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In this paper, microelectromechanical system (MEMS) has been utilized to make miniaturized ion optics required for making the portable all-in-one mass spectrometer. Four different ion optics components were fabricated using deep-reactive ion etching (DRIE) of n-doped silicon-on-insulator. These components are 1 mm Bradbury-Nielsen gate, 500 µm coaxial ring ion trap (CRITter), reflectron optics and 500 µm Einzel lens. The Bradbury-Nielsen gate was made using a pattern of alternating electrode wires which either allows ions to pass or stop through the gate. The CRITter was made using five trapping rings and two end caps to make mass selection ion optics and it was also used in testing the alignment capability of fabrication process. The reflectron optics was made using a assembly of fifteen rectangular elements arranged in series. The fourth ion optics component was assembled MEMS Einzel lens which consisted of three lenses. It was used to focus the ions beam to increase the ion current and detectability. All the components were tested using ion produced with 70 eV EI ionization. These assemblies were characterized in terms of breakdown voltage, durability, and alignment. For current devices, the breakdown voltage was reported 750 V. The CRITter was tested with 1% toluene at pressure of 1 x 10-4 Torr. The resolution was limited due to the alignment errors and also aberration in etched designs got more impactful as the size of the ion trap was reduced. Current reflectron optics was not capable of resolving the peaks of toluene. Therefore, in the future analyzer path length will be increased by using multiple reflectrons. These miniaturized components were assembled using an encoded piezo-manipulator with pick and place capability. Resolution and ion attenuation was found to be the greatest concern of the current design at present.
Fox, J.; Saini, R.; Tsui, K.; Verbeck, G., Microelectromechanical system assembled ion optics: An advance to miniaturization and assembly of electron and ion optics. Review of Scientific Instruments 2009, 80 (9), 093302.
In this paper, a soft landing (SL) instrument has been developed with capability of depositing the ions onto the substrate for preparative analysis. The two important components of this instrument are custom made drift tube and two split rings. The drift tube is consist of 18 concentric rings along with two split rings at the end. The drift tube was filled with an inert buffer gas like He and operated from 1 to 100 Torr of pressure. High pressure gas thermalized the cluster ions on collision to 0.01 to 1.0 eV kinetic energy and separate the clusters formed by laser ablation. This helped in further analysis of deposited clusters on mica surface. Two functions of split-ring are to direct the cluster ions towards either detector or a landing surface. This instrument works on the principle of narrowing the kinetic energy of ions going through drift tube to prevent the fragmentation on landing. The gating function of split ring was performed using a homemade pulsing circuit that changes the voltage across the split ring. The SL instruments was built with a quick door CF flange which reduced the number of gaskets required and helped in holding, adjusting and removing the detector and landing surface from the instrument without disassembling the instrument. A simple Faraday plate was used in SL instrument as detector. A 15 mm mica disk used for atomic force microscopy (AFM) was used as landing surface. Split ring pulsing helped in selecting and isolating the specific ion clusters. For initial experiments copper was used as analyte and it was ionized using laser ablation using ND-YAG laser. Mass spectrum of Copper was reported to have multiple peaks due to Cun+, CunOm+ ions formation in the presence of O2 as contaminant. After the cluster deposition on mica surface, surface was analyzed using AFM and was compared with the physical vapor deposition (PVD). In the future, other landing surfaces like gold, silicon, and highly ordered pyrolytic graphite can be used to better understand the deposition mechanism.
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