JP5004885B2 - Semiconductor structure processing method - Google Patents

Description

  The present invention relates to a method for processing a semiconductor structure including a wet etching process.

  Etching is known as a method for processing a semiconductor structure. As an etching method, there is wet etching using acid or alkali. Various etchants and etching methods are known for wet etching of group IV semiconductor structures such as Si and group III-V semiconductor structures such as gallium nitride GaN. Japanese Patent Application Laid-Open No. 2008-10608 discloses a method for wet etching of a group III nitride semiconductor structure using an alkaline solution.

JP 2007-101874 A

  On the other hand, in manufacturing a semiconductor structure using a ZnO-based semiconductor material such as ZnO, MgZnO, ZnOS, or ZnOSe, an appropriate wet etching method is required.

  An object of the present invention is to provide a new wet etching method for a ZnO-based semiconductor structure.

  According to one aspect of the present invention, a) a step of preparing a ZnO-based semiconductor structure having a + C surface, and b) a surface of the ZnO-based semiconductor structure is mixed with a mixed solution of ammonium fluoride and hydrofluoric acid. There is provided a semiconductor structure processing method comprising: an etching step; and c) a step of etching the surface of the ZnO-based semiconductor structure with a mixed solution of hydrochloric acid and nitric acid after the step b).

  According to the present invention, a ZnO-based semiconductor structure having a good etching surface can be provided.

  FIG. 1A is a schematic diagram of a typical wet etching method. As shown in the figure, the semiconductor structure 1 to be etched is placed in a grid-like holder 2 with a handle, the surface to be etched is exposed and immersed in the solution 4 in the beaker 3.

  FIG. 1B is a schematic view showing another method of wet etching. As shown in the figure, a method may be used in which an etchant is fluidly supplied using a pump 6 to a water tank 5 having a structure in which the etchant flows evenly in one direction, and the holder 2 containing the element 1 is immersed in the etchant.

  FIG. 2 shows a schematic cross-sectional view of a ZnO-based semiconductor structure. The ZnO-based semiconductor structure is a light-emitting layer 23 in which a buffer layer 21, an n-type ZnO-based semiconductor layer 22, a multiple quantum well (MQW), etc. are formed on a substrate 20 made of ZnO, MgZnO, or the like, a p-type ZnO-based semiconductor. A structure in which the layers 24 are stacked in this order. A ZnO-based substrate or a structure in which a ZnO-based semiconductor material is formed (single layer or stacked) on a ZnO-based substrate is also referred to as a ZnO-based semiconductor structure in a broad sense.

  In order to find a suitable etchant for a ZnO-based semiconductor structure, the inventors conducted a wet etching experiment using various etchants by the method shown in FIG. 1A. Each experimental condition and its result are described.

  FIG. 3A is a schematic cross-sectional view of a sample of a ZnO-based semiconductor structure used in the experiment, and FIG. 3B is a schematic plan view of the ZnO-based semiconductor structure. The sample used in the experiment is obtained by growing a ZnO-based film 8 of about 0.4 μm to 0.5 μm on a 10 × 10 mm square and 300 μm thick ZnO substrate 7 shown in FIG. The upper surface of the substrate 7 is a + C plane. On the surface of the ZnO-based film 8, a resist pattern 9 (thickness of about 1.3 μm to 1.5 μm) serving as a mask for wet etching is formed.

As shown in FIG. 3B, in the resist pattern 9, square resist films are arranged in a matrix on a ZnO-based film.
1) Hydrochloric acid The sample was immersed in a commercially available 36% by volume hydrochloric acid solution for 60 minutes.

FIG. 4 is an observation image of the sample surface after etching using an optical microscope. When etching was performed using hydrochloric acid, a large amount of etch pits 11 were generated as shown in the figure, and the etched surface could not be flattened. The etch pit is a small etching mark reflecting the crystal structure. Further, in the portion protected by the resist film, a so-called side etch 13 was confirmed at the edge of about 100 μm. The etch rate in the C-axis direction is 8.0 μm / hr.
2) Nitric acid The sample was immersed in a 61% by volume nitric acid solution for 60 minutes.

