Effective method for stress reduction in thick porous silicon films

Kim, Han-Su; Zouzounis, Eric C.; Xie, Ya-Hong
April 2002
Applied Physics Letters;4/1/2002, Vol. 80 Issue 13, p2287
Academic Journal
A promising stress control process is demonstrated to achieve near-zero stress levels in thick porous silicon (PS) films. Stress reduction is necessary for thick PS structures to be used for radio-frequency applications such as on-chip inductors and transmission lines that require very thick (> 100 µm) insulating materials. This study employs a standard sample structure with 50-µm-thick PS films formed on p[sup +] substrates through anodization under 50mA/cm² and 25% HF concentration. Mass spectroscopy analysis shows that the desorption of hydrogen is correlated with the stress evolution in PS films. As-prepared PS films are under compressive stress. Thermal annealing in N[sub 2] ambient turns it into tensile stress. Further annealing in an oxidizing ambient restores the compressive stress. Such stress evolution can be explained by the atomic structure changes on the pore walls of PS films: H[sub 2]-passivated and oxidized Si surfaces result in compression in PS films, whereas reconstructed state results in tension. These findings are used to fabricate thick PS films with negligible stress (< 1 MPa) at room temperature.


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