Fully Batch-fabricated Linear Quadrupole Mass Filters

We report the design, fabrication, and characterization of a fully batch-fabricated linear quadrupole mass filter. The device demonstrates for the first time, a microfabricated quadrupole mass filter that doesn’t require any post-fabrication electrode assembly. Utilizing a design with non-conventional square electrodes, a 30 mm quadrupole with an effective electrode diameter of ~1.7 mm, and integrated ion optics was fabricated. At a driving frequency of 2 MHz and a maximum applied voltage of 100 V, a mass range of 250 a.m.u. was achieved in the first stability region, and a halfheight peak-width of 0.7 a.m.u. was obtained in the second stability region at mass 28. INTRODUCTION Mass spectrometers are powerful analytical tools that can identify the composition of unknown compounds, and quantify compounds that are known. Scaling-down this device will permit faster analysis times, power savings through operation at higher pressures, portability, and reduced costs [1, 2]. A popular method of miniaturization is through the use of microelectromechanical systems (MEMS) fabrication technologies because it permits batch-processing. The development of micro-scaled gas analyzers will have applications in national security, environmental safety and monitoring, satellites and space exploration, industrial processing, as well as biological research and healthcare. Mass spectrometers comprise primarily of an ionizer to create charged species, a mass filter to sort the ions according to their mass-to-charge ratios, and a detector to measure the quantity of the filtered ions. Batch-fabricated ionizers and detectors have already been developed for a micro-gas analyzer system [3, 4]. In order to produce these systems inexpensively and efficiently, the mass filter component should also be completely batch-fabricated. Linear quadrupoles are popular mass filters due to their simplicity in design and usage. Researchers have demonstrated mass filtering with MEMS-based quadrupole mass filters (QMFs) [5-7]. Reported designs utilize metallic rods or metalized glass rods as electrodes, which required manual assembly of these electrodes after the MEMS components were fabricated. This methodology places a bottleneck on the manufacturing and sets a limit on the precision that can be achieved. We propose using a non-conventional square electrode geometry that can be fully batch-fabricated, thus taking advantage of the dimensional precision associated with MEMS technology. This paper reports the design, fabrication, and characterization of the micro-square electrode quadrupole mass filter (MuSE-QMF). The operating principles of a QMF will be described, followed by an overview of the optimization process. The fabrication process flow will be presented, and the experimental results discussed.