Study of a high efficiency optical MEMS transducer for the generation of narrowband laser ultrasound

In this paper we demonstrate an optically powered ultrasonic t ransducer. It has a high efficiency and was designed and fabricated using MEMS (microelectromechanical system) techniques. It can generate narrowband ultrasound from broadband laser excitation. It is a simple two-mask-level MEMS device with a micro-disc seated on a micro-stem. As a laser pulse is incident on the disc centre, the disc is excited into a `flapping' motion because of the thermomechanical interaction between the absorbing and nonabsorbing parts of the disc. This flapping motion is dominated by one of the resonances of the disc, coupling a narrowband longitudinal bulk wave propagating along the axis of the micro-stem into the sample. Experiments with these transducers have shown that narrowband ultrasonic waves with a high SNR (signal to noise ratio) were generated successfully. The device is simple to excite optically and generates higher amplitudes than by normal thermoelastic generation. No physical contact is required to excite the transducer, making it suitable for remote non-contact ultrasonic applications. 1.Introduction. Narrowband laser ultrasound is suitable for applications in industry [1], space technology [2, 3], medical treatment [4–7] and NDT [8, 9]. However, techniques to generate narrowband laser ultrasound have been limited [1, 10]. Other methods to optically generate narrowband ultrasound are those using complex optical arrays [11, 12] and special techniques [13, 14]. In this paper, an optical narrowband ultrasound generation transducer with high efficiency and a high SNR is presented. Fundamentally, it is a 3-D elastomechanical structure, a typical MEMS device, consisting of a micro-disc with diameter dd and thickness h, seated on a solid micro-stem with diameter ds and length hs, as shown by figure 1(a). Compared with other narrowband ultrasonic devices [10], transducer systems [14] or optical arrays [11, 12] in literature, the fundamental characteristics of this optical MEMS transducer are its small dimensions, high efficiency and wide application range by using a single general laser source with simple optical arrangements. In addition to these, it was potentially economic; it can be considered as to be disposable, and to be massless for most engineering applications. 2. Working principle In normal thermoelastic excitation where a laser pulse being incident on the surface of a sample the generated ultrasound is broadband [15-17]. When a laser pulse is incident on the micro-disc of the transducer (figure 1(a)), it produces localised temperature differences and stresses which excite the vibrational modes of the structure. In the case where laser light was well focused, a surface wave with a circular wavefront is actuated first in the centre of the disc (see figure 1(b) by FE (finite element)). As this surface wave propagates towards the edge of the disc, it mainly turns into an antisymmetric Lamb wave with a circular wavefront at the root (see figure 1(c)) because of the small thickness of the disc. After this, reflections of the Lamb wave start between the disc edge and the root because the step length ∆d of the flange is small, and soon turn the Lamb wave into a flapping motion of the flange part of the disc, as shown by part (d) of the figure. If the laser light is not well focused, the light spot can be close to or larger than the stem diameter of the transducer. In this case, the Lamb wave may be directly generated within the flange part of the disc. The micro-disc structure plays an important modulation part in the actuated disc motion frequency; the frequency of the laser pulse actuated disc motion always tends to be as close as possible to the first resonance of a 2-D axisymmetric micro-disc; higher modes may be slightly actuated, depending on the parameter of ∆d/h (see figure 1), pulse duration and stem diameter as well. Because the disc is elastically supported by the solid stem, this flapping motion of the disc couples an elastic surface wave travelling along the surface of the solid 15th International Conference on Photoacoustic and Photothermal Phenomena (ICPPP15) IOP Publishing Journal of Physics: Conference Series 214 (2010) 012103 doi:10.1088/1742-6596/214/1/012103 c © 2010 IOP Publishing Ltd 1 Figure 1. (a) Schematic design of the geometry of an optical generation transducer. It is a micro-disc seated on a microstem. As a laser pulse is incident on the centre of the micro-disc, a flapping motion of the disc flange is actuated with a frequency defined by one of the resonances of the disc, and couples a longitudinal bulk wave propagating along the axis of the micro-stem. (b) A surface wave is actuated first at the disc centre by a laser pulse, simulated by FE. (c) As the actuated surface wave travels from the disc centre towards the edge, it turns into an antisymmetric Lamb wave as it passes the flange root of the disc. (d) A disc flapping motion actuated by a laser pulse. micro-stem. Due to the axisymmetry of the micro-stem, the phase of the transverse wave is axisymmetric with respect to the cross section of the stem, resulting in a cancellation of the transverse wave due to the small diameter of the stem. However, the coupled longitudinal bulk wave components are in-phase around the micro-stem cross section. Therefore, the elastic coupling of the free disc flapping vibration generates an enhanced longitudinal bulk wave, travelling along the axis of the stem, with a frequency the same as that of the flapping motion of the disc; it is a narrowband longitudinal bulk wave with a frequency defined by the first resonance of the disc; components with higher frequencies may be included, depending on the transducer geometry, materials and the laser pulse duration; however, the amplitude corresponding the first resonance of the micro-disc must be dominant. Figure 2. Geometry design of the narrowband optical MEMS generation transducer. (a) Image of top view of the designed transducers with 1 MHz (left), 2 MHz (middle) and 3 MHz (right). (b) Three dimensional structure of a designed 3-MHz transducer. (c) and (d) Microscope images of the front and back of fabricated 1-MHz transducers with different stem diameters. (e) and (f) Similar microscope images of 3-MHz transducers. 3. Design and fabrication The basic task of designing the MEMS generation transducer is to define the geometry of the transducer over the required frequency parameter. The basic dimensions of the micro-disc of a transducer can be defined by using FE eigen analysis. The dimensions of the designed transducers are shown in Table I. Figure 2(a) shows the top view of the designed transducers. The left three in diagram (a) are the transducers with a frequency around 1 MHz, and the diameters of the stems are ds=100, 150 and 200 μm respectively. The middle three are with a frequency around 2 MHz and the right three are 3-MHz transducers. The 3-D structure for a 3MHz transducer is shown in figure 2(b). Two deep reactive ion etching processes were used to fabricate the transducer. Detailed fabrication process can be referred to at the website of MEMSCAP (refer to http://www. Memscap.com). Figures 2(c)~(f) show images of fabricated transducers under a microscope, where figure (c) shows the top view of two 1-MHz transducers and diagram (e) shows the top view images of two 3-MHz transducers. Figs. 2(d) and (f) are the backside images of the same transducers. 4. Experimental study In order to check if the designed MEMS transducers can be used to generate the ultrasound as expected, two experimental systems were configured, as shown in figure 3. Figure 3(a) shows a c-scan system which uses a laser vibrometer to measure out of plane motion. The head of the vibrometer was mounted on an x-y scan (a) (b) (c) (d)