Electrical Measurement Techniques for Nanometrology

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Electrical Measurement Techniquesfor
Nanometrology

• Speaker/Author Richard Timmons, P.Eng.
• President, Guildline Instruments
• richard.timmons_at_guildline.com
• Tel [phone] Fax [phone]

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Presentation Overview

• DC Electrical Measurements
• Nanoscale Range
• Low And High Resistances
• Low Currents
• Low Voltages
• Theoretical Frameworks
• Techniques And Tips To Improve Accuracy

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Electrical Standards - Resistance

• All Electrical Standards Traceable
• To National Metrology Institutes
• Via17025 Accredited Calibrations
• DC Resistance Standards
• 1 µ? (10-6) to 10 P? (10-16)
• Uncertainties Range from 0.2 to 5000 ppm
• Research Into 0.1 µ? and Smaller Values
• Temperature Stabilized Standards
• Better Than Traditional Oil Based Standards
• Best Uncertainties 0.2 ppm, Annual Drift lt 1.5
  ppm
• Temperature Coefficient lt 0.005 ppm
• Intrinsic Standard Is Quantum Hall at 12906.4035?

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Electrical Standards - Current

• Current
• Current Shunts 1 µAmp to 3000 Amps
• Best Uncertainties 1 ppm to 500 ppm
• Stable, Linear Performance With Respect to Power
• Primary Standard
• Current Balance Between 2 Coils of Known Mass and
  Dimensions With Uncertainty of 15 ppm
• Practical Realization of Ampere
• From 1A 1V / 1? With Better Than .001 ppm
  Uncertainties

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Electrical Standards - Voltage

• Voltage
• Typically 1 V to 10 V
• Best Uncertainties lt 1.0 ppm
• Intrinsic Standard Is Josephine Junction Array
• Typical Output In mV to 1V Range With Best
  Uncertainties In the 0.01 to 0.001 ppm Range
• Current Research on Stacked Josephine Junction
  Arrays to Get Higher Voltages
• Precision Voltage Dividers Used to Transfer To
  Range of Nanovolts to Kilovolts

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Resistance Measurements

• Source Current / Measure Voltage
• Source Voltage / Measure Current
• Low Resistance Measurements
• High Resistance Measurements

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Source Current / Measure Voltage

• Best for Low Resistance Measurements (lt 1kO)
• Voltage Sources Noisier Than Current Sources For
  Low Impedance
• The Johnson Voltage Noise At Room Temperature
  (270ºK)
• Simplifies to
• k Boltzmanns Constant, T Absolute
  Temperature of Source (ºK)
• B Noise Bandwidth (Hz), and R Resistance of
  the Source (O)
• As DUT Resistance (R) Decreases Noise Voltage
  Decreases

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Source Voltage / Measure Current

• Best for High Resistance Measurements
• gt 10 kO
• Voltage Sources More Stable When Driving High
  Impedance
• The Johnson Current Noise At Room Temperature
  (270ºK)
• B Noise Bandwidth (Hz), and R Resistance of
  Source (O)
• As DUT Resistance (R) Increases Noise Current
  Decreases

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Comparative Results Sourcing Current Versus
Sourcing Voltage

• Summary of 50 Measurements Made at Three
  Resistance Values Using a Guildline DCC Bridge
  Sourcing Both Current and Voltage

Test (?) Source Current Uncertainty (ppm) Source Voltage Uncertainty (ppm)
1k-1k 0.005 0.206
10k-10k 0.011 0.003
100k-100k 0.217 0.003
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LOW RESISTANCE MEASUREMENT 1k 1k

• Source Voltage Source Current
• 3V, 0.206 ppm Std. Dev. 3.16mA, 0.005 ppm Std.
  Dev.
• At 1k? and Lower, Sourcing Current Gives Much
  Better Measurements

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MEDIUM RESISTANCE MEASUREMENT 10k 10k

• Source Voltage Source Current
• 10V, 0.003 ppm Std. Dev. 1mA, 0.011ppm Std. Dev.
  
