JPH044750B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an etching solution, and more particularly to an etching method suitable for light-controlled etching in which etching is controlled by light irradiation.

Etching solid materials such as semiconductor crystals
Various techniques are known to do this. For example, in contrast to wet etching which uses liquid as an etching agent, dry etching or plasma etching which uses gas has recently attracted attention. The etching rate varies depending on the composition of the etching agent, and the etched surface has various properties such as a mirror surface and an etch pit. Conventionally, in order to perform desired etching, a method has generally been adopted in which an etching agent exhibiting desired etching characteristics is selected. On the other hand, a light-controlled vapor phase processing method for semiconductors has been proposed in Japanese Patent Publication No. 55-25497 (divisional application, Japanese Patent Publication No. 58-51421) filed by the same applicant.
By controlling the light irradiation pattern, selective processing can be performed without using a mask, and by using an infrared laser or the like, spots with high light intensity can be easily obtained. However, the gas phase processing method used in the above patent application uses light to control the promotion of gas phase reactions by heating. Further, a patterned vapor phase growth method using ultraviolet irradiation has been proposed in Japanese Patent Publication No. 39-2426, etc.

Etching is not only used to dig down a predetermined area to a predetermined depth, which is normally used in the manufacture of semiconductor devices, but also to perform various etching tests, such as checking the crystallinity (crystal integrity) of a grown layer. used for the purpose of Therefore, various etching techniques suitable for various purposes are desired.
In addition to selecting the composition, a widely known method for controlling liquid phase etching that is easy to operate is to electrochemically control etching by passing a current through the etching solution through the workpiece. There are limits to the range. In addition, the technology to control liquid phase etching using light was published in the Journal of the
Electrochemical Society, Vol. 119, No. 8, pp. 1063-1068 (F. Kuhn)
Kuhnenfeld) et al. This is done by using a (H ₂ SO ₄ :H ₂ O ₂ :H ₂ O) mixture or a 1% by volume methanol solution of bromine as the etching solution, and locally irradiating high-intensity light onto the object to be etched. This is to locally increase or decrease the etching speed. The object to be etched is gallium arsenide. It has been reported that irradiation with light increases the etching rate of n-type gallium arsenide and decreases the etching rate of p-type gallium arsenide compared to dark etching without irradiation of light. This method is reported to be suitable for observing dislocations and striations within crystals, but
The speed of dark etching is relatively high, and microstructures cannot be observed unless the amount of etching is large.

Epitaxial growth technology plays a very important role in the manufacture of semiconductor devices. The thickness of the epitaxial layer formed on the substrate is often very thin, less than 100 μm, and sometimes less than several μm.
When testing the crystallinity of such thin epitaxial layers, the required etching depth is 100 μm.
Such large etching methods are not practical for observing thin epitaxial layers. Of course, it is possible to grow a thick epitaxial layer for inspection and deeply etch it for inspection, but this cannot be said to be an inspection of the epitaxial layer actually used in the device. Therefore, there is a need for an etching technique that can inspect crystallinity with a very shallow etching depth.

An object of the present invention is to provide an etching method suitable for a novel liquid phase etching technique using optical control.

Another object of the present invention is to provide an etching method in which a chemical reaction is induced by light irradiation.

Another object of the present invention is to provide an etching method that induces a chemical reaction without heating by irradiating light with infrared rays removed.

The following description will be given based on Examples, but the Examples are illustrative and do not limit the present invention.

Gallium arsenide is the most widely used semiconductor in intergroup semiconductor devices, and is used in light emitting diodes, lasers, transistors, and the like.
The inventors proposed and developed a vapor pressure controlled temperature difference method as a compound semiconductor crystal growth method, and succeeded in obtaining a gallium arsenide crystal with excellent crystallographic, electrical, and electro-optical properties. . In order to examine the crystallinity of this gallium arsenide crystal, conventional liquid phase etching was performed, but no good results were obtained. For example, using an etching solution usually called a super oxidizer of HF49% aqueous solution: 30% H ₂ O ₂ :H ₂ O 1:1:4, gallium arsenide crystals doped with chromium are
When etched by light irradiation for a minute, the etched surface was covered with fine irregularities, and an etching pattern in which dislocations, striations, etc. could be observed could not be obtained. AB Etchiyanto (Journal)
of the Applied Physics Vol. 36 (1965), p. 2855), the etching pattern shown in Figure 1 was obtained with etching of approximately 100 μm or more; I don't get a pattern.

