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A multi-pronged approach to low-pressure Cu sintering using surface-modified particles, substrate and chip metallizations
Keywords: self-reducing, Cu particle sintering, surface modifications
Die-attach bonding is a key process to realize high-temperature operation of power semiconductor devices. With the increase in the usage of wideband gap (WBG) seminconductor devices, it is imperetive to find sustainable and reliable alternatives both on the economical and the technical fronts, able to fulfill the challenging requirements of the WBG semiconductor devices and to realize mass production. Our research focuses on developing Cu sintering as an alternnative to the widely reported Ag sintering. Cu is ~100 times cheaper than Ag and also more abundant and easily available. It has a lower coefficient of thermal expansion than Ag, a higher elastic and bulk modulus and nearly the same electrical and thermal conductivities as Ag. However, the higher melting point of Cu means that, the sintering temperatures are also slightly higher than in case of Ag. Application of high bonding pressure during the die-bonding process can lead to device cracking in semiconductor dies. One of the main reasons for the application of a bonding pressure is to achieve a closed packing structure in the joint. A solution widely reported to circumvent the use of high bonding pressure is to use hybrid pastes which are a combination of micro and nano particles [1]. Apart from the direct introduction of nanoparticles, we propose nanostructured surface modifications on micro particles. These nanostructured surface modifications serve two purposes namely; to increase surface contact between the particles and to increase surface energy due to the increased curvatures. Both these factors are well known to enhance sinterability. The affinity to fast oxidation under atmosphere is a major drawback for sintering of Cu particles as this is detrimental to the sinterability between the particles resulting in poor mechanical (shear strength) integrity and thermal performance of the joint [2][3][4]. We developed a hybrid Cu paste with combination of surface-modified micro particles, nano particles and poly-ethylene-glycol (PEG) as a reducing binder and performed sintering experiments under N2 atmopshere at 300C under the application of a bonding force. Analysis of the fracture surfaces after sintering, show that PEG has been successful in reducing the oxidized Cu particles and also prevent oxidation during the sintering process while sintering under N2 atmosphere as reported [5]. Initial observations show the formation of a protective outer layer around the joint and investigations are ongoing to study the impact of PEG in the formation of this layer and if this layer is able to protect the joint even when exposed to a high temperature environment (125C) for an extended period of time (~1000h). Substrate and chip metallizations are observed to play an important role while sintering. Three different types of surfaces have been investigated as part of our Cu sintering with the hybrid Cu paste, namely; Cu, Au and surface-modified substrates and chips. Preliminary shear tests on samples with surface modified substrate and chip are promising with results of ~21 MPa, which is greater by a factor 1,75 compared to sintering with Cu-Cu bonding surfaces and factor 1,5 compared to sintering with Au-Au bonding surfaces with the same Cu paste under sintering conditions of low bonding pressure (1MPa) at 300C for 60min under N2 atmosphere. With the aim to achieve a low-pressure Cu sintering process, we propose a multi-pronged approach namely; surface-modified substrates and chips, nanostructured surface modifications on micro-scale Cu particles and the use of a reducing binder in the Cu paste. The experiments are designed so as to ascertain the role of each of the above mentioned novelties towards realizing a good sintered joint. Sintering is conducted under the application of a low bonding pressure (1MPa) at 300C for 60min under N2 atmosphere. The study of the impact of sintering parameters (time, temperature, bonding pressure) are ongoing. First results show an improvement in shear strength by a factor 2 when sintering time is increased from 30min to 60min while keeping the bonding pressure and the temperature constant. With the bonding pressure and sintering time remaining constant, an increase in the bonding temperature from 275C to 300C shows an improvement in the shear strength by a factor 1,4. With sintering time (20min) and sintering temperature (300C) remaining constant, an increase in the bonding pressure from 0,1MPa to 10MPa shows a shear strength improvement by a factor 2. Cross-sectioning of the samples for microstructural analysis of the sintered joint for a deeper understanding of the sintering behaviour with varying sintering parameters is ongoing. In our research, we propose a multi-dimensional approach to low-pressure Cu sintering with hybrid Cu pastes, which are capable of sintering at 300C under the application of a bonding pressure on surface-modified substrates and chips. Temperature cycle tests to validate the quality of the interconnect under thermo-mechanical stress are ongoing and shall be presented during the submission of the full paper.
Sri Krishna Bogaraju, Research Associate
Institute of Innovative Mobility - Technische Hochschule Ingolstadt
Ingolstadt, Bavaria

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