MTL Annual Report

Figure 1: Electron-impact-ionization pump structure consists of a field-emission source (CNTs, an extractor gate and a focus gate), an electron-impact-ionization region (length of L) and an ion-implantation getter.

There is a need for microscale vacuum pumps that can be readily integrated with other MEMS and electronic components at the chip-scale level. Vacuum pumps exhibit favorable scaling and are promising for a variety of applications such as low-power portable mass spectrometers and sub-mm wavelength vacuum amplifiers. This project aims to develop the technology for a micro-fabricated electron-impact-ionizer pump.  The micropump consists of a field-emission electron source that is an array of double-gated isolated vertically aligned carbon nanotubes (VA-CNTs), an electron-impact-ionization region, and an ion implantation getter, as shown in Figure 1. The pump works as follows: first, electrons are field-emitted from the VA-CNT array; then, the electrons are accelerated at a bias voltage that maximizes the probability of collision with neutral gas molecules, this way achieving ionization by fragmentation of the molecules; finally, ions are implanted into the getter.

In a double-gated field-emitter array, the first gate (extractor) is used to extract electrons out of the tip, while the second gate (focus) is biased at a lower voltage than the first gate or emitter to focus the emitted electrons and collect the back-streaming ions, thus protecting the tip ^[1]. In this work, we are designing and simulating double-gated isolated VA-CNT field-emission arrays to study how the field-emitter tip position relative to the two gates affects the device performance. To quantify the effectiveness of the gates to affect the total emission current, the gate field factor (β_G) and the focus field factor (β_F) are examined using the commercial simulation software COMSOL. Figure 2 shows a solution of electric field for an emitter with 20 nm tip radius and 1 mm height with 0.24 mm aperture from a single gate. From this simulation, we obtain a gate field factor of 2.1×10⁵V/cm.

1. L.-Y. Chen, L. F. Velasquez-Garcia, X. Wang, K. Cheung, K. Teo, and A. I. Akinwande, “Design, Fabrication and Characterization of Double-Gated Vertically Aligned Carbon Nanofiber Field Emitter Arrays.” in Vacuum Nanoelectronics Conference, 2007, pp 82-83. [↩]