Method and apparatus for measurement of the thickness of thin layers by means of a measurement probe

Abstract

The invention relates to a method and an apparatus for measurement of the thickness of thin layers by means of a measurement probe (11) which has a housing (14) which holds at least one sensor element (17) whose longitudinal axis lies parallel to or on a longitudinal axis (16) of the housing (14), in which at least during the measurement process, a gaseous medium is supplied to a supply opening (21) of the measurement probe (11) on a measurement surface (28), and is supplied via at least one connection channel (24), which is connected to the supply opening (21), to one or more outlet openings (26) which are provided on an end face (29), pointing towards the measurement surface (28), of the measurement probe (11), and in which at least one mass flow, which flows out of one or more outlet openings (26), of the gaseous medium is directed at the measurement surface (28), and in which the measurement probe (11) is held in a non-contacting manner with respect to the measurement surface (28) during the measurement process.

Description

INCORPORATION-BY-REFERENCE OF FOREIGN PRIORITY DOCUMENT

Applicant herein incorporates by reference the following foreign priority document: German Appln. No. 10 2006 022 882.0, filed May 15, 2006.

BACKGROUND OF THE INVENTION

The invention relates to a method and an apparatus for measurement of the thickness of thin layers by means of a measurement probe, according to the precharacterizing clause of claim 1 and the precharacterizing clause of Claim 6.

DE 41 19 903 A1 discloses a method and an apparatus for measurement of the thickness of thin layers in which a non-destructive layer-thickness measurement is carried out. In this case, a sensor element is provided which is placed on the measurement surface via a cup attachment in order to subsequently carry out a measurement which is based on the eddy-current principle. Alternatively, magnet-inductive measurement methods may also be used, depending on the materials to be tested.

In tactile measurement methods such as these, the cup attachment rests on the measurement surface. This results in a defined distance from the measurement surface. This requires the provision of a clean layer which has a certain minimum hardness in order to carry out the measurement. However, further fields of application are known in which the requirement is to test a layer with a soft surface and with a moist or only partially cleaned layer surface.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of providing a method and apparatus for measurement of the thickness of thin layers by means of a measurement probe, which does not adversely affect the measurement surface.

According to the invention, this object is achieved by a method having the features of claim 1. The measurement probe is held at the measurement point, floating with respect to the measurement surface, by means of gaseous medium flowing out of the measurement probe, at least during the measurement process, without any contact with the measurement surface. This results in a non-contacting measurement, which allows soft layer thicknesses or coatings to be determined exactly without any mechanical deformation being introduced into the coating by contact with a measurement probe with a cup attachment. Particularly in the case of layers with thicknesses in the micrometre range, even minor contacts can lead to corruption of the measurement results if these measurement probes are not placed smoothly on the measurement surface. Furthermore, a non-contacting measurement such as this also allows a moist or wet measurement surface to be tested and the layer thickness to be recorded exactly. Dirty measurement surfaces, such as the adhesion of oil or lubricant layers, do not influence the measurement, either. This is achieved by supplying a gaseous medium via a supply opening to the measurement probe, at least during the measurement, which gaseous medium is passed at least via a connection channel to one or more outlet openings, where it emerges, with the at least one outlet opening being provided on an end face, pointing towards the measurement surface, of the measurement probe.

One preferred refinement to the method provides for the pressure of the emerging mass flow from the at least one outlet opening to be set as a function of the mass of the measurement probe such that a stable operating point is set for the measurement probe during the measurement with respect to the measurement surface. While the measurement probe is approaching the measurement surface, during which process the gaseous medium is already flowing out of the outlet opening, the measurement probe is first of all repelled. Beyond a certain distance for the measurement probe, the repulsion is reversed and a vacuum pressure is produced which moves the measurement probe towards the measurement surface. As the distance to the measurement surface decreases further, repulsion occurs again. The direct transition between the vacuum pressure, that is to say an attraction and repulsion as the distance between the measurement probe and the measurement surface is reduced further, represents the stable operating point. At this distance, the measurement probe holds itself in position with respect to the measurement surface, that is to say this results in a defined distance between the sensor element of the measurement probe and the measurement surface, for an exact measurement.

The measurement probe effectively remains at this distance from the measurement surface, without making contact. This stable operating point is a function of the mass of the measurement probe, with the mass of the measurement probe being taken into account by adjustment of the pressure of the emerging gaseous medium, in order to set the stable operating point.

