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Thermally induced ultrasonic emission from porous silicon26 August 1999 Nature 400, 853 - 855 (1999)
H. SHINODA, T. NAKAJIMA, K. UENO & N. KOSHIDA
Department of Electrical & Electronic Engineering, Tokyo
University of Agriculture & Technology, 2-24-16 Nakamachi, Koganei,
Tokyo 184-8588, Japan The most common
mechanism
The idea of a thermal sound generator (a "thermophone") was proposed 80
years ago6, in which the acoustic element was a simple
self-supporting, thin metal film. The photoacoustic effect in porous
silicon has also been reported7,8, but was used to characterize the porous
silicon itself from gas expansion in a closed space. One might think that
ultrasound generation by heat exchange is not possible, as the thermal
conduction is too slow. However, we report here that the experimental
device shown in Fig. 1 operates as an efficient ultrasound emitter. It
is composed of a patterned, thin aluminium film electrode (30 nm thick), a
microporous silicon layer (10 µm thick), and a p-type crystalline silicon
(c-Si) wafer. The porous silicon layer consists of many confined silicon
nanocrystallites with three-dimensional nanopores9. In our experiment, the porous silicon layer
(with porosity of 70%) was formed by a conventional anodization
technique in a solution of 55% HF:ethanol = 1:1 at a temperature of 20 °C
at a current density of 20 mA cm-2 for 8-40 min. The aluminium
electrode was used to input a sinusoidal current into the porous silicon
layer, the temperature of which was raised by Joule's heating. The emitted
acoustic pressure was measured as a function of output frequency by a
microphone type 4138, Brüel & Kjær, Denmark) placed at a position of
3.5 cm from the front surface.
where and C are respectively the thermal conductivity and heat capacity per unit volume of porous silicon, then the surface temperature change T0()exp(jt) is given by if the expansion of porous silicon and the heat flow into the air are neglected (see Fig. 2b). The temperature change induces an acoustic pressure P(x,)exp(jt) through the alternating thermal expansion of the air. Using the fundamental equations of photoacoustic analysis11, we find that where PA is atmospheric pressure, TA is room temperature, v is the sound velocity,
We now evaluate the product C, because it determines the efficiency
of the operation as suggested by The measured acoustic pressure amplitudes are plotted in Fig. 3 as a function of output frequency for a
sinusoidal Joule's heating power of 1 W cm-2. A significantly
high acoustic pressure is observed over a wide frequency range up to 100
kHz. The limit of upper measurement frequency at 100 kHz is simply due to
the specification of our measurement system; and the condition of
To compare (in Pa) the efficiency of our device with that of conventional methods is not straightforward, as the output acoustic pressure of our device is proportional to the input power per unit area, while that of piezoelectric or electrostatic devices is proportional to the input voltage. For conventional devices, an average membrane displacement of 12.6 nm V-1 at 1.7 MHz has been reported5: this corresponds to 55 Pa V-1, which we take as the highest efficiency at present. A PZT bimorph transducer with impedance matching cone (EFRTUB50K5, Matsushita) generates 2 Pa V-1 at 40 kHz when it is measured at distance of 30 cm. In the survey by Manthey et al.,1 typical efficiencies of both piezoelectric and (non-MEMS) electrostatic types are given as 0.1-1 Pa V-1 at 200 kHz when measured at a distance of 1 m. (Here MEMS indicates micro-electrical-mechanical systems.) As the impedance of our experimental device was 5 , a 1 cm2 device driven by a 5 V source gives a 0.4 Pa plane wave whose intensity is not much less than that of the conventional resonant device, though our device consumes more power. We note that the frequency range of our device is broad, unlike the resonant devices, and that its efficiency increases as the sound intensity increases. The thermo-acoustic coupling factor, which is defined as the square root of the ratio of the acoustic power output to the input electric power, is 0.03% at 1 W cm-2, while that of the conventional method reaches 10%. This low coupling factor is a present drawback of our non-optimized device. But the freedom to select its electrical impedance, and a high intensity free from the upper bound of conventional methods, are also advantages of our device, as are mechanical toughness, ideally flat frequency characteristics, and the potential to assemble a phased array of such devices with low crosstalk. As a result of the scaling principle in thermal conduction phenomena, the use of dot electrode structures (in which the heat exchange is concentrated in small islands) should greatly improve the efficiency and output acoustic pressure of our devices. Moreover, such construction could prove useful in two-dimensional array fabrication because it provides a large wiring space. On the other hand, as suggested by the data in Table 1, the use of oxidized porous silicon would be a promising and practical approach to enhancing the efficiency. Although we have not performed rigorous tests of its stability, this device is free from the often-quoted instability of porous silicon. The porous silicon layer is protected by the aluminium electrode from the atmosphere, and is not exposed to an intense electric field. (It is conceivable that oxidation might in fact upgrade the efficiency rather than degrade it.) We can confirm that this device worked well after three months at room temperature, humidity and pressure. Particular attention has been paid to the visible luminescence of porous silicon14,15,16. It is closely related to an optical bandgap widening, induced by strong quantum confinement in silicon nanocrystallites with the same band dispersions as c-Si (ref. 17). But complete carrier depletion in nanocrystallites associated with strong confinement, on the other hand, leads to extremely low values of and C. The ability to control these parameters over a wide range, and the process compatibility with ultra-large-scale-integration (ULSI) technology--including a large difference in the thermal properties between porous silicon and c-Si--suggests applications of porous silicon in acoustic integrated devices. Received 18 January;accepted 13 July 1999. References
Acknowledgements. We thank the late M. Fuchigami for his contribution to the analysis and experiments, and S. Ando who motivated this research. This work was partly supported by the Japan Society for the Promotion of Science, and a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. Correspondence and requests for materials should be addressed to H.S. (e-mail: shino@cc.tuat.ac.jp). |
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