Abstract:
The invention relates to a calibration standard, especially for the calibration of devices for the non-destructive measurement of the thickness of thin layers, with a carrier layer ( 12 ) consisting of a basic material and a standard ( 17 ) applied on the carrier layer ( 12 ), said standard having the thickness of the layer to be measured at which the device is to be calibrated, with the carrier layer ( 12 ) comprising a plane-parallel measuring surface ( 16 ) to its bearing surface ( 14 ), that the standard ( 17 ) comprises a bearing surface ( 18 ) plane-parallel with its measuring surface ( 19 ) for bearing on the measuring surface ( 16 ) of the carrier layer ( 12 ), and that the standard ( 17 ) is permanently provided on the carrier layer ( 12 ) by means of plating by rubbing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a calibration standard, especially for the calibration of devices for the non-destructive measurement of the thickness of thin layers with a carrier layer of a basic material and a standard applied on the carrier layer, said standard having the thickness of the layer to be measured at which the device is to be calibrated.  
         [0002]     For the non-destructive measurement of the thickness of thin layers by a magnetic induction method or an eddy current method depending on the basic material, it is necessary that—after a standardization—the devices are calibrated to the corresponding measuring task. By means of the calibration, a difference between nominal and actual values is established. This difference will be corrected, with the device being calibrated to the measuring task.  
         [0003]     For the correction of the difference between nominal and actual value, a measurement by means of the calibration standard is performed. To start with, a first value is registered on a carrier layer consisting of a basic material. Following that, another value is registered on a standard which is applied on the carrier layer. The difference will be the measured layer thickness. Due to the standard, the nominal value to be measured is determined. After several measurements, the averaged deviation between nominal and actual values will be registered and calibrated.  
         [0004]     The calibration standards known so far consist of a carrier layer of the basic material on which the standard is applied or glued on as a plastic film such as, for example, polyethylene terephthalate (PTFP). The calibration standards are used for the reproduction of layer thicknesses from e.g. 40 μm to 1,000 μm. With these calibration standards, it is problematic—in particular in the lower range of the layer thickness—that the application or especially the gluing can already result in minor build-ups by the bonding agent, with the reproduced layer thickness being distorted. The use of such films has the additional disadvantage that they are ductile and have a rough surface.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, this invention is based on the objective of providing a calibration standard which is simple to produce, having a high precision and repeating accuracy for the calibration of a device.  
         [0006]     This problem is solved by a calibration standard in which the standard is permanently provided by means of plating by rubbing on the carrier layer. Due to plating by rubbing the standard on the carrier layer, it will be possible to provide—without bonding agents or other additives—a connection between the standard and the carrier layer by adhesion. After plating by rubbing and positioning of the standard on the carrier layer, a type of cold welding will result with an increasing duration of time. This connection ensures a secure and plane-parallel configuration of the standard on the carrier layer. At the same time, a high-precision design of the calibration standard is achieved since no additional materials are intended between the standard and the carrier layer which might result in buckling or warping of the surface of the standard versus the carrier layer.  
         [0007]     According to an advantageous development of the invention, it is provided that the standard is positioned on the carrier layer by means of displacement movements with minor pressure. This can ensure that the layer of air between the bearing surface of the standard and a measuring surface of the carrier layer will be entirely displaced, and a direct adjacency of the bearing surface of the standard to the measuring surface of the carrier layer will be provided.  
         [0008]     The basic material of the carrier layer is preferably made of steel, iron or nonferrous metal. The basic materials steel and iron are used for the magnetic induction measuring method. For the eddy current method, the basic material for the carrier layer will be selected from a nonferrous metal, brass, copper or from aluminum or an aluminum alloy.  
         [0009]     The carrier layer preferably has a measuring surface which is polished or lapped. Thus, a surface with a low averaged surface roughness will be created so that uniform conditions are provided when the measuring probe is set onto the measuring surface. This surface is preferably hardened.  
