Semiconductor laser

Abstract

To provide an edge reflective film and a manufacturing method thereof that improve the performance of UV-B semiconductor lasers.SOLUTION: A reflective film used in a UV-B semiconductor laser is composed of a dielectric multilayer film made of tantalum oxide as a high refractive index material and a silicon oxide as a low refractive index material. The tantalum oxide film is formed to have an extinction coefficient of less than 0.0001 for light with a wavelength of 300 nm. In addition, the manufacturing method for the reflective film uses the ECR sputtering method, which reduces damage to the semiconductor laser end face.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to semiconductor lasers and manufacturing technologies thereof, and more particularly, to technologies effective when applied to semiconductor lasers that emit light in a wavelength range included in the ultraviolet region and manufacturing technologies thereof.

Background Art

[0002] Non-Patent Document 1 describes a technology related to a semiconductor laser that emits light in the UV-B wavelength range of the ultraviolet region.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] AlGaN-based ultraviolet semiconductor lasers have a wide range of applications in fields such as medicine and processing, and are expected to be put into practical use. Ultraviolet light is classified into three types: long-wavelength ultraviolet light (UV-A: wavelength greater than 315 nm and less than or equal to 380 nm), medium-wavelength ultraviolet light (UV-B: wavelength greater than 280 nm and less than or equal to 315 nm), and short-wavelength ultraviolet light (UV-C: wavelength greater than 200 nm and less than or equal to 280 nm), and room-temperature oscillation of semiconductor lasers has been reported in each wavelength range. Among these, the semiconductor laser that has been the most difficult to realize is the semiconductor laser that emits light in the UV-B wavelength range (hereinafter sometimes referred to as a UV-B semiconductor laser), and the inventors of this invention are diligently studying how to improve the performance of UV-B semiconductor lasers. In this regard, the development of high-performance reflective films is generally indispensable for improving the performance of semiconductor lasers, and from this perspective, the development of high-performance reflective films is also desired for UV-B semiconductor lasers. [Means for solving the problem]

[0005] In one embodiment, the semiconductor laser emits light in the wavelength range included in the ultraviolet region and is equipped with a reflective film on the end face opposite to the light emission surface. Here, the reflective film includes a tantalum oxide film, and the tantalum oxide film has an extinction coefficient of less than 0.0001 for light with a wavelength of 300 nm.

[0006] In one embodiment, the semiconductor laser emits light in the wavelength range included in the ultraviolet region and is equipped with an anti-reflective coating on the light emission surface. Here, the anti-reflective coating includes a tantalum oxide film, and the tantalum oxide film has an extinction coefficient of less than 0.0001 for light with a wavelength of 300 nm.

[0007] A semiconductor laser manufacturing method in one embodiment is a method for manufacturing a semiconductor laser that emits light in the wavelength range included in the ultraviolet region, and comprises a reflective film formation step in which a reflective film is formed on the surface opposite to the light emission surface. In the reflective film formation step, the reflective film is formed by using the ECR sputtering method.

[0008] A semiconductor laser manufacturing method in one embodiment is a method for manufacturing a semiconductor laser that emits light in the wavelength range included in the ultraviolet region, and comprises an anti-reflective film formation step in which an anti-reflective film is formed on the light emission surface. In the anti-reflective film formation step, the anti-reflective film is formed by using the ECR sputtering method. [Effects of the Invention]

[0009] According to one embodiment, the performance of a semiconductor laser that emits light in the wavelength range included in the ultraviolet region can be improved. [Brief explanation of the drawing]

[0010] [Figure 1] This diagram shows the device structure of a UV-B semiconductor laser. [Figure 2] This is a flowchart showing the manufacturing process of UV-B semiconductor lasers. [Figure 3] This diagram schematically illustrates the process of acquiring a laser bar from a semiconductor wafer and then acquiring a semiconductor chip from the acquired laser bar. [Figure 4] This is a schematic diagram showing an example of the configuration of the reflective film in the embodiment. [Figure 5] This figure shows a schematic configuration of an ECR sputtering apparatus. [Figure 6] This is a flowchart explaining the flow of the film deposition process. [Figure 7] This graph shows the wavelength dependence of the extinction coefficient of hafnium oxide formed by RF sputtering. [Figure 8] This graph shows the wavelength dependence of the extinction coefficient in various materials formed by the ECR sputtering method. [Figure 9] This graph shows the wavelength dependence of the extinction coefficient and refractive index for tantalum oxide films formed using RF sputtering and ECR sputtering, respectively. [Figure 10] This graph shows a magnified view of a portion of Figure 9. [Figure 11]Table showing the values of the attenuation coefficient and refractive index of a tantalum oxide film formed using the RF sputtering method and a tantalum oxide film formed using the ECR sputtering method at a wavelength of 300 nm. [Figure 12] Graph showing the wavelength dependence of the refractive index of various substances.

Best Mode for Carrying Out the Invention

[0011] In all the drawings for explaining the embodiments, the same members are generally denoted by the same reference numerals, and repeated explanations thereof are omitted. Note that, for clarity of the drawings, hatching may be added even to a plan view.

[0012] <Development History of UV-B Semiconductor Laser> Laser light is a light source that is coherent and has excellent directivity and convergence. Among them, semiconductor lasers have excellent performance represented by small size, high efficiency, long life, high productivity, etc. compared to other laser light sources. Semiconductor lasers were first realized in the infrared region with a long wavelength of light, and then in the visible light region (red, green, blue). These semiconductor lasers are used in many fields such as optical disks, laser printers, optical communications, laser processing machines, and measuring instruments. On the other hand, ultraviolet light is light in a wavelength range shorter than visible light, where the wavelength of light is greater than 200 nm and less than or equal to 380 nm. Since there are many application fields represented by medical treatment, environment, bioscience, sterilization, laser processing and microfabrication of metals, etc. for the light source in this ultraviolet wavelength range, the realization of a semiconductor laser in the ultraviolet wavelength range is desired.

