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High-temperature diamond capacitor
Diamond and Related Materials 8 (1999) 1875–1877 www.elsevier.com/locate/diamond
High-temperature diamond capacitor k W. Ebert a, *, M. Adamschik a, P. Gluche a, A. Flo¨ter b, E. Kohn a a Department of Electron Devices and Circuits, University of Ulm, Albert-Einstein-Allee 45, 89081 Ulm, Germany b Daimler Benz AG, W.-Runge-Str. 11, 89081 Ulm, Germany Received 16 September 1998; accepted 10 May 1999
Abstract Capacitors using diamond membranes as the dielectric were fabricated and evaluated up to 600°C using various MPCVD grown films. The films were grown on Si and were randomly oriented or highly oriented with different grain sizes. In part they were nitrogen doped. Admittance measurements at room temperature (RT ) show a dissipation factor <10−5 at 10 kHz, which was the resolution limit of the measurement system. The loss increases with the temperature with an activation energy of 1.0– 1.3 eV. This activation energy was found to be independent of the diamond CVD growth configuration. The level of the leakage is influenced by the grain size but not by the nitrogen doping concentration. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Capacitor; High temperature diamond; Membrane
1. Introduction High temperature electronic components are an important part of high temperature sensor and actuator systems, with their related energy supply and signal conducting circuits. Thus, besides diodes, transistors, and sensors, passive devices such as resistors and capacitors [1] will also be required. Furthermore, for ease of technology, the basic material of both active and passive devices should be the same. Due to the progress in high temperature sensors and electronics, diamond may be considered a material for high temperature capacitors [2–6 ].
2. Experimental The CVD diamond films were grown on silicon substrates using a novel biasing technique [7] which enables the growth of large area highly oriented and textured diamond films. Free-standing diamond membranes were curved out by wet chemical etching of the Si substrate in a 30% KOH solution at 80°C ( Fig. 1). k
Presented at the Diamond ’98 Conference, Crete, Greece, September 1998. * Corresponding author. Tel.: +49-731-5026-154; fax: +49-731-5026-155. E-mail address: [email protected] ( W. Ebert)
Fig. 1. CVD diamond capacitor.
They serve as the dielectric in the parallel plate capacitors. To investigate the influence of the nitrogen concentration and the grain boundaries, three different samples were used ( Fig. 2). After an oxygen plasma treatment of the diamond surface the contact metal (Au) was deposited by thermal evaporation onto both sides, patterned by standard lithography and structured by wet chemical etching. Au contacts on oxygen terminated surfaces as used here form barriers and act like blocking contacts; however, with grain boundaries reaching up to the metal interface. The area of the capacitors was 10 mm2. The C–V measurements were performed at frequencies between 10 kHz and 2 MHz with a HP 4192A impedance analyzer in the parallel RC mode, the I–V measurements with a HP 4142B DC source. All measurements were undertaken under atmospheric conditions in the temperature range between room temperature (RT ) and 600°C.
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W. Ebert et al. / Diamond and Related Materials 8 (1999) 1875–1877
Fig. 2. Investigated CVD diamond films.
3. Results and discussion For the characterization of capacitors, two measurement techniques are required. From AC measurements the capacitance and the relative dielectric constant e r may be extracted. For the investigated samples a constant e was observed, since their capacitances scaled r with their thickness. Furthermore, DC leakage measurements are necessary for the estimation of the dielectric loss. 3.1. DC leakage measurements The temperature-dependent I–V characteristics of the sample RON are shown in Fig. 3. The extracted leakage depends weakly on the applied voltage but depends strongly on the temperature. In the temperature range between 100°C and 500°C, the resistance varies over seven orders of magnitude. The extracted constant thermal activation energy of 1.3 eV at elevated temperature (T>350°C ) is also observed for the nominally undoped film (sample RO) and for the HOD film (sample HON ) ( Fig. 4). The observed value of the thermal activation energy and the fact that it is not related to the incorporated nitrogen concentrations (<1018 cm−3) is in good agreement with results published in Ref. [8], where an activation energy of 1.2 eV for undoped and 1.4–1.6 eV for heavily nitrogen doped (>1019 cm−3) CVD diamond films is reported. The fact that the thermal activation energy is also not related to the grain size (crystal quality) in the observed range between ~2 mm (samples RON and RO) and ~10 mm (sample HON ) is expected because the
Fig. 3. DC leakage current of the sample RON.
