Manufacturing Bits: March 2

Next-gen AFM
At the recent SPIE Advanced Lithography conference, Imec, Infinitesima and others described a new metrology tool technology called a Rapid Probe Microscope (RPM).

Infinitesima has shipped its first RPM 3D system, enabling three-dimensional (3D) metrology applications for leading-edge chips. The system was jointly developed with Imec.

In the IEDM paper, Imec and Infinitesima, along with the University of Twente and the University of Bristol, described RPM. Basically, the technology makes use of tomographic atomic force microscopy (AFM) using a novel multi-probe sensing architecture.

An AFM system is a common metrology type, which includes a laser, a diode, and a cantilever with a tiny tip. In an AFM system, a tiny tip is positioned over the surface of a sample. Then, the system generates images of the sample. The goal is to find defects in the sample at the nanoscale.

“A sharp tip is raster-scanned over a surface using a feedback loop to adjust parameters needed to image a surface,” according to Nanoscience Instruments. “Traditionally, most atomic force microscopes use a laser beam deflection system where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. Because the atomic force microscope relies on the forces between the tip and sample, these forces impact AFM imaging. The force is not measured directly, but calculated by measuring the deflection of the lever, knowing the stiffness of the cantilever.”

Generally, AFM systems provides detailed information at the nanoscale, but they are slow. So for years, the industry has been developing multi-probe sensing for AFMs in order to speed up the process, but there are some major challenges here.

In response, Imec, Infinitesima and others described a faster version of the technology. The technology is based on tomographic AFM or scalpel SPM (scanning probe microscopy). “The concept, often referred to as tomographic AFM or scalpel SPM, is based on use of a single-asperity nanocontact capable of sub-nm material removal, thus enabling a three-dimensional segmentation by alternating sensing and removal scans,” said Umberto Celano of Imec, Jenny Goulden of Infinitesima and others in the SPIE paper. “Here, as we aim to obtain a multiscale 3D analysis platform without compromising the high-resolution imaging, we propose a specific design, that can accomplish both accurate tip re-positioning, and a simple technique for switching and using multiple probes.”

The RPM system itself includes a scan head with a multi-probe switching system, and custom probe cassette. The custom probe cassette houses three independent AFM tips—a scalpel for tip-induced material removal; a tip for 3D profiling; and a conventional or other tip type.

The design is capable of accurate tip re-positioning. It makes use of a simple technique for switching and using multiple probes. “We show how the combined use of an interferometric detection system and strain gauge offers improved control for tip-induced material removal,” Celano, Goulden and others said in the SPIE paper.

“In summary, we have introduced a new microscope to perform tomographic sensing using scanning probe techniques. Starting from the baseline hardware of the Rapid Probe Microscope, we reported on the development of a custom scan head that is based on an in-situ, rapidly switchable, multi-probe hardware. To demonstrate the functionality of the RPM 3D, we sensed the conductive profiles in 3D for vertical poly-Si structures that mimic vertical channels of 3D NAND memory. This provides for the first time the potential to combine Scalpel SPM methodology with non-contact modes, including magnetic force microscopy of Kelvin probe force microscopy, among others,” they added. “Further development will explore non-contact modes available using this architecture and the options offered by the combined output of IDS and SG sensors for high speed, high data quality acquisition. This will drive fundamental materials research and site-specific analysis in nanoelectronics devices.”

Scanning probe lithography
Also at the SPIE event, Technische Universität Ilmenau and others presented a paper about a tip-based lithographic/metrology system that can fabricate and analyze tiny structures with high resolutions.

The system, called the Nano Fabrication Machine 100 (NFM-100), incorporates both an AFM and field-emission-scanning-probe-lithography (FESPL) in the same unit. Designed by the Technische Universitat Ilmenau, the system is manufactured by the SIOS Meßtechnik in cooperation with IMMS and nano analytic.

As stated, AFM uses a tiny tip to measure structures at the nanoscale. Scanning probe lithography, which is an R&D technology, uses tiny tips to pattern materials on structures.

With a working range up to 100mm in diameter, the NFM-100 system is an R&D tool for use in developing next-generation structures and materials with feature sizes below 10nm. The NFM-100 uses an active microcantilever with dimensions of L = 350µm x W = 140µm x T = 5µm.

In the system, the AFM can scan over long ranges at relatively high speeds. “With its large positioning range, the NFM-100 provides the possibility to analyze structures over long ranges and large areas. The NFM-100 offers an excellent accuracy and trajectory fidelity. Over a moving distance of 50mm the standard deviation perpendicular to the trajectory is as low as 1.5nm,” said Jaqueline Stauffenberg from Technische Universitat Ilmenau, in a paper at SPIE. Others contributed to the work.

Using the FESPL function, researchers pattered an SOI sample. The velocity of the microcantilever tip was set at 1 µm/s with a set point of 45 pA. With these settings, 20nm linewidths were achieved, according to researchers.

“In the future, particular focus will be placed on tip-based manufacturing on large areas with the use of the NFM-100. Here, a limiting factor is the duration of the writing process,” Stauffenberg said. “Tip-based processes are comparatively slow in fabrication and analysis, e. g. a structuring speed of about 1 µm/s on a surface of 100mm would currently result in a process time of more than 2000 hours. Due to this fact, new writing strategies have to be established and new tools have to be developed. Accordingly, tip wear is also a decisive factor. Here, the wear of the cantilever tip plays a role for the maximal permissible length of the structuring processes on large areas. However, new technologies such as parallel and multi-cantilever array applications can be used to shorten the time. Likewise, the use of diamond tips is a good way to reduce the wear of the ultra-sharp cantilever tip, which has already been proven.”

Litho books
Lithography, the art of patterning features on chips, is a complex subject. To help the industry get up to speed here, Harry Levinson, Andreas Erdmann, and Burn Lin recently discussed their latest or upcoming books on lithography.

Harry Levinson, editor-in-chief of SPIE’s Journal of Micro/Nanopatterning, Materials, and Metrology (JM3), has a new book, entitled, “Extreme Ultraviolet Lithography.” The book covers the many aspects of lithographic technology required to make EUV lithography ready for high-volume manufacturing.

Andreas Erdmann, head of computational lithography and optics at Fraunhofer IISB, discussed the materials for his upcoming book, called “Optical and EUV Lithography: A Modeling Perspective.” The book will explore various lithographic techniques for nanofabrication.

Burn Lin, a Distinguished Research Chair Professor at National Tsing Hua University and director of the Tsing Hua-TSMC Joint Research Center, discussed the material for his upcoming book, “Optical Lithography: Here is Why, Second Edition.” The book covers the image-formation physics of a lithographic system and provides an overview of the future of optical lithography and the many next-generation technologies that may enhance semiconductor manufacturing.

(For more information, contact: Daneet Steffens, public relations manager at SPIE. [email protected])