Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Reexamination Certificate
1998-11-03
2001-01-30
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
C250S283000, C250S397000
Reexamination Certificate
active
06180942
ABSTRACT:
TECHNICAL FIELD
This invention relates to low noise solid state charge integrating detectors. More particularly, it relates to single channel ion and electron detectors and to ion and electron measuring array detectors.
BACKGROUND ART
Mass spectroscopy is just one of several analytical techniques which require ion or charged particle detectors. Other applications in which ion or charged particle detection is required include electron energy analyzers, electron capture detectors, flame ionization detectors, photoionization detectors, ion mobility spectrometers, smoke and particle detectors or any application in which ions in solutions are collected and measured. Typically in applications which require an array it is necessary to use a costly and complex micro channel plate, phosphor-fiber optic-photodiode array assembly to detect ions directly. In single channel applications it is possible to detect ions directly with a multiplier device such as an electron multiplier, a channel electron multiplier (CEM) or a discrete dynode electron multiplier. It is also possible to use a phosphor to convert ions to photons, and then detect them with a photomultiplier. A Faraday cup collector and an electometer may also be used.
Replacement of channel electron multipliers or other detectors in, for example, quadrupole mass spectroscopy and in other applications would be of value in providing cost savings and improved performance. Preferably a detector should be insensitive to vacuum quality and should not be adversely affected by exposure to atmosphere. Further, if at all possible, it should not require high voltages, should not exhibit mass discrimination, and should not respond to neutral particles or low energy photons.
U.S. Pat. No. 5,386,115 to Freidhoff et al. discloses a solid state mass spectrograph which includes an inlet, a gas ionizer, a mass filter and a detector array all formed within a cavity in a semiconductor substrate. The detector array is a linear array oriented in the dispersion plane of the mass filter and includes converging electrodes at the end of the cavity serving as Faraday cages which pass charge to signal generators such as charge coupled devices formed in the substrate but removed from the cavity.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a charge sensitive detector that exhibits high sensitivity and dynamic range and which is insensitive to the gas pressure in the detector's environment.
In accordance with a first embodiment of the invention, high sensitivity and dynamic range can be achieved that are comparable to those achievable with electron multipliers, but without the incorporation of such a multiplier. Hence, in many applications that utilizes such a multiplier for signal amplification, the multiplier can be replaced with a much more cost effective charge sensitive detector in accordance with the invention without suffering significant loss in performance, while also reducing the requirement for high vacuum that may have been imposed by the use of the multiplier. The use of the detector in accordance with the invention also removes the requirement for high voltage that is otherwise required for operation of the electron multiplier. Further, and perhaps more significantly, such performance is achievable in environments where poor vacuum, or high gas pressures, exists, i.e., where electron multipliers can not be used due to their inherent requirement for a good vacuum environment. Hence, much higher sensitivities can be achieved in such high pressure environments than was previously achievable. Further, charged particles of much higher mass can be detected. In addition a detector according to the invention is insensitive to neutral atoms or molecules.
Using an integrated chip metallization for the pickup electrode, if of the order of 10 mm in diameter, would result in very high electrode capacitance, even with the thickest dielectric layers available in device manufacture. This high capacitance would make it impossible to achieve very low read noise levels.
A way to avoid this problem is to use an isolated pickup electrode, which may be made in any number of ways: machined metal, punched or electroformed metal, molded conductive material, conductively plated molded material, vapor deposited material, etc. An important aspect of the invention is to support the pickup electrode at sufficient distance from surrounding conductors, such as the device substrate, to reduce the electrode capacitance to a low value of, typically 1 pF or less. It is also necessary that the supporting structures have extremely low conductance, typically 10
13
ohms, and that the dielectric constants and geometry of the supports be consistent with the low electrode capacitance to the surroundings.
If this is done, other problems arise. At the very low detection limits required, such structures will tend to be extremely sensitive to (a) microphonics and (b) stray electrostatic fields that are time-varying.
Microphonics, or induced voltage variations on the pickup electrode due to mechanical vibrations of the electrode or surrounding conduction structures, can be reduced or eliminated by (1) making the electrode and all surrounding structures very rigid and (2) by arranging for there to be no net charge on the pickup electrode. The vibration induced voltage on the electrode is proportional both to the vibration induced capacitance variation between the electrode and its surroundings, and to the charge on the electrode. By surrounding the electrode with a “Faraday Cage” biased to the potential of the pickup electrode, charge on the electrode is minimized. There are at least 2 options here: (1) Put a fixed bias on the Cage, or shield, which is nominally equal to the reset potential of the pickup electrode. This will result in immunity from microphonics at zero or very small ion currents, but as the signal increases, the electrode will acquire some charge toward the end of each integration cycle, and will be subject to microphonic noise. However, it is precisely at low signal levels that the lowest noise is required, so this is generally acceptable. Option (2) is to bootstrap the Faraday Cage potential to that of the electrode, tracking it during each integration. The effect is essentially to eliminate the effective capacitance of the pickup electrode, so that all charge accumulation is in the MOS circuit itself. This option is more complex, but offers potentially better performance and, if done carefully, permits larger detector areas to be realized.
The influence of stray AC fields can also be controlled by placing the pickup electrode within a cage, with apertures or grids provided to allow entrance of the ion flux which is to be measured. If the fields are very large, multiple layers of shielding may be required. The mean potential on the cage must be held within 1 mV or better of the optimum value, and AC components at frequencies which would interfere with the detection process must be held to microvolt levels or less.
It is also vital that any electrical leakage paths from the pickup electrode to any other conducting surfaces be minimized. In particular, the RC time constants should be kept 2-3 orders of magnitude higher than the desired integration cycle times, depending on the desired charge measuring accuracy. Small amounts of leakage may be dynamically calibrated away. In particular, any contamination of surfaces by material in the sample or sample beam, or sputtered from the detector assembly or elsewhere by sample ions, must not be allowed to form conductive paths. This goal can be achieved by appropriate geometrical design which physically shields critical surfaces from direct contamination.
In a second embodiment of the invention, charge supplied by an ion or electron current in vacuum or gas is deposited on an integrated electrode, typically of dimensions 10 um by 1000 um. This electrode is connected to a MOSFET circuit capable of resetting the voltage on the electrode to a preset value and then reading out the charge (voltage) on the electr
Crockett Michael I.
Patkin Adam J.
Tracy David H.
Welkie David G.
Berman Jack
PerkinElmer Instruments LLC
St. Onge Steward Johnston & Reens LLC
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