Evaluation Engineering - December 2008 - (Page 30)

NANOELECTRONICS TEST away from the measuring circuit. You can’t stop a small amount of current from ﬂowing through test set-up insulation, but you can control where it ﬂows. In the figure, the high-impedance node connecting the ammeter and diode has been enclosed in a metal box driven at 15 V. A 1-GΩ leakage resistance has been assumed that, without the guard, would cause an additional 15 nA to ﬂow through the ammeter. With the guard, the 15 nA still flows, but from the very lowimpedance 15-V supply rather than from the high-impedance ammeterdiode node. The measurement error caused by the leakage current has been reduced to 0.2 pA, corresponding to a 0.2-mV burden voltage. Depending on the test setup, it may be more appropriate to drive the guard from a low-impedance buffered version of the measured output voltage. For example, the current leakage within a shielded cable driving a highimpedance voltmeter can be minimized through guarding. A buffered copy of the voltmeter output is used to drive the shield of the cable. Because the signal and the shield are at the same voltage, the leakage current is minimized. Beyond these considerations, many picoammeters average successive readings to reduce noise. The Agilent Technologies Model 4339B HighResistance Meter measures DC current from 60 fA to 100 µA and resistance from 1,000 Ω to 1.6 x 1016 Ω, using source voltages from 0.1 V to 1,000 V. Measurement precision and time are, 3, 4, and 5 digits requiring 10 ms, 30 ms, and 390 ms, respectively. Agilent also provides picoampere measurement capability in the Model B1500A Semiconductor Device Analyzer. This test platform has 10 slots that accept a range of source/monitor units, a multi-frequency capacitance measurement unit, a high-voltage pulse generator, and a waveform/fast measurement unit. Pulsed measurements are very important in nanoelectronic work, primarily because the structures are so small that they cannot support signiﬁcant currents and voltages at DC. 3 0 • E E • December 2008 Keithley’s Model 4200 Semiconductor Characterization System has parametric test functionality similar to the Agilent B1500A covering precision DC and pulse measurements. In addition, Keithley offers a range of standalone picoammeters and electrometers as well as the production-oriented S600 Parametric Test System with DC through RF capabilities. measure the output voltage, current measurement resolution of 0.5 pA results.” Nanovoltmeters Similar to the error currents that can affect low-current measurement accuracy, nanovolt measurements are subject to unintentional voltage drops. The most common solution to eliminate low-level voltage errors caused by the test set up is the use of four-wire or Kelvin connections. One set of leads sources the exciting current and the other set measures the voltage drop caused by the current. In a simple two-wire Ohms measurement, error voltages are created because the test current ﬂows through the measurement leads as well as the unknown resistance. Additional voltages may be generated by thermoelectric effects and cause a measurement offset. Typically, your test setup will require two or more connections between wires and terminals of different materials. If there is a temperature difference between parts of the test setup, small voltages will be generated—certainly nanovolts and possibly microvolts. The traditional way to minimize this effect is to make a voltage measurement with the current in one direction, then repeat the measurement with the current reversed. This technique gives the best results when the current can be reversed so quickly that the temperature has not changed between the two readings. Of course, this is only an approximation to a constant temperature situation, and may not be accurate for very sensitive measurements. A better approach is the so-called delta method that uses three successive measurements. This technique approximates any change in voltage between successive readings as a linear function. Using the delta method provides more accurate measurements even if the current direction is switched quickly. Agilent’s Model 34420A NanoVolt/ Micro-Ohm Meter is one of a few instruments with nanovolt sensitivity. This unit is interesting in several www.e v al u a ti o n e n g i n e e r i n g . c o m Engineers need to make accurate low-level stand-by or leakage measurements. Hilton Hammond Fluke Agilent and Keithley have the largest selection of picoammeters and related low-level instruments such as source/measure units (SMUs), the Keithley equivalent to Agilent’s source/monitor unit. With the addition of a remote preampliﬁer, Keithley’s Model 6430 SMU provides a 1-pA full-scale range with 10-aA resolution. One attoamp is equivalent to six electrons per second. This level of integration seems to have addressed a real need in the market as evidenced by the wide adoption of the SMU terminology. NI also offers an SMU, the Model PXI-4130, that can source, sink, and measure current to 1 nA. Other picoammeters include the Model AH401 from Sincrotrone Trieste that features 20-b measurements with 50-aA resolution and integration times from 1 ms to 1 s for full-scale ranges of 50 pA to 350 nA. This compact, four-channel unit transfers data via USB 2.0 and is intended for applications such as photodiode current measurement. According to NI’s Travis White, product manager, precision DC measurements, “A sensitive picoammeter can be based on the NI PXI-4022 Current Amplifier Module, which implements a feedback ammeter with less than 20 µV of burden voltage on its 100-nA current range. When combined with the NI PXI-4071 DMM to http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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