WO1990012769A2 - Heat-sink structures with increased heat dissipation capacity and methods for the production of such structures - Google Patents

Abstract

A porous or non-woven structure of high thermally conductive ceramic particles is embedded in a matrix of thermally conductive ceramics to provide a heat-sink substrate. Particular application to AlN heat-sinks. Embedding is provided by infiltration by dense ceramics under CVD conditions. In an embodiment the particles are whiskers coated with a diamond layer.

Description

HEAT-SINK STRUCTURES WITH INCREASED HEAT DISSIPATION CAPACITY AND METHODS FOR THE PRODUCTION OF SUCH STRUCTURES

The present invention concerns improved heat-sink substrate structures with enhanced heat dissipation capacity. The invention also concerns a method to manufacture such novel heat-sink structures, more particularly A1N ceramic substrates for electronic applications.

Heat-sinks are very important, to dissipate heat in elec¬ tronic equipment and other appliances because undissipated heat can unduly raise the operating temperature of electronic circuits and damage some of the sensitive components thereof. Hence it is essential that power circuits be coupled to effi¬ cient heat-sinks which will remove the excess heat generated during circuit operation, i.e. the circuits temperature will be maintained under safe limits.

Heat-sinks are generally boards or other shapes made of good thermally conductive materials. For electronic applica¬ tions, the thermally conductive materials should also possess appropriate electrical properties. Table I below lists some of the most common thermally conductive materials used for making heat-sinks together with their main electrical properties. The data also include single crystal silicon for comparison purposes.

As can be noted from the data of Table I, the properties of aluminium nitride as heat-sink material are particularly favorable.

Aluminum nitride (A1N) overcomes the main limitation as heat sink in electronic applications of alumina and beryllia ceramics, that is the thermal expansion mismatch between the ceramic and the deposited silicon which may result into dis¬ tortion or breakage under operating conditions. Yet, A1N has an average thermal coefficient of expansion (TCE) of 2.65 W/m°C which closely matches that of Si (3.6).

Hence, A1N offers a unique combination of properties, including high thermal conductivity and electrical resistivi¬ ty, which makes it a highly desirable ceramic for chip packag- ing, e.g. VLSI (very large scale integrated circuits) and VHSIC (very high scale integrated circuits).

In high power devices, EC-logic circuits and laser diode applications with less requirements under TCE matching, AIN nevertheless offers advantages over Al^O., in view of its much higher conductivity and over beryllia because of its easy and non-hazardous handling.

Heat-sinks made from ceramics with good thermal conducti¬ vity can be manufactured by different techniques including for instance the pressing and sintering of ceramic powders or the deposition of ceramic layers by chemical vapor deposition (CVD).

TABLE I Ceramic Thermal Resistivity Thermal Dielectric Conductivity (Λcm) Exp. constant (W/m°C) (10^"6/°C) (at 1 MHz)

Diamond 2000 >10 14 2.3 5.7

SiC

(monocryst.) 490 >10 13 4. 2 —

A1₂0₃ 10-35 >10 14 5-6 8. 5

Si_{3 4} 10-40 10^" 3. 1 -

Si

(monocryst.) 125 lo -3-lcr3 One of the requirements for good heat dissipation by a thermally conductive material is co pacity and optimalized density. Hence sintering techniques require high purity star¬ ting materials (very low residuel oxide concentration) and sophisticated manufacturing conditions under rather critical temperatures and pressures (see for instance J-59 101.889, TOKYO SHIBAURA); chemical vapor deposition (CVD) is an attrac¬ tive alternative.

AIN substrates can be manufactured in .high density shapes by high pressure sintering. However, this method has drawbacks: for instance the densifying aids used in sintering usually contain oxygen which decreases the thermal conducti¬ vity of the end product. On the other hand, if the densifying aids are decreased or suppressed to obviate the aforementioned drawback, sintering becomes difficult and expensive as densi- fication requires very high pressures and temperatures.

Some prior art is listed below which illustrates the making of high density, thermally conductive ceramics for heat-sinks by CVD. For instance J-62 113.770 (DENKI KAGAKU) discloses articles of AIN made by CVD having density of

3 3.2 g/cm or better and containing 99.9% Al nitride and 0.08% of oxygen. The articles are made by reacting gaseous Al ha- lides and ammonia at 600 - 1200°C under pressures below 100

Torr. Similar systems are disclosed in J-63 151.686 (TOSHIBA

CERAMICS); J-63 151.685 (TOSHIBA CERAMICS); FR-A-1.594.347

(MATSUSHITA ELECTRIC INDUSTRIES).

Trialkylaluminum compounds and nitrogen can also be used in the CVD making of AIN. For instance J-62 176.996 (SHARP) reports the formation of aluminum nitride on a substrate layer of β-SiC by reacting trimethyl-aluminum with nitrogen at 900 - 1500°C using hydrogen as a carrier.

Furthermore, J-63 151 686 (TOSHIBA) discloses making an AIN board by CVD on a supporting plate using a gaseous Al- compound and NH₃ at 600 - 1400°C under a pressure of 0.1 to 50 kPa. Then the board is separated from the base substrate. J-62 036 089 (TOSHIBA) also discloses a similar method. Here a crystalline ceramic film is formed by CVD on the surface of a base material having a desired shape; then the film is sepa- rated from the base and at least one additional film of the same ceramic material is grown by CVD on the surface of the separated film. This is applicable to many ceramics including AIN, Si_{3 4}, TiN, BN, ZrN, HfN, SiC, WC, TiC, 1₂0₃, Si0₂, Zr0₂, ZnO.

Other references concerning the preparation of heat-sink substrates by CVD are DE-A-36 20 329; J-60 057.993 (SUMITOMO); J-60 246.283 (TOSHIBA).

Although the aforementioned techniques have .merit, there always remains the problem of separating the initial film from the base plate. Very often the adherence of the AIN film or board and the base plate is significant which may cause a lot of broken rejects during separation. Also, easy handling of the items in the separation itself requires temporary shut¬ down of the CVD apparatus which is undesirable. Furthermore, if the ceramic substrate is grown in one step (i.e. no separa¬ tion is effected at the bait stage), only one side can be grown at a time which is slower than when two sides can be grown simultaneously.

