Electron Impact Ionizers

Current Collaborators: A. I. Akinwande (EECS), B. Gassend (Ph.D. EECS’07)Previous Collaborators: L-Y. Chen (Ph.D. EECS’07)
Funding: DARPA/MTO

Figure 1

Fig. 1 Schematic of a field electron impact ionizer: electrons that are produced by quantum tunneling collide with background gas molecules to produce ions.

We are interested in developing MEMS/NEMS enabled gas ionizers for portable mass spectrometry applications using carbon nanotube (CNT)-based field emission cathodes that produce ions through electron impact ionization because of the remarkable physical and chemical properties of CNTs. Mass spectrometers are powerful analytical tools that are helpful to quantitatively determine the chemical composition of a sample. Conventional mass spectrometry hardware is bulky and power hungry, which limits its applicability. The development of rugged scaled-down mass spectrometry (MS) systems would enable their portability and therefore, it would extend the use of mass spectrometry to a wide range of in-situ applications including geological survey, law enforcement, environmental monitoring, process control, and space exploration.

The power consumption, size, and weight of an MS system are driven by its vacuum requirements. Therefore, the relaxation of the vacuum level at which the MS components can operate would enable its portability. In addition, the use of micro- and nanofabrication technologies to implement miniaturized MS hardware would facilitate component batch-fabrication, further reduce the dimensions of the components, and potentially enable the implementation of systems with higher performance through better component integration. Our research has focused on the development of MEMS/NEMS enabled electron impact ionizers (EIIs) using vertically aligned multi-walled carbon nanotubes (MWCNTs) with a proximal gate as electron source. EIIs use electrons to fragment neutral gas molecules to produce ions (Fig. 1). In an EII the ratio between the ion current II(E) and the electron the current IE(E) is

Equation

where E is the energy of the electrons, P is the gas pressure, Kb is Boltzmann’s constant, T is the gas temperature, L is the ionization collision path length (i.e., the distance between the electron source and the ion collector), and σtotal is the total ionization cross-section along the ionization collision path length.

State-of-the-art EIIs for MS systems accomplish ionization using a thermionic electron source. Thermionic cathodes require high temperature (> 2000 K) to operate, which results in inefficient power consumption (typical thermionic ionizers consume 5 W), as well as in reliability issues if they are operated at high-pressure (>10-4 Torr) due to the chemical reactivity of the cathode surface, particularly if the gas mix contains oxygen. A better approach to implement EIIs for portable MS systems involves the use a cold cathode. Electrons are field emitted from the surface of metals and semiconductors when the potential barrier that holds electrons within the material (workfunction Φ) is deformed by the application of a high electrostatic field. High surface electrostatic fields are typically obtained by the application of a voltage to a high aspect-ratio structure with nanometer-scaled tip radius. Field emission is described by the Fowler-Nordheim (FN) model, which states that the electron current IE(VG) emitted from a tip biased at a voltage VG with respect to the gate has a strong dependence on the field factor β of the tip:

Equation