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Thermal resistance and electrical insulation of thin low-temperature-deposited diamond films
D IAMOND AND R|U TED / TERIALS ELSEVIER
Diamond and Related Materials 6 (1997) 298-302
Thermal resistance and electrical insulation of thin low-temperaturedeposited diamond films H. Verhoeven *, E. Boettger, A. F16ter, H. Reil3, R. Zachai Daimler-Ben',. A G, New Materials, Postfach 2360, 89013 Uhn, Germany
Abstract Diamond layers only a few microns thick were deposited by microwave plasma-assisted chemical vapour deposition (CVD) on silicon at different substrate temperatures (500, 550 and 800°C) using different methods for nucleation enhancement (ex situ mechanical pretreatrnent or in situ substrate biasing). The thermal resistance was measured for conduction normal to these thin layers, which span a wide range of structural properties flora random small-grained over columnar to highly oriented grain structures. It was shown that the thermal resistance normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The smallest thermal resistances were observed for bias-nucleated, highly oriented films deposited at 800°C with pronounced fibre textures. An upper limit for the effective thermal resistance of the diamond-silicon boundary of 1.8 x 10 - 9 m2K/W was determined for mechanically pretreated, columnar-grained films deposited at low temperatures, which suggests a small interfacial disorder for these films. Furthermore, the electrical insulation of the low-temperature deposited films was shown to be comparable with that of high-temperaturedeposited diamond. © 1997 Elsevier Science S.A. Keywords." Thern~tal resistance; Electrical insulation; Temperature; Diamond films
I. Introduction Diamond is well known for its unusual combination of high thermal conductivity and low electrical conductivity. The additional possibility of large-area deposition from the chemical vapour phase makes CVD diamond a promising material for device-adapted solutions for thermal management problems in high-power electronic systems. Apart from the attachment of higil thermal conductivity diamond substrates to the devices, which introduces substantial thermal resistances due to the interfacial bonding materials, one approach is the direct deposition of diamond on semiconductor materials at low temperatures ( T < 400°C). The latter method is most effective if a small thermal resistance for conduction normal to the interface layer is achieved. Up to now, only a few experimental investigations have dealt with the thermal conduction through the boundaries of CVD diamond with other materials [1-3]. A further insight into the dependence of the thermal resistance on structural properties is needed, especially for diamond films deposited at low temperatures. Furthermore, in view of * Corresponding author. 0925-9635/97/$17.00© 1997ElsevierScienceS.A. All rights reserved. PH S0925-9635 (96)00681-4
possible applications in electronics, the choice of the nucleation enhancement method and the deposition technique seems to play a major role. In the present paper we report on the influence of the orientation and size of the grains on the thermal resistance for conduction normal to CVD diamond layers only a few microns thick. Thermal resistance data for a variety of layers with distinct structural properties were obtained. Films were deposited on silicon by microwave plasma-assisted CVD at temperatures ranging from the relatively low value of 500°C (using an ex situ mechanical pretreatment for nucleation enhancement) to 800°C (in situ substrate biasing). Additionally, the electrical conductivity of the films was investigated.
