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Holographic Determination of Stress in Thin Films

IP.com Disclosure Number: IPCOM000042196D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 1 page(s) / 13K

Publishing Venue

IBM

Related People

Powers, JV: AUTHOR [+2]

Abstract

A hologram of the backsurface of a substrate is recorded on a holographic plate before a thin film is removed from (or deposited onto) the other side of the substrate. After the thin film is removed from (or deposited onto) the other side of the substrate, a hologram of the back surface of the substrate is again recorded on the same holographic plate. The double exposed holographic plate is then developed. The reconstructed holographic image contains optical interference fringes, which may then be counted to measure the deflection of the substrate caused by removal (or deposition) of the film. This technique has high sensitivity, resolution and dynamic range, and does not require that the substrate be flat. The measurement of stress in thin films is crucial to the successful fabrication of masks for X-ray lithography.

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Holographic Determination of Stress in Thin Films

A hologram of the backsurface of a substrate is recorded on a holographic plate before a thin film is removed from (or deposited onto) the other side of the substrate. After the thin film is removed from (or deposited onto) the other side of the substrate, a hologram of the back surface of the substrate is again recorded on the same holographic plate. The double exposed holographic plate is then developed. The reconstructed holographic image contains optical interference fringes, which may then be counted to measure the deflection of the substrate caused by removal (or deposition) of the film. This technique has high sensitivity, resolution and dynamic range, and does not require that the substrate be flat. The measurement of stress in thin films is crucial to the successful fabrication of masks for X-ray lithography. Distortions due to stress can render a mask unusable if the stress is not minimized. The ability to accurately measure stress is required, of course, before it can be controlled. All currently available methods of measuring stress involve measuring the distortion of a substrate caused by the stress in an overlying film and, from this value, calculating the stress. Since this distortion is proportional to the thickness of the film and inversely proportional to the thickness of the substrate squared, it is often a very small quantity. Therefore, high sensitivity is needed. The disclosed method has very high sensitivity. Another difficulty in making a stress measurement arises from the distortion of the substrate itself. The native distortion of a substrate is frequently an order of magnitude greater than the deflections applied by a stressed film onto the substrate. This makes interpretation of data extremely difficult. In the present technique, this background distortion is automatically cancelled out to leave only the desired str...