Abstract:
A device for generating infrared radiation includes at least one heating element and at least one radiating element for irradiating the infrared radiation. The heating element is a micromechanical, two-dimensional heater structure. The heating element may be applied on the radiating element using hybrid technology such that the radiation element has at least one side facing the heating element and at least one side facing away. The radiating element may be ceramic substrate.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to the infrared detection devices and more specifically to the detection of a gas or fluid using a micromechanical infrared sensor. 
   BACKGROUND INFORMATION 
   A spectroscopic measurement is used as a long-term stable and reproducible measuring principle in, for example, carbon dioxide sensors. The radiation of an infrared source in the MIR (mean infrared range) on a typical CO 2  absorption wavelength (e.g., 4.3 μm) is compared with the radiation of the infrared source for a reference wavelength (e.g., 4 μm). Information on whether and in what concentration CO 2  is present in the absorption path between the radiation source and the detector results from the comparison of the radiation intensity for the absorption wavelength and the reference wavelength. 
   Because of economic reasons, incandescent lamps, the irradiation spectrum of which reaches into the MIR range, are frequently used as the radiation source. The main limitation for the usability of the lamps is the MIR absorption of the lamp glass which limits the utilization of the incandescent lamps to the IR wavelength range&lt;4.5 μm. Special glass qualities also absorb a large portion of the IR radiation in the wavelength range of just 4 μm. Due to the irradiation maximum of the incandescent filament of the lamp in the range of visible light (wavelength&lt;1 μm), the largest portion of the generated radiation additionally represents a power loss which may also have a detrimental effect on the measuring accuracy. 
   Ceramic infrared radiators, special incandescent lamps made of silica glass, or devices known in the laboratory field as black-body radiators may represent alternatives to an incandescent lamp as an infrared source. If one intends to detect other gases, such as CO for example (absorption at 4.6 μm), using the above-mentioned spectroscopic measuring principle, one has to resort to these IR sources which are much more expensive and more complex. 
   An electrically operable tubular infrared radiator which is situated in a reflector is described in German Patent document No. 198 12 188. The ceramic infrared radiator has a carrier tube made of sintered Al 2 O 3  and a heater coil made of resistor wire which is wound on the carrier tube, the resistor wire being enclosed and held by a cover layer made of a ceramic compound which, at least on its surface, is dyed black. The reflector is designed as a molded body made of ceramic material having a focal line, the reflector being gold-plated on its side facing the carrier tube including the heater coil. The carrier tube including the heater coil is situated in the area of the reflector&#39;s focal line. 
   A carbon dioxide sensor having a substrate carrier with a heating element including power terminals attached to its bottom side is described in detail in German Patent document No. 44 37 692. Interdigital electrodes with a carbon dioxide-sensitive material on top are situated on the top side. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a device for generating infrared radiation, including at least one heating element and at least one radiating element for irradiating the infrared radiation. The heating element is a micromechanical, two-dimensional heater structure. This is a compact infrared radiator of small overall size and light weight, for specific applications, such as in the automotive field. 
   An example embodiment of the present invention provides that the heating element is applied on the radiating element using hybrid technology, the radiating element having at least one side facing the heating element and at least one side facing away from the heating element. Hybrid technology makes a particularly small overall size possible. 
   An example embodiment of the present invention provides that the radiating element is a ceramic substrate. Ceramic materials are characterized by a particularly high emissivity factor. 
   An example embodiment of the present invention provides that the ceramic is an aluminum oxide ceramic. 
   An example embodiment of the present invention provides that the side of the ceramic substrate facing away from the heating element is blackened, or is coated with a material that emits infrared radiation particularly well. A further increase in emissivity is thereby achieved. 
   An example embodiment of the present invention provides that ruthenium oxide is the material that emits infrared radiation particularly well. 
   An example embodiment of the present invention provides that the heater structure is made of platinum. Platinum is suited for the long-term stable generation of high temperatures and does not readily oxidize. 
   An example embodiment of the present invention provides that the heating element has at least one side facing away from the radiating element, and the side facing away from the radiating element is polished. The intensity of the infrared radiation emitted toward the side facing away from the radiating element is thereby further reduced. 
   An example embodiment of the present invention provides that the device is designed as a micromechanical element, the bottom side of the heating element being directly or indirectly applied on a diaphragm, and the top side of the heating element being directly or indirectly coated with an emission layer. The irradiations on the bottom side of the heating element are reduced due to the application above the diaphragm. The term “indirect” is to be understood as layers that are by all means not essential to the present invention but are required technologically such as, for example, a protective layer or an adhesion-enhancing layer, and that may be situated between the heating element and the diaphragm, and/or between the heating element and the emission layer, for exemplary purposes only. 
   An example embodiment of the present invention provides that the emission layer is made of ruthenium oxide. 
   An example embodiment of the present invention provides that there is a hollow space below the side of the diaphragm facing away from the heating element. A reduction in the irradiation toward this side is thereby achieved. 
   An example embodiment of the present invention provides that the diaphragm is designed using micromechanical silicon technology. 
   An example embodiment of the present invention provides that the device is used for analyzing gases. 
   An example embodiment of the present invention provides that the device is used within the scope of an optical carbon dioxide sensor for determining the carbon dioxide concentration in the air of a vehicle passenger compartment. 
   An example embodiment of the present invention provides that the infrared radiation is emitted to a selected side or direction. This makes it possible to focus the radiation in the direction toward the receiver (e.g., a radiation detector), needless irradiation in other directions being avoided or reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an example embodiment of the heating element of the infrared source. 
       FIG. 2  shows the applied emission layer. 
       FIG. 3  shows a lateral cross section through the micromechanical infrared source. 
       FIG. 4  shows the top view of a further example embodiment of the micromechanical infrared source. 
