Fabrication of two types of micro ion sources for a micro time-of-flight mass spectrometer

Abstract

This paper reports the fabrication of two types of micro ion sources for a micro time-of-flight mass spectrometer (TOFMS). The TOFMS is an analyzer that serves for the establishment of molecular weight by using the difference in the flight time of ions according to their mass-to-charge values. The micro TOFMS consists of a glass substrate with several nickel electrodes and a silicon substrate with an ion source. We have fabricated two types of ion sources using a hot filament and field emitters with a carbon nanoparticle structure. The overall size of the assembled device is 10 × 10 × 1 mm3. The hot filament is fabricated through the nickel electroplating process and some lithography processes. The carbon nanoparticle layer is grown by hot-filament chemical vapor deposition (HFCVD). The carbon nanoparticle field emitter has the triode structure with which we can control the emission current at low voltage. The maximum emission current of the hot-filament ion source is 0.28 μA. The emission current of the field emission ion source can be controlled from 0 to 18.5 μA when the gate voltage varies from 0 to 175 V.

CNT-Based MEMS/NEMS Gas Ionizers for Portable Mass Spectrometry Applications

“…For example, the electron current of our devices is several orders of magnitude larger than what was reported by Liu et al, with nanostructured boron nitride cathodes, and their operational voltage is visibly higher [63]. Also, the electron current that our CNT-based ionizer attained is several orders of magnitude larger than the electron current achieved by the MEMS thermionic ionizers developed by Yoon et al (7 nA using a tungsten filament [33] and 280 nA using a nickel filament [34]) or several orders of magnitude larger than the electron current from the CNT-based cold cathodes reported by Yoon et al (18.5 μA at 175 V [34]) or Hwang et al (80 μA at 1.1 kV [35]). Moreover, the ionization efficiency of our devices is almost four times larger than the ionization efficiency reported by Chen et al using arrays of double-gated isolated PECVD CNTs, with argon at 5.6 mtorr [48].…”

“…In an electron impact ionizer (EII), a stream of electrons is used to ionize neutral gas molecules by collision. Yoon et al reported a MEMS ionizer that uses a tungsten filament [33], a nickel filament [34], or carbon nanotubes (CNTs) [34] as electron source, although no ionization data were provided. Other researchers have developed CNT-based EIIs suitable for portable MS, including Hwang et al [35], Bower et al [36], Chen et al [37], Park et al [38], and Manohara et al [39].…”

Microchip technology in mass spectrometry

“…The ion source was made from three substrates (silicon, borosilicate, and oxidized silicon wafers), and DRIE, anisotropic etching, sputtering, and anodic bonding were applied during the microfabrication process. In another approach, an electron source with a hot filament was fabricated by sputtering tungsten onto silicon oxide and patterning a short filament by photolithography and etching (Yoon et al, 2002(Yoon et al, , 2007. The ion source contained also grid and acceleration electrodes, and a repeller from nickel to accelerate and focus ions towards the MS analyzer.…”

“…In addition, a variety of field emitters have been microfabricated for miniaturized mass spectrometers. These include cold-cathodes and their arrays (Felter, 1999;Kornienko et al, 2000), a ring emitter (Van Amerom et al, 2008), carbon nanopearls (Mouton et al, 2008), carbon nanoparticles (Yoon et al, 2007), carbon nanofiber arrays (Chen et al, 2007), and carbon nanotubes (Getty et al, 2007. The main advantage of field emitters over hot filaments is their better tolerance to the high pressure inside the ion source.…”

“…Namely, in most cases, the high mass resolution obtained with normal-sized analyzers decreases down to unit resolution or even worse because of the short flight path of miniaturized time-of-flight analyzers. In a micro-TOF analyzer fabricated on an n-type {1 0 0} silicon wafer by various microfabrication techniques (Yoon et al, 2002(Yoon et al, , 2007, the overall size of the ion source and mTOF analyzer was 10 mm  10 mm  1 mm, and the length of the flight path was approximately 5 mm (Fig. 22).…”