PeakForce - Scanning Microwave Impedance Microscopy

Teddy Huang1 and Oskar Amster2

1Sr. Application Scientist, Bruker Nano Surfaces, Teddy.Huang@Bruker.com

2Director of Sales and Marketing, PrimeNano Inc, amster@primenanoinc.com

PeakForce - Scanning Microwave Impedance Microscopy

Teddy Huang1 and Oskar Amster2

1Sr. Application Scientist, Bruker Nano Surfaces, Teddy.Huang@Bruker.com

2Director of Sales and Marketing, PrimeNano Inc, amster@primenanoinc.com

p-epi p-epi

n+ n+

n-LDD n-LDD

n-channel n-channel

p p p-epi

n+

n-LDD

n-channel

p

sMIM-C

Modulus

sMIM-R

Adhesion

Outline

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• Bruker nano-electrical measurements.

• PeakForce nano-electrical modes.

• Scanning Microwave Impedance Microscopy.

• Case studies:

Regular sMIM.

PeakForce sMIM.

• Summary.

AFM Nano-electrical Measurements

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Sample

Sample chuck

DC bias

Amplifier, filter and

gain stage

to A/D

AC bias

• Nano-electrical characterization: the probe and the

sample are parts of the electrical circuit.

• Bring macroscopic measurements to nanoscale.

• Bruker provides a versatile array of electrical

techniques for a multitude of applications.

Conductivity/Resistivity C-AFM, TUNA, SSRM

Electric Field EFM

Charge EFM, SCM

Surface Potential /

Work Function KPFM

Carrier Density SCM, SSRM

Piezoelectricity PFM

C-AFM: Conductive AFM

TUNA: Tunneling AFM

SSRM: Surface Spreading Resistance Microscopy

EFM: Electric Force Microscopy

KPFM: Kelvin Probe Force Microscopy

SCM: Scanning Capacitance Microscopy

PFM: Piezoelectric Force Microscopy

Bruker Nano-electrical Measurements

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SSRM, 1.2 x 0.6 um scan, cross-sectioned MOS transistors, log scale

KPFM, 2 x 1 um scan, potential map on InP nanowire with 3V electrical bias between contacts

SCM, 4 x 2 um scan, carrier diffusion of cross-sectioned double-diffused SiC MOSFET

PeakForce Tapping (2009, Bruker)

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• Probe is modulated at 1~2 kHz, allowing for imaging at high scan rate and high pixel resolution

• Feedback setpoint: maximum force or peak force of the tapping cycle.

• Sinusoidal ramping: direct force control of imaging forces with ultra-low setpoints (< 50 pN).

• Linear force control: automatic image optimization, ScanAsyst

• A triggered force curve at every tapping cycle: PeakForce QNM (Quantitative Nano Mechanics)

Adhesion Modulus Height

Brush polymer

2 nm

Sample courtesy: S. Sheiko, UNC, Chapel Hill

https://www.youtube.com/watch?v=wjguTT0rGXM

• Mechanical & Electrical properties are measured simultaneously

• Impossible in contact mode, as forces are too high.

• Higher resolution vs. contact mode.

PeakForce-enabled Electrical Measurements

11/30/2015 7 Bruker

Electrical

Leclere et. al. Nanoscale, 2012, 4, 2705

Integration with PeakForce Tapping for inaccessible, delicate

samples and adds correlated nanomechanical data PeakForce-TUNA

PeakForce-SSRM

PeakForce-KPFM

Height Adhesion Current

700x700 nm scan size

• Improve tip lifetime with hard samples

• Decrease sample wear with soft samples

• Improve resolution due to sharper tips & less sample damage

407 S California Ave, suite 5 Palo Alto, California 94306 Phone: +1 (650) 300-5115 Email: info@PrimeNanoInc.com

Oskar Amster

Introduction to Electrical Scanning Probe Microscopy Measurements Using

Microwave Impedance Microscopy

Who is PrimeNano, Inc?

• Instrument company focused on imaging and metrology for research and industry

• Founded in 2010 in Palo Alto, CA

• Patented technology spun out of Stanford University Applied Physics Dept.

• ScanWaveTM is an electrical module integrated with Bruker’s Icon AFM.

9

Probe the Nano Scale

STM

Atomic scale density of states

Conductive AFM

Spreading resistance

Electrostatic Force Microscope /Kelvin Probe Microscope

Work function / Capacitive Coupling

Scanning Gate Microscope

Current flow path

Scanning Capacitance Microscope

Doping level in semiconductor

Microwave

Microwave Impedance Microscope

Local (s, e) Probe of electrical properties

What is missing?

