- Email: [email protected]
Diamond vacuum field emission devices
Diamond and Related Materials 13 (2004) 975–981
Diamond vacuum field emission devices W.P. Kang, J.L. Davidson*, Y.M. Wong, K. Holmes Department of Electrical Engineering and Computer Science, Box 99, Station B, Vanderbilt University, Nashville, TN 37235, USA
Abstract Diamond for field emission has attracted considerable attention in vacuum microelectronics due to its low electron affinity for electron emission, hardness to withstand ion bombardment, and good thermal and electrical conductivity to handle high current. We have developed multiple processes to micro-pattern diamond films by mold transferring technique and achieve topologically managed diamond field emitters that optimize both the geometrical and quantum aspects of the Fowler–Nordheim governed emission. In this paper, we report the development of (a) vertical and (b) lateral diamond field emission devices. Vertically selfaligned gated diamond vacuum triodes were fabricated on a silicon-on-insulator (SOI) mold. This fabrication technique utilizes conventional silicon micro patterning and etching techniques to define the anode, gate, and cathode. The fabrication has achieved diamond field emitter triodes over practical wafer areas. The field emission of the triode array exhibits transistor characteristics with high d.c. voltage gain ;800 and good transconductance. Also, lateral diamond vacuum diodes were fabricated with a diamond patterning technique utilizing an oxide mask and lift-off. An anode–cathode spacing of less than 2 mm between the diamond anode and cathode was achieved. The lateral diode exhibits a low turn-on voltage of ;5 V (field approx. 3 Vymm) and a high emission current of 6 mA, from four diamond cathode ‘fingers’ configuration. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond film; Nanostructures; Field emission; Electronic device structures
1. Introduction The vacuum diode and triode in a vertical configuration are robust electronic devices with high-speed capability, high temperature and radiation tolerance. These diamond field emitters have superior transistor performance with low turn-on voltage, high emission current, good emission stability and durability. However, there are challenging obstacles to the development of a diamond field emitter triode due to the difficulty of diamond MEMS processing to achieve the three-terminal structure. The micro-construct of a diamond cold cathode diode and a self-aligned gate diamond emitter triode utilizing a silicon-on-insulator (SOI) wafer as a smart plate with a self-contained basic triode structure is described. The electrical characteristics of diamond field emission vertical devices are presented below. Lateral diamond field emission devices are also described. The lateral configuration has interest to vacuum microelectronics due to simpler fabrication, design versatility of electrode geometry, and precise spacing *Corresponding author. Tel.: q1-615-343-7886; fax: q1-615-3436614. E-mail address: [email protected] (J.L. Davidson).
between electrodes by fine photolithography control. A number of lateral field emitters utilizing materials other than diamond have been reported w1–7x, but their emission characteristics are marginal. We have recently developed a novel lateral diamond emitter fabrication technique utilizing an oxide patterning and lift-off process and determined its field emission characteristics, described below. 2. Device fabrication and experimental results 2.1. Vertical diamond emission devices The fabrication steps of the vertical diamond diode and gated triode have been described extensively in prior presentations and publications w8,9x. The SEM of the fabricated diamond field emission triode, Fig. 1, shows the nano-sharp apex, surrounded by a self-aligned silicon gate derived from the process. The emission current at the anode vs. anode voltage for several gate voltages (Ia –Va –Vg plots) of the self-aligned gate diamond triode (with 4 diamond tips) is shown in Fig. 2. The plots demonstrate the linear, saturation and cutoff behavior expected of a field emission transistor. Satu-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.103
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
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voltage, amplification factor and gate-cathode spacing, respectively. The amplification factor used in Eq. (2) is taken from the standard definition w13x. msy
Fig. 1. SEM of vertically self-aligned gated diamond vacuum triode.