[0013] Regarding this point, ultraviolet rays are classified into three types: long-wavelength ultraviolet rays (UV-A), medium-wavelength ultraviolet rays (UV-B), and short-wavelength ultraviolet rays (UV-C) from the longer wavelength side, and room-temperature oscillation of semiconductor lasers has been reported in each wavelength range. Here, the most difficult-to-realize semiconductor laser is the UV-B semiconductor laser. Since medium-wavelength ultraviolet rays have various application fields such as ultraviolet curing and ultraviolet adhesion / drying (UV curing), medical fields such as atopic treatment, and DNA sequencing, the realization of a UV-B semiconductor laser has great technical significance.

[0014] The realization of a UV-B semiconductor laser has been difficult due to reasons such as (1) the difficulty of fabricating high-quality AlGaN crystals that laser oscillate in the medium-wavelength ultraviolet wavelength range and (2) the difficulty of current injection required for laser oscillation. Regarding this point, a UV-B semiconductor laser that was difficult to realize has been realized by (1) a method of fabricating an AlN template by sputtering on a sapphire substrate and realizing high-quality AlGaN crystals on this AlN template and (2) applying the "polarization doping method" as a current injection method.

[0015] Here, the "polarization doping method" is a method of generating charge carriers (holes) that contribute to electrical conduction by utilizing the spontaneous polarization that occurs in nitride semiconductors with low crystal symmetry. Specifically, it is a technique of generating polarization-fixed charges (negative charges) by a compositionally graded AlGaN layer and inducing holes due to the presence of this polarization-fixed charge.

[0016] Hereinafter, a configuration example of a UV-B semiconductor laser will be described.

[0017] <Configuration of UV-B Semiconductor Laser> Figure 1 is a diagram showing the device structure of a UV-B semiconductor laser LD.

[0018] In FIG. 1, the UV-B semiconductor laser LD has a sapphire substrate 100, an aluminum nitride layer (AlN layer) 101 formed on the sapphire substrate 100, and an aluminum nitride layer (AlN layer) 102 formed on the aluminum nitride layer 101.

[0019] At this time, the aluminum nitride layer 101 is formed by using a sputtering method. On the other hand, the aluminum nitride layer 102 is formed by using a MOVPE method. The sapphire substrate 100 and the aluminum nitride layer 101 will form an AlN template substrate, and the aluminum nitride layer 102 is formed on this AlN template substrate. Here, the thickness of the aluminum nitride layer 102 is, for example, 1550 nm.

[0020] Next, as shown in FIG. 1, the UV-B semiconductor laser LD has a nanopattern 103 formed on the aluminum nitride layer 102. This nanopattern 103 has, for example, a pitch of 1000 nm, a diameter of 300 nm, and a depth of 1000 nm.

[0021] And the UV-B semiconductor laser LD has an Al 0.68 Ga 0.32 N layer 104 formed so as to embed the nanopattern 103. At this time, the thickness of the Al 0.68 Ga 0.32 N layer 104 is about 5 μm, and the surface of the Al 0.68 Ga 0.32 N layer 104 is flattened. Thus, in this embodiment, the Al 0.68 Ga 0.32 N layer 104 is formed so as to embed the pre-formed nanopattern 103. As a result, a high-quality Al 0.68 Ga 0.32 N layer 104 with few crystal defects can be formed. Note that the Al 0.68 Ga 0.32 N layer 104 is a non-doped layer into which conductive type impurities are not introduced.

[0022] Next, the UV-B semiconductor laser LD is Al 0.68 Ga 0.32 Al formed on N layer 104 0.62 Ga 0.38 It has an N layer 105. 0.62 Ga 0.38 The N layer 105 is an n-type semiconductor layer into which silicon (Si), an n-type impurity, is introduced, and the silicon impurity concentration is, for example, 6 × 10⁻⁶. 18 / cm 3 This Al 0.62 Ga 0.38 The thickness of the N-layer 105 is, for example, about 4 μm. 0.62 Ga 0.38 The N layer 105 functions as a cladding layer for confining laser light, as well as a contact layer with the n electrode 116.

[0023] And UV-B semiconductor lasers (LDs) are made of Al 0.62 Ga 0.38 Al formed on N layer 105 0.45 Ga 0.55 N layer 106 and Al 0.45 Ga 0.55 An active layer 107 formed on the N layer 106, and Al formed on the active layer 107 0.5 Ga 0.5 It has an N layer 108.

[0024] Here, Al 0.45 Ga 0.55 The N layer 106 functions as a lower guide layer that acts as an optical waveguide for the laser light, and its thickness is, for example, 50 nm. The active layer 107 is an emitting layer that emits laser light, and for example, is made of Al with a thickness of 4 nm. 0.3 Ga 0.7 A well layer consisting of N and an Al layer with a thickness of 8 nm. 0.45 Ga 0.55It has a barrier layer made of N. In particular, in this embodiment, the active layer 107 is composed of a combination of well layers and barrier layers for two periods. The well layers are layers with a smaller band gap than the barrier layer and are formed by reducing the Al composition ratio in AlGaN. On the other hand, the barrier layer is a layer with a larger band gap than the well layers and is formed by increasing the Al composition ratio in AlGaN.

[0025] Furthermore, Al formed on the active layer 107 0.5 Ga 0.5 The N layer 108 functions as an upper guide layer that constitutes the optical waveguide for the laser light, and its thickness is, for example, 50 nm.

[0026] Next, as shown in Figure 1, the UV-B semiconductor laser LD is Al 0.5 Ga 0.5 The device has an electron blocking layer (EBL layer) 109 formed on an N layer 108, and a two-stage composition gradient layer formed on the electron blocking layer 109. Here, the two-stage composition gradient layer is a layer for realizing "polarization doping" and is composed of a p-type AlGaN layer 110 and a p-type AlGaN layer 111. In this embodiment, "polarization doping" by the two-stage composition gradient layer is employed, thereby realizing the injection of hole current necessary for laser oscillation.

[0027] Next, as shown in Figure 1, the UV-B semiconductor laser LD has a p-type GaN layer 112 formed on a p-type AlGaN layer 111. This p-type GaN layer 112 functions as a contact layer with the p-electrode 114.

