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    EE 507: Micro- and Nano- fabrication

    Technology• Instructor: Wei Wu• ZHS 252, 2:00-3:20 MW• Email: [email protected]• Tel: 213-740-3085• Office: PHE632, Office hour: 5-6 pm Tue. Thurs.• TA: Yuhan Yao• Email: [email protected]• Office: VHE306, Office hour: 12 am - 2 pm Monday• Grader: Amber Garg• Email: [email protected]

    mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

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    House KeepingSpecial thank to• Prof. Bo Cui of U. of Waterloo for sharing his class notes.• Prof. Yong Chen of UCLA for sharing his class notes.

    Course text: “Fabrication Engineering at the Micro and Nanoscale”, by Stephen A.Campbell + handout• The books can be used as a reference book for fabrication related topics even

    after the class.Grading:10% homework, 20% oral presentation, 30% mid-term, 40% final exam.

    A: [Ave+ s, 100], A-: [Ave, Ave +s ), B+: [Ave- s, Ave),B: [Ave-2 s, Ave- s ), B-:[Ave-3 s, Ave-2 s ), …

    I know everyone has a tough life, but I need to be fair toeveryone too…

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    Goal of the Class• This is the 2nd class on Nanofabrication

    • The 1st is EE508 (lithography).• EE507: mainly non-lithography

    • Survey the landscape of the stat-of-the-artnano-fabrication technologies.

    • Understand the fundamental sciences behindnano-fabrication.

    • Provide the starting point of nano-fabricationresearch.

    • Exercise on problem solving in

    nanofabrication.

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    Thank you for coming, I am reallyflattered, however

    • Your friends might have told you that thescore is easy

    • This time it may not be true any more• For VLSI students

    • You don’t have to take this class as long asyou know the design rules.

    • But if you want to know where the designrule comes from, then this class is for you

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    Guest lectures

    • Prof. Qiang Huang: Nano-informatics• Prof. Han Wang: State-of-the-art device

    technologies•

    One speaker from industry

    Extra lecture to review EE508• Time?• How many attendance?

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    Scanning probe microscopy (SPM) and lithography

    1. Scanning tunneling microscopy.

    2. Atomic force microscopy (AFM) overview.1. AFM tip and its fabrication.2. Tapping mode AFM.3. Other forms of AFM (LFM, EFM, MFM, SCM…)

    3. Atom and particle manipulation by STM and AFM.4. AFM oxidation of Si or metals.5. Dip-pen nanolithography (DPN).6. Resist exposure by STM field emitted electrons.7. Indentation, scratching, thermal-mechanical patterning.8. Field evaporation, STM CVD, electrochemical deposition/etching.9. Scanning near field optical microscope (NSOM/SNOM) overview.10.Nanofabrication using SNOM

    “Scanning probe microscopy and spectroscopy” by Roland Wiesendanger is a good comprehensivereference book.

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    Scanning probe microscopy (SPM) overview

    Normally used for characterization of topographic, physical and chemical properties,though they can also be used as a lithography tool with high resolution yet low

    throughput.

    For imaging purpose, compared to SEM:• Extremely accurate in the z-dimension (

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    • Scanning Tunneling Microscopy(STM): topography, local DOS (density of state• Atomic Force Microscopy (AFM): topography, force measurement• Lateral Force Microscopy (LFM): friction• Magnetic Force Microscopy (MFM): magnetism• Electrostatic Force Microscopy (EFM): charge distribution• Scanning Capacitance Microscopy (SCM): dielectric constant, doping• Scanning Thermal Microscopy (SThM): temperature, conductivity• Spin-polarized STM (SP-STM): spin structure• Scanning Electro-chemical Microscopy (SECM): electro-chemistry• Scanning Tunneling Potentiometry: potential surface• Photon Emission STM (PESTM): chemical identification• Nearfield Scanning Optical Microscopy (NSOM): optical properties

    Scanning probe microscopy (SPM) family

    Variationsof STMand AFM

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    The first STM Instrumentation

    STM inventors Rohrerand Binnig, IBM, Zurich,Nobel Prize in Physics in1986.

    Exact copy of first Scanning TunnelingMicroscope of Binnig and Rohrer

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    Operation of an STM

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    Two basic scanning modes

    • Feedback off/constant height: Scan oversurface with constant z0 (piezo voltage),control signal changes with tip-surfaceseparation. For relative smooth surface,faster.

    • Feedback on/constant current: circuitregulates z piezo voltage to constant valueof control signal (constantly changes tip-surface separation). Irregular surfaces withhigh precision, slower.

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    • A voltage applied between two conducting bodies leads to an electrical current evenif the two bodies not quite touch: the tunneling current

    • Interaction: (tunneling-) current (down to pA)o Atomic scale surface topography of electrical conductorso Electronic properties of the surface (“conductivity”)

    • The tunneling current is strongly dependent on the distance of the two bodies: 1Åchanges the current by a factor of 10!

