Low density material such as hydrogen silsesquioxane (HSQ) can provide a lower dielectric constant than a conventional silicon dioxide insulator. However, the thermal stability of as-cured HSQ is about 400 degrees C. Both the leakage current and dielectric constant of HSQ rapidly increase with increasing annealing temperature. In this work, we study the enhancement of the thermal stability of the HSQ film by fluorine ion implantation treatment. The fluorine implantation step can enhance the thermal stability of the HSQ film as high as 500 degrees C. In addition, the implantation treatment after the curing step is more efficient in enhancing the thermal stability of the HSQ film than before the curing step.