• The 10 k? Resistance Level Is the Approximate
  Transition Point At Which Both Voltage and
  Current Methods Perform Equally Well With Respect
  to Measurement Noise

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HIGH RESISTANCE MEASUREMENT 100k 100k

• Source Voltage Source Current
• 32V, 0.003 ppm Std. Dev. 0.32mA, 0.217 ppm Std.
  Dev.
• At 100 k? and Higher Sourcing Voltage Gives Much
  Better Measurements

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Very Low Resistance Measurements

• 100 µ? Resistance Standard (Guildline 9334A)
• Below 1 mO Recommended to Use Current Range
  Extenders
• Up to 3000A
• Uncertainties of 10-8 ppm or Better

Serial 50A 75A 100A
68343 99.9871 99.9871 99.9888
69181 99.9803 99.9810 99.9826
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Very Low Resistance Measurements (cont)

• May Need Low Currents
• Saturation Current For Nanoscale Materials Often
  Very Low
• Self Heating Effects Create Measurement Errors
  and Excessive Heat Can Damage DUT
• Exception Is Super-Conducting Materials
• Current Comparator (CCC) Bridges Can Measure Down
  to 10-9 ? With Low Currents
• Thermal Stability Very Important
• For Both Resistance Standard and DUT
• Stable Air Baths (0.001 C)

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Very High Resistance Measurements

• DCC bridges measure up to 1 G?
• Provide Better Uncertainties At and Below 100 M?
• Best Uncertainties of 0.02 to 0.04 ppm For
  Multi-Ratio Bridges
• Teraohmmeters (i.e. electrometer based) Better
  Above 1G
• Measure From 1 MO up to 10 PO (1016) With Direct
  Measurement Uncertainty Ranging From 0.015 to 5
  Across This Range

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Very High Resistance Measurements (cont)

• Teraohmmeter With Multi-Ratio Direct Transfer
  Provides Best Uncertainties 1
• Transfers (25) To Known 1G, 10G and 100G
  Standards Using Known 100M Standard (Ratios Up to
  11000)
• Current Research to 1017O Using 1014O Standard.

Resistor Nominal Value (?) Charted Uncertainty (ppm) Direct Reading (ppm) Transfer Uncertainty (ppm)
100M 18 150 30.9
1G 41 200 33.7
10G 106 600 32.7
100G 94 800 46.6
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Low Current Measurements

• Generate or Measure Accurate and Traceable Low
  Value Currents
• Use Commercial Voltage Standard and Accurate High
  Value Resistance Standards
• Traceable Reference Currents Down to 50 fA (10-15
  A)
• Can be Verified Using a Teraohmeter 2

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Low Current Measurements(cont)
Resistor 9336/9337 Teraohmmeter Test Voltage Effective Current Uncertainty
100k 1 V 10 µA 0.025
1M 1 V 1 µA 0.025
10M 10 V 1 µA 0.025
100M 10 V 100 nA 0.015
1G 10 V 10 nA 0.02
10G 10 V 1 nA 0.06
100G 10 V 100 pA 0.08
1T 10 V 10 pA 0.1
10T 10 V 1 pA 0.2
100T 10 V 100 fA 0.3
1P 10 V 10 fA 1
10P 10 V 1 fA 5

• Guildline 6520 Teraohmmeter With Guildline
  9336/9337 Resistance Standards 2
• Uncertainties Can Be Improved by the Substitution
  Method 1

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Low Voltage Measurements

• In Order to Prevent Damage
• Unless Material Is Super-Conducting
• Nanovolt Meters Can Measure in the Picovolt
  (10-12) Range
• Johnson Noise (i.e. Motion of Charged Particles
  Due to Thermal Energy) Limits Accuracy of Low
  Voltage Measurements

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MEASUREMENT TECHNIQUES AND TIPS

• Temperature Effects
• Digital Filtering
• DC Reversal Techniques
• Humidity Effects
• Electromagnetic Interference (EMI)
• Connectors and Leads
• Guarding
• Grounding
• Settling Times
• Direct Measurement With No Amplification

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Temperature Effects

• 1.0 µ? Resistance Standard (Guildline 9334A)
• t/c of 8.5 ppm/C
• (8.5-12? or 8.5 p?)
• Best Thermometry Bridges lt 0.025 ppm
• Ruthenium Oxide Probe (RTD) For lt 1 ºK needs 75
  kO
• Stable Air Baths At lt 1 mK

Serial 21C µ? 23C µ? 25C µ?
68559 1.000048 1.000065 1.000083
68560 0.999997 1.000015 1.000032
68561 1.00033 1.00034 1.00035
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Digital Filtering

• Order of Magnitude of Additional Accuracy
• Large Number of Tests
• Reduces the Bandwidth of the Noise
• Ex Remove Outlier Measurements gt k3
• ( i.e. gt 3 x standard deviation)
• Dynamically Alter the Sampling Times
• Increase If Measurement Stable
• If Periodic, Synchronize To a Clock
• Telecommunications Industry
• Analyze Total Set of Test Results
• Post Experiment Analysis With PC

