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NANOSCAL SCRATCHING USING ATOMIC FORCE MICROSCOPY

Ampere A. Tseng Professor of Engineering

Arizona State University, Tempe, Arizona 85287 USAE-mail: ampere.tseng@asu.edu

Co-Workers: L.A. Abril-Herrera, A.O. Adebo, C. Chaidilokpattanakul , R. V. Edupuganti , Ravi Gupta, Ivan A. Insua, C. J. Johnson, J. D. Kaplan, M. H. Lee, Bharath Leeladharan,

Collaborators: Professor Tu Pei Chen, Nanyang Technological University, Singapore Professor Rudy Diaz, Arizona State University, USA

Dr. Shyankay Jou, National Taiwan University of Science and Technology, ROChina Dr. Gary Li, Freescale Semiconductor, USA

Dr. Zhuang Li, Chinese Academy of Sciences, PRChinaProfessor M. F. Luo, National Central University, ROChina

Dr. Andrea Notargiacomo, Roma TRE University, ItalyProfessor Jun-ichi Shirakashi, Tokyo University of Agriculture & Tech, Japan

Sponsors: US National Science Foundation, Freescale Semiconductor, Intel, Pacific Technology, Walsin Lihwa

Ravi Gupta, Ivan A. Insua, C. J. Johnson, J. D. Kaplan, M. H. Lee, Bharath Leeladharan, Bo Li, W. D. Nath, Jong S. Park, Masahito Tanaka, C. L. Thomas, George P. Vakanas

♦ AFM-based nanoscratch, also known as scratch nanolithograpy is using a functionalized cantilevered-tip for directly removing nanoscale amount of materials from target surfaces with sub-nanoscale precision under control manner.

♦ Nanoscratching aims at building nanoscale

WHAT IS AFM-BASED NANOSCRATCHING

♦ Nanoscratching aims at building nanoscale structures (0.1 – 100 nm), which can act as components, devices, or systems with desired properties and performance characteristics, in large quantities at low cost.• Methodology or Processing Technology• Toolbox (Equipment: hardware, software, system)• Physical Product (Components, device, system….)

♦ AFM-based nanofabrication is using an AFM-tip for performing nanofabrication of nanostructures (0.1 –100 nm), which can act as components, devices, or systems with desired properties and performance characteristics, in large quantities at low cost. • Nano-Manipulation: Atoms molecules

AFM Nanofabrication & Mechanical Scratching

• Nano Manipulation: Atoms molecules, nanoparticles, nanoclusters…• Material Removal: Mechanical scratch, etching..• Material Modification: Resist exposure, local anodic oxidation.• Material Deposition and Growth: Electrical field-induced mass-transfer, electrochemical deposition.

AFM: Mechanical Scratching

Gate

Source Drain

Submicro-scale Modification

Tunnel Barriers: Nano-Scale Modification

AFM Tip

Gate

Automation of AFM Nanofabrication* Predictive model * CNC control with sub-nanometer precision* Multiple tips

Typical Nanoscratch Apparatus

AFM scratching

Triangular pyramid tipDiamond coatedOperation in Contact mode

AFM Scratching of Silicon(in backward direction)

Effect of tip normal force on scratched depth and width

Scratch depth, d (Fn) = α1Ln (Fn/Ft1)Fn is the normal tip-sample forceα1 is scratch penetration depthFt1 is threshold forces based on depth data

A.A. Tseng et al., JAP (2009)

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AFM Scratching of Silicon AFM Scratching of Au-Pd FilmA Chinese poem (20 characters) of Tang Dynasty was scratched on gold-palladium film (with r.m.s. of 0.2 nm), which was deposited on cleaved mica by ioncleaved mica by ion sputtering. The scratch was performed by a Si AFM tip with a force constant of 35 N/m at a normal force of 20 μN. Scratched depth isabout 2 nm.

J. Song, Z. Liu, C. Li, H. Chen, H. He (1998),Appl. Phys. A 66, S715–S717.

Effect of tip normal force on scratched depth and width

AFM Scratching of Ni80Fe20(in backward direction)

Scratch depth, d (Fn) = α1Ln (Fn/Ft1)Fn is the normal tip-sample forceα1 is scratch penetration depthFt1 is threshold forces based on depth data

A.A. Tseng et al., JAP (2009)

Multiple Scratching of Ni80Fe20 in Upward directionEffect of multiple scratch on scratched depth and width

d(No) = M1(No)n1wf(No) = M2(No)n2

No is number of scratch or scanM is multiple machining coefficient n is multiple machining exponent

A.A. Tseng et al., JNN (2010)

