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Feature Articles

Nanotech Pushes Vision into the Extreme Ultraviolet

by Winn Hardin, Contributing Editor - AIA

 

Need for actinic metrology systems increases with the move to extreme ultraviolet (EUV) imaging solutions

There was a time when most semiconductor advances came out of Silicon Valley in California, Research Triangle Park, North Carolina, or Murray Hill, New Jersey. In the nanotechnology world, eyes often drift toward the small northeastern city of Albany, New York, and the nanotech testbed at the University of Albany.

Albany is home to the College of Nanoscale Science and Engineering (CNSE) of the University at Albany. CNSE’s research model involves not only traditional university research, but also features partnerships with corporate leaders in the fields of nanoelectronics, optoelectronics, telecommunications, defense, and nanobiotechnology. The Center’s campus includes five cleanrooms with another on the way and is often used as a testbed for new semiconductor, nanotechnology, and microelectromechanical (MEMs) design and manufacturing process development.

Metrology is a big part of nanotechnology development, and so is image processing. ‘‘We have an extensive set of SEMs [Scanning Electron Microscopes], optical microscopes, CDSEM [critical dimension SEM] as well as a KLA-Tencor bright-field inspection system (KLA-Tencor 2800) and a dark-field imaging system from Applied Materials,’‘ explains Dr. James Ryan, associate vice president of technology and professor of nanoscience at the College of Nanoscale Science and Engineering. ‘‘The metrology systems are critical for our success, whether you’re talking nanotechnology in general or nanoelectronics in particular. With nanotechnology, you have to understand what you’re making, and develop an understanding of the issues that can arise during fabrication. That understanding is key to our ability to continue to perfect both the structures and the process.’‘

Today, the smallest features on a microchip in volume production are created using 193-nm excimer laser sources focused through the scanner optical path that includes water between the lens and the wafer to generate critical dimensions (CD) as small as 65 nm (45 nm and 32 nm half-pitch technology nodes are currently in development). Within the next decade, next-generation extreme ultraviolet (EUV) light sources generate light with a wavelength of 13.5 nm – a move that will have to be mimicked by the metrology world if it wants to provide useful tools.

‘‘There’s a drive in the semiconductor industry to move toward actinic metrology, or metrology that essentially uses the same wavelength to measure the structure as was used to create it,’‘ explains Dr. Ryan.

Connecting Metrology to Production
Nanotechnology involves feature sizes and dimensions that are less than 100 nm. Some nanotechnology devices, such as digital light projection (DL) micromirrors and atomic force microscope (AFM) cantilever probes, have X,Y dimensions far below 100 nm, but also have vertical structures that are several orders of magnitude larger, generating depth of field issues for imaging systems capable of resolving the minimum CD. Other nanotechnology structures, such as molecules, carbon nanotubes and buckeyballs, among others, fall well under the 100 nm threshold. The diversity in sizes and shapes in the world of the ultrasmall is why nanotechnology requires such a wide variety of inspection and test devices.

The general rule of physics governing the ability to image an object is that an object or feature cannot be resolved if it is smaller than the wavelength of the illuminating light. Several methods exist to bend this rule, including those that allow 193 nm light sources to create 32 nm features. However, while the rules can be bent with great difficultly, they cannot be broken. Using the same wavelength for metrology techniques (actinic metrology) as is used in fabrication of a leading-edge device enables inspection of the smallest features on a wafer, photomask or other nanoscale structure. 

Today, bright field images collected from direct illumination and dark field imaging collected from scattered illumination that resolve nanometer-scale structures typically use Mercury-Xenon lamps filtered to the UV spectrum. More advanced systems can use the UV lasers to provide more intense illumination, but both systems face challenges from traditional optics.

UV light darkens silicon-based glass, so lithography source and metrology system manufacturers are forced to use various water-soluble salts, which are extremely difficult to mold into optics, or mixtures of Fluorite and Quartz, for illumination between 170 nm and 300 nm.

