You've said that you wanted to convert the extrusion to a surface, TC has two surface types. One is TC surface, that you will get if you set your property to "surface" or explode a solid a couple of times. TC surfaces have no capacity for curved surfaces, they're always faceted.
The other type is zero-thickness ACIS sheet solid, created if you extrude an open profile, or if you've an extrusion created from a closed curved profile, select and modify|create surface|from solid. If you need the object to be open rather than a closed-volume object, use facet editor to select and delete faces.