Thermal radiation source

Abstract

The present invention concerns a thermal radiation source (1) comprising a plate (10) and at least one heater (4) configured to heat via a current the plate (10) to an emission temperature such that an emitted radiation is emitted from the plate (10), 5 the heater (4) comprising a proximal portion (41) to the plate (10) and a distal portion (43) to the plate (3). The heater (4) is arranged so that a current density and / or a mechanical stress at the proximal portion (41) of the heater (4) is(are) less than the current density and / or the mechanical stress at the distal portion (43) of the 10 heater (4). This allows to improve the lifespan of the thermal radiation source (1). (Fig. 4)

Description

Thermal radiation sourceTechnical domain

[0001] The present invention concerns a thermal radiation source, and especially a microelectromechanical thermal radiation source that emits infrared and / or visible light through a thermal process.Related art

[0002] Thermal radiation sources are widely utilized in applications requiring precise infrared emission, such as spectroscopy, sensing, thermal imaging, etc. These sources typically comprise a plate, which serves as the radiating surface, and one or more heaters configured to heat the plate to a specific emission temperature.

[0003] The heaters, often based on resistive elements, generate heat by passing an electric current through materials with tailored thermal and electrical properties. The plate, heated to high temperatures, emits thermal radiation characterized by a spectrum dependent on its temperature and emissivity.

[0004] An example of a thermal radiation source is described in the documents W02020012042, WO2021144463, WO2021144464 or WO20231 52628, filed by the applicant.

[0005] Achieving a high emission temperature is critical for applications requiring intense or broadband thermal radiation. However, this comes at the cost of operational longevity. Prolonged exposure to elevated temperatures accelerates material degradation in both the plate and the heaters.4KMEMS-8-PCT

[0006] Oxidation, thermal fatigue, and creep are common mechanisms that reduce the operational life of these sources. For instance, stress and current can cause mass migration, resulting in voids or notches and higher resistance and further increases the current density, as well as local stress and heating.

[0007] In addition to material challenges, the thermal management of such systems presents significant design constraints. Uneven heating can lead to hotspots and thermal gradients on the heating elements, exacerbating mechanical stresses and further diminishing the source's lifespan. Repeated thermal cycling, which is common in many practical applications, compounds these issues, resulting in premature failure.

[0008] Thus, while current designs of the thermal radiation sources effectively provide thermal radiation at desired wavelengths and intensities, their limited lifespan remains a key drawback. There is a need to enhance the durability and operational life of thermal radiation sources. Addressing these challenges would significantly enhance the performance and reliability of thermal radiation sources in demanding applications.Short disclosure of the invention

[0009] An aim of the present invention is the provision of a thermal radiation source that overcomes the shortcomings and limitations of the state of the art.

[0010] Another aim of the invention is the provision of a thermal radiation source having an improved lifespan compared to known thermal radiation sources.

[0011] Another aim of the invention is the provision of a thermal radiation source alternative to known thermal radiation sources.4KMEMS-8-PCT

[0012] According to the invention, these aims are attained by the object of the attached claims, and especially by the thermal radiation source according to claim 1, preferred embodiments being given in the dependent claims.

[0013] The thermal radiation source according to the invention comprises a plate and at least one heater configured to heat via a current the plate to an emission temperature such that an emitted radiation is emitted from the plate, each heater comprising a proximal portion to the plate and a distal portion to the plate.

[0014] In one embodiment, each of the heater has a length, a width and / or a cross-sectional area which is(are) at least one order of magnitude smaller than the corresponding one of the plate.

[0015] In one embodiment, the heater has a substantially filamentary shape, i.e. its length is at least one order of magnitude greater than its width in the plane of the plate. This does not necessarily mean that it is straight, it may also be curved. In contrast, in one embodiment the plate has two in-plane dimensions of (i.e. its length and its width) of the same order of magnitude and may, for example, be square, polygonal, circular, etc.

[0016] In one embodiment, the plate has a lower electrical resistance in comparison to each of the heaters. Therefore, the heat generated by plate is less than the heat generated by the heather(s).

[0017] In this context, the expression "proximal portion" indicates the portion of the heater that is located closer to the plate. This portion is typically in direct or close thermal interaction and / or physical contact with the plate, facilitating efficient heat transfer to raise the plate's temperature.4KMEMS-8-PCT

[0018] In this context, the expression "distal portion" indicates the portion of the heater that is located far away from the plate. This portion is positioned away from a direct or close thermal interaction and / or physical contact with the plate. It may be connected to a connection pad or to other components. In one embodiment, connection pads are designed also to provide mechanical connection to a substrate, such that the plate is only supported relative to the substrate by the heaters and the pads. The connection pads could also provide electrical connection to the heaters.

[0019] According to the invention, the heater is arranged so that a current density and / or a mechanical stress at the proximal portion of the heater is(are) less than the current density and / or the mechanical stress at the distal portion of the heater.

[0020] The expression "current density" in this context refers to the amount of electric current flowing per unit cross-sectional area of the heater, i.e. the area in a plane perpendicular to the main plane of the plate. It is typically measured in amperes per square meter (A / m2).

[0021] The expression "mechanical stress" in this context refers to the internal forces per unit area within a heater's material that arise due in particular to thermal expansion and / or operational conditions. The mechanical stress could include various stress components, such as von Misses stress (used to predict yielding under complex loading) and hydrostatic stress (related to volumetric changes without shape distortion).

