Nondestructive investigation of 4-inch langasite wafer acoustic homogeneity

This paper reports an investigation of the free-surface SAW velocity homogeneity of 4-inch langasite wafers. The technique employed is fast, cheap, and nondestructive. The method is based on the determination of the SAW propagation time between two interdigital transducers (IDT), separated by a fixed distance. The measurement setup consists of an acoustic system comprising a sensor, the piezoelectric substrate under investigation, and a vector network analyzer HP-3577A. The sensor is fabricated on a nonpiezoelectric substrate, with two split-electrode IDTs. The piezoelectric substrate to be studied is placed in intimate contact with the transducer system. The complex frequency response of the acoustic system measured by the vector network analyzer is the product of the input and output IDT responses including the phase. It is then possible to determine the impulse response of the acoustic system accurately with the help of the Fourier transform. The total error (data processing and acoustic system imperfections) amounts to 35 ppm of the measured velocity. The method described was used to carry out an inspection of the SAW velocity uniformity of quartz, langasite and lithium niobate substrates. SAW velocity measurements on 3" and 4" langasite wafers have shown that the mean square deviation of SAW velocity is less than 120 ppm. Information on the velocity variation over the area of 3" and 4" langasite wafers has been used to improve the inhouse manufacturing process for the growth of large-size langasite crystals in batch production. A filter employing slanted fingers was designed and fabricated on wafers of LGS. This device required very small IDT dimensions and displayed exceptional repeatability of performance. The advantages of langasite devices are discussed.