A constant mass flow is preferably supplied to the supply opening at a measurement point, and emerges from the outlet opening, during the measurement process. This makes it possible to maintain an exact distance for each measurement, which in turn allows an exact measurement.

A mass flow of the gaseous medium is preferably applied to the measurement probe before the measurement probe approaches, or while it is approaching, a measurement point. This ensures that there is no contact between the sensor element of the measurement probe and the measurement surface.

Furthermore, it is preferable for the supply of the gaseous medium to be interrupted after completion of the measurement of a layer thickness, at the same time that the measurement probe lifted off the measurement surface, or after the measurement probe has been lifted off the measurement surface. This results in energy being saved, and prevents vortices from being formed by the emerging air.

According to the invention, the object is achieved by a measurement probe having the features of Claim 6. This measurement problem in particular for carrying out the method according to one of Claims 1 to 5, has the advantage that at least one outlet opening, pointing towards the measurement surface, of the measurement probe, which supplies at least one emerging mass flow of a gaseous medium to a measurement surface, allows non-contacting measurement with respect to the measurement surface at the intended measurement point. The at least one outlet opening is connected via at least one connection channel to at least one supply opening in the housing, via which outlet opening the gaseous medium is supplied. This arrangement keeps the measurement probe floating above the measurement surface, and the thickness of thin layers is measured at the same time without any disadvantageous influence on the measurement surface, owing to the floating arrangement.

One advantageous refinement of the measurement probe provides for the outlet opening to be provided on the longitudinal axis of the housing, and for the sensor element to be arranged concentrically with respect to the outlet opening. This refinement allows the mass flow of gaseous medium to emerge centrally with respect to the measurement surface, so that the emerging medium flows away uniformly in a radial direction along the measurement surface. This allows a simple design, in which the generally rotationally symmetrical measurement probes can retain their geometry and symmetry.

The connection channel of the outlet opening preferably has a first hole section, which is directed into the housing interior and whose length corresponds at least to the height of the sensor element. The method of operation of the sensor element can thus be retained, and the previous design can be used with the exception of the introduction of a hole section such as this. For example, pot cores can be used which hold coil formers and screen them from the outside. In this case, the inner pole of the pot core is provided with the hole section, with the magnetic field that emerges at the inner pole not being influenced by this hole section.

According to this first embodiment, the first hole section in the sensor element is advantageously connected to a lateral hole, which is adjacent to the sensor element or is located outside the sensor element, and to an annular channel or pierced hole which is connected to the at least one supply opening. This allows a simple design, allowing the measurement probe to be physically compact.

The sensor element is preferably held in the housing of the measurement probe by means of an air bearing. An air bearing such as this makes it possible to provide an arrangement between the sensor element and the housing that is not subject to tilting or canting, thus allowing the required distance that is suitable for the measurement to the measurement surface to be kept small, and the measurement probe to be provided such that it holds itself in position with respect to the measurement surface. This allows an exact measurement to be carried out.

According to a further advantageous refinement to the invention, the sensor element is supplied with a separate mass flow of a gaseous medium. The sensor element can thus be driven with its own mass flow independently of further elements, as will be described in the following text, thus allowing exact adjustment of the sensor element in order to measure the thickness of thin layers, irrespective of the other external conditions and further mass flows.

According to one advantageous refinement to the invention, the outlet opening, which is located on the longitudinal axis of the housing, is provided on a projection, which is in the form of a cup and points towards the measurement surface, on the end surface of the measurement probe. This has the advantage that the distance to the measurement surface increases immediately adjacent to the outlet opening, thus resulting in a vacuum pressure and counteracting the repulsion force of the emerging mass flow.

A supporting ring which extends in the radial direction is preferably provided on the housing of the measurement probe and has a plurality of outlet openings which point towards the measurement surface and are connected via preferably one annular connection channel to one or more supply openings. A supporting ring such as this has the advantage that it allows mass flows to emerge separately, in order to arrange the measurement probe such that it floats with respect to the measurement surface. Particularly in the case of large or heavy measurement probes or highly sensitive measurement surfaces, this makes it possible to form an air cushion in order to position the measurement probe at the stable operating point, in particular irrespective of the sensor element. This supporting ring can form an outer system which is supplied with a separate mass flow from the inner system, which is formed by the sensor element on an air bearing in the housing.

