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Field emission from diamond-like carbon films and fabrication of gated diamond-like carbon film emitter
Ultramicroscopy 73 (1998) 17—22
Field emission from diamond-like carbon films and fabrication of gated diamond-like carbon film emitter Sunup Lee!, Bokeon Chung!, Tae-Young Ko!, D. Jeon!,*, Kwang-Ryeol Lee", Kwang Yong Eun" ! Department of Physics, Myong Ji University, Yongin Kyunggi-Do, Seoul 449-728, South Korea " Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea Received 7 July 1997; received in revised form 24 October 1997
Abstract The emission from diamond-like carbon (DLC) films has been studied. The DLC films were prepared by sputtering a carbon target with Ar ions and collecting the carbon clusters on an indium tin oxide (ITO) film. The DLC films prepared this way exhibited a stable and reproducible emission. We also fabricated a gated field emitter using the DLC film as a cathode. The simple process consisted of the deposition of DLC, insulating, and gate layers, followed by back-etching of an array of holes until the DLC film at the bottom was exposed. The onset voltage of the completed DLC triode was 50 V and a stable anode current was achieved. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Field emission occurs when a high electric field is applied between two electrodes. Because of this, specimens for field emission and field ion microscopy need to be formed in a needle shape to enhance the electric field. For this reason, vacuum microelectronic devices which are based on the field emission phenomena require sharp electron emitters as cathodes. Until recently, the array of tips for vacuum microelectronic devices was made of metal or silicon. For example, molybdenum tips were
* Corresponding author. Tel.: #82 335 30 6172; fax: #82 335 35 9533; e-mail: [email protected].
formed by evaporation. However, as the size of molybdenum flux is limited, it is difficult to make a large size uniform panel. Silicon tips are fabricated using VLSI technology, but the processing is limited by the wafer size. Many difficulties of this vacuum microelectronics manufacturing problem arises from the fact that the emitters have to be sharp. If, instead, one can use a thin film as an emitter, the fabrication will be much easier and also costs less. Surfaces of some materials when treated specially exhibit very low electron affinity, so the electrons are emitted from flat surfaces, but these surfaces are very reactive and hence are not suitable for practical purposes [1]. The diamond surface, however, has been reported to exhibit negative electron affinity (NEA) properties without any surface treatment [2]. Although NEA properties of
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 3 0 - 7
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S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
diamond are yet to be confirmed, the emission at surprisingly low threshold voltage from thin films of diamond and diamond-like carbon (DLC) has been observed. The emission from a DLC film grown by chemical vapor deposition (CVD) has been reported by Wang et al. [3]. The key issue of their report was the comparison of the field and photoelectron emission patterns. From the result that two patterns coincided, they concluded that the emission from diamond was due to the low electronegativity. DLC films investigated for field emission so far have been prepared by CVD or laser ablation [4,5]. An interesting report for CVD DLC is that the emission was enhanced significantly when the arc occurred at the junction [6,7]. As for the reason for arcing, the smooth surface morphology and outgassing of hydrogen were proposed. Laser ablation-grown DLC exhibited a high emission current, but the problem was that the emission was not uniform and the film was difficult to reproduce [4]. There are a few other ways of growing DLC. In this paper, we report the emission from DLC films that were prepared by Ar ion sputtering of a high purity carbon target. In this method, we used two ion guns, one for sputtering a carbon target for DLC deposition and the other for cleaning the substrate and doping the substrate with nitrogen. Therefore, the advantage of this method is that the substrate and the DLC film can be modified in situ in real time. Unlike CVD DLC, sputter-grown DLC films exhibited a stable emission without arcing, and the emission showed a reproducible dependence on the surface roughness. The morphology or the roughness of the DLC film can also be controlled by changing the energy of Ar ions impinging on the target. The DLC film emitters have so far been used as a diode. In this configuration, the emission is controlled by the anode voltage. This configuration is not suitable for application to flat panel displays because the anode is a phosphor screen. To be able to control the emission from DLC films easily with low voltages, we fabricated a gated DLC emitter using a simple process of DLC insulating and gate layer deposition followed by back-etching. The onset voltage for the anode current of the gated DLC emitter was about 50 V.
