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Title: Anomaly in optical thickness monitoring of a quarterwave thin film multilayer stack — Its cause and how to avoid it
Authors: Basu, A
Keywords: Optical thickness monitoring;Thin film;Multilayer stack;Quarterwave;Turning points;Refractive index;Transmittance
Issue Date: Nov-2007
Publisher: CSIR
IPC Code: G01B11/06, G01B15/02, G01B21/00 121:02
Abstract: During the optical monitoring of a quarterwave thin film optical multilayer stack being deposited sequentially on a substrate in a deposition chamber, the intensity of the light beam transmitted through or reflected from the monitoring plate goes through a series of turning points (maxima and minima). The deposition of a film is stopped at a turning point in order to obtain a quarterwave thick film at the monitoring wavelength. In most of the cases, the maxima and minima occur sequentially. However, during the deposition of some quarterwave multilayer stacks a maximum is not followed by a minimum (or vice versa) during the deposition of successive films. Rather, the intensity may change slightly in the opposite direction to that which is expected or not change appreciably, making the monitoring of the deposited film thickness difficult, if not impossible. This occurs when the effective refractive index of the multilayer stack is very close to the refractive index of the film to be deposited next. Either an alternative method of monitoring the thickness of that particular film has to be adopted, or fresh monitoring plates have to be used at crucial stages during the sequential deposition of the multilayer stack.
Page(s): 920-925
ISSN: 0019-5596
Appears in Collections:IJPAP Vol.45(11) [November 2007]

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