JP2009184860A - Substrate and epitaxial wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a C-face single crystal substrate composed of a crystal of a hexagonal structure for epitaxial growth suited to be used for manufacturing a GaN-based semiconductor element, and an epitaxial wafer obtained by epitaxially growing a GaN-based semiconductor crystal on the substrate. <P>SOLUTION: The substrate is a C-plane single crystal substrate composed of a crystal of a hexagonal structure for epitaxial growth, wherein it has first split grooves and second split grooves formed on the surface for the epitaxial growth, the first split grooves and the second split grooves mutually intersect at right angle, and they are respectively parallel to the plane that bisects an angle formed by an M-plane and an A-plane of the crystal of the hexagonal structure. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a C-plane single crystal substrate for epitaxial growth made of a crystal having a hexagonal structure, and an epitaxial wafer formed by epitaxially growing a GaN-based semiconductor crystal on the substrate, which is suitable for use in manufacturing a GaN-based semiconductor device. About.

A GaN-based semiconductor is a compound semiconductor represented by the chemical formula Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1), and is a group III nitride semiconductor. Also called a nitride semiconductor. Research and development of various semiconductor elements using GaN-based semiconductors are active, but the light-emitting elements such as light-emitting diodes (LEDs) and laser diodes (LDs) have been put into practical use the earliest. A GaN-based semiconductor light-emitting element is manufactured by epitaxially growing a GaN-based semiconductor crystal thin film having a pn junction type light-emitting element structure on a wafer-size single crystal substrate. As the single crystal substrate, a C-plane substrate made of sapphire, SiC, GaN or the like having a hexagonal crystal structure is mainly used.

In the case of a GaN-based LED element using a sapphire substrate, as a method of cutting out a chip-like element (also called a die) from a wafer, a split groove is formed on the surface of the sapphire substrate using a scriber, and this force is applied by applying an external force. The mainstream method is to break the wafer starting from the groove. On the other hand, a method of forming a split groove by wet etching on a substrate before epitaxial growth of a GaN-based semiconductor crystal has been proposed (Patent Document 1). With wet etching, not only can a large number of split grooves be formed on a single substrate at a time, but also a large number of substrates can be processed at once, greatly reducing the time spent in the process of forming the split grooves. Can do.
JP 7-169715 A JP 2002-164296 A International Publication No. 2005/62392 Pamphlet

  In the invention disclosed in Patent Document 1, no special consideration is given to the direction of the split groove formed in the substrate. However, as a result of researches by the present inventors, when a split groove for cutting out a planar rectangular semiconductor chip is formed on the surface of the C-plane substrate (when the split groove is formed so as to form an orthogonal lattice pattern). If the direction is not taken into account, the direction of the two types of split grooves (vertical split grooves and lateral split grooves) forming the lattice is not crystallographically equivalent. It has been found that the epitaxial growth mode of crystals is significantly different. In such a case, much labor is required to find an epitaxial growth condition in which a crystal having desirable properties is formed in the vicinity of both split grooves. In addition, since the film thickness and surface shape of the crystals formed in the vicinity of each of the split grooves are different, the finally obtained element has a low shape symmetry, resulting in inconvenience depending on the purpose.

  Therefore, the present invention solves the above-mentioned problems of the prior art, and the crystal growth mode in the vicinity of the dividing groove differs depending on the direction of the dividing groove while having a dividing groove for cutting out a semiconductor chip having a rectangular shape on the surface. It is a main object to provide a C-plane substrate that does not occur. Another object of the present invention is to provide an epitaxial wafer obtained by growing a GaN-based semiconductor crystal on such a C-plane substrate.

The above problems can be solved by the following invention.
(1) A C-plane single crystal substrate for epitaxial growth made of a crystal having a hexagonal crystal structure, comprising a first split groove and a second split groove formed on the epitaxial growth surface, wherein the first The dividing groove and the second dividing groove are orthogonal to each other, and each is parallel to a plane that bisects the angle formed by the M-plane and the A-plane of the hexagonal crystal. substrate.
(2) The substrate according to (1), wherein the first split groove and the second split groove are V grooves formed by wet etching.
(3) The substrate according to (1) or (2), wherein a mask that inhibits growth of a GaN-based semiconductor crystal is formed on the wall surfaces of the first and second dividing grooves.
(4) The above-mentioned (1) to (1), wherein the surface for epitaxial growth has a region that is formed into a concavo-convex surface by processing in a region where the first and second split grooves are not formed. The substrate according to any one of 3).
(5) The substrate according to any one of (1) to (4), which is a sapphire substrate.
(6) An epitaxial wafer comprising the substrate according to any one of (1) to (5) and a GaN-based semiconductor crystal epitaxially grown on the surface for epitaxial growth of the substrate.

