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A description of the design and microfabrication of arrays of micrometer-scale cylindrical ion traps is offered. Electrical characterization and initial ion trapping experiments with a massively parallel array of 5 microm internal radius (r(0)) sized cylindrical ion traps (CITs) are also described. The ion trap, materials, and design are presented and shown to be critical in achieving minimal trapping potential while maintaining minimal power consumption. The ion traps, fabricated with metal electrodes, have inner radii of 1, 2, 5, and 10 microm and range from 5 to 24 microm in height. The electrical characteristics of packaged ion trap arrays were measured with a vector network analyzer. The testing focused on trapping toluene (C(7)H(8)), mass 91, 92, or 93 amu, in the 5 microm sized CITs. Ions were formed via electron impact ionization and were ejected by turning off the rf voltage applied to the ring electrode; a current signal was collected at this time. Optimum ionization and trapping conditions, such as a sufficient pseudopotential well and high ionization to ion loss rate ratio (as determined by simulation), proved to be difficult to establish due to the high device capacitance and the presence of exposed dielectric material in the trapping region. However, evidence was obtained suggesting the trapping of ions in 1%-15% of the traps in the array. These first tests on micrometer-scale CITs indicated the necessary materials and device design modifications for realizing ultrasmall and low power ion traps.

We have performed detailed SIMION simulations of ion behavior in micrometer-sized cylindrical ion traps (r 0 ϭ 1 m). Simulations examined the effects of ion and neutral temperature, the pressure and nature of cooling gas, ion mass, trap voltage and frequency, space-charge, fabrication defects, and other parameters on the ability of micrometer-sized traps to store ions. At this size scale voltage and power limitations constrain trap operation to frequencies about 1 GHz and rf amplitudes of tens of volts. Correspondingly, the pseudopotential well depth of traps is shallow, and thermal energies contribute significantly to ion losses. Trapping efficiency falls off gradually as q z approaches 0.908, possibly complicating mass-selective trapping, ejection, or quantitation. Coulombic repulsion caused by multiple ions in a small-volume results in a trapping limit of a single ion per trap. If multiple ions are produced in a trap, all but one ion are ejected within a few microseconds. The remaining ion tends to have favorable trapping parameters and a lifetime about hundreds of microseconds; however, this lifetime is significantly shorter than it would have been in the absence of space-charge. Typical microfabrication defects affect ion trapping only minimally. We recently reported (IJMS 2004, 236, 91-104) n parallel with advances in micromachining technology, numerous efforts are underway to produce miniaturized and microfabricated analytical instrumentation. Although portable instruments rarely match the performance of larger laboratory instruments, the benefits of low mass and power make small instruments well-suited for real-time in-field analytical applications. Mass spectrometers are attractive targets for miniaturization because they generally exhibit high sensitivity and chemical specificity. Many groups have focused on miniaturizing mass spectrometers, and small mass analyzers based on time-of-flight [1][2][3][4][5][6], magnetic sector [7], linear quadrupole [8], and hyperbolic and cylindrical ion traps [9 -17] have recently been described. The dimensions of some of these mass analyzers are about a millimeter or smaller. As mass analyzer dimensions decrease, vacuum requirements also typically decrease due to the shorter ion path and correspondingly shorter tolerable mean free path [11,18]. Similarly, power requirements typically decrease as the sizes of capacitive and inductive components are reduced, or as the required voltage needed for a given field strength decreases. Further reductions in size, for example, down to the micrometer range, may allow sufficiently small pumping, power, and electronics packages as to make a truly hand-held instrument.We recently reported [19] on the design considerations and development of microfabricated arrays of cylindrical ion traps for use as mass analyzers. These arrays contain up to 10 6 traps/cm 2 . The geometry of the cylindrical ion trap is amenable to multi-level microfabrication techniques, and the trap size and electrical requirements make it suitable for layout a...

We have investigated the field emission properties of free-hanging tungsten microelectromechanical system structures. These tungsten structures are designed to serve as electrodes in a Paul ion trap. Since the outer edges of the trap end cap electrodes are adjacent to the inner edges of the trap ring electrode and approximately 0.5μm apart, field emission may occur between these two edges when hundreds of volts are applied, thereby defining an edge field emitter. The arrays were tested under vacuum (10−5Torr) and at atmospheric pressure (625 Torr) to understand the field emission behavior. Vacuum tests show turn-on voltages of about 200 V for the 1 and 1.5μm radius traps and currents of ∼80nA for both sizes of trap with the largest array (106) at 6 MV/cm. The atmospheric tests showed lower turn-on voltages of approximately 150 V for both the 1 and 1.5μm radius traps. Currents up to a few μA were achieved at 6 MV/cm for smaller trap size (1μm) in the largest array indicating a gas ionization contribution. The measured current-voltage responses fitted the Fowler–Nordheim characteristics well, confirming that the current increase in vacuum was due to field emission. A stable emission current of 2.03 nA was obtained at 10 MV/cm for 11 min.

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This work describes the design, simulation, fabrication and characterization of a microfabricated thermal conductivity detector to be used as an extension of the µChemLab™. The device geometry was optimized by simulating the heat transfer in the device, utilizing a boundary element algorithm. In particular it is shown that within microfabrication constraints, a micro-TCD optimized for sensitivity can be readily calculated. Two flow patterns were proposed and were subsequently fabricated into ninepromising geometries. The microfabricated detector consists of a slender metal film, supported by a suspended thin dielectric film over a pyramidal or trapezoidal silicon channel. It was demonstrated that the perpendicular flow, where the gas directly impinges on the membrane, creates a device that is 3 times more sensitive than the parallel flow, where the gas passes over the membrane. This resulted in validation of the functionality of a microfabricated TCD as a trace-level detector, utilizing low power. The detector shows a consistent linear response to concentration and we are easily able to detect 100-ppm levels of CO in He. Comparison of noise levels for this analysis indicates that sub part per million (ppm) levels are achievable with the selection of the right set of conditions for the detector to operate under. This detector was originally proposed as part of a high-speed detection system for the petrochemical gas industry. This system was to be utilized as a process monitor to detect reactor "upset" conditions before a run away condition could occur (faster than current full-scale monitoring systems were able to achieve). Further outlining of requirements indicted that the detection levels likely achievable with a TCD detector would not be sufficient to meet the process condition needs. Therefore the designed and fabricated detector was integrated into a detection system to showcase some technologies that could further the development of components for the current gas phase µChemLab as well as future modifications for process monitoring work such as: pressurized connections, gas sampling procedures, packed columns. Component integration of a microfabricated planar pre-concentrator, gas-chromatograph column and TCD in the separation/detection of hydrocarbons, such as benzene, toluene and xylene (BTX) was also demonstrated with this system.

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