Abstract:
A cryogenic detector device includes a sensor based on a low-temperature effect and measures the temperature increase produced by the introduction of energy, such as an X-ray quantum. The smaller the thermal capacity of the sensor, the greater the temperature increase resulting from the introduction of energy and the higher the energy resolution of the sensor. Because the thermal capacity is temperature dependent, the sensor is operated in the range of comparatively small thermal capacities, i.e., in a range between 50 and 400 mK. Contrary to conventional assumptions, it was found that by keeping the three-dimensional size of the individual sensors sufficiently small and by increasing the effective sensor area, acceptable measurement results were achieved even at higher operating temperatures of the sensors in a range between 2.4 and 4.2 degrees Kelvin.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is filed under 35 U.S.C. §111(a) and is based on and hereby claims priority under 35 U.S.C. §120 and §365(c) from International Application No. PCT/EP2004/004080, filed on Apr. 16, 2004 and published as WO 2004/092770 A3 on Oct. 28, 2004, which in turn claims priority from German Application No. 103 17 888.0, filed on Apr. 17, 2003. This application is a continuation of International Application No. PCT/EP2004/004080, which is a continuation of German Application No. 103 17 888.0. International Application No. PCT/EP2004/004080 is pending as of the filing date of this application, and the United States is an elected state in International Application No. PCT/EP2004/004080. This application claims the benefit under 35 U.S.C. §119 from German Application No. 103 17 888.0, filed on Apr. 17, 2003, in Germany. The disclosure of each of the foregoing documents is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to cryogenic detector devices used in analytical applications for examining particles, radiation and fields. More specifically, the invention relates to a detector system with a sensor that is cooled by a mechanical cooler.  
       BACKGROUND  
       [0003]     Cryostats are used to cool sensors (often referred to as cryogenic detectors or cryodetectors) that are based on a low temperature effect These cryostats have typically included a first cooling means, as well as a second cooling means that is pre-cooled by the first cooling means. The sensor is thermally coupled to the second cooling means. In order to generate a temperature of approximately 4° K. (degrees Kelvin), the first cooling means typically consists of a coupled nitrogen/helium cooler. The second cooling means requires more space and involves more complex processes. The liquid coolant (nitrogen, helium) used by the first cooling means is both expensive and not available in every location. For this reason, the use of sensors based on a low-temperature effect is relatively expensive and generally unsuitable for industrial purposes.  
         [0004]     Another cooling means for a cryostat involves a refrigerating machine having the form of a pulse-tube cooler. Such a pulse-tube cooler is described in Info-Phys-Tech No. 6, 1996, from VDI Technologiezentrum, Physikalische Technologien. The pulse-tube cooler includes a pulse tube with a cold heat exchanger at one end and a hot heat exchanger at the other end. The cold head exchanger absorbs heat from the outside, and the hot heat exchanger releases heat to the outside. The refrigerating machine also includes a regenerator that serves as an intermediate heat reservoir, and a pressure oscillator that generates periodic pressure changes. At the end where the cold heat exchanger is located, the pulse tube is connected to the pressure oscillator. Lines connect the pulse tube to the pressure oscillator through the regenerator so that the working gas is periodically shifted between the pulse tube and the pressure oscillator.  
         [0005]     A cryodetector with a pulse-tube cooler is described in European Patent EP 1014056 A2. The cryodetector includes a first cooling means in the form of a two-stage pulse-tube cooler and second cooling means in the form of a demagnetization stage. A 3He/4He dilution refrigerator or a 3He cooler can be used for the second cooling means. Pre-cooling to approximately 4° K. is performed by the pulse-tube cooler. Further cooling to the operating temperature of the sensors based on a low-temperature effect in a range of 50-400 mK is performed by the second cooling means.  
         [0006]     The price of such a cryodetector device is considerable as a result of the complexity of the apparatus with the two cooling stages and extremely low operating temperatures. In addition, the diameter of the measuring probe is large to accommodate several infrared filters and shields around the probe, which are necessary due to the low operating temperatures.  
