Piezoelectric – Semiconductor Structures
The integration of Surface Acoustic Wave (SAW) Interdigital Transducers (IDT) in conventional Si processing technology is of high interest for on-chip solutions in wireless communication applications. Here, SAW elements are used as frequency filters in the interesting frequency range from several MHz up to the GHz regime. Fig. 1 shows the principle: The rf signal input of the sending transducer (IDT) on the left usually contains a wide frequency range. Only the frequency in resonance with the SAW element produces a periodic deformation in time and space of the piezoelectric film material. The resonance frequency of this modulation is determined by the distance between the “fingers” of the metallic IDT and the sound velocity of the film material. The resulting SAW propagates towards the receiving transducer on the right where it is converted again into an electric signal.
In addition, SAW devices may be applied for bio-molecular sensing and would allow for the fabrication of fully integrated sensor arrays. Individual sensors would operate according to the gravimetric principle. Bio-molecules attached to the surface cause a detuning of the resonance frequency f0 of the SAW device and may thus be determined by measuring the frequency shift Δf. Gravimetric sensors rely on the mass of the molecules to be analyzed and do not require an optical label (label free detection).
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Fig. 1: The principle of a SAW - frequency filter for wireless communication applications. |
The project aims to develop SAW devices based on piezoelectric layer systems which can be integrated in the back end of line (BEOL) process stream of mainstream Si microelectronic technologies (CMOS etc). For this purpose, the growth of a high-quality piezoelectric film with the desired crystallographic orientation must be achieved under the typical thermal budget constraints of BEOL processes. This task is a challenge which currently attracts world-wide research activities. Such integrated saw devices will also allow the development of real biochips, i.e. a biomolecular sensor platform that combines with integrated circuits capable for subsequent signal processing and wireless data transmission.