2013 USU Physics Colloquium Utah State University, Logan, UT Electrostatic Discharge in Solids Allen Andersen and JR Dennison Physics Department Utah State University, Logan, Utah

Supported through funding from the USU Howard L. Blood Fellowship and NASA Goddard Space Flight Center. Thank you! The entire USU Materials Physics Group

Electrostatic Discharge What is it and why do we care? Instrumentation and Procedures Determining material properties. Analysis and Modeling Understanding and comparing material behavior. Future Work Quantitative descriptions of microscopic physics. MotivationExperimentAnalysisConclusion Outline

MotivationExperimentAnalysisConclusion Motivation

MotivationExperimentAnalysisConclusion Insulating materials restrict the flow of charge in an electric field – up to a point. If E > E ESD → Electrostatic Discharge (ESD) Following an ESD: Insulator permanently damaged Large currents can flow In general THIS IS VERY BAD! What is Electrostatic Discharge or ESD?

MotivationExperimentAnalysisConclusion Examples of ESD ~1 MV/m in air MV/m in solids

MotivationExperimentAnalysisConclusion What to expect for solids (via some hand waving). Ionization energy ~10eV q e * Average distance between particles in air ~10 -5 m = ~10 6 V/m Ionization energy ~10eV q e *Average distance between defects in solids ~10 -8 m = ~10 9 V/m In Gas In Solids The electron must gain enough energy traveling through the E-field to ionize at the next collision to sustain an avalanche discharge.

MotivationExperimentAnalysisConclusion Spacecraft Charging The sun gives off high energy charged particles. These particles interact with the Earth’s atmosphere and magnetic field in interesting ways. High energy particles imbed charge into spacecraft surfaces.

MotivationExperimentAnalysisConclusion Spacecraft Charging As charge builds up in insulators the internal electric field can exceed the dielectric strength of the material. ESD accounts for majority of “anomalies” induced by spacecraft environment

MotivationExperimentAnalysisConclusion USU MPG More than two decades of Spacecraft Charging Research Charging of power grid components. Space Plasma Environment Simulations Physics Models

MotivationExperimentAnalysisConclusion Problems with Existing Grid 1. >10 % Power loss in transmission due to radiation and Joule heating. 2. The nation uses several separate out of phase regional power grids. 3. Existing grids are ageing and already being pushed past their limits.

MotivationExperimentAnalysisConclusion Possible Solutions Operate at Higher Voltages ~MV Currently HV wires operate at ~500kV in US. In Europe >1MV, in Japan and China>2 MV. Advantages Much higher efficiency for MV transmission. P=IV=V 2 /R Disadvantages Higher stress on insulating components resulting in leaks or failures. Coronal discharge for AC lines limits voltage.

MotivationExperimentAnalysisConclusion Coronal discharge

MotivationExperimentAnalysisConclusion Possible Solutions Use DC Transmission Advantages Reduces radiation and thermal resistance. More cost effective than AC over distances of hundreds of miles. HV transmission without coronal discharge up to 4 MV. Could connect the nation’s out of phase power grids. Disadvantages Requires expensive DC/AC converter stations. DC components in general are more expensive than AC. Tradition – War of the Currents. Higher electric field stress on materials. DC AC

MotivationExperimentAnalysisConclusion Benefits of Improving the Power Grid HVDC transmission improves efficiency. Transmission loss effectively halved. 500kV/1000kV ≈ ½ The energy saved if transmission losses were halved nationwide ~30 large coal burning power plants. This is quite possible!

MotivationExperimentAnalysisConclusion Electronics in General ESD is an important design consideration for all electronics. The problem does not scale linearly due to quantum tunneling. In Si/SiO 2 transistors the insulating layer is only a few atoms thick. d circuit ≈10 -3 m→ V ESD ≈ 10 4 V d MOFSET ≈10 -8 m→ V ESD ≈ 1V

MotivationExperimentAnalysisConclusion Experiment

MotivationExperimentAnalysisConclusion ESD SYSTEM USU MPG ESD chamber ~10 -6 torr. >10 5 times below Paschen minimum → No arcs through gas around samples.

