some novel concepts in scanning ... - university of exeter · probe technology workshop, june 11 th...

Zurich Research Laboratory

Probe Technology Workshop, June 11th 2009, Milan

Some Novel Concepts in Scanning Probe Technology

Abu Sebastian

Memory and Probe Technologies Group

Nanofabrication Group

2



Outline

Thermo-electric topography sensing

Modeling and experimental identification

Feedback enhanced thermo-electric sensing

Multi-scale resolution imaging

Magneto-resistive topography sensing

The basic concept

Experimental Results

Conductive probe technology

PtSi and encapsulated PtSi Conductive Probes

Resistance patterning on carbon thin films

3



Thermo-electric Topography Sensing





The basic concept





4



sensor bias voltage

current

substrate voltage for electrostatic actuation

Thermo-electricsensors

deflection signal

Scanning Probes with Thermo-electric Sensors

Originally designed for thermo-mechanical data storage

Micro-cantilevers with integrated thermo-electric sensors

Low doped regions, micro-heaters

The cantilevers can be actuated electrostatically

An optical deflection sensor can be used to measure the cantilever deflection in

addition to the thermo-electric sensors

5



Thermo-electric Sensors

Mode of Operation Thermo-electric sensors are micro-heaters

With the application of a DC voltage the sensors are heated to a certain temperature

Depending on the sensor-sample separation the temperature changes which also results in a change in the electrical resistance

Hence a change in the sensor-sample separation is measured as a current change Low Cost, Easily integratable sensor, can be used for a variety of applications

Significant interest in the experimental identification of sensitivity, bandwidth and resolution of thermo-electric sensors

“Sensing transfer function” characterizes the sensitivity and bandwidth

Map from cantilever deflection to current

Variation of sensitivity with frequency

Sensing Transfer Function

cantileverdeflection

current

THERMO-ELECTRIC SENSOR

Temp

Resistance

More cooling by substrate Less cooling by substrate

6



Model of Thermo-electric Sensing

An “operator” model of thermo-electric sensing Modeled by a linear thermal system in feedback with a memoryless nonlinear system: the nonlinear

temperature vs. resistance relationship

Thermal system: A function of the sensor-sample separation Change in sensor-sample separation perturbs the thermal system This manifests as a current fluctuation which is measured

bias voltage temp

temperature-resistance map

thermal system

V2/R

roomtemp

height fluctuations

current fluctuations

V/R

power

resistance

A. Sebastian and D. Wiesmann, “Modeling and Experimental Identification of Silicon microheater Dynamics: A systems

Approach”, J. of Microelectromech. Sys., 2008

7



Sensing Transfer Function from Electrical Measurements

For small changes in sensor-sample separation the operator model can be linearized Sensing transfer function given by the relationship between and The thermal system can be identified using electrical measurements From purely electrical measurements we can derive the sensing transfer function!

0~ =V

I~

02I

0/1 R 20I−

xTPΤ

)( 0Tg′0

0

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I−

+ +

+

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)(P

xK

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T~

R~

Τ′+

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xTP

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xxI PTgI

PTg

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0020

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)(1

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)(

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Linearized version of the thermal sensor model

system thermal theofGain :)(

map eTemperatur vsResistance :)(

System Thermal :

xK

TgxTPΤ

x~ I~

power Operating

resistance Operating

current Operating

0

0

0

===

P

R

I

8



Sensing Transfer Function from Electrical Measurements

Thermal Systemat different tip-sample

separations

temperature vs resistancerelationship

EXPERIMENTAL MEASUREMENTS ANALYTICAL SENSING TRANSFER FUNCTION

9



Direct Measurement of Sensing Transfer Function

Electrostatically actuate the cantilever and measure the deflection optically and using the thermo-electric sensor

Close to the sample surface we can use the thermo-electric sensor to measure the deflection with high enough SNR

From the simultaneous measurement of cantilever deflection using two sensors we can evaluate the “sensing transfer function” corresponding to the thermo-electric sensor

(Mechanical Frequency Response of the Cantilever)Transfer function from substrate voltage to

deflection measurement (optical)

tip-sample separation Substrate voltage

deflection (from optical sensor)

