High field breakdown characteristics of micrometric gaps in vacuum Xianyun Ma and T. S. Sudarshan Citation: Journal of Vacuum Science & Technology B 16, 745 (1998); doi: 10.1116/1.589896 View online: http://dx.doi.org/10.1116/1.589896 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/16/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Prebreakdown and breakdown investigation of needle-plane vacuum gaps in the micron/submicron regime J. Vac. Sci. Technol. B 18, 1222 (2000); 10.1116/1.591365 High field breakdown of narrow quasi uniform field gaps in vacuum J. Appl. Phys. 85, 8400 (1999); 10.1063/1.370687 High field characteristics of thin-film metal electrodes J. Vac. Sci. Technol. B 17, 769 (1999); 10.1116/1.590636 Prebreakdown and breakdown investigation of broad area electrodes in the micrometric regime J. Vac. Sci. Technol. B 16, 1174 (1998); 10.1116/1.590028 New perspectives in vacuum high voltage insulation. I. The transition to field emission J. Vac. Sci. Technol. A 16, 707 (1998); 10.1116/1.581051
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High field breakdown characteristics of micrometric gaps in vacuum Xianyun Ma and T. S. Sudarshana) Department of Electrical and Computer Engineering, University of South Carolina, Columbia, South Carolina 29208
~Received 18 August 1997; accepted 14 January 1998! In order to obtain the prebreakdown and breakdown data of plain vacuum gaps in the micrometric regime relevant to field emission displays, an automatic experimental system was developed. The dc breakdown characteristics of the gap in the micrometric regime, varying from 25 to 1000 mm, were extensively studied. The experimental results show that the highly finished broad-area metal electrodes withstand fairly high fields in relatively poor vacuum. The dc breakdown strength of a 50 mm gap was more than 220 V/mm at a pressure of 1025 Torr, while for a 1000 mm gap, the breakdown strength was about 44.5 V/mm for highly polished chrome-steel electrodes, 2 cm in diameter. The breakdown of a narrow (,200 m m) vacuum gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown. However, a wider gap ~500–1000 mm! was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. The breakdown characteristics of ideal gaps with very smooth solid electrodes, presented in this work, will define the theoretical limits to which actual gaps can be stressed in a field emission display. © 1998 American Vacuum Society. @S0734-211X~98!04902-6#
I. INTRODUCTION Dielectric spacer and plain vacuum-gap insulation for high fields are critical issues for field emission based vacuum microelectronic devices, specifically the field emission display ~FED!. For reliable operation of FEDs, an appropriate spacer design is of primary importance. The insulation consideration of the spacer becomes more complicated considering the operational environment of the FED—the presence of electron emission from regions surrounding the spacer, outgassing from the phosphor, and elevated operational temperature of the device. Assuming that flashover could be alleviated even at high operational fields, the voltage limitation of the device shifts to the breakdown of the active region of the device, which is the plain vacuum gap between the cathode and anode. Thus the issues related to the breakdown of plain vacuum microgaps are important, especially under the operating condition of the device. The breakdown strength of plain anode-to-cathode gaps can be used to define the theoretical limits to which actual gaps can be stressed in an FED. Although there is voluminous bibliography on discharge processes in plain vacuum gaps1,2 of spacing >1 mm, the breakdown and prebreakdown data of plain vacuum gaps is not available in the micrometric regime ~25 to few hundred mm!, relevant to FEDs. As part of the main investigation of our project, which is aimed at developing microspacer insulation for field emission displays, this article reports the issues related to the breakdown and prebreakdown characteristics of plain anode-to-cathode vacuum microgaps. II. EXPERIMENTAL TECHNIQUES The behavior of a vacuum gap under high voltage ~HV! stress depends on a large number of parameters including the type of electrode materials, the shape of electrode profile, a!