2. Experiment We made DLC films in a high-vacuum chamber equipped with two sputter ion guns. One gun was used to sputter-clean the substrate, and the other gun for sputtering a carbon target to desorb ion clusters. The substrate cleaning gun could also be used for doping the substrate with nitrogen. A schematic of the apparatus is illustrated in Fig. 1. ITO glass is used as a substrate. Before deposition, the substrate was cleaned by Ar ion sputtering for 1 min with a beam voltage of 400 V. For deposition, Ar ions sputtered a high purity carbon target at a beam voltage of 1 250 V. The thickness of the film varied with the growth time; for example, a 750 A_ thick film was deposited after 60 min for a given condition. The I—» data were measured in a vacuum chamber whose pressure was below 10~7 Torr. For measuring I—» data from a diode structure, pieces of 150 lm thick cover glass were inserted as a spacer between the DLC film and the molybdenum plate anode. The fabrication process for the gated DLC film emitter is simple as shown in Fig. 2. After depositing a DLC film on a ITO glass, we deposited a silicon oxide layer (8 000 A_ ) using atmospheric pressure CVD while the substrate was kept at
Fig. 1. A schematic of a DLC film preparation apparatus. Two ion guns are used, one for substrate cleaning and the other for sputtering a carbon target to grow a DLC film.
S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
19
350 °C. An aluminum gate film (3 000 A_ ) was then deposited using electron beam evaporation. The films were then etched back using a photoresistive mask until the DLC film was exposed so that an array of holes was formed. The aluminum and oxide layers were first etched by dry etching until the oxide layer became 500 A_ thick. The final etching was done by dipping the device in the buffered oxide etching solution. The hole diameter was 2 lm and the distance between the holes was 5 lm. For the I—» measurement of a triode structure, the anode was placed 1.5 mm above the gate layer and the voltage was applied to the gate.
3. Results and discussion The surface of CVD DLC film is extremely flat. On the contrary, the roughness of laser ablationgrown DLC films ranges several hundred angstroms depending on the growth condition [8]. The AFM image of the sputter-grown DLC, shown in Fig. 3a, indicates that the surface is rougher than CVD DLC but less rougher than laser ablation-grown DLC. In the present case, the surface
Fig. 2. A process to fabricate a gated DLC film emitter.
Fig. 3. (a) AFM image of a DLC film grown by sputtering. The film is 750 A_ thick and the average surface roughness is 45 A_ . (b) Raman spectrum of sputter-grown DLC, showing a typical DLC signal. (c) I—» data obtained from the sputter-grown DLC film using a diode configuration.
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roughness increased with film thickness and became maximum when the film was 750 A_ thick. For a larger thickness, the surface became smoother again and the morphology did not change with the thickness. The material properties of DLC films grown by ion beam sputtering are similar to those of films grown by laser ablation, i.e., the films do not contain hydrogen [9]. It is amorphous in terms of the structure, but its physical properties are diamond-like. Fig. 3b shows a Raman spectrum of our sputter-grown DLC film. The spectrum is a typical DLC Raman signal in that the D and G peaks typical to graphitized carbon film are not seen [9]. Several mechanisms have been suggested for the emission from DLC [10,11]. However, there are several reports that one could measure a large emission current from a CVD DLC after the surface morphology changed by arcing [6,7], while the emission from laser ablation-grown DLC was relatively stable [8]. These reports indicated that the emission from DLC films also depended on the roughness of the film. The emission from our sputter-grown DLC films also showed the dependence on the roughness. The best emission was achieved from the 750 A_ thick film (the roughest film) whose average roughness was 45 A_ , and the data obtained by diode configuration are shown in Fig. 3c. The threshold voltage for the emission is 1000 V for 150 lm gap, and this is equivalent to the field of 6.7 V/lm. The area of the emission was 8 mm2, so that the emission density was about 1.6 mA/cm2. The emission from this surface was stable and reproducible, and the microscopic inspection of the film after the emission measurement did not show any trace of arc-induced defect. We also cycled the anode voltage many times, but the I—» data followed the same curve. Most efforts to use thin films as a field emitter have concentrated on the diode structure which requires high anode voltage to induce the emission [12]. This is mainly because the patterning of diamond and DLC films is difficult. Using the process shown in Fig. 2, we fabricated a gated DLC field emitter so that we could control the emission by applying low voltage to the gate. A scanning electron microscope (SEM) image of the finished device is shown in Fig. 4. The edge of the gate holes is rough because the gate material is aluminum. For
Fig. 4. (a) SEM micrograph of the top view of the gate DLC film emitter. (b) Cross-sectional view.
