Near-Field Microwave Microscopy for 3D Surface Assessment of Manufactured Structures

Using near-field scanning microwave microscopy as a contact and non-contacting investigative tool for 3D surface metrology with three differing measurement modes, it has been possible to analyse structures that may be difficult for existing metrology systems. The system utilizes the small change in capacitance between a coaxial resonant probe (at around 2 GHz) ending in an open circuit tip, and the sample surface. This is measured in the frequency domain by the shift in the resonance frequency of the voltage transmission coefficient |S21|. It is also possible to investigate various materials (metallics, plastics etc.) owing to their differing dielectric properties. The probe has been tested on a computer-controlled 3D stage but is suitable for incorporation into a commercial co-ordinate measurement machine (CMM) to enhance its capability to inspect the inside surfaces of structures, e.g., threads in small bores.