JP2007227450A - Photo-electrochemical etching apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To selectively etch a group III nitride semiconductor by using a photo-electrochemical etching having advantages such as inexpensiveness of an etching apparatus itself, inexpensive running cost and excellent mass productivity. <P>SOLUTION: The etching apparatus is provided with a container 200 filled with an etchant 202; a tool 204 provided in the container and holding a substrate 110 having an etched layer and etching stopping layer; a current extracting line 212 to be connected to a current extracting electrode 60 provided on the substrate; a cathode line 208 provided in the container; a variable voltage source 214 and an ammeter 210 which are connected in series between the cathode line and the current extracting line; a mercury lamp 216 of a high-voltage mercury lamp or a low-voltage mercury lamp; and a filter 218 attached on the mercury lamp and selectively transmitting a light lower than the bandgap energy of the etching stopping layer and higher than the bandgap energy of the etched layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photoelectrochemical etching apparatus for selectively etching a group III nitride semiconductor.

  Currently, there are two types of group III nitride semiconductor etching: dry etching and photoelectrochemical etching.

  In dry etching, selective etching for a semiconductor substrate having an AlGaN layer and a GaN layer formed on the AlGaN layer has been reported (for example, see Non-Patent Document 1). However, in dry etching, the vicinity of the surface of the semiconductor substrate may be damaged by plasma ions or the like (see, for example, Non-Patent Document 2). In addition, the plasma etching apparatus itself is expensive, the running cost is high, and the etching rate ratio between the etching mask and the etching target is low.

In contrast to dry etching, photoelectrochemical etching has no risk of being damaged by plasma ions. Further, the etching apparatus is inexpensive, has a low running cost, and has advantages such as being advantageous for mass productivity. As selective etching by photoelectrochemical etching, an ultraviolet (UV) light source using a He—Cd laser has been reported (for example, see Non-Patent Document 3).
Yanjun HAN et al. "Highly Selective Etching of GaN over AlGaN Using Inductively Coupled Cl2 / N2 / O2 Plasma", Jpn. J. et al. Appl. Phys. , Vol. 42 (2003) p. L1139-1141 Chao-Yi FANG et al. , "Etching Damages on AlGaN, GaN and InGaN Caused by Hybrid Inductively Coupled Plasma Etch and Photoenhanced Chemical Wet Etch Schottky." J. et al. Appl. Phys. , Vol. 42 (2003) p. 4207-4212 P. Visonicc et al. "Highly selective photoenhanced wet etching of GaN for defect investing and device fabrication", Material Research Society Symposium, Materials Research Society. 639 (2001) G3.14.1

However, an etching apparatus using a He—Cd laser as a UV light source is difficult to put into practical use because the irradiation area of the He—Cd laser is usually as small as 1 cm ² or less. Here, if the irradiation area is widened with a lens or the like, the intensity of the irradiation light is attenuated, so there is a limit to the expansion of the irradiation area.

  When a mercury lamp is used as the UV light source, light in a broad wavelength range (200 nm to 600 nm) is emitted from the mercury lamp. For this reason, when only the GaN layer formed on the AlGaN layer is etched as an etching target layer, the AlGaN layer that is not the etching target is etched.

  In order to solve this problem, a configuration in which the wavelength of the irradiated light is selected by attaching a UV filter to the ultrahigh pressure mercury lamp is considered. In this case, the j-rays with wavelengths of 289 nm, 297 nm, 303 nm, and 313 nm are Only an intensity of about 60% is obtained with respect to the wavelength at which the maximum intensity is obtained.

  In contrast, when a high-pressure mercury lamp or a low-pressure mercury lamp is used as the mercury lamp, the intensity of irradiation light with a short wavelength (for example, a wavelength of 320 nm or 250 nm) is higher than that of the ultra-high pressure mercury lamp. Is effective for etching a short semiconductor.

  The present invention has been made in view of the above-mentioned problems, and an object of the present invention is an optoelectric device having advantages such as an etching apparatus being inexpensive, a running cost being low, and being advantageous for mass production. An object of the present invention is to provide a photoelectrochemical etching apparatus for selectively etching a group III nitride semiconductor having a short band gap wavelength by chemical etching.

