Materials And Methods

RESET

Gain

Gain

990 fF

10 fF

10 fF

Readout

"FARADAY ELECTRODE'

System

Figure 7.

The CMOS-CTIA is fabricated using the 0.35 micron process as a high input impedance amplifier with 10 fF and switch selectable 990 fF feedback capacitors. Read noise using multiple nondestructive readouts is below three electrons.

This manuscript evaluates the application of CMOS-CTIA technology in miniature portable IMS systems designed for detection of explosives in the field. The IMS ionizer/drift tube/high voltage power supply and gate circuitry fit in a volume 2"x1.56"x4" with a drift tube bore of 0.46" (Fig. 8). Miniaturization of the IMS causes a loss in sensitivity over systems with large volume ionizers and larger, longer drift tubes. This is obviously an application requiring improvements in sensitivity, and this need has been met by the CMOS-CTIA technology.

Figure 8. The miniature IMS system showing the motherboard containing the high voltage

FET gating circuitry, divider string, and drift tube assembly. The detector and readout circuitry are mounted on a daughterboard at the detector end of the tube.

Optimized single channel detectors have been developed with the ultimate goal of direct detection of single ions. Extremely high impedance CTIA amplifiers have been integrated with low capacitance collection elements to produce a detection system capable of direct measurement of charge with very high sensitivity. The small physical sizes of the collected elements and of the associated measurement circuitry resulted in a dramatic reduction in the input capacitance. Charge is accumulated on femtofarad-sized capacitors, whereas in a typical electrometer the input capacitance is tens of picofarads. The low relative capacitance produces a corresponding improvement in sensitivity. Read noise, using multiple nondestructive readouts [4,5], has been reduced to less than three electrons. Signals from a very low flux of ions can thus be reproducibly and quantitatively measured.

Figure 8. The miniature IMS system showing the motherboard containing the high voltage

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