DEVICE AND METHOD FOR MEASURING HEIGHT IN THE PRESENCE OF THIN LAYERS

Abstract

A device for measuring heights and/or thicknesses on a measurement object, includes (i) a first low-coherence interferometer for combining, in one spectrometer, a reference optical beam and a measurement optical beam originating from reflections of the light on interfaces of the measurement object, to produce a grooved spectrum signal with spectral modulation frequencies, (ii) apparatus for measuring an item of position information representative of the relative optical length, (iii) electronic and calculating apparatus arranged for determining at least one spectral modulation frequency representative of an optical path difference between the measurement optical beam and the reference optical beam, and for determining, by exploiting the item of information and the spectral modulation frequency, at least one height and/or thickness on the measurement object, and (iv) second optical apparatus for measuring distance and/or thickness with a second measurement beam incident on the measurement object on a second face opposite the measurement beam.

Claims

1. A device for measuring heights and/or thicknesses on a measurement object such as a wafer, comprising: a first low-coherence interferometer illuminated by a polychromatic light and arranged for combining, in one spectrometer, a reference optical beam originating from a reflection of said light on a reference surface and a measurement optical beam originating from reflections of said light on interfaces of the measurement object, so as to produce a grooved spectrum signal with spectral modulation frequencies; displacement means for varying the relative optical length of the measurement optical beam and the reference optical beam, and means for measuring an item of position information representative of said relative optical length; electronic and calculating means arranged for determining at least one spectral modulation frequency representative of an optical path difference between the measurement optical beam and the reference optical beam, and for determining, by exploiting said item of position information and said at least one spectral modulation frequency, at least one height and/or thickness on said measurement object; and second optical means for measuring distance and/or thickness with a second measurement beam incident on the measurement object on a second face opposite the measurement beam.

2. The device according to claim 1, comprising a measuring head with the reference surface, and means suitable for translational movement for relative displacement of said measuring head and the measurement object in a direction substantially parallel to an optical axis of the measurement optical beam.

3. The device according to claim 2, which comprises a reference surface in the form of a semi-reflective plate inserted in the path of the measurement optical beam.

4. The device according to claim 2, which comprises a measuring head with a beam-splitting optical element suitable for generating a separate measurement optical beam and a separate reference optical beam.

5. The device according to claim 4, which comprises a measuring head with a first interferometer of one of the following types: Mirau, Linnick, Michelson, for generating the measurement optical beam and the reference optical beam.

6. The device according to claim 1, which further comprises second translation means suitable for the relative displacement of the measurement optical beam and the measurement object in a plane substantially perpendicular to an optical axis of the measurement beam.

7. The device according to claim 1, which further comprises a support suitable for receiving the measurement object, and a reference object with a known height and/or known thicknesses arranged on or forming part of said support.

8. The device according to claim 1, which comprises a first low-coherence interferometer illuminated by a polychromatic light, which emits light in the visible spectrum.

9. The device according to claim 1, which comprises second optical means for measuring distance and/or thickness of one of the following types: spectral-domain low-coherence interferometer, chromatic confocal system.

10. The device according to claim 1, which comprises second optical means for measuring distance and/or thickness with a time-domain low-coherence interferometer.

11. The device according to claim 10, in which the time-domain low-coherence interferometer comprises a light source emitting in the infrared.

12. The device according to claim 10, in which the time-domain low-coherence interferometer comprises a double Michelson interferometer with an encoding interferometer and a decoding interferometer, and a measurement optical fibre with a collimator for generating the second measurement optical beam.

13. A method for measuring heights and/or thicknesses on a measurement object such as a wafer, using a first low-coherence interferometer illuminated by a polychromatic light and arranged for combining, in a spectrometer, a reference optical beam originating from reflection of said light on a reference surface and a measurement optical beam originating from reflections of said light on interfaces of the measurement object, so as to produce a grooved spectrum signal with spectral modulation frequencies, comprising: measuring an item of position information representative of the relative optical length of the measurement optical beam and the reference optical beam; determining at least one spectral modulation frequency representative of an optical path difference between the measurement optical beam and the reference optical beam; determining, by exploiting said item of position information and said at least one spectral modulation frequency, at least one height and/or thickness on said measurement object; and measuring a second item of information on height and/or thicknesses using second optical means for measuring distance and/or thickness with a second measurement beam incident on the object to be measured on a second face opposite the measurement beam, so as to determine an item of thickness information of said object to be measured.

