A method for forming a vertical channel device, the method comprising: forming a vertical semiconductor structure including an upper portion, an intermediate portion and a lower portion, by etching a semiconductor layer stack arranged on a substrate, the semiconductor layer stack including an upper semiconductor layer, an intermediate semiconductor layer and a lower semiconductor layer, wherein the intermediate semiconductor layer is formed of a material different from a material of the lower semiconductor layer and different from a material of the upper semiconductor layer, layer, wherein the forming of the vertical semiconductor structure comprises: etching the upper semiconductor layer to form said upper portion and the intermediate semiconductor layer to form said intermediate portion, detecting whether said etching has reached the lower semiconductor layer, and in response to detecting that said etching has reached the lower semiconductor layer, changing to a modified etching chemistry being different from an etching chemistry used during said etching of the intermediate semiconductor layer, and etching the lower semiconductor layer using the modified etching chemistry to form said lower portion, wherein the modified etching chemistry is such that the lower portion is formed to present, along at least a part of said lower portion, a lateral dimension gradually increasing along a direction towards the substrate, wherein said etching chemistries are dry etching chemistries each including a fluoride-including component and a carbon-including component, wherein a fluoride-to-carbon-ratio in the modified etching chemistry is smaller than a fluoride-to-carbon-ratio in the etching chemistry used during said etching of the intermediate semiconductor layer, and the method further comprising: forming a gate stack extending vertically along the intermediate portion to define a channel region of the vertical channel device. forming a vertical semiconductor structure including an upper portion, an intermediate portion and a lower portion, by etching a semiconductor layer stack arranged on a substrate, the semiconductor layer stack including an upper semiconductor layer, an intermediate semiconductor layer and a lower semiconductor layer, wherein the intermediate semiconductor layer is formed of a material different from a material of the lower semiconductor layer and different from a material of the upper semiconductor layer, layer, wherein the forming of the vertical semiconductor structure comprises: etching the upper semiconductor layer to form said upper portion and the intermediate semiconductor layer to form said intermediate portion, detecting whether said etching has reached the lower semiconductor layer, and in response to detecting that said etching has reached the lower semiconductor layer, changing to a modified etching chemistry being different from an etching chemistry used during said etching of the intermediate semiconductor layer, and etching the lower semiconductor layer using the modified etching chemistry to form said lower portion, wherein the modified etching chemistry is such that the lower portion is formed to present, along at least a part of said lower portion, a lateral dimension gradually increasing along a direction towards the substrate, wherein said etching chemistries are dry etching chemistries each including a fluoride-including component and a carbon-including component, wherein a fluoride-to-carbon-ratio in the modified etching chemistry is smaller than a fluoride-to-carbon-ratio in the etching chemistry used during said etching of the intermediate semiconductor layer, etching the upper semiconductor layer to form said upper portion and the intermediate semiconductor layer to form said intermediate portion, detecting whether said etching has reached the lower semiconductor layer, and in response to detecting that said etching has reached the lower semiconductor layer, changing to a modified etching chemistry being different from an etching chemistry used during said etching of the intermediate semiconductor layer, and etching the lower semiconductor layer using the modified etching chemistry to form said lower portion, wherein the modified etching chemistry is such that the lower portion is formed to present, along at least a part of said lower portion, a lateral dimension gradually increasing along a direction towards the substrate, wherein said etching chemistries are dry etching chemistries each including a fluoride-including component and a carbon-including component, wherein a fluoride-to-carbon-ratio in the modified etching chemistry is smaller than a fluoride-to-carbon-ratio in the etching chemistry used during said etching of the intermediate semiconductor layer, and the method further comprising: forming a gate stack extending vertically along the intermediate portion to define a channel region of the vertical channel device.