Versatile multilayer MCM-D structure for high reliability applications

Within the joint European submicron silicon initiative (JESSI) project Silicon Hybrids, Industrial Microelectronics Center (IMC) has developed and MIL-std 883 B qualified a highly versatile thin film structure suitable for high reliability MCM applications. The basis is a four-layer structure with aluminum conductors and benzocyclobutene (BCB) dielectric formed on a 5 silicon wafer. Optionally, a solderable top layer of Ni and Au can be added for flip chip assembly, and passive components can be integrated simply by adding a SiCr and a SiO x N y layer beneath the four-layer structure. To achieve a mechanically stable structure, a good understanding is required of how different parameters, such as conductor and dielectric material, and layer structure and dimensions affect the final result. The evaluation has been focused on mechanical stresses due to thermal cycling and shock. Finite Element Analysis and Scanning Electron Microscopy have been utilized in the evaluation. The results show that good adhesion between the different layers, careful stress control in the materials, and metal thickness dependent restrictions in the design rules are necessary to pass the MIL-std tests. With increasing demands on system integration, performance and low production cost, flip chip mounting of dies has become an attractive technology. For flip chip assembly, the chips often have solder balls, whence the substrates must have solderable assembly pads. However, all chips are not available with solder balls, and in these cases solder balls must be applied either to the chip or the substrate. IMC has developed a process to produce solder balls with well-controlled thickness and composition for flip chip assembly on Si substrates. Using specially designed MCM-D test vehicles, flip chip mounting of dies has been evaluated with respect to electrical and mechanical reliability, tested by temperature and power cycling tests. Electrical continuity tests show good results, and the structures have withstood thermal shocks with ΔT = 300 °C. The processes are currently used in a number of projects and some examples of applications will be given.