Effect of crystal plane orientation on the friction-induced nanofabrication on monocrystalline silicon
© Yu and Qian; licensee Springer. 2013
Received: 7 February 2013
Accepted: 14 March 2013
Published: 25 March 2013
Although monocrystalline silicon reveals strong anisotropic properties on various crystal planes, the friction-induced nanofabrication can be successfully realized on Si(100), Si(110), and Si(111) surfaces. Under the same loading condition, the friction-induced hillock produced on Si(100) surface is the highest, while that produced on Si(111) surface is the lowest. The formation mechanism of hillocks on various silicon crystal planes can be ascribed to the structural deformation of crystal matrix during nanoscratching. The silicon crystal plane with lower elastic modulus can lead to larger pressed volume during sliding, facilitating more deformation in silicon matrix and higher hillock. Meanwhile, the structures of Si-Si bonds on various silicon crystal planes show a strong effect on the hillock formation. High density of dangling bonds can cause much instability of silicon surface during tip disturbing, which results in the formation of more amorphous silicon and high hillock during the friction process. The results will shed new light on nanofabrication of monocrystalline silicon.
KeywordsFriction-induced nanofabrication Silicon crystal plane Atomic force microscope
Because of its excellent mechanical and electronic property, monocrystalline silicon has been widely used as the structural material in micro/nanoelectromechanical systems (MEMS/NEMS) [1, 2]. In the past years, photolithography served as a prevailing approach to fabricate various functional micro/nanostructures on silicon surface [3, 4]. However, the continuous scaling down of device size has eventually brought the photolithography to its limit in realizing high resolution [5, 6]. Development of new nanofabrication methods is always a significant issue of concern. Recently, the friction-induced nanofabrication was proposed to produce protrusive nanostructures on Si(100) surface by scanning a diamond tip on a target sample without any post-etching . Besides silicon, this method can also enable the fabrication on electrical insulators, such as quartz and glass. As a straightforward and maskless method, the friction-induced nanofabrication points out a new route in fabricating nanostructures on demand.
It is well known that monocrystalline silicon has three typical crystal planes, i.e., (100), (110), and (111). As a typically anisotropic material, monocrystalline silicon presents different elastic modulus on various crystal planes, namely 130 GPa on Si(100), 169 GPa on Si(110), and 188 GPa on Si(111), respectively . Experimental results showed that the cutting process and friction behavior of silicon were influenced by the crystal anisotropy [9, 10]. Based on pin-on-disk tests, the average friction coefficient measured on Si(100) wafer was about 80% higher than that on Si(110) and Si(111) wafers . Moreover, because of the difference in the density of dangling bonds and structure of back bonds, the etching rate of Si(100) or Si(110) was two orders of magnitude faster than that of Si(111) in alkaline solution [11, 12]. These anisotropic properties of monocrystalline silicon may induce the different nanofabrication behavior on silicon surfaces with various crystal planes. Therefore, even though the friction-induced nanofabrication enables producing protrusive nanostructures on Si(100) surface, it remains unknown whether the same nanofabrication method can be realized on other silicon crystal planes.
In the present study, the effect of crystal plane orientation on the friction-induced nanofabrication on monocrystalline silicon was investigated. To verify whether the friction-induced fabrication can be realized on various silicon crystal planes, scratch tests at a linearly increasing load were performed on Si(100), Si(110), and Si(111) surfaces, respectively. The effect of crystal plane orientation on the formation of friction-induced hillocks was further detected by scanning three silicon crystal planes under a constant normal load. Finally, the formation mechanism of the hillock on various silicon crystal planes was discussed based on their mechanical performance and bond structure.
Si(100), Si(110), and Si(111) wafers were purchased from MCL Electronic Materials Ltd., Luoyang, China. The surface root-mean-square roughness of the wafers was measured as less than 0.2 nm over a square of 2 × 2 μm2 by an atomic force microscope (AFM; SPI3800N, Seiko Instruments Inc., Tokyo, Japan). The mechanical properties of the wafers were detected by a triboindenter (TI750, Hysitron Inc., MN, USA). During the indentation tests, a spherical diamond indenter with the nominal curvature radius R = 1 μm was used, and the maximum indentation depth was set to 20 nm.
To investigate whether the friction-induced nanofabrication can be realized on silicon surfaces with various crystal planes, the scratches were performed on Si(100), Si(110), and Si(111) surfaces by a nanoscratching tester (NST; CSM Instruments SA, Peseux, Switzerland) in air. A diamond tip with R = 2 μm was employed, and the scratching distance was 200 μm. Since the minimum load applied by the tester was 0.3 mN and surface grooves can be produced on silicon wafers at 6.0 mN, the scratching test was performed under linear loading from 0.3 to 6.0 mN. Before the fabrication tests, the silicon wafers were ultrasonically cleaned with acetone, ethanol, and deionized water in turn to remove surface contamination.
