Kerry Cheung

An Application of 3-D MEMS Packaging: Out-of-Plane Quadrupole Mass Filters

This paper reports the design, fabrication, and characterization of low-cost out-of-plane quadrupole mass filters that use commercially available dowel pins as electrode rods. The quadrupoles implement a 3-D MEMS packaging technology that relies on deep-reactive ion etching (DRIE)-patterned deflection springs for alignment. Quadrupoles with rod diameter ranging from 0.25 to 1.58 mm and aspect ratio of 30 to 60 were built and tested at RF frequencies of 1.44, 2.0, and 4 MHz. Assembled devices operated in the first stability region achieved a maximum mass range of 650 amu, while a minimum half-peak width of 0.4 amu at mass 28 was obtained in the second stability region. Operation in the second stability region provides a means to higher resolution, smoother peaks, and removed peak splitting at the expense of transmission. The ultimate resolution of the reported quadrupoles is also discussed.

Performance Characteristics of a MEMS Quadrupole Mass Filter With Square Electrodes: Experimental and Simulated Results

Size reduction in quadrupole mass spectrometers (QMSs) is an ongoing requirement driven by the needs of space exploration, portable, and covert monitoring applications. Microelectromechanical systems (MEMS) technology provides a method of achieving this size reduction. A quadrupole mass filter (QMF) is one component of a QMS and is suitable for microfabrication. MEMS manufacturing techniques are more suitable to the production of rectilinear electrodes, instead of the more widely used circular electrodes. Present understanding of the performance characteristics of rectilinear electrodes and the dependence of these characteristics on electrode geometry are not well documented. In this paper, we report on the performance characteristics of a square-electrode QMF. Both the predicted performances obtained by computer simulation and experimental data are presented for operation in stability zone 1 (0.236, 0.706) and zone 3 (3.16, 3.23). A comparison between these results and the simulated data for equivalent devices constructed using hyperbolic and circular electrodes for operation in zone 1 is also made. This comparison demonstrates that, although the field produced by square electrodes is far from the “ideal,” it is still possible to achieve useful filtering action. Our results also show that, for operation in zone 3, performance comparable with that of hyperbolic and circular electrodes operating in zone 1 is achievable.

Design, fabrication and characterization of double-gated vertically aligned carbon nanofiber field emitter arrays

In this work, we designed and fabricated two types of double-gated isolated vertically aligned carbon nanofiber field emission arrays (VACNF FEAs) to study how the tip position relative to the gate affects the device performance. In the first type, the tip is in-plane with the gate, and in the second type, the tip is 0.9 mum below the gate. To quantify the effectiveness of the two gates to affect the total emission current, the gate field factor (betaG) and the focus field factor (betaF) are examined for both types of VACNF FEAs experimentally and by simulation.

Improvements on an out-of-plane MEMS quadrupole for portable mass spectrometry

Quadrupoles are mass filters composed of a set of four cylindrical rods in a symmetrical setup that are biased with a combination of DC and RF potentials. There has been an active interest in developing scaled-down quadrupole technology for over a decade. This has led to remarkable results in both miniaturized and MEMS-based quadrupoles. This paper reports the design, fabrication, and characterization of a second generation of MEMS out-of-plane quadrupoles mass filter, part of a portable mass spectrometer for gas sensing. The mass spectrometer is composed of a CNT-based ionizer array, a MEMS quadrupole mass filter, MEMS electrometer for charge sensing, and a MEMS displacement pump that provides the required vacuum level. Two key advantages of scaling down quadrupoles are device portability and the relaxation on the vacuum level required for operation. For a fixed rod aspect ratio the quadrupole weight roughly scales to the cube of the rod diameter. Also, scaled down quadrupoles can work at higher pressure provided the gas mean free-path is larger than the quadrupole length. We have experimentally demonstrated that higher working pressure, up to the mTorr level, is compatible with our CNT-based electron impact and field ionizers with no loss in ionizer performance, thus setting the maximum quadrupole length at 5 cm.

First principles optimization of mass producible microscaled linear quadrupoles for operation in higher stability regions

In recent years, there has been a desire to scale down linear quadrupoles. The key advantages of this miniaturization are the portability it enables, and the reduction of pump-power needed due to the relaxation on operational pressure. Various attempts at making linear quadrupoles on the micro-scale were met with varying degrees of success. This study was proposed to justify the work for a fully microfabricated, mass-producible, MEMS linear quadrupole mass filter. The study indicates that a MEMS quadrupole with square electrodes can function as a mass filter without significant degradation in performance if driven in higher stability regions. Successful implementation of such devices leads into arrayed configurations for parallel analysis, and aligned quadrupoles operated in tandem for enhanced resolution.