Semiconductor device

The present invention provides a semiconductor device realizing improved adhesion between a low-dielectric-constant material and a semiconductor material. The semiconductor device includes, on a semiconductor layer, an adhesion layer and a low-dielectric-constant material layer in order from the semiconductor layer side. The adhesion layer has a projection/recess structure, and the low-dielectric-constant material layer is formed so as to bury gaps in the projection/recess structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device provided with improved adhesion between a low-dielectric-constant material and a semiconductor.

2. Description of the Related Art

Power consumption of a vertical cavity surface emitting laser (VCSEL) is lower than that of an edge-emitting laser diode, and the VCSEL is possible to be directly modulated. Therefore, the VCSEL is used as a cheap light source for optical communication in recent years.

A VCSEL, generally, has a columnar mesa in which a lower DBR layer, a lower spacer layer, an active layer, an upper spacer layer, an upper DBR layer, and a contact layer are stacked in this order on a substrate. Either the lower DBR layer or the upper DBR layer is provided with a current narrowing layer having a structure that a current injection region is narrowed in order to increase the efficient of injecting current to the active layer and to decrease threshold current. An electrode is provided for each of the top face of the mesa and the rear face of the substrate. In the VCSEL, current injected from the electrode is narrowed by the current narrowing layer and then injected to the active layer, thereby generating light by recombination of electrons and holes. The light is reflected by the lower DBR layer and the upper DBR layer, laser oscillation occurs at a predetermined wavelength, and the resultant light is emitted as a laser beam from the top of the mesa.

The diameter of the mesa is at most tens and the area of the electrode on the mesa is extremely narrow. Due to this, an electrode pad for wire bonding, which is electrically connected to the electrode on the mesa is formed around the mesa. However, parasitic capacitance is generated between the electrode pad and the lower DBR layer. For direct modulation at high speed, the parasitic capacitance has to be suppressed, and a device such as insertion of a low-dielectric-constant material below the electrode pad is necessary.

The characteristics necessary for the low-dielectric-constant material are the following three points: (1) excellent heat resistance and moisture resistance at low dielectric constant, (2) easy formation of a thick film by spin coating or the like, and (3) easy patterning. Polyimide is a representative low-dielectric-constant material having all of the characteristics and is a common material formed below the electrode pad and a wiring in order to reduce parasitic capacitance.

SUMMARY OF THE INVENTION

Generally, in the case of forming a shape by polyimide, polyimide is applied in a state where polyamide acid as a precursor is dissolved on a semiconductor layer or an insulating layer, patterned, and dried and cured (imidized) at a high temperature of 300° C. or higher. During the curing, polyimide expands or contracts (in many cases, contracts). Consequently, the formed polyimide entails distortion. In addition, the thermal expansion coefficient of polyimide is different from that of a semiconductor layer and that of an insulating layer, so that there is case that a crack or peeling occurs in a subsequent process. There is also a case that a crack or peeling is caused by ultrasonic waves applied at the time of bonding a wire to the electrode pad.

Various methods to improve adhesion of an electrode have been proposed. For example, in Japanese Unexamined Patent Application Publication No. SHO59-35437, by providing the insulating layer below the electrode with a rough structure, the adhesion strength between the electrode and the insulating layer is improved. In Japanese Unexamined Patent Application Publication No. H01-308036, a method of partly cutting an insulating layer and a semiconductor substrate below an electrode to reduce parasitic capacitance is proposed. However, a general method of improving adhesion between a low-dielectric-constant material such as polyimide and a semiconductor material is not proposed.

It is desirable to provide a semiconductor device realizing improved adhesion between a low-dielectric-constant material and a semiconductor material.

According to an embodiment of the invention, there is provided a semiconductor device including, on a semiconductor layer, an adhesion layer and a low-dielectric-constant material layer in order from the semiconductor layer side. The adhesion layer has a projection/recess structure, and the low-dielectric-constant material layer is formed so as to bury gaps in the projection/recess structure.

In the semiconductor device of an embodiment of the invention, the adhesion layer having the projection/recess structure is provided between the semiconductor layer and the low-dielectric-constant material layer, and a low-dielectric-constant material layer is formed so as to bury the projection/recess structure of the adhesion layer. With the configuration, contact area between the adhesion layer and the low-dielectric-constant material layer increases.

