To miniaturize piezoresistive barometric pressure sensors we have developed a package using flip-chip bonding.
However, in a standard flip-chip package the different coefficients of thermal expansion (CTE) of chip and substrate and
strong mechanical coupling by the solder bumps would lead to stress in the sensor chip which is not acceptable for
piezoresistive pressure sensors. To overcome this problem we have developed a new ultra low stress flip-chip packaging
technology. In this new packaging technology for pressure sensors first an under bump metallization (UBM) is patterned
on the sensor wafer. As the next step solder bumps are deposited. After wafer-dicing the chips are flip-chip bonded on
copper springs within a ceramic cavity.
As sources of residual stress we identified the copper springs, the UBM and the solder bumps on the sensor chip.
Different CTEs of the silicon chip and the UBM/solder lead to creep strain in the aluminum metallization between UBM
and chip. As a consequence a temperature hysteresis can be measured.
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.
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.
Deep X-ray lithography with synchrotron radiation (DXRL) represents the technological core of the LIGA technique, which is a modem microfabrication technology facilitating the high volume production of micro products from a huge variety of materials. Since several applications make use of the high structure accuracy obtained in the primary lithography process, the demands of a detailed investigation of structure accuracy limiting aspects came to rise. Therefore, thorough theoretical and experimental research has been undertaken in order to understand the different radiation effects influencing the shape of the side walls and the lateral resolution to be obtained. Physical effects like diffraction, divergence of the synchrotron radiation beam, photo and Auger electrons, fluorescence and scattering have been calculated and are condensed in a computer code. The calculation results are discussed in detail with respect to
LIGA mask production by x-ray lithography as well as for deep x-ray lithography applications. The model can be partially extended to new irradiation techniques like tilted and rotated exposures. Different
absorber gradients due to various tilt angles have to be taken into account and the resulting dose contour lines resulting from inclined irradiations are compared with experimental data. In order to enhance the normal shadow printing process and to realize shaping in the third dimension, previous studies used the aligned multiple exposure technique realizing step like structures. We will discuss a novel approach using 500 m thick Beryllium mask blanks with free standing absorber structures (gold) on open windows for alignment purposes. First results show an overlay accuracy of about 0.4 tm using an internal alignment system installed in a DEX 2 JENOPTIK exposure apparatus.
Deep x-ray lithography (DXRL) makes use of synchrotron radiation (SR) to transfer an absorber pattern from a mask into a thick resist layer. For most applications the direction of the SR beam is perpendicular to the mask and the resist plane. Subsequent replication techniques, e.g. electroforming, moulding or hot embossing, convert the resist relief obtained after development into micromechanical, microfluidic or micro- optical elements made from metals, polymers or ceramic materials. This process sequence is well known as the LIGA technique. The normal shadow printing process is complemented and enhanced by advanced techniques, e.g. by tilting the mask and the resist with respect to the SR beam or aligned multiple exposures to produce step-like structures. In this paper a technology for the fabrication of multidirectional inclined microstructures applying multiple tilted DXRL will be presented. Instead of one exposure with the mask/substrate assembly perpendicular to the SR beam, irradiation is performed several times applying tilt and rotational angles of the mask/substrate assembly relative to the SR beam. A huge variety of 3-D structures can be obtained using this technique. Some possible applications will be discussed.