2. Experimental details Diamond layers with thicknesses between 0.5 and 4.0 ~tm were grown on (001) silicon substrates by microwave plasma-assisted CVD. Nucleation enhancement was achieved using both ex situ mechanical pretreatment by scratching with 0. l ~tm diamond powder and in situ
H. Verhoeven et al. / Diamond and Related Materials 6 (1997) 298-302
application of a - 2 0 0 V bias voltage between the substrate and the microwave plasma. The substrate temperature, To, during the diamond deposition was kept at 500 or 550°C for the mechanically pretreated films and for the bias-nucleated films at about 800°C. In the case of mechanical pretreatment, a series of diamond films was fabricated at 550°C under growth conditions as identical as possible. For protnoting lowtemperature growth of diamond, the mechanically pretreated films were deposited using oxygen-rich gas systems. In order to obtain fibre-textured films with epitaxially aligned grains, nitrogen with concentrations in the ppm-range was added to the gas mixture during the growth of the bias-nucleated films. For the photothermal measurements, the layers were metallized with 20 nm of titanium and 450 nm of gold. The thicknesses of the diamond and metal layer'; were determined using cross-sectional scanning electron microscopy (SEM). Measurements of the thermal resistance for conduction normal to the thin diamond layers and the diamond-silicon boundaries were performed at room temperature using a laser-heating method. This photothermal technique is described in detail in Ref. [1]. Briefly, it uses laser-reflectance thermometry to monitor the transient temperature at the gold surface for time scales as short as a few 100 ns after a brief laser-heating pulse. The thermal resistance is extracted by analyzing the shape of the measured response with a solution of the transient, one-dimensional thermal-conduction equation for the case of a three-layer system (metal/ diamond/silicon). The present analysis assumes for simplification that the thermal conductivity of each layer is homogeneous. Information about the structural properties of the films was obtained by means of SEM and micro-Raman spectroscopy~ Rr.man spectra were recorded at room temperature at a laser wavelcngth of 488 nm with a focal diameter of about l p m using a conventional Raman microspectroscopy triple system (ISA T64000). Confocal imaging yielded surface-sensitive Raman signals. Measurements of the electrical conductivity were performed at room temperature using a metal/insulator/semiconductor structure (MIS) with the silicon substrate as back contact. Metal contacts of diameter 620 pm were prepared by evaporation of titanium, platinum and gold onto the diamond films. To reduce surface conductivity, the samples were cleaned by oxygen plasma etching prior to contact formation. Film conductivities were measured with a source-measure unit (Keithley 237) by applying voltages up to + 200 V.
3, Results and discussion
Planar and cross-sectional SEM micrographs typical of the diamond layers investigated are shown in Fig. 1.
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Fig. 1. Planar and cross-sectional SEM micrographs showing the distinct grain structure of CVD diamond films grown with different nucleation pretreatment and substrate temperature: (a,b) mechanically pretreated fihn deposited at 550 C; (c-f ) bias-nucleated lihns deposited at 800 C.
The micrographs show that the samples differ signilicantly with respect to grain structure. The mechanically pretreated films exhibit columnar grain structures in the direction of growth with lateral grain dimensions increasing only slightly from the interface towards the diamond top surface. For example, the average grain size of the film shown in Figs. l(a) and l(b) is about 170 nm at the interface and 250 nm at the top surface. In the case of bias-nucleation, films with a random small-grained structure as well as films with grains highly oriented normal ( D i a . ( l l 0 ) / / S i ( l l 0 ) ; epitaxial alignment) and parallel (Dia.( 100)//Si( 100); fibre texture) to the growth direction were obtained. The degree of grain orientation achieved depends strongly on the details of the bias-nucleation process. The random small-grained film shown in Figs. l(c) and l(d) consists mainly of grains with dimensions less than 50 nm, which surround some isolated grains of larger sizes. A significant increase in ~he average grain size from less than 50 nm at the interface up to about 360 nm at the top surface is observable for the highly oriented film shown in Figs. l(e) and 1(f). Near the interface it is evident that
H. I/erhoel'en et aL 1 Dhmiond and Rehtted Materials 6 (1997) 298-302
300
