Holograms Fathom High-Aspect-Ratio Features

Direct-to-digital holography (DDH), a new technology developed at Oak Ridge National Laboratory (ORNL, Oak Ridge, Tenn.), enables the user to capture and record the intensity and phase of light at every pixel of a CCD camera. The system uses no film, plates, developer or darkroom, and requires no waiting. The actual object wave (phase and amplitude for each pixel) is extracted and stored using a fast Fourier transform. Additionally, the object wave phase or amplitude can be displayed directly on a 2-D display and stored for later use if desired.

Since a hologram stores amplitude and phase, with phase being directly proportional to wavelength and optical path length, DDH can serve as an extremely precise measurement tool for verifying shapes and dimensions of device features on a wafer. The capability to digitally store true holograms immediately provides a method for improved forms of digital holographic interferometry. Holograms of the same object, after some physical change (stress, temperature, micromachining, etc.), can be subtracted from one another (direct subtraction of phase) to calculate a physical measurement of the change in phase (phase being directly proportional to wavelength). Similarly, two ostensibly identical objects can be compared, and deviations can be identified and measured by subtracting their respective holograms.

International SEMATECH defect array with 60 nm surface defects. (Source: nLine)

While digital holographic interferometry (often practiced in the form of electronic speckle pattern interferometry) has been used for several years, it had not been possible to extract the original object wave. True digital holography — sufficiently adequate for this kind of a system — had to wait for digital video cameras, or cameras adaptable to digital media, to attain resolution capable of recording the very high spatial frequencies inherent in classical sideband holograms.

The Fathom combines the use of high-resolution video cameras, very small angle mixing of the holographic object and reference waves (mixing at an angle that results in at least two pixels per carrier frequency/sideband fringe), imaging of the object at the recording (camera) plane, magnification of the object as necessary for the required spatial resolution, and Fourier transform analysis of the heterodyne (sideband) hologram to make it possible to record holographic images. Additionally, an aperture stop must be used in the back focal plane of one or more lenses involved in focusing the object to prevent aliasing of any frequencies higher than can be resolved by the imaging system. No aperture is necessary if all spatial frequencies in the object are resolved by the holographic sideband imaging system. Once recorded, it is possible to play the holograms as 3-D phase or amplitude plots on a 2-D display.

DDH technology shows enormous promise as an imaging, mapping and quantitative measurement tool. Initial work on wafer defect detection with the DUV tool is extremely promising, particularly for the inspection of high-aspect-ratio structures.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.