Fig. 1.

Basic principles of SRIB. (a) Simple theory of SRIB. The surface is illuminated with the laser, and the reflected light is imaged on a CCD. Any additional height on the surface will change the optical path difference between the top surface and buried oxide–silicon interface. (b) Data acquisition. The sample reflectance is recorded as an intensity image by a camera at each wavelength in 1-nm steps, forming a reflectivity-vs.-wavelength curve at each pixel of the CCD. Different reflectivity-vs.-wavelength curves are formed as a result of different optical path differences. A high-contrast image is shown as the Inset, at a wavelength of 774 nm, where the slope of the curve is highest. The thickness at each pixel is found by processing these recorded curves and displayed to show the surface profile. (c) System calibration. The SRIB system was calibrated by imaging 25 squares with varying average depths on the chip and comparing the results with the measurements of commercial profilometers. These samples were prepared by standard photolithography. Average etch depth varies from 7.5 to 0.1 nm. Concentric circles are used to find the average height of the square (see Methods). (d) Line cut of the image indicated by the red arrow shown on above image.