Channel stop implant is a suitable ion implant under the field oxide which isolates the area between to adjacent MOS transistor. The field oxide is thick and in addition there is this channel stop implant to increase the threshold voltage beyond the VDD. Then if any metal line laying above this region and its voltage is VDD, there will be no channel will be formed in this region since the field effect is prevented at the highest applied voltage. In case of n MOS transistors one implants boron and in case of p MOS one implants phosphorus.
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