Cryogenic temperature sensor and cryogenic temperature measurement arrangement for temperature measurement in the cryogenic temperature range

Abstract

A cryogenic temperature sensor (01) for temperature measurement in the cryogenic temperature range, comprising a microtechnologically fabricated primary measuring resistor (03) and a secondary semiconductor measuring resistor (04) arranged on a common sensor carrier (02) and whose temperature-dependent resistance values ​​can be determined by a measuring circuit, wherein the sensor carrier (02) is formed by a dopable silicon chip, on one side of which the doped semiconductor measuring resistor (04) and on the other side of which the primary measuring resistor (03) are formed, and wherein the semiconductor measuring resistor (04) has an extremum in its resistance-temperature characteristic curve in the cryogenic operating range of the temperature sensor that can be set in the manufacturing process and which can be used as a sensor-specific fixed point for in-situ calibration of the primary measuring resistor (03).

Description

[0001] The invention relates to a cryogenic temperature sensor for measuring temperature in the cryogenic temperature range and a cryogenic temperature measurement arrangement using this sensor.

[0002] More recently, there has been an increasing number of technical applications that require very low ambient temperatures, which must be measured precisely.

[0003] Cryogens, or cryoliquids, are materials whose boiling point is at very low temperatures (helium 4.2 K; hydrogen 20.3 K; nitrogen 77.3 K; argon 87.1 K; oxygen 90.2 K). Solid cryogens also exist, such as frozen carbon dioxide with a sublimation point of 194.5 K. Cryogens are used, for example, to freeze organic materials, to cool superconductors, or as propellant for rocket engines. Cryoliquids are used in research laboratories and technical equipment to provide extremely low-temperature environments.

[0004] In connection with the present invention, a cryogenic temperature range is considered to be a range below 200 K, preferably below 100 K, particularly preferably below 70 K.

[0005] For precise temperature measurement, different sensors are used in state-of-the-art technology, depending on the accuracy requirements and temperature range. Common temperature sensors are often based on platinum resistance thermometers, which, according to DIN 60751, can be used for temperature measurement down to a minimum of approximately 73 K.

[0006] Below this temperature, the sensitivity of platinum measuring resistors is low, so they are only suitable to a limited extent for accurate, high-resolution temperature measurement.

[0007] An additional problem in the cryogenic temperature range is that platinum resistance thermometers must be calibrated for accurate measurements in this range, but readily available fixed points for such calibration are not readily available. The standardized fixed points of the International Temperature Scale (ITS-90) are formed by phase transition points of various metals. The Provisional Low Temperature Scale (PLTS-2000) serves as an extension of the ITS-90 into the low-temperature range and covers a range from 0.902 mK to 1 K. The PLTS-2000 uses the relationship between the melting pressure of the helium-3 isotope and temperature; this requires significant technical effort and can therefore only be implemented in a laboratory setting. In the cryogenic temperature range, such fixed points are hardly feasible in industrial applications.

[0008] EP 3 566 033 B1 describes a device for determining and / or monitoring the temperature of a medium. The device comprises a temperature sensor, a reference element for in-situ calibration and / or validation of the temperature sensor, and an electronic unit. The reference element consists at least partially of a material that exhibits a phase transition at a predetermined phase transition temperature within the temperature range relevant for calibrating the temperature sensor.

[0009] RU 2 602 400 C1 describes a measuring device that can be used for cryogenic temperatures. The measuring device comprises a reference resistor and a current source connected to the current input of the reference resistor. The resistance is measured in a 4-wire circuit under a constant current.

[0010] German patent application DE 10 2014 225 897 B3 describes a low-hysteresis, high-temperature sensor. This sensor consists of a thin, meandering conductor layer applied to a ceramic substrate. The conductor layer comprises two individual meander structures connected in parallel. At least two of these parallel meander structures are connected in series. The conductor layer is designed as a thin-film resistor and is made of platinum. The ceramic substrate is made of Al₂O₃.

