X-ray microbeam strain investigation on Cu–MEMS structures

The mechanical properties of simple metals in small dimensions have been the objects of many studies. Average stress measurements clearly show that yield stress increases significantly when the size decreases but local strain measurements indicate considerable grain-to-grain and intra-grain strain variations. The link between the macroscopic behaviour of microstructures and the corresponding local processes is still difficult to analyse.Copper is used in advanced microelectronic interconnects and the mechanical properties of narrow interconnects is a key issue for the reliability of devices. Average strain measurements performed on line arrays show a drastically different behaviour in narrow lines as compared with wide lines. ST microelectronics in Rousset developed a MEMS sensor integrated in the damascene process to follow the stress after processing. This microsystem can be a very efficient tool to monitor the average stress in interconnects on the chip. The gradient of strain in interconnects may lead to failures from void and hillocks and local stress repartition can be obtained through X-ray microdiffraction. Laue microdiffration is a unique non-destructive method to investigate the strain grain-by-grain in microstructures. The diffraction patterns were obtained at ESRF on BM32 beam line. We present the first steps of the investigation of a MEMS investigated by Laue microdiffraction. The sub-micron X-ray-beam allows a mapping of the microstructure with a step of around 1 μm, which is also the value of the estimated grain size. We present results on the orientation and stress at a sub-micron scale obtained from the Laue-diffraction measurements on the MEMS.