Sensing and Imaging with Motion in Structured Optical Illumination

Purdue University, West Lafayette IN

Investigators

Abstract

Title: Sensing and Imaging with Motion in Structured Optical Illumination Abstract Non-technical: Enhanced information about an object is available with optical measurements as a function of object position, with either natural motion or controlled adjustment of position. This concept is being exploited in an imaging and detection method having two facets. In one, speckled patterns of scattered laser light measured as a function of object position are being used to reconstruct the obscured moving object. The initial experiments use scanned dielectric objects between scattering acrylic sheets, allowing control over all relevant variables, which will enable applications including optical imaging in living subjects, such as cells and contrast agents deep within the brain, and imaging given environmental clutter. The safety, convenience, resolution, and information available from optical imaging in these applications are paramount. Of particular significance, the method will be effective for very heavy scatter, making it possible to image through substantial amounts of tissue. At a more general level, the effort relates to new opportunities for communication in a scattering environment that exploit natural motion and spatial diversity. The other aspect of the work involves use of optical measurements as a function of the precise position of a semiconductor wafer in a spatially varying optical intensity to find defects at an early stage of manufacture. This addresses a major need in industry, because there is no satisfactory method to find defects in three-dimensional devices that have become important in consumer electronics. This approach is being evaluated using example material arrangements with an introduced defect. Understanding from this aspect of the project will also be useful in other applications related to structure and material characterization. Technical: Laser optical speckle patterns as a function of object position are being used to form intensity correlations over position. This data has been shown to provide access to information about the moving object, and through a reconstruction method, to an image of the hidden object. For example, the image of a hidden aperture has been obtained. The key aspect being sought is evidence that the object's dielectric constant as a function of position can be obtained, thereby establishing the general principle of imaging in heavily scattering media. Experiments use scattering media with controlled properties and a translation stage. Information on the possible resolution is also being investigated. The other aspect of this project involves use of a model material arrangement representative of a semiconductor wafer with a defect, whereby the sample is scanned using a piezoelectric stage in an optical standing wave field created by illumination with a laser and reflection from a mirror. Detected light as a function of sample position, coupled with a model, allows the presence of a defect to be determined. The efficacy of this approach is being evaluated for application in the semiconductor wafer inspection industry. Because existing optical inspection methods are incapable of adequately detecting small defects in three-dimensional semiconductor structures, the prospect that this approach will provide a solution could address a multi-billion-dollar inspection market need.