The present invention relates to an ultrasonic microscope, and more particularly to an ultrasonic microscope in which a specimen to be observed by the ultrasonic microscope can be observed by an optical microscope incorporated therein.
In recent years, it has become possible to generate and detect an ultrasonic wave having an ultrahigh frequency as high as 1 GHz and hence to realize an ultrasonic wavelength of about 1.5 .mu.m in the water. As a result, an ultrasonic imaging equipment having a high resolving power has been proposed (see, for example, U.S Pat. No. 4,386,530). FIG. 1 shows the schematic construction of an ultrasonic microscope. Referring to the figure, the transmission, focusing and reception of an ultrasonic acoustic wave is made by an acoustic lens 1 including a cylindrical block of an acoustic propagating medium such as fused quartz one end face of which is polished to provide an optically flat face. On the polished face is disposed a laminated structure which includes upper and lower electrodes 3 with a piezoelectric thin film 2 of zinc oxide or the like sandwiched therebetween. An electric pulse signal 5 from a pulse oscillator 4 is applied to the piezoelectric thin film 2 to generate an ultrasonic acoustic wave 6 therefrom. The other end face of the acoustic lens 1 is provided with a semispherical recess having a diameter of 0.1 to 1.0 mm, and a space between the semispherical recess portion and a specimen 7 is filled with a medium 8 (for example, water) for propagating the ultrasonic wave 6 to the specimen 7. With such a construction, the ultrasonic wave 6 generated from the piezoelectric thin film 2 is propagated through the cylindrical block 1 in the form of a plane wave and is refracted at the semispherical recess portion in accordance with a difference in sound velocity between the fused quartz and the medium 8 so that the plane wave is focused on the specimen 7. The ultrasonic wave reflected from the specimen 7 is collected and converted into a plane wave by the acoustic lens 1. The plane wave is propagated to the piezoelectric thin film 2 to be converted into a radio frequency (RF) signal 9. The RF signal 9 is received by a receiver 10 which includes a diode detector for conversion into a video signal 11. The video signal 11 is used as the input signal of a CRT display unit 12. The specimen 7 is subjected to a two-dimensional scanning in an X-Y plane on the basis of a signal from a power source 13 for driving a specimen table 14. The variation of the intensity of the reflected ultrasonic wave from the specimen 7 thus scanned is two-dimensionally displayed on the screen of the CRT display unit 12.
The acoustic wave used in an ultrasonic microscope has an ultra-high frequency and the attenuation of the acoustic wave in a medium such as water increases in proportion to the second power of the frequency if the transmitting distance is constant. Therefore, it is required to maintain a gap between the acoustic lens 1 and the specimen 7 as small as possible in order to make the transmitting distance short. For example, in the case of an ultrasonic wave having a frequency of 1 GHz, the gap between the acoustic lens 1 and the specimen 7 is selected to be about 50 .mu.m, thereby minimizing the attenuation of the ultrasonic wave. On the other hand, if the semispherical recess portion of the acoustic lens 1 fabricated with high precision is damaged by collision with the specimen, etc. during an observation operation as may take place in an optical lens, the damaged acoustic lens is rendered useless. The above-mentioned requirements of maintaining the gap between the acoustic lens 1 and the specimen 7 very small makes it very difficult to visually ascertain the presence of the gap. The requirements involve a very high possibility that the acoustic lens 1 may be damaged by collision with the specimen 7 during a manipulation of the lens.