Vibration Compensated, Scanning White Light Interferometer for In Situ Depth Measurements in a Deep Reactive Ion Etcher

Abstract

A scanning white light interferometer with a long working distance and active vibration cancellation was designed and tested to enable in situ depth measurements in a deep reactive ion etcher. A superluminescent diode is used as the light source, enabling high-intensity illumination of the sample inside the etcher’s vacuum chamber. An additional laser beam co-propagates with the white light beam and is used to sense changes in the optical path length difference by observing the beam’s interference signal on a photodiode. The photodiode signal is used as feedback to control the position of the interferometer’s reference mirror which is mounted on a piezo actuator. By locking the position of the reference mirror to a zero crossing of the laser interference signal, during a scan, the reference mirror moves in discrete multiples of the laser wavelength. This design choice has the added advantage of tying the stability of the scan distance and step size to the laser wavelength stability. The prototype instrument is capable of imaging over a 1 cm $\times1$ cm field of view with a 1- ${\boldsymbol \mu }\text{m}$ depth resolution, 20- ${\boldsymbol \mu }\text{m}$ lateral resolution, and 30-cm working distance. The feedback system reduces the amplitude of vibrations by more than 10 dB out to 1 kHz. [2018-0263]