14. The method according to claim 13, which comprises a step of identifying the spectral modulation frequencies the value of which varies with a variation of the relative optical length of the measurement optical beam and the reference optical beam.

15. The method according to claim 13, which further comprises a step of varying the relative optical length of the measurement optical beam and the reference optical beam so as to obtain at least one spectral modulation frequency in a predetermined range of values.

16. The method according to claim 13, which further comprises steps of: calculating a spectral modulation signal representative of the amplitude of the Fourier transform of the grooved spectrum signal; and identifying amplitude peaks representative of spectral modulation frequencies in said spectral modulation signal.

17. The method according to claim 13, which further comprises a calibration step comprising measurement of height and/or thickness on a reference object of known height and/or thickness, so as to establish a relationship between at least one item of position information of the reference surface, at least one spectral modulation frequency, and at least one height and/or thickness.

18. The method according to claim 13, in which the measurement of a second item of information on height and/or thicknesses comprises steps of: generating the second measurement optical beam and a reference optical beam by means of a measurement optical fibre and a collimator; and determining optical path differences between the second measurement optical beam reflected on the measurement object and the reference beam, using a double Michelson interferometer with an encoding interferometer and a decoding interferometer provided with a time delay line.

Description

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0087] Other advantages and characteristics of the invention will become apparent on examination of the detailed description of an embodiment which is in no way limitative, and the attached diagrams, in which:

[0088] FIG. 1 shows an embodiment of the device according to the invention,

[0089] FIG. 2 shows an embodiment of the interferometer in the form of a Michelson interferometer,

[0090] FIG. 3 shows an embodiment of the interferometer in the form of a Mirau interferometer,

[0091] FIG. 4 shows (a) a grooved spectrum signal, and (b) a Fourier transform of the grooved spectrum,

[0092] FIG. 5 shows the steps of the method according to the invention,

[0093] FIG. 6 shows an embodiment of second optical measurement means.

[0094] It is well understood that the embodiments that will be described below are in no way limitative. In particular, it is possible to imagine variants of the invention comprising only a selection of the characteristics in no way limitative. It is possible to envisage variants of the invention comprising described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0095] In particular, all the variants and all the embodiments described can be combined together if such a combination is not ruled out technically.

[0096] In the figures, elements common to several figures retain the same reference.

[0097] A first embodiment of the device according to the invention for measuring heights or thicknesses of measurement objects 24 will be described, with reference to FIG. 1.

[0098] In the embodiment presented, the device according to the invention is intended more particularly for measuring measurement objects 24 in the form of wafers 24 while they are being processed.

[0099] As shown, these wafers 24 can comprise one or more thin layers 25 deposited on their surface.

[0100] These wafers 24 can for example comprise a thickness of silicon from 450 m to 700 m and a layer of polyimide, silicon oxide, silicon nitride or other transparent dielectrics from some tens of nanometres to some microns.

[0101] Usually these thin layers are at least partially transparent at visible wavelengths. Silicon is transparent at infrared wavelengths. However, depending on the samples, the layer of silicon can comprise opaque layers (component, transistors, metal layers or tracks etc.).

[0102] Under these conditions, as explained above, the known methods for measuring the total thickness of the wafer are not generally suitable for separating or resolving the interfaces of the thin layers, especially when they are transparent at the measurement wavelengths. Even if one does not wish to measure the thickness of these layers, but only the total thickness of the wafer 24, the measurement accuracy is limited by the uncertainty in the detection of the interfaces of the thin layers 25.

[0103] Conversely, these thin layers can be measured or their interfaces distinguished using techniques of low-coherence interferometry operating in the spectral domain, using a light source with a spectrum with a sufficiently wide range of frequencies. Nevertheless, these techniques cannot be used for measuring large optical thicknesses (such as 700 m of silicon, which corresponds to an optical thickness above 2 mm after taking into account the refractive index of silicon, which is of the order of 3.5) as the oscillations of the grooved spectrum become too close to be sampled by the detector.

[0104] Moreover, the wafers 24 to be measured can be greatly deformed, which requires a measurement system with a wide measurement range.

