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Chang Liu

Scanning Probe Microscopy and MEMS

Chang LiuMicro Actuators, Sensors, Systems Group

University of Illinois at Urbana-Champaign

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Chang Liu

Outline

• Introduction– various types of SPM technology

• Probe Fabrication• Probe position monitoring

– optical lever, integrated piezoresistive element, integrated piezoelectricelement

• Active probe lithography• A example active probe for data storage

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SPM Capability

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Useful Web Links• http://www.topometrix.com/spmguide/contents.htm• http://www.bell-labs.com/new/gallery/spm.html• http://users.erols.com/jwcross/spm/book.htm• http://www.mwrn.com/guide/scanning_probe/microscope.htm• http://www.mobot.org/jwcross/spm/• Nanotheater: http://www.di.com/Theater/Main.html• IBM gallary: http://www.almaden.ibm.com/vis/stm/gallery.html• thermal microscopes:

http://www.thermomicro.com/apps/index.htm• http://www.research.ibm.com/nanoscience/afm_oxidation.html

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SPM Defined• Scanning probe microscopes (SPMs) are a family of instruments used

for studying surface properties of materials from the atomic to themicron level. All SPMs contain the components illustrated in figurebelow.

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Scanning Tunneling Microscope• The scanning tunneling microscope (STM) is the ancestor of all scanning probe

microscopes. It was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBMZurich. Five years later they were awarded the Nobel prize in physics for its invention.The STM was the first instrument to generate real-space images of surfaces with atomicresolution.

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STM Specifics

• STMs use a sharpened, conducting tip with a bias voltage applied between the tip andthe sample. When the tip is brought within about 10Å of the sample, electrons from thesample begin to "tunnel" through the 10Å gap into the tip or vice versa, depending uponthe sign of the bias voltage. The resulting tunneling current varies with tip-to-samplespacing, and it is the signal used to create an STM image. For tunneling to take place,both the sample and the tip must be conductors or semiconductors. Unlike AFMs, whichare discussed in the next section, STMs cannot image insulating materials.

• Each mode has advantages and disadvantages. Constant-height mode is faster becausethe system doesn't have to move the scanner up and down, but it provides usefulinformation only for relatively smooth surfaces. Constant-current mode can measureirregular surfaces with high precision, but the measurement takes more time.

• As a first approximation, an image of the tunneling current maps the topography of thesample. More accurately, the tunneling current corresponds to the electronic density ofstates at the surface. STMs actually sense the number of filled or unfilled electron statesnear the Fermi surface, within an energy range determined by the bias voltage. Ratherthan measuring physical topography, it measures a surface of constant tunnelingprobability.

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Atomic Force Microscope• The atomic force microscope (AFM) probes the

surface of a sample with a sharp tip, a couple ofmicrons long and often less than 100Å indiameter. The tip is located at the free end of acantilever that is 100 to 200µm long. Forcesbetween the tip and the sample surface cause thecantilever to bend, or deflect. A detector measuresthe cantilever deflection as the tip is scanned overthe sample, or the sample is scanned under the tip.The measured cantilever deflections allow acomputer to generate a map of surfacetopography. AFMs can be used to study insulatorsand semiconductors as well as electricalconductors.

• Several forces typically contribute to thedeflection of an AFM cantilever. The force mostcommonly associated with atomic forcemicroscopy is an interatomic force called the vander Waals force. The dependence of the van derWaals force upon the distance between the tip andthe sample is shown in figure below.

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AFM System Diagram• In contact AFM mode, also known as repulsive

mode, an AFM tip makes soft "physical contact"with the sample.

• In non-contact mode, the system vibrates a stiffcantilever near its resonant frequency (typicallyfrom 100 to 400 kHz) with an amplitude of a fewtens of angstroms. Then it detects changes in theresonant frequency or vibration amplitude as thetip comes near the sample surface. Thesensitivity of this detection scheme provides sub-angstrom vertical resolution in the image, as withcontact AFM.

