1. Resistless Fabrication of Embedded Nanochannels by FIB Patterning, Wet Etching and Atomic Layer Deposition Zhongmei Han Marko Vehkamaki Markku Leskelä Mikko Ritala Laboratory of Inorganic Chemistry Department of Chemistry University of Helsinki, Finland E-mail: zhongmei.han@helsinki.fi UNIVERSITY OF HELSINKI FACULTY OF SCIENCE Introduction Nanochannels are interesting nanostructures for e.g. filters, transistors, nanofluidics, and molecule sensors. However, precise control of channel size and wall thickness is an issue for many nanochannel fabrication methods. Atomic layer deposition (ALD) is increasingly used in making 3D nanostructures due to the superior conformality and precise film thickness control [1]. In this study, we combined FIB patterning and chemical wet etching of silicon for nanofabrication of SiO 2 structures from thermal SiO 2 on Si substrates. The fabricated SiO 2 /Si line trenches were used as a template for subsequent ALD of Ta 2 O 5 to form embedded hollow nanochannels in contact with crystalline silicon (Fig.1). Fig.1. Schematic illustration of fabrication steps (FIB patterning to open a line on the top SiO 2, wet etching of Si under the SiO 2 layer, and ALD of conformal thin film) to form an embedded channel. Experimental Thermally oxidized Silicon (1000 ℃, 105 nm SiO 2 ) was patterned in a FIB-SEM by 30 keV gallium ion beam with 100 pA current and 90 % overlap. FIB patterned specimens were wet etched in three successive etchants: 1 mol/L:1 mol/L KOH/H 2 O 2 solution, 1 % HF and 1 mol/L KOH to make suspended SiO 2 structures with the FIB patterned features. ALD of Ta 2 O 5 was subsequently performed on the wet etched samples in a Microchemistry F-120 reactor at 250 ℃ with Ta(OEt) 5 and water as precursors. Results Fig. 2 shows SiO 2 structures formed by wet etching where FIB patterned structures were a circle (d = 10 μm), a ring (d outer = 20 μm, d inner = 19.8 μm), a square matrix (a = 500 nm) and a line set on a SiO 2 /Si piece before the chemical etching steps. The formed structures under SiO 2 shown in Fig. 2(a), (c) and (d) illustrate anisotropic etching of Si (100) and (111) planes while Fig. 2(b) shows larger underetching resulting from much higher etching rate of (221) and (331) planes than (100) plane [3]. Fig. 2. SEM images of suspended SiO 2 structures made by etching with KOH through different FIB patterned structures in the SiO 2 layer: (a) 10 μm-diameter circle, (b) 200 nm-wide ring (c) square matrix and (d) lines. Inset in (a) and (b) are corresponding tilted images with low voltage (3keV). For line trenches, the patterning ion dose and KOH etching time are significant for the resulting trench size. Fig. 3 shows top-views of four line sets patterned by FIB with various ion doses followed by wet etching in the KOH etchant for 0, 60, 80 and 120 min, respectively. The lateral etch distance increases with etching time, but is independent of ion dose. Fig. 3. SEM images of FIB patterned and wet etched line sets which were etched in KOH for (a) 0 min, (b) 60 min, (c) 80 min and (d) 120 min. The line separations were all set to 1.5 μm. The SEM images in Fig. 4 show the channels formed after ALD of Ta 2 O 5 thin film. The increased FIB patterning ion dose results in larger opening in the SiO 2 layer. More ALD cycles were required for closing wider line gaps, resulting in thicker channel walls (the wall thickness = 1/2 line gap). Therefore, the ion dose controls both the size of line trenches before ALD and wall thickness of channels after. Meanwhile, the KOH etching time determines the trench size and the resulting channel size. By using ALD to form nanochannels, a wide variety of channel wall materials become available. One can also have different materials on the inside and outside the channel, if a bilayer or multilayer is deposited. Fig. 4. Cross-section SEM images of nanochannels formed by FIB patterning, wet etching and ALD coating, corresponding to Fig. 3 (b), (c) and (d) respectively. Conclusion A resistless fabrication method was demonstrated for making embedded Ta 2 O 5 channels by FIB patterning, wet etching and ALD on SiO 2 /Si substrates. The channel size reaches 50 nm by this fabrication approach with a 60 min KOH etching time. The wall materials can be selected depending on the application such as metals, oxides and nitrides which are widely available in ALD processes, making this a highly versatile approach. Acknowledgements The authors wish to gratefully acknowledge financial support from China Scholarship Council (File No. 2011704017) and Finnish center of excellence in atomic layer deposition. References [1] M. Leskela and M. Ritala, "Atomic layer deposition chemistry: Recent developments and future challenges," Angew. Chem. Int. Ed. Engl., vol. 42, pp. 5548-5554, Nov 2003. [2] Z. Han, M. Vehkamäki, M. Leskelä, and M. Ritala, "Selective etching of focused ion beam implanted regions from silicon as a nanofabrication method," Nanotechnology, vol. 26, p. 265304, Jun 2015. [3] I. Zubel and I. Barycka, "Silicon anisotropic etching in alkaline solutions I. The geometric description of figures developed under etching Si(100) in various solutions," Sens. Actuators, A, vol. 70, pp. 250-259, Oct 1998. Silicon Thermal SiO 2 ALD thin film FIB Wet etch ALD (a) (b) (c) (d) 0 min KOH etched 60 min KOH etched 80 min KOH etched 120 min KOH etched 60min KOH etched 80min KOH etched 120min KOH etched (a) (b) (c)