulfate (ammonium persulfate, (NH4)2S2O8), or the like.

[0102] Also, S2O8 2- From SO4 -* The method for generating radicals is not particularly limited, and for example, at least one of irradiating the etching solution with light and heating may be used.

[0103] <Preferred embodiment of the present invention> Preferred embodiments of the present invention will be described below.

[0104] (Appendix 1) A step of preparing an etching target having at least a surface made of a group III nitride; a step of irradiating the surface of the etching object with ultraviolet light via the etching solution while the etching object is immersed in an etching solution containing peroxodisulfate ions, thereby generating sulfate ion radicals from the peroxodisulfate ions and generating holes in the Group III nitride, thereby etching the Group III nitride; and the step of etching the Group III nitride comprises making the etching solution acidic at the start of etching the Group III nitride, thereby maintaining the etching solution in an acidic state throughout the period during which the Group III nitride is etched.

[0105] (Appendix 2) 2. The method for producing a structure according to claim 1, wherein the decrease in pH of the etching solution at the end of the etching relative to the pH of the etching solution at the start of the etching is preferably 4 or less, more preferably 3 or less, even more preferably 2 or less, even more preferably 1.5 or less, and even more preferably 1 or less.

[0106] (Appendix 3) 3. The method for producing a structure according to claim 1, wherein a decrease in pH of the etching solution when all of the peroxodisulfate ions contained in the etching solution have been converted to sulfate ions, relative to the pH of the etching solution at the start of etching, is preferably 4 or less, more preferably 3 or less, even more preferably 2 or less, even more preferably 1.5 or less, and even more preferably 1 or less.

[0107] (Appendix 4) The etching solution has an acid dissociation constant pK of greater than 0 and less than 7. a 4. A method for producing the structure according to any one of claims 1 to 3, comprising a weak acid having the formula:

[0108] (Appendix 5) 5. The method for producing a structure according to claim 4, wherein the weak acid is phosphoric acid.

[0109] (Appendix 6) the acid dissociation constant pK of the weak acid at the pH of the etching solution at the starting point a The method for producing a structure according to Appendix 4 or 5, wherein the difference with respect to is preferably ±1 or less, more preferably ±0.5 or less.

[0110] (Appendix 7) The pH of the etching solution at the start point is determined by the acid dissociation constant pK a 7. The method for producing a structure according to any one of appendices 4 to 6, wherein the surface roughness is higher than

[0111] (Appendix 8) Acid dissociation constant pK of the weak acid a is higher than the pH of the etching solution when all of the peroxodisulfate ions contained in the etching solution are converted to sulfate ions.

[0112] (Appendix 9) 9. The method for producing a structure according to any one of claims 1 to 8, wherein the etching solution is not replaced during the period in which the group III nitride is etched.

[0113] (Appendix 10) 10. The method for producing a structure according to any one of claims 1 to 9, wherein the etching solution is not stirred during the period in which the group III nitride is etched.

[0114] (Appendix 11) The method for producing a structure according to any one of appendices 1 to 10, wherein the period during which the Group III nitride is etched is preferably 30 minutes, more preferably 60 minutes, even more preferably 90 minutes or more, and even more preferably 120 minutes or more.

[0115] (Appendix 12) 12. The method for producing a structure according to any one of claims 1 to 11, wherein the pH of the etching solution is measured during the period in which the group III nitride is etched.

[0116] (Appendix 13) 13. The method for manufacturing a structure according to claim 12, wherein, when measuring the pH of the etching solution, the probe of a pH meter is positioned so that a shadow cast by the ultraviolet light on the probe is not cast on the surface of the object to be etched.

[0117] (Appendix 14) 14. The method for producing a structure according to any one of claims 1 to 13, further comprising the step of: prior to the step of etching the Group III nitride, irradiating the etching solution with ultraviolet light in a state in which the object to be etched is not immersed in the etching solution, thereby converting some of the peroxodisulfate ions into sulfate ions, thereby lowering the pH of the etching solution.

[0118] (Appendix 15) 15. The method for manufacturing a structure according to any one of claims 1 to 14, wherein in the step of etching the group III nitride, the etching is performed in a state where a mask is formed on the surface (on a part of the surface).

[0119] (Appendix 16) 16. The method for manufacturing a structure according to claim 15, wherein the mask is a resist mask.

[0120] (Appendix 17) an etching target having at least a surface made of a group III nitride; a mask formed on the surface (or on a portion of the surface); and The intermediate structure is immersed in an acidic etching solution containing peroxodisulfate ions.

[0121] (Appendix 18) 18. The intermediate structure of claim 17, wherein the mask is a resist mask.

[0122] (Appendix 19) 19. The intermediate structure of claim 17 or 18, wherein the surface is irradiated with ultraviolet light via the etching solution.

[0123] (Appendix 20) preparing an etching target having at least a first layer made of a Group III nitride, and a second layer formed on the first layer and made of a Group III nitride having a composition different from that of the Group III nitride constituting the first layer (preferably a Group III nitride having a larger band gap than that of the Group III nitride constituting the first layer); a step of irradiating the second layer with light through the etching solution containing peroxodisulfate ions while the object to be etched is immersed in the etching solution to generate holes in the Group III nitride constituting the second layer, thereby etching the second layer; and A method for manufacturing a structure, wherein in the step of etching the second layer, the etching solution is made acidic at the start of etching the second layer, thereby maintaining the etching solution in an acidic state during the period in which the second layer is etched.

