Temperature-Dependent Nanofabrication on Silicon by Friction-Induced Selective Etching
© Jin et al. 2016
Received: 24 February 2016
Accepted: 14 April 2016
Published: 27 April 2016
Friction-induced selective etching provides a convenient and practical way for fabricating protrusive nanostructures. A further understanding of this method is very important for establishing a controllable nanofabrication process. In this study, the effect of etching temperature on the formation of protrusive hillocks and surface properties of the etched silicon surface was investigated. It is found that the height of the hillock produced by selective etching increases with the etching temperature before the collapse of the hillock. The temperature-dependent selective etching rate can be fitted well by the Arrhenius equation. The etching at higher temperature can cause rougher silicon surface with a little lower elastic modulus and hardness. The contact angle of the etched silicon surface decreases with the etching temperature. It is also noted that no obvious contamination can be detected on silicon surface after etching at different temperatures. As a result, the optimized condition for the selective etching was addressed. The present study provides a new insight into the control and application of friction-induced selective nanofabrication.
Owing to its excellent physical and mechanical properties and low cost to obtain, monocrystalline silicon has become a preferred semiconductor material . Silicon is now widely used for micro/nanoelectromechanical systems (MEMS/NEMS) , solar photovoltaic battery , template for nanoimprint , substrate for quantum dot growth and so on [5, 6]. Nanofabrication on silicon is essential to support these applications. At present, plenty of micro/nano-manufacture technologies have been used, including photolithography, nanoimprint lithography, electron beam lithography, probe-based anodic oxidation technology and so on . However, with the requirement for high resolution and low cost, the existing techniques encounter challenges and none of them can satisfy all the needs at the same time . How to fabricate silicon with low cost and high resolution is of much concern.
The probe-based technique enables both site-controlled fabrication and high-resolution measurement of nanostructure dimensions. With the advantages of simple processing, high flexibility, and minor cost, friction-induced nanofabrication provides a convenient method for producing nanostructures on silicon, quartz, glass, and GaAs surfaces more easily than photolithography with etching masks [9–15]. Yu et al.  proposed the friction-induced method to fabricate convex nanostructure on silicon surface with a diamond tip. Based on this, Guo et al. [10, 11] produced a series of protrusive hillocks with the height of hundreds of nanometers on silicon surface by the friction-induced selective etching. Furthermore, tribochemistry-induced selective etching can also produce defect-free nanostructures on silicon [12, 14]. The friction-induced selective etching method opens up ways for producing templates for nanoimprint lithography on silicon and quartz, and is expected to fabricate patterned nanochannels in micro/nano fluidic systems for drug delivery, ion transporters, DNA translocators and so on [12, 16]. It also provides a way to modify surface hydrophobicity or tribology properties.
However, it is far from full understanding of the friction-induced selective etching fabrication, and it is of importance to establish a controllable friction-induced selective etching process. It is known that the temperature has a great influence on the chemical reaction, but the effect of the temperature on the friction-induced selective etching remains unknown. Therefore, the present study will be focused on the temperature-dependent etching.
In this study, the effect of etching temperature on the height of the nanostructures, surface roughness and wetting performance was investigated. The hardness and elastic modules of the etched surface were studied by a nanomechanical test system. The chemical composition was studied by an X-ray photoelectron spectroscope (XPS). Based on above investigations, the rate for selective etching and condition for optimized fabrication was further addressed.
The Si(100) wafers, n-doped with boron, were purchased from MEMC Electronic Materials, Inc., USA. The surface root-mean-square (RMS) roughness was measured as about 0.10 nm over a 2 × 2 μm2 area by using an atomic force microscope (AFM; SPI3800N, Seiko Instruments Inc., Tokyo, Japan). Before fabricating, Si wafers were ultrasonically cleaned with acetone, ethanol, and deionized water for 3 min in turn to remove surface contamination. Then, the wafers were dipped in 5 wt.% HF solution for 2 min to etch away the native Si oxide layer, which can facilitate the direct etching of the Si substrate in the following etch process.
The fabrication process contains two steps, namely scratching and post-etching [11, 12]. With a 5-μm radius diamond tip on the homemade multi-probe instrument , a series of grooves with the depth of about 7 nm were produced on a silicon surface. The applied normal load used for the scratching was 20 mN. Then, the samples were etched in 20 wt.% KOH aqueous solution at different temperatures of 0, 25, 40, 50, 60, and 80 °C. To improve the etching quality, isopropanol alcohol (IPA) was added to the KOH solution with the volume ratio of 1:5 . The temperature was controlled via a thermostatic water bath with the accuracy of ±0.5 °C. After the post-etching, the fabricated nanostructures on Si were detected by AFM scanning with a Si3N4 tip (MLCT, Veeco Instruments Inc., Plainview, NY, USA) in vacuum. The nominal tip radius of the Si3N4 tip is 20 nm.
