Abstract:
The invention relates to an apparatus for measuring thickness and deviations from the thickness of thin conductive coatings on various substrates, e.g., metal coating films in semiconductor wafer or hard drive disks. The thickness films may be as small as fractions of microns. The apparatus consists of an inductive sensor and a proximity sensor, which are rigidly interconnected though a piezo-actuator used for displacements of the inductive sensor with respect to the surface of the object being measured. Based on the results of the operation of the proximity sensor, the inductive sensor is maintained at a constant distance from the controlled surface. Variations in the thickness of the coating film and in the distance between the inductive sensor and the coating film change the current in the inductive coil of the sensor. The inductive sensor is calibrated so that, for a predetermined object with a predetermined metal coating and thickness of the coating, variations in the amplitude of the inductive sensor current reflect fluctuations in the thickness of the coating. The distinguishing feature of the invention resides in the actuating mechanism of microdisplacements and in the measurement and control units that realize interconnection between the proximity sensor and the inductive sensor via the actuating mechanism. The actuating mechanism is a piezo actuator. Measurement of the film thickness in the submicron range becomes possible due to highly accurate dynamic stabilization of the aforementioned distance between the inductive sensor and the object. According to one embodiment, the distance is controlled optically with the use of a miniature interferometer, which is rigidly connected to the inductive sensor. According to another embodiment, the distance is controlled with the use of a capacitance sensor, which is also rigidly connected to the inductive sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of measurement of film thickness, more specifically, to measuring thickness of conductive coatings on various conductive substrates or on non-conductive substrates with electric properties different from those of the coating films. In particular, the invention may find use in measuring thickness of coating films on semiconductor wafers, hard drive disks, or the like. 
     BACKGROUND OF THE INVENTION 
     There exists a great variety of methods and apparatuses used in the industry for measuring thickness of coating films and layers applied or laid onto substrates. These methods and apparatuses can be classified in accordance with different criteria. Classification of one type divides these methods into direct and indirect. 
     An example of a direct method is measurement of a thickness in thin metal coating films by means of so-called X-ray reflectivity. One of these methods is based on a principle that X-rays and gamma-rays are absorbed by matter. When a beam of rays passes through a material, the amount of the beam absorbed depends on what elements the material consists of, and how much of the material the beam has to pass through. This phenomenon is used to measure the thickness or density of a material. The advantage of measuring in this way is that the gauge does not have to touch the material it is measuring. In other words, in thickness measurement, the surface of a web or strip product will not be scratched. The instrument for this method is e.g., RMS1000 Radiometric System produced by Staplethorne Ltd (UK). The instrument uses a suitable radiation source and one or more radiation detectors installed in a mechanical housing which also provides high quality radiological shielding. The source may be an X-ray tube or a radioactive source. The instrument also uses a set of beam defining collimators and one or more radiation detectors. The detectors measure the radiation absorbed within the object or flow being measured and output the signal data to a computer. For thickness gauging, the collimators usually define a single, narrow beam. This gives optimum spatial resolution. 
     A disadvantage of radiation methods is the use of X-ray or gamma radiation that requires special safety measures for protection of the users against the radiation. The instruments of this type are the most expensive as compared to metrological equipment of other systems. 
     Another example of direct measurement is a method of optical interferometry, described e.g., by I. Herman in “Optical Diagnostics for Thin Film Processing”, Academic Press, 1996, Chapter 9. Although the optical interferometry method produces the most accurate results in measuring the thickness of a coating film, it has a limitation. More specifically, for conductive films, to which the present invention pertains, this method is limited to measurement of extremely thin coating films which are thin to the extent that a nontransparent material, such as metal, functions as transparent. In other words, this method is unsuitable or is difficult to use for measuring conductive films thicker than 200 Å to 500 Å. 
     Another example of direct measurement methods is measuring thickness of a film in situ in the course of its formation, e.g., in sputtering, magnetron target sputtering, CVD, PVD, etc. These methods, which are also described in the aforementioned book of I. Herman, may involve the use of the aforementioned optical interferometry or ellipsometry. However, in this case measurement is carried out with reference to both the surface of the substrate and the surface of the growing layer. Therefore, this method is inapplicable to measuring thickness of the film that has been already deposited. 
     In view of the problems associated with direct methods, indirect non-destructive methods are more popular for measuring thickness of ready-made films. An example of a well-known non-destructive indirect method used for measuring thickness of a film is the so-called “four-point probe method”. This method is based on the use of four contacts, which are brought into physical contact with the surface of the film being measured. As a rule, all four contacts are equally spaced and arranged in line, although this is not a compulsory requirement. Detailed description of the four-point probe method can be found in “Semiconductor Material and Device Characterization” John Wiley &amp; Sons, Inc., N.Y., 1990, pp. 2-40, by D. Schroder. The same book describes how to interpret the results of measurements. This method is classified as indirect because the results of measurement are indirectly related to the thickness of the film. It is understood that each measurement of electric characteristics has to correlated with the actual thickness of the film in each particular measurement, e.g., by cutting a sample from the object and measuring the thickness of the film in a cross-section of the sample, e.g., with the use of an optical or electron microscope. Nevertheless, in view of its simplicity, low cost, and convenience of handling, the four-point probe method is the most popular in the semiconductor industry. 
