JP2000068608A - Manufacture of gallium nitride based semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable wet etching with little damage by etching chemically stable gallium nitride using reactive pyrophosphoric acid. SOLUTION: A gallium nitride growing substrate, a holding device or the like are pre-heated to the same temperature as an etching temperature. Then the gallium nitride growing substrate is soaked in pyrophosphoric acid to be etched. Optimum time to soak should be calculated by pre-conditioning. A temperature of pyrophosphoric acid should be also calculated similarly. Pyrophosphoric acid can be obtained by dehydrating orthophosphoric acid which is usually used. Or it can be obtained by adding water of a suitable ratio to phosphorus oxide. In addition, sufficient etching speed can be obtained by raising the temperature of the pyrophosphoric acid to a high temperature ranging from 210 to 220 deg.C or higher. Then after the time required for etching, materials are taken out, and the pyrophosphoric acid remaining on surfaces is removed using pure water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor device containing gallium nitride as a raw material.

[0002]

2. Description of the Related Art Semiconductor materials mainly composed of gallium nitride are:
Optical semiconductor devices such as blue semiconductor lasers and blue light emitting devices,
It is important as a material for electronic devices in high-temperature environments. This is because gallium nitride has a large band gap, is stable at high temperatures, and has strong physical and chemical strength. However, due to its features, there have been difficult points in the technology for manufacturing such optical semiconductor elements and electronic devices.

[0003] In other words, gallium nitride is a chemically stable material and has a high resistance to chemicals such as acids and alkalis, so that it has been difficult to process it by dissolving it in a liquid solution, that is, wet etching. For this reason, a processing technique using reactive ions created by a plasma technique or a reactive ion beam obtained by converting the ions into a beam, that is, dry etching, etc., has been used. In dry etching, reactive chlorine ions are created by making the reactive gas, such as chlorine gas, into a plasma state.
The ions are formed into a beam by the interaction between the electric field and the electric field, and the reactive chlorine ion beam is irradiated on a substrate for forming a semiconductor element containing gallium nitride to perform processing. On the irradiated semiconductor substrate, the lattice of gallium nitride is decomposed or destroyed by the physical energy or the chemical reaction force of the reactive ion beam. By this physical or chemical reaction, a desired semiconductor portion can be cut, that is, etched. Electronic devices and optical devices have been produced using this element formation technology. (Reference 1: Shin-ya Nunoue, Masahiro
Yamamoto, Mariko Suzuki, Chiharu Nozaki, Joji Nish
io, LisaSugiura, Masaaki Onomura, Kazuhiko Itaya, a
nd Masayuki Ishikawa, Reactiveion beam etching and
overgrowth process in the fabrication of InGaNinne
r stripe laser diodes, Jpn. J. Appl. Phys., Vol. 3
7, pp. 1470-1473part 1, no.3B, March 1998.)

However, in dry etching, since the sample is exposed to an atmosphere of ions having physically large energy, there is a disadvantage that a damaged layer is formed on the substrate surface. In particular, in the case of a fine structure of about several tens nm or the case where the surface is regrown after etching, the influence of the damage appears.

[0005] There is no wet etching technology,
Some examples use a sodium hydroxide solution, a potassium hydroxide solution, a hydrochloric acid solution, or the like. However, since gallium nitride is chemically stable, it is necessary to use another method in combination to obtain a practical speed at the time of fine processing such as etching.

When using a sodium hydroxide solution,
Etching is performed by attaching an electrode to the gallium nitride layer and anodizing the electrode. This is because, because gallium nitride is stable, etching does not occur simply by immersing it in a sodium hydroxide solution. Therefore, the method is to induce or accelerate etching by attaching an electrode to the gallium nitride layer and changing the electrochemical potential of the gallium nitride layer. (Reference 2: Mitsugu Oh
kubo, Effects of dissolved oxygen onanodic etching
of n-type GaN films using a sodium hydroxide elec
trolyte, Jpn. J. Appl. Phys., Vol. 36, pp. L955-L95
8, part 2., No.7B, July 1997)

However, this method poses a problem when the electrical resistance of the gallium nitride layer is large. In other words, when the concentration of impurities to be doped to lower the electric resistance is low, or when impurities or lattice defects that have the effect of increasing the electric resistance are present in the gallium nitride layer, the electric potential in the gallium nitride is reduced. Changes cannot be ignored, and uniform etching becomes impossible. Furthermore, in a non-doped gallium nitride layer or a semi-insulated gallium nitride layer, almost no current flows, so that the anodization technique is almost impossible.