According to one alternative embodiment of the measurement probe, a plurality of outlet openings are provided, and are arranged concentrically with respect to the longitudinal axis of the housing. These outlet openings are preferably distributed uniformly over the circumference thus allowing a uniform outlet flow in order to achieve the stable operating point. Outlet openings such as these may be designed such that the outlet flow direction of the mass flow is directed at right angles to the measurement surface. Alternatively, it is possible to provide for the outlet direction of the mass flow to be at an angle of other than 90° to the measurement surface. By way of example, all of the outlet openings can be provided at an angle to the outer edge area, so that the angle between the outlet direction of the mass flow from the outlet direction and the outlet-flow direction of the mass flow in the radial direction after striking the measurement surface is greater than 90°.

A circumferential annular gap is preferably provided on one end surface of the measurement probe, and connects the individual outlet openings to one another. In consequence, the mass flows which emerge from the outlet openings are also moved in the circumferential direction of the annular gap, thus resulting in an effectively annular outlet opening for the mass flow.

The annular gap which is arranged on the end surface of the measurement probe and connects the outlet openings to one another is preferably broader than the diameter of the outlet openings. This results in additional vortices, which assist the formation of a mass flow which emerges in an annular shape.

A sliding shoe is preferably provided on a lower face of the housing of the measurement probe and has a plurality of outlet openings which are connected to an annular gap, with this annular gap communicating with one or more connection channels in or on the housing of the measurement probe, to which the gaseous medium is applied via the at least one supply opening. By way of example, this makes it possible for the annular gap to be supplied via a supply opening and a connection channel with a mass flow which in turn supplies the mass flow to a plurality of outlet openings. Alternatively two or more connection channels can also be passed to the annular gap and are supplied with the gaseous medium via one or more supply openings.

An annular gap is advantageously likewise provided on the lower face of the sliding shoe that is preferably formed. This annular gap is formed analogously to the annular gap provided on the housing, and has the same advantages.

If a sliding shoe is provided on one lower face of a housing, an annular gap which communicates with the connection channel or channels can be provided either in the sliding shoe or on the housing, or the annular gap which in each case communicates with the connection channel or channels can be provided in the sliding shoe and in the housing.

In both of the abovementioned embodiments of the measurement probe, the sensor element is advantageously arranged fixed in the housing.

According to one alternative refinement to the invention, the at least one sensor element is mounted such that it floats with respect to the housing via an air cushion. This makes it possible for the mass to be held at the operating point to be less than would be the case with a sensor element arranged fixed in the housing. Further, in addition, tolerances in the positioning of the housing with respect to the measurement surface can automatically be compensated for by the floating bearing of the sensor element.

According to one advantageous development of this alternative embodiment, the sensor element has guide elements which project into the housing interior, in particular are in the form of pins and are guided in a housing hole with an air gap, with preferably radially circumferential depressions being provided on the guide element or on the housing hole. This makes it possible to improve the formation of the air cushion for the non-contacting air bearings.

The preferred embodiment advantageously has an annular gap between the air gap and the end-face outlet openings on the measurement probe of the sensor element, which is mounted such that it floats, and the volume of the annular gap is greater than the mass flow flowing through the air gap and is greater than the mass flow leaving the outlet openings. This results in a vacuum pressure in this annular gap, by which means the sensor element is arranged such that it holds itself in position with respect to the housing.

The sensor element, which is mounted such that it floats, preferably has outlet openings which are open at the edge and point towards the housing hole and the end face of the measurement probe. These on the one hand support the air bearing at the lower housing end between the sensor element and the housing hole, while on the other hand forming an air cushion in order to hold the sensor element in stable equilibrium with respect to the measurement surface.

At its end inside the housing, the guide element preferably has a conical, spherical or truncated-conical end section. The effective circular area is the reference area for the pressure which is supplied to the sensor element in the direction of the measurement surface. The mass flow flowing out of the outlet openings governs the resetting force acting on the sensor element, in order to arrange the sensor element such that it floats with respect to the measurement surface.

According to one preferred embodiment of the measurement probe, an outer edge area is provided on an end surface, pointing towards the measurement surface, of the housing or of the sliding shoe and is designed such that the distance between the end surface and the measurement surface increase outwards. An outer edge area such as this may be formed by a straight line, a rounded area or a curvature with a second- or third-degree function. This outer edge area is used to produce a vacuum pressure which counteracts the outlet-flow force of the mass flow from the outlet opening or openings and preferably results in an increase in the stable equilibrium.

