WO2009060195A1

WO2009060195A1 - Composite material

Info

Publication number: WO2009060195A1
Application number: PCT/GB2008/003740
Authority: WO
Inventors: Christopher John Howard Wort; Andrew John Whitehead; Richard Stuart Balmer
Original assignee: Element Six Limited
Priority date: 2007-11-06
Filing date: 2008-11-06
Publication date: 2009-05-14
Also published as: EP2218099A1; GB0721760D0

Abstract

The present invention relates to composite material comprising a layer of diamond material chemically bonded to a surface of a layer of silicon material, wherein the layer of diamond material comprises a mixture of diamond particles, silicon carbide and silicon and methods for producing the same.

Description

COMPOSITE MATERIAL

The present invention provides composite materials which are suitable as substrates for growing Group III nitride semiconductor devices. In particular, the present invention relates to composite materials comprising a layer of silicon material chemically bonded to a surface of a layer of diamond material and methods for producing the same.

Group III nitride semiconductor materials are used extensively in the electronics industry, in particular in light emitting diodes (LEDs) and high power, high frequency transistor assemblies. In addition, these materials have been recognized to be some of the most promising materials for fabricating optical devices in the visible short- wavelength and UV region. Examples of such materials include gallium nitride (GaN), aluminium nitride (AlN), which are both wide band gap semiconductor materials, and their alloys.

In order that the Group III nitride material has the desired properties, high quality epitaxial monocrystalline layers of the nitride are generally required. High quality crystals are difficult to synthesise and there are only a limited number of substrates upon which crystals of a suitable quality can be grown. Suitable substrates include {111 } silicon, silicon carbide and hexagonal basal plane (0001) sapphire (Al₂O₃).

Unwanted heat generation is a problem which is encountered increasingly frequently in the electronics industry. This is particularly the case in semiconductor assemblies which are typically subject to temperature cycling during their operation. For this reason, it is common for thermal management components to be included in such assemblies. Thermal management components generally comprise heat sinks used with or without discrete heat spreaders. Heat spreaders are made of materials with high thermal conductivity (typically > 170 Wm⁴K^"1.) and can greatly improve the overall efficiency of heat removal from a system.

Therefore, in order to optimize performance, the substrates upon which Group III nitride materials are grown need to act as a heat spreader. Silicon and hexagonal sapphire both have a comparatively poor thermal conductivity and thus the performance of a Group III nitride grown on such a substrate is thermally limited in the absence of any further thermal management component.

Whilst this problem can be reduced to an extent by using SiC substrates in place of Si substrates, such substrates are expensive and of limited availability, particularly as large size wafers.

Diamond is a material known to have a very high thermal conductivity and therefore lends itself to use in applications where there is a need to remove heat. For this reason, attempts have been made to use diamond as a substrate for Group III nitride crystal synthesis. However, due to a mismatch between the lattice of diamond and the lattice of Group III nitrides, the crystal structure of diamond does not permit the growth of high quality epitaxial single crystal layers of Group III nitrides thereon.

In view of this, work has focused on developing alternative ways in which diamond material can be employed to remove heat from electronic assemblies, in particular Group III nitride semiconductor assemblies.

WO2006/100559 describes a composite material, for use as a substrate, which comprises silicon and chemical vapour deposition (CVD) diamond. The CVD diamond layer is intimately attached to a silicon surface. In order to ensure that the heat spreading capabilities of the composite material are not limited by the comparatively poor thermal conductivity of the silicon layer, the thickness of the silicon layer must be kept to a minimum. However, this means that the overall thermal expansion coefficient of the material is dominated by the thicker CVD diamond layer. At any interface between materials having different thermal expansion coefficients, stress will be generated. As there is a large difference between the thermal expansion coefficients of GaN and diamond, it is not desirable for the thermal expansion coefficient of the composite material to be dominated by that of the diamond layer.

A similar solution is described on http://www.sp3inc.com, wherein a much thinner layer of CVD diamond is grown onto a layer of { 111 } silicon, which is then attached to a bulk, polycrystalline Si wafer. However, the improvement in heat spreading capabilities as compared to using SiC as a substrate are minimal because the diamond layer is so thin.

Thus, whilst these solutions employ the use of diamond material, they suffer from several drawbacks making them far from ideal. To date it has not been possible to simultaneously exploit both the thermal conductivity properties of diamond and thermal coefficient of expansion properties of silicon.

Hence, there is a need for a solution by which is it possible to exploit the high thermal conductivity of diamond material, in particular in Group III nitride semiconductor assemblies, without any ensuing loss in the quality of the Group III nitride semiconductor material and without introducing additional interfacial stresses into the resulting assembly.