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Conductive AFM• Conductive AFM is used for collecting simultaneous topography imaging and current

imaging. Specifically, standard conductive AFM operates in contact AFM mode.Variations in surface conductivity can be distinguished using this mode.

• Conductive AFM operates in contact AFM mode by using a conductive AFM tip. Thecontact tip is scanned in contact with the sample surface. Just like contact AFM, the Zfeedback loop uses the DC cantilever deflection signal to maintain a constant forcebetween the tip and the sample to generate the topography image. At the same time, aDC bias is applied to the tip. The sample is held at ground potential

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Magnetic Force Microscope• Magnetic force microscopy (MFM) images the spatial variation of magnetic forces on a sample

surface. For MFM, the tip is coated with a ferromagnetic thin film. The system operates in non-contact mode, detecting changes in the resonant frequency of the cantilever induced by the magneticfield's dependence on tip-to-sample separation. MFM can be used to image naturally occurring anddeliberately written domain structures in magnetic materials.

• An image taken with a magnetic tip contains information about both the topography and the magneticproperties of a surface

Hard-disk service. Field of viewis 30 µm.

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Scanning Hall Probe Microscopy• allows one to measure microscopic magnetic patterns by using a submicron Hall probe

to sense local magnetic fields. Fields as low as 0.1 gauss with 0.3um spatial resolutioncan be acheived. It competes with magnetic force microscopy which mig ht havesuperior resolution but is not calibrated and with scannig SQUID microscopy which hashigher magnetic field sensitivity but poorer spatial resolution

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Lateral Force Microscope

• Lateral force microscopy (LFM)measures lateral deflections(twisting) of the cantilever thatarise from forces on the cantileverparallel to the plane of the samplesurface. LFM studies are usefulfor imaging variations in surfacefriction that can arise frominhomogeneity in surfacematerial, and also for obtainingedge-enhanced images of anysurface.

• Lateral deflections of thecantilever usually arise from twosources: changes in surfacefriction and changes in slope.

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Force Modulation Microscopy• Force modulation microscopy (FMM) is an extension of AFM imaging that includes

characterization of a sample's mechanical properties. Like LFM and MFM, FMM allowssimultaneous acquisition of both topographic and material-properties data.

• In FMM mode, the AFM tip is scanned in contact with the sample, and the z feedbackloop maintains a constant cantilever deflection (as for constant-force mode AFM). Inaddition, a periodic signal is applied to either the tip or the sample. The amplitude ofcantilever modulation that results from this applied signal varies according to the elasticproperties of the sample.

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Electron Force Microscopy• Electrostatic force microscopy (EFM)

applies a voltage between the tip andthe sample while the cantilever hoversabove the surface, not touching it. Thecantilever deflects when it scans overstatic charges.

• EFM maps locally charged domains onthe sample surface, similar to howMFM plots the magnetic domains ofthe sample surface. The magnitude ofthe deflection, proportional to thecharge density, can be measured withthe standard beam-bounce system.EFM is used to study the spatialvariation of surface charge carrierdensity. For instance, EFM can map theelectrostatic fields of a electroniccircuit as the device is turned on andoff. This technique is known as"voltage probing" and is a valuable toolfor testing live microprocessor chips atthe sub-micron scale.

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Examining DVD RW

• Topography (left) and EFM (right) images of a DVD-RW.The EFM image clearly shows amorphous bits formedwith the phase change on the crystalline area.

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Scanning Thermal Microscopy

• Scanning thermal microscopy (SThM) measures the thermal conductivity of the samplesurface. Like MFM, LFM, and EFM, SThM allows simultaneous acquisition of bothtopographic and thermal conductivity data. Scanning thermal microscopy (SThM) canbe used in two different operating modes, allowing thermal imaging of sampletemperature and thermal conductivity. The heart of the scanning thermal microscope is athermal probe with a resistive element. The thermal control unit carries out thermalmapping and control thus producing temperature contrast imaging or conductivityimaging.

