A capacitance-to-digital converter for displacement sensing with 17b resolution and 20μs conversion time

In precision mechatronic systems, such as wafer steppers, the position of critical mechanical components must be dynamically stabilized with sub-nanometer precision. This can be achieved by a servo loop consisting of a displacement sensor and an actuator. Compared to optical interferometers, capacitive displacement sensors offer smaller size and lower cost. However, mechanical tolerances limit their electrode spacing to about 10μm [1], while the targeted resolution is below 100pmrms. This requires a capacitance-to-digital converter (CDC) with more than 17b resolution. Furthermore, its latency must be low enough (20μs) to avoid compromising servo-loop stability. Lastly, it should be stable enough to maintain measurement accuracy during the intervals between system calibrations.