Part II: Thin Film Characterization - physics.science.cmu ...physics.science.cmu.ac.th/courses/210441/lecture01.pdf · Part II: Thin Film Characterization 1. ... 210441 Physics of

210441 Physics of Thin Films

Part II: Thin Film Characterization

1. Introduction to Thin Film Characterization Techniques

2. Structural characterization: SEM, TEM, AFM, STM

3. Chemical characterization: AES, SIMS, RBS, XRD, XPS, XRF

4. Thickness characterization: Optical techniques, stylus profilometry,

etc.

Referenced Textbooks and Articles

1. M. Ohring, The Materials Science of Thin Films, Academic Press,

San Diego (1992)

2. J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and

E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis,

Plenum, New York (1981)

3. W.A. Pliskin and S.J. Zanin, in Handbook of Thin Films Technology,

eds. L.I. Maissel and R. Glang, McGraw-Hill, New York (1970)


General details of thin film characterization instruments

Size: - varies from a portable desktop interferrometer to the

50-ft long accerlerator and beam line of a Rutherford

backscattering (RBS) facility

Cost: - ranges from the modest cost of test instruments required

to measure electrical resistance of films to the approximate

$1,000,000 price tag of a commercial SIMS spectrometer.

Operating environment:

- varies from the ambient in the measurement of film thickness

to the 10-10-torr vacuum required for the measurement

of film surface composition.

Sophistication:

- At one extreme is the manual scotch-tape film peel test for

adhesion, and at the other is an assortment of electron

microscopes and surface analytical equipment where

operation and data gathering, analysis, and display are

essentially computer-controlled.


Film Properties

Common required film properties

- Film morphology and structure

- Film compositions

- Film thickness

Other properties related to specific applications

- hardness, adhesion, stress, electrical conductivity,

reflectivity, refractive index, magnetic domains,

electrical polarization, etc.


Structural Characterization

1. Scanning Electron Microscopy (SEM)

Figure 1. A picture of an ultra-high resolution SEM.(From Hitachi High-Technologies Canada, Inc.)

Specifications

Image resolution

0.5 nm @30kV guaranteed (magnification = 600,000x )

1.8 nm @ 1kV guaranteed (magnification = 150,000x )

(secondary electron image with test specimen)

Magnification: 60x - 2,000,000x

Accelerating voltage: 0.5 - 30 kV

Electron gun: cold field emission source


Figure 2. A schematic of the electron and x-ray optics of a combined SEM-EPMA (Electron Probe Microanalysis).

(From J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, Plenum, New York (1981))


Wehnelt Cylinder

Figure 3. Wehnelt cylinder.(From J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, Plenum, New York (1981))


Important Features of SEM

1. Electron gun

1.1 Tungsten hairpin electron gun

- size: ~100 µm wire

- tip radius: ~100 µm

- thermionic emission: resistive heating (if) is required.

- source size: ~30-100 µm

- usage: reliable, properties are well understood, relatively

inexpensive, not so high brightness (for low

magnification applications), stable high currents (for

X-ray microanalysis)

- At a temperature of 2700 K and a brightness of 105 A/cm2sr,

a typical lifetime is about 30-100 h in a reasonably good

vacuum (10-5 torr).


1.2 Lanthanum hexaboride (LaB6) electron gun

- most common thermionic electron source

- high brightness (5-10 times > tungsten)

- typical operating temperature: 1800 K

→ lower operating temperature increases the lifetime of

the filament.

- requires better vacuum condition (better than 10-7 torr)

→ LaB6 is extremely chemically reactive when hot and it

readily forms compounds with elements in the gas phase

that poisons the cathode and reduce its emission. The

volatile oxides of La and B which form in the presence

of water vapor or oxygen may cause a significant loss of

material.

- size of single crystal block: ~100 µm

- tip radius: 1-10 µm

- operation: activation period (slow heating of filament current)

is required to evaporate contaminants on the surface

of LaB6.

- usage: LaB6 is more expensive (10 times > tungsten) has longer

filament life.


1.3 Field emission electron guns

- resistive heating of filament (to eject electrons) not required

- electrons emitted (tunneled) by high applied electric field

(> 107 V/cm)

- made of wire of single crystal tungsten fashioned into a sharp

point and spot-welded to a tungsten hairpin.

- tip radius: < 100 nm

- Types of field emitter

(a) cold field emitter

→ source size is small that little demagnification of the

beam is necessary to form a small spot on the order of

1-2 nm.

→ energy spread is low which improves performance

for low-voltage operation.

→ require vacuum better than 10-10 torr

→ need to heat tip (from time to time) to about 2500 K

to clean off adsorbed gas atoms. The process is called

"flashing".

(b) thermal field emitter

→ heating a <100> single crystal tungsten field emitter

to 1800 K in a strong electric field, building up and

sharpening up the tip.

→ continuous heating of the tip at 1300-2000 K to

prevent gas molecules from adsorbing on tip's surface

→ may be operated in 10-9 torr vacuum condition.

(c) Schottky emitter

→ operated at 1800 K

→ uses a ZrO coating on a <100> tungsten facet at the

tip to reduce the work function and flat emitting area

provides good emission stability.

→ need greater demagnification to produce 1-2 nm spot.


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Figure 5. Schematic diagram of a directly heated LaB6 emitter thatdirectly replaces a tungsten filament assembly.


