Nondestructive nanofabrication on Si(100) surface by tribochemistry-induced selective etching

A tribochemistry-induced selective etching approach is proposed for the first time to produce silicon nanostructures without lattice damage. With a ~1 nm thick SiOx film as etching mask grown on Si(100) surface (Si(100)/SiOx) by wet-oxidation technique, nano-trenches can be produced through the removal of local SiOx mask by a SiO2 tip in humid air and the post-etching of the exposed Si in potassium hydroxide (KOH) solution. The material removal of SiOx mask and Si under low load is dominated by the tribochemical reaction at the interface between SiO2 tip and Si/SiOx sample, where the contact pressure is much lower than the critical pressure for initial yield of Si. High resolution transmission electron microscope (HRTEM) observation indicates that neither the material removal induced by tribochemical reaction nor the wet etching in KOH solution leads to lattice damage of the fabricated nanostructures. The proposed approach points out a new route in nondestructive nanofabrication.

and subsequent wet etching [20][21][22] . These findings open a door to develop new SPM-based nanolithography method for fabricating nanostructures with lower fabrication destruction.
In this paper, we report a simple and feasible nondestructive nanofabrication approach on monocrystalline silicon through tribochemistry-induced selective etching. With a ~1 nm thick SiO x film as etching mask grown on Si/SiO x surface by wet-oxidation technique, nano-trenches with required depths can be produced through the removal of local SiO x mask by a SiO 2 tip in humid air and the post-etching of the exposed Si in KOH solution. Experimental results show that both the ambient humidity and normal load of tip scanning during the fabrication process has remarkable effect on the fabrication depth. Different from the traditional material removal mechanism of the SPM-based mechanical scratching or cutting methods with a diamond tip, the material removal of Si/SiO x under low contact pressure (far from enough to result in the initial yield of silicon) in the present method is determined by the tribochemical reaction at the interface between SiO 2 tip and Si/SiO x sample. High resolution transmission electron microscope (HRTEM) observation indicates that in the present approach, neither the material removal induced by tribochemical reaction nor the wet etching in KOH solution leads to lattice damage beneath the fabrication area. The nondestructive fabrication capability of this approach is demonstrated by a series of nanostructures on Si(100) surface, such as array of nano-trenches, nanochannels and multilayered nanostructure.