the region between the highly oriented grains is filled with amorphous and fine-grained material. In order to display further differences with respect to the structural properties of the thin diamond layers under investigation, micro-Raman measurements were performed on the cleaved edges. Fig. 2 contrasts the depth-dependent behaviour of the inverse Raman phase purity of a mechanically pretreated film with columnar grain structure (1.9 pm thick; To = 500°C) and a biasnucleated film with highly oriented grain structure (2.7 pm thick; Tt)=800°C). The inverse Raman phase purity is defined by the ratio of the integrated intensity of the non-diamond carbon Raman lines to the integrated diamond Raman line intensity. Thus, it can be interpreted as a qualitative measure of the incorporation of non-diamond carbon into the diamond film and is hence a measure of film quality. In comparison with the highly oriented film, which exhibits a significant gradient in the inverse phase purity as a function of distance from the interface, the mechanically pretreated film shows a nearly homogeneous behaviour. For distances from the interface greater than 1 pm, the quality of the highly oriented film is considerably higher than that of the mechanically pretreated film. These observations are in close agreement with the results of the SEM investigations, which can be seen by comparing the Raman results with Figs. l(b) and l(f). It is worth mentioning that near the interface of the highly oriented film, the nanocrystalline diamond line was also observed in the Raman spectra. Therefore, we believe that the amorphous and fine-grained material filling the region between the highly oriented grains can be ascribed to non-diamond carbon and nanocrystalline diamond. The homogeneity of the mechanically pretreated films was also confirmed by further Raman investigations on the cleaved edges of films deposited at 550°C. The highest inverse phase purity of about 90 was found as expected
for the bias-nucleated film with the random smallgrained structure shown in Figs. 1(c) and l(d). The results of the thermal investigations are depicted in Fig. 3 together with data for bias-nucleated CVD diamond films deposited at 830°C from Ref. [1]. In agreement with theoretical considerations given in this reference, the main feature of the results is that the thermal resistance for conduction normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The columnar grain structure of the mechanically pretreated films deposited at low temperatures is favoured with respect to small thermal resistances in comparison with the random small-grained structure of the bias-nucleated films, although the higher substrate temperature during the growth of the latter films in principle provides more favourable growth conditions for CVD diamond. In order to obtain larger, oriented grains with biasenhanced nucleation, it is decisive to choose the nucleation conditions and the growth parameter ~ in such a way that non-oriented diamond nuclei and amorphous material will be overgrown by the highly oriented nuclei during the first stage of growth [4]. The highly oriented films exhibit a significant reduction in thermal resistance in comparison with all other films. Note that the thermal resistance of the thinnest highly oriented film is comparable with that of the thinnest mechanically pretreated films. However, the continuous improvement observed in structural quality of the highly oriented layers with increasing distance from the interface results in a reduced increase of thermal resistance with layer thickness in comparison with the mechanically pretreated films, which are nearly homogeneous with respect to the structural properties. We believe that the extremely low thermal resistances normal to the highly oriented films ~
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Fig. 2. Micro-Raman measurements on the cleaved edge of a biasnucleated film with highly oriented grain structure and a mechanically pretreated film with columnar grain structure.
Fig. 3. The~'mal resistance for conduction normal to thin metallized CVD diamond layers deposited with different nucleation pretreatments and substrate temperatures on silicon as a function of layer thickness. Also shown are thermal resistance data from Ref. [!]. The solid line is a linear regression of the thermal data of the mechanically pretreated films fabricated under identical growth conditions at 550°C.
301
H. Verhoeven et al. / Diamond and Related Materials 6 (1997) 298-302
result from the well-developed fibre texture, which enables long phonon mean free paths in the direction normal to these layers. However, the epitaxial alignment of the grains probably has no influence on the thermal resistance for vertical conduction. Transmission electron microscopy (TEM) investigations on highly oriented films fabricated in an identical manner reveal that the oriented grains have either a direct contact with the silicon substrate or are in contact with an interfacial layer of ]3-SIC with thicknesses less than 6 nm [5]. Due to the small thickness and the high bulk thermal conductivity of ~-SiC (>400 W/mK at 300 K; [6]), these interfacial SiC-layers would be expected to result in a small thermal resistance for heat flowing from an oriented grain into the silicon substrate. The boundary resistance due to the diffuse mismatch between gold, diamond and silicon can be calculated to be about 