According to a further advantageous embodiment of the invention, a rocker, preferably having an eddy-current brake, and on whose free arm the measurement probe is held, is provided for positioning the sensor element at a measurement point on the measurement surface. This makes it possible to position the measurement probe with respect to the measurement surface smoothly and without any jerking, while at the same time allowing the measurement probe to be positioned automatically at the stable operating point.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as further advantageous embodiments and developments of it will be described and explained in more detail in the following text with reference to the examples which are illustrated in the drawings. The features which can be found in the description and in the drawings may, according to the invention, be used individually in their own right or in any desired combination of one or more of them. In the figures:

FIG. 1 shows a schematic section illustration of a first embodiment according to the invention,

FIG. 2 shows a schematic section illustration of an alternative embodiment to that shown in FIG. 1,

FIG. 3 shows a schematic view from above of the embodiment shown in FIG. 2,

FIG. 4 shows a schematic view from underneath of the embodiment shown in FIG. 2,

FIG. 5 shows a schematic section illustration of a further embodiment according to the invention,

FIG. 6 shows a view from underneath of the embodiment shown in FIG. 5, and

FIG. 7 shows a schematic section illustration of a further alternative embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a measurement probe 11 which is connected via a connection line 12 to an apparatus 13 for measurement of the thickness of thin layers and for evaluation of the measurement data. The measurement probe 11 may alternatively be part of this apparatus 13, in the form of a stationary appliance or a handheld appliance. This measurement probe 11 is used for non-destructive and non-contacting layer-thickness measurement.

The measurement probe 11 has a housing 14 which, in particular is cylindrical. At least one sensor element 17 is arranged on a longitudinal axis 16 of the housing 14. This sensor element 17 is firmly connected to the housing 14 in the exemplary embodiment shown in FIG. 1.

The sensor element 17 is formed, for example, by a primary coil and a secondary coil with a magnet, or is in the form an unscreened dipole. A pot core 18 having at least one coil is provided on an inner pole 19 in the exemplary embodiment. A sensor element 17 such as this allows measurement based on the magnet-inductive process. The magnet-inductive measurement process is suitable for measurement of non-ferrous metallic layers, such as chromium, copper, zinc or the like on magnetic base materials, such as steel or iron, and for paint, lacquer and plastic layers on magnetic base material such as steel or iron. By way of example, the measurement range is up to a layer thickness of 1800 μm, preferably using a frequency of less than 300 Hertz.

Alternatively, it is possible to provide for the at least one sensor element 17 to comprise a coil. A sensor element 17 such as this makes it possible to use the eddy-current method, that is to say this makes it possible to measure the thickness of electrically non-conductive layers on non-ferrous metals, such as paints, lacquers, plastics, on aluminum, brass or stainless steel, or other anodized layers, with a radio-frequency alternating field. The measurement range may likewise be up to a layer thickness of 1800 μm.

The measurement probe 11 has a supply opening 21 on the housing 14, to which a connection 22 is fitted. This holds a flexible tube, which is not illustrated in any more detail, via which a gaseous medium is supplied to the supply opening 21 from a supply source, which is not illustrated in any more detail. Air is preferably used as the gaseous medium, in particular in a dust-free form and/or with oil having been removed from it. Alternatively, other non-explosive gases can also be used. The supply opening 21 is connected via a connection channel 24 to an outlet opening 26 which is provided on an end face 29, arranged with respect to the measurement surface 28, of the measurement probe 11, in particular of the sensor element 17. The connection channel 24 which is connected to the supply opening 21 is in the form of an annular channel 31, which merges into a lateral hole 32 in order that a first hole section 33 is supplied with a mass flow of the gaseous medium. This first hole section 33 is located on the longitudinal axis 16 of the sensor element 17 or of the housing 14, and allows the mass flow to emerge centrally from the measurement probe 11 via the inner pole 19. The first hole section 33 has a taper 34, which is in the form of a nozzle and merges into the outlet opening 26, in the lower area. The mass flow emerges from the outlet opening 26, and flows away radially along the measurement surface 28.

The outlet opening 26 is provided on a projection 36 in the form of a cup, thus resulting in the distance between the outlet opening 26 and the measurement surface 28 being increased adjacent to it, so that a vacuum-pressure area is created, adjacent to the outlet opening 26.