[0028] Furthermore, as shown in Figure 1, the UV-B semiconductor laser LD is Al 0.62 Ga 0.38It has an n - electrode 116 that contacts the N - layer 105 and a p - electrode 114 that contacts the p - type GaN layer 112. Also, a surface protective film 113 made of, for example, a silicon oxide film is formed on the surface region excluding the n - electrode formation part and the p - electrode formation part. Further, a pad electrode 117 is formed on the n - electrode 116. On the other hand, a pad electrode 115 is formed on the p - electrode 114. At this time, the n - electrode 116 is composed of a laminated film of V / Al / Ti / Au (20nm / 80nm / 40nm / 100nm). On the other hand, the p - electrode 114 is composed of a laminated film of Ni / Pt / Au (10nm / 10nm / 40nm). Also, the pad electrode 115 and the pad electrode 117 are composed of a laminated film of Ti / Au (50nm / 800nm).

[0029] Subsequently, in FIG. 1, the UV - B semiconductor laser LD has a resonator structure and has an emission surface 200A (one end surface) from which laser light is emitted and a reflection surface 200B (the region with dots: the other end surface) located on the side opposite to the emission surface 200A. An antireflection film is formed on the emission surface 200A, while a reflection film is formed on the reflection surface 200B.

[0030] In the above manner, the UV - B semiconductor laser LD is configured.

[0031] <Operation of the UV - B Semiconductor Laser> The UV - B semiconductor laser LD in the present embodiment is configured as described above, and its operation will be described below with reference to FIG. 1. First, a positive voltage is applied to the p - electrode 114 (pad electrode 115), and a negative voltage is applied to the n - electrode 116 (pad electrode 117). As a result, a forward current flows through the UV - B semiconductor laser LD from the p - electrode 114 toward the n - electrode 116. Thereby, holes are injected from the p - electrode 114 into the p - type GaN layer 112, and the injected holes flow into the two - step composition gradient layer (p - type AlGaN layer 110 and p - type AlGaN layer 111) from the p - type GaN layer 112. Then, the holes that flow into the two - step composition gradient layer are Al 0.5 Ga 0.5It passes through the inside of the N layer 108 and flows into the active layer 107.

[0032] On the other hand, from the n electrode 116, Al 0.62 Ga 0.38 Electrons are injected into the N layer 105, and the injected electrons pass through Al 0.45 Ga 0.55 The N layer 106 and are injected into the active layer 107.

[0033] In the active layer 107, an inversion distribution is formed by the injected holes and electrons, and when electrons transition from the conduction band to the valence band by stimulated emission, phase-aligned light is generated. Then, the light generated in the active layer 107 is mainly confined within the active layer 107 by the optical confinement effect of the two-stage gradient layer (upper cladding layer) with a refractive index lower than that of the active layer 107 and the Al 0.62 Ga 0.38 The N layer 105 (lower cladding layer). Then, the light confined within the active layer 107 is amplified by further stimulated emission by reciprocating in a resonator composed of an emission surface 200A and a reflection surface 200B formed parallel to the y-axis direction of the UV-B semiconductor laser LD. After that, laser light oscillates in the active layer 107, and the laser light is emitted from the emission surface 200A of the UV-B semiconductor laser LD. As described above, the UV-B semiconductor laser LD in the present embodiment operates.

[0034] <Manufacturing Method of UV-B Semiconductor Laser> Next, the manufacturing method of the UV-B semiconductor laser LD will be described.

[0035] FIG. 2 is a flowchart showing the manufacturing process of the UV-B semiconductor laser LD. <​​​First, an aluminum nitride layer 101 is formed on a sapphire substrate 100. The aluminum nitride layer 101 is formed, for example, by sputtering, and its thickness is 450 nm. Then, an AlN template substrate is fabricated by annealing at 1700°C for 3 hours in a nitrogen atmosphere. Next, an aluminum nitride layer 102 is formed on the aluminum nitride layer 101 by the MOVPE method. The thickness of this aluminum nitride layer 102 is 1550 nm.

[0037] Next, a nanopattern 103 is formed on the aluminum nitride layer 102. Specifically, a silicon oxide film is formed on the aluminum nitride layer 102 using a sputtering method. The thickness of this silicon oxide film is 420 nm. After that, a nanopattern with a pitch of 1000 nm and a diameter of 450 nm is formed on the resist film using a nanoimprint apparatus after coating with a resist film. Then, the exposed silicon oxide film is dry-etched, and subsequently, the silicon oxide residue is removed with buffered hydrofluoric acid. After etching the silicon nitride layer 102 to 1000 nm with chlorine gas, the silicon oxide film and resist film, which act as masks, are removed with buffered hydrofluoric acid. This allows for the formation of a nanopattern 103 on the aluminum nitride layer 102 with a pitch of 1000 nm, a diameter of 300 nm, and a depth of 1000 nm.

[0038] Next, by using the MOVPE method, the nanopattern 103 is made of Al 0.68 Ga 0.32 The N-layer 104 is embedded and planarized. At this stage, the thickness until complete planarization is achieved is approximately 5 μm. Subsequently, silane, the raw material for the donor Si, is supplied, and the silicon concentration is increased to 6 × 10⁻⁶. 18 / cm 3 Al 0.62 Ga 0.38 This Al forms the N layer 105. 0.62 Ga 0.38 The thickness of the N-layer 105 is 4 μm.

[0039] Next, Al as the lower guide layer 0.45 Ga0.55 After forming the N layer 106, the active layer 107 is formed. Specifically, the active layer 107 consists of a well layer made of Al 0.3 Ga 0.7 N with a thickness of 4 nm and a barrier layer made of Al 0.45 Ga 0.55 N with a thickness of 8 nm, and the active layer 107 is formed so as to be composed of a combination of two cycles of the well layer and the barrier layer. Then, on the active layer 107, an Al 0.5 Ga 0.5 N layer 108 is formed as the upper guide layer.

[0040] Thereafter, on the Al 0.5 Ga 0.5 N layer 108, an electron blocking layer 109 made of an Al 0.9 Ga 0.1 N layer is formed. Finally, after forming a two-step composition gradient layer (p-type AlGaN layer 110 and p-type AlGaN layer 111) on the electron blocking layer 109, a p-type GaN layer 112 is formed on this two-step composition gradient layer.