[0011] From JP H05-87643 A, a thermometer for extremely low (cryogenic) temperatures is known. The thermometer comprises an insulating substrate on which a finely crystallized semiconductor membrane is formed. Furthermore, the thermometer includes a pair of ohmic electrodes which are in contact with the semiconductor membrane. The semiconductor membrane forms an amorphous silicon semiconductor membrane resistor. The insulating substrate is a ceramic substrate. JP H05-87641 A, relating to a thermosensitive device, and JP H05-87642 A, relating to a device for measuring temperatures in the extremely low temperature range, initially exhibit an identical design.

[0012] Based on the prior art, one object of the invention is to provide an inexpensive, improved cryogenic temperature sensor that allows sufficiently accurate temperature measurement in the cryogenic temperature range in industrial environments. Furthermore, a cryogenic temperature measurement arrangement using this cryogenic temperature sensor is to be specified.

[0013] These tasks are solved by a cryogenic temperature sensor according to claim 1 or by a cryogenic temperature measurement arrangement according to claim 8.

[0014] The cryogenic temperature sensor is suitable for temperature measurement in the cryogenic temperature range, particularly at temperatures below 70 K. The invention is based, among other things, on the finding that semiconductor resistors exhibit higher sensitivity in the temperature range below 60 K than metallic resistors or other microtechnologically producible resistors, especially platinum resistors, and that they also show an extreme point in their characteristic curve in this temperature range. The characteristic curve of a semiconductor resistor in the temperature range of 4 K to 300 K typically shows two extreme points, a maximum and a minimum. In the cryogenic temperature sensor according to the invention, preferably only one extreme point is used for measurement; whether it is the maximum or minimum depends on the specific dopant used in the semiconductor. For example, for p-doped, e.g., boron-doped, semiconductor resistors, the maximum (high point) is suitable.In contrast, the minimum (low point) in n-doped, e.g., phosphorus-doped, semiconductor resistors could be utilized. The precise location of the extreme point can be precisely adjusted to a desired temperature during the manufacturing process using variable parameters (e.g., doping, tempering, substrate). The invention takes advantage of the fact that such an adjustable extreme point can be used as an (individual) sensor-specific fixed point to which a calibration measurement can be coupled, for example, for the in-situ calibration of a primary measuring resistor associated with the sensor.

[0015] It has been shown that not all semiconductor resistors are suitable for measurement in the cryogenic temperature range. For example, boron-doped or other p-doped semiconductor resistors with an implantation dose of 5 × 10⁻⁶ 13 cm -2They are not suitable as cryogenic temperature sensors because they exhibit exponential resistance growth below 90 K. However, it was surprisingly found that semiconductor resistors with an implantation dose of 10 15 cm -2 exhibit a different characteristic curve and are therefore suitable for thermometric purposes in the range of 8 K to 200 K.

[0016] The cryogenic temperature sensor according to the invention comprises at least one primary measuring resistor and at least one secondary semiconductor measuring resistor, which are arranged on a common sensor carrier and whose temperature-dependent resistance values ​​can be determined by a measuring circuit.

[0017] In the context of the invention, the primary measuring resistor is a microtechnologically producible measuring resistor. In the field, a microtechnologically producible measuring resistor is understood to be a resistor that can be manufactured using thin-film processes or semiconductor processes employed in microsystems technology, and whose RT characteristic allows its use as a temperature sensor. The range of suitable primary measuring resistors extends from metal resistors and semiconductor resistors to ruthenium oxide resistors and carbon resistors. Preferably, the primary measuring resistor is a metallic measuring resistor, in particular a platinum measuring resistor. Alternatively, the primary measuring resistor can also be another microtechnologically producible measuring resistor made of pure metal.The primary measuring resistor can also be formed by Kondo systems, for example AuFe, CuFe, RhFe as materials with intentionally introduced magnetic defects. Furthermore, the primary resistor can be a ruthenium oxide resistor (RuO2). Such primary measuring resistors, as components of the cryogenic temperature sensor according to the invention, can also be calibrated and used outside the specifications of DIN 60751.

