Nanoscale Chemical Characterization: Moving to 3 Dimensions

Nanoscale Chemical Characterization: Moving to 3 Dimensions Eric B. Steel Chemical Science & Technology Laboratory National Institute of Standards & Technology

Outline What is and why do we need chemical characterization and imaging? Current state of the art 1 and 2 D Why 2D is insufficient? What makes nanotechnology different How do we do 3D? What are the roadblocks? Is it worth it?

Nanoscale Chemical Characterization Spatially resolved on the nanoscale On the scale of the processes and components of nanodevices Chemical analysis Elemental Chemical species distribution Bonding and electronic structure Molecular Isotopic Electron Hologram courtesy Shirley Turner and John Bonevich

Measurement Research Approach No single right tool Multiple techniques must be used; All techniques have severe limitations Approaches Probes Photons, Ions, Electrons Spectroscopies Optical, Mass Spectrometry, X-ray, Electron Analytical Needs Required Information Molecular and/or Elemental and/or Isotopic Spatial Resolution 1D, 2D, 3D

Imaging & Chemical Information Atomic scale imaging Approaches: Scanned Probes (STM, AFM) Field Ion Microscope (Atom Probe) High Resolution Transmission Electron Microscopy Takes advantage of broad spectrum interaction Lacks specificity or requires specific sample Chemical Information Requires spectroscopy Inherently poorer signal to noise Selective interaction with very low cross section Chemical Imaging is MOST challenging

Drivers for Nanoscale Chemistry Industrial needs: Current needs Semiconductor Gate oxides, dopant levels, shrinking device dimensions Optoelectronics Nanoparticles/powders Catalysts, pigments, powder metallurgy, explosives, etc. Coatings and Nanocomposites Near future needs Biomedical/Biotech Microelectromechanical systems (MEMS) Unknowns? Government needs: Defense Department Homeland Security Biomedical - NIH

Example Need: Semiconductor Technology Dimension (nm) Today 2014 Gate length 120 22 <20% <20% Equiv gate dielectric 1.9-1.5 0.5-0.6 <4% <4% Sidewall spacer 72-144 3.7-7.5 <10% <10% Silicide thickness 55 12 Contact depth 75-145 15-35 Drain extension depth 42-70 8-13 Retrograde channel 21-35 4-8

Semiconductor Example Gate Dielectrics Insulator controlling electrical shorts in transistors Now only a few nanometers thick Chemically complex Si to Si-O-N to Si-O to Si Dopant Concentration & Location A few atoms of B, P, As, etc. control electrical properties Poly-Si Si-O-N Si Courtesy of John Henry Scott 2 nm

Gate Dielectric Variation of Si, O, N content along thickness of the gate is critical to electrical properties Must combine nanoscale structural and chemical information to understand the system Point analysis combined with morphology image O N Si Courtesy of

Chemical Imaging 1D, 2D, 3D One dimensional (point analysis related to general (nonspectroscopic) imaging system) Common, various resolutions, typically surface or projection image Manual or semiautomated Two dimensional (mapping) Common, various resolutions, speeds Automated Three dimensional Not common in nanodevices Common in medical imaging

2D Mapping Example Compositional Maps Qualitative Quantitative Solves many simple problems Complexity can be misinterpreted Analytical resolution can vary in x, y, z Sm map Co map C map Courtesy of John Henry Scott

Nanotech Measurement Challenge Increasing Sensitivity No. of Atoms 1.0E+18 1.0E+16 1.0E+14 1.0E+12 1.0E+10 1.0E+08 1.0E+06 1.0E+04 Number of Atoms vs. Size U3O8 Spheres Comfort zone for most analytical laboratories Current research 1.0E+02 1.0E+00 0.1 1 10 100 1000 10000 100000 Diameter in Nanometers New technology needed Increasing Spatial Resolution

Why 2D is insufficient? What makes nanotechnology different Most Common Nanotech Images: Surface Images Scanned Probes Scanning Electron Microscopes Inherently 2D or shallow 3D Projection Images Transmission Electron Microscopes Inherently 2D with 3D convoluted Chemical/Property Maps Inherently 2D Diagrams for cases we cannot image and measure We use a lot of diagrams in nanotechnology 3 µm Courtesy of John Henry Scott

Why Nanoscale 3D Chemical Imaging? Determine the relationship of components within complex nanodevices Many device components are now smaller than our analytical volume 3D morphology is not enough Need micrometer scale with nanometer resolution Needed by current and future nanotechnologies 3 µm Which surface or projection?

