DE102012103662B3 - Infrared radiation source e.g. infrared-thin film radiator of infrared absorption measurement system used for analysis of exhaust gas of passenger car, has emissivity-increasing layer formed on heating conductor resistor layer - Google Patents

Abstract

The infrared radiation source (1) has cantilevered layer structure that is formed from radiator element (2), heating conductor resistor layer (3) and emissivity-increasing layer (4). The emissivity-increasing layer is provided on heating conductor resistor layer so as to increase the optical emissivity of the radiator element. The heating conductor resistor layer and emissivity-increasing layer are made up of equaled material. The heat-conducting resistance layer is porous layer. An independent claim is included for method for manufacturing infrared radiation source.

Description

The present invention relates to an infrared radiation source which is formed as an infrared thin-film radiator, wherein an infrared radiating emitter element of the infrared thin-film radiator is formed as a cantilevered layer structure having at least one Heizleiter resistance layer and at least one emissive grain layer formed thereon. The invention further relates to a method for producing an infrared radiation source embodied in the form of an infrared thin-film radiator, wherein at least one emissivity gradient layer is provided on at least one heat conductor resistance layer to form an infrared radiation radiating element of the infrared thin-film radiator and the radiator element is patterned into a self-supporting layer structure.

Infrared absorption analysis is the leading method for detecting substances, especially fluids (i.e., gases and liquids), and their concentration. In particular, the spectral range λ = (1... 20) μm is of technical interest because here are the characteristic absorption bands of many compounds. Due to the high stability and reproducibility of the infrared gas analysis, this is mandatory for certain investigations by the legislator (eg car exhaust gas analysis). Gas detectors, which are based on the principle of infrared absorption spectrometry, are distinguished over solid-state sensors by a high resolution, good long-term stability and reproducibility and universal applicability. A distinction is made here between dispersive and non-dispersive infrared gas analyzers, the latter determining the market.

The history of non-dispersive-infrared (NDIR) gas analysis began in 1938 with the filing of the patent application DE 730478 A , An infrared gas analyzer essentially consists of an infrared radiation source, a cuvette with the gas to be measured and an infrared detector, which are mounted along an optical axis. As infrared radiation source, a thermal emitter is usually used, which emits electromagnetic radiation with a continuous spectrum due to its temperature. While in the field of infrared sensors a wide range of detectors with sufficient signal-to-noise ratio and low price are available (thermopiles, pyroelectric detectors), in the field of infrared radiators, there are no specified components with high radiant power, which has a long service life and have a high efficiency, are also electrically modulated and can be produced at a cost price.

An ideal, so-called black body radiator radiates at each wavelength λ the maximum amount of energy for all possible radiators and therefore has a wavelength-independent emissivity ε of one. The behavior of a blackbody is described by Planck's law of radiation. For its spectral specific radiation M _{λS the} following applies:

Here, c ₁ , c ₂ denote the Planck radiation constants and T the temperature. By integrating the spectral specific radiation over all wavelengths, one obtains the total specific radiation M _{S of} a black body radiator, which describes the radiation power emitted by a surface: M _S (T) = σ T ⁴ (2)

Here σ denotes the Stefan-Boltzmann constant. Real emitters have an emissivity of less than one, which is usually wavelength-dependent. The emissivity ε denotes the ratio of the spectral specific emission M _{λ of} an arbitrary temperature _emitter to the spectral specific emission M _{λS of} a black body _emitter of the same temperature:

This makes it possible to calculate the spectral specific radiation of any temperature radiator from the laws of the black body radiator, if its emissivity is known. For the total specific emission of my temperature radiator with wavelength-independent emissivity is therefore: M (ε, T) = εσT ⁴ (4)

The radiant power accordingly depends on the fourth power of the temperature and is directly proportional to the emissivity. For a high radiation power therefore a material must be used as a heating conductor, which can be operated at very high temperature. In addition, it must have a high and wavelength-independent emissivity that is as close to unity as possible.

The most widely used heating conductor materials are Kanthal and nickel-chromium alloys, as these can be operated in an open atmosphere. Kanthal stands for a trademark and refers to an alloy family of iron, chromium and aluminum (FeCrAl). It consists mainly of iron, with a typical chromium content of 20% -30% and an aluminum content of 4% -7.5%. These alloys are known for their very good temperature resistance and have a relatively high electrical resistivity. Therefore, they are used in a variety of resistance and high temperature applications and z. B. used as heating element often for heating elements. In these heating elements, electrical energy is converted into heat energy by passing an electric current through the heating conductor. Finally, heat is generated as a result of the resistance to current flow emanating from it. The resistance wire must be stable in air even at very high temperatures. Kanthal alloys form a thin layer of aluminum oxide on the surface, which protects them from corrosion and aging. Typical Kanthal alloys have a melting point of about 1200 ° C and can therefore be operated stably at temperatures of up to 1000 ° C in air. For use as infrared radiator disadvantageous is the low emissivity of the Kanthal alloys, whereby they have a low optical efficiency. In the technically interesting wavelength range λ = (1 ... 20) microns, the average emissivity is about 0.4 and is wavelength-dependent.

Nickel-chromium (NiCr) is a non-magnetic alloy of nickel and chromium that also contains small amounts of other elements, such as nickel. As silicon, zirconium, calcium, boron, iron, aluminum, rare earths, hafnium and production-related impurities may contain. A common alloy is 80% nickel and 20% chromium, which is often used as a resistance wire and is considered the oldest documented heating conductor alloy. Nickel-chrome is corrosion resistant even at very high temperatures. It has a melting point of about 1400 ° C, which allows it to operate at temperatures up to 1200 ° C. It has a relatively high specific electrical resistance for metals and is therefore very often used in electrical heating elements, eg. As hair dryer, electric oven, soldering iron, toaster. Another disadvantage is the very low optical efficiency by the, for metals usual, very low emissivity of typically 0.2 in the technically interesting wavelength range λ = (1 ... 20) microns.