     However, the four-point method has some disadvantages. The main problem associated with the aforementioned four-point probe method consists in that in each measurement it is required to ensure reliable contact in each measurement point. This is difficult to achieve since conditions of contact vary from sample to sample as well as between the four pointed contact elements of the probe itself in repeated measurement with the same probe. Such non-uniformity affects the results of measurements and makes it impossible to perform precision calibration. 
     Known in the art are also methods for measuring film thickness with the use of an inductive sensors. For example, U.S. Pat. No. 6,072,313 issued in 2000 to L. Li et al. describes in-situ monitoring and control of conductive films by detecting changes in induced eddy currents. More specifically, the change in thickness of a film on an underlying body such as a semiconductor substrate is monitored in situ by inducing a current in the film, and as the thickness of the film changes (either increase or decrease), the changes in the current are detected. With a conductive film, eddy currents are induced in the film by generating an alternating electromagnetic field with a sensor, which includes a capacitor and an inductor. The main idea of the apparatus of U.S. Pat. No. 6,072,313 consists in using a resistor and a capacitor in a parallel resonance circuit. The resonance is caused by means of an oscillator. The inductive coupling between the oscillation circuit and the Eddy current inducted in the coating is used for improving a signal/noise ratio and can be used for improving quality of measurements. In fact, this is a method well known in the radioelectronics for measuring under conditions of the electrical resonance. The above patent describes the aforementioned inductive method for measuring thickness of a film in chemical mechanical polishing (CMP). 
     A similar inductive method, which was used for measuring thickness of a slag, is disclosed in U.S. Pat. No. 5,781,008 issued in 1998 to J. Muller et al. The invention relates to an apparatus for measuring the thickness of a slag layer on a metal melt in a metallurgical vessel. The apparatus comprises a first inductive eddy current sensor which indicates the distance of the apparatus from the metal melt as it is moved toward the melt. A second sensor detects when the apparatus reaches a predetermined distance relative to or contacts the slag layer and triggers the inductive eddy-current sensor when such distance is attained. The sensors are arranged in a predetermined spatial relation, and the thickness of the slag layer is determined by an evaluation device, which analyzes the received signals. The apparatus permits measurement of the thickness of the slag layer without the need of additional equipment (e.g. mechanical lance movement or distance measurement). 
     The method and apparatus of U.S. Pat. No. 5,781,008 relate to macro-measurements of thick layers, and the sensors used in the apparatus of this invention are inapplicable of measuring thickness of thin-film coatings on such objects as semiconductor wafers and hard-drive disks. Furthermore, once the second sensor has detected that the apparatus reached a predetermined distance relative to or contacts the slag layer, this distance remains unchanged during the measurement procedure. This condition is unacceptable for measuring thickness of a thin film with microscopic thickness which moves relative to the sensor, e.g., for mapping, i.e., for determining deviations of the thickness over the substrate. 
     In order to understand why the use of known eddy-current sensor systems utilizing a measurement eddy-current sensor and a proximity sensor cannot be easily and directly applicable to measurement of microscopically-thin film coatings on conductive or non-conductive substrates, let us consider constructions and operations of the aforementioned known systems in more detail. 
     Generally speaking, all inductive sensors are based on the principle that in its simplest form an inductive sensor comprises a conductive coil which is located in close proximity to a conductive film to be measured and in which an electric current is induced. The conductive film can be considered as a short-circuited virtual coil turn with a predetermined electrical resistance. Since a mutual inductance exists between the aforementioned conductive coil and the virtual coil turn, an electric current is generated in the virtual coil turn. This current is known as eddy current or Foucault current. Resistance of the virtual coil turn, which depends on the material of the conductive film and, naturally, on its thickness, influences the amplitude of the alternating current induced in the virtual turn. It is understood that the amplitude of the aforementioned current will depend also on the thickness of the conductive film. 
     However, realization of a method and apparatus based on the above principle in application to thin films is not obvious. This is because such realization would involve a number of important variable parameters which depend on a specific mode of realization and which are interrelated so that their relationships not always can be realized in a practical device. 
     In order to substantiate the above statement, let us consider the construction of an inductive sensor of the aforementioned type in more detail. 
     FIG. 1 is a schematic view of a known inductive sensor  20  used, e.g., for positioning of an inductive sensor  22  relative to the surface S of an object  24 . Let us assume that the surface S of the object  24  is conductive. The inductive sensor comprises an electromagnetic coil  26  connected to an electronic unit  28 , which, in turn, is connected to a signal processing unit  30 . The latter can be connected, e.g., to a computer (not shown). The electronic unit  28  may contain a signal oscillator (not shown) which induces in the electromagnetic coil  26  alternating current with a frequency within the range from several kHz to several hundred MHz. 
     In a simplified form the sensor of FIG. 1 can be represented by a model shown in FIG.  2 . In this model, L 1  designates inductance of the electromagnetic coil  26 ; R 1  designates resistance of the coil  26 ; L 2  designates inductance of the aforementioned virtual coil turn; and R 2  is electrical resistance of the aforementioned virtual coil turn. M designates mutual induction between L 1  and L 2 . 
     It can be seen from the model of FIG. 2 that the amplitude of current I generated in coil  26  will depend on R 1 , L 12 , L 2 , R 2  and M. It is also understood that in this influence M is the most important parameter since it directly depends on a distance from the inductive sensor  22  to the surface S. 
     FIG. 3 is further simplification of the model of FIG.  2 . Parameters L and R are functions that can be expressed in terms of L 1 , L 2 , M, R 1 , and R 2 . Therefore, as shown in FIG. 3, these parameters can be considered as functions L(D) and R(D). 
