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FEATURE

Overlay Measurement Systems

Precision measurement technology in IC manufacturing

by Keiichi Hosoi

November 2002

While integrated circuit (IC) patterns continue to shrink, the capacity of dynamic random access memory (DRAM) in mass production is evolving from 128M to 256Mbit, and the minimum linewidth is now 130nm or finer. IC manufacturing relies on miniaturization technologies, and the role of steppers—the key machines in IC production—is crucial.

At the same time, continuing IC sophistication means more advanced measurement and inspection technologies are required to ensure that ICs are manufactured as designed. Simultaneous development of manufacturing and measurement/inspection technology guarantees a bright future for ICs.

In this issue, we focus on the Nikon NRM-3000/1000 series overlay measurement system (abbreviated as NRM herein), which features state-of-the-art measurement and inspection technologies. Essentially a measurement and inspection system, the NRM combines an optical microscope with exclusive optical system, a CCD camera and an image processing computer.

While the name gives little away, the NRM overlay measurement system is actually capable of handling a wide range of tasks. Let's take a closer look.

MEASURING AND INSPECTING ICS
In the manufacture of ICs, circuit patterns are formed in multiple layers on a silicon wafer. Reticles, the master images of the circuit patterns, are changed one after another and exposed repeatedly through the stepper to build up many layers of patterns. The 128M DRAM, for example, usually consists of 24 or 25 layers.