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copper composites produced by the pulse plasma sintering (PPS) method
Diamond & Related Materials 27–28 (2012) 29–35
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Synthesis and characterization of the diamond/copper composites produced by the pulse plasma sintering (PPS) method☆ M. Rosinski ⁎, L. Ciupinski, J. Grzonka, A. Michalski, K.J. Kurzydlowski Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, 02-507 Warsaw, Poland
a r t i c l e
i n f o
Available online 24 May 2012 Keywords: Diamond Copper/diamond composites Sintering Pulse plasma sintering PPS Thermal conductivity
a b s t r a c t The rapidly advancing miniaturization of micro-electronic devices leads to a considerable increase of the amount of heat evolved by electronic circuits. This, combined with the inexorable increase in the clock speed of running such devices, results in skyrocketing power densities in modern devices such as microprocessors and other high-performance chips. It is anticipated that, in the current decade, the power density will reach the limiting value possible to dissipate by the materials used at the present. As predicted by Patrick Gelsinger (Intel CTO), during the next few years the semiconductor industry will be “heating to a meltdown”, with the trend of power densities in modern microprocessors literally escalating toward levels found within a nuclear reactor. In order to enable the packing density of micro-electronic devices to be further increased, we need new materials of higher thermal conductivity. Another requirement is that these materials should have a thermal expansion coefficient comparable with that of the microelectronic substrate material so as to avoid damage to the heat sink/substrate joint due to the thermal stresses induced by cyclic temperature variation. These requirements can be satisfied by the diamond/metal composites with the metal matrix of high thermal conductivity, such as e.g. Cu. The thermal properties (conductivity, thermal expansion) of the composites can easily be tailored by modifying the metal/diamond proportion. However, within the temperature range of consolidation of these composites, diamond is a metastable phase and may, during the consolidation, be transformed into its stable phase i.e. graphite. This can be avoided by conducting the process under conditions of thermodynamic stability of diamond, i.e. by applying appropriately high consolidation pressure (4–5 GPa), which however increases the production costs. The authors of the present study experimented with producing copper/diamond composites with 50 vol.% of diamond particles under conditions of thermodynamic instability of diamond by consolidating the composite using the pulse plasma sintering (PPS) method. The process temperature was 900 °C, the pressure was 80 MPa and the sintering time was 10 min. The phase composition, density and microstructure of the composites thus obtained were examined. The Cu/diamond PPSconsolidated composites had a theoretical density of 96% and the diamond particles were distributed uniformly within the copper matrix. The major challenge in the development of this kind of composites is to obtain a well bonded interface between the copper and the diamond. To increase the interfacial bonding in the Cu/diamond composites the copper was alloyed with chromium to form Cu0.8Cr. The Cu0.8Cr/diamond composite had a theoretical density of 99.8% and was characterized by a strong bond between the diamond and the copper matrix, which was due to formation the interface between diamond and copper matrix. This paper presents the results of TEM examinations of this interface and describes the method of preparation of thin foils cut through it using a FIB technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Heat-dissipating components in electronic devices are at the present made of Cu–W, Cu–Mo, and Al–SiC [1] whose linear thermal expansion coefficient is close to that of the semiconductor materials used in microelectronics. In view however of the expected increase of the power ☆ Presented at the Diamond 2011, 22st European Conference on Diamond, DiamondLike Materials, Carbon Nanotubes, and Nitrides, Garmisch-Partenkirchen. ⁎ Corresponding author. Fax: + 48 22 2348446. E-mail address: [email protected] (M. Rosinski). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.05.008
density in microelectronic devices in the next decade, the thermal properties of these materials are not sufficiently good [2,3]. The material which has the highest thermal conductivity is monocrystalline diamond. Depending on the contamination degree its thermal conductivity ranges from 1500 to 2000 W/mK [4]. Diamond layers produced by the CVD method have good thermal conductivity (1000–1500 W/mK), but their application range is limited chiefly because of the great difference between their linear thermal expansion coefficient and the coefficient of semiconductor materials (diamond — 1.8 × 10 − 61/K, GaAs — 5.8 × 10− 61/K [4–8] which often leads to joint failure at the moment of switching on or off the microelectronic circuit [9,10]. Moreover,
