Thermal emitter, operating method for thermal emitter and MEMS gas / fluid sensor

Abstract

The invention relates to a thermal emitter, a method of operation for a thermal emitter, and a MEMS gas / fluid sensor. A method for operating a thermal emitter having a conductive semiconductor portion includes providing an activation signal having a first energy level (Joule heating) to the conductive semiconductor portion of the thermal emitter during an operation cycle to emit IR radiation, and providing a refresh signal having a second energy level to the conductive semiconductor portion of the thermal emitter during the refresh cycle, wherein the second energy level is different from the first energy level.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to a thermal emitter (Joule heater), a method for operating a thermal emitter having a conductive semiconductor portion (Joule heater structure), and a MEMS fluid (gas or liquid) sensor using the thermal emitter. The present disclosure also relates to improving the (long-term) operational stability of a thermal emitter by applying a refresh signal to the thermal emitter during a refresh cycle. Background Art

[0002] The sensing of environmental parameters such as noise, sound, temperature and gases, e.g., ambient gas composition, in the ambient atmosphere is becoming increasingly important when implementing appropriate sensors in mobile devices, home automation (such as smart homes), and the automotive industry. Harmful gas concentrations may occur due to pollution and / or equipment failures. However, the health of humans or animals is strongly influenced by the quality of the air. Therefore, precise and reliable gas detection in the ambient atmosphere by means of cheap, readily available and accurate sensors is a hot topic for the future.

[0003] In the field of monitoring ambient air quality, there are several types of gas sensing concepts, such as non-dispersive infrared (NDIR) sensors, chemical sensors and photoacoustic sensors (PAS = photoacoustic spectroscopy). The sensor effect commonly used is based on the excitation of gas molecules in a medium by (infrared) light of a specific wavelength. However, the currently available NDIR or PAS systems are relatively expensive due to their complex setup or special components. In addition, chemical sensors show relatively poor selectivity compared to single-wavelength or filter-containing optical systems. A typical optical sensor (e.g., a PAS sensor) comprises a radiation source, a filter element for wavelength selection, a detector and a sample area, where the light between the light source and the detector interacts with the ambient medium.

[0004] With the increasingly widespread use of gas sensors, there is a particular need to be able to operate such gas sensors with thermal emitters (MEMS heaters) as accurately and inexpensively as possible during the lifetime of the gas sensor.

[0005] Such need can be solved by a method for operating a thermal emitter according to claim 1 , a thermal emitter according to claim 10 and a MEMS gas or fluid sensor according to claim 15 .

[0006] Further specific implementations of the present concept for operating a thermal emitter are defined in the dependent claims.

[0007] Thus, the present disclosure describes a method for operating a thermal emitter, which may be, for example, part of a MEMS gas or fluid sensor capable of sensing at least one component of an ambient atmosphere, wherein the long-term stability of the emission characteristics of the MEMS heater may be improved by including a refresh cycle during or between measurement cycles of the sensor. The thermal emitter for infrared or mid-infrared radiation may be used in any MEMS device having an IR radiation source. Summary of the invention

[0008] According to one embodiment, a method for operating a thermal emitter having a conductive semiconductor portion includes providing an activation signal having a first energy level (Joule heating) to the conductive semiconductor portion of the thermal emitter to emit IR radiation during an operation cycle, and providing a refresh signal having a second energy level to the conductive semiconductor portion of the thermal emitter during a refresh cycle, wherein the second energy level is different from the first energy level.

[0009] According to one embodiment, a thermal emitter includes a heater element having a conductive semiconductor portion, and a driver element (e.g., an ASIC or a microcontroller) for providing an activation signal having a first energy level to the conductive semiconductor portion of the thermal emitter to emit IR radiation during an operation cycle, and providing a refresh signal having a second energy level to the conductive semiconductor portion of the thermal emitter during a refresh cycle, wherein the second energy level is different from the first energy level.

[0010] According to one embodiment, a MEMS gas or fluid sensor includes a thermal emitter (described above) for emitting thermal radiation, a measurement volume having a target gas or target fluid and providing an optical interaction path for the emitted thermal radiation, and an acoustic transducer or a direct thermal detector for providing a detector output signal based on the optical interaction of the emitted thermal radiation with the target gas or target fluid in the measurement volume.

[0011] Further embodiments are described in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Hereinafter, embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings, in which:

[0013] Figure 1 A schematic flow chart of a method for operating a thermal emitter (MEMS heater) according to an embodiment is shown;

[0014] Figure 2a-2b shows a schematic block diagram of emitter excitation with a refresh cycle according to an embodiment;

[0015] Figure 3Schematic diagram showing the "thermal" drift behavior of a MEMS transmitter during its lifetime (with refresh operation);

[0016] Figure 4 shows a schematic block diagram of a heat emitter device (Joule heater device) according to an embodiment;

[0017] Figure 5 1 shows a schematic block diagram of a fluid sensor using a thermal emitter (MEMS heater) according to an embodiment;

[0018] Figure 6 Detailed description of the invention: FIG. 1 is a schematic block diagram of a PAS gas (fluid) sensor according to an embodiment;

[0019] Figure 7 Detailed description of the invention: FIG. 1 is a schematic block diagram of an NDIR gas (fluid) sensor according to an embodiment;

[0020] Figure 8a-8b A schematic top view (plan view) and a schematic cross-sectional view of a thermal emitter are shown; and

[0021] Fig. 9 Schematic diagram showing the "thermal" drift behavior of a thermal (MEMS) emitter during lifetime (without refresh operation).