Figure 6. Schematic diagram comparing tip shapes for the Schottkyemitter, cold field emitter, and thermal field emitter.(adapted from Tuggle et al., 1985)


2. Electron lenses

- electromagnetic lenses

- demagnify the image of the crossover in the electron gun

(d0 ~ 10-50 µm for a thermionic gun) to the final spot size on the

specimen (1 nm - 1 µm)

- Types of lenses

(a) condenser lenses: demagnify electron-beam crossover diameter

(b) objective lenses: final lens in the column for probe formation

→ pinhole lens: - large specimen can be studied and its size

is limited by size of speciment chamber

below the lens.

- large variability working distance allows

excellent depth of field at long working

distance (and long focal length).

- working distance: 5-40 mm

- focal length: 10-40 mm

- subject to lens aberration and hence

reduction of image resolution.

→ immersion lens: - specimen is immersed in magnetic field

- focal lengths: 2-5 mm

- only a small specimen can be studied.

- lowest aberrations, smallest probe size,

highest image resolution.


Figure 7. A rotationally symmetric electron lens where the coil windingsare inside the iron shell and the field is produced across the lens gap between polepieces.(adapted from Hall, 1966)

Figure 8. Enlarged schematic of polepiece area showing the forces on anelectron that cause it to be focused.



Figure 9. Two objective lens configurations. (a) Asymmetrical pinholelens or conical lens, allowing a large speciment to be placedoutside the lens. (b) Symmetrical immersion lens, where a smallspecimen is placed inside the lens.



3. Electron-Specimen Interactions

3.1 Elastic scattering

- affect the trajectories of the beam electrons inside the specimen

without altering the kinetic energy of the electron.

- responsible for back-scattered electrons (0 ≤ EBSE ≤ E0 with

energy distribution peaking in the range 0.8-0.9E0 for targets

of intermediate and high atomic number.)

3.2 Inelastic scattering

- results in a transfer of energy from the beam electrons to

the atoms of the specimen.

- responsible for generation of

→ secondary electrons (0 ≤ ESE ≤ 50 eV with most

probable energy of 3-5 eV)

→ Auger electrons

→ characteristic and Bremsstrahlung (continuum) X-rays

→ electron-hole pairs in semiconductors and insulators

→ long-wavelength electromagnetic radiation in the

visible, ultraviolet and infrared regions of the spectrum

(cathodoluminescence)

→ lattice vibrations (phonons)

→ electron oscillations in metals (plasmons)


Figure 10. (a) electron and photon signals emanating from tear-shapedinteraction volume during electron-beam impingement on specimen surface. (b) Energy spectrum of electrons emittedfrom specimen surface. (c) Effect of surface topography on electron emission. (From Ref. [1])

Figure 11. Schematic illustration of the origin of two sources of secondaryelectrons in the sample. Incident beam electrons (B) generate secondary electrons (SEI) upon entering the sample. Backscatteredelectrons (BSE) generate secondary electrons (SEII) while leavingthe sample. λ is the mean free path for secondary electrons.


Figure 12. Monte Carlo electron-trajectory simulations of the interactionvolume in iron as a function of beam energy: (a) 10 keV, (b) 20 keV, (c) 30 keV(From J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, Plenum, New York (1981))


4. Detectors

4.1 Everhart-Thornley detector

- most commonly used in SEM

- used to detect secondary electrons and sometimes backscattered

electrons

Figure 13. Schematic diagram of the Everhart-Thornley detector:B, backscattered-electron trajectories; SE, secondary-electrontrajectoreis; F, Faraday cage (bias range -50 V to +250 V);S, scintillator, with thin metallic coating (high bias, +12 kV)supply to the scintillator coating; LG, light guide; PM,photomultiplier.


4.2 Dedicated backscattered electron detector

- usually placed near near the specimen

- Examples: scintillator detector, BSE-to-SE detector, solid-state

diode detector

Figure 14. (a) Schematic diagram of a solid-state BSE detector, showingseparation of electron-hole pairs by an applied external potential. (b) Typical installation of a solid-state detector onthe polepiece of the objective lens.


4.3 Electron-Beam-Induced Current (EBIC)

- use the specimen as a detector

- when the primary electron beam strikes the surface,

electron-hole pairs are generated and the resulting current

is collected to modulate the intensity of the CRT image.

5. Applications of SEM

- topographic contrast (surface imaging)

- compositional contrast

- elevated-pressure (1-10 torr) microscopy

- stereo microscopy

4.4 X-ray detectors

- gas-proportional counter


Figure 15. Magnification series of a fracture surface of high-purity iron, illustrating the rapid surveying capability of the SEM. The imagesare recorded at constant objective lens strength and workingdistance. Note that focus is maintained throughout the series andthere is no image rotation.


Figure 16. (a) Optical micrograph of the radiolarian Trochodiscus longispinus.(b) SEM micrograph of same radiolarian. The greater depth offocus and superior resolving capability are apparent.


Figure 17. High-resolution secondary electron images of the surface of themagnetic storage of a computer hard disc. Image recorded at35 keV on a field-emission SEM with signal collection from an E-T detector. Two magnifications of the same area are shown.


Figure 18. The specimen is Raney nickel, which contains multiple phasesthat produce atomic number contrast and surface defects thatproduce topographic contrast. (a) specimen is viewed witha positively biased E-T detector, showing predominantly topographic contrast. (b) Four-quadrant solid-state BSE detectorgives an atomic number contrast.


Figure 19. Atomic number (compositional contrast) observed in an aluminum-nickel alloy (Raney nickel). (a) Backscattered-electron image derived from a negatively biased Everhart-Thornley detector. (b) Direct specimen-current image of thesame region; note the contrast reversal compared to (a).

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Part II: Thin Film Characterization - physics.science.cmu ...physics.science.cmu.ac.th/courses/210441/lecture01.pdf · Part II: Thin Film Characterization 1. ... 210441 Physics of