The method of the present invention, as summarized in the claims, remedies the above deficiencies.

It has also been found that heat-sink substrates compri¬ sing acicular particles, e.g. monocrystal whiskers of thermal¬ ly conductive materials embedded in a compact phase of the same or another thermally conductive material have superior heat-dissipation properties. This is another key factor of the present invention as defined in the annexed claims. Re¬ garding the content of the claims, it should be mentioned that FR-A-2.014.148 (VARIAN) discloses a CVD method for making thick deposits of boron nitride on a substrate. The reactants are diborane and ammonia, the temperature is above 120°C and the pressure below atmospheric.

Also S.L. Spitz in Electronic Packaging & Production, 29 (1989), 36-41, discusses ceramic substrates as integrated circuit packages, inter alia aluminium nitride and hybrids comprising laminates of metals and ceramics. No ceramic substrate with embedded particulate ceramics or whiskers is however mentioned in this reference. US-A-4,256,792 (HONEYWELL) discloses a heat-sink substrate for micropackage assembly made of A1-0-. or other dielectric ceramics having embedded therein "needles" of AIN mostly oriented transversally to the substrate surface. Howe¬ ver the "needles" disclosed are not crystalline and cannot anticipate the substrates of the invention with embedded crys¬ talline acicular particles.

The method of preparation disclosed in this document (see col. 1, last paragraph) consists in the tape casting of a slurry of Al^O^ plus the AIN needles and, thereafter, sinter the tape cast film. In claim 1 of the present invention, the acicular particles, e.g. whiskers are tape cast alone, i.e. without the embedding matrix as, actually, the embedding ma¬ trix is provided afterwards by infiltration or otherwise.

US-A-4,725,456 (Industrial Science Agency) discloses a technique in which a mineral component is CVD deposited within the interstices of a porous structure by infiltration with gaseous reactants. This leads to the formation of whiskers or fine particles in the gaps of the porous structure or coatings on the inside surface of the pores. It is however not reported that this technique can lead to the filling of the pores with foreign material, i.e. to a fully densified structure no- longer porous.

Hence the achievements taught in this document are res¬ tricted to the preparation of a homogeneous mixture of a powder (whch may be presintered) plus whiskers or particles, not of a substrate containing whiskers embedded in a dense ceramic.

US-A-4,777,155 (NGK) discloses a sintered AIN base with whiskers of SiC. The material has improved heat conductivity and can be used as heatsink (col. 2, line 259. However, the components are co-sintered using densification aids which, obviously, will remain distributed within the matrix; hence such matrix with foreign materials distributed therein cannot be considered to be the same as the "compact dense thermally conductive ceramic" of claim 1 of the instant application which refers to a homogeneous phase without dispersed additio¬ nal sintering aids. The teaching of US-A-3,833,389 (TOKYO SHIBAURA) looks very similar to that of the previous document, as it discloses sintered compacts of AIN or Si₃N₄ with SiC, BN or C (possibly as whiskers, see col. 2, line 14) together with a third compo¬ nent (generally a metal oxide). It does not appear that the coiftposites disclosed in this document can be used as heat- sinks.

Basically, in the case of heat-sinks made with parti- culate ceramic, e.g. AIN, a thin porous web or core preform (defined hereafter as a bait) is pressed from AIN powder and/or whiskers and consolidated by infiltrating the pores with AIN made by CVD. Then, when the desired density for the bait has .been obtained, dense AIN is grown on the infiltrated bait up to the desired thickness. This method is economical because making a non-densified porous bait from AIN powder by pressing and, preferably sintering at relatively low tempera¬ ture, is not expensive and infiltrating the pores and even¬ tually growing dense AIN on the infiltrated bait can be done in the same CVD apparatus without shut-down.

In addition, in order to make porous pressed and sintered films, coarser grades (i.e. relatively cheap grades) of AIN powders can be used, e.g. in he range of about 0.5 to 5 μm gauge. Since the aim is to achieve a porous film bait, no densifying additives are required because no densification is required at this stage and also the temperatures and pressures can stay moderate, i.e. temperatures in the 800 - 1500°C range and pressures in the range of 0.1 - 2 T/cm 2 are sufficient.

Furthermore, a very convenient technique for making the porous bait, i.e. tape casting, can be used advantageously.

In this technique, a slurry of the coarse AIN powder and/or whiskers is made in an organic solvent, together with binders and/or plasticizers, and the slurry is cast over a flexible, plastic tape, for instance with a doctor blade to form of film of about 50 μm to 500 μm. Then the film is dried and cut to pieces of the desired sbstrate size, the organics therein are burned and the remaining green is heated around 1000 - 1500°C at ordinary pressure to consolidate the structure. The pieces can then be subjected to infiltration under CVD conditions.

More generally disclosed, the heat-sink substrate made with whiskers has the form of a laminate in which at least one composite core layer of non-woven monocrystalline ceramic whiskers is embedded in a dense ceramic matrix and integrally bound to at least one cladding layer of compact and dense thermally conductive ceramic.

As said before, the layer of non-woven whiskers can be made by conventional means, i.e. a slurry of the whiskers can be prepared in an appropriate solvent together with a binder and the slurry is molded or cast in the shape of the core laye , for instance by draining and drying on a porous support or on a tape (tape casting), or by pressing. The layer is therafter dried to form a green and heated to densify; if necessary the green can be heated to sintering temperature for further consolidation.

The dense matrix in which the whiskers are embedded as well as the cladding layer or layers can be achieved by lami¬ nating the core layer with one or more layers of powdered ceramics followed by pressing and sintering. A preferred me¬ thod however is to impregnate the core layer with a ceramic made by CVD and, therafter, build the cladding layer or layers thereon also by CVD. Basically, as in the case of non acicular particle ceramics, the consolidated core layer (defined hereafter as a preform or bait layer) is reinforced by infil¬ trating the voids between the whiskers with a ceramic depo¬ sited by CVD. CVD conditions suitable for infiltration are discussed hereafter. Possible conditions are also disclosed in the following references: US-A-4,576_f836, D.P. STINTON et al. Am. Ceram. Soc. Bull. 65 (1986), 347-50; R.D. VELTRI et al. J. Am. Ceram. Soc. _2 (1989), 478-80.