MotivationExperimentAnalysisConclusion ESD SYSTEM Simple parallel plate capacitor Fully automated system Applies up to 30 kV ~150 K<T<300K with ℓ-N 2 reservoir 1.98 cm 2 sample electrodes 6 electrode carousel

MotivationExperimentAnalysisConclusion ESD SYSTEM A)Copper electrodes B)Thermocouple electrodes C)Polycarbonate base D)Conductive sample plate E)Thermally conductive, electrically isolating layer F)Liquid Nitrogen reservoir G)Adjustable pressure springs H)Glassy sample I)Conductive padding J)Polymer sample A B C D E F G H I J

MotivationExperimentAnalysisConclusion ‘Typical’ Results As voltage is first applied to the sample no current flows through it. The insulator is doing its job. Once the critical voltage is reached the sample breaks down and current flows freely through the material. The slope of the graph is just the inverse of the current limiting resistance in the circuit. V=IR L I/V = R L -1 The discontinuity marks our breakdown voltage.

What ESD data tells us The breakdown voltage corresponds to a critical electric field. E esd =V esd /d Breakdown sites vary significantly depending on material*. 20 µm 10 mm MotivationExperimentAnalysisConclusion LDPE PolyimideePTFE (Gortex)

MotivationExperimentAnalysisConclusion LDPE Low Density Polyethylene – highly electrically insulating material. Inexpensive and comes in many forms. Common spacecraft insulator Common power line insulator Common everyday material.

MotivationExperimentAnalysisConclusion Polyimide Polyimide or Kapton TM – highly electrically insulating material. Relatively inexpensive and very robust chemically, thermally, and electrically. Common spacecraft insulator Common electronics insulator

MotivationExperimentAnalysisConclusion Glass Disordered Fused Silica SiO x –electrically insulating material. Expensive in thin sheets but otherwise inexpensive–very useful. Common semiconductor insulator Excellent optical coating Spacecraft solar array cover glass Window glass

MotivationExperimentAnalysisConclusion Analysis

MotivationExperimentAnalysisConclusion LDPE Data Breakdown voltage Ohmic slope I = V/R L x-intercept at origin Pre- breakdown arcing V 1 0 ≈0V

MotivationExperimentAnalysisConclusion Polyimide Data Breakdown voltage Ohmic slope I = V/R L Pre- breakdown arcing V 1 0 ≈0V

MotivationExperimentAnalysisConclusion Pre-Breakdown Arcs–Oscilloscope LDPE Pre- breakdown arcing

MotivationExperimentAnalysisConclusion Pre-Breakdown Arcs–Oscilloscope Polyimide Pre- breakdown arc

MotivationExperimentAnalysisConclusion Pre-Breakdown Arcs a) Defect sites can form in kinks of polymer chains. These low-energy defects can be thermally repaired since ε kinks ≥ 27 meV = k B T RM b) At higher voltages, electrons have enough energy to break bonds, creating permanent defect sites. ε bond >> 27 meV = k B T RM

MotivationExperimentAnalysisConclusion Pre-Breakdown Arcs F Energy Position ΔGΔG ΔGΔG ΔG+q e a o F qeaoFqeaoF aoao ΔG-q e a o F Under an applied electric field, charge can ‘hop’ between defect sites. The probability of a transition in a given time step depends upon temperature, well depth, and applied field.

MotivationExperimentAnalysisConclusion Conductivity Model Trap-to-trap Tunneling frequency Well depth Density of Defects Tests lasting only days can predict decades of behavior!

MotivationExperimentAnalysisConclusion Time Endurance - LDPE ~325 MV/m Breakdown time Pre-breakdown arcs

MotivationExperimentAnalysisConclusion Time Endurance - Polyimide ~320 MV/m Long wait time and higher current arcs indicate differences in defect populations.