In this regard, the present invention provides a composite material comprising a layer of diamond material chemically bonded to a surface of a layer of silicon material, wherein the layer of diamond material comprises a mixture of diamond particles, silicon carbide and silicon.

The present inventors have surprisingly found that it is possible to form a chemical bond between a layer of diamond material which is, itself a composite material, and a layer of silicon material. Thus, a composite material which overcomes the problems associated with the prior art is provided. More specifically, the diamond material which forms a part of the composite material of the present invention has both a high thermal conductivity, as a consequence of the presence of the diamond particles and, additionally, benefits from having a coefficient of thermal expansion which is very similar to that of silicon material. The coefficient of thermal expansion of the diamond material which forms a part of the composite material of the present invention is also substantially closer to the coefficients of thermal expansion of Group III nitride semiconductor materials than that of diamond. Therefore, advantageously, the composite material of the present invention, when combined with a { 111 } silicon surface, for example, lends itself to use as a substrate for growth of Group III nitride semiconductor layers. In addition, by use of a diamond material which is itself a composite, it is possible to control carefully the specific thermal properties of the composite material.

In addition, the present inventors have surprisingly found that a chemical bond between a layer of diamond material which is, itself a composite material, and a layer of silicon material can be formed using techniques which are known conventionally. In this regard, the present invention further provides a method for producing a composite material comprising chemically bonding a layer of silicon material to a surface of a layer of diamond material, wherein the diamond material comprises a mixture of diamond particles, silicon carbide and silicon. The present invention thus provides a simple and cost effective method for producing such composite materials.

Advantageously, the chemical bond can be achieved using silicon direct wafer bonding techniques i.e. for attaching a single crystal silicon wafer to another single crystal silicon wafer or for attaching a polycrystalline silicon wafer to another polycrystalline silicon wafer or for attaching a polycrystalline silicon wafer to another single crystal silicon wafer.

As described above, the composite materials of the present invention are particularly suitable for use as substrates for Group III nitride semiconductor growth. Accordingly, the present invention further provides the use of a composite material as defined above, as a substrate for growing a Group III nitride semiconductor device.

Once a Group III nitride semiconductor has been grown onto the composite material of the present invention, the resulting material is useful in the production of semiconductor assemblies, such as transistors and diodes. In this regard, the present invention is further directed to electronic devices comprising a composite material as defined herein.

The term "chemically bonded" as used herein is intended to require that a direct or indirect chemical bond is formed between a surface of the layer of diamond material and a surface of a layer of the silicon material. It is intended to include all types of chemical bonds which may be formed between two atoms, specifically, covalent bonds, ionic bonds, Van der Waals bonds and hydrogen bonds. There is no layer of a heterogeneous material, such as an adhesive, interposed between the surface of the layer of diamond material and the surface of the layer of silicon material. The two atoms between which the chemical bond is formed may, in the case of direct chemical bonding, be contained within the two layer(s) of material or may, alternatively, in the case of indirect chemical bonding, be an atom which functionalises the surface of the layer(s), such as, for example, an oxygen atom where either or both of the surfaces are, for example, oxygen terminated. In particular, the bonds formed between the layer of silicon material and a surface of the layer of diamond material may be Si-O-Si bonds.

The chemical bond formed is sufficiently strong that the two layers are adhered together and remain so, even during subsequent handling and processing. In this regard, the interfacial bond strength is preferably greater than about 50 MPa, preferably greater than about 100 MPa, preferably greater than about 200 MPa. Preferably the interfacial bond strength of the chemical bond is comparable with the bulk strength of the layer of silicon material. Whilst the term "chemically bonded" is intended to allow for the possibility that one or both of the surfaces of the two layers respectively has been chemically functionalised, e.g. hydrogen or oxygen terminated, it is intended to exclude the possibility that a further layer with a thickness of greater than about 1 nm, preferably greater than about 0.75 nm, preferably greater than about 0.5 nm is positioned between the layer of diamond material and the layer of silicon material.

Preferably the layer of silicon material is chemically bonded to a surface of the layer of diamond material across at least about 60%, preferably at least about 65%, preferably at least about 70%, preferably at least about 75%, preferably at least about 80%, preferably at least about 85%, preferably at least about 90%, preferably at least about 95%, preferably substantially all of the surface area over which the two layers are in contact.

The layer of diamond material of the composite material of the present invention is, in itself, a composite material. More specifically, the diamond material consists of three phases, specifically a diamond phase comprising diamond particles, a silicon carbide phase and an unreacted silicon phase. Preferably, the silicon carbide forms an interconnected skeletal material structure substantially surrounding each individual diamond particle and the silicon fills the remaining volume of the silicon carbide skeleton. For this reason, such diamond material is often referred to as "skeleton cemented diamond" or "ScD". Preferably, the resultant material is substantially free of porosity.