Four different types of thermal infrared radiators are currently used in previous applications in gas analysis: filament lamps, resistance coils, globars and thin-film radiators. Occasionally, light and laser diodes are also used for the near and medium infrared range. But these are not thermal radiators, which is why they do not obey the above radiation laws. Since they emit only radiation of a wavelength or in a very narrow wavelength range, they find practically little use in infrared spectroscopic devices, usually only when a specific gas to be detected, such. B. in the document DE 60216988 T2 described. Another disadvantage is the high price, which increases with increasing emission wavelength, and the usually insufficient long-term stability.

Filament lamps, such. B. incandescent tungsten filament, have a very high radiant power, since the temperature of the tungsten filament can be up to 3000 ° C. For this purpose, they must be operated in a protective gas atmosphere or in a vacuum. The technology for producing a tungsten filament in an evacuated glass bulb is so mature that these lamps are very inexpensive. However, the glass is no longer sufficiently transparent to infrared radiation above 4.5 microns wavelength, so this greatly limits the application area or glass bulb with special windows, z. As sapphire, must be used for improved infrared transmission. Because of the different thermal expansion coefficients, however, such materials are difficult to fuse with glass. Applications at wavelengths of more than 5 μm are therefore not feasible with commercial incandescent lamps. Filament lamps without glass housing, such. B. in the document US 5,438,233 A Although they are characterized by a large spectral bandwidth, they can only be modulated poorly and have a large design and a high price. Another disadvantage is the, for metals typical, very low Emissivity in the infrared range, whereby these emitters have a low optical efficiency and are not very efficient.

Spotlight with resistance coils of a thin, mostly meander-shaped structured metal heating foil, z. B. Kanthal or nickel-chromium ( US 5,939,726 A ), which can be operated in contrast to incandescent tungsten filaments in an open atmosphere, show a broadband infrared spectrum. These radiators are typically operated at temperatures of up to 900 ° C. However, they are not very efficient due to the low emissivity. In addition, their heat capacity is still too large, allowing them to modulate only a few hertz (typically <10 Hz). The technologies used to date for structuring (eg ion beam etching) also make them relatively expensive. Far more problematic, however, is the electrical contacting of the thin heating conductor foil, which is usually realized by means of resistance welding. The welded joint is a weak point because it is subjected to very high thermal loads. For this reason, these emitters have a reduced life.

A Globar is a ceramic glow rod, mostly made of silicon carbide ( DE 1123841 A ) or fused silica ( EP 372166 A2 ), with a typical diameter of 5 mm to 8 mm and a length of 20 mm to 50 mm. This type of emitter is very often used in infrared spectroscopic equipment. Globars have a relatively high emissivity (typically 0.8) and are electrically heated to temperatures of up to 1650 ° C, giving them high radiant power. In addition, they can be operated in an open atmosphere. Because of their large thermal mass, a long heating process is necessary, whereby they can not be electrically modulated and can only be operated with direct current radiation. For the realization of alternating radiation, therefore, the use of a mechanical chopper is required, which makes the overall system larger, more expensive and prone to failure. Another disadvantage is that Globare must be operated with water cooling, which is why they are not suitable for compact infrared spectroscopic devices. A much bigger problem is their short lifespan. The brittle material quickly cracks and breaks due to the high temperature load and by environmental factors such as vibration or even installation difficulties.

Thin-film radiators based on very thin resistance layers on silicon membranes, eg. B. known from the publication US 5,910,659 A , have a low thermal mass, which makes them good for electrical modulation. With a duty cycle of 50%, commercially available thin-film radiators achieve an acceptable modulation depth of up to about 100 Hz. However, they have too low a radiant power for many applications since their maximum operating temperatures are about 450 ° C to 750 ° C.

In order for these emitters to achieve high optical efficiency, an emissivity grade layer is usually deposited on the resistive layer. In the publication CH 695327 A5 In order to increase the optical emissivity proposed to prove the thermal infrared radiator with carbon nanotubes. However, this requires an evacuated lamp housing and thus has the disadvantages of conventional filament lamps described. In the publication JP 2001221689 A a layer of silicon carbide is applied to the resistive layer to increase the emissivity and in the document US 6,297,511 B1 The self-supporting resistance layer is completely surrounded by an insulating layer of silicon nitride, silicon dioxide or silicon carbide, which, in addition to increasing the emissivity, simultaneously protects the resistance layer from oxidation. This multi-component system consisting of heat conductor resistance layer and emitter layer has the disadvantage that the long-term stability and lifetime of the radiator are reduced by the different thermal expansion coefficients of the various materials or that the operating temperature must be reduced to achieve a required lifetime, which is why these types of radiators only a low radiant power have. In the publication US 5,939,726 B1 For example, an infrared radiation source is proposed in which a NiCr film is used as the heating conductor, which may be oxidized or otherwise coated to increase the emission.

