Monolithically Integrated Nanodiamond Vacuum Lateral
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Monolithically Integrated Nanodiamond Vacuum Lateral Microtriode Operable at Low Voltages W. P. Kang 1 , N. Ghosh 1 , S. H. Hsu 1 , S. Raina 1 , and J. H. Huang 2 1 Dept. of Electrical Engineering and Computer ScienceVanderbilt University, USA 2 Dept. of Materials Science and Engineering, National Tsing Hua University, Taiwan E-mail: [email protected] Abstract - We report a monolithic vacuum microtriode utilizing nanodiamond as the lateral multi-finger emitters. The structure is comprised of 140-fingerlike nanodiamond emitters with built-in nanodiamond gate and Si anode. A mixed lithography patterning approach is used to fabricate the three-terminal device structure. Triode characteristics, demonstrating gate controlled emission current modulation at low operating gate and anode voltages, are obtained. The realization of the efficient monolithic microtriode allows further development of robust vacuum integrated circuit for application in high temperature and radiation harsh environments. Keywords: Nanodiamond, Field Emission Microtriode. I. INTRODUCTION Chemical vapor deposited (CVD) nanodiamond structures are interesting electron emitters because of their superior electronic properties and tolerance to operate at much higher temperatures and radiation harsh environments [1]. Other advantages of nanodiamond as emitters include chemical and electrical stability, high breakdown voltage, low turn-on electric field and excellent thermal conductivity [1]. Vacuum field emission triodes have been proposed, simulated and fabricated in the past [2], but practical application wasn’t possible because of their design complexity, high turn-on field and poor choice of materials. Previously, we have reported a single-finger micro-patterned nanodiamond lateral triode [3]. This single-emitter structure has limited practical application because of its low emission current and high gate modulation voltage. Nonetheless, we have observed no measurable changes in device structure and electrical response after high neutron and X-ray irradiation [3], signifying an emerging electronics for harsh environment applications. In this work, a multifinger nanodiamond lateral vacuum microtriode is fabricated for the first time using electron beam lithography (EBL) and optical lithography. The goal is to overcome the previous limitations by providing higher emission current at lower gate and anode voltages. II. EXPERIMENTAL The monolithic lateral vacuum triode is comprised of an array of uniformly spaced, high aspect ratio, 140 finger-like nanodiamond emitters with integrated anode and gate. The fabrication process is based on multilevel mixed lithography approach. It started with the deposition of a uniform layer of nanodiamond (ND) film on silicon-on- insulator (SOI) substrate with 2 µm, 4 μm and 525 μm- thick of active, box and handle layer thickness respectively. Microwave plasma enhanced CVD system with CH 4 /H 2 /N 2 as precursor gases is used for nanodiamond deposition. Next, an aluminum (Al) layer is deposited on the ND by e- beam evaporation. Then a single-mask optical lithography is used to delineate the contact pads. The exposed Al is etched away at room temperature followed by photoresist removal. EBL is then used to delineate the device structure on e-resist coated Al pad. After developing the pattern, Al dry etch is performed to get an improved etch profile. Then ND RIE is performed using the patterned Al as a mask to get the emitter structure. Next, EBL is introduced for the second time to write a straight line over the edge of the ND fingers sitting on active silicon (Si) layer covered with the same e-resist used before. After developing the pattern, RIE of Si is performed to expose box silicon dioxide (SiO 2 ) layer selectively. Exposed SiO 2 is later etched away. This has exposed the handling Si layer of the SOI wafer to the emitter tips. Finally, all the mask material is removed and active Si layer is etched away to provide isolation between the electrodes hence realizing the desired device structure. III. RESULTS SEM image of the fabricated device structure is shown in Fig. 1. The lateral triode with 10 μm spacing between the ND electrodes and 8 μm spacing between the ND fingerlike emitter and exposed handling Si was characterized for field emission in a common emitter configuration in a vacuum condition of ~10 -7 Torr. The anode was used to turn-on the cathode which incorporates multiple nanodiamond fingered emitters, while the gate was employed to modulate the emission from the cathode to the anode. Taking the advantage of the device structure, the triode was operated in two modes of operation by interchanging the anode and gate. In the first mode of characterization the handling Si was used as anode and the other ND electrode as gate. Figure 2 displays the triode I-V characteristics of the nanodiamond lateral vacuum device at room temperature, indicating the anode current can be effectively controlled by low operating gate voltages. A large anode current (I a ) of ~27 μA was obtained at an Anode voltage (V a ) of ~60 V, when the gate bias (V g ) was 10 V. On the other hand, when negative 10 V was applied at the gate, I a suppressed to ~12 μA at same V a . For the 3-terminal triode operation, the resultant potential (V t ) at the cathode is consisted of two components; the potential field produced by the gate and Paper XX-XX-XX presented in IEEE Nanotechnology Materials and Devices Conference (IEEE NMDC 2013), October 7-9, 2013, National Cheng Kung University, Tainan, Taiwan.
Transcript of Monolithically Integrated Nanodiamond Vacuum Lateral
the field provided by the anode. The modified F-N [4] principle, in triode configuration, with the resultant Vt was used to analyze the emission data, Fig. 2 inset. The result confirms that the emission current is due to tunneling mechanism under applied electric fields. In the second mode of operation the handling Si was used as gate electrode and the other ND electrode as anode, while keeping the fingerlike emitters as cathode. Figure 3 displays the triode I-V characteristics. A larger Ia of ~40 µA was obtained at Va of ~60 V, when the Vg was 10 V. Where as, applying negative 10 V at the gate, suppressed Ia to ~20 µA at same Va. It is believed that the higher current attributes to the position of the gate which is 2 µm closer than the previous case. The total cathode voltage (Vt) was calculated as earlier and the device characteristics were also found to follow the modified F-N relationship, Fig. 3 inset. Further, the applied anode and gate voltages are one of the lowest reported for lateral device, especially with cathode-gate separation in microns. However, these values can be further improved by placing the gate closer to the emitter. It is also important to mention here that the gate-intercepted current is negligible to the anode current of the triode. The analysis based on the modified Fowler-Nordheim theory in triode configuration confirms the field emission mechanism and operating principle of the device. The geometry coefficient associated with the gate screening effect was estimated from the triode amplification factor extracted from the emission characteristics, showing consistency with the modeling results based on electrostatic theory.
IV. CONCLUSION The fabrication process of a monolithic nanodiamond microtriode has been successfully developed. The fabricated device exhibits clear triode behavior of gate- modulated emission current at low operating gate voltages. The analysis of triode parameters provides a viable approach to understand the field emission mechanism of the triode and for further design of the device structure with desired triode performance for vacuum IC.
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3. K Subramanian, PhD dissertation, Department of Electrical Engineering, Vanderbilt University, Nashville, TN, USA, 2008.
4. R. H. Fowler and L. W. Nordheim, Proc. of the Royal Society of London, Series A 119, 173-181, 1928.
Figure 1. SEM of the fabricated nanodiamond triode. Inset shows higher magnification image of the device.
Figure 2. Triode emission characteristics of the nanodiamond lateral device operating in mode 1 (handling Si as anode); Inset shows corresponding F-N plots.
Figure 3. Triode emission characteristics of the nanodiamond lateral device operating in mode 2 (handling Si as gate); Inset shows corresponding F-N plots.
Paper WP-1-6 presented in IEEE Nanotechnology Materials and Devices Conference (IEEE NMDC 2013), October 7-9, 2013, National Cheng Kung University, Tainan, Taiwan.
