Maskless and low-destructive nanofabrication on quartz by friction-induced selective etching
© Song et al.; licensee Springer. 2013
Received: 25 February 2013
Accepted: 12 March 2013
Published: 27 March 2013
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© Song et al.; licensee Springer. 2013
Received: 25 February 2013
Accepted: 12 March 2013
Published: 27 March 2013
A low-destructive friction-induced nanofabrication method is proposed to produce three-dimensional nanostructures on a quartz surface. Without any template, nanofabrication can be achieved by low-destructive scanning on a target area and post-etching in a KOH solution. Various nanostructures, such as slopes, hierarchical stages and chessboard-like patterns, can be fabricated on the quartz surface. Although the rise of etching temperature can improve fabrication efficiency, fabrication depth is dependent only upon contact pressure and scanning cycles. With the increase of contact pressure during scanning, selective etching thickness of the scanned area increases from 0 to 2.9 nm before the yield of the quartz surface and then tends to stabilise after the appearance of a wear. Refabrication on existing nanostructures can be realised to produce deeper structures on the quartz surface. Based on Arrhenius fitting of the etching rate and transmission electron microscopy characterization of the nanostructure, fabrication mechanism could be attributed to the selective etching of the friction-induced amorphous layer on the quartz surface. As a maskless and low-destructive technique, the proposed friction-induced method will open up new possibilities for further nanofabrication.
By virtue of its excellent chemical and physical properties, quartz has been widely used in micro/nanoelectromechanical systems (MEMS/NEMS), such as piezoelectric sensors , biochips , optical sensors , etc. Traditional lithographic fabrication on a quartz surface includes a complex process of mask deposition, exposure, etching and mask removal [4, 5]. As device dimension has been down to nanoscale, traditional lithography hardly provides feasible nanofabrication on the quartz surface because of its involute process and limited resolution .
Although electron beam lithography has a better resolution in nanofabrication , high-energy beam can cause undesirable amorphization on quartz . By virtue of their high precision, proximal probe methods based on scanning tunnel microscopy and atomic force microscopy (AFM) have been employed to fabricate nanostructures [9–13]. For example, AFM was used to fabricate a series of nanoscale grooves on polymers, metals and semiconductors by mechanical cutting . However, it is inappropriate for quartz surface because of crack generation during the cutting process on such hard, brittle materials . Although nanostructures can be fabricated on conductive surfaces by an anodic oxidation process at a given voltage, it is invalid for an insulator surface of quartz . Recently, hillock-like nanostructures can be fabricated on silicon and quartz surfaces by sliding a diamond tip with repeated scratching cycles under suitable low loads [12, 13]. However, since such hillocks are mainly generated from mechanical deformation of substrates, possible lattice damages may form on the surface of the hillocks and reduce their mechanical properties . Because the lattice damages are detrimental to the applications of quartz devices [15, 16], it is imperative to develop a straightforward and low-destructive nanofabrication method for the quartz surface.
In the present study, a novel nanofabrication method to produce three-dimensional nanostructures on the quartz surface has been developed by low-destructive scanning on a target area and post-etching in a KOH solution. The capability of this nanofabrication method was demonstrated by various nanostructures including slopes, hierarchical stages and chessboard-like patterns. The etching rate of the scanned area was tested at various temperatures. To produce deeper structures, refabrication was attempted on the existing nanostructures. The fabrication mechanism was discussed based on Arrhenius fitting of the etching rate and transmission electron microscopy (TEM) characterization of the nanostructure. In brief, the low-destructive friction-induced nanofabrication method may shed new light on nanotechnology.
