Abstract
This work concerns optimization of properties of aluminium (Al) thin films with respect to residual stress and thermal hysteresis. Controlling residual stress and minimizing the effect of thermal hysteresis is critical in a wide range of MEMS components.
Throughout the study we used wafer curvature, White Light Interferometry (WLI), High Resolution Scanning Electron Microscopy(HRSEM), X-Ray Diffraction (XRD) to assess mechanical and microstructural properties of Al thin
films as a function of deposition conditions and annealing.
The maximum thermal hysteresis in Al was measured to be below 500 MPa. Plastic yielding occurred from ca. 150 oC at a yield compressive stress of 120 MPa. The mean grain size increased from 250 to 850 nm when increasing sputter power
from 2 to 9 kW for annealed samples. A significant degree of temperature induced voiding and hillocking was observed on Al surface after annealing at 350oC. Integration of thin films of sputtered Ti and TiW on top of the Al film suppressed the hillock formation and is suggested as a possible solution to minimize thermal hysteresis.
Throughout the study we used wafer curvature, White Light Interferometry (WLI), High Resolution Scanning Electron Microscopy(HRSEM), X-Ray Diffraction (XRD) to assess mechanical and microstructural properties of Al thin
films as a function of deposition conditions and annealing.
The maximum thermal hysteresis in Al was measured to be below 500 MPa. Plastic yielding occurred from ca. 150 oC at a yield compressive stress of 120 MPa. The mean grain size increased from 250 to 850 nm when increasing sputter power
from 2 to 9 kW for annealed samples. A significant degree of temperature induced voiding and hillocking was observed on Al surface after annealing at 350oC. Integration of thin films of sputtered Ti and TiW on top of the Al film suppressed the hillock formation and is suggested as a possible solution to minimize thermal hysteresis.