[0105] The core of the measuring device according to the invention is constituted by a low-coherence interferometer integrated in a measuring head 10.

[0106] The measuring head 10 is fixed to displacement means 21 with a motorized translation stage which allows it to be displaced along an axis Z relative to the frame of the apparatus on which this translation stage is fixed. The translation stage is equipped with means for measuring an item of position information in the form of an optical ruler, enabling its displacement and its position to be measured accurately.

[0107] The interferometer is illuminated by a broadband light source 11, which emits polychromatic light 12 in the visible spectrum. In the embodiment presented, this source comprises a halogen source, or deuterium halogen source, with a spectrum extending to 300 nm in the ultraviolet.

[0108] The interferometer comprises a beam splitter 13, which directs the light from the source 11 to the object to be measured 24.

[0109] Part of the light is reflected on a reference surface 14 constituted by a semi-reflective plate 14, in order to form a reference optical beam 17.

[0110] Part of the light from the source is transmitted through the semi-reflective plate 14 in order to form a measurement optical beam 16. This measurement optical beam 16 is focused on the object to be measured 24 (wafer 24) by an objective or a lens 15.

[0111] The measurement optical beam 16 is positioned relative to the measurement object 24 so that its optical axis 19 is substantially perpendicular to the interfaces of this object 24. In the embodiment presented, this optical axis 19 is substantially parallel to the displacement axis Z of the displacement means 21.

[0112] The light of the measurement beam 16 is reflected on the interfaces of the object to be measured 24, and in particular, in the example shown, by the interfaces of the thin layer 25.

[0113] The reflected measurement beam 16 and reference beam 17 are directed through the beam splitter 13 to a detection spectrometer 18.

[0114] This spectrometer 18 comprises a diffraction grating, which scatters spatially as a function of the optical frequencies the combined light of the measurement beam 16 and reference beam 17, and a linear sensor (CCD or CMOS), each pixel of which receives the light originating from the diffraction grating corresponding to a particular range of optical frequencies.

[0115] The spectrometer is connected to electronic and calculating means 20 in the form of a computer 20.

[0116] The object to be measured 24, which in the embodiment shown is a wafer 24, is positioned on a support 23, which has the form of a wafer chuck 23.

[0117] The device further comprises a reference object 26 in the form of a portion of wafer 26 of known thickness. This reference object 26 is positioned on wafer chuck 23.

[0118] The wafer chuck 23 is fixed on second translation means 22 in the form of a translation stage 22 which ensures the displacement thereof (relative to the frame of the apparatus for example) in an X-Y plane substantially perpendicular to the optical axis 19 of the measurement beam 16.

[0119] These second translation means 22 make it possible to position the measurement beam 16 at every point of the surface of the wafer 24, and on the reference object 26.

[0120] The device according to the invention furthermore comprises second optical means for measuring distance and/or thickness 27 with a second measurement beam 28 incident on the object to be measured 24 on a second face opposite the measurement beam 16.

[0121] In the embodiment presented, these second optical measurement means 27 comprise a low-coherence interferometer 27 operating in the time domain, with a time delay line, which makes it possible to introduce a variable delay or variation in optical path.

[0122] Such interferometers are known to a person skilled in the art, so only the general principles will be recalled here.

[0123] The light originating from a wide-spectrum source is split into an internal reference beam and a measurement beam 28 incident on the object to be measured. The measurement beam 28 is reflected on interfaces of the object. Each reflection is subject to a delay proportional to the optical path to the interface under consideration. This delay is reproduced in the delay line so as to bring the measurement and reference beams back into phase and thus generate interference peaks during displacement of the delay line. The knowledge of the displacement of this delay line makes it is possible to determine the position of the interfaces giving rise to the interference peaks.

[0124] Preferably a light source in the infrared is used (around 1310 nm for example), which makes it possible to penetrate silicon and thus also perform measurements on layers inside the wafer if required.

[0125] FIG. 6 shows a diagrammatic representation of a low-coherence interferometer 27 of this kind, operating in the time domain.

[0126] The core of the interferometer 27 is a double Michelson interferometer based on single mode optical fibres, with an encoding interferometer 60 and a decoding interferometer 61. It is illuminated by a fibre light source 62, which is a superluminescent diode (SLD) central wavelength of which is of the order of 1300 nm to 1350 nm and the spectral width is of the order of 60 nm. The choice of this wavelength is in particular based on criteria of availability of the components.

