JP2005072148A - Nitride semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a GaN semiconductor device which is hardly deteriorated or decomposed by short wavelength light related to a GaN semiconductor element and has a die-bonding structure which has an improved reflection capability. <P>SOLUTION: A metal material is used for a bonding material 3, and the backside of a crystalline substrate S1 is metallized by a metal laminate 4 when die-bonding the GaN element 2. The metal laminate 4 at least includes a reflection layer 4a having a function to reflect light emitted inside the element structure or light targeted to be received which has passed through the element structure, to the element structure side, and a protection layer 4b existing between the reflection layer and the die-bonding material layer and having a function to protect the reflection layer from the die-bonding material layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a semiconductor device formed by bonding a nitride-based semiconductor element on a mounting substrate.

  The nitride-based semiconductor element is an element using a nitride-based semiconductor as a main part of its element structure, and includes various elements such as a light-emitting element, a light-receiving element, and a power device. For example, in the case of a light emitting element such as an LED or LD, it is possible to emit light having a short wavelength ranging from blue to ultraviolet by selecting a composition of a nitride-based semiconductor used in the light emitting layer (for example, a light emitting element). Patent Document 1) and Patent Document 2) as a light receiving element.

A nitride-based semiconductor is a compound semiconductor composed of a Group III nitride determined by the formula Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1). For example, those having an arbitrary composition such as GaN, InGaN, AlGaN, AlInGaN, AlN, and InN are exemplified. Hereinafter, “nitride-based semiconductor” is also referred to as “GaN-based semiconductor”.

  A GaN-based semiconductor element is a bare chip (also referred to as a “die”) that is separated from a semiconductor wafer as it is. To use this, the chip is fixed to a mounting substrate and connected to a circuit. Must. This is chip mounting, and fixing of the chip to the mounting substrate in this case is die bonding (die bonding). The die-bonded chip is electrically connected to a circuit on the mounting substrate by wire bonding or the like, and becomes a semiconductor device.

  When the chip is mounted on the mounting substrate, the orientation of the chip is such that the crystal substrate of the element is facing down and the back surface of the crystal substrate is bonded to the mounting substrate surface (mounting in a normal position). The electrode on the element structure laminated on the crystal substrate is joined to the circuit of the mounting substrate [upside down mounting]. In the upside down mounting, the die bond may not only be fixed but also serve as an electrical connection.

FIG. 4 is a diagram illustrating an example of a conventional GaN-based semiconductor device. In the example shown in the figure, a light emitting element (LED chip) 20 is bonded via a die bond material 30 on a To-18 stem base 10 which is a mounting substrate, with the crystal substrate 21 side (downside). Die bonding) to form a light emitting device. Illustration of wire bonding and molding to the electrodes P1 and P2 is omitted.
In the following description, “semiconductor element” and “semiconductor device” both use a GaN-based semiconductor as a main part, and the description of “GaN-based” may be omitted.

Conventionally, as a die bond material used for mounting a semiconductor element, for example, a silver paste containing an organic substance or a low melting point metal such as solder can be cited (Patent Documents 3 and 4).
JP 2000-101130 A JP 2002-280609 A JP 2003-069082 A Japanese Patent Laid-Open No. 11-8414

However, the inventors of the present invention have studied in detail about conventional bonding using silver paste. Silver paste has an advantage that it is easy to handle, but since the paste contains organic matter, Later, it was found that the following problems occurred.
The problem is that the GaN-based element mainly emits and receives blue to ultraviolet short-wavelength light, so that the organic matter in the paste is altered and decomposed by the short-wavelength light.
This altered / decomposed material absorbs the main light emission in the light emitting element, so that the normal orientation mounting impedes reflection of light upward, and in the light receiving element, the altered / degraded substance contaminates the inside of the package. The light to be received is blocked, and the light emission output and the light receiving sensitivity are lowered.

On the other hand, when a low-melting-point metal material such as solder is used as a bonding material, there is no problem due to organic matter as in the case of the silver paste. However, it has been found that this bonding method has the following problems to be improved with respect to reflectivity.
For example, when solder is used as a bonding material, metallization (providing a metal film) is previously provided on the back surface of the crystal substrate as a solder base in order to further improve the adhesion between the crystal substrate (such as sapphire) and the solder. It has been performed conventionally.