the I—» measurement from the gated DLC film, we placed an anode plate 1.5 mm above the gate and applied 300 V. The result is shown in Fig. 5 together with the Fowler—Nordheim plot. The onset voltage is 50 V and the emission current is 1.4 nA per hole emitter when the gate voltage is 120 V. This result is compatible with the field emitter triodes adopting needle-shaped cathodes. However, one problem was the relatively large gate current. This is because the cathode is wide and located much below the gate layer. Therefore, a new structure is required to avoid the leakage problem of the DLC film triode [13]. The insulating layer was destroyed when the gate voltage was further increased, and in Fig. 6 we show a SEM micrograph after the breakdown of the emitter when the gate voltage exceeded 120 V. Instead of
S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
21
depositing DLC first and then insulating the layer, we also tried to deposit DLC after the holes were formed through the gate and insulating layers. However, in this case, carbon clusters adsorbed on the side wall of the insulating layer causing a large leakage current to the gate through the surface of the insulating layer.
4. Conclusions We have investigated the field emission from DLC films grown using ion beam sputtering of a carbon target. The emission was stable and showed a dependence on the surface roughness of the DLC films. We also fabricated a gated DLC film triode emitter. Using this device, we could successfully control the anode current with the gate voltage.
Acknowledgements
Fig. 5. (a) I—» curve obtained from the gated DLC film emitter. (b) Corresponding Fowler-Nordheim plot.
This work was supported by the Korea Science and Engineering Foundation through Contract No. 961-0210-060-2 and by the Atomic Scale Surface Science Research Center at Yonsei University.
References
Fig. 6. SEM micrograph of a gated DLC film emitter after breaking down at the high gate voltage.
[1] D.T. Pierce, F. Meier, Phys. Rev. B 13 (1976) 5484. [2] F.J. Himpsel, J.A. Knapp, J.A. Van Vechten, D.E. Eastman, Phys. Rev. B 20 (1979) 624. [3] C. Wang, Q. Garcia, D.C. Ingram, M. Lake, M.E. Kordesch, Electron. Lett. 27 (1991) 1459. [4] C. Xie, C.N. Potter, R.L. Fink, C. Hilbert, A. Krishnan, D. Eichman, N. Kumar, H.K. Schmidt, M.H. Clark, A. Ross, B. Lin, L. Fredin, B. Baker, D. Patterson, W. Brookover, Technical Digest of 7th Int. Conf. Vacuum Microelectronics, Grenoble, France, 4—7 July 1994, p. 229. [5] A.A. Talin, T.E. Felter, T.A. Friedmann, J.P. Sullivan, M.P. Siegal, J. Vac. Sci. Technol. A 14 (1996) 1719. [6] J.D. Shovelin, M.E. Kordesch, Appl. Phys. Lett. 65 (1996) 863. [7] S. Lee, T.-Y. Ko, H. Cho, D. Jeon, K.-R. Lee, K.Y. Eun, J. Korean. Phys. Soc., in press. [8] H.S. Chung, private communication.
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[9] A. Grill, B.S. Meyerson, in: K.E. Spear, J.P. Dismukes (Eds.), Synthetic Diamond, Wiley, New York, 1996, pp. 100—102. [10] W. Zhu, G.P. Kochanski, S. Jin, L. Seibles, J. Appl. Phys. 78 (1995) 2707.
[11] N.S. Xu, R.V. Latham, Y. Tzeng, Electronics Lett. 29 (1993) 1596. [12] Z.L. Tolt, R.L. Fink, Z. Yaniv, Technical Digest of 10th Int. Vacuum Microelectronics Conf., Kyongju, Korea, 17—21 August.
