Non-destructive evaluation and defect detection studies on silicon wafer samples using laser shearography

• Main Author: Chugani, Hitesh Harish
• Other Authors: Murukeshan Vadakke Matham
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/159064
• Institution: Nanyang Technological University

The aim of this project is to use a non-destructive testing method, laser shearography, to examine defects in silicon wafers and composites. Soon after manufacturing, in the testing and inspection phase, parts are examined for defects. Non-destructive Testing (NDT) is used for examining, and testing components to find defects, or discontinuities on material’s surface and sub-surface level, while maintaining the serviceability of the component after inspection [1]. NDT is not only limited to the manufacturing process but is also used to detect defects in operational parts. It is a preferred method due to its ability to inspect components in a safe, reliable, and cost-effective manner. As digital technology evolves, transformation is observed through the use of high-speed computers, digital cameras that consist of high-resolution CCD sensors that allow real-time the capture of images and videos. These devices are utilised in advanced NDT techniques that are gaining popularity and is eventually replacing conventional NDT e.g.(visual, eddy current, magnetic particle, dye penetrant) that suffer from long inspection time and large false detection rates. Optical testing that makes use of laser is preferred due to its ability to penetrate through large depths with low radiation emission. Through the use of this method, defects are analysed by induced temperature change or mechanical deformation on the surface, when the sample is being loaded [2]. These methods are non-contact, have high detection accuracy and enables inspection of irregular and non-planar surfaces in real-time from macro to nanoscale [3]. Techniques such as thermography, holography or shearography fundamentally have unique flaw detection mechanisms. Shearography is able to measure a material’s mechanical response to stress while active thermography evaluates material heat transfer response to thermal excitation [4]. Both holography and shearography employ the interferometry technique the former uses two beams of the laser while the latter use internal referencing that allows results (fringe patterns) to be produced independently of the environment. During, the processing and handling phase, defects present in silicon wafers exponentially increase, resulting in a reduction in mechanical strength. Moreover, with the production of large diameter and thin silicon wafers, on top of subsurface defects, cracks are also observable. vii A literature review suggests using different techniques for non-destructive evaluation (NDE) on a silicon wafer. Amongst, NDE techniques utilised for subsurface and crack detection, shearography is actively explored given its benefit of being a whole field technique with ability to operate as a non-contact method for online inspection. The intent behind the project, is to explore the use of a commercial laser shearography system (ISI-SYS) to examine abnormalities in silicon wafers and fibre-reinforced plastic (FRP) composites. Samples of perfect wafers, wafers with induced cracks were qualitatively evaluated using the developed system. In order to allow speckle pattern to be formed the specimen needs to be held firmly on a fixture known as ‘JIG’. The JIG is designed and fabricated as part of this project using 3-D printing techniques of Fuse deposition modeling (FDM) and Low Force Stereolithography (SLA). The design of JIG is instrumental to the type of loading schemes that can be employed as such the JIG is made in such a way that the sample fits perfectly along its edges. The different loading mechanisms is utilised to centrally load the specimen such that out-of-plane deformation occurs. The ISI-SYS SE2 ESPI/Sherography system is calibrated where essential parameters such as shear distance, aperture setting, and focus are optimised to achieve accurate results. An aluminum reference plate is used for calibration and is tested in such a way that it can be captured in either real-time mode or double exposure mode. Most part of the experiment is performed in real-time mode. The image obtained is a modulo 2π pattern. After which, the filtering approach and phase unwrapping procedures are carried out. The ultimate aim is to get a complete 3-D deformation profile such that surface and subsurface defects can be easily identified. The samples considered in the study is an Aluminium, CFRP and GFRP plates. These are common materials used in the manufacturing of aircraft panels. After testing the samples, the system's ability to capture laser speckles from object surface is carried out and feasibility study is conducted. It is clear that a double bulls-eye sherography fringe is obtained that depicts the derivative of displacement. After the step is completed, with respect to the feasibility of using this system for generating slope fringes, the wafer is used as test target and is placed on the custom-made JIG. The JIG enables out-of-plane deformation by centrally loading the wafer from behind, the results obtained is detailed in chapter 4.