1. A method of controlling stability in a lithography process, comprising: (a) defining baseline performances of a lithography apparatus for use in the lithography process in a lithography model, wherein a first sub-model of the lithography model defining a first baseline performance is obtained under a first illumination condition, and a second sub-model of the lithography model defining a second baseline performance is obtained under a second illumination condition, the lithography model comprising one or more of illumination source pupil characteristics and lithography process response parameters; (b) monitoring illumination stability of the lithography apparatus under the first illumination condition by measuring a first temporal drift data of the illumination source pupil characteristics and using the first measured temporal drift data to maintain the illumination source pupil characteristics within or substantially close to the first baseline performance; and (c) monitoring lithography process response stability under the second illumination condition by measuring a second temporal drift data of the lithography process response parameter and using the second measured temporal drift data to maintain the lithography process response within or substantially close to the second baseline performance.