Durability MEMS

Aluminum possesses good electrical and thermal conductivity as well as its durability and availability makes it a preferable material for various microelectronics and MEMS applications. EMM of patterned A1 and Al-based alloys can be utilized in a wide range of applications, which includes energy storage devices microanalytical systems and microfluidic, optical, and microelectronic devices. EMM is economical and scalable to large geometric areas platform for microstructuring of A1 substrates. 

Cardiovascular disease, namely, coronary artery disease, remains the leading cause of death in the developed nations. Over the last few years, MEMS sensors have advanced the understanding of blood flow, namely, fluid shear stress, in arterial circulation. Fluid shear stress is defined as the frictional force acting tangentially on the surface of a blood vessel wall. Furthermore, the measurement of wall shear stress is important to study the durability of prosthetic valves and to monitor thrombosis or blood clots in cardiopulmonary bypass machines, artificial hearts, and left ventricular assist devices. Luminal shear stress measurement predicts the development of atherosclerotic plaque in patients at risk for acute heart attacks. In this context, the application of microscale hot-wire anemometry bridges fluid mechanics of blood flow with vascular biology. 

A system for application in a cell phone also based on MEMS technology was designed by Samsung [40] (Figure 23.4). Aspects such as the start-up time for the reaction temperature and the hydrogen flow, catalyst durability and volume of the total system were considered. The catalyst was introduced as small particles (50-100 tm) and a uniform packing was obtained. Maximum power output was 2.4 W, sufficient for a cell phone. 

All of the studies discussed above have shown that some silane SAMs are efficient in reducing the coefficient of friction, the work of adhesion, and stiction properties however, their wear resistance is not sufficient to provide high durability to the MEMS components [42]. One possible reason for the low wear durability of SAMs is the lack of a mobile portion in the lubricant. Hence, there is no replenishment in these layers as molecules are continuously removed from the contact area during the wear process. Moreover, the worn particles generated as a result of material removal act as a third body and further accelerate the wear of the film. Therefore, we proposed a lubrication concept of overcoating SAMs (bonded) with an ultrathin layer of per-fiuoropolyether (PFPE) (bound + mobile) to improve the wear durability of SAMs and hence that of the Si substrate (fig. 6.1) [43, 44]. The mobile PFPE is expected to lubricate and replenish the worn regions and hence enhance the wear durability. 

The present concept of overcoating SAMs with PFPE to improve the wear durability of SAMs of different functional groups has not been tried previously from the view of MEMS lubrication. The concept of overcoating PFPE onto epoxy nanocomposite bilayers with the purpose of improving the wear durability has been proposed by Julthong-piput [45] however, the results are not available. A similar concept of overcoating a mobile hydrocarbon-based lubricant onto hydrocarbon layers chemically bound onto... 

Si-based MEMS components has been used by Eapen et al. [46] to enhance the wear durability. We do not compare our results with those of Eapen et al. [46], as the top mobile layer and the SAMs they used were quite different than those used in our study. It is worth noting that both studies were carried out independently during the same period of time. In a recent study, Ahn et al. [47] have incorporated mobile paraffin oil molecules into polymer nanolayers on Si surfaces, which have shown high wear resistance because of self-repairability due to the migration of mobile molecules into the sliding contact. Once again, we do not compare the results of the present lubrication concept used in this chapter with those of Ahn et al. [47], as the mobile molecules used and the procedure of incorporating them into the ultrathin layers are very different. 

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