According to the semiconductor device of an embodiment of the invention, the contact area between the adhesion layer provided between the semiconductor layer and the low-dielectric-constant material layer and the low-dielectric-constant material layer increases, so that the low-dielectric-constant material layer does not easily peel off from the adhesion layer. Therefore, adhesion between the low-dielectric-constant material and the semiconductor may be improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Modes for carrying out the invention will be described in detail below with reference to the drawings. The description will be given in the following order.

2. Manufacturing Method

3. Action and effect

Configuration

FIG. 1is a perspective view of a laser diode1of a vertical cavity surface emitting type according to an embodiment of the present invention.FIG. 2illustrates an example of the sectional configuration taken along line A-A of the laser diode1ofFIG. 1.FIGS. 1 and 2are schematic views, and dimensions and shapes inFIGS. 1 and 2are different from actual ones.

The laser diode1of the embodiment has a stacked structure20in which a lower DBR layer11, a lower spacer layer12, an active layer13, an upper spacer layer14, an upper DBR layer15, and a contact layer16are stacked in order on one face side of a substrate10. In an upper part of the stacked structure20, concretely, in a part of the lower DBR layer11and in the lower spacer layer12, the active layer13, the upper spacer layer14, the upper DBR layer15, and the contact layer16, a columnar mesa17having, for example, a width of about 30 μm is formed.

In the embodiment, the lower DBR layer11corresponds to a concrete example of a “semiconductor layer” and a “first multilayer reflector” of the invention. The upper DBR layer15corresponds to a concrete example of a “second multilayer reflector” of the invention. The stacked structure20corresponds to a concrete example of a “resonator structure” of the invention.

The substrate10is, for example, an n-type GaAs substrate. Examples of n-type impurity are silicon (Si) and selenium (Se). The stacked structure20is made of, for example, an AlGaAs-based compound semiconductor. The AlGaAs-based compound semiconductor is a compound semiconductor including at least aluminum (Al) and gallium (Ga) in the elements of group 3B in the short periodical table, and at least arsenic (As) in the elements of group 5B in the short periodical table.

The lower DBR layer11is formed by alternately stacking a low-refractive-index layer (not illustrated) and a high-refractive-index layer (not illustrated). The low-refractive-index layer is made of, for example, n-type Alx1Ga1-x1As (0<x1<1) having a thickness of λ0/4n1(λ0is oscillation wavelength and n1denotes refractive index). The high-refractive-index layer is made of, for example, n-type Alx2Ga1-x2As (0<x2<x1) having a thickness of λ0/4n2(n2denotes refractive index).

The lower spacer layer12is made of, for example, n-type Alx3Ga1-x3As (0<x3<1). The active layer13is made of, for example, undoped Alx4Ga1-x4As (0<x4<1). In the active layer13, a region opposed to a current injection region18A which will be described later is a light emission region13A. The upper spacer layer14is made of, for example, p-type Alx5Ga1-x5As (0≦x5<1). Examples of the p-type impurity are zinc (Zn), magnesium (Mg), and beryllium (Be).

The upper DBR layer15is formed by alternately stacking a low-refractive-index layer (not illustrated) and a high-refractive-index layer (not illustrated). The low-refractive-index layer is made of, for example, p-type Alx6Ga1-x6As (0<x6<1) having a thickness of λ0/4n3(n3denotes refractive index). The high-refractive-index layer is made of, for example, p-type Alx7Ga1-x7As (0<x7<x6) having a thickness of λ0/4n4(n4denotes refractive index). The contact layer16is made of, for example, p-type Alx8Ga1-x8As (0<x8<1).

In the laser diode1, for example, a current narrowing layer18is provided in the upper DBR layer15. The current narrowing layer18is provided in place of the low-refractive-index layer in the portion of the low-refractive-index layer apart from the active layer13side by, for example, a few layers in the upper DBR layer15. The current narrowing layer18has a current narrowing region18B in the peripheral region of the layer and has the current injection region18A in the center region of the layer. The current injection region18A is made of, for example, p-type Alx9Ga1-x9As (0<x9≦1). The current narrowing region18B is made of, for example, aluminum oxide (Al2O3) and, as will be described later, is obtained by oxidizing high-concentration Al contained in a layer18D to be oxidized from a side. Consequently, the current narrowing layer18has a function of narrowing current. The current narrowing layer18may be formed, for example, in the lower DBR layer11, between the lower spacer layer12and the lower DBR layer11, or between the upper spacer layer14and the upper DBR layer15.

