Some Novel Concepts in Scanning ... - University of Exeter · Probe Technology Workshop, June 11 th...
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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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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0/1 R 20I−
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Linearized version of the thermal sensor model
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System Thermal :
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power Operating
resistance Operating
current Operating
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===
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R
I
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
Sensing Transfer Function from Electrical Measurements
Thermal Systemat different tip-sample
separations
temperature vs resistancerelationship
EXPERIMENTAL MEASUREMENTS ANALYTICAL SENSING TRANSFER FUNCTION
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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)
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Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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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xK
xK ′x~
T~
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+
FBK
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Τ′−−Τ′+
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=Τ)(1)(1
)()(
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020
000
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000
0
~~
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
Experimental Results
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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)
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
SensingDirection
Proof of Concept Experiment
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)
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Probe Technology Workshop, June 11th 2009, Milan
Vs+
Vs-
OUT+
OUT-
Proof of Concept Experiment
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
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Probe Technology Workshop, June 11th 2009, Milan
Proof of Concept Experiment
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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)
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Probe Technology Workshop, June 11th 2009, Milan
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
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Zurich Research Laboratory
Probe Technology Workshop, June 11th 2009, Milan
Resistance Patterning of a-C thin films
Resistance Patterning of Large
Surfaces
Topography Image
Resistance Image
Current Image
Resistance Signal (line scan)
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Probe Technology Workshop, June 11th 2009, Milan
Summary
Thermo-electric Topography Sensing
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
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Probe Technology Workshop, June 11th 2009, Milan
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