0.6 x 10 -9 m-'K/W at room temperature [7] and, therefore, likewise contributes little to the total thermal resistances measured here. An upper limit for the effective boundary resistance of 1.8 x 10-gm'-K/W can be deduced for the mechanically pretreated films from a linear regression of the thermal data of the sample series fabricated under identical growth conditions at 550°C. The fit of the data by a linear regression requires the assumption of homogeneity for these films, which is consistent with the experimental observations. The upper limit fo~" the effective boundary resistance de termined for these: films is only 3 times greater than the diffuse mismatch value. This suggests that the cont1~bution of interfacial disorder to the effective boundary resistance is small for the films deposited at low temperatures measured here. For comparison, Goodson et al. determined an effective boundary resistance greater than 1.0× 10-am-'K'W for the bias-nucleated fihns with random small-grained structure of Ref. [I]. For randomly oriented films consisting of 10 nm grains, the effective boundary resistance is even an order of magnitude higher [3]. To approach the minimum possible effective boundary resistance for CVD diamond, which is given by the diffuse mismatch limit, this work suggests that the interface area of direct contact of fibre-textured grains with the silicon substrate must be further increased. In order to evaluate the potential of these thin films for electrical insulation, the field strength dependence of the electrical conducti'vity was mcr_,:,ured. Results obtained on a highly oriented film a:~d mechanically pretreated films deposited at low temperatures are shown in Fig. 4. With a low-field conductivity below 10-14 (~¢'~.cm)-i and a dielectric strength above 7 × 10-s V cm- ~, the low-temperature-deposited films exhibit an electrical insulation which is comparable with that of high-temperature-deposited diamond [8]. The exponential increase in the electrical conductivity with field strength observed is typical for CVD diamond and
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Fig. 4. Field strength dependence of the electrical conductivity of a bias-nucleated film with highly oriented grain structure and mechanically pretreated films deposited at low temperatures.
can be interpreted in terms of Poole-Frenkel activation [9].
4. Conclusions
Measurements of the thermal resistance for conduction normal to thin CVD diamond layers on silicon were reported for a variety of samples with distinct structural properties. It was shown that the thermal resistance normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The nearly homogeneous columnar grain ,;tructure observed for films deposited at low temperatures of about 500°C is more favourable with respect to :~mall thermal resistances in comparison with random smallgrained structures. An upper limit for the effective thermal resistance of the diamond-silicon boundary of 1.8 x 10 -9 m2K/W was determined for the low-temperature-deposited films, which suggests a small interracial disot~der for these films. Highly oriented films with their pronounced fibre textures exhibit a further reduction in thermal resistance. This result can be understood in terms of the long phonen mean free path in the direction of fibre-textured grains, and in terms of the low thermal boundary resistance introduced by the highly oriented grains, which have either direct contact with the silicon substrate or are in contact with an interracial B-SiC layer only a few nanometers thick. However, up to now, highly oriented films can be deposited only at high temperatures of about 800C. In view of thermal managment applications, both a reduction in the deposition temperature as well as an increase in the area of direct contact of fibre-textured grains with the silicon substrate would be desirable. Additionally, it was shown that films only about 1 lam thick deposited at low temperatures exhibit an electrical insulation which is comparable with that of diamond deposited at high temperature.
302
H. Verhoeven et aL / Diamond and Related Materhds 6 (1997) 298-302
Acknowledgement The authors wish to thank G. Schulz for performing the SEM investigations, as well as A. Aleksov for the help with the electrical measurements.
References [1] K.E. Goodson, O.W. Kading, M. R6sler and R. Zachai, J. Appl. Phys., 77 (1995) 1385.
[2] K.E. Goodson, O.W. K~iding, M. R6sler and R. Zachai, Appl. Phys. Lett., 66 (1995) 3134. [3] H. Verhoeven, H. ReiB, H.-J. Fiifler and R. Zachai, Appl. Phys. Lett., 69 (1996) 1562. [4] C. Wild, R. Kohl, N. Herres, W. Mt~ller-Sebert and P. Koidl, Dkzmond Rela;. ?,later, 3 (1994) 373. [5] D. Wittorf, personal communication, 1996. [6] R. Berman, Thermal Comhwtion #l Solids, Oxford University Press, Oxford, 1976, Ch. 7. [7] E.T. Swartz and R.O. Pohl, Rev. Mod. Phys., 61 (1989) 605. [8] E. Boettger, X. Jiang and C.-P. Klages, Diamond Relat. Mater., 3 (1994) 957. [9] P. Gonon, Y. Boiko, S. Prawer and D. Jamieson, J. Appl. Phys., 79 (1996) 3778, and references cited therein.
