COMSOL SIMULATION STUDY OF THE EFFECT OF PROBE TIP SHAPE

ON THE MEASUREMENT OF AN ELECTRICAL FIELD GRADIENT GENERATED BY MICROELECTRONIC

TEST STRUCTURES

Jose CoronaAdvisors: Lin You and Joseph Kopanski

Engineering Physics Division, PMLSummer 2016

OUTLINE

Ø Theory

Ø Scanning Kelvin Force Microscopy (SKFM)

Ø Motivation

Ø COMSOL Model Builder

Ø Results

Ø Importance of tip shape

Ø Cantilever Effect

Ø Importance of Tip Shape

Ø Clearance Effect

Ø Differential Voltage

Ø Different Size Ratios

Ø Conclusions

Ø Future Work

Ø References

2

MOTIVATION

ØPrecise nano-scale measurements

ØUse of Scanning Kelvin Force Microscopy (SKFM)

ØElectric Field Measurements are HIGHLY dependent on the shape of the probe

ØDesign an Electrical Tip Shape Profiler Reference Material

Image from Semiconductor Manufacturing & Design Community

3

WORKING PRINCIPLES OF SKFM

ØTapping Mode vs. Mode LiftImage credit: Kaja

(Kaja’s PhD THESIS 2010)

Scanning Kelvin Force Microscope (SKFM)

4

15°

COMSOL MODEL BUILDER

Fig. 5

Domain Probe

Image credit: Kaja

(Kaja’s PhD THESIS 2010)

5

COMSOL MODEL BUILDER

Materials

6

Air

Copper

Glass

Electrostatics ØCharge Conservation ØZero ChargeØFloating PotentialØBiasingØGround

COMSOL MODEL BUILDER

7

+10V

Ground

Floating Potential

Meshing

COMSOL MODEL BUILDER

8

Parametric Sweep

COMSOL MODEL BUILDER

9

∠𝑡𝑖𝑝Clearance

ØThe Surface Potential dependency on the shape of the tip

ØSharper vs. Blunter tips

Ø5°Ø (-2.52 , 0.140)

Ø(2.66 , -0.149)Ø35°

Ø(-2.52 , 0.101)Ø(2.66 , -0.113)

IMPORTANCE OF TIP SHAPE

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-5 -4 -3 -2 -1 0 1 2 3 4 5

Surf

ace

Pote

ntia

l [V

]

Position [µm]

Surface Potential of Lateral Scan

5° 20° 35° 12.5° 27.5°

-0.2-0.15-0.1

-0.050

0.050.1

0.150.2

-5 -4 -3 -2 -1 0 1 2 3 4 5

Der

ivat

ive

Surf

ace

Pote

ntia

l [dV

]

Derivative Position [dµm]

Instantaneous Surface Potential at any Given Point

5° 12.5° 20° 27.5° 35°

10

FP

+10V

-10V

GRD

Glass

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

-15 -10 -5 0 5 10 15

Surf

ace

Pote

ntia

l [V

]

Position [um]

Cantilever Effect

5deg

25deg

45deg

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

-15 -10 -5 0 5 10 15

Surf

ace

Pote

ntia

l [V

]

Position [um]

Cantilever Effect

5deg

25deg

45deg

CANTILEVER EFFECT

11

CLEARANCE EFFECTS ON SURFACE POTENTIAL

0

0.1

0.2

0.3

0.4

0.5

0.6

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Surf

ace

Pote

ntia

l [V

]

Position [µm]

5º Cone Angle KFM Scan

10nm

43nm

77nm

110nm

144nm

177nm

210nm

244nm

277nm

310nm

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Surf

ace

Pote

ntia

l [V

]

Position [µm]

35º Cone Angle KFM Scan

10nm

43nm

77nm

110nm

144nm

177nm

210nm

244nm

277nm

12

Glass+10V

GRD

+10V Glass

GRD

CLEARANCE EFFECTS ON SURFACE POTENTIAL

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Surf

ace

Pote

ntia

l [V

]

Position [µm]

5º Cone Angle KFM Scan

10nm

20nm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Surf

ace

Pote

ntia

l [V

]

Position [µm]

35º Cone Angle KFM Scan

10nm

20nm

13

DIFFERENTIAL VOLTAGE

Ø 5º Cone Angle highest SP and most narrow widthØ Coherent

results as before

Ø Smaller lift height, higher SP

-0.02

-0.015

-0.01

-0.005

0

0.005

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Diff

eren

tial V

olta

ge [

dV]

Position [µm]

Determining the Width of the Tip

5°

35°

20°

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-15 -10 -5 0 5 10 15

Surf

ace

Pote

ntia

l [V

]

Position [µm]

Surface Potential of Rectangular Scan with Conical Tip

5°(10nm)

5°(20nm)

35°(20nm)

35°(10nm)

20°(20nm)

20°(10nm)

(20-10)nm

14

DIFFERENT SIZE RATIOS

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Surf

ace

Pote

ntia

l [V

]

Position [µm]

Lateral Scan of 5:1 Eccentric Cone

longside

shortside

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Surf

ace

Pote

ntia

l [V

]

Position [µm]

Lateral Scan of 2:1 Eccentric Cone

10nm longside

10nm shortside

Ø Noticeable gap

Ø Indicates the direction of scan

Long side

Short side

15

SUMMARY AND OUTLOOK

16

ØExtract: ØBase shapeØTip angleØHeight

ØFuture work: ØCompare

THANK YOU!

• How AFM Works: Scanning Kelvin Probe Microscopy (SKPM), Web. (http://www.parkafm.com/index.php/medias/nano-academy/how-afm-works#prettyPhoto).

• Khaled Kaja. Development of nano-probe techniques for work function assessment and application to materials for microelectronics. Physics. Universite Joseph-Fourier - Grenoble I, 2010. English. <tel-00515370>

• http://semimd.com/insights-from-leading-edge/2010/10/02/iftle-18-the-3d-ic-forum-at-2010-semicon-taiwan/

References

Any Questions?

17

CONCLUSIONS

Ø COMSOL’s ability to simulate SKFM

Ø Effect of Tip on the Surface Potential Measurement

Ø Seen through development of many DUTs

Ø IF SLOPE DETERMINED INSERT HERE

Ø Various lift heights affect Surface Potential

Ø Differential Voltage Produced

Ø V

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GOALS

• 3D COMSOL Simulation of Scanning Kelvin Force Microscopy (SKFM)• SKFM à Electric field measurements

• Determine the field distribution to design Electrical Tip Shape Profiler Reference Material• Cone Angle

• Base

• Height

19

THEORY OF SURFACE POTENTIAL

Electric Potential – work per unit charge to move a point charge

Coulomb’s Law – Electric force between charges

Gauss’s Law – Used to determine the Electric field of a Gaussian surface

Image from HyperPhysics

Image from HyperPhysics

Image from HyperPhysics

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html#c2

20

VAN DER WAAL’S FORCES

http://www.eng.usf.edu/~tvestgaa/ThinFilm/

Image by

21

http://mathworld.wolfram.com/FullWidthatHalfMaximum.html

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