Optics: measuring and testing – By configuration comparison – With photosensitive film or plate
Patent
1996-06-03
1998-04-28
Font, Frank G.
Optics: measuring and testing
By configuration comparison
With photosensitive film or plate
356381, 1566261, G01B 1106
Patent
active
057452407
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally concerned with measuring stresses in a thin film upon its deposition on a substrate, in order to control such stresses.
It is more particularly, although not necessarily exclusively, directed to the situation in which the substrate is a lens for spectacles or, to be more precise, the blank having a generally circular peripheral contour from which a spectacle lens can subsequently be obtained by trimming.
It is well known to apply a thin film to both faces of a spectacle lens, for example to confer anti-reflection properties on it to make it less noticeable and/or to improve resistance to scratching and abrasion, especially in the case of a spectacle lens made of organic material.
The thin film is usually deposited in a vacuum, for example by evaporation, carried out by electron bombardment in an appropriate processing chamber in practise containing a collective holder in the general shape of a bell, for example, and having a plurality of sites each adapted to receive an individual spectacle lens to be processed.
Any thin film deposited onto a substrate in this way is subject to internal stresses during its deposition on the substrate which, tending to expand or to contract it relative to the latter, tend to cause it to be detached or to render it more easily detachable in the event of external aggression if the adhesion forces that conjointly hold it onto the substrate are insufficient.
The internal stresses are both thermal in nature, resulting from an inevitable difference between the coefficient of thermal expansion of the materials used during the heating and cooling phases that are inherent to the processing employed, and intrinsic in nature, resulting from a structural rearrangement of the atoms that inevitably takes place as the layer is formed.
2. Description of Related Art
A first step towards controlling these internal stresses, and thereby improving the durability of the thin layer deposited, is to measure them.
U.S. Pat. No. 5,154,810 proposes the use for this purpose of a test sample carried by the collective holder on which the substrates to be processed are placed, a light source that directs a light beam onto the test sample and a sensor, in particular an optical sensor, to be more precise a displacement sensor, onto which the light beam reflected by the sample is directed.
As a thin film is deposited onto the sample, the stresses developing in the thin film cause curvature of the test sample which in turn causes deviation of the reflected light beam.
Measured by the displacement sensor, this deviation provides an assessment of the curvature of the test sample and therefore of the internal stresses in the thin film that it is carrying.
Apart from its relative simplicity, a method of this kind has the advantage of being usable in situ, the test sample being placed in the processing chamber.
In U.S. Pat. No. 5,154,810 the light source and the displacement sensor are outside the processing chamber, however.
This means that both the incident light beam and the reflected light beam have to pass through a porthole which becomes progressively less transparent as the processing proceeds, because of the deposit that is inevitably formed on the porthole itself, and this rapidly compromises the quality of the measurement conjointly effected.
What is more, the test sample being placed on a moving collective holder, the measurement is possible only when the test sample is facing the porthole through which the measurement light beams pass.
Because of this the measurement is discontinuous and necessarily comparative.
It also uses controls on the upstream and downstream side of the test sample.
Each measurement therefore requires three operations, one on the test sample and the others on the controls, and these operations have to be synchronized with the movement of the collective support, which significantly complicates the system.
SUMMARY OF THE INVENTION
A general object of the present invention is a method and
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C.M. Su et al., "In Situ Mechanical Relaxation of CU Films Growing on a SI Substrate", Applied Physics Letters, vol. 63, No. 25, Dec. 1993, New York, pp. 3437-3439.
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Bosmans Richard
Frakso Fatima
Nouvelot Luc
Essilor International "Compagnie Generale d'Optique"
Font Frank G.
Stafira Michael P.
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