Adaptive Focusing System for a Scanning Metrology Tool

A scanning metrology system comprising:

• a scanning metrology tool comprising an adaptive focusing system, the adaptive focusing system comprising:
• an optics module comprising at least one optical component;
• an optics module height positioner (OMHP);
• a position sensor operative to generate a position output indicating a height of said optics module;
• a predictive height estimator (PHE) operative to:
• generate an estimated height value of a sample at each site of a plurality of sites, said plurality of sites being located along a path defined by a movement of said optics module relative to said sample;
• and
• generate a desired optics module height output for each of said sites, said desired optics module height output being based at least partially on said estimated height value at said site;
• a regulator operative to generate, at least partially based on said desired optics module height output and a known height of said optics module, a sequence of optics module height control instructions for said plurality of sites;
• a driver operative to provide a sequence of control outputs to said OMHP, said sequence of control outputs being based on said sequence of optics module height control instructions;
• and
• a model predictive controller (MPC) operative to monitor differences between a reported height of said optics module, said reported height being at least partially based on said position output generated by said position sensor, and an MPC-expected height of said optics module, said MPC-expected height of said optics module being at least partially based on said optics module height control instructions, thereby to generate system amelioration values.