  A conventional typical metallization for solder has a two-layer structure of a base (Cr or Ni) and a surface layer Au. However, the inventors pay attention to the fact that Cr and Ni are not materials having preferable reflectivity with respect to short wavelength light peculiar to GaN-based elements. The reason why such a material is used is that the conventional metal rice is only for the purpose of further improving the bondability between the solder and the crystal substrate, and there is no idea of performing reflection on the metallized layer. Because.

  On the other hand, a technique of forming a metal film such as Al, Ag, Rh, etc. on the back surface of the crystal substrate and using it as a reflective layer is known, but if these metals are used as a metallizing material instead of Cr or Ni, solder and It was found that the alloying and adhesion with the crystal substrate decreased.

  The object of the present invention is to solve the above-mentioned problems, and to prevent short-wavelength light related to a GaN-based semiconductor element from being altered or decomposed, and to have a die bonding structure with improved reflectivity. It is to provide a semiconductor device.

The present invention has the following characteristics.
(1) A semiconductor device in which a nitride-based semiconductor element is die-bonded to a mounting substrate, the semiconductor element interposing a metal material as a die-bonding material in such a posture that the crystal substrate of the element is on the mounting substrate side Is die-bonded by
The back surface of the crystal substrate is metallized with a metal laminate,
(A) a reflective layer that functions to reflect light emitted within the element structure or light to be received that has passed through the element structure toward the element structure;
(B) a protective layer that is interposed between the reflective layer and the die bond material layer and functions to protect the reflective layer from the die bond material layer;
A nitride-based semiconductor device comprising at least the nitride-based semiconductor device.

(2) The nitride semiconductor device according to (1), wherein the metal material is solder.

(3) The nitride according to (2), wherein the solder is an alloy containing Au and Sn as main components, an alloy containing Au and Si as main components, or an alloy containing Au and Ge as main components. Semiconductor device.

(4) The nitride semiconductor device according to (1), wherein the reflective layer of the metal laminate is a layer made of one or more metal materials selected from Al, Ag, and Rh.

(5) The nitride semiconductor device according to (1), wherein the protective layer of the metal laminate is a layer made of one or more metal materials selected from Ti, Ni, Cr, Co, and Pt.

(6) The nitride semiconductor device according to (1), wherein an adhesion layer for enhancing the adhesion is further interposed between the reflective layer and the crystal substrate.

(7) The nitride semiconductor device according to (1), wherein the back surface of the crystal substrate is processed as an uneven surface, and the uneven surface is metallized with the metal laminate.

  In the present invention, first, a metal material (so-called brazing material) is used as a die-bonding material, thereby solving the problem of deterioration and decomposition of organic substances found in silver paste. Next, in order to use a metal material as a die bond material, metallization is necessary for the back surface of the crystal substrate. In the present invention, the metallized layer is a multilayer including a reflective layer, a protective layer, and an adhesion layer. By adopting a structure, it is possible to ensure both good reflectivity and good adhesion to the crystal substrate. With this die bond structure, particularly in a light-emitting element, reliable mounting and improvement of light extraction efficiency can be achieved.

Hereinafter, the present invention will be described using a case where a GaN LED chip is die-bonded on a stem as an example.
As shown in FIG. 1, the semiconductor device of the present invention has a structure in which a GaN-based semiconductor element 2 is die-bonded to a mounting substrate 1 with a die-bonding material 3. The die bond material is a metal material (particularly a low melting point metal having a melting point of 500 ° C. or lower) used for brazing. The element 2 has a crystal substrate S 1, and the back surface of the crystal substrate S 1 is metallized by a metal laminate 4.
The metal laminate includes at least the reflective layer 4a of (a) in (1) and the protective layer 4b of (b).

  There are no limitations on the types and functions of the semiconductor elements, and various elements such as light emitting elements such as LEDs and semiconductor lasers, light receiving elements, and other power devices may be used. In the present invention, since measures against blue light to ultraviolet light are given, the usefulness of the present invention is particularly remarkable when the element emits or receives light having a wavelength of 500 nm or less, particularly 420 nm or less.

