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Dipartimento di Fisica - Politecnico di Milano

[19/12/2018 - 15:00]

Prof. Anjam Khursheed (The Engineering Science Programme - The National University of Singapore)


"Graphene-based field emission sources for electron microscopy and lithograph"


mer 19/12/2018 - 15:00 - Aula Rossa, Dipartimento di Fisica



Over the past decades, the extensive research work carried out on carbon-based cathodes for cold field emission, such as Carbon Nanotubes (CNTs), has not as yet,  led to new viable electron sources for electron microscopy/lithography. Their most successful layout has typically been in form of dots arrays for large area field emission applications. Nano size emitter single point cathodes have proven to have even more severe problems than conventional single crystal tungsten cathodes: unmanageable stringent UHV requirements, relatively large current stabilities, and rapid emission decay in periods as short as one to two hours, requiring regular flashing (Joule heating).  These difficulties have prevented the widespread use of cold field emission electron sources for electron microscopy/lithography applications.  The more stable and reliable Shottky field emission source is often used, despite it having a lower reduced brightness and a larger energy spread than cold field emission cathodes.
Recently, the research group at the National University of Singapore have succeeded in using graphene field emission cathodes for electron microscopy/lithography applications. Their research has found that Graphene coated on a Ni sharpened tip dramatically lowers the work function of graphene (by over a factor of 4), enabling it to both provide stable field emission at cathode-tip electric field strengths as low as 0.5 V/nm, an order of magnitude lower than conventional single crystal tungsten point cathodes. This makes it possible to both operate the cathode in HV conditions (5×10-8 Torr) and use relatively large cathode-tip sizes (micron sizes), over three time larger than conventional single crystal tungsten tip sizes, sizes that are comparable to the Schotkky field emitter tip. The brightness and energy spread have been measured to be comparable or better than conventional tungsten cold field emitters, with the added advantages of working in HV conditions and having a very low work function. There are a wide range applications for the new graphene Ni coated source, ranging from pulsed emitters, miniature SEM columns, through to multi-beam electron inspection/lithographic systems. The NUS group have also  obtained stable field emission from a free-standing graphene ring structure, 5 μm in diameter and a wall thickness of around 3 nm. Emission currents of around 30 μA have been obtained at relatively low applied electric field (1.75 V/μm) in HV conditions (5×10-7 Torr). This ring-cathode emitter can directly image ring patterns, and has promising applications for electron beam lithography.



Professor Anjam Khursheed is Director of the Engineering Science Programme at the National University of Singapore (NUS), and also a member of staff in the Electrical and Computer Engineering Department. His area of research lies in the subject of developing new kinds of focused electron/ion beam microscopes and electron/ion sources. At present, his group have a research grant to find new ways to micro-fabricate electron sources for parallel electron beam microscopy/lithography applications. He also carries out research in the subjects of computational electron optics and the design of electron energy spectrometers for the Scanning Electron Microscope.
Anjam Khursheed obtained his BSC in Electronics and Physics from the University of Edinburgh, Scotland, in 1979, and went on to do a PHD there in the subject of Electron Energy Spectrometers for the Scanning Electron Microscope. Apart from being a Postdoctoral Fellow at Edinburgh University (1987-92), Anjam Khursheed has also worked as an applied physicist at the High Energy Particle Physics Accelerator organization of CERN in Geneva, Switzerland (1984-7), where he worked on the design of high frequency electromagnetic cavities. He also worked on the subject of atomic clocks at the Politecnico di Torino, Italy (1992-1994). In 1995, Anjam Khursheed joined the Electrical and Computer Engineering Department at the National University of Singapore, and became deputy director of the Engineering Science Programme since its inception in 2006, and subsequently its Director in 2016.

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