August_EDFA_Digital

edfas.org 29 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 3 inversely to transformmechanical vibrations into electri- cal signals. For inspections requiring a high resolution, the probe has an acoustic lens at its tip that is used to transmit a focused beam into a specimen. The beam spot size diameter in water d is given by Eq 2. [3] (Eq 2) Here, α is a constant, v is the acoustic velocity in water, F is the focal length of an acoustic lens, f is the frequency of the ultrasonic signal emitted from the ultrasonic probe, and D is the aperture diameter of the acoustic lens. As shown in Eq 2, there are three approaches to increasing the resolution used for ultrasonic inspection: increasing the aperture diameter, reducing the focal length, and increasing the frequency. However, in practice, the two approaches of increasing the aperture diameter and reducing the focal distance are sometimes ineffective for nondestructive inspection. When the aperture diameter is increased, according to Snell’s law, total reflection will occur during the process of propagating an ultrasonic signal into a specimen fromwater. As a result, even if the aperture diameter is increased, only the ultrasonic signal transmitted fromthe center of the lenswill be incident into the specimen. Reducing the focal length is only effective for detecting defects near the surface, but a signal with a reduced focal length cannot be focused on deep parts of a specimen and thus cannot be applied for the inspection of thick or stacked specimens. Finally, when the frequency is increased, the ultrasonic energy E z is attenuated according to Eq 3. (Eq 3) Here, b is a constant, E 0 is the initial ultrasonic energy, and z is the propagation distance of the ultrasonic signal. As shown in Eq 3, as the frequency is increased in order to increase the spatial resolution, the signal attenuation increases, and the defect-detection sensitivity decreases. Because the sensitivity of the defect detec- tion canbe increased throughdevelopments in hardware, for example through ultrasonic probe and electric circuit design, and soft- ware, such as through signal processing and transmission waveform design, increasing the ultrasonic frequency continues to be an effective approach to improving the resolution. This article describes the results of achieving high- resolution inspection by increasing the signal detection sensitivity in the high-frequency region by developing a highly sensitive ultrasonic probe that is an alternative to the piezoelectric one through hardware development and a signal processing technique using a pulse compression method through software development. HIGHLY SENSITIVE CMUT RECEIVING PROBE FOR THROUGH-TRANSMISSION SAT Capacitive micromachined ultrasound transducers (CMUTs) have advantages over piezoelectric transducers in terms of acoustic performance and circuit-integration capability. [4-5] CMUT technology can therefore be applied to fields such as medical diagnosis and NDI. So far, the potential of this technology has been demonstrated in terms of high resolution and sensitivity to the deep parts of the human body by using a CMUT as the probe for a medical ultrasonic diagnostic apparatus. [6] In this study, a CMUTwith a broadband characteristic and high receiver sensitivitywas applied as the receiving probe for through- transmission SAT in the NDI field and improved the lateral resolution. Figure 2 shows a schematic diagram of the typical structure of a CMUT cell and an IV converter. As for the structure of the CMUT cell, electrodes are formed above and below an evacuated gap covered with an insulating film. A membrane that includes an upper electrode is formed above the gap. To enable the CMUT cell to receive ultrasonic signals, a DC voltage is applied between the upper and lower electrodes, and the change in electro- static capacitance induced by vibration of the membrane generates a receive current signal I CMUT . The I CMUT is convert- ed into a voltage signal via an IV converter and outputted. The voltage signal in the reception of a sound pressure of 1 Pa corresponds to the receiver sensitivity of the CMUT Fig. 2 Schematic diagram of CMUT cell and IV converter.