The purpose of this experiment was to study the motion of fluorescent labeled latex beads in fused silica nanofluidic channels. The results of these experiments are used in device design. In particu- lar, methods of driving flow in nanometer size channels presented in the next chapter is based on the experimental results derived here. Another important result of these experiments is to demon- strate that particle and fluid mobility in sub-micron channels, although small, is large enough to be observable.
Electron-beam lithography was used to define arrays of lines from 100 nm (Figure 4.13) to 1
µm (Figure 4.15) in width. These patterns were then transferred to a metal etch mask by ion beam etching. The fused silica channels were then etched by reactive ion etching in a C2F6plasma
containing a small amount of argon.
Large connecting channels and fluid delivery reservoirs were defined using optical lithography and etched into the fused silica by hydrofluoric acid etching. Holes were drilled through the wafer using diamond drill bits and a slurry of abrasive aluminum oxide powder. Next the fused silica wafer was carefully cleaned using the RCA etch process and bonded to a similarly prepared cover
Figure 4.12: Surface map of channels in silicon dioxide obtained by atomic force microscopy.
Figure 4.13: Fused silica fluid channels used in fluorescent confocal microscopy measurements.
slip using the room temperature HF bonding technique.
A drop of test solution was placed in each hole leading to a fluid reservoir. The holes were covered with microscope cover slips to reduce fluid evaporation.
0 500 0 500 0 500 0 500 0 500 0 500 0 0.5 1 1.5 0 500 Time (Seconds)
Photon Counts per Millisecond
Figure 4.14: Fluorescent measurement on beads in the 100 nm wide, 100 nm deep channels shown in Figure 4.13.
Figure 4.15: Fused silica fluid channels used in fluorescent confocal microscopy measurements. The channels are one micron wide spaced five microns apart. The row of “+” marks across the middle of the image are used in aligning the channels with the confocal microscope.
Measurements of fluorescent labeled latex spheres 63 nm in diameter moving in fused silica fluid channels were performed using a confocal microscope. Confocal microscopy is a technique that illuminates and collects light from a small region within a sample volume [9, 10, 11, 12].
The confocal microscope setup in Figure 4.17 was designed and built by Andrew Berglund [13]. It is designed to excite and collect a fluorescent signal from a small volume in the sample under investigation.
Figures 4.14 and 4.16 show results of fluorescent measurements at one position in two different channels. The peaks correspond to movement of fluorescent particles through the observation vol-
0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 500 1000 0 0.5 1 1.5 0 500 1000 Time (S)
Photon Counts per Millisecond
Figure 4.16: Fluorescent measurements on 63 nm beads in the 1µm wide, 100 nm deep channels of Figure 4.15. Beam Splitter Sample Photo− Detector Microscope Objective Aperture Pin−Hole Excitation Laser Aperture Pin−Hole
Figure 4.17: Confocal microscope configuration.
ume of the fluorescent confocal microscope. These experiments were performed with no pressure differential driving the flow, all particle movement was caused by Brownian motion.
4.7
Conclusion
The specific processing techniques presented here resulted from experimentation with many different approaches. These are the techniques that were found to be most suitable for building nanofluidic devices.
References
[1] Ilesanmi Adesida and Thomas E. Everhart. Substrate thickness considerations in electron beam lithography. Journal of Applied Physics, 51(11):5994–6005, November 1980.
[2] S. Wolf and R. N. Tauber. Silicon Processing for the VLSI Era Volume 1 – Process Technology. Lattice Press, 1986.
[3] Evangelos Gogolides, Philippe Vauvert, George Kokkoris, Guy Turban, and Andreas G. Boudou- vis. Etching of SiO2 in fluorocarbon plasmas: A detailed surface model accounting for etching
and deposition. Journal of Applied Physics, 88(10):5570–5584, November 2000.
[4] K. Sasaki, H. Furukawa, K. Kadota, and C. Suzuki. Surface production of CF, CF2 and
C2 radicals in high-density CF4/H2 plasmas. Journal of Applied Physics, 88(10):5585–5591,
November 2000.
[5] Kaptonr polyimide flim bulletin gs-96-7. Technical report, Du Pont High Performance Mate- rials, P. O. Box 89, Route 23 South and Du Pont Road, Circleville, OH 43133, 2000.
[6] H. Nakanishi, T. Nishimoto, R. Nakamura, A. Yotsumoto, T. Yoshida, and S. Shoji. Studies on SiO2-SiO2bonding with hydrofluoric acid. room temperature and low stress bonding technique
for mems. Semsors and Actuators A, 79:237–244, 2000.
[7] Akihide Hibara, Takumi Saito, Haeng-Boo Kim, Manabu Tokeshi, Takeshi Ooi, Masayuki Nakao, and Takehiko Kitamori. Nanochannels on a fused-silica microstrip and liquid properties investigated by time-resolved fluorescence measurements. Analytical Chemistry, 74(24):6170– 6176, December 2002.
[8] Y. C. Lin and Everhart T. E. Study on voltage contrast in SEM. Journal of Vacuum Science and Technology, 16(6):1856–1860, November/December 1979.
[9] R. Oldenbourg, H. Terada, R. Tiberio, and S. Inou´e. Image sharpness and contrast transfer in coherent confocal microscopy. Journal of Microscopy, 172(1):31–39, October 1993.
[10] Jeff W. Lichtman. Confocal microscopy. In Science’s Vision: The Mechanics of Sight, pages 76–81. Scientific American, Inc., 1998.
[11] Marcus Dyba and Stephan W. Hell. Focal spots of size λ/23 open up far-field florescence microscopy at 33nm axial resolution. Physical Review Letters, 88(16), April 2002.
[12] M. Minsky. Memoir on inventing the confocal scanning microscope. Scanning, 10(4):128–138, 1988.
[13] A. J. Berglund, A. C. Doherty, and H. Mabuchi. Photon statistics and dynamics of fluorescence resonance energy transfer. Physical Review Letters, 89(6):art. no.–068101, 2002.
Chapter 5
Pumping Liquids at
Nanometer-Size Scales
5.1
Introduction
As the critical dimensions of fluid channels are reduced to sub-micron sizes, simply moving liquid through the channel becomes a substantial challenge. The traditional method of applying a pressure differential to either end of the channel to drive the flow is not very effective in nano-scale channels. This is due to increased interactions of fluid with the channel surfaces, and the fact that fluid viscosity in small dimensions differs dramatically from bulk fluids [1, 2]. For example, as the diameter of a fluid channel is decreased, the pressure differential necessary to maintain a certain flow velocity must increase. At some point the required pressure becomes unreasonably large [3].