Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Bhansali, Shekhar

Keywords

mass instability scan, voltage breakdown test, CYTOP, electroless plating, ultrasonic drilling, MEMS, DRIE

Abstract

This research presents a novel microfabrication approach and testing methodology for cylindrical ion trap (CIT) microarray tunable for mass- spectrometers. The growing interest in cylindrical ion trap (CIT) mass-spectrometers is primarily due to ease with which cylindrical geometry can be realized as compared to hyperbolic surfaces found in conventional quadrupole ion traps. Also due to the fact that the potential at the center of hyperbolic electrode in quadrupole ion trap and cylindrical electrode in cylindrical ion trap (CIT) does not differ significantly[2]. Since the RF voltage required to eject a given mass-to-charge ion scales as the square of the ion trap radius, a decrease in ion trap dimensions provides a significant reduction in electronics requirements, thereby providing a pathway for overall system miniaturization.

The reduction in sensitivity due to reduced ion storage capacity as a result of miniaturization can be improved by employing an array of identically sized ion traps. Microfabrication approach promises excellent uniformity in the fabrication of identically sized holes which in turn leads to low-cost high performance CIT microarray for mass spectrometers[1,2]. The criterion used for the determination of trap diameter was to ensure that the hole to be 1.09 times the wafer thickness to provide optimal potential to trap ions[1]. The end- plates were designed to optimize the electron and ion transmission into and out of the ion trap and provide a high quality electric field definition within each cylindrical ion trap (CIT)[3]. Two different approaches, namely deep reactive ion etching (DRIE) and mechanical drilling using ultrasonic disc cutter were proposed and used for the fabrication of ring-electrode which forms the main body of the ion trap.

Excellent uniformity in hole diameter was observed in both the approaches. The end-plates were fabricated using deep reactive ion etching (DRIE) which provided high transmission rigid grid structure for ions and electrons. Standard Bosch process was used for deep reactive ion etching (DRIE). The two electrodes were metallized using electroless plating which provides excellent uniformity of coating even on end-plate structures with 5micro m through holes. CYTOP[trademark], a cyclized perfluoro polymer, was used as an insulation layer and intermediate bonding layer between the ring electrode and end-plates. The breakdown voltage for a released 16 micro m thick CYTOP[trademark] layer was found to be 1.47KV. An assembly for testing miniature cylindrical ion trap (CIT) was designed and built. An electron impact ionization source was used for generation of ions. Mass selective instability scan was used to selectively eject ions with different mass-to-charge ratio.

A cylindrical ion trap (CIT) with 4mm diameter was fabricated and tested for analyte gases such as krypton and xenon.

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