Quadrupole mass spectrometers

Details Quadrupole mass spectrometers Quadrupole mass spectrometers

1997-10-03

2000-02-15

Berman, Jack I.

Radiant energy

Ionic separation or analysis

Cyclically varying ion selecting field means

H01J 4942

Patent

active

060255919

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

This invention concerns improvements in and relating to quadrupole mass spectrometers.
Quadrupole mass spectrometers may be used for residual gas analysis in the semiconductor industry, typically to determine gases present after processing, in order to ensure a clean chamber. Quadrupole mass spectrometers may also be used in chemical processes to detect presence of toxic gases.
A quadrupole mass spectrometer generally comprises an ion source, a mass filter and an ion detector. Conventionally circular section metal rods are used as the mass filter by being excited electrically. This electrical excitement produces a field that can be controlled to permit only ionised particles of interest to reach the ion detector, enabling their detection.
These spectrometers are generally expensive to purchase and to repair and recalibrate. There exists, therefore, a need for a low cost quadrupole mass spectrometer. Furthermore, size is a factor in determining where quadrupole mass spectrometers are used. Miniaturization of quadrupole mass spectrometers without compromising resolution is also desirable.
An object of this invention is to provide a quadrupole mass spectrometer which may be manufactured to be smaller than hitherto and at lower cost.
According to this invention there is provided a quadrupole mass spectrometer comprising a mass filter and an ion detector, characterized in that the mass filter comprises a plurality of metal coated fibres.
For many applications the quadrupole mass spectrometer of the invention will require an ion source but the quadrupole mass spectrometer may also be used in environments were ions are already generated.
The ion source preferably comprises an array of field emitters, such as tips or edges. In one preferred form the ion source comprises an array of relatively sharp tips which may be formed in a semiconductor material, such as silicon or gallium arsenide, or in a metal. The number of tips forming an array may be from 1 to 100,000, preferably around 10,000 over an area of about 0.25 mm.sup.2. The ion source emits electrons from said tips causing ionisation, by application of a negative potential thereto, of gas atoms or molecules in the vicinity thereof either by attachment (forming negative species) or by collision with the atoms or molecules (forming positive species).
In another preferred form the ion source comprises an array of relatively sharp edges which again may be formed in semiconductor material possibly in a similar fashion.
The ion source is preferably formed from a silicon wafer by depositing and etching oxide and metal layers to a desired pattern before isotropically etching substrate material to form ion emitter tips or edges. The oxide is preferably formed in a first layer before etching to remove oxide in discrete areas and then formed in a second layer whereby oxide thickness in those discrete areas is thinner than elsewhere. After metallisation of the oxide layer, etching may be used to remove oxide from said thinner areas but leaving mask pads for the emitter tips or edges to protect them during isotropic etching to form them. The subsequent isotropic etching of the substrate leaves emitter tips or edges protected by said mask pads which are then removed. Finally, the opposite side of the substrate is treated to provide one electrical contact i.e. by metallisation, the other electrical contact being made to the metal layer previously deposited on top of the oxide layer. This upper metal layer will normally be connected to ground or near ground potential e.g. 0 to 10V and the lower metal layer will be connected to a suitable negative potential, e.g. -40V.
The ion source may be arranged to emit electrons generally in the direction of a longitudinal axis of the quadrupole mass spectrometer. However, it is also possible to arrange for the ion source or a pair of facing ion sources to emit electrons transverse to the said axis. With these and in other ion source arrangements it may be advantageous to provide means for focussing produced ions in the direc

REFERENCES:
patent: 3699330 (1972-10-01), McGinnis
patent: 4985626 (1991-01-01), Margulies
patent: 5373157 (1994-12-01), Hiroki et al.
patent: 5401962 (1995-03-01), Ferran
patent: 5719393 (1998-02-01), Chutjian et al.

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