The advantages of coupling experimental methods and analytical modelling to fix deformation problems in devices conception and manufacturing
Résumé
In the Silicon 3D packaging field, designers need to stack Integrated Circuits and connect them vertically. To obtain a functional device, stresses and strains must be controlled during the steps of manufacturing. The automated loading of wafers into the process tools needs a bow lower than a critical value (close to +/- 300 mu m). Then, the cutting and the thinning of the substrate increases the strain leading to possible connection problems and consequently difficulties with alignment during assemblies made at high temperatures. In the case of extreme deformations, the integrity of the device can be questioned. In this article, we introduce a new technique of measurement composed of an optical module producing and measuring an array of parallel laser beams, a high resolution scanning stage, a rapid thermal processing (RTP) chamber and several accessorial gas control modules. The kSa MOS Thermalscan allows us to measure the mean stress in the deposited layer, by using curvature variation of the wafer before and after the deposition step and during thermal loads. In the case of evolving materials, we estimate significant mechanical transition temperatures (polymers) or the number of thermal cycling needed to stabilize the microstructure (metals). Furthermore, this methodology allows us to extract the thermal expansion coefficient (CTE) from the slope of the linear system of the stress-temperature curve. In addition to these experimental measurements, we developed an analytical model for the prediction of the stress distribution. With this approach, we consider the whole history of the multilayers system as well as processing conditions which are taken into account for each layer. This analytical model allows us to estimate the stress distribution in a multi-layered structure with no set conditions on the film vs. substrate thickness ratio [1].
