Electron Impact Ionizers

Figure 2

Fig. 2 A double-gated MWCNT electron impact ionizer. The inner gate biases a voltage to field emit electrons from the MWCNT tip, while the outer gate protects the field emitter from ion back streaming.

The field factor β relates the electric field at the tip surface to the applied gate voltage and it is to first order equal to the inverse of the emitter tip radius r. Field emission cathodes have a faster response than thermionic cathodes because the time lag in a thermionic source is controlled by its thermal inertia, while the time lag in a cold cathode is set by the RC time constant involved in charging the electrodes. Cold cathodes that use MWCNTs as field emitters are ideal to implement EIIs for portable mass spectrometry because CNTs have large field factors due to their nano-scaled diameters and very high aspect-ratios. In addition, the remarkable physical properties and good chemical resistance of CNTs address the back-ion bombardment-related and oxygen-related reliability issues in EIIs.

Figure 3

Fig. 3 A gated open architecture electron impact ionizer (left); top view of emitting die without the gate (right, A), detail of the gate-emitting substrate assembly structure (right, B), sparse MWCNT forests aligned to the gate apertures (right, C), and detail of the MWCNT tips (right, D).

We have developed two distinctive CNT-based electron impact ionizer technologies: (i) massive arrays of double gated VACNFs using a closed (i.e., well-like) architecture (Fig. 2), and massive sparse forests of MWCNTs using an open (i.e., no dielectric between adjacent MWCNTs, gate is effectively hovering on top of the field enhancers) architecture (Fig. 3). The doubled-gated well architecture ionizers were designed to emit current with very low voltage. These devices can turn-on (emit more than 10-11 A) with as little as 24 V vs. the typical 100 V startup voltage of the open architecture ionizers. However, the open architecture ionizer was designed to produce orders of magnitude more current  by visibly augmenting the array fabrication yield (number of emitters that work) through the use of a hovering gate structure. Both devices are capable of producing ions at mtorr-level pressure (Fig. 4, Fig. 5), although we have demonstrated better performance using the open architecture: we have shown orders of magnitude more electron current (mA-level), higher pressure operation (22 mtorr), and higher ionization efficiency (~20%) than the double-gated closed architecture.

Figure 4

Fig. 4 log-log plot of the ion current-to-electron current ratio versus pressure from an array of double-gated close architecture CNT-based electron impact ionizers. The data is in agreement with the electron impact ionization model. This technology is typically capable of μA-level electron currents and 5% ionization efficiency at 5 mtorr.

Figure 5

Fig. 5 log-log plot of the ion current-to-electron current ratio versus pressure from a gated open architecture CNT-based electron impact ionizer. The data is in agreement with the electron impact ionization model. This technology is typically capable of mA-level electron currents and 20% ionization efficiency at 22 mtorr.