Photolithography is a process commonly used in defining features during semiconductor wafer processing, such as used in the fabrication of integrated circuits (ICs).  Photolithography generally involves applying a photoresist material (e.g., resist) to a wafer, exposing the resist using a pattern of applied radiation (e.g., light), developing the resist, etching a material of the wafer through the patterned resist, and removing the resist after etching.  In photolithography, a critical dimension (CD) is a characteristic length that corresponds to various features critical to the IC performance that needs to be patterned on the surface, e.g., a minimum feature width and/or a minimum spacing between features.  CD control of lithography patterns is an important aspect of the lithography process to ensure that the end product meets the design specification. 
Typical methods to record lithography pattern critical dimensions, e.g., for CD control, are performed ex-situ.  Such ex situ methods start with exposing the pattern in the photoresist (e.g., resist), then developing the resist pattern, and finally measuring the pattern dimensions with the metrology tool such as a CD scanning electron microscope (SEM) or ellipsometer.  There are numerous drawbacks with such ex situ methods.  For example, there is a significant time delay between pattern exposure and obtaining the actual pattern critical dimensions, since the pattern is created using a lithography system and the actual pattern critical dimensions are later measured using separate metrology tools.  An additional drawback is that ex situ methods require extra tooling, such as the metrology tools that generate the pattering performance record.