A few years ago it was speculated whether the emission characteristics of thermal infrared radiators can be changed by means of photonic crystals and a higher spectral power density can be achieved than with a black body radiator. In this context it was also discussed whether the Planck radiation law is valid. The article "Thermal Radiation from Photonic Crystals: A Direct Calculation" (Phys. Rev. Lett., 93 (21), 2004, pp. 213905-1 to 213905-4) verified the accuracy of Planck's law of radiation for photonic crystals that is, they do not emit more than a blackbody radiator. Photonic crystals act like very precise optical filters. By a periodic arrangement of different dielectrics or materials with different dielectric constants, a photonic band structure is realized in which certain energy states in the photonic crystals are forbidden. Consequently, due to their structure, they only emit radiation of specific wavelengths. In the publication US 7,498,574 B2 such a radiation source is described. For many applications, however, a broadband radiation source is required.

However, photonic crystals can also be used to increase the optical efficiency of thermal infrared radiators in comparison to known thermal radiation devices and thus come close to the ideal of a black body radiator. A corresponding device is z. B. in the document DE 10 2005 008 077 A1 described. The need to use different dielectrics or materials with different dielectric constants to realize a photonic crystal, has the disadvantage that the lifetime of the radiator is significantly reduced by the different thermal expansion coefficients of the different materials, as by the strong thermal load during heating of the radiator Delamination of the layers is the result. Another significant disadvantage of photonic crystals lies in the complex, complicated and thus very expensive production of the corresponding structures. In order for the photonic crystals to function exactly and to achieve a high emissivity, the fine structures must be perfectly formed. This is technologically possible only with great effort (eg electron beam lithography), if at all feasible.

At present there is no technical solution for an efficient, electrically modulatable thermal infrared radiator with an emissivity close to the ideal of a black body radiator that can be operated at high temperatures to achieve high radiant power while having good long-term stability and long life. For use in infrared spectroscopic devices a variety of infrared radiation sources have been developed, most of which can only be operated at DC or at very low modulation frequencies due to their large thermal mass. Since electrical alternating signals can be processed much better than DC signals and the pyroelectric detectors most commonly used in infrared spectroscopic equipment are only changeable light sensitive, rectifier sources usually use mechanical choppers which temporally modulate the radiation flux of the infrared radiation source. However, these mechanical choppers make the overall system bigger, more expensive and prone to failure. Direct light emitters also have a very poor efficiency, because because of their large thermal mass more electrical power must be supplied as radiation power is emitted. Besides, they are relatively expensive. Of the thermal radiators listed, only thin-film radiators have the direct potential for microsystem technology production and thus for cost-effective mass production. Due to their low thermal mass, they can also be well modulated electrically. However, they have too low a radiant power for many applications, since their maximum operating temperatures are about 450 ° C to 750 ° C. Since a high radiation power is achieved by a high temperature, as a heat conductor therefore a material must be used that can be operated at very high temperature. For cost reasons, operation in an open atmosphere is advantageous. Here are essentially only high-melting metals and alloys, such. As Kanthal and nickel-chromium, in question. However, these have a very low emissivity in the infrared range, which is typically significantly less than 0.4. As a result, they are not very efficient. Conventional solutions for increasing the emissivity by applying an additional layer have the disadvantage that due to the high thermal load either the life and long-term stability of the radiator due to delamination can be significantly reduced or to achieve a required life, the operating temperature must be significantly reduced.

It is therefore the object of the present invention to provide an efficient and electrically modulatable thermal infrared radiation source with high radiation power and high lifetime. Furthermore, a suitable method for producing such an infrared radiation source should be provided.

The object is achieved, on the one hand, by an infrared radiation source of the abovementioned type, in which the heat conductor resistance layer and the emission gradient layer are formed from the same material, the emission grade layer, in contrast to the heat conductor resistance layer, being a porous layer, preferably a on the Heizleiter resistance layer deposited porous layer is.

Thus, in the infrared radiation source according to the invention, both the at least one heat conductor resistance layer and the at least one emission grade elevation layer formed thereon can be produced from the same high-temperature resistant material. As a result, an infrared radiator having high radiation performance and a long service life can be provided. In this case, a suitable for a high emission power can on the resistance of the non-porous Heizleiter resistance layer Temperature of the radiator element can be adjusted, while provided on the Heizleiter resistance layer provided porous, emissionsgrader-enhancing layer for a high emission of radiation.

Since the infrared radiation source according to the invention is designed as an infrared thin-film radiator whose radiating element is realized as a self-supporting layer structure, the infrared radiation source according to the invention has a low thermal mass and can thus be well modulated. Furthermore, the infrared radiation source according to the invention has the advantage that it has a simple structure of the radiator element, so that the infrared radiation source can be produced inexpensively in large numbers by conventional microelectronics or micromechanical manufacturing processes.

Due to the low heat capacity, the good thermal insulation and the high emissivity of the planar radiating element of the infrared radiation source according to the invention a very high efficiency and thus a very high overall efficiency of the infrared radiation source is achieved.

Preferably, in the case of the infrared radiation source according to the invention, the self-supporting layer structure is formed only from the at least one heat conductor resistance layer and the at least one emission gradient-increasing layer. Thus, the radiator element of the infrared thin-film radiator consists only of a single material. The problems that occur in the prior art through the use of different materials with different thermal expansion coefficients are thus not present in this embodiment of the infrared radiation source according to the invention. Delaminations or the like are therefore not to be feared. Accordingly, this infrared radiation source has a particularly good long-term stability.