[0041] <<Activation Annealing (S102)>> For the wafer grown by crystal growth as described above, activation annealing is performed by performing heat treatment at 550 °C for 10 minutes in an air atmosphere.

[0042] <<n-Electrode Formation (S103)>> Thereafter, first, patterning is performed with a resist film, nickel (Ni) is deposited at 100 nm and lifted off to form a mask for dry etching. Then, using this mask for dry etching, the wafer is dry-etched with chlorine gas until it reaches the Al 0.62 Ga 0.38 N layer 105. Subsequently, the dry-etching mask made of nickel is removed with chemicals. Next, after forming an n-electrode pattern with a resist film, V / Al / Ti / Au (20 nm / 80 nm / 40 nm / 100 nm) is sequentially deposited and lifted off, and then heat treatment is performed at 900 °C for 3 minutes in a nitrogen atmosphere to form the n-electrode 116.

[0043] <<Formation of Surface Protection Film (S104)>> Next, a surface protection film 113 is formed on the regions excluding the p - electrode formation region and the n - electrode formation region. The surface protection film 113 is formed of, for example, a silicon oxide film and can be formed by using a sputtering method.

[0044] <<Formation of p - electrode (S105)>> After that, after forming a p - electrode pattern with a resist film, Ni / Pt / Au (10 nm / 10 nm / 40 nm) is sequentially vapor - deposited and then lifted off, and then heat - treated at 700 °C for 1 minute in an oxygen atmosphere to form the p - electrode 114.

[0045] <<Formation of Pad Electrodes (S106)>> Then, after forming a pad - electrode pattern with a resist film, Ti / Au (50 nm / 800 nm) is sequentially vapor - deposited and lifted off to form the pad electrode 115 and the pad electrode 117.

[0046] <<Formation of End Faces (S107)>> Subsequently, by using dry etching and wet etching, laser end faces (one end face: emission face 200A and the other end face: reflection face 200B) are formed. Specifically, after patterning with a resist film, nickel (Ni) is vapor - deposited by 100 nm and lifted off to form a mask for end - face formation. Then, using the mask for end - face formation, dry etching is performed with chlorine gas until reaching the Al 0.62 Ga 0.38 N layer 105. Next, wet etching is performed for 5 minutes at 85 °C using a 25% aqueous solution of TMAH (Tetramethylammonium hydroxide) to form the laser end faces.

[0047] <<Formation of Laser Bars (S108)>> After that, by performing laser scribing and breaking, the wafer is cut into a plurality of laser bars.

[0048] <<Formation of reflective and anti-reflective coatings (S109)>> Then, an anti-reflective coating is formed on one end face of the acquired laser bar, for example, by using a sputtering method, while a reflective coating is formed on the other end face of the acquired laser bar, for example, by using a sputtering method. Subsequently, a UV-B semiconductor laser can be manufactured by cutting the laser bar into multiple semiconductor chips.

[0049] Furthermore, the manufacturing process after the laser bar formation process will be explained.

[0050] As shown in Figure 3, multiple laser bars LB, each in a strip shape, are formed on a semiconductor wafer WF on which multiple semiconductor laser device structures are formed by laser scribing and breaking. Each of the multiple laser bars LB has a roughly rectangular parallelepiped shape and has multiple chip regions CR arranged in a row. A semiconductor laser device structure is formed on each of these chip regions CR. Since the laser bars LB configured in this way have opposing end faces, first a reflective film is formed on one end face of the laser bar LB, and then an anti-reflective film is formed on the other end face of the laser bar LB. Subsequently, the multiple chip regions CR formed on the laser bar LB are separated to obtain multiple semiconductor chips CHP. On each of the multiple semiconductor chips CHP obtained in this way, a reflective film HR is formed on one end face of a pair of opposing end faces, while an anti-reflective film AR is formed on the other end face. In this way, semiconductor chips CHP on which semiconductor lasers are formed can be manufactured.

[0051] <<Example of reflective film configuration>> The following describes an example of the configuration of the reflective film formed on the end face.

[0052] Figure 4 is a schematic diagram showing an example of the configuration of the reflective film 300 in this embodiment.

[0053] In Figure 4, the reflective film 300 is composed of a dielectric multilayer film. Specifically, the reflective film 300 has a pair of films consisting of a low refractive index film 310 having a first refractive index and a high refractive index film 320 having a refractive index higher than the first refractive index. For example, the reflective film 300 is composed of eight stacked pair films. The reflective film 300 thus constructed can be formed, for example, by the following process. That is, the process of forming the reflective film 300 on the end face opposite to the light emission surface includes (1) the step of forming a low refractive index film 310 having a first refractive index on the end face, and (2) the step of forming a high refractive index film having a second refractive index higher than the first refractive index on the low refractive index film 310. The above-described steps (1) and (2) are carried out alternately and repeatedly. This makes it possible to form a reflective film 300 (dielectric multilayer film) having multiple pair films consisting of a low refractive index film 310 and a high refractive index film 320.

[0054] <The importance of film deposition technology at the end face> For example, the pair of end faces formed on a semiconductor laser function as resonators. That is, in order to easily achieve laser oscillation, it is necessary to minimize the loss of laser light from one end face (reflective surface) on which a reflective film is formed. Therefore, this reflective film is required to be a high-quality film with high reflectivity to laser light. On the other hand, from the viewpoint of efficiently emitting laser light from the other end face, the anti-reflective film formed on the end face (emission surface) on which the laser light is emitted is also required to be a high-quality film that can reduce reflection loss. Specifically, reflective and anti-reflective films are desired to have properties that can reduce light absorption. In other words, reflective and anti-reflective films are desired to be films with extremely small extinction coefficients.

[0055] Furthermore, since the end face (reflective surface) on which the reflective film is formed and the end face (emitting surface) on which the antireflection film is formed function as resonators, it is desirable to adopt a film-forming technique that can reduce the damage to the end face as a film-forming technique for the reflective film and the antireflection film on the end face. That is, for the high performance of the semiconductor laser, it is important that the characteristics of the end face constituting the resonator are good, and from the viewpoint of improving the characteristics of the end face, it is necessary to avoid a film-forming technique that damages the end face. Therefore, in this embodiment, we are considering adopting a film-forming technique that can reduce the damage to the end face. Hereinafter, the technical concept in this embodiment that can reduce the damage to the end face will be described.

