METHOD OF MONITORING THE THICKNESS OF THIN DIELECTRIC FILMS AND PERFORMANCE OF THE METHOD

Abstract: The thickness of thin dielectric films covering metal surfaces can be measured while the surface and the film are in motion. This is accomplished by arranging a prism close to the moving film, passing plane polarized light into said prism so that it is totally internally reflected at the surface of the prism adjacent the film. Thereafter the angle of incidence is vaired to determine when surface plasma oscillations on the film-metal layer interface are generated. This is indicated by attenuation of the reflectivity. By measuring the reflectivity as a function of the angle of incidence, that angle of incidence which produces the minimum reflectivity can be determined and by comparing that angle of incidence to previously established standards, the film thickness can be determined.

Description: METHOD OF MONITORING THE THICKNESS OF THIN DIELECTRIC FILMS The present invention relates to a method of monitoring the thickness of thin dielectric films on surfaces of metals. It has been known for a considerable length of time that when certain metals are placed very close to the surface of a prism, polarized light, which ordinarily would be totally internally reflected by that prism surface, will be reflected in less than totality. The light which is not reflected may be thought of as being converted into surface plasma oscillations at the surface of the metal. Attenuation of total internal reflection takes place with metals that are good conductors of electricity. Silver is best; gold and copper are also good. Magnesium, rhodium and aluminium also produce good results. The subject of surface plasma oscillations is discussed in Zeitschrift fur Physik 216, 398-410 (1968) by Otto. The same author shows a prism placed above a metal surface with a gap in between. This is in a publication entitled "The Surface Polariton Resonance in Attenuated Total Reflection" (Polaritons, Proceedings of the First Taormina Research Conference on the Structure of Matter, Ed. E. Burstein and F. de Mortini (Pergamon, New York, 1974). The effect of thin dielectric films on surface plasma resonance in gold is discussed in Optics Communications Vol. 22, No. 3, Page 374, published September 1977. "Surface Plasmon Spectroscopy of Organic Monolayer Assemblies" is shown in Surface Science, Vol. 74, pp. 237-244 (1978). Monitoring or measuring the thickness of thin dielectric films on metals can be done in a number of ways but all the ways of which we are aware have disadvantages. Particular difficulty is experienced in measuring or monitoring the thickness of a dielectric film on a moving metal carrier. Such a difficulty arises in the manufacture of aluminium based, magnetic storage disc since it is desirable to be able to monitor or measure the thickness of a lubricating film on the surface of the disc while the disc is in motion. The present invention is concerned with such difficulties and provides a solution thereto. Accordingly, the invention provides, a method of monitoring the thickness of a thin dielectric film on a flat surface of a high electrical conductivity metal element, said method being characterised by arranging a flat surface of an optical prism in a spaced parallel relationship with the flat surface of the metal element to form a gap therebetween less than 10,000 thick, passing monochromatic polarised light into the prism so that it is internally reflected at the said surface of the prism, varying the angle of incidence of the light on the said prism surface so as to cause the angle of incidence to pass through the range of angles of incidence at which surface plasma oscillations are generated in the film-metal layer interface and the intensity of the internally reflected light is temporarily substantially reduced, and monitoring the intensity of light emerging from the prism which has been internally reflected so as to detect occurance of the aforesaid range of angles of incidence. The foregoing method is further characterised by monitoring the intensity of emergent light to detect the critical angle, within the aforesaid range of angles, at which the intensity of emergent light is a minimum and using a calibration to determine the film thickness corresponding to the detected critical angle. The foregoing method is further characterised in that said metal element is moving in a direction parallel to the flat surface thereof and in that the aforesaid range of angles of incidence is detected during movement of the metal element. An example of a method of measuring the thickness of a thin dielectric film, which method embodies the invention, will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view (not to scale) of equipment suitable for performing the method to be described,and Fig. 2 is a typical graph obtained when reflectivity is measured as a function of the angle of incidence. Disclosure of the Invention In carrying out the method of the present invention, a prism is first placed in spaced parallel relationship to the metal surface which is covered by the thin dielectric film. This placing may be carried out either with the system at rest or aerodynamically while it is in motion. The prism should be placed so that there is a gap of from about 1000 A to about 10,000 A of thickness between the film surface and the prism plane polarized light is passed into the prism. The light is polarized in the plane of incidence. The light is passed into the prism and is incident at various angles of incidence on the internal reflecting surface thereof. Light incident at a specific angle, generates socalled surface plasma oscillations on the surface of the metal1 and the total internal reflection, which would occur in the absence of the metal, is thereby attenuated. The intensity or magnitude of the reflected light is measured. It will be found that reflectivity is a function of the angle of incidence. Sufficient measurements will be made so that that angle of incidence which produces the minimum reflectivity is determined. This minimum angle of incidence is then compared to a previously determined standard to measure the film thickness. Referring to Figure 1, it may be seen that light from some external source passes through a polarizer and then enters the prism. The prism is at a predetermined height above the surface of the metal. The light enters the prism and is incident on the internal reflecting surface thereof at a certain angle of incidence. In the absence of the metal, total reflectivity i.e. total internal reflection would take place. In the presence of metal, however, a portion of the light energy is converted from light energy into surface plasma oscillation. A detector is used to measure the intensity of the reflected light. Turning now to Figure 2, it will be seen that the amount of light which is converted into surface plasma oscillations depends upon the angle of incidence e of the light. Figure 2 shows a plot of the amount of reflected light compared with the angle of incidence. As may be seen, there is a range of angles of incidence, centre about a critical angle e p, at which surface plasma oscillations are generated in the film-metal layer interface and the intensity of the reflected light, i.e. the reflectivity is temporarily substantially reduced. There is one certain angle 0 p at which a maximum amount of light is converted to surface plasma oscillations. In other words, there is one particular angle at which the reflectivity is a minimum. The magnitude of this angle is a function of various parameter such as the properties of the prism and of the metal and of the spacings. The angle is particularly sensitive to the presence of dielectric films on the metal. The presence of dielectric films on the metal can be detected by measuring the shift in the angle of minimum reflectivity. For a predetermined known gap width, this shift can be quantitatively related to the film thickness. No dielectric film is shown in Figure 1 but in performance of the inventive method a dielectric film, probably between 5 i and 1000 would coat the metal layer. The gap width would be less than 10,000 To measure the thickness of a thin dielectric film on a metal, standards or other calibrations for these materials are first prepared. They consist of a series of metal films covered with different thicknesses of dielectric coatings over the range of coatings to be determined by this method. The same coating dielectric must be used for both the standards as the unknown layers. The standards could be measured either with the system at rest or in motion. It has been found that for any given system of materials, there is a straight line (linear) relationship between the angle producing minimum reflectivity and the thickness of the film layer. Because of this discovered linear relationship, only two standards are necessary. Because one of these could even be the uncoated metal film, actually only one coated standard is required. When these standards are determined and a plot of thickness vs. angle is drawn, the present system may be employed while the metal is in motion. A comparison of the angle of incidence to the plot of standards leads directly and rapidly to a determination of the thickness of the overcoating. It is a particular advantage of the present invention that it is useful when applied to moving materials. Most ordinary methods of measurement applicable to materials when they are standing still are simply not applicable to moving materials. This advantage is particularly valuable in regard to, for example, a magnetic disk which is part of a computer. Such disks are often treated with lubricants, and it often becomes important, particularly during the manufacture of the disks, to know the thickness of such a film of lubricant, while the disk is moving. The present invention provides a means of obtaining such measurement. Hereinbefore there has been described a method of measuring the thikcness of a dielectric film from about 5 to about 1000 A in thickness on the surface of a moving layer of a metal having high electrical conductivity, said method comprising the steps of providing a prism in a spaced relationship with the moving metal layer to form a gap from about 1,000 i to about 10,000 A thick, passing light which is polarized in the plane of incidence into said prism at various angles of incidence to generate surface plasma oscillations in the film-metal layer interface, and thereby attenuate relfectivity, measuing said reflectivity as a function of the angle of incidence, determining that angle of incidence which produces the minimum reflection, and comparing that angle of incidence with a previously determined standard to determine the film thickness.

