Mads Brincker

Effects of thermal cycling on aluminum metallization of power diodes

Abstract Reconstruction of aluminum metallization on top of power electronic chips is a well-known wear out phenomenon under power cycling conditions. However, the origins of reconstruction are still under discussion. In the current study, a method for carrying out passive thermal cycling of power diodes in a controlled environment is developed, thus eliminating possible contribution to degradation from electric current and humidity. The focus is centered on the structural changes in the top Al metallization layer of the power diodes, correlated with the change of sheet resistance. Since the atmosphere is controlled and the device is not subjected to a current load the observed degradation of metallization and corresponding increase of resistance is purely induced by thermo-mechanical stress. A correlation between number of cycles, micro-structural evolution, and sheet resistance is found and conclusions on the dominant role of thermo-mechanical stresses are achieved. Additionally, proposals are made on how the current thermal test setup can be further developed to study the role of corrosion.

Strength and reliability of low temperature transient liquid phase bonded Cu-Sn-Cu interconnects

Abstract As power electronic devices have tendencies to operate at higher temperatures and current densities, the demand for reliable and efficient packaging technologies are ever increasing. This paper reports the studies on application of transient liquid phase (TLP) bonding of Cu Sn Cu systems as a potential technology that could enable the realization of stacks with better thermal performance and reliability than those can be achieved using conventional soldering techniques. Low temperature TLP bonded Cu Sn Cu samples are fabricated, and the strength of the achieved bonds is measured by shear testing. Micro-sectioning and optical microscopy studies of the samples reveal that the TLP bonds show good homogeneity with a small number of voids at the interface. Energy dispersive X-ray analysis is applied to examine at what rates Sn is converted into Cu Sn intermetallics since a full conversion is critical for achieving a strong and high temperature resistant bond. Finally, initial results from a thermal cycling test are presented and it is concluded that the achieved TLP bonding is a promising candidate for the fabrication of reliable interconnects in power electronics.

Mechanisms of metallization degradation in high power diodes

Abstract Under operation the topside metallization of power electronic chips is commonly observed to degrade and thereby affecta devices electrical characteristics. However, the mechanisms of the degradation process and the role of environmental factors are not yet fully understood. In this work, we investigate the metallization degradation by passive thermal cycling of unpackaged high-power diode chips in different controlled atmospheres. The electrical degradation of the metallization is characterized by sheet resistance measurements, while the microstructural damage is investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). To study the evolution of the chemical composition of the metallization, energy dispersive X-ray spectroscopy (EDX) is also applied. Since the degradation depends on the initial microstructure of the metallization, the film texture and grain size distribution is determined using electron backscatter diffraction (EBSD). The obtained data show that the type of atmosphere plays a minor role in the degradation process, with a slight tendency that cycling in dry nitrogen atmosphere accelerates the degradation compared to the experiments in ambient atmosphere with a controlled relative humidity of 50 and 95%.

Microstructured gradient-index lenses for THz photoconductive antennas

A new type of substrate lens for photoconductive antennas (PCA’s) based on sub-wavelength microstructuring is presented and studied theoretically by the use of Greens function integral equation methods (GFIEM’s). By etching sub-wavelength trenches into a flat substrate, the effective dielectric constant can be designed to function like a gradient index (GRIN) lens. The proposed GRIN substrate lenses have sub-mm dimension, which is smaller than the dimensions of a typical hyper-hemispherical substrate lens (HSL), and could enable fabrication of arrays of closely packed PCA’s with individual lenses integrated directly into the PCA substrate. The performance of different GRIN lenses is compared to a HSL and shown to be comparable with regards to the terahertz radiation extraction efficiency, and it is shown that the collimating properties of these GRIN lenses can be tailored by changing the parameters used for microstructuring.

Low temperature transient liquid phase bonded Cu-Sn-Mo and Cu-Sn-Ag-Mo interconnects – A novel approach for hybrid metal baseplates

Abstract Transient liquid phase (TLP) bonding is one of novel techniques, which is an alternative to conventional soldering used in power electronics packaging. In this paper, we report on the formation and analysis of TLP bonded Cu-Sn-Mo and Cu-Sn-Ag-Mo systems for production of hybrid metal baseplates. The obtained bonds of intermetallic compounds are characterised by shear testing, optical microscopy and energy dispersive X-ray spectroscopy. Cu-Sn-Mo systems show low bond strength due to severe void and crack formation while Cu-Sn-Ag-Mo interfaces demonstrate high homogeneity and considerable shear strength. Thus, it is concluded that the low temperature TLP bonding using Cu-Sn-Ag-Mo can be a promising candidate for the fabrication of hybrid Cu-Mo baseplates. However, deeper studies of thermal reliability of these systems are required.