[0022] The applicant has discovered that, moving the current density and / or the mechanical stress away from the heat zone (i.e. the proximal portion of the heaters) to the cold zone (i.e. the distal portion of the heaters) of the heater improves the lifetime of the thermal radiation source.4KMEMS-8-PCT

[0023] The lifetime of the claimed thermal radiation source could be improved till a factor ten or even more, compared to knowns thermal radiation sources.

[0024] The applicant has found several different ways to achieve this result, that can be used in an independent or combined way.

[0025] In one embodiment, the stiffness of the proximal portion of the heater is higher than the stiffness of the distal portion of the heater. In this embodiment, the mechanical deformation is moved to the distal portion, which is colder.

[0026] In one embodiment, the stiffness is increased by a geometry adjustment, and in particular by increasing the width and / or the crosssection of the heater.

[0027] In one embodiment, the heater has a variable width and / or cross section, and in particular a width and / or a cross section thinner at the distal portion and thicker at the proximal portion.

[0028] In this context, a width of a heater is its transversal dimension (i.e. its dimension perpendicular to the direction of a main dimension of the heater) in the thermal radiation source's plane (e.g. the plate's plane).

[0029] Other measures are possible to increase the stiffness, including increasing the thickness only of the heater, adding reinforcing elements, like ribs or beams or using material having a high Young's modulus.

[0030] In one embodiment, the heater's length is higher than 40 pm, for example higher than 60 pm or higher than 80 pm. In fact, the applicant has discovered that making the heaters longer reduces the current density and the mechanical stress.4KMEMS-8-PCT

[0031] In one embodiment, the heater and the plate have a thickness belonging to the range 1 pm - 5 pm, the maximum dimension of the plate (e.g. its dimeter if it has a circular shape) belongs to the range 100 pm - 400 pm, and / or the length of the heater lengths belongs to the range 40 pm - 160 pm.

[0032] Let's assume a thermal radiation source having four heaters of a same specific length. If their length is increased, e.g. it is doubled, the thermal radiation source's mechanical stress and the current density (increase in resistance) will be reduced. The result is a softer device that operates at lower power (assuming a constant voltage). Both the power and stiffness can be compensated by adding additional heaters, which will each last longer as the individual heaters all have the same current density and mechanical stress.

[0033] In one embodiment, the heater is single bended.

[0034] In one embodiment, the bend is between the proximal portion and the distal portion of the heater. In one embodiment, the bend is around the middle of the path between the two ends of the heater.

[0035] In one embodiment, the bending is smooth, i.e. devoid of sharp conner(s).

[0036] In one embodiment, the proximal portion and the cold portion form a bending angle comprised in the range 80° - 100°, and in particular 85° - 95°, for example 90°. In one embodiment, the bending is achieved by using a hyperbolic tangent heater shape of the type tanh(x). Other mathematical functions could be used to realize a single bend, for example the arctangent function (arctan(x)), a logistic function (1 / (1 +ekx)), where a steep transition can model the bending to a target angle, depending on the parameter k, a piecewise linear function with smoothing, as combining straight segments with smoothly interpolated curves can produce controlled bending to a specific angle, or in general a non-linear function4KMEMS-8-PCTf(x), for example f(x) = a if x < Xi, f(x) = bx2- ex3if Xi < x < X2 and f(x) = d if x < X2, wherein Xi, X2, a, b, c and d are real numbers.

[0037] In one embodiment, the heater is double bended. In this case, the heater comprises a central portion separating the proximal portion from the distal portion, the proximal portion and the central portion forming a first bending angle and the central portion and the distal portion forming a second bending angle. In one embodiment, the first and / or the second bending angle are comprised in the range 80° - 100°, and in particular 85° - 95°, for example 90°.

[0038] In one embodiment, a first bend is around at a first third of the path between the two ends of the heater, and the second bend is around at a second third of the path between the two ends of the heater.

[0039] In one embodiment, the double bending is achieved by using a hyperbolic tangent heater shape of the type tanh(x - Xi), wherein Xi is a real number. Other mathematical functions could be used to realize a double bend, comprising the functions used for the single bending wherein x is replaced by (x - Xi).

[0040] In one embodiment, Xi is a multiple of xend2, wherein the heater is drawn as a function of f(x) between x = xendi (for example x= 0, at the proximal portion) and x = xend2 (at the distal portion).

[0041] In one embodiment, the distal portion comprises at least one flexure spring, in particular a folded flexure spring, i.e. a flexure spring with a folded or zigzag-like geometry, often consisting of multiple segments connected in a folded pattern.

[0042] Using a flexure spring, and in particular a folded flexure spring, allows mechanical stress to be redistributed and concentrated specifically at the location where it is positioned. This embodiment ensures that the4KMEMS-8-PCTmechanical stress is absorbed by the flexure spring, improving the overall structural performance and durability of the thermal radiation source. The same applies for the current density.

[0043] In one embodiment, the flexure spring is substantially perpendicular to the main direction of the heater, which can have a linear shape. In other words, the heater can comprise at least one flexure and / or at least one bend at the distal portion and / or a straight proximal portion.

[0044] In one embodiment, the attachment angle of the heater is less or equal to 0°.

[0045] In one embodiment, the attachment angle of the heater belongs to the range -35° to -15°.

