JP2012054509A - Dry etching method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dry etching method of a substrate to be processed whose at least surface layer is composed of gallium nitride, which can obtain a smooth etched-surface and can perform etching with good reproducibility.SOLUTION: There is provided a method of etching a substrate to be processed whose at least surface layer is composed of gallium nitride by exciting a chlorine gas into a plasma state, including a step of etching the substrate to be processed by the plasm generated from the chlorine gas in the presence of a material containing aluminum nitride (AlN) that generates aluminum radicals and aluminum chloride radicals.

Description

  The present invention relates to a dry etching method for a gallium nitride compound semiconductor.

  Hydrochloric acid, sulfuric acid, hydrofluoric acid, or a mixture thereof is often used to wet-etch III-V compound materials, but gallium nitride hardly dissolves in these solutions. Therefore, the etching of gallium nitride is generally performed by dry etching instead of wet etching. In particular, dry etching using reactive plasma has a high etching rate and is practical, and therefore has been actively researched and developed.

In dry etching using reactive plasma, the composition of the reaction gas is an important factor that determines the etching speed and quality. Reactive gases that can be used for etching gallium nitride semiconductors include CF ₄ gas, CCl ₂ F ₂ gas, CCl ₄ gas, BCl ₃ gas, SiCl ₄ / Cl ₂ mixed gas, Cl ₂ / H ₂ mixed gas, BCl ₃ / Ar mixed gas, BCl ₃ / SiCl ₄ mixed gas, SiCl ₄ gas, SiCl ₄ / SiF ₄ mixed gas, etc. have been reported. Among these reaction gases conventionally used for dry etching, some etching residues and etch pits are likely to be generated, and a smooth etching surface cannot be obtained. On the other hand, according to Patent Document 1, the above problem is solved by using a mixed gas of SiCl ₄ / Cl ₂ , and a smooth etching surface is obtained.

JP 08-017803 A

However, in general, when dry etching of a gallium nitride compound semiconductor is performed by plasma of chlorine gas, if the evacuation in the reaction chamber is insufficient, the etched surface is roughened due to residual oxygen, residual moisture, or gallium nitride crystal defects. Etch pits are likely to occur. In order to prevent this, it was necessary to create a high vacuum of the order of 10 ⁻⁴ to 10 ⁻³ Pa in the reaction chamber before the plasma treatment.

  A problem to be solved by the present invention is a method of dry etching a substrate to be processed having at least a surface layer made of gallium nitride, and a dry etching method capable of obtaining a smooth etching surface even at a relatively low degree of vacuum (high pressure). Is to provide.

  In order to solve the above problems, the present invention provides a dry etching method for etching a substrate to be processed having at least a surface layer made of gallium nitride by exciting chlorine gas into a plasma state. Etching is performed in the presence of a material containing aluminum nitride (AlN).

  In this case, etching is performed using a material containing aluminum nitride in at least a part of the constituent members of the tray that is disposed in the etching reaction chamber for transporting the substrate to be processed, or at least one of the constituent members of the etching reaction chamber. Etching may be performed using a material containing aluminum nitride in the portion.

  The surface layer of the substrate to be processed is preferably doped with at least one of aluminum and indium.

The inventors of the present invention, when generating chlorine gas plasma and performing dry etching of a gallium nitride compound semiconductor, the presence of aluminum radicals and aluminum chloride radicals planarizes the etched surface of gallium nitride, resulting in a low degree of vacuum. It was found that even under (high pressure), a smooth etched surface free from roughness, residue, etch pits and the like can be obtained.
Therefore, the present inventors, if a substance containing aluminum nitride is present in the chlorine gas plasma, the substance is etched by the chlorine gas plasma and aluminum (Al) is ejected, and this aluminum is radical (for example, chlorine radical). The present invention has been achieved by utilizing the fact that an aluminum radical or an aluminum chloride radical is produced by combining with the above.

  Thus, according to the present invention, when a substrate to be processed having at least a surface layer made of gallium nitride is etched by exciting chlorine gas into a plasma state, a substance containing aluminum nitride is present in the chlorine gas plasma. Thus, the etching can be efficiently performed even under a low degree of vacuum (high pressure).

The schematic block diagram which shows the example of the dry etching apparatus used in order to implement the dry etching method of this invention. The figure which shows schematically the cross-sectional structure of the element substrate used in order to implement the dry etching method of this invention. The SEM image of the etching surface obtained by the process of an Example is shown, (a) is the SEM image seen from 45 degrees diagonally, (b) is the SEM image seen from right above. The SEM image which looked at the etching surface obtained by the process of the comparative example from 45 degrees diagonally.

  The target substrate to be etched by the dry etching method of the present invention is a gallium nitride compound semiconductor substrate having at least a surface layer composed of a gallium nitride layer (GaN layer). In this case, the surface GaN layer may be doped with aluminum (Al) or indium (In). The gallium nitride compound semiconductor structure of the substrate to be processed may be a single layer structure or a multilayer structure. In the case of a multilayer structure, all layers may be composed of a gallium nitride compound semiconductor, but at least the surface layer may be composed of a gallium nitride compound semiconductor. In the present invention, the gallium nitride based semiconductor structure of the substrate to be processed can be satisfactorily etched regardless of whether it is a single layer structure or a multilayer structure.

  As such a substrate to be processed, for example, a double heterostructure in which a GaN buffer layer, an n-type GaN layer, an n-type GaAlN layer, an InGaN layer, a p-type GaAlN layer, and a p-type GaN layer are sequentially stacked on a sapphire substrate. And a semiconductor substrate. The gallium nitride based semiconductor substrate having such a structure has been put into practical use as a blue LED light emitting chip.