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because of the columnar structure of the CVD diamond layers, contrasted with the isotropic structure of sintered composites, heat is here dissipated mainly in one direction. The material that fulfils the requirement of intensive heat dissipation without any disadvantageous side effects is the diamond/Cu composite. In a composite material it is possible to control the thermal conductivity and linear thermal expansion coefficient by modifying the volumetric proportion between of diamond and copper (αdiamond ~ 1.8 × 10− 6 1/K, αCu ~ 17.6 × 10 − 6 1/K). Among the problems faced in sintering the Cu/diamond composites, besides the graphitization of diamond at high temperatures the main difficulty is that copper does not wet diamond and that no chemical reactions take place between them, which would permit a good bond to form [11,12]. The phase interfaces in the Cu/diamond composite for thermal management applications should ensure a strong bond and a minimum thermal resistance. This can be achieved by adding carbide forming elements, e.g. Cr or B to the copper matrix. During the sintering process these additives form an interface between the diamond and matrix, which ensures the strong bond and a low thermal resistance [13]. Ke Chu [14] sintered Cu/diamond composites using MBD6 diamond, with a particle size of 100 μm, covered with a chromium layer 1 μm thick by vacuum evaporation technique. The SPS sintering processes were conducted in the temperature range from 820 to 940 °C for 5 min at a heating rate of 100 °C /min under a load of 30 MPa. The chromium coating of the diamond particles increased the density of the Cu/diamond composite by about 3% irrespective of the sintering temperature. SEM observations showed that the bond between the chromium-coated diamond particles and the matrix was good, contrary to the Cu/diamond composites with uncoated diamond particles where there was no bond between the diamond particles and the matrix and voids were visible. Irrespective however of the grade of diamond and the parameters of the SPS sintering process, the density of the composite did not exceed 96% of theoretical density (TD). The authors of Ref. [15] produced Cu/diamond composites by the consolidation of their constituents at a temperature between 1150 and 1200 °C. In order to avoid graphitization, the consolidation was conducted under pressure of 4.5 GPa, which ensured the thermodynamic stability of diamond. The composites thus produced had the density near the theoretical value and their thermal conductivity exceeded that of copper depending on the diamond content. With 50 vol.% of diamond the thermal conductivity was 520 W/mK and with 70 vol.% of diamond it increased to 742 W/mK. This method is however expensive since, in the consolidation process, appropriate pressed compacts are required and specialized presses suitable for high-temperature sintering under high pressure (‘belt’-type presses [16]) must be available. Prior to the consolidation, the powders have to be placed in a metal capsule. At a sintering temperature of 900 °C diamond is a metastable phase and undergoes graphitization. Within the temperature range from 700 to 1400 °C and in high vacuum (low oxygen partial pressure) the rate at which the diamond graphitization proceeds is very low and it only takes place on the surfaces of the diamond particles [17–19]. It is only above 1400 °C when graphitization proceeds rapidly in the entire volume of diamond. This is why, to avoid graphitization, the sintering process should be conducted at a low temperature in high vacuum and for a short time. In the present study we propose a new method of sintering the Cu/diamond composites by using the pulse plasma sintering (PPS) method. The PPS method has been used for sintering a wide variety of materials, such as nanocrystalline cemented carbides [20], WC/TicBN [21], WCCo/diamond [22], nanocrystalline sinters [23–25] and, in combination with the SHS reaction, for fabricating high-melting ceramics [26–28]. The sintering processes of Cu/diamond composites were conducted at a pressure of 80 MPa under the conditions of thermodynamic instability of diamond contrary to the processes used thus far, where the sintering proceeds under diamond thermodynamic equilibrium conditions at a pressure of 4.5 GPa [15]. We also add
Fig. 1. SEM image of the cubo-octahedral MBD4-grade synthetic diamond powder.
same amount of Cr to ensure good wettability of diamond by copper and to achieve a strong bond between the diamond particles and the matrix, but the manner of introducing Cr differs from that employed by Ke Chu who coated the diamond particles with chromium [14], whereas in our method the matrix is made of Cu0.8Cr. The examination of the interface between diamond and copper matrix is also one of the aims of our study. 2. Experimental methods The Cu/diamond composite containing 50 vol.% of diamond particles was produced using a mixture of a Cu powder or a Cu0.8Cr powder (with a particle size of 40 μm and 15 μm respectively) added with an MBD4 diamond powder (particle size — 180 μm) (Fig. 1) delivered by the Luoyang High-Tech Qiming Superhard Materials Co. The selection of a diamond powder with the large particle size was based on the results obtained by K. Hands and K. Yoshida in their studies on Cu composites added with diamond particles, which showed that the thermal diffusivity and thermal conductivity of the composites increase with increasing diamond particles size and also with increasing diamond volume fraction. These studies have also shown that the thermal properties of diamond/copper composites increase with increasing volume fraction of diamond, but the stronger affecting factor is the diamond particle size [29,30]. The mixtures of the Cu and diamond powders were prepared in a turbular mixer and the mixing operation lasted for 10 h. The powder mixture was heated at a temperature of 220 °C for 1.5 h in a hydrogen atmosphere in order to reduce CuO according to the reaction CuO + H2 → Cu + H2O. The probability of the occurrence of the CuO
Fig. 2. Plots of the process parameters during the pulse plasma sintering of the Cu/ diamond and Cu0.8Cr/diamond composites.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
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Hence the thermodynamic potential of the reaction was: 0
ΔG298 ¼ −86; 65−298⋅48; 78 ¼ −14623; 09½J=mol:
Fig. 3. Surface of the Cu0.8Cr/diamond composite after grinding and polishing.