[0046] In this context, the attachment angle is the angle of attachment of a heater relative to the plate. In general, it is the angle between the normal to the plate at the attachment point of the heater to the plate (e.g. the radial direction at the attachment point, if the plate is circular), and the main direction of the heater at the attachment point. It is negative if the angle is formed by a heater inclined clockwise with respect to the plate at the attachment point with the plate, zero if the heater is perpendicular at the point of attachment, and positive if the inclination is counterclockwise with respect to the plate at the attachment point with the plate. This is particularly true when the heater has not a straight or linear shape.

[0047] In one embodiment, the thermal radiation source comprises more than four heaters, e.g. more than six heaters.

[0048] In one embodiment, the thermal radiation source comprises at least one petal.4KMEMS-8-PCT

[0049] In this context, the term "petal" refers to the additional plate material that extends beyond the attachment point of the heater in a radial direction, like the petal of a flower. In this embodiment, the plate comprises at least one and preferably more plate's recess extending from the plate's periphery inward and arranged to host a heater. In other words, the petal is the material portion of the plate situated between two adjacent recesses.

[0050] In one embodiment, the heater is monobloc, i.e. realized in a monolithic way.

[0051] In one embodiment, the thermal radiation source is monobloc, i.e. realized in a monolithic way. In one embodiment, the heater and the plate are made by the same material, in particular a refractory material, i.e. a material with a melting point above 2000 °C. Examples of refractory materials are Tungsten, Titanium, Hafnium, Zirconium, Tantalum, Molybdenum and their Nitrides and Carbides.

[0052] In one embodiment, the thermal radiation source is arranged to operate at a temperature equal or higher than 1500 K.

[0053] In one embodiment, the heater arranged to support the plate.

[0054] In one preferred embodiment, the thermal radiation source comprises a plurality of heaters.

[0055] In one embodiment, the heaters all have the same dimensions and shape.

[0056] In one preferred embodiment, the heaters are equidistant.

[0057] All the cited embodiments could be combined, taking at least two at a time. As a non-limitative example, a heater can have both a single-4KMEMS-8-PCTbended shale and a flexure spring, or a heater can have a double-bended shape, a stiffness of the proximal portion higher than the stiffness of the distal portion and a length higher than 80 pm.Short description of the drawings

[0058] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:Figure 1 A illustrates a top view of a reference thermal radiation source according to one embodiment of the invention (circular heater shape with heater length = 80 pm).Figure 1 B illustrates a top view of a detail of a thermal radiation source according to one embodiment of the invention, for showing the meaning of wa and wb.Figure 1C illustrates in a schematic way what the attachment angle is.Figure 2 illustrates a top view of a first thermal radiation source according to one embodiment of the invention.Figure 3 illustrates a top view of a thermal radiation source according to a third embodiment of the invention.Figure 4 illustrates a top view of a fourth embodiment of the invention.Figure 5A illustrates a top view of a fifth embodiment according to the invention.4KMEMS-8-PCTFigures 5B and 5C illustrates a top view of two other possible heater's bending.Figure 6 illustrates a top view of a sixth embodiment according to one embodiment of the invention.Figure 7A shows a graphic representing in an indicative way the behaviour of the current density exponential term of the Mean Failure Rate (MFR) and the behaviour of the temperature of the first embodiment, between the two ends of a heater, for a radiation source with four heaters having wa = wb = 4 pm and for a radiation source with eight heaters having wa = wb = 2 pm, based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the first embodiment.Figure 7B shows a graphic representing in an indicative way the behaviour of the current density and of the temperature of the first embodiment (f(x) = Ax3heater type), between the two ends of a heater, for a radiation source with four heaters having wa = wb = 4 pm and for a radiation source with eight heaters having wa = wb = 2 pm, based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the first embodimentFigure 7C shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR and of the temperature of the first embodiment, between the two ends of a heater, for a radiation source with four heaters having wa = wb = 4 pm and for a radiation source with eight heaters having wa = wb = 2 pm, based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the first embodiment.4KMEMS-8-PCTFigure 7D shows a graphic representing in an indicative way the behaviour of the von Mises stress and of the temperature of the first embodiment, between the two ends of a heater, for a radiation source with for four heaters having wa = wb = 4 pm and for a radiation source with eight heaters having wa = wb = 2 pm, based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the first embodiment.Figure 8A shows a graphic representing in an indicative way the behaviour of the current density exponential term of the MFR and the behaviour of the temperature of the fourth embodiment (double bend heater type), between the two ends of a heater, for a radiation source with six heaters having wa = wb = 2 pm and for a radiation source with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the fourth embodiment.Figure 8B shows a graphic representing in an indicative way the behaviour of the current density and of the temperature of the fourth embodiment, between the two ends of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the fourth embodiment.Figure 8C shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR and the behaviour of the temperature of the fourth embodiment, between the two ends of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the4KMEMS-8-PCTtests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the fourth embodiment.Figure 8D shows a graphic representing in an indicative way the behaviour of the von Mises stress and of the temperature of the fourth embodiment, between the two ends of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant. The graphic comprises values plotted along the length 400 of an individual heater of the fourth embodiment.Figure 9 shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR, the behaviour of the von Mises stress and the behaviour of the temperature of the sixth embodiment (folded flexure spring heater type), along the exterior edge 400 of the heater of the sixth embodiment from the base (cold) to the plate (hot), based on the tests performed by the applicant.Examples of embodiments of the present invention

[0059] In the framework on the present invention, the applicant has considered that the total vacancy flux in a thermal radiation source is given by the sum of different terms, notably:- the diffusive flux,- the vacancy flux due to electric field,- the vacancy flux due to temperature gradient, and- the vacancy flux due to hydrostatic stress gradient.