[0022] Before discussing the present embodiment in further detail using the drawings, it should be pointed out that in the drawings and the specification, the same elements and elements with the same functions and / or the same technical or physical effects generally have the same figure marks or are identified by the same names, so that the descriptions of these elements and their functions shown in different embodiments can be interchangeable or can be applied to each other in different embodiments. DETAILED DESCRIPTION

[0023] In the following description, embodiments are discussed in detail, however, it should be understood that the embodiments provide many applicable concepts that can be implemented in a variety of thermal emitters. The specific embodiments discussed only illustrate the implementation and specific ways of using the present concepts, and do not limit the scope of the embodiments. In the description of the following embodiments, the same or similar elements or elements with the same functions are provided with the same figure marks or identified with the same names, and repeated descriptions of elements provided with the same figure marks or identified with the same names are generally omitted. In the following description, a number of details are set forth to provide a more thorough explanation of the embodiments of the present disclosure.

[0024] However, it will be apparent to those skilled in the art that other embodiments may be practiced without these specific details. In other cases, well-known structures and devices are shown in block diagram form rather than in detail to avoid obscuring the examples described herein. In addition, unless otherwise specifically stated, the features of the different embodiments described herein may be combined with each other.

[0025] It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intervening elements. Conversely, when an element is referred to as being "directly," "connected" or "coupled" to another element, there are no intervening elements. Other terms used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," "on" versus "directly on," etc.).

[0026] For the convenience of describing different embodiments, the drawings include a Cartesian coordinate system x, y, z, wherein the xy plane corresponds to (i.e. is parallel to) the first main surface area of ​​the substrate (=reference plane=xy plane), wherein the direction vertically upward relative to the reference plane (xy plane) corresponds to the "+z" direction, and wherein the direction vertically downward relative to the reference plane (xy plane) corresponds to the "-z" direction. In the following description, the term "lateral" refers to a direction parallel to the x-direction and / or the y-direction (i.e., parallel to the xy plane), wherein the term "vertical" refers to a direction parallel to the z-direction.

[0027] In the following description, the thickness of an element generally indicates the vertical dimension of such element. In the drawings, different elements are not necessarily drawn to scale. Therefore, the illustrated dimensions of different elements are not necessarily drawn to scale.

[0028] In the description of the embodiments, terms and text passages placed in parentheses (located next to the described elements or functions) should be understood as further explanations, exemplary configurations, exemplary additions and / or exemplary substitutions of the described elements or functions.

[0029] As about Figures 1 to 7 As described, various embodiments of the present disclosure relate to improved operation of a thermal heater (Joule heater or MEMS emitter), wherein a refresh signal is applied to the thermal emitter during a refresh cycle (mode) to improve the (long-term) operational stability of the thermal emitter. Thus, a fluid (gas or liquid) sensor using such a MEMS heater can achieve higher sensing accuracy during its lifetime.

[0030] However, before describing the various embodiments of the present disclosure, the inventors' findings and observations regarding the physical properties and long-term behavior of Joule heater structures and the resulting technical impact on gas sensors (e.g., long-term effects or drift during sensor lifetime) are summarized.

[0031] Figure 8a shows a schematic top view (parallel to the xy plane) of an exemplary thermal emitter 10 (MEMS heater=Joule heater), Figure 8b A schematic cross-sectional view of the heat emitter 10 along section line AA' is shown (parallel to the xz plane). The operating principle of a MEMS heater is generally referred to as "Joule heating", also known as resistive, resistive or ohmic heating. Joule heating of a MEMS heater is the process by which heat is generated by an electric current (in response to an applied drive signal, such as a voltage) passing through an electrical conductor of the MEMS heater.

[0032] like Figure 8a-8b As exemplarily shown in FIG. 1 , the thermal emitter 10 includes, for example, a Joule emitter structure 12 in the form of a conductive semiconductor portion, which may include a doped polysilicon material. The thermal emitter 10 may include a membrane 14 supported by a substrate 16 (e.g., a bulk Si substrate), wherein the membrane 14 includes a conductive portion 12 (Joule heater portion), such as a conductive semiconductor material. Figure 8a-8b As shown, the Joule emitter structure 12 can be arranged on a film 14 (e.g., including an insulating material such as SiN, and / or a Si material such as polysilicon). Alternatively, the Joule emitter structure 12 can be a part of the film 14, such as a highly doped polysilicon region 12 of the polysilicon film 14.

[0033] The Joule heater structure 12 may be arranged on a cavity 18 in a substrate 16. The substrate 16 may include a bulk semiconductor substrate (e.g., a bulk Si substrate). Based on the configuration of the Joule heater structure 12, such a radiation source 10, e.g., for a gas or fluid sensor, may provide a relatively high thermal resistance (= low thermal mass) to achieve a relatively high energy efficiency (= high temperature increase per watt of input power), wherein due to this arrangement the thermal emitter may be operated at a relatively high operating temperature (e.g., 600° C. or higher).

[0034] According to one embodiment, the conductive portion 12 may optionally form branch current paths separated by the central portion 12-1. Figure 8a As exemplarily shown in , the conductive part 12 may form a ring-shaped heater element of width W, which generates heat during an activated state (= energization of the conductive part 12), which heat may also diffuse into / through the (optional) central part 12-1.