reduction was estimated based on the value of the thermodynamic potential under standard conditions, ΔG0298 , which was calculated from the relationship: 0
0
0
ΔG298 ¼ ΔH 298 −298ΔS298 : Enthalpy and entropy of the reaction were calculated from: 0
ΔH 298 ¼
0
0
∑ ΔH 298 − ∑ ΔH298 ¼ −241; 95 þ 0−ð−155; 3 þ 0Þ
products
substrate
¼ −86; 65½J=mol 0
ΔS298 ¼
0
0
∑ ΔS298 − ∑ ΔS298 ¼ 33; 32 þ 188; 81−42; 70−130; 65
products
substrate
¼ 48; 78½J=molK:
Under standard conditions the thermodynamic potential appears to be below zero, ΔG0298 b0 and, thus, in the presence of hydrogen CuO is reduced to Cu. These thermodynamic conditions are necessary for the reduction process to proceed but they do not define its rate. The rate at which the thermodynamic equilibrium is reached depends on its characteristic activation energy which, in this case, is the energy of the formation of a copper nucleus. The samples were sized at 15 mm in diameter and 6 mm in height and were sintered in a graphite die at a heating rate of 100 °C/min. The sintering process was conducted by the PPS method under a load of 80 MPa at a pressure of 5 × 10 − 5 mbar and a temperature of 900 °C for 10 min. Fig. 2 shows plots of the process parameters during the pulse plasma sintering of the Cu/diamond composite. The effect of the pulse heating on the microstructure and properties of the Cu/diamond composite, in particular on the structure of the diamond/matrix interface, which has not been investigated in details yet, required careful study. Because of big differences in sputtering rates of Cu and diamond so that it is very difficult to prepare electron-transparent TEM specimens using conventional methods. That is why the Hitachi FB-2100 FIB (40 kV) was used for thin foil preparation with using a special 3D holder. This holder allowed to rotate sample and thinning it from different directions, which is impossible with a standard holder. Fig. 3 shows the surface of a Cu0.8Cr/ diamond composite after grinding and polishing. In order to achieve a relatively flat surface suitable for thinning by the FIB technique, the material was polished using diamond-embedded resins. Having approximately flat surface, the thin foil from interface between Cu matrix and diamond particle was prepared. The particular
Fig. 4. Process of FIB preparation of thin foil from the Cu0.8Cr/diamond composite. The images describe particular steps of FIB preparation starting from showing the place of cutting (a) with zoom (b), sample attachment to 3D holder (c), additional protection W layer deposition at the side of sample (d), view of thin foil after ion polishing from two directions in cross-section (e) and top (f) views.
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100µm
200µm
20µm
20µm
Fig. 5. SEM image of the surface of a fracture of the Cu/diamond composite.
steps of FIB preparation are illustrated in Fig. 4. The place of thin foil cutting where the protection tungsten layer was deposited (a) with its zoom (b) is visible. The next step was sample attachment to 3D
holder (c), deposition of additional protection W layer at the side of sample (d). At Fig. 4e and f the final view of thin foil after ion polishing from two directions is shown in cross-section and top views
100µm
50µm
100µm
10µm
Fig. 6. SEM image of a fracture of the Cu0.8Cr/diamond composite.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
Fig. 7. BF STEM photograph of the Cu0.8Cr/diamond composite.
respectively. In the case of FIB preparation we can obtain better quality of thin foil when material which is more difficult to sputter is directly under protective layer than in the situation when boundary between materials is perpendicular to it and non-uniform sputtering occurs. Thanks to application of 3D holder and polishing sample from two directions the transparent sample was possible to obtain. The microstructure of thus prepared samples was observed in a Hitachi Spherical Aberration Corrected STEM HD2700 (200 kV) scanning transmission electron microscope. The density of the sintered samples was measured by the Archimedes method and their phase composition was examined using a Philips PW 1140 diffractometer. The theoretical densities of pure copper (8.95 g/cm 3) and diamond (3.3 g/cm 3) were used to calculate the theoretical density (TD) of the samples. 3. Results and discussion Fig. 5 shows an image of the surface of a fracture of the Cu/ diamond composite sintered at a temperature of 900 °C under the load of 80 MPa for 10 min. It can be seen that diamond particles have been torn out from the copper matrix quite easily and that the fracture propagates either through diamond/copper interface or Cu matrix. Delaminations at the interface between diamond and copper
33
matrix are also observed. The density of the sintered composite was 96.0% of the theoretical density (determined from the mixing rule), whereas the density of the composite sintered at the same sintering parameters but with Cu0.8Cr matrix was almost 4% higher. These results indicate that the addition of chromium to the Cu matrix increases the density of the sintered composite to a value near TD. The use of the Cu0.8Cr alloy as the matrix material resulted in a change of the morphology of the diamond/matrix interfacial boundary. Also the density of the sintered composite increased to nearly the theoretical value (99.8% of TD). Fig. 6 shows a SEM image of the surface of a fracture in the Cu0.8Cr/diamond composite matrix. One can see that the diamond particles are well bound with the Cu0.8Cr matrix, and no pores are present around them. On the surface of the composite fracture, the SEM photograph reveals transcrystalline fractures of the diamond particles. The presence of transcrystalline fractures of the diamond particles indicates that the bonding forces between the diamond particles and the Cu0.8Cr matrix exceed the matrix cohesion. Improvements in the properties of the composites were achieved thanks to the use of a copper matrix alloyed with chromium that increased the interfacial bonding between the Cu0.8Cr matrix and diamond. This can be achieved thanks to the formation of the interface between diamond and copper matrix. Fig. 7 is a BF (bright field image) STEM photograph of the Cu0.8Cr/diamond composite sintered at a temperature of 900 °C under a load of 80 MPa during 10 min. The STEM analyses revealed a well-defined discontinuous layer about 100–200 nm thick located at the interface between the Cu0.8Cr matrix and the diamond particle surface. W protection layer is coming from FIB process. STEM analyses of the Cu0.8Cr/diamond interface revealed its specific features: the diamond particles appeared to be well bonded to the Cu0.8Cr matrix, no interfacial voids or debonding were observed, and an interfacial layer between a diamond particle and the Cu0.8Cr matrix is visible. This interfacial layer has a decisive influence on the thermal properties of the composite. We suppose that the examinations of the phase composition of the interface between diamond and copper matrix would permit us to find how the composition of this interface in the Cu0.8Cr/diamond composite depends also on the sintering parameters. The present experiments have shown that with the increase of the sintering time from 5 to 10 min the number of trans-crystalline fractures of the diamond particles increases and the number of thorn-out diamond particles decreases. The Cu0.8Cr matrix well adhering to diamond particles is visible on their surfaces. When observing the diamond particles at greater magnifications (Fig. 8) one can see surface regions occupied by a binding phase.