    Quantum mechanical tunneling

    Atom Surface STM

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    Quantum mechanical tunneling

    Tunneling through a rectangular barrier

    Elastic tunneling vs. inelastic tunnelingElastic: energy of tunneling electrons conserved.Inelastic: electron loses a quantum of energy within the tunneling barrier.

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    Why atomic resolution?

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    Bias polarity : probing filled and empty states

    Highest occupied molecular orbital (HOMO)Lowest unoccupied molecular orbital (LUMO)

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    The resolution is determined by:• Dimension of probe• Distance of probe to sample

    Tip is the key

    Oxide or insulating contamination layersof thickness several nanometers canprevent vacuum tunneling.This may lead to mechanical contact

    between tip and sample. (the servo will force thetip to collide in an effort to achieve the set-point current)Tunneling through the oxide orcontamination layer may damage tip.

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    STM tip preparation

    Very sharp tips can beobtained, ideally terminatedby a single atom.

    How to make sharp STM tips?• Wire of W or Pt-Ir, with 200 m

    diameter.• Cut or etch to 40nm diameter tip.• Hand-made, no micro-fabrication

    process.• Can be sharpened by focused ion

    beam milling.

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    • Surface geometry• Molecular structure• Local electronic structure• Local spin structure• Single molecular vibration• Electronic transport• Nano-fabrication• Atom manipulation• Nano-chemical reaction

    Applications of STM

    Surface Structure with atomic resolution

    Various reconstructions of Ge(100)-2x1

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    k ijk ij E d u

    Piezoelectric tube scanner

    Displacement electric field

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    Inverse piezoelectric effectDiscovered in 1880 by Pierre and Jacques CurieMost common material: PZT

    • Piezoelectric materials have an asymmetric unit cell like a dipole.• If these crystals are grown in the presence of a strong electric field

    then the crystal grains will align and the piezoelectric effect is created.• Typical achievable strain ratio: 1/1000, e.g. 1μm stroke for 1mm PZT.

    Piezo driving technology: the basics

    Piezoelectric effect:changing the size of an objectresults in a voltage generated bythe object .

    PZT: Lead zirconium titanate

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    Scanning probe microscopy (SPM) and lithography

    1. Scanning tunneling microscopy.2. Atomic force microscopy (AFM) overview.

    1. AFM tip and its fabrication.2. Tapping mode AFM.3. Other forms of AFM (LFM, EFM, MFM, SCM…)

    3. Atom and particle manipulation by STM and AFM.

    4. AFM oxidation of Si or metals.5. Dip-pen nanolithography (DPN).6. Resist exposure by STM field emitted electrons.7. Indentation, scratching, thermal-mechanical patterning.8. Field evaporation, STM CVD, electrochemical deposition/etching.9. Scanning near field optical microscope (NSOM/SNOM) overview.10. Nanofabrication using SNOM

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    Digital Instruments (DI,later Veeco, now Bruker)

    multi-mode head,scanner and base

    For DI multi-mode head, sample is put on piezo stage.

    For DI dimension 3000 head, tip is put on piezo stage.

    P b l i t ti d d t ti t

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    Probe-sample interaction and detection system

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    Probe-sample interaction detection systemDetect deflection in z-direction(to maintain constant force fornormal AFM operation)

    Detect defection in the x-y direction,for lateral force/friction microscopy.

    Photo-diode(divided intofour parts)

    Measure(A+B-C-D)/(A+B+C+D)

    Measure(A+C-B-D)/(A+B+C+D)

    F db k l f t t f AFM

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    Feedback loop for constant force AFM

    Z is equivalent to the topography of the sample

    Tiny deflection of cantilever leads to large shift of the beam spot position on thephoto-diode, so extremely sensitive for z-dimension detection (sensitivity Z

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    Interactions between sample and tip in force microscopy

    Close (

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    AFM tip-sample interaction

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    Atomic Force Microscope (AFM)

    Sample: conductor, nonconductor, etcForce sensor: cantileverDeflection detection: photodiode

    Two basic AFM Modes:Contact mode (no vibrating tip)Tapping mode (vibrating tip)

    Many variations on Scanning Force Microscopy:Liquid AFMMagnetic Force Microscopy (MFM)Latteral Force Microscopy (LFM)Intermitant and non-contact AFMForce Modulation Microscopy (FMM)Electrostatic Force Microscopy (EFM)

    Here tip on piezo-stage, alsopossible sample on piezo-stage.

    S i b i (S ) d li h h

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    Scanning probe microscopy (SPM) and lithography

    1. Scanning tunneling microscopy.2. Atomic force microscopy (AFM) overview.

    1. AFM tip and its fabrication.2. Tapping mode AFM.3. Other forms of AFM (LFM, EFM, MFM, SCM…)

    3. Atom and particle manipulation by STM and AFM.

    4. AFM oxidation of Si or metals.5. Dip-pen nanolithography (DPN).6. Resist exposure by STM field emitted electrons.7. Indentation, scratching, thermal-mechanical patterning.8. Field evaporation, STM CVD, electrochemical deposition/etching.

