Development and Applications of Laser-ultrasonic Testing in Nuclear Industry
A laser-based maintenance system, which includes the las er-ultrasonic testing as a non-destructive testing technology, the laser-peening as a surface stress imp rovement technology and the laser welding as a repair technology, for the in-service maintenance of n uclear reactor internal components is presented. Regarding the laser-ultrasonic testing, surface acoustic wave generated and detected by lasers is used to detect surface-breaking cracks. As the benefits of broad -ban feature of the laser-ultrasonics, this method allows to detect very small cracks having the depth of les s than 0.1mm. Also, a frequency response analysis technique of the surface acoustic wave, which propagates through the crack, is developed to size the depth of the crack. The error of this depth measurement thod is estimated at less than 0.2 mm through a series of experiments using stress corrosion cracki ng. A part of the laser-based maintenance system has been already applied to the actual maintenance works in Japanese nuclear power plants . The developed system and its operation are presented. Finally, m ore recent work to detect and visualize small defects on the weld bead by using leakly-Rayleigh waves is introduced.
Sizing of Micro Cracks Using Laser-induced Broad-band Surface Waves.
A new method to estimate crack depth based on laser-induced surface acoustic wave (SAW) is proposed. Laser ultrasonic technique that uses laser beams for generation and detection of ultrasonic waves allows remote and non-contacting inspection. Moreover, since laser-induced SAW has a wide frequency band, it also offers crack-depth measurement using a frequency analysis. To confirm the performance, some feasibility tests using artificial slits were performed. Simulated surface cracks were machined on the type 304 stainless steel plates; each of them had a width of about 100μm and a depth of from 0.2mm to 1.5mm. The location and the depth of the slits were estimated from the detected waves by both the pulse-echo measurement and a frequency analysis. Results of the frequency analysis show that a surface slit behaves as the low pass filter to the laser-induced SAW and the step response time of the transfer function has linear relationship with the slit-depth. Consequently, it is possible to detect small cracking having 0.2mm depth and good agreement can be achieved between measured and nominal depths.
Bevel RIE application to reduce defectivity in copper BEOL processing
Bevel etch used during wafer fabrication for semiconductor devices is discussed. In this case, the bevel etch process was utilized in middle of the line processing (MOL) to reduce back end of line (BEOL) defectivity. Tungsten and titanium nitride films, from the formation of contacts to the transistors, extend into the bevel region of the wafer and have been shown to initiate arcing. This can spread foreign material defects on the front side of the wafer in various BEOL post processing steps including subsequent reactive ion etching, SEM metrology and film depositions. The bevel etch tool confines a plasma etch to the outer edge of the front side of the wafer (which can be individually adjusted) and can remove dielectric, organic and Ti/TiN/W films using a fluorine based chemistry.