Research/MEMS Power Gate Switch for Low Power Electronics

2019-05-03 (金) 06:39:15 (467d)

Integrated MEMS Switch for Low Power Electronics


We propose a new MEMS (Micro Electro Mechanical systems) switch for power gating the LSI processors in order to eliminate the standby leakage and to realize low power operation. Technology scaling has led to higher performance and integration density. However, the sub-threshold leakage increases exponentially as the transistor threshold voltages have been scaled down. Suppression of the leak current is inevitable for low-power mobile electronics. Power-gating switch is a commonly used circuit technique to reduce the leak power, by cutting off the unused part of electronic blocks. Conventional power gating switches use solid-state transistors between the circuit block and the ground network. Transistor switches are simple but have fundamental limitations in their effectiveness. At this point, MEMS switches are expected to eliminate the leak current thanks to the high isolation at the off-state and low resistance at the on-state. We pursue an integrated MEMS approach to build-in the micromechanical power gating switch onto a pre-fabricated CMOS circuitry.




The number of transistors are integrated into a semiconductors chip doubles every two years according to Moore’s Law, LSI has achieved rapid development, now is a lot of transistors integrated on a single circuit. Thanks to the development, portable systems like cell-phones have become extremely popular. However, a system spends most of the time in its standby mode. In a recent article it has been estimated that it spends almost 500 hour in the standby mode and 15 hour in talk time. As a result, the leakage power in the standby mode is especially important issue.



Common technique for reducing the leakage current is power gating, where the power source to the circuit is shut off by means of a switch. The most common switch is a transistor. Transistor switches are easy to implement and can yield a large leakage reduction, but the transistors themselves have leak current, and the switches have an effective electrical resistance. MEMS switches have good characteristics for ideal power gating switches. The switches have near perfect isolation when off, and almost no resistance when on. In this paper, we propose a MEMS switch for power gating in order to eliminate the leakage current.

Switch Structure



Impedance matching has been the limitation in designing the conventional RF-MEMS switches, and hence it usually lead to large drive voltage to make sufficient contact pressure. In our power gating switch, on the other hand, such limitation associated to the RF design does not exist, as it is supposed to handle DC-level voltages. It is, therefore, possible to allocate relatively large footprint for a switch to lower the drive voltage as low as 3.3V.

Fabrication Process



We have developed a CMOS-compatible low-temperature process to post-process the power gating switch onto an LSI circuit wafer. An electroplating method has been adopted to construct micromechanical structure of plated gold, while using plated copper as a sacrificial layer.

Pull-in Voltage Analysis



Three-dimensional FEM analysis has discovered that a use of a thin plastic membrane (1 micron) as a suspension would lower the electrostatic operation voltage for the micro bridge structure to be as low as 3.3V.

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