The pressure of the mass flow which emerges and is directed at the measurement surface 28 is used to adjust the distance between the sensor element 17, which is arranged such that it floats with respect to the housing 14 or has a central air bearing, and the measurement surface 28, creating an equilibrium between the mass resulting from the weight of the measurement probe 11 and the vacuum pressure acting on it, as a result of the mass flow flowing away on the one hand and the resetting force of the mass flow on the other hand, which results from the mass flow striking the measurement surface 28.

A non-contacting measurement of the thickness of the layer at a defined distance from the measurement surface 28 can be carried out at the stable operating point. In consequence, dirty surfaces, such as oily or greasy surfaces, as well as moist or wet surfaces, have no disadvantageous effect on the layer-thickness measurement. Furthermore, recordings can also be made on soft coatings without mechanical deformation of the measurement surface 28. Layers which are not yet completely cured can likewise be measured.

A supporting ring 38 is provided on the lower housing edge of the housing 14, preferably as a separate outer system for the sensor element 17 which surrounds the inner system, and in the central or outer edge area has outlet openings 52 which are arranged on a lower face of the supporting ring 38 pointing towards the measurement surface 28. A mass flow of the gaseous medium is supplied to the supporting ring 38 via a supply opening 51 and, via an annular gap which is in the form of a connection channel 24, supplies a plurality of outlet openings 52, which are preferably distributed uniformly over the circumference. By way of example, one supply opening 51 is sufficient to supply a mass flow to a plurality of outlet openings 52 via the annular gap. If an increased mass flow is required, a plurality of supply openings 51 may also be provided, in order to feed the gaseous medium to the connection channel or channels 24.

The separation between the outer system and the inner system makes it possible for the measurement probe 11 to be held by the outer system such that it floats above the measurement surface 28, and for the sensor element 17 to assume a self-holding measurement position, independently of this, in which position the distance to the measurement surface is considerably less than in the case of a lower face of the supporting ring 38 of the outer system. In consequence, the magnetic field is passed via the inner pole 19 to the measurement surface of the measurement object without spreading out, thus resulting in good resolution in order to increase the measurement accuracy. The central supply of the mass flow via the hole sections 33 and 26 in the inner pole 19 in this case does not influence the magnetic field, or influences it only to a negligible extent.

An outer edge area 39 of the supporting ring 38, which points towards the measurement surface 28, is designed such that the distance to the measurement surface 28 increases outwards. This makes it possible to produce a vacuum pressure which can be used to assist the assumption of the stable operating point.

An alternative embodiment to that shown in FIG. 1, which is not illustrated in any more detail, may, in addition to the first hole section 33 or as an alternative to the first hole section 33, have one or more hole sections which run parallel in the outer edge area between the housing 14 and the sensor element 17, or the outer edge area of the sensor element 17, such that further outlet openings 26, pointing towards the measurement surface 28, are supplied with a mass flow via the annular channel 31.

An alternative embodiment to that shown in FIG. 1, which is likewise not illustrated in any more detail, can be provided by providing a fixed arrangement of the sensor element 17 with respect to the housing 14, instead of the sensor elements 17 being mounted in a floating manner, with the supply of the mass flow via the supply opening 21 at the connection 22 as well as that of the supply opening 51 then being matched to one another in order to assume an optimum measurement distance in a self-holding manner, with the sensor element 17 being held with respect to the measurement surface 28, without making contact with it.

FIG. 2 shows a schematic section illustration of one alternative embodiment of the invention shown in FIG. 1. FIG. 3 shows a view from above. In this embodiment, the sensor element 17 is likewise provided fixed in the housing 14. Instead of a mass flow emerging centrally on the longitudinal axis 16 of the housing 14, a plurality of outlet openings 26 are alternatively provided concentrically with respect to the longitudinal axis 16 of the housing and result in the gaseous medium emerging approximately, and in particular ideally, in an annular form. At least one supply opening 21 is provided on the housing 14, and carries the mass flow to an annular gap 4.2 via the connection channel 24. According to the exemplary embodiment, this annular gap 42 may be provided in a sliding shoe 41, which is screwed, adhesively bonded, clipped or latched into the housing 14, or is directly integrated on a lower face of the housing 14. A plurality of outlet openings 26 are provided in the sliding shoe 41, preferably distributed uniformly over the circumference, as is illustrated by way of example in FIG. 4. These outlet openings 26 are supplied through the annular channel 40. In order to supply the gaseous medium to the measurement probe 11 more uniformly, the exemplary embodiment shown in FIG. 2 has a second supply opening 21 with a second connection channel 24, which runs separately from the first connection channel 24. The outlet openings 26 open into an annular gap 40 which is of the same size as, or preferably larger than, the diameter of the outlet openings 26. This results in additional vortices. At the same time, this makes it possible to assist the formation of an annular air cushion or air outlet.

