An Introduction to mems (Micro-electromechanical Systems)
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| An Introduction to MEMS
Prime Faraday Technology Watch – January 2002 26
Despite certain successful high volume applications such as the airbag accelerometer and the disposable blood pressure sensor, high yields are difficult with MEMS devices due to their mechanical complexity and their integration with the necessary microelectronics. Assembling and packaging complex microscopic parts is also extremely difficult. As conventional automated assembly and packaging is not suited to such a microlevel, to date, many MEMS devices require individual handling. As a result, the final cost of a device may be up to 100 times the cost of the actual component. For their successful commercial production these challenges have to be overcome.
Over the years different approaches have been developed for the integration of the electronic interface. These include hybrid integration using conventional wire bonding and flip-chips (described later in Section 3.8) and monolithic integration. Monolithic integration offers superior system integration performance to hybrid systems but at an overall higher price in terms of involved technology and processing. Monolithic integration can be carried out in three ways:
i) IC before MEMS Monolithic integration by IC first has proved to be successful and relatively cheap; an example is the technology in Texas Instrument’s DMD (Figure 10). The process relies heavily on bulk micromachining and the addition of new layers through electroplating. It is a relatively simple integrated system but suffers from residual stresses within the device materials. To date, refractory metals need to be used within the IC components in order to withstand the high temperature annealing cycles required to relieve the stress in structural polysilicon.
ii) Mixed MEMS-IC fabrication A typical example of MEMS and microelectronics being fabricated side by side is the airbag accelerometer (Figures 4 and 5). Monolithic processing of this device as well as the reduced number of parts enable a very compact device with high reliability at a very low cost. The trade-off lies within its complexity as this process leads to a very rigid and constrained process flow which is expensive, thus requiring very high volumes.
iii) MEMS fabricated prior to IC The most promising monolithic integration technique includes fabricating the MEMS device prior to the microelectronics. Using technology known as iMEMS (Integrated Micro- electromechanical Systems) patented by Sandia National Laboratories, USA, MEMS components are fabricated in trenches on a silicon substrate and then the standard electronics are processed onto the same substrate as shown in Figure 27 [2].
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