Fig. 8. Microstructure of the Cu0.8Cr/diamond composite.
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Fig. 9. HAADF STEM image of Cu matrix/ diamond particle interface (a) with corresponding EDS chemical concentration maps of copper (b), chromium (c) and carbon (d).
The bond between the diamond and the copper matrix in the Cu0.8Cr/diamond composite appeared to be strong which was due probably to the chromium carbide layer formation. An EDS analysis of these regions (Fig. 9) confirmed the presence of chromium there. The EDS line scans through the interface confirmed these findings. The C, Cr, Cu signals (Fig. 10), registered along the line perpendicular to the Cu0.8Cr/diamond boundary show an increase of chromium signal at the interface between diamond and copper matrix. This suggests formation of chromium compounds at the interface, especially carbides. The formation of interfacial carbides in this type of materials was observed by Schubert et al. [12] who identified the carbide phase as Cr3C2. However, the X-ray diffraction pattern (Fig. 11) does not confirm the occurrence of the chromium carbide phase in the Cu0.8Cr/diamond composite sintered at a temperature of 900 °C
during 10 min. Graphite phase was not detected, either. The use of the Cu0.8Cr alloy as the matrix material resulted in a change of the density of the sintered composite increased to nearly the theoretical value (99.8% of TD). Copper does not wet diamond and no chemical reactions take place between them. Increase in density of Cu0.8Cr/ diamond composite is related to formation of well adhering interface between diamond and copper matrix. Addition of Cr results in good wettability of diamond by the alloyed copper. The Cu0.8Cr/diamond composite fracture surface reveals diamond particles with well adhering pellets (see Fig. 10). Chemical analysis of these pellets, performed using EDS, confirmed the presence of chromium. Only two phases: diamond and copper were identified in x-ray diffraction experiments. This observation suggests that the interfacial layer of chromium carbide is probably too thin to be detectable by the
Fig. 10. EDS line scan (line on the left image) of the interface in the Cu0.8Cr/diamond composite.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
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throughout the matrix. The addition of Cr to the matrix resulted in the formation of chromium carbide at the surfaces of diamond particles resulting in high bonding forces between the diamond and the matrix. Acknowledgments This work was supported by the project nos. 3548/B/T02/2008/35 from the Polish Ministry of Science and Higher Education and UDAPOIG.01.01.02-00-97/09-01 from the European Commission and the Innovative Economy of Polish National Cohesion Strategy. References
Fig. 11. Diffraction spectrum obtained for the Cu0.8Cr/diamond composite sintered at 900 °C.
XRD technique. Comparing our results with those reported by Ke Chu et al. [14], it must be noticed that the thickness of the chromium carbide layer obtained by those investigators differ significantly from our results. In K Chu's study the diamond particles were pre-coated with chromium. In our experiments the interfacial phase formed during the composite synthesis. Based on the results of the microstructure analyses (see Figs. 7 and 8), the thickness of the chromium carbide layer in our composite can be estimated at about 50–200 nm whereas in the K Chu's experiments it is one micron thick, which is enough to be detected by XRD. 4. Conclusions The pulse-plasma sintering method, developed at our laboratory, permits producing Cu/diamond composites and Cu0.8Cr/diamond composites with a theoretical density of 96.0% and 99.8%, respectively. In the composites of both types the diamond is uniformly distributed
[1] C. Zweben, Electron Packaging Prod. 42 (9) (2002) 37. [2] M.J. Ellsworth, The Ninth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, 2, 2004, p. 707. [3] G. Marsh, Mater. Today 6 (5) (2003) 28. [4] J.E. Fidel, The Properties of Natural and Synthetic Diamond, Academic Press, 1992. [5] R.S. Sussmann, J.R. Brandon, S.E. Coe, C.S. Pickles, C.G. Sweeney, A. Wasenczuk, C.J. Wort, C.N. Dodge, Ind. Diamond Rev. 58 (1998) 578. [6] P.M. Fabis, Microelectron. Reliab. 42 (2002) 233. [7] Technical Brochure from P1 Limited, Mountain View, CA, 1999, p. 7. [8] O. Beffort, S. Vaucher, F.A. Khalid, Diam. Relat. Mater. 13 (2004) 1834. [9] J.W. Tomm, A. Gerhardt, T. Elsaesser, D. Lorenzen, P. Henning, Appl. Phys. Lett. 81 (17) (2002) 3269. [10] D. Lorezen, P. Hennig, Proc. SPIE 4945 (2003) 174. [11] Q. Sun, O.T. Inal, Mater. Sci. Res. B41 (1996) 261. [12] T. Schubert, B. Trindate, T. Weissgoearber, B. Kieback, Mater. Sci. Eng. A., 475 (2008) 39. [13] Y. Katsuhito, M. Hideaki, Microelectron. Reliab. 44 (2004) 303. [14] K. Chu, Z. Liu, Ch. Jia, H. Chen, X. Liang, W. Gao, W. Tian, H. Guo, J. Alloys Compd. 490 (1–2) (2010) 453. [15] W. Behr, J.F. Luy, IEEE Electron. Device Lett. 11 (1990) 206. [16] H.T. Hall, Rev. Sci. Instrum. 31 (2) (1960) 174. [17] D.V. Fedoseev, V.L. Buhovest, S.P. Vnukov, Surface 1 (1980) 92 (in Russian). [18] W.Z. Shao, V.V. Ivanov, L. Zhen, Y.S. Cui, Y. Wang, Mater. Lett. 58 (2003) 146. [19] G.B. Bokii, G.N. Bezrukov, U.A. Klyuev, Natural and Synthetic Diamond, Nauka, Moscow, 1986, p. 222, (in Russian). [20] J. Kupczyk, A. Michalski, P. Siwak, M. Rosinski, J. ASTM Int. 8 (2011) 2. [21] M. Rosinski, A. Michalski, J. Szawlowski, Key Eng. Mater. 484 (2011) 130–134. [22] A. Michalski, M. Rosiński, J. Am. Ceram. Soc. 91 (11) (2008) 3560. [23] A. Michalski, J. Jaroszewicz, M. Rosiński, D. Siemiaszko, K.J. Kurzydłowski, Solid State Phenom. 114 (2006) 227. [24] A. Michalski, M. Rosinski, D. Siemiaszko, J. Jaroszewicz, K.J. Kurzydłowski, Solid State Phenom. 114 (2006) 239. [25] M. Rosinski, A. Michalski, Solid State Phenom. 114 (2006) 233. [26] A. Michalski, J. Jaroszewicz, M. Rosiński, D. Siemiaszko, Intermetallics 16 (2006) 603. [27] A. Michalski, J. Jaroszewicz, M. Rosinski, Int. J. Self-Propag. High-Temp. Synth. 12 (2003) 237. [28] D. Roy, S. Singh, R. Mitra, M. Rosinski, A. Michalski, W. Lojkowski, I. Mann, Philos. Mag. Philos. Mag. Lett. 89 (12) (2009) 1051. [29] K. Hanada, K. Matsuzaki, T. Sano, J. Mater. Process. Technol. 153 (2004) 514. [30] K. Yoshida, H. Morigami, Microelectron. Reliab. 44 (2004) 303.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Synthesis and characterization of the diamond/copper composites produced by the pulse plasma sintering (PPS) method☆ M. Rosinski ⁎, L. Ciupinski, J. Grzonka, A. Michalski, K.J. Kurzydlowski Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, 02-507 Warsaw, Poland
