AFM Flashcards
AFM
NanoFabrication & Nanostructres Characterization
Requirements:
- a highly doped Si cantilever and probe
- Conductive coating (Au, Ir, Pt etc…) *A conductive probe.
A closed circuit (tip-sample) - Sample Holder with clips for contacting electrodes
- Built-In I-V “meter” compatible with the feedback system
Techniques
- Conductive AFM (CAFM)
- Electric Force Microscopy (EFM)
- Kelvin Probe Force Microscopy (KPFM)
- Piezoresponse Force Microscopy (PFM)
Contact-mode: topography
-No cantilever oscillation
-Record z-deflection signal
-Repulsive force regime
-Adhesion forces larged
-Tip in firm contact with sample
C-AFM Conductive AFM
We want to measure current through/on a sample to determine local conductivity variations:
1. Sample is Imaged in Contact Mode
2. Tip is grounded and sample is biased
3. Current through the tip is measured
C-AFM Conductive AFM outcomes
❖ Topography + Current maps
Channels: Height, Deflection, Current
❖Localized I-V curves
C-AFM allows for
- Current mappings
- I-V on specific spots
- I profiles
C-AFM disadvantages
-Extend electrodes for helping accessing and biasing the sample
- Contact Mode makes it harder to image nanostructures or soft materials…
-Tip coating easily removed by usage… Tip wearout!
EFM
The presence of electrostatic forces (due to electric field or charge accumulation) induce shifts and
changes on the tip oscillation (amplitude and phase). These effects are enhanced or minimized by applying an electric potential. The charge distribution on the sample surface can be obtained.
EFM – Electrostatic Force Microscopy
2 pass scanning mode:
- First Scan , topography is measured in tapping mode
- In the second pass line a fixed height is applied and the tip follows the recorded topography accessing the Electric Field
EFM caracteristicas
qualitative contrast with minimal setup quantitative
interpretation is challenging.
The measured signals have a complex dependence on tip
and sample geometry, charge distribution, surface
chemistry, and even atmospheric conditions.
EFM outcomes
❖ Topography + phase disturbance
Channels: Height, Amplitude, nap phase
❖Electric Field variations
KPFM – Kelvin Probe Force Microscopy
2 pass scanning mode:
- First Scan , topograhy in tapping mode
- Second Scan Potential mapping of the surface following the topography raised by a known
distance.
–Tip is raised a fixed lift height
Cantilever is forced with an AC signal
–Bias applied between Sample and tip required to cancel the tip oscillation
KPFM outcomes
❖ Topography + Surface Potential
Channels: Height, Amplitude, V
❖Contact potential difference between tip and sample
KPFM descrição random
When the AFM tip is brought close to the sample surface, an electrical force is generated between the tip and sample surface, due to the differences in their Fermi energy levels.
KPFM can:
-give contrast where topography does not
-measure work function of metals and semiconductors
PFM – Piezoelectric Force Microscopy
Characterization of the electromechanical response of piezoelectric materials.
–A conductive cantilever is scanned over the sample surface in contact mode.
–While scanning the surface, an AC bias is applied to the tip. The electric field causes a strain in the
surface which in turn causes a periodic deflection of the cantilever.
Nano Fabrication - Nanolithography
-Exposure of Resists by Field Electron Emission
-Quite easy to achive such high fields (tip-sample distances < 10 nm) even at low Bias Voltages
-This establishes the basis for electron exposure of resists using an AFM or STM
Nano Fabrication - Nanolithography pt2
-Very thin resist layer (<50 nm) has to be used. emitted e-’s have kinetic energy of only a few tens of volts.
They cannot travel far in resist
-Very divergent beam that depends on the distance between tip and sample and on the applied voltage
-Low energy exposure that in STM case leads to high exposure dose than e-beam lithography
Nano Manipulation
Tip can cut, push and transport materials, creating patterns (or modifying them) or change the location in the
substrate
AFM Local Anodic Oxidation (LAO)
A voltage pulse is applied between the AFM tip and the sample. The BIAS induces the formation of a water bridge if the amplitude of the voltage pulse is above a threshold voltage.
The liquid medium induces an oxidation reaction, destroying the covalent bonds in H2O molecules.
Factors affecting LAO process
- Applied Voltage
- Dwelling time
- Ambient humidity
Dip-Pen Nanolithography
Transference of materials from the tip to the surface of a sample under high electrical feld
Factors affecting Resolution of a DPN pattern:
- the grain size of substrate,
- surface chemisorption properties which determines the diffusion of molecules after deposition;
- tip dwelling time on the surface,
- humidity level which affect the size of meniscus formed between the tip and substrate
Nano Fabrication – Mechanical Scratching
-Force-assisted AFM nanolithography
-Tip can scratch, remove material
-We can use it to pattern at the nanoscale
-Also requires very thin resist if we want to use as mask
-Scratching hard surface can wear the tip and reduce its
lifetime:
-Coating with hard materials like carbon or Si3N4 or
using diamond tips
Factors affecting Resolution of mechanical scratching
- Size and geometry of the tip
- Depth of scratch
- Applied Force
Mechanical Scratching advantages
-Since the tip geometry is fixed it represents one of the ighest
resolution among all SPM litography techniques
-Even a large tip can produce a small scratch by controlling
the force and depth.
-The mechanical scratch patterns made with standard AFM
tips are no more than a few nanometers deep.