  In order to achieve the above-described object, a photoelectrochemical etching apparatus according to the present invention includes an etch stop layer of a first group III nitride semiconductor, and a first group III nitride semiconductor formed on the etch stop layer. 2 is a photoelectrochemical etching apparatus for selectively etching a layer to be etched of a second group III nitride semiconductor to be etched having a low bandgap energy and a group III nitride semiconductor substrate having a current extraction electrode. The photoelectrochemical etching apparatus includes a container filled with an etchant, a jig for holding a group III nitride semiconductor substrate provided in the container, a current extraction line connected to a current extraction electrode, and a container. Attached to the installed cathode wire, variable voltage source and ammeter connected in series between the cathode wire and current lead wire, high pressure mercury lamp or low pressure mercury lamp, high pressure mercury lamp or low pressure mercury lamp A group III nitride semiconductor that selectively transmits light having energy lower than the band gap energy of the first group III nitride semiconductor and higher than the band gap energy of the second group III nitride semiconductor. And a filter for irradiating the substrate.

  According to the photoelectrochemical etching apparatus of this invention, the etching apparatus itself is inexpensive, the running cost is low, and further, the group III nitride semiconductor is formed by photoelectrochemical etching having advantages such as mass production. Selective etching can be performed. In addition, when a high-pressure mercury lamp or a low-pressure mercury lamp is used as the light source, the band gap wavelength is long among the first group III nitride semiconductor and the second group III nitride semiconductor both having a band gap wavelength shorter than 350 nm. It is suitable for selectively etching only one of them.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the composition (material) and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiment.

(Group III nitride semiconductor substrate)
With reference to FIG. 1, a group III nitride semiconductor substrate for performing selective etching with the photoelectrochemical etching apparatus of the present invention will be described. FIG. 1 is a schematic cross-sectional view for explaining a group III nitride semiconductor substrate. Here, elements belonging to Group III are boron (B), aluminum (Al), gallium (Ga), and indium (In). The group III nitride semiconductor includes BN, AlN, GaN, and InN, and mixed crystals of them, such as AlGaN and InGaN. Formula III nitride semiconductor _{is, Al x In y Ga z B} 1-x-y-z N ( However, 0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 ≦ z ≦ 1, x + y + z ≦ 1) It is. In addition, when x, y, z, and 1-xyz are 0, it shows that each element of Al, In, Ga, and B is not contained, respectively.

  The group III nitride semiconductor substrate 110 includes an ohmic electrode 22, an etching mask 50, and a current extraction electrode 60 on the epitaxial substrate 10.

  The epitaxial substrate 10 is configured by laminating a buffer layer 12, a heterostructure layer 13, and a cap layer 18 in this order on a base substrate 11. The buffer layer 12, the heterostructure layer 13, and the cap layer 18 are formed by, for example, a metal organic chemical vapor deposition (MOCVD) method or an electron beam epitaxy (MBE) method.

  As the base substrate 11, for example, a silicon carbide (SiC) substrate, a sapphire substrate, or a silicon (Si) substrate is used.

  The buffer layer 12 is made of, for example, GaN or AlN. The buffer layer 12 is provided in order to produce a lattice relaxation effect between the base substrate 11 and the heterostructure layer 13.

  The heterostructure layer 13 includes a channel layer 14 formed on the buffer layer 12 and a carrier supply layer 16 formed on the channel layer 14. The channel layer 14 is formed of a group III nitride semiconductor, for example, GaN. The carrier supply layer 16 is made of a first group III nitride semiconductor, for example, AlGaN. The junction between the channel layer 14 and the carrier supply layer 16 is a heterojunction and has an energy band structure having a potential well. Electrons confined in this potential well have no freedom of movement in the direction perpendicular to the junction surface of the heterojunction (hereinafter referred to as the heterojunction surface), and are called two-dimensional electron gas. This two-dimensional electron gas has a high electron mobility, and the current caused by the two-dimensional electron gas flowing between the source and the drain is controlled by the voltage applied to the gate.

