Method of measuring curvature of reflective surface and related optical device

Abstract

The invention relates to a method for measuring the curvature of a reflective surface and a related optical device. The field of the invention relates to a method for measuring a deformation of a reflective surface (10) of an object. The measuring device comprises an illumination pattern (21) comprising light spots (22), a camera device (30, 31) and an image analysis device (40), the illumination pattern and camera device being arranged such that, in the position of measurement, the illumination pattern is visible to the detector of the camera device via a virtual or real image (23) of the surface, said image representing a deformation of the illuminated area (11). The method according to the invention comprises the following steps:-measuring the distance between images of two spots; calculating a ratio between the measured distance and a reference distance; -calculating magnification in a predetermined direction as a function of said ratio; and calculating the deformation of the reflective surface in the predetermined direction.

Description

[0001] This application is a divisional application of the PCT patent application entered into China with Chinese patent application number 201880042299.3, invention name “Method for measuring the curvature of a reflective surface and related optical device”, and application date May 23, 2018. Technical Field

[0002] The field of the present invention relates to optical devices for measuring the deformation of reflective surfaces. These measuring devices can be used in particular for measuring the deformation of semiconductor wafers. The measuring device according to the present invention allows the wafer to be inspected during operations for depositing material layers required for the production of electronic components. The measuring device also allows the wafer to be inspected after deposition or after any type of material processing that can cause wafer deformation, thus enabling ex-situ monitoring or inspection. Background Art

[0003] When vacuum deposition operations are performed to deposit material layers on semiconductor wafers, for example by molecular beam epitaxy, stresses are generated in the deposited layers and mechanical stresses are induced in the wafer. Wafers are typically very thin, ranging from 100 to 700 microns. Under the stresses they experience, they may deform to varying degrees. Knowledge of these deformations provides information about the magnitude, nature, and location of the stresses, making it possible to determine whether the deposition being performed is proceeding correctly and to identify the atomic mechanisms causing these stresses.

[0004] Wafers are typically reflective. This property is exploited to measure deformation, and the measurement devices used are optical. All of these devices consist of a light source and a receiver of known geometry. The light source and receiver are arranged so that the light emitted by the light source can be observed by the receiver via a reflective surface. The receiver thus observes an image of the light source through the surface of the wafer. If the wafer is a perfectly flat mirror, this image will not be deformed within the uncertainty of the measurement system. If the wafer deforms under stress, the image of the light source will deform. Measuring this deformation allows the deformation of the reflective wafer to be determined.

[0005] Typically, the light source used has a simple geometry or consists of light spots arranged in a known geometry. For example, the light spots can be generated using a laser beam that is reflected multiple times within a parallel plate. Multiple transmitted parallel light beams form an illumination pattern. US Patent No. 5,912,738, entitled "Measurement of the curvature of a surface using parallel light beams," describes this type of measurement device. US Patent No. 9,070,590, entitled "Workpiece breakage prevention method and apparatus," describes another type of measurement device for applications other than measuring wafer properties: measuring thermal stress.

[0006] A limiting factor for this type of measurement setup is that deposition occurs within a vacuum reactor, where optical components are not readily available. In this case, light must be transmitted and received outside the reactor through a transparent window. However, wafers can be quite large, with wafers as large as 250 mm in diameter. To inspect wafers of this size, the light source and receiver can be moved to perform various measurements. However, maintaining measurement accuracy and quality is understandably challenging, given that inspections must occur in real time to enable action if deposition operations are not proceeding as expected. Furthermore, in most cases, the small size of the window makes such movement to change the light source difficult or impossible. Summary of the Invention

[0007] The measuring method and the associated measuring device according to the invention do not have these disadvantages. They are based on the fact that, even in the case of large dimensions of the reflecting surface, the illuminated area of ​​the reflecting surface can be moved without moving the receiver. More precisely, a first subject of the invention is a method for measuring the deformation of at least one reflecting surface of an object using a measuring device, said measuring device comprising at least one illumination pattern containing light points, a photographic device and an image analysis device, said illumination pattern and the photographic device being arranged so that, at the position where the deformation of the surface is measured, the illumination pattern is visible to a detector of the photographic device via a virtual or real image of the surface, said image representing the deformation of the area of ​​the surface illuminated by the illumination pattern, characterised in that the method for performing the measurement comprises the following steps:

[0008] Step 1: measuring at least one distance between the images of two light spots;

[0009] Step 2: Calculating a ratio between the measured distance and at least one reference distance;

[0010] Step 3: Calculate the magnification in the defined direction according to the ratio;

[0011] Step 4: Calculate the deformation of the reflective surface in the defined direction.

