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Definitions

• the invention relates to a process for producing a diamond-containing composite material.
• Diamond-containing composite materials have been used for a long time as cutting tool materials.
• they are also potentially interesting materials for heat sinks.
• the thermal conductivity of diamond is from 1000 to 2000 W/(m.K), with the content of nitrogen and boron atoms on lattice sites being of special importance for determining the quality.
• Heat sinks are widely used in the production of electronic components. Apart from the heat sink, semiconductor components and a mechanically stable encapsulation are the essential constituents of an electronic package.
• the terms substrate, heat spreader or support plate are frequently also used for the heat sink.
• the semiconductor component comprises, for example, single-crystal silicon or gallium arsenide. This is connected to the heat sink, usually using soldering methods as joining technique.
• the heat sink has the function of conducting away heat produced during operation of the semiconductor component.
• Semiconductor components which produce a particularly large quantity of heat are, for example, LDMOS (laterally diffused metal oxide semiconductor), laser diodes, CPU (central processing unit), MPU (microprocessor unit) or HFAD (high frequency amplify device).
• the geometric configurations of the heat sink are specific to the application and may vary widely. Simple forms are flat plates. However, substrates having a complex configuration with recesses and steps are also used.
• the heat sink itself is in turn joined to a mechanically stable encapsulation.
• the coefficients of thermal expansion of the semiconductor materials used are low compared to other materials and are reported in the literature as from 2.1 ⁇ 10 ⁇ 6 K ⁇ 1 to 4.1 ⁇ 10 ⁇ 6 K ⁇ 1 for silicon and from 5.6 ⁇ 10 ⁇ 6 K ⁇ 1 to 5.8 ⁇ 10 ⁇ 6 K ⁇ 1 for gallium arsenide.
• Ceramic materials are Al 2 O 3 with a coefficient of expansion of 6.5 ⁇ 10 ⁇ 6 K ⁇ 1 or aluminum nitride having a coefficient of expansion of 4.5 ⁇ 10 ⁇ 6 K- ⁇ 1 .
• EP 0 521 405 describes a heat sink which has a polycrystalline diamond layer on the side facing the semiconductor chip.
• the absence of plastic deformability of the diamond layer can lead to cracks in the diamond layer even during cooling from the coating temperature.
• U.S. Pat. No. 5,273,790 describes a diamond composite material having a thermal conductivity of >1700 W/(m.K) in the case of which loose diamond particles brought to shape are converted into a stable shaped body by means of subsequent diamond deposition from the gas phase.
• the diamond composite produced in this way is too expensive for commercial use in mass-produced parts.
• WO 99/12866 describes a process for producing a diamond-silicon carbide composite material. It is produced by infiltration of a diamond skeleton with silicon or a silicon alloy. Owing to the high melting point of silicon and the resulting high infiltration temperature, diamond is partly converted into carbon or subsequently into silicon carbide. Owing to the high brittleness, the mechanical forming of this material is highly problematical and costly, so that this composite material has hitherto not yet been used for heat sinks.
• U.S. Pat. No. 4,902,652 describes a process for producing a sintered diamond material.
• An element from the group of transition metals of groups 4a, 5a and 6a, boron and silicon are deposited onto diamond powder by means of physical coating methods in this process.
• the coated diamond grains are subsequently joined to one another by means of a solid-state sintering process.
• Disadvantages are that the product formed has a high porosity and a coefficient of thermal expansion which is too low for many applications.
• U.S. Pat. No. 5,045,972 describes a composite material in which diamond grains having a size of from 1 to 50 ⁇ m and also a metallic matrix comprising aluminum, magnesium, copper, silver or an alloy thereof are present.
• a disadvantage is that the metallic matrix is bound only unsatisfactorily to the diamond grains, so that, as a result, the thermal conductivity and mechanical integrity are not sufficient.
