WO2011052395A1

WO2011052395A1 - Epitaxial wafer, method of producing epitaxial wafer, light-emitting element wafer, method of producing light-emitting element wafer, and light-emitting element

Info

Publication number: WO2011052395A1
Application number: PCT/JP2010/068097
Authority: WO
Inventors: Kimio Ichikawa
Original assignee: Fujifilm Corp
Current assignee: Fujifilm Corp
Priority date: 2009-10-30
Filing date: 2010-10-07
Publication date: 2011-05-05
Anticipated expiration: 2012-04-30
Also published as: JP2011096935A; TW201125164A

Abstract

The present invention provides an epitaxial wafer including: a monocrystalline substrate; and a semiconductor crystal layer formed on the monocrystalline substrate by crystal growth, an element forming region of a surface of the semiconductor crystal layer being partially surface roughened in comparison with a crystal growth surface.

Description

DESCRIPTION

EPITAXIAL WAFER, METHOD OF PRODUCING EPITAXIAL WAFER, LIGHT-EMITTING ELEMENT WAFER, METHOD OF PRODUCING LIGHT-EMITTING

ELEMENT WAFER, AND LIGHT-EMITTING ELEMENT

TECHNICAL FIELD

[0001] The present invention relates to an epitaxial wafer, a method of producing an epitaxial wafer, a light-emitting element wafer, a method of producing a light-emitting element wafer, and a light-emitting element.

BACKGROUND ART

[0002] A light-emitting element such as a light-emitting diode (LED) or the like is produced by cutting an epitaxial wafer obtained by epitaxially growing a semiconductor crystal layer on a monocrystalline substrate into individual pieces element by element. In general, an LED is low in light-emitting efficiency. This is because the refractive index of a semiconductor crystal layer on a light extraction side is higher than that of air and emitting light is reflected at an interface with air to result in lowering light output efficiency. For example, the refractive index n of gallium nitride (GaN) is 2.4.

[0003] An LED where, in order to improve the light extraction efficiency, a fine irregular structure of sub-micrometer order (0.1 μπι or more and less than 1 μπι) is formed on a surface of a semiconductor crystal layer is variously proposed (see "Higher Luminescence LED Using Nanostructured Surface Fabricated by Self- Assembled Block Copolymer Lithography", TOSHIBA REVIEW Vol. 60 No. 10 p32 - p35 (2005)). Furthermore, other than LEDs, an optical device that makes use of a fine irregular structure such as a photonic crystal structure or the like has been variously proposed (see, Japanese Patent Application Laid-Open (JP-A) Nos. 11-330619, 2001-308457 and 2002-62554).

[0004] The fine irregular structure can be formed by forming a resist film on a surface of a semiconductor crystal layer, followed by subjecting the resist film to photolithography. In the photolithography, technologies with high accuracy such as electron beam exposure, Deep UV laser exposure (for example, wavelength λ = 248 nm), super-resolution exposure with a semiconductor laser and the like are used. The resist film as well has to be uniformly formed with a thickness of sub-micrometer order. In order to form such a thin resist film, a spin coat process where a centrifugal force is used to thinly expand a coating liquid is preferred.

DISCLOSURE OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] However, when a spin coat process is used to form a resist film, a coating defect called a comet defect is generated. In general, when a flow of a coating liquid has been disturbed by dust attached onto a surface being coated, the comet defect is generated. Since a coating defect section extending in fan with an attached point of dust as a starting point looks like a comet, the coating defect is called a "comet defect".

[0008] When an epitaxial wafer 10 is rotated around a center point 36 and a spin coat process is used to form a resist film 42 as shown in FIG. 11, in some cases, plural comet defects 70 are generated. A coating liquid is fed in the vicinity of the center point 36.

Accordingly, the nearer the starting point is to the center point 36, the larger the comet defect 70 expands. In the example, three comet defects 70_\ to 70₃ are generated, and the comet defect 70] of which starting point is near the center point 36 becomes largest.

[0009] In a coating defect section owing to the comet defect, a fine irregular structure is not formed on a surface of the semiconductor crystal layer of the epitaxial wafer. Accordingly, there is a problem that, when the comet defect has been generated in the resist film, it is difficult to form a uniform fine irregular structure on a surface of a semiconductor crystal surface. Furthermore, when the epitaxial wafer is cut into individual pieces element by element to produce LEDs, there is a problem that performance fluctuates between produced LEDs.

[0010] The present inventors studied hard and obtained following findings. That is, normally, a crystal growth surface is a mirror surface and defects such as protrusions or the like are not generated. However, it was found that, in the case of an epitaxial wafer used in the production of LEDs, on a surface of a semiconductor crystal layer, there are protrusions of a diameter from several micrometers to several hundreds micrometers and the protrusions generate the comet defects. A semiconductor crystal layer of the epitaxial wafer necessitates a high level of technology to manage and control a crystal growth step and is likely to have various crystal defects. The protrusion as well is one of the crystal defects.

