Low-power electrical characterization of CNTs and nanoscale devices
04/01/2006
Jonathan Tucker, Keithley Instruments Inc., Cleveland, Ohio
Manufacturers of carbon nanotubes (CNTs) or other low-power nanoscale devices continue to face challenges as potential uses for these devices grow. One issue is the difficulty of electrically characterizing extremely small circuit elements, not only in the current generation of semiconductors, but in next-generation nanoscale electronics, as well.
A second challenge is how to characterize these next-generation devices when power limitation is critical. The scaling of devices and components to the nanoscale forces researchers to limit the levels of electrical signals that can be applied for characterization.
Lastly, with standard gate dimensions of <90nm and space budgets shrinking continuously, the smallest probe pad dimensions required for most probe systems remain fixed at ~50µm. This limitation is largely the result of the inaccuracy of probe movements and the size of the probe tips. This problem is being solved with new probing tools that offer nanometer movement precision with probe tip diameters of <50nm and current measuring capability better than 1pA. This article focuses on measurement techniques that can be applied to characterizing CNTs and low-power devices, and on ways to overcome various sources of measurement error.
Methods and techniques
Consumers are demanding faster, more feature-rich products in ever-smaller form factors. As electronics decrease in size, components will have limited power-handling capability. As a result, when electrically characterizing components, the test signals need to be kept small to prevent breakdown or other damage. Current vs. voltage (I-V) characterization of nanoscale devices (Fig. 1) may require the measurement of very small voltages because of the need to apply a very small current to control power or to reduce Joule-heating effects. Therefore, low-level voltage measurement techniques become important for I-V characterization of devices and can also be extended to resistance measurements on highly conductive materials and components. This power limitation presents a challenge for researchers and electronics industry test entineers in characterizing modern devices and materials and future devices.
 Figure 1. I-V curve on a carbon nanotube. |
Unlike I-V curve generation on macro- and micro-scale components and materials, measurements on CNTs and nanoscale devices require special care and techniques. General-purpose I-V curve characterizations are often performed using a two-point electrical measurement technique. The problem with this method is that the voltage is measured across the device, as well as across the test leads and contacts. If the goal is to measure the resistance of a device using a typical ohmmeter that measures resistances greater than a few ohms, this added resistance is usually not an issue. However, when measuring low resistances on conductive nanoscale materials or components, obtaining accurate results with a two-point measurement technique may be a problem.
 Figure 2. Four-point measurement schematic. |
If the I-V characterization or resistance measurement involves low voltage or low resistance, such as with molecular wires, semiconducting nanowires, and CNTs, a four-wire, or Kelvin, measurement technique with a probe station is preferred and will yield more accurate results. With Kelvin measurements, a second set of probes is used for sensing. Negligible current flows in these probes from high impedances associated with the sensing inputs; therefore, only the voltage drop across the device under test (DUT) is measured (Fig. 2). As a result, resistance measurement, or I-V curve generation, is more accurate. Source and measurement functions for this measurement technique are typically provided by source-measure units (SMU), i.e., electronic instruments that source and measure DC voltages and currents.
Sources of error
Low-power electrical characterization of CNT-based devices and other nanoscale components can be fraught with measurement error. Offset voltage and noise sources that can normally be ignored when measuring higher signal levels can introduce significant error into low-voltage, low-current, low-power measurements. The following factors can affect measurement performance and accuracy:
Offset voltages. Ideally, when a voltmeter is connected to a relatively low-impedance circuit in which no voltages are present, it should read zero. However, a number of error sources in the circuit may show up as a non-zero voltage offset. These sources include thermoelectric EMFs, offsets generated by rectification of RFI (radio frequency interference), and offsets in the voltmeter input circuit. Steady offsets can generally be nulled out by shorting the ends of the test leads together, and then enabling the instrument’s zero (relative) feature. However, canceling the offset drift may require frequent re-zeroing or using specific measurement techniques, particularly in the case of thermoelectric EMFs.
