| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 94 === In this thesis, we will separate three parts: first: introduce the growth of single vertically-aligned carbon nanotube (CNT), secondly: field emission measurement and analysis with various shape of single CNT, and finally: the application of scanning probe microscopy and triode structure device with single CNT.
First, the vertically aligned CNTs were fabricated by a combination of EBL and ICP-CVD deposition. The positive photoresist of polymethylmehtacrylate (PMMA) 35 nm thickness was coated on a <100> p-type silicon substrate, followed by EBL. The spacing between dots were 1~3 μm apart. The acceleration voltage was 20 KV, and the exposure time was varied between 5 ms and 10 ms to control the size of holes after development.The minimum and average sizes of the nickel dots after lift-off ranged were 30 nm and 60 nm, respectively. Nickel, as the catalyst, was then deposited either by E-gun evaporation or radio frequency sputter, which thickness was about 10 nm. An ICP-CVD system was used to grow vertically aligned CNTs under the following process conditions: ICP power of 1000 W, substrate RF bias of 300 W, feed gas mixture of C2H2/H2/Ar with 8/24/0.5 sccm flow rates, and total pressure of 20 mtorr. The substrate temperature was about 550o C and the growth time was 10 minutes. To separate the field emission characteristics with various shape of CNT, two different types (24 cm and 4cm) of graphite electrodes supporting silicon substrates were used. In additionally, the aspect ratio of CNT is directly proportional to the exposure time of electron beam after using EBL and ICP-CVD.
Secondly, we have designed and constructed the three-axis nano-positioning device for carbon nano-tip assembly and FE measurement inside a SEM chamber in order to investigate the field emission characteristics with various shape and aspect ratio of single CNT (Free standing VACNTs of various aspect ratios (height/radius) were explained at first part). This device consists of two parts: inertial walker for x-y-axis and inchworm unit for the z-axis. The x and y stages are independent and actuated with piezoelectric stack respectively. The dimension of the x-y stage is 25 mm × 25 mm. The dimension of the inchworm is 48 mm × 20 mm. Based on the results of experiment, it clearly shows that the lengths and diameters of CNT are varied with the exposure times (the larger the Ni dot size, the large the CNT diameter and the faster the CNT growth rate). It was believed that the increasing growth rate for larger Ni dot size is attributed to the increasing carbon flux around Ni. The large height and small radius of CNT lead to high aspect ratios and thus better enhancement of the electric field at the tip. That means required turn-on electric filed is substantially reduced as the height increases or radius is decreases due to the increased geometrical field enhancement factor. Furthermore, the turn-on field was also showed a result of FE simulation at the apex of the CNT emitter with various aspect ratio by using a commercial code, SIMION 7.0.The SIMION code simulation conditions are: (1) diameter of anode probe of 3 μm,(2) spacing between anode and cathode 200 nm, and (3) anode voltage 24 V. The results were also showed that the maximum field enhancement factor and field strength was related to the aspect ratio.
Finally, we have constructed the scanning probe microscopy and triode structure device with single vertically-aligned CNT emitter source by using electron beam lithography and inductively-couple plasma (ICP) chemical vapor deposition. In the scanning probe microscopy measurement part, it was apparent that the silicon pillar array with height 150 nm and length 1.25~1.5 μm have been completely scanning by the single CNT probe. In the triode measurement, the field emission was the synthesized effect of the gate structure; in other words, in addition to the geometric factor of CNTs, the field strength at the CNT apex in triode structure also depends on the gate hole size, position of CNT at gate hole, and the height of CNT tip relative to the gate height (gate-to-cathode spacing). In this study, the gate hole size, gate-to-cathode spacing, and the position of CNT were fixed (i.e. CNT is positioned at gate hole center). Thus the emphasis was on the effect of CNT length and the consequent difference of CNT tip to gate distance. It was evident that the optimized CNT height for highest field at the CNT apex is that equal to the gate-to-cathode spacing. Furthermore, the turn-on field was also showed a result of FE simulation at the apex of the CNT emitter with various CNT heights by using a commercial code, SIMION 7.0. The simulation conditions are: (1) diameter of aperture hole and anode probe of 1.5 μm and 4 μm respectively, (2) applied gate voltage -10 V and anode voltage 0 V, and (3) cathode voltage -27.7 V. The results were also showed that the maximum field strength was related to the CNT length. Based on the above results, it was evident that in the triode measurement, although the difference of field emission is “controlled” by varying the “CNT length”, the difference is not simply due to the aspect ratio, but also due to the difference in distance between CNT tip height and gate height.
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