[0035] The central portion 12-1 may include a low-doped or undoped semiconductor (e.g., polysilicon) material, such that the conductive portion 12 forms branch current paths separated by the central portion 12-1. In addition, the two regions 12, 12-1 may have the same layer thickness. Thus, the central portion 12-1 and the conductive portion 12 may together form a final heating structure or heating element for emitting or dissipating heat, for example, into a test body, in the form of thermal (e.g., infrared) radiation. Thus, the heated annular conductive portion 12 may provide a relatively uniform temperature distribution in the heating structure.

[0036] According to another embodiment, the central portion (central area) 12-1 may also include a material doped similarly to the heater (conductive portion) 12, but separated by a narrow partition (partition line) 12-2, which provides electrical isolation between the central portion 12-1 and the conductive portion 12, but maintains sufficient thermal conductivity to the conductive portion (heater) 12.

[0037] According to another embodiment, the central part (central area) 12-1 may also include an insulating material (e.g., SiN) having a thermal expansion coefficient similar to (within the range of ±5% or ±2%) that of the material of the heater (conductive part) 12 and maintaining sufficient thermal conductivity to the conductive part (heater) 12.

[0038] According to another embodiment, the conductive portion 12 may also include one of a circular, square, oval, and elliptical shape (or any convex polygon), wherein this list of shapes of the conductive portion 12 should not be considered exhaustive.

[0039] The (set) conductor width W of the conductive portion 12 (see Figure 8a ) may depend on the drive voltage for which the Joule heater structure 12 of the MEMS heater 10 is designed. For lower drive voltages, the conductor width W (and thus the footprint of the heater 12) is typically larger, wherein the temperature non-uniformity of the Joule heater structure 12 is also typically larger for lower drive voltages.

[0040] Typically, MEMS Joule heaters need to reach temperatures above 600°C to generate enough optical output power for gas sensing during gas measurement operation. For voltage driven heaters, the resistance of the heater has to be reduced to reduce the required driving voltage. Geometry optimization of low voltage emitters favors distributed planar heater designs, i.e., with a wider footprint, which results in (relatively) larger temperature variations (= clamping losses) over the electrically active heaters due to the different proximity to the membrane boundaries. An increase in the thickness of the polysilicon layer has a similar effect, as the thermal conduction losses through the polysilicon heater also increase.

[0041] The inventors of the present disclosure have also recognized the following physical relationship: in polysilicon, when the temperature is above the dopant activation energy (~600°C), high temperature variations (i.e., non-uniformity) change the local dopant distribution over time. More specifically, the wider or larger the Joule heater structure (Joule line) 12 (e.g., R squared and * film thickness), the faster the resistor structure 12 degrades, i.e., the faster the material properties (characteristics) of the Joule heater structure 12 change, and thus the faster the electrical characteristics drift.

[0042] The inventors of the present disclosure have recognized that under the described operating conditions of the thermal heater 10 (MEMS emitter or Joule heater), a systematic (upward) drift of the resistance of the thermal emitter may be observed.

[0043] Fig. 9 A schematic diagram of the "thermal" drift behavior of a MEMS emitter during lifetime (without refresh operations) is shown. For example, due to temperature non-uniformity over the width W of the Joule heater structure 12, the resistance of the Joule emitter structure 12 may drift systematically (upward). Due to this (upward) drift in the resistance of the emitter 12, the emitter 12 consumes less electrical power and emits less light (IR radiation) at a given voltage (drive signal). As a result, gas measurement readings on a calibrated sensor with such an emitter are detuned. As a result, the PPM accuracy (PPM = parts per million) or detection accuracy of a gas sensor using the emitter 10 degrades during lifetime. In other words, as the resistance of the heater changes during the lifetime of the sensor, the optical output power of the emitter will also change, resulting in a deviation from the target specification, thereby adversely affecting the long-term performance of a sensor using the emitter.

[0044] The inventors of the present disclosure have recognized that the physics behind this behavior can be explained as follows. When a voltage is applied to the MEMS Joule heater, the Joule heater itself reaches different local temperatures. This non-uniformity is due to different thermal isolation of corresponding areas on the MEMS Joule heater, which is a result of the emitter design. For semiconductor materials, such as polycrystalline silicon (poly-Si) or silicon carbide (SiC) as conductive materials, different local temperatures above the activation energy of dopant diffusion (about 600°C) can lead to local different electrical drift behaviors of the thermal emitter (MEMS Joule heater). This effect / behavior is called temperature-dependent "dopant segregation". The conductivity of polycrystalline silicon changes (i.e., decreases) due to the segregation of dopant atoms to grain boundaries, where the atoms themselves are trapped and become electrically inactive. This effect (temperature-dependent dopant segregation) increases with the temperature non-uniformity over the width of the Joule heater structure.

[0045] Figure 1 A schematic flow chart of a method 100 for operating a thermal emitter 10 (MEMS heater) having an electrically conductive semiconductor portion 12 (Joule heater structure) according to an embodiment is shown.

[0046] like Figure 1 As exemplarily shown in FIG. 1 , the operation method 100 may include: step 110, during the operation cycle (mode), providing the conductive semiconductor portion 12 of the thermal emitter 10 with a first energy level E 1 (The first Joule heating energy level = the first energy E provided 1 ) activation signal S ACT to emit IR radiation, such as pulsed IR radiation; and step 120, during the refresh cycle (mode), providing the conductive semiconductor portion 12 of the thermal emitter 10 with a second energy level E 2 (Second Joule heating energy level = provided second energy E 2 ) of the refresh signal S REF , where the second energy level E 2 Different from the first energy level E 1 .