     The model of FIG. 3 can also be characterized by a quality factor Q, which is directly proportional to the frequency of the current in the sensor coil  26 , to inductance of the sensor of FIG. 3, and is inversely proportional to a distance D (FIG. 2) from the sensor coil  26  to the surface S. The higher is the value of Q, the higher is stability of the measurement system and the higher is the measuring accuracy. Thus it is clear that in order to achieve a higher value of Q, it is necessary to operate on higher frequencies of the alternating currents in the inductance coil  26 . Analysis of relationships between Q, L, and R for a fixed distance D was made by S. Roach in article “Designing and Building an Eddy Current Position Sensor” at http://www.sensormag.com/articles/0998/edd0998/main.shtml. S. Roach introduces an important parameter, i.e., a ratio of D to the diameter of the sensor coil  26 , and shows that R does not practically depend on the above ratio, while the increase of this parameter leads to the growth in L and Q. When distance D becomes equal approximately to the diameter of the coil  26 , all three parameters, i.e., L, Q, and R are stabilized, i.e., further increase in the distance practically does not change these parameters. In his important work, S. Roach generalized the relationships between the aforementioned parameters and showed that, irrespective of actual dimensions of the sensor, “the rapid loss of sensitivity with distance strictly limits the range of eddy current sensor to about ½ the coil diameter and constitutes the most important limitation of this type of sensing”. 
     The impedance of the coil also depends on such factors as film thickness, flatness of the film, transverse dimensions, temperature of the film and coil, coil geometry and DC resistance, operating frequency, magnetic and electric properties of the film, etc. 
     As far as the operating frequency of the inductive coil is concerned, the sensor possesses a self-resonance frequency, which is generated by an oscillating circuit formed by the power-supply cable and the capacitor. As has been shown by S. Roach, in order to improve sensitivity, it is recommended to increase the quality factor Q and hence the frequency. However, the sensor must operate on frequencies at least a factor of three below the self-resonant frequency. Thus, practical frequency values for air core coils typically lie between 10 kHz and 10 MHz. 
     The depth of penetration of the electromagnetic field into the conductive film is also important for understanding the principle of operation of an inductive sensor. It is known that when an alternating electromagnetic field propagates from non-conductive medium into a conductive medium, it is dampened according to an exponential law. For the case of propagation through the flat interface, electric and magnetic components of the alternating electromagnetic field can be expressed by the following formulae: 
     
       
           E=E   0  exp (−α x ) 
       
     
     
       
           H=H   0  exp (−α x ), 
       
     
     where α=(πfμσ) ½ , f is oscillation frequency of the electromagnetic field, σ is conductivity of the medium, and μ=μ 0 =1.26×10 −6  H/m (for non-magnetic materials). 
     Distance x from the interface, which is equal to 
     
       
           x =δ1/α=1/(πfμσ) ½   (1) 
       
     
     and at which the amplitude of the electromagnetic wave decreases by e times, is called the depth of penetration or a skin layer thickness. Based on formula (1), for copper on frequency of 10 kHz the skin depth δ is equal approximately to 650 μm, on frequency of 100 kHz to 200 μm, on frequency of 1 MHz to 65 μm, and on frequency of 10 MHz to 20 μm. 
     The above values show that for the films used in the semiconductor industry, which are typically with the thickness on the order of 1 μm or thinner, the electromagnetic field can be considered practically as uniform. This is because on any frequency in the range from 10 KHz to 10 MHz the electromagnetic waves begin to dampen on much greater depth than the thickness of the aforementioned films. Similar trend is observed in the films made from other metals, where the skin layer is even thicker because of lower conductivity. At the same time, deviations from uniformity in the thickness of the conductive coating films used in the semiconductor industry, e.g., copper or aluminum layers on the surface of silicon substrates, should not exceed 5%, and in some cases 2% of the average thickness of the layer. In other words, the deviations should be measured in hundreds of Angstroms. It is understood that conventional inductive sensors of the types described above and used in a conventional manner are inapplicable for the solution of the above problem. Furthermore, in order to match conditions of semiconductor production, such sensors must have miniature constructions in order to be installed in close proximity to the measurement site. The distance between the measurement element of the inductive sensor and the surface of the film being measured also becomes a critical issue. Due to high sensitivity, the sensor becomes very sensitive to the influence of the environment, especially, mechanical vibrations, variations in temperature, etc. 
     The applicants are not aware of any existing inductive sensors capable of solving the above problems. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide an apparatus and method for measuring thickness and thickness fluctuation in conductive coatings with sensitivity as high as several hundred Angstroms. Another object is to provide the aforementioned apparatus which has a miniature construction, can be installed in closed proximity to the surface of the film being measured, is sensitive even to minute variations in the film thickness and makes it possible to record the aforementioned variations while performing relative movements between the sensor and the object. Another object is to provide the apparatus of the aforementioned type suitable for mapping distribution of thickness variation over the surface of the coated object. A further object of the invention is to provide the apparatus of the aforementioned type in which a proximity sensor that measures the distance from the inductive sensor to the object is embodied as an integrated circuit. Still another object is to provide the apparatus of the aforementioned type which is built into a spinstand for testing hard disks/magnetic heads as an integral unit for measuring thickness of final coating layers such as carbon layers, or layers of SiC, GaAs, etc. 
