1. A process control method for use in a heating apparatus comprising a plurality of heating zones, the method comprising: obtaining a baseline CD (critical dimension) map W by measuring CD at predetermined locations within the substrate processed with the heating apparatus at a baseline setting; calculating an error CD map ΔW by subtracting CD value of a target CD map from that of the baseline CD map at each of the predetermined locations within the substrate; obtaining a CD map W i for each heating zone i by measuring CD at the predetermined locations within the substrate processed with the heating apparatus at a setting where the temperature of the heating zone i is deviated from an original setting by a predetermined value ΔT i ; calculating a basis function P i for each heating zone i by subtracting CD value of an original CD map from that of the perturbed CD map at each of the predetermined locations within the substrate; expanding the error CD map ΔW by a collection of the basis functions with a corresponding expansion coefficient c i for each basis function P 1 ; and adjusting the baseline setting of each heating zone i of the heating apparatus according to the error CD map 66 W, the expansion coefficient c i , and the basis functions.