Diamond and Related Materials 6 (1997) 298-302
Thermal resistance and electrical insulation of thin low-temperaturedeposited diamond films H. Verhoeven *, E. Boettger, A. F16ter, H. Reil3, R. Zachai Daimler-Ben',. A G, New Materials, Postfach 2360, 89013 Uhn, Germany
Abstract Diamond layers only a few microns thick were deposited by microwave plasma-assisted chemical vapour deposition (CVD) on silicon at different substrate temperatures (500, 550 and 800°C) using different methods for nucleation enhancement (ex situ mechanical pretreatrnent or in situ substrate biasing). The thermal resistance was measured for conduction normal to these thin layers, which span a wide range of structural properties flora random small-grained over columnar to highly oriented grain structures. It was shown that the thermal resistance normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The smallest thermal resistances were observed for bias-nucleated, highly oriented films deposited at 800°C with pronounced fibre textures. An upper limit for the effective thermal resistance of the diamond-silicon boundary of 1.8 x 10 - 9 m2K/W was determined for mechanically pretreated, columnar-grained films deposited at low temperatures, which suggests a small interfacial disorder for these films. Furthermore, the electrical insulation of the low-temperature deposited films was shown to be comparable with that of high-temperaturedeposited diamond. © 1997 Elsevier Science S.A. Keywords." Thern~tal resistance; Electrical insulation; Temperature; Diamond films
I. Introduction Diamond is well known for its unusual combination of high thermal conductivity and low electrical conductivity. The additional possibility of large-area deposition from the chemical vapour phase makes CVD diamond a promising material for device-adapted solutions for thermal management problems in high-power electronic systems. Apart from the attachment of higil thermal conductivity diamond substrates to the devices, which introduces substantial thermal resistances due to the interfacial bonding materials, one approach is the direct deposition of diamond on semiconductor materials at low temperatures ( T < 400°C). The latter method is most effective if a small thermal resistance for conduction normal to the interface layer is achieved. Up to now, only a few experimental investigations have dealt with the thermal conduction through the boundaries of CVD diamond with other materials [1-3]. A further insight into the dependence of the thermal resistance on structural properties is needed, especially for diamond films deposited at low temperatures. Furthermore, in view of * Corresponding author. 0925-9635/97/$17.00© 1997ElsevierScienceS.A. All rights reserved. PH S0925-9635 (96)00681-4
possible applications in electronics, the choice of the nucleation enhancement method and the deposition technique seems to play a major role. In the present paper we report on the influence of the orientation and size of the grains on the thermal resistance for conduction normal to CVD diamond layers only a few microns thick. Thermal resistance data for a variety of layers with distinct structural properties were obtained. Films were deposited on silicon by microwave plasma-assisted CVD at temperatures ranging from the relatively low value of 500°C (using an ex situ mechanical pretreatment for nucleation enhancement) to 800°C (in situ substrate biasing). Additionally, the electrical conductivity of the films was investigated.
2. Experimental details Diamond layers with thicknesses between 0.5 and 4.0 ~tm were grown on (001) silicon substrates by microwave plasma-assisted CVD. Nucleation enhancement was achieved using both ex situ mechanical pretreatment by scratching with 0. l ~tm diamond powder and in situ
H. Verhoeven et al. / Diamond and Related Materials 6 (1997) 298-302
application of a - 2 0 0 V bias voltage between the substrate and the microwave plasma. The substrate temperature, To, during the diamond deposition was kept at 500 or 550°C for the mechanically pretreated films and for the bias-nucleated films at about 800°C. In the case of mechanical pretreatment, a series of diamond films was fabricated at 550°C under growth conditions as identical as possible. For protnoting lowtemperature growth of diamond, the mechanically pretreated films were deposited using oxygen-rich gas systems. In order to obtain fibre-textured films with epitaxially aligned grains, nitrogen with concentrations in the ppm-range was added to the gas mixture during the growth of the bias-nucleated films. For the photothermal measurements, the layers were metallized with 20 nm of titanium and 450 nm of gold. The thicknesses of the diamond and metal layer'; were determined using cross-sectional scanning electron microscopy (SEM). Measurements of the thermal resistance for conduction normal to the thin diamond layers and the diamond-silicon boundaries were performed at room temperature using a laser-heating method. This photothermal technique is described in detail in Ref. [1]. Briefly, it uses laser-reflectance thermometry to monitor the transient temperature at the gold surface for time scales as short as a few 100 ns after a brief laser-heating pulse. The thermal resistance is