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745
J. Vac. Sci. Technol. B 16„2…, Mar/Apr 1998
surface preparation and condition, residual gas and other contaminants, such as vacuum pump oil.3,4 In order to establish firmly, the relationship between various independent parameters associated with the vacuum gap, and its insulation capability, we adopted the test arrangement shown in Fig. 1. The test setup is mainly designed to study the behavior of a HV vacuum gap under controlled laboratory conditions with special consideration of low level prebreakdown current measurement and protection of the electrometer. A pair of highly polished ~surface roughness arithmetic average ,0.08 m m! chrome-steel spheres, 2 cm in diameter, was used as electrodes. Numerical electric field analysis using a field computation program based on the boundary element method ~BEM! showed that the geometrical field enhancement of such an electrode setup was less than 1.06 at gaps in the range 10–1000 mm. Hence, the gap can be assumed to produce a near-uniform electric field. The electrode arrangement is shown in Fig. 1. Each ball was held by a cylindrical holder, which was attached to the translator through a ceramic support. The fixed part of the translator was mounted on the flange of the vacuum chamber and the moving part was driven by a stepping motor. Changes in the ambient chamber pressure, temperature, and friction associated with feedthrough cause errors in the gap setup, and thus we should pay special attention to the change in gap spacing during an experimental run. The gap distance was mainly determined by the stepping motor with a resolution of 1 mm. An optical system, which was realized with the combination of a telescope and a zoom lens, was used to observe the test gap on a monitor. The system also enabled us to watch any change of the gap spacing directly. A light emitting diode ~LED! was used to illuminate the gap which was imaged by the optical system and finally captured by a CCD camera. Figure 2 is an image of the gap at 10 mm. The gap spacing was found to change
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©1998 American Vacuum Society
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Xianyun Ma and T. S. Sudarshan: High field breakdown of micrometric gaps in vacuum
FIG. 1. Schematic of the test system for dc studies.
more than 4 mm during the experiment at gap spacings in the 50–500 mm range. Before setting up the gap, the electrode balls were submerged in methanol and cleaned in an ultrasonic cleaner three times, 15 min each. During setting up the gap, the balls were dealt with using ultra low-lint wipes. The electrode surfaces were checked and imaged using a microscope before and after testing of each setup of the gap. The test chamber was evacuated by a turbomolecular pump backed by an oil-free diaphragm pump and the final pressure in the chamber was ;831026 Torr. An extra low noise HV supply was used to produce voltages from 100 V to 50 kV. Since any discharge ~corona! along the HV lead will cause a large noise in the low level current measurement, the entire HV system should be discharge free. Capacitor C c ~500 pF! and Tektronix oscilloscope TDS 744A were used to monitor any noise along the HV lead, which could also be used to determine the occurrence of relatively large prebreakdown currents and breakdown of the gap. The resistor R f and capacitor C f were used to filter out noise in the high voltage system. The noise of the entire HV system was less than 1 V in the voltage range from 100 V to 50 kV.
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The current was measured by a Keithley 6512 programmable electrometer, which is capable of measuring current as low as 10216 A. In our experimental system, the associated external electrical circuitry was designed to measure very low prebreakdown currents with a sensitivity less than 10214 A. A large value resistor R m (50 GV) was used for this reason which improved the noise performance of the electrometer.5 To protect the electrometer against a flashover of the gap, it is usually desirable to shunt the electrometer by some form of protection device such as a pair of biased-off zener diodes connected back–back.2 But this method will cause extra noise and shunt some current past the electrometer. We developed another method in which the structure housing the large value resistor R m was designed to withstand HV only to about 7 kV. When gap breakdown occurred and the HV across the resistor (R m ) housing exceeded 7 kV, the resistor would automatically be short circuited to ground, thus protecting the electrometer. A current-limiting resistor R 1 ~200 MV! was used to prevent excessive current flow through the gap in case of a breakdown. Sub-pico ampere current measurement under HV environment requires use of doubly-shielded coaxial cables, single point grounding, and the use of Faraday cage where all the measurement equipment is housed. The whole experimental system was computerized. After setting up the gap, the data collection was made automatically. III. RESULTS AND DISCUSSIONS With the above experimental system, the I – V characteristics of the gap in the micrometric regime varying from 25 to 1000 mm were extensively studied. Detailed investigation and theoretical analysis of these data are made in another report.6 The results of breakdown characteristics obtained in these investigations, in the micrometric vacuum gap regime, are presented here. A. Effect of electrode separation