• For SThM, there are different types of cantilever available. For example, a cantilevercomposed of two different metals is used. (Or, a thermal element made up of two metalwires can be used.) The materials of the cantilever respond differently to changes inthermal conductivity, and cause the cantilever to deflect. The system generates a SThMimage, which is a map of the thermal conductivity, from the changes in the deflection ofthe cantilever. A topographic non-contact image can be generated from changes in thecantilever's amplitude of vibration. Thus, topographic information can be separated fromlocal variations in the sample's thermal properties, and the two types of images can becollected simultaneously.

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Near Field ScanningOptical Microscope (NSOM)

• NSOM is a scanning optical microscopytechnique that enables users to work withstandard optical tools beyond thediffraction limit that normally restricts theresolution capability of such methods. Itworks by exciting the sample with lightpassing through a sub-micron apertureformed at the end of a single-mode drawnoptical fiber. Typically, the aperture is afew tens of nanometers in diameter. Thefiber is coatd with aluminum to preventlight loss, thus ensuring a focused beamfrom the tip.

• In a typical NSOM experiment, the probeis held fixed while the sample is scannedemploying the same piezo technologyfound in most commercial scanning probemicroscopes.

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Melt-Drawn NSOM Tip• Melt drawn from a single optical fiber with the core material

already removed.

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NSOM Probe

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More on NSOM ...• Conventional optical microscopy (or far-field optical

microscopy) is one of the most straightforward, versatiletechniques employed to produce magnified images ofspecimens. The standard arrangement consists of asystem of lenses which focus light from the sample intoa virtual, magnified image. Unfortunately, utilisinglenses in image formation introduces diffraction effectsand places a limit on the resolution of the technique(Abbé diffraction barrier).

• The operational principle behind near-field opticalimaging involves illuminating a specimen through a sub-wavelength sized aperture whilst keeping the specimenwithin the near-field regime of the source. Broadlyspeaking, if the aperture-specimen separation is keptroughly less than half the diameter of the aperture, thesource does not have the opportunity to diffract before itinteracts with the sample and the resolution of thesystem is determined by the aperture diameter as opposeto the wavelength of light used. An image is built up byraster-scanning the aperture across the sample andrecording the optical response of the specimen through aconventional far-field microscope objective.

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Nano Lithography• Normally an SPM is used to image a surface without damaging it in any way. However,

either an AFM or STM can be used to modify the surface deliberately, by applyingeither excessive force with an AFM, or high-field pulses with an STM. Not onlyscientific literature, but also newspapers and magazines have shown examples ofsurfaces that have been modified atom by atom. This technique is known asnanolithography.

• Example: IBM local oxidation(http://www.research.ibm.com/nanoscience/afm_oxidation.html)

• The substrate locally oxidizes upon moving the tip in contact mode across the surface.The oxidant for the chemical reaction is provided by OH- ions in the water droplet that isformed between the tip and the sample. Thus, the lateral resolution of the AFM oxidationprocess depends strongly on the humidity in the air.

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IBM Single ElectronTransistor

• Metal oxide (e.g. TiO2)forms when lateral currentflows through the surfaceof metal (Ti)

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Micro Nano Manipulation• IBM nanotube

manipulation forpositionnanotube ontransistors.

• http://www.research.ibm.com/nanoscience/nanotubes.html

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CNT - Carbon Nano Tubes

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Results of Nanotube manipulation• Smallest transistor with sub-

micron gate length.• IBM can change the amount

of current (ISD) flowingthrough the nanotube channelby a factor of 100,000 bychanging the voltage appliedto a gate (VG), as can be seenin our data below.

