Carbon nanotube mechanical relays for electronics

Developing electronics that operate in extreme environments is an area of strategic importance to NASA's future planetary missions. Instruments that may someday be sent to explore Venus, where surface temperatures can reach as high as 486^°C, or to investigate high-radiation Jovian environments such as Jupiter will require significant advances in electronics beyond what is currently available. The invention of the silicon transistor more than five decades ago revolutionized the electronics industry and transformed many aspects of our society, including current communications technologies as well as information storage and processing. However, according to Moore's law, continued miniaturization of these solid-state switches and circuits will limit their ultimate performance. Thinner gate oxides and closely spaced wiring cause substantial power dissipation due to excessive leakage currents, even in standby or static mode. Estimates suggest static losses alone could consume 100% of a device's power budget within a few years.¹ Solid-state switches are also inherently more susceptible to radiation, and their operation at thermal extremes is compromised since silicon reverts to its intrinsic behavior (i.e., a significant alteration of its electrical properties) at either high or low temperatures. For space applications, power consumption, ruggedness, and size are all critical metrics, more so than in consumer electronics or even military applications.

Figure 1. (a) Low-magnification scanning electron microscope (SEM) image of a nanoelectromechanical (NEM) device. The inset shows the device schematic. (b) High-magnification SEM image showing a single-walled carbon nanotube (SWNT) bridging the nanotrench beneath it. PECVD: Plasma-enhanced chemical vapor deposition. SiO₂: Silicon dioxide. Nb: Niobium. Au/Ti: Gold/titanium.

Figure 2. Actuation voltage measurements of the NEM device shown in Figure 1b over multiple cycles. From the I-V characteristic, the 4-orders-of-magnitude rise in current is seen at ∼2.4V, indicating well defined ON and OFF states. Hysteresis is also seen between the increasing and decreasing voltage paths. The inset shows the ON-state voltage to be similar in the forward-bias (pull electrode grounded) and reverse-bias (pull electrode positive) regimes, indicating that field emission is an unlikely possibility at these voltages.

Figure 3. (a) Isolation of the RF CNT NEM switch up to 100GHz. (b) Insertion loss up to 10GHz. The inset at right shows insertion loss on an expanded scale.

Unlike solid-state switches, nanoelectromechanical (NEM) devices have low OFF-state currents because their conducting paths are physically isolated. In particular, carbon nanotubes (CNTs) have remarkable mechanical and electrical properties that make them excellent candidates for the design of NEMs. They are extremely stiff,² they accommodate very large mechanical strains,³ and their low mass and chemical inertness make them very promising for enabling low-power, low-leakage, and high-speed switching devices that also have the potential to function under extreme environments. Nanotube-based NEMs have already been used in applications ranging from nanotweezers⁴ to memory devices.⁵

At the Jet Propulsion Laboratory (JPL), we are exploring several architectures for the formation of NEM switches, using bottom-up techniques, for their potential application in extreme-environment electronics. These NEM switches are based on carbon nanotubes that are synthesized using chemical vapor deposition (CVD). One such architecture consists of single-walled carbon nanotubes (SWNTs) that are grown at 850^°C using a predefined iron catalyst, where the tubes bridge nanotrenches formed over a metal base electrode.⁶ Since nanotube growth is poisoned by the presence of certain metals that inhibit catalytic activity, niobium is used as a refractory metal due to its compatibility with the high-temperature CVD synthesis of SWNTs. The scanning electron microscope (SEM) micrograph in Figure 1 depicts a completed device with a single nanotube shown crossing a nanotrench.⁷

Actuation voltage measurements of these devices showed well-defined OFF and ON states, as indicated by the I-V (current-voltage) characteristic in Figure 2. An increase of 4 orders of magnitude between the low- and high-current states was observed as the voltage between the top and base electrode was increased. Typical power dissipation per switching event was very low, in the hundreds of nanowatts. The inset in Figure 2 shows that switching occurred in both the forward- and reverse-bias modes, which suggests that field emission, a polarity-dependent phenomenon, is unlikely to occur at these voltages. In addition, the switching times of these devices were measured to be only a few nanoseconds.⁸

The radio frequency (RF) performance of these NEM switches was also evaluated, since microelectromechanical systems (MEMS) switches have far-improved characteristics compared to P-I-N diodes and field-effect transistors at high frequencies.⁹ The inductive, capacitive, and DC resistance was computed using quantum parameters of the CNT,¹⁰ and an equivalent circuit of our structure was constructed and simulated up to 100GHz.¹¹ The isolation in the shunt configuration was found to be inhibited by the large tube inductance and the high DC series resistance. An alternative architecture was conceived where the high tube inductance is used in such a way that it actually enhances RF performance. In this series switch configuration, a portion of the tube is metalized. In the OFF state with no DC bias applied, the high tube inductance prevents any parasitic coupling between the RF lines and the tube, resulting in isolation as high as 20dB up to 100GHz, as shown in Figure 3(a). In the ON state, the metalized portion of the tube is in contact with underlying metal pads that allow RF signals to be transmitted with low loss (<0.5dB at 100GHz), as shown in Figure 3(b). The RF CNT NEM switch appears to have exceptional RF performance characteristics. It also has the potential for operability at low actuation voltages (<10V) and high speeds (>μs), both of which are difficult to achieve concurrently with MEMS switches.

Besides the promising high-frequency communication applications of such switches, we at JPL are also interested in devising techniques to control the mechanical characteristics of CNTs using electric field-assisted growth. The overall objective is to develop switches for use in nonvolatile memory and logic circuits with characteristics that will allow the deployment of instruments for the exploration of the extreme environments encountered during space exploration.

We thank Richard Baron, Elizabeth Kolawa, and Martin I. Herman for useful discussions, and Randy T. Odle for programmatic support. This research was carried out at the JPL, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) and was partly funded through the internal Research and Technology Development (R&TD) program.