Fig. 4. Temperature dependent resistivity.
main mechanism of the conductivity should be the same. A second indicator of the presence of one dominant transport mechanism is the smooth approximation of the observed frequency dependent resistance to the DC resistance at high temperature (also mentioned in Ref. [9]), as shown in Fig. 5. In Refs. [9,10] this mechanism is identified as hopping conductivity at grain boundaries. Under that assumption of grain boundary related leakage current, the observed higher resistivity of the sample HON compared with the sample RON may be explained. 3.2. C–V measurement In Fig. 6 typical C–V characteristics of the CVD diamond capacitors made in this study are shown. There is no significant bias dependence of the capacitance over
Fig. 5. Frequency dependent resistance (sample RO).
W. Ebert et al. / Diamond and Related Materials 8 (1999) 1875–1877
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for glassy ceramic. At elevated temperature, for both amorphous ceramic material as well as CVD diamond, thermal activation of leakage and corresponding losses become apparent. In both cases the activation energy is approximately 1.3 eV.
4. Conclusions
Fig. 6. C–V characteristics of sample A after RIE.
the entire temperature range, indicating that partial depletion by space charge layers can be neglected. The changes at T≥450°C are due to the increased leakage current at high temperature and at high bias. The temperature dependent increase of the capacitance is thought to be caused by the parallel RC equivalent circuit of the measurement system which is oversimplified. A more accurate equivalent circuit would need a locus diagram analysis (see Ref. [11]). A commonly used figure of merit for a capacitor is the dissipation factor (DF ). It is a measure for the dielectric loss of the capacitor and should therefore be as low as possible. Fig. 7 shows a comparison between the temperature dependent dissipation factor of CVD diamond and glassy ceramic [1], another high temperature material. It becomes apparent that at lower temperature the dissipation factor of the diamond capacitor is up to two orders of magnitude lower than the value
Fig. 7. Temperature dependent dissipation factor.
For the further improvement of the performance of diamond capacitors it is absolutely necessary to reduce the high thermal activation energy and the RT conductivity of the CVD diamond films. Future investigations need therefore be aimed in two directions: $ in situ passivation of the grain boundaries by a modified growth technique; $ reduction of the grain boundary density by optimized growth of large grains, that can only be achieved by at least 15 mm thick films. However, the required thin dielectric membranes can then only be obtained by thinning of the diamond films from the backside. References [1] L. Mandelcom, S.R. Gurkovich, K.C. Radford, Trans. 3rd Int. High Temperature Electronics Conf., Albuquerque, NM, 9–14 June Vol. VI (1996) 3–13. [2] M.A. Plano, M.I. Landstrass, M.A. Moreno, M.D. Moyer, Proc. 14th Capacitor and Resistor Technology Symp. (1994) 3–8. [3] P.B. Kosel, D. Wu, O.P. Kosel, S.F. Carr, A. Garscaddern, P.R. Emmert, R.L.C. Wu, Proc. Applied Diamond Conf., Gaithersburg, MD (1995) 37s–40s. [4] J.L. Davidson, T.A. Roppel, Proc. First Int. Symp. on Diamond and Diamond-Like Films, The Electrochemical Society, Pennington, NJ, 1995, pp. 306–316. [5] M.M. Freeman, A.L. Barton, H.L. Marcus, Proc. Applied Diamond Conf., Gaithersburg, MD (1995) 785–788. [6 ] R. Ramesham, P.E. Pehrsson, T.I. Smith, M.F. Rose, J. Mater. Sci. (1997) 69–72. [7] A. Flo¨ter, paper presented at the Diamond ’97 Conf., 4–8 August, Edinburgh, UK. [8] E. Boettger, A. Bluhm, X. Jiang, L. Scha¨fer, C.-P. Klages, J. Appl. Phys. 77 (1995) 6332–6337. [9] B. Fliegl, R. Kuhnert, M. Ben-Chorin, F. Koch, Appl. Phys. Lett. 65 (1994) 371–373. [10] S. Nath, J.I.B. Wilson, Diamond Relat. Mater. 5 (1996) 65–75. [11] W. Ebert, A. Vescan, E. Kohn, Diamond Relat. Mater. 3 (1994) 887–890.