[0060] In the framework on the present invention, the applicant has considered two parameters for defining a Mean Time to Failure (MTTF) for4KMEMS-8-PCTa thermal radiation source, i.e. the current density exponential term and the mechanical stress exponential term.

[0061] The current density exponential term of the MTTF (MTTFj) is given by the formula:whereinAo is a constant that depends on material properties and specific device characteristics j is the current densityEais the activation energy associated with the failure mechanism (typically in eV) n is an exponent that can be determined experimentally and that depends on the used material, typically 2 k is the Boltzmann constantT is the temperature

[0062] The mechanical stress exponential term of the MTTF (MTTFO) is given by the formula:MTTF, exp ) (2) whereinBo is a constant that depends on material properties and specific device characteristics cr is the mechanical (Von Mises) stressEb is the activation energy associated with the failure mechanism (typically in eV) m is an exponent that can be determined experimentally and that depends on the used material, typically between 1 and 5.4KMEMS-8-PCT

[0063] In this context, the Mean Failure Rate MFR is the multiplicative inverse of the MTTF. The MFR has also current density and mechanical stress exponential terms, wherein MFRj= 1 / MTTFj and MFRO= 1 / MTTFn.

[0064] The MFR should be minimized, so as to improve the lifespan of the thermal radiation source.Reference thermal radiation source

[0065] Figure 1 A illustrates a top view of a reference thermal radiation source 1 according to a one embodiment of the invention. The thermal radiation source comprises a plate 10 and six heaters 4 configured to heat via a current the plate 10 to an emission temperature such that an emitted radiation is emitted from the plate 10.

[0066] As better visible in Figure 1 B, each heater 4 comprises a proximal portion 41 to the plate 10 and a distal portion 43 to the plate 10, i.e. a portion 41 of the heater located closer to the plate 10, typically in direct or close thermal interaction with the plate 10, and a portion 43 of the heater located farther away from the plate 10.

[0067] In the embodiment of Figure 1 A, the portion 43 is connected to a connection pad 3 designed to provide mechanical connection to a substrate, such that the plate 10 is only supported relative to the substrate by the heaters and the pads 3. The pad 3 provides also electrical connection to the heaters 4.

[0068] The circular shape of the connection pads 3 of Figure 1 A, and also of the other Figures, should be considered as non-limitative.

[0069] In the embodiment of Figure 1A, all the heaters 4 have the same circular shape, the same dimensions and are equidistant. Although this is often the case, it should be considered that the present invention is not4KMEMS-8-PCTlimited to equidistant heaters 4 having the same shape and dimensions, as long as they are arranged so that a current density and / or a mechanical stress at the proximal portion of the heaters is(are) less than the current density and / or the mechanical stress at the distal portion of the heaters.

[0070] The width of each heater 4 at the proximal portion 41 is named in the following wa and the width of the heater 4 at the distal portion 43 is named in the following wb.

[0071] The proximal portion 41 could also be named "hot portion" and the distal portion 43 "cold portion", since the temperature at the proximal portion 41 is in general higher than the one at the distal portion 43.

[0072] Each heater 4 is drawn as a (same) function f(x) between x = xendi (i.e. at the plate 10, e.g. x = 0) and x = xend2 (i.e. at the connection pad 3).

[0073] The thermal radiation source 1 of Figure 1A comprises a petal 100 for each heater 4.

[0074] Figure 1C illustrates in a schematic way what the attachment angle 0 is, in particular for a linear heater 4.

[0075] For their tests, the applicant has considered a thermal radiation source 1 with the geometry of Figure 1 A and having also the following features:- wa = 3 pm- wb = 3 pm- attachment angle 0 = -35° for circular arm shape- heater length = 80 pm.First embodiment -= -6E8xA34KMEMS-8-PCT

[0076] Figure 2 illustrates a top view of a thermal radiation source 1 according to a first embodiment of the invention, having a heater shape defined by the function f(x) = -6E8xA3 and 0 = -30°.

[0077] The thermal radiation source 1 of Figure 2 is a non-limitative example of a thermal radiation source 1 having a heater shape defined by the function f(x) = AxA3, wherein A is a real number, and in general of a thermal radiation source 1 having a heater shape defined by a polynomial function f(x).

[0078] Tests performed by the applicant have shown that, for the heater shape f(x) = -6E8xA3, the current density and / or the mechanical stress at the proximal portion of the heater(s) is(are) less than the current density and / or the mechanical stress at the distal portion of the heater(s).

[0079] The applicant has performed tests for the embodiment of Figure 2, by considering different combinations of widths, i.e. wa = 2 pm and wb = 2 pm, wa = 3 pm and wb = 2 pm and wa = 3 pm and wb = 3 pm. All the tests' results have been normalized with the geometry of the reference thermal radiation source 1 of Figure 1A.

[0080] The normalized results for the arm shape f(x) = -6E8A3 and arm width combination wa = 2 pm and wb = 2 pm show better results compared to of the reference thermal radiation source 1 of Figure 1A in terms of current density and mechanical stress. A dependency has been also observed regarding the vacancy flux values versus the attachment angle values.