[0056] <Basic idea in the embodiment> The basic idea in this embodiment is the idea of using the ECR sputtering method as a method of forming a reflective film and an antireflection film used in a UV-B semiconductor laser on the end face. That is, the basic idea is not to use the RF sputtering method but to use the ECR sputtering method as a method of forming a reflective film or an antireflection film on the resonance surface. According to this basic idea, the damage to the end face of the UV-B semiconductor laser can be reduced.

[0057] Hereinafter, first, the ECR sputtering apparatus will be described, and then, according to the ECR sputtering apparatus, the mechanism that can reduce the damage to the end face will be described.

[0058] <Configuration of an ECR (Electron Cyclotron Resonance) sputtering apparatus> FIG. 5 is a diagram showing a schematic configuration of an ECR sputtering apparatus 1.

[0059] In FIG. 5, an ECR sputtering apparatus 1 has a chamber 10 which is a film formation chamber. In this chamber 10, a holding part 11 is arranged, and by this holding part 11, a film formation object SUB typified by, for example, a substrate is held. The holding part 11 is connected to a mechanism part 12 arranged close to the chamber 10 and is configured to be operable by the mechanism part 12. In this chamber 10, a gas inlet 10a and a gas exhaust port 10b are provided.

[0060] Next, in the chamber 10, a plasma generation part 13 is provided at a position facing the film formation object SUB held by the holding part 11. This plasma generation part 13 is configured to generate plasma, and around the plasma generation part 13, for example, a magnetic field generation part 14 composed of a coil is arranged. Further, a waveguide 15 is connected to the plasma generation part 13, and microwaves propagating through the waveguide 15 are introduced into the plasma generation part 13. Furthermore, between the holding part 11 and the plasma generation part 13 and at a position close to the plasma generation part 13, for example, a target TA having a cylindrical shape is arranged, and this target TA is electrically connected to a high-frequency power supply 16. Thereby, the target TA is configured such that a high-frequency voltage from the high-frequency power supply 16 is applied. This target TA is fixed by a fixing part 17.

[0061] As described above, the ECR sputtering apparatus 1 is configured.

[0062] <Film formation operation in the ECR sputtering apparatus> Subsequently, the film formation operation in the ECR sputtering apparatus 1 will be described.

[0063] FIG. 6 is a flowchart for explaining the flow of the film formation operation.

[0064] First, in Figure 5, argon gas is introduced into the plasma generation unit 13. When a magnetic field is generated from the magnetic field generator 14 located around the plasma generation unit 13, the electrons contained in the argon gas introduced into the plasma generation unit 13 are subjected to the Lorentz force and undergo circular motion. At this time, when microwaves (electromagnetic waves) having the same period (or frequency) as the period (or frequency) of the electrons' circular motion are introduced into the plasma generation unit 13 from the waveguide 15, the circularly moving electrons and the microwaves resonate, and the energy of the microwaves is efficiently supplied to the circularly moving electrons (electron cyclotron resonance phenomenon) (S201 in Figure 6). As a result, the kinetic energy of the electrons contained in the argon gas increases, and the argon gas becomes argon ions (Ar + It separates into argon ions (Ar + An argon plasma consisting of ) and electrons is generated (S202 in Figure 6).

[0065] Next, in Figure 5, a high-frequency voltage is supplied to the target TA from the high-frequency power supply 16. In this case, a positive potential and a negative potential are alternately applied to the target TA to which the high-frequency voltage is supplied. Here, of the argon ions and electrons that make up the argon plasma, only the lighter electrons can follow the high-frequency voltage applied to the target TA, while the heavier argon ions cannot. As a result, the positive potential that attracts the following electrons is canceled out by the negative charge of the electrons, while the negative potential remains, so the average value of the high-frequency power shifts from 0V to a negative potential. This means that even though a high-frequency voltage is applied to the target TA, it can be considered as if a negative potential is applied to the target TA. As a result, argon ions with positive ions are attracted to the target TA, which is considered to have a negative potential applied to it on average, and collide with the target TA (S203 in Figure 6).

[0066] Subsequently, when argon ions collide with the target TA, the target particles constituting the target TA receive part of the kinetic energy of the argon ions and fly out into the internal space of the target TA or the chamber 10 (S204 in FIG. 6). Then, part of the target particles that have flown out into the internal space of the chamber 10 adhere to the surface of the film formation target object SUB held by the holding unit 11 (S205 in FIG. 6). And as such a phenomenon is repeated, as a result of a large number of target particles adhering to the surface of the film formation target object SUB, a film is formed on the surface of the film formation target object SUB (S206 in FIG. 6).

[0067] As described above, the film formation operation in the ECR sputtering apparatus 1 is realized.

[0068] For example, when the target TA is made of silicon (Si), the target particles become silicon atoms, and the film formed on the film formation target object SUB becomes a silicon film. However, when the above-described film formation operation is performed while introducing oxygen gas from the gas inlet 10a provided in the chamber 10 of the ECR sputtering apparatus 1 shown in FIG. 5, a silicon oxide film can be formed on the surface of the film formation target object SUB.

[0069] Similarly, for example, when the target TA is made of tantalum (Ta), the target particles become tantalum atoms, and the film formed on the film formation target object SUB becomes a tantalum film. However, when the above-described film formation operation is performed while introducing oxygen gas from the gas inlet 10a provided in the chamber 10 of the ECR sputtering apparatus 1 shown in FIG. 5, a tantalum oxide film can be formed on the surface of the film formation target object SUB.

[0070] Therefore, by alternately repeating the process of forming a silicon oxide film and the process of forming a tantalum oxide film while replacing the target TA, a reflective film made of a "silicon oxide / tantalum oxide multilayer film" can be formed.