  The cap layer 18 is a second group III nitride semiconductor having a band gap energy E2 (<E1) lower than the band gap energy E1 of the first group III nitride semiconductor constituting the carrier supply layer 16, for example, GaN. Formed with.

  The second group III nitride semiconductor cap layer 18 becomes an etching target layer to be subjected to photoelectrochemical etching. Further, the first group III nitride semiconductor carrier supply layer 16 becomes an etch stop layer that is not etched.

  On the epitaxial substrate 10 described above, the ohmic electrode 22 is formed by evaporating titanium (Ti) or aluminum (Al). An etching mask 50 is formed on the epitaxial substrate 10 and the ohmic electrode 22. The etching mask 50 is formed, for example, by depositing silicon oxynitride or aluminum oxynitride by a CVD method, a plasma CVD method, or a sputtering method.

  The etching mask 50 has an opening in the selective etching region 30 corresponding to a portion of the epitaxial substrate 10 that is selectively etched. Similarly, an opening is provided in the contact region 32 where current is taken in and out of the epitaxial substrate 10. The current extraction electrode 60 in contact with the contact region 32 of the epitaxial substrate 10 is formed by metal deposition. The opening of the selective etching region 30 and the contact region 32 of the etching mask 50 is performed by, for example, any suitable well-known wet etching.

  Photoelectrochemical etching is performed on the group III nitride semiconductor substrate 110 in which the ohmic electrode 22, the etching mask 50, and the current extraction electrode 60 are formed on the epitaxial substrate 10 described above.

(Photoelectrochemical etching equipment)
A photoelectrochemical etching apparatus according to the present invention will be described with reference to FIG. FIG. 2 is a schematic configuration diagram schematically showing a photoelectrochemical etching apparatus. A photoelectrochemical etching container such as a beaker (hereinafter also referred to simply as a container) 200, an etchant 202, an alkali such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), or hydrochloric acid (HCl) or the like A solution in which the acid is dissolved in water, glycol or the like is filled.

  A jig 204 that holds a group III nitride semiconductor substrate (hereinafter also simply referred to as a substrate) 110 is provided in the container 200. The substrate 110 including the current extraction electrode 60 is fixed to the jig 204. The current lead line 212 is connected to the current lead electrode 60 provided on the substrate 110. A cathode line 208 is provided in the container 200, and a variable voltage source 214 and an ammeter 210 are connected in series between the cathode line 208 and the current lead line 212. As the cathode wire 208, a material having a high ionization tendency such as platinum, gold, silver, and carbon is used.

  The photoelectrochemical etching apparatus includes a mercury lamp 216 with a filter 218 attached as a light source. The mercury lamp 216 has a large irradiation area and can irradiate the substrate 110 with uniform light (indicated by an arrow I in the figure). The filter 218 irradiates the substrate with light E (lower than the band gap energy E1 of the first group III nitride semiconductor and higher than the band gap energy E2 of the second group III nitride semiconductor). Light having E2 <E <E1) is selectively transmitted. As will be described later, it is preferable to use a high pressure mercury lamp or a low pressure mercury lamp as the mercury lamp.

(Photoelectrochemical etching)
With reference to FIGS. 3A and 3B, selective etching in photoelectrochemical etching will be described. FIGS. 3A and 3B are schematic diagrams for explaining the mechanism of selective etching, and FIG. 3A shows a case where GaN is irradiated with ultraviolet light (UV light) having a wavelength of 340 nm. FIG. 3B illustrates a case where Al _0.25 Ga _0.75 N is irradiated with UV light having a wavelength of 340 nm.

The band gap energy of GaN is 3.42 eV, and the corresponding band gap wavelength is 363 nm. The band gap energy of Al _0.25 Ga _0.75 N is 4.158 eV, and the corresponding band gap wavelength is 298 nm.

  Here, when GaN is irradiated with UV light having a wavelength of 340 nm, the wavelength of the UV light is shorter than the band gap wavelength, that is, the energy of the UV light is higher than the band gap energy of GaN. And excited in the conduction band (FIG. 3A).

