Surface Acoustic Wave Devices for Harsh Environment

Abstract

There is an increasing demand of electronic components for aerospace, aircraft industries, sensors, automotive, chemical and material processing applications, to name just a few, able to operate reliably and for long time at high-temperature. Measurements reliability requires the electronic components to be placed directly inside the extreme environment, and to withstand temperatures of several centigrade degrees with lifetimes of several hours. The device mounting and packaging, but first of all the device materials must be stable with the working temperature, otherwise temperature-induced stress may result in device’s failures. Electroacoustic devices based on surface and bulk acoustic wave (SAW and BAW) technology must satisfy the requirements of low cost, high frequency, high-Q, low loss, large piezoelectric coupling and zero temperature coefficient of delay (TCD) to be key devices in the communication and sensor fields. The temperature stability of the piezoelectric crystal is an essential characteristic because of its direct link with the temperature sensitivity of the electroacoustic device operation frequency. The high operation frequency is an essential characteristic for SAW and BAW devices to be used in mobile phones, cordless headphones, alarm and security systems, military equipment, sensors, etc. The temperature stability and the high operation frequency demands can be met through a proper choice of the piezoelectric substrate crystal cut, new piezoelectric materials and/or multilayer configurations. The use of temperature stable cuts of single crystal bulk piezoelectric materials or temperature compensated multilayers represents two possible solutions to the temperature stability requirement. The use of high-resolution lithography techniques and/or of high SAW velocity materials is required in order to extend the upper limit of the electroacoustic device frequency range. Submicron feature sized interdigital transducers (IDTs) are required to implement GHz range SAW devices on slow piezoelectric materials, while micron feature sized IDTs can still be used on fast materials, since the SAW device centre frequency, f = v/λ, depends on both the phase velocity of the propagating medium, v, and on the acoustic wavelength λ, being the IDT’s period p = λ/2. Conventional piezoelectric substrates, such as quartz, lithium niobate (LiNbO3), and lithium tantalate (LiTa03) crystals, cannot be used above 500°C. Quartz ST cut is a temperature stable material but it shows an alpha-beta transition at 573°C, which causes the loss of piezoelectricity, and results in a non-operable device. SAW devices implemented on LiNb03 have been studied for a temporary usage at 400°C [1]; however the LiNbO3 acoustic wave properties are highly dependent on temperature since it is a pyroelectric