Plasma potential formation with electron cyclotron heating is investigated in a fully-ionized collisionless plasma flow along converging magnetic-field lines (monotonously increasing magnetic field) in the presence of a single electron cyclotron resonance (ECR) point. When the ECR point is located in a region of good curvature of the magnetic line, a potential hump (plug potential) with a potential dip (thermal barrier) is generated around the ECR point. This potential structure persists in the steady state, working as a plasma-flow dike potential. In case that the ECR point is located in a region of bad curvature, on the other hand, the dike potential is only transiently formed, and collapses gradually as low frequency curvature-driven instabilities grow.