High Performance Carbon Nanotube Ring Oscillator

Owing to their excellent electrical properties, single wall carbon nanotubes are found to be one of the most promising candidates for post-CMOS applications. While the individual device geometry of single nanotube transistors has been improved over the last few years and results in dc characteristics which can outperform silicon devices, very few attempts have been carried out to probe the ac performance of carbon nanotubes. In this paper, we demonstrate a high performance ring oscillator based on one individual single wall carbon nanotube. This is up-todate the most sophisticated circuit implemented on a single molecule and provides the potential to probe the intrinsic switching speed of carbon nanotubes. The frequency performance of our ring oscillator has been improved by 5-6 orders of magnitude, compared with previous ring oscillators fabricated on multiple nanotubes. "12 As shown in fig. 1, we have fabricated a 5-stage CMOS ring oscillator entirely wired up on one single wall carbon nanotube. The nanotube was CVD grown on 100nm SiO2 substrate with -2nm diameter and 18ptm in length. Palladium source/drain contacts were defined on top of the nanotube, followed by the deposition of an A1203 gate dielectric and metal gate on top of each transistor channel. The 5-stage CMOS ring oscillator consists of 5 p-type FETs and 5 n-type FETs. The circuit layout is designed such that the same type FETs from contiguous inverter stages share the same source/drain contacts, which makes the circuit rather compact. To avoid any interference from the measurement set-up, we added an identical CMOS inverter stage right next to the ring oscillator and its output is directed to a spectrum analyzer. The complete circuit, including the aforementioned 6 inverter stages, is 9pm in width along the nanotube length. One more inverter stage is fabricated on the same nanotube to investigate the electrical properties of the nanotube such that the ideal parameter set for the ring oscillator measurement can be determined beforehand. In order to realize the CMOS scheme on a single carbon nanotube, we used different work function metal gates, i.e. palladium (Pd) for the p-FET and aluminum (Al) for the n-FET. The FET output characteristics obtained from the test inverter are shown in fig.2. To simplify the process, we used Pd for the source/drain contacts for both pand n-FETs. The p-FET shows small contact resistance at small drain voltages and decent saturation currents at high drain biases. In comparison, the n-FET carries much smaller currents at small voltages due to the same Pd source/drain contacts which favor hole carrier injection. However, at high drain bias the n-FET reaches similar high current levels as the p-FET, which is one of the key factors in ensuring a high oscillation frequency. The threshold voltages, Vth=-0.4V for the p-FET and Vth=OV for the n-FET, are successfully controlled by our work function scheme of the metal gates. In addition to these metal gates, we used the highly doped silicon substrate as an additional way to control both, the p-type and n-type FETs' threshold voltages simultaneously. Fig.3 shows inverter characteristics measured at 3 different silicon back-gate voltages, Vbg: negative Vbg shift the Vth for both p-type and n-type FETs to positive values. The highest inverter gain of 9 is obtained at Vbg=-6V. The measured inverter characteristics agree very well with the expected behavior extracted from the output characteristics of individual pand n-FETs. Improving the saturation currents in both, the pand n-FETs will improve the inverter gain in future ring oscillator generations as will the use of low work function metals as source/drain contacts for the n-FETs. Using a spectrum analyzer with a fixed 50Q input impedance, we measured the output of the ring oscillator as a function of the supply voltage, Vdd, as shown in fig.4. At Vdd=0.56V, the ring oscillator operates at 17MHz, and the oscillation frequency increases to 72MHz at Vdd=1.04V. The signal heights ranging from 34piV to 390piV for different Vdd are rather small compared to the supply voltages. This is simply due to the mismatch of the -1MQ output impedance of the ring oscillator circuit and the 50Q input impedance of the spectrum analyzer. Fig.5 shows the resonance frequency of the ring oscillator as a function of the supply voltage Vdd (left axis) and the corresponding stage delay time for each voltage value (right axis). The rising frequency is a direct result of the increasing current of the individual FETs. At Vdd=1.04V, the ring oscillator operates at a stage delay time of 1.4ns. This is to our knowledge the fastest stage delay measured for a ring oscillator built on any nanostructure including nanowires3. The parasitic capacitance per stage is estimated to be Cstage 1.8fF. Taking the current, I 1.7[tA at Vdd=1.04V, into account, the stage delay time can be calculated as X = 1.Ins which is very close to the measured value of 1.4ns indicating that the present performance of our ring oscillator is entirely dominated by the parasitics rather than the intrinsic nanotube performance. In summary, we have demonstrated the highest performance of a single molecule ring oscillator with the shortest stage delay time reported to date. While this frequency response is still far from the predicted THz4 performance of carbon nanotubes, our results are a critical step towards an all nanotube based nanoelectronics.