‘‘To achieve 100-nm resolution, parts must be imaged at the 248 nm DUV wavelength,’‘ explains William Bridson, director of new product development at Navitar (Rochester, New York). ‘‘You have Fluorite and Quartz to work with, and a few rare earth salts, but they don’t make good lenses. And it’s extremely difficult to make well corrected lenses using only two materials.’‘

Most microscopes used to image nanometer-scale structures use visible light with standard glass objectives to find the feature in question. The operator then switches to the UV filtered light with special step objectives to spread reflected light from a 100 nm structure across two pixels of the detector, which can measure up to 40 microns or more. A pair of pixels is the minimum requirement for a image sensor to resolve a feature in an image.

Last fall, Navitar prototyped the NanoVue 248 zoom lens to ease inspection of nanometer-scale structures. The DUV operating wavelength effectively doubles the resolution ability of conventional visible-light microscopes. The lens is designed to work with 248 nm DUV infinity-corrected microscope objectives to enable optical inspection to under 0.1 micron; the lens has a zoom range of 0.75X to 3X and when combined with a 100X objective corrected at a 200 mm tube length, the NanoVue 248 can deliver a zoom range between 75X to 300X.

‘‘If you’re doing some quality assurance and you’re looking for lines and breaks in a 80-nm trace, the only way you can see that is with an electron microscope, and they’re destructive,’‘ Bridson says. ‘‘RIT [Rochester Institute of Technology] has been doing a lot of semiconductor metrology development, and right now, they can only use an SEM. They’d be a lot happier if they could see the trace in real time.’‘

Electrons and Nanotechnology
For resolving a structure that measures only a few nanometers, visible light microscope imaging techniques fail, but the image processing algorithms of machine vision continue to press on.

Transmission electron microscopes (TEMs), traditional SEMs and in-line SEMs with focused ion beams (FIB) for on-the-fly wafer inspection are a critical component of any nanotechnology development facility, according to Albany’s Dr. Ryan. ‘‘Electron microscopes give you 3D information on a level that visible systems cannot,’‘ Ryan explains. ‘‘In-line SEMs with focused ion beam capability like the Applied Materials SEMVision allow you to make a cross section of the wafer with the FIB and study it with the SEM, but continue to process it in the fabrication line. At the end of the process flow, the samples are also examined with traditional SEMs and TEMs where the sample preparation techniques require the wafers to be cleaved, so in order to gather all the structural information required, you need various types of electron microscopes.’‘

FEI Company (Hillsboro, Oregon) is using a combination of automated image processing and improved engineering to make these electron microscopes more user-friendly to support the growing nanotech industry. Looking forward, Stacey Stone, Technologist at FEI's NanoElectronics Fab Division, notes that, ‘‘Our customers are driving for more automated and easy to use systems for TEM preparation and imaging. FEI is leading the way with our use of pattern recognition and edge finding algorithms used in conjunction with our leading edge FIB [focused ion beam], SEM [scanning electron microscopy], and TEM [transmission electron microscopy] technology to automate tasks that have historically been done manually. Also, customers have an increased need for higher resolution and material contrast on the 45 nm and 32 nm semiconductor process nodes. This need is being driven by limitations in conventional SEM imaging techniques for contrast and resolution at the nanoscale. Further, the material characterization in the semiconductor industry is starting to be driven by the size of transition regions where concentration changes of specific material components change, leading up to an interface.’‘

Through a combination of ‘machine vision’ automation, improved optics and visible imaging systems, and advanced electron microscopes, scientists are answering the new challenges posed by nanotechnology. ‘‘Metrology is a critical enabler for nanotechnology programs,’‘ explains CNSE’s Dr. Ryan, ‘‘but I think that the companies and scientists engaged in it are continuing to make progress. When EUV production begins, actinic metrology should be there, too.’‘

 

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