Fundamentals of Nanoscale Measurement

Electrical measurements of nanoscale components—voltage, resistance, and current—present unique challenges and are inherently error-prone. The development of materials and components at the quantum level imposes limitations on the electrical measurements that can be made. In any measurement, the theoretical limit to sensitivity is set by the noise generated by the resistance in the circuit. Voltage noise[1] is proportional to the square root of the resistance, bandwidth, and absolute temperature. High source resistance limits the theoretical sensitivity of voltage measurements[2]. While it is possible to measure a 1mV signal with a source impedance of 1W, it is realistic to measure the same 1mV signal with a source impedance of 1 terahertz. Even if the source resistance is reduced to 1MW, measuring a 1mV signal approaches the theoretical limit and becomes difficult with a conventional digital multimeter (DMM).
 
In addition to the lack of voltage or current sensitivity, many DMMs have high input offset currents when measuring voltage, and their input resistance is too low for the more sensitive low-level DC measurements[4] that nanotechnology often requires[3]. These characteristics increase measurement noise and introduce unnecessary interference into the circuit, which can cause measurement errors.
 
Once the system is built, its performance must be verified and potential sources of error eliminated. Sources of error can include cables, connections, probes[5], contamination, and heat. Some ways to reduce these errors are discussed in the following sections.
 
Safety is always a serious consideration. Electrical measurement tools can output dangerous or even lethal voltages and currents. It is extremely important to understand when these situations may occur while using the instrument so that people can take appropriate safety precautions. Please read and follow the safety instructions that come with each tool.