[0071] <Advantages of the ECR sputtering apparatus> In the ECR sputtering apparatus 1, target particles are ejected by colliding argon ions with the target TA and then attached to the object to be deposited SUB, forming a film on the object SUB with the target particles as constituent material. This mechanism is the same in the RF sputtering apparatus. However, in the RF sputtering method, the kinetic energy of the target particles is greater than in the ECR sputtering method. As a result, for example, when a reflective or anti-reflective film is formed on the edge of a semiconductor laser using the RF sputtering method, the damage to the edge is greater due to the high kinetic energy of the target particles. In other words, the ECR sputtering method has the advantage of reducing damage to the edge because the kinetic energy of the target particles is lower than in the RF sputtering method.

[0072] Furthermore, in the ECR sputtering apparatus 1, the magnetic field generating unit 14 is provided only around the plasma generation unit 13. Therefore, in the ECR sputtering apparatus 1, the magnetic field strength is high inside the plasma generation unit 13 surrounded by the magnetic field generating unit 14, while the magnetic field strength is weak near the region where the film to be deposited SUB is placed (near the region away from the magnetic field generating unit 14). In other words, in the ECR sputtering apparatus 1, a magnetic field gradient is generated from the plasma generation unit 13 toward the region where the film to be deposited SUB is placed. To put it another way, in the ECR sputtering apparatus 1, a divergent magnetic field (non-uniform magnetic field) is formed inside the chamber 10.

[0073] Furthermore, the electrons present in the plasma generation unit 13 are undergoing circular motion and therefore possess a magnetic moment. Consequently, the circularly moving electrons move along the magnetic field gradient due to the interaction between the magnetic moment and the magnetic field. In other words, the electrons present in the plasma generation unit 13 move slowly from the plasma generation unit 13 toward the film deposition target object SUB and adhere to it, thereby suppressing damage to the film deposition target object SUB caused by these electrons.

[0074] In other words, the ECR sputtering apparatus 1 generates plasma by utilizing the cyclotron resonance phenomenon caused by electrons in circular motion, which is why it can suppress damage to the film-forming object SUB caused by electrons. This is because electrons in circular motion have a magnetic moment, and if a magnetic field gradient exists, it is possible to move electrons along the magnetic field gradient. In other words, the ECR sputtering apparatus 1 does not require the application of a new electric field to move electrons, thus suppressing the acceleration of electrons by the electric field and the resulting damage to the film-forming object SUB caused by electron collisions. Thus, with the ECR sputtering apparatus 1, for example, when forming a reflective or anti-reflective film on the end face of a semiconductor laser, which is the film-forming object SUB, the damage to the end face can be reduced by the mechanism described above. As a result, in this embodiment, the performance of the semiconductor laser can be improved by using the ECR sputtering method instead of the RF sputtering method as a method for forming a reflective or anti-reflective film on the resonant surface.

[0075] <Novel findings discovered by the inventors> As described above, the basic concept of this embodiment is to use the ECR sputtering method as a method for forming a reflective film on the end face used in UV-B semiconductor lasers. According to this basic concept, the advantages of the ECR sputtering method described above make it possible to reduce damage to the end face when depositing a reflective film on the end face of a UV-B semiconductor laser. Based on the premise of using the ECR sputtering method as a method for forming a reflective film on the end face, the inventors investigated how to construct the reflective film from a film having an extremely small extinction coefficient, and as a result obtained the following novel findings, which will be explained below.

[0076] For example, in the reflective film 300 shown in Figure 4, the low refractive index film 310 in the UV-B region, which includes a wavelength range of approximately 300 nm, is composed of, for example, a silicon oxide film. On the other hand, in the UV-B region, which includes a wavelength range of approximately 300 nm, it is being considered that the high refractive index film 320 be composed of a hafnium oxide film or a tantalum oxide film.

[0077] In this regard, Figure 7 is a graph showing the wavelength dependence of the extinction coefficient of hafnium oxide formed by the RF sputtering method. In Figure 7, it can be seen that when a hafnium oxide film is formed by the RF sputtering method, the extinction coefficient at a wavelength of 300 nm can be made extremely small. Therefore, since the extinction coefficient of a hafnium oxide film formed by the RF sputtering method is very small in the wavelength region of about 300 nm, it can be considered desirable to use a hafnium oxide film formed by the RF sputtering method as a high refractive index film 320 in the wavelength region of about 300 nm. In particular, in the RF sputtering method, the kinetic energy of the target particles is large, which tends to easily eject impurities and create strong bonds. For this reason, as mentioned above, it is presumed that when a hafnium oxide film is formed by the RF sputtering method, hafnium oxide with an extremely small extinction coefficient can be formed.

[0078] However, as mentioned above, ECR sputtering is superior to RF sputtering in terms of reducing damage to the edge surface where the reflective film is formed. Therefore, the inventors investigated whether an extremely small extinction coefficient could be obtained even when forming a hafnium oxide film by ECR sputtering. Specifically, the inventors investigated the extinction coefficient of hafnium oxide formed by ECR sputtering.

[0079] Figure 8 is a graph showing the wavelength dependence of the extinction coefficient for various materials formed by the ECR sputtering method. Specifically, Figure 8 shows the wavelength dependence of the extinction coefficient for hafnium oxide, tantalum oxide, silicon oxide, aluminum oxide, aluminum nitride, and aluminum oxynitride.

[0080] In Figure 8, focusing on hafnium oxide, we see that in the wavelength range of approximately 300 nm, the extinction coefficient of hafnium oxide formed by ECR sputtering is greater than that of hafnium oxide formed by RF sputtering (see Figure 7), indicating that light absorption exists in this wavelength range. In other words, in the wavelength range of approximately 300 nm, the extinction coefficient of hafnium oxide formed by ECR sputtering is greater than that of hafnium oxide formed by RF sputtering.

[0081] From this, it can be seen that in the UV-B region, which includes wavelengths of around 300 nm, it is difficult to reduce the extinction coefficient of the high refractive index film 320 while simultaneously reducing damage to the end face forming the reflective film when the high refractive index film 320 is constructed from a hafnium oxide film. In other words, to achieve both reduction of damage to the end face by the ECR sputtering method and reduction of the extinction coefficient in the UV-B region, which includes wavelengths of around 300 nm, using hafnium oxide as the high refractive index film 320 is not the best option.

