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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!