The sliding shoe 41 preferably has an outer edge area, analogous to that of the supporting ring 38.

An aperture 43 is provided in the housing cover through which the connecting lines or connection lines 12 are passed out of the housing 14 between the sensor element 17 and the apparatus 13.

This embodiment has the advantage that the sensor element 17 is provided in a separate accommodation area in the housing 14, allowing the supply line 12 to be passed out in a simple form.

FIGS. 5 and 6 show one alternative embodiment of a measurement probe 11. In this embodiment the sensor element 17 is arranged such that it floats above an air bearing with respect to the housing 14. The sensor element 17 has a guide element 44 which projects into the housing interior and is guided in a housing hole 46. The sensor element 17 is arranged at the lower end of the guide element 44. An air gap is formed between the guide element 44 and the housing hole 46, thus allowing friction-free movement of the sensor element 17 relative to the housing 14. Outlet openings 26 are provided adjacent to the sensor element 17 and are in the form of slots, which are open at the edge and open in an annular channel 48. This annular channel 48 is supplied with gaseous medium via the air gap between the guide element 44 and the housing hole 46, and, together with this, forms the connection channel 24. The increase in the volume in the annular channel 48 in comparison to the air gap and the reduction in the cross section of the outlet openings 26 result in a vacuum pressure in this area, by which the sensor element 17 is held in the housing 14. At the same time, the mass flow striking the measurement surface 24 creates a resetting force on the end surface 29 of the sensor element 17, so that this sensor element 17 is held such that it floats with respect to the measurement surface 28.

The outlet openings 26, which are open at the edge, on the sensor element 17 may alternatively be in the form of holes which are open or closed at the edge. The number, the alignment and the geometry of the outlet openings 26 are all matched to the required mass flow in order that the sensor element 17 can assume a stable operating point with respect to the measurement surface 28.

FIG. 7 shows one alternative embodiment to that shown in FIGS. 5 and 6. In this embodiment, the sensor element 17 is likewise held such that it can be moved or is mounted in a floating form with respect to the housing 14. A supply opening 21 supplies a gaseous medium to a connection channel 24, which is formed by an annular channel 31 in a first section, from which a plurality of hole sections 33 branch off, and lead to the outlet openings 26. These outlet openings 26 surround the inner sensor element 17, preferably at a uniform distance, with this sensor element 17 being arranged on the longitudinal axis 16 of the housing 14.

Two annular channels 49 are provided in the sensor element 17, parallel to one another but separated by a distance which corresponds essentially to the diameter of the supply opening 21, so that the gaseous medium is applied to both annular channels 49, and an equilibrium is formed.

Analogously to the supporting ring 38, the sensor element 17 has an outer edge area 39, in order to achieve the same effect and advantages.

All of the features described above are each in their own right significant to the invention and may be combined with one another as required.

Claims

1. Method for measurement of the thickness of thin layers by means of a measurement probe which has a housing which holds at least one sensor element whose longitudinal axis lies parallel to or on a longitudinal axis of the housing characterized in that, at least during the measurement process, a gaseous medium is supplied to a supply opening of the measurement probe on a measurement surface, and is supplied via at least one connection channel, which is connected to the supply opening, to one or more outlet openings which are provided on an end face, pointing towards the measurement surface, of the measurement probe, and in that at least one mass flow, which flows out of one or more outlet openings, of the gaseous medium is directed at the measurement surface, and in that the measurement probe is held in a non-contacting manner with respect to the measurement surface during the measurement process.

2. Method according to claim 1, characterized in that the pressure of the emerging mass flow is set for a stable operating point of the measurement probe, which operating point is a function of the mass of the measurement probe and of a resetting force of the mass flow emerging onto the measurement surface.

3. Method according to claim 1, characterized in that a constant mass flow is supplied to the supply opening, and emerges from the outlet opening, during the measurement process.

