An interface for eddy current displacement sensors with 15-bit resolution and 20 MHz excitation

This paper presents a high-performance interface for eddy current displacement sensors, intended for use in IC fabrication equipment. The interface consists of a low-power front-end oscillator and a synchronous demodulator. With an excitation frequency of 20 MHz, it enables the use of targets with only a few tens of μm thickness. A ratio-metric measurement approach is applied to suppress the oscillator´s noise contribution. The demodulator is auto-zeroed to suppress its 1/f noise. The interface has been realized in a 0.35μm BiCMOS technology and consumes about 18 mW. Measurement results demonstrate a linearity of 0.4%, and a resolution of 15.3 bits (75 nm on a 3mm range) within a 1 kHz signal bandwidth.