The semiconductor element 2 illustrated in FIG. 1 is an LED, and an n-type GaN contact layer S2, a light emitting layer (active layer) S3, a p-type AlGaN cladding layer S4, and a p-type GaN contact layer S5 are sequentially formed on a crystal substrate S1. It has a laminated structure formed by epitaxial growth.
The composition of each layer and the number of stacked layers are examples, and the contact layer and the clad layer may be separate or combined, and the light emitting layer may have a stacked structure such as a multiple quantum well structure. Further, an additional GaN-based crystal layer may be added as necessary. In addition, the stacked body S may partially include a structure (such as a SiO ₂ mask pattern described later) made of a material other than a GaN-based material.

The crystal substrate of the device may be a crystal substrate on which a GaN-based crystal can be epitaxially grown. Examples of the material include sapphire (C plane, A plane, R plane), SiC (6H, 4H, 3C), GaN, AlN. , Si, spinel, ZnO, GaAs, NGO and the like.
A structure necessary for growing a high-quality GaN-based crystal layer on the crystal substrate may be included as appropriate. For example, a buffer layer (especially a so-called low-temperature growth buffer layer in which GaN, AlN, etc. are grown at a low temperature) is interposed, a SiO ₂ mask pattern or unevenness is formed on the crystal substrate surface, and a GaN crystal is laterally grown or faceted. Examples include a structure that grows and lowers the dislocation density.

  The die bond material used in the present invention may be any metal material that can function as a die bond material by being interposed between the mounting substrate and the element. Examples of such a metal material include In, for example, Pb—Pd, Au—Ga, Au—Ge, Au—Si, Al—Zn, Al, as exemplified as the bonding material in the above-mentioned patent document. -Ge, Al-Mg, Au-In, Ag-Mg, Al-Cu, Al-Si, Al-Si-Mg, Al-Si-Zn, Al-Pd, Al- In-based, Cu-Ge-based, Ag-Ge-based, Cu-In-based, Au-Zn-based, or Au-Sn-based alloys can be given.

Among the die bond materials, metal materials having a melting point of 500 ° C. or less, particularly metal materials having a melting point of 400 ° C. or less are preferable. An example of such a metal is solder. Preferable solder includes an alloy mainly composed of Au and Sn, an alloy mainly composed of Au and Si, or an alloy mainly composed of Au and Ge. Among these, eutectic solder is particularly preferable since it has a lower melting point.
If it is a low melting point metal as described above, (i) bonding can be performed at low temperature and manufacturing efficiency is good. (B) Because of bonding at low temperature, the reaction between the die bond material and the reflective layer is sufficiently prevented by a thin protective layer. If the GaN-based device includes a layer containing In (eg, an InGaN layer), the decomposition temperature of the layer is relatively low and heat-sensitive. For example, such a feature that the layer is not thermally damaged can be obtained.
Even if the die bond material is a low melting point metal as described above, sufficient heat resistance can be ensured for general applications.

  The mounting substrate may be any substrate for die-bonding the element, and examples thereof include a light emitting element, a light receiving element stem, and a planar mounting ceramic substrate.

The reflective layer (a) contained at least in the metal laminate is preferably made of a material that can preferably reflect short-wavelength light related to light emission and light reception. Examples of such materials include Al, Ag, and Rh.
The thickness of the reflective layer is preferably 10 nm or more, particularly preferably 50 nm or more. The upper limit of the thickness is not limited, but about 100 μm or less is preferable for a normal element.

On the other hand, among the metal materials that can be used as the reflective layer, there are those that are not preferable as the metallized layer material because of poor adhesion to the crystal substrate. For such materials, it is preferable to interpose a metal thin film (adhesion layer) for enhancing adhesion between the reflective layer and the crystal substrate.
Examples of the material for the adhesion layer include Ti, Ni, Cr, Co, and Pt. The thickness of the adhesion layer is preferably 0.1 nm to 100 nm, and particularly preferably 1 nm to 10 nm so that the light to be reflected can be transmitted.

Examples of the material for the protective layer include Ti, Ni, Cr, Co, and Pt.
Further, the thickness of the protective layer is preferably 1 nm to 1000 nm, particularly preferably 10 nm to 100 nm.