High-temperature diamond capacitor k W. Ebert a, *, M. Adamschik a, P. Gluche a, A. Flo¨ter b, E. Kohn a a Department of Electron Devices and Circuits, University of Ulm, Albert-Einstein-Allee 45, 89081 Ulm, Germany b Daimler Benz AG, W.-Runge-Str. 11, 89081 Ulm, Germany Received 16 September 1998; accepted 10 May 1999
Abstract Capacitors using diamond membranes as the dielectric were fabricated and evaluated up to 600°C using various MPCVD grown films. The films were grown on Si and were randomly oriented or highly oriented with different grain sizes. In part they were nitrogen doped. Admittance measurements at room temperature (RT ) show a dissipation factor <10−5 at 10 kHz, which was the resolution limit of the measurement system. The loss increases with the temperature with an activation energy of 1.0– 1.3 eV. This activation energy was found to be independent of the diamond CVD growth configuration. The level of the leakage is influenced by the grain size but not by the nitrogen doping concentration. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Capacitor; High temperature diamond; Membrane
1. Introduction High temperature electronic components are an important part of high temperature sensor and actuator systems, with their related energy supply and signal conducting circuits. Thus, besides diodes, transistors, and sensors, passive devices such as resistors and capacitors [1] will also be required. Furthermore, for ease of technology, the basic material of both active and passive devices should be the same. Due to the progress in high temperature sensors and electronics, diamond may be considered a material for high temperature capacitors [2–6 ].
2. Experimental The CVD diamond films were grown on silicon substrates using a novel biasing technique [7] which enables the growth of large area highly oriented and textured diamond films. Free-standing diamond membranes were curved out by wet chemical etching of the Si substrate in a 30% KOH solution at 80°C ( Fig. 1). k
Presented at the Diamond ’98 Conference, Crete, Greece, September 1998. * Corresponding author. Tel.: +49-731-5026-154; fax: +49-731-5026-155. E-mail address: [email protected] ( W. Ebert)
Fig. 1. CVD diamond capacitor.
They serve as the dielectric in the parallel plate capacitors. To investigate the influence of the nitrogen concentration and the grain boundaries, three different samples were used ( Fig. 2). After an oxygen plasma treatment of the diamond surface the contact metal (Au) was deposited by thermal evaporation onto both sides, patterned by standard lithography and structured by wet chemical etching. Au contacts on oxygen terminated surfaces as used here form barriers and act like blocking contacts; however, with grain boundaries reaching up to the metal interface. The area of the capacitors was 10 mm2. The C–V measurements were performed at frequencies between 10 kHz and 2 MHz with a HP 4192A impedance analyzer in the parallel RC mode, the I–V measurements with a HP 4142B DC source. All measurements were undertaken under atmospheric conditions in the temperature range between room temperature (RT ) and 600°C.
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 1 4 6- 6
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W. Ebert et al. / Diamond and Related Materials 8 (1999) 1875–1877
Fig. 2. Investigated CVD diamond films.
3. Results and discussion For the characterization of capacitors, two measurement techniques are required. From AC measurements the capacitance and the relative dielectric constant e r may be extracted. For the investigated samples a constant e was observed, since their capacitances scaled r with their thickness. Furthermore, DC leakage measurements are necessary for the estimation of the dielectric loss. 3.1. DC leakage measurements The temperature-dependent I–V characteristics of the sample RON are shown in Fig. 3. The extracted leakage depends weakly on the applied voltage but depends strongly on the temperature. In the temperature range between 100°C and 500°C, the resistance varies over seven orders of magnitude. The extracted constant thermal activation energy of 1.3 eV at elevated temperature (T>350°C ) is also observed for the nominally undoped film (sample RO) and for the HOD film (sample HON ) ( Fig. 4). The observed value of the thermal activation energy and the fact that it is not related to the incorporated nitrogen concentrations (<1018 cm−3) is in good agreement with results published in Ref. [8], where an activation energy of 1.2 eV for undoped and 1.4–1.6 eV for heavily nitrogen doped (>1019 cm−3) CVD diamond films is reported. The fact that the thermal activation energy is also not related to the grain size (crystal quality) in the observed range between ~2 mm (samples RON and RO) and ~10 mm (sample HON ) is expected because the
Fig. 3. DC leakage current of the sample RON.