• U.S. Pat. No. 5,783,316 describes a process in which diamond grains are coated with W, Zr, Re, Cr or titanium, the coated grains are subsequently compacted and the porous body is infiltrated, for example, with Cu, Ag or Cu—Ag melts.
• the high coating costs limit the uses of composite materials produced in this way.
• EP 0 859 408 describes a material for heat sinks whose matrix is made up of diamond grains and metal carbides, with the interstices of the matrix being filled by a metal.
• metal carbides mention is made of the carbides of metals of groups 4a to 6a of the Periodic Table.
• TiC, ZrC and HfC are particularly emphasized in EP 0 859 408.
• Ag, Cu, Au and Al are said to be particularly advantageous filler metals.
• a disadvantage is that the metal carbides have a low thermal conductivity, which in the case of TiC, ZrC, HfC, VC, NbC and TaC is in range from 10 to 65 W/(m.K).
• the metals of groups 4a to 6a of the Periodic Table have a degree of solubility in the filler metal, for example silver, as a result of which the thermal conductivity of the metal phase is greatly reduced.
• EP 0 893 310 describes a heat sink comprising diamond grains, a metal or a metal alloy having a high thermal conductivity from the group consisting of Cu, Ag, Au, Al, Mg and Zn and a metal carbide of the metals of groups 4a, 5a and Cr, with the metal carbides covering at least 25% of the surface of the diamond grains.
• EP 0 898 310 also describes techniques, for example an infiltration process, for producing a heat sink. Alloys comprising a metal having a high thermal conductivity and a carbide-forming metal from the group of the elements of groups 5a, 6a and Cr are used for this purpose.
• liquidus temperature of a eutectic Ag—Cu alloy (Ag-30% by weight of Cu) is 780° C.
• Cu—Ag—Ti alloys mentioned in EP 0 898 310 have a liquidus temperature of from 830 to 870° C.
• the process of the invention comprises a shaping step carried out under atmospheric pressure or with the aid of pressure to produce an intermediate.
• the intermediate comprises diamond powder having a mean particle size of the diamond grains of from 5 to 300 ⁇ m. A preferred particle size range is from 60 to 250 ⁇ m. Fine diamond grains and thus a large interfacial area to adjoining neighboring phases reduce the thermal conductivity.
• Pressureless processes are, for example, pouring processes, vibratory introduction processes or slip casting.
• Pressure-aided techniques are, for example, die pressing, isostatic pressing and powder injection molding.
• the proportion of diamond after the shaping process is from 40 to 90%, based on the total volume.
• the remainder comprises pores and/or binder and/or metallic components having a high thermal conductivity.
• An incorporated binder makes it possible to increase the density of the green body or reduces the die friction.
• Diamond powder and binder are for this purpose mixed in customary mixers or mills.
• Suitable binders are, for example, those based on polymer or wax.
• Advantageous proportions of binder are in the range from 1 to 20% by weight. It is advantageous to remove at least part of the binder by means of a chemical or thermal process prior to the infiltration step. In the case of a thermal process, it can be advantageous to carry out the process so that residues of pyrolized carbon remain on the diamond surface and react with part of the infiltrate to form a carbide. Thermal binder removal can also be integrated into the infiltration process.
• Metallic components having a high thermal conductivity which may be mentioned are Cu, Al, Au and alloys thereof.
• the infiltration process can be carried out under atmospheric pressure or with the aid of pressure. The latter is usually referred to as squeeze casting.
• the infiltrate alloy has a eutectic or near-eutectic composition. Near-eutectic alloys encompass compositions which have a liquidus temperature below 950° C.
• the infiltrate alloy comprises at least one metallic component having a high thermal conductivity and comprising an element or an alloy from the group consisting of Cu, Ag, Au and at least one element from the group consisting of Si, Y, Sc, rare earth metals. It has been found that the use of infiltrate alloys according to the invention leads to very good wetting of the diamond grains and to a high interface strength between the diamond grains and the surrounding phases.