Field emission from diamond-like carbon films and fabrication of gated diamond-like carbon film emitter Sunup Lee!, Bokeon Chung!, Tae-Young Ko!, D. Jeon!,*, Kwang-Ryeol Lee", Kwang Yong Eun" ! Department of Physics, Myong Ji University, Yongin Kyunggi-Do, Seoul 449-728, South Korea " Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea Received 7 July 1997; received in revised form 24 October 1997
Abstract The emission from diamond-like carbon (DLC) films has been studied. The DLC films were prepared by sputtering a carbon target with Ar ions and collecting the carbon clusters on an indium tin oxide (ITO) film. The DLC films prepared this way exhibited a stable and reproducible emission. We also fabricated a gated field emitter using the DLC film as a cathode. The simple process consisted of the deposition of DLC, insulating, and gate layers, followed by back-etching of an array of holes until the DLC film at the bottom was exposed. The onset voltage of the completed DLC triode was 50 V and a stable anode current was achieved. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Field emission occurs when a high electric field is applied between two electrodes. Because of this, specimens for field emission and field ion microscopy need to be formed in a needle shape to enhance the electric field. For this reason, vacuum microelectronic devices which are based on the field emission phenomena require sharp electron emitters as cathodes. Until recently, the array of tips for vacuum microelectronic devices was made of metal or silicon. For example, molybdenum tips were
* Corresponding author. Tel.: #82 335 30 6172; fax: #82 335 35 9533; e-mail: [email protected].
formed by evaporation. However, as the size of molybdenum flux is limited, it is difficult to make a large size uniform panel. Silicon tips are fabricated using VLSI technology, but the processing is limited by the wafer size. Many difficulties of this vacuum microelectronics manufacturing problem arises from the fact that the emitters have to be sharp. If, instead, one can use a thin film as an emitter, the fabrication will be much easier and also costs less. Surfaces of some materials when treated specially exhibit very low electron affinity, so the electrons are emitted from flat surfaces, but these surfaces are very reactive and hence are not suitable for practical purposes [1]. The diamond surface, however, has been reported to exhibit negative electron affinity (NEA) properties without any surface treatment [2]. Although NEA properties of
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 3 0 - 7
18
S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
diamond are yet to be confirmed, the emission at surprisingly low threshold voltage from thin films of diamond and diamond-like carbon (DLC) has been observed. The emission from a DLC film grown by chemical vapor deposition (CVD) has been reported by Wang et al. [3]. The key issue of their report was the comparison of the field and photoelectron emission patterns. From the result that two patterns coincided, they concluded that the emission from diamond was due to the low electronegativity. DLC films investigated for field emission so far have been prepared by CVD or laser ablation [4,5]. An interesting report for CVD DLC is that the emission was enhanced significantly when the arc occurred at the junction [6,7]. As for the reason for arcing, the smooth surface morphology and outgassing of hydrogen were proposed. Laser ablation-grown DLC exhibited a high emission current, but the problem was that the emission was not uniform and the film was difficult to reproduce [4]. There are a few other ways of growing DLC. In this paper, we report the emission from DLC films that were prepared by Ar ion sputtering of a high purity carbon target. In this method, we used two ion guns, one for sputtering a carbon target for DLC deposition and the other for cleaning the substrate and doping the substrate with nitrogen. Therefore, the advantage of this method is that the substrate and the DLC film can be modified in situ in real time. Unlike CVD DLC, sputter-grown DLC films exhibited a stable emission without arcing, and the emission showed a reproducible dependence on the surface roughness. The morphology or the roughness of the DLC film can also be controlled by changing the energy of Ar ions impinging on the target. The DLC film emitters have so far been used as a diode. In this configuration, the emission is controlled by the anode voltage. This configuration is not suitable for application to flat panel displays because the anode is a phosphor screen. To be able to control the emission from DLC films easily with low voltages, we fabricated a gated DLC emitter using a simple process of DLC insulating and gate layer deposition followed by back-etching. The onset voltage for the anode current of the gated DLC emitter was about 50 V.