[0012] Advantageously, the method comprises a step 5 in which steps 1 to 4 are performed on images of a plurality of light points in order to measure the magnification in a plurality of given directions and to calculate the anisotropy of the deformation of the reflecting surface.

[0013] Advantageously, the illumination pattern comprises a set of discrete light points distributed in a matrix array.

[0014] Advantageously, the illumination pattern comprises at least one circle of light or one ellipse of light, the measurement being performed on the image of points belonging to this circle of light or this ellipse of light.

[0015] Advantageously, the method comprises the step of performing at least one second measurement, the second measurement comprising emitting a second illumination pattern, the means for performing the two measurements being arranged so that a first area of ​​the surface illuminated by the first illumination pattern associated with the first measurement is different from a second area of ​​the surface illuminated by the second illumination pattern associated with the second measurement, the photographic device remaining stationary between the two measurements.

[0016] A second subject of the invention is a device for measuring the deformation of at least one reflective surface of an object, the measuring device comprising at least one illumination pattern containing light points, a camera device, and an image analysis device, the illumination pattern and the camera device being arranged such that, at the location where the deformation of the surface is measured, the illumination pattern is visible to a detector of the camera device via a virtual or real image of the surface, said image representing the deformation of the area of ​​the surface illuminated by the illumination pattern, characterised in that the image analysis device comprises:

[0017] - means for measuring at least one distance between the images of two light points;

[0018] - first calculation means for calculating a ratio between the measured distance and at least one reference distance;

[0019] - second calculation means for calculating a magnification in a defined direction based on the ratio;

[0020] - third calculation means for calculating the deformation of the reflecting surface in said defined direction.

[0021] Advantageously, the device comprises means for performing at least two measurements, each measurement comprising emitting an illumination pattern, the means for performing the two measurements being arranged so that a first illumination pattern associated with a first measurement illuminates a first area of ​​the surface different from a second illumination pattern associated with a second measurement, the camera device remaining stationary between the two measurements.

[0022] Advantageously, the measuring device comprises means for shifting, deforming or magnifying the illumination pattern.

[0023] Advantageously, the means for performing the measurements comprise means for moving the object in a defined plane between two measurements and means for measuring said movement.

[0024] Advantageously, the means for moving the object in said plane are rotational or translational means.

[0025] Advantageously, the measuring device comprises a display screen and graphics means for generating said illumination pattern on said display screen.

[0026] Advantageously, the illumination pattern is a matrix array of discrete light points.

[0027] Advantageously, the lighting pattern is a circle or an ellipse, or a series of circles or ellipses.

[0028] Advantageously, the measuring device comprises a light source illuminating an opaque screen comprising holes arranged to form an illumination pattern.

[0029] Advantageously, the measuring device comprises a semi-transmissive, semi-reflective planar beam splitter, which is arranged so that the light spot pattern forms an image on a detector of the photographic device after being transmitted by the beam splitter, reflected from a surface and reflected from the beam splitter, or forms an image on a detector of the photographic device after being reflected from the beam splitter, reflected from a surface and transmitted by the beam splitter.

[0030] Advantageously, the measuring means comprise means for performing a plurality of measurements to generate a complete map of the deformation of said surface.

[0031] Advantageously, the local concave or convex curvature radius of the deformation varies between a few millimeters and tens of kilometers.

[0032] Advantageously, the object is a semiconductor wafer and the reflective surface is one of the side surfaces of said wafer.

[0033] The invention also relates to measuring concave reflective surfaces using a measuring device such as defined above, characterized in that the illumination pattern and the camera device are arranged such that the image of the illumination pattern reflected by the concave reflective surface is located near the lens of the camera device.

[0034] Advantageously, the device is used for monitoring a process causing a deformation of a reflective surface of an object in a growth reactor, characterized in that the measurements are performed during the deposition of at least one layer of material on said reflective surface.