[0011] Furthermore, in the field of semiconductor elements, in order to make a

semiconductor element thinner, mechanical polishing, chemical mechanical polishing (CMP) or the like is used to polish a substrate or to flatten an insulating film. The polishing step is normally conducted uniformly on a two-dimensional plane with an entire substrate held under pressure. An epitaxial wafer having protrusions on a surface of the semiconductor crystal layer is likely to warp in so-called parabola or propeller. Accordingly, when, in the polishing step, an entirety of substrate is held under pressure, the substrate may be deformed to result in polishing a normal surface other than the defect or in generating cracks.

[0012] A surface of a wafer obtained by crystal growth is normally a mirror surface;

accordingly, there is no need of conducting a mirror polishing step. A polishing step of a wafer surface is rather conducted to roughen a surface. An object of roughening a wafer front surface (or back surface) is to differentiate a front and back surface, to chamfer a peripheral portion, to mark a production number and the like (see JP-A Nos. 2007-42748, 2009- 182341 and 2008- 181972). When a front surface is differentiated from a back surface, a back surface side is roughened.

[0013] The present invention was conducted in view of the above-mentioned situations and an object of the invention is to provide an epitaxial wafer capable of forming a resist film having a uniform film thickness on a surface of a semiconductor crystal layer and a production method thereof. Furthermore, another object of the present invention is to provide a light-emitting element wafer where on a surface of semiconductor crystal layer a resist film having a uniform film thickness or a uniform fine irregular structure is formed and a production method thereof. Still furthermore, still another object of the invention is to provide a light-emitting element of which light extraction efficiency is improved by a uniform fine irregular structure formed on a surface of a semiconductor crystal layer.

MEANS TO SOLVE THE PROBLEMS

[0014] In order to achieve the objects, inventions involving the respective claims are characterized by having following configurations.

[0015] <1> An epitaxial wafer, including: a monocrystalline substrate; and a semiconductor crystal layer formed on the monocrystalline substrate by crystal growth, an element forming region of a surface of the semiconductor crystal layer being partially surface roughened in comparison with a crystal growth surface.

[0016] <2> The epitaxial wafer of <1>, wherein the roughened surface has an arithmetic surface roughness Ra of from 0.1 μπι to 10 μπι.

[0017] <3> The epitaxial wafer of <1> or <2>, wherein an area of the roughened surface is 0.1% or less of a total area of a surface of the semiconductor crystal layer.

[0018] <4> The epitaxial wafer of <1> to <3>, wherein the element is a light-emitting diode

(LED).

[0019] <5> The epitaxial wafer of any one of <1> to <4>, wherein the semiconductor crystal layer is any one selected from the group consisting of a gallium nitride (GaN) semiconductor, a gallium arsenide (GaAs) semiconductor, an indium-gallium-aluminum-phosphorus

(InGaAlP) semiconductor and a zinc oxide (ZnO) semiconductor.

[0020] <6> A method of producing the epitaxial wafer of <1> to <5>, including:

detecting a protrusion section present on the element forming region of the surface of the semiconductor crystal layer;

removing the detected protrusion section by polishing; and

partially surface roughening the element forming region.

[0021] <7> The method bf <6>, wherein the protrusion section is polished so as to be substantially the same in height as that of a surface of the semiconductor crystal layer.

[0022] <8> The method of <6> or <7>, wherein the protrusion section is polished with a pencil grinder.

[0023] <9> A light-emitting element wafer formed by further processing the epitaxial wafer of <1> to <5>, including: a resist film formed on a surface of the epitaxial wafer.

[0024] <10> A light-emitting element wafer formed by further processing the epitaxial wafer of <1> to <5>, including: a porous structure formed by arranging holes having a diameter of 0.05 μπι or more and less than 1 μιη at a pitch of sub-micrometer order on a surface of the epitaxial wafer.

[0025] <11> A method of producing the light-emitting element wafer of <9>, including forming the resist film by spin coating.

[0026] <12> A method of producing the light-emitting element wafer of <10>, including: forming a photoresist film on a surface of the epitaxial wafer; and forming a porous structure where holes having a diameter of 0.05 μπι or more and less than 1 μπι are arranged at a pitch of sub-micrometer order on a surface of the epitaxial wafer by photolithography with the photoresist film.

[0027] < 13> A light-emitting element including a chip obtained by sectioning the light-emitting element wafer of <10>.

