The microphase structures of ultrathin symmetric polystyrene/poly(methyl methacrylate) diblock copolymer films on silicon substrate under variation of film thicknesses in the nanometer range were investigated by using AFM. The microphases were identified by simultaneously measuring the topography and the phase-shift image in TappingMode and by comparing the results with the topography after selectively etching the poly(methyl methacrylate) phase. The thinnest spin-cast film showing complete wetting was only 1.8 nm thick. However, even for this film a microphase separation in the range of the correlation length (L) of the diblock bulk material was observed. For this minimum film thickness, the calculated PMMA microdomain volume approaches the volume of a single PMMA coil, which may be discussed as the limiting microdomain volume necessary as a critical wetting condition. Geometrical considerations lead to an average PMMA/silicon substrate contact angle of 46 degrees in the spin-cast and vacuum-dried ultrathin diblock copolymer films. Annealing of these films at 130 degreesC reveals a second minimum thickness, at which the microphases show coplanar lamellar ordering with the PMMA at the substrate and the PS at the surface. This minimum thickness of coplanar ordering is about 40% higher than the calculated width of the interface between PS and PMMA in the bulk.