  In the above example, the metal laminate has two or three layers, but the number of layers is not limited. For example, when die bonding is performed with a eutectic solder containing Au, an Au layer (for example, 4c in FIG. 1) for improving adhesion with the eutectic solder and other necessary functional layers are optionally added 4 It may be a five-layer structure.

  As a preferred embodiment of the present invention, as shown in FIG. 2, there is an embodiment in which the uneven surface f is processed on the back surface of the crystal substrate to form an uneven surface, and the uneven surface is metallized by the metal laminate. When the die bond material enters the irregularities, the die shear strength (bonding strength between the chip and the mounting substrate) is improved. Since the metallized layer (metal laminate) is actually thinner than the unevenness, the illustration thereof is omitted in FIG.

The arrangement pattern of the irregularities when the irregular surface is viewed is not limited, and is a pattern in which dot-like concave portions are regularly or irregularly arranged, a stripe pattern in which groove-like concave portions are arranged in parallel, or the like. Good. The cross-sectional shape of the unevenness may be a rectangular wave shape, a sawtooth wave shape, a sine curve shape, or the like.
The amplitude of the unevenness (= recess depth) and the dimensions of the individual recesses and protrusions are not limited, but it is preferable that the unevenness remains even after the metallized layer is formed and the die-bonding material sufficiently enters. For example, taking a dimension example with a stripe pattern having a cross-sectional shape of a rectangular wave, the amplitude of the unevenness is preferably about 1 μm to 100 μm, and the width of the concave portion and the width of the convex portion are preferably about 1 μm to 100 μm, respectively.
Although the processing method of unevenness | corrugation is not specifically limited, The processing method by a dicing saw, an etching, etc. is mentioned.

  For the method itself for forming the metal laminate on the back surface of the crystal substrate, a known technique may be referred to. For example, a vacuum deposition method, a sputtering method, a plating method and the like are preferable. A layer in contact with the back surface of the crystal substrate is preferably formed by a sputtering method because adhesion is improved.

Example 1
In this embodiment, as shown in FIG. 3, a metal laminate 4 including an Al layer 4a, a Ti layer 4b, and an Au layer 4c is formed on the back surface of the sapphire substrate of the GaN-based light receiving element (chip stage) in this order from the substrate side. By doing so, the back surface of the sapphire substrate was metallized.
The Al layer acts as a reflective layer, and the Au layer acts as a layer that improves adhesion with the AuSn eutectic solder. The Ti layer between the Al layer and the Au layer serves as a protective layer that prevents the reaction between Al and Au by heat during die bonding.
This chip was die-bonded to a stem base using AuSn eutectic solder as a die-bonding material, and sealed in a can package to produce a light receiving device (example product).

[Structure of light receiving element]
The light receiving element is a Schottky type and has a laminated structure in which an n-type GaN contact layer S2 and an i-type GaN light receiving layer S3 are epitaxially grown in order on a sapphire substrate S1, as shown in FIG. An ohmic electrode P2 is formed on the surface of the n-type GaN contact layer S2, and a transparent Schottky electrode P1 is formed on the surface of the i-type GaN light receiving layer S3. The light receiving element 2 is mounted on the stem base 1 via a die bond material 3 in a normal posture with the sapphire substrate facing down.

In FIG. 3, an AlN low-temperature buffer layer for reducing the dislocation density is interposed between the sapphire substrate S1 and the n-type GaN contact layer S2, but the illustration is omitted.
[Production of Example Products]
At the stage where the light receiving element is a semiconductor wafer, the metallized layer 4 comprising the Al layer 4a (thickness 230 nm) / Ti layer 4b (thickness 20 nm) / Au layer 4c (thickness 200 nm) is formed on the back surface of the sapphire substrate by sputtering. Formed.
The semiconductor wafer is divided to obtain a chip, which is mounted on a TO-8 stem base via AuSn solder pellets, and the stem 3 is heated and cooled to melt and solidify the solder 3 to obtain a light receiving element chip. Die-bonded on the stem base.
Finally, in a nitrogen atmosphere, as shown in FIG. 3, the cap with a glass window for light reception was canned, and the light receiving device of the present invention was obtained.