In general, a sapphire substrate is used as a substrate for manufacturing a gallium nitride-based semiconductor device, but this sapphire substrate is an insulator. On the other hand, when manufacturing an element, it is essential to use a large substrate of several inches or more in order to reduce production costs. Therefore, if it is necessary to use anodizing technology in the production process, even if the electrical resistance of the semiconductor layer such as a gallium nitride layer is low, the substantial electrical distance becomes large, so that a large number of electrodes are used. It needs to be provided on a substrate. This is a serious problem in practical use. Such a processing process not only lowers the quality of the product but also lowers the yield and the like.

Anodization becomes more difficult when used during the processing process, that is, when used on a substrate that has already been subjected to some fine processing. This is, for example, in the etching of a part having a shape on the order of several μm that is processed so as to be insulated from other parts. In this case, since it is electrically insulated from other parts, it is necessary to form an anodizing electrode inside a shape on the order of several μm. However, even with such an electrode forming technique, it is very difficult to apply a potential to the portion from outside.

[0010] There is also an etching example using a potassium hydroxide solution or a hydrochloric acid solution. In this case, the anodization technique is indispensable, as in the above-described etching method using a sodium hydroxide solution. Furthermore, these solutions are less reactive than sodium hydroxide. Therefore, this is a method in which the etching rate is promoted by irradiating ultraviolet light.
(Reference 3: MSMinsky, M. White, and ELHu, Room-
temperaturephotoenhanced wet etching of GaN, Appl.
Phys. Lett., Vol. 68, No. 11, pp.1531-1533, March 1
996)

In this method, since the anodization technique is also used, problems occur in the uniformity and practicality as described above. Further, since it is necessary to additionally perform light irradiation, there arises a problem that an ultraviolet light source must be prepared.
While the etching rate depends on the intensity of ultraviolet light, 1 mm
Light intensity of several mW or more is required for all directions. Therefore, when processing a substrate of several inches, it is necessary to uniformly irradiate using an ultraviolet light source of several watts or more, and in some cases, tens of watts or more.

[0012]

The first object of the present invention is to develop a wet etching which does not cause damage. At present, dry etching is generally used. However, dry etching has problems such as formation of a damaged layer. It is necessary to develop wet etching with less damage.

The second problem is the development of wet etching which does not require anodizing technology or light irradiation. The processes of anodization technology and light irradiation are unnecessary in nature and cause unnecessary problems.

A third problem is to improve the etching rate. The conventional wet etching method has a low etching rate even if anodization technology, light irradiation, or the like is used. That is, in the conventional wet etching, the etching rate is low, and a practical etching rate is obtained only when many defects exist in the gallium nitride layer.

A fourth problem is the development of an etching mask for enabling patterning and the like. The etching technique requires a thin film having a resistance to an etching solution.

A fifth problem is the development of etching for obtaining a vertical end face used for an optical device or the like. In an optical device, particularly a semiconductor laser, it is necessary to manufacture a reflecting mirror to form a resonator structure for confining light. There was no wet etching method for this.

[0017]

In order to etch chemically stable gallium nitride, reactive pyrophosphoric acid is used. Pyrophosphoric acid is obtained by dehydrating a commonly used orthophosphoric acid. It can also be obtained by adding an appropriate ratio of water to phosphorus oxide. Since pyrophosphoric acid is a strong acid, it has a sufficient etching effect without anodization or light irradiation.

The temperature of pyrophosphoric acid is increased from 210 ° C. to 2 ° C.
By setting the temperature to 20 ° C. or higher, a sufficient etching rate can be obtained. Thus, a speed of etching a gallium nitride film having a thickness of several hundred nm or more per minute can be obtained.
Further, the etching rate depends on the temperature, and the etching rate can be controlled by controlling the temperature.

In patterning, a mask for patterning is important. Since pyrophosphoric acid has a large reaction force, it does not act as a mask with ordinary metals or low-quality oxide films. Therefore, a high-quality silicon oxide film formed by a thermal CVD method or the like is used. If the thickness of the silicon oxide film is increased to several hundreds nm or more, it functions as a sufficiently resistant mask.

When a gallium nitride-based semiconductor film formed on a sapphire substrate or the like is etched using a sufficiently thick mask as described above, a substantially vertical side surface of the semiconductor film is formed. This side surface forms a surface perpendicular to a substrate such as sapphire, and serves as a surface of a reflecting mirror forming an optical device, particularly a laser resonator.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the etching of pyrophosphoric acid will be described. Pyrophosphoric acid is a colorless and transparent liquid, and is obtained by heating orthophosphoric acid to 210 to 220 ° C. or higher, or by adding diphosphorus pentoxide to a suitable amount of water followed by heating. It is also possible to use a commercially available one.