a r t i c l e
i n f o
Available online 24 May 2012 Keywords: Diamond Copper/diamond composites Sintering Pulse plasma sintering PPS Thermal conductivity
a b s t r a c t The rapidly advancing miniaturization of micro-electronic devices leads to a considerable increase of the amount of heat evolved by electronic circuits. This, combined with the inexorable increase in the clock speed of running such devices, results in skyrocketing power densities in modern devices such as microprocessors and other high-performance chips. It is anticipated that, in the current decade, the power density will reach the limiting value possible to dissipate by the materials used at the present. As predicted by Patrick Gelsinger (Intel CTO), during the next few years the semiconductor industry will be “heating to a meltdown”, with the trend of power densities in modern microprocessors literally escalating toward levels found within a nuclear reactor. In order to enable the packing density of micro-electronic devices to be further increased, we need new materials of higher thermal conductivity. Another requirement is that these materials should have a thermal expansion coefficient comparable with that of the microelectronic substrate material so as to avoid damage to the heat sink/substrate joint due to the thermal stresses induced by cyclic temperature variation. These requirements can be satisfied by the diamond/metal composites with the metal matrix of high thermal conductivity, such as e.g. Cu. The thermal properties (conductivity, thermal expansion) of the composites can easily be tailored by modifying the metal/diamond proportion. However, within the temperature range of consolidation of these composites, diamond is a metastable phase and may, during the consolidation, be transformed into its stable phase i.e. graphite. This can be avoided by conducting the process under conditions of thermodynamic stability of diamond, i.e. by applying appropriately high consolidation pressure (4–5 GPa), which however increases the production costs. The authors of the present study experimented with producing copper/diamond composites with 50 vol.% of diamond particles under conditions of thermodynamic instability of diamond by consolidating the composite using the pulse plasma sintering (PPS) method. The process temperature was 900 °C, the pressure was 80 MPa and the sintering time was 10 min. The phase composition, density and microstructure of the composites thus obtained were examined. The Cu/diamond PPSconsolidated composites had a theoretical density of 96% and the diamond particles were distributed uniformly within the copper matrix. The major challenge in the development of this kind of composites is to obtain a well bonded interface between the copper and the diamond. To increase the interfacial bonding in the Cu/diamond composites the copper was alloyed with chromium to form Cu0.8Cr. The Cu0.8Cr/diamond composite had a theoretical density of 99.8% and was characterized by a strong bond between the diamond and the copper matrix, which was due to formation the interface between diamond and copper matrix. This paper presents the results of TEM examinations of this interface and describes the method of preparation of thin foils cut through it using a FIB technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Heat-dissipating components in electronic devices are at the present made of Cu–W, Cu–Mo, and Al–SiC [1] whose linear thermal expansion coefficient is close to that of the semiconductor materials used in microelectronics. In view however of the expected increase of the power ☆ Presented at the Diamond 2011, 22st European Conference on Diamond, DiamondLike Materials, Carbon Nanotubes, and Nitrides, Garmisch-Partenkirchen. ⁎ Corresponding author. Fax: + 48 22 2348446. E-mail address: [email protected] (M. Rosinski). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.05.008
density in microelectronic devices in the next decade, the thermal properties of these materials are not sufficiently good [2,3]. The material which has the highest thermal conductivity is monocrystalline diamond. Depending on the contamination degree its thermal conductivity ranges from 1500 to 2000 W/mK [4]. Diamond layers produced by the CVD method have good thermal conductivity (1000–1500 W/mK), but their application range is limited chiefly because of the great difference between their linear thermal expansion coefficient and the coefficient of semiconductor materials (diamond — 1.8 × 10 − 61/K, GaAs — 5.8 × 10− 61/K [4–8] which often leads to joint failure at the moment of switching on or off the microelectronic circuit [9,10]. Moreover,
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M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