On the other hand, when UV light with a wavelength of 340 nm is irradiated onto Al _0.25 Ga _0.75 N, the wavelength of the UV light is longer than the bandgap wavelength, that is, the energy of the UV light is Al _0.25 Ga _0.75 N. Therefore, electrons in the valence band are not excited by the conduction band (FIG. 3B).

The cap layer 18 to be etched is formed of GaN as the second group III nitride semiconductor, and the carrier supply layer 16 to be the etch stop layer is Al _0.25 Ga ₀ as the first group III nitride semiconductor. _The substrate 110 formed of _.75 N is photoelectrochemically etched.

  The substrate 110 is fixed to the jig 204 of the photoelectrochemical etching apparatus, and the current drawing line 212 and the current drawing electrode 60 are connected. Thereafter, the mercury lamp 216, which is a UV light source, irradiates the substrate 110 with UV light, and applies a positive voltage to the substrate 110 from the variable voltage source 214. The energy E of the UV light is set higher than the band gap energy E2 of the second group III nitride semiconductor and lower than the band gap energy E1 of the first group III nitride semiconductor. For example, the wavelength of the UV light is set to 340 nm, which is a wavelength corresponding to energy E that satisfies E2 <E <E1.

  Since the energy E of the UV light satisfies E2 <E, the valence band electrons of the GaN constituting the cap layer 18 are excited to the conduction band by the irradiation of the UV light. The excited electrons are carried to the cathode line 208 via the current extraction line 212, the variable voltage source 214, and the ammeter 210. Since the cathode wire 208 uses a material having a strong ionization tendency, it is ionized by receiving electrons and dissolves in the solution. In addition, GaN that has lost the electrons of the layer to be etched is decomposed by the following chemical formula.

2GaN + 6h ⁺ → 2Ga ³⁺ + N ₂
Here, h ⁺ means a hole due to loss of electrons. By the reaction described above, GaN in the etching target layer is etched.

On the other hand, since the energy E of the UV light satisfies E <E1, the electrons in the valence band of Al _0.25 Ga _0.75 N constituting the carrier supply layer 16 are in the conduction band even when irradiated with the UV light. Is not excited. Therefore, Al _0.25 Ga _0.75 N of the carrier supply layer 16 is not etched.

Thus, higher than GaN band gap energy, _and, by irradiating UV light of Al _{0.25 Ga 0.75} N lower energy than the band gap energy of, GaN and _Al 0.25 _{Ga 0.75} N Of these, only GaN can be selectively etched.

  Note that the UV light having a wavelength of 340 nm described here is obtained by transmitting only light having a wavelength in the vicinity of 340 nm through the filter 218, which is output from the mercury lamp 216 serving as a light source. As the filter 218, any suitable known ultraviolet filter can be used. Here, as the filter 218, a band-pass filter that transmits only light in the vicinity of a specific wavelength range may be used, or a filter that blocks light having a wavelength shorter than the specific wavelength range may be used.

The transmission wavelength of the filter 218 can be determined according to the composition of the group III nitride semiconductor of the etched layer and the etch stop layer. Table 1 shows bandgap energy (eV), bandgap wavelength (nm), and suitable filter wavelength (nm) with respect to the composition of AlGaN (see Jpn. J. Appl. Phys., 41 (2002) 73). ). The filter wavelength in Table 1 indicates the filter wavelength when AlGaN or AlN is used as the etch stop layer when the layer to be etched is GaN. For example, when the layer to be etched is Al _0.125 Ga _0.875 N and the etch stop layer is Al _0.25 Ga _0.75 N, the filter wavelength is set to a wavelength between 298 nm and 329 nm. That's fine.

  Similarly, Table 2 shows bandgap energy (eV), bandgap wavelength (nm), and suitable filter wavelength (nm) for InGaN composition (Jpn. J. Appl. Phys., 40 (2001)). 3157). The filter wavelength in Table 2 indicates the filter wavelength when GaN is used as the etch stop layer when the layer to be etched is InGaN.