In an advantageous variant of the invention, the heating conductor resistance layer is a thin layer. According to the invention, a thin layer is understood as meaning a layer deposited on an auxiliary substrate or sacrificial substrate, which can be produced, for example, by a physical layer deposition method or a chemical layer deposition method. The layer thickness of thin films typically moves below 1 μm, for example in a range of 200 nm to 600 nm. These layer thicknesses are large enough to provide a suitable heat conductor for the infrared radiation source according to the invention, but also thin enough to accommodate the thermal To keep the mass of the radiator element low and thus to achieve a good modulation of the infrared radiation source according to the invention.

According to another option of the present invention, the heating conductor resistance layer may also be a foil. This film can be suitably prefabricated. Films differ from thin layers, in particular in the layer thickness, which in the case of films typically moves above 1 μm, for example in the range from 2 μm to 5 μm. Such films have the advantage that they are mechanically more stable compared to thin films and thus are particularly well suited for the formation of self-supporting structures. So it is when it turns out that to achieve a necessary mechanical stability of the Heizleiter resistance layer must have a thickness of several microns, possibly more economical to use a film instead of a deposited Heizleiter resistance.

It has proved to be particularly favorable if the heat conductor resistance layer and the emission grade increasing layer are formed from a material having a melting temperature at normal pressure of ≥ 1200 ° C. This is called high-melting materials. These materials have the advantage that their maximum operating temperature is very high, that is ≥ 900 ° C, adjustable. Since, as explained above, the fourth power temperature is included in the radiant power, very high radiant powers can be provided for the infrared radiation source according to the invention when using refractory materials for the heat conductor resistance layer and the emission gradient layer.

Particularly suitable materials for the formation of the heat conductor resistance layer and the emission grade increasing layer are, for example, nickel-based alloys, such as Ni _x Cr _1-x alloys with 0 ≤ x ≤ 1, or materials such as tungsten, molybdenum, carbon or platinum. These materials are characterized by particularly high melting temperatures, so that they can be used at very high operating temperatures. Accordingly, particularly high radiation powers of the infrared radiation source according to the invention can be generated when using the materials mentioned. Also of particular interest is the use of nickel-based superalloys, which are specifically made for high temperature applications, to form the heat conductor resistance layer and the emission grade enhancing layer. These superalloys have a complex composition, the main constituent being always nickel. By adding high-melting elements, in particular chromium, cobalt, tungsten, carbon, molybdenum, rhenium, ruthenium, niobium, tantalum and / or titanium, the high-temperature resistance is further improved. Furthermore, the elements cobalt, rhenium, tantalum, ruthenium, vanadium and niobium are very high temperature stable and Thus, they can be used to make the heater resistor layer and the emission grade enhancing layer. But titanium-based alloys are also an alternative. Further, ceramics such as tantalum carbide, tantalum hafnium carbide and hafnium carbide having a higher melting point than tungsten are excellent for the formation of the heat conductor resistance layer and the emission grade elevation layer. In addition to the above-mentioned materials or elements, alloys of these materials are also suitable for producing the heat conductor resistance layer and the emission gradient-enhancing layer. For example, vitallium, which is an alloy of cobalt, chom and molybdenum and has high temperature stability, can be used as such an alloy. Kanthal alloys are also particularly well suited for the formation of the heat conductor resistance layer and the layer that increases the emission grade. In principle, however, other, not mentioned, refractory materials for the formation of the heat conductor resistance layer and the emissions grading layer can be used.

In a preferred embodiment of the infrared radiation source according to the invention, the total layer thickness of at least one Heizleiter resistance layer ≤ 500 nm. In this case, the Heizleiter resistance layer is formed as at least one thin layer, resulting in a low thermal mass of the Heizleiter resistance layer, due to which a good modulatable infrared radiation source can be provided. The alternating signals generated with such an infrared radiation source are easily processed further in a measuring arrangement, so that, for example, small, inexpensive and reliable infrared absorption measuring systems can be made available with the aid of the infrared radiation source according to the invention.

In one embodiment of the present invention, the total layer thickness of the at least one emission grading layer is ≦ 3 μm, preferably ≦ 2 μm. Due to the relatively small layer thickness and the porosity of the at least one emission grading layer results in a low heat capacity of this layer, whereby a total of the infrared radiation source can be made quickly modulated when using a thin film as Heizleiter resistance layer.

To ensure a good electrical modulation even at high frequencies, it has proved to be particularly advantageous if the flat radiator element of the infrared radiation source has a very low heat capacity, which is achieved by a very small total thickness of the radiator element of ≤ 5 microns and the porous structure of the emissionsgraderhöhenden layer becomes.

In another variant of the present invention, the total layer thickness of the at least one emission grading layer may also be ≥ 5 μm. Such layer thicknesses are of interest, for example, for the formation of terahertz emitters which can be used, for example, in body scanners.

In a preferred embodiment of the invention, the radiating element of the infrared radiation source has a meandering shape. In this embodiment, therefore, the at least one heating conductor resistance layer as well as the at least one emission grade increasing layer formed thereon are formed meander-shaped. On the shape and the thickness of the radiator element whose electrical resistance and thus the radiation power can be adjusted appropriately. The structuring of the layers of the radiating element can take place, for example, by means of a mask technique in an etching process.

In an equally favorable embodiment of the invention, the radiator element can also be structured in three dimensions to form internal cavities and / or pits in the heat conductor resistance layer and / or the emission grade elevation layer. This results in the possibility, in particular, to reduce the thermal mass of the heating conductor resistance position, in order to achieve a good modulability of the infrared radiation source according to the invention and nevertheless to realize a high mechanical stability of the radiator element.

