DE2524982A1 - METHOD OF DETERMINING THE RELATIVE THICKNESS OF PARTS OF A TRANSLUCENT FILM DEPOSITED ON A PLANE REFLECTIVE SURFACE - Google Patents

Description

Patent attorneys Dipl.-Ing. KWeickmann,

Dipl.-Ing. H.Weickmann, Dipl.-Phys. Dr. K. Fincke Dipl.-Ing. F. A. Weickmann, Dipl.-Chem. B. Huber

XI

8 MUNICH 86, DEN

PO Box 860 820

MÖHLSTRASSE 22, CALL NUMBER 98 39 21/22

DECCA LIMITED

Decca House, 9 Albert Embankment,

London, SE1 7SW, England

Method of determining the relative thickness of portions of a light transmissive film deposited flat on a plane specular reflection

The invention relates to methods for determining or determining the actual and relative thickness of areas a translucent or transparent film, which is on a flat specular reflective surface a carrier is deposited.

The invention is based on the object of showing a way in which the actual and relative thicknesses of areas of a transparent film in a method of the foregoing Kind can be determined.

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The above object is achieved in a method for determining the relative thickness of parts of a translucent film deposited on a flat specular reflective surface of a support according to the invention in that the upper surface of the film is illuminated by a light diffuser in such a way that almost all of it is on the film incident light is passed through the diffuser with the formation of interference fringes due to a structural or structural interference between the light reflected from the specular reflection surface of the carrier and the light reflected from the upper surface of the film _? and that the stripe density is determined in areas of this part.

According to one aspect of the invention, the method is for determining the thickness of a portion of a translucent layer deposited on a specular reflective surface of a substrate Film, characterized in that the upper surface of the film in question is illuminated in such a way that interference fringes by constructive interference between that reflected from the specular reflective surface of the film Light and the light reflected from the top surface of the film are formed that the carrier about a first axis is rotated, which passes through the illuminated upper surface of the film and perpendicular to this, such that a point lying in an interference fringe lies on a certain reference line which is perpendicular to the relevant one Axis runs that the distance of the point in question is measured from said first axis that the carrier is rotated about a second axis passing through the carrier and perpendicular to said first axis,

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that the carrier is rotated about the second axis in such a way that the interference fringe encompassing the region in question disappears for a certain length of light and then occurs again that the recurrence of the interference fringe is determined for the particular wavelength and that the angle through which the carrier has been rotated about the second axis in the event that the interference fringe is measured occurs again for the particular wavelength.

With reference to drawings, the invention is exemplified below explained in more detail.

Fig. 1 shows a light diffuser which is arranged to have a film deposited on a flat support illuminated.

Fig. 2 shows a typical fluorescent light spectrum. Fig. 3 shows schematically part of an apparatus for determining the thickness of a film.

In Fig. 1, a light diffuser is shown, which is designed as a funnel made of translucent material and the is arranged with its wider end on a translucent or transparent film 2. This movie 2 is in turn applied to a flat specular reflective surface of a carrier 3. A camera 4 is arranged so that the upper surface of the film 2 is to be photographed allowed, through the small opening of the funnel 1. A luminous tube 5 is coaxial with the diffuser arranged.

If one observes the film 2 under illumination by means of a fluorescent light with the spectrum shown in FIG. 2,

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f 11 111 I ι

so colored bands that are predominantly green and purple can be fixed in the plane of the film will. The ligaments are generally smooth; on a circumference, and the number of bands depends on differences in film thickness.

These bands are interference fringes. The visibility of the interference fringes as green and purple bands is a Result of the lighting conditions, the spectrum of the fluorescent filter, the properties of the film and the Imaging mechanism of the eye. If one uses the eye to

light
a disk under fluorescence / or. When viewing fluorescent light from a distance greater than 50 cm, at least some of the light received from any point on the surface has been specularly reflected, the light causing illumination from all angles. Some of this reflected light will be reflected on or on the surface of the film, and some of the light will be reflected on or on the flat specular reflective surface of the carrier. A light collection system having a small light collection angle, as is formed by the pupil of the eye, is required to collect light that is reflected at at least one angle in order to ensure the visibility and / or detectability of the interference fringes. Accordingly, all of the light from that point satisfies a single interference condition.

For sufficient remote viewing and for sufficiently thin films, the angle of reflection for all points is on the reflective reflection surface of the carrier 3 effectively constant;

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accordingly, the strips are strips of equal thickness. For a certain wavelength of light there is thus an interference fringe represents a thickness contour for the film.

For a given wavelength of light and viewing at a fixed viewing angle, there are a number of Thicknesses that meet the conditions for constructive interference. The row in question increases in equal steps, wherein the step size is determined by the wavelength of the light for a given viewing angle.

Accordingly, the stripe density in an area of a film indicates the degree of change in the thickness of the film.