     SUMMARY OF THE INVENTION 
     The invention relates to an apparatus for measuring thickness and deviations from the thickness of thin conductive coatings on various substrates, e.g., metal coating films in semiconductor wafer or hard drive disks. The films may have a thickness as small as fractions of microns. The apparatus consists of an inductive sensor and a proximity sensor, which are rigidly interconnected though a piezo-actuator used for displacements of the inductive sensor with respect to the surface of the object being measured. Based on the results of the operation of the proximity sensor, the inductive sensor is maintained at a constant distance from the controlled surface. Variations in the thickness of the coating film and in the distance between the inductive sensor and the coating film change the current in the inductive coil of the sensor. The inductive sensor is calibrated so that, for a predetermined object with a predetermined metal coating and thickness of the coating, variations in the amplitude of the inductive sensor current reflect fluctuations in the thickness of the coating. The distinguishing feature of the invention resides in the actuating mechanism of microdisplacements and in the measurement and control units that realize interconnection between the proximity sensor and the inductive sensor via the actuating mechanism. The actuating mechanism is a piezo actuator. Measurement of the film thickness in the submicron range becomes possible due to highly accurate dynamic stabilization of the aforementioned distance between the inductive sensor and the object. According to one embodiment, the distance is controlled optically with the use of a miniature interferometer or a fiber-optic proximity sensor, which is rigidly connected to the inductive sensor. According to another embodiment, the distance is controlled with the use of a capacitance sensor, which is also rigidly connected to the inductive sensor. To achieve a certain level of accuracy during environment temperature variations, it is recommended to provide the proximity sensor with a thermocouple for temperature control. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a known inductive sensor. 
     FIG. 2 is the sensor of FIG. 1 in a simplified form. 
     FIG. 3 is a model representation of the sensor of FIGS. 1 and 2. 
     FIG. 4 is a schematic side view of the mechanical part of the apparatus of the invention with optical control of the distance. 
     FIG. 5 is a main operating unit of the apparatus of FIG.  4 . 
     FIG. 6 is a schematic view illustrating a signal processing and control unit for the embodiment of the apparatus utilizing an optical measurement system. 
     FIG. 7 is a schematic view illustrating general arrangement of the embodiment with electrical measurement of the displacement. 
     FIG. 8A is a view illustrating an example of a PC board mounting a combination of both a capacitive sensor and an inductive sensor. 
     FIG. 8B is an electric circuit of the film thickness measurement system that incorporates the inductive sensor. 
     FIG. 8C is a view similar to FIG. 8B for an embodiment of the invention with the use of a precalibrated spacer for maintaining the distance between the conductive film and the inductive coil constant. 
     FIG. 9 is a fragmental three-dimension view of an integrated-circuit microchip that contains an inductive sensor and a capacitive sensor. 
     FIG. 10 is a schematic view of a signal processing and control circuit for the embodiment of the invention with the use of a capacitive sensor for measuring variations in distance X. 
     FIG. 11 is an example of actual measurements of a thickness of a copper film on a semiconductor wafer by apparatus of the invention; for comparison, the same drawing contain results of measurements by a four-point probe method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG.  4 —General Arrangement of the Embodiment with Optical Measurement of the Displacement 
     For the simplicity of description and for ease of understanding, the apparatus of the invention will be first described in general in a very simplified form, and then individual units of the apparatus will be described in more detail. FIG. 4 is a simplified schematic side view of the mechanical part of the apparatus of the invention with optical control of the distance. The apparatus, which in general is designated with reference numeral  30 , has a rigid C-shaped frame  32  that consists of a base plate  34 , a column  36 , and a rigid hangover cantilever portion  38 . The base plate  36  supports a sample table  40  for securing a sample to be measured, e.g., a semiconductor wafer W coated with a conductive film F the thickness of which is an object of measurement. The sample table  40  may have a two-coordinate drive and may comprise a standard device such as Mod. MS-4M-F-50 Mod. table produced by Newport Co., CA, USA. The same table  40  may be equipped with a vacuum chuck for flattening thin objects during measurement. 
     Attached to the vertical surface of the hangover portion  38  is a piezo-actuator  42 , which may be a standard device, e.g., piezo-actuator Mod. 17PA2015 produced by Melles Griot Co. CA, USA. With the supply of a variable voltage signal to the piezo actuator  42 , the latter changes its dimensions in proportion to the voltage signal. The piezo actuator  42  has rather quick response characteristics (up to several tens of kHz). Attached to the side of the piezo actuator  42 , which faces the sample W, are a micro-interferometer  46  and an inductive sensor  44 . The micro-interferometer  46  is an optical device, which is sensitive to microdisplacements. Some models of microinterferometers may detect displacements as small as 200 Angstroms. An optical interferometer is an optical instrument, which is based on the phenomenon of interference of electromagnetic waves. The inductive sensor  44  may be a conventional inductive sensor of the type described with reference to FIGS. 1-3 and may comprise a flat spiral coil having a 1.5 to 3 mm outer diameter with about 50 turns. The coil can be formed on a PC board (not shown). The inductive sensor may comprise a standard device, e.g., a sensor of EX-501 type produced by Keyence Corporation of America, NJ., USA. 
