Surface Analysis Microscopy and Spectroscopy. Fundamentals of Electrochemistry CHEM*7234 CHEM 720 Lecture 11. ANALYSIS. SYNTHESIS. Microscopy. Spectroscopy. Areas of Application. ELECTROCHEMISTRY. Current Microscopy Example. “Advanced Plating Chemistry for 65 nm Copper Interconnects”
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Fundamentals of Electrochemistry
“Advanced Plating Chemistry for 65 nm Copper Interconnects”
Semiconductor International May 2003
Mounding when filling trenches with electrochem deposited Cu using two component electrolyte system.
No mounding with three component system.
Utilized Fast Ion Beam (FIB) Microscope for these images.
“Structural Studies in Lithium Insertion into SnO-B2O3 Glasses and Their Applications for All-Solid-State Batteries”, Katada et al., JES 150, A582 (2003)
Ex Situ Experiment: An experiment performed on a sample after it has been removed from the location wherein it was formed.
• wider range of experimental techniques available.
In Situ Experiment: An experiment performed on a sample while it is still located in its native environment.
• less risk of altering the sample’s true properties.
• loss of electrochemical control
• loss of solvent, ion atmosphere
• risk contamination, oxidation
• the electrolyte solution can strongly absorb the various probe particles which might be used to perform different analyses.
• cell design needs to account for refraction
Two approaches to image formation
Incident wave scatters from surface features, interfering with itself and forming a diffraction pattern. When diffracted wave is refocused, it produces an image of the surface.
•Entire image formed simultaneously
•Resolution limited by wavelength
Incident wave focused to a small point and rastered across surface. Signal is acquired from each point on surface.
•Image formed sequentially
•Resolution determined by spot size
Buildings vibrate (motors, air conditioners, walking, vehicles).
Resonances between 1 and 100 Hz. Amplitudes in micron range.
• build microscope rigid
• couple to building loosely
• provide multiple stages with alternate rigid/loose coupling
• shield acoustically for very precise measurements
• a diffraction experiment
• basic lens components
• coarse/fine focus
• Mon/Bin/Tri ocular schemes
• working distance
• adjust interpupillary distance
• quantitation with reticle
• image recording
A good web site for a brief introduction to optical microscopes can be found below.
Select the correct combination of lenses for your task.
• Rayleigh equation
d = 0.61 (l / N.A.)
d is distance between objects that can still be distinguished
l is wavelength of light
N.A. is numerical aperture of lens = n sin(Qvertex)
Trochodiscus longispinus in OM and SEM. Note improved depth of field and resolving capability of the SEM experiment.
Both can be used, different information, different detection scheme.
Encode some chemical information
Very low energy
Better structural contrast
The probability of an incident electron being scattered varies as the square of the atomic number of the atom and inversely as the incident kinetic energy.
• Greater depth of penetration for low Z materials (e.g. Al vs. W)
• BSE emission branch increases with Z
Equation d prop to W V2/Z rho
Microstructural Development and Surface Characterization of Electrodeposited Nickel/Yttria Composite Coatings, Cunnane et al., JES 150, C356 (2003)
Changing the Y content in the Ni electrolyte bath from 1 to 5 g/L. Preferential growth directions are altered as the nucleation rates are changed by the co-depositing material.
Uses same e-beam source as SEM. Energy analyzes electrons emitted in 100 - 1000 eV range (higher than secondary, lower than backscattered).
Provides unique atomic identity information.
Very surface specific (10 nm)
Chemical maps of surfaces
Auger process. Chemical maps. Hemispherical Electron Analyzer. Secondary scattering in samples.
BS electrons are also scattered into the neighbouring regions of the sample with sufficient energy to further excite atoms not in the original excitation volume.
Spatial resolution degraded 2 to 5 times over that of the corresponding SEM resolution.
Tunneling gap ~ 5 Å
Tunneling Current 10 pA - 10 nA
Tunneling Electron Current
Every substance has a complex electronic structure. At every energy, there are a certain number of electronic states. The number is so large for bulk material, that one reports the number of states per unit energy – the Density of States or DOS.
Tunneling can occur between states of the same energy; the electron’s energy does not change during the tunneling event.
There’s still a vacuum gap, even in water!
Shield tip to minimize faradaic processes. Melted wax or plastic to coat shank of tip. Expose last few nanometers only. Tunneling current must be large compared to faradaic current.
Adlayer of 1,10-phenanthroline on Cu(111) in acidic solution
Itaya, et al. J.E.S. 150 E266 (2003).
Monitored molecular orientation on surface in real time
Create an ultramicroelectrode and use the faradaic current as the control signal.
Signal modulated by proximity to surface.
Depends on forces (repulsive or attractive) between atoms.
Sharpened Cantilevered Tip
Repulsive force between surface atoms and tip atoms, lead to cantilever deflection, altering of relected beam path.
Sample is rastered and moved vertically to maintain constant cantilever deflection.
Can damage delicate samples.
Frictional force measurement. During scan, frictional forces on surface will tend to twist the cantilever.
Use Signal = 1+3 - (2+4) as feedback/imaging signal.
Chemically sensitive: –CH3 covered surface vs. –COOH covered surface
Important when dealing with delicate samples.
Can achieve atomic resolution.
Vibrate tip at resonant frequency (100’s of kHz).
As tip approaches surface, the attractive forces between the substrate and the tip alter the resonance condition.