Preferably, with whiskers of SiC, AIN, BeO, Si₃N., A1₂0₃ or mixtures thereof, one uses CVD deposited ceramics selected from AIN, SiC and BeO. For convenience, in the following des¬ cription, reference will mainly be made to AIN by CVD but it is understood that other ceramics deposited by CVD are conve¬ nient as well. In addition to using preforms obtained from whiskers or blends of whiskers in neat form, the use of whis- kers subjected before hand to surface treatment, in whole or part, is also possible. For instance, the whiskers can be coated, partly or totally before use, with a layer or with crystals of property modifying materials, for instance mate¬ rials with particular hardness, or particularly good thermal conductivity, or other physical properties. In an embodiment, the whiskers to be used for achieving the mat preform are first subjected to diamond coating by standard techniques. One preferred technique is diamond deposition by pla.sma activated CH₄/H₂ mixtures. This will be detailed hereafter in this specification. In this embodiment diamond crystals grow on the surface of the whiskers which modification further enhances heat dissipation properties and also helps consolidating the preform by entanglement. It should be further remarked that the composites made with entangled diamond coated whiskers packed in a compact ceramic have outstanding cutting and abrading properties and can be also used for high duty machi¬ ning and grinding purposes e.g. super hard grinder wheels and cutting tool surfaces.

After infiltration, when the desired density for the infiltrated non-woven bait has been obtained, dense ceramic is grown on the infiltrated bait up to the desired thickness, this being brought about on one side or on both simultaneous¬ ly. This method is economical because making a non-densified, non-woven mat from ceramic whiskers by tape casting, draining or moulding by pressing and preferably sintering at relatively low temperature, is not expensive and infiltrating the voids and eventually growing dense ceramics on the infiltrated bait can be done in the same CVD apparatus without shut-down. Also the pressures used for shaping and the temperatures for conso¬ lidating the structure can stay moderate, i.e. pressures in

2 the range of 0.1 - 2 T/cm and temperatures in the 800 -

1500°C range are sufficient.

Furthermore, as said before, a very convenient technique for making the non-woven bait, i.e. tape casting, can be used advantageously.

In this technique, a slurry of the ceramic whiskers is made in an organic solvent, together with binders and/or plasticizers, and the slurry is cast over a flexible plastic tape, for instance with a doctor blade to form a film of about 50 μm to 500 μm. Then the film is dried and cut to pieces of the desired substrate size, the organics therein are burned and, if desired, the remaining green is heated around 1000 - 1500°C at ordinary pressure to consolidate the structure. The pieces can then be subjected to infiltration reinforcement and embedment in dense ceramics under CVD conditions.

The invention will now be described in more detail with reference to the annexed drawing.

Figure 1 is a schematic representation of an apparatus for CVD operation which is used for infiltration and embedment purposes and for growing dense ceramic layers on the infil¬ trated preforms.

Figure 2 is a schematic side view of a holder for holding the baits subjected to embedment by infiltration and subse¬ quent growing.

Figure 3 is a plan view of the holder of figure 2. Figure 4 is a graph showing the rate of deposition of AIN made by CVD plotted against the % (v/v) of HCl in the input gases, the remaining operational parameters (i.e. A1C1-, vapor 4.6%, NH₃ 6.9%, H₂ balance, pressure 10 Torr, temperature 935°C, total flow 500 cm /min) being kept constant.

Figure 5 is a schematic view of a microwave apparatus for growing diamond crystals on monocrystalline ceramic whiskers.

Figure 6 is a microphograph of a ceramic whisker whose surface is coated with diamond crystals.

The apparatus represented in figure 1 comprises a tubular oven enclosure 1 made of heat resisting materials, e.g. quartz, steel or ceramics, surrounded by an electric heating mantle 2. This heating mantle can include an electric resistor heater or a high frequency heating coil acting on a susceptor within the enclosure. The susceptor can be used to hold the samples to be CVD treated. The enclosure is equipped with gas inlets 3a and 3b, a gas outlet 4 and a perforated holder plate 5 supported by a rod 6. In the case AIN deposition is desired, the gas inlet 3a communicates with an A1C1₃ source 7 which is swept by a current of gas (H₂) schematized by arrow 8 and which can be heated to a desired temperature by a mantle 9 in which a heating fluid is circulated (arrows). By changing the temperature of the A1C1₃ source and the gas flow over it, the feeding rate of A1C1₃ vapors can be changed. Inlet 3b is connected to a source of ammonia schematized by arrow 10. Ammonia and hydrogen are supplied from pressure cylinders (not shown) via flow-meters to control gas supply rate.

The output 4 which is used to remove the exhaust gases is connected to a vacuum pump. The pressure is controlled by an electrically driven valve 11 and by a needle valve 12 which by-passes the electric valve 11.

The temperature of the enclosure 1 can be controlled by means of thermocouple 12 and a regulated supply 13. The tempe¬ rature of the plate 5 can also be measured by a gauge 14, so hat the CVD temperature near the samples to be infiltrated or coated can be checked. Instead of gauges, optical pyrometers can be used.

The holder plate 5 which, in case of HF induction heating i ^' made of an electrically conductive material such as gra¬ phite or TiN, supports a holder frame 21 (see figs 2 and 3) which consists of a refractory hollow body with top openings 22 and side openings 23. These openings 22 and 23 correspond to through-holes in holder 21. The preform baits of pressed non-acicular AIN particles or of non-woven whiskers 24 to be infiltrated and subsequently coated with dense AIN are placed to rest over the openings 22. Unused openings 22 can be plugged with removable plugs (not shown) , if desired. In the embodiment represented in figures 2 and 3, the holder frame is rectangular and smaller in size than plate 5. Hence the CVD gases in the enclosure can penetrate through openings 23 and reach the underside of the substrates 24 from below as well as the top thereof from above. Hence, in this embodiment, infil¬ tration (and coating) can proceed on both sides simultaneous¬ ly.

