Flip-chip packaging for smart MEMS

The cointegration of IC microsensors, actuators and readout circuit leads to smart Micro Electro Mechanical Systems (MEMS) which are superior in many aspects to their conventional discrete counterparts. However, the packaging of such devices is still a challenge and a major factor of the overall production cost. On one hand MEMS need protection against mechanical contact and media. On the other hand, the encapsulation of the transducer must be partially permeable to the environment. We developed a packaging method which successfully addresses these challenges. Thereby the number of steps needed to electrically contact and partially seal the MEMS are reduced by combining them using flip-chip technology. An opening in the substrate is aligned with the transducer, and enables the interaction with external media. Concurrently with the electrical connections, a frame plated onto the microsystem is soldered to a corresponding structure on the substrate. This frame seals the rest of the chip from the medium interacting with the transducer. Using passive test chips we evaluate the performance of the new packaging method. Various underbump metal and solder deposition techniques were investigated. Both ceramic and flexible organic substrate materials were used. The combination Ni/Au bumps/InPb40 solder/ceramic substrate showed the following mechanical and electrical parameters: For 98 % of the tested chips, the helium leakage rate of the sealing frame surrounding the sensor is below the threshold of the used mass spectrometer (5x10(-7) Pa I s(-1)). For flip-chip pads ranging from 200 to 300 mm square, the bump resistances are smaller than 2 m Omega. The approach, is illustrated with three successfully packaged MEMS for the measurement of humidity, gas flow, and volatile organic compounds, respectively. They all contain integrated readout circuitry providing digital output.