         [0007]     A cryogenic detector device is sought that is less complex and costly and that has a smaller measuring probe.  
       SUMMARY  
       [0008]     A sensor based on a low-temperature effect and employed in a cryogenic detector device measures the temperature increase produced by the introduction of energy, e.g., of an X-ray quantum. By keeping the thermal capacity of an individual sensor low, the energy resolution of the sensor is increased. The smaller the heat capacity of a sensor, the greater the temperature increase resulting from the introduction of energy and the higher the energy resolution of the sensor. The heat capacity is temperature dependent as opposed to constant. Thus, in order to achieve small heat capacities, the sensor is conventionally operated in a temperature range between 50 and 400 mK. But providing such low temperatures is very expensive and technically complicated.  
         [0009]     Contrary to conventional assumptions, it was found that by keeping the three-dimensional size and structural volume of individual sensors sufficiently small and increasing the effective sensor area, acceptable measurement results can still be achieved even at higher operating temperatures of the sensors in a range between 2.4 and 4.2 degrees Kelvin (° K.). Preferably, the operating temperature of the sensor is selected to be in a temperature range between 2.5 and 3.5 K. Even more preferred is a range between 2.6 and 2.9 K.  
         [0010]     By enabling the higher operating temperatures of the sensors, less absorptive infrared filters are necessary in the inlet window. As a result, the efficiency of the sensors increases. Because the absorption of the radiation to be detected scales exponentially with the thickness of the filters, the efficiency of the sensors is especially high when low energies of radiation are to be detected. Moreover, reducing the amount of filters allows the diameter of the probe of the cryogenic detector device to be smaller. The smaller probe allows the cryogenic detector device to get closer to the sample, for example, in an electron microscope, and to achieve increased efficiency.  
         [0011]     A cryogenic detector device includes an absorber that absorbs and thermalizes particles and radiation, and a sensor. The sensor is, for example, a phase transition thermometer and is positioned below or adjacent to the absorber.  
         [0012]     The higher operating temperature of the cryogenic detector device allows the cooling to be achieved with a single, one-stage or multi-stage mechanical cooler. A second cooling stage that must be pre-cooled is not required. Therefore, the refrigerating capacity of the mechanical cooler can be reduced by about a factor of five. This leads to a less expensive and more compact design.  
         [0013]     An X-ray lens is used as a means for enlarging the sensor area. This constitutes a uncomplicated and cost-effective way of increasing the active sensor area. As an alternative, the effective sensor area can also be increased using a sensor array comprised of a plurality of individual sensors. A one-stage or multi-stage pulse-tube cooler or a Gifford-McMahon cooler is used as the mechanical cooling means.  
         [0014]     Various devices can be used as the sensors. Superconducting tunnel diodes, magnetic calorimeters, resistance thermometers, and phase transition thermometers can all be used as the sensors. Phase transition thermometers are preferred because the measurement of introduced energy can be detected directly through the increase of the temperature.  
         [0015]     A temperature compensator stabilizes the operating temperature of the sensors. This is particularly advantageous when a pulse-tube cooler is used as the mechanical cooler. The final temperature in the range around 2.5 K generated by the pulse-tube cooler fluctuates slightly owing to the gas pulses. The temperature compensation can be achieved passively through limited thermal coupling between the mechanical cooler and the sensors, or actively through a heater element that stabilizes the sensors to an operating temperature T S &gt;T minK .  
         [0016]     A pre-amplifier is held at the same temperature as the sensors. This allows the length of the measurement signal lines between the sensors and the pre-amplifier to be reduced. The pre-amplifier and the sensors are part of the same integrated circuit. Thus, measurement errors that result from exposed measurement signal lines between the sensors and the pre-amplifier, such as bonding wires, are avoided.  
         [0017]     Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.  
         [0019]      FIG. 1  is a schematic diagram of a detector system according to an embodiment of the invention.  
         [0020]      FIG. 2  is a schematic diagram of a detector system according to another embodiment of the invention.  