Substrate voltage Current (from thermo-electric sensor)

Sensing Transfer Function:deflection (from optical sensor)

current (from thermo-electric sensor)

10



Comparison

Sensing transfer function obtained using three different methods Direct measurement using simultaneous measurement of cantilever deflection using the optical sensor and the

thermo-electric sensor

Using electrical measurements and analytical relations

Using ANSYS simulation

Remarkable agreement between all three

Very good understanding of the underlying sensing mechanism

H. Rothuizen et al., “Design of power-optimized thermal cantilevers for scanning probe topography sensing”, IEEE MEMS, 2009

Sensing Transfer Function

11



Feed-back Enhanced Thermo-electric Sensing

Observation: Sensing is faster than the thermal system Reason: Inherent thermo-electric feedback How about an external feedback to further shape the sensing transfer function?

V~

I~

02I

0/1 R 20I−

xTPΤ

)( 0Tg′0

0

R

I−

+ +

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0)(

)(P

xK

xK ′x~

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R~

+

FBK

( )xx

x

x

TPFB

TP

TP

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TgIR

KPTgI

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I

xK

xK

Τ′−−Τ′+

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=Τ)(1)(1

)()(

)(

020

000

20

000

0

~~

12



Simulation Results

Low power reader identified The sensing transfer function is simulated for varying values of feedback gain: KFB

Steady increase in sensitivity and bandwidth Need not correspond to an increased resolution

Depends on the noise sources

Simulated Sensing Transfer Function

13




There is a dramatic impact on sensing bandwidth There is an increased sensitivity as predicted by the analysis Can arbitrarily shape the sensing transfer function using filters instead of constant

feedback gain Illustration of a scenario where feedback control is used to shape the “sensing transfer

function”

A. Sebastian et al., “Feedback enhanced thermo-electric topography sensing”, Transducers, 2009

Cantilever Mechanical Frequency Response Identified Sensing Transfer Function

14



Multi-scale Resolution Topography Imaging

Topography sensing using scanning probes with optical deflection sensors

Tip In contact: Variation in sample topography results in cantilever deflection

Lateral resolution defined by the tip dimensions (nano-scale)

Tip Out of contact: No cantilever deflection and hence no information

No scope for multi-scale resolution

Scanning probe systems are usually assisted by a micro-scope with micro-scale resolution

Scanning probe based manipulation system

assisted by an optical microscope(courtesy: CMU)

Topography sensing using thermo-electric sensors

In contact: Nanoscale resolution provided by the tip

Out of contact:

The sensor measures the absolute sensor-sample separation

Scope for multi-scale resolution: Sample topography can be measured with lateral resolution defined by the area of the heater

Thermo-electric sensors can provide multi-scale resolution!

deflection signal

In contact Out of contact

15



Off-Contact Thermal Imaging (Hovercraft)

The thermo-electric sensors provide information on the tip-sample separation

The cantilever can “hover” over the sample surface maintaining a fixed tip-sample separation

Images obtained with a lateral resolution in the order of the dimension of the sensor

Z control

Controller

Tip-sampleseparation

cantilever

reference+-

X-Y-Z Scanner

Calibration sample (2 µµµµm) deepPitch: 10 µµµµm

thermo-electric sensor2 µm X 1.5 µm

lateral dimension

“Hovercraft” Image obtained by maintaining

a 500 nm tip-sample separation

16



Lateral and Vertical Resolution

Highly localized heating

essential for the scheme to

work

Simulations show the extent of

heat confinement

Silicon sample with 200 nm

trenches of varying pitch was

imaged off contact

Experimental results show the

capability to easily resolve ≈ 50

nm tall features with < 10 µm

pitch

Finite Element Simulation Showing the Heat Flux (H. Rothuizen)

