Characterization of PillarHall test chip structures using a reflectometry technique

Thin film samples where one of the thin layers consists of a vacuum or air are called PillarHalls due to their support structure in silicon wafers. Custom PillarHall samples were provided by Chipmetrics Ltd and characterized by reflectometry with a Cary 7000 spectrometer. Data at 8° of angle of incidence were collected with p-polarization of the incident light within the wavelength range of 550-1800 nm. These data were then analyzed with a dedicated MATLAB code, using fitting software accompanying the transfer matrix method for calculation of the reflectance spectrum. Layer thicknesses and unknown refractive indices were chosen as fitted parameters. The oscillating reflectance spectrum of the PillarHall test chip yielded an air gap thickness of 86 nm with an estimated standard uncertainty of 5 nm. This is close to the nominal value of 100 nm. The results demonstrate that reflectometry data are sensitive to the thickness of the thin air layer deep inside the silicon structure.