Fabrication of Nanopatterns on Silicon Surface by Combining AFM-Based Scratching and RIE Methods
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This study presents an atomic force microscope (AFM)-based scratching combined with reactive ion etching (RIE) method to fabricate nanopatterns on the silicon surface. All AFM-based scratching processes are performed on the polymethylmethacrylate (PMMA) film to create features in order to protect the AFM tip from wearing and enlarge its life time. Two types of resist films are selected for the RIE process. One is only PMMA thin-film resist, and the other one contains two-layer thin-film resist which are PMMA and aluminum thin films. The nanopatterns on the silicon after etching using RIE with these two types of resist film are compared. Results show that when using PMMA film, the width of the nanopattern on silicon is larger than on the PMMA film, and the material pile-ups formed during the scratching process cannot be considered as a resist. On the contrary, the aluminum thin films can improve the quality of the transfer. It indicates that the thin-film resist with bilayers can guarantee the transfer accuracy of the features, which can expand the application of the AFM tip-based scratching technique.
KeywordsAtomic force microscopy Reactive ion etching Scratching Nanopattern
With the rapid development of nanotechnology, nanostructures have been used in various fields, especially on the silicon- and carbon-based materials [1, 2, 3, 4]. Nowadays, there are several nanofabrication approaches to create nanostructures on the silicon or carbon-based materials, such as electron beam lithography (EBL) , focused ion beam (FIB) lithography , and electrochemical machining . However, the disadvantages of low throughput, complex operation and/or cost of implementation greatly hinder the development of these nanofabrication techniques.
Since atomic force microscopy (AFM) was invented in 1986 by Binning et al. , it has been considered as a nanoscale profilometer. When the interaction force between the tip and the sample is enlarged, the AFM tip can serve as a small cutting tool to fabricate nanostructures on the sample surface . Nanogrooves, two-dimensional and even three-dimensional nanostructures, were fabricated on various material successfully, including metal, polymer and semiconductor materials . Because of the low hardness of the polymer material, the silicon tip is always selected to conduct nanoscratching process [11, 12, 13]. For the metal and semiconductor materials, the diamond tip with a large stiffness cantilever is usually chosen as the cutting tool [14, 15]. However, the tip wear is the main problem when implementing long-distance scratching test, especially for machining on the semiconductor materials . Thus, some scholars utilized the AFM tip first to scratch on a resist thin-film surface, and then transferred the patterns to the materials with large hardness, such as silicon or quartz, by wet etching or reactive ion etching (RIE) process [16, 17, 18, 19]. For the wet etching process, no specific equipment is needed and this method is easy to use [16, 17]. However, the dimensions of the fabricated structure are difficult to be controlled, and the transverse etching effect can lead to reducing the transfer accuracy. For the RIE process, the dimension of the etched patterns can be accurately controlled by the etching time, especially for the nanoscale desired depth. Peng et al.  used a diamond AFM tip to scratch on the aluminum thin-film resist directly due to the large hardness of the metal material as mentioned above. However, the large radius of the diamond tip can enlarge the width of the machined groove on the aluminum resist, which will result in low machining accuracy. Zhang et al. utilized high-rate tunable ultrasonic force nanomachining process to scratch on the PMMA/aluminum bilayers thin-film resist following RIE process to create nanogrooves on silicon substrate . Because of the low hardness, the tip wear is relatively small when first scratching on the PMMA thin-film resist, even for the silicon tip. However, the difference between the influence of the single and bilayers resists on the etching results and the effect of the thickness of the resist are rarely studied.
Therefore, in the present study, an AFM tip-based scratching combined with RIE method is used to fabricate nanopatterns on silicon surface. Two types of resist films are selected for the RIE process, including PMMA and PMMA/aluminum thin-film resists. The influence of thickness of the PMMA thin film on the etched results is studied. In addition, experiments are conducted to compare the two types of thin-film resists on the etching results.
2 Experimental Detail
In this study, a commercial AFM system (Dimension Icon, Bruker Company, Germany) is employed for all nanoscratching processes. All scratching tests and measurement processes are conducted with single crystal silicon tips (TESPA, Bruker Company, Germany). The radius of the new tip is approximately 10 nm, which is evaluated by the blind reconstruction method using a titanium roughness sample (RS-15M, Bruker Company, Germany) . The force constant of the cantilever is 36.5 N/m, which is calibrated according to the vibration spectrum of the cantilever presented by Sader .
Two types of resist films are selected for the RIE process. For the thin-film resist only containing PMMA, four thin films with different thicknesses are selected, including 10 nm, 25 nm, 48 nm and 120 nm. PMMA (Mw = 100,000 from Aladdin as 1, 2, 4 and 6% dilution in anisole) thin films are obtained by spin-coating on a cleaned silicon substrate for 50 s at 7000 rpm and baked at 160° for 20 min to increase the adhesion of the interface between films and substrates. A single crystal silicon tip is employed to scratch directly on the PMMA thin-film surface. Then, RIE process is used to transfer the features of the PMMA thin film on to the surface of the silicon substrate. In the measurement process, a new silicon tip is used in tapping mode for all image processes.
From previous studies, in the dry anisotropic etching process, SF6/O2 gas mixtures were found to anisotropically etch silicon [23, 24]. Some researchers also found that the addition of CHF3 to the SF6/O2 plasma produced smooth etch surfaces. In the case of the aluminum layer considered as a resist in the RIE process, this resist could not react with the etch gas and the ion bombardment has little influence on the surface of this resist. Thus, SF6/O2/CHF3 gas mixtures are selected as the etching gas for the aluminum layer resist. However, for the PMMA thin-film resist, oxygen plasma can react with the PMMA thin-film material. Moreover, the surface of the PMMA thin film can be bombarded by CF x + which is introduced by CHF3. This can result in a relatively low etch selectivity. Thus, when using PMMA thin film as a resist, only SF6 is chosen as the etching gas.
