What Is An SMU? SEP 2016
Agenda SMU Introduction Theory of Operation (Constant Current/Voltage Sourcing + Measure) Cabling : Triax vs Coax Advantages in Resistance Applications (vs. DMMs) Advantages in I-V Applications (vs. Power Supplies) Concept of 4-Quadrant Operation (Sourcing and Sinking Modes) Concept of Pulse I-V Technique (Pulse mode vs DC mode) Summary and Q&A
SMU Introduction
Decades of Leadership in SMU Technology Series 23x SMUs 1989 Series 2600 System SourceMeter Instruments Series 2400 SourceMeter Instruments 1995 2000 2005 >20 patents issued for SMU-specific technology Numerous industry awards, including R&D 100, T&MW, and more Thousands and thousands of customers large and small in R&D and mfg. Serving Semiconductor, Electronic Components, Optoelectronics, Automotive, Mil/Aero, Medical, Research & Education, and many more electronics industries Series 2600A System SourceMeter Instruments 2012 Series 2450 SourceMeter Instruments 2014
SMU Applications Span Electronics R&D and Industry Resistivity of Materials Solar Cells Capacitor Leakage Open and Short Testing of Cables, Connectors Cell/Battery Charge and Discharge Profiling Optoelectronic Devices Including LEDs/Lasers Electrochemistry Applications such as Potentiometry and Galvanometry Nanotechnology Applications such as Nano Device Testing and Material Characterization DC to DC Converters and Regulators Semiconductor Devices: Transistors, Diodes, etc. (Wafer, chip, device level) Medical Implantable Devices such as Pacemakers, Defibrillator, Pain Mgt. Precision Resistor Calibration/Binning
SMU Theory of Operation
What is an SMU? SMU STANDS FOR SOURCE MEASURE UNIT. A precision instrument used for I-V characterization on a wide variety of devices and materials An SMU can source current or source voltage An SMU can measure both current and voltage
What is an SMU? An SMU is a single instrument that can source voltage or current and measurement voltage and current. Source V, Measure I Source I, Measure V
Simplified SMU Circuit One SMU can replace entire rack of equipment!
Feedback Ammeter RF is GΩ and greater to generate mv and higher voltages VB is <1mV (VB or Vin for an ideal op amp is 0V)
Shunt Ammeter if RS/Rshunt = 100, measurement error due to the shunt is 1%
Keithley SMU Simplifies I-V Testing Precision Power Supply (source) DMM (measure I, V, and R) Current Source Electronic Load Source Measure Unit (SMU)
Why is a Keithley SMU needed to Simplify Device Testing? Typical Equipment Rack for Device Testing Picoammeter Power Supply Current Source Digital Multimeter Electronic Load
Coaxial Cables Can Complete the Shield from DUT to the Meter
Positive: Coax Cables Shield the Signal Trade-Off: Leakage Current Reduces Accuracy For common applications, the cable insulation is near infinite so leakage current not significant. But when measuring pa or lower, the cable leakage can be significant
Eliminate Cable leakage with Triax Cable and Guarding Coax Cable Triax Triax Cable adds an Inner Conductor, the Inner Shield
Eliminate Cable leakage with Triax Cable and Guarding The guard amplifier maintains the inner shield V at same V as center conductor. Thus there is no leakage current in measurement path. Outer shield provides electrostatic shielding and also a safety shield Ground Leakage current thru RL2 exists but is supplied by guard and not in measurement path
Making Accurate Low Current Measurements: Eliminating Cable Leakage When measuring pa or lower, the cable leakage can be significant No cable leakage current: Guard applies VOUT to inner conductor so ΔV = 0V
When to Take the Measurement Stimulus Response Measure (Delay) Capacitance (and sometimes inductance) in the system often require that you delay measurements after a change in stimulus to let the system stabilize.
Making Accurate Measurements: Eliminating 50Hz Noise A/D Integrates over a PLC 1msec 116mV 50Hz noise DCV level 115.5mV 116mV 1msec 114mV 1msec Average = 0 DCV Level (115.5mV) 1 PLC 1/50 sec = 20 msec 50Hz noise
SMU Vs DMM
SMU vs. DMM VS. Model 2450 SourceMeter SMU Feature SMU Model 7510 DMM DMM Combines several functions into one unit DC Current, DC Voltage, Resistance Sources Current and Voltage Yes, user has full control of Sources Current or Voltage both current and voltage when measuring resistance, source including sweeping. but user has no control over output. Measures Low Current Yes, SMUs can usually measure in the picoamp range (10e-12) or less. DC and AC Current and Voltage, Resistance DMMs typically can measure in the microamp range (10e-6) or less.