EFFECTS OF THE INVENTION [0028] According to the present invention, an epitaxial wafer capable of forming a resist film having a uniform film thickness on a surface of a semiconductor crystal layer and a production method thereof are provided. Furthermore, a light-emitting element wafer where on a surface of semiconductor crystal layer a resist film having a uniform film thickness or a uniform fine irregular structure is formed and a production method thereof are provided. Still furthermore, a light-emitting element of which light extraction efficiency is improved by a uniform fine irregular structure formed on a surface of a semiconductor crystal layer is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 A is a diagram for explaining an LED wafer used as a raw material;

FIG IB is a diagram for explaining an LED wafer used as a raw material;

FIG. 1C is a diagram for explaining an LED wafer used as a raw material;

FIG 2A is a perspective view showing an aspect where a defect portion is present on a surface of an LED wafer;

FIG 2B is an enlarged diagram of the defect portion of FIG. 2 A;

FIG. 3 is a schematic diagram showing an aspect where a resist film is formed by spin coating on a surface of the LED wafer;

FIG. 4A is a sectional diagram showing an aspect where a fine irregular structure is non-uniformized by generation of a comet defect;

FIG. 4B is a sectional diagram showing an aspect where a fine irregular structure is non-uniformized by generation of a comet defect;

FIG 4C is a sectional diagram showing an aspect where a fine irregular structure is non-uniformized by generation of a comet defect;

FIG 5 A is a process chart showing a production process of an LED wafer involving an embodiment of the present invention;

FIG. 5B is a process chart showing a production process of an LED wafer involving an embodiment of the present invention;

FIG. 6 is a plan diagram of an LED wafer wherefrom protrusions involving an embodiment of the present invention are removed;

FIG. 7 is a schematic diagram showing an aspect where a protrusion is removed by polishing to form a rough surface section;

FIG. 8 A is a sectional diagram showing an aspect where when an LED wafer from which a protrusion has been removed is used a fine irregular structure is uniformly formed;

FIG. 8B is a sectional diagram showing an aspect where when an LED wafer from which a protrusion has been removed is used a fine irregular structure is uniformly formed;

FIG 8C is a sectional diagram showing an aspect where when an LED wafer from which a protrusion has been removed is used a fine irregular structure is uniformly formed;

FIG. 9 is a sectional diagram schematically showing a structure of an LED element involving an embodiment of the present invention;

FIG. 10 is a partially enlarged diagram of a surface of a semiconductor crystal layer of an LED element; and

FIG 11 is a plan view for describing a comet defect generated by spin coating.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] In what follows, examples of embodiments of the present invention will be described with reference to the drawings.

[0031 ] <Epitaxial wafer for LED element>

FIG. 1 A is a perspective view showing an appearance of an epitaxial wafer for light-emitting diode, which is used as a raw material in an embodiment of the present invention. FIG IB is a schematic diagram showing an aspect when LED chips are individualized from the epitaxial wafer shown in FIG. 1 A. FIG. 1C is a sectional view showing a laminate structure of the epitaxial wafer shown in FIG 1 A.

[0032] As shown in FIGs. 1 A and 1C, an epitaxial wafer for LEDs (hereinafter, referred to as "LED wafer") 10 is a wafer obtained by epitaxially growing a semiconductor crystal layer 30 constituting an LED element on a monocrystalline substrate 22. A surface 14 of the semiconductor crystal layer 30 is a front surface side of the LED wafer 10. The LED wafer 10 is almost circular in plan view and a diameter (wafer size) thereof is from 2 inch to 12 inch depending on a material. For example, in an LED wafer 10 where a GaN semiconductor crystal layer is disposed on a sapphire substrate, a wafer size of 2 inch (53.8 mm) is usually used.

[0033] To the LED wafer 10, a straight notch called an orientation flat (OF) 12 is disposed to distinguish a crystal direction. An OF 12 is used also in various alignments. In some cases, in place of the OF 12, a V-shaped notch called a notch is disposed. Furthermore, in some cases, a back surface side of the LED wafer 10 is roughened to form a so-called cloudy surface. For example, this is a case when a back surface side is roughened to distinguish a back surface from a front surface or a case when a substrate side is polished to make thin.

[0034] As shown in FIG. 1 B, the LED wafer 10 is divided into plural LED chips 20. A magnitude of each of LED chips 20 may be about 360 μπι x 360 μιη. For example, as was shown with dotted lines, when the LED wafer 10 is patterned in directions parallel and vertical to the OF 12, followed by dividing along the pattern, plural LED chips 20 rectangular in plan view can be obtained. For example, in the case where the OF 12 is formed in parallel with a cleaved surface of a crystal, by forming a scribe groove having a predetermined depth on the pattern by laser scribing or the like, followed by dividing along the scribe groove, the LED wafer can be readily divided.

[0035] The LED wafer 10 is roughly circular in plan view and a part thereof is notched to dispose an OF 12. Accordingly, in a peripheral section of a surface 14 of the LED wafer 10, an unused region 16 (hatched portion) that is not used to form the LED chips 20 is generated. On the other hand, a region used to form LED chips 20 is called an "element forming region 18". The LED chips 20 obtained from the LED wafer 10 are used to form individual LED elements. A structure of the LED element will be described below.

[0036] As shown in FIG. 1C, the LED wafer 10 has a semiconductor crystal layer 30 formed on a monocrystalline substrate 22. In the semiconductor crystal layer 30, an n-type semiconductor layer 24, a light-emitting layer 26 and a p-type semiconductor layer 28 are stacked in this order from a monocrystalline substrate 22 side. The p-type semiconductor layer 28 is exposed on the surface 14 of the LED wafer 10. For example, in a GaN based LED wafer, on a sapphire substrate, an n-GaN layer, a light-emitting layer and a p-GaN layer are stacked. The semiconductor crystal layer 30 can be formed by the use of a known epitaxial growth method such as a metalorganic vapor phase epitaxy method (MOVPE), a molecular beam epitaxial growth method (MBE) or the like.