The strips can be photographed by means of the camera 4, to provide a permanent record of the strips. Alternatively, a television camera or photomultiplier can be used used to identify the streaks. Filters (not shown) can too. Selection of the interference bands can be used, which result at a certain wavelength of the incident light.

The use of a light diffuser ensures that in the In the event that the film surface 6 is photographed by the camera 4, the photograph does not have reflections from the light source 5 shows. The imaging system of the camera 4 must have a small aperture and be spaced appropriately be arranged by the carrier 3. The imaging system in the camera 4 above the carrier is preferably of this type centered that changes over the film due to the

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are minimized over the flat support changing angle of reflection.

The actual thickness of a film in an area within a stripe can be measured as follows. The planar support is arranged as shown in FIG. An area within a stripe on the thin film is selected, and the planar support is rotated about an axis OC (see Fig. 3) _? until the selected area is next to a transparent scale (not shown). The radial distance R is then read from the scale and recorded. The carrier 3 is then rotated about the axis AB until the color of the strip changes to another color (violet or green in the case of light with the spectrum shown in FIG. 2) and then back to the original color.

The angle of inclination of the disk from its horizontal i ' ₂ (not shown) is measured. If the conditions of constructive interference are fulfilled for two angles of incidence i ^ and i ~ (not shown), but not for an angle i, so that the relationship i ^ i-OU is fulfilled, then the thickness d of a film is given by:

2n

Here, λ means the wavelength at which the constructive interference takes place, η is the refractive index of the film, and 0 (i ^), 0 (i ₂ ) mean the difference between the

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Phase changes on / of the reflection on the specular reflective surface of the carrier and the upper surface of the transparent film 2. The quantities 0 Ci ₁ ) and 0 (ip) can be calculated from the knowledge of the existing materials and the polarization of the incident light. If 0 (i.) Has a significant quantity for values in the range of i to be used, then a single polarization of the illumination, which lies either in the carrier circle or in the plane lying perpendicular to the plane of the carrier 3, should be selected for the observation will. The simple polarization can be achieved by inserting a polarizing filter in the light path through the film 2 and the camera 4.

In this case i _{2 is} the angle of incidence at which the disk is inclined at _{an angle i '2.} Accordingly, one obtains

sin i

cos i ' ₂

² (1 + R ² / _T 2) ^1/2

For simplicity, curves of i ¹ versus d can be denoted for a given series of R, L, η and Λ so that the film thickness for a given angle can be read from the curve.

The methods and apparatus described can be used to measure the actual and relative thickness of a thermoplastic Films are used on a conductive

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Carrier part is deposited, as described in more detail elsewhere (GB-PS 1 373 511), or any film, deposited by a device described in more detail elsewhere (see British patent application No. 25420/74).

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Claims (5)

Claims 1.j method for determining the relative thickness of parts - "'of a transparent film deposited on a flat specular reflective surface of a support, characterized in that the upper surface of the film (2) is illuminated by a light diffuser (1) in such a way that that almost all of the light incident on the film (2) through the light diffuser (1) with the formation of differential stripes due to a constructive interference between the light reflected from the specular reflective surface of the carrier (3) and that from the surface of the film (2) reflected light penetrates, and that the stripe density in the area of the relevant part of the film (2) is determined. 2. The method according to claim 1, characterized in that the The stripe density is determined by photographing the stripes. 3. The method according to claim 1 or 2, characterized in that the light diffuser (1) with a non-uniform light spectral intensity is illuminated. 4. The method according to any one of claims 1 to 3, characterized in that that a funnel made of a translucent material is used as the light diffuser (1). 5. Method of determining the thickness of an area of a on a specular reflective surface of a Translucent film applied to the carrier, in particular according to one of claims 1 to 4, characterized in that 509881/0355 - ίο - draws that the upper surface of the film (2) is illuminated in such a way that interference fringes through constructive Interference between the light reflected from the specular reflective surface of the film and that from the light reflected from the upper surface of the film (2) is formed in that the carrier (3) is rotated about a first axis, which runs through the illuminated upper surface of the film (2) and at right angles to this surface, such that an in a point lying on an interference fringe lies on a certain reference line which is perpendicular to said line first axis that the distance of the point in question is measured from said first axis that the carrier (3) is rotated about a second axis which runs through the carrier (2) and at right angles to the first axis, that the carrier (3) is rotated about the second axis in such a way that it encompasses the relevant area Interference fringes at a certain wavelength of light disappears and occurs again that the recurrence of the interference fringe at the particular wavelength is determined and that the angle is measured by which the carrier (3) is rotated about the second axis until the Interference fringes reappear for the particular wavelength. A method according to claim 5, characterized in that the upper surface of the film (2) is illuminated by that light from a light source (5) with a non-uniform spectral intensity through a funnel Translucent material formed light diffuser (1) is guided in such a way that almost all of it is on the upper Surface of the film (2) incident light is passed through the light diffuser (1). 509881/0355