By way of example, the heat conductor resistance layer and / or the emission grade elevation layer may have a honeycomb structure. The walls of the honeycomb structure are placed in the thickness direction of the layers of the radiator element, so that the characteristic, polygonal honeycomb shape results in particular in a plan view of an underside of the radiator element. Such structures have a very high mechanical stability and a low mass, so that on the one hand a good modulability and on the other hand a high long-term stability of the infrared radiation source can be achieved.

In a suitable embodiment of the present invention, the cantilevered layer structure is suspended by means of connecting elements in the form of connecting arms or connecting network structures on a support frame of the infrared thin-film radiator. Through the connecting arms or Connecting network structures, a sufficient mechanical connection between the radiating element and the connecting and supporting part of the infrared radiation source is made. In this case, the heat loss is minimized by heat transfer from the radiator element to the other regions of the infrared radiation source, resulting in a high efficiency of the infrared radiation source according to the invention.

It is advantageous if at least two of the connecting elements are provided as electrical conductors or as a carrier for electrical conductors for electrically connecting the radiator element with contact surfaces on the support frame or a surface of the infrared thin-film radiator connected to the support frame. Thus, at the same time as the mechanical connection, electrical connections between the radiating element and the surrounding areas of the infrared radiation source according to the invention can be produced.

The object stated above is also achieved by a method of the type mentioned in the introduction, in which the emission-grade increasing layer is deposited from the same material as the heat conductor resistance layer, but as a porous layer.

In the method according to the invention, the infrared radiation emitting surface of the infrared thin-film radiator according to the invention is formed as a self-supporting structure of at least one thin heat conductor resistance layer and at least one thin, on the Heizleiter resistance layer formed emission grading layer. In this case, the same materials are used for producing the at least one heat conductor resistance layer as well as for producing the at least one emission grading layer. This has the advantage that the heat conductor resistance layer and the emission grade increasing layer have the same thermal expansion coefficients. The latter in turn guarantees a high long-term stability of the trained radiating element even when using high or frequently changing temperatures.

In addition, the method according to the invention provides the possibility of selecting a material with high temperature resistance both for the heating conductor resistance layer and for the emission gradient-increasing layer, so that infrared radiators with high radiation power can be made available. In addition, the method according to the invention can be implemented using typical process steps of microelectronics or micromechanics so that well-modulatable infrared thin-film radiators can be produced in large quantities and at low cost with the method according to the invention. It is particularly favorable in the method according to the invention that in this case the planar radiator element of the infrared radiation source to be formed is formed as a cantilevered one-component system, whereby no delamination of the radiator element can occur.

By virtue of the fact that the at least one heating conductor resistance layer is formed as a non-porous layer and the at least one emissivity-increasing layer is formed as a porous layer, different physical properties can be assigned to the individual layers despite the use of the same materials. Due to the porosity of the emission grading layer, its emissivity in the infrared region is significantly increased, so that an infrared radiation source with a high efficiency can be provided.

Thus, the inventive method allows the production of thermally well insulated, infrared radiation emitting surface elements with low heat capacity and high emissivity of only one material. As a result, high operating temperatures and consequently high radiation powers with a long service life and good electrical modulability of the infrared radiation source formed by the method are possible. The method is characterized by its simplicity and effectiveness, such as the manufacturability of the radiator elements of infrared radiation sources in the wafer assembly, thereby enabling cost-effective mass production of very powerful infrared thin-film radiators.

According to the invention, a porous layer is understood to mean a layer in which pores, gaps, cavities or interstices are present which have a size in the range from 10 ^-9 to 10 ^-6 m. This results in a reduced density of the porous layer compared to conventional, non-porous or homogeneously closed layers, such as the heat conductor resistance layer.

In a particularly suitable embodiment of the method according to the invention, the emission-grade increasing layer is deposited by evaporation of material under high vacuum conditions at an angle of incidence to the surface normal of the heat conductor resistance layer, during which the infrared thin film radiator to be formed rotates. It is recommended to use an angle of incidence of ≥ 70 °, preferably ≥ 80 ° to the surface normal of the heating conductor resistance layer. Such a technique is for example for depositing an absorption layer of a thermal radiation receiver in the patent DE 10 2008 007 674 B3 described. When using this method, formation of the emission-grading layer results in the formation of lobe-like nanorods whose density and number decreases with increasing layer thickness, whereas the diameter and spacing of the nanorods increase during layer growth. As a result, the formed emission grade increasing layer has a porosity that increases along the growth direction of the nanorods. However, the emission-grade increasing layer is mechanically stable and resistant to moisture and solvents.

In principle, however, other process variants for forming the porous emission grading layer according to the present invention come into consideration.

It has proven to be particularly advantageous if the heat conductor resistance layer and the emission grade layer of nickel-base alloys, of nickel-base superalloys, of Ni _x Cr _1-x alloys with 0≤x≤1, of tungsten, of molybdenum, of carbon, platinum, tantalum, vanadium, titanium based alloy, rhenium, niobium, cobalt, or an alloy of at least two of these materials. These materials are characterized by high melting points, so that when using these materials, the manufactured infrared thin-film radiator can be operated at high temperatures to allow a high radiant power. Of the materials mentioned nickel-chromium alloys are particularly well suited according to the invention, since nickel-chromium alloys have a relatively high electrical resistivity and thus are particularly good heat conductors and beyond can be operated in an open atmosphere and not as tungsten or carbon evacuated spotlight housing require. Furthermore, nickel-chromium alloys, but especially also pure chromium, are readily thermally evaporable, so that these materials can be used particularly well in the process according to the invention. With NiCr operating temperatures of an infrared radiation source of up to 1200 ° C are possible. This means a doubling of the operating temperature compared to commercial thin-film radiators and thus an approximately sixteen times higher radiant power. The use of NiCr guarantees a long service life and good long-term stability of the infrared radiation source to be formed. In addition, a variety of other materials are suitable for the formation of the heat conductor resistance layer and the emissions grading layer, which have already been mentioned above.