  Mercury lamps include ultra high pressure mercury lamps, high pressure mercury lamps and low pressure mercury lamps. The spectral distribution of each mercury lamp will be described with reference to FIG. 4A, 4B, and 4C are characteristic diagrams showing spectral distributions of output light from ultra high pressure mercury lamps, high pressure mercury lamps, and low pressure mercury lamps manufactured by USHIO INC., Respectively. 4A, 4B, and 4C show the wavelength on the horizontal axis. 4A and 4B show the relative intensity (%) when the intensity of the wavelength of the maximum intensity is taken as 100% on the vertical axis, and FIG. 4C shows the maximum intensity on the vertical axis. The relative intensity when the intensity at the wavelength of 1 is taken as 1 is shown.

  As shown in Table 1, the band gap wavelengths of AlGaN and AlN are shorter than 330 nm. Here, when an ultra-high pressure mercury lamp is used, the j-rays with wavelengths of 289 nm, 297 nm, 303 nm, and 313 nm can only obtain an intensity of about 60% with respect to the wavelength at which the maximum intensity can be obtained (FIG. 4A). On the other hand, when a high-pressure mercury lamp is used, the light intensity at a wavelength near 300 nm reaches about 80% of the wavelength at which the maximum intensity can be obtained (FIG. 4B). When a low-pressure mercury lamp is used, the light intensity around the wavelength of 250 nm is the maximum intensity (FIG. 4C).

  Thus, the use of a high-pressure mercury lamp or a low-pressure mercury lamp is particularly suitable for selective etching of a group III nitride semiconductor having a short band gap wavelength.

  According to the photoelectrochemical etching apparatus of the present invention, the group III nitride semiconductor is selected by photoelectrochemical etching which has the advantages that the apparatus itself is inexpensive, has low running cost, and is advantageous for mass production. Can be etched.

(Example)
With reference to FIGS. 5 to 7, an example in which a group III nitride semiconductor substrate is selectively etched using photoelectrochemical etching will be described.

As a group III nitride semiconductor substrate, a cap layer includes a GaN layer having a thickness of 6 nm, a carrier supply layer includes an Al _0.25 Ga _0.75 N layer having a thickness of 25 nm, and a channel layer includes a GaN layer having a thickness of 2 μm. A substrate is used. In the following description, Al _0.25 Ga _0.75 N may be expressed as AlGaN.

  As an etchant, an aqueous potassium hydroxide solution having a concentration of 0.015 mol / l is used. The voltage of the variable voltage source is set to 2 V, and etching is performed for 60 seconds using a 1500 W high-pressure mercury lamp.

  FIGS. 5A and 5B are diagrams showing surface images obtained by a scanning electron microscope (SEM). FIG. 5A shows the result when no filter is used, while FIG. 5B shows the result when a filter is used. In FIGS. 5A and 5B, a portion indicated by reference numeral I is an etched portion, and its periphery is an etching mask. In FIG. 5A, the etched portion has a rough surface state, a portion where AlGaN is exposed (portion indicated by symbol II in the drawing) and a portion where GaN is exposed (reference symbol III in the drawing). The part indicated by) coexists. On the other hand, in FIG. 5B, the surface of the etched portion is flat.

  6 (A) and 6 (B) are diagrams showing the result of measuring the level difference measured using a palpation level difference meter. FIG. 6A shows the result when no filter is used, while FIG. 6B shows the result when a filter is used. 6A and 6B, the horizontal axis indicates the operation length (μm) which is the horizontal length of FIGS. 5A and 5B, and the vertical axis indicates the etching depth. (Nm) is shown.

  In FIG. 6A, a one-dot broken line (indicated by symbol I) represents an initial state, that is, a step meter measurement result before performing the etching process. In FIG. 6A, a dotted line (indicated by reference numeral II) represents a step meter measurement result when etching is performed for 20 seconds. In FIG. 6A, a solid line (indicated by reference numeral III) represents a measurement result of the level difference when etching is performed for 40 seconds. When the dashed line I and dotted line II in FIG. 6A are compared with each other, the difference is 6 nm or more. Therefore, by etching for 20 seconds, the 6 nm thick cap layer that is the etching target layer is penetrated, and the etching stop layer is It can be seen that a certain carrier supply layer is etched. Further, when the dotted line II and the solid line III in FIG. 6A are compared, it can be seen that the etching stop layer is etched deeper by performing etching for 20 seconds, that is, etching for 40 seconds as a whole. . Since FIG. 5A is a surface image obtained as a result of etching for 60 seconds, the GaN shown in FIG. 5A is the channel layer exposed by the penetration of the etch stop layer. 14 GaN.