[0037] The semiconductor crystal layer 30 can be constituted of a gallium nitride (GaN) semiconductor, a gallium arsenide (GaAs) semiconductor, an

indium-gallium-aluminum-phosphorus (InGaAlP) semiconductor, a zinc oxide (ZnO) semiconductor or the like. A semiconductor constituting the semiconductor crystal layer 30 is appropriately selected in accordance with a desired emission wavelength, emission brightness and the like of LED. An emission wavelength of the LED can be broadly selected from an UV region (for example, 200 nm) to an infrared region.

[0038] FIG. 2 A is a perspective view showing an aspect when a surface of the LED wafer is partially enlarged and observed. FIG. 2B is a partially enlarged view of a surface of FIG. 2A. As shown enlarged in FIG. 2B, when a part 32 of a surface 14 of the LED wafer 10 was observed in detail, it was found that a protrusion 34 is present in the part 32. In the case where a resist film is formed on a surface 14 of the LED wafer 10 by spin coating, a comet defect is generated owing to the protrusion 34. Here, a protrusion 34 found at one position (part 32) is illustrated. However, in actuality, there are also cases where plural protrusions 34 are present scattered on a surface 14.

[0039] <Comet defect and inhomogenization of fine irregular structure>

FIG 3 is a schematic diagram showing an aspect when a resist film is formed on a surface of the LED wafer by spin coating. As shown in FIG. 3, in the case where a resist film is formed by spin coating on a surface 14 of the LED wafer 10, the LED wafer 10 is rotated around a rotation axis L of the LED wafer 10 (for example, in a direction of an arrow mark A). The rotation axis L is an axis that goes through a center point 36 of the LED wafer 10 and is vertical to a surface 14 of the LED wafer 10.

[0040] Furthermore, as is schematically shown with a dotted line, on the upper side of the LED wafer 10, a dispenser 38 for feeding a resist film forming coating liquid (photoresist) 40 is disposed. In the vicinity of a center point 36 of the LED wafer 10, a coating liquid 40 from the dispenser 38 is fed. The coating liquid 40 that is fed is thinly stretched from an inner side toward an outer side by a centrifugal force owing to a rotation of the LED wafer 10. Thereby, the resist film is formed into a thin film of sub-micrometer order.

[0041] Viscosity of a coating liquid 40 used to process a photonic crystal structure described below is 0.1 Pa - s or less, preferably 0.05 Pa - s or less. The lower limit of the viscosity is preferably 0.005 Pa- s or more and more preferably 0.01 Pa* s or more.

[0042] FIGs. 4A to 4C are sectional views showing an aspect when a fine irregular structure is inhomogenized owing to generation of comet defects. The drawings describe

conventional problems. As is mentioned above, in the case where a protrusion 34 is present on a front surface 14 (that is, a surface of a semiconductor crystal layer 30) of the LED wafer 10, because of the protrusion 34, a comet defect is formed in the resist film. A height of the protrusion 34 is larger than a thickness of a resist film 42 and the protrusion 34 is present so as to punch through the resist film 42.

[0043] As shown in FIG. 4 A, when a resist film is formed by spin coating, a flow of the coating liquid 40 from the inside toward the outside is disturbed by the protrusion 34, thereby a coating defect section called a comet defect is generated (see FIG. 11). As the result thereof, on a side inner than the protrusion 34, a thick resist film 42 is formed and on a side outer than the protrusion 34, a thin resist film 42 is formed. That is, the resist film 42 is not formed uniformly.

[0044] As shown in FIG. 4B, a resist film 42 is patterned so as to form a fine irregular structure on a surface 14 of the LED wafer 10. As will be described below, the fine irregular structure here is a photonic crystal structure where circular holes are regularly bored. When the resist film 42 is patterned, owing to mask exposure and development of the resist film 42, circular holes 43 are regularly bored in the resist film 42. On a side more inside than the protrusion 34, deep circular holes 43 are formed, and on a side more outside than the protrusion 34, shallow circular holes 43 are formed. In a portion where circular holes 43 are formed, a surface 14 of the LED wafer 10 is exposed. That is, the resist film 42 is not uniformly patterned.

[0045] As shown in FIG. 4C, when the resist film 42 where circular holes 43 have been bored is used to dry etch, a surface 14 of the LED wafer 10 is partially removed. The surface 14 exposed by formation of circular holes 43 is removed by dry etching. Thereby, on a surface of the semiconductor crystal layer 30, a fine irregular structure where circular holes 44 corresponding to the circular holes 43 have been regularly bored is formed. On a side more inside than the protrusion 34, deep circular holes 44 are formed and on a side more outside than the protrusion 34, shallow circular holes 44 are formed. That is, the fine irregular structure is not uniformly formed. Here, illustration is simplified by assuming that the protrusion 34 portion is not etched.