It is possible to control the thermal properties of the diamond material by selecting the specific proportions of diamond particles, silicon carbide and silicon which are present. Examples of suitable diamond materials for the layer of diamond material of the composite material of the present invention are disclosed in EP1337497.

Preferably the diamond material comprises diamond particles in an amount of about 55% by volume or more, preferably about 60% by volume or more, preferably about 65% by volume or more, preferably about 70% by volume or more, preferably about 75% by volume or more. Preferably, the diamond material comprises diamond particles in an amount of about 85% by volume or less, preferably about 80% by volume or less, preferably about 75% by volume or less. The amount of diamond particles in the layer of diamond material of the composite material of the present invention is preferably in the range from about 55% to about 85% by volume, preferably from about 58% to about 81% by volume, preferably from about 60% to about 80% by volume, preferably from about 65% to about 75% by volume. This is advantageous as it ensures that the diamond material has a suitable thermal conductivity.

Preferably at least about 50% by weight, preferably at least about 55% by weight, preferably at least about 60% by weight, preferably at least about 65% by weight, preferably at least about 70% by weight of the diamond particles in the diamond phase of the layer of diamond material have a diameter of about 80 μm or more. Preferably, the at least about 50% by weight, preferably at least about 55% by weight, preferably at least about 60% by weight, preferably at least about 65% by weight, preferably at least about 70% by weight of the diamond particles in the diamond phase of the layer of diamond material which have a diameter of about 80 μm or more, have a nitrogen content of about 300 ppm or less, preferably about 250 ppm or less, preferably about 200 ppm or less, preferably about 150 ppm or less, preferably about 100 ppm or less, preferably about 50 ppm or less. In order to ensure that the packing of the diamond particles within the layer of diamond material is appropriate, preferably the diamond particles in the diamond phase have a minimum diameter of about 6 μm.

Preferably the diamond particles in the diamond phase of the layer of diamond material are approximately equiaxed.

Advantageously, the diamond phase comprises at least two different fractions wherein the average (mean) diameter of the diamond particles in each fraction is different. This is advantageous as it means that the packing of the diamond particles in the layer is sufficiently close to provide a high diamond concentration and hence high thermal conductivity and thermal diffusivity.

Preferably the diamond material comprises silicon in an amount of less than about 45%, preferably less than about 40% by volume, preferably less than about 35% by volume, preferably less than about 30% by volume. Preferably, the diamond material comprises silicon in an amount of at least about 0.5% by volume, at least about 1% by volume, at least about 2% by volume.

Preferably the diamond material of the present invention has a silicon carbide content of about 2% by volume or more, preferably about 4% by volume or more, preferably about 5% by volume or more, preferably about 10% by volume or more, preferably about 15% by volume or more. Preferably, the diamond material of the present invention has a silicon carbide content of about 45% by volume or less, preferably about 40% by volume or less, preferably about 35% by volume or less, preferably about 30% by volume or less.

Preferably, the layer of diamond material of the composite material of the present invention has a silicon carbide content in the range from about 2% to about 45% by volume, preferably from about 3 % to about 41% by volume, preferably from about 4% to about 40% by volume, preferably from about 5% to about 35% by volume, preferably from about 10% to about 30% by volume. Diamond has a coefficient of thermal expansion which is lower than the coefficient of thermal expansion of silicon. In contrast, silicon carbide has a coefficient of thermal expansion which is higher than that of silicon. By including the diamond, silicon and silicon carbide in the amounts defined above, a balance of thermal expansion coefficients is achieved which means that the coefficient of thermal expansion of the diamond material is closely matched to the coefficient of thermal expansion of the layer of silicon material.

Advantageously the diamond material has a high thermal conductivity which ensures that the removal of heat from a device into which the composite material is incorporated is minimised. In addition, the diamond material of the composite material of the present invention has a particularly good thermal conductivity because the silicon carbide grows epitaxially on the diamond particles. This means that phonon transport, which is the most effective way of transporting heat through a body, is enhanced.

Preferably the diamond material has a thermal conductivity at about 300 K of about 400 Wm^-1K-Or more, preferably about 450 Wm^-1K-Or more, preferably about 500 Wm^-1K^-1 or more, preferably about 550 Wm^-1K^-1 or more.

