PHOTOMULTIPLIER

Abstract: A photomultiplier device is made from a number of layers (2, 4, 6) bonded together. The layers include a photocathode layer (2) which emits photoelectrons when a photon is incident upon it, an electron multiplying layer (4) which multiplies the electrons emitted from the photocathode (2), and an anode layer (6) upon which the electrons emitted from the electron multiplying layer (4) are incident. The different layers (2, 4, 6) may be spaced from each other by spacer members (3, 5) and the cavity between the layers (2, 4, 6) evacuated. The photocathode layer (2) may be formed from a photoresponsive coating on a transparent substrate (1). The electron multiplying layer (4) may be a microchannel plate. The different layers (2 4, 6) may be in the form of wafers, in which case the manufacture of the device includes a step of dicing the wafers after the layers have been bonded together to form a plurality of discrete devices.

Description: PHOTOMULTIPLIER Technical Field of the Invention The present invention relates to a photomultiplier device. Background Art In a conventional photomultiplier tube, photons impinge upon a photocathode, which usually includes a photosensitive alkali metal, which emits photoelectrons in response to the incident photons. The photoelectrons strike a first dynode, which emits secondary electrons, thereby multiplying the original photoelectrons. The emitted electrons impinge on subsequent downstream dynodes, each emitting secondary electrons, thereby further multiplying the electrons. The potential difference between adjacent dynodes is typically 100 V, and this multiplies the electrons by a factor of around 10 for each dynode. The electrons emitted from the array of dynodes are detected by an anode, which gives an output which is a function of the intensity of the radiation incident upon the photocathode. All of the components are provided in a vacuum envelope, normally made of metal, ceramic or glass, which includes a transparent face through which photons pass to the photocathode. The material from which the photocathode is made depends on the radiation which is to be detected, for example whether the photomultiplier is to detect visible light, infra-red, ultra-violet, X-ray or gamma-ray radiation. Typical photo ultipliers have a gain of about l million, although this can be increased by the provision of additional dynodes. The problem with conventional photomultiplier tubes is their large size. Accordingly photodiodes and other solid state detectors have been increasingly used. These devices have the advantage of being smaller, less expensive, and do not need as high a voltage as photomultiplier tubes, however they are not as sensitive as photomultiplier tubes unless they are cooled to cryogenic temperatures. Summary of the Invention According to a first aspect of the present invention, a photomultiplier device comprises a plurality of layers bonded together, the layers including a photocathode layer for emitting photoelectrons when a photon is incident upon it, an electron multiplying layer for multiplying the electrons emitted from the photocathode, and an anode layer upon which the electrons emitted from the electron multiplying layer are incident. By forming the photomultiplier device from a plurality of layers bonded together, rather than from discrete components provided in a vacuum envelope as with conventional devices, the device can be miniaturized. Further, microengineering techniques can be used in the manufacture of the device, making production costs similar to those for other solid state devices. It is preferred that the photocathode layer, the electron multiplying layer and the anode layer are separated from each other by spacer members, and that the cavity between the layers is evacuated. This gives improved transmission of electrons between the layers. The photocathode layer is preferably formed by coating a photoresponsive layer on a transparent substrate layer. The transparent substrate layer forms a window through which photons pass to the photocathode. In forming the photocathode layer, it is beneficial to etch discrete window portions in the transparent substrate before coating the substrate with the photoresponsive material. After forming the photocathode layer, the coating may be etched to remove selectively the photoresponsive material and allow bonding of spacers or the electron multiplying layer to the substrate layer. Where the device is to be used in a vacuum, there is no requirement for a window, and the photocathode may be coated directly onto the electron multiplying layer. The electron multiplying layer is advantageously a microchannel plate. This has the advantage of being thin and flat compared to an array of dynodes. A plurality of electron multiplying layers may be provided in series to increase the amplification of electrons. It is preferred that the photocathode layer, the electron multiplying layer and the anode layer are each formed on or from a wafer which, when bonded together, may be diced to form a plurality of discrete devices. It is advantageous for the anode layer to be a resistive anode or a segmented anode. This allows the photomultiplier to give positional information relative to the incident radiation. According to a second aspect of the present invention, a method of manufacturing a photomultiplier device comprises forming a photocathode layer for emitting photoelectrons when a photon is incident upon it, an electron multiplying layer for multiplying electrons emitted by the photocathode layer, and an anode layer, and bonding