To understand the mechanism of the low-destructive friction-induced nanofabrication, the microscopic structure of the scanned area of the quartz sample was detected by cross-sectional TEM (XTEM, JEOL JEM-2100 LaB6, JEOL Ltd., Tokyo, Japan) before and after KOH etching. The XTEM samples were prepared using a Quanta 3D FEG focused ion beam (FIB, FEI Company, Hillsboro, OR, USA) miller from the scanned area on quartz. In order to facilitate the FIB cutting across the scanned area, linescratch areas in the length of 200 μm were produced on the quartz samples by a nanoscratch tester (CSM Instruments, Peseux, Switzerland) with a spheral diamond tip having a radius of 20 μm. The low-destructive area was scratched under the normal load of 45 mN (the corresponding Hertzian contact pressure is 4.9 GPa), and the groove in a depth of 23 nm was scratched under the normal load of 95 mN.
When the scan size was adjusted during repeated scanning, the hierarchical structure (as shown in Figure 2c) with five stages can be fabricated. Here, under a constant load of 5 μN and with a fixed centre point, the scanned area was successively set as 7 × 7, 5.6 × 5.6, 4.2 × 4.2, 2.8 × 2.8 and 1.4 × 1.4 μm2. Consequently, the overlapped areas were scratched by repeated scans. After selective etching for 3 h, the hierarchical structure was created, where the fabrication depth D of the stages increased from 1.2 to 3.5 nm with the increase of the number of repeated scanning cycles from 1 to 5. By programming the tip trace, the demanded patterns can also be produced at a target area. The chessboard-like patterns in Figure 2d were produced by Fn = 12 μN and N = 1. The scan size was 1 μm, and the interval between adjoining scan centres was 2 μm. After etching for 3 h, the separated convexity with 2.1 nm in height was formed.
The results in Figure 3 also suggested that different from the continuous etching in traditional lithography, the selective etching of the scanned area mainly occurred before the turning point. After the turning point in each curve, the etching rate was very low and the etching depth was quite close to the final fabrication depth. Therefore, a linear fit was used for estimating the average etching rate η before the turning point in each curve. Clearly, the rise of etching temperature can significantly improve the etching rate. With the increase of temperature from 273 to 328 K, the etching rate rose from 0.17 to 7.0 nm/h under the scan load of 8 μN. However, since etching pits appeared on the original surface at the etching temperature of 328 K in this experiment, the highest etching temperature for efficiency improvement of fabrication should be limited to 328 K. It was also noted that even though the etching rate η varied dramatically with temperature, the final fabrication depth D f only relied on the scan load or the contact pressure. For instance, D f was always about 1.6 nm for Fn = 8 μN regardless of the etching temperatures. The results have implied that the fabrication is indeed friction-controlled.
From Figure 2b, it was noted that the fabrication depth increased with the increase of the scanning load or the contact pressure on the wearless scanned area. To analyse the effect of contact pressure on nanofabrication in a more detailed manner, a scratch consisting of a wearless area and groove was performed on the quartz surface under progressive loads from 0 to 40 μN by AFM. As shown in Figure 4, the cross-sectional profile of the scanned area (dashed line) revealed that the surface was wearless below Fn = 15 μN, where the corresponding contact pressure Pc was below 5.1 GPa. With the further increase of the normal load from 15 to 40 μN, the groove was formed on the surface and the wear depth increased from 0 to 1.7 nm (dashed line). After etching for 6 h at 293 K, the etched thickness increased from 0 to 2.9 nm on the wearless scanned area and then tended to stabilise on the groove. Therefore, the upper limit of the contact pressure for low-destructive scanning was 5.1 GPa, and the critical fabrication depth was 2.9 nm after single low-destructive scanning and post-etching. The fabrication of deeper nanostructure can be realised through repeated scanning or refabrication process (Figures 2c and 5).
Fabrication depths under various scanning loads and etching temperatures (nm)
Scanning load (μN)
In brief, the contact pressure played a significant role in the low-destructive friction-induced nanofabrication. With the increase of contact pressure, the fabrication depth increased from 0 to 2.9 nm before the yield of quartz surface. Since the superficial layer of the scanned area can be etched selectively, it was reasonable to speculate that such layer may reveal a unique etching behaviour and microstructure. To understand the fabrication mechanism of the proposed method, it is essential to analyse the reaction kinetics of selective etching and detect the microstructures of the scanned area.
