Fabrication Of High Performance PMN-PT Ultrasonic Microscope Transducer Using A Short Pulse 355nm Laser

With its excellent resolution and great penetration,scanning acoustic microscope（SAM）plays an important part in the field of defects detection of chips,advanced materials research,characterization of welding properties,biomedical science and et al.The high-frequency（>15 MHz）transducer,as a key component of SAM,directly determines the image quality.However,due to its small size,complex processing technology and interdisciplinary design theory,the transducer has become the bottleneck of SAM technology development.Compared with traditional ultrasound microscope transducer,the Fresnel array has advantages of long imaging depth,high resolution and better signal-to-noise ratio（SNR）.On the other hand,using the Pb(Mg_1/3Nb_2/3)O₃-x%Pb Ti O₃（PMN-x%PT）with the highest piezoelectric properties(d₃₃>1200 p C/N)to prepare Fresnel array ultrasonic transducer has the potential to obtain better performance.However,the technology of single crystal transducer is tightly controlled by foreign well-known manufacturers,such as Philips,GE,Samsung and et al,and it is of great significance to solve the stress damages during the micromachining of single crystal.Ultrashort pulses laser micromachining,especially for femtosecond and picosecond techniques,has shown great application in micropatterning of nano-sized materials,ceramics,glass and semiconductor materials.In order to develop PMN-PT ultrasonic microscope transducer with high performance,a 50 MHz Fresnel array transducer was fabricated using a 355nm laser,and the results are as follows:First,the theoretical model of short pulse laser micromachining was established.The relationships between input energy and parameters of laser system,such as average power,repetition frequency,speed,scanning cycles,etc.,have been studied.We found that the energy is directly proportional to average power,and inversely proportional to repetition frequency.Second,we studied the kerf profile as a function of laser parameters systematically,revealing that 1)the micron kerf is clean with well-defined edges and fewer extended micro-cracks;2)the ablated depth is proportional to power and increases slightly after a certain number of scanning cycles;3)the scanning cycles and speed affect the ablated width to a certain extent,because the incident energy varies with power and repetition frequency;4)the smallest width of 15μm and the largest depth–width ratio of 8 were obtained.The domain morphology of micromachined PMN–31%PT was thoroughly analyzed to validate the superior piezoelectric performance maintained near the kerfs.After optimizing the laser parameters,the domain structures along the kerf agree well with those of the unprocessed samples;a high piezoresponse（～1200 pm/V）is also observed near the kerfs（～10μm）.These results imply that the ultrahigh piezoelectric properties of the materials are still maintained after micromachining into ultrasmall elements（<50μm）.Last but not least,a 50 MHz Fresnel ultrasonic microscope transducer with an aperture of 2.4mm has been designed and fabricated.A transducer testing system and a SAM with 8 channels were developed,which provide the hardware for the study of high-performance ultrasonic technology.In conclusion,the results present new micromachining guidance for developing superior PMN–PT Fresnel ultrasonic microscope transducer（50 MHz）that can solve the challenging issue of large reduction in piezoelectric properties.Moreover,the principles described in this study can be extended to the development of other piezoelectric microdevices.