[0081] The normalized results for the heater shape f(x) = -6E8xA3 and heater width combination wa = 3 pm and wb = 2 pm still show better results compared to the reference thermal radiation source 1 of Figure 1A. However, at high positive values of the attachment angle, the values of tracked parameters related to the current density and the mechanical stress increase more than in the case of width combination wa = 2 pm and wb = 24KMEMS-8-PCTm, meaning that the performance of the thermal radiation source 1 with such attachment angle is worse. A dependency is always observed regarding the vacancy flux values versus the attachment angle values. Higher positive values of attachment angles show increased fluxes, especially for angles of 15° and 30°.

[0082] In terms of the mechanical stress, the most favourable are negative attachment angles and again the difference between best and worst angle is close to two times higher.

[0083] The normalized results for the heater shape f(x) = -6E8xA3 and heater width combination wa = 3 pm and wb = 3 pm show again better results compared to the reference thermal radiation source 1 of Figure 1A, however only up to attachment angle equal to 15°. The performance of the thermal radiation source 1 with highly positive attachment angle is even three times worse compared to the reference thermal radiation source 1 of Figure 1A.

[0084] In terms of current density exponential term of formula (1), the attachment angle equal to 0° or negative is the most favourable.

[0085] The comparison of different heater width combinations for the same attachment angle (for example 0 = -30°) revealed that in terms of the vacancy fluxes the heater width combination where the heater is wider at the proximal portion of the heater and narrower at the distal portion of the heater is favourable compared to the combinations presented. This is as well true for the current density.

[0086] In terms of mechanical stress, there is no big difference between the width combinations wa = 2 pm and wb = 2 pm and wa = 3 pm and wb = 2 um, however compared to width combination wa = 3 pm and wb = 3 pm a significant improvement is observed.4KMEMS-8-PCTSecond embodiment - Circular heater with heater

[0087] The applicant has also performed tests on the geometry of the reference thermal source device 1 of Figure 1A, which is a non-limitative example of a thermal radiation source 1 having a circular heater shape, having a certain length, by varying wa and wb.

[0088] The normalized results for the circular heater shape (heater length = 80 pm) and heater width combination wa = 2 pm and wb = 2 pm show better results compared to the reference thermal radiation source 1 of Figure 1A in terms of current density and mechanical stress. A dependency is observed regarding the vacancy flux values versus the attachment angle values. Higher positive values of attachment angles show increased fluxes.

[0089] The normalized results for the circular heater shape (heater length = 80 pm) and heater width combination wa = 3 pm and wb = 2 pm still show better results compared to the reference thermal radiation source 1 of Figure 1A.

[0090] The tracked parameters for the thermal radiation source 1 with highly positive attachment angles have higher values, meaning that the higher the angle, the worse performance in terms of MTTF. The cause is similar as in the first embodiment of Figure 2.

[0091] The comparison of different heater width combinations for the same attachment angle shows that - in terms of the vacancy fluxes - the heater width combination where the heater is wider at the proximal portion of the heater and narrower at the distal portion of the heater is favourable compared to other heater width combinations presented.Third embodiment - Circular heater with heater le4KMEMS-8-PCT

[0092] Figure 3 illustrates a top view of a third embodiment of the thermal radiation source 1 (circular heater shape with heater length equal to 93 pm).

[0093] It is a non-limitative example of a thermal radiation source 1 having a circular heater shape, having a high length.

[0094] In fact, the current density respectively the mechanical stress decrease with increasing the heater length. The loss in power (for constant voltage) and stiffness can be compensated by additional heaters, without increasing the stresses and current density in each individual heater.

[0095] The normalized results for the circular heater shape (heater length = 80 pm) and heater width combination wa = 2 pm and wb = 2 pm show better results compared to the reference thermal radiation source 1 of Figure 1A in terms of current density and mechanical stress. For this heater width combination, no specific dependence is observed regarding the vacancy flux values versus the attachment angle values.

[0096] The normalized results for the circular heater shape (heater length = 80 pm) and heater width combination wa = 3 pm and wb = 2 pm still show better results compared to the reference thermal radiation source 1 of Figure 1 A. Again, no clear dependency is observed regarding the vacancy flux values versus the attachment angle values for that heater width combination. In terms of current density, negative attachment angles are most favourable.

[0097] For the heater shape with width combination wa = 3 pm and wb = 3 pm, highly positive attachment angles could lead to worse performance in terms of MTTF. However very negative angles are not beneficial as well, the good angle seems to be for this embodiment within range -35° to -15°. The observed increase in Total Vacancy Flux is similar as in previous geometry.4KMEMS-8-PCT

[0098] The comparison of different heater width combinations for the same attachment angle shows that - in terms of the vacancy fluxes - the heater width combination where the heater is wider at the proximal portion of the heater and narrower at the distal portion of the heater is favourable compared to other heater width combinations presented.Fourth embodiment - Heater*(x-0.5xlim))*20

[0099] Figure 4 illustrates a top view of a fourth embodiment of the invention (heater's shape f(x) = tanh(300000*(x-0.5xlim))*20), wherein the heater is drawn as a function of f(x) between x = xendi = 0 and x = xend2 = xlim, wherein xlim is a parameter (real number).

[0100] It is a non-limitative example of a thermal radiation source 1 having a double-bended heater shape.

[0101] Tests performed by the applicant have shown that, by applying a double bend on the shape function, the mechanical stress is minimized both at the proximal portion of the heater and as well at the distal portion of the heater.

