Abstract:
An array of ion detectors ( 30 ) comprising a plurality of pickup electrodes ( 20, 34 ) for receiving ions; a substrate ( 32 ); a plurality of insulators ( 22, 35 ) positioned respectively between said pickup electrodes ( 20, 34 ) and said substrate ( 32 ); a plurality of charge storage areas ( 12, 38 ) for storing charge received by said pickup electrodes ( 20, 34 ), wherein each area ( 12, 38 ) is connected to a particular pickup electrode ( 20, 34 ) and means ( 44 ) for determining the amount of charge collected by each charge storage area.

Description:
This application claims benefit to Provisional Application 60/015,303 filed Apr. 12, 1996. 
    
    
     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&#39;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 electrode after a set integration time. The capacitance of the electrode and its associated circuit (FET gate, etc.) is typically 0.5 to 1 pF. A read noise level as low as several electrons is achieved. Alternatively CCD circuits can be used to detect the charge and provide signals indicative of the amount of charge present on the electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged perspective view of a single channel detector in accordance with the invention. 
     FIG. 2 is a further enlarged cross-sectional view of the detector of FIG.  1 . 
     FIG. 3 is an enlarged perspective view of an array detector in accordance with the invention. 
     FIG. 3A is a further enlarged portion of the array detector of FIG.  3 . 
     FIG. 4 is an enlarged schematic cross sectional view of the array of FIG.  3 . 
     FIG. 5 is a simplified schematic diagram of the array of FIG.  3 . 
     FIG. 6 is a diagram of the manner in which a large array may be constructed from several small arrays. 
     FIG. 7 is a block diagram of a spectrometer using an array detector in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows schematically a first embodiment of the invention in the form of a charge integrating detector chip shown generally as  10  formed on a silicon substrate  11  with an integrated charge detection circuit  12  having a small exposed bonding signal pad  14  for signal pickup. There are also a number of normal bonding pads  16  for external connections to the chip. A large area metallization region, or other conductive surface, is provided as the bottom part of a Faraday shield. It may be interrupted by the circuit traces, or it may be a second metallization covering substantially all the chip area except for the signal pad  14 . A pickup electrode  20  is shown suspended above the chip, with a bonding wire  22  attaching it to the signal node. Supports for the pickup electrode  20  and a top shield are not shown in FIG.  1 . 
     The pickup electrode  20  may be in the form of a flat plate of any shape, a cup, or of other suitable form. To reduce scattering and secondary electron emission, it may have a multiwell configuration, such as a honeycomb, so that ions are actually trapped within deep columnar structures and any scattered or secondary ions will require multiple bounces to escape. 
     FIG. 2 shows a cross section of the complete detector, including the electrode supports  22  and the top shield  24 , which is electrically connected to the bottom shield metallization region  18 , which, as indicated previously, is electrically biased to a potential very near the signal node potential (the potential of pad  14 ). Electrode supports  22  may be of many forms: The electrode  20  may be supported by one or more globs of low dielectric constant, highly insulating polymer or other material, such as an open cell foam adhesive. Alternatively, electrode  20  may be supported by three or more low cross section solid dielectric legs or supports, or by a sufficiently thick (many mm) solid dielectric layer. The dimensions shown, an 8 mm diameter pickup located 2 mm from both the top and bottom of the Faraday Cage, result in a pickup capacitance of approximately 0.5 pF. The dimensions can be adjusted to achieve lower capacitance, or similar capacitance with larger pickup area. 
     Referring to FIG. 3, the principles of the present invention are applied to a second embodiment of the invention in the form of an integrated array detector shown generally as  30 . Array detector  30  is formed on a silicon substrate  32 . The array includes numerous detector elements  34  which may be formed of aluminum, or other conductive material, or by reactive ion etching of a more refractory material such as tungsten which is resistant to sputtering. As may be seen in FIG. 3A, each detector element  34  is connected by an extension  36  of its metalization to a charge storage circuit  38 . The array elements may be of any shape,and spaced uniformly or non-uniformly. Charge storage circuit  38  may be a MOSFET circuit or a CCD circuit as described above. 
     The output of each charge storage circuit  38  is connected to an output line  40 . Lines  40  form a signal bus  41 . The voltage output of each line  40  is multiplexed with a multiplexer circuit (not shown in FIG. 3) to an analog to digital converter  42  and eventually sent to a computer  44  which acquires, stores and analyzes the data obtained by array detector  30 . 
     Referring to FIG. 4, each detector element  34  has an insulating material  35  under it and therefore disposed between detector element  34  and substrate  32  to prevent voltage breakthrough and to minimize the capacitance to substrate  32 . Insulating material 35 may be, for example, a 3 μm thick layer of silicon dioxide. Other thicknesses or materials such as silicon nitride or a polyimide may also be used, alone or in combination. 
     Each detector element  34  may have a length of two millimeters, be spaced 12.5 μm center to center with a 2 μm gap between adjacent elements  34 . A typical array may include 1,024 electrodes. However, smaller or larger numbers may be present and an array may include 4,000 or more detector elements. 
     Each charged storage circuit  38  may be operated in a quadruple correlated sampling (QCS) or double correlated double sampling (DCDS) mode. Thus, the stored voltage is measured before sampling when no signal has been accumulated and after sampling. Each measurement is further compared to a first reference (the reset voltage) to eliminate both thermodynamic (kTC) ½  reset noise and 1/f amplifier noise. Thus, four measurements are taken: 
     (1) While reset switch is on (clamped to bias reference) 
     (2) After reset 
     (3) At end of integration 
     (4) During reset clamp 
     The difference between measurements (1) and (2) is the kTC reset noise q v . The difference between measurements (3) and (4) is the final charge Q F . 
     Thus, the detected charge is: 
     
