1. Tools & Techniques
SEM, TEM, AFM, EDX, FIB...

2. Analytical Techniques
Used in production and R&D
Crude view: High magnification microscope and high sensitivity titration (composition analysis)
Basic Principles, capabilities, limitations
Recent advances
22-Sep-18

3. Index Microscopy Scanning Probe Microscopy (SPM)
e.g. STM, AFM and their variations
Secondary Electron Microscopy (SEM)
Analytical
XPS(ESCA), AES, AAS, FTIR,
SIMS,ICP-MS
...
Others
Thickness measurements
Ellipsometry, Interferometry, Four point Probe
FIB
22-Sep-18

4. SPM Scanning Probe Microscopy
Atomic Force Microscopy (more commonly used now)
Scanning Tunneling Microscopy (First)
A 3 dimensional picture, at the best with atomic resolution
Graphite
X and Y scales 30 A, each bump is an atom
DVD 10 um x 10 um image
(color by software)
©Photometrics
Images for STM are from ©Leiden Univ, Netherlands
22-Sep-18

5. STM: Principle Basics Note Prob Location Quantum Physics
Probability of locating an electron in any region can be calculated
Note
The probability of finding an electron at a particular ‘point’ is zero
To find the probability in a small region DxDyDz
integrate the PDF
The probability to find it rapidly decreases, as one moves from the ‘center’ point
Prob
Location
22-Sep-18

6. STM: Principle Basics If two conductors are brought close together
close means 1 nm (approx 3 atomic distances)
and a DC bias is applied
prob of finding an electron on the ‘other’ side is non-zero
Some of the electron ‘tunnels’ through the barrier
Not the same as AC voltage
ac current can pass through, because of the varying field
Classical physics sufficient for AC voltage
Not the same as breakdown (arcing) in DC
If the field is strong, electrons overcome the barrier, not tunneling
22-Sep-18

7. ©Leiden Univ, Netherlands
STM: Principle
Metal with electrons in conduction band. Electrons are at Ef and need work function F to escape from the metal
When probe and tip are brought together, they still need F to ‘jump’ above the barrier
If the distance is small and a voltage is applied, electron can ‘tunnel’ through the barrier
©Leiden Univ, Netherlands
22-Sep-18

8. ©Leiden Univ, Netherlands
STM: Principle
Tunneling depends very strongly (exponential) on the distance
3 A distance corresponds to 1000 times change in current
Directly proportional to voltage (approx)
Work function etc remain constant for a material
©Leiden Univ, Netherlands
22-Sep-18

9. Scanning Tunneling Microscopy
History
In IBM, Binnig & Rohrer invented STM and were awarded Nobel Prize
Design/Working
Similar to Record player
Sharp tip (probe)
not touching the sample
Applied voltage (few mV to few V)
Piezo element for precise movement of tip (up/down)
Precision galvanometer
©Leiden Univ, Netherlands
22-Sep-18

10. Scanning Tunneling Microscopy
If tip touches the sample (conducting sample), there will be (significant) current
If tip is far away from sample, zero current
At very small (1 nm or less) distances, tunneling current
pico amp or nano amp
Very strong dependence on distance
e.g. If the distance increases by about 3 Angstroms, the current decreases by 1000 times!
Feed back loop with piezo electric element
Apply voltage to piezo electric element to change the length (and hence the distance between the probe and the sample)
22-Sep-18

11. STM: Operation z y x Operation:
Apply a voltage and bring the tip down, until a certain amount of current is measured (eg 1nA)
Move the tip in X direction (horizontal)
(actually the sample is moving in horizontal plane)
using piezo electric material
for very precise movement
If the sample is closer
tunneling current increases
a pre-amplifier shows increasing current
converts it to a voltage
A circuit checks it with a reference voltage (corresponding to some preset current, for example 1 nA)
Difference in voltage is amplified , recorded and supplied to piezo tube
tip is pulled back z y x
22-Sep-18

12. ©Leiden Univ, Netherlands
STM: Operation
Electronics
Also uses filters, PID controls in the circuits
©Leiden Univ, Netherlands
22-Sep-18

13. STM : Operation Another piezo tube moves the sample in the x direction
the operation is repeated
moving the sample is better than moving the whole assembly of tip+piezo and electronic
however, moving sample, in a precise manner is not very easy
x and y resolutions are not as good as z resolution
normal sample size is about an inch
At the end of one x scan, the tip is brought back to the beginning, stepped a bit in y direction (again using piezo) and another ‘line scan’ begins
The voltage applied to each piezo element (for x, y and z) are recorded
used to create the final map
22-Sep-18

14. STM: Miscellaneous Graphite in atomic resolution
What is meant by atomic resolution?
Electronic Fermi level map
Works for conductors
Practically, for non conductors, can coat a thin film of gold
one will not get atomic level resolution of original surface, but still very good resolution
Easier to use in air/vacuum, more difficult in liquid medium
22-Sep-18

15. STM: Summary Atomic level resolution
nm level resolution achieved repeatedly
Better suited for conductors
Instrumentation requirements
Capability to detect very small current precisely
Capability to move the tip in a very controlled and precise manner ( 0.1 A is the current advertised capability)
Robust feed back electronics
Sharp tip
need not be exactly conical
High aspect ratios are preferred
Blunt tip will ‘convolute’ the image
22-Sep-18