Since pyrophosphoric acid already contains moisture in the air or absorbs moisture depending on the humidity at the place where the etching is performed, in such a case, the pyrophosphoric acid is reduced to the etching temperature. It is necessary to perform etching after holding for a certain period of time, for example, one hour. As a result, excess water evaporates or, conversely, absorbs water in accordance with the humidity of the place, thereby achieving an equilibrium state, and the etching rate is stabilized.

In order for pyrophosphoric acid to etch gallium nitride, it is necessary to keep pyrophosphoric acid at a temperature at which etching occurs. The effect of the etching has a temperature dependency and a threshold temperature. The threshold temperature is between 210 ° C and 220 ° C. Even if the temperature is lower than that, etching occurs slightly, and it is effective for peeling about several molecular layers on the surface,
A practical etching rate effective for patterning or the like cannot be obtained. If it is a damaged layer on the surface, it may be etched. Conversely, above the threshold, a large etching rate can be obtained.

FIG. 2 shows a measurement example of the etching depth and the etching time when using pyrophosphoric acid at about 230 ° C.
This is a measurement result when a hexagonal gallium nitride layer containing a little lattice defect and the like grown on a sapphire substrate is etched. It can be seen that the etching is not performed for the first one minute or so, and the etching is performed after two minutes or more.

There is little etching during the first minute or two, but the details are not yet clear. One consideration is that there is some kind of chemically stable ultrathin film on the surface, which stops the etching of gallium nitride by pyrophosphoric acid for a certain period of time. It is believed that the chemically stable ultra-thin film is removed within the first fixed time, after which gallium nitride etching is performed at a substantially constant rate. The same occurs when the etching is stopped halfway and the etching is performed again.
This requires further study on the details of the chemical reactions.

In the example shown in FIG. 2, the etching rate after elapse of 2 minutes or more is about 100 nm / min. The etch rate depends on the quality of the gallium nitride material. When the number of lattice defects is small, the etching rate is low, and when the number of defects is high, the etching rate is high. The etching rate also depends on other properties such as doping density. Therefore, it is necessary to check in advance the etching characteristics of the gallium nitride material actually used in the process. For example, it should be performed after accurately measuring the temperature dependence of the etching rate, the time during which almost no etching is performed at the beginning of etching, and the like.

At present, there is no substrate having a lattice constant equal to that of gallium nitride, and instead, a sapphire substrate or the like is used. Therefore, a large number of lattice defects and the like exist in the semiconductor layer. The defect density has changed.
Therefore, it is impossible at present to accurately determine the etching rate. Therefore, it is considered essential to determine the conditions in advance using a dummy material or the like.

The following points should be noted regarding containers and equipment. Since high-temperature pyrophosphoric acid has a large etching effect, ordinary glass or the like is etched. Also, many metals are easily etched. Care must be taken to prevent corrosion of the container of pyrophosphoric acid, tools for holding gallium nitride material when placed in pyrophosphoric acid, and thermometers for measuring the temperature in pyrophosphoric acid. Therefore, it is preferable to use a container or holding tool made of quartz, or a material made of platinum. Since the thermocouple and the like are also etched, it is necessary to cover with a quartz tube or the like.

FIG. 1 is a diagram illustrating an etching process which can be used in the method for manufacturing a gallium nitride based semiconductor device of the present invention. In step S1, a gallium nitride material that needs to be etched, that is, a gallium nitride growth substrate, is attached to a transfer device, and in step S2, the gallium nitride growth substrate, the holding device, and the like are preheated. That is, it is set to the same temperature as the etching temperature. This is because if the temperature of the holding device or the gallium nitride growth substrate material is different from the temperature of pyrophosphoric acid, the temperature of pyrophosphoric acid changes when the gallium nitride growth substrate or the like is placed in pyrophosphoric acid.

Next, in step S3, the gallium nitride growth substrate or the like is immersed in pyrophosphoric acid and etched. For the immersion time, conditions are determined in advance to determine the optimal time. The same applies to the temperature of pyrophosphoric acid. It is necessary to keep the temperature of pyrophosphoric acid constant by a heater such as an oven or a heating furnace. A well-known conventional method for keeping the temperature constant may be used. However, when the substrate is large or when the volume of the liquid is large and the temperature of pyrophosphoric acid in the container is not constant, it is necessary to stir.