       
         Q D =Q F −Q N =( 3 )−( 4 )−[( 2 )−( 1 )] 
       
     
     Measurement ( 4 ) can be used as measurement ( 1 ) for next integration interval. 
     Referring to FIG. 5, a multiplexing circuit  48  connected to signal bus  41  is used to successively read out the voltages stored on charge storage circuits  38  by supplying the output voltage to A to D converter  42 . 
     Referring to FIG. 6, a plurality of array detectors  30 A,  30 B,  30 C and  30 D are tiled together to form an extended array. Each detector is shown in plan view and the beam of ions is perpendicular to the plane of the figure. The ion beam is assumed to be at least twice as wide as the detectors so that one half of the beam impinges on detectors  30 A and  30 C while the other half impinges on detectors  30 B and  30 D. This results in a reduction in sensitivity by a factor of approximately two. However, this arrangement avoids the “black line” problem of having spaces in the extended array so that when it is used as a detector in an image plane application there are locations at which no charged particles are detected. 
     Referring to FIG. 7, a Mattauch-Herzog mass spectroscopy system (which may be combined with a gas chromagraph) utilizes a detector  50  in accordance with the invention placed in an image plane  51 . The signals from the elements of detector  50  are provided to an image readout circuit  52  analogous to the analog to digital converter circuit  42  described above. A computer  54  is analogous to computer  44  previously described, but may also be used to control other functions within the spectrometer such as the Z-lens voltage circuit  56 . 
     As is well known in the art, computer  54  may also control an ion source voltage circuit  56  and an electrostatic analyzer voltage circuit  58 . In a mass spectrometer the electrostatic analyzer provides a charged particle beam of relatively constant energy so that subsequent sorting by momentum will translate into sorting by mass as energy is held substantially constant. 
     Other signals may be exchanged between computer  54  and a mechanical roughing pump  60  which backs a turbomolecular pump  62  which also interchanges signals with computer  54 . However, if the detector  50  in accordance with the invention is used, in many applications turbomolecular pump  62  will not be necessary. 
     Pump  60  (and possibly turbomolecular pump  62 ) evacuate a chamber  64  which includes an ion source  66 . The ions may be those eluted from the column of a gas chromatograph  68 . These ions pass through an object slit  70  and form an ion beam shown generally as  72 . Beam  72  passes through an electric sector  74  and a Z focus lens  76  before entering a magnetic sector  78 . In magnetic sector  78  the ions are dispersed according to the square root of their mass thus producing a spectrum of mass versus position in the image plane  51 . 
     The detector  50  of the present invention has a dynamic range of approximately six orders of magnitude. Greater dynamic range can be achieved by modulation of the ion current or by changing the rate of readout which may be in the order of 100 times per second, but can be varied depending upon the application. Further, the detector according to the invention is sensitive to a mass range of 1 to at least 1,000 atomic mass units, but in principle the mass range may be especially unlimited. 
     Various engineering considerations will occur to those skilled in the art. For example, those portions of the array associated with on-chip charge storage transfer, applification and digitization should be shielded from ion and photon bombardment. This can be accomplished by suitable passivation and metalization coatings or external shields. Further, charge buildup between electrodes and other metalizations can be minimized by the use of suitable guard rings. Finally an array in accordance with the invention can be mounted in an integrated circuit chip package for ease of handling or onto a custom purpose package for ease of positioning in the image plane  51 .