For a given electrode material and geometry, the breakdown voltage V b was not a simple function of gap spacing d. Moreover, vacuum gaps that were apparently prepared identically exhibited significant scatter, even as high as 50% in the first breakdown voltage value V b1 . Figure 3 shows V b1 vs the gap distance d for vacuum gaps formed between highly polished chrome-steel sphere electrodes. For gaps between 25 and 200 mm, V b1 increased steadily with voltage according to V b 5373d 0.37 kV, where d is in mm, while for gaps 200 m m,d220 kV/mm for a 50 mm gap to >45 kV/mm for a 1000 mm gap. Thus the insulation capability per unit length decreased with increasing gap spacing. Even though, for a perfect vacuum gap, the theoretical breakdown field is 6.5 MV/ mm, the experimental results show that the highly polished broad-area metal electrodes could withstand rather high fields in relatively poor vacuum. It is shown that the investigated gap prebreakdown current is dominated by field emission electrons.6
C. Conditioning effect of breakdown
In small gaps (,200 m m), a microdischarge could lead to breakdown, causing damage to the electrodes. As a result, in each subsequent test run the gap held off a lower voltage compared to the previous breakdown value. Thus the vacuum gap performance degraded after each breakdown. Conversely, in larger gaps (.500 m m), each breakdown caused an improvement ~conditioning! and, after several breakdowns, the breakdown voltage was substantially higher than the first breakdown voltage (V b1 ). For a 500 mm gap, V b1 was as low as 20 kV, while V c , the conditioned voltage, >34 kV. Thus for gaps >500 m m, in order to reach the full insulation capability, a vacuum gap is subjected to conditioning, a process of in situ cleaning whereby the sources of prebreakdown current and microdischarges are safely quenched so that the sources of instability that contribute to the breakdown of the gap are reduced. If the electrodes were conditioned at a larger gap distance, the I – V characteristics of the gap, from 50 to 1000 mm, were more stable and repeatable, in addition to exhibiting improved insulation capability. But with this kind of conditioning, the issue was more complicated since the surface of the anode was usually damaged after conditioning ~see the next section!.
D. Role of electrode surface condition
FIG. 4. Insulation capability of micrometric gaps. 3: experimental data; –: best curve fit of experimental data.
As stated before, although the conditioning at large gaps improves the insulation capability of the electrode gap, the anode was usually damaged after conditioning. Figure 5 shows the anode surface condition after five breakdowns while the cathode suffered no damage. In spite of the damage to the anode, we obtained very stable I – V data and good insulation capability ~about 35 kV across 500 mm!. It might be concluded that the high-energy electrons cause surface damage of the anode; however, the I – V and the breakdown characteristics of the gap are mainly determined by the cath-
JVST B - Microelectronics and Nanometer Structures
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748
Xianyun Ma and T. S. Sudarshan: High field breakdown of micrometric gaps in vacuum
ode. In order to investigate the effect of the surface roughness, we have performed experiments in which the highly polished electrode spheres were polished with 1 mm diamond paste. The insulation capability of the gap with electrodes polished by 1 mm diamond paste did not differ significantly from that of a highly polished one, but the I – V characteristics were unstable. Extensive investigations related to the influence of the electrode surface treatment on the breakdown characteristics of micrometric gaps will be reported in another article. E. Summary
A study of the breakdown characteristics of micrometric vacuum gaps is necessary in order to design electrical insulation in vacuum microelectronic devices, specifically, FEDs. Although practical FEDs use a planar ITO anode and a planar cold-cathode array, the present investigations are useful in defining the theoretical limits to which ideal micrometric vacuum gaps can be stressed. Our group is extending these broad-area solid-electrode breakdown measurements to planar metal-film electrodes including ITO anode structures. The breakdown characteristics of these structures will be reported in the near future. IV. CONCLUSIONS High field breakdown characteristics of broad-area micrometric gaps in vacuum were revealed in this study. Highly polished metal electrodes can withstand very high fields in
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relatively poor vacuum. The dc breakdown strength of a 50 mm gap was more than 220 V/mm at a pressure of 1025 Torr, while for a 1000 mm gap, the breakdown strength was about 44.5 V/mm for highly polished stainless-steel electrodes. The breakdown of a narrow (,200 m m) gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown, while a wider gap ~500–1000 mm! was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. However, this conditioning causes damage of the anode surface. ACKNOWLEDGMENTS This research was supported from DARPA Grant No. N000149610845, managed by ONR. The authors are grateful to Dr. C. Wood for his interest and support of this research. They gratefully acknowledge the assistance of Dr. V. P. Madangarli, Dr. J. D. Kim, and Ian Carlton in the experimental system setup, and of M. McLester and P. G. Muzykov in the experimental work. H. C. Miller, IEEE Trans. Electr. Insul. 26, 949 ~1991!. R. Latham, High Voltage Vacuum Insulation ~Academic, London, 1995!. R. Hawley and A. Maitland, Vacuum as an Insulator ~Chapman and Hall, London, 1967!. 4 R. Hawley, Vacuum 18, 383 ~1968!. 5 Model 6512 Programmable Electrometer Instruction Manual, Keithley Instruments Inc., 1994. 6 X. Ma and T. S. Sudarshan, Conference on Electrical Insulation and Dielectric Phenomena, Minneapolis, Oct. 1997. 1 2 3