AFM Scratchability of Si and Ni80Fe20

V = kL(Fn/H)

Archard’s wear equation

where V is the volume loss caused by wear, k is wear coefficient, L is sliding distance, Fn is applied normal force, and H is hardness of material to be tested

k = k/H = V/(LF ) = dw /F

Scratch coefficient, ks

A.A. Tseng et al., JAP (2009)

where L becomes the scratch distance. Since the cross-section of the scratched groove is in a “V” shape, its cross-section area can be approximated by d x wf and V = dwfL.

ks = k/H = V/(LFn) = dwf/Fn

ks = βsLn (Fn/Fts)where βs is the scratch penetration area

Property Single crystal silicon

Ni80Fe20 thin film

Scratchability rating#

Scratch penetration depth, α1 0.78 [nm] 4.96 [nm] 636%Mean scratch coefficient, ks 2.42 [nm2/μN] 26.43 [nm2/μN] 1,092%Scratch penetration area, βs 1.418 [nm2] 18.079 [nm2] 1,275%

Hardness, H 10.4 [GPa] 8.0 [GPa] 130%

SCRATCHING OF Ni80Fe20NANOWIRE

Nano-Constriction

A.A. Tseng et al., JAP (2009)

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Mechanical Scratching and Etching by AFMDepletion of 2-D Electron Gas (2DEG) of GaAs/AlGaAS hetero-structure scratched by AFM-tip with normal force of 50 μN at scan speed of 100μm/s: a) measured resistance against number of scans during scratching on GaAs/AlGaAs layers, the inset showing the schematic of the scanning and measuring process (Schumacher et al., 1999), b) Periodic hole arrays in a quantum well fabricated on InAs/AlSb surface by direct AFM mechanical scratching (Rosa et al., 1998). AFM images of V-shaped

grooves on mica substrate scratched by Si tip at

contact mode: a) 2-nm deep and 60-nm wide

grooves with one scratch at normal force of 500 nN,

b) 20-30 nm groove by five repeated scratches with a

normal force of 5 μN (Tseng et al., 2007).

AFM SCRATCHING OF Ni

A.A. Tseng et al., JVSB (2010)

Hamaker (1937) performed the integration of the interaction

∫−= 216

112 )/()( dVdVzAzPv ρρ

where V1 and V2 are the volume of the plane and sphere, respectively. If the distance between one element (dV2) in the sphere and one element (dV1) in the plane is (x2 +z2)1/2 where dV1 is located in a ring volume of 2πxdxdz, the above equation, the above equation becomes

2

3212

0 0

322112 12

2)z(x/2)( ∫∫ ∫ ∫ −

∞ ∞

−=+−= dVzdVdzdxxAzPvρπρ

ρπρ

By considering a circular section of area (πx2dz’),where z’ varies from 0 to r0, the integration of thewhole sphere interaction with the plane can be found.Si 2 2 ’2 d ’ + th b tiintegration of the interaction

potential of Eq. (1) to calculate the total interaction of the sphere-plane geometry. By assuming that the total interaction can be obtained by the pairwise summation of the individual contributions and the summation can be replaced by an integration over the volumes of the interacting elements with a uniform density of the plane, ρ1and and the sphere, ρ2, the total potential for a sphere interacting with a smooth plane, Pv, should be

Since x2 = r02 – z’2 and z = z’ + zd the above equation

becomes

⎥⎦

⎤⎢⎣

⎡+

++−=+−−= ∫− ........

21

6'])'/()'[(

6),'(

0

3220

121

do

d

d

ohr

r ddv zrz

zrAdzzzzrAzzP o πρπρ

Since the associated wdW force (Fv) can be found to be the gradient of that potential, i.e Fv = -∇Pv(zd)

)6/()( 2dohdv zrAzF −=

22

3

)2(3)(

odd

ohdv rzz

rAzF

+−=

If zd << ro, the above equation can be reduced to

where Ah is the well-known Hamaker constant equal to A1π2ρ1ρ2.

Hertz contact theory: the contact area:Ac = π(3FnR/4Ec)2/3

The maximum contact stress:σn = 1.5Fn/Ac = [6FnEc

2/(π3R2)]1/3 and1/Ec = (1- υd

2)/Ed + (1- υn2)/En

where the subscripts n and c denote the property related to diamond and nickel.

E = 1143 GPa and E = 190 GPa

AFM SCRATCHING OF Ni

Ed = 1143 GPa and En = 190 GPa υd = 0.07 and υn = 0.24. Ec = 171 GPa.