because of the columnar structure of the CVD diamond layers, contrasted with the isotropic structure of sintered composites, heat is here dissipated mainly in one direction. The material that fulfils the requirement of intensive heat dissipation without any disadvantageous side effects is the diamond/Cu composite. In a composite material it is possible to control the thermal conductivity and linear thermal expansion coefficient by modifying the volumetric proportion between of diamond and copper (αdiamond ~ 1.8 × 10− 6 1/K, αCu ~ 17.6 × 10 − 6 1/K). Among the problems faced in sintering the Cu/diamond composites, besides the graphitization of diamond at high temperatures the main difficulty is that copper does not wet diamond and that no chemical reactions take place between them, which would permit a good bond to form [11,12]. The phase interfaces in the Cu/diamond composite for thermal management applications should ensure a strong bond and a minimum thermal resistance. This can be achieved by adding carbide forming elements, e.g. Cr or B to the copper matrix. During the sintering process these additives form an interface between the diamond and matrix, which ensures the strong bond and a low thermal resistance [13]. Ke Chu [14] sintered Cu/diamond composites using MBD6 diamond, with a particle size of 100 μm, covered with a chromium layer 1 μm thick by vacuum evaporation technique. The SPS sintering processes were conducted in the temperature range from 820 to 940 °C for 5 min at a heating rate of 100 °C /min under a load of 30 MPa. The chromium coating of the diamond particles increased the density of the Cu/diamond composite by about 3% irrespective of the sintering temperature. SEM observations showed that the bond between the chromium-coated diamond particles and the matrix was good, contrary to the Cu/diamond composites with uncoated diamond particles where there was no bond between the diamond particles and the matrix and voids were visible. Irrespective however of the grade of diamond and the parameters of the SPS sintering process, the density of the composite did not exceed 96% of theoretical density (TD). The authors of Ref. [15] produced Cu/diamond composites by the consolidation of their constituents at a temperature between 1150 and 1200 °C. In order to avoid graphitization, the consolidation was conducted under pressure of 4.5 GPa, which ensured the thermodynamic stability of diamond. The composites thus produced had the density near the theoretical value and their thermal conductivity exceeded that of copper depending on the diamond content. With 50 vol.% of diamond the thermal conductivity was 520 W/mK and with 70 vol.% of diamond it increased to 742 W/mK. This method is however expensive since, in the consolidation process, appropriate pressed compacts are required and specialized presses suitable for high-temperature sintering under high pressure (‘belt’-type presses [16]) must be available. Prior to the consolidation, the powders have to be placed in a metal capsule. At a sintering temperature of 900 °C diamond is a metastable phase and undergoes graphitization. Within the temperature range from 700 to 1400 °C and in high vacuum (low oxygen partial pressure) the rate at which the diamond graphitization proceeds is very low and it only takes place on the surfaces of the diamond particles [17–19]. It is only above 1400 °C when graphitization proceeds rapidly in the entire volume of diamond. This is why, to avoid graphitization, the sintering process should be conducted at a low temperature in high vacuum and for a short time. In the present study we propose a new method of sintering the Cu/diamond composites by using the pulse plasma sintering (PPS) method. The PPS method has been used for sintering a wide variety of materials, such as nanocrystalline cemented carbides [20], WC/TicBN [21], WCCo/diamond [22], nanocrystalline sinters [23–25] and, in combination with the SHS reaction, for fabricating high-melting ceramics [26–28]. The sintering processes of Cu/diamond composites were conducted at a pressure of 80 MPa under the conditions of thermodynamic instability of diamond contrary to the processes used thus far, where the sintering proceeds under diamond thermodynamic equilibrium conditions at a pressure of 4.5 GPa [15]. We also add
Fig. 1. SEM image of the cubo-octahedral MBD4-grade synthetic diamond powder.
same amount of Cr to ensure good wettability of diamond by copper and to achieve a strong bond between the diamond particles and the matrix, but the manner of introducing Cr differs from that employed by Ke Chu who coated the diamond particles with chromium [14], whereas in our method the matrix is made of Cu0.8Cr. The examination of the interface between diamond and copper matrix is also one of the aims of our study. 2. Experimental methods The Cu/diamond composite containing 50 vol.% of diamond particles was produced using a mixture of a Cu powder or a Cu0.8Cr powder (with a particle size of 40 μm and 15 μm respectively) added with an MBD4 diamond powder (particle size — 180 μm) (Fig. 1) delivered by the Luoyang High-Tech Qiming Superhard Materials Co. The selection of a diamond powder with the large particle size was based on the results obtained by K. Hands and K. Yoshida in their studies on Cu composites added with diamond particles, which showed that the thermal diffusivity and thermal conductivity of the composites increase with increasing diamond particles size and also with increasing diamond volume fraction. These studies have also shown that the thermal properties of diamond/copper composites increase with increasing volume fraction of diamond, but the stronger affecting factor is the diamond particle size [29,30]. The mixtures of the Cu and diamond powders were prepared in a turbular mixer and the mixing operation lasted for 10 h. The powder mixture was heated at a temperature of 220 °C for 1.5 h in a hydrogen atmosphere in order to reduce CuO according to the reaction CuO + H2 → Cu + H2O. The probability of the occurrence of the CuO
Fig. 2. Plots of the process parameters during the pulse plasma sintering of the Cu/ diamond and Cu0.8Cr/diamond composites.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
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Hence the thermodynamic potential of the reaction was: 0
ΔG298 ¼ −86; 65−298⋅48; 78 ¼ −14623; 09½J=mol:
Fig. 3. Surface of the Cu0.8Cr/diamond composite after grinding and polishing.