[0127] The light from the source 62 is directed through a coupler 60, which constitutes the encoding interferometer 60, and a measurement optical fibre 67 to a collimator 66, in order to constitute the second measurement beam 28.

[0128] A part of the beam originating from the source 62 is reflected in the measurement fibre 67 at the collimator 66, in order to constitute the internal reference beam. More precisely, in the embodiment presented, the reference beam is generated by the Fresnel reflection at the interface between the end of the measurement optical fibre 67 and the air in the collimator. This reflection is usually of the order of 4%.

[0129] The retroreflections originating from the interfaces of the wafer 24 are coupled in the fibre 67 and directed with the reference wave to the decoding interferometer 61 constructed around the fibre coupler 61. This decoding interferometer functions as an optical correlator the two arms of which are, respectively, a fixed reference 64 and a time delay line 65. The signals reflected at the reference 64 and the delay line 65 are combined, via coupler 61, on a detector 63, which is a photodiode. The function of the delay line 65 is to introduce an optical delay between the incident and reflected waves, which is variable over time in a known manner. This delay is obtained for example by the displacement of a mirror 68 in translation along the axis of the optical beam.

[0130] The length of the arms 64 and 65 of the decoding interferometer 61 is adjusted so as to make it possible to reproduce, with the delay line 65, the optical path differences between the reference wave reflected at the collimator 66 and the retroreflections from the object to be measured 24. When this optical path difference is reproduced for a position of the mirror 68, an interference peak shape and width of which depend on the spectral characteristics of the source 62 (the wider the spectrum of the source 62, the narrower the interference peak) is obtained on the detector 43.

[0131] Thus, the measurement range is determined by the difference in optical length between the arms 64 and 65 of the decoding interferometer 61, and by the maximum length of the delay line 65. Interferometers of this type thus have the advantage of allowing wide measurement ranges. Moreover, as the successive interfaces of the object to be measured 24 appear as successions of interference peaks separated by the optical distances separating these interfaces (as reproduced for example by the travel of the mirror 68), stacks of numerous layers can be measured unambiguously.

[0132] By implementing a double interferometer system, with an encoding interferometer 60 and a decoding interferometer 61, and generating the reference at the end of the measurement fibre 67, it is possible to make the system insensitive to the perturbations in the measurement fibre 67. Thus, the true optical distances between the collimator and the interfaces of the object to be measured 24 can be measured very accurately.

[0133] Moreover, this configuration with a measurement optical fibre 67 makes it possible to move the interferometer 27 away. Thus, only the collimator 66 is in the proximity of the object to be measured 24. This is an important advantage when the object to be measured 24 is a wafer 24 on a wafer chuck 23, for which access via its face on the wafer chuck 23 is more difficult.

[0134] The use of two measurement beams 16, 28 on either side of the object to be measured 24 in a calliper configuration makes it possible to perform measurements of thickness on this object 24 by measuring the distances of its faces on either side relative to the measurement systems. It is thus possible to determine the thickness of the object 24 in all cases, whether it is transparent, opaque, or partially opaque at the measurement wavelengths used.

[0135] Of course, the second translation means 22 also allow the second measurement beam 28 to be positioned at any point of the second surface of the wafer 24, and on a second face of the reference object 26 opposite the first measurement beam 16.

[0136] It should be noted that the combination of: [0137] a technique of low-coherence interferometry operating in the spectral domain and using a light source with a very broad spectrum [0138] and a technique of low-coherence interferometry operating in the time domain in the infrared, [0139] in a calliper configuration as described above,

[0140] allows very complete characterization of samples such as wafers with dielectric thin layers, owing to the great complementarity of these measurement techniques.

[0141] FIGS. 2 and 3 show variants of embodiments of the interferometer that have the advantage of spatially separating the measurement beam 16 and reference beam 17. These configurations in particular make it possible to increase the working distance between the interferometer and the object to be measured 24 without increasing the optical path difference between the measurement beam 16 and reference beam 17.

[0142] FIG. 2 shows a configuration of a Michelson interferometer. The light from the source is split by a splitter cube 31 in order to form a measurement beam 16 directed onto the object 24 and a reference beam 17 directed onto a reference surface in the form of a mirror 14. The measurement and reference beams are substantially perpendicular.