[0018] The semiconductor measuring resistor has at least one selected extremum in its characteristic curve within the cryogenic operating range of the temperature sensor. This defined extremum can be used as a sensor-specific fixed point for calibrating the primary measuring resistor (e.g., platinum measuring resistor), provided that both measuring resistors are at the same temperature, which is achieved through their physical coupling on the sensor substrate.

[0019] Preferably, the extreme point in the characteristic curve is determined during the manufacturing process by selecting a suitable doping concentration for the semiconductor material. Alternatively, other parameters can be used to define the extreme point at a desired location in the resistance-temperature characteristic curve. Besides doping, the base substrate can be selected, or temperature steps can be performed, either alternatively or cumulatively, which affect the shape of the characteristic curve. Further adjustable parameters for the production of semiconductor measuring resistors are known to those skilled in the art.

[0020] The properties of the semiconductor measuring resistor are determined, among other things, by the following parameters: the doping concentration and the substrate used. If any of these parameters are changed, the shape of the RT characteristic curve changes. The characteristic curve of the semiconductor measuring resistor to be set within the scope of the present invention must be suitable for sensor applications (sufficient sensitivity and uniqueness in combination with the primary measuring resistor).

[0021] The cryogenic temperature sensor has an integrated design, eliminating the need for a separate assembly step. In this case, the sensor carrier is formed by a silicon chip, on one side of which is the doped semiconductor measuring resistor, and on the other side is the microtechnically fabricated primary measuring resistor, preferably a platinum measuring resistor. This allows the temperature sensor to be manufactured simply using established semiconductor process technologies.

[0022] For example, the following layer sequence results from bottom to top: implantation layer, substrate (Si) of the semiconductor measuring resistor as sensor carrier, oxide layer (SiO2) as insulating layer, possibly an adhesive layer (e.g. titanium), a platinum track as platinum measuring resistor.

[0023] A key advantage of the cryogenic temperature sensor is that the integrated combination of predominantly platinum and semiconductor measuring resistors allows the usable operating range of the platinum resistor to be extended to temperatures below 100 K, and in some cases even down to the single-digit Kelvin range. The semiconductor measuring resistor provides a sensor-specific fixed point in this low temperature range, complementing and extending the technically far more complex process of establishing natural fixed points (phase reversal points). By selecting appropriate parameters during the manufacturing process, particularly the choice of suitable semiconductor doping, this sensor-specific fixed point can be precisely positioned within the desired operating range of the cryogenic temperature sensor. These measures are technically easy to implement, thus keeping the manufacturing costs of the cryogenic temperature sensor low.

[0024] According to a preferred embodiment, the extreme point of the resistance-temperature characteristic of the semiconductor measuring resistor lies in the range of 4 K to 100 K, preferably in the range of 50 K to 70 K. Such resistance maxima occur with suitable doping concentrations or doses for numerous substrate-doper combinations. Particularly preferred embodiments of the doped semiconductor measuring resistor are, for example, n-SiB, p-SiP, p-InSb, p-GaAs, or p-GaSb. By suitable selection, the semiconductor measuring resistor can thus be given a resistance extremum at a predetermined temperature in the cryogenic temperature range. This extremum is then used as a fixed point or individual reference point for the calibration of the platinum measuring resistor.

[0025] In a non-inventive embodiment of the cryogenic temperature sensor, the two measuring resistors are mounted on a ceramic substrate, which forms the sensor carrier. This is preferably achieved by means of a metallurgical bond, in particular using glass solder or a chip adhesive, which, however, is less temperature-stable. In this embodiment, it is particularly advantageous if the semiconductor measuring resistor is mounted symmetrically to the primary measuring resistor.

[0026] Generally, it is important to ensure good thermal conductivity between the measuring resistors. If an adhesive is used, air inclusions should be avoided. This ensures that both measuring resistors measure nearly the same temperature.