On the top face of the mesa17(the top face of the contact layer16), an annular-shaped top electrode21having an aperture (light emitting port21A) in a region opposed to the current injection region18A is formed. An insulating layer22is formed on side faces and the surface of the periphery of the mesa17. On the surface of the insulating layer22, an electrode pad23for bonding a wire (not illustrated) and a connection part24are provided. The electrode pad23and the top electrode21are electrically connected to each other via the connection part24. A bottom electrode25is provided on the rear side of the substrate10.

The insulating layer22is made of an insulating material such as oxide or nitride. The top electrode21, the electrode pad23, and the connection part24are obtained by stacking, for example, titanium (Ti), platinum (Pt), and gold (Au) in this order, and are electrically connected to the contact layer16on the mesa17. The bottom electrode25has a structure obtained by stacking, for example, an alloy of gold (Au) and germanium (Ge), nickel (Ni), and gold (Au) in order from the substrate10side, and is electrically connected to the substrate10.

In the embodiment, for example, as illustrated inFIG. 2, a base26is provided just below the electrode pad23. The base26is formed between the foot of the mesa17(a no-formation region of the mesa17) in the lower DBR layer11and the insulating layer22. The base26is formed, for example, as illustrated inFIG. 2, in a part (11B) lower than a formation part (11A) of the mesa17. The base26has a projection/recess part26A and a burying part26B.

In the embodiment, the projection/recess part26A corresponds to a concrete example of an “adhesion layer” of the present invention. The burying part26B corresponds to a concrete example of a “low-dielectric-constant material layer” of the present invention.

FIG. 3Aillustrates an example of a sectional configuration taken along line A-A of the laser diode1ofFIG. 2while omitting the burying part26B and the insulating layer22. The projection/recess part26A is formed in contact with the lower DBR layer11and is formed integrally (continuously) with the lower DBR layer11. As illustrated inFIG. 3A, for example, the projection/recess part26A has a projection structure in which a plurality of columnar projections27are disposed at predetermined intervals. The projections27are preferably coupled to each other at the bottom of the projection/recess part26A. Preferably, the projection/recess part26A does not have an aperture which comes into contact with the part just below the projection/recess part26A in the lower DBR layer11. That is, preferably, the projection/recess part26A spatially isolates the burying part26B and the lower DBR layer11.

The projection/recess part26A may have a projection structure other than the structure illustrated inFIG. 3A.FIGS. 3B and 3Cillustrate other examples of the sectional configuration taken along line A-A of the laser diode1ofFIG. 2while omitting the burying part26B and the insulting layer22. The projection/recess part26A may have a projection structure in which a plurality of plate-shape projections28extending in a plane parallel to the substrate10are disposed at predetermined intervals. Preferably, the projections28are coupled to each other on the bottom of the projection/recess part26A. The projection/recess part26A may have, for example, as illustrated inFIG. 3C, a projection structure in which a plurality of annular projections29extending in a plane parallel to the substrate10are disposed concentrically at predetermined intervals. The projections29are preferably coupled to each other on the bottom of the projection/recess part26A.

The projection/recess part26A is made of an insulating oxide semiconductor. The projection/recess part26A is formed by, as described above, forming the lower DBR layer11, the lower spacer layer12, the active layer13, the upper spacer layer14, and the upper DBR layer15in a projection/recess structure by selective etching and, after that, oxidizing the projection/recess structure from side faces. The projection/recess structure corresponds to a projection/recess part26D (refer toFIG. 5A) which will be described later.

Width D of the projection/recess part26A (refer toFIGS. 3A to 3C) is preferably set to a degree that the entire projection/recess part26A may be oxidized (insulated) in an oxidizing process which will be described later. The width D is, for example, preferably, equal to or less than the half of the width of the mesa17(for example, 10 μm or less). With such a configuration, the projection/recess part26D is allowed to be oxidized simultaneously with the layer18D to be oxidized, and the manufacturing process is simplified. The projection/recess part26A is preferably as tall as possible from the viewpoint of reducing the parasitic capacitance generated by the electrode pad23and the lower. DBR layer11, and preferably has a height equal to that of the mesa17. The height of the projection/recess part26A is, for example, about 0.1 μm to 5 μm.