extracted by analyzing the shape of the measured response with a solution of the transient, one-dimensional thermal-conduction equation for the case of a three-layer system (metal/ diamond/silicon). The present analysis assumes for simplification that the thermal conductivity of each layer is homogeneous. Information about the structural properties of the films was obtained by means of SEM and micro-Raman spectroscopy~ Rr.man spectra were recorded at room temperature at a laser wavelcngth of 488 nm with a focal diameter of about l p m using a conventional Raman microspectroscopy triple system (ISA T64000). Confocal imaging yielded surface-sensitive Raman signals. Measurements of the electrical conductivity were performed at room temperature using a metal/insulator/semiconductor structure (MIS) with the silicon substrate as back contact. Metal contacts of diameter 620 pm were prepared by evaporation of titanium, platinum and gold onto the diamond films. To reduce surface conductivity, the samples were cleaned by oxygen plasma etching prior to contact formation. Film conductivities were measured with a source-measure unit (Keithley 237) by applying voltages up to + 200 V.
3, Results and discussion
Planar and cross-sectional SEM micrographs typical of the diamond layers investigated are shown in Fig. 1.
(
);,:~,
~
%
299
......
.,~o.
Fig. 1. Planar and cross-sectional SEM micrographs showing the distinct grain structure of CVD diamond films grown with different nucleation pretreatment and substrate temperature: (a,b) mechanically pretreated fihn deposited at 550 C; (c-f ) bias-nucleated lihns deposited at 800 C.
The micrographs show that the samples differ signilicantly with respect to grain structure. The mechanically pretreated films exhibit columnar grain structures in the direction of growth with lateral grain dimensions increasing only slightly from the interface towards the diamond top surface. For example, the average grain size of the film shown in Figs. l(a) and l(b) is about 170 nm at the interface and 250 nm at the top surface. In the case of bias-nucleation, films with a random small-grained structure as well as films with grains highly oriented normal ( D i a . ( l l 0 ) / / S i ( l l 0 ) ; epitaxial alignment) and parallel (Dia.( 100)//Si( 100); fibre texture) to the growth direction were obtained. The degree of grain orientation achieved depends strongly on the details of the bias-nucleation process. The random small-grained film shown in Figs. l(c) and l(d) consists mainly of grains with dimensions less than 50 nm, which surround some isolated grains of larger sizes. A significant increase in ~he average grain size from less than 50 nm at the interface up to about 360 nm at the top surface is observable for the highly oriented film shown in Figs. l(e) and 1(f). Near the interface it is evident that
H. I/erhoel'en et aL 1 Dhmiond and Rehtted Materials 6 (1997) 298-302
300
the region between the highly oriented grains is filled with amorphous and fine-grained material. In order to display further differences with respect to the structural properties of the thin diamond layers under investigation, micro-Raman measurements were performed on the cleaved edges. Fig. 2 contrasts the depth-dependent behaviour of the inverse Raman phase purity of a mechanically pretreated film with columnar grain structure (1.9 pm thick; To = 500°C) and a biasnucleated film with highly oriented grain structure (2.7 pm thick; Tt)=800°C). The inverse Raman phase purity is defined by the ratio of the integrated intensity of the non-diamond carbon Raman lines to the integrated diamond Raman line intensity. Thus, it can be interpreted as a qualitative measure of the incorporation of non-diamond carbon into the diamond film and is hence a measure of film quality. In comparison with the highly oriented film, which exhibits a significant gradient in the inverse phase purity as a function of distance from the interface, the mechanically pretreated film shows a nearly homogeneous behaviour. For distances from the interface greater than 1 pm, the quality of the highly oriented film is considerably higher than that of the mechanically pretreated film. These observations are in close agreement with the results of the SEM investigations, which can be seen by comparing the Raman results with Figs. l(b) and l(f). It is worth mentioning that near the interface of the highly oriented film, the nanocrystalline diamond line was also observed in the Raman spectra. Therefore, we believe that the amorphous and fine-grained material filling the region between the highly oriented grains can be ascribed to non-diamond carbon and nanocrystalline diamond. The homogeneity of the mechanically pretreated films was also confirmed by further Raman investigations on the cleaved edges of films deposited at 550°C. The highest inverse phase purity of about 90 was found as expected