Silicon sample with trenches of vaying pitch

3 σ resolution

17



Demonstration of Multi-scale resolution Imaging

10 um1 um

2 um

Nanowire attached to four

electrodes imaged using a

cantilever with integrated

thermo-electric sensors

Images of the electrodes down to

the nanowire are obtained

showing the multi-scale

resolution

sample being imaged

“Hovercraft” Image

High resolution image obtained using the tip

location of the nanowire

18








The basic concept





19



MR Sensor

Magneto-resistive Topography Sensing

Basic concept: Translate topography

variations to modulation of a magnetic

field strength (HX) which is sensed by a

Magneto-resistive (MR) sensor

An MR sensor is attached to the cantilever

A micro-magnet is fixed to a structure

which is independent of the cantilever

motion

As the tip traverses the topography, the

magnetic field seen by the MR sensor

changes and the topography signal is

generated

The MR Sensor and the micro-magnet can

reverse positions

MR SensorHX

HX

MAGNET

MAGNET

SAMPLE

SAMPLE

20



Y

Z

X

Magnet

MR sensor

Simulations

Magnetization along X-axis

Sensing motion along Z-axis

NbFeB type magnetic material

Magnetic field decays in the order of the

size of the magnet

MR sensor senses the X-component of the

magnetic field from the magnet

Working distance is in the order of the

dimension of the magnet

S N

Magnetic Field

Hx (Oe)

21



Proof of Concept Experiment

R : Giant magneto-resistive sensors (GMR)FC : Flux concentratorsR0 : Reference GMR sensors (Isolated from magnetic field)FS : Flux shield

SensingDirection

Commercial GMR sensor (NVE Corp.)

Used to measure the magnetic field strength along

the sensing direction

Four sensors arranged in a Wheatstone bridge

configuration

Two GMR elements are exposed to magnetic field

The other two GMR elements are shielded from

external magnetic field

FC FC

FCFC

R0 R0

RVs+

Vs-

OUT+

OUT- RFS FS

22



SensingDirection


Bandwidth >1MHz

Resistance = 5 kΩ

Magneto-resistive ratio = 20%

Sensitivity = 2.5 mV/Oe (5V supply)

Saturation Field = 100 Oe

Hysteresis is small for small variation in magnetic field

Anti-ferromagnetic (AF) coupled

Sensor Characteristics (similar)

Applied Magnetic Field (Oe)Outp

ut Voltage (V)

23



Vs+

Vs-

OUT+

OUT-


Micro-Magnet glued on to a micro-cantilever

Magnet: NbFeB, Diameter ≈ 10 µm, Magnetized in the lateral direction

Tip height ≈ 10 µm

The GMR Sensor is used as the sample

Objective: Demonstrate magneto-resistive topography sensing

10-12 µm

Cantilever

MagnetTop View

Front View

Magnetization direction,

Sensing direction

24




Sensitivity: 130 mV/µm Resolution: 1.4 nm over 10kHz

Approach Curve

Noise power spectral density

Topography Image (Magneto-resistive Sensing)

Topography Image

(Using Commercial AFM)

A topography image of the sensing

element was obtained

25



Magneto-resistance based Topography Sensing

Significant potential for a high bandwidth, high resolution integratable sensor

Very favorable for size scaling

Compares well with thermo-electric sensing

Thermo-electric sensor

Senses resistance change ∆R versus temperature T

Sensitivity: ∆R/R ≈ 10-4/nm

Resolution: < 1 nm over 100 KHz

Limitation in bandwidth

Novel magneto-resistive sensor

Senses resistance change ∆R versus magnetic field B

Sensitivity: ∆R/R > 10-3/nm

Resolution comparable to thermo-electric sensor

Bandwidth in excess of 1 MHz

26








The basic concept





27



Conductive-mode AFM

Conductive-mode AFM is a powerful tool for

nano-scale electrical characterization

Resistance Mapping

Apply a constant voltage between the tip

and a bottom electrode

Scan the tip over the sample surface

Resistance map of the sample surface

obtained with nano-scale resolution

Local transformation of material properties

A high enough voltage signal applied

between tip and bottom electrode

Resulting current flow induces highly

localized change of material properties

Deflection signal used to maintain constant

loading force while performing these operations

deflection signal

X

YZ

VoltageSource

X/Y/Z Scanner

28



Conductive Probes

Commercial conductive probes are typically Si tips with a conductive coating

Poor wear characteristic (especially since high loading forces are typically required for reliable and repeatable

conduction)

