Anisotropic Langmuir-Blodgett films of phthalocyanine derivatives have been patterned using an atomic force microscope to scan the surface of the film at high scan rate (>100 mu m/s). The depth of the periodic grooved lines, of width down to 50 nm, formed perpendicular to the fast scan direction can be controlled to the monolayer level through the effect of the load (a few nanonewtons) applied to the tip. It is shown that the patterning results from an oscillation in the feedback loop resulting in a periodic force modulation of a few nanonewtons along each scan line. Using the same principle, we show that the microscope can be modified to write patterns of arbitrary form by the superposition of a given voltage waveform onto the Z piezo voltage.