As a result of various studies, for gallium arsenide (GaAs), 49% HF aqueous solution: 30% H ₂ O ₂ : H ₂ O 1:
By using a 1:10 etching solution and irradiating it with light, a clean etching pattern clearly showing crystallinity was obtained with an extremely shallow etching depth. FIG. 2 shows an outline of the apparatus used for etching.

Using a 500W tungsten lamp as light source 1,
A synthetic resin beaker 2 containing an etching solution (HF49% aqueous solution: 30% H ₂ O ₂ :H ₂ O 1:1:10) is placed at a distance of about 20 cm below it to prevent the temperature of the etchant from rising. A water filter 3 with a depth of about 10 cm was placed as an infrared absorbing filter for this purpose. The water filter 3 is placed on the glass holder 4. Etching is stopped with running water 5. A gallium arsenide crystal 7 to be inspected is placed in a basket 6 made of synthetic resin or platinum, and is submerged into an etchant. The light source 1 is turned on and the crystal 7 is irradiated with light while the etching solution is gently stirred using a synthetic resin stirring rod or a magnetic stirrer coated with a synthetic resin. After photo-etching for a predetermined period of time,
The crystal 7 is pulled up together with the basket 6 and the etching is stopped with running water 5.

In this example, in the case of non-doped GaAs in the light irradiated area, the etching rate was approximately 1 μm/1 minute. The etching rate tends to be slow for low-resistance crystals, and for n ⁺ type GaAs, the etching rate is approximately
It was 3000 Å/min.

As for the composition ratio of hydrofluoric acid, hydrogen peroxide, and water, when the amount of water is increased, the reactivity decreases, and when the amount of water is decreased, the reactivity increases.
The composition may be changed as appropriate depending on the semiconductor to be etched.

In this way, various gallium arsenide (GaAs)
_The composition of the crystals is approximately HF49% aqueous solution: 30% _H2O2 :
The results of photoetching using a ₁ :1:10 H 2 O etching solution are shown in the Nomarski micrographs of FIGS. 3a to 3f. The magnification is approximately 180x for Figure 3e and approximately 65x for the others.

FIG. 3a is a photograph of the etched surface of the {100} plane of n ⁺ type GaAs, in which a striped etch pattern corresponding to striations and protrusions in the depressions corresponding to dislocations are observed. Since there are reports that impurities precipitate around dislocations, it is thought that the depressions around protrusions are related to the impurity density distribution.

FIG. 3b is a photograph of the etched surface of the {100} plane of a semi-insulating GaAs substrate crystal doped with chromium. Although the contrast is weak, an etching pattern showing striations and dislocations is observed as in FIG. 3a. In this case, no depressions are observed around the protrusions corresponding to dislocations.

Figure 3c shows a non-doped product with no added impurities.
This is a photograph of an etching pattern of a GaAs crystal grown by liquid phase epitaxial growth. The contrasting vertical stripes are evidence of melting. No striations are observed in this epitaxial layer.

FIG. 3d is a photograph of an etching pattern of a non-doped GaAs crystal grown by vapor phase epitaxial growth. Similar to Fig. 3a, protrusions are observed within the depressions.

FIG. 3e is a photograph of an etching pattern of GaAs grown by vapor phase epitaxial growth, and it is thought that stacking faults were observed. That is, it can be seen that cross-shaped depressions are formed corresponding to incomplete dislocations at the ends of the rectangular stacking fault.

FIG. 3f shows a non-doped GaAs liquid phase epitaxial growth on a chromium-doped GaAs crystal substrate, etching the cleavage planes, and observing the boundaries. A line running horizontally through the center of the crystal indicates the interface (junction surface) between the substrate and the epitaxial layer. In this case, etching stopped in about 15 seconds. Note that the vertical streaks are flaws during cleavage.

As is clear from the above results, according to this example, a clear etching pattern corresponding to the bonding surface or dislocation can be easily obtained with an etching depth of 1 μm or less. The reactivity is extremely low when using only an etching solution, and there is almost no etching when using dark etching, making it easy to control. If light is irradiated in a pattern, selective etching will proceed only in the areas that are irradiated with light, and the areas that are not irradiated with light can be kept as they are.

