**5. Conclusion**

344 Microelectromechanical Systems and Devices

heat dissipation to the underlying substrate, permits the single layer structure to exhibit

increased robustness to the laser irradiation compared to the multilayered structures. The optical-thermal effects can also explain the temperature discontinuities observed in the temperature measurements of laser irradiated cantilevers and actuators (Serrano & Phinney, 2007; Serrano et al., 2009), shown in Fig. 9. As predicted above, the discontinuity corresponds to the presence of the peak in the absorptance curve. The surface temperature increases rapidly by 200 K as the peak is encountered. The temperature-power relationship regains a linear relation after the temperature reaches the opposite side of the absorptance peak. Numerical simulations of this experiment, utilizing the non-linear absorptance and known material and geometrical parameters for the irradiated structure, are in good agreement with the measured values, reproducing temperature discontinuity. This type of sudden increase in the temperature makes predicting a threshold power for laser damage in polysilicon structures extremely challenging without accurate knowledge of optical and

Fig. 10. Measured and modeled temperatures of a polysilicon MEMS structure measured irradiated with an λ = 808 nm laser. The discontinuity in the temperature results from a peak in the absorptance of the irradiated layer due to thin film interference effects

dimensional properties.

(Serrano et al., 2009).

Understanding the thermal and optical response of laser-irradiated microsystems requires careful consideration of not only the individual thermal, optical, and mechanical parameters, but also the coupling that exists between them. Of particular importance is the impact that the change in the optical properties with temperature can have in the performance and reliability of these structures. To gain insight into the role that temperature and geometry play in the optical performance of these devices, one must utilize the basic optical relations in a way that is compatible with thermal analyses of a laser-heated structure. The LTR method has proven to be a very useful technique in these types of analyses since it can easily incorporate temperature dependant optical properties and readily provide the interlayer absorptances for the irradiated structures.

Once the temperature and optical fields are coupled in the analysis, a more accurate picture emerges of the thermal and optical behavior of the irradiated device. These coupled optical-thermal effects give rise to non-linear absorptance that can, in some instances, lead to increased resistance to laser damage by dynamically reducing the absorptance as the incident laser power is increased, while in other cases, the nonlinear effects compound to enhance absorptance of the incident laser energy producing rapid temperature increases that eventually lead to device damage. A quantitative estimation of device robustness to determine in which regime of damage susceptibility a particular structure resides in therefore requires a complete description of the overall irradiating conditions as well as the device composition. For polysilicon-based devices, this type of analysis has shown reasonable agreement with the experimentally-observed thermal behavior, and can explain the observed damage trends of the laser-irradiated structures.

### **6. Acknowledgment**

The authors would like to acknowledge the help and assistance of Allen Gorby, James Rogers, Wayne Trott, and Jaime Castaneda. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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**Part 4** 

**MEMS Based Actuators** 

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**Part 4** 

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**14** 

*Portugal* 

J. Pérez de la Cruz

**Piezoelectric Thick Films:** 

**Preparation and Characterization** 

*INESC Porto - Institute for Systems and Computer Engineering of Porto, Porto,* 

Sol-gel technology allows the deposition of 1m thick oxide films onto a variety of substrates at temperatures well below those conventionally used for bulk ceramic processing. Thin film processing temperatures as low as 500-700oC are typically used allowing ceramic materials to be incorporated into the silicon processing stages (Ohno et al., 2000; Wu et al., 1999). In addition, low processing temperatures reduce the inter-diffusion of atomic species between the different thin film layers and ionic vaporization, such as: lead in lead zirconate titanate oxide (PZT) films (Wu et al., 1999; Jeon et al., 2000). PZT thin films have been largely deposited in order to produce several types of devices, such as: membrane sensors, accelerometers and micromotors (Barrow et al., 1997). However, devices requiring larger actuation forces (i.e. high frequency transducers, vibration control devices) require thicker piezoelectric films (Tsurumi et al., 2000). In these cases, it is not practical to produce thick PZT films using standard sol-gel techniques, because of the increased cracking risk due to shrinkage nor is it desirable to produce thick films by a repetitive single layer

deposition process due to the time required (Barrow et al., 1995; Zhou et al., 2000).

thickness range that is difficult to achieve by other deposition techniques.

The interest in ferroelectric lead zirconate titanate thick films for device applications, including high-frequency ferroelectric sonar transducers (Bernstein et al., 1997), microelectromechanical system devices (Polla & Schiller, 1995; Myers et al., 2003; (Akasheh et al., 2004), elastic surface wave devices (Cicco et al., 1996), hydrophones (Chan et al., 1999) and sensors (Xia et al., 2001), has increased in the last decades because PZT ferroelectric thick films possess the merits of both bulk and thin film materials (Barrow et al., 1997; Ledermann et al., 2003). PZT thick films devices not only work at low voltage and high frequency, as they are compatible with semiconductor integrated circuit, but also possess superior electric properties approaching near-bulk values. Naturally, processing of PZT thick films has also become an increasingly popular research field. Some approaches that have recently been studied to process thick films include electrophoretic deposition (Corni et al., 2008), pulsed laser deposition (Yang et al., 2003), screen printing (Walter et al., 2002) and sol-gel (Xia et al., 2001; Wang et al., 2003; He et al., 2003). Among them, the hybrid sol-gel technique has a special interest due to its low preparation cost and excellent stoichiometric control. Sol-gel also offers the capability to lay down thick layers anywhere between 0.1m and 100m, a

**1. Introduction** 