[0027] Furthermore, the choice of materials used in the cryogenic temperature sensor should ensure good overall temperature stability. Since the temperature sensor is used in the cryogenic temperature range, preferably only a few different materials are combined that have similar coefficients of thermal expansion in this temperature range, so that only minimal mechanical stresses are induced within the sensor during temperature changes.

[0028] In the aforementioned non-inventive embodiment, in which the measuring resistors are connected to each other or to the sensor carrier by means of an adhesive, these requirements can be met if the base material of the primary measuring resistor is, for example, an Al₂O₃ ceramic on which a resistive track (e.g., a platinum track) preferably runs in a meandering pattern, with a predefined resistance value of preferably 1,000 ohms (at 0°C). The substrate material of the doped semiconductor measuring resistor is silicon. The cryogenic temperature sensor thus designed therefore has a layer sequence from bottom to top as follows: ceramic (Al₂O₃); vapor-deposited platinum track as a Pt1000 platinum measuring resistor; potting compound; a glass solder for joining the measuring resistors applied to the ceramic; above this, a semiconductor substrate (silicon) in which an implantation is placed.

[0029] A further developed embodiment is characterized in that several microtechnologically manufactured primary measuring resistors, preferably several platinum measuring resistors, and / or several semiconductor measuring resistors form the cryogenic temperature sensor. This is particularly easy to implement in the design where the measuring resistors are mounted on opposite sides of a chip. This results, among other things, in redundancy in the sensor, thus providing higher reliability.

[0030] The cryogenic temperature sensor combines two different temperature-dependent measuring resistors: at least one primary measuring resistor, preferably a platinum resistor, and at least one doped semiconductor measuring resistor. The inherent ambiguity in the region of an extremum of the resistance-temperature characteristic of the doped semiconductor resistor is compensated for by the measuring circuit through the one-to-one correspondence of the characteristic of the primary measuring resistor (platinum resistor). In this way, a measurement uncertainty in the range of ±0.4 K (k=1) can be achieved. Due to the decreasing sensitivity of the RT characteristic of the doped semiconductor resistor, the measurement uncertainty can be significantly greater than ±0.4 K (k=1) in the region of the extremum and between 200 K and 300 K.

[0031] It should be noted that the extremum in the RT characteristic curve, which can be used as a fixed point for calibration, can be set not only by the doping of the substrate material of the semiconductor measuring resistor, but also by other parameters. Several sub-goals can be pursued when defining the extremum. For example, it may be desirable for this temperature reference point to be reproducibly achievable within a tolerance of ±0.2 K (k=1) across a sample. In the manufacturing process, the technological parameters that can be varied, in particular, are the doping concentration and profile, substrate material, crystal orientation, dopant, and external magnetic field, in order to shift the position of the temperature reference point (extremum), primarily to temperatures below 60 K, and especially preferably below 50 K.According to an advantageous embodiment, the extreme point in the RT characteristic curve of the semiconductor resistance is determined by setting the aforementioned parameters at or near the phase transition point of hydrogen (14.025 K).

[0032] The cryogenic temperature measuring arrangement according to the invention for temperature measurement in the cryogenic temperature range comprises the cryogenic temperature sensor in one of the previously described embodiments and a measuring circuit configured to supply the measuring resistors of the temperature sensor with a measuring current, to determine the measuring voltages across the measuring resistors, and to determine the temperature-dependent resistance values ​​from these. Calibration values ​​for the platinum measuring resistor, previously determined by an initial calibration measurement, are taken into account. The in-situ calibration measurement is performed at an extreme point in the resistance-temperature characteristic curve of the semiconductor measuring resistor. This in-situ calibration can be repeated as often as necessary, for example, whenever the extreme point is detected.

[0033] To derive a temperature value, the measuring circuit—after storing a calibration curve or corresponding calibration values ​​in an associated memory—determines the corresponding temperature value from a resistance value. Preferably, the measuring resistors are operated in a 4-wire circuit, with a constant current of, for example, 100 µA, followed by voltage measurement and calculation of the resistance using Ohm's law.