FIG. 5 is an observation image of the sample surface after etching using an optical microscope. When etching with nitric acid, a large amount of etch pits 11 were generated as in the case of hydrochloric acid. The etch rate in the C-axis direction was very slow at 0.3 μm / hr.
3) Phosphoric acid The sample was immersed in phosphoric acid (phosphoric acid: water = 15 ml: 150 ml) for 60 minutes.

FIG. 6 is an observation image of the sample surface after etching using an optical microscope. When etching with phosphoric acid, a large amount of etch pits 11 were generated as in the case of hydrochloric acid. The etch rate in the C-axis direction was 2.5 μm / hr.
4) Hydrofluoric acid A ZnO sample without a resist film was immersed in a 10% by volume hydrofluoric acid solution for 10 minutes.

FIG. 7 is an observation image of the sample surface after etching using an optical microscope. As shown in the figure, a large amount of etch pits 11 also appeared when hydrofluoric acid was an etchant. The etching rate in the C-axis direction was 1.5 μm / hr.
5) Ethylenediaminetetraacetic acid · 2Na (hereinafter referred to as EDTA): ethylenediamine (hereinafter referred to as EDA) = 20: 1 (volume ratio) mixed solution The sample was immersed in an EDTA: EDA solution for 10 minutes.

FIG. 8 is an observation image of the sample surface after etching using an optical microscope. In the case of this solution, almost no etch pits were observed, but it was not isotropically etched, and anisotropic etching in which etching progressed in a biased direction was obtained. A portion 14 shown in the figure is a region etched along the C-plane off direction (C-plane off region 14). The etch rate in the C-axis direction was 0.62 μm / hr.
6) Buffered hydrofluoric acid Samples were mixed into a mixed aqueous solution (PH = 5, referred to as buffered hydrofluoric acid) of 15% by volume of ammonium bifluoride (NH ₄ F · HF) and 28% by volume of ammonium fluoride. It was immersed for 30 minutes.

FIG. 9 is an observation image of the sample surface after etching using an optical microscope. As shown in the drawing, a large amount of irregularly shaped irregularities 15 were observed in the etching region 10. The etch rate in the C-axis direction was 1.0 μm / hr. In addition, the round part 16 in the figure is the electrode 16.
7) Aqua regia The sample was immersed in aqua regia (hydrochloric acid: nitric acid = 3: 1 (vol)) for 60 seconds.

  FIG. 10A is an observation image of the sample surface after etching by an optical microscope, and FIG. 10B is a graph showing the result of measuring the sample cross section with a stylus type surface shape measuring instrument (DEKTAK).

  When etching with aqua regia, few etch pits were observed, but side etch 13 appeared near the interface between the resist protection region 12 and the etching region 10. The etch rate in the C-axis direction is 7.2 μm / hr, and the etch rate of the side etch 13 (A-axis or M-axis direction) is 900 μm / hr. Therefore, the side etching progress rate is much faster than the etching in the depth direction. As shown in FIG. 10B, the side etch region was actually 15 μm, and it was found that the side etch progressed very much.

  Up to this point, the wet etching for the ZnO film has been described. The inventors examined the effect of the example also in the wet etching for the MgZnO film.

For MgZnO, etching experiments 8) and 9) were performed using each of buffered hydrofluoric acid and aqua regia as etchants.
8) Etching MgZnO film with buffered hydrofluoric acid MgZnO film 11 of about 0.3 μm to 0.5 μm on a 10 mm × 10 mm square, 300 μm thick ZnO substrate 7 having a shape as shown in FIGS. 3A and 3B The sample with the resist pattern 9 provided thereon was immersed in the same buffered hydrofluoric acid as in Experiment 6) for 20 minutes.

FIG. 11 is an image observed by an optical microscope on the surface of the MgZnO film after etching. As shown in the figure, as in Experiment 6), a large amount of unevenness 15 was confirmed. At this time, the etch rate in the C-axis direction was 1.2 μm / hr.
9) Etching MgZnO film with aqua regia A sample similar to experiment 8) was immersed in aqua regia similar to experiment 7) for 60 seconds.