Comparative Example 1
A chip having the same specifications as in Example 1 above is die-bonded to a stem base using Ag paste as a die-bonding material as before, encapsulated in a can package, a light receiving device (comparative example product) is produced, and compared with the example product. went.
[Production of comparative example products]
A light receiving element that was not metallized on the back surface of the sapphire substrate was mounted on a TO-8 stem base via Ag paste, and the light paste was die-bonded on the stem base by baking the Ag paste in an oven.
Finally, in a nitrogen atmosphere, the cap with a glass window for light reception was canned, and a light receiving device as a comparative product was obtained.

[Evaluation]
The product of the example and the comparative example were irradiated with laser light having a wavelength of 193 nm using an ArF excimer laser device, and the change in light receiving sensitivity with time was examined.
In the product of the example, there is no decrease in light receiving sensitivity due to the lifetime of the element, and the light receiving surface of the element (light incident surface into the element) and the inner surface of the light receiving window of the can package are contaminated. There was no decrease in light receiving sensitivity caused by the die bonding material.
On the other hand, in the comparative product, the light receiving sensitivity of the element decreased to 50% of the initial value after 180 seconds from the start of irradiation. When this light-receiving device was disassembled after the experiment, the silver paste was discolored black, and the organic matter contained in the silver paste was altered to contaminate the light-receiving surface of the element and the inner surface of the light-receiving window of the can package. It has been found that this is an obstacle to the incidence of light.

Example 2
In this example, AuSn eutectic solder was used as a die bond material, and a GaN-based LED chip was die-bonded to a stem base to produce a light emitting device (example product). Similarly to Example 1, only the example product was metallized on the back surface of the crystal substrate of the light-emitting element by Al / Ti / Au in order from the back surface.

[Element structure of light emitting element chip]
As shown in FIG. 1, the prepared chip is obtained by epitaxially growing an n-type GaN contact layer S2, a light emitting layer (active layer) S3, a p-type AlGaN cladding layer S4, and a p-type GaN contact layer S5 in this order on a sapphire substrate S1. It has a laminated structure. An n-type ohmic electrode P2 is formed on the surface of the n-type GaN contact layer S2, and a p-type ohmic electrode P1 is formed on the surface of the p-type GaN contact layer S5. The light emitting element 2 is mounted on the stem base 1 via the die bond material 3 with the sapphire substrate S1 facing downward.
The light emission wavelength of the element is 405 nm.

[Production of Example Products]
When the light-emitting element is a semiconductor wafer, metallization comprising an Al layer 4a (thickness 230 nm) / Ti layer 4b (thickness 20 nm) / Au layer 4c (thickness 200 nm) is sequentially performed on the back surface of the sapphire substrate by sputtering. A layer was formed. The Al layer 4a functions as an adhesion / reflection layer, the Ti layer 4b functions as a protective layer that prevents the reaction between Al and Au, and the Au layer 4c functions as a layer that improves solder wettability.

  The semiconductor wafer is divided to obtain a GaN-based LED chip 2, which is mounted on the TO-18 stem base 1 through AuSn solder pellets, and the stem 3 is heated and cooled to melt and solidify the solder 3. The LED chip 2 was die-bonded on the stem base 1 to obtain the light emitting device of the present invention.

Comparative Example 2
A light emitting device was manufactured in the same manner as in Example 2, except that the metallized layer was an Al layer (thickness: 230 nm) in Example 2.

Comparative Example 3
A light emitting device was manufactured in the same manner as in Example 2 except that the metallized layer was a base Cr (thickness 20 nm) / surface layer Au (thickness 200 nm) in Example 2.

[Evaluation]
When a continuous lighting test for 4000 hours was performed using the product of Example 2 and the products of Comparative Examples 2 and 3, the product of Example 2 had a decrease in light emission intensity due to the element lifetime (5 for 1000 hours). However, there was no change in the die bond material part, no absorption at that part, no contamination of the element surface, and no reduction in light emission intensity caused by die bonding. It was.
On the other hand, in the product of Comparative Example 2, the initial value of the light emission intensity of the device was 60% of the product of the example. When this light emitting device is disassembled, Al is alloyed with the bonding material solder and discolored, and this is an obstacle to light extraction from the element as a layer that absorbs light emitted downward. I understood.
In the product of Comparative Example 3, the initial value of the light emission intensity of the device was 80% of that of the Example product. The reason is that, as described in the description of the prior art, the Cr layer is a layer that absorbs light emitted downward, and is an obstacle to light extraction from the element.