2. Experiment We made DLC films in a high-vacuum chamber equipped with two sputter ion guns. One gun was used to sputter-clean the substrate, and the other gun for sputtering a carbon target to desorb ion clusters. The substrate cleaning gun could also be used for doping the substrate with nitrogen. A schematic of the apparatus is illustrated in Fig. 1. ITO glass is used as a substrate. Before deposition, the substrate was cleaned by Ar ion sputtering for 1 min with a beam voltage of 400 V. For deposition, Ar ions sputtered a high purity carbon target at a beam voltage of 1 250 V. The thickness of the film varied with the growth time; for example, a 750 A_ thick film was deposited after 60 min for a given condition. The I—» data were measured in a vacuum chamber whose pressure was below 10~7 Torr. For measuring I—» data from a diode structure, pieces of 150 lm thick cover glass were inserted as a spacer between the DLC film and the molybdenum plate anode. The fabrication process for the gated DLC film emitter is simple as shown in Fig. 2. After depositing a DLC film on a ITO glass, we deposited a silicon oxide layer (8 000 A_ ) using atmospheric pressure CVD while the substrate was kept at
Fig. 1. A schematic of a DLC film preparation apparatus. Two ion guns are used, one for substrate cleaning and the other for sputtering a carbon target to grow a DLC film.
S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
19
350 °C. An aluminum gate film (3 000 A_ ) was then deposited using electron beam evaporation. The films were then etched back using a photoresistive mask until the DLC film was exposed so that an array of holes was formed. The aluminum and oxide layers were first etched by dry etching until the oxide layer became 500 A_ thick. The final etching was done by dipping the device in the buffered oxide etching solution. The hole diameter was 2 lm and the distance between the holes was 5 lm. For the I—» measurement of a triode structure, the anode was placed 1.5 mm above the gate layer and the voltage was applied to the gate.
3. Results and discussion The surface of CVD DLC film is extremely flat. On the contrary, the roughness of laser ablationgrown DLC films ranges several hundred angstroms depending on the growth condition [8]. The AFM image of the sputter-grown DLC, shown in Fig. 3a, indicates that the surface is rougher than CVD DLC but less rougher than laser ablation-grown DLC. In the present case, the surface
Fig. 2. A process to fabricate a gated DLC film emitter.
Fig. 3. (a) AFM image of a DLC film grown by sputtering. The film is 750 A_ thick and the average surface roughness is 45 A_ . (b) Raman spectrum of sputter-grown DLC, showing a typical DLC signal. (c) I—» data obtained from the sputter-grown DLC film using a diode configuration.
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roughness increased with film thickness and became maximum when the film was 750 A_ thick. For a larger thickness, the surface became smoother again and the morphology did not change with the thickness. The material properties of DLC films grown by ion beam sputtering are similar to those of films grown by laser ablation, i.e., the films do not contain hydrogen [9]. It is amorphous in terms of the structure, but its physical properties are diamond-like. Fig. 3b shows a Raman spectrum of our sputter-grown DLC film. The spectrum is a typical DLC Raman signal in that the D and G peaks typical to graphitized carbon film are not seen [9]. Several mechanisms have been suggested for the emission from DLC [10,11]. However, there are several reports that one could measure a large emission current from a CVD DLC after the surface morphology changed by arcing [6,7], while the emission from laser ablation-grown DLC was relatively stable [8]. These reports indicated that the emission from DLC films also depended on the roughness of the film. The emission from our sputter-grown DLC films also showed the dependence on the roughness. The best emission was achieved from the 750 A_ thick film (the roughest film) whose average roughness was 45 A_ , and the data obtained by diode configuration are shown in Fig. 3c. The threshold voltage for the emission is 1000 V for 150 lm gap, and this is equivalent to the field of 6.7 V/lm. The area of the emission was 8 mm2, so that the emission density was about 1.6 mA/cm2. The emission from this surface was stable and reproducible, and the microscopic inspection of the film after the emission measurement did not show any trace of arc-induced defect. We also cycled the anode voltage many times, but the I—» data followed the same curve. Most efforts to use thin films as a field emitter have concentrated on the diode structure which requires high anode voltage to induce the emission [12]. This is mainly because the patterning of diamond and DLC films is difficult. Using the process shown in Fig. 2, we fabricated a gated DLC field emitter so that we could control the emission by applying low voltage to the gate. A scanning electron microscope (SEM) image of the finished device is shown in Fig. 4. The edge of the gate holes is rough because the gate material is aluminum. For
Fig. 4. (a) SEM micrograph of the top view of the gate DLC film emitter. (b) Cross-sectional view.