• frequency shift
• phase shift
For contact mode
For LFM and non-contact mode
The Electrochemical Reaction of Lithium with Tin Studied By In Situ AFM, Dahn et al., JES 150, A419 (2003).
Li is driven into Sn electrochemically which leads to a swelling of the Sn grains. SFM images were used to measure the grain sizes as the potential changed, contributing to a model rgarding Li incorporation in the Sn film.
Visible wavelength optical microscope.
Also called Non-contact Profilometry.
Nanometer resolution vertical to surface.
Uses interferometry to measure surface profile.
Large dynamic range.
Instrument. Interference technique. Computational process. VSI mode. PSI mode. Angle of acceptance. Terraced surface vs. rough surfaces.
In-phase reflections are bright; out-of-phase are dark
First reflecting surface
Structured reflecting surface
Recombined, reflected light is directed to image plane of CCD camera.
Points on surface that are separated from lens by an integer number of wavelengths is bright; those a half-integer are dark.
Interference is strong only when reflected light is in focus; the sample-lens distance is at the focal position.
Scan sample-lens distance around the focal length.
Each pixel will strongly modulate its intensity when the lens reaches the focal position corresponding to each point on the surface.
High resolution position information comes from a linear variable differential transformer (LVDT) connected to the lens scanning drive.
Vertical Scanning Interferometry
Phase Shifting Interferometry
Scan objective over range of µm.
Record image frames sequentially.
Search each pixel through frames and locate frame where intensity modulation is greatest.
Assign height information by correlating frame number to LVDT.
Alter optical path length in series of steps.
This causes fringe pattern to shift laterally.
The series of shifted fringe patterns are combined to form interferograms from which height information is calculated
Interference can occur only if light is reflected back into objective lens. If surface angle is inclined beyond acceptance angle of lens, no interference is observed.
2.5x obj. 2°
10x obj. 10°
50x obj. 25°
Preparing Au substrates on mica for use in forming nanostructured electrodes from self-assembled monolayers. Heat treatment created mounds on surface.
Raman spectroscopy is molecular vibrational spectroscopy. Microscope uses a focused laser beam as the excitation source. The detector can be tuned to look for a particular spectral peak and this can be used to produce a chemical map - now based on molecular and not just atomic features.
Incident laser impinges on sample. Scattered light is shifted slightly to longer wavelengths; small amount of photon energy is left in molecules to excite vibrations. This scattered light, looking for loss of energy, correlates with molecular vibrational spectrum.
Distribution of beclamethasone dipropionate (BDP) and salbutamol in an allergy medication. Particle size is important for effectiveness.
Much faster than mapping. Uses bandpass filters instead of dispersive grating detection. Entire image passes through filter and exposed to CCD camera at once. Image keyed to the radiation intensity passing through the bandpass. This is selected for a particular molecular transition.
Raman image can pick out the 5 differfent layers very easily. From a forensics study of a car.
Fuel cell development. Troubled by contamination with NO+ in solid oxide fuel cell electrolyte, which poisoned process. IR is weak and overlapped by CO2.
What is on the surface? (Atoms or Molecules or Bulk)
What is the structure of the surface layer?
How are they oriented?
What is their oxidation state?
How do these properties change with potential? with time? with additional participants in the electrolyte solution?
Done in conjunction with SEM. Name shifting to EDS.
Add an X-ray detector. Emitted X-rays identify atomic species in excitation volume.
Detector analyzes X-ray photons by energy, rather than wavelength.
Can be used to chemically map a surface.
Can also be done in wavelength dispersion mode. Higher resolution (10 eV compared to 100 eV), but more complex. Getting better. Also higher sensitivity. Order of magnitude better.
Cooled in LN2 temps, Si crystal converts X-ray photon into charge by ionization. Charge is integrated through the FET and is proportional to X-ray energy.
Concave mirror crystal is key to the process. Can be LiF, thallium acid phthalate, or multilayered structures such as W/C, W/Si, or Mo/B.
A cast iron sample
Irradiate sample with monochromatic X-ray beam and energy analyze the photoelectrons which are ejected. (Kind of opposite of EDS).
High resolution (< 1eV) allows chemical state identification (Si, Si2+, Si4+, SiO2 compared to SiTe2.
Vacuum required to be able to detect the electrons.
New instruments can focus X-ray to a few µm in diameter. The beam can be scanned to do imaging XPS.
Electron beam (15 kV) strikes an anode (Mg or Al). Emits x-rays. Tuned to maximize for narrow emission range (example, Mg Ka).
Hemispherical analyzer. Electron lens systems adjusts incoming electron energy to particular kinetic energy. Only specific energy passes through the hemispherical path to reach detector.
Detector is electron multiplier. Can be multichannel.
Spectrum for Yttrium
Ag spectrum showing the spin-orbit splitting of the 3d peaks. The instrumental linewidth of 0.82 eV is also shown.
Vibrational information about molecules. Valuable because of surface selection rules
P-polarized: electric vector amplified at surface.
S-polarized: electric vector cancels at surface.
Phase shifts 180° upon reflection
Subtractively Normalized Interfacial Fourier Transform Infared Spectroscopy
Thin Film Electrolyte (2 µm)
Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy
Electrolyte (D2O) µm
Organic layer nm
Electronically modulate polarization at 150 kHz.
Pyridine bound to Au(111) changes orientation with cell potential