Nanofabrication on monocrystalline silicon through friction-induced selective etching of Si3N4 mask
© Guo et al.; licensee Springer. 2014
Received: 8 April 2014
Accepted: 5 May 2014
Published: 16 May 2014
A new fabrication method is proposed to produce nanostructures on monocrystalline silicon based on the friction-induced selective etching of its Si3N4 mask. With low-pressure chemical vapor deposition (LPCVD) Si3N4 film as etching mask on Si(100) surface, the fabrication can be realized by nanoscratching on the Si3N4 mask and post-etching in hydrofluoric acid (HF) and potassium hydroxide (KOH) solution in sequence. Scanning Auger nanoprobe analysis indicated that the HF solution could selectively etch the scratched Si3N4 mask and then provide the gap for post-etching of silicon substrate in KOH solution. Experimental results suggested that the fabrication depth increased with the increase of the scratching load or KOH etching period. Because of the excellent masking ability of the Si3N4 film, the maximum fabrication depth of nanostructure on silicon can reach several microns. Compared to the traditional friction-induced selective etching technique, the present method can fabricate structures with lesser damage and deeper depths. Since the proposed method has been demonstrated to be a less destructive and flexible way to fabricate a large-area texture structure, it will provide new opportunities for Si-based nanofabrication.
KeywordsFriction-induced selective etching Si3N4 mask Silicon
Nanostructures of silicon have been widely used in micro/nanoelectromechanical systems (MEMS/NEMS) , photovoltaic devices [2–4], nanoimprint lithography template , and so on. As a typical nanofabrication method on silicon, photolithography technique involves complex systems and multiple steps [6, 7]. Although it has a huge merit in mass production, photolithography is not suitable for flexible fabrication of micro-mold and prototype fabrication of microsystems . Therefore, it remains essential to develop a simple and flexible nanofabrication technique to meet the requirements of nanoscience and nanotechnology.
Due to its simplicity, flexibility, and high resolution, scanning probe microscope (SPM)-based techniques have been demonstrated to hold great potential in fabricating nanostructures [9–14]. Among various SPM-based techniques of silicon, local anodic oxidation  and friction-induced selective etching  have attracted much attention from researchers. However, local anodic oxidation process strongly relies on the experimental parameters such as voltage, humidity, tip dwell time, and gaseous ambient environment . Compared to local anodic oxidation technique, the friction-induced selective etching method has a more straightforward process and a lower requirement to environment. Without any additional facility, patterns can be easily fabricated by directly scratching a diamond tip on silicon substrate along the target trace and post-etching . In this method, an affected layer is formed on the scratched area. Due to its resistance to alkaline solution, the affected layer can serve as an etching mask (defined as tribo-mask) for fabricating protrusive structures [17, 18]. However, the etching selectivity of tribo-mask/Si(100) in KOH solution is low and uncontrollable . When etching for a long time, the collapse may occur in the upper part of the structure . Due to the restriction by the above factors, the maximum fabrication depth is generally less than 700 nm, which to some extent limits the application of the fabricated nanostructures . To broaden the range of fabrication depth to micron scale, it is necessary to develop new fabrication methods with a high-quality mask. Since the etching selectivity of Si(100)/Si3N4 in KOH solution is about 2,600:1, the Si3N4 mask may be a good candidate by virtue of its excellent resistance to chemical attack .
In this paper, the friction-induced selective etching behavior of the Si3N4 mask on Si(100) surface was investigated. Effect of normal load and KOH etching period on fabrication depth was separately clarified. Based on the scanning Auger nanoprobe analysis, the fabrication mechanism of the proposed method was discussed. Finally, a large-area texture pattern with depth of several microns was attempted on Si(100) surface. The results may provide a simple, flexible, and less destructive way toward patterning a deep structure on silicon surface.
Si(100) wafers coated with low-pressure chemical vapor deposition (LPCVD) Si3N4 films (Si/Si3N4) were purchased from Hefei Kejing Materials Technology, Hefei, China. X-ray photoelectron spectroscopy (XPS; XSAM800, Kratos, Manchester, UK) detection revealed that the deposited films were stoichiometric Si3N4. Scanning Auger nanoprobe (PHI 700, ULVAC-PHI, Inc., Kanagawa, Japan) detection indicated that the thickness of Si3N4 films was about 50 nm. Using an atomic force microscope (AFM; SPI3800N, Seiko, Tokyo, Japan), the root-mean-square (RMS) roughness of the Si/Si3N4 samples was measured to be 0.4 nm over a 2 μm × 2 μm area. The elastic modulus of the Si3N4 film was estimated to be 240 GPa by nanoindentation with a spherical diamond tip .
During the fabrication process, scratching was conducted on Si/Si3N4 samples by a nanoscratch tester (TI750, Hysitron Inc., Eden Prairie, MN, USA) using a spherical diamond tip with a nominal radius R of 1.5 μm. The large-area fabrication was realized by a self-developed microfabrication apparatus, on which the maximum fabrication area of 50 mm × 50 mm can be achieved . During scratching process, the temperature was controlled at 22°C and the relative humidity ranged between 40% and 45%. In etching process, 2 wt.% HF solution was used for selective etching of the scratched Si/Si3N4 sample and removal of the residual Si3N4 layer; a mixture of 20 wt.% KOH solution and isopropyl alcohol (IPA) (volume ratio = 5:1) used for selective etching of the exposed silicon. The etching temperature was set to be 23 ± 1°C. All of the AFM images were scanned in vacuum by silicon nitride tips (MLCT, Veeco Instruments Inc., Plainview, NY, USA) with a spring constant k = 0.1 N/m. The morphology of large-area textured surface was observed by a scanning electron microscope (SEM; QUANTA200, FEI, Hillsboro, OR, USA). The contact angle of textured surface was tested by an optical contact angle measuring device (DSA-100, KIUSS, Hamburg, Germany).
Results and discussion
Friction-induced selective etching of Si3N4 mask in HF solution
Effect of scratching load and KOH etching period on nanofabrication
Fabrication of nanostructures on Si(100) surface
Based on the friction-induced selective etching of the Si3N4 mask, a new nanofabrication method was proposed to produce nanostructures on monocrystalline silicon. Experimental results suggest that HF solution can selectively etch the scratched Si3N4 mask and then provide the gap for KOH deep etching. The patterning structures with designed depth can be effectively fabricated on the target area by adjusting the scratching load and KOH etching period. Due to the excellent masking ability of the Si3N4 film, the maximum fabrication depth of 2.5 μm can be achieved. Compared to the traditional friction-induced selective etching, the advantage of the present method is to fabricate nanostructure with lesser damage and deeper depth. As a simple, flexible, and less destructive technique, the proposed method will provide new opportunities for Si-based nanofabrication.
atomic force microscope
low-pressure chemical vapor deposition
scanning electron microscope
scanning probe microscope
X-ray photoelectron spectroscopy.
The authors are grateful for the support from the Natural Science Foundation of China (91323103 and 51375409). Jian Guo wants to thank the 2013 Doctoral Innovation Funds of Southwest Jiaotong University and the Fundamental Research Funds for the Central Universities, the Cultivation Project of Sichuan Province Science and Technology Innovation Seedling Project (20132077).
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