Guy Lefranc

Improvements in electron microscopy by application of superconductivity

Resolution tests on amorphous carbon foils were carried out in an electron microscope with a superconducting system containing 4 lenses including a shielding lens at 200 kV beam voltage. Due to the mechanical and electrical stability of the system and the absence of contamination of the specimen the highest space frequencies transferred at vertically incident beam were 6 nm-1 corresponding to a resolution of 0.17 nm, a value which approaches the theoretical resolving power of the electron optical system. It should also be feasible to apply such a lens system for microprobe analysis without strongly reducing the theoretical resolution limit, if the construction of the shielding lens is slightly changed.

Superconducting lens design

Abstract A survey of various types of superconducting lenses is given. Cryogenic problems, particularly material problems, are discussed which must be solved in order to construct lens systems applicable for electron microscopy. The results obtained up to now with the few superconducting lens systems in operation prove a better resolution than is achievable with conventional lenses. The specimen has a temperature of approximately 4 K since it is completely surrounded by walls directly cooled by liquid helium, and the stability of the superconducting lens system permits long exposure times of the micrographs which make it possible to minimize the heating of the object by the beam. For this reason due to cryoprotection one can take advantage of a drastic reduction of radiation damage by the beam.

Localisation of electrical-insulation- and partial-discharge failures of IGBT modules

The partial discharge (PD) and insulation resistance is very important in view of the increasing operating voltages of IGBT modules. Partial discharge spectroscopy showed that the PDs from metallisation edges and interfaces in silicone gel were the main sources of PD at high voltages. It also allows these types of PD to be clearly distinguished. As the PDs from interfaces in silicone gel increase strongly at high voltages, it is especially important for the silicone gel to adhere well to the ceramic.

Reliability of AlN substrates and their solder joints in IGBT power modules

Abstract The reliability of IGBT modules was investigated with respect to the metallized ceramic (substrate) and the solder layer between the substrate and copper baseplate. Thermal cycles were performed between −55°C and +150°C on substrates based on different technologies and from various manufacturers. An incipient delamination of the metallization could be predicted from the mechanical resonance frequency. The warping of the substrates after cycling due to crack propagation and the adhesion of the metallization were determined. Thermal and active-power cycles were performed on 1200 A / 3.3 kV IGBT power modules to investigate the reliability of the solder joint between substrate and baseplate.

IGBT module technology with high partial discharge resistance

The high operating voltages of 6.5 kV IGBT modules place additional demands on the insulation and partial-discharge resistance. The most important component affected here is the metallized aluminum nitride ceramic which is embedded in a silicone gel. A high electric field strength can cause a local electric discharge in the silicone gel known as partial discharge, leading ultimately to electric insulation failure and reducing the reliability of the IGBT module. For a 6.5 kV IGBT module, the insulation test must be performed up to a voltage of 10.5 kV rms. Technological steps have been carried out to reduce the maximum electric field strength along the edge of the copper metallization. The edge of the ceramic was coated with a high-impedance layer of doped amorphous silicon. The electric current along the edge of the ceramic homogenizes the electric field strength. The partial discharge was determined up to 11 kV and a considerable reduction was observed compared to standard modules. Without an a-Si:H coating, the partial discharge already increases strongly at low voltages of 3-4 kV. At high voltages, the interface between the silicone gel and the substrate is a major source of partial discharge. The a-Si:H coating reduces electric field peaks and the partial discharge does not exceed 10 pC up to a voltage of 10 kV.

Reliability testing of high-power multi-chip IGBT modules

Abstract Power-cycling tests are among the most important tools used for evaluating the reliability of power modules. They are in most cases carried out at the rated module current and during a relatively short cycle time, i.e. under worst-case operating conditions. Test conditions must be defined which also permit information to be obtained about failure mechanisms in the various parts of the module. This paper describes the measurement of the temperature distribution, the test conditions, the rates of temperature change in modules with 36 semiconductor components as well as the results of power-cycling tests in which the thermomechanical stress principally affects the substrate-baseplate interface.

Binary and multilevel diffractive lenses with submicrometer feature sizes

Binary and multilevel diffractive lenses with submicrometer feature sizes are realized on silicon and galliumphosphide wafers using CAD methods, direct write e-beam lithography, reactive ion etching, antireflection coating, and wafer dicing. Measurements prove aberration-free imaging and maximum diftraction efficiencies of 70% for lenses with high numerical aperture (NA) of 0.5. Comparisons to other miniature and micro-optical elements are carried out with respect to laser-to-single-mode-fiber coupling and show the competitive performance of the diffractive lenses. Arrays for 18-channel parallel receiver modules are fabricated in on-axis and off-axis versions. Optical crosstalk is estimated.

Electron microscopical results on cryoprotection of organic materials obtained with cold stages

Abstract Electron microscopy work on radiation damage performed with cold stages as described in the literature is reported. The gain in tolerable exposure dose due to cooling is of the order of five. There are indications that this gain will increase with decreasing temperature. The advantages of cold stages are discussed.

The interpretation of radiation damage measurements with electron diffraction of organic materials at very low temperatures

Abstract At very low object temperatures organic specimens suffer not only radiation damage but, to a higher extent, charging and heating during electron irradiation due to very low electrical and thermal conductivities of organic materials. The fading of electron diffraction patterns is seriously influenced by the superposition of both effects. For this reason an exact determination of the progress in radiation damage is nearly impossible by direct observations of the diffraction patterns, particularly for obtaining the cryoprotection factors of organics. Charging and subsequently heating cause intrinsic motions of the irradiated areas which also seem to lead to a destruction of the crystallinity. With a suitable preparation of organic specimens, charging and heating effects at very low temperatures could be avoided, and it could be demonstrated that the true cryoprotection was relatively good.

AlSiC composite materials in IGBT power modules

The reliability of IGBT modules is limited by thermal fatigue of soft solder layers due to different coefficients of thermal expansion. A thermally matching conducting material can be produced from Al matrix composites containing high volume fractions of SiC particulates. Variously processed prototypes of AlSiC baseplates were investigated with a view to their suitability for IGBT power modules. The volume fraction of SiC particulates as well as their thermal conductivity, heat capacity, thermal expansion and mechanical properties were determined. The thermal conductivity of AlSiC increases with the SiC content to reach more than 200 W/mK, which exceeds that of the matrix alloy of about 180 W/mK. The remarkable plastic elongation of the material during initial heating indicates that it originally contained internal stresses which relax during heating by the plastification of the matrix. The mechanical and physical properties of the tested AlSiC materials are appropriate for applications in high-power IGBT modules. Annealing treatment is recommended for AlSiC to reduce the internal stresses.