  FIG. 12A is an image observed by an optical microscope on the surface of the MgZnO film after etching, and FIG. 12B is a graph showing the result of measuring the sample cross section with a stylus type surface shape measuring instrument (DEKTAK).

  Referring to FIG. 12A, when the MgZnO film is etched with aqua regia, the etch pits are not observed as much as when the ZnO film is etched with aqua regia, but in the vicinity of the interface between the etching region 10 and the resist protection region 12. A large side etch 13 was observed. The etch rate was 5.2 μm / hr in the C-axis direction, and the side etch was 1300 μm / hr.

  The examination of 1) -7) is summarized. When the etchants 1) to 4) are used, a large amount of etch pits are generated on the etched surface. In the case of 5), although the etch pit is hardly seen, anisotropic etching is performed. In the case of 6), irregularities different from the etch pits are generated on the etched surface. In the case of 7), the etched surface is a relatively flat surface, but the side etch becomes large.

  Based on the above investigation, the inventors have devised a wet etching method capable of obtaining a good etching surface by using a combination of the etchants of 6) and 7). That is, a two-stage etching process is performed in which etching is performed with buffered hydrofluoric acid and then with aqua regia.

  In addition, the results of Experiments 8) and 9) are not significantly different from the results of Experiments 6) and 7). Therefore, it is considered that the MgZnO film, like the ZnO film, can be etched with a flat etching surface and less side etching by etching with buffered hydrofluoric acid and then with aqua regia for a short time. .

  Example 1 will be described in detail. As shown in FIG. 1, the semiconductor structure 1 was set in the holder 2 and immersed in the solution in the beaker 3. Here, the semiconductor structure 1 is obtained by cutting a ZnO-based film surface on a ZnO substrate having a thickness of 300 μm with a resist pattern into a 5 mm square. The resist pattern forming method is, for example, a contact exposure method. A resist is applied to the surface of the ZnO-based film with a spin coater and then pre-baked to form a resist film. A photomask is contacted with the resist film and exposed to transfer the pattern to the resist. The resist was developed, and the exposed pattern was removed to form a resist pattern.

  As a first stage of etching, the semiconductor structure 1 was immersed in 100 ml of buffered hydrofluoric acid (solution A) in a 200 ml beaker 3 for 20 minutes.

  As a second stage of etching, the semiconductor structure 1 pulled out from the solution A was immersed in 200 ml of aqua regia (150 ml of hydrochloric acid and 50 ml of nitric acid) in a 300 ml beaker for 5 seconds.

  FIG. 13 is an optical microscope image of the surface of the ZnO film after etching according to Example 1. As shown in the figure, no etch pits or irregularities were seen in the etching region 10, and side etching was hardly seen.

  FIG. 14 is an enlarged sectional view near the interface between the etching region 10 and the resist protection region 12. When the vicinity of the interface is enlarged, some side etch 13 can be confirmed, but the size is 1.2 μm. It can be said that side etch was reduced as much as possible by shortening the immersion time in aqua regia to 5 seconds.

  As in Example 1, first, etching in the depth direction (C-axis direction) is performed using the solution A, and then using aqua regia, the surface unevenness is removed and the etching of the ZnO film with less side etching is performed. Can be done.

  In Example 2, a method of forming an isolation trench for element isolation in a ZnO-based semiconductor structure will be described.

  As shown in FIG. 15A, n-type and p-type conductive films 1502 and 1503 are sequentially grown on a + C-plane ZnO substrate 1501 using a ZnO-based material. The growth method may be a method obvious to those skilled in the art, such as MOCVD and MBE. Thereafter, electrodes 1504 and 1505 for n-type and p-type are deposited by, for example, electron beam (EB) vapor deposition, and then annealed at an appropriate temperature. Titanium and gold are used for the n-side electrode 1504, and nickel and gold are used for the p-side electrode 1505. A resist mask 1506 is formed by photolithography so as to cover the electrode 1504, the back surface of the substrate 1501, and the electrode 1505. Thus, the semiconductor structure A is formed.

  As shown in FIG. 15B, the semiconductor structure A is immersed in an etchant solution in the same manner as in Example 1 and then immersed in pure water for 90 seconds to wash away the etchant, thereby forming a separation groove 1507 (formation of semiconductor structure B). .