• the infiltrate alloys according to the invention have the advantage that their solidus temperatures are significantly below those of Cu, Au or Ag alloys with the metals of groups 4a/5a of the Periodic Table or Cr, as can be seen from Table 1. This makes it possible to use two-component alloys instead of multicomponent alloys, which has a favorable effect on the thermal conductivity.
• the solidus temperatures of the infiltrate alloys according to the invention are below 870° C. This ensures that unacceptably high reaction of the diamond does not occur during the infiltration process.
• Table 1 also shows that the infiltrate alloys according to the invention have a very low solvent capability for Y, Si and rare earth metals at the eutectic temperature or at 400° C.
• This has the advantage that the Cu—, Ag— or Au-rich phase formed by the eutectic conversion has a very high purity and thus thermal conductivity.
• Alloys of Ag or Au with Cu or up to 3 atom % of Ni likewise have a sufficiently high thermal conductivity which is not reduced to an unacceptable extent by small amounts of undissolved Si, Y, Sc or rare earth metal. Proportions of graphite also do not reduce the thermal conductivity to an unacceptable extent.
• Y, Sc, Si and the rare earth metals not only reduce the solidus temperature of Cu, Au and Ag but also produce good wetting and bonding of the Cu—, Au— or Ag-rich phase to the diamond grains.
• Ag—Si an Si—C compound having a thickness in the nanometer range was able to be found. Owing to the low proportion, these phases do not produce any significant deterioration in the thermal conductivity.
• the thermal conductivity of Si—C of about 250 W/(m.K), which is very high compared to the metal carbides of the elements of groups 4a and 5a of the Periodic Table and chromium carbide. The good wetting behavior ensures that the pores of the intermediate are filled to an extent of at least 97%.
• the wetting behavior can be improved still further by addition of Ni, Cr, Ti, V, Mo W, Nb, Ta, Co and/or Fe, but the total content of these elements must not exceed 3 atom %, since otherwise they result in an unacceptably large reduction in the thermal conductivity.
• the advantages of the infiltrate alloy according to the invention also become apparent when hot pressing is used as densification process.
• an intermediate comprising from 40 to 90% by volume of diamond grains having a mean particle size of from 5 to 300 ⁇ m and from 10 to 60% by volume of a eutectic or near-eutectic infiltrate alloy which has a solidus temperature of ⁇ 900° C.
• At least one metallic component of high thermal conductivity which comprises an element or an alloy from the group consisting of Cu, Ag, Au and at least one element from the group consisting of Si, Y, Sc, rare earth metals and optionally ⁇ 3 atom % of one or more elements from the group consisting of Ni, Cr, Ti, V, Mo, W, Nb, Ta, Co, Fe which promote wetting, with near-eutectic alloys encompassing compositions which have a liquidus temperature of ⁇ 950° C., is homogenized by mixing or milling.
• a die of a hot press e.g. a graphite die, is filled with the intermediate.
• the intermediate is subsequently brought to a temperature which is above the solidus temperature of the infiltrate alloy but below 1000° C., for example by conductive heating of the die, and densified, with the pressure being applied by moving the punch.
• a temperature which is above the solidus temperature of the infiltrate alloy but below 1000° C., for example by conductive heating of the die, and densified, with the pressure being applied by moving the punch.
• the infiltrated intermediate can be subjected to a heat treatment so that constituents which have been trapped in solution are precipitated, as a result of which the thermal conductivity is improved.
• This heat treatment can also have a favorable effect on the interface strength between the diamond particles and the surrounding constituents.
• This heat treatment step can also be integrated into the cooling process of the infiltration step.
• Diamond-containing composite materials produced according to the invention have a sufficiently good mechanical formability due to the very ductile Ag, Au or Cu microstructure constituents. It is also advantageous for inexpensive production that the high thermal conductivity of the Ag—, Au— or Cu-rich microstructure constituents enables the diamond content to be reduced.
• Variation of the diamond and metal phase content make it possible to produce heat sinks for a variety of requirements to be tailored in respect of thermal conductivity and thermal expansion.