10

What is sMIM? – A mode for AFM

• Direct measurement of electrical properties – Image local variation of e (permittivity) and s

(conductivity) – Measure carrier type and carrier concentration

for doped semiconducting samples

• Compatible with all materials – Images dielectrics, insulators, semiconductors &

metals

• Compatible with typical AFM imaging modes: – contact, tapping mode, and PeakForce Tapping

• Sub-surface sensitivity – Can image through ~100+ nm overlayers

11 11

Why Microwaves?

• Optics (light) have poor contrast

Silicon (poor conductor, s = 0.0016)

Aluminum (good conductor, s = 3.5e7)

Sapphire (Al2O3, e ~ 9) Glass(SiO2, e ~ 4) sMIM of SiO2 in Si2N3

• Microwaves have high contrast

sMIM of Al dots on SiO2

High Contrast between metals and insulators

SiO2 SiO2

12

Basic Theory (electrical model)

C = e*(A/d) R = (1/s)*(d/A)

How do do we get to this from Microwaves?

13

3. Relation of physical parameters (ε,σ) to lumped element model

4. Changes in ε and σ are seen as changes in

C & R respectively

2. Probe-sample impedance, Ztip, can be

expressed as a lumped element model.

1. Probe/sample interface has its own impedance Metal probe Oxide interface

Sample

(leaky capacitor) capacitor and

resistor in parallel

Z Z Z

Z Z Z

Contrast Mechanism (How Does it Work?)

1. 3GHz Microwave is generated by the ScanWave electronics

Complex impedance is made up of 2 components:

Ztip = Z(Re) + Z(Im)

2. At the probe/sample interface there is a microwave reflection due to a variation in the system

impedance from 50 ohm.

Z(Re) Resistance Z(Im) Capacitance

ScanWave presents these two components as output signals from

the electronics, (sMIM-R and sMIM-C)

By interfacing with the scanning AFM we can synchronize the the output sMIM signals with the

topography and display two images representing the variation in permittivity and conductivity

Ztip

14

Contrast Mechanism (How Does it Work?)

1. 3GHz Microwave is generated by the ScanWave electronics

2. At the probe/sample interface there is a microwave reflection due to a variation in the system

impedance from 50 ohm.

Image Reflection as tip scans.

Real reflection

x

MIM-Re MIM-Im

Imaginary reflection

y

15

Shielded Cantilever Probe

(e,s) (e,s)

20 μm

Shielding is important to reduce stray coupling Low loss, low capacitance Sharp tip (~ 50 nm) Batch fabrication

16

sMIM: Basic Operation

17

• 6 Channels of data available simultaneously with Topography • sMIM-C; sMIM-R • dC/dV Phase & Amplitude • dR/dV Phase & Amplitude

• sMIM-C; sMIM-R Non-contact imaging (dC/dZ; dR/dZ) • sMIM-C; sMIM-R with PeakForce Tapping

6 output channels

Compatibility of Materials

Semiconductors

Silicon FinFETs

Semiconductor/Metallic

Tre

nch

gat

e

Gat

e o

xid

e la

yer

Emitter region

Gate contact

Emitter region

Common emitter/Source metal

Insulators/Dielectrics

Oxide buried under SiN

Compact disk

SiO2

SiO2

SiO2

SiO2

18

Series of bulk insulator samples

• sMIM-C is proportional to ε (permittivity)

– Measure dielectric constant over wide range

sMIM-C proportional to Capacitance

C R C = e*(A/d)

Im(z)

Geometric term = probe tip & distance

0

0.2

0.4

0.6

0.8

1

1.2

1 10 100 1000

sMIM vs Dielectric Constant

Prox. Mod Results

System Model

Dielectric Constant

sMIM

Sig

nal

(V

)

19

0.1

0.2

0.3

0.4

0.5

1E+15 1E+16 1E+17 1E+18 1E+19 1E+20

sMIM vs Doping Level

Acceptor Concentration (/cm3)

sMIM

Sig

nal

(V

)

sMIM-C proportional to Capacitance

IMEC staircase studies

• Linear response with log doping concentration

– Sensitivity from 1014 atoms-cm3 – to 1020 atoms-cm3

Metal

Oxide

Semiconductor

Depletion

layer Vgate

20

ScanWave Electronics Capacitance Sensitivity

Measure Au dot on thickest oxide layer

Smallest Au area – 0.1fF

Sensitivity of 0.15af/mV (calculated)

Measured NIST Capacitance standard using ScanWave

sMIM Image

Line profile from capacitance standard

0.3aF RMS

Schematic of NIST C standard

• 4 different Au dot sizes

• 4 different SiO2 step sizes

• Resulting in 16 different Capacitors

21

.1fF

ScanWave on Bruker ICON Instrument

PrimeNano bracket

Probe

interface

module

Microwave

Electronic

Module

• ScanWaveTM has easy integration with the Bruker instruments

• PrimeNano has made a custom probe interface module for the ICON

• Probe interface bracket uses same scanner placement and screw holes as the SCM electrical module

• PrimeNano probe interface module uses standard Bruker pins to secure to the scanner

22

Case Studies: Contact sMIM

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• Buried Structures.