[0046] As obvious from examples of FIGs. 4A to 4C, when a protrusion 34 is present on a surface 14 of the LED wafer 10, a coating defect section is generated in the resist film 42; accordingly, it is difficult to homogeneously form a fine irregular structure on a surface of a semiconductor crystal layer 30. Furthermore, in what was mentioned above, as shown in FIG. 4A, an example where a thin resist film 42 is formed on the outside than the protrusion 34 is described. However, there is also a case where the resin film 42 is not formed more outside than the protrusion 34. In this case, on the outside more than the protrusion 34, circular holes 43 and circular holes 44 are not formed. That is, since there is not a resist film 42, a surface of the semiconductor crystal layer 30 is flatly removed by etching to result in not forming a fine irregular structure.

[0047] In the present embodiment, generation of the comet defect when a resist film was formed by spin coating is pointed out as a problem. However, also in an inkjet process or a spray coating process, in the case where there is a defect owing to the protrusion, liquid dripping or abnormality in a drying step is generated and a thickness of a resist film around the protrusion varies, and thereby the similar problem is caused.

[0048] Furthermore, when a semiconductor layer or a metal layer higher in the etching resistance such as a silicon oxide (Si0₂) layer or the like is appropriately disposed as a lower resist layer between an LED wafer and a resist film, the LED wafer can be more preferably etched. A thickness of the lower resist layer is preferably from 0.01 μπι to 1 μπι, more preferably from 0.02 μιη to 0.5 μπι, and still more preferably from 0.05 μιη to 0.3 μπι. Even when the lower resist layer is formed, a protrusion-like defect generated on a surface of the LED wafer cannot be completely covered, that is, the comet defect cannot be improved.

[0049] <Removal of protrusion>

FIGs. 5 A and 5B each show a process chart showing a production step of an LED wafer involving an embodiment of the present invention. An LED wafer involving an embodiment of the present invention is what has been obtained by removing protrusions that generate comet defects from the LED wafer that becomes foregoing raw material.

[0050] Firstly, as shown in FIG. 5A, defect portions 46 owing to protrusions on a surface 14 of the LED wafer 10 are detected. In some cases, plural (from several defect portions to several tens defect portions) defect portions 46 are detected. In this example, three defect portions 461 to 46₃ are detected. The defect portions 461 to 46₃ each are different in a distance from the center point 36 and in area. The defect portion 46 owing to the protrusions has a diameter in the order from several micrometers to several hundred micrometers. In particular, the defect portions 46 of about several hundred micrometers in diameter are high in the likelihood of forming comet defects. However, in a majority of the LED wafers, in the case where a total area of the defect portions 46 is represented by "S_d" and a total area of the surface 14 is represented by "S_t", an occupied area rate of the defect portions 46 (= S_d/S_t x 100) is substantially 0.02% or less by percentage.

[0051] In some cases, these defect portions 46 can be visually confirmed. In this case, detected defect portions 46 can be marked with a pencil by circling the defect portions 46 or the like. Furthermore, by the use of an L -shaped scale 45 in contact with the OF 12 and an outer periphery of the LED wafer 10, with the center point 36 as a reference point, a position of the defect portion 46 may be specified. Still furthermore, though not shown in the drawing, the LED wafer 10 is disposed on a predetermined stage, a surface 14 of the LED wafer 10 is photographed by a stationary camera, and the photographed image may be used to specify a position (coordinates) of the defect portion 46 from an X-Y coordinate system set relative to the camera.

[0052] In the next place, as shown in FIG. 5B, the protrusion 34 present in the detected defect portion 46 is mechanically polished and removed. Examples of polishing machines include known rotary polishing machines such as a pencil grinder, a drum grinder, a spindle grinder and the like. Among these, a pencil grinder appropriate for polishing a narrow range is preferable. Furthermore, in the case where the outermost periphery section of the LED wafer 10 warps 20 μπι or more from a reference surface, the LED wafer 10 is deformed by holding under pressure; accordingly, also from the viewpoint that there is no need of holding the LED 10 under pressure, a pencil grinder is preferable.

[0053] In an example shown in FIG 5B, as a polishing machine, a pencil grinder 48 equipped with a polishing section 48A where a disc-like grind stone rotates is used. When, with a polishing section 48A brought into contact with the defect portion 46, the polishing section 48 A is rotated, the protrusion 34 present in the defect portion 46 is polished and removed. In order to remove dust generated by polishing, after polishing, a surface 14 of the LED wafer 10 is preferably washed and dried.