In another embodiment, the holder frame 21 is shaped like plate 5, which means that the passage of the gases through plate 4 is impeded by these obstacles and by creating a pres¬ sure differential between the top and bottom parts of the reactor 1, the input gases can be forced through the structures, i.e. through the meshes of the porous substrate or non-woven preforms 24 to reach (through the perforated plate 5) the bottom of the enclosure 1 and the output opening 4. The reasons for this kind of geometry will be explained hereafter with the operation of the apparatus.

The preform 24 of agglomerated non-acicular AIN particles and/or of non-woven whiskers to be infiltrated are made by standard methods, e.g. press-moulding and sintering or tape casting as mentioned above. Then thr> porous baits 2* a,r- placed on holder 21 as shown in figures 2 and 3 and the holder is rested on the plate 5 in the enclosure 1. Then, according to a first embodiment of the invention, conditions are set up to provide densification by infiltration of the bait with AIN by CVD. The conditions are:

Temperature of the substrates in the enclosure 800 - 1000 °C

Pressure inside the enclosure 2 - 50 Torr ^• (con- trolled by a pump connected to output

4 through ^■valve 12)

CVD gases H₂; NH₃; A1C1₃ vapoιr, HCl

% (v/v) of reactants in H₂ 1 - 15% % of HCl in gas mixture 1 - 20%

The addition of dry hydrogen chloride is required to limit the reaction rate of AIN formation and thus promote infiltration into the voids between the particles and/or the whiskers, i.e to prevent the AIN produced by the reaction of A1C1₃ and NH- vapors to only deposit on the external part of the particles or whiskers.

Figure 4 illustrates the relation between hydrogen chlo¬ ride volume content in the input gases and deposition rate of AIN (mg/hr and μm/hr), the other operational parameters (ex¬ cept H₂%) being constant, and show that preferred volume con¬ centrations of HCl range between 1 and 20%. Naturally, since the amount of HCl can also be zero for dense AIN growing purposes, the concentration of HCl can also vary between zero and 1.

Upon establishing the foregoing operational conditions, the A1C1₃ source 7 is heated and swept by hydrogen, whereby A1C1₃ vapors are entrained in correct proportion through input 3a. Simultaneously, the correct proportions of NH₃ and H₂ are fed through input 3b (via flow-meters not shown) and the composition is delivered into the enclosure, whereby it comes into contact with the samples 24 resting on holder 21. The gas mixture also penetrates the inside of holder 21 through ope¬ nings 23 and contacts the underside of the non-woven core preforms 24, whereby infiltration can proceed from both sides of the samples. The residual gases then will cross plate 5 through the perforations and be drawn by pumps (not shown) through valve 12 or valves 11 and 12. Naturally, traps to collect corrosive reactants or products (not shown) are inser¬ ted on the exhaust line 4 so that any deleterious substance can be retained therein for preserving the environment.

The overall reaction here is 1C1₃ + NR, —> AIN + 3HC1 which occurs to a significant extent within the voids between the particles or whiskers of the preforms. Hence the AIN formed progressively fills the voids in the open structure and progressive densification occurs.

Under the above range of conditions, infiltration to a high material density (about 3-3,2) required about 10 - 50 hrs for 2 to 12 samples 100 - 500 μm thick of 2 - 20 cm² total surface and 40 - 60% porosity, i.e. void volume, in a 1 to 4 liter enclosure supplied with 0.2 - 1 liter of input gases/min.

After reaching the desired extent of infiltration, the densified bait can be coated on one or both sides with cry¬ stalline AIN grown by CVD. For this, the HCl flow is strongly reduced or cut off completely, while the input of A1C1₃ vapor and NH₃ are increased by at least about 50% to about 3 to 6 fold. The temperature and pressures need not be modified but can be changed if desired. Under these conditions AIN growing rates of 50 - 100 μm/hr can be obtained, the AIN provided 15

further advantage of the invention.

Also whiskers predominant orientation in the preforms has an influence on optimalizing the heat dissipation capacity; this is probably due to some preferential direction of heat migration within the crystals of the whiskers. The orientation of the whiskers in the core preform can be controlled to some extent by the casting or molding conditions, i.e. centrifugal casting will result in partial whiskers alignement. Also for¬ cing the whiskers slurry through a narrow tube.will produce some alignment in the direction of flow.

Diamond deposition can be carried out in an apparatus schematically illustrated in figure 5.

This apparatus comprises a holder tube 31 housing a refractory plate 32 which supports mono crystal whiskers on which diamond is to be deposited. The whiskers 33 are placed as a loose heap on the plate 32 so that methane and hydrogen (see arrow) can flow freely through the piled whiskers.

The apparatus also comprises a microwave guide 34 crossed by tube 31 in such a position that the microwave energy in wave-guide 34 resonates and creates a gas discharge in tube 31 to which the whiskers 33 are subjected. The microwave guide 34 is powered by a generator 35, the output of which is tuned by a series of plungers 36a, 36b, 36c. The reflected power is detected by probes 37 and measured by gauges 38. Optimized energy transfer (by correct adjustment of plungers 36a to 36c) results in minimal reflected power as monitored on gauges 38. Element 39 is an insulator.

Figure 6 illustrates the results obtained after 7 hrs operation on SiC whiskers under the following conditions:

frequency 2.45 GHz; generated power 400 W; consumed power 370 W; gas mixture H₂ with 1% CH., flow rate 20 ml/min; pressure 15 Torr.

The whiskers were glowing dark red. The diamond crystals were a few μm size.

The following Examples illustrate the practical aspects of the invention. structure and improves impregnation efficiency.

After infiltration, the preforms are thickened by growing thick dense AIN under the conditions mentioned before. It should be noted that the coarse surface condition of the preform bait after infiltration which derives from using coar¬ se AIN powders and/or whiskers for making the bait results in excellent adhesion thereto of the dense AIN reinforcing layer ultimately grown in the third phase of the method, this being an additional advantage thereof.