To study the effect of crystal plane orientation on the hillock formation on silicon, the fabrication was performed on three silicon samples by AFM with a vacuum chamber under a constant load (Fn) of 50 μN both in air and in vacuum (<5.0 × 10−6 Torr). A diamond tip (Micro Star Technologies, TX, USA) with R = 500 nm was used. The normal spring constant (k) of the cantilever of the AFM diamond tip was calibrated as 194 N/m through a calibration cantilever (CLFC-NOBO, Veeco Instruments Inc., NY, USA) . The line-shaped hillocks were produced at the sliding velocity of 40 μm/s. The number of scratch cycles (N) was 100 or 200. To study the effect of pressed volume on the hillock formation, a sharp diamond tip (R = 250 nm) was employed to perform the fabrication test on Si(100) surface in air. The topography of the scratches produced by the NST and the hillocks by the AFM was observed using the silicon nitride tips (MLCT, Veeco Instruments Inc.) with R = 20 nm and k = 0.1 N/m. During the entire experimental process, the temperature was set to 25 ± 2°C, and the relative humidity was between 50% and 55%.
Realization of friction-induced nanofabrication on various silicon crystal planes
Comparison of hillock formation under the constant load
Effect of the mechanical property on the hillock formation
The transmission electron microscope observation indicated that the friction-induced hillock on Si(100) surface contained a thin superficial oxidation layer and a thick disturbed (amorphous and deformed) layer in the subsurface [17, 18]. It was suggested that the mechanical interaction through amorphization was the key contributor to hillock formation on Si(100) surface. Although the silicon wafers with various crystal planes present different elastic modulus, all these wafers consist of Si-I phase (diamond-like structure) regardless of crystallographic orientations. During the sliding process, the transformation of Si-I to amorphous structure may occur on three silicon crystal planes, which will further induce the formation of hillock on these silicon surfaces. However, under the same loading conditions, the height of hillock on various silicon crystal planes was different as shown in Figures 2, 3 and 4. The results suggested that the crystal plane orientation of silicon had a strong impact on the friction-induced nanofabrication on the silicon surface.
Comparison of the contact of a diamond tip on various silicon crystal planes
Contact area A (nm2)
8.86 × 103
7.61 × 103
7.17 × 103
Pressed volume V (nm3)
2.49 × 104
1.83 × 104
1.63 × 104
Effect of the surface bond structure on the hillock formation
With two dangling bonds on each silicon atom, the (100) plane has the highest density of dangling bonds compared with the other crystal planes. Although only one dangling bond is attached to one silicon atom, the nonequilibrium in bonding state is further increased by the in-plane bonds on (110) plane . Even with the similar dangling bond number per atom as the (110) plane, the atom on the (111) plane is supported by three equivalent Si-Si backbonds, which enhance the mechanical stability of the Si(111) surface [21, 24]. Therefore, under the same loading condition, the highest hillock was generated on Si(100), while the lowest hillock was formed on Si(111) either in air or in vacuum. However, the disturbance from the tip was reduced because of the protective effect of the adsorbed water, oxidation layer, and contamination in air. As a result, a little lower hillock was produced on silicon in air compared to that in vacuum.
In summary, the friction-induced nanofabrication can be realized on different silicon crystal planes, with the contact pressure less than the hardness. At the same normal load, the silicon crystal plane with low elastic modulus or high density of dangling bonds can facilitate the formation of friction-induced hillock. Because of the configuration of Si-Si bonds, crystal silicon reveals different mechanical properties on various crystal planes, which eventually result in the variation of hillock formation in the present study. These findings may provide possibilities to control the hillock formation on monocrystalline silicon and help understand the subtle mechanism.
Friction-induced nanofabrication can be realized on Si(100), Si(110), and Si(111) surfaces, respectively. The crystal plane orientation has a significant effect on the hillock formation on silicon surface. Under the same loading condition, the highest hillock was generated on Si(100), while the lowest hillock was formed on Si(111) either in air or in vacuum.
The mechanical performance of silicon shows a strong effect on the hillock formation on various silicon crystal planes. The crystal plane with the lower elastic modulus can lead to larger pressed volume, which facilitates more deformation in silicon matrix and higher hillock.
The structures of Si-Si bonds play a key role in the hillock formation on various silicon crystal planes. High density of dangling bonds can cause much instability, facilitating the formation of more amorphous silicon and high hillock during nanoscratching.
atomic force microscope
applied normal load
normal spring constant
number of scratch cycles
The authors are grateful for the financial support from the National Basic Research Program (2011CB707604), Natural Science Foundation of China (90923017 and 51175441).
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