Application of near-field optics to critical dimension metrology

During the manufacture of microelectronic devices and circuits, it is necessary to accurately measure the sizes of the features being patterned. For measurement of extremely small features (<0.25 /spl mu/m), scanned probe microscopes are commonly used, but these techniques are typically quite poor at measuring absolute length scales. The reason for this is the extreme nonlinearity of the piezoelectric element used to provide the fine motion of the probe. We have designed and built a near-field scanning optical microscope that corrects for these problems. This technique takes advantage of the fact that the probe is emitting light, by imaging the aperture on a position-sensitive photodetector. This technology is enabled by the recent invention of all-electrical force feedback using an oscillating quartz tuning fork. This allows the design of a tip-scanning microscope as opposed to a sample-scanner. The optical probe is a length of metallized single mode optical fiber, whose end has been tapered to form a small point. The probe is positioned by a piezoelectric tube, which can provide motion in three dimensions.