The apparatus illustrated in figures 1, 2 and 3 has been disclosed in connection with the CVD infiltration and deposi¬ tion of AIN, this being for simplifying the discussion. It is however evident that the same kind of apparatus can be used with minor modifications for the CVD production of other embedding dense ceramics, e.g. Si₃N., TiN, BN, SiC, A1„0₃ and the like. Of course when the CVD deposition of ceramics diffe¬ rent from AIN is contemplated, different CVD reagents are used in conformity with the teaching of the prior-art regarding such ceramics, i.e. for Si_gN. there is used silicon haloge- nides and ammonia, BC1₃ and ammonia for BN, SiCl. and hydro¬ carbons like methane for SiC, A1C1₃ and water for A1₂0 and corresponding other reagents for other dense ceramics. The deposition conditions are then set up in function to the desired results in conformity with usual practice for the deposition of these materials. This need not be discussed further here because the appropriate conditions can be easily adapted by skilled operators from the teaching of the prior art depending on the desired needs.

In the present invention, although the particulate mate¬ rial described is AIN, the whiskers need not be selected from only one type of whiskers but they can be made of blends or admixtures of two or more whiskers components. Changing the composition of the whisker blends may vary the heat dissipa¬ tion capacity of the heat-sink structure, hence by adjusting the proportions in the blend, skilled ones can devise heat- sink substrates with predetermined heat dissipation capacity. Also, particles of ceramics, e.g. AIN, can be admixed with the whiskers prior to making the starting preforms. This is one 13

being very dense, of very high thermal conductivity and excel¬ lent electrical insulating properties.

When it is desired to infiltrate an AIN whisker mat preform with SiC by CVD instead of AIN, the A1C1-, supply shown in figure 1 is replaced by a wash-bottle filled with a liquid silicon compound, e.g. (CH₃)₂SiCl₂ and swept with argon. Also only hydrogen is introduced via input 3b. The (CH₃)₂SiCl₂ bottle is kept at a given temperature by a thermostat control¬ led bath. The input rate of the vapors of the Si compound can be controlled by the temperature of the heating bath. In this case, the reaction is:

(CH₃)₂SiCl₂ SiC + 2HC1 + CH_χ + (2 - x/2)H₂,

formula in which CH represents hydrocarbons including CH_Δ

The preferred conditions for SiC deposition are: tempera¬ ture of reactor 800 - 1200°C; pressure 2 - 200 Torr; percent of (CH₃)₂S Cl₂ in the carrier gase 1 - 15% v/v; flow rate 5 - 50 standard ml/min per cm of preform material.

According to a second embodiment, the holding plate 5 and the samples to be infiltrated are arranged in the enclosure to create a pressure drop between the upper and lower parts of the enclosure. This becomes possible when the holder plate 5 matches with the inner cross-section of the enclosure 1 which geometry impedes the flow of gases at the sample site. Using an input gas pressure of 150 - 200 Torr, a pressure gradient of 100 - 190 Torr across the samples to be infiltrated can be established. This pressure differential forces the gaseous reactants through the porous structure and speeds-up the in¬ filtration procedure. The input gases compositions and tempe¬ ratures are in the same range as for embodiment 1 above.

In a third embodiment, the needle valve 12 is closed and the electric valve controllably closes and opens at a given rhythm, thus providing a pulsating variable reduced pressure in the enclosure. This pulsed pressure variation establishes a correspondingly varying pressure gradient in the enclosure which helps the reactants to penetrate the preform porous Example 1

There was prepared a slurry of AIN powder by milling together for 18 hrs in a ball-mill containing 200 g of tungsten carbide (WC) balls:

82 g AIN powder (To uyama F; 1 μm grade; less than 1% oxygen)

1 g Fish oil

7 g Dioctyl-phthalate plasticizer (Fluka 80032)

8 g PEG (polyethylene glycol) plasticizer 12 g PVB (polyvinylbutyral) binder (Hoechst) 50 g Trichlorethylene

50 g Ethanol

The viscosity of the slurry after milling was about 4600 cP.

A 100 - 300 μm layer of this paste was deposited by mean of an applicater (doctor's blade) on a 20 mm wide flexible Mylar strip and the solvent was evaporated in air at 25°C. Under evaporation, the sintering composition was converted to a flexible film which was peeled off the Mylar strip and cut into slabs 24, 10 - 40 mm long. The slabs ere placed on a refractory holder 21 of the kind illustrated in fig 2 and 3.

This holder consists of a frame 1 of porous refractory material, for instance of clay or china, provided with holes 22 and 23. The holder is supported by the refractory base plate 5. Removable ceramic plugs (not indicated in the draw¬ ing) are inserted in the holes 23, the upper surface of which is flush with the upper surface of the holder frame 21. The slabs 24 are layered over the holes 23 with the edges resting over the frame surface and the body resting over the plug surface. This arrangement prevents warping of the slabs during calcination.

The frames with slabs were brought into an oven and heated slowly (10°C/hr) up to 500°C under a controlled atmos¬ phere (N- + 0„) containing only 10% of oxygen so that the organic components of the slab were burned although preventing oxidation of the AIN component.

The burned slabs are fragile but, with the help of the present holder, they can be further processed with minimal risks of breaking.

At this stage the slabs (having a porosity of about 40% by volume) can be subjected to mechanical consolidation by presintering at 1500°C under an inert atmosphere (N_?); however due to the handling performances of the present holder, this step can be omitted if desired and the preforms 24 can be infiltrated directly.

The holder with burned slabs 24 was then transferred to a CVD apparatus but before so, the frame 1 with the slabs was lifted from the under-plate 5 retaining the plugs, thus emp¬ tying the holes 23 from the underside. By this operation, both sides of slabs 24 become available to be contacted by the CVD gaseous reactants.

For the infiltration treatment, a total flow input of 500 3 cm was used comprising A1C1₃ vapor 1%, NH₃ 1.5%, HCl 3%, H-

94.5%. Temperature 940°C; pressure about 10 Torr. Infiltration was considered sufficient after a period of 10 - 15 hrs of operation for a slab about 200 μm thick and with a porosity of about 40%.

