# Micro-Nano Machining TiO\({}_{2}\) Patterns without Residual Layer by Unconventional Imprinting
## 3 Results and Discussion  
The scheme of fabricating TiO\({}_{2}\) lines without residual layer is shown in Figure 1. Firstly, PMMA loaded with titanium salt (PMMA/Ti\({}^{4+}\)) in the xylene solution was spin-coated on the surface of PDMS template. In the spin-coating process, the solution is filled into the template by capillary force, forming the meniscus between air and liquid within the concave field [19; 21; 22; 23]. Secondly, the hydroxyl was modified on the surface of the PMMA/Ti\({}^{4+}\) film and the substrate by the O\({}_{2}\) Plasma. Then, the PMMA/Ti\({}^{4+}\) film deposited on the surface of PDMS was contacted with the substrate, and then was transferred to the substrate surface. The hydrogen bonding effect between the PMMA/Ti\({}^{4+}\) film and the substrate enhance the interfacial bond strength (\(\tau\)), resulting in that \(\tau\) (PMMA/Ti\({}^{4+}\)-Substrate) is higher than \(\tau\) (PMMA/Ti\({}^{4+}\)-PDMS), which is the necessary interface condition to realize FRTI. Finally, the PMMA/Ti\({}^{4+}\) samples with patterns were dealt with calcination and hydrothermal reaction to obtain TiO\({}_{2}\) patterns without residual layer. It's worth noting that the PMMA/Ti4+ solution with low concentration easily aggregated to the edge of the concave field in the spin-coating process, conforming to the \"solvent flow\" effect [19; 23], which induces TiO2 patterns with higher resolution than PDMS template, as shown in Figure 1b. For patterning PMMA/Ti4+, the coating speed of PMMA/Ti4+ film, the contact time between the template and substrate, and the solution concentration were investigated. The AFM image of PDMS template with 1.5 \(\upmu\)m line features spaced by 2.5 \(\upmu\)m has been introduced in the previous work of our group. Under certain conditions of the PMMA/Ti4+ concentration (1.5%) and the contact time (60 s), the coating speeds of 2000, 4000 and 8000 rpm were adjusted. The cross-section analysis of AFM images in Figure 2 shows that the average height of PMMA/Ti4+ lines increase along with decreasing coating speed. One reason is that less amount of solution is splashed away out of the depressed patterns when the speed is low. The other reason is that the solvent volatilizes slowly under the low coating speed, and then the more solute (PMMA/Ti4+) can move into the PDMS template based on the capillary force. Therefore, the slower the coating speed, the higher the resolution ratio of PMMA/Ti4+ lines. Meanwhile, it is noted that the \"Rabbit ear\" phenomenon of PMMA/Ti4+ lines is the most obvious with the minimum curvature radius as the highest coating speed of 8000 rpm. This is due to that the compressed degree of air in the template cavity is reduced along with increasing coating\(-\)speed, resulting in a smaller curvature radius. Due to that the glass transition temperature of PMMA is 105 \({}^{\circ}\)C, the traditional hot imprinting (HI) needs to keep PMMA as mobile phase to fully move into the template for  
Figure 1: Schematic illustration of the fabrication process of TiO2 lines without residual layer. (a) When the concentration of PMMA/Ti4+ solution is high. (b) When the concentration of PMMA/Ti4+ solution is low. Figure 2: AFM images of PMMA/Ti4+ lines obtained with different coating speed: (a) 2000 rpm, (b) 4000 rpm and (c) 8000 rpm. 20-30 min, which takes a long time and is not conducive to the efficiency of patterning PMMA and its composite [24; 25]. In order to explore the patterning efficiency, the influence of the contact time on the PMMA/Ti\({}^{4+}\) topography was investigated. Although the coating speed of 2000 rpm, mentioned in the previous paragraph, is most suitable for the sufficient addition of PMMA/Ti\({}^{4+}\) into the template, the low coating speed would cause the nonuniform of PMMA/Ti\({}^{4+}\) film. Therefore, 4000 rpm is selected as the optimal coating speed with 1.5% PMMA/Ti\({}^{4+}\), and different contact time (5 s, 60 s and 180 s) is investigated.