9.9 A 0.6nm resolution 19.8mW eddy-current displacement sensor interface with 126MHz excitation

Displacement sensing with sub-nanometer resolution is required in advanced metrology and high-tech industry, e.g., to measure the lens position in wafer scanners. Linear encoders and interferometers are often used for this purpose, but they are bulky and costly. Capacitive sensors [1], though compact, are sensitive to environment and require electrical access to the target. Eddy-current sensors (ECSs) do not have these disadvantages, but their resolution and stability are limited by the skin-effect [2-5]. For sub-nm measurements, this can be alleviated by using excitation frequencies >100MHz. This calls for stable flat sensing coils (to minimize parasitics) in close proximity to the ECS interface, whose power dissipation must then be low enough to avoid self-heating and displacement errors due to thermal expansion [2,6].