Nanoscale surface tracking of laser material processing using phase shifting diffraction interferometry
Spotlight summary: Chemical vapor deposition (CVD) is one of the most popular techniques for growing nanoscale ultrathin layers on various kinds of substrates by using decomposition and/or combination of the CVD gaseous compounds on the substrate. These chemical reactions are activated only at relatively high temperature and are further increased with increasing temperature, particularly for the formation of a layer only at very small region of the substrate. Therefore, selective control of heating with focused laser irradiation is essential. In this case, laser-assisted heating in CVD is generally referred to as laser-based chemical vapor deposition (LCVD). The relationship between the thickness of the deposited layer and the fluence of the laser can be easily obtained experimentally. However, as described in this Optics Express article by Guss et al. the use of this relationship is not sufficient for controlling the thickness of the layer with a precision of nanometers for some special applications in optics, such as repairing and monitoring local damage of optical components in a high energy laser system; in-situ measurements of the thickness of deposited layer in real time are then required.
To achieve nanometer-scale precision for damage mitigation of fused silica optics using a LCVD process, the authors employ phase shifting diffraction interferometry (PSDI) for real time monitoring, because of its many advantages over other surface metrology techniques such as confocal microscopy, atomic force microscopy, and stylus profilometry, because of its wide field of view with an axial resolution of a few nanometers and quick acquisition time. The authors’ strategy to monitor a LVCD healing process at a local damage of fused silica optics is as follows. They first place a single mode fiber-based PSDI measurement system in the backside of the bulk of fused silica sample, and then monitor and control the morphological profile of the laser-treated front surface from the inside of the bulk fused silica by surface profile retrieval and feedback control of LCVD in real time using Fourier optics. This configuration can successfully avoid effects from thermal and gaseous environments on the laser-treated site. During the retrieving process, the authors also carefully minimize the errors resulting from unavoidable heating of the sample during the LCVD process by considering a thermally induced optical path length increase due to the expansion and refractive index change of the sample.
In summary, the authors successfully demonstrate a special technique for real-time in-situ monitoring and controlling of morphological profile of the fused silica sample during a LCVD process by the implementation of PSDI. As the authors note, this technique can be potentially useful to gain insight into the hidden dynamics of CVD and thermo-mechanical material response even in extreme environmental conditions.
--Taek Yong Hwang
Technical Division: Optical Design and Instrumentation
ToC Category: Instrumentation, Measurement, and Metrology
|OCIS Codes:||(120.6650) Instrumentation, measurement, and metrology : Surface measurements, figure|
|(310.1860) Thin films : Deposition and fabrication|
|(110.3175) Imaging systems : Interferometric imaging|
|(240.6648) Optics at surfaces : Surface dynamics|
07/21/2014 1:11 AM posted by Thalangunam Krishnaswamy S.
It made a good reading of the paper.
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