Proceedings Article | 19 May 2009
KEYWORDS: Silica, Electrodes, Microfluidics, Acoustics, Silicon, Dielectrics, Sputter deposition, Reactive ion etching, Quartz, Polishing
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
gases2,3 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
damage4,5 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.