[0124] (Appendix 21) the first layer is a channel layer of a high electron mobility transistor; 21. The method of manufacturing a structure according to claim 20, wherein the second layer is a barrier layer of a high electron mobility transistor.

[0125] (Appendix 22) 22. A method for manufacturing a structure according to claim 21, wherein the object to be etched further has a cap layer formed on the barrier layer.

[0126] (Appendix 23) 23. The method for manufacturing a structure according to claim 21 or 22, wherein the step of etching the second layer forms a gate recess of the high electron mobility transistor.

[0127] (Appendix 24) 24. The method for manufacturing a structure according to any one of claims 20 to 23, wherein the second layer is made of aluminum gallium nitride.

[0128] (Appendix 25) 25. The method for manufacturing a structure according to any one of claims 20 to 24, wherein the first layer is made of gallium nitride or aluminum gallium nitride.

[0129] (Appendix 26) The method for manufacturing a structure according to any one of Appendices 20 to 25, wherein in the step of etching the second layer, recesses are formed by etching the second layer to an intermediate depth so that the arithmetic mean roughness Ra of the bottom of the recesses is 10 nm or less (more preferably 5 nm or less, and even more preferably 1 nm or less).

[0130] (Appendix 27) A method for manufacturing a structure according to any one of Appendices 20 to 26, wherein in the step of etching the second layer, etching of the second layer is automatically stopped (when the second layer has been etched to an intermediate depth).

[0131] (Appendix 28) 28. The method for manufacturing a structure according to any one of appendixes 20 to 27, wherein the second layer forms a two-dimensional electron gas in the first layer.

[0132] (Appendix 29) 29. The method for manufacturing a structure according to any one of claims 20 to 28, wherein in the step of etching the second layer, a mask defining an area to be etched is formed on the second layer.

[0133] (Appendix 30) A step of preparing an etching target having a first layer made of a group III nitride containing Al (preferably, for example, AlGaN); a step of irradiating the first layer with light through the etching solution containing peroxodisulfate ions while the object to be etched is immersed in the etching solution to generate holes in the Group III nitride constituting the first layer, thereby etching the first layer; and A method for manufacturing a structure, wherein in the step of etching the first layer, the etching solution is made acidic at the start of etching the first layer, thereby maintaining the acidic state of the etching solution during the period in which the first layer is etched. [Explanation of symbols]

[0134] 10...etching object, 11...substrate, 12...group III nitride layer, 12a...nucleation layer, 12b...channel layer, 12c...barrier layer, 12d...cap layer, 20...etching surface 20, 21...area to be etched, 30...cathode pad, 50...mask, 100...processing object, 110...recess (gate recess), 120...bottom, 150...structure (high electron mobility transistor), 151...source electrode, 152...gate electrode, 153...drain electrode, 154...protective film, 160...element isolation trench, 200...PEC etching apparatus, 201...etching solution, 202...top surface of etching solution, 210...container, 220...light source, 221...UV light, 230...pH meter, 231...probe, 240...support

Claims

1. preparing an etching target having at least a first layer made of a Group III nitride, and a second layer formed on the first layer and made of a Group III nitride having a composition different from that of the Group III nitride constituting the first layer; a step of irradiating the second layer with light through the etching solution containing peroxodisulfate ions while the object to be etched is immersed in the etching solution to generate holes in the Group III nitride constituting the second layer, thereby etching the second layer; having In the step of etching the second layer, the etching solution is made acidic at the start of etching the second layer, thereby maintaining the acidic state of the etching solution during the period in which the second layer is etched; the first layer is a channel layer of a high electron mobility transistor; the second layer is a barrier layer of a high electron mobility transistor; In the step of etching the second layer, a recess is formed by etching the second layer to an intermediate depth such that the arithmetic mean roughness Ra of the bottom of the recess is 10 nm or less. A method for manufacturing a structure.

2. The method for manufacturing a structure according to claim 1 , wherein the object to be etched further comprises a cap layer formed on the barrier layer.

3. The method for manufacturing a structure according to claim 1 or 2, wherein the step of etching the second layer forms a gate recess of the high electron mobility transistor.

4. The method for manufacturing a structure according to any one of claims 1 to 3, wherein the second layer is made of aluminum gallium nitride.

5. The method for manufacturing a structure according to any one of claims 1 to 4, wherein the first layer is made of gallium nitride or aluminum gallium nitride.

6. 6. The method for manufacturing a structure according to claim 1, wherein in the step of etching the second layer, etching of the second layer is automatically stopped.

7. preparing an etching target having at least a first layer made of a Group III nitride, and a second layer formed on the first layer and made of a Group III nitride having a composition different from that of the Group III nitride constituting the first layer; a step of irradiating the second layer with light through the etching solution containing peroxodisulfate ions while the object to be etched is immersed in the etching solution to generate holes in the Group III nitride constituting the second layer, thereby etching the second layer; having In the step of etching the second layer, the etching solution is made acidic at the start of etching the second layer, thereby maintaining the acidic state of the etching solution during a period in which the second layer is etched; In the step of etching the second layer, a recess is formed by etching the second layer to an intermediate depth such that the arithmetic mean roughness Ra of the bottom of the recess is 10 nm or less. A method for manufacturing a structure.

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