Method of controlling a fabrication process using an iso-dense bias

A method of controlling a fabrication process using iso dense bias, the method comprising:

• defining a grating layer, the grating layer comprising a dense area with a plurality of dense features and an isolated area with at least one isolated feature;
• forming the grating layer on a first substrate using a fabrication cluster;
• providing the grating layer on the first substrate to a metrology tool;
• exposing the grating layer to electromagnetic energy;
• measuring a first diffraction signal from the electromagnetic energy diffracted by the grating layer and establishing a first iso-dense bias;
• forming the grating layer on a second substrate using the fabrication cluster;
• providing the grating layer on the second substrate to the metrology tool;
• exposing the grating layer to electromagnetic energy;
• measuring a second diffraction signal from the electromagnetic energy diffracted by the grating layer and establishing a second iso-dense bias;
• transmitting the first iso-dense bias and the second iso-dense bias to the fabrication cluster wherein the fabrication cluster was used to create the grating layer on the first substrate and the second substrate, the fabrication cluster having process parameters and equipment settings;
• and
• adjusting one or more process parameters or equipment settings of the fabrication cluster based at least on the difference between the first iso-dense bias and the second iso-dense bias.