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Designer hand draws geometries which specify transistors and other devices for an integrated circuit. Designer must be an expert in VLSI (Very Large Scale Integration) design.
Can acheive very hgh transistor density (transistors per square micron); unfortunately, design time can be very long (multiple months).
Involves the creation of a a completely new chip, which consists of about a dozen masks (for the photolitographic manufacturing process). Mask creation is the expensive part.