**4. Conclusion**

This chapter is focused on two important applications of the acoustic-wave based MEMS devices; (1) biosensors and (2) telecommunications. Only few types of acoustic wave devices could be integrated in microfluidic systems without significant degradation of the quality factor. The acoustic wave based MEMS devices reported in the literature as biosensors are film bulk acoustic wave resonators (FBAR) and SAW resonators and delay lines. The Love mode SAW devices are often used as biosensor because the acoustic energy is confined to the sensing surface resulting in higher sensitivity to surface perturbations. The experimental results demonstrate that Love mode biosensors have high detection sensitivity.

Acoustic waves offer a promising technology platform for the development of biosensors and small-sized, low power RF-MEMS filters and resonators. MEMS acoustic wave biosensors are characterized by high sensitivity, small size and portability, fast responses, ruggedness and robustness, high accuracy, compatibility with integrated circuit (IC) technology, and excellent aging characteristics. Sensors based on this technology can be manufactured using standard photolithography and hence can be produced as relatively inexpensive devices. Integration of acoustic elements and electronic circuitry on a single silicon chip allows smart acoustic microsensors with advanced signal processing capabilities to be realized. Acoustic waves based biosensors offer the possibility of observing real-time binding events of proteins and other important biological molecules at relevant sensitivity levels and at low cost.

Acoustic wave MEMS devices used in telecommunications applications are also presented in this chapter. Telecommunication devices have different requirements compared to biosensors, where acoustic wave devices operating as a filter or resonator are expected to operate at high frequencies (GHz), have high quality factors and low insertion losses. Traditionally, SAW devices have been widely used in the telecommunications industry, however with advancement in lithographic techniques, FBARs are rapidly gaining popularity. FBARs have the advantage of meeting the stringent requirement of telecommunication industry of having Qs in the 10,000 range and silicon compatibility.

Currently, there is the concern that ZnO film, widely used as piezoelectric substrate for acoustic wave devices employed as biosensors and for telecommunications is very reactive, and unstable in liquid or air. Therefore, the stability and reliability of these devices become a problem. To solve this problem, the deposition of a thin protection layer such as Si3N4 on top of the ZnO film could be considered. Compared to ZnO, AlN shows a slightly lower piezoelectric coupling. However, AlN films have excellent piezoelectric properties. The Rayleigh wave phase velocity in (0 0 1) AlN is much higher than ZnO, which suggests that AlN is preferred for high frequency and high sensitivity applications (Gorla et al., 1999). AlN is a hard material with bulk hardness similar to quartz, and is chemically stable at temperatures less than about 700 ºC. Therefore, using AlN could be an alternative and lead to the development of acoustic devices operating at higher frequencies, with improved sensitivity and performance in insertion loss and resistance in harsh environments (Mason et al., 1972)

The popularity of portable communication gadgets has increased the demand and necessity of small-sized, low power RF-MEMS filters and resonators. Passive acoustic wave resonators fulfill this market niche of low-power, radio frequency and silicon-compatible resonators and filters. Surface acoustic wave devices (filters and resonators) have long been popular in the communications industry. To improve silicon compatibility, efforts have been made to implement the SAW resonator using standard CMOS process with minimal postprocessing. Results indicate that while this device shows promise, significant improvement is required before the CMOS SAW resonator can meet the stringent communication requirements. In this aspect, FBARs have shown better performance in terms of quality factor (6000) and low insertion losses. However, complete CMOS-compatibility has not yet been achieved and the device still requires bond wires for connections to the circuitry.

#### **5. References**

82 Microelectromechanical Systems and Devices

This chapter is focused on two important applications of the acoustic-wave based MEMS devices; (1) biosensors and (2) telecommunications. Only few types of acoustic wave devices could be integrated in microfluidic systems without significant degradation of the quality factor. The acoustic wave based MEMS devices reported in the literature as biosensors are film bulk acoustic wave resonators (FBAR) and SAW resonators and delay lines. The Love mode SAW devices are often used as biosensor because the acoustic energy is confined to the sensing surface resulting in higher sensitivity to surface perturbations. The experimental results demonstrate that Love mode biosensors have high detection

Acoustic waves offer a promising technology platform for the development of biosensors and small-sized, low power RF-MEMS filters and resonators. MEMS acoustic wave biosensors are characterized by high sensitivity, small size and portability, fast responses, ruggedness and robustness, high accuracy, compatibility with integrated circuit (IC) technology, and excellent aging characteristics. Sensors based on this technology can be manufactured using standard photolithography and hence can be produced as relatively inexpensive devices. Integration of acoustic elements and electronic circuitry on a single silicon chip allows smart acoustic microsensors with advanced signal processing capabilities to be realized. Acoustic waves based biosensors offer the possibility of observing real-time binding events of proteins and other important biological molecules at relevant sensitivity

Acoustic wave MEMS devices used in telecommunications applications are also presented in this chapter. Telecommunication devices have different requirements compared to biosensors, where acoustic wave devices operating as a filter or resonator are expected to operate at high frequencies (GHz), have high quality factors and low insertion losses. Traditionally, SAW devices have been widely used in the telecommunications industry, however with advancement in lithographic techniques, FBARs are rapidly gaining popularity. FBARs have the advantage of meeting the stringent requirement of telecommunication industry of having Qs in the 10,000 range and silicon compatibility. Currently, there is the concern that ZnO film, widely used as piezoelectric substrate for acoustic wave devices employed as biosensors and for telecommunications is very reactive, and unstable in liquid or air. Therefore, the stability and reliability of these devices become a problem. To solve this problem, the deposition of a thin protection layer such as Si3N4 on top of the ZnO film could be considered. Compared to ZnO, AlN shows a slightly lower piezoelectric coupling. However, AlN films have excellent piezoelectric properties. The Rayleigh wave phase velocity in (0 0 1) AlN is much higher than ZnO, which suggests that AlN is preferred for high frequency and high sensitivity applications (Gorla et al., 1999). AlN is a hard material with bulk hardness similar to quartz, and is chemically stable at temperatures less than about 700 ºC. Therefore, using AlN could be an alternative and lead to the development of acoustic devices operating at higher frequencies, with improved sensitivity and performance in insertion loss and resistance in harsh environments (Mason

The popularity of portable communication gadgets has increased the demand and necessity of small-sized, low power RF-MEMS filters and resonators. Passive acoustic wave resonators fulfill this market niche of low-power, radio frequency and silicon-compatible resonators and filters. Surface acoustic wave devices (filters and resonators) have long been

**4. Conclusion** 

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**Part 2** 

**MEMS Characterization and Micromachining** 