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Data Storage• A suspected defect in a

100 µm x 100 µm area ona compact disk isexamined in detail

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NSOM Lithography• Data storage density in current optical disk

drives using 780 nm diode lasers is limited bydiffraction of light to ~ 500 Mbits/ Sq. In.However, the diffraction limited resolutioncan be improved by near field scanningoptical microscopy (NSOM) where a sub-wavelength sized optical source is rasterscanned in close proximity of a sample toproduce an image. The source aperture can befabricated by pulling an optical fiber in thepresence of heat to a conical shape andevaporating Al on the fiber at an angle toallow an open aperture at the end of the fiber.Using such a tapered fiber, magnetoopticdomains as small as 60 nm have beenoptically written and imaged in a 14 nm thickCo/Pt multilayer magnetooptic film presentingthe possibility of 45 Gbits/ Sq. In. storagedensities with this technique. The figureshows 100 nm domains written with ~ 6 mWof power in the fiber and imaged with thistechnique. As a comparison, spots written byconventional far-field techniques are alsoshown. These spots were imaged usingNSOM.

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Silicon Micromachined Probe

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EBD (Electron Beam Deposited) Super Tip

• Standard silicon nitirde pyramidal tips which are availablecommercially are not always sharp enough for someexperiments. By focusing the electron beam in a scanningelectron microscope onto the apex of the unmodified pyramidtip, a sharp spike of any desired length can be grown.

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Mounting Carbon Nano tube• How to mount nanotube onto micro probes• http://cnst.rice.edu/mount.html

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“Mystery” Tips• STPtips.com• Magnetic tips

• Tip with highperformance is agrowth art form ofengineering.

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High Density Probe Design and Fabrication

• High density– 5 tips within 1 mm2

– Average distance between probes is 300 µm.• Requires electrical addressing

– electrical leads must be done with little complication– through-wafer connection is the optimum configuration for dense

array• High uniformity of probe profile

– low stress is required - single crystal silicon is optimum

• E. Chow, et al., Stanford University, “Fabrication of highdensity cantilever arrays and through wafer interconnects”, HH98, p. 220.

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High Density Probe Configuration

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Fabrication Process

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Fabrication Process

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Through Wafer Interconnects• (a) silicon wafer with two side coated with oxide; (b) patterning

on back side and anisotropic etching; (c ) deposition andpatterning of metal; (d) remove oxide; (e and f) deposit andpattern metal on back side.

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Disadvantage for This Scheme

• Foot print penalty– the holes on the backside of the wafer is much greater than the

opening on the front, therefore each through-wafer hole takes upmuch larger area.

• Alternative– use vertical profile for the through wafer via hole.

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Vertical Through Wafer Interconnects

• Start with regular siliconwafer (orientation notcritical)

• etch deep holes using reactiveion etching

• deposit oxide using thermaloxidation– oxygen reacts with silicon to

form silicon dioxide– the oxide growth is highly

uniform and covers allsurfaces.

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Fabrication Process (continued)

• Chemical vapor deposition ofpolycrystal silicon– conformal deposition, again

covering all layers• The polysilicon serves as a

seedlayer to grow metal (Cu)by CVD– it is impossible to deposit

metal into the verticalsurfaces of the holes;

– the copper is used to furthergrow thicker copper byelectroless plating.

• Grow of photoresist byelectroplating.

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Fabrication Process (Continued)

• Pattern photoresist on bothtop and bottom surfaces;

• etch the metal not covered bythe photoresist;

• remove photoresist usingacetone.

• Done!

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Dip Pen Nanolithography (DPN)

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Single Crystal Silicon Probes

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Silicon Probe Arrays - Fabrication

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Thermal Bimetallic Actuation

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Thermal Bimetallic Probe Picture

heater

Heatingpatch

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Silicon Nitride Probe Arrays - Fabrication

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IBM Thermal Mechanical Data StorageMillipede

• http://www.research.ibm.com/journal/rd/443/vettiger.html

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IBM Millipede

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Millipede

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SPM as Bio Sensors• http://www.me.berkeley.edu/mtpl/mtl.html