[0047] Activation signal S ACT It may include a transient (or pulsed) signal shape, where the refresh signal S REFTransient (or pulsed) signal shapes may also be included. In the context of this specification, a transient signal may have a signal shape that is continuous or stepless between a maximum value and a minimum value, or may have a pulsed signal shape with a fast (steep) rising edge between a minimum value and a maximum value and a fast (steep) falling edge between a maximum value and a minimum value.

[0048] According to an embodiment of the present disclosure, an additional refresh cycle of the thermal emitter 10 is introduced into the gas measurement readout routine. The refresh cycle counteracts the resistance drift of the polysilicon Joule heater 10 caused by the target voltage operation during the measurement. The resistance drift may be affected by temperature-dependent "dopant segregation", in which the conductivity of the polysilicon is changed (i.e., reduced) due to the segregation of dopant atoms to the grain boundaries, where the atoms themselves are trapped and become electrically inactive.

[0049] The energy level provided during the refresh cycle differs in energy (voltage and time) from the operating measurement, so that the drift produced by the refresh cycle itself cancels the electrical drift of the target voltage operation. By adjusting (optimizing) the energy / joule heating (voltage and time and therefore the conductor temperature) of the conductive semiconductor portion 12 of the thermal emitter 10 during the refresh cycle, an additional rearrangement of the dopants inside the semiconductor is triggered after the gas measurement or after each gas measurement to cancel the drift produced by the target operation (e.g., upward drift). For example, since the resistivity of the Joule heater structure 12 of the thermal emitter 10 can be maintained at substantially the same value, dopant segregation that causes, for example, a systematic (upward) drift in the resistance of the thermal emitter 10 can be offset or reversed with the help of the refresh cycle. (One or more) refresh cycles can reverse or at least reduce dopant segregation at the grain boundaries of the polysilicon material of the Joule heater structure 12 of the thermal emitter 10.

[0050] The temperature dependence of the dopant segregation mechanism allows to define a refresh cycle. Thus, the dopant segregation caused by the activation signal during the operation cycle can be effectively offset by means of the refresh signal(s) during the operation cycle(s). As a result, a drift of the resistance of the conductive semiconductor portion 12 from a target value can at least be reduced or even avoided.

[0051] The refresh cycle concept can be implemented for thermal heaters (essentially) without hardware adaptation, since the same bill of materials (BOM) can be used with the adjusted firmware on the control unit (e.g. PASCO 2 Therefore, the ppm accuracy of the gas sensor improves over its lifetime due to reduced transmitter resistance drift.

[0052] For example, a thermal emitter 10 for a gas or fluid sensor may include a relatively high thermal resistance (= low thermal mass) to achieve relatively high energy efficiency (= high light output power increase per electrical input power increase = high temperature increase per Watt of input power) and may operate with reduced systematic (upward) drift of the electrical characteristics of the thermal emitter to achieve high PPM accuracy over the lifetime of the gas sensor.

[0053] According to one embodiment, the energy level E 2 (Joule heating) and thus the final refresh temperature of the Joule heater structure 12 during the refresh cycle is set to a value for counteracting dopant segregation in the Joule heater structure 12 (the conductive semiconductor portion) of the thermal emitter 10 .

[0054] According to one embodiment, the energy level of the refresh signal (which is provided to the thermal emitter 10 during the refresh period) is set to a value between 3% and 100% or between 5% and 50% higher than the energy level of the activation signal of the Joule heater structure 12 provided to the thermal emitter 10 during the activation period.

[0055] Therefore, according to the higher energy level E 2 The (pulse) temperature T of the Joule heater structure 12 of the heat emitter 10 during the refresh cycle 2 Correspondingly higher than the (pulse) temperature T of the Joule heater structure 12 during the activation period 1 Therefore, the temperature T of the Joule heater structure 12 during the refresh cycle 2 is set to be higher than the temperature T of the Joule heater structure 12 during the activation period (operation period). 1 The final temperature T of the Joule heater structure 12 of the heat emitter is, for example, between 3% and 30%. 1 , T 2 Depending on the energy level E provided by the activation signal or the refresh signal 1 、E 2 .

[0056] The temperature T of the Joule heater structure 12 during the refresh cycle 2 and the temperature T during the activation period 1 The temperature difference ΔT(=T 2 -T 1 ) is set to a value that effectively counteracts dopant segregation in the Joule heater structure (conductive semiconductor portion) 12 of the thermal emitter 10. The adjusted temperature difference ΔT may depend on the geometry of the Joule heater structure 12, the semiconductor material used, and the doping material and doping level of the semiconductor material of the Joule heater structure 12.

[0057] The temperature T of the Joule heater structure 122 The temperature T of the Joule heater structure 12 is higher (eg, during a refresh cycle) than during an active cycle (operation cycle). 1 Between 3% and 30% higher may be suitable for situations where high temperatures effectively cause the dopant to re-diffuse to become electrically active, segregation-dominated.

[0058] According to another embodiment, the energy level of the refresh signal (which is provided to the thermal emitter 10 during the refresh period) is set to a value between 3% and 70% or between 5% and 50% lower than the energy level of the activation signal of the Joule heater structure 12 provided to the thermal emitter 10 during the activation period.

[0059] Therefore, according to the lower energy level E 2 The (pulse) temperature T of the Joule heater structure 12 of the heat emitter 10 during the refresh cycle 2 Correspondingly lower than the (pulse) temperature T of the Joule heater structure 12 during the activation period 1 Therefore, the temperature T of the Joule heater structure 12 during the refresh cycle 2 is set to be higher than the temperature T of the Joule heater structure 12 during the activation period 1 The final temperature T of the Joule heater structure 12 of the heat emitter is, for example, between 3% and 30%. 1 , T 2 Depending on the energy level E provided by the activation signal or the refresh signal 1 、E 2 .