The diamond material of the composite material of the present invention preferably has a coefficient of thermal expansion (CTE) in the range from about 2 to about 4 pprnK^'1, preferably in the range from about 2.5 to about 3.5 ppmK^"1. The CTE of such diamond material is much more closely matched to the CTE of the layer of silicon material, thus minimising generation of strain and subsequent bending of the device during operation.

Methods for producing the layer of diamond material of the composite material of the present invention are described in US 6,447,852, US 6,709,747 and US 2004/247873. These methods generally involve the steps of forming a green body of the desired dimensions from a blend of diamond particles and silicon carbide, heating the green body under controlled temperature conditions to create a desired amount of carbon by graphitisation of diamond particles, infiltrating molten silicon into the partially graphitised body and reacting the molten silicon and graphite to form silicon carbide. In order to avoid the diamond material being "lossy" at RF frequencies i.e. the generation of currents therein during use and subsequent consumption of energy, it is preferred that a high purity silicon source is used during the preparation of the layer of diamond material. In this regard, the present invention provides a method for forming diamond material comprising: forming a green body from a blend of diamond particles and silicon carbide; heating the green body to cause a controlled amount of graphitisation; infiltrating molten high purity silicon into the green body to react with non-diamond carbon produced by graphitisation, to form silicon carbide. As used herein, the term "high purity silicon" refers to a silicon source in which the content of impurities such as Al, N and B in the silicon source are minimised.

Preferably, the levels of impurities (that is combined content of Al, N and B) in the silicon source are less than about 20 ppm, preferably less than about 15 ppm, preferably less than about 10 ppm, preferably less than about 5 ppm. High purity silicon is widely available. An example of a supplier of a suitable material in powder or lump form is ESPI Metals, Ashland, Oregon, USA.

The present invention also further relates to diamond material comprising a mixture of diamond particles, silicon carbide and silicon, wherein the combined content of Al, N and B impurities in the silicon is less than about 40 ppm, preferably less than about 30 ppm, preferably less than about 20 ppm, preferably less than about 10 ppm.

As a consequence of its method of manufacture, a polycrystalline silicon layer is formed on outer surfaces of the diamond material where the molten silicon infiltrates and exits the green body. Conventionally, this silicon layer is removed by grit blasting as its presence is understood to be undesirable. However, the present invention is based on the inventor's realisation that the presence of this silicon layer is advantageous. More specifically, as a surface of the diamond material is actually a continuous silicon surface, it is possible to chemically bond a layer of silicon material thereto.

The thickness of the layer of diamond material is preferably about 80 μm or more, preferably about 200 μm or more, preferably about 300 μm or more. Preferably the thickness of the layer of diamond material is about 5 mm or less, preferably about 2 mm or less, preferably about 1 mm or less.

Preferably the surface of the diamond material to which the layer of silicon material is chemically bonded is a continuous silicon surface. The term "continuous" as used herein refers to a surface where there is a layer of silicon across the entire surface area thereof. The term "continuous" does not require that the layer of silicon must have a uniform thickness across the surface area.

Although the dimensions of the layer of diamond material will depend on the ultimate use of the composite material, in order to maximise the heat spreading capabilities of the composite material, preferably the layer of diamond material has a longest dimension of about 15 mm or more, preferably about 20 mm or more, preferably about 25 mm or more, preferably about 35 mm or more, preferably about 40 mm or more, preferably about 45 mm or more.

In the composite material of the present invention, a layer ("wafer") of silicon material is chemically bonded to a surface, preferably a surface which is a continuous silicon surface, of the layer of diamond material.

Preferably the layer of silicon material is single crystal silicon. This is preferred because single crystal silicon material provides a good substrate for growing a Group III nitride semiconductor on a surface thereon due to a good lattice match. Where the silicon material is single crystal silicon, it is preferred that it is { 111 } silicon as this provides a particularly good lattice match. Alternatively, the single crystal silicon may be { 100}, { 110} or {311 } silicon, which is advantageous where a device is to be formed in the layer of silicon material. As will be described in detail below, single crystal silicon may be chemically bonded to a surface of the layer of diamond material by techniques known in the art, in particular silicon direct wafer bonding. In this regard, preferably the layer of silicon material is bonded, preferably wafer bonded, to a surface, preferably a continuous silicon surface, of the diamond material. The thickness of the layer of silicon material, prior to any post attachment processing, is preferably in the range from about 20 μm to about 500 μm, preferably from about 50 μm to about 300 μm.

Preferably the layer of silicon material has a longest dimension which is approximately equal to the longest dimension of the layer of diamond material.

