100‐&mgr;m silicon‐on‐insulator structures have been achieved by first utilizing silicon molecular beam epitaxial (Si MBE) growth on porous silicon and subsequently oxidizing the porous silicon through the patterned Si MBE film windows. A Si beam method is used for the low‐temperature surface cleaning of porous silicon prior to Si MBE growth. By using a two‐step growth technique, the Si MBE film shows good crystallinity checked by Rutherford backscattering channeling spectroscopy and cross‐sectional transmission electron microscopy. An electron mobility of 1300 cm2 V−1 s−1with a doping concentration of 6×1015cm−3has been achieved.