[0060] The temperature T of the Joule heater structure 12 during the refresh cycle 2 and the temperature T during the activation period 1 The temperature difference ΔT(=T 1 -T 2 ) is set to a value that effectively counteracts dopant segregation in the Joule heater structure (conductive semiconductor portion) 12 of the thermal emitter 10. The adjusted temperature difference ΔT may depend on the geometry of the Joule heater structure 12, the semiconductor material used, and the doping material and doping level of the semiconductor material of the Joule heater structure 12.

[0061] In the case of an operation where the dopant is over-driven resulting in over-diffusion, the temperature T of the Joule heater structure 12 2 (e.g., during a refresh cycle) than the temperature T of the Joule heater structure 12 during an active cycle. 1 A temperature between 3% and 30% lower may be appropriate. In this case, the lower temperature T 2 (ie, a lower refresh voltage) will effectively counteract the over-diffusion.

[0062] According to one embodiment, the refresh signal may have a transient or pulse signal shape (process), wherein the activation signal may have a transient or pulse signal shape (process) for achieving a pulsed emission of IR radiation by the thermal emitter 10 .

[0063] According to one embodiment, the energy level E of the activation signal 1 The activation signal strength (activation voltage V 1 ), activation length (or pulse length) and the number of activation states (or the number of pulses) to adjust at least one (or two or all). In addition, the energy level E of the refresh signal 2 The signal strength of the refresh signal (refresh voltage V 2 ), activation length (or pulse length) and number of activation states (or number of pulses) to adjust at least one (or two or all).

[0064] Figure 2a-2b A schematic block diagram of thermal emitter excitation with a refresh cycle according to an embodiment is shown.

[0065] like Figure 2a As exemplarily shown in , a refresh cycle may be performed after each operating cycle (e.g., gas measurement), after each second operating cycle, after each third operating cycle, etc. Thus, a refresh cycle may be performed (intermittently or periodically) between two operating cycles, or may be performed each time between two operating cycles of the thermal emitter.

[0066] like Figure 2a As shown exemplarily in FIG. 1 , the operating cycle can be divided into a preparation mode (e.g., measurement preparation of the sensor) and an activation mode (e.g., gas measurement of the sensor), wherein a refresh cycle is performed after the operating cycle (with the preparation mode and the activation mode). For example, the preparation mode (measurement preparation) can include two to eight or five operating "hot" pulses, wherein the activation mode (gas measurement) can include a refresh cycle with an activation voltage V 1 Ten to twenty-five or seventeen operating heat pulses.

[0067] The refresh cycle may include multiple heat pulses (activation pulses) or a refresh voltage V 2 For example, a refresh cycle may include one to one hundred or one to twenty thermal pulses, or may include a single DC pulse with a refresh voltage V 2 As described above, the energy level provided by the refresh cycle is higher than the energy level E provided by the activation signal during the operation cycle. 1 High between 3% and 100% or High between 5% and 50% or Low between 3% and 70% or Low between 5% and 50%.

[0068] like Figure 2b As exemplarily shown in , the refresh cycle may be included in a preparation routine (eg, a measurement preparation routine of a sensor) of the thermal emitter 10. Thus, the refresh cycle may be performed before an operation cycle of the thermal emitter 10 or before each of them (intermittently or periodically).

[0069] For example, an adapted preparation mode (adapted measurement preparation) may include a measurement module with an activation voltage V 1 of two to eight or five operating "hot" pulses, and additionally or alternatively, including a refresh cycle having one to twenty or two to ten hot pulses or having a refresh voltage V 2 A single DC pulse, wherein the active mode (gas measurement) may include an activation voltage V 1 As described above, the energy level provided by the refresh cycle is higher than the energy level E provided by the activation signal during the operation cycle. 1 Between 3% and 100% higher or between 5% and 50% higher, or between 3% and 70% lower or between 5% and 50% lower.

[0070] Hence, according to one embodiment, the activation signal may include a plurality of DC activation pulses, wherein the refresh signal may include at least one or more DC refresh pulses.

[0071] Embodiments of the present concept describe improvements of a thermal emitter 10, for example for a gas sensor, wherein the thermal emitter 10 comprises a Joule heater structure 12 (conductive semiconductor part), for example, the Joule heater structure 12 comprises polysilicon or SiC material. Based on the described method for operating such a thermal emitter 10, drifts (resistance drifts) during the lifetime due to thermal inhomogeneities on the heater structure under operating conditions can be reduced or even avoided. According to embodiments, an additional refresh cycle can be included in each operating cycle, for example in each operating gas measurement block. The electrical parameters of the refresh cycle (such as the signal strength (activation voltage V 1 ), activation length (pulse length) and number of activation states (pulse number)) are selected so that the drift observed under operating conditions is offset by the drift caused by the refresh cycle itself. The refresh cycle can be a DC pulse at the refresh voltage or a set of multiple pulses (AC excitation).

[0072] Figure 3 Schematic diagram showing the "thermal" drift behavior of a thermal emitter during lifetime with refresh operation compared to thermal emitter operation without refresh operation. PPM accuracy measurements with refresh cycles can be made by firmware adaptation, e.g. for PAS CO 2 Sensor firmware adaptation.