  In the C-plane substrate provided by the present invention, the first split groove and the second split groove formed on the surface are orthogonal to each other, but the extension direction is crystallographically equivalent. In addition, the crystal epitaxial growth mode is substantially the same in the vicinity of the first split groove and in the vicinity of the second split groove. Therefore, the labor for finding an epitaxial growth condition for forming a crystal having desirable properties in the vicinity of both split grooves is reduced. Further, since the film thickness and the surface shape of the crystals formed in the vicinity of each of the split grooves are substantially the same, the problem that the finally obtained element has a low shape symmetry is solved.

  In the epitaxial wafer provided by the present invention, a GaN-based semiconductor crystal is epitaxially grown in substantially the same manner in the vicinity of the first dividing groove and in the vicinity of the second dividing groove. Therefore, in the process of cutting out elements from the wafer, the way of breaking the wafer along each split groove is substantially the same, so that the conditions in this process can be easily optimized.

[Reference experiment example]
A SiO ₂ film having a thickness of 0.5 μm was formed as an etching mask by plasma CVD on the mirror-finished surface of a C-plane sapphire substrate having one surface mirror-finished for epitaxial growth. The SiO ₂ film was provided with a stripe-shaped window using a photolithography technique. The direction of the window portion was the M-axis direction and the A-axis direction of sapphire, and the width of the window portion was a designed value of 5 μm. An etching solution prepared by heating a mixed acid of sulfuric acid / phosphoric acid mixed at a ratio of H ₂ SO ₄ : H ₃ PO ₄ = 4: 1 (volume ratio) to 300 ° C. is used for the window portion of the SiO ₂ film. The exposed sapphire surface was etched (wet etching).
By etching, a V-groove having a symmetric V-shaped cross section (hereinafter also referred to as “V-groove in the M-axis direction”) extending in the M-axis direction of sapphire is formed in the window portion formed in the M-axis direction of sapphire. It was done. The inclination of the two wall surfaces of the V-groove was 60 degrees / 60 degrees. On the other hand, in the window portion formed in the A-axis direction of sapphire, a V-groove (hereinafter also referred to as “A-axis direction V-groove”) extending in the A-axis direction of sapphire and having an asymmetric V-shaped cross section is formed. It was. The inclination of the two wall surfaces of the V-groove was 67 degrees / 40 degrees.
After etching, the SiO ₂ film was removed using buffered HF.

Following the formation of the V-groove, an SiO ₂ film having a film thickness of about 0.5 μm to 1 μm was formed on the wall surface of the V-groove as a growth inhibition mask for inhibiting the growth of the GaN-based semiconductor. The growth inhibition mask was patterned by a subtractive method. That is, after the SiO ₂ film was formed so as to cover the entire surface of the substrate, the portion except the portion formed on the wall surface of the V groove was removed by RIE using CF ₄ gas. At this time, the removal of the photoresist film residue used to protect the SiO ₂ film on the wall surface of the V-groove was performed simultaneously with the removal of the photoresist film residue used as an etching mask when forming the PSS region described later. . The photoresist film residue was removed by exposure to oxygen plasma.

After the formation of the V-groove and the growth inhibition mask on the wall surface, the region where the V-groove is formed (the “WS line” below) and the regions each having a width of 5 μm (the “WS space” below) The surface of the substrate except for “)” was processed into a concavo-convex surface on which facet structure growth of GaN-based semiconductor crystals (Patent Document 2) was possible. Specifically, the 1 μm portion is removed from the surface of the substrate by RIE (reactive ion etching) except for the portion to be left as a convex portion, and the upper surface has a diameter of 2 μm and a height as shown in FIG. An uneven surface formed by regularly arranging 1 μm frustoconical convex portions so as to form a pattern having six convex portions closest to each convex portion was formed. Note that the distance between the centers of the upper surfaces of the convex portions closest to each other was 4 μm.
Hereinafter, a region where the V-groove is formed on the substrate surface is referred to as a “WS line”, and a region where an uneven surface is formed by the RIE process is referred to as a “PSS region”. In addition, an unprocessed region sandwiched between the WS line and the PSS region on the substrate surface is referred to as “WS space”. A substrate on which WS lines and PSS regions are formed is called “WSPSS”.
FIG. 2 shows a schematic cross-sectional view of WSPSS.

