Characterization of Pt/AlN/Pt-based structures for high temperature, microwave electroacoustic devices applications

Abstract

Highly c-axis oriented AlN films, 3.15 μm thick, were grown by rf reactive sputtering technique at 200 °C on bare and Pt-covered Si(100) substrates previously oxidized to a thickness of about 2 μm in wet oxygen atmosphere. A Pt film, 2200 Å thick, was then sputtered on the free surface of the AlN/Pt/SiO₂/Si multilayer at 200 °C without breaking the vacuum in order to avoid any oxidation effects of the layers. The multilayers were then annealed in air at 900 °C for different time lengths up to 32 h in order to test the materials' resistivity to harsh environment. The influence of this high temperature annealing (HTA) on the thin films' crystallinity, as well as on the c-AlN piezoelectricity and Pt sheet resistivity was investigated at room temperature before and after each annealing. X ray diffraction investigations revealed that the films' crystallinity was improved by the HTA: the full width of half maximum of the AlN(002) and Pt(111) peaks decreases from 0.39° to 0.24°, and from 0.42° to 0.28° after 32-hours-HTA. Scanning electron microscopy, four points probe and piezoelectricity tests revealed that the morphology and the sheet resistivity (in the range from 0.6 to 0.5 Ω/sq) of the outer Pt film, as well as the AlN piezoelectric constants d₃₃ (in the range from 6.2 to 7.4⋅10⁻¹² C/N) was quite unaffected by the HTA even after 32 h of annealing.

Introduction

There is an increasing demand of high-temperature electronic components for aerospace, aircraft industries, sensors and automotive applications. Measurements reliability requires the electronic controls to be placed directly inside the extreme environment, and to withstand temperatures of several centigrade degrees with lifetimes of many hours. Both the device mounting and packaging, and the device materials must be stable with the working temperature, otherwise temperature-induced stress may result in device's failures. Piezoelectric crystals of the langasite (LGS) family group [1], [2] and GaPO₄ substrates [3] have been widely investigated for the implementation of electroacoustic devices able to work at high temperature. LGS does not undergo any phase transformations up to its melting temperature (1470 °C) and has a higher electromechanical coupling coefficient and lower acoustic losses than those of quartz. GaPO₄ has twice the sensitivity of quartz and many physical constants stable up to about 1000 °C. Unfortunately these two materials show quite high acoustic losses and low acoustic wave velocity that prohibit their use in the microwave range. Aluminum nitride (AlN) is a promising piezoelectric material able to maintain its piezoelectricity up to 1200 °C. Its high surface and bulk acoustic wave (SAW and BAW) velocities make it the ideal candidate for microwave electroacoustic devices implementation. Its high thermal conductivity and resistance to high temperature and to caustic chemicals guarantee the stability of the AlN-based electroacoustic devices when they are in contact with extreme environments [4]. Furthermore, AlN can be grown in thin film form onto non piezoelectric substrates, such as silicon or sapphire: this fact allows AlN to recover the double role of electroacoustic transducer material and protective layer with respect to the metal interdigital transducers (IDTs) when located at the substrate/film interface. Moreover, if the AlN film is sandwiched between the IDTs and the ground electrode, four piezoelectric coupling configurations can be obtained by placing the IDTs at the substrate/film interface or at the film surface, with and without the floating electrode opposite the IDTs. These four configurations show frequency dispersive characteristics (i.e., SAW velocity, electroacoustic coupling efficiency, temperature coefficient of delay, and IDT radiation resistance and capacitance) highly sensitive to the electrical boundary conditions; thus high-frequency, enhanced coupling, thermally compensated elctroacoustic devices can be designed at the proper films thickness values. Due to its high temperature coefficient of resistance (TCR), Pt thin film is the material of choice for metallic components (the IDTs and ground electrode) that have to withstand oxidation. In the present paper Pt and AlN films were deposited on oxidized Si wafers by rf sputtering technique at 200 °C and Si/SiO₂/Pt/AlN/Pt structures were obtained. The investigation of the effects of the thermal annealing on the morphology, structural properties and room-temperature sheet resistivity of the Pt film, and on the piezoelectric constants d₃₃ of the AlN films allowed to assess the AlN and Pt films sustainability in high temperature applications.