1. Guildline Instruments Limited 2007 2007 Techniques for Ultra-Precise Resistance Measurements Speaker/Author: Richard Timmons, P.Eng. President, Guildline Instruments richard.timmons@guildline.com Tel: 1.613.283.3000; Fax: 1.613.283.6082

2. Guildline Instruments Limited 2007 Presentation Overview DC Electrical Measurements Resistance Standards Resistance Standards Resistance Measurements Resistance Measurements Theoretical Frameworks Very Low Resistance Very High Resistance Techniques And Tips To Improve Accuracy

3. Guildline Instruments Limited 2007 Electrical Standards - Resistance All Electrical Standards Traceable To National Metrology Institutes To National Metrology Institutes Via17025 Accredited Calibrations Via17025 Accredited Calibrations DC Resistance Standards 1 µΩ (10 -6 ) to 10 PΩ (10 -16 ) 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 Research Into 0.1 µΩ and Smaller Values Temperature Stabilized Standards 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 Intrinsic Standard Is Quantum Hall at 12906.4035Ω

4. Guildline Instruments Limited 2007 Resistance Measurements Source Current / Measure Voltage Source Voltage / Measure Current Low Resistance Measurements High Resistance Measurements

5. Guildline Instruments Limited 2007 Source Current / Measure Voltage Best for Low Resistance Measurements (< 1kΩ) Voltage Sources Noisier Than Current Sources For Low Impedance Voltage Sources Noisier Than Current Sources For Low Impedance The Johnson Voltage Noise At Room Temperature (270ºK) Simplifies to: 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

6. Guildline Instruments Limited 2007 Source Voltage / Measure Current Best for High Resistance Measurements > 10 kΩ > 10 kΩ Voltage Sources More Stable When Driving High Impedance 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

7. Guildline Instruments Limited 2007 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 (Ω) Source Current Uncertainty(ppm) Source Voltage Uncertainty(ppm) 1k-1k0.0050.206 10k-10k0.0110.003 100k-100k0.2170.003

8. Guildline Instruments Limited 2007 LOW RESISTANCE MEASUREMENT 1k – 1k Source VoltageSource Current 3V, 0.206 ppm Std. Dev.3.16mA, 0.005 ppm Std. Dev. At 1kΩ and Lower, Sourcing Current Gives Much Better Measurements

9. Guildline Instruments Limited 2007 MEDIUM RESISTANCE MEASUREMENT 10k – 10k Source VoltageSource 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

10. Guildline Instruments Limited 2007 HIGH RESISTANCE MEASUREMENT 100k – 100k Source VoltageSource 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

11. Guildline Instruments Limited 2007 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 Serial # 50A75A100A 6834399.987199.987199.9888 6918199.980399.981099.9826

12. Guildline Instruments Limited 2007 Very Low Resistance Measurements (cont) May Need Low Currents Saturation Current For Nanoscale Materials Often Very Low Saturation Current For Nanoscale Materials Often Very Low Self Heating Effects Create Measurement Errors and Excessive Heat Can Damage DUT Self Heating Effects Create Measurement Errors and Excessive Heat Can Damage DUT Exception Is Super-Conducting Materials Exception Is Super-Conducting Materials Current Comparator (CCC) Bridges Can Measure Down to 10 -9 Ω With Low Currents Current Comparator (CCC) Bridges Can Measure Down to 10 -9 Ω With Low Currents Thermal Stability Very Important For Both Resistance Standard and DUT For Both Resistance Standard and DUT Stable Air Baths (0.001 °C) Stable Air Baths (0.001 °C)

13. Guildline Instruments Limited 2007 Very High Resistance Measurements DCC bridges measure up to 1 GΩ Provide Better Uncertainties At and Below 100 MΩ 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 (10 16 ) With Direct Measurement Uncertainty Ranging From 0.015% to 5% Across This Range Measure From 1 MΩ up to 10 PΩ (10 16 ) With Direct Measurement Uncertainty Ranging From 0.015% to 5% Across This Range

14. Guildline Instruments Limited 2007 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 10 17 Ω Using 10 14 Ω Standard. Resistor Nominal Value (Ω) ChartedUncertainty(ppm)DirectReading(ppm) Transfer Uncertainty (ppm) 100M1815030.9 1G4120033.7 10G10660032.7 100G9480046.6

15. Guildline Instruments Limited 2007 Low Current Measurements Generate or Measure Accurate and Traceable Low Value Currents Use Commercial Voltage Standard and Accurate High Value Resistance Standards Use Commercial Voltage Standard and Accurate High Value Resistance Standards Traceable Reference Currents Down to 50 fA (10 -15 A) Traceable Reference Currents Down to 50 fA (10 -15 A) Can be Verified Using a Teraohmeter [2] Can be Verified Using a Teraohmeter [2]