DMM Resistance Measurements KEITHLEY MODEL 7510 OHMMETER SPECS DMM measures resistance at specified test current. User has no control of test current. Some applications require the user to have control of test current: - Device must be tested at specific current - DUTs prone to heating effects - Nano structures (too much current can damage) - Superconductors (causes device to heat up)
SMU vs. DMM: Measuring Resistance DMM Current Source VoltMeter Measuring Resistance as a function of Current DMMs usually measure resistance by sourcing a fixed current and measuring the voltage drop across the resistor: The user has no control over the test current.
SMU vs. DMM: Measuring Resistance AN SMU CAN MEASURE RESISTANCE BY SOURCING CURRENT OR VOLTAGE. The user has control over how much current or voltage to apply to device. Result of measuring the current as a function of a sweeping voltage: I Current V Variable Voltage Source R Resistance Ammeter Measuring Resistance as a function of Voltage An SMU can even measure resistance as a function of the applied current or voltage.
SMU vs. DMM: Ammeter Specs TYPICALLY, AN SMU HAS A MUCH MORE SENSITIVE DC AMMETER THAN A DMM Example: What instrument can measure 50nA more accurately, the Model 2450 SourceMeter or the Model 7510 DMM? Keithley Model 7510 DMM Specs: Uncertainty = 406pA Model 2450 SourceMeter Uncertainty = 130pA
SMU Vs Power Supply
SMU Compare to a Power Supply? IN GENERAL, SMUS HAVE GREATER SPEED AND PRECISION THAN POWER SUPPLIES. SMUs measure current and voltage as well as source current and voltage. Power supplies usually only source voltage. Function SMU Power Supply Source Voltage Yes Yes, Limited Ranges Source Current Yes Sometimes, Limited Ranges microseconds milliseconds Source Sweeping Yes Limited to list sweep Sweep through 0 Yes No must physically switch test leads Measure Voltage Yes No Measure Current Yes No 4 Quadrant 1 Quadrant Source Settling Time Quadrants of Operation
SMU vs. Power Supply: Precision IN GENERAL, SMUS ARE MORE PRECISE AND HAVE GREATER RESOLUTION THAN POWER SUPPLIES. Typical SMU Typical Power Supply
SMU vs. Power Supply: Speed IN GENERAL, THE VOLTAGE OUTPUT OF AN SMU CAN SETTLE MUCH FASTER THAN A POWER SUPPLY. Faster settling times. Typical SMU Typical Power Supply
SMU vs. Power Supply: Waveform Generation SMUS HAVE BUILT-IN WAVEFORM GENERATION (SWEEP) FUNCTIONALITY: DC Pulse Custom
SMU vs. Power Supply: 1 vs. 4 Quadrant SMUS HAVE TREMENDOUS FLEXIBILITY THAT A POWER SUPPLY DOES NOT HAVE: Power Supply 1 Quadrant Source Only SMU 4 Quadrant Source and Sink resistive devices semiconductors IR testing resistive devices semiconductors IR testing Reverse leakage tests SMUs can source and sink current and voltage. SMUs act as both power supply and electronic load. SMUs are useful for testing energy generating devices. solar cells batteries
SMU : Pulse mode vs DC mode
Joule Heating (Device Self Heating) In DC testing the device heats itself which changes it s electrical characteristics. + - DC V Use pulsing to minimize self-heating! Ω Temp
Self Heating in MOSFETs MOSFET Output Characteristics 20 18 16 Current does not stay constant in the saturation region. 14 Ids (Amps) 12 Vgs = 4.5V Vgs = 5V 10 Vgs = 5.5V Vgs = 6V 8 Vgs = 6.5V 6 4 2 0 0 2 4 6 Vds (Volts) 8 10 DC
Self Heating in MOSFETs VGS = 6.5V for both curves. MOSFET Output Characteristics 20 18 current caused Drop in by heat16 from testing with more points. Vgs = 4.5V 14 Vgs = 5V Vgs = 5.5V Ids (Amps) 12 Vgs = 6V Vgs = 6.5V 10 Vgs = 4.5V 8 Vgs = 5V Vgs = 5.5V 6 Vgs = 6V Vgs = 6.5V 4 2 0 0 2 4 6 Vds (Volts) DC 8 10 DC