reduction was estimated based on the value of the thermodynamic potential under standard conditions, ΔG0298 , which was calculated from the relationship: 0
0
0
ΔG298 ¼ ΔH 298 −298ΔS298 : Enthalpy and entropy of the reaction were calculated from: 0
ΔH 298 ¼
0
0
∑ ΔH 298 − ∑ ΔH298 ¼ −241; 95 þ 0−ð−155; 3 þ 0Þ
products
substrate
¼ −86; 65½J=mol 0
ΔS298 ¼
0
0
∑ ΔS298 − ∑ ΔS298 ¼ 33; 32 þ 188; 81−42; 70−130; 65
products
substrate
¼ 48; 78½J=molK:
Under standard conditions the thermodynamic potential appears to be below zero, ΔG0298 b0 and, thus, in the presence of hydrogen CuO is reduced to Cu. These thermodynamic conditions are necessary for the reduction process to proceed but they do not define its rate. The rate at which the thermodynamic equilibrium is reached depends on its characteristic activation energy which, in this case, is the energy of the formation of a copper nucleus. The samples were sized at 15 mm in diameter and 6 mm in height and were sintered in a graphite die at a heating rate of 100 °C/min. The sintering process was conducted by the PPS method under a load of 80 MPa at a pressure of 5 × 10 − 5 mbar and a temperature of 900 °C for 10 min. Fig. 2 shows plots of the process parameters during the pulse plasma sintering of the Cu/diamond composite. The effect of the pulse heating on the microstructure and properties of the Cu/diamond composite, in particular on the structure of the diamond/matrix interface, which has not been investigated in details yet, required careful study. Because of big differences in sputtering rates of Cu and diamond so that it is very difficult to prepare electron-transparent TEM specimens using conventional methods. That is why the Hitachi FB-2100 FIB (40 kV) was used for thin foil preparation with using a special 3D holder. This holder allowed to rotate sample and thinning it from different directions, which is impossible with a standard holder. Fig. 3 shows the surface of a Cu0.8Cr/ diamond composite after grinding and polishing. In order to achieve a relatively flat surface suitable for thinning by the FIB technique, the material was polished using diamond-embedded resins. Having approximately flat surface, the thin foil from interface between Cu matrix and diamond particle was prepared. The particular
Fig. 4. Process of FIB preparation of thin foil from the Cu0.8Cr/diamond composite. The images describe particular steps of FIB preparation starting from showing the place of cutting (a) with zoom (b), sample attachment to 3D holder (c), additional protection W layer deposition at the side of sample (d), view of thin foil after ion polishing from two directions in cross-section (e) and top (f) views.
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M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
100µm
200µm
20µm
20µm
Fig. 5. SEM image of the surface of a fracture of the Cu/diamond composite.
steps of FIB preparation are illustrated in Fig. 4. The place of thin foil cutting where the protection tungsten layer was deposited (a) with its zoom (b) is visible. The next step was sample attachment to 3D
holder (c), deposition of additional protection W layer at the side of sample (d). At Fig. 4e and f the final view of thin foil after ion polishing from two directions is shown in cross-section and top views
100µm
50µm
100µm
10µm
Fig. 6. SEM image of a fracture of the Cu0.8Cr/diamond composite.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
Fig. 7. BF STEM photograph of the Cu0.8Cr/diamond composite.