After the time required for the etching has elapsed, in step S4, the material is taken out and the pyrophosphoric acid remaining on the surface is removed using pure water. As soon as the sample is taken out of the pyrophosphoric acid at high temperature to room temperature, the temperature of the sample drops to room temperature, so that the etching is almost stopped. Similarly 2
The temperature is lowered by immersing in pure water of about 0 ° C., and the etching is completely stopped to further clean the pyrophosphoric acid. Thereafter, in step S5, the next process is started by blowing and drying pure water as necessary.

Next, a method for finely processing a semiconductor layer made of gallium nitride or the like formed on a sapphire substrate or the like by metal organic chemical vapor deposition or molecular beam epitaxy will be described. FIG. 3 illustrates a method of manufacturing a gallium nitride based semiconductor device of the present invention using the etching process shown in FIG. Since silicon oxide and the like are not easily melted by pyrophosphoric acid, it is effective as a mask for fine processing. As shown in FIG. 3A, first, a silicon oxide thin film having a thickness of 200 nm is formed on a semiconductor layer (gallium nitride film 2) which is laminated on a substrate (sapphire substrate 3) and needs to be finely processed. 1 is formed into a thin film by using a conventional film forming technique such as a thermal CVD method or a plasma CVD method. Since pyrophosphoric acid is a highly effective liquid, sufficient care is taken so that minute holes are not formed in silicon oxide during film formation.
Therefore, the silicon oxide film 1 formed by the thermal CVD method or the like works effectively.

Thereafter, as shown in 2) of FIG. 3, the silicon oxide is patterned into a desired shape by using photolithography or electron beam lithography using a resist together with an etching solution containing hydrofluoric acid or the like. I do. The silicon oxide thin film formed at this time is used as a mask at the time of fine processing of the semiconductor layer made of gallium nitride or the like.

Next, etching is performed, as shown in 3) of FIG. The details are as shown in FIG. The temperature of pyrophosphoric acid is 230-240
By maintaining the temperature at about ° C, an etching rate of about 20-2000 nm / min can be obtained. However, the etching rate is
Since it differs depending on the film quality of the semiconductor layer, it is necessary to determine conditions in advance using a dummy sample or the like. Finally, if necessary, the silicon oxide film or the like is removed, and the process proceeds to the next process.

[0035]

The wet etching using pyrophosphoric acid has features such as less damage as compared with dry etching. Also, by increasing the temperature, it is possible to obtain an etching rate higher than that of dry etching. This can be an important technique in the preparation and polishing of a substrate of gallium nitride itself.

Further, since neither light irradiation nor anodization technology is required, the present invention is effective in developing a device on a large substrate and a device having a fine structure. In particular, in the anodization technique, a process of forming an electrode on the element surface is required, and in some cases, a process of removing the electrode is also required. Therefore, the number of steps in the processing process can be reduced. Further, since a vertical end face can be formed, a great effect is produced in the production of diodes, semiconductor lasers, and the like.

Further, since etching using pyrophosphoric acid does not contain sodium ions, potassium ions, etc., it is important as a processing technique at the time of manufacturing an element or an integrated circuit using a gallium nitride-based material. This is evident in view of the fact that metal ions such as sodium ions and potassium ions are major factors that cause the yield of elements and the deterioration of production facilities and equipment.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an etching process that can be used in a method for manufacturing a gallium nitride based semiconductor device of the present invention.

FIG. 2 is a diagram showing a measurement example of an etching depth and an etching time when using pyrophosphoric acid.

FIG. 3 is a diagram illustrating a method for manufacturing a gallium nitride based semiconductor device of the present invention using the etching process shown in FIG. 1;

[Explanation of symbols]

 Reference Signs List 1 silicon oxide film 2 gallium nitride film 3 sapphire substrate 4 patterned silicon oxide film 5 patterned gallium nitride film

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Junichi Shiragashi 1-1-4 Umezono, Tsukuba, Ibaraki Pref. Electronic Technology Research Institute (72) Inventor Masanobu Watanabe 1-4-1, Umezono, Tsukuba, Ibaraki F-term (Reference) in the Electronic Technology Research Laboratory, National Institute of Industrial Science 5F041 CA40 CA74 5F043 AA13 BB06 CC05 DD07 FF10 GG10 5F073 CA02 DA23

Claims (3)

[Claims] 1. A method of manufacturing a gallium nitride-based semiconductor device containing gallium nitride as a material, wherein the gallium nitride material is processed by wet etching using pyrophosphoric acid. 2. The method according to claim 1, wherein a mask of a silicon oxide film is used for processing and patterning the gallium nitride material. 3. The semiconductor device according to claim 1, wherein the semiconductor device is an optoelectronic device requiring a vertical end face, and a vertical etching surface is obtained by wet etching using the pyrophosphoric acid. For manufacturing gallium nitride based semiconductor device