[0035] Advantageously, the device is used for inspecting semiconductor wafers and is characterized in that measurements are performed consecutively on at least two different objects. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The invention will be better understood and other advantages will become apparent on reading the following description, given in a non-limiting manner, and with the help of the accompanying drawings, in which:

[0037] Figure 1 A first embodiment of a measuring device according to the invention is shown, said device comprising a turntable;

[0038] Figure 2 and Figure 3 The principle of optical measurement of wafer deformation is shown;

[0039] Figure 4 A second embodiment of a measuring device according to the invention is shown, said device comprising a display screen in the form of an illumination pattern;

[0040] Figure 5 A variant of this second embodiment of the measuring device according to the invention is shown;

[0041] Figure 6 and Figure 7 A third embodiment of the measuring device according to the invention and variants of said embodiment are shown;

[0042] Figure 8 A fourth embodiment is shown;

[0043] Figure 9 An embodiment of the device according to the invention is shown, which is suitable for measuring concave reflecting surfaces;

[0044] Figure 10 shows the change in magnification according to the radius of curvature of the reflecting surface;

[0045] Figure 11 and Figure 12 Images of illumination patterns containing concentric circles and illumination patterns formed by curved reflective surfaces are shown. DETAILED DESCRIPTION

[0046] As previously mentioned, the measuring device can be used to measure the deformation of a reflective surface of an object. The measuring device is particularly suitable for measuring the deformation of semiconductor wafers. The following examples all relate to this technical field, although this should not be considered to indicate that the present invention is limited thereto.

[0047] By way of a first non-limiting example, Figure 1 A first embodiment of an apparatus according to the present invention is shown for measuring deformation of wafer 10. In this figure and the following figures, the wafer is represented by a bold arc to illustrate the deformation. Furthermore, the path of light emitted from a specific point in the illumination pattern is represented by a thin dashed line, and the field of view covered by the objective lens of the camera is represented by a thick dashed line.

[0048] Typically, wafers have a thickness between 100 and 700 microns. Their diameters typically range from 25 to 250 millimeters. By appropriately modifying the measurement device's structure, it can measure localized deformations, such as concave or convex curvatures, with radiuses ranging from a few millimeters to tens of kilometers. For example, these substrates are made of gallium arsenide.

[0049] The measurement device includes a device 20 for creating an illumination pattern 21 of a known shape. This pattern can be generated using discrete components, such as a light source illuminating a transparent symbol on an opaque screen. Alternatively, a display screen can be used to display the illumination pattern. In this case, it is easy to modify or copy the illumination pattern, move it around on the display screen, or even modify its brightness or color.

[0050] The use of monochromatic or spectrally limited radiation can be advantageous for limiting parasitic light. In this case, the photosensitive receiver is equipped with a spectral filter which transmits only the emitted radiation.

[0051] Geometric lighting patterns are generally formed by light points that can be configured in the form of a matrix array. As an example, Figure 3 An array of this type is shown comprising nine light spots 22 arranged in a matrix array comprising three rows and three columns. Figure 3 In Figure 1, light spots are represented by circular disks. As you will see, the use of light spots facilitates signal processing. A matrix array containing more spots can be used to achieve higher precision. Increasing the number of light spots improves measurement accuracy but increases image processing time accordingly. However, in some applications, working in real time with a limited number of light spots is advantageous.

[0052] For example, the diameter of the light spot is about 500 microns, and the distance between two spots is about a few millimeters.

[0053] When the measurement device is used with a vacuum chamber, the illumination pattern is external to the chamber. The illumination patterns are spaced on the wafer at intervals of several tens of centimeters. The measurement device can operate at various tilt angles θ between the line connecting the center of the illumination pattern and the center of the illuminated area and the normal to the wafer surface. However, if it is desired to operate the measurement device at normal or near normal incidence, it is necessary to modify it, as will be seen in the remainder of the description, to separate the transmit and receive channels.

[0054] One advantage of the device according to the present invention is that it can operate at any tilt angle. It is important to note that the sensitivity of the device increases with increasing tilt angle θ. At low curvatures, it varies approximately as the inverse of the cosine of the tilt angle and is therefore greatest at grazing incidence. Therefore, using larger tilt angles is advantageous. The only limitation is that as the tilt angle increases, the projection of the illumination pattern on the reflective surface covers a larger area of ​​the reflective surface. Typically, to benefit from this advantage, the tilt angle can be within an angular range of 60 to 89 degrees.

[0055] The wafer forms an image 23 of the illumination pattern 21 by reflection. Figure 1 and are drawn with dotted lines in the following figures.

