Method for measuring a heated surface, in particular an electric hot plate, by thermography.

Abstract

The invention relates to a method for measuring a heated surface, such as a metallic surface, a ceramic surface, and in particular an electric hot plate (2), by thermography. The invention consists in the following: a homogeneous layer of a coating material of constant thickness is deposited on the surface to be measured; the surface is heated to a predetermined temperature; and a thermal image of said coated surface is captured using a thermal measuring device, the coating material having a detectable emissivity at the predetermined temperature within the spectral range of the thermal measuring device (5). Figure 1.

Description

Title of the invention: Method for measuring by thermography a heating surface, in particular an electric heating plate.

[0001] The present invention relates to a method for measuring by thermography a surface such as a heating surface, in particular an electric heating plate, as well as an installation for implementing said method.

[0002] Heating metal surfaces such as electric heating plates are used in particular by equipment manufacturers of deposition machines for the semiconductor industry. These plates allow the heating of silicon wafers, known by the English term "wafer", wafers on which the substrate required for etching electronic chips is deposited.

[0003] These devices are very highly technical and must therefore be very precisely thermally characterized. Therefore, the criterion of thermal homogeneity on the surface of the heated wafers is crucial for the quality of the deposition of this substrate. It is therefore important to propose a heating plate with thermal homogeneity allowing homogeneous heating of said wafers.

[0004] Also, during the manufacture of these heating plates, it is appropriate to set up a control of the thermal homogeneity of the surface of the heating plates, which allows both precise and repeatable measurement.

[0005] In order to measure the thermal homogeneity of a heated metal surface, various methods are known. Among these, there are contact measurement methods using thermocouples or resistance probes (RTD) and remote measurement methods using an infrared camera or pyrometer, therefore without contact.

[0006] Contact measurement methods are implemented using thermocouples, resistance probes in contact with the surface to be tested, which is relatively intrusive and can, in particular, locally modify the temperature of the plate, thus impacting the accuracy of the measurement. Variable contact resistances depending on the location of the measurement and the pressure exerted on the sensor can also affect the repeatability of the measurements. In addition, one must wait for the object to reach a temperature at the point of contact to obtain a correct reading, a phenomenon which “pumps” heat from the object and can therefore modify the temperature of the measurement point. Consequently, there is a risk of measurement drift over time, particularly when using thermocouples.

[0007] Non-contact measuring methods offer the advantage of a non-intrusive method. Therefore, the measurement has no influence on the surface temperature. of the tested plate.

[0008] Thermal imaging cameras or pyrometers are used for non-contact temperature measurements in industrial applications. Pyrometers and thermal imaging cameras work on the same principle: they detect infrared radiation and convert it into a temperature measurement. However, thermal imaging cameras have several advantages over pyrometers. A pyrometer only displays one value, while thermal imaging cameras produce a complete image.

[0009] Thus, if a pyrometer measurement method is used, the temperature of a single point is measured and mapping the plate then requires carrying out a large number of successive measurements over a period of time during which the plate must remain stable in temperature, which can prove difficult to implement.

[0010] Thermography carried out with an infrared camera makes it possible to establish the complete temperature field of a metal surface to be checked in a single measurement and without contact.

[0011] However, in order to carry out a remote measurement of this type, a problem is encountered in that the metal surfaces have a low emissivity at the wavelengths to which the usual infrared sensors are sensitive, i.e. between 0.76 qm and 1000 qm, which is unfavorable to the measurement accuracy.

[0012] In order to overcome the aforementioned drawbacks, the subject of the invention is a method for measuring by thermography a heating surface such as a metal surface, a ceramic surface, in particular an electric heating plate, the invention consisting in that: a homogeneous layer of a coating material of constant thickness is deposited on the surface, the surface is heated to a determined temperature; and capturing a thermal image of said surface coated with said heated layer of coating material using a thermal measuring device, the coating material having a detectable emissivity in the spectral range of the thermal measuring device.