• microstructure constituents do not worsen the property to an unacceptable degree as long as their content does not exceed 5% by volume.
• these microstructure constituents increase the thermal conductivity slightly, they in the case of C and Si have a favorable effect on the coefficient of thermal expansion by reducing the latter. In addition, they can sometimes only be avoided completely with a relatively high degree of difficulty in terms of the production process.
• Particularly advantageous contents of Ag—, Au— or Al-rich phase are from 7 to 30% by volume.
• diamond powder can be processed within a wide particle size spectrum. Apart from natural diamonds, it is also possible to process more inexpensive synthetic diamonds. Excellent processing results have also been achieved using the customary coated diamond types. As a result, the most inexpensive type in each case can be employed.
• the highest thermal conductivity values can be achieved by the use of Ag at contents of from 7 to 30% by volume.
• the composite material of the invention can also be used as heat sink in other applications, for example in the aerospace field or in engine construction.
• Natural diamond powder of the grade IIA (Micron+SND from Element Six GmbH) having a mean particle size of 80-150 ⁇ m was introduced into a graphite mold having the dimensions 35 mm ⁇ 35 mm ⁇ 5 mm. The bulk density was brought to 65% by volume by mechanical shaking. The diamond powder was subsequently covered with a film composed of a eutectic Ag—Si alloy having an Si content of 11 atom % and, to carry out the infiltration, was heated in a furnace to a temperature of 860° C. under reduced pressure, with the hold time being 15 minutes. The subsequent gas pressure infiltration using helium was carried out at 1 bar for 15 minutes. After cooling to room temperature with a hold point at 400° C. for about 10 minutes, the volume contents of the phases present were determined by means of quantitative metallography.
• the value for silicon carbide was about 1% by volume, with the silicon carbide mostly enveloping the diamond grains uniformly. Owing to the low thickness of this silicon carbide shell, the modification of the silicon carbide phase could not be determined.
• the microstructure comprises an Ag-rich phase with embedded Si precipitates which have been formed by the eutectic reaction.
• the proportion by volume of the Ag-rich phase was about 12%, and that of Si was about 1%. No further constituents apart from Ag could be detected in the Ag-rich phase by means of EDX, so that it can be assumed on the basis of the applicable detection limit that the proportion of Ag is greater than 99 atom %.
• the plate was processed by means of a laser and erosion. A mean value of 500 W/(m.K) was measured for the thermal conductivity at room temperature. The determination of the coefficient of thermal expansion gave a mean value of 8.5 10 ⁇ 6 K ⁇ 1 .
• Synthetic diamond powder of the grade Micron+MDA from Element Six GmbH having a mean particle size of 40-80 ⁇ m was processed as described in Example 3, but without a hold phase at about 400° C. for 15 minutes being carried out during cooling from the infiltration temperature.
• the mean thermal conductivity at room temperature of the composite material produced in this way was 440 W/(m.K), and the mean coefficient of thermal expansion was 8.5 ⁇ 10 ⁇ 6 K ⁇ 1 .
• Natural diamond powder of the grade IIA (Micron+SND from Element Six GmbH) having a mean particle size of 40-80 ⁇ m was mixed with 7% by volume of a binder based on epoxy resin.
• the precursor or intermediate produced in this way was pressed by means of die pressing at a pressure of 200 MPa to give a plate having the dimensions 35 ⁇ 35 mm ⁇ 5 mm.
• the porosity of the plate was about 15% by volume.
• This plate was subseq uently covered with a film composed of a eutectic Cu—Y alloy having a Y content of 9.3 atom % and, to carry out the infiltration, was heated in a furnace to a temperature of 900° C. under reduced pressure, with the hold time being 15 minutes.
• the plate was processed by means of a laser and erosion. A mean value of 410 W/(m.K) was measured for the thermal conductivity at room temperature. The determination of the coefficient of thermal expansion gave a mean value of 7.7 10 ⁇ 6 K ⁇ 1 .

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