     One indispensable requirement for realization of the apparatus  30  of the invention is stability of distance X (FIG. 4) between the end face F 1  of the inductive sensor  44  and the surface S of the film F. As will be explained below in connection with operation of the apparatus  30 , the very principle of the invention is based on maintaining the distance X constant. Since the micro-interferometer  46  and the inductive sensor  44  both are rigidly connected to the piezo actuator  42 , the distance L between the micro-interferometer  46  and the surface S of the film F also must be maintained constant. 
     The device that maintains the distances X and L constant is the aforementioned piezo-actuator  42 , which is controlled by an electronic unit  48  shown in the form of a block in FIG.  4 . Reference numeral  50  designates another electronic unit, which contains an oscillator for the inductive sensor  44  and measures the current in the coil of the inductive sensor  44 . Variations in this current are proportional to variations in the thickness of the conductive coating film F. Reference numeral  52  designates a data processing and analyzing unit, e.g., a computer, which receives information from the electronic unit  50 . 
     In order to compensate for temperature variations which may be essential for high-accuracy measurements, the inductive sensor can be equipped with a thermocouple  45 . 
     Some units and parts of the apparatus of FIG. 4 will be further considered in more detail. 
     FIG.  5 —Main Operating Unit of the Apparatus of FIG.  4   
     A main operating unit of the apparatus  30  of the invention consists of the piezo-actuator  42 , the inductive sensor  44 , and the micro-interferometer  46 . FIG. 4 showed the interconnection between these devices in a very schematic manner. More realistic presentation of this interconnection is given in FIG.  5 . The assembly comprises a deformable body  54  in the form of a four-link parallelogram  54  made from spring steel. In the embodiment shown in FIG. 5, the parallelogram  54  comprises a monolithic rectangular body with a central rectangular-shaped opening  55 . The “hinged” links  56 ,  58 ,  60 , and  62  are formed by drilling holes  64 ,  66 ,  68 , and  70  in the corners of the rectangular body. 
     One link  58  is rigidly connected, e.g., by bolts  57  and  59 , to the vertical wail  37  of the hangover part  38  of the frame  30 , which in FIG. 4 was shown conventionally. As a result, when the parallelogram  54  deforms under the effect of a vertical force applied from the piezo-actuator  42 , the link  62  always moves parallel to the link  58  and to the aforementioned vertical wall  37 . 
     The vertical link  62  rigidly supports the measuring optical part of the interferometer  46  (FIGS. 4 and 5) and the inductive sensor  44 . L and Δx have the same meanings as defined with reference to FIG.  4 . In a specific case these distances may be selected equal. Similar to FIG. 4, reference numeral W designates in FIG. 5 an object, such as a semiconductor wafer, with a coating, e.g., a conductive film F. Reference numeral  40  designates a moveable table. 
     For coarse movement to the operation position in a vertical direction, the assembly consisting of the parallelogram  54  with the inductive sensor  44  and the micro-interferometer can be shifted vertically in guides  72  of the frame  38  and fixed in the selected position by a screw  74 . 
     The piezo-actuator  42  is supported by the link  56  via a body of rotation, such as roller  76  having a longitudinal axis in the direction perpendicular to the plane of the drawing of FIG.  5 . The piezo-actuator  42  is constantly pressed to the link  56  by a compression spring  78 , and the roller  76  is moveable in the gap  80  formed by this roller between the lower surface of the piezo-actuator and the upper surface of the link  56 . Thus, the gap  80  and the taper angle of the gap are increased as the roller  76  is shifted towards the vertical surface  37 . FIG. 5 also shows that the inductive sensor  44  is connected by a feeder line  82  to the electronic unit  48 . The use of the roller  76  makes it possible to adjust displacements of the link  62  of the deformable parallelogram  54  so, that with the same control voltages of the piezo-actuator  64 , the link  62  that supports the inductive sensor  44  and the microinterferometer  46  can be set to different working amplitudes of movement of the link  62 . In other words, the dynamic range of the link displacements i.e., a ratio of maximum displacement to the minimum one caused by the piezo-actuator  42  can be about ten. 
     FIG.  6 —Signal Processing and Control Unit for Embodiment of the Apparatus Utilizing an Optical Measurement System 
     FIG. 6 is a more detailed view of the electronic unit  48  together with micro-interferometer  46 . The electronic unit  48  processes optical signals obtained from the micro-interferometer  46  and controls operation of the piezo-actuator so as to maintain distances L and X constant. The unit  48  comprises a light source, e.g., a laser light source  84 , which generates a laser beam B 1  sent via a beam collimator  86  to a beam splitter  88 . The latter divides beam B 1  into a measurement beam B 2 , which is reflected from the beam-splitting surface  90  of the beam splitter  88  and is directed towards the measured film F, and the reference beam B 3  which passes through the beam-splitting surface  90  toward a reflective mirror M 1 . The beam B 3  is then sent to a reflective mirror M 2  and is directed therefrom to a semitransparent mirror M 3 . The interferometric part of the device also contains an objective  92  and a double-based or dual photo-diode  94  located on the optical path of the beam B 3  reflected from the semitransparent mirror M 3 . 
     The output of the photo-diode  94  is electrically connected to a differential amplifier  96  and with another amplifier  98  via a proportional integral derivation regulator (PID)  100 . The PID regulator is a process controller that monitors the analog input process variable, compares the input of the desired set point, and calculates the analog output control variable based on the control algorithm programmed by the regulator. The output of the amplifier  98  is connected to the piezo-actuator  42  (FIGS. 4,  5 , and  6 ). In FIG. 6, the piezo-actuator  42  is shown in the form of a box, and rigid connection between the piezo-actuator and the micro-interferometer  46  is shown by a solid line  102 . 