    9. Scanning near field optical microscope (NSOM/SNOM) overview.10. Nanofabrication using SNOM

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    Force sensor: cantilever

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    Cantilever fabrication – silicon micro-machined probe

    Silicon nitride

    AFM tip fabrication (another way)

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    T. Wakayama, T. Kobayashi, N. Iwata, N. Tanifuji, Y. Matsuda, and S.Yamada, Sensors and Actuators a-Physical, vol. 126, pp. 159-164, 2006.

    AFM tip fabrication (another way)

    1. SiO2 mask

    2. RIE Si dry-etch

    3. KOH Si wet-etch

    4. SiO2 mask

    5. RIE Si dry-etch

    6. SiO2 mask on backside

    7. KOH Si wet-etch, passivation on front-side

    8. BHF (buffered HF) SiO2 wet-etch

    9. RIE Si dry-etch

    10. Release of cantilever in BHF

    AFM tip fabrication

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    AFM tip fabrication(another one)

    Use EDP instead of KOH.Add oxidation sharpening.

    EDP: ethylene-diamine pyrocatechol,is an anisotropic etchant solution forsilicon, consisting of ethylene-diamine, pyrocatechol, pyrazine andwater.

    Pyrocatechol

    Pyrazine

    Ethylene-diamine

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    KOH etched

    Si-mould

    Polymer SU-8 tip fabrication

    Released tip

    Probe (tip cantilever) summary

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    Probe (tip, cantilever) summary

    Tip array for fast lithography tip for tapping mode AFM tip for contact mode AFM

    l b d d

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    Standard silicon nitride pyramidal tips which are available commercially arenot always sharp enough for some experiments.By focusing the electron beam in a scanning electron microscope onto theapex of the unmodified pyramid tip, a sharp spike of any desired length canbe grown.(i.e. growth of carbon from contamination by focused electron beaminduced deposition, not necessarily very sharp, but with very high aspectratio to reach deep holes/trenches.)

    Electron beam deposited super tip

    Using carbon nanotube to improve resolution

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    Using carbon nanotube to improve resolution

    Vibration problem: need short tube 0.2 m

    Scanning probe microscopy (SPM) and lithography

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    Scanning probe microscopy (SPM) and lithography

    1. Scanning tunneling microscopy.2. Atomic force microscopy (AFM) overview.

    1. AFM tip and its fabrication.2. Tapping mode AFM.3. Other forms of AFM (LFM, EFM, MFM, SCM…)

    3. Atom and particle manipulation by STM and AFM.

    4. AFM oxidation of Si or metals.5. Dip-pen nanolithography (DPN).6. Resist exposure by STM field emitted electrons.7. Indentation, scratching, thermal-mechanical patterning.8. Field evaporation, STM CVD, electrochemical deposition/etching.

    9. Scanning near field optical microscope (NSOM/SNOM) overview.10. Nanofabrication using SNOM

    Scanning modes of AFM

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    Scanning modes of AFM

    Not popular

    Non contact mode imaging

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    Raspberry polymer

    Non-contact mode imaging

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    • Vibration of cantilever around its resonance frequency (often hundreds of kHz)• Change of frequency due to interaction between sample and cantilever

    Vibrating cantilever (tapping) mode: most popular

    Cantilever oscillate and ispositioned above the surfaceso that it only taps the surfacefor a very small fraction of itsoscillation period.When imaging poorlyimmobilized or soft samples,

    tapping mode may be a farbetter choice than contactmode.

    But for the AFM we have, we operate at

    Resonance frequency:keff = k0 - dF/dz (F is force)

    f eff =1

    2 /

    0 300kHz

    Vibrating cantilever (tapping) mode

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    Free oscillationLarge amplitude Hitting surfaceLower amplitude

    Vibrating cantilever (tapping) mode

    Both amplitude and phase canbe used

    • Cantilever oscillates at resonant frequency and“taps” sample surface, where feedback loop

    maintains constant oscillation amplitude.• Reduces normal (vertical) forces and shear

    (lateral) forces, thereby reducing damage tosofter samples.

    • Can image surface with weak adhesion.• But much slower than contact mode.

    Ph i gi g

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    Drive s ignal

    Cantilever s ignal

    Topography Phase

    Polymer blend(Polypropylene & EDPM)

    Measure relative elastic properties of complex samples

    Phase imaging

    • Measure the phase lag of the cantilever driving frequencyvs. actual oscillation.

    • Contrast depends on the physical properties (Young’smodulus and damping) of the material.

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    AFM (contact mode):Au(111) polycrystalline filmon a glass substrate

    AFM (non-contact mode):Atomic resolution on Si(111)7x7

    Atomic resolution AFM (in ulta-high vacuum)

    l b l l

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    Many types: DNA and RNA analysis, protein-nucleic acid complexes,

    chromosomes, cellular membranes, proteins and peptides, molecularcrystals, polymers and biomaterials, ligand-receptor binding.Bio-samples have been investigated on lysine-coated glass and micasubstrate, and in buffer solution (SEM… all in vacuum).By using phase imaging technique one can distinguish the different

    components of the cell membranes.

    Applications to biological system

    A li i bi l i l

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    Applications to biological system