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Digital Filtering(cont)

• Sophisticated Techniques Include Profiling Noise,
  Excitation Effects, Systematic Errors, and Other
  Effects With a Suitable Mathematical Model
• Use Weighted Coefficients
• Ex Closure Error For a Multi-Ratio Guildline DCC
  bridge 3

Correction Method Relative Improvement (ppm)
Uncorrected (Baseline Measurement) 0.000
Rounding 0.050
Linear Interpolation 0.061
Logarithmic Weighting 0.084
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DC Reversal Techniques

• Polarity Reversal
• Eliminate Thermal EMFs
• Reduces the Effect of White Noise
• Increases the Signal-To-Noise Ratio
• Can Be Optimized
• Faster When Measured Parameter Is Changing
• Slower When Measured Parameter Is Stable

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Humidity Effects

• Make Measurements In a Controlled, Low Humidity
  Environment
• Essential If DUT Absorbs Water
• Use High Quality Insulators
• Teflon, Polyethylene, Sapphire

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Electromagnetic Interference (EMI)

• EMI Noise In Most Laboratories
• Florescent Lights, Cell Phones, Fixed Point
  Temperature Furnaces, Electric Motors, AC
  Electrical Power Lines
• Ambient EMI Noise Often Higher Than Nanoscale
  Electrical Measurements
• Instruments Have Built-in EMI Noise
• Display Screens, Microprocessors /
  Microcontrollers, Power Supplies
• EMI Shielding For Both Measurement Circuitry and
  DUT
• High Quality Air Baths Provide Both EMI Shielding
  and Temperature Stability
• Power Line Filters

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Connectors and Leads

• 4 Terminal Mode
• Most Accurate Method for Measuring Small
  Resistances
• Corrects For Lead Resistance
• Allows Longer Test Leads
• Current Supply Compliance Important
• Very Low Resistances May Have Greater Voltage
  Drop Across Leads and Connectors Then Across
  Shunt
• Condition of Connectors, Cleanliness Important
• Poor Measurements From Cracked Terminals, Dirty
  Contacts, Moisture Absorbed By Standards and DUTs
  
• Errors As High As 10 ppm
• High Resistance Needs Very Good Insulation

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Guarding

• Conductor Connected To Low Impedance Point In
  Circuit That Is At Nearly Same Potential As High
  Impedance Lead Being Guarded
• Reduces Leakage Currents and Noise In Test /
  Measurement Circuits
• Very Important For High Resistance Measurements
• Measurement Instruments Should Provide Guarded
  Connection Terminal
• Reduces Effect Of Shunt Capacitance

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Grounding

• Single Point Ground For All Components In Test
  Setup Including DUT
• Avoids Ground Loop Currents Between Measurement
  Circuit and DUT, or Measurement Circuit and Test
  Fixture
• Noisy Power Lines
• Largest Contributor Is Typically PCs
• NOT Good Measurement Practice To Connect
  Different Components Of Test Setup To Different
  Power Outlets
• Power Line Grounds May Not Be At Same Electrical
  Potential, Thus Creating Spurious Currents
• NOT Good To Connect Instruments Common Ground To
  Chassis Ground (i.e. Power Line Ground)

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Settling Times

• Needed To Overcome Capacitance Effects,
  Self-Heating Effects, Dielectric Absorption
• Present In Measurement Instruments, Standards,
  Cabling, DUT
• Longer Settling Times Very Important For
  Resistances gt 100 k?

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Direct Measurement With No Amplification

• NOT Recommended To Use Operational Amplifiers or
  Other Techniques To Increase the Measured Signal
• Will Proportionally Increase Noise
• Operational Amplifiers Or Other Circuitry Will
  Introduce Additional Noise
• Need Instruments Capable Of Directly Measuring
  Electrical Properties At Very Low Values

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References

• 1 Mark Evans and Nick Allen, Guildline
  Instruments Limited, Evaluation of a Concept for
  High Ohms Transfers at Ratios gt 101, 2007
  Conference Proceedings of the NCSL International
  Annual Workshop and Symposium.
• 2 Mark Evans, Application of the Guildline
  Model 6520 Teraohmmeter for the Nuclear Power
  Industry, White Paper, Guildline Instruments
  Limited.
• 3 Mark Evans and Xiangxiao Qiu, P. Eng.,
  Guildline Instruments Limited, Application of
  Software Enhanced DCC Bridge Measurement, 2005
  Conference Proceedings of the NCSL International
  Annual Workshop and Symposium.

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Providing PrecisionMeasurement Solutions

• Guildline Instruments Limited