ABSTRACT
For a load of 70 µN, some plastic deformation occurred, which was then removed. The maximum height of the processed area was about 4.5 nm [22]. Since the von Mises stress increases with load, the plastically deformed part generated in front of the indenter is removed instead of the whole protuberance in processed area [22]. In this case, the oxide layer is formed by the same mechanochemical action based on the reaction with water and oxygen as that in protuberance processing. The oxidized portion of the processed area acts as a mask in the KOH solution etching of silicon. Figure 9.5 shows an example of a small portion of a large sample formed by oxidative deterioration under certain conditions being used as a mask in an etching process with KOH solution [22]. In addition, acceleration and deceleration of the rate of etching are possible by changing the scanning density of tip sliding. In the example mentioned above, nine areas of 1 × 1 µm2were scanned with a high scanning density of tip sliding, and oxidative layers were formed due to mechanochemical action. Following this, the processed surface was scanned over an area of 5 × 5 µm2 at a low scanning density of tip sliding and then processed by etching with KOH solution as shown in Fig. 9.5 [23]. In the case of a short etching time with KOH solution, the area processed at a high scanning density and the unprocessed area covered with a natural oxidative layer acted as masks and were not etched. In this way, mechanochemical action enables various types of nanofabrication processes to be realized. Nanoscale local oxidation based on mechanochemical action can be expected to be applied to nanodevice processes and nanolithography technology in the future. 9.4 Nanofabrication and its Application based
9.4.1 Nanoprocessing of Layered Crystal Materials at the Layer UnitUtilizing the mechanical anisotropy of crystal materials, if layered crystal materials having periodically weak bonds are used as processed workpieces, processing at the level of single layers can be realized [9]. Mica (Muscovite: Muscovite mica), graphite,
molybdenum disulfide (MoS2), and boron nitride have a layered structure, and there is little interaction between the cleavage planes
existing in the basal plane of these materials. Moreover, it is easy to image the atoms on the basal plane, where the processed shape at the atomic order can also be observed. By applying AFM, the nanomachining characteristics of the layered crystal materials MoS2, highly-oriented pyrolytic graphite (HOPG) and mica have been investigated using a superhard film tip. MoS2 has a hexagonal structure [24], and there are van der Waals forces between adjacent S layers. The interval between the cleavage planes is 0.616 nm. HOPG has a structure similar to a single crystal, and the interval between graphite layers is 0.34 nm. Mica has a structure in which O, Al, OH, and K are stacked with a K layer sandwiched between SiO4 cleavage planes. The layer period is 0.7 nm for the SiO4 layer and 1 nm for the K layer, and there are van der Waals forces between K and SiO4 layers. A DLC-coated Si tip with a tip radius of less than 50 nm was used as a tool for the nanoprocessing of layered crystalline materials. Upon loading a certain atomic force on the DLC-coated Si tip, nanoprocessing is performed by controlling the tip scanning path with a computer. Figure 9.6a shows an example of processing MoS2 to obtain latticed grooves. The depth of the grooves is about 0.6 nm. This approximately corresponds to the cleavage plane interval of MoS2 of 0.616 nm, i.e., single-layer processing can be realized. Compared with MoS2, as shown in Fig. 9.6b, mica can be comparatively easily processed at the layer-unit scale. In contrast, HOPG is difficult to process using the DLC nanofilm tip because the bond strength of the basal plane of HOPG is large.