ration is seen for various gate voltages at anode voltage above ;120 V. The triode exhibits a very low gate turn-on voltage of 10 V and a high emission current of 6 mA at an operating voltage of 20 V at anode voltage ranging from 150 to 400 V. The threshold current value observed as the turn-on voltage is 10 nA per tip. Furthermore, the diamond triode has stable emission current at a given anode and gate voltage. The emission current, at a fixed gate and anode voltage, fluctuates within 10% over extended time (a typical emission stability test is conducted for 3 days). The gate turn-on voltage is exceptionally low for electron field emitter triode (cathode-gate spacing of 1 mm). The tip sharpening, sp2 content, and boron doping are believed to contribute to this low operating voltage and high emission current of the diamond triode. A detailed discussion of the effect of sp2 content and boron doping on the emission characteristics of diamond tips has been presented w10–12x. The Fowler–Nordheim (F–N) equation was applied to characterize the emission data of the diamond emitter triode:
dVa dVg
)
(3) Iasconstant
The amplification factor is a parameter used to enumerate how much effect the gate voltage has on the anode emission current over anode voltage. For a vacuum field emitter triode, the gate voltage has more effect on emission current than the anode voltage per the form of the electric field at the emitter. The gate has much more effect on the electric field at the emitter than does the anode because of its close proximity to the emitter. Thus, the amplification factor describes how the gate voltage influences the cathode electric field more so than the anode voltage, depicted in Eq. (2). The amplification factor is an important parameter for small signal applications because it defines the limit of voltage gain as can be implied from Eq. (3). The simplest way to estimate the amplification factor from emission data is to graphically interpret it from the standard definition in Eq. (3). For example, from Fig. 2, at a constant Ia of 4 mA, if Va changes from 350 to 400 V, Vg is changed from ;19.2 to 19 V. Thus, the estimated m is 50y0.2s 250. This high value for the amplification factor is very good for a field emission transistor and indicates that the diamond field emission transistor provides high voltage gain when operated as an amplifier. Furthermore, transconductance (gm) of the diamond triode was found to be ;2.5 mS (for four tips) at a low operating gate voltage of 20 V. This high transconductance of the diamond field emission triode confirms the dominant control of the gate voltage over the anode voltage for
Ln(IyE2)sLn(A=K1=b2 yF)y(K2 =F1.5 yb)(1yE)
(1a)
where K1 and K2 are constants: K1s1.54=10y6 A eVy V2, K2s6.83=107 Vy(cm eV3y2), I is the emission current collected by the anode, F is the work function of the emitting surface (in eV), b is the field enhancement factor, A is the emitting area, and E is the composite macroscopic applied electric field from the gate and the anode (Vycm) which is given by Es(VgqVa ym)yd
(2)
where Vg, Va, m and d are the gate voltage, anode
Fig. 2. Field emission characteristics of diamond vacuum triode.
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tips participate in emission but smaller average beta. Additional testing is proceeding. 2.2. Packaging
Fig. 3. Indented anode diode configuration.
Of practical relevance, we have developed a vacuum cavity package, construct shown in Fig. 5, for the diamond devices. Fig. 6 is a photograph of said package (without lid in place), the total package size is 4=4 cm and the enclosed cavity is 2=2 cm. Fig. 7 is an optical picture of the cathode–anode ‘stack’. The cathode aligns to the anode on supports that set on their designated contact areas on the cathode. Note the diamond tip array of the cathode. Fig. 8 is the electrical operation of the packaged diode, illustrating forward, reverse and Fowler–Nordheim type emission. 2.3. Lateral diamond vacuum devices
the anode emission current. Therefore, the diamond field emission transistor offers excellent performance for small signal amplification applications and other possibilities. The two terminal diode behavior has been examined for cathode current density. An ‘indented anode’ configuration Fig. 3, was examined and I–V measured, Fig. 4. A forward current of ;0.1 A, at the limit of the power supply, is observed. The high forward bias is the result of a large cathode anode spacing (field approx. 10 Vy mm). The F–N curve is inset. Two slopes appear, suggesting at low fields, less tips emit, therefore, a smaller emission area but high beta. At high field, more
Lateral diamond diodes are fabricated from an SOI wafer by utilizing oxide patterning and lift-off. The fabrication steps for the lateral diamond emitter are shown in Fig. 9. The process begins with the growth of 1 mm SiO2 on a SOI wafer followed by patterning the anode and cathode structures on the SiO2 layer. The exposed SiO2 was then etched away and the diamond grown on the silicon area by PECVD with bias enhanced nucleation. The SiO2 masking layer was etched away. The patterned diamond anode and cathode served as a mask for etching silicon to yield isolation between the electrodes as well as setting the anode–cathode spacing.
Fig. 4. I–V plot of the diamond vacuum diode with indented anode and spacer (inset: F–N plot of the corresponding data).
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W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
Fig. 5. Vacuum cavity package for diamond emission devices.