[0102] Tests performed by the applicant have shown that, with this double bending, the heater width plays bigger role in optimizing the performance of the thermal radiation source 1 than the attachment angle.

[0103] In terms of the vacancy fluxes, the heater width combination where the heater is wider at the proximal portion of the heater and narrower at the distal portion of the heater is favourable compared to other heater width combinations presented. When the heater is wider at the proximal portion and narrower at the distal portion of the heater, the current density at the proximal portion of the heater is reduced and its maximum is placed at the distal portion of the heater.4KMEMS-8-PCTFifth embodiment - Heaterim

[0104] Figure 5A illustrates a top view of a fifth embodiment of the invention (heater shape f(x) = tanh(300000*x)*xlim).

[0105] It is a non-limitative example of thermal radiation source 1 having a single-bended heater shape.

[0106] The single-bended heater shape allows to minimize the stress at the proximal portion of the heater and to move the maximum peak of current density away from the proximal portion.

[0107] Figures 5B and 5C illustrates a top view of two other possible heater's bending. In particular, in those embodiments, the heater comprises a single proximal portion, which can ne straight, and two distal portions. In the embodiment of Figure 5B, the two distal portions are straight and extend in two opposite directions. In the embodiment of Figure 5C, the two distal portions are curved and extend in two opposite directions.Sixth embodiment - Heaterof folded flexure

[0108] Figure 6 illustrates a top view of a sixth embodiment of the thermal radiation source 1.

[0109] It is a non-limitative example of thermal radiation source 1 having heaters comprising a flexure spring 8 at their distal portion (a folded flexure spring in the illustrated embodiment).

[0110] The (folded) flexure spring of the heater is used for the purpose of minimizing the magnitude of mechanical stress and moving its peak to the distal portion of the heater.4KMEMS-8-PCT

[0111] In the illustrated embodiment, the proximal portion of the heater is straight: this helps too with moving the maximum peak of current density further away from the proximal portion of the heater.

[0112] Tests performed by the applicant have shown that, with this folded spring geometry, the mechanical stress is reduced, and the maximum is moved away to the distal portion of the heater.

[0113] In one embodiment, the heater portion between the flexure spring and the plate has a straight or linear shape. In one embodiment, the heater portion between the flexure spring and the plate is wider closer to the plate.

[0114] The best performing heater geometries in terms of both current density and mechanical stress are the ones including at least one bend in the heater (Figures 4 and 5), and the (folded) flexure spring (Figure 6).

[0115] The heater shape function f(x) = tanh(300000*x)*xlim (one bend in the heater as shown in Figure 4) and an heater with a folded flexure spring (Figure 6) are most favourable for the figures of merit related to current density, like vacancy flux due to electric field (electromigration) or exponential term of current density. It is because of the straight portion of the heater allows for more even distribution of current density, compared to other heater shape functions that have a bended starting portion of the heater causing the current to accumulate at the bend.

[0116] For the exponential term for the mechanical stress (creep term) according to formula (2), the heater shape function f(x) = tanh(300000*(x- 0.5xlim))*20 (two bends in the heater as shown in Figure 4) and an heater with a folded flexure spring (Figure 6) are most favourable. For these4KMEMS-8-PCTgeometries the magnitude of von Mises stress is lower, and maximum is moved away from the proximal portion.

[0117] The heater width combination where heater is wider at the proximal portion of the heater and narrower at the distal portion of the heater is favourable for all geometries.

[0118] The favourable attachment angle range is 0 < 0° for all the simulated geometries.

[0119] The applicant has found also that reducing stiffness overall is good, but it is better to maintain high stiffness at the proximal portion so that mechanical deformation occurs at the distal portion.

[0120] The applicant has found also that making the heaters longer reduced the strain and the current density. The current density associated figures of merit reduces even more than the mechanical stress figures of merit.Heaters' number and width influence on MFR - Current density

[0121] Figure 7A shows a graphic representing in an indicative way the behaviour of the current density exponential term of the MFR and the behaviour of the temperature of the first embodiment, along the length of a heater, for a radiation source 1 with four heaters having wa = wb = 4 pm and for a radiation source 1 with height heaters having wa = wb = 2 pm, based on the tests performed by the applicant.

[0122] The temperature decreases from the proximal portion of the heater (hot portion) to the distal portion of the heater (cold portion) in a substantially identical way for both heaters' configurations (from around 2200 K to less than 400 K in Figure 7A).4KMEMS-8-PCT

[0123] After a peak at the proximal portion of the heater, the MFR exponential term due to current density decreases from the proximal portion of the heater to the distal portion of the heater in both heaters' configurations.

[0124] The arrow in Figure 7A indicates that, the MFR exponential term due to current density is not substantially affected by the number of heaters.

[0125] Figure 7B shows a graphic representing in an indicative way the behaviour of the current density and of the temperature of the first embodiment (f(x) = Ax3heater type), along the length of a heater, for a radiation source 1 with four heaters having wa = wb = 4 pm and for a radiation source 1 with height heaters having wa = wb = 2 pm, based on the tests performed by the applicant.

[0126] Again, the temperature decreases in the same way of Figure 7A.

[0127] The current density has first peak neat the hot portion of the heater and then it is substantially constant along the heater, with a second peak at the cold portion.