     FIG.  7 —General Arrangement of the Embodiment with Electrical Measurement of the Displacement 
     FIG. 7 is a simplified schematic side view of the mechanical part of the apparatus of the invention with electrical control of the distance. In fact, the arrangement of the embodiment of FIG. 7 is similar to the one for the embodiment of FIG.  4  and differs from it in that variations in distances Lg 1  and X 1  are measured with the use of a capacitive sensor instead of the microinterferometer. The apparatus of this embodiment, which in general is designated with reference numeral  104 , has a rigid C-shaped frame  106  that consists of a base plate  108 , a column  110 , and a rigid hangover cantilever portion  112 . The base plate  108  supports a sample table  114  for securing a sample to be measured, e.g., a semiconductor wafer W 1  coated with a conductive film F 1 , the thickness of which is an object of measurement. The sample table  114  may have a two-coordinate drive and may comprise a standard device such as Newport table Mod 4M-F-50 produced by Newport Co., CA, USA. 
     Attached to the vertical surface  111  of the hangover portion  112  is a piezo actuator  116 , which may be a standard device of the type 17PAZ015 produced by Melles Griot. With the supply of a variable voltage signal to the piezo actuator  116 , the latter changes its dimensions in proportion of the voltage signal. The piezo actuator  116  has rather quick response characteristics (up to several tens of kHz). Attached to the side of the piezo actuator  116 , which faces the sample W 1 , are a capacitive sensor  118  and an inductive sensor  120 . The capacitive sensor  118 , which is known also as a dilatometer, is a device having a sensitive element in the form of a capacitor plates included into an electrical oscillation circuit. If the aforementioned capacitor plates are located in close proximity to the conductive film F 1 , variations in the magnitude of distance X 1  will change the resonance frequency of the sensor  118  approximately in a square-root relationship with the aforementioned distance variation. 
     The device that maintains the distances X 1  and L 1  constant is the aforementioned piezo-actuator  116 , which is controlled by an electronic unit  117  shown in the form of a block in FIG.  7 . Reference numeral  119  designates another electronic unit, which contains an oscillator for the inductive sensor  120  and measures the current in the coil of the inductive sensor  120 . 
     Some models of dilatometers (which operate in a microwave frequency range and are equipped with special compensators of environment fluctuations) may detect displacements as small as tens of Angstroms. General-purpose dilatometers have sensitivity of several hundreds of Angstroms. An example of such a sensor is shown in FIG. 8A, which is a simplified schematic view of the sensor. The capacitive sensor  118  has a pair of circular capacitor plates  122  and  124  which are spaced from each other and surround a spiral inductive coil  126  of the inductive sensor  120  (FIGS. 7 and 8A) located in the center of the internal capacitor plate  126 . The entire capacitive sensor  118 , including the inductive coil  126  of the inductive sensor  120 , can be formed on a PC board and may have overall dimensions not exceeding several millimeters. Miniature dimensions of the capacitive sensor  118  and the inductive sensor  120  are important for localization of measurements. Terminals  128  and  130  are output terminals of the inductive coil  126 . Thus, it is understood that though in FIG. 7 the inductive sensor  120  and the capacitive sensor  118  are shown as separate units, they can be combined into one small assembly of the type shown in FIG.  8 A. An electric circuit of the film thickness measurement system that incorporates the inductive sensor  120  is shown in FIG.  8 B. As can be seen from FIG. 8B, the coil  126  is maintained at constant distance X from the surface S of the coating film F. The coil  126  is connected in series with a capacitor  127 , a measurement resistor  129 , and an high-frequency current generator  131  to form an electric oscillating circuit operating on the frequency of the aforementioned generator  131 . A measurement instrument, such as an analog or digital voltmeter  133 , is connected in parallel to the measurement resistor  129 . 
     If necessary, the unit that consists of the inductive sensor  120  and the capacitive sensor  118  can be formed as an integrated-circuit microchip, e.g., on a silicon substrate. Such an arrangement is shown in a three-dimensional view of FIG. 9, where  132  is an inductive sensor, and  134  designates a capacitive sensor. This combined integrated sensor is manufactured in accordance with MEM&#39;s (Micro-Electro-Mechanical) technique by etching silicon coated, e.g., with a Si 3 N 4 /SiO 2 /Si 3 N 4  stack with a metallic planar coil and capacitor. The aforementioned coil and capacitor are formed by photolithography. In FIG. 9, reference numeral  135  designates contacts of the inductive coil, and reference numeral  137  designates contacts of the capacitor. 
     If necessary, the inductive sensor  120  may be installed separately and comprise, as in the embodiment of FIG. 5, a conventional inductive sensor, e.g., a sensor of EX-501 type produced by Keyence Corporation of America, NJ., USA. 
     The piezo-actuator  116  of the embodiment of FIG. 7 may be the same as the one described in connection with the optical embodiment of FIG.  4  and may incorporate the same parallelogram arrangement as shown in FIG.  5 . Therefore the description of mechanisms used for attachment of the piezo-actuator  116  is omitted. 