16. Guildline Instruments Limited 2007 Low Current Measurements (cont) Guildline 6520 Teraohmmeter With Guildline 9336/9337 Resistance Standards [2] Uncertainties Can Be Improved by the Substitution Method [1] Resistor9336/9337Teraohmmeter 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 %

17. Guildline Instruments Limited 2007 MEASUREMENT TECHNIQUES AND TIPS Temperature Effects Digital Filtering DC Reversal Techniques Humidity Effects Electromagnetic Interference (EMI) Connectors and Leads GuardingGrounding Settling Times Direct Measurement With No Amplification

18. Guildline Instruments Limited 2007 Temperature Effects 1.0 µΩ Resistance Standard (Guildline 9334A) t/c of 8.5 ppm/°C (8.5 -12 Ω or 8.5 pΩ) (8.5 -12 Ω 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 Serial # 21°CµΩ23°CµΩ25°CµΩ 685591.0000481.0000651.000083 685600.9999971.0000151.000032 685611.000331.000341.00035

19. Guildline Instruments Limited 2007 Digital Filtering Order of Magnitude of Additional Accuracy Large Number of Tests Reduces the Bandwidth of the Noise Reduces the Bandwidth of the Noise Ex: Remove ‘Outlier’ Measurements > k3 Ex: Remove ‘Outlier’ Measurements > k3 ( i.e. > 3 x standard deviation) ( i.e. > 3 x standard deviation) Dynamically Alter the Sampling Times Increase If Measurement Stable Increase If Measurement Stable If Periodic, Synchronize To a Clock Telecommunications Industry Telecommunications Industry Analyze Total Set of Test Results Post Experiment Analysis With PC Post Experiment Analysis With PC

20. Guildline Instruments Limited 2007 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 Rounding0.050 Linear Interpolation 0.061 Logarithmic Weighting 0.084

21. Guildline Instruments Limited 2007 DC Reversal Techniques Polarity Reversal Eliminate Thermal EMFs Eliminate Thermal EMFs Reduces the Effect of White Noise Reduces the Effect of White Noise Increases the Signal-To-Noise Ratio Increases the Signal-To-Noise Ratio Can Be Optimized Faster When Measured Parameter Is Changing Faster When Measured Parameter Is Changing Slower When Measured Parameter Is Stable Slower When Measured Parameter Is Stable

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

23. Guildline Instruments Limited 2007 Electromagnetic Interference (EMI) EMI Noise In Most Laboratories Florescent Lights, Cell Phones, Fixed Point Temperature Furnaces, Electric Motors, AC Electrical Power Lines 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 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 High Quality Air Baths Provide Both EMI Shielding and Temperature Stability Power Line Filters

24. Guildline Instruments Limited 2007 Connectors and Leads 4 Terminal Mode Most Accurate Method for Measuring Small Resistances Most Accurate Method for Measuring Small Resistances Corrects For Lead Resistance Corrects For Lead Resistance Allows Longer Test Leads 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 Poor Measurements From Cracked Terminals, Dirty Contacts, Moisture Absorbed By Standards and DUTs Errors As High As 10 ppm Errors As High As 10 ppm High Resistance Needs Very Good Insulation

25. Guildline Instruments Limited 2007 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 Very Important For High Resistance Measurements Measurement Instruments Should Provide Guarded Connection Terminal Measurement Instruments Should Provide Guarded Connection Terminal Reduces Effect Of Shunt Capacitance

26. Guildline Instruments Limited 2007 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 Avoids Ground Loop Currents Between Measurement Circuit and DUT, or Measurement Circuit and Test Fixture Noisy Power Lines Largest Contributor Is Typically PCs 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 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)

27. Guildline Instruments Limited 2007 Settling Times Needed To Overcome Capacitance Effects, Self- Heating Effects, Dielectric Absorption Present In Measurement Instruments, Standards, Cabling, DUT Present In Measurement Instruments, Standards, Cabling, DUT Longer Settling Times Very Important For Resistances > 100 kΩ Typically Do NOT Use Initial Measurements For Example, Ignore First 50 DCC Bridge Readings For Example, Ignore First 50 DCC Bridge Readings

28. Guildline Instruments Limited 2007 Direct Measurement With No Amplification NOT Recommended To Use Operational Amplifiers or Other Techniques To Increase the Measured Signal Will Proportionally Increase Noise Will Proportionally Increase Noise Operational Amplifiers Or Other Circuitry Will Introduce Additional Noise Operational Amplifiers Or Other Circuitry Will Introduce Additional Noise Need Instruments Capable Of Directly Measuring Electrical Properties At Very Low Values

29. Guildline Instruments Limited 2007 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.

30. Guildline Instruments Limited 2007 2007 Questions