[0013] Advantageously, by depositing on the surface such as a heated metal surface to be tested a material whose emissivity is detectable, preferably greater than that of the metal surface for a given temperature, in the spectral range of the infrared thermal measuring device used, preferably an infrared thermal camera, it is then possible to recover a thermal image in a simple and reliable manner. This emissivity is preferably high and for example greater than 70% in the wavelength range of standard infrared cameras, i.e. from 7.5 to 14 pm.

[0014] Advantageously, the determined heating temperature is chosen from a temperature range corresponding to the operating temperatures of the heating surface. In the case where the heating surface is metallic and constitutes an electric heating plate, the temperature is chosen from the range of 350 to 450°C, preferably 400°C, the operating temperature of said surface.

[0015] In particular, the emissivity of the coating material is higher than that of a metal surface, thus making it possible to carry out a measurement without the risk of divergent, unstable and unreliable measurements due to the low emissivity of metals in the spectral range of an infrared thermal camera.

[0016] The contactless measurement method according to the invention makes it possible to carry out a surface temperature measurement instantly, achievable in milliseconds, and without mechanical action (by contact of a sensor) on the surface to be measured.

[0017] The invention proposes a method for measuring the metallic heating surface by thermography which is simple to implement and also makes it possible to limit measurement errors on the surface of a heated metal plate, for example between 350 and 450°C.

[0018] Indeed, the method proposes to deposit a smooth and homogeneous coating layer which makes it possible to obtain the most homogeneous surface condition possible. This surface condition thus makes it possible to have a uniform, reliable and repeatable measurement. It is indeed possible that the metal surface has stains, roughness in places, traces of machining or oxidation (welds, previous heating) and that the infrared thermal camera then interprets them as zones of different temperatures. This is particularly problematic when one wants to evaluate the thermal homogeneity of a heating plate, each place on the plate being able to have a different emissivity due to variations in surface conditions.The presence of a coating material in the form of a smooth and homogeneous layer therefore advantageously makes it possible to mask such surface defects and make the measurements repeatable, avoiding variations in emissivity and therefore in temperatures read by the thermal measuring device.

[0019] The method of the invention also applies advantageously to ceramic heating surfaces. Indeed, the method also applies to any surface made of a material whose emissivity is sufficient to be measured by thermography but whose measurement can however be disturbed due to the surface condition. Thus, if the ceramic surface, if there are variations in surface condition (stains, scratches, etc.) which result in variations in emissivity and therefore in temperatures read by the thermal measuring device, the deposition of the material such as kaolin will standardize the surface condition and therefore the emissivity which will be the same at all points, avoiding a variation in the temperatures read. The invention thus advantageously makes it possible to promote uniformity of the emissivity of the surface to be measured.

[0020] According to the invention, the coating material is an inert, non-toxic material which, in addition, is easily removable from the metal surface after measurement, while offering a high emissivity sufficient to carry out the measurement in a spectral range of 7.5 to 14 pm of a standard infrared thermal camera.

[0021] According to a preferred form of the invention, such a material is made up of a white clay such as kaolin made up of an aluminosilicate, for example based on kaolinite of formula Al2Si2O5 (OH)4; or based on Al2SiO7. 2 H2O with a density of 470 kg / m3, offered under the trade name “Kaolin Blanc” from the company Merck.

[0022] This white clay is mixed with water, preferably in the following proportions of 40% white clay and 60% water. This mixture is then deposited, sprayed onto the heating plate to be tested to form a homogeneous coating layer, of constant thickness, with a smooth and opaque surface. The layer is deposited to be homogeneous and opaque, and preferably has a constant thickness of between 50 μm and 100 μm.

[0023] Such a material is particularly advantageous because the white clay layer has a high emissivity, of 0.96 at a temperature of 400°C, between 5 and 14 pm, much higher than that of the bare metal surface, which makes it possible to use standard type infrared cameras having a spectral range of 7.5 to 14 pm for example.