A plurality of GaN-based semiconductor crystal layers are epitaxially grown on the surface of the WSPSS formed on the side where the V-groove is formed so that a pn junction type LED structure is formed using the MOVPE method. A GaN-based semiconductor film of about 8 μm was formed. Specifically, first, an undoped GaN layer was grown so as to have a flat surface through an AlGaN low-temperature buffer layer. In the process of forming the GaN layer, facet structure growth was generated so that the recesses in the PSS region were filled with GaN crystals. In order to generate facet structure growth, the growth temperature may be set low enough to form a hexagonal pyramid-shaped GaN crystal on the convex portion of the PSS region during the growth.
Following the formation of the undoped GaN layer, a Si-doped GaN contact layer, an InGaN / GaN multiple quantum well active layer, a Mg-doped AlGaN cladding layer, and a Mg-doped AlGaN contact layer were sequentially grown and laminated.

3 to 6 show SEM images of an epitaxial wafer obtained by epitaxially growing a GaN-based semiconductor film on WSPSS as described above.
FIG. 3 is an SEM image obtained by observing an epitaxial wafer having an exposed cross section substantially perpendicular to the V-groove in the M-axis direction from an oblique direction, and FIG. 4 is an enlarged view of the magnification.
FIG. 5 is an SEM image obtained by observing an epitaxial wafer having a cross section that is substantially orthogonal to the V-groove in the A-axis direction from an oblique direction, and FIG. 6 is an enlarged view of the magnification.
From comparison between FIG. 3 and FIG. 5 and comparison between FIG. 4 and FIG. 6, the epitaxial growth mode of the GaN-based semiconductor crystal is completely different between the vicinity of the V groove in the M-axis direction and the vicinity of the V groove in the A-axis direction. You can see that they are different.

[Example]
In the above-described reference experiment example, a WSPSS having a V groove in the M-axis direction and a V groove in the A-axis direction was created. However, in this embodiment, instead, the angle formed by the M-plane and the A-plane of sapphire is second order. A WSPSS having an orthogonal lattice-shaped split groove pattern composed of only V grooves parallel to the plane to be divided was prepared. The direction of the V groove on the surface of this WSPSS is schematically shown in FIG. FIG. 7 shows the direction of the V-groove on the surface of the C-plane sapphire substrate. The double arrow made of a solid line shows the direction of the V-groove in the WSPSS of this embodiment, and the double arrow made of a broken line. This is the direction of the V groove in the WSPSS of the reference experimental example.
An epitaxial wafer was produced in the same manner as in the reference experiment example except that the direction of the V groove on the surface of WSPSS was changed in this way.

SEM images of the epitaxial wafer according to this example are shown in FIGS.
FIG. 8 is an SEM image obtained by observing an epitaxial wafer with a cross-section substantially orthogonal to one V-groove exposed from an oblique direction, and FIG. 9 is an enlarged view of the magnification.
FIG. 10 is an SEM image obtained by observing the epitaxial wafer from which the cross section substantially orthogonal to the cross section shown in FIG. 8 is exposed from an oblique direction, and FIG. 11 is an enlarged view of the magnification.
From FIG. 9 and FIG. 11, it can be seen that each of the V grooves perpendicular to each other is a V groove having an asymmetric V-shaped cross section similar to the V groove in the A-axis direction in the reference experimental example.
From comparison between FIG. 8 and FIG. 10 and comparison between FIG. 9 and FIG. 11, the epitaxial growth mode of the GaN-based semiconductor crystal is very similar in the vicinity of the V-grooves orthogonal to each other. I understand that
These results clarified from the SEM observation are considered to be caused by the crystallographically equivalent directions of the respective V grooves.

  After the epitaxial growth, the thickness of the epitaxial wafer was reduced to about 80 μm by lapping the back surface of the sapphire substrate, and by applying an external force, the epitaxial wafer could be divided at the position of the WS line. Although the scribe line was not formed on the surface of the sapphire substrate by a scriber, the wafer was broken with the V groove formed by wet etching before the epitaxial growth as a split groove, and a square shape with a side of about 350 μm without any chipping. A semiconductor chip could be obtained with a high yield.

[Preferred embodiment]
The substrate of the present invention is preferably made of a sapphire C-plane substrate, but is not limited, and a C-plane substrate made of SiC, GaN, or any other crystal having a hexagonal crystal structure is used in the present invention. It can be used as a material for the substrate. Note that the C-plane substrate referred to in the present invention may have an off-angle.

  The substrate of the present invention is preferably WSPSS, particularly preferably WSPSS having WS space, but is not limited thereto. That is, it may be a WSPSS that does not have a WS space, or may be a substrate that does not have a PSS region (for example, a substrate having a V groove formed by wet etching on a flat surface). Further, in the substrate of the present invention, the dividing groove may be formed by a method other than wet etching, or the dividing groove may have a cross-sectional shape other than the V-shape. Examples of the groove forming method other than wet etching include plasma etching, dry etching processing such as RIE, machining using a dicing blade, a scriber, and laser processing.

