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Digital holography interferometry (DHI) and digital holographic interference microscopy (DHIM) are tools that provide whole field information of the wavefront interacting with the object. This imaging modality can be an ideal tool for quantitative phase imaging of technical objects such as semiconductor samples. The phase information numerically reconstructed from the holograms can lead to extraction of the thickness/height of the sample. Usually DHI and DHIM are used in transmission mode for determination of optical thickness of the sample under investigation. In reflection mode this imaging techniques provide height of the sample.
For the semiconductor industry determination of height/thickness of object structures as well as the quantification of defects in the object structures is an important issue. The thickness and defect determination can be that of semiconductor thin films, micro/nano-pillars, LED displays, liquid crystal panels, the cover glasses used for protection of these structures etc. Digital holographic interferometric method (both reflection and transmission) can be used to design devices that can act as a fast, single shot technique for quantitative phase imaging of such samples. Such devices can provide more information about the sample compared to intensity based measurement systems. Also compact the digital holographic interferometric systems can be deployed in the manufacturing line of such devices to provide real time information.
We are involved in the design and development of digital holographic devices for inspection of semi-conductor wafers, thin films, displays and glass plates covering such samples. We have implemented digital holographic devices in the lens-less mode (in the case of DHI) and also with the use of an imaging lens (in the case of DHIM) both in reflection and transmission mode. DHI provides field of view equivalent to the sensor size, while DHIM technique was implemented with different magnifications, thereby providing varying field of views of the sample. Also in the case of DHI a propagation from the hologram plane (the plane at which the digital array for recording the hologram was situated) to the best focus plane (object plane) was realized by numerical implementation of diffraction integral. In DHIM, the digital array used for recording the hologram was at the image plane of the magnifying/de-magnifying lens. So the whole numerical reconstruction process reduced to Fourier fringe analysis, making the technique less computationally exhaustive, fast and quasi real-time. The developed devices were calibrated using known objects and then tested on different samples. The obtained results are found to be encouraging. In this paper, we describe our efforts in design, development and fabrication of digital holographic devices for the inspection of semiconductor samples.
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