In a preferred embodiment of the method according to the invention, the heat conductor resistance layer and the emission grade elevation layer are deposited in successive steps of one and the same layer deposition process. Preferably, this layer deposition process is a high vacuum process. As a result, the infrared thin-film radiator can be produced with very high quality and particularly effective. Since it is provided according to the invention to form both the heat conductor resistance layer and the emission gradient layer from the same material, the layer deposition process in which both layers are deposited in successive steps is therefore also particularly easy to design.

It is recommended in a variant of the invention to first deposit the heat conductor resistance layer on a sacrificial material, wherein the sacrificial material for exposing the radiator element is removed after deposition of the emission grading layer on the heat conductor resistance layer. The sacrificial material thus initially forms only an auxiliary backing on which the layer deposits can be made. However, for the function of the infrared radiation source to be formed, the sacrificial material is not required, but rather annoying, so that it is subsequently removed. The removal of the sacrificial material can be done for example by means of etching.

In another variant of the method according to the invention, the heating conductor resistance layer is first provided as a prefabricated film and applied to a sacrificial material. Thereafter, the emission grade increasing layer is deposited on the heat conductor resistance layer and subsequently the sacrificial material is removed to expose the radiating element. This method can also be used for mass production, wherein inexpensive films with a suitable thickness can be used to form the radiating element of the infrared radiation source.

Preferably, in accordance with an embodiment of the method according to the invention, the heat conductor resistance layer and the emission gradient-increasing layer are structured in meandering fashion using a masking technique. By this structuring the shaping of the radiator element takes place. Due to the shape of the meander, the electrical resistance of the infrared radiation source to be formed is adjustable.

In a further embodiment of the present invention, the heat conductor resistance layer can also be etched honeycomb-shaped. In this way, the thermal mass of the heating conductor resistance position can be suitably adjusted in order to achieve good modulability of the infrared radiation source to be formed.

Preferred embodiments of the present invention, their structure, function and advantages are explained in more detail below with reference to figures, wherein

1 schematically shows a cross section through an embodiment of an infrared radiation source according to the invention;

2 to 6 schematically show a possible process sequence for forming an infrared radiation source according to the invention;

7 schematically shows a plan view of a variant of an infrared radiation source according to the invention;

8th schematically shows a plan view of another embodiment of an infrared radiation source according to the invention; and

9 schematically shows a plan view of a further embodiment of an infrared radiation source according to the invention.

1 shows schematically in a cross section a possible embodiment of an infrared radiation source according to the invention 1 , The infrared radiation source 1 is designed in the form of an infrared thin-film radiator. The infrared radiation source 1 has a radiating element emitting infrared radiation 2 which is formed as a cantilevered layer structure.

The self-supporting layer structure is in the in 1 illustrated embodiment of a heat conductor resistance layer 3 and an emissive grade enhancing layer formed thereon 4 together. Here are the heat conductor resistance 3 and the emission-grade increasing layer 4 made of the same material. However, the emission grading layer is 4 , in contrast to the heat conductor resistance 3 , a porous layer. That is, the emission-grade increasing layer 4 has pores, inclusions, interstices or voids, or columns, of the order of 10 ^-9 to 10 ^-6 m, and thereby the emission-grade increasing layer 4 a porosity and one compared to the Heizleiter resistance layer 3 give reduced density.

In the in 1 illustrated embodiment, the emissionsgraderhöhende layer 4 a special porous structure in the form of club-like nanorods, whose density and number increase with increasing layer thickness of the emission-grading layer 4 reduce, whereas the diameter and distance of the nanorods increase with increasing layer thickness. In other embodiments of the present invention not shown, the porosity of the emission grade enhancing layer may be increased 4 also differently, as it is known, for example, in porous silicon to be formed.

In the in 1 shown embodiment, both the heating conductor resistance layer 3 as well as the emission grading layer 4 made of a nickel-chromium alloy. In other, not shown embodiments of the present invention, the heat conductor resistance layer 3 and the emission-grade increasing layer 4 be formed from other suitable, preferably high-melting materials such as chromium, tungsten, molybdenum, carbon, platinum or other materials already mentioned above. It is recommended for the training of Heizleiter resistance 3 and the emission grading layer 4 to choose a material which has a melting temperature of ≥ 1200 ° C at atmospheric pressure. In principle, however, other materials with lower melting temperatures come into consideration.

Although in the embodiment of 1 only one heat conductor resistance layer 3 and an emissions-grade increasing layer 4 are shown, each of the layers may also be formed from a layer sequence. So can the heat conductor resistance 3 for example, of several superimposed individual layers of the same material and the emissions grading layer 4 be formed of several superimposed individual layers of the same material.

Like it out 1 it can be seen that is from the Heizleiter resistance layer 3 and the emission grading layer 4 composite radiating element 2 self-supporting on a support frame 5 assembled. Here is the radiator element 2 via suitable connecting elements with the support frame 5 connected. Here is the radiator element 2 on the sides to which it is attached to the support frame 5 is suspended, electrically by means of bonding wires 6 contacted. The bonding wires 6 connect the radiator element 2 with electrical connection areas on a carrier substrate 7 the infrared radiation source 1 ,

As it further looks like 1 shows is the radiator element 2 structured. By structuring the radiator element 2 its electrical resistance is adjustable. Possible structuring of the radiator element 2 can be found for example in 7 and 8th , which are explained in more detail below.