Fig. 4. Temperature dependent resistivity.
main mechanism of the conductivity should be the same. A second indicator of the presence of one dominant transport mechanism is the smooth approximation of the observed frequency dependent resistance to the DC resistance at high temperature (also mentioned in Ref. [9]), as shown in Fig. 5. In Refs. [9,10] this mechanism is identified as hopping conductivity at grain boundaries. Under that assumption of grain boundary related leakage current, the observed higher resistivity of the sample HON compared with the sample RON may be explained. 3.2. C–V measurement In Fig. 6 typical C–V characteristics of the CVD diamond capacitors made in this study are shown. There is no significant bias dependence of the capacitance over
Fig. 5. Frequency dependent resistance (sample RO).
W. Ebert et al. / Diamond and Related Materials 8 (1999) 1875–1877
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for glassy ceramic. At elevated temperature, for both amorphous ceramic material as well as CVD diamond, thermal activation of leakage and corresponding losses become apparent. In both cases the activation energy is approximately 1.3 eV.
4. Conclusions
Fig. 6. C–V characteristics of sample A after RIE.
the entire temperature range, indicating that partial depletion by space charge layers can be neglected. The changes at T≥450°C are due to the increased leakage current at high temperature and at high bias. The temperature dependent increase of the capacitance is thought to be caused by the parallel RC equivalent circuit of the measurement system which is oversimplified. A more accurate equivalent circuit would need a locus diagram analysis (see Ref. [11]). A commonly used figure of merit for a capacitor is the dissipation factor (DF ). It is a measure for the dielectric loss of the capacitor and should therefore be as low as possible. Fig. 7 shows a comparison between the temperature dependent dissipation factor of CVD diamond and glassy ceramic [1], another high temperature material. It becomes apparent that at lower temperature the dissipation factor of the diamond capacitor is up to two orders of magnitude lower than the value
Fig. 7. Temperature dependent dissipation factor.
For the further improvement of the performance of diamond capacitors it is absolutely necessary to reduce the high thermal activation energy and the RT conductivity of the CVD diamond films. Future investigations need therefore be aimed in two directions: $ in situ passivation of the grain boundaries by a modified growth technique; $ reduction of the grain boundary density by optimized growth of large grains, that can only be achieved by at least 15 mm thick films. However, the required thin dielectric membranes can then only be obtained by thinning of the diamond films from the backside. References [1] L. Mandelcom, S.R. Gurkovich, K.C. Radford, Trans. 3rd Int. High Temperature Electronics Conf., Albuquerque, NM, 9–14 June Vol. VI (1996) 3–13. [2] M.A. Plano, M.I. Landstrass, M.A. Moreno, M.D. Moyer, Proc. 14th Capacitor and Resistor Technology Symp. (1994) 3–8. [3] P.B. Kosel, D. Wu, O.P. Kosel, S.F. Carr, A. Garscaddern, P.R. Emmert, R.L.C. Wu, Proc. Applied Diamond Conf., Gaithersburg, MD (1995) 37s–40s. [4] J.L. Davidson, T.A. Roppel, Proc. First Int. Symp. on Diamond and Diamond-Like Films, The Electrochemical Society, Pennington, NJ, 1995, pp. 306–316. [5] M.M. Freeman, A.L. Barton, H.L. Marcus, Proc. Applied Diamond Conf., Gaithersburg, MD (1995) 785–788. [6 ] R. Ramesham, P.E. Pehrsson, T.I. Smith, M.F. Rose, J. Mater. Sci. (1997) 69–72. [7] A. Flo¨ter, paper presented at the Diamond ’97 Conf., 4–8 August, Edinburgh, UK. [8] E. Boettger, A. Bluhm, X. Jiang, L. Scha¨fer, C.-P. Klages, J. Appl. Phys. 77 (1995) 6332–6337. [9] B. Fliegl, R. Kuhnert, M. Ben-Chorin, F. Koch, Appl. Phys. Lett. 65 (1994) 371–373. [10] S. Nath, J.I.B. Wilson, Diamond Relat. Mater. 5 (1996) 65–75. [11] W. Ebert, A. Vescan, E. Kohn, Diamond Relat. Mater. 3 (1994) 887–890.