         [0021]      FIG. 3  is a diagram of a configuration of a sensor and a pre-amplifier that may be employed in the detector systems of  FIGS. 1 and 2 .  
         [0022]      FIG. 4  is a diagram of another configuration of a sensor and a pre-amplifier that may be employed in the detector systems of  FIGS. 1 and 2 .  
         [0023]      FIG. 5  is a diagram of an X-ray lens used to enlarge the area of the sensor of  FIGS. 3 and 4 .  
         [0024]      FIG. 6  is a diagram of a sensor matrix, including a plurality of individual sensors, used to enlarge the area of the sensor of  FIGS. 3 and 4 .  
         [0025]      FIG. 7  is a side view of the structure of a cryogenic detector device that may be employed in the detector systems of  FIGS. 1 and 2 .  
         [0026]      FIG. 8  is a diagram of yet another structure of a cryogenic detector device that may be employed in the detector systems of  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION  
       [0027]     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0028]      FIG. 1  shows a detector system  9  with a cryogenic detector device  10  including a sensor  12  that is based on a low-temperature effect and that includes an active sensor area. Sensor  12  can be a superconducting tunnel diode, a magnetic calorimeter, a resistance thermometer, or a phase transition thermometer. Phase transition thermometers are preferred because the measurement of introduced energy can be detected directly through an increase in temperature. Depending on the device used for sensor  12 , sensor  12  detects particles, radiation or fields. Cryogenic detector device  10  is connected to a pre-amplifier  16  through measurement signal lines  14 . Pre-amplifier  16  and cryogenic detector device  10  are thermally coupled to mechanical cooler  20 . Pre-amplifier  16  and cryogenic detector device  10  are arranged on a cold plate  22  having the minimum cooling temperature T minK . The thermal coupling between cold plate  22  on one side and cryogenic detector device  10  and pre-amplifier  16  on the other side is relatively limited. A temperature compensator  24  or temperature buffer is placed between cold plate  22  on one side and cryogenic detector device  10  and pre-amplifier  16  on the other side.  
         [0029]     In one embodiment, mechanical cooler  20  is a two-stage pulse-tube cooler. Temperature compensator  24  compensates and equalizes the slight fluctuations of the minimum cooling temperature T minK  caused by the gas pulses of the pulse-tube cooler. Temperature compensator  24  reduces the fluctuations of the minimum cooling temperature T minK  such that the operating temperature of sensor  12  stays within a range of +/−100 mK. In a preferred embodiment, temperature compensator  24  reduces the fluctuations of the minimum cooling temperature T minK  to stabilize the operating temperature to within a range of +/−1 mK.  
         [0030]     Sensor  12  outputs measurement signals onto the measurement signal lines  14 . Pre-amplifier  16  amplifies the measurement signals output by sensor  12 . Mechanical cooler  20 , pre-amplifier  16  and cryogenic detector device  10  are arranged in a vacuum vessel  30 . The particles, radiation or fields  31  to be measured or detected enter detector system  9  through a first inlet window  32  in vacuum vessel  30 . Pre-amplifier  16  is held at the same temperature as sensor  12 . This allows the length of the measurement signal lines  14  between sensor  12  and pre-amplifier  16 , which are sensitive to disturbances, to be reduced.  
         [0031]     A SQUID (Superconducting QUantum Interference Device) or a SQUID array is used as pre-amplifier  16 . The SQUID or SQUID array is adapted as a pre-amplifier in a low-impedance electrical circuit. The temperature change of the thermometer is evident from the change of resistance that produces a change of current in the exciting coil of the SQUID. The voltage signal generated by the SQUID is then the starting point for determining the energies of the incident particles or radiation. Pre-amplifier  16  and sensor  12  can be part of an integrated circuit and arranged on a common chip. Thus, measurement errors owing to exposed measurement signal lines between sensor and pre-amplifier, such as bonding wires, are avoided. In the case where the pre-amplifier  16  and the sensor  12  are on separate substrates, the measurement signal lines  14  are rigidly connected to cold plate  22  in order to avoid disturbances of the measurement results resulting from mechanical movement of the measurement signal lines  14 .  
