Called the DualBeam line, the semiconductor metrology machines let researchers and engineers poke around submicron structures. The machines use a scanning charge-particle beam to generate secondary electrons, which the system discerns via a special secondary electron detector. The detector output gets converted to a raw video signal, meaning it contains no synchronization signals. "The synchronization signals have to be generated by the beam control system, so that it acts like a camera in this application," explains Steve Rosenberg, director of engineering at the Beam Control Group of FEI Co., the Hillsboro, Oreg., maker of DualBeam machines.
The machine's image acquisition must also reliably handle scan (pixel) rates which fluctuate from slow (100
m sec) to fast (100 nsec) within each application. This happens because the secondary detector is quite sensitive and thus generates a lot of noise.
A Viper-Quad four-channel frame grabber from Coreco Imaging does the necessary processing. It passes images to a PC, where they are controlled, processed, analyzed and output in hardcopy format.
One input of the frame grabber is dedicated to acquiring images from a CCD composite video source. The other three inputs acquire images from a slow-scan video source using an external vertical/horizontal sync and pixel clock or a fast scan video source using an external vertical/horizontal sync. As only one VS/HS input is provided for the latter three, they are, by definition, synchronized.
To help increase image fidelity, Coreco developed a hardware-based noise reduction algorithm that processes images in real time. This pixel averaging function "has definitely been one of the most helpful features in ensuring image quality and reliability," says Rosenberg.