Integration and Miniaturization Trends of MEMS Device High Voltage Driving Power Supply
Microelectromechanical systems devices increasingly require high voltage drive for actuation, sensing, and other functions. The integration of high voltage generation with MEMS devices and the miniaturization of the power supply components enable compact systems with enhanced capabilities. The trends toward integration and miniaturization drive innovation in power supply design, component technology, and system architecture.
MEMS devices use mechanical structures fabricated by semiconductor processing techniques to perform various functions. Actuators move structures for positioning, switching, or other mechanical functions. Sensors detect physical parameters through mechanical response. The devices often require high voltage for electrostatic actuation, piezoelectric drive, or capacitive sensing. Typical voltage requirements range from tens to hundreds of volts.
Electrostatic MEMS actuators use electric field forces to move structures. Parallel plate actuators have movable plates that are attracted by voltage applied to fixed plates. Comb drive actuators use interdigitated fingers that are attracted by voltage. The force depends on the voltage squared and the geometry. Higher voltages provide stronger forces, enabling larger displacements or higher forces from smaller devices.
Piezoelectric MEMS devices use piezoelectric materials that deform under electric field. The deformation provides actuation for positioning or vibration. The voltage required depends on the piezoelectric coefficient and the desired deformation. Thin film piezoelectric materials may require high voltage to achieve significant deformation due to the thin film thickness.
Traditional high voltage power supplies are discrete units separate from the MEMS devices. The separation requires interconnects between the power supply and the device, adding complexity and potential reliability issues. The separate power supply also occupies volume that may be significant in compact systems. Integration of the power supply with the MEMS device reduces these issues.
Integrated high voltage generation places the power supply components on the same substrate as the MEMS device or in the same package. The integration eliminates external interconnects and reduces the system size. The integration may use the same fabrication processes as the MEMS device, or may use separate processes with subsequent assembly.
On chip high voltage generation uses circuit elements fabricated on the MEMS substrate. Charge pump circuits use capacitors and switches to step up the voltage from a lower supply voltage. The capacitors can be fabricated as integrated capacitors using dielectric layers. The switches can be fabricated as transistors using integrated circuit processes. The on chip generation provides high voltage without external components.
Charge pump efficiency depends on the capacitor values, the switching frequency, and the load current. Larger capacitors provide more charge transfer per cycle, improving efficiency. Higher frequencies enable faster voltage buildup but may increase switching losses. The load current drains the pumped voltage, requiring continuous pumping to maintain the voltage. The charge pump design must meet the MEMS requirements for voltage, current, and response time.
Dielectric integration uses high voltage capacitors fabricated with thick dielectric layers to withstand the high voltage. The dielectric thickness must be sufficient to prevent breakdown at the operating voltage. The dielectric material must have adequate permittivity to provide the required capacitance in the available area. Specialized dielectric materials and processes enable integrated high voltage capacitors.
Packaging integration places the high voltage components in the same package as the MEMS device. The package provides the interconnections between components and the interface to external circuits. The package must withstand the high voltage, requiring appropriate insulation and spacing. Advanced packaging technologies including system in package and chip scale packaging enable compact integration.
Miniaturization of high voltage components reduces the size of the power supply elements. Smaller capacitors occupy less area while providing the required capacitance and voltage rating. Smaller inductors or transformers, if needed, occupy less volume. Miniaturization enables power supplies that fit within the size constraints of MEMS systems.
Component miniaturization technologies include thick film capacitors, thin film inductors, and integrated transformers. Thick film capacitors use printed dielectric layers to build up capacitance and voltage capability. Thin film inductors use patterned metal layers to create planar coil structures. Integrated transformers use coupled planar coils for voltage transformation. These technologies enable components that are compatible with MEMS fabrication and packaging.
Design challenges for integrated and miniaturized power supplies include thermal management, reliability, and high voltage isolation. The small size concentrates heat generation, requiring effective thermal paths. The integrated components must have reliability compatible with MEMS devices. The high voltage must be isolated from low voltage circuits and external connections. The design must address these challenges within the size and integration constraints.
Application drivers for integration and miniaturization include portable devices, implantable medical devices, and distributed sensor systems. Portable devices require compact power supplies to fit within the device envelope. Implantable medical devices require small, reliable power supplies for safe operation. Distributed sensor systems may require many small power supplies for individual sensors. The application requirements drive the trends toward integration and miniaturization.
