MEMS structures for stress measurements for thin films deposited using CVD

Abstract

Mechanical stress in thin films is an important reliability issue in microelectronic devices and systems. The presence of large stresses can lead to the formation of defects that can cause device failure. The ability to control the magnitude of stress during film formation is, therefore, crucial to the fabrication of defect-free and reliable electronic devices and systems. However, the origin of stress in thin films is still a subject of intense debate. The development of a detailed understanding of the origin of stress hinges on our ability to make accurate stress measurements during and after film deposition. To this end, two novel MEMS structures were developed to measure the stress of thin films deposited using chemical vapor deposition (CVD). Buckled SOI membranes were designed and fabricated for the measurement of the stress in thin films deposited on them. A simple analytic model was developed and calibrated to assess the pre-deposition and post-deposition buckling of the structures. By measuring the changes in mechanical responses upon film deposition, it was successfully demonstrated that stress in thin films can be accurately measured. In particular, the stresses in evaporated chromium films measured using the SOI membranes and the curvature method were found to agree to within 5%. Stress pointers were designed and fabricated for in-situ stress measurements of CVD thin films. The design was based on mechanically amplified rotation and the novel concept of "footprinting". Stress is recorded during film formation in the form of beam rotation, which is also dependent on film thickness. As a result, stress can be measured as a function of film thickness in post-deposition analyses, without using real-time measurements during deposition. The fabricated structures were found to bend down and stick to the substrate. PECVD films were found to deposit non-uniformly underneath the beams, thereby complicating the mechanical responses of the stress pointers. A new and improved design is proposed.