Claims: CLAIMS 1. A method of monitoring the thickness of a thin dielectric film on a flat surface of a high electrical conductivity metal element, said method being characterised by arranging a flat surface of an optical prism in a spaced parallel relationship with the flat surface of the metal element to form a gap therebetween less than 10,000 A thick, passing monochromatic polarised light into the prism so that it is internally reflected at the said surface of the prism, varying the angle of incidence of the light on the said prism surface so as to cause the angle of incidence to pass through the range of angles of incidence at which surface plasma oscillations are generated in the film-metal layer interface and the intensity of the internally reflected light is temporarily substantially reduced, and monitoring the intensity of light emerging from the prism which has been internally reflected so as to detect occurance of the aforesaid range of angles of incidence. 2. A method as claimed in claim 1, further characterised by monitoring the intensity of emergent light to detect the critcal angle, within the aforesaid range of angles, at which the intensity of emergent light is a minimum and using a calibration to determine the film thickness corresponding to the detected critical angle. 3. A method as claimed in claim 2, further characterised by establishing a calibration scale for predetermined parameter (e.g. gap width) using at least one film of known thickness. 4. A method as claimed in claim 3, further characterised in that said scale is linear and is established by detecting the aforesaid critical angle when no film is present on the metal surface and when a film of predetermined thickness is present on the metal surface. 5. A method as claimed in any one of claims 1 to 4, further characterised in that said metal element is moving in a direction parallel to the flat surface thereof and in that the aforesaid range of angles of incidence is detected during movement of the metal element. 6. A method as claimed in any one of claims 1 to 5, further characterised in that gap width is 1000 or greater than 1000 A and that the I thickness of the film is between 5A and lOO0 A. 7. A method as claimed in any one of claims 1 to 6, further characterised in that the metal is silver, gold, copper, magnesium, aluminium or rhodium. 8. A method as claimed in any one of claims 1 to 7, further characterised in that the dielectric film is a lubricant film. 9. A method for measuring the thickness of a dielectric film from about 5 to about 1000 in thickness on the surface of a moving layer of a metal having high electrical conductivity, said method comprising the steps of: (1) providing a prism in a spaced relation with the moving metal layer to form a gap from about 1,000 A to about 10,000 i thick, (2) passing light which is polarized in the plane of incidence into said prism at various angles of incidence to generate surface plasma oscillations in the film-metal layer interface, and thereby attenuate reflectivity, (3) measuring said reflectivity as a function of the angle of incidence, (4) determining that angle of incidence which produces the minimum reflection, and (5) comparing that angle of incidence with a previously determined standard to determine the film thickness. 10. In the manufacture of a magnetic storage disc, performance of a method as claimed in any one of claims 1 to 8 to monitor or measure the thickness of a film of lubricant on the disc.

Inventors: GORDON II, JOSEPH GROVER, PHILPOTT, MICHAEL RONALD, SWALEN, JEROME DOUGLAS
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1980
Main classification: G01B11
IPC codes: G01B11, G01N21