The final structure consists of patterned diamond anode and cathode, supported by a silicon layer underneath, sitting on the SiO2 layer on the silicon substrate. Fig. 10 shows a lateral device. The diamond layer was characterized using Raman spectroscopy, a sharp sp3 peak at 1332 cmy1 was observed. The boron doping concentration was 1018 cmy3. The fabricated diamond field emission devices were then tested for field emission behavior in 10y6 Torr vacuum. A Keithley picoammeter model 485 was used to measure the emission current collected by the anode. Computerized data acquisition facilitated the analog output of the picoammeter to a computer. The current–voltage plot of emission characteristics of the lateral diamond emitter is shown in Fig. 11. A low turn-on voltage of 5 V and a high emission current of 6 mA, from 4 diamond fingers, at anode voltage of 25 V is observed. The threshold current value to determine the turn-on voltage is 10 nA per finger. This turn-
Fig. 6. Photograph of vacuum cavity package.
on voltage is lower than that of lateral silicon field emitters that utilized ultra sharp apex and submicron anode–cathode spacing w6,7,14,15x. Furthermore, the lateral diamond emitter has reasonably stable emission current at a given anode voltage. A typical current vs. time plot of the lateral diamond emitter is shown in Fig. 12. The emission current, at a fixed anode voltage, fluctuates less than 10% over time (approx. 3 days). The Fowler–Nordhiem (F–N) equation was applied to characterize the emission data of the diamond emitter triode: Ln(IyE2)sLn(A=K1=b2 yF)y(K2 =F1.5 yb)(1yE)
(1b)
where K1 and K2 are constants: K1s1.54=10y6 A eVy V2, K2s6.83=107 Vy(cm eV3y2), I is the emission current, F is the work function of the emitting surface (in eV), b is the field enhancement factor, A is the
Fig. 7. Optical picture of the cathode–anode ‘stack’.
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Fig. 8. I–V plot of packaged diamond vacuum diode–diode behavior, zero reverse leakage (inset: F–N plot confirms field emission).
emitting area, and EsVyd is the macroscopic applied electric field. V is the applied anode voltage and d is anode–cathode spacing. The F–N plot of the lateral diamond emitter is inset to Fig. 11. The emission current of the lateral emitter conforms to F–N behavior and thus confirms that the observed current is electron field emission. The shallow F–N slope of approximately 8.9 Vymm implies that the lateral diamond emitter diode has a high field emission enhancement factor. From the SEM micrograph observation, it would seem that the lateral diamond emitter
Fig. 9. Fabrication process for lateral diamond emitter utilizing silicon-on-insulator wafer.
Fig. 10. Scanning electron micrograph of lateral diamond emitter diode structure (1500=).
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W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
Fig. 12. Typical current–time (I–t) plot of lateral diamond emitter diode.
Fig. 11. Current–voltage (I–V) plot of lateral diamond emitter diode (inset: Fowler–Nordheim plot).
diode would have a low geometrical field enhancement factor due to the apparently large radius curvature of the diamond finger cathode. Thus, the observed low turn-on electric field and shallow F–N slope may arise from other components of the field emission enhancement factor such as that contributed by the inclusion of sp2 content, the boron dopant in the diamond film w16– 18x or cathode tip topology at a much more local region than the lithographic pattern. The results demonstrate that due to excellent emission properties of diamond, lateral diamond emitters exhibit outstanding emission characteristics. Application of sub-micron photolithography patterning is of interest. More elaborate lateral configurations are conceived and under evaluation, such as ‘comb’ geometry in Fig. 13.
circuit (IC) fabrication technology. Lateral diamond field emitters were fabricated by a diamond patterning technique that utilizes oxide patterning and lift-off process on a silicon-on-insulator (SOI) wafer. An anode– cathode spacing of -2 mm between the diamond anode and cathode was achieved. The fabricated lateral diamond emitter diode exhibits excellent emission characteristics with a low turn-on voltage of ;5 V and a high emission current of 6 mA, from four diamond fingers, at an anode voltage of 25 V. The emission current is stable over time, even at high emission current. The low turn-on voltage (turn-on field approx. 3 Vymm) and high emission characteristics are among the best of reported lateral field emitter structures. The lateral diamond field emitter has potential applications in vacuum microelectronics, sensors and microelectromechanical systems (MEMS).
