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Characterizing nanoscale surface roughness
using transmission electron microscope
Subarna Khanal, Abhijeet Gujrati, Tevis D. B. Jacobs
University of Pittsburgh
Pittsburgh, PA, USA
STLE 72nd Annual Meeting & Exhibition
24 May 2017
Outline 2
➢ Motivating the study of roughness
➢ Introduction to roughness models, need for small-scale topography
➢ Techniques for nanoscale surface roughness measurements
– Transmission electron microscopy (TEM)
– Sample preparation for roughness measurement
– Image analysis for roughness measurement
➢ Results and implications for roughness models
➢ Conclusions
➢ Acknowledgments
3
Brand, et al., Tribol. Lett. (2013)
source
drain
Kim, Ku, Song, et al., Nature Comm. (2013)
• Key example: micro-/nanoelectromechanical (MEMS, NEMS) devices and switches
Characterizing roughness for advanced devices
• The nature of the contact depends in part on the small-scale topography
Rezvanian, et al., J. MEMS (2007) Jackson, et al., J. Appl. Phys. (2015)
4
Persson, Surf. Sci. Rep. (2006)
ζ=1
ζ=10ζ=100
➢ While some properties (such as contact stiffness) depend on the coarse
structure,
➢ Some key properties (such as adhesion) depend on the finest-scale structure
Multi-scale contact models link roughness to properties
Persson, Tribol. Lett. (2015)
➢ The Power Spectral Density (PSD)
enables us to decompose contributions
from different length scales
log(Wavevector [1/m])
log
(PSD
[m
4])
Persson, Surf.
Sci. Rep. (2006)
5While nanoscale topography is critically important, it is not accessible with conventional techniques
➢ Our group has reported, using virtual surfaces and experimental
measurements, the presence of a “reliability cut-off” in PSDs at which
artifacts dominate.
• Paper: T. Jacobs, et al, Quantitative characterization of surface topography using spectral analysis. Surface
Topography: Metrology & Properties, arXiv:1607.03040 (2016)
Measured surface
input PSD (used to create virtual surface)
“measured” PSD (virtually scanned, 40-nm radius tip)
artifact-induced self-affine scaling (𝐶 𝑞 ∝ 𝑞−4)
Reliability cut-off adapted from
Church & Tacaks, SPIE, (1990)
where ℎ𝑟𝑚𝑠′′ 𝑞𝑐 ~1/𝑅𝑡𝑖𝑝
Virtual surface
6
• High Resolution Images 1,000,000 X
• Geometry and morphology
• Crystallinity
• Chemical composition
• Dislocations and stacking faults
➢ Transmission Electron Microscopy (TEM)
Benefits
Techniques: Brief introduction to TEM
Viewing axis
< 100 nm
sample
thickness
Electron beam
Image
7
➢ Sample Preparation
1. For deposited materials, apply a coating to a thin structure
e.g. an atomic force microscope probe
Electron
beam
Techniques: Sample preparation for roughness measurement - Coating
e.g. a microfabricated thin wedgeTEM Images
8
Cannot use FIB-based approach: sample surface is damaged and/or hidden
under “protective layer”
Techniques: Sample preparation for roughness measurement – FIB-based Cross-sections
1 mm
2 µm100 nm
protective
platinum
surface of
interest
9
2. Cross-section sample preparation
5 mm
4 mm
Dummy Dummy
Specimen
5 mm
4 mm
Wafer bonding Slicing
Disc cutting & thinning
0.5 mm
3 m
m
Dimple grinding & TEM milling
Ar ion
Techniques: Sample preparation for roughness measurement – Surface preserving cross-sections
Techniques: Sample preparation for roughness
measurement – Surface preserving cross-sections10
Surface of interest
-4º to -3º -4º to -3º
Original
surface
never
sees
ions
-4ºto -3º
<100 nm
Low magnification TEM image of
rough surfaces
2. Cross-section sample preparation
11
1 mm
5 µm
Optical
image
after
dimpling
Optical
image
after ion
milling
Low Magnification Optical and TEM Images
This is the
region
where ion
beam
applied
Low mag
TEM
images UNCD
UNCD
Glue
12
500 nm 50 nm
2 nm
TEM-based topography measurements – AFM Probe
Electron
beam
AF
M-P
robes
TEM-based topography measurements – Si-Wedge
250 nm
13
50 nm
2 nm
Electron
beam
14
UNCDGlue
TEM-based materials analysis
➢ Zoom in TEM images of Cross-Section UNCD Sample
15
100 nm 20 nm
2 nm
TEM-based topography measurements – Cross-Section
Sample
TEM-based materials analysis 16
(220)
(311)
(110)
UNCD
A TEM image of UNCD grains (dotted
lines) near the edge surface
Diffraction pattern of UNCD region
5 nm
17
(111)
(110)
(111)
(110)
UNCD Region:
sp3 diamond,
Lattice spacing:
0.356 nm2 nm
Also enables TEM-based materials analysis
18
Low magnification TEM Image
Many topography measurements, various magnifications
19
10-24
10-28
10-32
10-22
Pow
er
Spectr
al D
ensity [m
3]
104 105
Wavevector, Τ2𝜋𝜆 [m-1]
106 107 108 109 1010
1 mm 1 𝜇m 1 nm
Wavelength, 𝜆
10-26
10-30Atomic Force
Microscopy
(AFM)
Stylus
Profilometry
Transmission
Electron Microscopy
(TEM)
Filling in the small-scale spectral information
20
• Eliminates tip-based artifacts that limit the accuracy of atomic force microscopy
• Enables multi-scale characterization of surface roughness, from millimeters to
Ångströms
• Provides more complete inputs for analytical roughness models
Filling in the small-scale spectral information
21Conclusions
➢ Filled in small-scale spectral information, which is required by models
➢ Developed techniques for surface-preserving sample preparation
➢ Performed TEM-based observation and topography measurement
2 nm
AFM Probe Si-Wedge Cross-Section Sample
22Acknowledgements
Assistance on this project is gratefully acknowledged from:
• Pawel Nowakowsk, James J. Schlenker ( Fischione Instrument)
• Susheng Tan (U. Pittsburgh)
Funding for this project is gratefully acknowledged from:
• NSF CMMI #15-36800
• U. Pitt. Central Research Development Fund
Questions? 23
Thank you for your attention!
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