The layer of silicon material of the composite material of the present invention provides a surface on which a Group III nitride semiconductor may be grown. Accordingly, the composite material of the present invention may further comprise a layer of a Group III nitride semiconductor chemically bonded to a surface of the layer of silicon material. Preferably the layer of Group III nitride semiconductor is chemically bonded to the surface of the layer of silicon material which is opposite the surface which is chemically bonded to the layer of diamond material.

Examples of suitable Group III nitride materials include those selected from the group consisting of GaN, AlN, InN, Al_xGai_-xN and In_xGai_-xN, wherein 0 > x > 1. Preferably the Group III nitride semiconductor is a layer of GaN or AlN because these Group III nitride materials have wide band gaps and exhibit excellent semi-conducting properties, making them useful in a variety of electronic applications, in particular in LEDs and transistor assemblies. Where the composite material of the present invention is used in such applications, the benefits of both the high thermal conductivity of the layer of diamond material and the excellent semi-conducting properties of the Group III nitride semiconductor layer are exploited.

Preferably, where it is included, the thickness of the layer of Group III nitride semiconductor is not limited and will ultimately depend on the end application to which the composite material of the present invention is to be put. For example, the layer of Group III nitride semiconductor may have a thickness of between about 1 nm and about 1 μm. Preferably the longest dimension of the layer of Group III nitride semiconductor is approximately equal to the longest dimension of the layer of diamond material and/or the longest dimension of the layer of silicon material.

The present invention further provides a method for forming a composite material comprising chemically bonding a layer of silicon material to a surface of a layer of diamond material, wherein the diamond material comprises a mixture of diamond particles, silicon carbide and silicon.

The layer of diamond material and layer of silicon material are as described above. The techniques used to chemically bond the two layers will depend on the nature of the silicon material.

Where the silicon material is single crystal silicon, preferably {111 } silicon, preferably chemical bonding is achieved by silicon direct wafer bonding.

Silicon direct wafer bonding is a technique which is well known in the art. It is, for example described in M. A. Schmidt, Proc IEEE, 1998, 86, No.8; M. A. Schmidt, Proc Solid State Sensor and Actuator Workshop, Hilton Head SC; and W. H. Ko et al. Micromachining and Micropackaging of Transducers, Elsevier Science Publishers. In brief, in this technique, the two surfaces to be chemically bonded are brought into contact and a pressure is applied to the centre of one of the two surfaces to create an initial point of contact between the two surfaces, while the two layers to be attached remain physically separated by means of mechanical spacers. The mechanical spacers are then gradually retracted to create a single bonding wave which propagates out from the centre of the layers. The surfaces are then heated which results in the formation of a chemical bond between the two surfaces.

The process of silicon direct wafer bonding relies initially on intermolecular Van der Waals attractive forces between the surfaces of the two layers, respectively when brought into contact. Van der Waals forces are short range forces and thus, to maximise these forces, it is advantageous to ensure that the surface roughness and flatness of the two surfaces is as low as possible. In this regard, prior to the step of chemically bonding the two layers, the method of the present invention may include a step wherein a surface of the layer of diamond material and/or a surface of the layer of silicon material is processed to a surface roughness R_a of about 20 nm or less, preferably about 10 nm or less, preferably about 5 nm or less, preferably about 3 nm or less, preferably about 2 nm or less, preferably about 1 nm or less, preferably about 0.5 nm or less. Alternatively or in addition, one or both of the surfaces may be polished prior to the step of chemical bonding. Surface roughness R_a may be measured according to British Standard BS 1134 Part 1 and Part 2, using a stylus profilometer, such as a Taylor Hobson FormTalysurf 50, available from Taylor Hobson Ltd., Leicester, UK.

Preferably, prior to the step of chemically bonding the two layers, a surface of the layer of diamond material and/or a surface of the layer of silicon material is processed to a flatness such that any deviation from ideal flatness is about 1 μm or less, preferably about 0.5 μm or less, preferably about 0.2 μm or less, preferably about 0.1 μm or less measured over an area of at least about 100 mm², preferably at least about 200 mm². It is possible to process the surface(s) of the layer(s) to such flatnesses by use of conventional techniques. The flatness of the surface may be measured using a reflection interferometer, for example, a Zygo Mark GPI Interferometer which is available from Zygo Corporation, Middlefield, CN, USA.

After chemical bonding of the layer of silicon material to a surface of the layer of diamond material, preferably the thickness of the layer of silicon material is reduced. Preferably the thickness of the layer of silicon material is reduced to less than about 50 μm, preferably less than about 40 μm, preferably less than about 30 μm, preferably less than about 20 μm, preferably less than about 10 μm, preferably less than about 5 μm, preferably less than about 2 μm, preferably less than about 1 μm. It is preferable to minimise the thickness of the layer of silicon material in order to ensure that it is the thermal properties of the layer of diamond material which dominate the thermal behaviour of the composite material of the present invention.