4. Method according to claim 1, characterized in that a mass flow of the gaseous material is applied to the measurement probe before the measurement probe approaches, or while it is approaching, a measurement surface.

5. Method according to claim 1, characterized in that the supply of the gaseous medium is interrupted after the measurement has been carried out, at the same time that the measurement probe is lifted off the measurement surface, or after the measurement probe has been lifted off the measurement surface.

6. Measurement probe for an apparatus for measurement of the thickness of thin layers, having a housing which holds at least one sensor element whose longitudinal axis lies on or parallel to a longitudinal axis of the housing, characterized in that the measurement probe has a supply opening to which a connection for supplying a gaseous medium is fitted and which has at least one outlet opening which is provided on an end face, pointing towards the measurement surface, of the measurement probe and has at least one connection channel, which connects the at least one supply opening to one or more outlet openings.

7. Measurement probe according to claim 6, characterized in that the outlet opening is provided on the longitudinal axis of the housing, and the sensor element is arranged concentrically with respect to the outlet opening.

8. Measurement probe according to claim 7, characterized in that the connection channel has a first hole section, which is directed from the outlet opening into the housing interior and whose length corresponds at least to the height of the sensor element.

9. Measurement probe according to claim 8, characterized in that the first hole section opens into a lateral hole and is connected to an annular channel in order to form a connection channel, or in that the first hole section is connected to a pierced hole which opens into the supply opening.

10. Measurement probe according to any one of claims 6 to 9, characterized in that the sensor element is arranged in the housing such that it is moved longitudinally by means of a central air bearing.

11. Measurement probe according to claim 6, characterized in that the sensor element is supplied with a separate mass flow of gaseous medium.

12. Measurement probe according to claim 8, characterized that the outlet opening is provided on a projection, in the form of a cup, on one end surface of the sensor element.

13. Measurement probe according to claim 6, characterized in that a supporting ring which extends in the radial direction is provided on the housing and has a plurality of outlet openings which point towards the measurement surface and are connected, preferably via one annular connection channel, to one or more supply openings.

14. Measurement probe according to claim 6, characterized in that a plurality of outlet openings which are arranged concentrically with respect to the longitudinal axis of the housing are provided on one end face of the measurement probe.

15. Measurement probe according to claim 14, characterized in that a circumferential annular gap is provided on one end surface of the measurement probe and connects the individual outlet openings to one another.

16. Measurement probe according to claim 15, characterized in that the annular gap is broader than the diameter of the outlet openings.

17. Measurement probe according to claim 6, characterized in that a sliding shoe is provided on a lower face of the housing and has a plurality of outlet openings which are connected to one another by means of an annular gap, with the annular gap communicating with one or more connection channels, to which the gaseous medium is applied via at least one supply opening.

18. Measurement probe according to claim 17, characterized in that the sliding shoe has an annular gap, which connects the outlet openings, on an end face which points towards the measurement surface.

19. Measurement probe according to claim 14, characterized in that the at least one sensor element is arranged fixed to the housing.

20. Measurement probe according to claim 6, characterized in that the sensor element is mounted such that it floats with respect to the housing, by means of a cushion of gaseous medium.

21. Measurement probe according to claim 20, characterized in that the sensor element has a guide element which projects into the housing interior and is guided in a housing hole with a gap for gaseous medium.

22. Measurement probe according to claim 20, characterized in that an annular gap is formed between the gap and the end-face outlet openings of the measurement probe of the sensor element, which is mounted such that it floats, and the volume of the annular gap is designed to be greater than the mass flow flowing through the air gap and greater than the mass flow leaving the outlet openings.

23. Measurement probe according to claim 20, characterized in that outlet openings which are open at the edge and point towards the end face and towards the housing hole are provided on the sensor element, which is mounted such that it can float.

24. Measurement probe according to claim 20, characterized in that the guide element has a conical, spherical, or truncated-conical end section at its end inside the housing.

25. Measurement probe according to claim 6, characterized in that an outer edge area whose distance from the measurement surface increases towards the outside is provided on an end surface, pointing towards the measurement surface, of the housing or of the sliding shoe or of the housing and the sliding shoe.

26. Measurement probe according to claim 6, characterized in that a rocker, having an eddy-current brake, and on whose free arm the measurement probe is held, is provided for positioning the sensor element at a measurement point on the measurement surface.