the plurality of layers together. Preferably, the photocathode layer is formed by coating a photoresponsive material on a transparent substrate, the photocathode layer subsequently being bonded to one face of the electron multiplying layer. Optionally, the photocathode layer may be bonded to a spacer which is in turn bonded to the electron multiplying layer, and the electron multiplying layer may be bonded to a further spacer which is in turn bonded to the anode layer. By carrying out at least the last bonding step in a vacuum, this creates evacuated cavities between the layers of the device which improve the device characteristics. It is preferred that the transparent substrate, the electron multiplying layer, and the anode layer are in the form of wafers, in which case the method may include the additional step of dicing the wafers after bonding to form a plurality of discrete devices. Brief Description of the Drawings Figure 1 shows a first example; Figure 2 shows a second example; Figure 3 shows a third example; and Figure 4 shows a resistive anode for use in any of the examples. Detailed Description Figure 1 shows a first example of a photomultiplier device according to the present invention. A transparent substrate 1 of glass, quartz or sapphire is provided, and is etched to form a number of window recesses. The recesses are formed by a dissolution in a liquid or flux of energetic or reactive atoms, or ions. A photoresponsive material 2 is then coated in each window recess. The photocathode material 2 may be, for example gallium arsenide, indium phosphide, or other mixture of alkali metals and their compounds which emit electrons when radiation is incident upon them. If necessary, the photocathode material 2 may be selectively etched to allow for bonding of spacer members 3 to the transparent substrate 1. The spacer members 3 are bonded using any suitable bonding method, for example by adhesive, fusion, solder or anodic bonding. A microchannel plate 4 is bonded to the spacer members 3 by a suitable bonding method in spaced apart, confronting relation to the photocathode 2. The microchannel plate is conventional in construction and comprises an array of channels, each having a diameter of around lOμm. Each channel has a semiconductor photosensitive inner surface. An electric field is applied along the length of each channel. When a photon is incident on the semiconductor surface, secondary electrons are emitted. These secondary electrons impinge on the semiconductor surface, causing further secondary electrons to be emitted, causing multiplication of the photoelectron. Appropriate microchannel devices are available commercially from Hamamatsu Photonics KK (Shizuoka-Ken, Japan) and others. The channels of the microchannel plate may be provided at an angle with respect to the face of the plate. This prevents positive ions from residual gases in the photomultiplier from being accelerated along the channels, and causing stray electron emissions. Spacer members 5 are bonded to the face of the microchannel plate 4 opposite the photocathode 2, and an anode layer 6 is bonded onto these spacers 5. The volumes between the photocathode 2 and the microchannel plate 4, and between the microchannel plate 4 and the anode 6 are evacuated. The bonded layers are cut, or diced, perpendicular to the layers to divide the layers into a plurality of discrete devices. In use, photons are transmitted through the window 1, and onto the photocathode layer 2. The photocathode layer 2 emits photoelectrons in response to the photons incident upon it, and these are transmitted to the microchannel plate 4. The microchannel plate multiplies the photoelectrons striking it, and transmits electrons towards the anode layer 6, where they are detected. The photocathode 2 , microchannel plate 4 and anode layer 6 are all biased with a voltage to ensure the electrons generated are accelerated towards the anode layer 6. Figure 2 shows a second example of the present invention. In this example, the photocathode layer 2 is bonded directly onto one face of the microchannel plate 4, and the anode layer 6 is bonded directly onto the opposite face of the microchannel plate 4. In this case, the transparent substrate 1 is not etched to form recessed before the formation of the photocathode layer 2. In both examples of the present invention, the transparent substrate 1 can be omitted, providing the device is to be used in a vacuum. To improve the sensitivity of the device, a plurality of microchannel plates may be bonded together in series. Such a device is shown in Figure 3. For positional detection of incident radiation, a segmented anode may be provided. In this case, the voltage of each segment can be monitored to identify the position. Alternatively, as shown in Figure 4, a resistive anode 10 can be used. Such an anode 10 is a resistive sheet connected to a positive supply via resistors R, at various locations on the anode 10. When electrons are incident on the anode 10 at a point 11, current radiates from the point of incidence 11. The current flows uniformly in all directions, and experiences a voltage drop due to the resistive anode 10 depending on the distance it flows. Therefore, the voltage across each resistor R will depend on the distance of the resistor from the point of incidence 11, the greater the distance the lower the voltage. By monitoring the voltage across at least two resistors, the point of incidence can be determined. Alternatively, position detection may be achieved by providing an array of detectors. The array may be a one or two dimensional array.