Thermoelectric voltages. Thermoelectric voltages, or thermoelectric EMFs, are the most common source of errors in low-voltage measurements. These voltages are generated when different parts of a circuit are at different temperatures and when conductors made of dissimilar materials are joined together. Constructing circuits using the same material for all conductors minimizes thermoelectric EMF generation.
Measurements at cryogenic temperatures pose special problems because the connections between the sample in the cryostat and the voltmeter are often made of metals with lower thermal conductivity than copper, such as iron, which introduces dissimilar metals into the circuit. In addition, because the source may be near 0K while the meter is at 300K, there is a large temperature gradient. By matching the composition of the wires between the cryostat and the voltmeter and by keeping all dissimilar metal junction pairs at the same temperature, nanovolt measurements can be made with good accuracy.
Another approach to controlling thermoelectric voltages is to use a delta measurement technique. A constant thermoelectric voltage may be canceled using voltage measurements made at positive and negative test currents. Alternating the test current also increases noise immunity by increasing the signal-to-noise ratio. Over the short-term, thermoelectric drift may be approximated by a linear function. The difference between consecutive voltage readings is the slope or the rate of change, of the thermoelectric voltage. This slope is constant, so it may be canceled by alternating the current source three times to make two-delta measurements, one at a negative-going step and one at a positive-going step. For the linear approximation to be valid, a current source must alternate quickly and the voltmeter must make accurate voltage measurements within a short time interval. If these conditions are met, a three-step delta technique yields an accurate voltage reading of the intended signal unimpeded by thermoelectric offsets and drifts.
Device heating. Small amounts of heat introduced by the measurement process itself can raise the DUT’s temperature, skewing test results or even destroying the device. Device heating is a consideration when making I-V measurements on temperature-sensitive devices such as nanoscale components or materials.
The power dissipation in a device is given by P = I2R, which means that the power dissipated increases by a factor of four each time the current doubles. One way to minimize the effects of device heating is to use the lowest current possible while maintaining the desired voltage across the device being tested.
Current sources that offer pulse measurement capability can also minimize the amount of power dissipated into a DUT. Pulse measurement tools allow users to program the optimal pulse current amplitude, pulse interval, pulse width, and other pulse parameters to reduce potential device heating and control the energy applied to the device. Combined with a synchronized nanovoltmeter, the combination can synchronize the pulse and measurement and thus reduce device heating.
Contaminated probes. Test signal integrity when probing CNTs or nanoscale devices depends on a high-quality probe contact, which is directly related to contact resistance (Fig. 3). Probe contact resistance has become increasingly important as signal voltages drop and contact pressures decrease.
 Figure 3. SEM photo of a carbon nanotube attached to the (S100) probes. |
During use, probe needles can become contaminated. Contamination that builds up on the tip and tip wear can cause an increase in contact resistance. The best way to enhance long-term performance of probe tips is to incorporate periodic cleaning procedures in the test protocol. While regularly scheduled cleaning removes contaminants before they cause test yield problems, this gain must be weighed against its cost.
Testing standards
As newer electronic devices are created using CNTs or other nanoscale materials, the need for testing standards becomes more evident. Consistency in measurement technique and reporting of data is critical for new manufacturing processes to be consistent. The Institute of Electrical and Electronics Engineers (IEEE) created P1650-2005, the world’s first measurement standard for the electrical characterization of CNTs. P1650 and future standards and recommended guidelines will permit semiconductor manufacturers and materials manufacturers of CNTs and nanoscale materials to precisely manufacture and fabricate the next generation of electronic components.
Conclusion
Traditional measurement techniques can still be applied to CNTs and nanoscale devices, but as dimensions shrink and power limitations concerns grow, the measurement techniques must be tailored to achieve the desired results, and new measurement tools now becoming available must be adopted to address the many issues. In addition, professional organizations must continue working on developing new measurement standards so that the measurement results can be made, compared, and verified with confidence.
Acknowledgment
P1650-2005 is a trademark of The Institute of Electrical and Electronics Engineers (IEEE).
Contact Jonathan Tucker at Keithley Instruments Inc., 28775 Aurora Road, Cleveland, OH 44139; ph 440/248-0400, e-mail [email protected].