MetrologyScanning profilometry (contact and non-contact)

SEM

AFM

Contact angle

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By Emok - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3783176

Contact Profilometry

A tip, called a stylus, is scanned over the surface of a sample with a given applied force and measures variations in the surface topology

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• Stylus radius ranges from 20 nm

to 50 mm. (Typical 2-15 mm)

• Can measure heights ranging

from 10 nm to 1 mm.

• Extremely quick measurement

• Very common tool – present in

most cleanrooms

• Typically only scans a line

across the sample

• 3D imaging possible, but at

much lower resolution

Contact

Profilometry

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Contact Profilometry Data

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Optical Profilometry

• Non-contact profilometer based on

optical feedback

• Much faster than contact (stylus)

based profilometry

• 3D mapping is more practical than

with a stylus based system

• Lateral resolution ranges from a few

microns to sub micron

Most common types of optical

profilometers

• Optical Interference

• Confocal Aperture

• Focus Detection

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http://www.laserfocusworld.com/articles/print/volume-46/issue-1/features/optical-surface-profiling.html

Interference Profilometry

• Sample moves up and down

• Light from sample and a

reference mirror recombine at

the detector to produce

interference fringes.

• Height at which the

interferogram is in best focus is

recorded for each point

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The optical path difference = height difference

Each transition (dark/light) =1/2 wavelength

Confocal Aperture

http://www.nanoscience.com/technology/optical-profiler-technology/confocal-wli-pattern-projection-focus-detection/confocal-profilometry/ 7

1. Illumination wavelength is 409

nm.

2. 0.5 nm linear scale module (Z-

resolution).

3. Resolution Repeatability, Z-axis:

≤10 nm.

4. Smallest Feature Detection: ≤10

nm.

5. Spatial Resolution (X/Y): ≤ 180

nm.

6. Motorized Stage: X-Y-Z ; 100 mm

x 100 mm range min.

Optical Resolution

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Practical limit obtained when imaging very small objects

by magnification

diffraction causes blurring of objects when imaging

smaller than 200-500 nm

(diffraction limit)

“broadening” of a point caused by diffraction is

known as the “point spread function, PSF” ()

x-y = (0.61 )/( sin())

= refractive index medium

= half-cone angle of focused light

=𝜆

2 𝑁𝐴

Optical Profilometry Examples

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Stylus versus Optical Profilometry

http://www.filmetrics.com/opticalprofilers/profilm3d-vs-stylus10

Scanning Electron Microscopy

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http://www.ammrf.org.au/myscope/sem/background/

Light Microscope versus SEM

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Scanning Electron Microscopy (SEM)

X-rays

SEM detected electrons

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1. Tungsten filament

2. Solid state crystal (Cerium hexaboride or Lanthanum

hexaboride)

3. Low energy (50 eV)

4. Image formed by inelastic scattering

Secondary Electron Image

1. Secondary electrons are knocked out of the sample by the primary electron beam.2. Escape Depth = ~2 nm. 3. SE image is an image of surface topography

i.e. reveals topographical contrast.

SE generation indicates the shape, edges produce more electrons – better contrast, looks brighter

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Back Scattered Electrons• BSE are primary electrons that scatter back out of the sample.

• BSE signal arises from a much deeper depth than secondary electrons

• Images show less surface topology, but more atomic number contrast

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SE vs BSESecondary

Electron Image

Backscattered

Electron Image

Solder on a circuit board16

ChargingA buildup of negative charge on a specimen irradiated with an electron beam.

http://www.ammrf.org.au/myscope/sem/practice/principles/troubleshooting.phphttp://classes.mst.edu/civeng120/extra/galena_rose/index.html

Sample preparation considerations:• In a traditional SEM, non-conductive samples should

be coated with metal, carbon to prevent charging (a thin conductive film)

• An environmental SEM (ESEM) can be used to prevent charging (allows wet samples visualization, water vapor is an imaging gas)

gaseous secondary electron detector

https://itg.beckman.illinois.edu/microscopy_suite/equipment/downloads/how_it_works.pdf17

Charging

http://www.ammrf.org.au/myscope/sem/practice/principles/troubleshooting.phphttp://classes.mst.edu/civeng120/extra/galena_rose/index.html

Sample preparation considerations:

• In a traditional SEM, non-conductive samples should be coated with metal to prevent charging

• An environmental SEM can also be used to prevent charging

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Atomic Force Microscopy

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https://en.wikipedia.org/wiki/Atomic-force_microscopy#/media/File:Atomic_force_microscope_block_diagram.svg

AFM Schematic

Tapping mode

• Most common imaging mode

• Tip oscillates at its resonant frequency and

taps along the surface

• Helps to prevent damage to the tip/sample

Contact mode

• A constant force is applied between the tip

and the surface.

• The tip will move up and down to track the

surface topology and keep the force

constant

• Can cause damage to the tip/sample

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https://en.wikipedia.org/wiki/Atomic-force_microscopy#/media/File:AFM_(used)_cantilever_in_Scanning_Electron_Microscope,_magnification_1000x.JPG

AFM Tips

• Wide variety of AFM tips:

• Different materials (Si, Si4N3)

• Different spring constants

• Different geometries

• Different surface chemistries

• AFM tips are much sharper

(end radius of <10 nm) than

profilometer tips

Tips with high aspect ratios may be required to

image nanochannels

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AFM of Nanochannels

FIB milled nanochannel

Resin stamp produced from FIB milled nanochannel

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Advantages of AFM• Creates a 3D map of surface

• Can calculate surface roughness

• Many other types of experiments possible (force curves, imaging

in fluid, electrical measurements, etc.)

• Can achieve atomic lateral resolution and sub nm resolution in z

Graphenehttp://blogs.lt.vt.edu/sustainablenano/2015/09/28/atomic-force-microscopy-application-in-nanoscience/

Disadvantages• Scans take much longer than

SEM (5-10 minutes per image)

• AFM artifacts can cause

distortions

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AFM Artifacts• The AFM image is a combination of surface topology and tip geometry.

Tip is not sharp enough for the feature.

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https://upload.wikimedia.org/wikipedia/commons/7/71/Afm_artifact2.pnghttp://www.nanophys.kth.se/nanophys/facilities/nfl/afm/fast-scan/bruker-help/Content/Service%20and%20Apps/Troubleshooting/AFM%20Image%20Quality.htm

AFM Artifacts

Tip is wider than

the feature.

Tip is dirty.

25Dull/dirty tip

Double tip

http://www.ramehart.com/images/ca2.jpg

Contact Angle

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http://www.ramehart.com/images/ca2.jpg

Contact Angle

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ssg = ssl + slg • cos q

θ = contact angleσsg = surface free energy of the solidσsl = interfacial tension between liquid and solidσlg = surface tension of the liquid

Young’s Equation

Contact Angle for Treated COC Surfaces

Can we add functionality by UV/Ozone treatment?-C-OH -COOH

Data from Colleen O’Neil, Soper lab

Polyethylene (PE)Cyclic Olefin Copolymer (COC)

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Thank You!

Any Questions?

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