Methods of processing a surface of the layer of silicon material in order that it is "epi-ready", that is processing the surface such that it provides a suitable surface for epitaxial growth of a Group III nitride semiconductor layer thereon, include chemical- mechanical processing ("CMP") and the use of the well-known cleaning processes widely referred to as "SCl" and "SC2". These cleaning processes are described in the Handbook of Semiconductor Wafer Cleaning Technology, Ed W. Kern, Noyes Publications (1993), Park Ridge, NJ, USA, pages 21-22.

The method of the present invention may comprise a further step of growing a Group III nitride semiconductor layer on a surface of the layer of silicon material. Preferably, the layer of Group III nitride semiconductor layer is an epitaxial layer. The Group III nitride semiconductor may be grown on the surface of the layer of silicon material by any technique known in the art. For example, where the silicon material is single crystal silicon, the Group III nitride semiconductor may be grown on the surface thereof by Molecular Beam Epitaxy (MBE) or Metal Organic Vapour Phase Epitaxy (MOVPE) techniques.

Alternatively, rather than using the composite material as a substrate for Group III nitride semiconductor growth, a device may be formed in the layer of silicon material by conventional techniques.

The method of the present invention provides a simple and cost effective way in which composite materials such as those of the present invention may be formed.

The invention will now be described further by reference to the following figures and examples which are in no way intended to limit the scope of protection claimed.

Figures l(a) to l(e) are a schematic representation of a method of forming the composite material of the present invention.

In figure l(a), a green body (2) is formed by compacting together a blend of diamond particles and silicon carbide. The green body (2) is then heated to cause graphitisation and the formation of non-diamond carbon. The green body is then infiltrated with molten silicon. With reference to figure 1 (b), the molten silicon enters the green body at the infiltration surface (4) to form a layer of diamond material (6) which comprises diamond, silicon carbide and silicon ("ScD"). In addition, a layer (8) of polycrystalline silicon is formed on the outer surfaces of the layer of diamond material.

The surfaces of the layer of diamond material which have a layer of polycrystalline silicon thereon are then processed by chemical-mechanical processing (CMP) to provide a polycrystalline surface which is flat and has a low roughness (10), as shown in figure l(c).

As illustrated in figure 1 (d), a layer of silicon material, preferably single crystal { 111 } silicon (12) is then chemically bonded to the polycrystalline silicon surface (10) of the layer of diamond material (6).

With reference to figure l(e), the thickness of the layer of silicon material is then reduced by, for example, chemical -mechanical processing to form a surface (14) on which a Group III nitride semiconductor may be grown.

Examples

Example 1

Example 1 describes the production of a diamond-silicon carbide-silicon material having a polycrystalline diamond surface layer.

A mixture of 100 g synthetic diamond particles obtained from Element Six Limited, Shannon, Republic of Ireland was prepared with the following size characteristics: all particles substantially equiaxed in shape; 65 weight% of particles with a mean size of 420 μm; 25 weight% of particles with a mean size of 50 μm;

10 weight% of particles with a mean size of 7 μm; and

1 weight% or less of particles with a size smaller than 6 μm. Approximately 5 ml of an organic binder phase (25 volume% phenol formaldehyde resin in ethanol) was added to the mixture of diamond particles and the diamond-binder mixture was thoroughly mixed using a commercial "Z-blade" mixer (Winkworth Machinery Ltd, Reading, UK). Mixing was continued for about 20 minutes.

After completion of the mixing, samples of the diamond-binder mixture were pressed to form squat cylinders of about 25 mm in diameter and 5 mm in height. The density of the samples was about 78% of the theoretical density.

After pressing, the samples were dried for about 1 hour at approximately 70⁰C and then hardened for about 1 hour at approximately 150°C.

The samples were then vacuum heat treated to decompose the binder phase and provide controlled graphitisation of the diamond particles. The heat treatment conditions used were: pressure less than 5 x 10^"4 mbar; temperature of approximately 1550⁰C; time at temperature approximately 15 minutes; and rate of temperature increase of approximately 50⁰C per minute.

The conditions for controlled graphitisation were empirically determined to ensure that the amount of diamond converted to graphite was such that all the graphite would subsequently be converted to silicon carbide.

