Evaluation Engineering - December 2008 - (Page 31)

FOR MORE INFORMATION Agilent Technologies EADS North America Defence Test and Services Fluke Keithley Instruments National Instruments Scitec Signal Recovery Sincrotrone Trieste Stanford Research Systems Enter www.rsleads.com/812ee-XXX Model 34420A NanoVolt/Micro-Ohm Meter Model 1830 Test Platform Model 8808A DMM Model 2182A Nanovoltmeter Model PXI-4130 SMU Model 450S Lock-in Ampliﬁer Model 7124 Lock-in Ampliﬁer Model AH401 Picoammeter Model SR850 Digital Lock-in Ampliﬁer aspects. As with the company’s 4339B Meter, resolution is traded against integration time. Here, DC readings with 5½-, 6½-, and 7½-digits resolution require 1, 20, or 200 power line cycles (plc), respectively. In a country with 60-Hz power, 200 cycles are equivalent to 3.33 s. The speciﬁcation is written in this way because the type of ADC used in the nanovoltmeter integrates an integer number of power line cycles to reduce power line noise. That this technique is effective can be seen from the 110-dB normal-mode rejection resulting from 200-plc integration. Integrating for one plc gives 60 dB. The instrument has 0.1-nV resolution, but the most sensitive full-scale range is 1 mV. On the 1-mV range, the peak-peak noise observed during a 2-minute period was 8 nV. Over a 24-hour period, the peak-peak value increased to 12 nV. No doubt, the noise spectrum is limited by the instrument’s bandwidth, but nevertheless the increase simply conﬁrms the generally unbounded nature of Gaussian noise. Keithley’s Model 2182A Nanovoltmeter emphasizes both speed and sensitivity, making a 15-nV peak-to-peak measurement in 1 second. Slightly less sensitive 40-nV to 50-nV measurements take only 60 ms. The delta mode thermoelectric drift cancellation measurement scheme is built in. The Model 2182A also is capable of operating synchronously with the Models 6220 or 6221 Current Sources. Both of these instruments can be operated in pulse mode and can switch current polarity at a 24-Hz rate. A Keithley white paper discusses the improved performance represented by the combination of models 2182A and 6220 compared to a very good lock-in ampliﬁer. Voltage noise is lower than the lock-in amplifier by a factor of 7.2.2 This is important because lower noise supports a higher SNR with lower signal power. And, with nanoelectronics and nanomaterial research, it’s always necessary to minimize test signal power. DC noise performance is comparable to the Agilent ﬁgures, but expressed differently. For the Keithley 2182A, noise was observed for a 2-minute period after the meter had settled to an input step. The 10-mV range has 1-nV resolution, and on this range integrating for 5 plc and with a 75-reading digital ﬁlter, the peak-to-peak noise was 6 nV. Summary The ubiquitous 3-digit $49 DMM is a long way from coping with the scale of measurements required in nanoelectronics and nanomaterial research. Instead, much more specialized equipment is needed that has orders of magnitude higher input impedance and sensitivity. Many of the same kinds of measurements are made in semiconductor parameter testing, and the Keithley and Agilent test platforms are good examples of this type of product. In addition to the hardware, different test techniques are needed, and are supported in the newer test instruments. Several companies are involved in building low-level test systems with a wide range of capabilities. For exam- ple, Charles Greenberg, senior product marketing manager at 209 EADS North America Defense 210 Test and Services, described 211 a wafer probing application that required four-wire con212 nections, pulsed operation to 213 reduce device heating, and 214 six-wire guarded Ohms mea215 surements. In addition, because many 216 points needed to be tested, the 217 good low-level performance had to be maintained when the instrumentation was switched among many channels. The company’s new Model 1830 Test Platform combines switching with multiple SMUs and supports low-level measurements. It’s also apparent that even if a bench top DMM can’t match the sensitivity of a picoammeter or nanovoltmeter, its capabilities are being extended in those directions. Fluke’s marketing manager Hilton Hammond commented that with so many consumer products being battery powered, engineers need to make accurate low-level stand-by or leakage measurements during design and manufacturing. This is one reason that the Fluke Model 8808A uses a feedback ampliﬁer-based current measurement circuit with higher accuracy and less loading at low currents. Nanoelectronics and nanomaterial measurements have special needs. Help is available in the form of white papers, application notes, and a dedicated low-level measurement handbook.3 For this kind of work, understanding how a measurement must be approached to get the desired result is at least as important as having the right instruments. References 1. “Low Current Measurements”, Keithley Instruments, Application Note Series, Number 100, 2007. 2. Daire, A., et al, “New Instruments Can Lock Out Lock-ins,” Keithley Instruments, white paper, 2005. 3. Nanotechnology Measurement Handbook, Keithley Instruments, 2007. www. ev alua t ion e n gin e e rin g.com December 2008 • EE • 31 http://www.rsleads.com/812ee-XXX http://www.rsleads.com/812ee-209 http://www.rsleads.com/812ee-210 http://www.rsleads.com/812ee-211 http://www.rsleads.com/812ee-212 http://www.rsleads.com/812ee-213 http://www.rsleads.com/812ee-214 http://www.rsleads.com/812ee-215 http://www.rsleads.com/812ee-216 http://www.rsleads.com/812ee-217 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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