Whole lot of shakin' goin' on

Minimizing vibration and noise betters image quality in inspection applications. Harrison H. ChinApplied Concept Research Inc. When it comes to high-resolution inspection as in scanning for semiconductor defect review, image stability is paramount. Consider a scanning electron microscope (SEM), which can provide image resolution of 1 nm in theory. In practice, external noise often makes this resolution impractical. Shrinking design rules and nanometer-size defects force equipment designers to focus on the impact that clean-room noise has on the tool. Common clean-room noise sources include floor vibration, acoustical noise, and EMI. The environmental effect is even more apparent in applications exceeding 25,000X magnification where a small disturbance can severely harm image quality. All high-resolution inspection tools incorporate some method of reducing sensitivity to clean-room noise. Isolation systems to attenuate floor vibration are widespread and magnetic shielding, active cancellation, and image processing can eliminate EMI. Using a sealed enclosure around the tool reduces transmission of acoustic noise. Unfortunately, these enclosures are not always used since they require additional clean-room space, complicate tool service, and increase cost. Because it's difficult to completely eliminate the transmission of clean-room noise, equipment designers should examine its effect on the mechanical structure and positioning system. In addition, the operation of the positioning system can also degrade image quality and should also be carefully reviewed. Typically, several resonances in the optics-wafer mechanical structural loop can have a hand in degrading images. The way to keep the image stable is to avoid any relative motion between the loop end points. In practice, there is relative motion when the frequency of a noise source coincides with a mechanical resonance. The amount of relative motion depends on the input noise level and the mechanical gain or amplification at resonance. The mechanical structure can be modeled as a second-order, mass-damper-spring system. Designers measure