[0024] Advantageously, since white clay is soluble in water, the mixture is deposited in the following proportions: 40% kaolin and 60% water by volume, by manual spraying onto the metal surface, for example using a manual paint gun.

[0025] Preferably, this same mixture can be deposited, as sprayed, on the heated metal surface using an automatic deposition device such as a robot with XY movement and regulated pressure.

[0026] In addition, it is easily removable by washing with water without risk of damage to the metal surface. It is important that no residue of the coating material remains on the surface, once the measurement has been carried out.

[0027] Once the mixture has been deposited on the surface, it is dried, either in the open air or in an oven at 150°C.

[0028] A homogeneous coating, of constant thickness and having a high emissivity in the spectral range of a thermal measuring device such as a standard infrared thermal camera therefore advantageously makes it possible to overcome the uncertainties linked to the imperfections of the metal surface to be controlled, in particular for metals with low to medium emissivity.

[0029] The invention also relates to an installation for implementing the method of measuring by thermography a metal surface such as an electric heating plate, according to the invention, characterized in that it comprises: a station for depositing a homogeneous layer of constant thickness of a coating material on the metal surface; a thermal measuring station at a distance from said coated and heated surface, using a thermal measuring device.

[0030] The depositing station is a station for depositing by spraying the mixture of white clay and water consisting of either a manual depositing device such as a paint gun or an automatic depositing device such as a robot with XY movement and regulated pressure.

[0031] The depositing station also includes a drying station either in the open air or provided with a drying device, such as an oven.

[0032] The thermography measuring station consists of a chassis comprising means for suspending the coated surface to allow the positioning of said surface in the upper part of the chassis, means for heating the surface and means for supporting a thermal measuring device, the suspension means being arranged so that the coated and heated metal surface is suspended with said surface directed towards the lower part of the gantry, i.e. in the direction of the ground on which the chassis is erected, the support means being arranged so that the thermal measuring device is directed towards the suspended coated heated surface.Thus, an infrared thermal camera is placed on the ground using support means while the coated and heated surface is suspended above said infrared thermal camera, which advantageously makes it possible to limit the effects of convection on the heated metal surface and therefore the turbulence which could disturb the measurement.

[0033] In order to make the measurement even more reliable, the measuring station also includes a protective screen surrounding the measurement area between the coated metal surface and the infrared thermal camera in order to mitigate the parasitic effects of the environment. Indeed, the humidity of the air can absorb part of the radiation, lighting or other nearby sources can interfere with the infrared camera or even the ventilation within the station. The protective screen advantageously makes it possible to control these parameters, which contributes to obtaining a reliable, non-intrusive and repeatable remote thermal measurement by the infrared camera.

[0034] The invention will now be described in more detail with reference to the drawing in which is shown:

[0035] [Fig. 1] a side perspective view of a measuring station of an installation for implementing the method according to the invention.

[0036] In order to produce a metal heating plate, a metal wafer such as steel or nickel-based alloys is machined. Machining thus makes it possible to form a metal surface on the back of which an electric heating cable is housed.

[0037] Due to their use in the field of electronics, these plates must be very precisely thermally characterized. In particular, the criterion of thermal homogeneity on the surface is crucial.

[0038] Also, during the manufacture of these heating plates, it is important to be able to control this criterion of thermal homogeneity using a measurement of the surface temperature which must be precise and repeatable, making it possible in particular to limit the measurement errors on the surface of a plate heated to 400°C, a temperature which corresponds to the temperature in use.

[0039] For this purpose, a contactless temperature measurement of the surface of the heating plate is carried out using an infrared thermal camera, thus making it possible to obtain, remotely and without contact, an image of the entire surface of the heating plate.