[0056] The measuring device also includes a photosensitive receiver 30. This is a camera device. The receiver comprises an objective lens 31 with a focal length of a few centimeters and a matrix array of photoreceivers (not shown in the figures). For example, an objective lens with a focal length of 50 mm or 100 mm can be used. The aperture of the objective lens usually defines the depth of field. The matrix array of photoreceivers does not necessarily have to have a high resolution. Figure 1 As shown, the optical axis of the camera is arranged so that the final image 24 of the image 23 of the illumination pattern reflected by the wafer is located substantially at the center of the camera's field of view. Therefore, the camera's position is symmetrical to the position of the illumination pattern about the normal to the wafer surface. The field of view of the camera, equipped with an objective lens, must be able to observe the entire image of the pattern. As mentioned above, the camera's optical components may include spectral filters tailored to the emission spectral band of the illumination pattern to reduce parasitic light.

[0057] For example Figure 2 The illumination pattern 21 shown and comprising nine light spots 22, after being reflected from the wafer and focused by the objective lens 31, is finally obtained as shown in FIG. Figure 3 An image 24 is shown comprising nine light spots 22. The image comprises nine light spots represented by disks 26.

[0058] If the wafer were perfectly planar, this image 24 would consist of a circular disk drawn with a dotted line 25. This would be a perfect image of the illumination pattern.

[0059] If the wafer is deformed, the image will be deformed and consist of thickly drawn disks 26. By analyzing the image using image analysis device 40, the center position of each image 26 of each light point 22 can be determined with higher accuracy than with a light detection matrix array.

[0060] For this purpose, a so-called "upscaling" technique can be used, which artificially increases the resolution of the image. Typically, for this type of application, the upscaling is performed with a factor of 8.

[0061] Therefore, if Figure 3 As shown, the X-axis distance x1 and the Y-axis distance y1 between the light points 26 at time t1 can be determined very accurately in a two-dimensional coordinate system (X, Y) and compared with the distances x0 and y0 between the light points 25 at time t0 on a reference surface. The ratio between the average values ​​of these distances allows the magnification in a defined direction to be determined. Geometric optics principles can be used to deduce deformation from these magnification measurements. Using the same optical principles, the anisotropy of the deformation can be derived by studying the magnification in multiple directions. Combined with the brightness invariance with incident angle, these principles can be applied at any angle between the illumination pattern and the normal to the wafer surface.

[0062] Image processing requires a computing device that is fully compatible with the performance of current desktop computers in terms of required computing resources and memory and can be performed in real time, ie within a time interval between two measurements (ie a few hundredths of a second).

[0063] The deformation measured is that of the region 11 of the wafer illuminated by the illumination pattern. If the wafer is of larger dimensions, this region only partially covers wafer 10. Therefore, the measuring device according to the invention comprises means for performing at least two measurements, each measurement comprising emitting an illumination pattern, the means for performing the two measurements being arranged so that a first illumination pattern associated with the first measurement illuminates a first region of the wafer, and a second illumination pattern associated with the second measurement illuminates a second region of the wafer, different from one another, the camera device remaining stationary between the two measurements. An alternative approach is to map the curvature using an array of fixed points covering the entire screen, with the image of the array of fixed points covering the entire area of ​​the surface under investigation.

[0064] In this case, the measurement device comprises a turntable 50 placed beneath the wafer 10. The turntable's axis of rotation is parallel to the normal to the wafer surface. This arrangement is advantageous because, for reasons of uniformity in deposited layers, most vacuum chambers include this type of turntable. Therefore, to produce a complete map of the wafer, it is sufficient to record a series of measurements corresponding to various rotation angles of the wafer. If the deformation of the wafer is uniform and isotropic, the measurement device can continuously measure the deformation, even in the presence of rotation, with the same sensitivity as when the wafer is stationary.

[0065] Under typical conditions for depositing atomically thin films on wafers, the camera setup must have an acquisition frequency of at least 10 Hz in order to maintain sensitivity on the scale of a single layer while employing measurements with an optimized signal-to-noise ratio.

[0066] The wafer position during measurement must be precisely known. Various techniques can be used to determine this position. One possible technique is to calibrate the wafer before deposition. This calibration has the advantage of accounting for all system imperfections. Therefore, during measurement, any deviations measured correspond only to deformations caused by the deposition on the wafer.

