Fabrication and characterization of well-aligned and ultra-sharp silicon nanotip array
© Wu et al; licensee Springer. 2012
Received: 18 November 2011
Accepted: 13 February 2012
Published: 13 February 2012
Well-defined, uniform, and large-area nanoscaled tips are of great interest for scanning probe microscopy and high-efficiency field emission. An ultra-sharp nanotip causes higher electrical field and, hence, improves the emission current. In this paper, a large-area and well-aligned ultra-sharp nanotip arrays by reactive ion etching and oxidation techniques are fabricated. The apex of nanotips can be further sharpened to reach 3-nm radius by subsequent oxidation and etching process. A schematic model to explain the formation of nanotip array is proposed. When increasing the etching time, the photoresist on top of the nanotip is also consumed, and the exposed silicon substrate is etched away to form the nanotip. At the end, the photoresist is consumed completely and a nanotip with pyramid-like shape is developed. The field emission property was measured, and the turn-on field and work function of the ultra-sharp nanotip was about 5.37 V/μm and 4.59 eV, respectively. A nanotip with an oxide layer capped on the sidewall is also fabricated in this paper. Comparing to the uncapped nanotip, the oxide-capped sample exhibits stable and excellent field emission property against environmental disturbance.
Ultra-sharp nanotips are drawn a lot of attention because of their promising applications in many fields such as electronics, microscopy, nanolithography, and biology [1–3]. More recently, application of nanotip in high-efficiency field emission [FE], flat panel displays, scanning probe microscopy, and scanning tunneling microscopy are intensively investigated . The efficiency of these techniques strongly depends on the characterization of the tip. For instance, a high-brightness and quick-response FE display can be obtained by a well-aligned nanotip array. FE is a quantum phenomenon that electrons are emitted from the cathode by a large electric field and tunneled through the surface of the tip array. This technique is a potential method for the manufacture of high-quality display with features of thin panel thickness, wide view angle, low power consumption, and high tolerance. Generally, the tip-end size, as well as the arrangement of the nanotip array, plays an important role in performing an effective FE . A tip with smaller apex can induce higher electrical field and, hence, significantly enhances the emission current [6, 7].
Various materials are employed to form the nanotips [8–10]. Among them, silicon is one of the most promising candidates to fabricate nanotips because it is the most used materials and ease of fabrication in the micro-electronic field [11–13]. Numerous technologies have been developed recently for the preparation of silicon nanotips, including evaporation deposition, electroplating, anisotropic wet etching method, and dry etching method [14–18]. Huang et al. used porous anodic alumina membrane as the mask and obtained a Si nanotip array by removing the silicon oxide [SiO2] islands which were formed during anodization of the Al/Si interface ; Cheng et al. fabricated silicon nanotips through high-density hydrogen plasma etching ; Hsu et al. proposed a one-step and self-masked dry etching technique for fabricating uniform and high-single-crystal silicon nanotips . These methods demonstrate simple processes to fabricate ultra-sharp nanotips. The alignment of nanotip array is, however, unsolvable issue because the nanotips are formed randomly by these methods. Linn et al. developed a microfabrication-compatible technology using inverted silicon pyramidal pits to fabricate the periodic gold nanopyramids with nanoscale sharp tips . Using this method, aligned silicon tip is obtained, but it is difficult to fabricate nanotips with high aspect ratio and sharp end since only pyramidal shape structures are provided.
In this work, Si nanotip arrays by combining the photolithography and reactive ion etching technology are fabricated. The apex of the nanotip can reach down to 3 nm in radius. By using this method, a large-area, well-aligned, and patternable nanotip array with high aspect ratio, ultra-sharp tip end can be achieved. In addition, we propose a formation scenario and model to explain the experiment results. The FE properties of Si nanotip arrays are investigated. The results indicate that the FE properties of nanotip arrays are improved when sharpening the tip end by oxidation process. We also demonstrate an oxide-capped nanotip which is only tip-end exposed. The oxide-capped sample exhibits stable and excellent field emission property against environmental disturbance.
Fabrication of the nanotip
The ultra-sharp nanotip array was fabricated using a commercially available 6-in., (100)-oriented, p-type silicon wafers. After standard (NH4OH/H2O2 = 3:1, then HCl/H2O2 = 3:1) and hydrofluoric acid [HF] cleaning, a 700-nm-thick photoresist [PR] was coated using TEL Clean Track Model-MK8 system (Tokyo Electron Limited, Tokyo, Japan), followed by 300 × 300 nm2 square array was defined using the optical exposure (Canon FPA-3000i5 stepper, Canon, Tokyo, Japan) systems. The plasma etching system (Lam Research TCP9400SE, Lam Research Corporation, Fremont, CA, USA) was then employed to form the pyramid-like tips. The etching power and pressure were 300 W and 12 mTorr, respectively. The gas flow was Cl2/HBr = 35:125 sccm.
To further sharpen the tip, a SiO2 was thermally grown to oxidize the sidewall of the tips, and then, samples were immersed into a buffer oxide etch [BOE] (NH4F/HF = 6:1) solution to fully remove the silicon oxide.
Fabrication of oxide-capped nanotip
Property analysis of the nanotip
Microstructure of the nanotip array was examined using SEM. The field emission characteristics were measured at 1E-6 Torr using a Keithley 237 high-voltage semiconductor parameter analyzer (Keithley Instruments, Inc., Cleveland, OH, USA). The silicon nanotips served as the lower electrode, and the tungsten probe approached the nanotips gradually to 100 nm. The tungsten probe was then applied a positive voltage ranging from 0 to 1,000 V and measured the emission current.
Results and discussion
Shape conversion of the tip at various etching times can be observed by top-view images. The shape of the nanotip significantly influences the field emission property. Interestingly, the exterior appearance of the tip is transforming with etching time. As seen in Table 1, the top-view appearance of the initial PR is rectangular, while after 4-min etching time, the tip is turning into round-corner pyramid shape; at 6-min etching time, the corner of the pyramid shape becomes acute; after 10-min etching time, the tip transforms to recessed pyramid shape. Although the mechanism needs to be further studied, this phenomenon is imaginable from the point of view of etch probability.
where A and B are constants equal to 1.56 × 10-10 A eV V-2 and 6.83 × 103 eV-3/2 V μm-1, respectively. The field enhancement factor is β, and ϕ is the work function; β and ϕ could be extracted by fitting the straight line from the ln(J/E2) versus 1/E plot. Figure 3b illustrates the FN plot of the pyramid-like tip and sharpened nanotip. The FN plots show a linear relationship, implying that the quantum tunneling effect is the main mechanism for the FE. The extracted field enhancement factors from the FN plots are 711 and 818 for the pyramid-like tip and sharpened nanotip, respectively.
We have fabricated a large-area and well-aligned ultra-sharp nanotip array by photolithography and reactive ion etching techniques. The apex of the nanotip can reach to 3 nm in radius. The mechanism of nanotip formation is that the remained photoresist on top of the tip is gradually consumed during the etching process, and the exposed silicon substrate is etched away to form the nanotip. The field emission property of the ultra-sharp nanotip is measured, and the turn-on field and work function of the ultra-sharp nanotip was estimated about 5.37 V/μm and 4.59 eV, respectively. The oxide-capped nanotip was also fabricated and demonstrated its excellent property against contamination.
This work was supported by Taipei Medical University under the contract number TMU99-AE1-B25.
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