         [0010]     The standard is advantageously designed as an insulation layer. This insulation layer is preferably made of a semiconductor material. Here, element semiconductors—such as germanium or silicon—can be used, or compound semiconductors which consist, for example, of compounds of the elements of the main group III and V, such as GaAs, InSb, etc. or of the main group II and IV, such as CdS, ZnS, etc. These semiconductor materials are provided in wafers and brought to the desired form of the standard. The standard can be angular or round in form. This semiconductor material has the advantage that the wafers will be produced in exact layer thicknesses. This is achieved by individual atomic layers growing up in a defined manner, with defined layer thicknesses being achieved thereby. Semiconductor materials may be used which are doped, partially doped or which are defective in terms of the implementation of individual elements or dopings. Dopings of the semiconductor material have no effect on the measurement. The surface of such wafers is of a high strength; it has minor roughness and it is gleaming bright and thus provides identical setting conditions for a measuring probe over the entire measuring surface of the standard. Moreover, this material is nearly wear-proof; for example, with a total of 60,000 measurements at the same measuring point, a deviation of 0.8 μm was determined with a thickness of the standard of 200 μm.  
         [0011]     According to another advantageous embodiment of the invention, it is provided that an auxiliary pole is allocated to the standard. This auxiliary pole is provided on a basic body at a pre-determined distance to the standard so that a device for measuring the thickness of thin layers—especially a manual measuring instrument—can be simply and safely calibrated with a base point or contact point or, respectively, an abutment.  
         [0012]     Advantageously, it is provided that a carrier layer taking up several standards is positionable on a bearing surface on the basic body, with the bearing surface on the basic body being designed plane-parallel with the auxiliary pole. Thus, an exact alignment of the measuring surfaces of the carrier layer and the standards and the bearing surfaces on the basic body can be enabled.  
         [0013]     According to a first embodiment of a calibration standard which comprises several standards and auxiliary poles allocated to them, it is provided that the contact surface of the auxiliary pole is taken up steplessly by an adjustment device, particularly by a thread in the basic body and is adjustable to the respectively allocated standard. Thus, it will be enabled that the contact surfaces of the auxiliary pole and those of the standard are in one plane. Thus, a tilt-free measurement can be performed. Preferably, adjacent to the auxiliary pole, a bearing surface is provided on the basic body which is in a plane with the measuring surface of the carrier layer of the associated standard so that first the measuring device is set onto a bearing surface—on the basic body—which is level with the measuring surface of the carrier layer to register a zero position and, subsequently, it is set onto the auxiliary pole and the associated standard, to measure the layer thickness and to perform the calibration of the measuring device.  
         [0014]     According to another advantageous embodiment of the calibration standard, it is provided that—in the embodiment of at least one standard and one auxiliary pole allocated to it—the measuring surfaces of the standard are in one plane and one bearing surface of the basic body is in the same plane as the standard, and that graded auxiliary poles are provided on the basic body which are allocated to the graded measuring surfaces of the carrier layers. Thus, in this embodiment, the auxiliary poles are provided at different heights, as well as the measuring surface of the carrier layers, with the standards and the auxiliary poles being in a joint plane. An alternative embodiment provides for a reversal. In such a reversal, the carrier layer is provided in one plane with the bearing surface on the basic body. The auxiliary poles are provided graded in analogy with the measuring surfaces of the standard.  
         [0015]     It is preferably provided that the standard is surrounded by a frame which preferably directly borders on the measuring surface of the standard. This embodiment has the advantage that the standard—in its planar extension—can be designed very small for setting on a sensor element of the measuring probe and that the following frame presents an enlargement of the contact surface for a probe bush of the measuring probe. The sensor element of the measuring probe is mostly kept movably in a probe bush. It will thus be rendered possible that the standard can be designed very small in its size and costs can thus be saved, with the supplementary frame having the same function as with a large-surface standard.  
