The European 3D heterogeneous integration platform has been established by the consortium of the Integrated Project e-BRAINS [1], where technologies of the following relevant main categories of 3D integration are provided to enable future applications of smart sensor systems:3D System-on-Chip Integration - 3D-SOC: TSV technology for stacking of thinned devices or large IC blocks (global level),3D Wafer-Level-Packaging - 3D-WLP: embedding technology with through-polymer vias (TPV) for stacking of thinned ICs on wafer-level (no TSV), and3D System-in-Package - 3D-SIP: 3D stacking of packaged devices or substrates
*definitions according to [2]
Regarding TSV performance, the applications do not need ultra-high vertical interconnect densities as for 3D stacked Integrated Circuits – 3D-SIC*. Nevertheless, the lateral sizes of the TSVs are preferably minimized to allow for place and route for small “open” IC areas. Smaller TSVs are also preferred in order to reduce thermo-mechanical stress. e-BRAINS' focus is on how heterogeneous integration and sensor device technologies can be combined to bring new performance levels to targeted applications with high market potentials. The consortium, under coordination of Infineon and technical management by Fraunhofer EMFT, is composed of major European system manufacturers (Infineon, Siemens, SensoNor, 3D PLUS, Vermon and IQE), SMEs (DMCE, Magna Diagnostics, SORIN and eesy-ID), the large research institutions CEA Grenoble, Fraunhofer (EMFT Munich & IIS-EAS Dresden), imec, SINTEF, Tyndall and ITE Warsaw, and universities (EPFL Lausanne, TU Chemnitz and TU Graz). Target applications include automotive, ambient living and medical devices, with a specific focus on wireless sensor systems. Concerning the enabling 3D Heterogeneous Integration Platform, the e-BRAINS partners are working close together, where Infineon, Fraunhofer EMFT, imec and SINTEF are focusing mainly on 3D-SOC and 3D-WLP, and the French system manufacturer 3D PLUS and Tyndall on 3D-WLP and 3D-SIP technologies.
The focus of this paper is on low-temperature bonding processes for highly reliable 3D integrated sensor systems. One of the key issues for heterogeneous systems production is the impact of 3D processes to the reliability of the product, i.e. the high built-in stresses caused by e.g. the CTE mismatch of complex layer structures (thin Si, ILDs, metals etc.) in combination with elevated bonding temperatures. As consequence, extensive project work was dedicated in the developments of reliable low-temperature bonding processes. Mainly intermetallic compound (IMC) bonding with Cu/Sn metal systems supported by ultrasonic agitation (Fraunhofer EMFT) was successfully introduced in 3D integration technology (see Fig. 2). A copper/tin solid-liquid interdiffusion (SLID) system was investigated using ultrasonic agitation to reduce the assembly temperature below the melting point of tin. Cleaning procedures are important shortly before joining the samples; dry cleaning has best results due to removal of thin oxide layers. Figure 2 shows a cross section of US supported Cu/Sn bonding at 150C. The intermetallic compounds Cu3Sn and Cu6Sn5 as well as pure tin easily can be identified. Due to low temperature assembly the most stable intermetallic compound (IMC) Cu3Sn has a minor share of the metal system. Most importantly there is no gap between top and bottom part of the joint despite the macroscopic assembly temperature is far away from the melting point of tin. But maybe the ultrasonic agitation brings enough energy to the interfaces, so locally melting can occur. In this way robust IMC bonding technology at 150C could be demonstrated with shear forces of 17 MPa and an alignment accuracy of 3 μm, well-suited for 3D integration.
Figure 2: Low-temperature IMC bonding technology using ultrasonic agitation (Fraunhofer EMFT)
Reliability for SLID contacts is certainly a very challenging objective especially looking for robust solutions in automotive applications. Thermally induced mechanical stress is the main reason for early fails during temperature cycling. Cross sectioned samples were investigated and methods like nanoindentation, Raman spectroscopy, fibDAC, and high local resolution x-ray scattering were applied to measure the intrinsic stresses. It can be shown that low temperature bonding is the right approach to avoid excessive stress cracking the interface or even fracturing the silicon. Also fatigue of metals can be reduced in a range that plastic deformation is no lifetime limiting factor.