Effect of surface machine roughness on detectability of sub-millimetre reflectors in polycrystalline materials using ultrasonic immersion testing

摘要

The detection of sub-millimetre defects in the high-purity polycrystalline materials used during the manufacture of sputtering targets is an important quality requirement imposed by the semiconductor industry. Discontinuities ranging from 0.25 mm to 1.0 mm should be detected, resolved and reported. Typically, the cast ingots and fabricated plates used during target manufacture have tightly-controlled grain texture and crystallography. Since the texture is controlled, only two major variables affecting the detectability of minute flaws during ultrasonic immersion testing remain to be addressed: surface roughness and equipment-related electronic noise. It is determined that the duration of front-surface echo ringing depends on the level of regularity in the machine finish pattern and the depth of the machining marks. Experiments with different depths of machining marks analysed with Ry yield that roughness with an Ry less than 6 μm does not contribute to noise. Machine surface roughness between Ry = 8 μm and Ry = 27 μm is accompanied by an increase in front-surface echo ringing, which extends the 'dead zone' duration. Exceeding Ry = 27 μm results in the formation of a strong interference diffraction pattern with a noticeably extended duration of the front-surface echo. This pattern has a tendency to penetrate into the gated test region and increase the noise level. The observed interference diffraction phenomenon is explained using the Huygens-Fresnel principle applied to a highly regular pattern of machine valleys, which serve as secondary point sources for ultrasonic emission.