the I—» measurement from the gated DLC film, we placed an anode plate 1.5 mm above the gate and applied 300 V. The result is shown in Fig. 5 together with the Fowler—Nordheim plot. The onset voltage is 50 V and the emission current is 1.4 nA per hole emitter when the gate voltage is 120 V. This result is compatible with the field emitter triodes adopting needle-shaped cathodes. However, one problem was the relatively large gate current. This is because the cathode is wide and located much below the gate layer. Therefore, a new structure is required to avoid the leakage problem of the DLC film triode [13]. The insulating layer was destroyed when the gate voltage was further increased, and in Fig. 6 we show a SEM micrograph after the breakdown of the emitter when the gate voltage exceeded 120 V. Instead of
S. Lee et al. / Ultramicroscopy 73 (1998) 17—22
21
depositing DLC first and then insulating the layer, we also tried to deposit DLC after the holes were formed through the gate and insulating layers. However, in this case, carbon clusters adsorbed on the side wall of the insulating layer causing a large leakage current to the gate through the surface of the insulating layer.
4. Conclusions We have investigated the field emission from DLC films grown using ion beam sputtering of a carbon target. The emission was stable and showed a dependence on the surface roughness of the DLC films. We also fabricated a gated DLC film triode emitter. Using this device, we could successfully control the anode current with the gate voltage.
Acknowledgements
Fig. 5. (a) I—» curve obtained from the gated DLC film emitter. (b) Corresponding Fowler-Nordheim plot.
This work was supported by the Korea Science and Engineering Foundation through Contract No. 961-0210-060-2 and by the Atomic Scale Surface Science Research Center at Yonsei University.
References
Fig. 6. SEM micrograph of a gated DLC film emitter after breaking down at the high gate voltage.
[1] D.T. Pierce, F. Meier, Phys. Rev. B 13 (1976) 5484. [2] F.J. Himpsel, J.A. Knapp, J.A. Van Vechten, D.E. Eastman, Phys. Rev. B 20 (1979) 624. [3] C. Wang, Q. Garcia, D.C. Ingram, M. Lake, M.E. Kordesch, Electron. Lett. 27 (1991) 1459. [4] C. Xie, C.N. Potter, R.L. Fink, C. Hilbert, A. Krishnan, D. Eichman, N. Kumar, H.K. Schmidt, M.H. Clark, A. Ross, B. Lin, L. Fredin, B. Baker, D. Patterson, W. Brookover, Technical Digest of 7th Int. Conf. Vacuum Microelectronics, Grenoble, France, 4—7 July 1994, p. 229. [5] A.A. Talin, T.E. Felter, T.A. Friedmann, J.P. Sullivan, M.P. Siegal, J. Vac. Sci. Technol. A 14 (1996) 1719. [6] J.D. Shovelin, M.E. Kordesch, Appl. Phys. Lett. 65 (1996) 863. [7] S. Lee, T.-Y. Ko, H. Cho, D. Jeon, K.-R. Lee, K.Y. Eun, J. Korean. Phys. Soc., in press. [8] H.S. Chung, private communication.
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[9] A. Grill, B.S. Meyerson, in: K.E. Spear, J.P. Dismukes (Eds.), Synthetic Diamond, Wiley, New York, 1996, pp. 100—102. [10] W. Zhu, G.P. Kochanski, S. Jin, L. Seibles, J. Appl. Phys. 78 (1995) 2707.
[11] N.S. Xu, R.V. Latham, Y. Tzeng, Electronics Lett. 29 (1993) 1596. [12] Z.L. Tolt, R.L. Fink, Z. Yaniv, Technical Digest of 10th Int. Vacuum Microelectronics Conf., Kyongju, Korea, 17—21 August.