Patterned wafer inspection using laser holography and spatial frequency filtering

An automatic patterned wafer inspection system for use in integrated circuit fabrication is described. The system is based on the techniques of optical spatial frequency filtering and laser holography. The system can locate defects as small as 0.5 μm over a 150 mm patterned silicon wafer in ∼30 min. The wafer under inspection is placed in front of a lens and is flood illuminated through beam splitting optics with a collimated laser beam. The light reflected and diffracted from the wafer is collected by the lens through the beam splitter. In the back focal plane of the lens, the Fourier transform (spatial frequency) pattern of the wafer circuit is formed. A spatial filter placed in this plane effectively blocks the transmission of the light from the repetitive circuit features of all dies of the wafer. The wave front transmitted through the filter is then recorded in a hologram. The conjugate to the recorded wave front is later reconstructed from the hologram. The reconstructed wave front thus reverse ray traces the filter and the lens, forming a real image of the wafer. In this image, the defects are greatly intensified relative to the filtered circuit features. An analysis of the optical wave fronts in the system is provided. The defect information is obtained by scanning a two-dimensional photodetector array through the holographic image and recording the size and location of each point of light above a predetermined threshold. Test wafers containing electron beam programmed defects in a trilevel resist structure were prepared and inspected. Performance results are described.