When the dividing groove in the substrate of the present invention is a V-groove formed by wet etching or the like, the width of the V-groove is not particularly limited, and may be appropriately determined according to the size of the semiconductor chip to be manufactured. it can.
In the case of manufacturing a GaN-based semiconductor chip of 300 μm to 500 μm square and a thickness of 120 μm or less using a C-plane sapphire substrate, if a V groove is formed to a depth of 2.5 μm or more, this is used as a split groove. It becomes possible. The larger the area of the GaN-based semiconductor chip to be manufactured, the smaller the depth required for the dividing groove. When manufacturing a chip having a larger area than 1 mm square, the thickness of the C-plane sapphire substrate is 250 μm. Even so, the V groove having a depth of 3 μm functions as a split groove.
Considering the time required for wet etching, the preferred width of the V-groove is 20 μm or less, more preferably 10 μm or less.

  The substrate of the present invention preferably has a growth inhibition mask on the wall surface of the dividing groove, but it is not essential. When providing a growth inhibition mask, the material of the mask used in the selective growth of the GaN-based semiconductor crystal can be used as appropriate. Specifically, in addition to silicon oxide, silicon nitride, silicon oxynitride, titanium oxide , Zirconium oxide, tungsten and the like.

  The surface of the PSS region in WSPSS is preferably a concavo-convex surface capable of growing a facet structure of a GaN-based semiconductor crystal. As a specific uneven surface pattern, a pattern in which convex portions having shapes such as a cylinder, a truncated cone, a prism, and a truncated pyramid are regularly distributed, or concave portions having these shapes are regularly distributed. Examples thereof include a pattern, a pattern in which striped concave portions and convex portions having a rectangular or trapezoidal cross section are alternately arranged. The height of the convex portion or the depth of the concave portion on the uneven surface can be, for example, 0.1 μm to 5 μm, and preferably 0.5 μm to 2 μm. The period in the case of forming a pattern in which convex portions or concave portions are periodically arranged can be set to 1 μm to 10 μm, for example.

  As described above, the substrate of the present invention is preferably a WSPSS having a WS space, and the width of the WS space in the WSPSS can be, for example, 1 μm to 50 μm, and preferably 1 μm to 10 μm.

  The present invention is not limited to the embodiments explicitly shown above, and various modifications can be made without departing from the spirit of the invention.

It is a SEM image of the PSS area | region of the board | substrate which concerns on an Example and a reference experiment example. It is a schematic cross section of WSPSS. It is the SEM image which observed the cross section of the epitaxial wafer which concerns on a reference experiment example from the diagonal direction. It is the SEM image which observed the cross section of the epitaxial wafer shown in FIG. It is the SEM image which observed the cross section of the epitaxial wafer which concerns on a reference experiment example from the diagonal direction. 6 is an SEM image obtained by observing a cross section of the epitaxial wafer shown in FIG. It is a figure for demonstrating the relationship between the direction of the V-groove provided in the board | substrate which concerns on an Example, and the direction of the V-groove provided in the board | substrate which concerns on a reference experiment example. It is the SEM image which observed the cross section of the epitaxial wafer which concerns on an Example from the diagonal direction. FIG. 9 is an SEM image obtained by observing a cross section of the epitaxial wafer shown in FIG. It is the SEM image which observed the cross section of the epitaxial wafer which concerns on an Example from the diagonal direction. It is the SEM image which observed the cross section of the epitaxial wafer shown in FIG.

Claims (6)

A C-plane single crystal substrate for epitaxial growth made of a crystal having a hexagonal crystal structure, comprising a first split groove and a second split groove formed on the epitaxial growth surface, the first split groove and The substrate characterized in that the second dividing grooves are orthogonal to each other and are parallel to a plane that bisects the angle formed by the M-plane and the A-plane of the hexagonal crystal. The substrate according to claim 1, wherein the first split groove and the second split groove are V-grooves formed by wet etching. The substrate according to claim 1, wherein a mask that inhibits growth of a GaN-based semiconductor crystal is formed on the wall surfaces of the first and second dividing grooves. The said surface for epitaxial growth has the area | region made into the uneven | corrugated surface by the process in the area | region in which the said 1st dividing groove and the 2nd dividing groove are not formed. The substrate described. The board | substrate in any one of Claims 1-4 which is a sapphire board | substrate. An epitaxial wafer comprising: the substrate according to claim 1; and a GaN-based semiconductor crystal epitaxially grown on the surface for epitaxial growth of the substrate.