for the bias-nucleated film with the random smallgrained structure shown in Figs. 1(c) and l(d). The results of the thermal investigations are depicted in Fig. 3 together with data for bias-nucleated CVD diamond films deposited at 830°C from Ref. [1]. In agreement with theoretical considerations given in this reference, the main feature of the results is that the thermal resistance for conduction normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The columnar grain structure of the mechanically pretreated films deposited at low temperatures is favoured with respect to small thermal resistances in comparison with the random small-grained structure of the bias-nucleated films, although the higher substrate temperature during the growth of the latter films in principle provides more favourable growth conditions for CVD diamond. In order to obtain larger, oriented grains with biasenhanced nucleation, it is decisive to choose the nucleation conditions and the growth parameter ~ in such a way that non-oriented diamond nuclei and amorphous material will be overgrown by the highly oriented nuclei during the first stage of growth [4]. The highly oriented films exhibit a significant reduction in thermal resistance in comparison with all other films. Note that the thermal resistance of the thinnest highly oriented film is comparable with that of the thinnest mechanically pretreated films. However, the continuous improvement observed in structural quality of the highly oriented layers with increasing distance from the interface results in a reduced increase of thermal resistance with layer thickness in comparison with the mechanically pretreated films, which are nearly homogeneous with respect to the structural properties. We believe that the extremely low thermal resistances normal to the highly oriented films ~
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Fig. 2. Micro-Raman measurements on the cleaved edge of a biasnucleated film with highly oriented grain structure and a mechanically pretreated film with columnar grain structure.
Fig. 3. The~'mal resistance for conduction normal to thin metallized CVD diamond layers deposited with different nucleation pretreatments and substrate temperatures on silicon as a function of layer thickness. Also shown are thermal resistance data from Ref. [!]. The solid line is a linear regression of the thermal data of the mechanically pretreated films fabricated under identical growth conditions at 550°C.
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result from the well-developed fibre texture, which enables long phonon mean free paths in the direction normal to these layers. However, the epitaxial alignment of the grains probably has no influence on the thermal resistance for vertical conduction. Transmission electron microscopy (TEM) investigations on highly oriented films fabricated in an identical manner reveal that the oriented grains have either a direct contact with the silicon substrate or are in contact with an interfacial layer of ]3-SIC with thicknesses less than 6 nm [5]. Due to the small thickness and the high bulk thermal conductivity of ~-SiC (>400 W/mK at 300 K; [6]), these interfacial SiC-layers would be expected to result in a small thermal resistance for heat flowing from an oriented grain into the silicon substrate. The boundary resistance due to the diffuse mismatch between gold, diamond and silicon can be calculated to be about 0.6 x 10 -9 m-'K/W at room temperature [7] and, therefore, likewise contributes little to the total thermal resistances measured here. An upper limit for the effective boundary resistance of 1.8 x 10-gm'-K/W can be deduced for the mechanically pretreated films from a linear regression of the thermal data of the sample series fabricated under identical growth conditions at 550°C. The fit of the data by a linear regression requires the assumption of homogeneity for these films, which is consistent with the experimental observations. The upper limit fo~" the effective boundary resistance de termined for these: films is only 3 times greater than the diffuse mismatch value. This suggests that the cont1~bution of interfacial disorder to the effective boundary resistance is small for the films deposited at low temperatures measured here. For comparison, Goodson et al. determined an effective boundary resistance greater than 1.0× 10-am-'K'W for the bias-nucleated fihns with random small-grained structure of Ref. [I]. For randomly oriented films consisting of 10 nm grains, the effective boundary resistance is even an order of magnitude higher [3]. To approach the minimum possible effective boundary resistance for CVD diamond, which is given by the diffuse mismatch limit, this work suggests that the interface area of direct contact of fibre-textured grains with the silicon substrate must be further increased. In order to evaluate the potential of these thin films for electrical insulation, the field strength dependence of the electrical conducti'vity was mcr_,:,ured. Results obtained on a highly oriented film a:~d mechanically pretreated films deposited at low temperatures are shown in Fig. 4. With a low-field conductivity below 10-14 (~¢'~.cm)-i and a dielectric strength above 7 × 10-s V cm- ~, the low-temperature-deposited films exhibit an electrical insulation which is comparable with that of high-temperature-deposited diamond [8]. The exponential increase in the electrical conductivity with field strength observed is typical for CVD diamond and
>I--> I0 a
10-1o
i
i
i
i
10-111 10-12
z
Eo 10-13 0 0 C~ 10.14 ._1 < lO-~S 0 n, !10-16 0 LU ._1 LU
!