[0073] Figure 3 Curves 1 and 3 of exemplarily show the electrical drift observed at the PAS system level without a refresh cycle or with a non-optimized energy level of the refresh cycle (resulting in a non-optimal ΔT), which results in an upward drift in resistance, thereby causing a drift in the resistance of the Joule heater structure 12 and, in turn, a drift in the PPM accuracy of the gas sensor within a typical target specification channel range. Curve 2 shows the resistance of the Joule heater structure 12 of a thermal emitter with an "optimized" refresh cycle, wherein the resistance drift of the Joule heater structure 12 is substantially reduced, and, therefore, the drift in the PPM accuracy of the gas sensor during lifetime is reduced.

[0074] Figure 4 1 shows a schematic block diagram of a heat emitter device 20 according to an embodiment. The heat emitter device 20 comprises a heat emitter 10 having a Joule heater structure (conductive semiconductor portion) 12 and a driver element 22, such as an ASIC or a microcontroller, for providing the conductive semiconductor portion 12 of the heat emitter 10 with a first energy level E during an operation cycle. 1 The activation signal S ACT to emit IR radiation R and provide the conductive semiconductor portion 12 of the thermal emitter 10 with a second energy level E during the refresh cycle 2 The refresh signal S REF , where the second energy level E 2 Different from the first energy level E 1 .

[0075] According to one embodiment, the driver element 22 is arranged to provide an activation signal S ACT and refresh signal S REF , in order to carry out Figures 1 to 3 The method steps of the method 100 for operating the thermal emitter 10 are exemplarily described in . According to one embodiment, the conductive semiconductor portions 12 may form branched current paths separated by the central portion 12 - 1 of the thermal emitter 10 .

[0076] According to one embodiment, the conductive semiconductor portion 12 of the thermal emitter 10 may be arranged on a cavity 18 in the substrate 14 to which the heater element is attached. According to one embodiment, the conductive semiconductor portion 12 of the thermal emitter 10 may include polysilicon or SiC material.

[0077] Figure 5 According to an embodiment (for example, according to Figure 4) is a schematic block diagram of a fluid sensor 50 (50-1, 50-2) using a thermal emitter 10 (a Joule heater MEMS heater). According to one embodiment, the MEMS gas or fluid sensor 50 includes a thermal emitter 10 for emitting thermal radiation R, such as Figure 1-Figure 4 and Figure 8- Fig. 9 The target gas G TAR Or target fluid F TAR and providing a measurement volume 30 for the emitted thermal radiation R to provide an optical interaction path 32; and for providing a detector output signal S based on the optical interaction of the emitted thermal radiation R with a target gas or target fluid in the measurement volume 30 OUT acoustic transducer 40 or direct thermal detector 42.

[0078] According to one embodiment, the fluid sensor may be formed as a PAS (Photo Acoustic System) gas detector (fluid detector) 50-1, wherein the thermal emitter device 20 comprises a heater element 10 (Joule heater) having a conductive semiconductor portion 12, and a driver element 22 (e.g., an ASIC or a microcontroller) for implementing the method 100. According to another embodiment, the fluid sensor 50 may be formed as an NDIR (NDIR=Non-Dispersive Infrared) gas detector 50-2, wherein the thermal emitter device 20 comprises a (heater) element (Joule heater) 10 having a conductive semiconductor portion 12, and a driver element 22 (e.g., an ASIC or a microcontroller) for implementing the method 100.

[0079] Figure 6 FIG. 5 shows a schematic block diagram of a fluid (gas or liquid) sensing device 50 - 1 in the form of a PAS gas sensor according to another embodiment. Figure 6 As shown, the PAS gas sensor 50-1 includes a thermal emitter 10 arranged in a package 11, such as an acoustically sealed package. The package 11 can provide an acoustically sealed chamber (absorption chamber) isolated from external noise. Therefore, the package 11 can form a closed mid-infrared emitter package for emitting (filtered) radiation R into the measurement volume. The thermal emitter device 20 includes a (heater) element 10 having a conductive semiconductor portion 12. The heater element 10 can be activated by an actuation signal S ACT The actuation, for example, of the heater element 10 may be chopped periodically.The package 11 may comprise an optical filter 13 for wavelength selection of the radiation R emitted.

[0080] The gas sensor device 50-1 also includes a sample region (interaction region) 30 in which the light or radiation R between the light source (heater element) 10 and the detector 40 interacts with the ambient medium. The filter 13 is arranged to filter wavelengths associated with a specific gas, e.g. for CO2 , wavelength λ=4.26 μm. The detector 40 may be formed as, for example, a (capacitive or piezoelectric) MEMS microphone. In the interaction region 30, the target gas G TAR molecules (e.g., CO 2 The molecules) absorb the light R emitted from the heater element 10 and filtered by the filter 13. The light absorption causes a periodic local variation of the temperature in the interaction region 30, which causes a corresponding pressure variation in the interaction zone 30. The periodic pressure variation is detected by an acoustic detector 40 (e.g., a MEMS microphone), which can be optimized for low frequency operation. The acoustic detector 40 can be read out by means of an ASIC 42, for example. In addition, the closed interaction region can be accessed through the fluid port 34 for fluid (gas or liquid) diffusion.

[0081] Figure 7 1 shows a schematic block diagram of a fluid (gas or liquid) sensing device in the form of an NDIR gas sensor 50-2 having a thermal emitter 10 according to another embodiment. Figure 7 , the NDIR sensor device 50-2 comprises a thermal emitter 10, a sample chamber or light pipe 11, and a closed package 15 for a detector 42. The package 15 may include a filter element 13 for wavelength selection and a sample area (interaction area) 30, which can be accessed by means of a gas inlet 34 and a gas outlet 36. In the interaction area 30 (for providing an interaction path 32), light R between the light source (thermal emitter) and the detector interacts with the ambient medium (e.g., the target gas G TAR )interaction.

