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Large-Scale Fabrication of Carbon Nanotube Probe Tips for Space Imaging and Sensing Applications
 

An innovative approach has been developed that combines nanopatterning and nanomaterials synthesis with traditional silicon micromachining technologies for large-scale fabrication of carbon nanotube (CNT) probe tips. The CNT probes are strong, wear-resistant, and capable of high resolution and critical dimension imaging and sensing which are vital to space exploration.

Benefit

Carbon nanotube (CNT) AFM probing tips provide a new class of high performance nanoprobes. Innovative ultra-small probing and sensing systems are vital to future space explorations and missions. Carbon nanotube AFM probing tips provide a new class of high performance nanoprobes for in-space in-situ imaging and sensing applications. After being developed for more than two years at Ames, this technology has been brought to very high maturity, has very low risk, and is close to commercialization.

Research Overview

At NASA Ames Research Center, we have developed an innovative approach that combines nanopatterning and nanomaterials synthesis with traditional silicon micro-machining technologies for large-scale fabrication of carbon nanotube probe tips. The nano-micro integration is achieved through catalyst nanopatterning and registration at wafer scale and through effective nanocatalyst protection and release schemes before and after silicon micromachining. CNT tip locations and diameters are defined by e-beam lithography. A nitride protection layer and specialty protective chemicals are deposited on wafers prior to microfabrication. Cantilevers are fabricated from Silicon-On-Insulator (SOI) wafers through a series of photolithographic patterning, dry, and wet etch process steps.

Figure 1. A carbon nanotube AFM probe tip.Figure 1. A carbon nanotube AFM probe tip.

CNT probe tips are grown at the end of the fabrication process from the defined nanocatalyst spots on cantilever beams. CNT length, orientation, and crystalline quality are controlled by plasma enhanced chemical vapor deposition (PECVD). Our batch fabrication process has produced 244 well-aligned singular CNT probe tips per 4-in. wafer with precise control over tip characteristics. In PECVD, an electric field is present in the plasma discharge to direct the nanotubes to grow and align parallel to the electric field. Due to the crystalline morphology of our PECVD-grown CNTs, there is no need in our process to conduct post fabrication treatment to remove and/or to shorten the CNT tips. With effective catalyst protection schemes, this fabrication process is very similar to conventional approach for fabricating wafer scale silicon AFM probe tips. Process control is therefore feasible and the overall yield is greatly improved.

CNT probe tips with diameters ranging between 40-80 nm and lengths between 2-6 m m have been produced in our lab. They are found to be functional probe tips with no need for post-growth treatment. Using the optimized catalyst formulation and growth conditions, for released CNT AFM cantilever tips, we have achieved an individual CNT growth yield of 85-90% from 100 to 200 nm catalyst sites. This reliable and true bottom-up wafer scale integration and fabrication process provides a new class of high performance nanoprobes. The probe tips made from this method display good image acquisition characteristics. Preliminary AFM imaging results show that the CNT probe tips are strong, wear-resistant, and capable of high resolution and critical dimension imaging.

Background

A key hurdle in nanoscience and nanotechnology is the integration of nanoscale materials with micron scale electronics and structures to form workable devices and detectors. While there are numerous efforts targeting the growth, structures, and related properties of nanomaterials, studies addressing the issue of nano-micro integration for device and sensor fabrication are scarce. This work presents the first example of such integration. We have demonstrated a truly innovative bottom-up wafer scale fabrication methodology for the reliable fabrication of AFM probe tips by integrating carbon nanotubes with silicon cantilevers for critical dimension imaging and sensing for space applications.

Cross-sectional TEM images of an individual CNT on a cantilever beam with the desired bamboo morpholFigure 2. Cross-sectional TEM images of an individual CNT on a cantilever beam with the desired bamboo morphology. (a) CNT cantilever 90-degree side view with ¥1500 magnification. The CNT is 60 nm in tip diameter, 5 µm in length, and has a tilt angle of 13º with respect to the cantilever beam surface normal. (b) CNT tip with ¥15K magnification. (c) CNT tip body part with ¥100K magnification. Multi-wall CNT wall structures with bamboo-like periodic crossover parts are clearly visible along the stem. (d) CNT tip end with ¥100K magnification. Ni catalyst is wrapped with thin graphite layers at the tip end.

Carbon nanotubes and related nanostructures possess remarkable electrical, mechanical, and thermal properties. They show tremendous promise in a wide variety of applications including chemical sensors, biosensors, electronics, interconnects, field emitters, and scanning probes. The intrinsic nanometer scale diameter, high aspect ratio, and strong mechanical robustness of CNTs make them ideal for high lateral resolution imaging and deep trench/via critical dimension imaging and sensing for space exploration and applications. CNT probes are also highly desired in space biological and chemical sensing applications where gentle probe-sample interactions are required. When CNT probes approach the sample surface during AFM tapping mode imaging, the CNT buckles elastically which restricts the maximum force that can be applied to soft samples. CNT tips can also be functionalized at the tube open ends. They can be made into multi-purpose nanoprobes by imaging and sensing at the same time, and probing and manipulating materials at the same time.