[0054] In the case where the defect portion 46 can be visually confirmed, the protrusion 34 can be manually removed with a pencil grinder 48. Furthermore, in the case where an image taken with a camera is used to specify a position of the defect portion 46, a polishing operation can be also automatically conducted by disposing a polishing machine movable in three-dimensional directions (3 axis directions of X-axis/Y-axis/Z-axis) on a stage, followed by moving the polishing machine to a specified position and by moving in an up and down direction (Z axis direction). Since the defect portion 46 is polished until a height

substantially the same as that of the surface 14 of the LED wafer 10 is achieved, a position in an up-and-down direction is automatically determined. Here, the height "substantially" the same means that as to a polishing height of the protrusion 34 a process error of micrometer order can be allowed because a thickness of the resist film is sub-micrometer order. As will be described below, this is because an aim here is achieved as long as a flat resist film is formed by polishing the protrusion 34 to the extent where the comet defect is not generated.

[0055] Furthermore, by disposing a monitoring means to track a progress status of the polishing, whether the polishing came to an end or not can be determined to automatically finish the polishing. As the monitoring means like this, an optical monitoring unit that makes use of reflective light or an image processing unit that processes a photographic image can be used. Furthermore, in the case where the defect portion 46 can be visually confirmed, since also whether the polishing came to an end or riot can be visually determined, this is most efficient.

[0056] <LED wafer from which protrusion is removed>

FIG 6 is a plan view of an LED wafer from which a protrusion involving an embodiment of the present invention has been removed. An LED wafer 1 OA from which protrusions have been removed by polishing has rough surface portions 50 of which surface is roughened in comparison with a crystal growth surface (mirror surface) partially in an element forming region of the LED surface 14 A. In this example, three detected defect portions 46_\ to 46₃ are polished (see FIGs. 5A and 5B) and thereby three rough surface sections 50i to 50₃ are formed on the surface 14A.

[0057] Here, the rough surface section 50 is a surface portion of which arithmetic surface roughness Ra is from 0.1 μπι to 10 μπι. A region of which arithmetic surface roughness Ra is 0.1 μηι or more is obviously different from a crystal growth surface that is a mirror surface and can be also visually distinguished. When the arithmetic surface roughness Ra is made 10 μιη or less, the comet defect can be remarkably reduced. Around the rough surface section 50, in some cases, a thickness of the resist film may fluctuate. In order to avoid the variation of a thickness of the resist film, the upper limit value of the arithmetic surface roughness Ra is preferably as small as possible and 1 μιη or less is more preferable.

[0058] In addition, the arithmetic surface roughness Ra is the arithmetic average surface roughness according to JIS B0601 and a measurement length is from 10 μπι to 100 μπι.

Furthermore, a measurement method may be either an optical measurement method or a stylus type measurement method.

[0059] FIG. 7 is a schematic diagram that shows an aspect where a protrusion is polished and removed to form a rough surface section. It is preferred to polish to a broad range containing a defect portion 46 owing to the protrusion 34 so that a height the substantially same as a surface 14 of an LED wafer 10 may be obtained. For example, when a diameter of the defect portion 46 is assigned to Rj and a diameter of the rough surface section 50 is assigned to R₂, the roughening process is preferably conducted in the range where the diameter R₂ may be 0.5 to 10 times the diameter R_\ by polishing. On the other hand, there is no need of removing a small protrusion that does not disturb the formation of a resist film and a protrusion present in an unused region 16 (see FIG. IB). Accordingly, a total area of the rough surface section 50 is in the range of 0.1 to 10 times a total area of the defect portion 46.

[0060] Even when the roughening process has been exceedingly conducted, an occupied area rate of the defect portion 46 is such small as 0.02% or less. Accordingly, when a total area of the rough surface section 50 is assigned to "S_c" and a total area of the surface 14 is assigned to "S_t", an occupied area rate of the rough surface section 50 (= S_c/S_t x 100) is 0.1% or less by percentage.

[0061] FIGs. 8 A to 8C each are a sectional diagram that shows an aspect where when an LED wafer 10A from which a protrusion has been removed is used, a fine irregular structure is homogeneously formed. As shown in FIG. 8 A, on a surface 14 A of the LED wafer 10A, the defect portion 46 is polished and thereby a rough surface section 50 is formed. When a resist film is formed by spin coating, a coating liquid 40 expands from the inside toward the outside.

[0062] As obvious when compared with a case where a protrusion 34 shown in FIG. 4A is present, since the protrusion 34 has been removed, a flow of the coating liquid 40 is not disturbed and thereby a resist film 42 having a constant film thickness can be formed from the inside toward the outside. That is, the resist film 42 is homogeneously formed. Also after the protrusion 34 has been removed by polishing, in the rough surface section 50 and the surroundings thereof, a fine irregularity remains. However, the fine irregularities do not form the comet defect. This because the resist liquid has a property of undergoing "liquid film flattening" where owing to "leveling" during coating and drying, the resist liquid dries and solidifies along a tilt surface and thereby a flat resist film is formed.

[0063] As shown in FIG. 8B, the resist film 42 is patterned so as to form a fine irregular structure on a surface 14A of an LED wafer 10A. In the resist film 42, circular holes 43 are regularly bored. As obvious when compared with the case where a protrusion 34 shown in FIG. 4B is present, a rough surface section 50 is formed at a height substantially the same as the surface 14 A; accordingly, circular holes 43 having a constant depth are formed from the inside toward the outside. That is, the resist film 42 is homogeneously patterned.