[0034] According to a preferred embodiment, a Howland current source is used to provide a constant measuring current for each measuring resistor. The control of the measurement sequence and the processing of the measured values ​​can be carried out, for example, using a microcontroller that is part of the measuring circuit.

[0035] Further advantages and details of the invention will become apparent from the following description of preferred embodiments, with reference to the drawing. The drawing shows: Fig. 1 a top view and a perspective view of a first embodiment of a cryogenic temperature sensor according to the invention; Fig. 2 a schematic representation of a second embodiment of the cryogenic temperature sensor in several views; Fig. 3 a simplified block diagram of a measuring circuit of a cryogenic temperature measuring arrangement according to the invention.

[0036] Fig. Figure 1 shows a simplified top view and a perspective view of a cryogenic temperature sensor 01 according to a first embodiment. The cryogenic temperature sensor has a sensor carrier 02 made of ceramic. A platinum measuring resistor 03, which can be passivated with glass solder, for example, is mounted on the sensor carrier 02. A semiconductor measuring resistor 04 is attached to the sensor carrier with a bonding material, for example, glass solder, so that the semiconductor measuring resistor 04 is in good thermal contact with the platinum measuring resistor 03.

[0037] Fig. Figure 2 shows a basic design of a second embodiment of the cryogenic temperature sensor 01 in several views. In this case, the sensor carrier 02 is formed by a section of a wafer, in particular a silicon chip. In the illustration, the chip 02 is shown cut along an axis of symmetry to make the two sides of the chip 02 visible. In reality, the chip 02 is preferably designed as a single piece. The platinum measuring resistor 03 is applied to the right-hand side of the chip 02 (in the right-hand figure). The doped semiconductor measuring resistor 04 is attached to the left-hand side of the chip 02 (in the right-hand figure). In this embodiment, a redundant system is created by doubling the semiconductor measuring resistor 04 and the platinum measuring resistor 03, which increases the reliability.

[0038] For the appropriate doping of the semiconductor measuring resistor 04, the person skilled in the art can initially refer to numerous known materials described in the literature (see, e.g., LONG, D.; ZOOK, J.; CHAPMAN, P.; TUFTE, O.: Electrical Properties and Resonance Scattering in Heavily Doped Semiconductors, Solid State Communications, Vol. 2, pp. 191-195, Pergamon Press, 1964). The doping concentrations are then to be determined in the manner already described above. For example, for a boron-doped semiconductor measuring resistor 04 in silicon, an implantation dose of 1 × 10⁻⁶ can be used. 15 cm -2 selected, which corresponds to a doping concentration of approximately 1.2·10 19 cm -3 corresponds (depending on the tempering).

[0039] The implantation dose significantly influences the characteristic curve of doped semiconductor resistance resistors. Changing the doping concentration through annealing or altering the implantation dose results in a different characteristic curve shape and thus also shifts the maximum (extreme point) in the curve. Another parameter in the characteristic curve is the trace width, which does not affect the curve shape but does influence the measured resistance value. The larger the trace width, the lower the resistance value. Furthermore, the characteristic curve shape can be influenced by the selected substrate material, the crystal orientation, the dopant used (n- or p-doped), or an external magnetic field applied during the process.

