SAW-grade SiO 2 for advanced microfluidic devices

Abstract

Acoustoelectronic devices based on surface acoustic wave (SAW) technology are primarily used in radio frequency filters, delay lines, duplexers, amplifiers and RFID tags. Thereby, SAW's are excited at the surface of piezoelectric materials (e.g. Quartz, LiTaO₃, LiNbO₃) by an RF signal applied via interdigital transducers (IDTs)¹. Novel SAW applications that emerged recently in the field of microfluidics such as the handling of minimum quantities of fluids or gases^2,3 require a fluid compatible design approach, high power durability and long lifetime of the devices. However, conventional SAW devices with finger electrodes arranged on top of the chip surface experience acoustomigration damage^4,5 at high power input and/or higher operating temperature leading to failure of the device. Additionally, inappropriate material systems or chip surface topography can limit their performance in microfluidic application. To overcome these limitations the electrodes can be buried in an acoustically suited ("SAW-grade") functional layer which moreover should be adjustable to the specific biotechnological task. Depending on the properties of this layer, it can suppress the acoustomigration impact⁶ and improve the power durability of the device. Also, a reduction of the thermally-induced frequency shift is possible⁷. The present paper describes a novel SAW based chip technology approach using a modular concept. Here, the electrodes are buried in surface polished SAW-grade SiO₂ fabricated by means of reactive RF magnetron sputtering from a SiO₂- target. This approach will be demonstrated for two different metallization systems based on Al or Cu thin films on 128° YX-LiNbO₃ substrates. We also show the application of the SiO₂-layer with respect to compensation of thermallyinduced frequency shift and bio /chemical surface modification. Investigations were carried out using atomic force microscopy, laser-pulse acoustic measurement, glow-discharge optical emission spectroscopy, spectral reflectometry, variable angle ellipsometry and x-ray photoelectron spectroscopy. The electrode edge covering of sputter deposited SiO² layers and the reactive ion etching of the SiO₂ layers are also discussed. This modular technology gives the possibility to improve the compatibility of surface acoustic wave devices to microfluidics and generally allows the integration of SAW driven actuators (pumps and mixing devices) and sensors (sensitive to surface mass change or complex viscosity change) together with other microfluidic elements (e.g. electrophoresis, heating elements) on one chip.