Hereinafter, the dry etching method of the present invention will be specifically described.
FIG. 1 is a schematic configuration diagram showing an example of a dry etching apparatus used for carrying out the dry etching method of the present invention. This apparatus is an inductively coupled type (ICP), and a flat plate-like lower electrode 12 is placed in an upper part (outside) of the reaction chamber 11 in a sealed etching reaction chamber (hereinafter simply referred to as “reaction chamber”) 11. Are respectively provided with excitation coils 15 via a quartz plate 14. The excitation coil 15 is a three-dimensional spiral (inverted tornado type) coil, which supplies high-frequency power from the center of the coil and is grounded at the outer periphery of the coil. The element substrate 20 to be etched is placed on the lower electrode 12 via the transfer tray 16. The lower electrode 12 is connected to a high frequency power source 13. The lower electrode 12 has a built-in cooling mechanism for cooling the element substrate 20 and is controlled by the cooling control unit 17.

Using the above apparatus, an experiment for etching by the dry etching method according to the present invention was conducted. FIG. 2 schematically shows a cross-sectional configuration of the element substrate 20 used in this experiment. This element substrate 20 has an n-type GaN layer 22 (thickness 5.0 μm), a GaN active layer 23 (thickness 0.1 μm) having a multiple quantum well structure (MQW), a p-type GaN layer 24 (on a sapphire substrate 21). A mask layer 25 made of SiO ₂ and having a thickness of 0.2 to 0.3 μm is laminated. In FIG. 2, the illustration of the etching window of the mask layer 25 is omitted.

Experiments were performed using the above apparatus as follows. First, an aluminum nitride (AlN) transfer tray 16 is placed on the lower electrode 12 of the reaction chamber 11, and the element substrate 20 is placed thereon. Then, the air in the reaction chamber 11 was discharged, and the pressure in the reaction chamber was set to 2 × 10 ⁻³ Pa. Thereafter, Cl ₂ gas and Ar gas were supplied to the reaction chamber 11 at flow rates of 5 sccm and 3 sccm, respectively, and the gas pressure in the reaction chamber 11 was set to 0.3 Pa. Then, high-frequency power (excitation coil 15 / lower electrode 12 = 120/100 (W)) was supplied to the excitation coil 15 and the lower electrode 12 for 10 minutes to generate a plasma 18 of a reactive gas. As a result of etching using this plasma 18, the p-type GaN layer 24 and the mask layer 25 were etched at an etching rate of 86.0 nm / min. The selection ratio (p-type GaN layer / mask layer) between the p-type GaN layer 24 and the mask layer 25 was 1.0. Etching progressed from the surface of the p-type GaN layer 24 to a depth of 0.86 μm by etching for 10 minutes, and the bottom of the etching reached halfway through the n-type GaN layer 22.

  After etching as described above, the element substrate 20 was taken out of the reaction chamber 11 and the etched surface was observed with a scanning electron microscope (SEM). FIG. 3 shows an SEM image (a) of the etched surface as viewed from obliquely above 45 °, and an SEM image (b) of the etched surface as viewed from directly above. As can be seen from these images, the walls of the etched surfaces of the p-type GaN layer 24, the GaN active layer 23, and the n-type GaN layer 22 are almost vertical, and the etching proceeds with high anisotropy. In addition, an extremely smooth etched surface with no roughness, residue, etch pits or the like was observed on the etched bottom surface of the n-type GaN layer 22.

(Comparative Example 1)
Example except that a transfer tray made of alumina (Al ₂ O ₃ ) is used instead of the transfer tray 16 made of AlN, and Cl ₂ gas and Ar gas are supplied as reaction gases at flow rates of 5 sccm and 3 sccm, respectively. Etching was performed in the same manner. As a result, an etching rate of 111.0 nm / min was obtained for the p-type GaN layer 24, and the selectivity between the p-type GaN layer 24 and the mask layer 25 (p-type GaN layer / mask layer) was 0.68. At this time, an SEM image of the etching surface of the element substrate 20 taken out from the reaction chamber 11 is shown in FIG. FIG. 4 is an SEM image of the etched surface viewed from above at an angle of 45 °. From these images, a number of pillars (columns) were generated on the bottom of the n-type GaN etched.

  In the above embodiment, dry etching by ICP is performed, but the present invention can also be implemented by other dry etching methods such as RIE.

DESCRIPTION OF SYMBOLS 10 ... Plasma etching apparatus 11 ... Reaction chamber 12 ... Lower electrode (support stand)
DESCRIPTION OF SYMBOLS 15 ... Excitation coil 18 ... Plasma 20 ... Element substrate 21 ... Sapphire substrate 22 ... N-type GaN layer 23 ... GaN active layer (multiple quantum well structure)
24 ... p-type GaN layer 25 ... SiO ₂ mask layer

Claims (4)

In a dry etching method for etching a substrate to be processed having at least a surface layer made of gallium nitride by exciting chlorine gas into a plasma state,
A dry etching method characterized by etching in the presence of a substance containing aluminum nitride (AlN) in the chlorine gas plasma.   2. The dry etching according to claim 1, wherein at least a part of a component of a tray for conveying the substrate to be processed, which is disposed in an etching reaction chamber, is etched using a material containing aluminum nitride. 3. Method.   The dry etching method according to claim 1, wherein etching is performed using a substance containing aluminum nitride in at least a part of the constituent members of the etching reaction chamber.   4. The dry etching method according to claim 1, wherein the surface layer of the substrate to be processed is doped with at least one of aluminum and indium.