Example 3
In this example, the metal layer formed as the metallized layer in Example 2 was sequentially formed from the back surface of the sapphire substrate: Ni layer (thickness 3 nm) / Ag layer (thickness 200 nm) / Ti layer (thickness 20 nm) / Au layer A light-emitting device was manufactured in the same manner as in Example 2 except that a four-layer structure (thickness: 200 nm) was used.
When this example product was compared with the example product obtained in Example 2, the emission intensity equivalent to that of the Example product according to Example 2 was obtained.
In this embodiment, Ni having good adhesion is used for the metal in contact with the sapphire substrate. The reflectance of Ni is lower than that of Al used in Example 2, but the thickness of the Ni layer is 3 nm, which is sufficiently thin to transmit light, so that the light transmitted through the Ni layer is an Ag layer with good reflectance. It is considered that high luminescence intensity was obtained by reflection.

Example 4
In this example, except that the back surface of the sapphire substrate of the LED chip used in Example 2 was subjected to a striped concavo-convex process having a width of 10 μm, a depth of 10 μm, and a pitch of 10 μm using a dicing saw. A light emitting device was manufactured in the same manner as described above.
When this example product was compared with the example product obtained in Example 2, the die shear strength (adhesion strength of the chip) of this example product was 750 g or more, and the die shear strength of Example 2 (500 g). The strength was 1.5 times or more. This is presumably because the adhesive strength was increased by the solder material flowing into the concavo-convex portion.

As described above, the semiconductor device according to the present invention does not change the bonding material even if the element emits and receives light having a short wavelength (blue light to ultraviolet light) peculiar to GaN-based semiconductors. The problem of the metallized layer found when using a low melting point metal as a bonding material is solved, and the reflectivity and adhesion are improved.
According to the present invention, deterioration of sensitivity is improved in the light receiving element, light extraction efficiency is improved in the light emitting element, and in any case, mounting reliability is improved.

It is a figure which shows an example of the structure of the GaN-type semiconductor device of this invention. It is a figure which shows the aspect which provides an unevenness | corrugation in the back surface of a crystal substrate in the GaN-type semiconductor device of this invention. It is a figure which shows the structure of the GaN-type semiconductor light-receiving device manufactured in Example 1 of this invention. Illustration of wire bonding is omitted. It is a figure which shows the structure of the conventional GaN-type semiconductor device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate for mounting 2 Nitride semiconductor element 3 Die bond material 4 Metal laminate

Claims (7)

A semiconductor device in which a nitride-based semiconductor element is die-bonded to a mounting substrate, and the semiconductor element is formed by interposing a metal material as a die-bonding material so that the crystal substrate of the element is on the mounting substrate side. Die-bonded,
The back surface of the crystal substrate is metallized with a metal laminate,
(A) a reflective layer that functions to reflect light emitted within the element structure or light to be received that has passed through the element structure toward the element structure;
(B) a protective layer that is interposed between the reflective layer and the die bond material layer and functions to protect the reflective layer from the die bond material layer;
A nitride-based semiconductor device comprising at least the nitride-based semiconductor device.   The nitride semiconductor device according to claim 1, wherein the metal material is solder.   The nitride semiconductor device according to claim 2, wherein the solder is an alloy mainly composed of Au and Sn, an alloy mainly composed of Au and Si, or an alloy mainly composed of Au and Ge.   The nitride semiconductor device according to claim 1, wherein the reflective layer of the metal laminate is a layer made of one or more metal materials selected from Al, Ag, and Rh.   The nitride semiconductor device according to claim 1, wherein the protective layer of the metal laminate is a layer made of one or more metal materials selected from Ti, Ni, Cr, Co, and Pt.   The nitride-based semiconductor device according to claim 1, wherein an adhesion layer for enhancing the adhesion is further interposed between the reflective layer and the crystal substrate. The nitride semiconductor device according to claim 1, wherein the back surface of the crystal substrate is processed as an uneven surface, and the uneven surface is metallized by the metal laminate.

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