Claims: CLAIMS 1. A photomultiplier device comprising a plurality of layers bonded together, the layers including a photocathode layer (2) for emitting photoelectrons when a photon is incident upon it, an electron multiplying layer (4) for multiplying the electrons emitted from the photocathode (2) , and an anode layer (6) upon which the electrons emitted from the electron multiplying layer (4) are incident. 2. A device according to claim 1, in which the electron multiplying layer (4) and the anode layer (6) are separated from each other by spacer members (5) and the cavity between the layers is evacuated. 3. A device according to claim 1 or 2 , in which the electron multiplying layer (4) is a microchannel plate. 4. A device according to claim 3, in which the electron multiplying layer (4) comprises a plurality of microchannel plates. 5. A device according to claim 3 or 4, in which the channels of the or each microchannel plate (4) extends at an angle with respect to the face of the plate (4) . 6. A device according to any one of the preceding claims, in which the photocathode layer (2) comprises a photoresponsive material coated on a transparent substrate (1) • 7. A device according to claim 6, in which the photocathode layer (2) includes a plurality of discrete window portions formed by etching the transparent substrate (1) . 8. A device according to any one of the preceding claims, in which the anode layer (6) is a resistive or segmented anode arranged to give positional information relating to the incident radiation. 9. A method of manufacturing a photomultiplier device comprising the steps of : forming a photocathode layer (2) for emitting photoelectrons when a photon is incident upon it; forming an electron multiplying layer (4) for multiplying electrons emitted by the photocathode layer (2); forming an anode layer (6) ; and, bonding the plurality of layers (2,4,6) together. 10. A method according to claim 9, in which the step of forming the photocathode layer (2) includes the step of coating a photoresponsive material on a transparent substrate (1) , the photocathode layer (2) subsequently being bonded to one face of the electron multiplying layer (4) . 11. A method according to claim 9 or 10, in which the photocathode layer (2) is bonded to a spacer (3) which is in turn bonded to the electron multiplier layer (4) . 12. A method according to any one of claims 9 to 11, in which the electron multiplying layer (4) is bonded to a spacer (5) which is in turn bonded to the anode layer (6). 13. A method according to claims 11 or 12, in which the or each bonding step is carried out in a vacuum, thereby creating evacuated cavities between the layers (2,4,6) of the device. 14. A method according to any one of claims 9 to 13, in which the transparent substrate (1) , the electron multiplying layer (4) , and the anode (6) layers are in the form of wafers, the method further comprising a step of dicing the wafers after bonding to form a plurality of discrete devices.

Inventors: SKARDA VLADIMIR, WESTLAND DUNCAN JAMES, SKARDA, VLADIMIR, WESTLAND, DUNCAN, JAMES
Applicants: ERA PATENTS LTD, SKARDA VLADIMIR, WESTLAND DUNCAN JAMES, ERA PATENTS LIMITED, SKARDA, VLADIMIR, WESTLAND, DUNCAN, JAMES
Application year: 1996
Main classification: H01J43
IPC codes: H01J43