[0067] For example, if the turntable rotates at 12 revolutions per minute and the camera measures at 30 records per second, a series of records corresponding to areas spaced 2.4 degrees apart is obtained. Since these areas are angularly close, the curvature of the turntable at any angle of rotation can be perfectly determined by interpolating between two consecutive angles. Knowing the angle at which the measurement was taken, any subsequent measurement can be compared with a reference value derived from the interpolated value at the same angle.

[0068] The rotational motion of the wafer is well suited for "in-sita" characterization, ie, characterization during deposition of a layer on the wafer.

[0069] It is also possible to perform translational motion of the wafer within its plane to perform complete wafer qualification. Once again, perfect knowledge of the wafer's linear motion is sufficient, either by direct measurement of the motion or from prior qualification. Measurements made via linear motion are well-suited for qualification purposes, such as determining the flatness of raw wafers before deposition or checking the surface finish of finished wafers after deposition. One of the main advantages of this technique is that measurements can be performed "ex situ" outside the vacuum chamber, an environment with far fewer environmental constraints than a vacuum chamber.

[0070] One of the advantages of performing measurements continuously is that even in the case of large deformations of the wafer, where the image of the pattern is severely deformed, the evolution of this deformation can always be tracked so that the measured points are not blurred.

[0071] By way of a second non-limiting example, Figure 4 A second embodiment of the apparatus for measuring deformation of a wafer 10 according to the present invention is shown. Figure 4 The reference numerals used are the same as those in Figure 1 The reference numerals used are the same. The nature of the camera used is the same. In this second embodiment, the wafer 10 remains stationary. In order to obtain a movement of the measurement area, the illumination pattern is moved. There are various ways to achieve this movement of the pattern. The simplest and most easily reproducible method is to move the pattern on the display screen. Figure 4In the 1980s, the movement was represented by a V-shaped symbol arranged along two different axes. Therefore, in this configuration, no mechanical parts are movable. In addition, not only can the lighting pattern be easily moved, but it can also be copied, enlarged or modified. It is also easy to know perfectly the position of the light points forming the lighting pattern on the display. The brightness and resolution of modern displays are sufficient to produce lighting patterns of smaller sizes. For example, the brightness of the points is 200 cd / m². 2 to 500cd / m 2 The average screen resolution is between 100 and 500 dots per inch (DPI).

[0072] Again, by performing a series of measurements, a complete map of the deformation of the wafer is determined.

[0073] exist Figure 5 In one variant shown, multiple wafers 10a, 10b and 10c may be measured in a given series of measurements, for example to check the reproducibility of a deposition operation. This type of inspection is typically performed ex situ under more favorable environmental conditions.

[0074] like Figure 1 、 Figure 4 and Figure 5 As shown in Figure 1, if the incident angle θ of the emitted light beam remains constant (e.g., greater than a few degrees), the portion generating the illumination pattern will naturally separate from the receiving camera. This situation is different when the incident angle θ is small or zero, that is, when measurements are performed at normal or near normal incidence on the wafer.

[0075] To solve this problem, the measurement setup includes a semi-transparent and semi-reflective planar beam splitter, such as Figure 6 and Figure 7 As shown. The beam splitter 60 is arranged so that the dot pattern forms an image on the detector of the camera after being transmitted by the beam splitter, reflected from the wafer, and reflected from the beam splitter. It is also possible to reverse the illumination pattern and the camera. In this case, the dot pattern forms an image on the detector of the camera after being reflected from the beam splitter, reflected from the wafer, and transmitted by the beam splitter.

[0076] Of course, this setup can be used to Figure 7 The illustrated illumination pattern can be moved on the display screen or the measurement area can be moved by moving or rotating the wafer.

[0077] like Figure 8 As shown, a lighting device with a large illumination pattern and a large number of illumination points can also be used to inspect multiple wafers simultaneously and obtain a map of the instantaneous deformation.