[0040] In order to carry out a reliable and repeatable measurement from one plate to another, the method according to the invention proposes, in an installation according to the invention, to deposit a layer of inert coating material such as a white clay, preferably kaolin, on the surface of the heating plate to constitute a homogeneous coating, with a smooth surface and constant thickness, on said metal surface.

[0041] Such a coating provides a homogeneous surface condition which allows reliable and repeatable measurement using an infrared camera.

[0042] Thus, the stains and roughnesses that may be present in places on the metal surface are masked by the layer of material deposited, which prevents the infrared camera from interpreting them as zones of different temperatures. The coating layer therefore makes it possible to carry out a measurement that allows the thermal homogeneity to be evaluated without the risk that a place on the plate may have a different emissivity due to variations in surface conditions, which would thus give “erroneous” results with regard to the real homogeneity criterion.

[0043] The chosen coating material consists of a white clay, such as kaolin, comprising an aluminosilicate (Al2SiO7.2H2O with a density of 470 kg / m3). The emissivity of white clay or kaolin is very high in the spectral range between 5 and 14 pm, corresponding to that of the standard type infrared thermal camera used. This layer of coating material allows the use of an infrared camera of standard spectral range (therefore for long wavelengths and a temperature of 400°C.)

[0044] White clay or kaolin being easily soluble in water, a mixture is prepared in the following proportions: 40% kaolin and 60% water by volume. This The mixture is then easily sprayed onto the heating plate to create the coating layer with a smooth, opaque and homogeneous surface.

[0045] This mixture is then deposited in a depositing station either using a manual type paint gun, in a regular manner by crossing the passes, at approximately 40 cm from the surface to be treated.

[0046] Alternatively, and preferably, the deposition station comprises an automatic deposition device such as a robot with XY movement and regulated pressure, which cross-hatches the heating plate at a distance of 30 to 50 cm in order to obtain a constant thickness repeatedly.

[0047] Preferably, the thickness of kaolin deposited is in a thickness range between 50 μm and 100 μm. A thickness chosen in this range is optimal to avoid any risk of flaking while being sufficiently opaque. Once the mixture has been deposited, the coated heating plate is dried, preferably in an oven at 150°C. Thus, the white clay dissolved in water is deposited in a thin layer in a manner that is relatively easy to control, masking the surface condition of the plate which is in a way reset.

[0048] Once the deposit is dry, the coating is sufficiently adherent so that the treated heating plate can be handled without the risk of the coating layer peeling off. The deposited coating layer thus obtained has an emissivity of 0.96 at 400°C.

[0049] This value is determined beforehand by comparison with a stainless steel sheathed K-type thermocouple with a diameter of 0.5 mm measuring the surface temperature of the plate at a specific location. The thermal camera used measures the same location in parallel, the emissivity entered into the camera is then modified until the camera and thermocouple indicate the same value. This double measurement is carried out numerous times at different locations on the plate. The deposit (coating) is stable up to 600°C, so it allows thermal homogeneity tests to be carried out on any support up to this temperature level.

[0050] Thus, the infrared thermal camera is reliably set to engage a control measurement chain, which allows reliable control measurements of the thermal homogeneity of heating plates to be carried out.

[0051] The kaolin-coated heating plate is placed in a thermal measuring station. This station comprises a frame 1 consisting of three vertical uprights 11 resting by a lower part or foot 11a on a support surface, such as the floor S of a measuring room. The upper end 11b of the upright 11 carries a crosspiece 12, the three crosspieces 12 at their junction having means 3 for suspending the heating plate 2 and means for heating the plate, such as electrical connection means for heating the plate 2.

[0052] These suspension means are notably made up of a base 31 fixed to the crosspieces 12 and a barrel 32 on which the electric heating plate 2 is fixed with its kaolin-coated surface facing the ground.

[0053] Support means 4 of the infrared thermal camera 5 are positioned on the ground, with the lens of the infrared thermal camera 5 facing the coated surface suspended above. Holding the heating plate 2 by the suspension means 3 of the frame 1 with its heating surface directed towards the ground and above the infrared thermal camera 5 positioned close to the ground and directed towards said heated metal surface, makes it possible to limit the turbulence linked to the effects of convection on the surface of the plate 2.

