AIN Acoustic Wave Sensors Using Excimer Laser Micromachining Techniques
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AlN Acoustic Wave Sensors Using Excimer Laser Micromachining Techniques Feng Zhong, Changhe Huang, and Gregory W. Auner Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202 ABSTRACT Aluminum Nitride (AlN) is a promising piezoelectric material for Acoustic Wave (AW) sensor application due to its high acoustic velocity, linear thermal coefficient and high electromechanical coupling coefficient. Epitaxial AlN thin films were successfully grown on the Sapphire C plane substrate by Plasma Source Molecular Beam Epitaxy (PSMBE). Standard twoport resonator structures were fabricated on the AlN/Sapphire by standard photolithography. Excimer laser micromachining techniques were utilized to fabricate microgroove gratings on the surface of thin film. Beside Surface Acoustic Wave (SAW), Surface Transverse Wave (STW) propagation was also found on the devices with those microgroove and classical metal strip gratings. Laser micromachinined microgroove gratings were found to enhance the propagation of STW. Further, sensors based on STW showed little attenuation in the liquid environment, while sensors based on SAW suffered excess loss when exposed to liquid. INTRODUCTION Aluminum Nitride (AlN) is a promising piezoelectric material for acoustic device application due to its high acoustic velocity and electromechanical coupling coefficient [1]. There are many applications using AlN as high frequency (up to 2.4 GHz) band pass filter in high speed mobile and wireless communication system [2,3]. On the other hand, AlN also can be used as a highly sensitive detector for gases or organic components. It serves as the frequencydetermining element of an oscillator circuit. The mass change of the coating arising from the absorption of an analyte leads to a proportional shift of the oscillation frequency. AlN is highly stable in humid and high temperature environments, which make it very suitable for biochemical sensing. Currently most acoustic sensors are based on Surface Acoustic Wave (SAW) devices, which suffer excess attenuation when expose to liquid sensing environment as in the case of biosensing. In this paper, we report the results of preliminary investigation of a biochemical dual mode sensor on AlN on C plane Sapphire that can be switched from SAW to Surface Transverse Wave (STW) devices. EXPERIMENT Materials growth AlN thin films were grown on (1000) Sapphire by Plasma Source Molecular Beam Epitaxy (PSMBE). PSMBE uses a unique hollow cathode plasma source lined with MBE grade aluminum. High quality AlN epitaxial layers have been grown by using this system [4, 5]. The base pressure of the system is maintained in the range of 10-9-10-10 Torr. Radio Frequency (RF) power is supplied to generate plasma inside the hollow cathode. The dynamic pressure during deposition is maintained at 1× 10-3 Torr. Negative acceleration bias is applied to the substrate. The growth temperature, acceleration bias, N2 flow and RF power level can be adjusted to get optimum thin films. G11.28.1
C-plane Sapphire was ultr
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