Shape Memory Capacitors for Next Generation Embedded Actives (2006)

Integral discrete passives occupy nearly 45% of the board real estate. By embedding passives we have the following advantages: improving packaging efficiency & electrical performance, reducing the use of printed wiring board real estate, eliminating assembly to board, minimizing solder joint failure and enhanced reliability. Embedded systems are getting more attention in digital area applied to noise reduction and system performance vs cost enhancement while maintaining EMI suppression. Decoupling capacitors play an important role in high speed system due to the inductance associated with the lead free interconnects. Decoupling can be achieved using embedded capacitance materials that make use of the capacitance of closely spaced power and ground planes. The approach improves electrical performance, frees surface real estate, and eliminates solder connections, which can lead to improved reliability. RF circuits embedded capacitors play a critical role in determining the resonant frequency. Drift in resonant frequency results in distortion in signal quality. In order to over come this frequency drift due to temperature and other environmental conditions, we propose a novel embedded shape memory capacitor (SMC) which can vary with applied stimulus, and subsequently the RF circuit can be tuned. In this paper, we characterize the performance of a novel embedded shape memory capacitor fabricated using shape memory polymers dielectric material, with one metal electrode fabricated on a low strength back plane to allow the electrode to flex . The shape memory dielectric contracts and expands when heated. A step by step process is adopted to transfer the shape memory dielectric to the electrode using lithography technology. A variable frequency DC current is applied to platinum electrode fabricated above the top capacitor plate. The DC voltage not only provides a resonance current, it also allows the platinum electrode to heat the capacitor. The heat deforms the shape memory dielectric thereby changing the capacitance. This variable capacitor is tuned by changing the current and applied frequency. The compression effect on the shape memory also causes a thickness variation there by making the already submicron thick dielectric vary in thickness. The thinner dielectric makes it suitable for high frequency capacitance application. By changing the thickness of the dielectric and varying the frequency, the capacitor becomes very suitable to work over a broad range of RF frequencies and capacitances. In this paper we model, characterize the shape memory dielectric, fabricate and test the novel shape memory capacitor. The data collected from a test bed used to implement this novel capacitor for RF frequency tuning. This characteristics including material components and construction, dielectric properties and topography at metal-dielectric interfaces are optimized for a wide frequency band width. Attention is given to frequency dependence of the different material properties. Effects of the material characteristics on electrical performance, including capacitance and power and ground plane impedance, are examined. Finally, processing and fabrication issues are discussed. A detail description of how it is implemented in RF circuits was presented