After this period, the input gas composition was changed to: A1C1₃ 4% (by increasing correspondingly the temperature of the A1C1₃ source) , H₃ 7% (by increasing the rate from the NH-. pressure cylinder); H₂ 89%. The rates of the input components can be controlled individually by means of rotameters (not shown on the drawing) . The total gas input was unchanged as well as the enclosure pressure and temperature. Under these conditions, the deposition rate was about 80 μm/hr. So the deposition process was stopped after about 3 hrs, whereby the initial preform bait thickness had increased by about 500 μm (0.7 mm final thickness). The thermal conductivity was excel¬ lent. Example 2

There was proceeded as in Example 1, making a slurry from

100 g AIN (Tokuyama F)

2 g Fish oil

8 g Camphor

200 g 1:3 mixture of tert.BuOH/petroleu -ether

After milling for 16 hrs with 200 g of balls, the suspen¬ sion was evaporated under vacuum at 70°C (Rotovap) and the dry powder residue was sieved on a 300 μm mesh grating.

Then the powder was pressed into disks of 20 mm diameter,

2 100 μm thick under 1.1 T/cm . The disks (porosity about 30%) were held on a holder and presintered for 1 hr at 1500°C and thereafter placed in a vacuum enclosure for infiltration.

The sample holder shape matched with the inner walls of the enclosure thus providing a barrier to the flowing of gases therein. Hence a pressure gradient could be installed within the enclosure by proper adjustment of the suction pressure at the valve 12 and the input of supply gases at inputs 3a and

3b. Under this gradient of pressure, the reactant gases were forced through the porous structure of the sintered preforms.

The, infiltration conditions were as follows:

Temperature 890°C; initial pressure in upper part of reactor 140 Torr; pressure at output = 2 Torr; gradient = 138

' Torr; input gas flow = 150 cm3; gas composition A1C1-. 0.2%,

NH₃ 0.4%, HCl 2.0%, H₂ 97.4%; infiltration time about 20 - 30 hrs, at the end of which the pressure gradient had increased to 188 Torr.

After infiltration, some of the plugs in the sample holder were removed to equilibrate the pressures, and CVD was resumed using 10 Torr pressure, 910°C in the enclosure and a gas composition of A1C1₃ 6%, H₃ 6%, HCl 7%, H₂ 81% (all by volume). Flow rate 500 cm /mm. The dense AIN growing rate was about 90 μm/hr, whereby a plate about 1 mm thick was obtained after about 5 hrs time. Example 3

Tape cast preforms were prepared as described in Example 1 (porosity 40%, thickness about 0.2 μm) and placed on a holder in the CVD vacuum enclosure. Gas flow between the upper and lower parts of the enclosure were not impeded, hence upper and lower pressures were virtually identical in a matter of seconds.

The operating conditions were set up as follows:

₃

Temperature 900°C; total flow rate 250 cm /min. The in¬ side pressure was periodically varied by acting on the elec¬ tric valve 11 (main valve 12 is closed) , this being controlled by an external electronic regulator not shown. Actually the pressure was set to 50 Torr for 1 min, followed by 30 sec at 7 Torr. The composition of input gases changed in accordance with the pressure changes, i.e. although NH₃ (2%) and HCl (4%) remained constant, A1C1₃ varied from 0.5% (lower pressure stage) to 8.8% (higher pressure stage). The carrier gas was H₂ as usual. After about 50 hrs under the above conditions, the infiltration was considered complete.

Then the overall conditions were altered to:

T = 910°C; pressure = 10 Torr; flow-rate = 500 cm /min and gas composition: A1C1₃ 6%, NH₃ 6%, HCl 1%, H₂ 87%.

Under these conditions, the dense AIN growing rate on both sides of the infiltrated preform was about 90 μm/hr. The operations were stopped after 3 hrs, whereby a 750 μm thick disc of excellent thermal conductivity was obtained.

Example 4

Preparation of ceramic whiskers

In the following Examples, it was possible to use commer¬ cially available whiskers; however in-house made whiskers were preferred for better end-product properties.

For making the whiskers, a vertical cylindrical quartz reactor about 250 mm long and 80 mm diameter surrounded by a HF coil was used. The reactors ends were capped with copper closures with central openings; the lower opening was for initial evacuation (vacuum pump) and the upper opening for exhaust of reaction products. The bottom closure was provided with input ducts for reactant gases opening into the bottom of a vertically oriented nozzle consisting of two coaxial tubes, a central tube connected to one input duct and an external annular tube connected to another input duct.

The reactor contained a central substrate tube for the deposition of whiskers about 180 mm long and 50 mm diameter surrounded by a graphite susceptor and supported on a zirconia washer placed slightly above the upper opening of the afore¬ mentioned nozzle. The central opening in the washer was adap¬ ted to ensure proper mixing of the gas components issuing from the nozzle and passing through it. The zirconia washer rested on a ceramic tubular spacer held on an internal flange of the lower copper closure.

For making silicon carbide whiskers, the central tube was of iron, one input was fed with argon and the other with a mixture of hydrogen and dimethyl-dichlorsilane (DMDS).

Table II below indicates suitable operating conditions for producing SiC whiskers which formed on the surface of the inner tube and were removed afterwards by scraping.

A B E II

T Gas phase Λr + FLo Duration Yield Conversion

^•c V. DHDS X H₂ seem min 9 X

1250 1 5 2000 60 0.6 28

1200 2 10 4000 60 1.4 16

1150 2 10 4000 60 2.0 23

1150 2 10 4000 90 4.4 34

1150 2 10 4000 60 3.1 36

1150 4 20 4000 60 4.8 28 Example 5

There was prepared a slurry of SiC whiskers by stirring together the following ingredients :

82g SiC whiskers (prepared according to the aforemen¬ tioned directions and marked with a A in Table II) lg Fish oil

7g Dioctyl-phthalate plasticizer (Fluka 80032) 8g PEG (polyethylene glycol) plasticizer

12g PVB (polyvinylbutyral) binder (Hoechst)

50g Trichlorethylene (solvent)

50g Ethanol (solvent)

A 100 - 300 μm layer of this slurry was deposited by means of an applicator (doctor's blade) on a 20 mm wide flexi¬ ble Mylar strip and the solvent was evaporated in air at 25°C. By evaporation, the slurry composition was converted to a flexible non-woven layer which was peeled off the Mylar strip and cut into mats 24, 10 - 40 mm long. The mats were placed on a refractory holder fixture 21 of the kind illustrated in figures 2 and 3.

