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Abstract

An apparatus for both deflecting a beam of light illuminating a spot on a surface and varying the size of the spot, electronically, without changing any system components. The apparatus includes an acousto-optic deflector driven with a linear FM signal produced by a chirp signal generator. The linear FM signal is characterized with a dispersion rate, and the chirp signal generator includes a chirp dispersion selector to vary the dispersion rate. A beam of collimated light passes through the acousto-optic deflector and appropriate focusing optics image the beam onto a spot in a nominal focal plane. The chirp dispersion selector sets the dispersion rate in accord to a nominal rate, resulting in the beam illuminating a spot in the focal plane. Generally, the focal plane coincides with a wafer surface, of the type having periodic and non-periodic features on it. The spot size may be varied from that of a diffraction limited spot to a spot whose maximum size is system dependent. The spot size varies as a result of changing the dispersion rate of the chirp signal. The spot size may vary as it is scanned, or may remain fixed during the inspection of a wafer. In this manner, inspection by periodic feature comparison may be implemented when it proves advantageous. Alternatively, a larger spot may be obtained when periodic feature comparison would provide no benefit, and spatial filtering would achieve an enhanced signal/background.

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