To investigate the effect of etching temperature on the performance of silicon surface, a series of detections were conducted on unscratched silicon surface before and after etching at different temperatures. The mechanical properties including hardness and elastic modulus of the etched Si surface were investigated by the in situ nanomechanical test system (TI900, Hysitron Inc., USA). A Berkovich tip was used for the test of hardness H and elastic modulus E. The indentation test was conducted under a depth-controlled mode. To obtain reliable data, at least three repetitive tests were performed under each indentation depth. The nano-indentation elastic modulus was obtained by analyzing the indentation force-depth (F-d) curves with the Oliver-Pharr method [18, 19]. The test was calibrated by a fused quartz wafer with E = 69.6 GPa ± 5 % and H = 9.25 GPa ± 10 % (Hysitron Inc., USA). The chemical composition of the etched silicon surface was detected by an X-ray photoelectron spectroscopy (XPS; Escalab 250xi, Thermo Fisher Scientific Inc., USA). The contact angle vs. deionized water of the etched Si surface was measured by a drop analysis system (DSA, Kruss Corp., Germany).
Results and Discussions
Effect of Etching Temperature on Height of Nanostructures
After immersing the Si surface with groove-shaped scratches in KOH-IPA solution, the protrusive nanostructures were piled up from the scratches. The formation of the protrusive nanostructures (hillocks) was ascribed to the difference in etching rate of the scratched area and original silicon surface, where the scratched area acted as a resist mask against etching [11, 12]. It is also noted that the native oxide layer-covered Si surface can delay the selective etching, where the etching started at about 30 min in 20 wt.% KOH solution . In this study, the native oxide layer was removed by HF solution before consequent etching in KOH-based solution, and hence, obvious selective etching began within much less immersing time.
Temperature-Dependent Selective Etching Rate
During the friction-induced selective etching process, the chemical reaction of Si and H2O takes place in the catalysis of hydroxyl ion OH−, and Si(OH)4 and H2 will be produced as the products . The pre-scratching on silicon can lead to the deformation of silicon crystal matrix, and this will result in a slower etching rate than that on original silicon surface . However, mask effect will disappear when the Si surface is etched for a long time or at a high temperature as shown in Fig. 1. Therefore, we will discuss here the temperature-dependent selective etching based on results from short-time etching up to 40 min.
Effect of Etching Temperature on Surface Roughness
Effect of Etching Temperature on Mechanical Properties
The reason for the temperature-dependent elastic modulus and hardness can be ascribed to the variation in surface rough microstructures. During the etching, surface bonding will be destroyed by the reactants, leaving a rough surface with some sub-structures, such as micro holes, micro walls, and asperities. By contrast to the original surface with compact bonding structure, the etched surface with the sub-structures is expected to degrade the mechanical performance of the Si surface. The etching at a higher temperature can lead to rougher surface with more sub-structures, and hence lower value of elastic modules and hardness . It is also noted that there is an obvious indentation size effect on the tests of elastic modulus E and hardness H, where the measured value decreases with the indentation depth in the nanoscale. Similar results have been reported on metal or polymer surface [24, 25]. This may be ascribed to the effect of geometrically necessary dislocations in the nanoscale test .
To sum up, the effect of etching temperature on the height of protrusive hillocks, surface roughness of silicon, surface wetting, mechanical performance, and chemical composition was comparatively investigated. Firstly, the removal of surface oxide enables the selective etching, and the hillock height can be controlled by choosing appropriate etching time and temperature as shown in Fig. 2. Secondly, the cooperative effect of etching time and temperature on the fabrication quality should be taken into account for real fabrication. Although high temperature enables a rapid selective etching rate, resulting in high protrusive hillocks, it also causes a rough surface. Considering the collapse of the hillock by excessive etching (Figs. 1 and 2), the temperature below 50 °C is recommended in the present study for selective etching. Since a longer etching time can also cause a rougher silicon surface (Figs. 4 and 5), the etching time should be selected based on the requirement for hillock height and surface roughness. Finally, the selective etching results in a clean surface without contaminations, and the chemical reaction can be predicted by the Arrhenius equation. It should be highlighted that based on the Arrhenius equation, the selective etching process can be further controlled by adjusting the solution concentration, etching temperature, and time. The present study is expected to provide an insight into the understanding, controlling, and application of friction-induced selective nanofabrication.
The temperature has an obvious effect on the height of the nanostructures and surface roughness produced by friction-induced selective etching. Before the collapse of the hillock, the height of the hillock increases with temperature for the same etching time. The temperature-dependent selective etching process can be fitted well by the Arrhenius equation.
Silicon surface becomes rougher with the increasing etching temperature or etching time. The rougher surface produced by selective etching leads to lower contact angles, elastic modulus, and harness than the original surface. No obvious contamination can be detected on the silicon surface after etching at different temperatures
Since a high hillock but rough surface can be produced by selective etching at a high temperature or a long time, the temperature below 50 °C is recommended for friction-induced selective etching, and a short etching time can facilitate forming a smooth surface.
The authors are grateful for the support by the Natural Science Foundation of China (51305365) and the Fundamental Research Funds for the Central Universities (2682015CX037).
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