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  • Metrology System Advances Photomasks
Aug 2010
JENA, Germany, Aug. 16, 2010 — Sematech and the Semiconductor Metrology Systems (SMS) division of Carl Zeiss announced recently that Zeiss’ next-generation photomask registration and overlay metrology system has passed a key development milestone. The jointly developed system, called Prove, demonstrated advanced photomask measurement capability for the 32-nm node and below. In a series of test runs, the key specifications — 0.5-nm repeatability and 1.0-nm accuracy in image placement, registration and overlay measurement — were verified.

“The mask pattern placement metrology tool project builds upon a partnership between Carl Zeiss SMS and Sematech for past work on mask Aerial Image Metrology Systems (AIMS) tool platforms. The partnership has resulted in a working metrology tool that is meeting specifications for repeatability, reproducibility and accuracy at the 32-nm half-pitch node,” said Bryan Rice, director of lithography at Sematech.

“The industry now has the capability to determine smaller image placement errors than could be measured before. Achieving these specifications is a major milestone toward enabling the International Technology Roadmap for Semiconductors (ITRS) mask requirements for the 32-nm node and below. This accomplishment will help to advance the development of photomasks with tighter overlay requirements, demanded by memory devices and double patterning methods.”

The performance targets of the tool were driven by the requirements for advanced memory and double-exposure/double-patterning mask pattern placement and overlay that will help extend 193-nm lithography, according to the ITRS.

“To achieve the performance specification of the Prove system is a major milestone in the project and crucial for our customers in the mask making industry. The system is based on a completely new developed platform, enabling in-die and subnanometer pattern placement metrology in a most versatile way. The measurements can be done on arbitrary production features in the active area of the photomask for accurate and cost-efficient metrology and is extendable to EUV technology,” said Dr. Oliver Kienzle, managing director of Carl Zeiss SMS. “We will now roll out the Prove product with deliveries to our customers.”

This technology represents a significant improvement over previous capability due primarily to the incorporation of high-resolution, 193-nm-wavelength imaging optics, a flexible illuminator that maximizes image contrast, a highly versatile in-die registration analysis algorithm, and a state-of-the-art metrology platform. The system can be fully extended to measure EUV photomasks. The tool will play a vital role in enabling next-generation mask-making technology, said the company.

Carl Zeiss was chosen to develop the system in April 2007 by an evaluation team of mask makers and Sematech member companies. Carl Zeiss’ proposal included a novel design allowing mask manufacturers to measure position deviation of photomask features with high precision and accuracy. Since that time, Carl Zeiss and Sematech engineers have partnered to develop the concept into a working metrology tool.

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image contrast
Also referred to as image visibility, the contrast of an image is the variation in the intensity of an image formed by an optical system, where the image pixels are defined on a gray scale scheme of black being the dimmest and white being the brightest intensity values. Image contrast is then quantified by the expression: (I_max –I_min) / (I_max + I_min), where I_max and I_min are the maximum and minimum points of intensity in the image.
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