16. AFM: Principle Repulsion Force Distance Attraction
When a probe attached to a very fine spring is brought near a surface
if the probe is far, there is not much force
when the probe comes near, there is an attractive force
when the probe comes very near, there is repulsion
If one can monitor the force and keep it constant...
Similar to keeping the ‘tunneling current’ constant in STM...
And record the voltage applied to piezo, then one can get the topography
Repulsion
Force
Distance
Attraction
22-Sep-18

17. AFM: Construction Detail
A very flexible spring puts very little load on the tip
AFM spring constant is 0.1 N/m
achieved used cantilever
If spring constant is low, it is flexible
If spring constant is high, then the tip will not ‘respond’ quickly if it is dragged over a surface with topography
Hence tip with high spring constant will slow down the data acquisition (if one wants to do it correctly)
Tip with low spring constant and mass is preferred
corresponds to high resonant frequency
© US Navy Res Lab
22-Sep-18

18. AFM: Construction Detail
© US Navy Res Lab
Sample Tip images
Normal
‘super’ tip
‘ultra’ lever
(home made)
SEM images of tips
The end radius (~ 30 nm for normal & super tips and 10 nm for ultra)
Limits the resolution (similar to wavelength in optics)
22-Sep-18

19. AFM: Construction Detail
AFM: Tip structure
Cantilever
Note: STM had a ‘rigid’ probe structure
Measurement of force
optical lever
©Leiden Univ, Netherlands
©Muller Institute, Swiss
© US Navy Res Lab
22-Sep-18

20. AFM: Construction Detail
Detector Schematic
Position Sensitive Detector (usually segmented detector)
4 or 2 segments
Intensity (i.e. Detected current) must be equal in all the four segments
Laser, cantilever and detector are put together in a rigid mount
If cantilever position moves, reflected beam will move
Four segment (quadrant) can be used to detect lateral forces (measure torsion) also
Lateral Force Microscopy (LFM)
22-Sep-18

21. AFM: Construction Detail
Piezo construction
Original construction with 3 orthogonal bars
Newer construction with a tube
Inner cylinder is one electrode, outer cylinder is segmented (typically 4)
Constant potential between inner and outer cylinders will move the piezo in z direction (compression or tension)
Potential difference between outer plates will move the piezo in (mainly) x or y direction
©Leiden Univ, Netherlands
22-Sep-18

22. AFM: Operation Sample is moved in x and y direction
Cantilever moves up or down, depending on sample topography
Lever movement is detected by segmented photo detector
signal is amplified and compared with ‘reference’ or ‘pre-set’ value
difference is amplified and fed back to the piezo
cantilever (tip) is pulled back (for example)
© US Navy Res Lab
*PID controls included, filters to prevent ‘oscillation’
*When the tip is first brought close to the sample, one can see the tip getting pulled towards the sample (using a microscope)
(attractive force) before repulsion
22-Sep-18

23. AFM: Operation Modes
1. Constant Force Mode: (with feed back control). a.k.a. Height mode
(a) Contact Mode:
The probe is in ‘contact’ with the sample
working in repulsion regime
better (more accurate) reproduction of surface topography
tip wears off quickly, (tip is dragged over suface and lateral forces are significant)
soft surface may get damaged
(b) Non contact mode
probe is bit farther from the sample (attraction regime?)
resolution is worse than that in contact mode
sample not damaged and tip life is longer (soft samples)
however, tip may ‘jump’ to contact mode (e.g.water on the surface etc)
22-Sep-18

24. AFM: Operation Modes
1. Constant Force Mode: (with feed back control). a.k.a. Height mode
(c) Tapping mode (®Digital Instruments)
Probe oscillates at a particular frequency
AC signal at the detector (in each segment) detected
probe is probably in both the regimes (attractive and repulsion)
better resolution than non contact mode, but longer tip life etc...
2. Constant Height Mode:(with out feed back control). a.k.a. Deflection mode
Works for surface with out too much topography
Tip is brought close to the sample and once a preset force is reached, tip is stopped
sample scanned (line by line) and tip position is recorded (from the laser position). Tip is not moved up or down
Tip can crash if the sample is rough!
Relatively lower resolution
Faster image acquisition (since tip need not move in z direction)
22-Sep-18

25. AFM: Tip Effects Compression effect
If the sample is soft, it may get compressed when the tip comes close
(compare the elasticity of probe vs surface). Probe has low spring constant
Can actually be used (with slight modification) to measure surface hardness
Note: The force is low , in nano Newton
However, pressure can be MPa!
Image broadening
Due to large tip
(Tip convolution)
© US Navy Res Lab
© Univ. Bristol
22-Sep-18

26. AFM: Tip Effects Interaction Forces If other forces come into play
e.g. Chemical , magnetic, etc
If one is aware of it, more information can be obtained
If not, image obtained is incorrectly interpreted!
Aspect Ratio
If sample has vertical features
At the best, features with ‘probe aspect ratio’ can be measured accurately
sharper features will still show ‘probe aspect ratio’
© Univ. Bristol
22-Sep-18

27. AFM: Miscellaneous STM not used regularly in the fab; more in R&D
AFM applicable for insulators and conductors; used in fab and R&D labs
Used after dep and CMP to measure ‘roughness’
RMS of the height
Or any other feature (e.g. Poly after etch), to find side-wall slope
aspect ratio issue can be overcome on one side by tilting the sample
Tips are easy to change and are ‘reasonably’ expensive
22-Sep-18