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Electrical Measurement Techniques for Nanometrology

Electrical Measurement Techniques for Nanometrology. Speaker/Author: Richard Timmons, P.Eng. President, Guildline Instruments richard.timmons@guildline.com Tel: 1.613.283.3000; Fax: 1.613.283.6082. Presentation Overview. DC Electrical Measurements Nanoscale Range Low And High Resistances

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Electrical Measurement Techniques for Nanometrology

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  1. Electrical Measurement Techniquesfor Nanometrology Speaker/Author: Richard Timmons, P.Eng. President, Guildline Instruments richard.timmons@guildline.com Tel: 1.613.283.3000; Fax: 1.613.283.6082 2007

  2. Presentation Overview • DC Electrical Measurements • Nanoscale Range • Low And High Resistances • Low Currents • Low Voltages • Theoretical Frameworks • Techniques And Tips To Improve Accuracy 2007

  3. Electrical Standards - Resistance • All Electrical Standards Traceable • To National Metrology Institutes • Via17025 Accredited Calibrations • DC Resistance Standards • 1 µΩ (10-6) to 10 PΩ (10-16) • Uncertainties Range from 0.2 to 5000 ppm • Research Into 0.1 µΩ and Smaller Values • Temperature Stabilized Standards • Better Than Traditional Oil Based Standards • Best Uncertainties 0.2 ppm, Annual Drift < 1.5 ppm • Temperature Coefficient < 0.005 ppm • Intrinsic Standard Is Quantum Hall at 12906.4035Ω 2007

  4. 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 / 1Ω With Better Than .001 ppm Uncertainties 2007

  5. Electrical Standards - Voltage • Voltage • Typically 1 V to 10 V • Best Uncertainties < 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 2007

  6. Resistance Measurements • Source Current / Measure Voltage • Source Voltage / Measure Current • Low Resistance Measurements • High Resistance Measurements 2007

  7. Source Current / Measure Voltage • Best for Low Resistance Measurements (< 1kΩ) • Voltage Sources Noisier Than Current Sources For Low Impedance • The Johnson Voltage Noise At Room Temperature (270ºK) • Simplifies to: • k = Boltzmann’s Constant, T = Absolute Temperature of Source (ºK) • B = Noise Bandwidth (Hz), and R = Resistance of the Source (Ω) • As DUT Resistance (R) Decreases Noise Voltage Decreases 2007

  8. Source Voltage / Measure Current • Best for High Resistance Measurements • > 10 kΩ • 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 (Ω) • As DUT Resistance (R) Increases Noise Current Decreases 2007

  9. 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 2007

  10. LOW RESISTANCE MEASUREMENT1k – 1k • Source Voltage Source Current • 3V, 0.206 ppm Std. Dev. 3.16mA, 0.005 ppm Std. Dev. • At 1kΩ and Lower, Sourcing Current Gives Much Better Measurements 2007

  11. MEDIUM RESISTANCE MEASUREMENT10k – 10k • Source Voltage Source Current • 10V, 0.003 ppm Std. Dev. 1mA, 0.011ppm Std. Dev. • The 10 kΩ Resistance Level Is the Approximate Transition Point At Which Both Voltage and Current Methods Perform Equally Well With Respect to Measurement Noise 2007

  12. HIGH RESISTANCE MEASUREMENT100k – 100k • Source Voltage Source Current • 32V, 0.003 ppm Std. Dev. 0.32mA, 0.217 ppm Std. Dev. • At 100 kΩ and Higher Sourcing Voltage Gives Much Better Measurements 2007

  13. Very Low Resistance Measurements • 100 µΩ Resistance Standard (Guildline 9334A) • Below 1 mΩ Recommended to Use Current Range Extenders • Up to 3000A • Uncertainties of 10-8 ppm or Better 2007

  14. 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 Ω With Low Currents • Thermal Stability Very Important • For Both Resistance Standard and DUT • Stable Air Baths (0.001 °C) 2007

  15. Very High Resistance Measurements • DCC bridges measure up to 1 GΩ • Provide Better Uncertainties At and Below 100 MΩ • Best Uncertainties of 0.02 to 0.04 ppm For Multi-Ratio Bridges • Teraohmmeters (i.e. electrometer based) Better Above 1G • Measure From 1 MΩ up to 10 PΩ (1016) With Direct Measurement Uncertainty Ranging From 0.015% to 5% Across This Range 2007

  16. 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 1:1000) Current Research to 1017Ω Using 1014Ω Standard. 2007

  17. 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] 2007

  18. Low Current Measurements(cont) Guildline 6520 Teraohmmeter With Guildline 9336/9337 Resistance Standards [2] Uncertainties Can Be Improved by the Substitution Method [1] 2007

  19. 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 2007

  20. 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 2007

  21. Temperature Effects 1.0 µΩ Resistance Standard (Guildline 9334A) t/c of 8.5 ppm/°C (8.5-12Ω or 8.5 pΩ) Best Thermometry Bridges < 0.025 ppm Ruthenium Oxide Probe (RTD) For < 1 ºK needs 75 kΩ Stable Air Baths At < 1 °mK 2007

  22. Digital Filtering • Order of Magnitude of Additional Accuracy • Large Number of Tests • Reduces the Bandwidth of the Noise • Ex: Remove ‘Outlier’ Measurements > k3 • ( i.e. > 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 2007

  23. 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] 2007

  24. 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 2007

  25. Humidity Effects • Make Measurements In a Controlled, Low Humidity Environment • Essential If DUT Absorbs Water • Use High Quality Insulators • Teflon, Polyethylene, Sapphire 2007

  26. 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 2007

  27. 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 2007

  28. 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 2007

  29. 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 Instrument’s Common Ground To Chassis Ground (i.e. Power Line Ground) 2007

  30. 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 > 100 kΩ 2007

  31. 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 2007

  32. References • [1] Mark Evans and Nick Allen, Guildline Instruments Limited, Evaluation of a Concept for High Ohms Transfers at Ratios > 10:1, 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. 2007

  33. Providing PrecisionMeasurement Solutions Guildline Instruments Limited 2007

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