Journal of SMT Article
ACOUSTIC MICRO IMAGING IN THE FOURIER DOMAIN FOR EVALUATION OF ADVANCED PACKAGING
Company: Sonoscan, Inc.
Date Published: 1/1/2003 Volume: 16-1
Presently, the most common method of imaging involves time gating of a selected echo or echoes on an A-scan, which is a single point depth profile through the sample, to select a specific depth in the sample for display/analysis. This is known as time domain imaging. There is a direct relationship between frequency and resolution in AMI. The higher the frequency the shorter the wavelength and the higher the resolution potential.
With the evolution of microelectronic devices to smaller sizes and/or higher I/O counts, the sizes of the features have become increasingly smaller and layer thicknesses increasingly thinner. This has pushed the development of higher frequency imaging in AMI to increase the available resolution in both the spatial (x-y) and axial (z) dimensions. However, there is a point at which further methods are needed to extract additional information beyond what can be done by standard time domain (C-Scan) acoustic imaging.
Recently, studies have been done in AMI using Fourier analysis of echo waveform distortions to measure minute thickness variations in materials. More recently imaging in the frequency domain has been used to enhance features and bring out information previously unavailable when using time domain AMI.
In the work done for this study the method involves mathematically converting the captured waveforms within a time gate to the frequency domain. Specific frequencies are selected from the FFT spectra, and an image is then reconstructed from the frequency information. This paper will present case studies where FFT frequency domain imaging has been used to reveal features down to only Angstroms in thickness, which is substantially below the accepted wavelength limit of the resolution, in applications such as wafer bonding and flip chips.
Key words: AMI, frequency domain imaging.
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