This holder consists of a frame 21 of porous refractory material, for instance of alumina, mullite or alumino-sili- cate, provided with holes 22 and 23. The holder is supported by the refractory base plate 5. Removable ceramic plugs (not indicated in the drawing) are inserted in the holes 23, the upper surface of which is flush with the upper surface of the holder frame 21. The mats 24 are layered over the holes 23 with the edges resting over the frame surface and the body resting over the plug surface. This arrangement prevents war¬ ping of the mats during calcination.

The frame with mats was brought into an oven and heated slowly (10°C/hr) up to 500°C under a controlled atmosphere (N« + 0₂) containing only 10% of oxygen so that the organic compo¬ nents of the mats were burned although preventing oxidation of the AIN component. It should be noted that if in the above composition the binders are replaced by other binders, e.g. polyisobutylene, which can be volatilized under vacuum or in an inert atmosphere, the aforementioned burning step can be avoided.

The burned mats are fragile but, with the help of the present holder, they can be further processed with minimal risks of breaking.

At this stage the mats 24 (having a porosity of about 40% by volume) can be subjected to mechanical consolidation by presintering at 1500°C under an inert atmosphere (N„); however due to the handling performances of the present.holder, this step can be omitted if desired and the mats 24 can be infil¬ trated directly.

The holder with burned mats 24 was then transferred to a CVD apparatus but, before so, the frame 21 with the mats was lifted from its under-plate 5 retaining the plugs, thus emp¬ tying the holes 23 from the underside. By this operation, both sides of mats 24 become available to be contacted by the CVD gaseous reactants.

For the infiltration treatment, a total flow input of 3 500 cm per minute was used comprising A1C1₃ vapor 1%, NH₃

1.5%, HCl 7%, H₂ 90.5%. Temperature 940°C; pressure about 10 Torr. Infiltration and densification were considered suffi¬ cient after a period of 10 - 20 hrs of operation for a mat about 200 μm thick and with a mesh corresponding to about 60% solids (by volume).

After this period, the HCl input was turned off and the input gas composition was changed to : A1C1, 5% (by increasing correspondingly the^'temperature of the A1C1₃ source), NH₃ 7% (by increasing the rate from the NH₃ pressure cylinder); - 88%. The rates of the input components can be controlled individually by means of flow-meters (not shown on the draw¬ ing) . The total gas input was unchanged as well as the enclo¬ sure pressure and temperature. Under these conditions, the deposition rate was about 40 μm/hr. So the deposition process was stopped after about 6 hrs, whereby the initial preform bait thickness had increased by about 500 μm (0.7 mm final thickness). The thermal conductivity was excellent and ex¬ ceeded that of a compact AIN substrate of comparable size. Example 6

There was proceeded as in Example 5, making a slurry from

lOOg AIN whiskers (obtained conventionally: see for instance VINIGRIBKOV et al, Izvestiya Akademii Nauk SSSR 13 (1977), 1775-78; JP 63-75.000 (TOYOTA); H. ITOH et al., J. Crystal Growth 94 (1989), 387-91) 2g Fish oil 8g Camphor 200g 1:3 mixture of tert.BuOH/petroleum-ether

The suspension was drained in a filter under vacuum at

70°C (Rotovap) and the resulting mats (disks of 20 mm diame-

2 ter), 100 μm thick were pressed under 1.1 T/cm . The disks

(porosity about 30%) were held on a holder and presintered for

1 hr at 1500°C and thereafter placed in a vacuum enclosure for infil ration.

The sample holder shape matched with the inner walls of the enclosure thus providing a barrier to the flowing of gases therein. Hence a pressure gradient could be installed within the enclosure by proper adjustment of the suction pressure at the valve 12 and the input of supply gases at inputs 3a and 3b. Under this gradient of pressure, the reactant gases were forced through the porous structure of the non-woven preforms.

The infiltration conditions were as follows :

Temperature 870°C; initial pressure in upper part of reactor 120 Torr; pressure at output 2 Torr; gradient 138 Torr; input gas flow 150 cm /min; gas composition A1C1₃ 0.2%, NH₃ 0.4%, HCl 2.0%, H₂ 97.4%; infiltration time about 20 - 30 hrs, at the end of which the pressure gradient had increased to 370 Torr.

After infiltration, some of the plugs in the sample holder were removed to equilibrate the pressures, and CVD was resumed using 10 Torr pressure, 910°C in the enclosure and a gas composition of A1C1₃ 6%, NH₃ 6%, HCl 7%, H₂ 81% (all by volume). Flow rate 500 cm /min. The dense AIN growing rate was about 50 μm/hr, whereby a plate about 1 mm thick was obtained after about 8 hrs time. The thermal conductivity exceeded that of a comparable compact AIN substrate without embedded AIN whiskers.

Example 7

Silicon carbide whiskers with deposited diamond (see figure 6) were converted to non-woven mat preforms using the technique of Example 5.

Subsequent impregnation was carried out under conditions adapted for SiC deposition, i.e. using dimethyl-dichlorsilane vapors in a carrier gas. The specific condition were as fol¬ lows : temperature 900°C; pressure 100 Torr; gas flow 180 sml/min; gas composition (CH₃)₂SiCl₂ 0.2%, H₂ 1%, Ar 98.8%.

After about 10 hrs operation, infiltration of the preform with SiC was considered complete and the deposition was conti¬ nued for about 20 hrs to build up a compact SiC layer on both sides of the consolidated preform.

The heat conductivity of the obtained substrate was ex¬ cellent.

If in the above Example SiC CVD deposition conditions were replaced by AIN CVD infiltration and growing conditions described in Examples 1 and 2, heat-sink substrates with excellent heat dissipation were obtained.

If in the foregoing Examples the SiC or AIN whiskers were replaced by whiskers of TiC, TiN, Ti(C,N), Si₃ ., TiB₂ or A1₂0₃ (coated or not with diamond), substrates with excellent physical properties (heat conductivity, hardness, etc.) were obtained.