The 2 to 6 schematically show a possible process sequence for the production of the infrared radiation source 1 out 1 , In principle, however, other process sequences or changes of individual steps of the process sequence described below by way of example are also possible.

According to 2 First, a substrate is provided, which later the support frame 5 formed. The substrate is placed in a deposition chamber, preferably under high vacuum conditions. In the deposition chamber, for example by means of physical deposition, such as by evaporation, a vertical particle incidence on the surface of the substrate, wherein on the surface of the substrate over the entire surface of a thin Heizleiter resistance layer 3 which is deposited in the in 2 illustrated embodiment is a nickel-chromium alloy. In the exemplary embodiment shown, the deposition of the heating conductor resistance layer takes place 3 under constant rotation of the substrate.

In the in 2 As shown, the layer thickness of the heating conductor resistance layer 3 about 500 nm. In other, not shown embodiments of the present invention may also be another layer thickness of the Heizleiter resistance layer 3 to get voted. In still other embodiments of the present invention, the heat conductor resistance layer 3 also a not deposited on the substrate, but be applied to the substrate, prefabricated film. Layer thicknesses of such films are typically significantly thicker than the deposited thin films. For example, the layer thickness of such a film can be between 2 and 5 μm.

In a subsequent process step, preferably in the same deposition chamber and directly following the deposition of the heating conductor resistance layer 3 is executed, as in 3 shown on the heat conductor resistance layer 3 an emission grading layer 4 deposited. The emission grading layer 4 is also like the heat conductor resistance layer 3 in the illustrated embodiment, a nickel-chromium alloy.

While the heat conductor resistance 3 is produced under conventional Schichtabscheidebedingungen, that is, with normal particle incidence on the surface of the substrate, the emissivity increasing layer 4 as shown schematically by the arrows in 3 with particle incidence at an angle to the surface normal of the substrate 5 on the heat conductor resistance layer 3 generated. Preferably, the angle of incidence to the surface normal of the substrate is> 80 °. The substrate continues to rotate at a constant speed. Due to the high angle of incidence, a porous layer forms, the porosity of which can be influenced by controlling the angle of incidence. The resulting porous layer has the property that it absorbs infrared radiation very well. Since, according to the Kirchhoff radiation law, good absorbers are also good emitters, the trained emission-grading-enhancing layer possesses 4 a high emissivity in the infrared range. The deposition of the emission grading layer 4 can be done, for example, with a deposition rate of 1 to 5 nm per second.

As a result of this form of deposition, an emission-grade increasing layer results 4 which has a porosity. The porosity is due to the composition of the emission grading layer 4 from a multiplicity of side-by-side upright club-like nanorods, between which intermediate spaces are formed whose size increases along the direction of growth of the nanorods.

Like it out 4 is apparent in a further process step on the emission grading layer 4 a paint mask 8th applied, over which the emission grading layer 4 and the underlying heating conductor resistance layer 3 For example, be patterned by means of ion beam etching. This will be the radiator element 2 and the contacting surfaces of the infrared radiation source 1 defined with the Kontaktierungsarmen.

As in 5 see below is a mask on the back of the substrate 9 applied over which the substrate for exposing the radiator element 2 is etched away. At the same time, chip etching can take place with the etching process. Alternatively, this can also be done by subsequent sawing of the substrate material. By exposing the radiator element 2 A self-supporting, structured Straherer element is created 2 on the support frame 5 is attached from substrate material via Kontaktierungsarme and connected to contacting surfaces, where the electrical contact is made.

The setting of a meaningful for the operation electrical resistance via the layer thickness of the heating conductor resistance 3 as well as through the structuring of the radiator element 2 that is, by structuring the heat conductor resistance layer 3 and the emission grading layer 4 , in at least two dimensions. As it is in the 7 and 8th is shown, this can be done for example in the form of a meander.

6 schematically shows the structure 5 after the etching step, that is, as a singulated chip. The chip out 6 can then, as it is in 1 is shown, for example with an adhesive layer 10 with a carrier substrate 7 get connected. Furthermore, by bonding of bonding wires 6 electrical contacts between the radiator element 2 and contact surfaces on the carrier substrate 7 getting produced. The in 6 For example, the chip shown can also be incorporated in a standard transistor housing.

7 schematically shows a possible embodiment of an infrared radiation source according to the invention 1 in which the radiating element 2 is formed in a meandering shape. In the presentation of 7 are the electrical connections of the radiator element 2 on one side of the carrier substrate 5 intended.

In an alternative embodiment, the in 8th is shown schematically in a plan view, is the radiator element 2 ' the infrared radiation source shown 1' also formed in meandering, with the electrical contacts of the radiator element 2 ' on opposite sides of the support frame 5 are located.

9 shows schematically in plan view yet another possible embodiment of an infrared radiation source according to the invention 1'' , In the presentation of 5 is the heat conductor resistance layer 3 of the radiating element 2 '' three-dimensionally, here honeycomb-shaped, etched. The honeycombs can be, for example, by means of ion beam etching of the heating conductor resistance layer 3 from the back of the in 9 be formed structure shown. Over the etched heat conductor resistance layer 3 becomes the emission grading layer 4 deposited flat, with the shape of the emission grading layer 4 the etched shape of the heat conductor resistance layer 3 adapts. The radiator element 2 '' from 9 is on opposite sides with the support frame 5 the infrared radiation source 1'' mechanically connected. The mechanical connection is in this case, inter alia, by the honeycomb walls of the radiator element forming a connection network structure 2 '' realized.

