AVS 47th International Symposium
    MEMS Wednesday Sessions
       Session MM-WeP

Paper MM-WeP1
Characterizing the Thermal Behavior of Thin Films Using Micromachined Cantilevers

Wednesday, October 4, 2000, 11:00 am, Room Exhibit Hall C & D

Session: Poster Session
Presenter: H.-C. Tsai, National Tsing Hua University, Taiwan
Authors: H.-C. Tsai, National Tsing Hua University, Taiwan
W. Fang, National Tsing Hua University, Taiwan
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The thermal stress is a very important factor of fracture for microstructure. In this paper, we exploit the micromachined cantilever to investigate the thermal behavior of the thin films. The thermal behaviors of thin films were characterized under two thermal loads including the thermal cycle load and the isothermal load. Thus, the variation of the thermal stress of thin film with the thermal loading time is studied. Moreover, the variation of the thermal strain rate of thin film with the film thickness is also discussed. In summary, the strain rate of thin film is proportion to the film thickness after the isothermal load test. On the contrary, the strain rate of thin film is in inverse proportion to the film thickness after the thermal cycle load test. In this study, the silicon dioxide cantilevers were fabricated through the bulk micromachining. The film to be characterized was then deposited on top of the cantilever to form a bi-layer beam. Since the thermal expansion coefficient (CTE) of the film and that of the silicon dioxide are different, the film will subject to stress during the thermal load test. The strain of the deposited film is determined by the deflection of the bi-layer beam. For the isothermal load test, the bilayer beam was heated by a heating stage. As to the thermal cycling load test, the sample was placed into a chamber that can set the variation of the temperature with time. In application, the thermal behavior of the sputtered Al film is studied. The Al film is under compression during heating since its CTE is larger than that of the silicon dioxide. As a result of isothermal load, the total compressive strain of the Al film was increased drastically in the beginning. However, the compressive strain of the Al film was gradually decreased after a certain heating time. It was obtained that the decreasing rate of the strain was proportional to the film thickness. For the thermal cycle load test, the variation of the strain of Al film with the number of thermal cycle was measured. We obtained that the compressive strain of Al film was drastically increased for the first 200 cycles, and then gradually decreased. According to this study, the thermal behaviors of thin films under the static and the dynamic loads are discussed. In addition, the film thickness will be an important factor for the thermal behaviors of thin film.