The burying part26B is formed between the projection/recess part26A and the insulting layer22so as to be in contact with the projection/recess part26A and the insulating layer22. The burying part26B is formed so as to bury the gaps in the projection/recess part26A, and the top face of the burying part26B is a flat face. The burying part26B is made of an insulting low-dielectric-constant material. As the insulating low-dielectric-constant material used for the burying part26B, for example, a material which easily buries the gaps in the projection/recess part26D by spin coating or the like and which is easily patterned is preferable. It is also important that the insulating low-dielectric-constant material has high heat resistance and high moisture resistance. Examples of the insulting low-dielectric-constant material having all of such characteristics are polyimide, BCB resin (benzocyclobutene), and the like.

Manufacturing Method

For example, the laser diode1of the embodiment may be manufactured as follows.

FIGS. 4A and 4BtoFIGS. 6A and 6Billustrate the manufacturing method in process order. Each ofFIGS. 4A and 4BtoFIGS. 6A and 6Bis a cross section taken along line A-A ofFIG. 1, illustrating the configuration of a device in a manufacturing process.

A compound semiconductor layer on the substrate10made of GaAs is formed by, for example, MOCVD (Metal Organic Chemical Vapor Deposition). As the material of a III-V group compound semiconductor, for example, trimethyl aluminum (TMA), trimethyl gallium (TMG), trimethyl indium (TMIn), or arsine (AsH3) is used. As the material of donor impurity, for example, H2Se is used. As the material of acceptor impurity, for example, dimethyl zinc (DMZ) is used.

Concretely, first, on the substrate10, the lower DBR layer11, the lower spacer layer12, the active layer13, the upper spacer layer14, the upper DBR layer15, and the contact layer16are stacked in this order (FIG. 4A). At this time, the layer18D to be oxidized is formed in a part of the upper DBR layer15. The layer18D to be oxidized is a layer which is oxidized in a oxidizing process to be described later to become the current narrowing layer18, and contains, for example, AlAs.

Next, on the surface of the contact layer16, a resist layer (not illustrated) having an aperture corresponding to a region where the electrode pad23is to be formed in a later process is formed. Subsequently, for example, by reactive ion etching (RIE), the contact layer16and the upper DBR layer15are selectively removed using the resist layer as a mask. The etching is stopped just before reaching the layer18D to be oxidized. As a result, a step30is formed in a region where the projection/recess part26D is to be formed in a later process (FIG. 4B). After that, the resist layer is removed.

Next, a circular resist layer (not illustrated) having a diameter equal to that of the mesa17is formed on the surface of the contact layer16. Further, a resist layer (not illustrated) having a pattern similar to that of the base26illustrated inFIGS. 3A to 3Cis formed on the surface of a part exposed by the preceding process in the upper DBR layer15. At this time, the width of the pattern is set to, for example, the half of the diameter of the mesa17or less in consideration of oxidization rate in the oxidizing process to be described later.

Next, for example, by RIE, using the resist layer as a mask, a part of the lower DBR layer11, the lower spacer layer12, the active layer13, the upper spacer layer14, the upper DBR layer15, the contact layer16, and the layer18D to be oxidized are selectively removed (FIG. 5A). By the operation, the mesa17is formed just below the circular-shaped resist layer. At this time, the layer18D to be oxidized is exposed from the side faces of the mesa17. Just below the resist layer having a pattern similar to that of the base26illustrated inFIGS. 3A to 3C, the projection/recess part26D having a shape in which the pattern is reflected is formed. The projection/recess part26D is formed in the part (11B) lower than the part (11A) where the mesa17is formed. After that, the resist layer is removed. The mesa17and the projection/recess part26D may be formed simultaneously or separately.

Next, the oxidizing process is performed at high temperature in water-vapor atmosphere to selectively oxidize Al included in the layer18D to be oxidized from the side faces of the mesa17and selectively oxidize Al included in the projection/recess part26D. By the oxidization, the outer edge region of the layer18D to be oxidized becomes an insulating layer (aluminum oxide) and the current narrowing layer18is formed (FIG. 5B). Further, the projection/recess part26D narrower than the current narrowing layer18is completely oxidized, thereby forming the insulating projection/recess part26A (FIG. 5B).