  As shown in FIG. 15C, the resist mask 1506 is removed. As a removal method, an ultrasonic wave is applied to peel off the semiconductor structure B in acetone. After the resist mask 1506 is peeled off, the resist mask 1506 is immersed in boiling isopropyl alcohol (IPA) to evaporate moisture, and then lifted and dried in the atmosphere of IPA.

  As shown in FIG. 15D, for example, a scribe line 1508 or the like is formed in the separation groove to cut the semiconductor structure D and separate into individual semiconductor elements. The cut may be dicing.

  FIG. 16A shows IV characteristics of a semiconductor structure in which an isolation trench is formed according to Example 2 and an element is isolated. FIG. 16B shows IV characteristics when a separation groove is formed only with the solution A as a reference example. As shown in FIG. 16A, in the element formed according to Example 2, good IV characteristics with a steep rise of current can be obtained. On the other hand, as shown in FIG. 16B, in the element in which the isolation trench is formed only by the solution A, a leak current is generated due to current concentration due to the unevenness, and therefore the rise of the current is duller than that of Example 2. -V characteristics.

  In Example 3, a method of mirror processing of the surface of a semiconductor structure will be described. In the third embodiment, the same etchant as in the first and second embodiments is used. The difference from Example 1 is that a + C plane ZnO substrate was used instead of a ZnO-based film grown on a ZnO substrate as a semiconductor structure. Further, since the surface of the substrate is mirror-finished, a resist mask is not used.

  The surface of the ZnO substrate is rough when nothing is processed. According to the embodiment, since a substrate having a flat surface can be formed, it is possible to perform flat epitaxial growth with good crystallinity.

  FIG. 17A shows a micrograph of the actual state of the epitaxial layer surface of the ZnO-based semiconductor structure in which the ZnO substrate was actually mirror-finished by Example 3 and epitaxially grown. FIG. 17B shows a micrograph of the actual state of the epitaxial layer surface of a ZnO-based semiconductor structure epitaxially grown using a substrate that has not been processed. When comparing FIG. 17A and FIG. 17B, the surface of the ZnO-based semiconductor structure according to Example 3 was very flat compared to a substrate on which no processing was performed. In addition, generation of innumerable defects due to substrate processing damage as shown in FIG. 17B (innumerable pits generated in the shape corresponding to the polishing scratches and appearing cloudy) was not observed. Furthermore, the surface of the epitaxial layer having a ZnO-based semiconductor structure epitaxially grown using a substrate whose surface was etched with only hydrofluoric acid was also observed. White turbidity due to unevenness due to pits generated during surface etching was observed. Such turbidity was not observed on the surface of the ZnO-based semiconductor structure according to Example 3. That is, a flat epitaxial film with good crystallinity can be grown by using the mirror-finished ZnO substrate according to the embodiment.

Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. For example, examples of the ZnO-based material to which the embodiment can be applied include a composition formula Mg _x Zn _1-x O _1-yz S _y Se _z (0 ≦ x <0.5, 0 ≦ y <0.1, 0 ≦ A semiconductor material represented by z <0.2) may be mentioned. This is because the semiconductor material represented by this composition formula is considered to have the same etching characteristics because the crystal form is not different from that of ZnO.

  Although aqua regia is used as the etchant in the examples, a mixed solution of hydrochloric acid: nitric acid = 1 to 10: 1 (volume ratio) may be used instead of aqua regia. About the buffered hydrofluoric acid used for the etchant in an Example, the mixed solution of ammonium fluoride: hydrofluoric acid = 1-10: 1 may be sufficient.

  It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.