  In FIG. 6B, a one-dot broken line (indicated by reference numeral IV) represents an initial state, that is, a step meter measurement result before performing the etching process. In FIG. 6B, a dotted line (indicated by a symbol V) represents a step meter measurement result when etching is performed for 30 seconds. In FIG. 6 (B), a solid line (indicated by reference numeral VI) represents a step gauge measurement result when etching is performed for 60 seconds. It can be seen that the GaN layer having a thickness of 6 nm is almost etched by the etching for 30 seconds. Further, since the etching depth does not change by etching for 30 seconds, that is, etching for 60 seconds as a whole, it can be seen that the AlGaN layer functions as an etch stop layer.

  FIG. 7A is a diagram illustrating a surface image obtained by an atomic force microscope (AFM). FIG. 7A shows the surface of the substrate etched for 60 seconds using a filter. The silicon nitride etching mask is removed with hydrofluoric acid. FIG. 7B is a diagram showing the result of cross-sectional measurement using a palpation step meter, as in FIGS. 6A and 6B. In FIG. 7B, the horizontal axis indicates the horizontal length (μm) of FIG. 7A, and the vertical axis indicates the etching depth (nm). As a result of the cross-sectional measurement, it is shown that the etching stops at a depth of 6 nm from the surface of the GaN layer.

  In FIG. 7B, the surface corresponding to AlGaN having an etching depth of about −4 nm is rougher than the portion corresponding to GaN having an etching depth of about 2 nm. The roughness of the surface is very similar to the surface of the AlGaN carrier supply layer when there is no GaN cap layer. Therefore, it can be seen from this point that the GaN cap layer is etched and the AlGaN carrier supply layer is exposed.

It is a schematic sectional view for explaining a group III nitride semiconductor substrate. It is the schematic block diagram which showed the photoelectrochemical etching apparatus typically. It is a schematic diagram for demonstrating the mechanism of selective etching. It is a characteristic view which shows the spectral distribution of a mercury lamp. It is a figure which shows the surface image by a scanning electron microscope. It is a figure which shows a level difference measurement result. It is a figure which shows the result of the surface image and cross-section measurement by an atomic force microscope.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Epitaxial substrate 11 Base substrate 12 Buffer layer 13 Heterostructure layer 14 Channel layer 16 Carrier supply layer 18 Cap layer 22 Ohmic electrode 30 Selective etching region 32 Contact region 50 Etching mask 60 Current extraction electrode 110 Group III nitride semiconductor substrate 200 Etching vessel 202 Etchant 204 Jig 208 Cathode Line 210 Ammeter 212 Current Lead Line 214 Variable Voltage Source 216 Mercury Lamp 218 Filter

Claims (1)

Etch stop layer of first group III nitride semiconductor, and second group III nitride semiconductor to be etched having a band gap energy lower than that of the first group III nitride semiconductor formed on the etch stop layer A photoelectrochemical etching apparatus for selectively etching the layer to be etched of a group III-nitride semiconductor substrate comprising a layer and a current extraction electrode,
A container filled with an etchant;
A jig for holding the group III nitride semiconductor substrate provided in the container;
A current extraction line connected to the current extraction electrode;
A cathode wire provided in the container;
A variable voltage source and an ammeter connected in series between the cathode line and the current lead line;
High pressure mercury lamp or low pressure mercury lamp,
It is attached to the high-pressure mercury lamp or the low-pressure mercury lamp, and has energy lower than the band gap energy of the first group III nitride semiconductor and higher than the band gap energy of the second group III nitride semiconductor. A photoelectrochemical etching apparatus comprising: a filter configured to selectively transmit light having the light to irradiate the group III nitride semiconductor substrate.