Claims (22)

Infrared radiation source ( 1 . 1' . 1'' ), which is formed as an infrared thin-film radiator, wherein an infrared radiation emitting emitter element ( 2 . 2 ' . 2 '' ) of the infrared thin-film radiator is designed as a self-supporting layer structure which has at least one heat conductor resistance layer ( 3 ) and at least one formed thereon, for an increase in the optical emissivity of the radiator element ( 2 . 2 ' . 2 '' ) providing an emission-increasing layer ( 4 ), characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 ) are formed of the same material, but the emission-grade increasing layer ( 4 ) is a porous layer. Infrared radiation source according to claim 1, characterized in that the cantilevered layer structure only from the at least one Heizleiter resistance layer ( 3 ) and the at least one emission grading layer ( 4 ) is trained. Infrared radiation source according to claim 1 or 2, characterized in that the heating conductor resistance layer ( 3 ) is a thin film. Infrared radiation source according to claim 1 or 2, characterized in that the heating conductor resistance layer ( 3 ) is a foil. Infrared radiation source according to one of the preceding claims, characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 ) are formed of a material having a melting temperature at normal pressure of ≥ 1200 ° C. Infrared radiation source according to claim 5, characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 nickel-based alloy, nickel-base superalloy, Ni _x Cr _1-x alloy of 0≤x≤1, tungsten, molybdenum, carbon, platinum, tantalum, vanadium, titanium-based alloy, is formed from rhenium, niobium, cobalt or an alloy of at least two of these materials. Infrared radiation source according to one of the preceding claims, characterized in that the total layer thickness of the at least one heat conductor resistance layer ( 3 ) ≤ 500 nm. Infrared radiation source according to one of the preceding claims, characterized in that the total layer thickness of the at least one emission grading layer ( 4 ) ≤ 3 μm. Infrared radiation source according to one of claims 1 to 7, characterized in that the total layer thickness of the at least one emission grading layer ( 4 ) is greater than 5 microns. Infrared radiation source according to one of the preceding claims, characterized in that the radiator element ( 2 . 2 ' ) has a meandering shape. Infrared radiation source according to one of the preceding claims, characterized in that the radiator element ( 2 '' ) three-dimensionally forming internal cavities and / or pits in the heat conductor resistance layer ( 3 ) and / or the emission grading layer ( 4 ) is structured. Infrared radiation source according to claim 11, characterized in that the heating conductor resistance layer ( 3 ) and / or the emission-grade increasing layer ( 4 ) has a honeycomb structure. Infrared radiation source according to one of the preceding claims, characterized in that the cantilevered layer structure by means of connecting elements in the form of connecting arms or connecting network structures on a support frame ( 5 ) of the infrared thin-film radiator is suspended. Infrared radiation source according to claim 13, characterized in that at least two of the connecting elements as electrical conductors or as a carrier for electrical conductors for electrically connecting the radiator element ( 2 . 2 ' . 2 '' ) with contact surfaces on the support frame ( 5 ) or one with the support frame ( 5 ) are provided connected surface of the infrared thin-film radiator. Method for producing an infrared radiation source (in the form of an infrared thin-film radiator) ( 1 . 1' . 1'' ), wherein to form an infrared radiation emitting emitter element ( 2 . 2 ' . 2 '' ) of the infrared thin-film radiator on at least one heat conductor resistance layer ( 3 ) at least one, for an increase in the optical emissivity of the radiator element ( 2 . 2 ' . 2 '' ) providing an emission-increasing layer ( 4 ) and the radiator element ( 2 . 2 ' . 2 '' ) is structured into a self-supporting layer structure, characterized in that the emission-grade increasing layer ( 4 ) made of the same material as the heat conductor resistance layer ( 3 ), but as a porous layer, is deposited. Method according to Claim 15, characterized in that the emission-grade increasing layer ( 4 by evaporating material under high vacuum conditions at an angle of incidence of ≥ 80 ° to the surface normal of the heating conductor resistance layer ( 3 ) is deposited, wherein rotates during the evaporation of the trainee infrared thin-film radiator. Method according to claim 15, characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 nickel-based alloy, nickel-base superalloy, Ni _x Cr _1-x alloy of 0≤x≤1, tungsten, molybdenum, carbon, platinum, tantalum, vanadium, titanium-based alloy, of rhenium, niobium, cobalt or an alloy of at least two of these materials. Method according to one of claims 15 to 17, characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 ) are deposited in successive steps of one and the same layer deposition process. Method according to claim 18, characterized in that the heating conductor resistance layer ( 3 ) is deposited on a sacrificial material, whereby after deposition of the emission-grading layer ( 4 ) on the heating conductor resistance layer ( 3 ) the sacrificial material for exposing the radiator element ( 2 . 2 ' . 2 '' ) Will get removed. Method according to one of claims 15 to 17, characterized in that the heating conductor resistance layer ( 3 ) is provided as a film and applied to a sacrificial material, then the emissive grade layer ( 4 ) on the Heizleiter resistance layer ( 3 ) is deposited and subsequently the sacrificial material to expose the radiator element ( 2 . 2 ' . 2 '' ) Will get removed. Method according to one of claims 15 to 20, characterized in that the heating conductor resistance layer ( 3 ) and the emission-grade increasing layer ( 4 ) are meandered using a masking technique. Method according to one of claims 15 to 20, characterized in that the heating conductor resistance layer ( 3 ) is honeycomb-etched.

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