[0078] It has been seen that in the case of flat or slightly curved surfaces, the sensitivity of the device can be increased by increasing the angle of incidence, with the sensitivity increasing sharply at grazing incidence. There is a second way to increase the sensitivity of the device, this time in the case of a curved reflecting surface. The measuring device according to the invention is able to measure the curvature of a surface by observing the deformation of the image of an object passing through the reflecting surface. To this end, the magnification between the image of the illumination pattern and the illumination pattern itself is measured. For a given deformation of the reflecting surface, the greater the change in magnification, the higher the sensitivity of the measuring device. It is therefore advantageous to find a configuration that allows for the best magnification sensitivity. These configurations are obtained when the image of the pattern is located near the optical elements of the photographic device. This condition can only be achieved for concave reflecting surfaces. In this case, if we denote the distance d from the pattern to the center of the reflecting surface, the distance d' from the lens of the photographic device to the same center, and the radius of curvature of the surface as R, then in order to achieve the maximum magnification sensitivity, the distances d and d' must obey the following equations:

[0079] d·d' / (d+d')=R / 2

[0080] A simple arrangement that can achieve this higher amplification sensitivity is to place the illumination pattern at the center of the curvature of the reflecting surface. Figure 9 As shown. The reference numerals used in this figure are the same as those used in the previous figures. In this case, the distance d is equal to the radius R of the reflective surface 10, and the distance d' is also equal to the radius R. In order to separate the light emitted by the pattern from the light reflected by the reflective surface 10, as shown in FIG. Figure 6 、 Figure 7 and Figure 8 Like the aforementioned device, a half-transparent half-reflective mirror 60 is used.

[0081] Figure 10 The curve shows the variation of the magnification γ according to the curvature κ of the surface when the distances d and d' are equal to 1 meter. Figure 10 In the equation, the curvature κ varies from -5 to +5, and the magnification γ varies from -4 to +4. When the curvature κ of the surface is equal to 1 meter, that is, when its radius of curvature is equal to 1 meter, the above equation is followed, and the magnification γ diverges, as Figure 10 As shown. Maximum sensitivity is then achieved. Any change in the radius of curvature above this value will result in a very large change in magnification.

[0082] This last arrangement is only suitable for concave reflective surfaces. In the case of semiconductor wafers, a planar wafer can be used that has been prestressed to achieve the desired curvature. This stress can be easily achieved, for example, by performing deposition on the backside, which will cause the wafer to bend. Deposition on the front side that introduces a slight change in the radius of curvature will result in a substantial change in the magnification observed by the camera.

[0083] Generally speaking, the further the luminous object and the camera device are from the reflecting surface, the better the sensitivity of the measuring device.

[0084] The curvature of a reflective surface is not necessarily the same in every direction. This is especially true when depositing thin crystal films on semiconductor wafers. For example, during the crystal growth of anisotropic materials, anisotropic deformations are observed, where one direction is more curved than another. When the illumination pattern is composed of Figures 2 to 8 When composed of individual light spots as shown, anisotropy information is obtained simultaneously in two orthogonal directions for each analyzed image. However, a single image is not sufficient to determine the anisotropy axis. To determine the anisotropy axis, the wafer needs to be rotated a full circle around its axis.

[0085] To determine anisotropy information, a more suitable illumination pattern than a matrix array of light spots is required. Therefore, if circular or elliptical light is used in a patterned manner, or even a series of concentric circles or a series of concentric ellipses, all wafer anisotropy information can be determined from a single image. Figure 11 This type of lighting pattern 21 is shown, which consists of 9 concentric circles of light. Figure 12 The image of these concentric circles after reflection from a reflective wafer is shown. The elliptical deformation of the circles and the tilt of the ellipse axis represent the anisotropy of the reflective wafer.

[0086] There are no special problems in generating these specific circular or elliptical lighting patterns.

Claims

1. A method for measuring the deformation of at least one reflective surface of an object using a measuring device, the measuring device comprising at least one illumination pattern comprising light points arranged in a matrix array, a camera device, and an image analysis device, the light from the light point is reflected by the reflective surface, the reflective surface forms a virtual image of the illumination pattern, the virtual image produced by the reflective surface being located behind the reflective surface, the illumination pattern and the camera arrangement being arranged such that, at the location where the deformation of the surface is measured, a virtual image of the illumination pattern formed by reflection of the illumination pattern on the reflective surface is visible to a detector of the camera arrangement, The camera device is arranged at a position symmetrical to the illumination pattern relative to the normal of the reflective surface, the virtual image representing a deformation of a region of the surface illuminated by the illumination pattern, the camera arrangement being configured to form a final image of the virtual image of the illumination pattern on a detector of the camera arrangement, in, The method for performing the measurement comprises a step of analyzing the final image, the step of analyzing comprising the following sub-steps: Sub-step 1: measuring at least one distance between images of two light spots of the illumination pattern; Sub-step 2: determining a ratio between the measured distance and at least one reference distance corresponding to the distance between images of two light points obtained by reflection on a reference surface; Sub-step 3: determining a magnification in a defined direction according to the ratio; Sub-step 4: Determine the deformation of the reflective surface in the defined direction.