At Fn = 1 µN with R = 120 nm, σn = 7.33 GPa

H = Pmax/Ac = 7.0 to 7.7 GPa The permanent indentation is achieved only if

the indentation pressure or contact stress is exceeding a certain limit. Very often, its hardness is considered as the limit. The

hardness of Ni thin films can be found from 7.0 to 7.7 GPa. The contact stress, σn, is

consistent with the experimental hardness

d (Fn) = α1Ln (Fn/Ft1)

where α1 is scratch penetration depth.Ft1 is threshold forces based on depth data.

Pauli Repulsion Between Atoms or MoleculesThe repulsive forces between an AFM tip and a sample can be obtained by finding the corresponding pairwise potentials first. The potential of the repulsion interaction (pr) between neutral atoms or molecules can normally be approximated as

ns zBzp /)( 1=

where B1 is a proportional constant and the exponent n is higher than 10

where Plj is the Lennard-Jones potential including both the repulsive and the attractive terms, P0 is the depth of the potential well, z0 is the distance at which the inter-particle potential is zero, and z is the distance between the centers of the paired particles. The basic shape of Lennard-Jones potential curve is common to almost all empirical potentials and shown below:

and the exponent n is higher than 10 for most of materials (Magomedov, 2006). Note that the main repulsion between atoms is not due to electrostatic repulsion between the nuclei, but due to the Pauli exclusion principle for their electrons. If n = 12, the above potential can incorporate with the potential of the vdW interaction to form the well-known Lennard-Jones potential (1924):

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−⎟⎠

⎞⎜⎝

⎛=6

012

004)(

zz

zz

PzPlj

As shown, at short range (small z) the potential energy is very large and positive, revealing that this is a very unfavorable arrangement of the atoms, which indicates that the two atoms are strongly overlapping.

Lennard-Jones Potential and ForcesAlthough the attractive vdW potential is derived from dispersion interactions, the exponent 12 in describing the Pauli repulsion has no theoretical justification and was chosen because of its simplicity and good agreement with experimental data for most gases. The z−12 term describes Pauli repulsion at short ranges due to overlapping electron orbits. The typical values of P0 and z0 for O-O interaction are 1.079×10 −21 J and 0.355 nm while P0 = 0.8336×10 −21J and z0 = 0.193 nm for C-H2.

If the tip can be approximated by a circular cone, Du and Li (2007) found that the combined repulsive and attractive interaction force can be expressed as

−⎥⎦

⎤⎢⎣

⎡+++

−= 9

222

7

2

)tan/(tantan936tan

1260)(

db

ddbb

d

hdlj zr

zzrrz

BzF

γγγγ

⎥⎦

⎤⎢⎣

⎡

+++

−− 3

222

3

2

)tan/(tantan33tan

6 db

ddbb

d

h

zrzzrr

zA

γγγγ

where γ is the semi-aperture of the cone and rb is the radius of the circular cone base. If rb >> zd, the above

By integation, the interaction force between a sphere and a plane using the Lennard-Jones potential can be found as

∫− +−+−

=

or

r dd

dlj

dzzzAzzAzr

xzP

0

']})'(6/[)'(45/[){'(

6)(

31

92

220

21

π

ρπρ

where the two constant of P0 and z0 introduced in Eq. (11) have been substituted by A1 and A2. Again, if zd << ro, the interaction forces based on Lennard-Jones potential can be found as

)6/()180/()( 28dohdohdlj zrAzrBzF −=

where the constant Bh is also known as the repulsive Hamaker constant equal to A2π2ρ1ρ2.

)6/(tan)1260/(tan)( 272dhdhdlj zAzBzF γγ −=

equation can be reduced to

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AFM Scratch Coefficient and Volume??

Scratch coefficient, ks = V/(LFn) = dwf/Fn

Assume the cross-section of scratched groove is in a “V” shape, V = dwfLAg = d wf

The cross-section of the tip end segment, At:

At = R2(θ – sinθ) where θ = 2 cos-1(1 - di/R).di = the impressed depth of the semispherical tip into the sample, and di = d.R = 120 nm

A.A. Tseng et al., JVSB (2010)

AFM Scratchability of Si and NiScratchability property Silicon Nickel thin film Scratchability

Scratch penetration depth, α1 0.78 [nm] 3.94 [nm] 506%

Mean scratch coefficient, ks 2.42 [nm2/μN] 18.16 [nm2/μN] 750%

Scratch penetration area, βs 1.418 [nm2] 14.597 [nm2] 1,029%

Hardness 10.35 GPa 7.35 GPa2 141%

• Ni thin film possesses excellent scratchability and one d f it d hi h th th t f Si lth h NiForder of magnitude higher than that of Si, although NiFe

and Si have the same level of hardness.