[0040] Fig.Figure 3 shows a simplified block diagram of a measurement circuit for an embodiment of a cryogenic temperature measurement arrangement, symbolizing both the basic circuit units and important steps in the execution of a measurement. The measurement circuit comprises an analog block 10 and a digital block 20. The analog block 10 has a current source 11 that supplies a constant measuring current of, for example, 100 µA to a measuring resistor, across which a voltage drop occurs. The voltage value is supplied to an amplifier stage 12. The digital block 20 contains an analog-to-digital converter 21, which digitizes the amplified voltage value. A microprocessor 22 calculates the resistance value R from the digitized values. PW of the platinum measuring resistor 03 and the resistance value R HWof the semiconductor measuring resistor 04. Calibration values ​​or characteristic curves for the temperature range to be covered are stored in a memory, which were recorded during a previous calibration (e.g., an initial calibration). Since the values ​​of the semiconductor measuring resistor 04 are not unique in the area of ​​the extreme points due to the characteristic curve, the resistance value R, which must be determined unambiguously in each case, serves as the basis for the calibration. PW of the platinum resistor 03 for selecting the appropriate resistance values ​​R HW of the semiconductor resistor 04. In a next step, the exact temperature value can then be calculated, taking the selected values ​​into account. The temperature value can then be output and / or recorded. The measurement steps are repeated at defined intervals to enable continuous temperature measurement. Reference sign 01 Cryo-temperature sensor 02 Sensor carrier 03 Primary measuring resistor / Platinum measuring resistor 04 Semiconductor measuring resistor 10 Analog block 11 Power source 12 amplifier stages 20 digital blocks 21 Analog-to-Digital Converters 22 Microprocessor

Claims

[1] Cryogenic temperature sensor (01) for temperature measurement in the cryogenic temperature range, comprising a microtechnologically manufactured primary measuring resistor (03) and a secondary semiconductor measuring resistor (04) arranged on a common sensor carrier (02) and whose temperature-dependent resistance values ​​can be determined by a measuring circuit, wherein the sensor carrier (02) is formed by a dopable silicon chip, on one side of which the doped semiconductor measuring resistor (04) and on the other side of which the primary measuring resistor (03) are formed, and wherein the semiconductor measuring resistor (04) has an extremum in its resistance-temperature characteristic curve in the cryogenic operating range of the temperature sensor that can be set in the manufacturing process and which can be used as a sensor-specific fixed point for in-situ calibration of the primary measuring resistor (03). [2] Cryo-temperature sensor according to claim 1, characterized by, that the semiconductor measuring resistor (04) has a predetermined doping by which the extremum in its resistance-temperature characteristic is defined. [3] Cryo-temperature sensor according to claim 2, characterized by , that the semiconductor measuring resistor (04) delivers an implantation dose of 10 15 cm -2 or more. [4] Cryo-temperature sensor according to one of claims 1 to 3, characterized by , that the extreme point of the resistance-temperature characteristic of the semiconductor measuring resistor (04) lies in the range of 4 K to 100 K, preferably in the range of 50 to 70 K. [5] Cryo-temperature sensor according to one of claims 1 to 4, characterized by , that the semiconductor measuring resistor (04) is formed by one of the following doped substrates: n-SiB, p-SiP, p-InSb, p-GaAs or p-GaSb. [6] Cryo-temperature sensor according to any one of claims 1 to 5, characterized by, that the primary measuring resistor (03) is selected from the following group of microtechnologically producible measuring resistors: - metallic measuring resistors, - Platinum measuring resistors (03), - Kondo system measuring resistors, -Ruthenium oxide measuring resistors, - Carbon resistors. [7] Cryo-temperature sensor according to any one of claims 1 to 6, characterized by that it includes several primary measuring resistors (03) and / or several semiconductor measuring resistors (04). [8] Cryogenic temperature measurement arrangement for temperature measurement in the cryogenic temperature range, comprising a cryogenic temperature sensor (01) according to any one of claims 1 to 7 with at least one microtechnologically producible primary measuring resistor (03) and at least one secondary semiconductor measuring resistor (04), and further comprising a measuring circuit which is configured to supply the measuring resistors (03; 04) of the temperature sensor (01) with a measuring current, to determine the measuring voltages at the measuring resistors (03; 04) and to determine the temperature-dependent resistance values ​​therefrom. [9] Cryo-temperature measuring arrangement according to claim 8, characterized by , that the primary measuring resistor (03) and the semiconductor measuring resistor (04) are each connected to a Howland current source, which each supplies a constant measuring current to the measuring resistor (03; 04).

Images