Results
Fabrication process of the tribochemistry-induced selective etching. Figure 1 shows the fabrication process of the tribochemistry-induced selective etching on Si(100) surface. Firstly, SiO x film with thickness of ~1 nm was generated on the H-passivated Si(100) surface through the wet-oxidation technique with hot HNO 3 solution (Fig. 1a) 23 . Secondly, the local SiO x film on the target area of Si(100) substrate was removed through the scanning of SiO 2 tip under low load in humid air to expose the Si(100) substrate (Fig. 1b). It has been clarified that under the mechanical shear by the SiO 2 tip in humid air, the tribochemical wear occurs [24][25][26] . During the wear process, Si-O-Si bridges firstly form at the contact interface, then water molecules hydrolyze and dissociate the strained Si-O-Si bonding bridges easily to produce the silane compounds that can be removed by tip scanning. As a result, both the SiO x layer and silicon substrate on the scanned area can be gradually removed by such tribochemical reaction. Thirdly, the exposed Si was selectively etched by KOH solution to form a deep nano-trench (Fig. 1c). Due to its lower etching rate in KOH solution than Si(100), the SiO x on the non-scanned area was served as a mask to protect the Si(100) substrate below from etching. Under the present experimental condition, the etching selectivity of SiO x :Si(100) was estimated as about 1:250, which was close to the reported selectivity of SiO 2 :Si(100) in KOH solution (less than 1:185 27 ).
Effect of humidity and normal load on the fabrication depth. Experimental results suggested that the fabrication depth depended on not only the KOH etching period but also the relative humidity (RH) and the normal load F n during tip scanning process. The in-situ three-dimensional topographies of the fabrication areas under various humidity and experiment of load conditions were characterized by atomic force microscope (AFM, Figure S1 and Figure S2 in Supplementary Section 1).
The fabrication depth (before and after post-etching) versus RH curves are plotted in Fig. 2a. It indicated that under the given load of 3 μ N, (i) when RH was 0%, in the absence of tribochemical reaction, no appreciable wear could be observed on the scanned area of SiO 2 tip from the AFM image. However, because of the slight disturbance of the SiO x structure by mechanical interaction, the disturbed SiO x could be selectively dissolved in KOH solution slowly to obtain a shallow nano-trench; (ii) The tribochemical reaction would be intensified with the increase of RH in certain range (0%-50%), resulting in the increasing material removal (tribochemical wear) of Si/SiO x ; (iii) There was a threshold RH (~20%) above which the SiO x on the scanned area could be removed thoroughly to expose the Si substrate. Then the exposed Si could be etched by KOH solution to fabricate deeper nano-trench. It was revealed that the tribochemistry-induced nanofabrication could be realized at common humidity condition, without the need for a higher humidity. Figure 2b shows the load dependence of the fabrication depth. The result suggested that load could intensify the tribochemical wear of Si/SiO x , and at present experimental condition, a condition of F n ≈ 1 μ N and one scanning cycle was enough to realize the removal of SiO x mask with ~1 nm in thickness. Once the Si substrate on tip scanning area was exposed, deeper nano-trenches could be produced quickly through the selective etching of Si in KOH solution.
Since the maximum Hertzian contact pressure during the scanning process by SiO 2 tip (calculated as about 0.48-1.2 GPa) was far below the critical pressure for initial yield of Si (~11.3 GPa 26 ), such contact between SiO 2 tip and Si substrate should be elastic. Moreover, the anisotropic etching of Si in KOH solution is pure chemical behavior 28 , which does not bring lattice distortion beneath the etched nano-trench. Therefore, the subsurface of the fabricated trench was deduced to be composed of damage-free monocrystalline silicon.
HRTEM characterization on the tribochemistry-induced nanofabrication area. In order to verify whether the tribochemistry-induced selective etching nanofabrication brings destruction or not, both the tip scanning area and KOH post-etching area were observed by cross-sectional transmission electron microscope (XTEM), as shown in Fig. 3. Figure 3a shows the XTEM observation of the scanned area with the SiO x mask removed thoroughly. The observed nano-trench with depth of 18 nm and width of 118 nm (XTEM image, in the middle of Fig. 3a, the cross-section is (110) crystal face of Si) was fabricated on a Si(100)/SiO x sample by line-scanning mode under the conditions of F n = 2 μ N, number of scanning cycles N = 500 and RH = 50%. After the fabrication, the SiO x film and partial silicon substrate beneath the SiO x film on the scanned area were removed because of the tribochemical wear mentioned before. As shown in the HRTEM lattice fringe images, the structure of the scanned area is monocrystalline silicon, no amorphous silicon layer and lattice distortion layer are observed. Figure 3b shows the XTEM observation of the post-etched area by KOH solution. Firstly, a shallow nano-trench was fabricated by line-scanning mode with a SiO x tip under the same fabrication conditions as those of the nano-trench shown in Fig. 3a. Secondly, this sample was dipped into KOH solution etching for 10 min to obtain the final nano-trench with depth of 147 nm and width of 220 nm (XTEM image, in the middle of Fig. 3b). The slope sidewall of nano-trench is (111) crystal face of Si, which is well known to be the etch-stop crystal face due to its ultralow etching rate in KOH solution 28 . It can be clearly observed that the post-etched area is also