• AFM Scratching can be a promising technique for creating or repairing Ni-based masks, stamps and molds

• AFM Scratching can be used to scratch electroplated Ni structures that can act as major components and devices in different MEMS/NEMS and masks for NGL

CORRELATION OF GROOVE DEPTH WITH APPLIED NORMAL FORCE

AFM Scratchability??

Si

Al/As2S3

SiNi

Ni80Fe

PGMA

Polymeric photoresist

Zr55Cu30Al10Ni5

A.A. Tseng, Surface Science (2010)

Parameter Si (100)

Si(100)

Ni80Fe20thin film

Ni thin film

Al/As2S3bilayer

Zr55Cu3

0Al10Ni5

PGMA

Spring const. [N/m] 42.0 239. 42.0 42.0 165. 239. 0.68Scratch speed [μm/s] 0.10 30. 0.10 0.10 5.0 30. 10.Scratch coeff. α1 [nm] 0.765 38.65 4.96 3.94 80.51 18.09 4.538

Threshold force, Ft1 [μN] 1.01 52.66 1.09 1.20 11.57 45.35 0.037Coefficient R2 0.96 0.97 0.98 0.95 0.98 0.96 0.96

Comparison of Scratch Properties

Scratch ratio, α1/Ft1[nm/μN]

0.756 0.734 4.555 3.272 6.962 0.399 123.44

Scratch ratio rating [%]* 100 97 603 433 921 53 16,328Hardness, H [GPa] 10.0 10.0 8.0 7.35 1.00 4.90 --

Rating based on hardness* 100% 100% 125% 136% 1,000% 204% --Rating comparison ratio+ 1.00 0.97 4.82 3.18 0.92 0.26 --

Data source present [15] [3] [16] [17] [18] [19]* With respect to Si properties+ Scratch ratio rating/Hardness rating

A.A. Tseng, Surface Science (2010)

SCRATCH DIRECTION OR TIP SHAPE

a) Upward b) Forward c) Downward d) Backward

SCRATCH DIRECTION OR TIP SHAPE

a) Upward b) Forward c) Downward d) Backward

Tip Normal Force = 9μΝ

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AFM Scratching Versus Oblique Cutting

SEM image of 6 scratched grooves with chips using Fn= 1, 2, 3, 5, 7 and 9 µN

SEM image of 5 machined grooves

with chips using Fn = 9 µN with number of

scan (No) equal to 1, 3, 5, 7, and 9

Finite Element Modeling of AFM Scratching

Static Model for normal AFM tip (negative rake

angle)

Dynamic model

For better tip design

(positive rake angle)

Mechanical Scratching and Etching by AFMDepletion of 2-D electron gas (2DEG) of GaAs/AlGaAS hetero-structure scratched by AFM-tip with normal force of 50 μN at scan speed of 100μm/s: a) measured resistance against number of scans during scratching on GaAs/AlGaAs layers, the inset showing the schematic of the scanning and measuring process (Schumacher et al., 1999), b) Periodic hole arrays in a quantum well fabricated on InAs/AlSb surface by direct AFM mechanical scraching(Rosa et al., 1998). AFM images of V-shaped

grooves on mica substrate scratched by Si tip at

contact mode: a) 2-nm deep and 60-nm wide

grooves with one scratch at normal force of 500 nN,

b) 20-30 nm groove by five repeated scratches with a

normal force of 5 μN (Tseng et al., 2007).

Polymeric Materials by AFM Scratch

AFM images of trenches of on polymer films by Si tip at contact mode: 150-225nm deep with one scratch on PMMA film using normal

• Scratched resist thin film can be used as masked for subsequent etching or deposition processes. • Material removal rate can be increased by aiding chemical with chemical etching.

PMMA film using normal force of 17.5 μN, b) 150-nm deep with one scratch on PI film using normal force of 8.5 μN.

The scratched groove possesses a V-shaped cross-section with ridges or bulges observed along the trench banks. The ridge is partially formed by the pile-up of debris, which is the material chips plowed from the trench. It could also be the material squeezed by the tip during scratch due to viscoplastic deformation,

which is similar to those phenomena observed in nano-indentation of polymers. At relatively high scratching forces, the plowed debris can cover the trench, which can greatly not only deteriorate the uniformity of the trench profile but

also reduce the material removing rate.

