Specific Process Knowledge/Characterization/Optical characterization: Difference between revisions
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[[image:Prism_coupler.jpg|275x275px|right|thumb|Prism coupler: positioned in cleanroom 2]] | [[image:Prism_coupler.jpg|275x275px|right|thumb|Prism coupler: positioned in cleanroom 2]] | ||
[[Image:Prism_coupler_tech.jpg|500x500px|left]] | [[Image:Prism_coupler_tech.jpg|500x500px|left]] | ||
The sample to be measured see above is brought into contact with the base of a prism by means of a pneumatically-operated coupling head, creating a small air gap between the film and the prism. A laser beam strikes the base of the prism and is normally totally reflected at the prism base onto a photodetector. At certain discrete values of the incident angle , called mode angles, photons can tunnel across the air gap into the film and enter into a guided optical propagation mode, causing a sharp drop in the intensity of light reaching the detector. | |||
To a rough approximation, the angular location of the first mode determines film index, while the angular difference between the modes determines the thickness, allowing thickness and index to be measured completely independently. | |||
Measurements are made using a computer-driven rotary table which varies the incident angle , and locates each of the film propagation modes. As soon as two of the mode angles are found, film thickness and index can be calculated. The entire measurement process is fully automated and requires approximately twenty seconds. | |||
The number of modes supported by a film of given index increases with film thickness. For most film/substrate combinations, a thickness of 1000-2000 Å is required to support the first mode, while films in the one-micron range can support as many as four or five modes. If the film is thick enough to support two or more propagation modes (typically 3000- 4800 Å), the Prism coupler calculates thickness and index for each pair of modes, and displays the average and standard deviation of these multiple estimates. | |||
The standard deviation calculation, unique to the prism coupling technique, is an indication of measurement self-consistency and a powerful means of confirming the validity of the measurement. | |||
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==Comparison of the three methods== | ==Comparison of the three methods== | ||
For comparison of the three methods, see here: [[Specific Process Knowledge/Characterization/Measurement of film thickness and optical constants#Film_thickness_and_optical_constants_of_optical_transparent_films|Film thickness and optical constants of optical transparent films]] | For comparison of the three methods, see here: [[Specific Process Knowledge/Characterization/Measurement of film thickness and optical constants#Film_thickness_and_optical_constants_of_optical_transparent_films|Film thickness and optical constants of optical transparent films]] | ||