NUS

Quantitative dopant contrast measurements inside the Scanning Electron Microscope (SEM) have so far been limited to the probing of idealized test pn junction specimens, where only step dopant concentrations have been quantified. Lack of precision and reliability have been attributed to errors caused by a native oxide layer, beam-induced contamination, and surface electric fields. In addition, dopant contrast mechanisms are not well understood. There is no agreement about which one is more accurate, how they relate to one another, or how each of them is linked to dopant concentration. This paper shows how these problems can be overcome by using a high signal-to-noise band-pass electron energy spectrometer attachment inside the SEM to quantify changes in the shape of a Secondary Electron (SE) energy spectral signal.

Experimental SE energy spectral signals show that there are at least two different contrast mechanisms contained within the shape of the SE energy spectrum, both can be used to map dopant concentration variations separately, but together, they provide more information about the specimen. A relative change in the spectral signal’s peak height is related to electron affinity information, while a change in its most probable energy is linked to the presence of space-charge regions. These lattice and electronic material properties of the sample can be simultaneously superimposed on to the SEM’s topographical image. Lateral profiling was carried out on a test sample across the edges of  p-stripe regions in n-substrate, and showed good agreement with data derived from the Spread Resistance Profiling (SRP). Depth profiling was carried out on a cleaved solar cell precursor device, and was found to be in good agreement with data obtained by Electrochemical Capacitance-Voltage (ECV) profiling.

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: unmanageably 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 Schottky 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, led by A. Khursheed, have succeeded in using graphene field emission cathodes for electron microscopy/lithography applications. They have 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. This ring-cathode emitter can directly image ring patterns, and has promising applications for electron beam lithography.

Another development is the discovery 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 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 Schottky field emitter tip. There is no obvious need for cathode-tip flashing.  These developments are expected to greatly extend the use of cold field emission electron sources for electron microscopy and lithography applications. 

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: unmanageably 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 Schottky 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, led by A. Khursheed, have succeeded in using graphene field emission cathodes for electron microscopy/lithography applications. They have 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. This ring-cathode emitter can directly image ring patterns, and has promising applications for electron beam lithography.

Another development is the discovery 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 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 Schottky field emitter tip. There is no obvious need for cathode-tip flashing.  These developments are expected to greatly extend the use of cold field emission electron sources for electron microscopy and lithography applications.