Evaluation Engineering - December 2008 - (Page 26)

NANOELECTRONICS TEST Nano-Measurements Need Mega-Care by Tom Lecklider, Senior Technical Editor anoelectronics and nanomaterial research typically involves sourcing and measuring very small voltages and currents. In addition, it’s common to require a means of holding and contacting the samples being examined. These capabilities are built into semiconductor test equipment used to determine the electrical parameters of wafer test structures. General-purpose test instruments such as picoammeters, lock-in amplifiers, and nanovoltmeters often are integrated into this equipment and used separately with tools such as probe stations and nanopositioners. N Quantum Dot Nanocrystals Deposited on a Silicon Substrate Courtesy of Nikon Small World, photography by Seth A. Coe-Sullivan, Massachusetts Institute of Technology 2 6 • E E • December 2008 An integrated solution will include many of the precautions necessary when working with very small signals. Low-level signals will be kept as far as possible from large digital signals to reduce coupling. Capacitive coupling is further minimized by electrostatic shielding. Material thickness isn’t as important as conductivity, copper being more effective than steel at very high frequencies. Twisting a signal wire with its return wire minimizes the enclosed loop area, which reduces inductive coupling. Magnetic shielding to reduce inductive coupling requires either high-permeability material or a thicker shield if the permeability is lower. More shielding information can be found in Ron Brewer’s February 2008 EE-Evaluation Engineering article “3,000 dB and Rising.” Whether you use a test station or build your own test setup, noise will be a major consideration. Regardless of how well a test setup has been constructed, broadband Johnson noise cannot be avoided. This is thermal noise that every resistance generates. Some types of resistive materials develop noise in excess of Johnson noise levels. Typically, this and similar excess noise generated by semiconductors have a 1/f spectrum. This means that measurements made at low frequencies and DC will be more affected by noise. Differential ampliﬁcation is the preferred procedure for handling lowlevel signals. In a balanced differential circuit, the signal and return lines are Continued on page 28 www.e v al u a ti o n e n g i n e e r i n g . c o m 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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