It should be noted that the aforementioned techniques of depositing diamond on ceramic whiskers (see fig. 5) can be advantageously applied directly to non-woven whiskers preforms of the kind disclosed in the previous examples. In this case the casting of the non-woven preform precedes the step of diamond coating; otherwise the manufacturing steps of the substrates are alike.

Claims

1. A method of manufacture of a heat-sink substrate consisting essentially of crystalline acicular particles, i.e. particles in the form of fibers or whiskers of thermally conductive ceramics, and optionally in admixture therewith non-acicular ceramic particles, embedded in a compact matrix phase of non-porous dense thermally conductive ceramics, which comprises the steps of : a) slurrying the particles of ceramics in a solvent in the presence of at least a binder for binding the particles and shaping the slurry into a porous mat¬ like structure of inter-connected particles by mold¬ ing or casting followed by drying and, optionally, consolidation by firing; b) filling the voids between the particles with a dense compact phase of thermally conductive ceramics brought in the solid or the gas phase.

2. The method of Claim 1, in which the particles are whiskers and step (b) is brought about by applying finely powdered ceramics over or into said mat of whiskers, then pressing and sintering.

3. The method of Claim 1, in which step (b) is brought about by subjecting said mat of whiskers to infiltration by chemical vapor deposition (CVD).

4. The method of Claim 3, in which infiltration by CVD comprises contacting the mat in an enclosure at high tempera¬ ture with a gaseous composition of reactants whose reactions in the gas phase will produce said matrix of dense thermally conductive ceramics, this being continued for a time suffi¬ cient to provide full densification to a composite core of whiskers embedded in said matrix.

5. The method of Claim 4, in which the CVD treatment is continued after densification, this resulting in the gro¬ wing over said composite core of at least one layer of homoge¬ neous compact CVD deposited ceramics.

6. The method of claim 3 in which there are used diamond coated ceramic whiskers which, after being embedded in a matrix of compact ceramics, will provide a substrate for making machining tools of high speed and efficient, heat dissipation capacity.

7. The method of Claim 3, in which there is used in the porous structure a mixture of acicular and non-acicular parti¬ cles.

8. A heat-sink substrate consisting essentially of crys¬ talline acicular particles of ceramic, e.g. monocrystalline whiskers of thermally conductive ceramics and, optionally, non-acicular ceramic particles embedded in a compact matrix phase of dense thermally conductive ceramics.

9. The heat-sink substrate of Claim 8, in which the ceramics of the whiskers have a different or same chemical composition as that of the dense ceramics of the compact phase.

10. The heat-sink substrate of Claim 8 in the form of a laminated structure comprising at least one composite core layer consisting essentially of ceramic particles comprising monocrystalline heat-conductive ceramic whiskers and, optio¬ nally, non-whisker ceramic particles embedded in a compact matrix phase of dense thermally-conductive ceramics and, inte¬ grally bound to said core layer, at least one cladding layer of dense thermally-conductive ceramics.

11. The heat-sink substrate of Claim 9, in which the ceramics of the whiskers and of the optional non-whisker particles are selected from Si₃N., SiC, AIN, BeO, A1„0-, and mixtures thereof and the ceramics of the compact matrix are selected from AIN, Si₃N₄, TiN, TiC, Ti(C,N), SiC, A1₂0₃, Zr0₂, BeO and 1₂0₃ and mixtures thereof.

12. The heat-sink substrate of Claim 9, in which the whiskers are oriented statistically at random or following a preferred direction in the compact matrix.

13. The heat-sink of Claim 12, in which such preferred direction is substantially parallel or perpendicular to the surface of the substrate.

14. The heat-sink of Claim 10, in which the weight pro¬ portion of particles in said composite core layer is from about 20 to 80% .

15. The heat-sink substrate of Claim 8, in which the whiskers surface is at least partially coated with a deposited thermally conductive material different from that of the whis¬ kers and of the compact phase.

16. The heat-sink substrate of Claim 15, in which the material deposited for coating at least partially the whiskers surface is diamond.

17. A method for making dense AIN substrates of high thermal conductivity to be used in electronics, which comprises the steps of: a) pressing AIN powder into a thin porous preform bait using only moderate pressures and tempera¬ tures, b) densifying said porous bait by infiltrating the pores with AIN under CVD conditions, i.e. by contacting it with a gaseous composition con¬ taining volatile nitrogen and aluminum com¬ pounds under conditions whereby these compounds react together by forming AIN which penetrates into the pores and deposits therein in order to fill the pores with AIN, then when a desired density is attained, c) growing on both sides of the densified bait layers of dense crystalline AIN by the CVD technique.

18. The method of claim 17, in which said gaseous compo¬ sition comprises A1C1₃ vapors and NH₃ carried by hydrogen carrier gas and containing a proportion of gaseous HCl for infiltration (b) and less or no HCl for growing dense crystal¬ line AIN (c) .

19. The method of claim 18, in which the volume ratio of HCl to the total of A1C1₃ vapor and NH₃ is from about 0.5:1 to 10:1 .

20. The method of claim 19, in which the proportion of HCl by volume in the gaseous composition is from about 1 to 20%.

21. The method of claim 18, in which the gas composition is forced through the porous preforms during the infiltration stage (b) .

22. The method of claim 21, in which a pressure differen¬ tial is established across the porous preforms which causes the gases to be forced into the pores thereof.

23. The method of claim 18, in which for carrying out phase (c), the proportion of A1C1₃ and NH₃ in the gaseous composition is 5 - 10%, respectively, and the quantity of HCl is zero to 3%, the temperature is 800 to 1000°C and the pres¬ sure is 5 - 20 Torr, whereby the deposition rate of dense crystalline AIN is 50 to 120 μm/hr.

24. The method of claim 18, in which during the infiltra¬ tion stage (b), the pressure within the enclosure varies pe¬ riodically according to a predetermined scheme.

25. The method of claim 24, in which the pressure is raised and lowered by pulses.

26. The method of claim 25, in which the pressure is raised to a few tens of Torr and lowered to a few Torr.

27. The method of claim 27, in which the pressure diffe¬ rential is from about 50 to 200 Torr, The pressure in the bottom part of the enclosure being about 1 - 5 Torr.