[0143] FIG. 3 shows a configuration of a Mirau interferometer. The light from the source is split by a semi-reflective plate 32 approximately perpendicular to the optical axis 19 of the incident beam in order to form a measurement beam 16 directed onto the object 24 and a reference beam 17 directed onto a reference surface in the form of a mirror 14. In this case the reference mirror 14 is on the optical axis 19 of the incident beam, forming a central obscuration thereof.

[0144] FIG. 4(a) shows a grooved spectrum signal 41 such as is obtained at the output of the spectrometer 18.

[0145] This signal represents a spectral intensity I(v) expressed as a function of the optical frequency v. This intensity I(v) can be represented as a sum of i harmonic functions each corresponding to an interference signal between two waves incident on the spectrometer 18:

I(v)A.sub.0(v)+.sub.i{A.sub.i(v)cos [(2n/c)2L.sub.iv+.sub.i]}

[0146] where A.sub.0 and A.sub.i are intensity coefficients, .sub.i is a phase coefficient, c is the speed of light, and 2 L.sub.i is the optical path difference between the two interfering waves.

[0147] The frequency of spectral modulation of each of these harmonic functions (which in fact has a dimension of time and corresponds to the delay between the two waves that interfere) can be written:

.sub.i=(2L.sub.i/c).

[0148] This frequency of spectral modulation is therefore representative of the optical path difference 2L.sub.i between the two waves that interfere.

[0149] In order to analyse the signal of spectral intensity I(v), a Fourier transform is performed thereon, and an amplitude spectrum or spectral modulation signal 42 is obtained, as shown in FIG. 4(b). It should be noted that this spectral modulation signal 42 is representative of an envelope of the temporal autocorrelation function of the measurement 16 and reference 17 beams. It comprises an amplitude peak 43, 44, 45 for each delay .sub.i corresponding to an optical path difference 2L.sub.i between two waves that interfere.

[0150] The spectral modulation signal 42 shown in FIG. 4(b) corresponds qualitatively to the situation shown in FIG. 1, in which one has a measurement object 24 with a thin layer 25.

[0151] Of course, the signals shown in FIG. 4(a) and FIG. 4(b) are purely illustrative.

[0152] The spectral modulation signal 42 comprises a first peak 43 centred on a delay corresponding to the optical path difference 2E, where E is the optical thickness of the thin layer 25. This first peak 43 therefore corresponds to interference between two components of the measurement beam 16 reflected on the two interfaces of the object 24 situated on either side of the thin layer 25.

[0153] It also comprises a second peak 44 and a third peak 45 corresponding respectively to interferences between the reference beam 17 and the components of the measurement beam 16 reflected on the respective interfaces of the object 24 situated on either side of the thin layer 25.

[0154] Only these second and third peaks 44, 45 and the associated spectral modulation frequencies are representative of an optical path difference between the measurement optical beam 16 and the reference optical beam 17. Therefore only these second and third peaks 44, 45 contain information on the absolute height of the object.

[0155] It is therefore necessary, for performing a height measurement on the object 24, to be able to discriminate the peaks 43 due solely to interferences between components of the measurement beam 16 and the peaks of interest 44, 45 that are due to interferences between the reference beam 17 and the measurement beam 16 and which alone contain useful information.

[0156] To this end, the measuring head 10 is displaced relative to the object to be measured 24 with the displacement means 21, which varies the optical path difference between the measurement beam 16 and the reference beam 17. Only the peaks of interest 44, 45 due to interferences between the reference beam 17 and the measurement beam 16 are displaced in the measurement range, making it possible to distinguish them from the others that remain stationary. Moreover, it is therefore possible to position them in a preferred zone of the measurement range where they can be distinguished and measured under good conditions. To this end, the peaks of interest 44, 45 are positioned: [0157] in the exploitable measurement range (in terms of delays or optical path differences 2L). This measurement range extends from zero (zero delay) to delays for which the spectral modulation frequencies can no longer be sampled owing to the spectral resolution of the spectrometer. [0158] preferably in a zone of the measurement range corresponding to delays or optical path differences 2L greater than those corresponding to the thickness of the thin layers 25 of the object 24.