[0082] Thus, the main components of the NDIR sensor 50-2 are the thermal emitter 10, the sample chamber or light pipe 11, the optical filter 13 and the infrared detector 42 in the package 15. IR light (radiation) R is directed through the sample chamber 11 towards the enclosed detector 42. The target gas in the sample chamber 11 causes absorption of a specific wavelength, and the intensity change at that wavelength is measured by the detector 42 to determine / derive the target gas concentration. The optical filter 13 in front of the detector 42 is arranged (optically) to eliminate all light except the wavelengths that the target gas molecules can absorb.

[0083] By using Figure 1 The illustrated method 100 for operating a thermal emitter 10 (MEMS heater) having a conductive semiconductor portion 12 (Joule heater structure) may enable effective drift stabilization of a PAS system or NDIR system.

[0084] In physical gas sensors such as photoacoustic spectroscopy (PAS) and non-dispersive infrared (NDIR) sensors, the improved operating characteristics (improved drift behavior) of the transmitter 10 used as a subcomponent of these systems also improves the long-term drift stability of these gas sensor systems. When the transmitter unit is operated by voltage control, the electrically stable transmitter unit 100 achieves a stable ppm reading of the gas sensor. Therefore, the transmitter 10 with improved electrical stability will improve system performance. This is especially true for gas sensor systems with lower ohmic polysilicon MEMS heaters to achieve low voltage domains (e.g., 3V).

[0085] Additional embodiments and aspects are described that may be used alone or in combination with the features and functionality described herein.

[0086] According to one embodiment, a method for operating a thermal emitter having a conductive semiconductor portion includes providing an activation signal having a first energy level (Joule heating level) to the conductive semiconductor portion of the thermal emitter to emit IR radiation during an operation cycle, and providing a refresh signal having a second energy level to the conductive semiconductor portion of the thermal emitter during a refresh cycle, wherein the second energy level is different from the first energy level.

[0087] According to one embodiment, the second energy level of the refresh cycle is set to a value for counteracting dopant segregation in the conductive semiconductor portion of the thermal emitter.

[0088] According to one embodiment, the energy level of the refresh signal is set to a value between 3% and 100% higher than the energy level of the activation signal, or wherein the energy level of the refresh signal is set to a value between 3% and 70% lower than the energy level of the activation signal.

[0089] According to one embodiment, the temperature T2 of the Joule heater structure during the refresh cycle is set to a value between 3% and 30% higher than the temperature T1 of the Joule heater structure during the operating cycle, or wherein the temperature T2 of the Joule heater structure during the refresh cycle is set to a value between 3% and 30% higher than the temperature T1 of the Joule heater structure during the operating cycle. 2 is set to be greater than the temperature T of the Joule heater structure during the operating cycle 1 Low values ​​between 3% and 30%.

[0090] According to one embodiment, the refresh signal has a transient (or pulsed) signal shape, and wherein the activate signal has a transient (or pulsed) signal shape.

[0091] According to one embodiment, the method also includes setting the first energy level by setting at least one of the signal strength of the activation signal, the activation length (or pulse length), and the number of activation states (or pulses), and setting the second energy level by setting at least one of the signal strength of the refresh signal, the activation length (or pulse length), and the number of activation states (or pulses).

[0092] According to one embodiment, the method further comprises performing a refresh cycle between two operation cycles.

[0093] According to one embodiment, the method further comprises performing a refresh cycle each time between two operation cycles.

[0094] According to one embodiment, the method further comprises carrying out a refresh cycle before or each time before an operation cycle of the thermal emitter.

[0095] According to an embodiment, the activation signal comprises a plurality of DC activation pulses, and wherein the refresh signal comprises at least one DC refresh pulse.

[0096] According to one embodiment, a thermal emitter device includes a thermal emitter having a conductive semiconductor portion, and a driver element (e.g., an ASIC or a microcontroller) for providing an activation signal having a first energy level to the conductive semiconductor portion of the thermal emitter to emit IR radiation during an operation cycle, and providing a refresh signal having a second energy level to the conductive semiconductor portion of the thermal emitter during a refresh cycle, wherein the second energy level is different from the first energy level.

[0097] According to an embodiment, the driver element is arranged for providing an activation signal and a refresh signal to the thermal emitter for implementing the method steps of the method for operating a thermal emitter.

[0098] According to one embodiment, the conductive semiconductor portions form branched current paths separated by a central portion of the heat emitter.

[0099] According to one embodiment, the electrically conductive semiconductor portion of the thermal emitter is arranged on a cavity in the substrate, the thermal emitter being attached to the cavity.

[0100] According to one embodiment, the electrically conductive semiconductor portion of the thermal emitter comprises polysilicon or SiC material.

[0101] According to one embodiment, a MEMS gas or fluid sensor includes a thermal emitter for emitting thermal radiation, a measurement volume having a target gas or target fluid and providing an optical interaction path for the emitted thermal radiation, and an acoustic transducer or a direct thermal detector for providing a detector output signal based on the optical interaction of the emitted thermal radiation with the target gas or target fluid in the measurement volume.

