DECEMBER 5--nLine Corp. (Austin, TX; www.nline.com) has introduced its Fathom patterned-wafer-defect inspection tool, which uses Direct-to-Digital Holography (DDH) technology to detect defects within high-aspect-ratio (HAR) structures, which have a height-to-width ratio of greater than four to one. Bob Owen, nLine president and CEO, said: "Inspection of HAR structures is a growing unfilled niche within the overall patterned-wafer-inspection market. Conventional optical inspection tools cannot detect defects at the bottom of HAR structures, and electron-beam tools are simply too slow for production process control."
DDH is an imaging technology invented by nLine cofounder and chief technical officer C. E. "Tommy" Thomas while at Oak Ridge National Laboratory in Tennessee. DDH enables the digital recording of a hologram of the wafer surface on a CCD camera. The hologram image encodes information about the amplitude and phase of light reflected from the wafer surface. The amplitude information can be used to create a conventional black-and-white image. Because phase is related to beam path length, the phase information can be used to create a topographic map of the wafer surface. No other defect-inspection tool uses topographic information to detect defects.
Unlike conventional imaging techniques, phase measurement is not diffraction-limited. Resolution in the vertical direction can be less than 1/100 of the wavelength of the illuminating light. Therefore a DDH tool can detect defects as small as one-fifth the size of the minimum defect detectable by a conventional brightfield inspection tool operating at the same wavelength and camera pixel size. In addition, very low signal strength is required to make the phase measurement because DDH uses a heterodyne signal amplification technique. To inspect HAR structures, very little light needs to get to the bottom of and then back out of the structure to create a detail rich image. This allows DDH to detect defects at the bottom of unfilled 12:1 aspect ratio contacts, something that no other technique can do.