J. Vac. Sci. Technol. B, Vol. 16, No. 2, Mar/Apr 1998
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High field breakdown characteristics of micrometric gaps in vacuum Xianyun Ma and T. S. Sudarshana) Department of Electrical and Computer Engineering, University of South Carolina, Columbia, South Carolina 29208
~Received 18 August 1997; accepted 14 January 1998! In order to obtain the prebreakdown and breakdown data of plain vacuum gaps in the micrometric regime relevant to field emission displays, an automatic experimental system was developed. The dc breakdown characteristics of the gap in the micrometric regime, varying from 25 to 1000 mm, were extensively studied. The experimental results show that the highly finished broad-area metal electrodes withstand fairly high fields in relatively poor vacuum. The dc breakdown strength of a 50 mm gap was more than 220 V/mm at a pressure of 1025 Torr, while for a 1000 mm gap, the breakdown strength was about 44.5 V/mm for highly polished chrome-steel electrodes, 2 cm in diameter. The breakdown of a narrow (,200 m m) vacuum gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown. However, a wider gap ~500–1000 mm! was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. The breakdown characteristics of ideal gaps with very smooth solid electrodes, presented in this work, will define the theoretical limits to which actual gaps can be stressed in a field emission display. © 1998 American Vacuum Society. @S0734-211X~98!04902-6#
I. INTRODUCTION Dielectric spacer and plain vacuum-gap insulation for high fields are critical issues for field emission based vacuum microelectronic devices, specifically the field emission display ~FED!. For reliable operation of FEDs, an appropriate spacer design is of primary importance. The insulation consideration of the spacer becomes more complicated considering the operational environment of the FED—the presence of electron emission from regions surrounding the spacer, outgassing from the phosphor, and elevated operational temperature of the device. Assuming that flashover could be alleviated even at high operational fields, the voltage limitation of the device shifts to the breakdown of the active region of the device, which is the plain vacuum gap between the cathode and anode. Thus the issues related to the breakdown of plain vacuum microgaps are important, especially under the operating condition of the device. The breakdown strength of plain anode-to-cathode gaps can be used to define the theoretical limits to which actual gaps can be stressed in an FED. Although there is voluminous bibliography on discharge processes in plain vacuum gaps1,2 of spacing >1 mm, the breakdown and prebreakdown data of plain vacuum gaps is not available in the micrometric regime ~25 to few hundred mm!, relevant to FEDs. As part of the main investigation of our project, which is aimed at developing microspacer insulation for field emission displays, this article reports the issues related to the breakdown and prebreakdown characteristics of plain anode-to-cathode vacuum microgaps. II. EXPERIMENTAL TECHNIQUES The behavior of a vacuum gap under high voltage ~HV! stress depends on a large number of parameters including the type of electrode materials, the shape of electrode profile, a!
Electronic mail: [email protected]
745
J. Vac. Sci. Technol. B 16„2…, Mar/Apr 1998
surface preparation and condition, residual gas and other contaminants, such as vacuum pump oil.3,4 In order to establish firmly, the relationship between various independent parameters associated with the vacuum gap, and its insulation capability, we adopted the test arrangement shown in Fig. 1. The test setup is mainly designed to study the behavior of a HV vacuum gap under controlled laboratory conditions with special consideration of low level prebreakdown current measurement and protection of the electrometer. A pair of highly polished ~surface roughness arithmetic average ,0.08 m m! chrome-steel spheres, 2 cm in diameter, was used as electrodes. Numerical electric field analysis using a field computation program based on the boundary element method ~BEM! showed that the geometrical field enhancement of such an electrode setup was less than 1.06 at gaps in the range 10–1000 mm. Hence, the gap can be assumed to produce a near-uniform electric field. The electrode arrangement is shown in Fig. 1. Each ball was held by a cylindrical holder, which was attached to the translator through a ceramic support. The fixed part of the translator was mounted on the flange of the vacuum chamber and the moving part was driven by a stepping motor. Changes in the ambient chamber pressure, temperature, and friction associated with feedthrough cause errors in the gap setup, and thus we should pay special attention to the change in gap spacing during an experimental run. The gap distance was mainly determined by the stepping motor with a resolution of 1 mm. An optical system, which was realized with the combination of a telescope and a zoom lens, was used to observe the test gap on a monitor. The system also enabled us to watch any change of the gap spacing directly. A light emitting diode ~LED! was used to illuminate the gap which was imaged by the optical system and finally captured by a CCD camera. Figure 2 is an image of the gap at 10 mm. The gap spacing was found to change
0734-211X/98/16„2…/745/4/$15.00
©1998 American Vacuum Society
745
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746
Xianyun Ma and T. S. Sudarshan: High field breakdown of micrometric gaps in vacuum
FIG. 1. Schematic of the test system for dc studies.