[0102] Although some aspects are described as features in the context of an apparatus, it is obvious that such description can also be seen as a description of corresponding features of a method. Although some aspects are described as features in the context of a method, it is obvious that such description can also be seen as a description of corresponding features related to the functionality of the apparatus.

[0103] In the above detailed description, it can be seen that in order to simplify the present disclosure, various features are grouped together in the examples. This method of disclosure should not be interpreted as reflecting the intention that the examples claimed for protection require more features than those explicitly cited in each claim. On the contrary, as reflected in the following claims, the subject matter may be present in less than all the features of a single disclosed example. Therefore, the following claims are incorporated into the detailed description, where each claim can serve as a separate example. Although each claim can serve as an example alone, it should be noted that although a dependent claim may refer to a specific combination with one or more other claims in a claim, other examples may also include a combination of a dependent claim with the subject matter of other dependent claims, or a combination of each feature with other dependent or independent claims. Unless it is expressly stated that a specific combination is not intended, such a combination will be proposed herein. In addition, it is also intended to include the features of a claim in any other independent claim, even if the claim is not directly subordinate to the independent claim.

[0104] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that various alternatives and / or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present embodiments. This application is intended to cover any adaptation or variation of the specific embodiments discussed herein. Therefore, it is intended that the embodiments are limited only by the claims and their equivalents.

Claims

1. A method (100) for operating a thermal emitter (10) having an electrically conductive semiconductor portion (12), comprising: During an operation cycle, an activation signal (S) having a first energy level (E1) is provided (110) to the conductive semiconductor portion (12) of the thermal emitter (10). ACT ) to emit IR radiation (R), and During a refresh cycle, a refresh signal (S) having a second energy level (E2) is provided (120) to the conductive semiconductor portion (12) of the thermal emitter (10). REF ), wherein the second energy level (E2) is different from the first energy level (E1).

2. The method (100) of claim 1, wherein the second energy level (E2) of the refresh cycle is set to a value for counteracting dopant segregation in the conductive semiconductor portion (12) of the thermal emitter (10).

3. The method according to claim 1 or 2, The refresh signal (S REF ) is set to be higher than the activation signal (S ACT ) has an energy level (E1) higher than a value between 3% and 100%, or The refresh signal (S REF ) is set to be higher than the activation signal (S ACT ) has an energy level (E1) lower than a value between 3% and 70%.

4. The method (100) according to any one of the preceding claims, wherein during the refresh cycle, the temperature T2 of the Joule heater structure (12) is set to a value between 3% and 30% higher than the temperature T1 of the Joule heater structure (12) during the operating cycle, or Wherein during the refresh cycle, the temperature T2 of the Joule heater structure (12) is set to a value between 3% and 30% lower than the temperature T1 of the Joule heater structure (12) during the operation cycle.

5. The method (100) according to any of the preceding claims, wherein the refresh signal has a transient (or pulsed) signal shape, and wherein the activate signal has a transient (or pulsed) signal shape.

6. The method (100) according to any one of the preceding claims, further comprising: The first energy level (E1) is set by setting at least one of the signal strength, activation length (or pulse length) and number of activation states (or pulses) of the activation signal, and The second energy level (E2) is set by setting at least one of a signal strength, an activation length (or pulse length), and a number of activation states (or pulses) of the refresh signal.

7. The method (100) according to any one of the preceding claims, further comprising: The refresh cycle is implemented between two operating cycles or every time between two operating cycles.

8. The method (100) according to any one of the preceding claims, further comprising: The refresh cycle is performed before or each time before an operation cycle of the heat emitter (10).

9. The method (100) according to any one of the preceding claims, wherein the activation signal (S ACT ) comprises a plurality of DC activation pulses, and wherein the refresh signal (S REF ) includes at least one DC refresh pulse.

10. A heat emitter device (20), comprising: A thermal emitter (10) having a conductive semiconductor portion (12), and A driver element (22) for During an operation cycle, an activation signal (S) having a first energy level (E1) is provided (110) to the conductive semiconductor portion (12) of the thermal emitter (10). ACT ) to emit IR radiation (R), and During a refresh cycle, a refresh signal (S) having a second energy level (E2) is provided (120) to the conductive semiconductor portion (12) of the thermal emitter (10). REF ), wherein the second energy level (E2) is different from the first energy level (E1).

11. The thermal emitter device (20) according to claim 10, wherein the driver element (22) is arranged for providing the activation signal and the refresh signal to the thermal emitter for performing the method steps of the method (100) according to claims 1 to 9.

12. The heat emitter device (20) according to claim 10 or 11, wherein the electrically conductive semiconductor portions (12) form branched current paths separated by a central portion (12-1) of the heat emitter (10).

13. The heat emitter device (20) according to any one of claims 10 to 12, wherein the electrically conductive semiconductor portion (12) of the heat emitter (10) is arranged on a cavity (18) in a substrate (16), the heat emitter (10) being attached to the cavity.

14. The thermal emitter device (20) according to any one of claims 11 to 13, wherein the electrically conductive semiconductor portion (12) of the thermal emitter (10) comprises polysilicon or SiC material.

15. A MEMS gas or fluid sensor (50; 50-1; 50-2), comprising: A heat emitter device (20) for emitting thermal radiation according to any one of claims 10 to 14, a measurement volume (30) having a target gas or a target fluid and providing an optical interaction path (32) for the emitted thermal radiation (R), and An acoustic transducer (40) or a direct thermal detector (42) for providing a detector output signal (S) based on an optical interaction of the emitted thermal radiation (R) with the target gas or the target fluid in the measurement volume (30) OUT ).

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