Scratch of Nanochannel on Gold Films and Wires

Metric 2010 2012 2014Feature Position Accuracy

[nm]50 25 5

Feature Size Variation [% of dimension]

10 3 1

Heterogeneity in Parameter: Size Shape or Orientation

Two values of one parameter

Five values of two parameters

Continuous control over two

Roadmap for Tip-Based Nanofabrication*Developed in 2007 for Commercialization

Size, Shape, or Orientation one parameter two parameters control over two parameters

Feature Fabrication Rate (No. of structure/min/tip with

array tips)

1/min Single tip

5/min/tip Five-tip array

60/min/tip Thirty-tip array

Tip Shape Variation [% of dimension with required operations]

Height < 10%, Radius < 20% 100 operations

Height < 5%, Radius < 10%

1000 operations

Height < 1%, Radius < 3%

106 operationsIn-situ Tip Height Sensing

[nm]20 10 2

* A. A. Tseng, S. Jou, A. Notargiacomo, T. P. Chen, J. Nanosci. Nanotech., May 2008

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• Develop closed-loop control strategy to reduce the geometric errors induced by equipment including hysteresis creep by piezo scanner, AFM tip, mechanical drift, thermal drift…, - To reduce the hysteresis creep, feedback signal uses direct measuring non contact capacitive position sensors

Equipment to Have Precision of ~1 nm

direct-measuring, non-contact capacitive position sensors (high motion linearity, long-term stability, phase fidelity)- To reduce mechanical drift, positioning and scanning use frictionless flexure guidance (within 0.01 nm/s)- To reduce thermal drift, invar or low thermal expansion materials is used for the stage (within 0.01 nm/s)- The total inaccuracy should be less than 1 nm/mm and 0.01 nm/s.

• Dimension and accuracy of patterns are highly affected by operating parameters: contact mode, tip material, scanning speed, humidity, applied voltage….

• Accuracy of patterning scheme requires further refining, and better processing procedures need to be developed.

DESIGN SOFTWARE FOR PRECISION PARTERNING

• Height and width modulation by adjusting the phase and the duration time of pulsed voltage (short pulse voltage restricting lateral diffusion while high voltage pulse producing fast growing in height).

• Develop CAD software for the design and control of SPM (including AFM, STM, SNOM) oxidation and scratching patterning.

Parallel Processing & Multi-Resolution(IBM Millipede and Special Tip Design)

Schematics and SEM image of a cantilever-SWNT-based sensing device. The white arrows indicate the location of the

Millipede chip: the electrical wiring for addressing the 1,024 tips etched

out in a square of 3mm by 3mm (The chip's size is 7 mm by 14 mm

http://domino.research.ibm.com/Comm/bios.nsf/pages/millipede.html

location of the suspended SWCNT. Deflections of the cantilever induce strains in the CNT leading to a change in its resistivity (Stampfer et al., 2006).

Sensing and Actuating for Arrays of AFM Tip

A key element when developing probe array devices is a convenient read-out system for measurements of the probe deflection. Such a read-

out should be sufficiently sensitive, resistant to the working environment, and compatible with the operation of large number of

probes working in parallel.

Parallel Processing of AFM Tip Arrays

Schematic of three main classes of sensory used for deflection detection in AFM

1) external sensor: sensor is not integrated in the cantilever,

2) self sensing: sensor is integrated in the cantilever,

3) Tip-surface interaction sensor: other tip-surface than force interactions are detected, e. g. thermal or electrochemical interactions (Courtesy of J. Polesel-Maris).

AFM

• Tool/Workpiece Interaction (physical, chemical, biological…….)Relevant techniques: Science with Manufacturing

• Tool/Workpiece Manipulation (position, movement, force, ……)

FACTORS IN NANOSCRATCHING

Relevant techniques: CNC, Automation

• Desired Product Requirements (properties of surface, shape, ……) Relevant techniques: On-line monitoring and sensing

• Productivity(reliability, system……)Relevant techniques: Equipment design, management

Vehicle &StockerC t ll

ManufacturingExecution System

Computer Integrated Manufacturing for TBN

Desk Top Based System

MaterialControlSystem

Controllers

Material HandlingEquipment

ExecutionControlView Dispatch ListsMoveQueueDispatchJob Control/Trigger

Real-timeScheduler/Dispatcher

Decision Support SystemsProd. EquipmentStationControllers APC SPCWafer

control

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Life Is Limited! L i I U li it d!

For All of Us in Nano Worlds

Learning Is Unlimited!

Thank You!!

US National Science Foundation has made predictions of the of nanotechnology by 2015

General Nanotechnology Related Markets

• $340 billion for nanostructured $materials,

• $600 billion for electronics and information-related equipment,

• $180 billion in annual sales from nanopharmaceuticals