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On Chip Diffusion Bonding Creates Stable Interconnections Usable at Temperatures Over 300C
Keywords: Diffusion Bonding , Reliability, Interconnection Technology
In this paper, we present a conceptual design of an on-chip solder stack to connect silicon devices faster and more reliable. Almost all electric devices rely on solder layers to provide electrical. mechanical and thermal connections between components. We improve the solder connection at industry standard solder parameters of 300�C and some minutes of solder time. An ideal solder connection is composed of intermetallic phases (IMPs) at the interfaces between device and solder, and substrate and solder. Typically, a thin region of tin-based solder remains between the two IMP layers at the interfaces. IMPs of copper (Cu) and tin (Sn) are Cu6Sn5 and Cu3Sn. The development of IMPs is decisive for a good mechanical connection because of their higher melting point and mechanical stability. To cover these requirements we implement the solder stack as a transient liquid phase bonding (TLPB) system to realize interconnections. For this we use the diffusion of a high melting component in a second component that is liquid at solder process temperature. Ongoing diffusion leads to the formation of IMPs with a melting point above process temperature, resulting in a solidified connection at a constant temperature. By this isothermal solidification, the solder contact becomes more durable against mechanical and thermal load and can be used at high temperatures exceeding 300C. For the construction of the solder layer directly onto a silicon chip we aimed for an atomic ratio of three parts copper and one part tin to build the Cu3Sn phase. Required time of diffusion depends on soldering temperature and diffusion length, which is achieved through the layer thickness. The creation of extremely thin layers accelerates the fast formation of intermetallic phases in solder processes. By using this effect, a dramatic increase in the diffusion dynamic can be observed. We calculated the solder stack according to Ficks's law of diffusion using diffusion constants of copper and tin to deliberately form the phase Cu3Sn. For an application in power electronics we deposited chromium as an adhesion layer via magnetron sputtering on a silicon chip followed by the copper-tin solder stack. The next copper layer (500nm) transforms 311nm tin into Cu3Sn. Approximately half of the next 500nm copper layer diffuses into the lower tin layer and transforms the rest of the 465nm tin layer into Cu3Sn phase. Theoretically, the other half of the copper diffused into the upper tin layer, transforms half of it into Cu3Sn. This was repeated until the final layer thickness is reached. The rest of the subsequent tin layer wetted the DCB (direct copper bonded)-substrate and builds Cu3Sn with DCB copper. We sputtered the layers consecutively without intermediate exposure to oxidizing atmosphere with a deposition rate of 0,5nm/s for copper and tin with 2nm/s. Experimental results show a combination of columnar copper growth followed by a uniform growth of tin results in a layer structure. This multilayer enables a rapid soldering process. The phenomena we see here are very promising for a fast and reliable creation on IMPs for a connection. To ensure a good wetting, a solder thickness of several micrometers is needed to compensate for DCB surface roughness. We built a solder stack of copper and tin that covers all aforementioned requirements of the solder connection. The designed copper-tin-stack connects silicon chips with copper substrates by TLPB for a higher reliability in power electronics applications. The generated connection is capable of enduring high mechanical and thermal load. It is applicable at temperatures over 300C without melting and with improved mechanical stability compared to an ordinary solder joint. The on chip creation of intermetallic phase precursors reduces process steps in preparation of soldering and decreases solder time. In our experiments, the formation of the IMP can be realized directly during sputtering of copper layer and following tin layer. Because of the good mixture, phase building is approximately completed after depositing the layer. The solder stack bonds only at the interface between copper of the DCB and subsequent tin layer. With minor modifications in a next series of experiments it may be possible to create IMPs at every layer interface at process temperature and a more stable solder connection. With a reduced deposition rate of copper, the layers should mix less and the building of IMPs and solidification of the solder stack will take longer. Thereby it should be possible to prevent IMP formation before soldering. In general, the standard soldering interconnection can be improved by using TLPB for solder connections. The solder stack is a fast method to implement this more reliable interconnection. The process is easily implemented in industrial soldering system due to the standard process temperature and duration. It creates a connection with the advantage of a stable solder connection usable at temperatures over 300C.
Jessica Richter, PhD student, scienfific assistant
Faculty of Electrical Engineering and Information Technology, Department of Microelectronics, HSD University of Applied Sciences Duesseldorf, Germany
Duesseldorf, NRW

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