         [0016]     An advantageous embodiment of the standard with a frame surrounding the standard provides for the frame being formed of a nonferrous metal, of plastic, glass, ceramic. Such materials are more economical than the materials used for the standard. Moreover, any layer thicknesses, as well as geometries for taking up the standard can be provided in such materials. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     In the following, the invention as well as additional advantageous embodiments and developments of the same will be explained and described in detail on the basis of the examples presented in the drawings. The features which can be gathered from the description and the drawings can be used individually by themselves or in a plurality in any combination according to the invention. It is shown in:  
         [0018]      FIG. 1 a  diagrammatic side view of the calibration standard,  
         [0019]      FIG. 2 a  diagrammatic top view of the calibration standard,  
         [0020]      FIG. 3   a  a perspective presentation of a calibration standard with an auxiliary pole allocated to it,  
         [0021]      FIG. 3   b  a perspective presentation of an alternative embodiment of a calibration standard of  FIG. 3   a,    
         [0022]      FIG. 4 a  perspective presentation of an alternative embodiment of  FIG. 3 , and  
         [0023]      FIG. 5 a  perspective presentation of another alternative embodiment of  FIGS. 3 and 4 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]      FIG. 1  presents diagrammatically the calibration standard  11  according to the invention. It comprises a carrier layer  12  with a bearing surface  14  for positioning the calibration standard  11  on a measuring table or a work area. On the opposite side, a measuring surface  16  of the carrier layer  12  is provided which is produced plane-parallel with the bearing surface  14 . This measuring surface  16  is preferably high-gloss polished. On this measuring surface  16 , the standard  17  is provided. It rests with one bearing surface  18  on the measuring surface  16 . On the opposite side, one measuring surface  19  of the standard  17  is provided, with the bearing surface  18  being aligned plane-parallel with the measuring surface  19 .  
         [0025]     The carrier layer  12  comprises a basic material, for example steel and iron for the magnetic induction layer thickness measuring method and a non-ferromagnetic basic material, e.g. non-magnetic steel, aluminum and their alloys for the layer thickness measurement by the eddy current method.  
         [0026]     The standard  17  is produced as an insulating layer from a semiconductor material. For this, the standard  17  is produced from a wafer of semiconductor material, especially silicon or germanium.  
         [0027]     For applying the standard  17  on the carrier layer  12 , the measuring surface  16  is at least rubbed clean with a lint-free cloth and freed from dust. Additionally, the measuring surface  16  can be cleaned with alcohol or the like. In the same manner, the bearing surface  18  of the standard  17  will be cleaned. Additionally, the bearing surface  18  can be polished. Subsequently, the standard  17  is applied on the measuring surface  16  by pushing on or rubbing on. Preferably, the standard  17  is positioned at a desired position on the carrier layer  12  with light pressure and a sliding movement. By repeatedly moving the standard  17  back and forth about the desired point, the standard  17  can be positioned to the carrier layer  12 . After positioning the standard  17  on the carrier layer  12 , the standard  17  remains permanently on the carrier layer  12  due to adhesion forces and a beginning cold welding setting in.  
         [0028]     On the carrier layer  12 , the nominal value of the layer thickness for the standard  17  is still additionally indicated, as well as the basic material of the carrier layers  12  or a layer/carrier combination.  
         [0029]     The standard  17  can be provided and applied in layer thicknesses of up to 1,000 μm. Even layer thicknesses of e.g. 10 μm of the standard  17  can be provided by pushing on the carrier layer  12  for producing the calibration standard  11  according to the invention. The measurement of the thickness of the standard  17  can be traceably performed with mechanical precision methods which in turn are calibrated.  