0
~
I
2x10 5
i
I
4x10 s
i
I
i
6x10 5
ELECTRICAL FIELD S T R E N G T H [Wcm]
Fig. 4. Field strength dependence of the electrical conductivity of a bias-nucleated film with highly oriented grain structure and mechanically pretreated films deposited at low temperatures.
can be interpreted in terms of Poole-Frenkel activation [9].
4. Conclusions
Measurements of the thermal resistance for conduction normal to thin CVD diamond layers on silicon were reported for a variety of samples with distinct structural properties. It was shown that the thermal resistance normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The nearly homogeneous columnar grain ,;tructure observed for films deposited at low temperatures of about 500°C is more favourable with respect to :~mall thermal resistances in comparison with random smallgrained structures. An upper limit for the effective thermal resistance of the diamond-silicon boundary of 1.8 x 10 -9 m2K/W was determined for the low-temperature-deposited films, which suggests a small interracial disot~der for these films. Highly oriented films with their pronounced fibre textures exhibit a further reduction in thermal resistance. This result can be understood in terms of the long phonen mean free path in the direction of fibre-textured grains, and in terms of the low thermal boundary resistance introduced by the highly oriented grains, which have either direct contact with the silicon substrate or are in contact with an interracial B-SiC layer only a few nanometers thick. However, up to now, highly oriented films can be deposited only at high temperatures of about 800C. In view of thermal managment applications, both a reduction in the deposition temperature as well as an increase in the area of direct contact of fibre-textured grains with the silicon substrate would be desirable. Additionally, it was shown that films only about 1 lam thick deposited at low temperatures exhibit an electrical insulation which is comparable with that of diamond deposited at high temperature.
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H. Verhoeven et aL / Diamond and Related Materhds 6 (1997) 298-302
Acknowledgement The authors wish to thank G. Schulz for performing the SEM investigations, as well as A. Aleksov for the help with the electrical measurements.
References [1] K.E. Goodson, O.W. Kading, M. R6sler and R. Zachai, J. Appl. Phys., 77 (1995) 1385.
[2] K.E. Goodson, O.W. K~iding, M. R6sler and R. Zachai, Appl. Phys. Lett., 66 (1995) 3134. [3] H. Verhoeven, H. ReiB, H.-J. Fiifler and R. Zachai, Appl. Phys. Lett., 69 (1996) 1562. [4] C. Wild, R. Kohl, N. Herres, W. Mt~ller-Sebert and P. Koidl, Dkzmond Rela;. ?,later, 3 (1994) 373. [5] D. Wittorf, personal communication, 1996. [6] R. Berman, Thermal Comhwtion #l Solids, Oxford University Press, Oxford, 1976, Ch. 7. [7] E.T. Swartz and R.O. Pohl, Rev. Mod. Phys., 61 (1989) 605. [8] E. Boettger, X. Jiang and C.-P. Klages, Diamond Relat. Mater., 3 (1994) 957. [9] P. Gonon, Y. Boiko, S. Prawer and D. Jamieson, J. Appl. Phys., 79 (1996) 3778, and references cited therein.