Stress measurement of thin wafer using reflection grating method

Flatness/Curvature measurement is critical in many Si-wafer based technologies ranging from micro-electronics to MEMS and to the current PV industry. As the thickness of the wafer becomes smaller there is an increased tendency for it to warp and this is not conducive to both patterning as well as dicing. Monitoring of curvature/flatness is thus necessary to ensure reliability of device and its uses. A simple whole-field curvature measurement system using a novel computer aided phase shift reflection grating method has been developed and this project aims to take it to the next step for residual stress measurement. The system was developed from our earlier works on Computer Aided Moiré Methods and Novel Techniques in Reflection Moiré, Experimental Mechanics (1994) in which novel structured light approach was shown for surface slope and curvature measurement. This method uses similar technology but coupled with a novel phase shift system to accurately measure slope and curvature. In this research, the system is calibrated with reference to stress measurement equipment from KLA-Tencor. Some initial results based on a joint project with Infineon Technologies are re-examined. The stress distribution of the wafers are derived with the aid of Stoney's equation. Finally, the results from our proposed system are compared and contrasted with data obtained from KLA-Tencor equipment.