As a result of measuring the temperature dependence of photoetching in this example, the excitation energy for etching was approximately 0.35 eV (approximately 8 Kcal/mol). Therefore about
It is thought to respond to light with a wavelength of 3.5 μm or less.
It is thought that etching can be excited by light with a wavelength of approximately 3.5 μm or less, which is not absorbed by the etching solution or absorbed in the optical path in the middle, but in order to prevent heating of the etching solution, the wavelength must be approximately 1.1 μm or less. It is preferable to use light in the near-infrared and visible regions with a wavelength of . It is also possible to perform selective etching by selectively irradiating tiny parts with light using a light source such as a He--Ne laser, Ar laser, or xenon lamp, or to control the etching rate by irradiating pulsed light. Further, in the etching according to this example, the etched surface was etched with 98% H ₂ SO ₄ :30% H ₂ O ₂ :H ₂ O.
The etching pattern can be easily erased by mirror etching such as 10:1:1.

An example of the results is shown in Figures 4a and 4b. FIG. 4a is a microscopic photograph of the surface after the above-mentioned photoetching was performed for about 1 minute, and dislocations are observed. then 98%
After etching with H ₂ SO ₄ :30% H ₂ O ₂ :H ₂ O 10:1:1 for about 1 minute, the same surface becomes as shown in Figure 4b, and the protrusions that appeared in Figure 4a are removed. It disappeared. The method is not limited to etching gallium arsenide crystals. Other examples will be described below.

A 49% HF aqueous solution of the cleavage plane (110 plane) of a chip made by epitaxially growing a gallium aluminum arsenide phosphorus mixed crystal on a gallium arsenide (GaAs) substrate:
When etching was carried out in an etching solution of 30% H ₂ O ₂ :H ₂ O 1:1:10 for about 15 seconds by light irradiation as described above, an etching pattern suitable for interface observation was obtained.

HF:H ₂ O ₂ :H ₂ O-based etching solutions have lower etching reactivity with gallium phosphide (GaP) than with GaAs. For GaP, HF49
% aqueous solution: 30% H ₂ O ₂ :H ₂ O 1:1:4 etching solution was used to carry out light irradiation etching in the same manner as in the previous example to obtain a similar etching pattern.

Similarly, for indium gallium phosphide (InGaP) mixed bulk crystal, HF49% aqueous solution: 30%
Fine grain boundaries could be observed by light irradiation etching using an etching solution of H ₂ O ₂ :H ₂ O, 1:1:4.

In this way, the irradiated crystal surface can be etched by using a low-reactivity etching solution in which etching does not substantially proceed with the etching solution alone, and by irradiating light with a predetermined photon energy or higher. This technique is particularly suitable for observing thin crystal layers such as epitaxial layers. Although it was difficult to evaluate the epitaxial layer of a compound semiconductor grown in a liquid phase using the vapor pressure controlled temperature difference method using conventional etching techniques, it has now become possible to appropriately evaluate it using the aforementioned etching technique. As described above, the method of the present invention is particularly effective for evaluating crystals with excellent crystallinity. Good results were obtained when a hydrofluoric acid-based etching solution, particularly an HF:H ₂ O ₂ :H ₂ O-based oxidizing etching solution, was used as the etching solution for crystallinity evaluation. Appropriate etching can be carried out by selecting the proportion of water in the etching solution so that etching does not substantially proceed with the etching solution alone, and synergistically with light irradiation. For intergroup compound crystals containing gallium arsenide as one of the main components, the volume ratio of H ₂ O to 49% HF aqueous solution is approximately 8.
The above is preferable. Contains gallium phosphorous as one of the main components - HF49% for intergroup compound crystals
The volume ratio of H ₂ O to aqueous solution is preferably about 3 or more.

[Brief explanation of drawings]

FIG. 1 is a micrograph of an etched surface using a conventional AB etchant, and FIG. 2 is a schematic diagram for explaining an etching method according to an embodiment of the present invention.
Figures 3a to 3f are micrographs of surfaces etched according to the embodiment of the present invention, and Figures 4a and 4b show how the surface pattern produced by the etching method of the present invention is erased by mirror etching. FIG.

Claims (1)

[Claims] 1. An intergroup compound semiconductor crystal mainly composed of GaAs is exposed to light using an etching solution consisting of a mixture of 49% HF, 30% H ₂ O ₂ and H ₂ O in a volume ratio of approximately 1:1:10. A low-reactivity etching method characterized in that etching is performed while being irradiated with the light, substantially no etching is performed when the light is not irradiated, and etching is performed while being irradiated with the light.