Gap detection system and method, and focal plane correction method

A gap inspection system, configured to inspect global gap data between a mask and a substrate, comprising:

• a support frame;
• a mask frame, comprising an autofocus module for real-time monitoring the gap data between the mask and the substrate;
• a wafer stage module, comprising a wafer chuck for adjusting a position and an orientation of the substrate;
• a front-side surface topography inspection module, mounted on an upper substrate of the support frame for inspecting the substrate, to acquire primary surface topography parameters;
• a folding surface topography inspection module, switchable between an inspection position and a retract position for inspecting the mask, to acquire secondary surface topography parameters;
• and
• a control system, configured to acquire position parameters of the mask and the substrate and calibration parameters of the front-side surface topography inspection module and the folding surface topography inspection module, and configured to calculate global gap data between the mask and the substrate in conjunction with the position parameters, the calibration parameters, the primary surface topography parameters, and the secondary surface topography parameters after measuring a tilt angle between the mask and the substrate.