STUDY OF THE CRACKING OF HIGHLY POROUS P+ TYPE SILICON DURING DRYING

The origin of the cracking of highly porous silicon layers during dryi ng is investigated. Optical and scanning electron microscopy observati on allow us to observe the cracking occurrence, In situ x-ray diffract ion experiments, under controlled vapor pressure of pentane, reveal th at large capillary stresses occur at a vapor pressure P during the co ntrolled drying. These stresses lead to the cracking of the highly por ous layer, which occurs for samples thicker than a critical thickness h(c). Taking into account the mechanical properties of the material, a model based on energy balance is presented. This model predicts a lay er thickness h(c) of cracking occurrence, showing that h(c) varies as (1-p)(3)/gamma(LV)(2) (where gamma(LV) is the surface tension of the d rying liquid and p is the porosity). This model is in good agreement w ith experimental data obtained with two liquids, water, and pentane, w hich have very different surface tension and also for two different po rosities. (C) 1996 American Institute of Physics.