Self Heating in MOSFETs MOSFET Output Characteristics 20 Higher max current Flat curves in the saturation region 18 16 Vgs = 4.5V 14 Vgs = 5V Vgs = 5.5V Vgs = 4.5V Vgs = 6V Vgs = 5V Vgs = 6.5V Vgs = 5.5V Vgs = 4.5V Vgs = 6V Vgs = 5V Vgs = 6.5V Vgs = 5.5V Ids Ids (Amps) (Amps) 12 10 8 6 Vgs = 6V Vgs = 6.5V 4 2 0 0 2 4 6 Vds (Volts) (Volts) Vds 8 10 10 DC Pulse
On-State Characterization Challenges TEST REQUIREMENTS Pulsed stimulus requires low resistance, low inductance cabling rated for the maximum test current Low inductance ensures good pulse fidelity Low resistance ensures desired voltage at the DUT For on-wafer testing, ensure wafer is adequately prepared so that there is low contact resistance to the chuck For best results, ensure high current signal lines are isolated to the device under test
On-State Characterization Challenges EXAMPLE: HIGH CURRENT FET TEST CONNECTIONS
On-State Characterization Challenges HIGH CURRENT PULSE STIMULUS On-state measurements frequently require pulsed stimulus but desire is DC measurement
On-State Characterization Challenges HIGH GAIN & OSCILLATION Use a resistor in series with the gate SMU to combat oscillation Tune resistance value to dampen oscillation without significantly impacting switching time of device
On-State Characterization Challenges HIGH GAIN & OSCILLATION High gain and high switching speed transistors often suffer from oscillation Can result in device destruction. Can result in inconsistent and erroneous measurements Large changes in device impedance when device changes state results in drain voltage variation.
SMU Applications
LED Testing LIGHT-EMITTING DIODES (LEDS) ARE BASICALLY P-N JUNCTIONS (DIODES) THAT ACT AS A LIGHT SOURCE. LEDs are used in many applications including: ü Consumer Electronics ü Electronic Instrumentation ü Lighting ü Displays ü Sensors I-V testing on LEDs is performed in all phases of the development process: Research and design Quality assurance On-wafer Production Test Flexible OLED Blue LEDs
LED Testing Researchers will want to measure the entire I-V curve. Production users will only want to measure specific points on the curve. Requires positive and negative sourcing Requires ability to sweep voltage through 0V Two points require sourcing a known current and measuring voltage: Forward Voltage: Vf Breakdown Voltage: VR One point requires sourcing a voltage and measuring current: Leakage Current: IL
LED I-V Testing Source Voltage, Measure I Current Source Voltage, Measure I Current This method used for the leakage current (IL) and the reverse breakdown voltage (VR) tests. This method used for the leakage current (IL) and the reverse breakdown voltage (VR) tests.
LED Testing ONE SMU SIMPLIFIES TESTING: sources both current and voltage source either polarity without switching leads measures both current and voltage simplifies test set-up and programming since only one instrument is required
Solar Cell Testing Solar Cells convert the energy of light into Electricity.
Solar Cell Testing I-V CHARACTERIZATION OF THE SOLAR CELL IS USED TO DETERMINE IT S EFFICIENCY. Test: Source V, Measure I Determine: Maximum Current, Imax Maximum Voltage, Vmax Maximum Power, Pmax Open Circuit Voltage, Voc Short Circuit Current, Isc
Solar Cell Testing The solar cell is basically a diode which has a large area. When light hits the solar cell, photons are absorbed and electrons are released. When a Load is connected to the output of the cell, a current will flow.
Solar Cell Testing When the illuminated photovoltaic cell is connected to the output of the SMU, the SMU will sink the current. The measured current is negative.
Solar Cell Testing 0.5 0.4 I-V Characteristics of Solar Cell Measured by Keithley SMU -0.5 0.3 0.2 Dark Cell 0.1-0.3-0.1 0 0.1 0.3 0.5-0.1-0.2-0.3-0.4-0.5 Illuminated Cell (SMU sinks current)
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