[0082] In this regard, as shown in Figure 8, in the ultraviolet region (greater than 200 nm and less than or equal to 380 nm), in the wavelength range from approximately 300 nm to 380 nm, the extinction coefficient of tantalum oxide formed by the ECR sputtering method is significantly smaller than that of hafnium oxide formed by the ECR sputtering method. Therefore, when the high refractive index film 320 is constructed from a tantalum oxide film formed by the ECR sputtering method in the wavelength range from approximately 300 nm to 380 nm, it is possible to reduce the extinction coefficient of the high refractive index film 320 while simultaneously reducing damage to the end face forming the reflective film. In other words, to achieve both reduced damage to the end face by the ECR sputtering method and reduced extinction coefficient in the wavelength range from approximately 300 nm to 380 nm, it is desirable to use tantalum oxide as the high refractive index film 320.

[0083] Let's explain in more detail. Figure 9 shows the results of measuring the wavelength dependence of the extinction coefficient and refractive index of a tantalum oxide film using a spectroscopic ellipsometer. Specifically, Figure 9 shows the wavelength dependence of the extinction coefficient and refractive index for a tantalum oxide film formed using RF sputtering and a tantalum oxide film formed using ECR sputtering. In Figure 9, the wavelength dependence of the extinction coefficient and refractive index dependence for the tantalum oxide film formed using RF sputtering and the tantalum oxide film formed using ECR sputtering appear to be almost the same. That is, in the wavelength region greater than 298 nm and less than or equal to 380 nm, both the extinction coefficients for the tantalum oxide film formed using RF sputtering and the tantalum oxide film formed using ECR sputtering are small.

[0084] In this regard, Figure 10 shows a magnified view of the wavelength region around 300 nm. As shown in Figure 10, when the wavelength region around 300 nm is magnified, it can be seen that the wavelength dependence of the extinction coefficient differs between tantalum oxide films formed using the RF sputtering method and tantalum oxide films formed using the ECR sputtering method. Specifically, as shown in Figure 10, in the tantalum oxide film formed using the RF sputtering method, the extinction coefficient increases sharply in the wavelength region smaller than 301 nm. In contrast, in the tantalum oxide film formed using the ECR sputtering method, the extinction coefficient increases sharply in the wavelength region smaller than 298 nm. This means that in the wavelength region greater than 298 nm and less than 301 nm, the extinction coefficient of the tantalum oxide film formed using the ECR sputtering method is significantly lower than that of the tantalum oxide film formed using the RF sputtering method. For example, as shown in Figure 11, the extinction coefficient of a tantalum oxide film formed using the RF sputtering method is 0.0001 for light with a wavelength of 300 nm, whereas the extinction coefficient of a tantalum oxide film formed using the ECR sputtering method is less than 0.0001 for light with a wavelength of 300 nm, and is very close to "0".

[0085] Considering the above, tantalum oxide films formed using the ECR sputtering method can exhibit extremely low extinction coefficients in the wavelength range within the ultraviolet region, specifically in the wavelength range greater than 298 nm and less than or equal to 380 nm.

[0086] Furthermore, if we limit the scope to the UV-B wavelength range (wavelengths greater than 280 nm and less than or equal to 315 nm), tantalum oxide films formed using the ECR sputtering method can exhibit extremely low extinction coefficients in the UV-B wavelength range, specifically in the wavelength region greater than 298 nm and less than or equal to 315 nm.

[0087] Considering the above-mentioned findings discovered by the inventors, it is desirable to adopt a tantalum oxide film formed by ECR sputtering as the high refractive index film 320, from the viewpoint of achieving both reduction of damage at the end face and extremely low extinction coefficient in the wavelength region greater than 298 nm and less than or equal to 380 nm within the ultraviolet region. Furthermore, limiting the ultraviolet region to the UV-B region, from the viewpoint of achieving both reduction of damage at the end face and extremely low extinction coefficient in the wavelength region greater than 298 nm and less than or equal to 315 nm within the UV-B wavelength region, it is desirable to adopt a tantalum oxide film formed by ECR sputtering as the high refractive index film 320.

[0088] Next, Figure 12 is a graph showing the wavelength dependence of the refractive index of various materials. Specifically, Figure 12 shows the wavelength dependence of the refractive index of hafnium oxide, tantalum oxide, silicon oxide, aluminum oxide, aluminum nitride, and aluminum oxynitride.

[0089] As shown in Figure 12, it can be seen that the refractive index of tantalum oxide is greater than that of hafnium oxide in a wide wavelength range from 250 nm to 450 nm. Here, in the reflective film 300 composed of dielectric multilayer films, a larger refractive index difference between the low refractive index film 310 and the high refractive index film 320 is advantageous from the viewpoint of improving reflectivity (the larger the refractive index difference, the fewer the number of paired films stacked to achieve high reflectivity) and the stopband width (the width of the wavelength range in which high reflectivity can be achieved) can be increased. Therefore, it is even more desirable to use a tantalum oxide film formed by the ECR sputtering method as the high refractive index film 320.

[0090] <Features of the Embodiment> From the above, the distinctive feature of this embodiment is that, based on the fundamental idea of ​​using the ECR sputtering method to form a reflective film on the edge face of a semiconductor laser that emits light in the wavelength range included in the ultraviolet region, a tantalum oxide film formed by the ECR sputtering method is adopted as the high refractive index film 320 of the reflective film 300, which is composed of a dielectric multilayer film. Expressed as an invention, the distinctive feature of this embodiment is that a tantalum oxide film is adopted as the high refractive index film 320 of the reflective film 300, which is composed of a dielectric multilayer film, and this tantalum oxide film has the characteristic that its extinction coefficient for light with a wavelength of 300 nm is less than 0.0001. In other words, by forming the tantalum oxide film by the ECR sputtering method, it is possible to realize a tantalum oxide film that has the characteristic that its extinction coefficient for light with a wavelength of 300 nm is less than 0.0001.

