Interdigital Transducer Design

The electrical impedance of the IDT depends on a variety of factors including the electromechanical coupling coefficient (K ), the dielectric permittivity of the substrate (e ), and the geometry of the IDT electrode width, spacing, number of finger pairs, and acoustic aperture (i.e., IDT finger overiap length). Table 

To efficiently couple power into and out of the IDT, thereby promoting 

Center Frequency, MHz Periodicity (WavelengUi), fim No. of finger pairs Finger endM finger spmmg, a Acoustic aperture, A- Acoustic path length. 

For SAW resonators (either one- or two-port), design criteria are very similar to those discussed above for delay lines, with an important caveat the number of fingers utilized for the IDT must be significantly fewer than for delay lines in order to couple to the acoustic cavity (the ridge array) at the proper signal level, and simultaneously achieve the desired impedance. The result is that while 

---

FIGURE 3.14 The standard interdigital transducer for SAW excitation, where the aspect ratio or relative aperture (I /A) is a design parameter. 

As it was mentioned earlier, a much larger range of micro/nanofabrication tools are utilized in the design and fabrication of micro/nanotransducers than applied in the fabrication of microfluidic systems. Standard photolithography as described above is only one of the many techniques used. Three examples will be described here the fabrication of freestanding transducers, UV-photolithography with lift-off technique for preparation of interdigitated ultramicroelectrode arrays (IDUAs), and more traditional techniques for microtransducer preparations. 

For a SAW waveguide modulator, an interdigitated, single periodic pattern of metal fingers are deposited directly onto the waveguide surface, as shown in (Fig. 3.14), where the transducer has a design... 

---