1. A method of calibrating a manufacturing process model comprising: measuring, using a scatterometer, a diffraction pattern generated by illuminating a target structure, the target structure having been produced by a layout for the target structure and a manufacturing process; providing a manufacturing process model for simulation of the manufacturing process, the manufacturing process model comprising manufacturing process parameters; determining structural information corresponding to the target structure using the manufacturing process model; determining a model diffraction pattern using the structural information; comparing the model diffraction pattern to the measured diffraction pattern and calculating a merit function based on a result of the comparison; generating calibrated manufacturing process parameters based on a result of the merit function; and calibrating the manufacturing process model using the generated calibrated manufacturing process parameters.