SAW-grade SiO2 for advanced microfluidic devices

SAW-grade SiO2 for advanced microfluidic devices Andreas Winkler*, Siegfried Menzel, Hagen Schmidt IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany 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, SAWs 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 SiO2target. 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, 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.

Keywords: SAW, surface acoustic wave, microfluidics, SiO2, PVD, sputtered SiO2 1. INTRODUCTION 1.1

Surface Acoustic Wave (SAW) devices in microfluidics

Surface acoustic waves (SAWs) were theoretically described in 1885 by Lord Rayleigh8, but the breakthrough for their technological use was achieved first by White & Voltmer in 19659. They discovered, that SAWs can be excited at the surface of piezoelectric materials (e.g. Quartz, LiTaO3, LiNbO3) by applying an RF signal via a set of conductive finger electrodes, the so called interdigital transducer (IDT). This SAW technology has found various applications in the fields of radio frequency filters, delay lines, duplexers, amplifiers and radio frequency identification (RFID) tags1. Besides the simple bidirectional λ/4-IDT structures (Figure 1) numerous specific IDTs were developed, e.g. split finger, slantedfinger, unidirectional or apodization IDTs5. Depending on their application it is possible to excite waves of different polarizations, e.g. longitudinal-horizontally polarized shear waves or sagittally-polarized Rayleigh waves.

Smart Sensors, Actuators, and MEMS IV, edited by Ulrich Schmid, Carles Cané, Herbert Shea Proc. of SPIE Vol. 7362, 73621Q · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.822290

Proc. of SPIE Vol. 7362 73621Q-1

piezoelectric substrate

damping mass

reflector

I DT

I DT

reflector

I DT

Figure 1: Sketches of typical SAW devices: a) delay line, b) resonator

Two layouts are in general used for SAW devices: the delay line (Figure 1a) and the resonator (Figure 1b). In a delay line structure the acoustic wave is excited by a first IDT. Under vacuum or gas atmosphere, the wave is then propagating over centimetres of length with only low damping and divergence. A second IDT is used to reconvert the acoustic wave to an adequate electric RF signal. In a SAW resonator the SAW is bidirectionally reflected by sets of reflector stripes arranged perpendicularly to the SAW propagation path (Figure 1b). In both structures, environmental conditions influence the behaviour of the SAW (resonance frequency, signal phase and amplitude), that can be electrically monitored with high precision. While the main field of application for SAW-based devices is still telecommunication, the applicability of sagittally or horizontally polarized SAWs in the field of micro-/nanofluidics, i.e. the handling (actuation) or observation (sensing) of minimum quantities of complex fluids or gases, has been proven recently2. Here, examples are the acoustic mixing of small fluid volumes10,11, the pumping of droplets on the surface12 or the pumping of fluids in open channels13. Thereby, different wave polarizations determine different interactions with the fluid medium: Out-of-plane polarized waves dissipate energy into the fluid and are therefore suited for fluid actuation. For sensors, in-plane polarized (shear) waves are preferred, because of lower damping and energy loss. 1.2

Concept for advanced microfluidic SAW devices (Al & Cu)

The layout of conventional SAW devices shows several drawbacks when applied to fluidic technology: (1) The IDTs which are generally arranged on top of the chip surface have to be covered with a dielectric material to avoid short circuits and the contact of the fluid with the piezoelectric material. Here, the dielectric of choice is SiOx-based material, because of its long tradition as substrate in surface bio-/chemistry and biotechnology. However, the properties of the film (e.g. dielectric constant, Young’s modulus, stoichiometry, impurities) have a great influence on the SAW propagation and in many cases films of minor quality and poor long term stability (e.g. produced by CVD processes) are applied. (2) The surface of SAW devices shows protruding features (i.e. the IDT electrodes, reflection gratings, feed electrodes), which will influence the wave propagation and their interaction with fluids. This is even more critical for bio-/sensor applications, where flat surfaces are required. (3) The dominating electrode materials for fluidic SAW devices (polycrystalline Al and Au films) show high acoustomigration damage which limits the maximum power input and, consequently, the driving force for fluid actuation as well as the operating temperatures of the devices14,15,16. Thus, for high power devices the usage of acoustomigration-resistant electrode materials, e.g. Cu, highly textured films or alloys, is necessary4,5. The technology approaches mentioned in this work are capable to overcome these limitations and to extend the fluidic applicability of SAW devices. The modular character of these approaches makes it easy to change the materials for substrate, IDT metallization and functional top layer or to adapt it to the corresponding fluid.

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(Underlayer deposition)

Metal deposition

Si02-deposition

Wet or dry etching

Wet etching & wire Bonding

Chem.-mech. polishing

Figure 2: Technology scheme for design approach 1 (e.g. for Al-, Au-, Pt- or Ru-based electrode systems)

The first technology approach (Figure 2) is suitable for electrode materials which do not need a diffusion barrier, e.g. for Al-, Au-, Ru- or Pt-based materials. Here, the IDT electrodes should be fabricated first either a) by deposition of the electrode material and post structuring via resist lithography and etching, or b) by in SAW technology well-accepted Lift-Off technology. An adhesion-promoting or texture-defining layer can be deposited between the piezoelectric substrate and the metallization. The covering dielectric film is then deposited on top of the electrodes and polished by chemical-mechanically polishing (CMP) to create a flat surface with low roughness (