The samples were infiltrated with molten silicon obtained by melting lump silicon of commercial purity (e.g. 99.95% purity lump Si from Goodfellow Cambridge Limited, Huntingdon, UK). The infiltration was carried out in a vacuum furnace (pressure less than 5 x 10^"3 mbar) at a temperature of approximately 1450⁰C (which is above the nominal melting point of silicon which is 1420⁰C). The solid lump silicon was placed in a crucible and the samples were placed on top of the solid lump silicon to allow infiltration by capillary action once the silicon melted. The infiltration process resulted in the formation of a polycrystalline silicon layer on the outer surface of the sample. On completion of the infiltration process, the phase composition of one of the samples was assessed by preparing a cross section and determining the areas of the different phases in the cross section (equivalent to the volume% of each phase), with the following results: diamond 75%, silicon carbide 14%, silicon 11%, and porosity <1%.

The thermal conductivity of a slice of the material having a thickness of about 1 mm, taken from the centre of one of the samples was measured and found to be greater than 400 Wm-¹K-¹.

A surface of one of the samples was mechanically lapped and polished to a R₃ of less than 5 nm. The R₃ was measured according to British Standard BS 1134 Part 1 and Part 2, using a stylus profilometer (Taylor Hobson FormTalysurf 50, Taylor Hobson Ltd, Leicester, UK). After mechanical lapping and polishing, the deviation of the surface from ideal flatness was measured using a reflection interferometer (Zygo Mark GPI Interferometer, Zygo Corporation, Middlefield, CN, USA) and was determined to be better than 0.1 μm over a central area of 15 mm x 15 mm as indicated by the fact that no fringes were visible in the interferogram obtained using 633 nm illumination from a He- Ne laser. At this point the surface of the diamond material comprised a layer of polycrystalline silicon. The electrical resistivity of the material was measured to be approximately 0.015 Ωcm.

Example 2

A series of samples were prepared using the same process as Example 1 except that in the infiltration step, the silicon used had a combined content of Al, B and N impurities of less than 20 ppm (lump silicon, 99.995% purity, Goodfellow Cambridge Limited, Huntingdon, UK). The phase composition and the thermal conductivity of the sample were the same as the material of Example 1. The electrical resistivity of the material was measured to be greater than 1 x 10 Ωcm. Example 3

In this example, the diamond-silicon carbide-silicon material with processed surface polycrystalline surface layer as prepared in Example 1 was direct wafer bonded to a flat, parallel { l l l }-oriented 1" (2.54 cm) diameter silicon single crystal wafer (nominally 400 μm thick, obtained from Sil-Mat, Landsberg-am-Lech, Germany).

As a first step, the diamond material and the silicon wafer were cleaned to remove debris and particulates. The final step of the cleaning process utilised ultra-pure, particulate- free water. The diamond material and silicon wafer were dried using dry, particulate- free nitrogen.

The diamond material and silicon wafer were then loaded into a wafer bonding system (Applied Microengineering Ltd, Didcot, UK) and joined at room temperature using an applied load of approximately 0.2 MPa in vacuum (pressure less than 5 x 10^"6 mbar);

The diamond material and the silicon wafer were then heated to a temperature of approximately 1000°C, again under vacuum and with the applied load maintained, for a duration of approximately five minutes to form a direct chemical bond.

Upon completion of the direct wafer bonding process, the exposed surface of the silicon wafer was processed using conventional CMP technology to reduce the thickness of the silicon wafer from 400 μm to between 2 μm and 3 μm.

Once the silicon wafer was at the correct thickness, the diamond material carrying the silicon wafer was cleaned and was suitable for epitaxial deposition of, for example, GaN.

Example 4

In this example, the diamond-silicon carbide-silicon material with processed surface polycrystalline surface layer prepared in Example 2 was direct wafer bonded to a flat, parallel { l l l }-oriented silicon single crystal wafer and subsequently processed according to Example 3. The result was a diamond-silicon carbide-silicon composite material in which all the silicon and silicon-containing phases had an impurity level with respect to Al, B and N of less than 20 ppm, chemically bonded to a { l l l }-oriented single crystal silicon layer with a thickness of approximately 2-3 μm. The composite material obtained was suitable for use as a substrate for GaN epitaxial deposition.

Claims

1. A composite material comprising a layer of diamond material chemically bonded to a surface of a layer of silicon material, wherein the layer of diamond material comprises a mixture of diamond particles, silicon carbide and silicon.

2. A composite material according to claim 1, wherein the layer of silicon material is covalently bonded to a surface of the layer of diamond material.

3. A composite material according to claim 2, wherein the layer of silicon material is silicon direct wafer bonded to a surface of the layer of diamond material.

4. A composite material according to any preceding claim, wherein the silicon material is single crystal silicon.

5. A composite material according to claim 4, wherein the single crystal silicon is { 111 } silicon.

6. A composite material according to any preceding claim, wherein the diamond material comprises diamond particles in an amount of about 55 % by volume or more.