Nondestructive nanostructures fabricated on Si(100).
Based on the tribochemistry-induced nanofabrication method, a series of nondestructive nanostructures were fabricated on Si(100) surface. Figure 4a shows a high-density 5 × 5 nano-trench array with depth of ~100 nm, which was fabricated by area-scanning under the condition of RH = 50%, F n = 3 μ N and t (post-etching period of KOH solution) = 5 min. Figure 4b shows four nanochannels with the same length of 8 μ m and width of 0.5 μ m, different depths of about 0.7, 3.8, 7.8 and 15 nm. Such nanochannels were directly produced by area-scanning of SiO 2 tip under RH = 50%, F n = 1 μ N and the changed N of 1, 2, 4 and 8, respectively. Figure 4c shows a multilayered nanostructure produced on Si(100) surface by carrying out twice tribochemistry-induced selective etching fabrication. Firstly, the first nano-trench with depth of ~190 nm was produced on a Si(100)/SiO x sample under F n = 3 μ N and t = 10 min in the 1 st fabrication period. Secondly, after new SiO x mask was grown on the trench area, the second nano-trench with ~130 nm in depth within the first nano-trench was generated under F n = 3 μ N and t = 5 min in the 2 nd fabrication period. This fabrication case suggested that nondestructive refabrication on the fabricated surface could be realized through the repetition of tribochemistry-induced nanofabrication process, by which the complex multilevel nanostructures could be fabricated easily.
The fabrication process of tribochemistry-induced selective etching is conductivity-independent and straightforward. Without the need for any additional field (such as electrical field) and template, nano-trenches with arbitrary shape can be precisely patterned on the specified location through programming the trace of tip. By virtue of the near atomic scale tribochemistry wear, the minimum fabrication depth can be below 1 nm (several atoms layer). Different from the traditional SPM-based nanolithography methods for monocrystalline silicon, where the material removal is mainly realized through mechanically scratching or cutting under high load, the tribochemistry-induced material removal process is dominated by tribochemical reaction under much lower load and then does not lead to the lattice damage beneath the fabrication area. A contrasting example of the two different material removal behaviors is shown in the Supplementary Information (Supplementary Section 2). The tribochemistry-induced approach can be used for fabricating nondestructive silicon nano-textures in order to improve or regulate the surface hydrophobicity or tribology properties [29][30][31] . In addition, the nanochannels free of lattice damage are expected to be applied as the key components in micro/nano fluidic systems 32 , which are widely used in drug delivery, ion transporters, DNA translocators, and so on [33][34][35][36] . The nondestructive tribochemistry-induced nanofabrication method is also available for other chemical reactive surface, such as GaAs, to produce defect-free or well-ordered nanostructures 37 .

Conclusion
In summary, we have proposed a simple and feasible nanofabrication approach to produce damage-free monocrystalline silicon nanostructures. When the contact pressure does not result in the initial yield of Si, the material removal of Si/SiO x is determined by the tribochemical reaction between the SiO 2 tip and Si/SiO x sample. Experimental results suggest that the fabrication depth is mainly dependent on the humidity, the normal load during tip scanning process and the etching period in KOH solution. Results of HRTEM observation reveal that the nanostructures fabricated by tribochemistry-induced selective etching are free of lattice damage. Such nondestructive nanostructures are very difficult to be fabricated by conventional SPM-based mechanical cutting or friction-induced nanofabrication methods.

Experimental
Materials and methods. Experiments were performed on p-type Si(100) wafers with thickness of 0.5 mm (MEMC Electronic Materials, Inc., USA). Before the fabrication, samples were dipped into 10 wt.% HF solution for 2 min to remove the superficial native oxide layer, and then the Si surface became H-passivated 38 . Because of its strong oxidizing and low viscosity, HNO 3 solution was used to produce SiO x mask on Si substrate 23 . The H-passivated Si samples were soaked in ~65 wt.% HNO 3 solution for 30 min at 80 °C, then SiO x film with ~1 nm in thickness (characterized by scanning Auger nanoprobe (PHI 700, ULVAC-PHI, Inc., Japan), see Figure S5 in Supplementary Section 3) was generated on Si(100) substrate. With an AFM (SPI3800N, Seiko, Japan), the root-mean-square roughness of the Si/SiO x sample was measured to be less than 0.08 nm over an area of 500 nm × 500 nm. Spherical SiO 2 tips (Novascan Technologies, USA) with radii of ~1.0 μ m were employed for the scanning to remove the local SiO x mask. The normal spring constant of the cantilever was 14 N/m. In post-etching procedure, a mixture of 20 wt.% KOH solution and isopropyl alcohol (volume ratio = 5:1) was used as an etchant for selective etching of the exposed Si. The etching temperature was set to be 23 ± 2 °C. AFM and XTEM characterization. All of the AFM images were scanned by silicon nitride probes (MLCT, Bruker Corp., USA) with a spring constant of 0.1 N/m. The microscopic structural feature of the fabricated area on silicon sample was detected by XTEM (Tecnai G2 F20, FEI, USA). The XTEM samples of the fabricated area were prepared using a FIB (NanoLab 400, FEI, USA) miller. Before FIB cutting, an epoxy polymer passivation layer was deposited on the sample surface to protect the surface from damage in subsequent FIB cutting process.