[0091] As a result, the ECR sputtering method reduces damage to the edge surface, and the tantalum oxide film formed by the ECR sputtering method can significantly reduce the extinction coefficient in the wavelength range between 298 nm and 380 nm within the ultraviolet region, or between 298 nm and 315 nm within the UV-B wavelength range. This synergistic effect makes it possible to achieve both reduced film formation damage and reduced extinction coefficient at the edge surface forming the reflective film 300. Consequently, according to this embodiment, the performance of a semiconductor laser emitting light in the wavelength range included in the ultraviolet region can be improved.

[0092] <Variation> The basic concept in this embodiment is to use ECR sputtering as a method for forming a reflective film on the end face of a semiconductor laser that emits light in the wavelength range included in the ultraviolet region. This basic concept can also be applied to a method for forming an anti-reflective film on the exit surface of a semiconductor laser. That is, to reduce damage when forming an anti-reflective film on the exit surface of a semiconductor laser, the anti-reflective film can be formed by ECR sputtering. The anti-reflective film is formed from a multilayer film in which multiple pairs of films are formed, each consisting of a low refractive index film having a first refractive index and a high refractive index film having a second refractive index greater than the first refractive index. For example, the low refractive index film is composed of a silicon oxide film. On the other hand, the high refractive index film is composed of a tantalum oxide film. As a result, by forming the tantalum oxide film by ECR sputtering, it is possible to realize an anti-reflective film containing a tantalum oxide film that has the characteristic of having an extinction coefficient for light with a wavelength of 300 nm smaller than 0.0001.

[0093] Based on the above, the synergistic effect of reducing film formation damage on the laser beam emission surface where the anti-reflective coating is formed, and the fact that tantalum oxide films formed by the ECR sputtering method can significantly reduce the extinction coefficient in the wavelength range between 298 nm and 380 nm within the ultraviolet region, or between 298 nm and 315 nm within the UV-B wavelength range, makes it possible to improve the performance of semiconductor lasers that emit light in the wavelength range included in the ultraviolet region.

[0094] It should be noted that "reflective film" and "anti-reflective film" are basically the same type of film. When a "reflective film" is composed of a laminated film of a low refractive index film and a high refractive index film, if the wavelength of the light to be reflected is λ, the thickness of the low refractive index film is adjusted to λ / 4n, and the thickness of the high refractive index film is also adjusted to λ / 4n (where n is the refractive index). On the other hand, when an "anti-reflective film" is composed of a laminated film of a low refractive index film and a high refractive index film, if the wavelength of the light to be reflected is λ, the thickness of the low refractive index film is adjusted to λ / 2n, and the thickness of the high refractive index film is also adjusted to λ / 2n (where n is the refractive index).

[0095] Although the present invention has been specifically described above based on its embodiments, it goes without saying that the present invention is not limited to the above embodiments and can be modified in various ways without departing from its essence. [Explanation of Symbols]

[0096] 1 ECR sputtering apparatus 10 Chambers 10a Gas inlet 10b Gas exhaust port 11 Holding part 12 Mechanism 13 Plasma generation section 14 Magnetic field generation unit 15 Waveguide 16 High frequency power supply 17 Fixed part 100 sapphire substrates 101 Aluminum Nitride Layer 102 Aluminum Nitride Layer 103 Nanopattern 104 Al 0.68 Ga 0.32 N layer 105 Al 0.62 Ga 0.38 N layer 106 Al 0.45 Ga 0.55 N layer 107 Active layer 108 Al 0.5 Ga 0.5 N layer 109 Electron Block Layer 110 p-type AlGaN layer 111 p-type AlGaN layer 112 p-type GaN layer 113 Surface protective film 114p electrode 115 Pad electrodes 116 n electrode 117 Pad electrodes 200A injection surface 200B reflective surface 300 Reflective film 310 Low refractive index film 320 High refractive index film AR anti-reflection coating CHP semiconductor chip CR chip area HR reflective film LB Laser Bar LD UV-B semiconductor laser SUB Film Deposition Target TA Target WF Semiconductor wafer

Claims

1. A semiconductor laser that emits light in the wavelength range included in the ultraviolet region, The end face opposite to the light emission surface is provided with a reflective film, The reflective film includes a tantalum oxide film. The tantalum oxide film is a semiconductor laser whose extinction coefficient for light with a wavelength of 300 nm is less than 0.0001.

2. In the semiconductor laser according to claim 1, The semiconductor laser is characterized in that the reflective film is composed of a laminated film of a first film having a first refractive index and a tantalum oxide film having a second refractive index higher than the first refractive index.

3. In the semiconductor laser according to claim 2, The semiconductor laser is characterized in that the reflective film is composed of a multilayer film having a plurality of laminated films, each consisting of the first film and the tantalum oxide film.

4. In the semiconductor laser according to claim 3, The first film is a silicon oxide film, in a semiconductor laser.

5. In the semiconductor laser according to claim 1, The aforementioned wavelength range is greater than 298 nm and less than or equal to 380 nm, and is a semiconductor laser.

6. In the semiconductor laser according to claim 5, The aforementioned wavelength range is greater than 298 nm and less than or equal to 315 nm, and is a semiconductor laser.

7. A semiconductor laser that emits light in the wavelength range included in the ultraviolet region, The light emission surface is provided with an anti-reflective coating, The anti-reflective coating includes a tantalum oxide film. The tantalum oxide film is a semiconductor laser whose extinction coefficient for light with a wavelength of 300 nm is less than 0.0001.

8. In the semiconductor laser according to claim 7, The anti-reflective coating is composed of a laminated film of a first film having a first refractive index and a tantalum oxide film having a second refractive index higher than the first refractive index, in a semiconductor laser.

9. In the semiconductor laser according to claim 8, The semiconductor laser is characterized in that the anti-reflective film is composed of a multilayer film having a plurality of laminated films, each of which consists of the first film and the tantalum oxide film.

10. In the semiconductor laser according to claim 9, The first film is a silicon oxide film, in a semiconductor laser.

11. In the semiconductor laser according to claim 7, The aforementioned wavelength range is greater than 298 nm and less than or equal to 380 nm, and is a semiconductor laser.

12. In the semiconductor laser according to claim 11, The aforementioned wavelength range is greater than 298 nm and less than or equal to 315 nm, and is a semiconductor laser.

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