         [0032]     Pulse-tube cooler  20  includes a first stage  202  and a second stage  204 . First stage  202  provides a cooling temperature of approximately 70 degrees Kelvin (K). Second stage  204  then provides the minimum cooling temperature T minK  in a range between 2.4 and 4.0 K. First stage  202  includes a first pulse tube  206  and a first regenerator  208 . Second stage  204  includes a second pulse tube  210  and a second regenerator  212 . Second cooling stage  204  includes a 77-K shield  214 . In another embodiment, mechanical cooler  20  is a one-stage or multi-stage Gifford-McMahon cooler.  
         [0033]     The 77-K shield  214  protects cold plate  22  from “warm” infrared radiation. Thus, cold plate  22  is exposed only to “cold” radiation below 77 K. A second inlet window  216  is located in the 77-K shield  214  between cryogenic detector device  10  and first inlet window  32 . The particles, radiation or fields  31  to be detected arrive at cryogenic detector device  10  after passing through second inlet window  216 . For additional information on the construction and the operation of a pulse-tube cooler, see Info-Phys-Tech No. 6, 1996, from VDI Technologiezentrum, Physikalische Technologien.  
         [0034]     The active sensor area of sensor  12  is less than 50,000 μm 2 . In comparison to conventional sensors having an area of approximately 80,000 μm 2 , the smaller area of sensor  12  of cryogenic detector device  10  results in a reduced thermal capacity, which in turn allows the operating temperature to be increased to the desired range. When the structural volume of sensor  12  and the size of the active sensor area is reduced to enable an operating temperature range between 2.6 and 2.9 K, the active sensor area would no longer be sufficient to achieve reasonable measurement results without some additional improvements. Therefore, a sensor area enlargement means is provided to make a sufficiently large effective sensor area available. The sensor area enlargement means can be implemented, for instance, through an X-ray lens or through a sensor matrix having a plurality of individual sensors. Other implementations of the sensor area enlargement means are also possible. An X-ray lens is used, for example, where detector system  9  detects X-ray radiation. In another implementation, a sensor with a limited volume but with an increased effective sensor area measures energy in a scanning electron microscope at an operating temperature between 2.4 and 4.2 K.  
         [0035]      FIG. 2  shows a second embodiment of detector system  9  in which pre-amplifier  16  is not located inside vacuum vessel  30 . In this embodiment, pre-amplifier  16  is outside vacuum vessel  30  and is coupled to cryogenic detector device  10  by the measurement signal lines  14 . In this embodiment, the measurement signal lines  14  are rigidly attached to mechanical cooler  20  so as to avoid mechanical vibrations of the measurement signal lines  14  and disturbances of the measurement signal.  
         [0036]     As in the embodiment of  FIG. 1 , temperature compensator  24  is placed between cold plate  22  and cryogenic detector device  10 . In the embodiment of  FIG. 2 , however, temperature compensator  24  is an active heating means  240 . The heat provided by active heating means  240  slightly increases the minimum cooling temperature T minK  and thus stabilizes the operating temperature range to a particular temperature. The operating temperature of the cryogenic detector device is stabilized slightly above the minimum cooling temperature T minK . In the embodiment of  FIG. 1 , it is also possible to achieve active temperature compensation using active heating means  240  instead of achieving the passive temperature compensation provided by limited thermal coupling. It is also possible to apply passive temperature compensation through limited thermal coupling in the embodiment of  FIG. 2 .  