respectively. In the case of FIB preparation we can obtain better quality of thin foil when material which is more difficult to sputter is directly under protective layer than in the situation when boundary between materials is perpendicular to it and non-uniform sputtering occurs. Thanks to application of 3D holder and polishing sample from two directions the transparent sample was possible to obtain. The microstructure of thus prepared samples was observed in a Hitachi Spherical Aberration Corrected STEM HD2700 (200 kV) scanning transmission electron microscope. The density of the sintered samples was measured by the Archimedes method and their phase composition was examined using a Philips PW 1140 diffractometer. The theoretical densities of pure copper (8.95 g/cm 3) and diamond (3.3 g/cm 3) were used to calculate the theoretical density (TD) of the samples. 3. Results and discussion Fig. 5 shows an image of the surface of a fracture of the Cu/ diamond composite sintered at a temperature of 900 °C under the load of 80 MPa for 10 min. It can be seen that diamond particles have been torn out from the copper matrix quite easily and that the fracture propagates either through diamond/copper interface or Cu matrix. Delaminations at the interface between diamond and copper
33
matrix are also observed. The density of the sintered composite was 96.0% of the theoretical density (determined from the mixing rule), whereas the density of the composite sintered at the same sintering parameters but with Cu0.8Cr matrix was almost 4% higher. These results indicate that the addition of chromium to the Cu matrix increases the density of the sintered composite to a value near TD. The use of the Cu0.8Cr alloy as the matrix material resulted in a change of the morphology of the diamond/matrix interfacial boundary. Also the density of the sintered composite increased to nearly the theoretical value (99.8% of TD). Fig. 6 shows a SEM image of the surface of a fracture in the Cu0.8Cr/diamond composite matrix. One can see that the diamond particles are well bound with the Cu0.8Cr matrix, and no pores are present around them. On the surface of the composite fracture, the SEM photograph reveals transcrystalline fractures of the diamond particles. The presence of transcrystalline fractures of the diamond particles indicates that the bonding forces between the diamond particles and the Cu0.8Cr matrix exceed the matrix cohesion. Improvements in the properties of the composites were achieved thanks to the use of a copper matrix alloyed with chromium that increased the interfacial bonding between the Cu0.8Cr matrix and diamond. This can be achieved thanks to the formation of the interface between diamond and copper matrix. Fig. 7 is a BF (bright field image) STEM photograph of the Cu0.8Cr/diamond composite sintered at a temperature of 900 °C under a load of 80 MPa during 10 min. The STEM analyses revealed a well-defined discontinuous layer about 100–200 nm thick located at the interface between the Cu0.8Cr matrix and the diamond particle surface. W protection layer is coming from FIB process. STEM analyses of the Cu0.8Cr/diamond interface revealed its specific features: the diamond particles appeared to be well bonded to the Cu0.8Cr matrix, no interfacial voids or debonding were observed, and an interfacial layer between a diamond particle and the Cu0.8Cr matrix is visible. This interfacial layer has a decisive influence on the thermal properties of the composite. We suppose that the examinations of the phase composition of the interface between diamond and copper matrix would permit us to find how the composition of this interface in the Cu0.8Cr/diamond composite depends also on the sintering parameters. The present experiments have shown that with the increase of the sintering time from 5 to 10 min the number of trans-crystalline fractures of the diamond particles increases and the number of thorn-out diamond particles decreases. The Cu0.8Cr matrix well adhering to diamond particles is visible on their surfaces. When observing the diamond particles at greater magnifications (Fig. 8) one can see surface regions occupied by a binding phase.
Fig. 8. Microstructure of the Cu0.8Cr/diamond composite.
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Fig. 9. HAADF STEM image of Cu matrix/ diamond particle interface (a) with corresponding EDS chemical concentration maps of copper (b), chromium (c) and carbon (d).
The bond between the diamond and the copper matrix in the Cu0.8Cr/diamond composite appeared to be strong which was due probably to the chromium carbide layer formation. An EDS analysis of these regions (Fig. 9) confirmed the presence of chromium there. The EDS line scans through the interface confirmed these findings. The C, Cr, Cu signals (Fig. 10), registered along the line perpendicular to the Cu0.8Cr/diamond boundary show an increase of chromium signal at the interface between diamond and copper matrix. This suggests formation of chromium compounds at the interface, especially carbides. The formation of interfacial carbides in this type of materials was observed by Schubert et al. [12] who identified the carbide phase as Cr3C2. However, the X-ray diffraction pattern (Fig. 11) does not confirm the occurrence of the chromium carbide phase in the Cu0.8Cr/diamond composite sintered at a temperature of 900 °C
during 10 min. Graphite phase was not detected, either. The use of the Cu0.8Cr alloy as the matrix material resulted in a change of the density of the sintered composite increased to nearly the theoretical value (99.8% of TD). Copper does not wet diamond and no chemical reactions take place between them. Increase in density of Cu0.8Cr/ diamond composite is related to formation of well adhering interface between diamond and copper matrix. Addition of Cr results in good wettability of diamond by the alloyed copper. The Cu0.8Cr/diamond composite fracture surface reveals diamond particles with well adhering pellets (see Fig. 10). Chemical analysis of these pellets, performed using EDS, confirmed the presence of chromium. Only two phases: diamond and copper were identified in x-ray diffraction experiments. This observation suggests that the interfacial layer of chromium carbide is probably too thin to be detectable by the
Fig. 10. EDS line scan (line on the left image) of the interface in the Cu0.8Cr/diamond composite.
M. Rosinski et al. / Diamond & Related Materials 27–28 (2012) 29–35
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throughout the matrix. The addition of Cr to the matrix resulted in the formation of chromium carbide at the surfaces of diamond particles resulting in high bonding forces between the diamond and the matrix. Acknowledgments This work was supported by the project nos. 3548/B/T02/2008/35 from the Polish Ministry of Science and Higher Education and UDAPOIG.01.01.02-00-97/09-01 from the European Commission and the Innovative Economy of Polish National Cohesion Strategy. References
Fig. 11. Diffraction spectrum obtained for the Cu0.8Cr/diamond composite sintered at 900 °C.
XRD technique. Comparing our results with those reported by Ke Chu et al. [14], it must be noticed that the thickness of the chromium carbide layer obtained by those investigators differ significantly from our results. In K Chu's study the diamond particles were pre-coated with chromium. In our experiments the interfacial phase formed during the composite synthesis. Based on the results of the microstructure analyses (see Figs. 7 and 8), the thickness of the chromium carbide layer in our composite can be estimated at about 50–200 nm whereas in the K Chu's experiments it is one micron thick, which is enough to be detected by XRD. 4. Conclusions The pulse-plasma sintering method, developed at our laboratory, permits producing Cu/diamond composites and Cu0.8Cr/diamond composites with a theoretical density of 96.0% and 99.8%, respectively. In the composites of both types the diamond is uniformly distributed
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