MotivationExperimentAnalysisConclusion Time Endurance 1 min. 1 hr. 1 day 1 week

MotivationExperimentAnalysisConclusion Fused Silica Breakdowns Conductive Polyimide pads under fused silica ESD

MotivationExperimentAnalysisConclusion Fused Silica Data Breakdown Voltage Non-ohmic slope 80µm V 1 0 ≈1000V R 1 >R L

MotivationExperimentAnalysisConclusion Fused Silica Data Breakdown Voltage Non-ohmic slope 2 nd non-ohmic slope What is going on?! R 1 >R 2 >R L V 2 0 ≈200V V 1 0 ≈1000V 80µm

MotivationExperimentAnalysisConclusion Fused Silica Data Actually, we’d already seen this before. 65µm coating V 2 0 ≈80V V 1 0 ≈200V R 1 >R 2 >R L

MotivationExperimentAnalysisConclusion Fused Silica Data Relatively low critical field – we can deal with that. Tunneling current seems to be a bigger factor for thinner coatings No noticeable pre-breakdown arcs – for SiO 2 we don’t have polymer chains to “kink.” Non-ohmic post breakdown slope – possibly only broken down part way through the sample. Transitions to secondary slopes – marks increasing partial breakdowns

MotivationExperimentAnalysisConclusion We shouldn’t expect the same behavior for different structures!

MotivationExperimentAnalysisConclusion Density of States a) Delta function b) Constant c) Linear d) Power law e) Exponential f) Gaussian Each of these has different transport properties. LDPE – Linear Polyimide – Exponential SiO 2 – Exponential+Gaussian

MotivationExperimentAnalysisConclusion Transport Equations

MotivationExperimentAnalysisConclusion Transport Equations

10/7/13 ISU Colloquium46 Modified Joblonski diagram VB electrons excited into CB by the high energy incident electron radiation. They relax into shallow trap (ST) states, then thermalize into lower available long-lived ST. Three paths are possible: (i)relaxation to deep traps (DT), with concomitant photon emission; (ii)radiation induced conductivity (RIC), with thermal re-excitation into the CB; or (iii)non-radiative transitions or e - -h + recombination into VB holes. Complementary Responses to Radiation 1.92 eV 2.48 eV 2.73 eV 4.51 eV eV --41 meV EFEF eff --24 meV E

10/7/13 ISU Colloquium47 Modified Joblonski diagram VB electrons excited into CB by the high energy incident electron radiation. They relax into shallow trap (ST) states, then thermalize into lower available long-lived ST. Four paths are possible: (i)relaxation to deep traps (DT), with concomitant photon emission; (ii)radiation induced conductivity (RIC), with thermal re-excitation into the CB; (iii)non-radiative transitions or e - -h + recombination into VB holes; or (iv)avalanche effect as CB electrons excite more VB electrons into the CB, causing ESD. Complementary Responses to Radiation and Electric Field Stress 1.92 eV 2.48 eV 2.73 eV 4.51 eV eV --41 meV EFEF eff --24 meV E E

MotivationExperimentAnalysisConclusion Conclusion

MotivationExperimentAnalysisConclusion Conclusions Polymer and glass structural differences are manifest in ESD measurements of pre-breakdown arcing and post-breakdown slopes. Pre-breakdown arcing can be understood in terms of thermally recoverable and irrecoverable defect generation. The onset, magnitude, and frequency of pre-breakdown arcing depends on the density of states for a given material. The performance of insulating materials under electric field stress over time also depends on the density of defects.

MotivationExperimentAnalysisConclusion Conclusions Am I really talking about ESD? The lines between different charge transport phenomena blur. This is charge transport on an extreme scale!

MotivationExperimentAnalysisConclusion Future Work Acquire and analyze more ESD data for LDPE and Polyimide. Breakdown image analysis – look for patterns. Pursue fused silica test options and test borosilicate glass in the meantime. Cast ESD phenomena in terms of the USU MPG charge transport formalism. Merge ESD, CVC, SVP, SEY, RIC, cathodoluminescence. Derive t breakdown for the various DOS distributions and fit to data. Pursue applications for power grid technologies.

Questions?