In this letter, the effects of pre-CF4 plasma treatment on Si for sputtered HfO2 gate dielectrics are investigated. The significant fluorine was incorporated at the HfO2/Si substrate interface for a sample with the CF4 plasma pretreatment. The Hf silicide was suppressed and Hf-F bonding was observed for the CF4 plasma pretreated sample. Compared with the as-deposited sample, the effective oxide thickness was much reduced for the pre-CF4 plasma treated sample due to the elimination of the interfacial layer between HfO2 and Si substrate. These improved characteristics of the HfO2 gate dielectrics can be explained in terms of the fluorine atoms blocking oxygen diffusion through the HfO2 film into the Si substrate.