   

   DETAILED DESCRIPTION 
   The use of a micromechanical sensor element or a sensor element manufactured using hybrid technology represents a cost-effective radiation source for a spectroscopic gas sensor or fluid sensor having an irradiation maximum in the desired MIR range. This makes it possible to omit material that absorbs in this wavelength range, such as glass, for example, in the radiation path of the IR source configured in this way. 
   The power consumption and the overall size of the radiation source are reduced since the predominant portion of the radiation is generated in the required wavelength range. For spectroscopic sensors, the radiation proportion of IR sources (e.g., incandescent lamps) generated in the more short-waved range is dispensed with as undesirable power loss. The yield of the desired infrared radiation is increased by omitting a cover for the radiation source having a material that absorbs in the relevant wavelength range. 
   A radiation source manufactured using hybrid technology is described below as the first exemplary embodiment. 
   A heater structure, such as the heater structure of chemical sensor elements, is applied on a ceramic substrate, e.g., aluminum oxide ceramic. For example, a platinum structure is used for this purpose. This platinum structure as the heater structure may generate long-term stable temperatures up to a range of approximately 450° C. (725 K). 
   According to Wien&#39;s displacement law, the maximum of the radiant power of a black-body radiator is reached at a wavelength of approximately 2890/T [μm K], where T indicates the temperature of the radiator. A desired radiation maximum at a wavelength of 4 μm results in a required temperature of 722 K (450° C.); a desired radiation maximum at a wavelength of 4.3 μm results in a required temperature of 672 K (400° C.). 
   Real radiation sources differ from the ideal for a black-body radiator by their emissivity factor which, depending on the material, deviates to a greater or lesser extent downward from the maximum irradiation of the black-body radiator. This means that the irradiated power of a radiation source is dependent on the absorption factor of the radiator material at the selected temperature. The absorption factor is at the same time the emissivity factor according to Kirchhoff&#39;s law. The absorption factor and the emissivity factor show dispersion, i.e., they are temperature-dependent. Metals, in particular bare metals, have emissivity factors at around 10%; in contrast, ceramic materials such as, for example, AlO have emissivity factors close to 90%. 
   A directional effect of the IR radiation source is established by applying a platinum heater structure (e.g., over the full surface) on one side of an aluminum oxide carrier ceramic and using the opposite (back) side of the carrier ceramic without metallic plating as the irradiation surface. As the directional effect becomes stronger, the more the emissivity factors of the materials used differ. Therefore, it is advantageous to select a metal, such as Pt, which does not oxidize readily, for the metal plating of the heater side since, in the long term, a poorly emitting, bare metallic surface may be ensured even at the required temperatures of up to 450□C. Polishing of this metal surface also has an advantageous effect. 
   Blackening of the surface or coating the ceramic with a particularly well emitting material likewise has an advantageous effect on the ceramic side used as the irradiation surface. Coatings with ruthenium oxide, a material which is also used in thick film technology for manufacturing resistors, have been shown to be favorable. 
   In a second example embodiment, the radiation source is configured using a micromechanical element. 
   Similar to the above-described configuration using hybrid technology, a micromechanical configuration may also be implemented in which a thin diaphragm for thermally insulating the infrared radiation source is created, for example, using micromechanical silicon technology, a Pt heater structure, as a heater coil or a wave-shape structure, for example, being applied on the diaphragm. Comparable to the micromechanical configuration of a chemical sensor or to the micromechanical configuration of an IR detector chip, an emission layer, e.g., the above-described ruthenium oxide, is applied over this heater structure using a dispensing method. 
   The emission layer has the function of improving the otherwise poor emissivity of silicon and platinum in the infrared range. 
   The assembly of the micromechanical radiator takes place similar to the assembly of micromechanical sensor elements, in premolded housings, for example, the covers of which are provided with an opening for the emission of the infrared radiation. The directional effect of the infrared radiation may be achieved via the shape and size of the cover opening. 
   The present invention is described below in conjunction with  FIGS. 1 through 4 . 
     FIGS. 1 through 3  refer to the second example embodiment, and  FIG. 4  refers to the first example embodiment. 
     FIG. 1  shows a wave-shape heater structure  100  which is heated to the required radiator temperature by current flow (generation of ohmic heat). The heater is a platinum structure, as an exemplary embodiment. For a temperature-controlled operation, a second platinum structure may optionally be utilized as a temperature detector (Pt has a positive temperature coefficient, i.e., the specific resistance increases with rising temperature). Also for temperature control, the heater structure itself (the resistance of the heater arises from the ratio (applied voltage)/current) may be utilized for temperature restoration. 
     FIG. 2  shows emission layer  200  applied on the chip shown in  FIG. 1  via a micro-dispensing method. 
   A lateral cross section through a micromechanical configuration is illustrated in  FIG. 3 . A thin diaphragm  302  (made of silicon dioxide for example) is applied over a cavern or hollow space  300  which is etched out of substrate material  301 . An adhesion-enhancing layer  303  is situated on top of diaphragm  302 . This layer  303  provides for the cohesion between the diaphragm  302  and the layers applied on it. This is a protective layer  304 , heater structure  305  being applied on it. Four segments of heater structure  305  are shown in cross section in  FIG. 3 . The two outer segments are slightly wider since the bonding contacts must also be applied to them. Furthermore, passivation layer  306  and emission paste  307  are indicated in  FIG. 3 . Another protective layer  308  may be situated between emission paste  307  and the heater structure (as in the embodiment according to  FIG. 3 ). 
   A radiation source according to the first example embodiment is illustrated in  FIG. 4 .  FIG. 4  shows a wave-shape heater structure  400  on a ceramic substrate  401 . In one embodiment, it is possible to configure the conductors of the heater structure to be slightly wider and the insulating gaps between the conductors to be narrower.