[0128] The arrow in Figure 7B indicates that, the maximum of current density is substantially not affected by the number of heaters (at constant voltage).Heaters' number and width influence on MFR - mechanical stress

[0129] Figure 7C shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR and of the temperature of the first embodiment, along the length of a heater, for a radiation source 1 with four heaters having wa = wb = 4 pm and for a4KMEMS-8-PCTradiation source 1 with height heaters having wa = wb = 2 m, based on the tests performed by the applicant.

[0130] Figure 7D shows a graphic representing in an indicative way the behaviour of the von Mises stress and of the temperature of the first embodiment, along the length of a heater, for a radiation source 1 with four heaters having wa = wb = 4 pm and for a radiation source 1 with height heaters having wa = wb = 2 pm, based on the tests performed by the applicant.

[0131] For the temperatures illustrated in Figures 7C and 7D, the same considerations made for Figures 7A and 7B apply.

[0132] The von Mises stress (Figure 7D) has two maxima at the ends of the heater. It can be seen that, the amplitude of those maxima reduces a lot, e.g. it more than halves, in the configuration of eight heaters having wa = wb = 2 pm compared to the configuration of four heaters having wa = wb = 4 pm.

[0133] This reduction is more visible in the von Mises stress exponential term of the MFR of Figure 7C. This is related to the exponential term m of formula (2). In Figure 7C, it has been considered that m = 3, such that the Mises stress exponential term of the MFR reduces by more than 8.Heater width influence on MFR - Current density versus heater width

[0134] Figure 8A shows a graphic representing in an indicative way the behaviour of the current density exponential term of the MFR and the behaviour of the temperature of the fourth embodiment (double bend heater type), along the length of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant.4KMEMS-8-PCTT1

[0135] Figure 8B shows a graphic representing in an indicative way the behaviour of the current density and of the temperature of the fourth embodiment, along the length of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters having wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant.

[0136] The temperature decreases from the proximal portion of the heater (hot portion) to the distal portion of the heater (cold portion) in a similar way for both heaters' configurations.

[0137] A significant reduction of the maximum of MFRj is visible in Figure 8A for the heater's configuration wa = 3 pm and wb = 1 pm compared to the heater's configuration wa = wb = 2 pm.

[0138] In other words, the widening of the heater results in a drop of the current density at the hot portion of the heater, as visible in Figure 8B.

[0139] The highest current density, visible in Figure 8B, is at the second heater bend, which is located away from the plate and therefore occurs at a lower temperature.

[0140] The lowering the current density by a factor around two reduces the MFR by factor of around four in the illustrated examples. This is related to the exponential term n of formula (1). In Figure 8A, it has been considered that n = 2. Similar considerations could be made for other embodiments of the invention.

[0141] By making the proximal portion wider and the distal portion of the heater narrower, the maximum of the current density is moved away from the proximal portion, possibly leading to an exponential improvement in the current density exponential term MTTF, given by the formula (1).4KMEMS-8-PCT

[0142] In general, the power density generated in the heater is proportional to rho, where rho is the heater's resistivity. Typically, it is wanted more power close to the plate and far from the attachment, as heat generated close to the attachment is lost to the base.

[0143] Therefore, a narrow section is wanted closest to the plate 10. Here the opposite is proposed, for reducing the current density in the proximal region. Typically, this may have negative effects on performance, however, as the resistivity increases strongly with temperature (factor 5-10 for tungsten reaching high temperatures) even if the width doubles or more, the power density remains highest closest to the hotplate. This is a property of refractory material as tungsten, or most metals.Heater width influence on MFR - Mechanical stress versus heater width

[0144] Figure 8C shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR and the behaviour of the temperature of the fourth embodiment, along the length of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters with wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant.

[0145] Figure 8D shows a graphic representing in an indicative way the behaviour of the von Mises stress and of the temperature of the fourth embodiment, along the length of a heater, for a radiation source 1 with six heaters having wa = wb = 2 pm and for a radiation source 1 with six heaters with wa > wb (wa = 3 pm and wb = 1 pm), based on the tests performed by the applicant.

[0146] The temperature decreases from the proximal portion of the heater (hot portion) to the distal portion of the heater (cold portion) in a similar way for both heaters' configurations.4KMEMS-8-PCT-The widening of the heaters results in a drop of the stress density at the hottest location

[0147] As visible in Figure 8D, the highest stress occurs in the cold portion, with no contribution to the MFRn, as visible in Figure 8C.

[0148] The MFROhas a first peak at the hot portion for the constant thickness beam, as indicated by the arrow on the left side in Figure 8D, and a second at the bend, as indicated by the arrow on the right in Figure 8D.

[0149] Again, since formula (2), a factor F of reduction in stress reduced the MFRa by factor Fm.

[0150] In general, by making the proximal portion wider and the distal portion of the heater narrower, the maximum of the von Misses stress is moved further along the heater away from the proximal portion, resulting in a decrease of creep strain rate, leading to an exponential improvement in the stress exponential term MTTF, given by the formula (2).

[0151] Similar considerations apply to the hydrostatic stress.

[0152] Figure 8D shows in particular is that the stress at the hottest point is reduced (left arrow) The beak is still further down the bend.