more than 4 mm during the experiment at gap spacings in the 50–500 mm range. Before setting up the gap, the electrode balls were submerged in methanol and cleaned in an ultrasonic cleaner three times, 15 min each. During setting up the gap, the balls were dealt with using ultra low-lint wipes. The electrode surfaces were checked and imaged using a microscope before and after testing of each setup of the gap. The test chamber was evacuated by a turbomolecular pump backed by an oil-free diaphragm pump and the final pressure in the chamber was ;831026 Torr. An extra low noise HV supply was used to produce voltages from 100 V to 50 kV. Since any discharge ~corona! along the HV lead will cause a large noise in the low level current measurement, the entire HV system should be discharge free. Capacitor C c ~500 pF! and Tektronix oscilloscope TDS 744A were used to monitor any noise along the HV lead, which could also be used to determine the occurrence of relatively large prebreakdown currents and breakdown of the gap. The resistor R f and capacitor C f were used to filter out noise in the high voltage system. The noise of the entire HV system was less than 1 V in the voltage range from 100 V to 50 kV.
746
The current was measured by a Keithley 6512 programmable electrometer, which is capable of measuring current as low as 10216 A. In our experimental system, the associated external electrical circuitry was designed to measure very low prebreakdown currents with a sensitivity less than 10214 A. A large value resistor R m (50 GV) was used for this reason which improved the noise performance of the electrometer.5 To protect the electrometer against a flashover of the gap, it is usually desirable to shunt the electrometer by some form of protection device such as a pair of biased-off zener diodes connected back–back.2 But this method will cause extra noise and shunt some current past the electrometer. We developed another method in which the structure housing the large value resistor R m was designed to withstand HV only to about 7 kV. When gap breakdown occurred and the HV across the resistor (R m ) housing exceeded 7 kV, the resistor would automatically be short circuited to ground, thus protecting the electrometer. A current-limiting resistor R 1 ~200 MV! was used to prevent excessive current flow through the gap in case of a breakdown. Sub-pico ampere current measurement under HV environment requires use of doubly-shielded coaxial cables, single point grounding, and the use of Faraday cage where all the measurement equipment is housed. The whole experimental system was computerized. After setting up the gap, the data collection was made automatically. III. RESULTS AND DISCUSSIONS With the above experimental system, the I – V characteristics of the gap in the micrometric regime varying from 25 to 1000 mm were extensively studied. Detailed investigation and theoretical analysis of these data are made in another report.6 The results of breakdown characteristics obtained in these investigations, in the micrometric vacuum gap regime, are presented here. A. Effect of electrode separation
For a given electrode material and geometry, the breakdown voltage V b was not a simple function of gap spacing d. Moreover, vacuum gaps that were apparently prepared identically exhibited significant scatter, even as high as 50% in the first breakdown voltage value V b1 . Figure 3 shows V b1 vs the gap distance d for vacuum gaps formed between highly polished chrome-steel sphere electrodes. For gaps between 25 and 200 mm, V b1 increased steadily with voltage according to V b 5373d 0.37 kV, where d is in mm, while for gaps 200 m m,d220 kV/mm for a 50 mm gap to >45 kV/mm for a 1000 mm gap. Thus the insulation capability per unit length decreased with increasing gap spacing. Even though, for a perfect vacuum gap, the theoretical breakdown field is 6.5 MV/ mm, the experimental results show that the highly polished broad-area metal electrodes could withstand rather high fields in relatively poor vacuum. It is shown that the investigated gap prebreakdown current is dominated by field emission electrons.6
C. Conditioning effect of breakdown
In small gaps (,200 m m), a microdischarge could lead to breakdown, causing damage to the electrodes. As a result, in each subsequent test run the gap held off a lower voltage compared to the previous breakdown value. Thus the vacuum gap performance degraded after each breakdown. Conversely, in larger gaps (.500 m m), each breakdown caused an improvement ~conditioning! and, after several breakdowns, the breakdown voltage was substantially higher than the first breakdown voltage (V b1 ). For a 500 mm gap, V b1 was as low as 20 kV, while V c , the conditioned voltage, >34 kV. Thus for gaps >500 m m, in order to reach the full insulation capability, a vacuum gap is subjected to conditioning, a process of in situ cleaning whereby the sources of prebreakdown current and microdischarges are safely quenched so that the sources of instability that contribute to the breakdown of the gap are reduced. If the electrodes were conditioned at a larger gap distance, the I – V characteristics of the gap, from 50 to 1000 mm, were more stable and repeatable, in addition to exhibiting improved insulation capability. But with this kind of conditioning, the issue was more complicated since the surface of the anode was usually damaged after conditioning ~see the next section!.