     FIG. 10 is a schematic view of a signal processing and control circuit for the embodiment of the invention with the use of a capacitive sensor for measuring variations in distance X. In other words, FIG. 10 shows a signal processing and control circuit for the embodiment of FIG.  7 . In this drawing, reference numerals  122  and  124  designate capacitive plates of the capacitor sensor  118  which form, together with the inductance L 1 ′, a high-frequency generator  138 . Reference numerals  140  and  142  designate a frequency meter and a frequency-voltage converter, respectively. The output of the frequency-voltage converter  142  is connected to the piezo-actuator  116  via a PID  144  and an amplifier  146 . Resistor R is an active resistance of the measurement circuit, which is connected in series into the oscillation circuit  148 . 
     FIGS.  4 ,  5 ,  6 —Operation of the Apparatus of the Invention with Optical Measurement and Control of Distance Variations 
     It is assumed that the inductive sensor  44  has been calibrated for operation with a predetermined material of the conductive coating and for a predetermined measurement distance X from the inductive sensor  44  to the surface of the film F. A sample W coated with a conductive film F is secured to the sample table  40 , the inductive sensor  44  and the microinterferometer  46  are energies, and the point of interest of the object is axially aligned with the position of the inductive sensor  44  and the microinterferometer  46 . The table  40  is adjusted vertically by moving it in the vertical slides  72  (FIG. 5) and is fixed by bolt  74  in a position, in which the object is spaced from the inductive sensor  44  at the distance X. In accordance with the invention, the inductive sensor provides the highest resolution of measurements when X in FIG. 4 (which is distance D in FIG. 1) is about half of external outer diameter of the coil. For coils of large diameters, e.g., for standard sensors of Keyence Corporation of America, this distance may be greater than have of the coil diameter. 
     When the laser light source  84  is activated, the laser beam B 1  is sent via the beam collimator  86  to the beam splitter  88 , where beam B 1  is divided by the beam splitting surface  90  into the measurement beam B 2 , which is sent to the film F and is reflected therefrom and the reference beam B 3 . The measurement beam B 2  reflected from the film F passes through the beam-splitting surface  90 , the semitransparent mirror M 2 , the objective  92 , and propagates to the double-based or dual photo-diode  94  located on the optical path of this beam. At the same time, the reference beam B 3  passes through the beam splitting surface  90 , and, after being reflected sequentially from the mirrors M 1 , M 2  and mirror M 3 , it is conjugated with the optical path of the measurement beam B 2 , so that it also falls onto the double-based or dual photo-diode  94 . Since both the measurement beam B 2  and the reference beam B 3  are emitted from the same light source, they have correlated phases. The phase shift is determined by difference in the length of the optical path. Thus, by changing the length of the optical path of one of the beams, it is possible to adjust the phase difference between the beams, e.g., in the focal plane of the objective  92 , which is located on the receiving surface of the double-based or dual photo-diode  94 . 
     It is understood that as a result of the events described above, a localized interference of the beams B 2  and B 3  will take place. The aforementioned receiving surface of the double-based or dual photo-diode  94  is divided into two areas  94   a  and  94   b . The micro-interferometer  46  is adjusted so that one interference fringe overlaps two areas  94   a  and  94   b  simultaneously so that electric signals from the outputs of the double-based or dual photo-diode  94  are equal. When such equal signals are supplied to the input of the differential amplifier  96 , the output of this amplifier is zeroed. Even a slightest variation in the distance L causes, because of variation in the phase difference of the beams B 2  and B 3 , geometrical displacement of the fringe to one or to the other area  94   a  or  94   b . This, in turn, causes a positive or negative voltage signal on outputs of the differential amplifier  96 . This signal is processes by the PID  100  into a form matched via the amplifier  98  with control elements of the piezo-actuator  42 . The latter immediately responds to the signal and shifts the link  56  (FIG. 5) of the parallelogram  54  with respect to the surface of the film F to restore the reference distance L. For the system described above, the entire process of microadjustment occurs approximately during 1 to 10 milliseconds. 
     The operation described above is repeated each time when distance L is changed. Although the distance stabilization system described above is capable of maintaining distance L with accuracy up to 100 Angstroms, with special modification of the interferometer and environment compensation features it is possible to reach accuracy as high as tens of Angstroms. 
     Thus, it is assumed that due to the use of the aforementioned optical system that stabilizes distance L, the inductive sensor  44  (FIG. 4) or  120  (FIG.  8 A and FIG. 8B) always operates at a constant distance X from the film F. It is also understood that distance X is maintained constant when the sample table  40  moves from one measurement point to another. Since the inductive sensor has been calibrated for each predetermined material of the conductive film and for each predetermined thickness of this film, readings of the analog or digital voltmeter  133  will correspond to the predetermined values of the film thickness. The thicker is the coating film, the lower is electrical resistance in the conductive film, which can be considered as a virtual turn with the current induced by the coil  126 . If thickness of the conductive film F changes, the amplitude of the current in the oscillating circuit formed by the coil  126 , capacitor  127 , resistor  129 , and generator  131  also will be changed. Variation in the aforementioned amplitude expresses variations in the film thickness. Calibration of the inductive sensor for various values of the current amplitudes makes it possible to interpret readings of the voltmeter  133  directly in terms of the film thickness. 
     The capacitor  127  is shown as a variable capacitor that can be adjusted for optimization of a frequency ratio between the frequency of the generator and the resonance frequency of the oscillating circuit. Such an optimization is needed to provide maximum sensitivity of measurements. 
     By repeating measurements of the film thickness in different measurement points, it is possible to map distribution of thickness variations over the entire surface of the coated object. 