Etching parameters selected in RIE process
3 Results and Discussion
3.1 Effect of the PMMA Thin-Film Thickness on the Etched Grooves
In the RIE etching process, the etching ratio of the PMMA thin film is a critical parameter which is related to the thickness of the PMMA thin film. In order to investigate the effect of the thickness of the PMMA thin film on the etching ratio, the samples with four thicknesses of the PMMA thin film are selected, namely of 10 nm, 25 nm, 48 nm and 120 nm. In this section, the etching time is set as 50 s to over etch all the resist films. The power, pressure and flow are set to 100 W, 7 Pa and 50 sccm, respectively. After the etching process, the obtained surface of the silicon substrate is scanned by the tapping mode of AFM employing a new silicon tip to estimate the etching quality. From the measured 20 μm × 20 μm AFM images of the etched surfaces, the obtained values of the roughness Ra corresponding to the etching parameters mentioned above are 4.3 nm, 5.1 nm, 4.8 nm and 5.4 nm, respectively. This indicates that the silicon surfaces obtained by different etching parameters display similar quality and these values of Ra are much larger than the polished silicon surface before etching (less than 1 nm). The possible reason for this is explained as follows. For relatively time-consuming etching process, the raise of sample temperature results in softening the thin-film. Thus, the decrease in the thin-film thickness may be inconsistent in different ranges of the resist surface when processing under the action of ion physical bombardment. This may lead to some ranges of the resist being etched completely but other ranges of the resist still remaining on the substrate, which could enlarge the roughness of the etched silicon surface. It can also be observed that the values of Ra obtained with different thicknesses of the resist are similar. It can thus be concluded that there is little influence of the thickness of the resist on the etching quality with the over etching condition.
In this study, the etch selectivity is defined as the ratio between the thickness of the thin film to the depth of the etched groove on the substrate. Thus, the etch selectivity for the PMMA thin films with the thicknesses of 10 nm, 25 nm, 48 nm and 120 nm can be obtained as 4.4, 4.8, 5.6 and 5.8, respectively. It can be concluded that the larger the thickness of the thin film, the higher of the etch selectivity. The thin film with relatively small thickness (10 nm and 25 nm) can be easily over etched resulting in exposing the substrate in the reaction gas, which can enlarge the roughness of the substrate compared with the original surface. While, for the thin film with relatively large thickness (120 nm), it is difficult to be cut through and the stick-slip behavior occurring during the scratching process can lead to low etching quality. Therefore, a moderate thickness of the thin film is required in the tip-based scratching and RIE combination approach, which is chosen as 48 nm in this study. Furthermore, it can be observed from the AFM images of the scratched grooves on the PMMA thin films that there are some materials accumulated on both sides of the grooves. However, there is no pile-up structure on the sides of the grooves obtained by the RIE over etching process. The possible reason can be explained as follows. The molecular chains of the PMMA thin film can be cut off by the AFM tip to form the material pile-up . The etching resistance for the material pile-up may be much weaker than the original PMMA surface, which disappears by ion cluster bombardment with relatively higher energy. Thus, the material pile-up in this study cannot work as a resist to protect the substrate.
3.2 Fabrication of Nanostructures by RIE Process with One Layer Thin-Film Resist
3.3 Fabrication of Nanostructures by RIE Process with Bilayers Thin-Film Resist
The effect of the thickness of the resist thin-film on the etching results is studied in detail. The widths of the etched grooves can be greatly enlarged when utilizing PMMA thin films with the thicknesses of 10 nm and 25 nm as the resists. A relatively large stress between the resist thin-film and the silicon substrate may be the possible reason. For the thickness of 120 nm, the stick-slip behavior of the polymer material leads to discontinuous grooves, while, for the thickness of 48 nm, the increase of the groove width is much smaller than those obtained by the resists with the thicknesses of 10 nm and 25 nm, which shows the best etching result.
For the single layer resist, the PMMA thin film with the thickness of 48 nm is chosen as the resist. The etched grooves show poor uniformity for the widths at different locations along the length direction. The low etching resistance of the intermittent material pile-ups adhered on the sides of the groove may be the possible reason. In addition, the depths of etched circle arrays show great difference at different locations, which results from the effect of the change in scratching direction when machining circle patterns.
For the bilayers thin-film resist, the thicknesses of the PMMA and aluminum thin films are selected as 48 nm and 40 nm, respectively. The depths and widths of the etched grooves show good consistency. In order to overcome the effect of the machined depth difference caused by the various scratching directions, the vibration-assisted scratching method is used to scratch circle patterns on the PMMA thin film, and better etching results are obtained. Moreover, experiment results indicate that the double resist layer approach is the preferred technique to overcome the negative effect of the pile-ups introduced during the scratching process.
The throughput of the AFM scratching combining RIE approach to fabricate nanostructure on the semiconductor materials mainly depends on the efficiency of the AFM machining process. The scratching velocity selected in this study is set as 3 µm/s, which is relatively low. However, if we use the piezoelectric actuator-based AFM tip-based high-speed machining system proposed by the authors in Ref. (25) in the further work, the theoretical cutting speed over 5 m/min can be achieved, which will enhance the throughput of the method presented in this study.
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51705104, 51675134), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51521003), Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education (Harbin Institute of Technology No. 2017KM005), China Postdoctoral Science Foundation (No. 2017M610206 and 2018T110289) and the National Program for Support of Top-notch Young Professors.