Areas of heavier material cause the electrons to scatter backwards away from the wafer surface, while more transparent sections pass the electrons through to image the mask on the resist. The small masks presented a major concern with the system. In order to image a full 300-mm wafer, the system has to scan back and forth across the wafer's surface. Throughput concerns also caused by the mask's small size were dealt with by scanning the electron beam back and forth to image a larger area as it cuts across the wafer surface. The final contender, ion projection, uses an excited helium gas multicusp source.
Magnetic fields extract ions, keeping the beam in a very tight spread.
The ions pass through a silicon mask. Holes in the mask, representing features, allow the ions to pass through to the wafer's surface, while the presence of even a thin layer of silicon in the mask stops the ions, creating the image effect on the wafer surface. In addition to significantly lower source costs, the system should also offer high throughputs.