D. Role of electrode surface condition
FIG. 4. Insulation capability of micrometric gaps. 3: experimental data; –: best curve fit of experimental data.
As stated before, although the conditioning at large gaps improves the insulation capability of the electrode gap, the anode was usually damaged after conditioning. Figure 5 shows the anode surface condition after five breakdowns while the cathode suffered no damage. In spite of the damage to the anode, we obtained very stable I – V data and good insulation capability ~about 35 kV across 500 mm!. It might be concluded that the high-energy electrons cause surface damage of the anode; however, the I – V and the breakdown characteristics of the gap are mainly determined by the cath-
JVST B - Microelectronics and Nanometer Structures
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.210.236.20 On: Mon, 04 May 2015 09:13:55
748
Xianyun Ma and T. S. Sudarshan: High field breakdown of micrometric gaps in vacuum
ode. In order to investigate the effect of the surface roughness, we have performed experiments in which the highly polished electrode spheres were polished with 1 mm diamond paste. The insulation capability of the gap with electrodes polished by 1 mm diamond paste did not differ significantly from that of a highly polished one, but the I – V characteristics were unstable. Extensive investigations related to the influence of the electrode surface treatment on the breakdown characteristics of micrometric gaps will be reported in another article. E. Summary
A study of the breakdown characteristics of micrometric vacuum gaps is necessary in order to design electrical insulation in vacuum microelectronic devices, specifically, FEDs. Although practical FEDs use a planar ITO anode and a planar cold-cathode array, the present investigations are useful in defining the theoretical limits to which ideal micrometric vacuum gaps can be stressed. Our group is extending these broad-area solid-electrode breakdown measurements to planar metal-film electrodes including ITO anode structures. The breakdown characteristics of these structures will be reported in the near future. IV. CONCLUSIONS High field breakdown characteristics of broad-area micrometric gaps in vacuum were revealed in this study. Highly polished metal electrodes can withstand very high fields in
748
relatively poor vacuum. The dc breakdown strength of a 50 mm gap was more than 220 V/mm at a pressure of 1025 Torr, while for a 1000 mm gap, the breakdown strength was about 44.5 V/mm for highly polished stainless-steel electrodes. The breakdown of a narrow (,200 m m) gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown, while a wider gap ~500–1000 mm! was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. However, this conditioning causes damage of the anode surface. ACKNOWLEDGMENTS This research was supported from DARPA Grant No. N000149610845, managed by ONR. The authors are grateful to Dr. C. Wood for his interest and support of this research. They gratefully acknowledge the assistance of Dr. V. P. Madangarli, Dr. J. D. Kim, and Ian Carlton in the experimental system setup, and of M. McLester and P. G. Muzykov in the experimental work. H. C. Miller, IEEE Trans. Electr. Insul. 26, 949 ~1991!. R. Latham, High Voltage Vacuum Insulation ~Academic, London, 1995!. R. Hawley and A. Maitland, Vacuum as an Insulator ~Chapman and Hall, London, 1967!. 4 R. Hawley, Vacuum 18, 383 ~1968!. 5 Model 6512 Programmable Electrometer Instruction Manual, Keithley Instruments Inc., 1994. 6 X. Ma and T. S. Sudarshan, Conference on Electrical Insulation and Dielectric Phenomena, Minneapolis, Oct. 1997. 1 2 3
J. Vac. Sci. Technol. B, Vol. 16, No. 2, Mar/Apr 1998
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