A new interferometer has been developed using an engineered low coherence LED illumination system, where both the spectral and spatial coherence are tailored to allow single surface interference from plane parallel transparent optical components such as optical windows, glass wafers, glass computer disk substrates, etc. without the need to paint the rear surface to suppress interference from that surface. Only the reflection from a surface at a specific optical path difference can interfere with the reference beam eliminating spurious interference. While the interference region due to spectral coherence is engineered to provide interference within a 280 micron zone, diffraction at surface defects and dust within this zone produces secondary wavefronts which can interfere with the test and reference wavefronts, producing phase artifacts which can limit measurement accuracy in standard coherent laser-based interferometers; these phase errors are eliminated by using a spatially extended LED source. For this low coherence interferometer, the “purity” of interference between the test surface and a high quality calibrated reference surface, along with high spatial resolution phase measurements, offers the ability to review waviness and surface polish variations to sub-nanometer height resolutions over a large 4 inch field-of-view. Examples of polish process variations of optical components, along with power spectral density plots (PSD) are shown where the data were acquired with OptoFlat, a downward-looking LED-based low coherence interferometer newly developed for the rapid measurement of precision polished flat surfaces including thin transparent optical components without special surface preparation.
With their high capacity and low cost, magnetic disks remain competitive for digital data storage. The low flying heights of the read/write head in the single nm-range place stringent requirements on the disk substrate topography. Interferometric surface metrology can provide the required topography data. For the measurement of state-of-the-art disk substrates with Angstrom-level waviness, the interferometer noise needs to be extremely low. A robust and cost-effective approach uses an LED as a light source providing temporally and spatially low-coherence illumination. The low temporal coherence leads to axially localized fringes, and clean single-surface fringes and topography maps are obtained even for transparent substrates. The low spatial coherence suppresses coherent noise, or speckle noise, in the fringe intensities and topography maps. Coherent noise cannot be determined by a simple repeatability measurement, since it is stable over some time for a given disk alignment. It does not appear in difference maps, but nevertheless is present in all acquired maps. An upper limit of low-level coherent noise is determined by looking at speckle decorrelation with increasing tilt of the test surface. In this presentation, the coherence issues are discussed together with the characterization of coherent noise and waviness filtering. Disk measurement examples are shown where the data were acquired with OptoFlat, an LED-based interferometer newly developed for the measurement of flat surfaces like disks and wafers.
Wafer dimensional metrology, used to qualify substrates for lithography at the appropriate critical dimensions (CDs), historically reports shape and flatness. While these metrics have enabled several generations of educated wafer procurement, the high numerical aperture (NA) lithography required for sub-wavelength CDs now in production is becoming sensitive to front surface topography that is not reported by either shape or flatness. SEMI Standard M43, Guide for Reporting Wafer Nanotopography, is now published. According to this guide, 'Nanotopography is the non-planar deviation of the whole front wafer surface within a spatial wavelength range of approximately 0.2 to 20 mm and within the fixed quality area.' These nanometer scale non-planar deviations lead to within-die, die-to-die, and wafer-to- wafer variation that contributes to the overall focal budget. Several advanced CMOS device manufactures are no specifying incoming wafer nanotopography. These manufacturers all produce complex, high-speed, large die- size chips. In the following we detail wafer nanotopography metrology and nanotopography quantification. We also explore several known correlations of nanotopography to leading edge process integration issues.
According to industry standards (SEMI M43, Guide for Reporting Wafer Nanotopography), Nanotopography is the non- planar deviation of the whole front wafer surface within a spatial wavelength range of approximately 0.2 to 20 mm and within the fixed quality area (FQA). The need for precision metrology of wafer nanotopography is being actively addressed by interferometric technology. In this paper we present an approach to mapping the whole wafer front surface nanotopography using an engineered coherence interferometer. The interferometer acquires a whole wafer raw topography map. The raw map is then filtered to remove the long spatial wavelength, high amplitude shape contributions and reveal the nanotopography in the filtered map. Filtered maps can be quantitatively analyzed in a variety of ways to enable statistical process control (SPC) of nanotopography parameters. The importance of tracking these parameters for CMOS gate level processes at 180-nm critical dimension, and below, is examined.
The methods of phase shifting interferometry have been applied to a number of unique optical metrology instruments which demonstrates the power of phase detection.
KEYWORDS: Video, Video processing, Computing systems, Interferometers, Optical testing, Signal processing, Interferometry, Digital signal processing, CCD cameras, Phase shifts
Modern interferometric measurements have evolved beyond simple interference fringe tracing to automatic phase measurement over large data array sizes. Through the use of video digitizers and inexpensive PC computers, phase shifting interferometry has become fairly commonplace, with various commercially available instruments on the marketplace. As data arrays grow in size, the number of calculations increases to where the PC computers are stretched for performance. Real time manipulation and display of results are beyond the capabilities of even workstation class computers. In response to the desire to rapidly manipulate large data sets from interferometric measurements, special electronics were developed to operate in conjunction with a high performance interferometer to aid in the testing of large optics.
Interferometric testing of large optics over long path lengths has been hampered by vibration in the test set-up. The precision of phase measuring interferometry has not been able to provide measurements in vibration environments due to the time required to perform the required phase shift between multiple images of the interferogram. The simultaneous phase shift interferometer (SPSI) has eliminated effects of vibration from phase measurements by creating four separate phase shifted interferograms simultaneously, viewed with four CCD cameras. The CCD cameras provide electronic shutter exposure control which effectively 'freezes' the interference patterns producing high contrast interferograms even with severe vibration. Polarization optics are used to maintain the appropriate phase relationships between the four interferograms. Four separate synchronized video digitizers are used to digitize the interferograms to a maximum resolution of 380 by 240 pixels by 8 bits per pixel. The phase at each pixel in the interferogram is calculated by a PC/486 based microcomputer which also provides complete analysis and graphics of the measurement. Averaging of multiple measurements to reduce the effects of air turbulence is done automatically.
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