3. Summary Diamond field emission devices have been fabricated and evaluated in vertical and lateral configurations. The fabrication techniques developed are practical and efficient. Vertical diode structures show high cathode current potential and a packaging approach is demonstrated. Vertically self-aligned gated diamond vacuum triodes were fabricated on a silicon-on-insulator (SOI) mold. This fabrication technique utilizes existing silicon micro patterning and etching techniques to define the anode, gate and cathode early in the SOI mold fabrication stage prior to diamond deposition. The fabrication has achieved diamond field emitter triodes over a large area. The fabrication of lateral diamond emitter by the diamond patterning techniques is demonstrated. The processing flow is simple and compatible with integrated
Fig. 13. SEM picture of ‘comb’ type lateral diamond field emitter diode.
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
Acknowledgments The Microelectronics Laboratory of Auburn University provided important help in processing and packaging activity. The Tennessee Valley Authority and the US Air Force have been importantly fiscally supportive. References w1x H. Shin, S. Yang, T. Hwang, S. Han, J. Lee, J.D. Lee, Microprocessor Nanotechnol. Conf. 134 (1999). w2x M.-S. Lim, C.-M. Park, M.-K. Han, Y.-I. Choi, J. Vac. Sci. Technol. B 17 (1999) 635. w3x V. Milanovic, L. Doherty, D.A. Teasdale, C. Zhang, S. Parsa, V. Nguyen, et al., IEEE Electron Device Lett. 21 (2000) 271. w4x W.-J. Zang, J.-H. Lee, J.-H. Lee, Y.-H. Bee, C.-A. Choi, S.-H. Hahm, J. Vac. Sci. Technol. B 18 (2000) 1006. w5x M.-S. Lim, C.-M. Park, M.-K. Han, Y.-I. Choi, Vac. Microelectron. Conf. 11 (1998) 113. w6x C.-S. Lee, J.-D. Lee, C.-H. Han, IEEE Electron Device Lett. 21 (2000) 479. w7x M.Y.A. Turner, R.J. Roedel, M.N. Kozicki, J. Vac. Sci. Technol. B 17 (1999) 1561.
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w8x W.P. Kang, J.L. Davidson, D.V. Kerns Jr., Mold method for forming vacuum field emitters and methods for forming diamond emitters, United States Patent Number: 6 132 278. w9x W.P. Kang, T. Fisher, J.L. Davidson, New Diamond Frontier Carbon Technol. 11 (2) (2001) 129. w10x W.P. Kang, A. Wisitsora-at, J.L. Davidson, Q. Li, J.F. Xu, C.K. Kim, et al., J. Vac. Sci. Technol. B 16 (1998) 684. w11x A. Wisitsora-at, W.P. Kang, J.L. Davidson, Q. Li, J.F. Xu, D.V. Kerns, Appl. Surf. Sci. 146 (1999) 280. w12x A. Wisitsora-at, W.P. Kang, J.L. Davidson, Y. Gurbuz, D.V. Kerns, Diamond Relat. Mater. 8 (1999) 989. w13x W.J. Orvis, C.F. McConaghy, D.R. Ciarlo, J.H. Yee, E.W. Hee, IEEE Trans. Electron Device 36 (1989) 2615. w14x S.-S. Park, D.-I. Park, S.-H. Hahm, J.-H. Lee, H.-C. Choi, J.H. Lee, IEEE Trans. Electron Device 46 (1999) 1283. w15x C.-M. Park, M.-S. Lim, M.-K. Han, IEEE Electron Device Lett. 18 (1997) 538. w16x W.P. Kang, A. Wisitsora-at, J.L. Davidson, Q. Li, J.F. Xu, C.K. Kim, et al., J. Vac. Sci. Technol. B 16 (1998) 684. w17x A. Wisitsora-at, W.P. Kang, J.L. Davidson, Q. Li, J.F. Xu, D.V. Kerns, Appl. Surf. Sci. 146 (1999) 280. w18x A. Wisitsora-at, W.P. Kang, J.L. Davidson, Y. Gurbuz, D.V. Kerns, Diamond Relat. Mater. 8 (1999) 989.