7. A composite material according to any preceding claim, wherein the diamond material comprises diamond particles in an amount in the range from about 55% to about 85% by volume.

8. A composite material according to any preceding claim, wherein the diamond material comprises silicon carbide in an amount of about 2% by volume or more.

9. A composite material according to any preceding claims, wherein at least 50% by weight of the diamond particles in the layer of diamond material have a diameter of at least about 80 μm.

10. A composite material according to claim 9, wherein the nitrogen content of the diamond particles in the layer of diamond material which have a diameter of 80 μm or more is about 300 ppm or less.

11. A composite material according to any preceding claim, wherein the layer of diamond material comprises at least two different fractions of diamond particles, wherein the average diameter of the diamond particles in each fraction is different.

12. A composite material according to any preceding claim, wherein the layer of diamond material comprises less than about 45% by volume of silicon.

13. A composite material according to any preceding claim, wherein the silicon which forms a part of the diamond material is high purity silicon.

14. A composite material according to claim 13, wherein the silicon which forms a part of the layer of diamond material has a combined content of Al, N and B impurities of less than about 40 ppm.

15. A composite material according to any preceding claim, wherein a device is formed in the layer of silicon material.

16. A composite material according to any one of claims 1 to 14, further comprising a layer of a Group III nitride semiconductor material chemically bonded to a surface of the layer of silicon material.

17. A composite material according to claim 16, wherein the Group III nitride semiconductor is selected from the group consisting of GaN, AlN, InN, Al_xGa) _-XN and In_xGai_-xN, wherein 0>x >1.

18. A composite material according to claim 17, wherein the Group III nitride semiconductor is GaN.

19. Use of a composite material as defined in one of claims 1 to 14, as a substrate for growing a Group III nitride semiconductor device.

20. An electronic device comprising a composite material as defined in any one of claims 1 to 18.

21. A transistor comprising a composite material as defined in any one of claims 1 to 18.

22. A diode comprising a composite material as defined in any one of claims 1 to

18.

23. A method for producing a composite material comprising chemically bonding a layer of silicon material to a surface of a layer of diamond material, wherein the diamond material comprises a mixture of diamond particles, silicon carbide and silicon.

24. A method according to claim 23, wherein the surface of the layer of diamond material is a continuous silicon surface.

25. A method according to claim 23 or claim 24, wherein the silicon material is single crystal silicon.

26. A method according to claim 25, wherein the single crystal silicon is {111 } silicon.

27. A method according to claim 25 or claim 26, wherein the single crystal silicon is chemically bonded to a surface of the layer of diamond material by silicon direct wafer bonding.

28. A method according to claim 27, wherein a surface of the layer of diamond material is processed to a surface roughness R₃ of less than about 10 nm prior to silicon direct wafer bonding.

29. A method according to any one of claims 23 to 28, comprising a further step after chemically bonding the layer of silicon material to a surface of the layer of diamond material, of reducing the thickness of the layer of silicon material.

5 30. A method according to claim 29, wherein the thickness of the layer of silicon material is reduced to a thickness of less than about 50 μm.

31. A method according to any one of claims 23 to 30, further comprising a step of polishing a surface of the layer of silicon material to a surface roughness R_a of about

10 20 nm or less.

32. A method according to any one of claims 23 to 31, comprising a further step after chemically bonding the layer of silicon material to a surface of the layer of diamond material, of growing a layer of a Group III nitride semiconductor on a surface

15 of the silicon material.

33. A method according to claim 32, wherein the Group III nitride semiconductor layer is grown by Molecular Beam Epitaxy (MBE) or Metal Organic Vapour Phase Epitaxy (MOVPE). 0

34. A method according to claim 32 or claim 33, wherein the Group III nitride semiconductor is selected from the group consisting of GaN, AlN, InN, Al_xGai_-xN and In_xGai _-XN, wherein 0>x >1. 5

35. A method according to claim 34, wherein the Group III nitride semiconductor is GaN.

36. Diamond material comprising a mixture of diamond particles, silicon carbide and silicon, wherein the combined content of Al, N and B impurities in the silicon is less 0 than about 40 ppm.

37. A method for forming diamond material comprising forming a green body from a blend of diamond particles and silicon carbide; heating the green body to cause a controlled amount of graphitisation; infiltrating molten high purity silicon into the green body to react with non-diamond carbon produced by graphitisation, to form silicon carbide.

38. A method according to claim 37, wherein the levels of impurities (that is combined content of Al, N and B) in the silicon source are less than about 20 ppm.