A SU-8 liquid cell for surface acoustic wave biosensors L. A. Francis a,b , J.-M. Friedt c , C. Bartic b and A. Campitelli b a PCPM, Universit´ e catholique de Louvain, Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium; b Biosensors Group, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium; c LMN, Universit´ e de Franche-Comt´ e, La Bouloie, 25030 Besan¸ con, France ABSTRACT One significant challenge facing biosensor development is packaging. For surface acoustic wave based biosensors, packaging influences the general sensing performance. The acoustic wave is generated and received thanks to interdigital transducers and the separation between the transducers defines the sensing area. Liquids used in biosensing experiments lead to an attenuation of the acoustic signal while in contact with the transducers. We have developed a liquid cell based on photodefinable epoxy SU-8 that prevents the presence of liquid on the transducers, has a small disturbance effect on the propagation of the acoustic wave, does not interfere with the biochemical sensing event, and leads to an integrated sensor system with reproducible properties. The liquid cell is achieved in two steps. In a first step, the SU-8 is precisely patterned around the transducers to define 120 μm thick walls. In a second step and after the dicing of the sensors, a glass capping is placed manually and glued on top of the SU-8 walls. This design approach is an improvement compared to the more classical solution consisting of a pre-molded cell that must be pressed against the device in order to avoid leaks, with negative consequences on the reproducibility of the experimental results. We demonstrate the effectiveness of our approach by protein adsorption monitoring. The packaging materials do not interfere with the biomolecules and have a high chemical resistance. For future developments, wafer level bonding of the quartz capping onto the SU-8 walls is envisioned. Keywords: surface acoustic wave, biosensor, liquid cell, SU-8, packaging, protein, fibrinogen 1. INTRODUCTION Initially developed for filters and delay lines in the telecommunication field, surface acoustic wave (SAW) devices have also attracted a growing interest in the sensor field. 1 The sensing principle is based on the perturbation of an elastic field propagating along the surface of a planar substrate. When the SAW is shear polarized, like SH-SAW and Love-mode sensors, the acoustic energy loss is low if a liquid is present on the surface. 2, 3 These sensors are adequate as biosensors where the perturbation is caused by the adsorption of biomolecules present in solution. 4, 5 The biochemical recognition results in a SAW velocity change that is monitored in real-time as an electrical delay phase through the transducers. The transduction is obtained on piezoelectric substrates thanks to interdigital transducers 6 (IDTs) that consist in a set of interdigitated electrodes with alternative electrical polarity. The separation between the IDTs defines the sensing area as sketched in Figure 1. The transfer function of the device, which relates the amplitude of an AC electrical signal at the output transducer to its value at the input transducer, is affected when a liquid with a high dielectric permittivity value is partly or fully contacting the transducers. 7 As example, Figure 2 shows the effect of deionized water in contact with any of the transducers on a quartz substrate. The attenuation observed in the amplitude and the phase of the transfer function is mainly due to an extra capacitance loading of the transducers; more details related to this effect are given in Appendix A. The intensity of the attenuation is function of the value of the electromechanical coupling constant K 2 of the piezoelectric substrate. It is more intense for low K 2 substrates such as quartz than for high K 2 substrates such as lithium niobate or lithium tantalate. For Love-mode sensors, it also depends of the thickness of the guiding dielectric overlayer that acts as shielding and reduces the interaction with liquids. Further author information: L. A. Francis: E-mail: francis@pcpm.ucl.ac.be SPIE USE, V. 1 5455-43 (p.1 of 11) / Color: No / Format: A4/ AF: A4 / Date: 2004-03-26 10:23:36 Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.