Diamond vacuum field emission devices W.P. Kang, J.L. Davidson*, Y.M. Wong, K. Holmes Department of Electrical Engineering and Computer Science, Box 99, Station B, Vanderbilt University, Nashville, TN 37235, USA
Abstract Diamond for field emission has attracted considerable attention in vacuum microelectronics due to its low electron affinity for electron emission, hardness to withstand ion bombardment, and good thermal and electrical conductivity to handle high current. We have developed multiple processes to micro-pattern diamond films by mold transferring technique and achieve topologically managed diamond field emitters that optimize both the geometrical and quantum aspects of the Fowler–Nordheim governed emission. In this paper, we report the development of (a) vertical and (b) lateral diamond field emission devices. Vertically selfaligned gated diamond vacuum triodes were fabricated on a silicon-on-insulator (SOI) mold. This fabrication technique utilizes conventional silicon micro patterning and etching techniques to define the anode, gate, and cathode. The fabrication has achieved diamond field emitter triodes over practical wafer areas. The field emission of the triode array exhibits transistor characteristics with high d.c. voltage gain ;800 and good transconductance. Also, lateral diamond vacuum diodes were fabricated with a diamond patterning technique utilizing an oxide mask and lift-off. An anode–cathode spacing of less than 2 mm between the diamond anode and cathode was achieved. The lateral diode exhibits a low turn-on voltage of ;5 V (field approx. 3 Vymm) and a high emission current of 6 mA, from four diamond cathode ‘fingers’ configuration. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond film; Nanostructures; Field emission; Electronic device structures
1. Introduction The vacuum diode and triode in a vertical configuration are robust electronic devices with high-speed capability, high temperature and radiation tolerance. These diamond field emitters have superior transistor performance with low turn-on voltage, high emission current, good emission stability and durability. However, there are challenging obstacles to the development of a diamond field emitter triode due to the difficulty of diamond MEMS processing to achieve the three-terminal structure. The micro-construct of a diamond cold cathode diode and a self-aligned gate diamond emitter triode utilizing a silicon-on-insulator (SOI) wafer as a smart plate with a self-contained basic triode structure is described. The electrical characteristics of diamond field emission vertical devices are presented below. Lateral diamond field emission devices are also described. The lateral configuration has interest to vacuum microelectronics due to simpler fabrication, design versatility of electrode geometry, and precise spacing *Corresponding author. Tel.: q1-615-343-7886; fax: q1-615-3436614. E-mail address: [email protected] (J.L. Davidson).
between electrodes by fine photolithography control. A number of lateral field emitters utilizing materials other than diamond have been reported w1–7x, but their emission characteristics are marginal. We have recently developed a novel lateral diamond emitter fabrication technique utilizing an oxide patterning and lift-off process and determined its field emission characteristics, described below. 2. Device fabrication and experimental results 2.1. Vertical diamond emission devices The fabrication steps of the vertical diamond diode and gated triode have been described extensively in prior presentations and publications w8,9x. The SEM of the fabricated diamond field emission triode, Fig. 1, shows the nano-sharp apex, surrounded by a self-aligned silicon gate derived from the process. The emission current at the anode vs. anode voltage for several gate voltages (Ia –Va –Vg plots) of the self-aligned gate diamond triode (with 4 diamond tips) is shown in Fig. 2. The plots demonstrate the linear, saturation and cutoff behavior expected of a field emission transistor. Satu-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.103
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
976
voltage, amplification factor and gate-cathode spacing, respectively. The amplification factor used in Eq. (2) is taken from the standard definition w13x. msy
Fig. 1. SEM of vertically self-aligned gated diamond vacuum triode.
