Recent technology for particle detection on patterned wafers

Laser scanning techniques for detection of particles on patterned wafers are mainly used as in-line particle monitors in the semiconductor manufacturing line. However, as pattern sizes decrease, the complicated signal from the patterned surface is difficult to separate from the signals caused by small particles on the wafers. To reduce such pattern noise in the detection of smaller particles, the following new techniques have been developed. (1) Automatic spatial filtering in the back-focal plane (the Fourier Transform plane) for blocking repetitive pattern noise. (2) Die-to-die signal comparison to cancel non-repetitive pattern noise. (3) Variable threshold level setting for high-level pattern noise. The physics of each method and their effects on reduction of the pattern noise are described. The total performance of the particle inspection system is also discussed.