Gregor Feiertag

Packaging of MEMS Microphones

To miniaturize MEMS microphones we have developed a microphone package using flip chip technology instead of chip and wire bonding. In this new packaging technology MEMS and ASIC are flip chip bonded on a ceramic substrate. The package is sealed by a laminated polymer foil and by a metal layer. The sound port is on the bottom side in the ceramic substrate. In this paper the packaging technology is explained in detail and results of electro-acoustic characterization and reliability testing are presented. We will also explain the way which has led us from the packaging of Surface Acoustic Wave (SAW) components to the packaging of MEMS microphones.

Chip scale package of a MEMS microphone and ASIC stack

Most MEMS microphone systems on the market are packaged by conventional chip bonding and wire bonding.. A significant step towards miniaturization was achieved earlier by applying flip-chip bonding to MEMS microphone packaging. This technology is called chip scale MEMS package (CSMP). Thereby the package size could be reduced to 2.8 × 2.05 × 0.9 mm³ compared to a standard size of 3.76 × 2.95 × 1.1 mm³. In this paper the next step in miniaturization is presented. A further reduction to a total size of only 2.05 × 2.05 × 0.95 mm³ was achieved by placing the ASIC directly under the microphone chip inside a cavity in the substrate. This design is called stacked CSMP.

A method for improving the drop test performance of a MEMS microphone

Most micro electro mechanical system (MEMS) microphones are designed as capacitive microphones where a thin conductive membrane is located in front of a rigid counter electrode. The membrane is exposed to the environment to convert sound into vibrations of the membrane. The movement of the membrane causes a change in the capacitance between the membrane and the counter electrode. The resonance frequency of the membrane is designed to occur above the acoustic spectrum to achieve a linear frequency response. To obtain a good sensitivity the thickness of the membrane must be as small as possible, typically below 0.5 μm. These fragile membranes may be damaged by rapid pressure changes. For cell phones, drop tests are among the most relevant reliability tests. The extremely high acceleration during the drop impact leads to fast pressure changes in the microphone which could result in a rupture of the membrane. To overcome this problem a stable protection layer can be placed at a small distance to the membrane. The protective layer has small holes to form a low pass filter for air pressure. The low pass filter reduces pressure changes at high frequencies so that damage to the membrane by excitation in resonance will be prevented.