[0153] By making the proximal portion wider and the distal portion of the heater narrower, the maximum of the hydrostatic stress is moved further along the heater from the proximal portion. The vacancy concentration is dependent on hydrostatic stress and temperature. High tensile hydrostatic stress will increase the vacancy concentration. The same goes for temperature, the higher temperature the larger vacancy concentration. Moving the maximum of the hydrostatic stress to lower temperatures works in favour of reducing vacancy concentration at the4KMEMS-8-PCTproximity part of the heater, resulting in minimizing fluxes responsible for atomic migration.

[0154] Figure 9 shows a graphic representing in an indicative way the behaviour of the von Mises stress exponential term of the MFR, the behaviour of the von Mises stress and the behaviour of the temperature of the sixth embodiment (folded flexure spring heater type) along the exterior edge of the heater from the base (cold) to the plate (hot) for a radiation source 1 with six heaters, based on the tests performed by the applicant.

[0155] Is it possible to see that the peak of the von Mises exponential term of the MFR is not at the proximal portion of the heater.Attachment anqle influence on MTTF

[0156] Tests performed by the applicant show that for the embodiment of Figure 6, negative attachment angles are favourable for von Mises stress and for hydrostatic stress, compared to positive angles.

[0157] Electric field is most evenly distributed for the attachment angle equal to 0°. In fact, the distribution of the electric field is most even when it flows perpendicularly to the hotplate. If a large angle is present, the field tends to accumulate on the curved part of the attachment leading to a higher concentration: this is why it is important to move the bend away from the hotplate and towards the distal portion of the heater.By applying either negative attachment angles or angles equal to 0°, the value of the current density and stresses can be reduced, compared to positive attachment angles, possibly leading to an improvement in MTTF assuming formulas (1) and (2), and an improvement in terms of the total vacancy flux leading to atomic migration which is influenced by the hydrostatic stress.4KMEMS-8-PCTReference numbers used in the Figures1 Thermal radiation source3 Connection pad4 Heater 8 Folded spring10 Plate41 Proximal portion of the heater43 Distal portion of the heater100 Petal 400 Line of the heater used for some applicant's tests4KMEMS-8-PCT

Claims

Claims1. A thermal radiation source (1) comprising a plate (10) and at least one heater (4) configured to heat via a current the plate (10) to an emission temperature such that an emitted radiation is emitted from the plate (10), the heater (4) comprising a proximal portion (41) to the plate (10) and a distal portion (43) to the plate (3), characterised in that the heater (4) is arranged so that a current density and / or a mechanical stress at the proximal portion (41) of the heater (4) is(are) less than the current density and / or the mechanical stress at the distal portion (43) of the heater (4).

2. The thermal radiation source (1) of claim 1, wherein the proximal portion (41) is in direct thermal interaction and / or physical contact with the plate (3), facilitating efficient heat transfer to raise the plate's temperature.

3. The thermal radiation source (1) of one of claims 1 or 2, wherein the distal portion (43) is located positioned away from a direct thermal interaction and / or a physical contact with the plate.

4. The thermal radiation source (1) of one of claims 1 to 3, wherein the heater (4) has a substantially filamentary shape, its length being at least one order of magnitude greater than its width in the plane of the plate (3), and / or wherein the plate (3) has a length and a width of the same order of magnitude.

5. The thermal radiation source (1) of one of claims 1 to 4, wherein the plate (3) has a lower electrical resistance in comparison to each of the heaters (4),4KMEMS-8-PCT6. The thermal radiation source (1) of one of claims 1 to 5, wherein the stiffness of the proximal portion (41) of the heater (4) is higher than the stiffness of the distal portion (43) of the heater (4).

7. The thermal radiation source (1) of one of claims 1 to 6, wherein the width of the heater and / or the cross section of the heater is thinner at the distal portion (43) than the proximal portion (41).

8. The thermal radiation source (1) of one of claims 1 to 7, wherein the heater's length is higher than 40 pm.

9. The thermal radiation source (1) of one of claims 1 to 8, wherein the heater (4) is single bended.

10. The thermal radiation source (1) of claim 9, wherein the bend is between the proximal portion (41) and the distal portion (43) of the heater (4).11.The thermal radiation source (1) of one of claims 1 to 8, wherein the heater (4) is double bended.

12. The thermal radiation source (1) of claim 11, wherein the heater (4) comprises a central portion separating the proximal portion (41) from the distal portion (43), the proximal portion (41) and the central portion forming a first bending angle and the central portion and the distal portion (43) forming a second bending angle.

13. The thermal radiation source (1) of one of claims 1 to 12, wherein the distal portion (43) comprises at least one flexure spring (8), in particular a folded flexure spring.

14. The thermal radiation source (1) of claim 13, wherein the flexure spring (8) is substantially perpendicular to the main direction of the heater (4).4KMEMS-8-PCT15. The thermal radiation source (1) of one of claims 1 to 14, wherein an attachment angle (0) of the heater (4) is less or equal to 0°, for example belonging to the range -35° to -15°, wherein the attachment angle is the angle between the normal to the plate at the attachment point of the heater to the plate, and the main direction of the heater at the attachment point.

16. The thermal radiation source (1) of one of claims 1 to 15, comprising more than four heaters (4).

17. The thermal radiation source (1) of one of claims 1 to 16, comprising at least one petal (100), the petal being an additional plate material that extends beyond the attachment point of the heater in a radial direction.

18. The thermal radiation source (1) of one of claims 1 to 17, being monobloc.

19. The thermal radiation source (1) of one of claims 1 to 18, comprising a plurality of equidistant heaters (4), having the same dimensions and shape.4KMEMS-8-PCT

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