Super-Resolution Optical Material Characterization

Purdue University, West Lafayette IN

Investigators

Abstract

This project entails the study of a means to determine information about object features at nanometer length scales, and in situations where the environment does not allow direct imaging of the object, using laser light. The approach is based on relative motion between a structured illumination pattern, formed from coherent light, and the object or material system, where either the light or the object is scanned in small precise spatial steps. To date, because the achievable resolution is generally limited to about one half of the optical wavelength without prior information, the approach in technology to improve spatial resolution or optical memory capacity has been to reduce the wavelength. This faces increasing challenges in moving to shorter wavelengths. On the other hand, the use of relative motion of a sample with a spatially varying optical intensity provides information about the sample that can be used for super-resolution imaging of features far smaller than the wavelength. Consequently, it should become possible to detect small structures of importance in the semiconductor industry and in microscopy. Furthermore, a suite of material inspection and imaging situations have significant background clutter, exacerbating the challenges. For example, it is currently difficult or impossible to detect small defects in the three-dimensional semiconductor processing steps used in building vertical solid-state memory, with multi-billion-dollar market ramifications. With laser inspection following the deposition of films, random scatter due to material roughness produces speckle, and this seemingly worsens the situation. During this project, an inspection and imaging method for extracting information about such defects and other objects by using speckle is being developed. More broadly, the approach enables a means to image objects hidden in a randomly scattering environment such as fog or biological tissue using light. The associated research is forming the basis of theses for two Ph.D. students, and the project involves undergraduate research students. A mathematical learning module is being created for junior high school students. Existing optical inspection methods are incapable of adequately detecting small defects in three-dimensional semiconductor structures like those in vertical memory. This project offers a path to finding such defects through two key objectives: (i) Numerical modeling to develop sensing and imaging with relative field motion; and (ii) Evaluative application-oriented experiments. Simulations with background laser beam interference fringes and random speckle fields are being used to investigate the relationship between far-subwavelength material geometric variables and the measured intensity as a function of relative position change, to provide a physical forward model for cost-function-based inversion, and to aid in the design of experiments. Two types of coherent optical sensing experiments are being investigated to illustrate technology applications with relative motion between an object of interest (to be characterized) and the background field. One involves speckle generated from a randomly scattering medium and statistical extraction of the relevant features. Another utilizes membrane films translated through interference fringes, with the use of a forward model to determine the associated parameters. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.