MTL Annual Report

Figure 1: A proposed design of a field-ionizer unit consisting of an isolated vertical CNT and a high-transparency gate with electric field created between them.

Micro vacuum-pump technology, which refers to vacuum-pump systems with miniaturized functional parts, currently attracts a noticeable interest from researchers due to the potential cost savings and energy consumption reduction that they would introduce ^{[1] [2] [3] [4]}, which would enable portability of many devices that require less-than-atmospheric pressure to operate.  In this project we propose a MEMS/NEMS field-ionization micropump that is able to operate at pressures as high as 30 torr and evacuate small (1 mm³) volumes to reach an mtorr-level vacuum.  The field-ionization micropump can be integrated with other MEMS/NEMS subsystems to span a pressure range between atmospheric pressure and high vacuum (10^-6 torr) to satisfy a wide range of applications that require portable vacuum sources.  The field-ionization pump employs arrays of isolated vertically aligned carbon nanotubes (VA-CNTs) that have a proximal gate to generate local high-electric fields (10⁸ V/cm) with low voltage, continuously ionizing gas molecules by field ionization. Positive gas ions then are attracted and implanted into the cathode getter, which is typically biased at 1kV.

Figure 2: Vertical CNT arrays on silicon columns with 5-um pitch. Catalyst nanodots used for isolated CNT synthesis were defined by the photolithography method.

Our field-ionization pump consists of three major parts: an anode field-ionization array (i.e, the field ionizer), a cathode getter, and a gas inlet/outlet. The schematic diagram of the field-ionizer unit is shown in Figure 1. The fabrication of the field-ionization pump involves the synthesis of isolated VA-CNT arrays at the wafer level. Instead of using low-throughput methods such as e-beam lithography to define the catalyst pads for VA-CNT growth, we have developed a technique that uses projection photo-lithography, which increases the fabrication throughput by four orders of magnitude. Figure 2 is a high-resolution SEM picture of an array of isolated VA-CNTs defined by the projection photo-lithography technique. Simulations of the pump predict an ultra fast air-extracting rate (3 seconds) for a 56-mm³ volume from ionization arrays consisting of 1 million isolated VA-CNTs, while the pump consumes less than 1W. ^[5]

1. N. Nguyen, X. Huang, and t. Chuan, “MEMS-Micropumps: A Review,” ASME J. Fluids Eng., vol. 124, pp. 384–392, 2002. [↩]
2. P. Woias, “Micropumps—Summarizing the First Two Decades,” Proc. SPIE, vol. 4560, pp. 39–52, 2001. [↩]
3. D. Laser and J. Santiago, “A Review of Micropumps,” J. Micromech. Microeng., vol. 14, pp. R35–R64, 2004. [↩]
4. S. Shoji and M. Esahi, “Micro Flow Devices,” IEEE Fifth International Symposium on Micro Machine and Human Science, pp. 89–95, 1994. [↩]
5. J. Hauschild, E. Wapelhorst, and J. Mueller, “A Fully Integrated Plasma Electron Source for Micro Mass Spectrometers,” in Ninth International Conference on Miniaturized Systems for Chemistry and Life Sciences, pp. 476–478, 2005. [↩]