A fabrication technique for preparing nanogap electrodes, such as a gold (Au) nano electrode, using conventional silicon (Si) processes-photolithography, etching, thermal oxidation and deposition-is proposed. Stencil substrates are prepared using the Si processes. Then, without requiring complicated technology, nanogap structures can be formed using the technique. Numerous kinds of materials can be selected as an electrode. The mass production of a sensing device for the detection of deoxyribonucleic acid (DNA), or a so-called DNA chip, can be realized at a low cost.
This paper proposes an amplitude-modulated and phaseinverted (AMPI) ultrasonic driving signal for accurate distance measurements. Undesirable part of the ultrasonic wave was canceled using the phase-inverted wave and its active canceling effect was enhanced by the amplitude-modulation, so that a unique ultrasonic waveform having a sharp envelope could be generated even with a narrow-band transducer of frequency 40 kHz (wavelength λ ∼ = 9 mm). The sharp envelope generated by the AMPI signal determined a zero-crossing time without any uncertainty. As a result, an accuracy better than 0.02 mm was achieved in the range of 0.1 − 0.5 m.
New nondestructive inspection methods with high spatial resolution are expected to support the evaluation and enhancement of the reliability of microjoints on printed circuit boards. An X-ray microtomography system, the SP-μCT has been developed at the Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, the SP-μCT was first applied to the nondestructive evaluation of thermal fatigue phenomena, namely microstructure evolution (i.e., phase growth) and microcrack propagation, appearing in actual solder microbumps of flip chip interconnects due to thermal cyclic loading. In addition, a refraction-contrast imaging technique was simultaneously applied to visualize the fatigue cracks with an actual opening of less than 100 nm. The observed specimen has a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 150 μm in diameter. Consequently, the process of phase growth and crack propagation was determined via observation of consecutive computed tomography (CT) images obtained in the same plane of the same specimen. As the thermal cycle proceeded, remarkable phase growth was clearly observed, followed by the appearance of fatigue cracks in the corners of the interfaces between the solder bump and Cu pad. Moreover, the CT images also enabled us to evaluate the fatigue lifetime of the bumps, as follows. The lifetime to fatigue crack initiation was estimated by quantifying the increase in the phase growth. The crack propagation lifetime to failure was then determined by measuring the average crack propagation rate. Such results have not been obtainable at all by X-ray CT systems for industrial use and demonstrate the possibility of nondestructive inspection by a synchrotron radiation X-ray microtomography system.
In high-density packaging technology, one of the most important issues is the reliability of the microjoints connecting large scale integrated circuit chips to printed circuit boards electrically and mechanically. The development of nondestructive testing methods with high spatial resolution is expected to enhance reliability. An X-ray microtomography system called SP-μCT has been developed in Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied in the nondestructive evaluation of microstructure evolution, that is, the phase growth due to thermal cyclic loading in solder ball microjoints. Simulating solder microjoints used in a flip chip, specimens were fabricated by joining a Sn–Pb eutectic solder ball 100 μm in diameter to a steel pin in the usual reflow soldering process. The phase growth process was determined by observing the computed tomography (CT) images obtained consecutively at the fixed point of the target joining. In the reconstructed CT images, the distribution of the constituent phases in the Sn–Pb eutectic solder was identified based on the estimation value of the X-ray linear attenuation coefficient. Consequently, the microstructure images obtained nondestructively by SP-μCT provided us with the following useful information for evaluating the reliability of the solder microjoints. First, each phase involves not dispersing particles but a three-dimensional monolithic structure like a sponge. Second, the phase growth proceeds in such a way that the average phase size to the fourth power increases proportionally to the number of cycles. Finally, in the vicinity of the joining interface, more rapid phase growth occurs compared to the other regions because local thermal strain due to the mismatch of thermal expansion leads to a remarkable phase growth.
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