ration is seen for various gate voltages at anode voltage above ;120 V. The triode exhibits a very low gate turn-on voltage of 10 V and a high emission current of 6 mA at an operating voltage of 20 V at anode voltage ranging from 150 to 400 V. The threshold current value observed as the turn-on voltage is 10 nA per tip. Furthermore, the diamond triode has stable emission current at a given anode and gate voltage. The emission current, at a fixed gate and anode voltage, fluctuates within 10% over extended time (a typical emission stability test is conducted for 3 days). The gate turn-on voltage is exceptionally low for electron field emitter triode (cathode-gate spacing of 1 mm). The tip sharpening, sp2 content, and boron doping are believed to contribute to this low operating voltage and high emission current of the diamond triode. A detailed discussion of the effect of sp2 content and boron doping on the emission characteristics of diamond tips has been presented w10–12x. The Fowler–Nordheim (F–N) equation was applied to characterize the emission data of the diamond emitter triode:
dVa dVg
)
(3) Iasconstant
The amplification factor is a parameter used to enumerate how much effect the gate voltage has on the anode emission current over anode voltage. For a vacuum field emitter triode, the gate voltage has more effect on emission current than the anode voltage per the form of the electric field at the emitter. The gate has much more effect on the electric field at the emitter than does the anode because of its close proximity to the emitter. Thus, the amplification factor describes how the gate voltage influences the cathode electric field more so than the anode voltage, depicted in Eq. (2). The amplification factor is an important parameter for small signal applications because it defines the limit of voltage gain as can be implied from Eq. (3). The simplest way to estimate the amplification factor from emission data is to graphically interpret it from the standard definition in Eq. (3). For example, from Fig. 2, at a constant Ia of 4 mA, if Va changes from 350 to 400 V, Vg is changed from ;19.2 to 19 V. Thus, the estimated m is 50y0.2s 250. This high value for the amplification factor is very good for a field emission transistor and indicates that the diamond field emission transistor provides high voltage gain when operated as an amplifier. Furthermore, transconductance (gm) of the diamond triode was found to be ;2.5 mS (for four tips) at a low operating gate voltage of 20 V. This high transconductance of the diamond field emission triode confirms the dominant control of the gate voltage over the anode voltage for
Ln(IyE2)sLn(A=K1=b2 yF)y(K2 =F1.5 yb)(1yE)
(1a)
where K1 and K2 are constants: K1s1.54=10y6 A eVy V2, K2s6.83=107 Vy(cm eV3y2), I is the emission current collected by the anode, F is the work function of the emitting surface (in eV), b is the field enhancement factor, A is the emitting area, and E is the composite macroscopic applied electric field from the gate and the anode (Vycm) which is given by Es(VgqVa ym)yd
(2)
where Vg, Va, m and d are the gate voltage, anode
Fig. 2. Field emission characteristics of diamond vacuum triode.
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
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tips participate in emission but smaller average beta. Additional testing is proceeding. 2.2. Packaging
Fig. 3. Indented anode diode configuration.
Of practical relevance, we have developed a vacuum cavity package, construct shown in Fig. 5, for the diamond devices. Fig. 6 is a photograph of said package (without lid in place), the total package size is 4=4 cm and the enclosed cavity is 2=2 cm. Fig. 7 is an optical picture of the cathode–anode ‘stack’. The cathode aligns to the anode on supports that set on their designated contact areas on the cathode. Note the diamond tip array of the cathode. Fig. 8 is the electrical operation of the packaged diode, illustrating forward, reverse and Fowler–Nordheim type emission. 2.3. Lateral diamond vacuum devices
the anode emission current. Therefore, the diamond field emission transistor offers excellent performance for small signal amplification applications and other possibilities. The two terminal diode behavior has been examined for cathode current density. An ‘indented anode’ configuration Fig. 3, was examined and I–V measured, Fig. 4. A forward current of ;0.1 A, at the limit of the power supply, is observed. The high forward bias is the result of a large cathode anode spacing (field approx. 10 Vy mm). The F–N curve is inset. Two slopes appear, suggesting at low fields, less tips emit, therefore, a smaller emission area but high beta. At high field, more
Lateral diamond diodes are fabricated from an SOI wafer by utilizing oxide patterning and lift-off. The fabrication steps for the lateral diamond emitter are shown in Fig. 9. The process begins with the growth of 1 mm SiO2 on a SOI wafer followed by patterning the anode and cathode structures on the SiO2 layer. The exposed SiO2 was then etched away and the diamond grown on the silicon area by PECVD with bias enhanced nucleation. The SiO2 masking layer was etched away. The patterned diamond anode and cathode served as a mask for etching silicon to yield isolation between the electrodes as well as setting the anode–cathode spacing.
Fig. 4. I–V plot of the diamond vacuum diode with indented anode and spacer (inset: F–N plot of the corresponding data).
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W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
Fig. 5. Vacuum cavity package for diamond emission devices.
The final structure consists of patterned diamond anode and cathode, supported by a silicon layer underneath, sitting on the SiO2 layer on the silicon substrate. Fig. 10 shows a lateral device. The diamond layer was characterized using Raman spectroscopy, a sharp sp3 peak at 1332 cmy1 was observed. The boron doping concentration was 1018 cmy3. The fabricated diamond field emission devices were then tested for field emission behavior in 10y6 Torr vacuum. A Keithley picoammeter model 485 was used to measure the emission current collected by the anode. Computerized data acquisition facilitated the analog output of the picoammeter to a computer. The current–voltage plot of emission characteristics of the lateral diamond emitter is shown in Fig. 11. A low turn-on voltage of 5 V and a high emission current of 6 mA, from 4 diamond fingers, at anode voltage of 25 V is observed. The threshold current value to determine the turn-on voltage is 10 nA per finger. This turn-
Fig. 6. Photograph of vacuum cavity package.