[0064] As shown in FIG. 8C, when dry etching is conducted with the resist film 42 in which circular holes have been bored, the surface 14A exposed by the formation of the circular holes 43 is removed by dry etching, and thereby a fine irregular structure where circular holes 44 are regularly bored is formed. As obvious when compared with the case where a protrusion 34 shown in FIG. 4C is present, since circular holes 43 having a constant depth are formed, on the surface 14A of the LED wafer 10A, circular holes 44 having a constant depth are formed from the inside toward the outside. Also in the rough surface section 50, circular holes 44 having substantially the same shape are formed. That is, a fine irregular structure is homogeneously formed.

[0065] As obvious from examples of FIGs. 8 A to 8C, in the case where an LED wafer 10A from which the protrusion has been removed is used, the resist film 42 is homogeneously formed and thereby a fine irregular structure is homogeneously formed on a surface of a semiconductor crystal layer 30. Thus, as long as the LED wafer 10A is provided with a homogeneously formed fine irregular structure on a surface 14A thereof, even when LEDs are produced by dividing into plural LED chips 20, performances of produced LEDs do not fluctuate.

[0066] <LED elements>

In the next place, a structure of an LED element prepared with the LED wafer 10A will be described. FIG. 9 is a sectional view schematically showing a structure of an LED element (light-emitting element) involving an embodiment of the present invention. As shown in FIG. 9, an LED element 20A includes a monocrystalline substrate 22, a

semiconductor crystal layer 30, a contact layer 52, a p-side electrode 54, an n-side electrode 56 and a protective film 58. The contact layer 52 can be constituted of a transparent conductive material such as indium tin oxide (ITO) or the like. The p-side electrode 54 and n-side electrode 56 can be constituted of a metal such as gold (Au) or the like. The protective film 58 can be constituted of an insulating material such as a metal oxide or the like.

[0067] The semiconductor crystal layer 30 is constituted of an n-type semiconductor layer 24, a light-emitting layer 26 and a p-type semiconductor layer 28. The n-type semiconductor layer 24, the light-emitting layer 26 and the p-type semiconductor layer 28 are formed by sequentially epitaxially growing on the monocrystalline substrate 22. Furthermore, the semiconductor crystal layer 30 is formed in mesa by partially removing by dry etching the n-type semiconductor layer 24, the light-emitting layer 26 and the p-type semiconductor layer 28 to the middle of the n-type semiconductor layer 24.

[0068] On a surface of the n-type semiconductor layer 24 exposed by etching, the n-side electrode 56 is formed. On the other hand, on the p-type semiconductor layer 28, the p-side electrode 54 is formed via the contact layer 52. Furthermore, the protective film 58 is formed so as to cover a surface of the n-type semiconductor layer 24, light-emitting layer 26, p-type semiconductor layer 28 and contact layer 52 and a side surface of the p-side electrode 54 and n-side electrode 56.

[0069] FIG. 10 is a partially enlarged diagram of a surface of a semiconductor crystal layer of an LED element. An LED element 20 A prepared with the LED wafer 1 OA has a fine irregular structure formed on a surface of the semiconductor crystal layer 30, that is, on a surface of the p-type semiconductor layer 28. A part 60 of a surface of the p-type semiconductor layer 28 is enlarged and the fine irregular structure will be detailed therewith. As shown in FIG 10, on a surface of the p-type semiconductor layer 28, a photonic crystal structure where circular holes 62 are regularly bored is formed. In this example, a two-dimensional photonic crystal structure where circular holes 62 are arranged in square lattice is formed.

[0070] When a photonic crystal structure is formed on a surface of a p-type semiconductor layer 28, a light extraction efficiency of an LED element 20A is improved. For example, when a lattice arrangement of a photonic crystal structure is designed so that an aperture diameter X of the circular hole 62 may be 1 μηι or less and, when a pitch of the circular hole 62 is assigned to Y, a value of an XY ratio (= aperture diameter X/pitch Y) may be 1 or less, a light extraction efficiency of the LED element 20A is remarkably improved. A depth Z of the circular hole 62 can be set to, for example, about 0.1 μη to 0.2 μπι.

[0071] In the foregoing embodiment, a case where a light-emitting element is an LED was described. However, as long as a light-emitting element is prepared by individualizing an epitaxial wafer, a light-emitting element may be another semiconductor light-emitting element such as a semiconductor laser or the like.

EXAMPLES

[0072] <Example 1>

An LED wafer obtained by depositing a GaN semiconductor crystal layer on an m surface of a sapphire substrate having a diameter of 2 inches (trade name: "ES-WQBL", manufactured by Epistar Corporation) was prepared. It was confirmed that, on a surface of the LED wafer (p-GaN layer surface), protrusion defects having a diameter of 100 μπι and a diameter of 200 μηι, respectively, were 10.