Next, for example, by spin coating, a photosensitive resin material such as photosensitive polyimide is applied on the entire surface including the top face of the projection/recess part26A. The photosensitive resin material is applied to a degree that the gaps in the projection/recess part26A are buried and the surface becomes almost flat. Next, by patterning, only a part corresponding to the top face of the projection/recess part26A in the applied photosensitive resin material is left. The left part is dried and solidified. The insulating burying part26B is formed on the top face of the projection/recess part26A and, as a result, the base26is formed (FIG. 6A).

Next, on the entire surface, for example, the insulating layer22made of an insulating inorganic material such as silicon oxide (SiO2) is formed (FIG. 6B). Subsequently, a resist layer (not illustrated) having an annular aperture is formed on the top face of the mesa17and, after that, the insulating layer22is selectively removed using the resist layer as a mask by, for example, RIE. By the operation, the aperture22A is formed in a part where the top electrode21is to be formed (FIG. 6B). After that, the resist layer is removed.

Next, for example, by vacuum deposition, the above-described metal materials are stacked on the entire surface. After that, for example, by selective etching, the annular top electrode21is formed so as to bury the aperture22A, the electrode pad23is formed in a part just above the base26in the insulating layer22and, further, the connection part24is formed between them (FIG. 2). Further, the rear face of the substrate10is properly polished to adjust the thickness of the substrate10, and the bottom electrode25is formed on the rear face of the substrate10(FIG. 2). In such a manner, the laser diode1of the embodiment is manufactured.

The action and effect of the laser diode1of the embodiment will now be described.

Action and Effect

In the laser diode1of the embodiment, when a predetermined voltage is applied across the bottom electrode25and the top electrode21, current is injected to the active layer13via the current injection region18A in the current narrowing layer18, and light is emitted by recombination of electrons and holes. The light is reflected by the pair of lower and upper DBR layers11and15, laser oscillation is generated at a predetermined wavelength, and the light is emitted as a laser beam from the light emitting port21A to the outside.

In the embodiment, the projection/recess part26A having the projection/recess structure is provided between the lower DBR layer11made of semiconductor material and the burying part26B made of the low-dielectric-constant material. Further, the burying part26B is formed so as to bury the projection/recess structure of the projection/recess part26A. As the contact area between the projection/recess part26A and the burying part26B becomes large, occurrence of a crack between the projection/recess part26A and the burying part26B is suppressed, and peeling-off of the burying part26B from the projection/recess part26A is suppressed. Since the projection/recess part26A is formed integrally (continuously) with the lower DBR layer11, in a normal process, a crack does not occur between the projection/recess part26A and the lower DBR layer11, and the projection/recess part26A does not peel off from the lower DBR layer11.

Consequently, a crack does not occur between the lower DBR layer11made of semiconductor material and the burying part26B made of low-dielectric-constant material, and the burying part26B does not peel off from the lower DBR layer11. Therefore, as compared with the case where the insulating low-dielectric-constant material is in direct contact with the flat semiconductor material, adhesion between the semiconductor material and the low-dielectric-constant material is improved.

In the embodiment, the insulating base26is provided between the electrode pad23and the lower DBR layer11. With the configuration, the magnitude of parasitic capacitance formed between the electrode pad23and the lower DBR layer11is reduced.

Modifications

Although the present invention has been described by the embodiment, the invention is not limited to the embodiment but may be variously modified.

For example, although the projection/recess part26A is made of the insulating oxide semiconductor in the embodiment, it may be made of another material such as conductive semiconductor. In this case, for example, as illustrated inFIG. 7, a projection/recess part27A provided in place of the projection/recess part26A may be constructed by a part of the lower DBR layer11. In this case, the projection/recess part27A is made of the same material as that of the lower DBR layer11. The projection/recess part27A may have projections27,28, or29as illustrated inFIGS. 3A,3B, and3C.

Although the present invention has been described using the AlGaAs-based compound laser diode as an example in the foregoing embodiment, it is also applicable to another compound laser diode such as a laser diode made of compound semiconductor which is oxidizable. Although the case of applying the present invention to a VCSEL has been described, obviously, the invention is also applicable to another semiconductor device.

The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-026673 filed in the Japan Patent Office on Feb. 6, 2009, the entire content of which is hereby incorporated by reference.