Fitting results in Figure 6
A(c) α (nm/h)
Based on the mentioned discussions, the possible fabrication mechanism can be proposed as follows. In the KOH solution, the defective Si-O nets of friction-induced amorphous SiO2 can help the KOH solutes to preferentially diffuse into the scanned area and induce both higher concentrations of reactants c and a large number of colliding between reactants A in the scanned area [21, 26]. According to the collide theory, the KOH etching of the scanned area depends on the total collision number between the KOH solutes and Si-O microscopic structure. Compared to the original surface, the scanned area has a faster etching rate and will be etched selectively. The etched depth may be decided by the thickness of the amorphous layer, and the etching rate is determined by the extent of amorphization. Higher contact pressure and repeated scanning can provide more gross energy W for the interaction between the tip and the quartz surface and consequently lead to faster etching rates and deeper structures. For example, under the conditions of N = 1 and Fn = 5 μN, the fabrication depth D was 1.2 nm and the etching rate η was 0.68 nm/h. The gross energy W dissipated during the scanning was calculated as 2.0 × 10−9 J . To produce a deeper structure in depth of 1.6 nm, it needed 3.2 × 10−9 J by increasing Fn to 8 μN or 4.0 × 10−9 J by scanning one more cycle. At the same time, the etching rate η increased to 1.05 nm/h by increasing Fn to 8 μN, which meant that the increment of η was 0.31 nm/h per 10−9 J. As a comparison, the etching rate η increased to 1.21 nm/h by scanning one more cycle and the corresponding increment of η was 0.27 nm/h per 10−9 J. Therefore, compared to the number of scanning cycles, it seems that the scan load reveals a relatively stronger effect on the etching depth and etching rate. Nevertheless, the competitive relation between the load and scanning cycles should be investigated more thoroughly in the future.
In summary, a novel nanofabrication method based on friction-induced selective etching is proposed, by which three-dimensional nanostructures on demand can be created on a target quartz surface. This method enables nanofabrication more easily than photolithography with etching masks. This proximal probe technique makes it possible to fabricate at specified locations and to measure the dimensions of nanostructures with high precision. To overcome the critical etching thickness in a single fabrication flow, refabrication can be conducted on the existing structures. The friction-induced process under low contact pressure facilitates nanoscale material removal in a low-destructive way. Considering such advantages and potential applications, this method can open up new opportunities for future nanofabrication.
In conclusion, we have presented a maskless and low-destructive method for nanofabrication on quartz based on AFM. Various nanostructures including slopes, hierarchical stages and chessboard-like patterns can be fabricated by changing the loading mode and programming the scan traces. Under the given experimental conditions, the surface of the scanned area is wearless below the critical contact pressure 5.1 GPa and the etched thickness of the scanned area goes up from 0 to 2.9 nm with the increase of contact pressure. Even though the rise of etching temperature can improve the efficiency of fabrication, the fabrication depth is controlled by the contact pressure and scanning cycles. Refabrication can be realised to overcome the critical etching thickness in a single fabrication flow. TEM observation shows that the formation of distortion does not contribute to the etching thickness. Analysis suggests that the fabrication mechanism could be attributed to the selective etching of friction-induced amorphous layer on the quartz surface. The proposed method based on friction-induced selective etching will provide new opportunities for nanofabrication on quartz surface.
Atomic force microscopy
Focused ion beam
Transmission electron microscopy
The authors are grateful for the financial support from the National Basic Research Program (2011CB707604), Natural Science Foundation of China (90923017 and 51175441) and from the Royal Society under the China-UK Science Networks Scheme. Chengfei Song wants to thank the 2011 Doctoral Innovation Funds of Southwest Jiaotong University and the Fundamental Research Funds for the Central Universities.
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