A vertical furnace includes a heat treatment tube, at least one reactive gas inlet, first adiabatic plates and second adiabatic plates. The at least one reactive gas inlet is disposed at or near a bottom end of the heat treatment tube. The first adiabatic plates are stacked in the heat treatment tube, each of the first adiabatic plates having a flow channel structure for allowing a gas to pass through, in which all the corners in the flow channel structure are rounded. The second adiabatic plates are stacked below the first adiabatic plates in the heat treatment tube.