• Cobalt Surface-Modified γ-Fe2O3.

• Inverted Gate Bipolar Transistor (IGBT).

• Static Radom Access Memories (SRAM).

• CMOS Image Sensor.

sMIM-C

SiO2

SiO2

SiO2

SiO2

sMIM-C sMIM-R

Phase overlays on height sensor

Polished Si3N4 film, buried SiO2 patterned structures

11/30/2015 24 Bruker

• Surface was polished to eliminate residual

topographic features.

• SiO2 and Si3N4 are both insulating, no variation in

sMIM-R over the sample.

• Permittivity difference between SiO2 (ε = 3.9) and

Si3N4 (ε = 7.5), sMIM-C shows different capacitive

response. sMIM-R

SiO2 SiO2

sMIM-C SiO2

SiO2

sMIM is a near-field technique: long range sensitivity to local permittivity variation

sMIM on SRAM Sample

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sMIM-C sMIM dC/dV Amplitude sMIM dC/dV Phase Height Sensor

n+ n+

n-LDD n-LDD

n-channel n-channel

p p

n+

n-LDD

n-channel

p • Image variation of local permittivity.

• SCM modulates sample without

knowing the DC properties. Phase overlays on Height Sensor

)(

)(

Vdd

AVC

dep

Si

oxox

oxMOS

e

ee

• Capable of resolving semiconductor specification.

sMIM on SRAM Sample

11/30/2015 26 Bruker Confidential

sMIM dC/dV Phase sMIM dC/dV Amplitude sMIM-C

Phase overlays on height sensor

sMIM on SRAM Sample

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Phase overlays on height sensor

• Sharp phase contrast on the

topographically featureless region.

• sMIM resolves transitions in carrier

type < 20 nm.

sMIM on Vertical Insulated Gate Bipolar Transistor (IGBT)

11/30/2015 28 Bruker

• sMIM-C shows device structure.

• sMIM carrier profiling resolves electronic structures.

• sMIM provides a level of information that usually

requires both the SEM and SCM to provide the full level

of device structural detail.

sMIM dC/dV Amplitude sMIM dC/dV Phase

p

n

SEM capability N-Substrate

Poly

-Si

(p-type)

base

(n-

type)

Tren

ch gate

Gate

oxid

e laye

r

Emitter region

Gate contact

sMIM-C

1 um

1 um 1 um

p

n

n-Type

p-Type

Highly

Or

Un-

doped

Phase x Amplitude

1 um

This IGBT was prepared by ChipWorks

Substrate

Base

sMIM on CMOS Image Sensor

11/30/2015 29 Bruker Confidential

N-type cathode

P-type implant

P-type pinning

• Small feature size

• Doping gradient on p-doped region

• Pinning layer thickness and spacing

0.7 um

1 um

0.8 um

P-type pinning

140 nm

~100 nm

sMIM dC/dV Phase

sMIM dC/dV Amplitude

~75 nm

sMIM-C Doping gradient

Doping gradient

This CMOS sensor was prepared by ChipWorks

Cobalt Surface-Modified γ-Fe2O3

11/30/2015 30 Bruker

960 nm x 960 nm

1.Magnetic materials

2. Inhomogeneous conductivity

Height Sensor sMIM-C sMIM-R

PeakForce-TUNA MFM

Cobalt Surface-Modified γ-Fe2O3

11/30/2015 Bruker 31

sMIM-R sMIM dR/dV Amplitude

sMIM-C sMIM dC/dV Amplitude sMIM dC/dV Phase

• Carrier profiling for

nanoparticle film.

• dC/dV shows phase

domains.

sMIM dR/dV Phase

• dR/dV shows phase

domains.

• Nonlinear dielectric

properties.

Cobalt Surface-Modified γ-Fe2O3

11/30/2015 Bruker 32

sMIM-C

sMIM-R

sMIM dC/dV Phase

sMIM dR/dV Phase

sMIM dC/dV

Phase on Height

sensor

sMIM dR/dV

Phase on Height

sensor

• Sharp transition from one phase to another: ~ 10 nm.

• dC/dV and dR/dV do not necessarily have the same phase

distribution.

• Different electrical properties within one particle domain.

Voltage Sweeping: sMIM-C vs Voltage (C-V characteristics). sMIM-R vs Voltage (R-V characteristics).