[0073] A surface of the LED wafer was polished with a pencil grinder and thereby the protrusion defects having a diameter of about 50 μπι or more were removed. The polishing with a pencil grinder was conducted with an elastic rubber grindstone having a diameter of 2 mm and #800 grain size rotating at 4000 rpm. From a result of optical microscope observation of a surface of the LED wafer, a total area of a protrusion defect portion was 0.002% of a total area of the LED wafer and a total area of the ground portion was 0.01%.

[0074] On a surface of the LED wafer, a photoresist liquid having the viscosity of 0.01 Pa* s was coated by a spin coater (trade name: 'Ή-Γ', manufactured by Mikasa Corporation) and thereby a resist film having a thickness of 0.1 μπι was formed. The photoresist liquid was obtained by dissolving 2 g of the compound having a salt structure (in which the molar ratio of anion : cation is 1 : 1/2) shown below in 100 ml of tetrafluoropropanol. By polishing the protrusions, on a spin-coated resist film, a comet defect was not found.

[0075]

[0076] On an entire surface of the resist film, by the use of an exposure device equipped with a laser optical system having an wavelength of 405 nm and NA = 0.85 (trade name: "NEO-1000", manufactured by Pulse Tech Co., Ltd.), laser processing was conducted so that circular holes having a diameter of an opening of 0.15 μπι may be formed at a pitch of 0.4 μητ,.

[0077] A surface of a laser-processed LED wafer was etched by the use of a parallel plate type RIE system (trade name: "10NL", manufactured by SUMCO Co., Ltd.) with SF6 gas to remove the resist film. When a surface of an LED wafer from where the resist film was removed is observed with an AFM, it was confirmed that a photonic crystal structure where circular holes having a diameter of an aperture of 0.2 μπι and a depth of 0.2 μπι are formed at a pitch of 0.4 μηι is obtained. When an in-plane emission distribution of the resulting LED wafer was measured, a light amount distribution at a forward current of 3.5 V was from 4.5 mW to 6.0 mW.

[0078] <Comparative example 1>

In a manner similar to example 1 except that an LED wafer from which protrusion defects were not removed was used, a resist film was formed, followed by conducting a photonic crystal processing with the resist film. At the time when the resist film was formed, it was confirmed that comet defects were formed in the surroundings of the protrusion. The circular holes formed thereafter by etching were formed indefinite and deformed in shape depending on positions and a depth of the comet defect portion was several tens nm. It was the same as that a fine irregular structure was not formed. When an in-plane emission distribution of the resulting LED wafer was measured, a light amount distribution at a forward current of 3.5 V was from 4.0 mW to 6.0 mW. It is found that in comparison with example 1, a light amount distribution is broad and a light amount fluctuates more.

Claims

1. An epitaxial wafer comprising:

a monocrystalline substrate; and

a semiconductor crystal layer formed on the monocrystalline substrate by crystal growth, an element forming region of a surface of the semiconductor crystal layer being partially surface roughened in comparison with a crystal growth surface.

2. The epitaxial wafer of claim 1 , wherein the roughened surface has an arithmetic surface roughness Ra of from 0.1 μπι to 10 μηι.

3. The epitaxial wafer of claim 1, wherein an area of the roughened surface is 0.1% or less of a total area of a surface of the semiconductor crystal layer.

4. The epitaxial wafer of claim 1, wherein the element is a light-emitting diode (LED).

5. The epitaxial wafer of claim 1, wherein the semiconductor crystal layer is any one selected from the group consisting of a gallium nitride (GaN) semiconductor, a gallium arsenide (GaAs) semiconductor, an indium-gallium-aluminum-phosphorus (InGaAlP) semiconductor and a zinc oxide (ZnO) semiconductor.

6. A method of producing the epitaxial wafer of claim 1 , comprising:

removing the detected protrusion section by polishing; and

partially surface roughening the element forming region.

7. The method of claim 6, wherein the protrusion section is polished so as to be substantially the same in height as that of a surface of the semiconductor crystal layer.

8. The method of claim 6, wherein the protrusion section is polished with a pencil grinder.

9. A light-emitting element wafer formed by further processing the epitaxial wafer of claim 1, comprising:

a resist film formed on a surface of the epitaxial wafer.

10. A light-emitting element wafer formed by further processing the epitaxial wafer of claim 1, comprising:

a porous structure formed by arranging holes having a diameter of 0.05 μπι or more and less than 1 μπι at a pitch of sub-micrometer order on a surface of the epitaxial wafer.

11. A method of producing the light-emitting element wafer of claim 9, comprising: forming the resist film by spin coating.

12. A method of producing the light-emitting element wafer of claim 10, comprising: forming a photoresist film on a surface of the epitaxial wafer; and

forming a porous structure where holes having a diameter of 0.05 μπι or more and less than 1 μπι are arranged at a pitch of sub-micrometer order on a surface of the epitaxial wafer by photolithography with the photoresist film.

13. A light-emitting element comprising :

a chip obtained by sectioning the light-emitting element wafer of claim 10.