[0159] If the thickness of a thin layer 25 of the object 24 is sufficiently large, the measuring head 10 can also be positioned relative to the object 24 so that the length of the optical path of the reference optical beam 17 is intermediate between the lengths of the optical paths of the measurement beam 16 as reflected by the respective interfaces of the thin layer 25. In this case, the reference surface 14 appears optically as being between the interfaces of the thin layer 25, and the peaks of interest 44, 45 are located at delays (or optical path differences 2L) less than that corresponding to the thickness of the thin layer 25 of the object 24.

[0160] It should be noted that: [0161] the total measurement range is thus determined essentially by the stroke of the displacement means 21, and [0162] the resolution, i.e. the ability to discriminate close interfaces, is determined by the resolution of spectral detection.

[0163] As explained above, the interferometer makes it possible to determine optical path differences 2L.sub.i between the reference beam and the measurement beam reflected by the interfaces of the object 24. It therefore makes it possible to determine the optical heights L.sub.i of these interfaces relative to an origin defined by an equality of optical path in the interferometer.

It will be recalled that optical distances or heights correspond to geometric distances or heights multiplied by the refractive index of the media traversed. In the embodiment in FIG. 1, these heights L.sub.i correspond to the optical distance between the reference surface 14 and the interfaces of the object 24 along the Z axis.

[0164] In order to calculate the optical height Hu.sub.i of the interfaces of the object 24 relative to an origin of a coordinate system (X, Y, Z) as shown in FIG. 1, it is necessary to take into account the position P.sub.H of the interferometer or of the measuring head 10 along the Z axis. This position P.sub.H is given by the position measuring means of the translation stage 21, after calibration. Considering a position P.sub.H and optical heights Hu.sub.i oriented along the Z axis, the optical height Hu.sub.i of the interfaces of the object 24 is given by the relationship:

Hu.sub.i=P.sub.HL.sub.i.

[0165] It is also possible to obtain measurements of optical height HI.sub.j of the interfaces of the measurement object 24 on its opposite face in a similar manner with the second optical measurement means 27. Preferably, these measurements of optical height HI.sub.j are measured relative to the same origin of the coordinate system (X, Y, Z).

[0166] The optical thicknesses T of the object can then be determined by adding (or subtracting, depending on the sign conventions) the optical heights Hu and HI obtained on the two faces of the object 24.

[0167] With reference to FIG. 5, a method for measuring distances and/or thickness that uses the device of the invention will now be described.

[0168] In order to perform a measurement: [0169] the measurement beam is positioned on the surface of the object to be measured 24 by means of the second translation means 22 (step 50); [0170] the measuring head 10 is displaced in the Z direction relative to the object to be measured 24 with the displacement means 21 for varying the optical path difference between the measurement beam 16 and the reference beam 17 (step 51); [0171] the peak or peaks of interest 44, 45 are identified as explained above and/or they are positioned in a preferred zone of the measurement range (step 52); [0172] the optical path difference or differences L.sub.i corresponding to these peaks of interest 44, 45 are measured in the measurement range of the interferometer (relative to the zero delay corresponding to equality of optical paths of the measurement beam 16 and the reference beam 17) (step 53); [0173] the optical height Hu.sub.i of the interfaces of the object 24 is calculated, taking into account the position P.sub.H of the interferometer as described above (step 54); [0174] in order to calculate a thickness of the object, a measurement of the optical height HI.sub.j of the interfaces of the measurement object 24 is also performed on its opposite face with the second optical measurement means 27, and the optical heights Hu and HI are combined in order to determine the (optical) thickness T (step 55).

[0175] The measurement beams can then be moved to another point of the surface of the object 24 in order to perform another measurement and thus produce a mapping or topology of the object 24.

[0176] Step 51 of displacement of the measuring head 10 can be omitted between the measurement points at the surface of the object if identification of the peaks of interest is retained.

[0177] The method according to the invention also comprises a calibration step 56 that makes it possible to determine the value of the position P.sub.H of the interferometer or measuring head 10 along the Z axis. To this end, one or more measurements are performed on the reference object 26 the height Hu of which is known, and the value of the position P.sub.H is deduced therefrom. In a similar way it is also possible to calibrate the second optical measurement means 27.

[0178] This calibration procedure can be carried out once before performing a set of measurements on the surface of an object 24.

[0179] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without exceeding the scope of the invention.