on voltage is lower than that of lateral silicon field emitters that utilized ultra sharp apex and submicron anode–cathode spacing w6,7,14,15x. Furthermore, the lateral diamond emitter has reasonably stable emission current at a given anode voltage. A typical current vs. time plot of the lateral diamond emitter is shown in Fig. 12. The emission current, at a fixed anode voltage, fluctuates less than 10% over time (approx. 3 days). The Fowler–Nordhiem (F–N) equation was applied to characterize the emission data of the diamond emitter triode: Ln(IyE2)sLn(A=K1=b2 yF)y(K2 =F1.5 yb)(1yE)
(1b)
where K1 and K2 are constants: K1s1.54=10y6 A eVy V2, K2s6.83=107 Vy(cm eV3y2), I is the emission current, F is the work function of the emitting surface (in eV), b is the field enhancement factor, A is the
Fig. 7. Optical picture of the cathode–anode ‘stack’.
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
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Fig. 8. I–V plot of packaged diamond vacuum diode–diode behavior, zero reverse leakage (inset: F–N plot confirms field emission).
emitting area, and EsVyd is the macroscopic applied electric field. V is the applied anode voltage and d is anode–cathode spacing. The F–N plot of the lateral diamond emitter is inset to Fig. 11. The emission current of the lateral emitter conforms to F–N behavior and thus confirms that the observed current is electron field emission. The shallow F–N slope of approximately 8.9 Vymm implies that the lateral diamond emitter diode has a high field emission enhancement factor. From the SEM micrograph observation, it would seem that the lateral diamond emitter
Fig. 9. Fabrication process for lateral diamond emitter utilizing silicon-on-insulator wafer.
Fig. 10. Scanning electron micrograph of lateral diamond emitter diode structure (1500=).
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Fig. 12. Typical current–time (I–t) plot of lateral diamond emitter diode.
Fig. 11. Current–voltage (I–V) plot of lateral diamond emitter diode (inset: Fowler–Nordheim plot).
diode would have a low geometrical field enhancement factor due to the apparently large radius curvature of the diamond finger cathode. Thus, the observed low turn-on electric field and shallow F–N slope may arise from other components of the field emission enhancement factor such as that contributed by the inclusion of sp2 content, the boron dopant in the diamond film w16– 18x or cathode tip topology at a much more local region than the lithographic pattern. The results demonstrate that due to excellent emission properties of diamond, lateral diamond emitters exhibit outstanding emission characteristics. Application of sub-micron photolithography patterning is of interest. More elaborate lateral configurations are conceived and under evaluation, such as ‘comb’ geometry in Fig. 13.
circuit (IC) fabrication technology. Lateral diamond field emitters were fabricated by a diamond patterning technique that utilizes oxide patterning and lift-off process on a silicon-on-insulator (SOI) wafer. An anode– cathode spacing of -2 mm between the diamond anode and cathode was achieved. The fabricated lateral diamond emitter diode exhibits excellent emission characteristics with a low turn-on voltage of ;5 V and a high emission current of 6 mA, from four diamond fingers, at an anode voltage of 25 V. The emission current is stable over time, even at high emission current. The low turn-on voltage (turn-on field approx. 3 Vymm) and high emission characteristics are among the best of reported lateral field emitter structures. The lateral diamond field emitter has potential applications in vacuum microelectronics, sensors and microelectromechanical systems (MEMS).
3. Summary Diamond field emission devices have been fabricated and evaluated in vertical and lateral configurations. The fabrication techniques developed are practical and efficient. Vertical diode structures show high cathode current potential and a packaging approach is demonstrated. Vertically self-aligned gated diamond vacuum triodes were fabricated on a silicon-on-insulator (SOI) mold. This fabrication technique utilizes existing silicon micro patterning and etching techniques to define the anode, gate and cathode early in the SOI mold fabrication stage prior to diamond deposition. The fabrication has achieved diamond field emitter triodes over a large area. The fabrication of lateral diamond emitter by the diamond patterning techniques is demonstrated. The processing flow is simple and compatible with integrated
Fig. 13. SEM picture of ‘comb’ type lateral diamond field emitter diode.
W.P. Kang et al. / Diamond and Related Materials 13 (2004) 975–981
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