[0054] A peripheral protective screen 6 surrounds the measurement area and thus makes it possible to attenuate the parasitic effects of the environment.

[0055] Once the temperature of plate 2 has stabilized, it is possible to perform infrared thermography with a repeatability of ± 1°C at 400°C. This makes it possible to carry out a reliable performance test at the end of manufacturing and to ensure that there is no drift in production. At the end of the test, the white clay can be easily removed by washing with water, for example using a jet of demineralized water.

Claims

Claims

1. Method for measuring by thermography a heating surface such as a metal surface, a ceramic surface, in particular an electric heating plate, to control the thermal homogeneity of the heating surface, characterized in that: a homogeneous layer of a coating material of constant thickness is deposited on the surface to be measured, the surface is heated to a determined temperature; and a thermal image of said surface coated with said layer of coating material, heated, is captured using a thermal measuring device, the coating material having a detectable emissivity at the determined temperature in the spectral range of the thermal measuring device (5).

2. Method according to claim 1, characterized in that the emissivity of the material is greater than that of the heating surface at the determined temperature, in the spectral range of the thermal measuring device (5).

3. Method according to one of claims 1 or 2, characterized in that the heating surface is a metal surface such as an electric heating plate, heated to a determined temperature in a range of 350 to 450°C.

4. Method according to one of claims 1 to 3, characterized in that the coating material is a white clay such as kaolin consisting of an aluminosilicate, namely Al2SiO7. 2 H2O with a density of 470 kg / m3.

5. Method according to claim 4, characterized in that the white clay is mixed with water, preferably in the following proportions of 40% white clay and 60% water, then the mixture is deposited to form a layer of constant thickness between 50 μm and 100 μm.

6. Method according to claim 5, characterized in that the mixture is deposited by spraying onto the surface either using a manual deposition device such as a manual paint gun, or using an automatic deposition device such as a robot with XY movement and regulated pressure.

7. Method according to claim 5 or 6, characterized in that once the mixture deposited on the surface, it is dried either in the open air or in an oven at 150°C.

8. Method according to one of claims 4 to 7, characterized in that the white clay layer has a high emissivity, of 0.96 at a heating temperature of 400°C, in a spectral range between 5 and 14 pm.

9. Method according to one of claims 4 to 8, characterized in that the coating material is easily removable by washing with water without risk of damage to the metal surface.

10. Installation for implementing a method of measuring by thermography a heating surface such as a metal surface, a ceramic surface, in particular an electric heating plate, according to one of the preceding claims, characterized in that it comprises: a station for depositing a homogeneous layer of constant thickness of a coating material on the surface;and a thermal measuring station at a distance from said coated and heated surface, using a thermal measuring device, consisting of a chassis (1) comprising means (3) for suspending the coated surface (2) to allow the positioning of said surface (2) in the upper part of the chassis (1), means for heating the surface (2) and means (4) for supporting a thermal measuring device, the suspension means (3) being arranged so that the coated and heated metal surface (2) is suspended with said surface directed towards the lower part of the gantry, i.e. in the direction of the ground (S) on which the chassis (1) is erected, the support means (4) being arranged so that the thermal measuring device is directed towards the suspended coated heated surface (2).;

11. Installation according to claim 10, characterized in that the depositing station is a depositing station by spraying onto the surface a mixture of white clay and water, consisting either of a manual depositing device such as a paint gun, or of an automatic depositing device such as a robot with XY movement and regulated pressure.

12. Installation according to claim 11, characterized in that the depositing station also comprises a drying apparatus, such as an oven.

13. Installation according to claim 10, characterized in that the measuring station also comprises a protective screen (6) surrounding the measuring zone between the coated surface (2) and the measuring device thermal (5).

Images