         [0030]      FIG. 3   a  presents a calibration standard  11  which is provided in particular for devices which have—in addition to the measuring probe—a contact point or abutment by means of which the measuring device is first set onto a measuring surface before the measuring probe is set onto the layer thickness to be measured. To calibrate in particular such measuring devices which are designed, for example, as manual instruments, a calibration standard  11  is proposed which comprises at least one standard  17  and at least one auxiliary pole  21  allocated to the standard  17 . In the exemplary embodiment according to  FIG. 3 , three standards  17  are exemplarily combined to one calibration standard  11 . These standards  17  stand for different layer thicknesses such as, for example, 50 μm, 100 μm and 500 μm. The auxiliary pole  21  is provided on a basic body  25 . This basic body  25  comprises a bearing surface  26  which comprises a surface quality in analogy to the measuring surface  16  of the carrier layer  12 . The basic body  25  takes up the carrier layer  12 , with a plane-parallel bearing surface being created for the bearing surface  26  on which the carrier layer  12  rests.  
         [0031]     In this exemplary embodiment, the measuring surface  16  lies in a plane with the bearing surface  26 . Thus, the measuring device can first be set on with a contact point on the measuring surface  26  to subsequently set the measuring probe onto the measuring surface  16 . For calibration of the measuring device, a contact point of the measuring device is subsequently set onto a bearing surface  23  of the auxiliary pole  21 , and the measuring probe is set on the measuring surface  19  of the allocated standard  17 . The bearing surface  23  of the auxiliary pole  21  lies plane-parallel with the allocated standard  17  in the same plane.  
         [0032]     By means of such a calibration standard  11 , a tilt-free measurement can be carried out with such manual instruments and an exact calibration can be performed. The number of the standards  17  and their allocated auxiliary poles  21  provided on the calibration standard  11  are only exemplary. In this embodiment, the auxiliary poles  21  are provided by a thread in the basic body  25  and can be adjusted to the thickness of the standard  17 .  
         [0033]      FIG. 3   b  presents an alternative embodiment of a calibration standard  11  to  FIG. 3   a . In this embodiment, the standards  17  are reduced in size. To achieve a sufficiently large bearing surface for a probe bush of the measuring probe, the standard is surrounded by a frame  31 . This frame  31  advantageously borders directly on the standard  17 . One bearing surface  32  of the frame  31  turns preferably flush into the bearing surface  19  of the standard  17 . However, it is not absolutely required that this bearing surface  32  lies in a plane with the bearing surface  19  since the sensor element is slidably provided versus the probe bush and can plunge into the probe bush. This frame  31  is preferably made of a more economical material than the standard  17 . For example, a nonferrous metal, glass, ceramic or especially plastic material can be provided. The form of the frame  31  may be round or angular. A recess in the frame  31  for taking up the standard  17  is adjusted to the geometry of the standard  17 .  
         [0034]     The allocation of a frame  31  to a standard  17  can be provided with a calibration standard  11  according to the embodiments in the  FIGS. 3   a ,  4  and  5 , as well as with an embodiment according to the  FIGS. 1 and 2 . Such a frame  31  can additionally feature a protective function to protect a transfer between the adjoining bearing surfaces  16  and  18  from the carrier layer  12  and the standard  17 .  
         [0035]      FIG. 4  presents an alternative embodiment of  FIG. 3 . Instead of the adjustable auxiliary poles  21 , contact surfaces  23  are provided which are worked into the basic body  25  by milling. The bearing surface  26  of the basic body  25  is, in turn, provided plane-parallel with the measuring surface  16  of the carrier layer  12  and lying in the same plane.  
         [0036]      FIG. 5  presents an alternative embodiment of  FIG. 3  and  4  and in particular a reversal of the embodiment of  FIG. 4 . The measuring surfaces  19  of the standard  17  are in a joint plane. In this plane, the auxiliary pole  21  is also provided with a contact surface  23  on the basic body  25  for taking up the contact point of the measuring device. The measuring surface  16  of the pertinent carrier layer  12  is displaced versus the contact surface  23  by the thickness of the standard  17 . The same applies analogously for the bearing surface  26 .  
         [0037]     These calibration standards  11  according to the invention are highly precise for layer thickness measurements.  
         [0038]     All of the above mentioned features are essential for the invention and can be randomly combined with each other.