1A and 1B are schematic views of a typical wet etching method. FIG. 2 is a schematic cross-sectional view of a ZnO-based semiconductor structure. FIG. 3A is a schematic cross-sectional view of a sample of a ZnO-based semiconductor structure used in the experiment, and FIG. 3B is a schematic plan view of the ZnO-based semiconductor structure. FIG. 4 is an observation image of the sample surface after etching using an optical microscope. FIG. 5 is an observation image of the sample surface after etching using an optical microscope. FIG. 6 is an observation image of the sample surface after etching using an optical microscope. FIG. 7 is an observation image of the sample surface after etching using an optical microscope. FIG. 8 is an observation image of the sample surface after etching using an optical microscope. FIG. 9 is an observation image of the sample surface after etching using an optical microscope. FIG. 10A is an observation image of the sample surface after etching by an optical microscope, and FIG. 10B is a graph showing the result of measuring the sample cross section with a stylus type surface shape measuring instrument (DEKTAK). FIG. 11 is an image observed by an optical microscope on the surface of the MgZnO substrate after etching. FIG. 12A is an image observed by an optical microscope on the surface of the MgZnO film after etching, and FIG. 12B is a graph showing the result of measuring the sample cross section with a stylus type surface shape measuring instrument (DEKTAK). FIG. 13 is an optical microscope image of the surface of the ZnO film after etching according to Example 1. FIG. 14 is an enlarged sectional view near the interface between the etching region 10 and the resist protection region 12. 15A to 15D are schematic cross-sectional views showing the element manufacturing process according to the second embodiment. FIG. 16A is an IV characteristic of a semiconductor structure in which an isolation groove is formed according to Example 2, and an element is isolated, and FIG. 16B is an IV characteristic when an isolation groove is formed only with solution A as a reference example. It is. 17A and 17B are surface micrographs of an epitaxial layer having a ZnO-based semiconductor structure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor structure 2 Holder 3 Beaker 4 Solution 5 Water tank 6 Pump 7, 20 Substrate 8 ZnO system film 9 Resist pattern 10 Etching area 11 Etch pit 12 Resist protection area 13 Side etching 14 C surface Off area 15 Concavity and convexity 16 Electrode 21 Buffer layer 22 n-type ZnO-based semiconductor layer 23 light-emitting layer 24 p-type ZnO-based semiconductor layer

Claims (10)

a) preparing a ZnO-based semiconductor structure whose surface is a + C plane;
b) etching the surface of the ZnO-based semiconductor structure with a mixed solution of ammonium fluoride and hydrofluoric acid;
c) A method for processing a semiconductor structure, comprising a step of etching the surface of the ZnO-based semiconductor structure with a mixed solution of hydrochloric acid and nitric acid after the step b). Said steps b) and c)
bc-1) forming a desired resist pattern on the surface of the ZnO-based semiconductor structure;
bc-2) etching the surface of the ZnO-based semiconductor structure with a mixed solution of ammonium fluoride and hydrofluoric acid using the resist pattern as a mask;
The step of bc-3) comprising, after the step bc-2), etching the surface of the ZnO-based semiconductor structure with a mixed solution of hydrochloric acid and nitric acid using the resist pattern as a mask. Processing method. In the step bc-3),
An etching pattern formed by etching is an element isolation groove,
After step bc-3)
The semiconductor structure processing method according to claim 1, further comprising: d) isolating the ZnO-based semiconductor structure along the element isolation trench.   4. The method of processing a semiconductor structure according to claim 3, wherein a method of isolating the ZnO-based semiconductor structure is scribe.   4. The method of processing a semiconductor structure according to claim 3, wherein the method of separating the ZnO-based semiconductor structure is dicing.   The semiconductor structure processing method according to claim 1, wherein the ZnO-based semiconductor structure is a substrate including a ZnO layer.   The semiconductor structure processing method according to claim 1, wherein the ZnO-based semiconductor structure is a substrate including an MgZnO layer. The material of the ZnO-based semiconductor structure has a composition formula Mg _x Zn _1-x O _1-yz S _y Se _z (0 ≦ x <0.5, 0 ≦ y <0.1, 0 ≦ z <0. The semiconductor structure processing method according to claim 1, wherein the semiconductor material is represented by 2).   The method for processing a semiconductor structure according to claim 1, wherein the mixed solution of ammonium fluoride and hydrofluoric acid is mixed at ammonium fluoride: hydrofluoric acid = 1 to 10: 1 (volume ratio).   The method for processing a semiconductor structure according to claim 1, wherein the mixed solution of hydrochloric acid and nitric acid is mixed with hydrochloric acid: nitric acid = 1 to 10: 1 (volume ratio).