     FIGS.  7 ,  8 A,  10 —Operation of the Apparatus of the Invention with Electrical Measurement and Control of Distance Variations 
     As has been mentioned above, the inductive sensor  120  (FIG. 7) has been calibrated for operation with a predetermined material of the conductive coating and for a predetermined measurement distance X from the inductive sensor  120  to the surface of the film F 1 . 
     A sample W 1  coated with a conductive film F 1  is secured to the sample table  114 , the inductive sensor  120  and the capacitive sensor  118  are energies, and the point of interest of the object is axially aligned with the position of the inductive sensor  120  and the capacitive sensor  118 . The table  114  is adjusted vertically by moving it in the vertical slides (not shown for this embodiment) and is fixed in a position, in which the object is spaced from the inductive sensor  120  at the distance X 1 . In accordance with the invention, the inductive sensor provides the highest resolution of measurements when X 1  in FIG. 7 (which is distance D in FIG. 1) is about half of external outer diameter of the coil. For coils of large diameters, e.g., for standard sensors of Keyence Corporation of America, this distance may be greater than half of the coil diameter. 
     If the capacitive sensor has a construction of the type shown in FIG. 8A, i.e., the inductive sensor  120  and the capacitive sensor  118  are mounted on the same substrate, distance X 1  is equal to distance Lg 1 ′, as shown in FIG.  10 . When the measurement circuit shown in FIG. 10 is energized and operates at resonance conditions, the amplitude of the current in this circuit (which is formed by the inductance L 1 ′, resistor R′, AC generator  138 , and the capacitor  118  formed by the capacitive plates  122 ,  124  of the capacitive sensor  118  and the coating film F 1 ′) is stabilized and is determined by the Q factor. If the distance Lg 1 ′ changes, capacitance of the capacitor  118  also will change. As a result, resonance conditions will be violated, and the amplitude of the oscillations in the aforementioned oscillating circuit will drop. 
     Measurements of variations in the distance Lg 1  are carried out by restoring resonance conditions by adjusting the excitation frequency of the AC generator  138 , and then the difference between the restored resonance frequency and the previous one will be measured to determine the variation in the film thickness. It should be noted that variation in the resonance frequency will be proportional to the square root of variation in Lg′. The second resonance frequency is restored automatically in real time since the waveband of the excitation frequency of the AC generator  138  is sufficiently wide and the shift of the resonance frequency occurs within the boundaries of the aforementioned waveband. 
     The signal from the measurement resistor R′ is sent to the frequency meter  140  and then to frequency-voltage converter  142  (FIG.  10 ). From the output of the frequency-voltage converter  142  the signal is sent to the piezo-actuator  116  via the PID  144  and the amplifier  146 . 
     The piezo-actuator  116  and the inductive sensor  120  will operates exactly in the same manner as has been described for the embodiment of FIGS. 4,  5 ,  6  with the optical data processing and control of variations in the distance. 
     It is understood that for simplification of operation, during measurement in all aforementioned embodiments the inductive sensors  44  and  120  can be brought in contact with the surface of the conductive film F (F 1 ) via a spacer  121  of a calibrated thickness. In this case distance X (X 1 ) will always be constant. This measurement condition is shown in FIG. 8C which is a fragmental view of the circuit similar to the one shown in FIG.  8 B. It can be seen that the inductive coil  126 ′ is in contact with the spacer  121  which is placed onto the conductive film F 2 . The coil  126 ′ is pressed to the spacer  121  with a predetermined force T by means of a load  125 . 
     Such an application makes the use of the distance stabilization mechanisms, i.e., the microinterferometer  46  and the capacitive sensor  118  unnecessary. However, all the advantages of the non-contact measurement will be lost. Furthermore, contact measurements will require the use of special means for stabilization of contact pressure. 
     FIG. 11 is an example of actual measurements of a thickness of a copper film on a semiconductor wafer by apparatus of the invention. For comparison, the same drawing shows results of measurements by a four-point probe method. The curve I corresponds to the results obtained with the apparatus of the invention, and the curve  11  corresponds to the results obtained by the four-point probe contact method. It can be seen that both results are well correlated and that the curve obtained by the apparatus of the invention is characterized by a significant decrease in scattering of measurement data. 
     Thus it has been shown that the invention provides an apparatus and method for measuring thickness and thickness fluctuations in conductive coatings with sensitivity as high as several hundred Angstroms. The aforementioned apparatus has a miniature construction, can be installed in closed proximity to the surface of the film being measured, is sensitive even to minute variations in the film thickness and makes it possible to record the aforementioned variations while performing relative movements between the sensor and the object. The apparatus is suitable for mapping distribution of thickness variation over the surface of the coated object. The proximity sensor that measures the distance from the inductive sensor to the object can be embodied as an integrated circuit. 
     The device of the invention can be built into a spinstand for testing hard disks/magnetic heads as an integral unit for measuring thickness of final coating layers such as carbon layers. 
     Although the invention has been shown and described with reference to specific embodiments, it is understood that these embodiments should not be construed as limiting the areas of application of the invention and that any changes and modifications are possible, provided these changes and modifications do not depart from the scope of the attached patent claims. For example, the distance between the surface of the film and the inductive sensor can be stabilized with the use of an atomic force microscope, by measuring frequency with the use of a special capacitive sensor that operates on a microwave frequency, or by utilizing an optical-fiber proximity sensor.