11/30/2015 Bruker 33

• Direct measurement of the DC signals allows for local CV and RV sweeping.

• Reversed phases expect reversed slopes, confirmed by CV and RV sweeping.

sMIM dC/dV Phase

Sample Bias (V)

-1.0 -0.5 0.0 0.5 1.0

sMIM

-C (

mV

)

-10

-5

0

5

10

1

2

1

2

sMIM dR/dV Phase

Sample Bias (V)

-1.0 -0.5 0.0 0.5 1.0sM

IM-R

(m

V)

-20

-10

0

10

1 3

1

3

Case Studies: PeakForce-sMIM

11/30/2015 34 Bruker

1) First trace/retrace: cantilever tracks surface topography with

PeakForce tapping.

2) Cantilever ascends to Lift height.

3) Second trace/retrace: cantilever profiles topography while collecting

sMIM data.

Contact time

Force vs. time

sMIM vs. time

• Capture sMIM data averaged over a full tapping circle.

• Capture sMIM signal during contact time are measured

• Tip oscillates at 1kHz. Contact time is typically 20 – 200 µs

One-path measurement, similar to PeakForce TUNA

Two primary approaches to integrate PeakForce tapping with sMIM

Interleave scanning, similar to PeakForce-KPFM

PeakForce-sMIM on Carbon Nanotube

11/30/2015 35 Bruker

• Convenient for delicate samples.

• Differentiation of CNTs with insulating, semiconducting,

and metallic properties .

• No need to make electrical contacts.

• Simultaneously mapping mechanical properties.

sMIM-R

PeakForce-sMIM on IGBT

11/30/2015 36 Bruker

N-Substrate

Poly-

Si

(p-type)

base

(n-type)

Common emitter/source

metal

• PeakForce Tapping eases the scanning on the rough region,

challenging for contact mode.

• PeakForce-sMIM increases tip lifetime.

• Higher resolution due to a sharper tip.

6 µm

Height Sensor

sMiM-C

Emitter

Base

Ga

te o

xid

e

Tre

nc

h g

ate

Common emitter/source metal

Topography from

contact-sMIM

PeakForce-sMIM on SRAM

11/30/2015 37 Bruker

• PeakForce Tapping advantages.

• High data quality, no flattening on sMIM-C channel.

• Resolve detailed electronic variations on sMIM-C and dC/dV channels.

sMIM-C overlays on Height Sensor dC/dV Phase overlays on Height Sensor dC/dV Amplitude overlays on Height Sensor

sMiM-C

n-channel

n-LDD

p

n+

PeakForce-sMIM on SRAM

11/30/2015 38 Bruker

sMIM dC/dV Phase

sMIM dC/dV Amplitude

sMIM-C

• High lateral resolution in the dC/dV phase image.

• The p-n junction is well defined.

• The depletion region edges are resolved even

below the probe tip radius.

Summary

11/30/2015 39 Bruker Confidential

• Bruker AFMs are platforms for comprehensive electrical measurements, and PeakForce

Tapping extends the applications of these nano-electrical modes.

• sMIM is a powerful tool for direct measurement of material electric properties on various

materials with resolution in the 10’s of nm’s.

• The integration of sMIM with PeakForce Tapping expands electrical measurement to

otherwise inaccessible, delicate samples and adds correlated nanomechanical data.

n+

n-LDD

n-channel

p

11/30/2015 40 Bruker

If you need Bruker AFM help:

Atomic Force Microscope Technical Support Group

Phone: +1 800-873-9750

E-mail: AFM.Support@bruker.com

Website: www.bruker.com

Bruker Support: http://brukersupport.com/

Resources: www.nanoscaleworld.bruker-axs.com

Bruker Probes: www.brukerafmprobes.com

Expert Training: http://www.bruker.com/service/education-training/training-courses/afm-optical-training-courses.html

Teddy Huang, Ph.D.

Sr. Applications Scientist, Electricity and Electrochemistry

Bruker Surfaces Business

112 Robin Hill Road, Santa Barbara, CA 93117

Email: Teddy.Huang@bruker.com

Tel: (805) 967-2700 x2431; Fax: (805) 967-7717

Thank you for your attention!

Oskar Amster

Director of Marketing

PrimeNano, Inc.

407 S. California, suite 5

Palo Alto, CA 94306

Tel: 650-300-5115; Fax: 650-300-5200

If you need PrimeNano sMIM help:

PrimeNano Technical Support

Phone:650-300-5115

E-mail: info@primenanoinc.com

Website: www.primenanoinc.com

Cross-section analysis

11/30/2015 Bruker 41

sMIM dC/dV Phase sMIM dR/dV Phase