         [0037]      FIG. 3  shows one configuration of cryogenic detector device  10  and pre-amplifier  16 . Sensor  12  includes an active sensor area  121  on a sensor semiconductor substrate  124 . Measurement signal lines  126  originate from active sensor area  121  and lead to bonding pads  128 . Pre-amplifier  16  includes electronic circuits  164  on a pre-amplifier semiconductor substrate  162 . Thus, both sensor  12  and pre-amplifier  16  are in the form of integrated circuits. Measurement signal supply lines  166  originate from the electronic circuits  164  and lead to bonding pads  128 . The two semiconductor substrates  124  and  162  are arranged in the immediate vicinity of each other so that the distance between the bonding pads  128  on the two semiconductor substrates  124  and  162  is as small as possible. The bonding pads  128  are electrically connected to each other through bonding wires  130 . Thus, the measurement signal lines  14  of  FIG. 1  are formed by the measurement signal connection lines  126 , the bonding wires  130 , and the measurement signal supply lines  162 . This configuration of cryogenic detector device  10  and pre-amplifier  16  allows the length of the measurement signal lines  14  that is exposed and thus capable of vibration to be reduced. The bonding wires  130  correspond to the exposed portion of the measurement signal lines  14 .  
         [0038]      FIG. 4  shows another configuration of pre-amplifier  16  and cryogenic detector device  10 . The length of the measurement signal lines  14  that is exposed and thus capable of vibration can be eliminated altogether by avoiding the short bonding wires  130  of the configuration of  FIG. 3 . In  FIG. 4 , both sensor  12  and pre-amplifier  16  are located on one integrated circuit on a common semiconductor substrate  170 . In this configuration, the measurement signal lines  14  are implemented as traces or conductor lines of the integrated circuit.  
         [0039]      FIG. 5A  shows a sensor area enlargement means  122 . The structural volume of sensor  12  and the active sensor area  121  are reduced to enable an operating temperature range between 2.4 and 4.2 K. By using sensor area enlargement means  122 , an operating temperature range between 2.5 and 3.5 K can be achieved, and a range between 2.6 and 2.9 K was preferred.  FIG. 5A  shows an X-ray lens  180  used as sensor area enlargement means  122  in the detection of X-ray radiation. The X-ray radiation from a radiation source  190  is focused by X-ray lens  180  onto sensor  12  of cryogenic detector device  10 . The dispersion angle of radiation source  190  using X-ray lens  180  covers and area greater than active sensor area  121 .  
         [0040]      FIG. 5B  shows in comparison the smaller dispersion angle of radiation source  190  in a configuration of detector system  9  that does not use an X-ray lens. The typical amplification factors achievable with X-ray lenses are between 10 and 100.  
         [0041]      FIGS. 6A and 6B  show an alternative sensor area enlargement means  122 . The sensor area enlargement means  122  is implemented as a sensor matrix  182  having a plurality of individual sensors  12 - i . The individual sensors are arranged in a matrix of rows and columns. The overall effective sensor area of sensor matrix  182  is the sum of the active sensor areas  121  of the individual sensors  12 - i .  FIG. 6A  also shows that the dispersion angle of radiation source  190  that can be sensed is increased. This means that sensor matrix  182  allows detector system  9  to detect incident radiation  31  that exhibits a greater dispersion angle.  
         [0042]      FIG. 7  is a side view of the structure of one embodiment of cryogenic detector device  10 . Cryogenic detector device  10  includes an absorber  220  and sensor  12 . Sensor  12  is a phase transition thermometer  222  in this embodiment. Absorber  220  which is located on sensor  12  which is based on a low-temperature effect. Absorber  220  and phase transition thermometer  222  are arranged on a membrane  224 , which in turn is thermally coupled to sensor semiconductor substrate  124 . Phase transition thermometer  222  is electrically coupled to the measurement signal lines  14 . An aperture  226  is located adjacent to absorber  220  and collimates the incident radiation, particles or fields  31  onto the absorber  220 . Aperture  226 , preferably surrounded by platinum, thus defines active sensor area  121 .  
         [0043]      FIG. 8  is a side view of the structure of another embodiment of cryogenic detector device  10 . Unlike the structure of  FIG. 7 , the embodiment of  FIG. 8  lacks and absorber. In this structure, the incident radiation, particles or fields  31  are absorbed directly by the phase transition thermometer  222  and thermalized.  
         [0044]     Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.