Stefan Stegmeier

Optimization of the work function response of CO 2 -sensing Polysiloxane layers by modification of the polymerization

Gas sensors based on the work function read out of (Hetero-) Polysiloxane sensing layers can be used for the detection of CO2. To investigate the reaction with CO2, different polymers based on modified Polysiloxanes are examined. It is shown that the sensitive layers can be systematically modified to evoke a preferred and specified chemical reaction with CO2. This may open up the possibility for new ambient-temperature-CO2-sensors, for which a fast response time, a high long-term stability as well as a high sensitivity between 400 ppm CO2 (background in atmosphere) and 4000 ppm CO2 (bad air) are characteristic.

FE analyses and power cycling tests on the thermo-mechanical performance of silver sintered power semiconductors with different interconnection technologies

The paper reports on thermo-mechanical performance analyses of power semiconductors. Realistic transient temperature loadings as well as mechanical stresses were simulated by fully coupled electro-thermal-mechanical finite element analyses for power cycling loads. Power cycling tests were run in parallel to the theoretical investigations. The failure modes observed by testing were analyzed and adjusted to FE results. Some of the failures need a sophisticated evaluation strategy, as failure initiates at bi-material free edges, which obey a mechanical stress singularity. Damage mechanical modelling by means of the cohesive zone method (CZM) was adopted along with the coupled finite element analysis (FEA) in those cases. Applications of the methodology are presented for a SiC Mosfet testing sample operating at medium power and a high voltage inverter module with insulated gate bipolar transistors (IGBTs) and diodes, operating at high power. Both modules use silver sintering technology on directly bonded copper (DBC) substrates. Top interconnects are made by wire bonding for the Mosfet test sample but by an electroplating based planar technology for the inverter. Considering electro-thermal results it was calculated that stacks with planar copper interconnects outperform the wire bonded versions by 15–30% dependent on layout and current concerning thermal performance. For the die bonds, networks of cracks in the DCB copper and the silver layer replace the creep-ratchetting mechanism dominant for soft-soldered dies. This failure mode could be attributed to high cyclic in-plane normal stresses leading to subcritical crack growth at high power cycle numbers. The failure mode wire bond lift-off, characteristic for heavy Al wires, was investigated by CZM. The CZM methodology was also adopted to evaluate planar metallization delamination. For the latter, a parametric study has been made to optimize the materials choice and the layout of the metallization.

Electro-thermal-mechanical analyses on stress in silver sintered power modules with different copper interconnection technologies

Increasing demands for higher energy efficiency and operating at harsh environments lead to the development of new compact power electronics, which is complemented by new interconnection technologies. Investigations were made on a planar copper interconnection technology. The characteristic difference to other technologies can be seen in the replacement of bonding wires by planar copper interconnects and the high voltage applicability of the resulting modules. A high voltage and temperature resistant polymeric foil provides the insulation. Electrical connection is made by structured electrodeposited copper structures, which allow for additional heat spreading from top of the dies. Investigations on the thermo-mechanical behavior of prototype inverter modules, which use silver sintering and copper wire bonding technology or, alternatively, planar copper interconnection technology are reported. Fully coupled electrical-thermal-mechanical finite element (FE-) simulations were used to get realistic transient temperature loadings as well as mechanical stresses, also including wire heating or heating of the planar metallization, respectively. Improved thermal performance of the planar technology could be shown. A parametric FE-study was made to minimize delamination failure risks of planar structures based on cohesive zone modeling. Studies on processing dependent properties of the key materials sintered silver, electroplated copper, and dielectric foils are reported, which are indispensable for simulation input.

Stepwise improvement of room temperature VOC sensing layers by addition of catalysts on micro- and nanoscale

Polycrystalline Ga2O3/platinum-thick films can be used as a novel sensing layer for VOC detection in sensors based on the readout of the work function by GasFET devices. Room temperature operation can be used if an intermittent short thermal activation at 175°C is employed. This paper reports on how the stepwise addition of catalytic activity using platinum as catalyst incrementally improves the gas sensitivity of this material system towards VOCs. The best variants are capable to detect small concentrations even below the odor or medical exposure threshold.