2. The measurement method according to claim 1, wherein: The analyzing step comprises a sub-step 5, wherein sub-steps 1 to 4 are performed on the images of the plurality of light points to measure the magnification in a plurality of given directions and determine the anisotropy of the deformation of the reflective surface.

3. The measurement method according to claim 1, wherein: The method comprises the step of performing at least one second measurement, the second measurement comprising emitting a second illumination pattern, the means for performing the two measurements being arranged such that a first area of ​​the surface illuminated by the first illumination pattern associated with the first measurement and a second area of ​​the surface illuminated by the second illumination pattern associated with the second measurement are different, the photographic device remaining stationary between the two measurements.

4. A device for measuring the deformation of at least one reflecting surface (10) of an object, the measuring device comprising: - at least one illumination pattern (21) comprising light points (22) arranged in a matrix array, - a camera, and - an image analysis device (40), the light from the light point is reflected by a reflective surface, which forms a virtual image (23) of the illumination pattern, the virtual image produced by the reflective surface being located behind the reflective surface, the illumination pattern and the camera being arranged so that, at the position where the deformation of the surface is measured, the virtual image of the illumination pattern formed by the reflection of the illumination pattern on the reflective surface is visible to a detector of the camera, the camera being arranged in a position symmetrical to the illumination pattern with respect to the normal to the reflective surface, the virtual image representing the deformation of the area of ​​the surface illuminated by the illumination pattern, the camera being configured to form a final image of the virtual image of the illumination pattern on the detector of the camera, The image analysis device includes a processing unit, and the processing unit is configured to: - measuring at least one distance between the images of two light points; - determining the ratio between this measured distance and at least one reference distance corresponding to the distance between the images of two light points obtained by reflection on a reference surface; - determining the magnification in a defined direction based on the ratio; - determining a deformation of the reflecting surface in said defined direction.

5. The measuring device according to claim 4, wherein The device is configured to perform at least two measurements, each measurement comprising emitting an illumination pattern, the device for performing the two measurements being arranged such that a first area of ​​the surface illuminated by a first illumination pattern associated with a first measurement and a second area of ​​the surface illuminated by a second illumination pattern associated with a second measurement are different, the camera device remaining stationary between the two measurements. The measuring device according to claim 4 , wherein: The measuring device is configured to shift, deform or amplify the illumination pattern.

7. The measuring device according to claim 4, wherein The apparatus is configured to perform at least two measurements by moving the object in a defined plane between the two measurements and measuring the movement.

8. The measuring device according to claim 7, wherein The object is moved in said plane by means of a rotation device or a translation device.

9. The measuring device according to claim 4, wherein The measuring device comprises a display screen and a graphics device for generating the illumination pattern on the display screen.

10. The measuring device according to claim 4, wherein The measuring device comprises a light source that illuminates an opaque screen containing apertures arranged to form an illumination pattern.

11. The measuring device according to claim 4, wherein The measuring device comprises a semi-transmissive, semi-reflective planar beam splitter, which is arranged so that the light spot pattern forms an image on a detector of a photographic device after being transmitted through the beam splitter, reflected from a surface and reflected from the beam splitter, or forms an image on a detector of a photographic device after being reflected from the beam splitter, reflected from a surface and transmitted through the beam splitter.

12. The measuring device according to claim 4, wherein The measuring means comprises means for performing a plurality of measurements to generate a complete map of the deformation of the surface.

13. The measuring device according to claim 4, wherein The local concave or convex curvature radius of the deformation varies from a few millimeters to tens of kilometers.

14. The measuring device according to claim 4, wherein The object is a semiconductor wafer and the reflective surface is one of the side surfaces of the wafer.

15. Use of the measuring device according to claim 4 for measuring a concave reflective surface, wherein: The illumination pattern and the camera are arranged such that an image of the illumination pattern reflected by the concave reflective surface is located near a lens of the camera.

16. Use of the measurement device according to claim 4 for monitoring a process causing deformation of a reflective surface of an object in a growth reactor, wherein Measurements are performed during deposition of at least one layer of material on the reflective surface.

17. Use of the measuring device according to claim 4 in an apparatus for inspecting a semiconductor wafer, wherein: The measurements are performed consecutively on at least two different objects.

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