Need for 3D Chemical Reconstruction Projection and Surface Images are Limiting Currently most used approach is 2D projection or surface morphologic imaging with limited chemical mapping This approach can easily lead to misinterpretation Chemical 3D information is now often required to determine true nature of working nanodevices and their failure modes Drawing by John O Brien, The New Yorker Magazine (1991)

Current State-of-the-Art: 3D Current 3D nanoscale methods X-ray tomography at beamlines for morphology Electron tomography for morphology Energy Filtered Transmission Electron Microscopy (EFTEM) tomography Atom Probe SPM Confocal methods

Attaining 3D Serial sectioning Ultramicrotomy Biology FIB Materials 2D mapping of each section (many techniques) Depth profiling Surface milling, ablation, or etching 2D mapping over time (Atom Probe, SIMS, FIB, Auger, XPS, etc.) Tomography Tilt series with hyperspectral imaging

EFTEM (Energy Filtered TEM) Example of catalyst particles in zeolite MCM 41 with catalyst nanoparticles, 35 degrees tilt, field of view ~150nm 53 HAADF images acquired at 2 degree intervals from +60 to -48 degrees From M.Weyland, P.A.Midgley and R.E Dunin-Borkowski

Atom Probe Boils off atoms at surface and sends through imaging TOF MS, then recombines the many first surface images to reconstruct 3D chemical images Difficult sample preparation (even by TEM standards) Limited field of view Conducting or semiconducting samples Cu - Red, Ag -Blue Courtesy of Imago

Roadblocks to 3D Nano Improve Sensitivity Increase Spatial Resolution These two needs are often incompatible Improve probe Increase Intensity Increase interaction with specimen Improve detectors Change detector efficiency Improve Probe Reduce size Reduce interaction volume Improve environment Reduce interference New technology is needed to break through this incompatibility

Improving Spatial Resolution SIMS Cluster Ions Monoatomic primary ion bombardment creates extensive subsurface damage resulting in reduced sensitivity and no compositional depth information for organics and reduced depth resolution for elemental depth profiling. SRIM Simulation of Ion Impacts on PMMA film -normal incidence 25 kev Ga + Range 25 = kev 40 Ga nm + Cluster ions offer: Lower penetration depth Higher sputter rate Higher secondary ion yields Reduced accumulation of beam damage 25 kev C + Range 60 = 4 nm Each C = 417 ev Range = 4 nm 0 PMMA 80 nm Courtesy of Greg Gillen

New X-ray Technology Microcalorimeters Few ev resolution, multichannel Improves spectral resolution allowing chemical mapping Silicon Drift Detectors Large area, high count rate Improves efficiency, sensitivity, speed µcal EDS Counts (0.16 ev bins) 2000 1500 1000 500 0 O Kα C Kα NIST K3670 glass Si(Li) EDS NIST µcal EDS (real-time analog processing) Ni Lα Fe Lα Zn Lα Mg Kα Al Kα Si Kα 0 500 1000 1500 2000 Energy (ev) Courtesy of Dale Newbury 15000 10000 5000 Si(Li) EDS Counts (10 ev bins)

Improving Sensitivity & Spatial Resolution Implemented New Technologies Mass Spectrometry (SIMS) Order of magnitude improvement in depth resolution Electron Microscopy Several orders of magnitude improvement in electron beam current and X-ray collection efficiency Improved spatial resolution by factor of five or more Optical Spectroscopy (NSOM) 10-50 time improvement in spatial resolution

3D Chemical Imaging well positioned to move forward but have major technical roadblocks Higher Speed At nm sized pixels a 1 X 1 µm area would take 1000pixels x 1000pixels x ~1sec = 12 days Higher Sensitivity From 100 s of atoms to single atom Zeptogram spectroscopy and analysis High resolution in x, y, and z Larger volumes Higher Spatial Precision Stages need atomic level precision in 6 axes Insensitivity to environment Better measurement environments Move from 1D and 2D to 3D Spectroscopic nanotomography

Courtesy David Muller, Cornell 3D Chemical Imaging The holy grail of characterization and chemical measurement: Know each atom and relationship to all others Where one or more atoms well placed or misplaced can make or break a nanodevice

Metric Units Name 10 24 yotta 10 21 zetta 10 18 exa 10 15 peta 10 12 tera 10 9 giga 10 6 mega 10 3 kilo 10 2 hecto 10 1 deka Symbol Y Z E P T G M k h da Factor Name 10-1 deci 10-2 centi 10-3 milli Symbol d c m 10-6 micro µ 10-9 nano n 10-12 pico p 10-15 femto f 10-18 atto a 10-21 zepto z 10-24 yocto y Nanotechnology requires zeptogram level chemical analysis