Cannot sustain high currents

All Metal Probes and Diamond Probes

Tip diameter is usually compromised

Difficult to mass produce and hence very expensive

Significant need for highly reliable nano-scale conductive probes which can sustain high currents

image taken after 25mm of scanning. conduction stopped after 3 mm of

scanningTip before an experiment

29



Wear Characteristics

(on TAC)

Platinum Silicide Tips

Platinum Silicide is formed at the apex of a

silicon tip

Moderate Improvement in wear

Most importantly very good electrical

conduction Instant, sustainable conduction

Can sustain high currents

Nanoscale resolution not compromised

PtSiSi

ElectricalConduction

(on Au)

H. Bhaskaran, A. Sebastian, M. Despont, “Nanoscale PtSi tips for conducting probe technologies”, IEEE Trans. on NanoTech., 2009

ElectricalConduction

(on Au)

30



N17:Before Wear Exp

N17:After Wear Exp

Encapsulated Platinum Silicide Tips

Encapsulated tips with a conductive core

Should significantly improve the wear characteristics

Long term conduction and wear reliability of these tips were evaluated thoroughly

Wear Characteristics

oxidePtSi

conductive core

H. Bhaskaran, A. Sebastian, U. Drechsler, M. Despont, “Encapsulated tips for reliable nanoscale conduction in scanning probe technologies”, NanoTech., 2009

31



Simultaneous Conduction and Deflection Measurement (HOPG)

Encapsulated Platinum Silicide Tips

Experiments confirm the excellent contact quality and spatial confinement of the

conductive core

Increased tip-sample contact area and the subsequent increase in adhesive forces also

enable reliable operation in the retraction mode (regulation on negative deflection)

X position (nm)

CURRENT IMAGE

2000 4000 6000

50010001500

HOPG Image obtained using an encap PtSi tip

32



Localized enhancement of conductivity in a-C thin films

Sample: 40nm TiN/20nm C

(Prepared by A. Pauza,

Plarion)

Field dependent change in

resistivity

Threshold switching behavior

Permanent change in resistance

thresholdswitching

permanent changeIn resistance

Field-dependent change in resistivity

Two consecutive I-V curves at the same location

Si

CondTip

TiN

carbonV

33



Localized enhancement of conductivity in a-C thin films

No perceivable change in sample

topography

Significant change in resistance

Switching possible even with sub-

microsecond pulses

Topography Image

Resistance Image

Voltage and Current Signals

34



Amplitude and Thickness Dependence

The resistance contrast increases with increasing current

The threshold switching voltage increases with increasing carbon thickness

40 nm TiN/20 nm a-C, varying amplitude I-V Varying thickness of a-C

35



Resistance Patterning of a-C thin films

Resistance Patterning of Large

Surfaces

Topography Image

Resistance Image

Current Image

Resistance Signal (line scan)

36



Summary


Using an operator model, the sensors can be characterized in terms of bandwidth and sensitivity

Using feedback control the sensitivity and bandwidth can be enhanced

Thermo-electric sensors have the potential for multi-resolution imaging

Magneto-resistive Topography Sensing

Topography information translated to an equivalent magnetic field gradient

Preliminary experimental results show great promise towards the realization of a high

bandwidth, high resolution integratable topography sensor.

Conductive Probe Technology

Significant need for highly reliable conductive probes that can sustain high currents

PtSi and Encapsultad PtSi conductive probes could meet this demand and are powerful tools for

nanoscale electrical characterization

Conductive probe based nanoscale resistance patterning was demonstrated on carbon thin films

37



Acknowledgements

Colleagues at IBM

Harish Bhaskaran

Rachel Cannara

Deepak R. Sahoo

Hugo Rothuizen

Peter Baechtold

Ute Drechlser

Walter Haeberle

Michel Despont

Haris Pozidis

Evangelos Eleftheriou

ProTeM Partners

Andrew Pauza, Plarion Ltd.

David Wright, Univ. of Exeter

some novel concepts in scanning ... - university of exeter · probe technology workshop, june 11 th...

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