Design and Fabrication of a Strain-Based Nano-Compass

Fabrication and Testing of a Strain-Based Carbon Nanotube Magnetometer

Structure

J. A. Brame1, J. E. Goodsell 1, S. A. Getty2 *, Y. Zheng 3, and D. D. Allred1*, 1.

Department of Physics and Astronomy, Brigham Young University, Provo, UT 2.

Code 541, GSFC, NASA, Greenbelt, MD; 3. Code 553, GSFC, NASA, Greenbelt,

MD * corresponding author, [email protected] [email protected].

ABSTRACT

A test structure for a prototype nanoscale magnetometer exploiting the strain sensitivity of Single Walled Carbon Nanotubes (SWCNTs) has been fabricated. The nanotube magnetometer would boast increased operating temperature and reduced dimensions, mass and power requirements compared to a Fluxgate magnetometer.

Dramatic resistance increase with strain has been previously reported for individual nanotubes, and this magnetometer design concept seeks to extend this strain-resistance property to an "as grown" ensemble of SWCNTs. Measurements of a test structure show a correlation between applied magnetic field and device conductivity. This correlation indicates an increase in conductivity with strain to the network of nanotubes; c. andidate mechanisms for this behavior are discussed.

INTRODUCTION

A magnetometer is an instrument for measuring magnetic fields. There are two basic categories of magnetometers: scalar magnetometers, which measure the total strength of a magnetic field, and vector magnetometers, which measure both the strength and the along a known direction of a magnetic field. Some magnetometers can measure magnetic fields below one nanotesla. However, many magnetometers lack robustness, operate at extremely high or low temperatures and demand bulky mechanical structures or high power inputThe objective of this work is to develop a new vector magnetometer technology that operates in a wide range of temperatures with lower power consumption, within a smaller footprint, and with higher spatial resolution than conventional magnetometer technologies, such as fluxgate or SQUID magnetometers.

Space exploration provides a wide range of use for magnetometer devices.

Magnetometers are used for scientific measurement, such as mapping the interaction of solar wind with planetary magnetospheres and probing the interior of planets. In planetary exploration, magnetometers can provide large and small scale geologic mapping and are critical for orienting spacecraft and instruments. In the future, nanoscale magnetometers may prove useful for astronaut and rover orientation.

Because of their remarkable electronic and mechanical properties, carbon nanotubes, particularly single-walled carbon nanotubes (SWCNTs), have been the subject of intensive research efforts over the past decade. The bonding structure of carbon nanotubes is predominantly sp2, as in graphite. SWCNTs can be thought of as a thin strip of graphene (single layer graphite) rolled up to form a seamless tube.1 The aspect ratio

(ratio of length to width) of these tubes can be in the millions> 10 6, and SWCNTs are stronger per weight than steel.

An additional property, the focus of this investigation, is that as the tubes are stretched or strained bent their electrical resistance increases by orders of magnitude for very small strains.1-5 Many researchers have already suggested that this large strain- induced response in electronic resistance, or piezoresistance, can be exploited in strain- sensing devices.6-8

An as-grown ensemble of SWCNTs was selected to develop one such strain- sensing device for magnetic field detection. Although a SWCNT ensemble will be less sensitive to strain than an isolated SWCNT, a networks-grown ensembles of SWCNTs areis much easier to produce and manipulate than individual nanotubes or purified batches of nanotubesused to promote device uniformity and ease of device integration.

The proposed magnetometer design consists of a microscopic microscale iron needle suspended over a trench and mechanically coupled to by a mat of electrically contacted nanotubes (see figure 1). In the presence of a magnetic field the iron needle will torque to align with the field similar to a traditional compass. This torque will strain the nanotubes on which the needle is suspended, thereby changing their resistance. Upon calibration this change in resistance will correspond to a magnetic field strength. Device dimensions would allow for 10,000 devices to fit onto a 1 cm x 1 cm instrument, enabling high spatial resolution of both magnetic field strength and direction.

This SWCNT-based magnetometer compares favorably with currently available devices for space applications, s. Table 1such as compares the theoretical performance of the magnetometer with that of a the traditional fluxgate magnetometer, magnetoresistive devices (MR and anisotropic magnetoresistor, AMR), and an optically pumped scalar instrument. Compared to these instruments, the SWCNT magnetometer operates at lower power, occupies a smaller footprint, and permits orders of magnitude higher spatial resolution, which may prove useful in studies of the magnetic microstructure on other planets in performance areas important to space flight. The SWCNT-based magnetometer should reliably operate in temperatures much higher than those typically experienced by space craft. Further, the SWCNT magnetometer will be thousands of times smaller and more energy efficient than the fluxgate magnetometer; such compactness, low mass and low power requirements are critical in space-flight hardware.

Finally, the SWCNT magnetometer promises nanoscale resolution, sensitivity and robustness necessary for future space exploration applications.

A 2 x 2 array of proof-of-concept magnetometer test structures has been successfully fabricated. This paper details the fabrication process for the array of magnetometer test structures. Magnetic field testing was performed on one successful test structure to determine the preliminary strain-resistance response of the prototypes.

Initial test results are presented and discussed. Finally, future refinements to the next iteration of magnetometer test structures are outlined.

METHODS

A schematic of the test structure processing is shown in Figure 1. An ensemble of

SWCNTs is grown on a Si/SiO2 wafer. Gold contact pads are deposited on the wafer surface, followed by an iron needle between the gold pads. Finally, a trench is etched under the needle, which is supported by the SWCNTs, allowing the needle to rotate and align itself with a magnetic field. Details for each processing step follow.

The substrate was a 1cm by 1cm (100) silicon chip cut from a 500 micron thick wafer with a 500 nm oxide over-capping layer. A dense ensemble growth of SWCNTs was achieved using chemical vapor deposition (CVD) and an indirect catalyst evaporation.8-9, 11 and chemical vapor deposition (CVD). Patterns for the contact pads were generated using electron-beam lithography

(EBL). The sample was coated with a bi-layer electron-beam resist (EBR) of methyl methacrylate/methacrylic acid (MMA/MAA) followed by polymethyl methacrylate

(PMMA). The bi-layer resist is commonly used in semiconductor processing to provide an undercut to facilitate removal of metal by liftoff outside the desired pattern area. The resist-coated sample was placed in a scanning electron microscope (SEM) with an in-situ

EBL controllerfitted with a Nanometer Pattern Generation System. Four identical sets of gold contact patterns were generated using Nabity’s “Nano-Pattern Generation System”

(NPGS) and were written on the sample. The exposed sample was developed by sequential dipping in a solution of toluene:-isopropyl alcohol (IPA) (1:3) for 5 seconds,

IPA rinse for 10 seconds, and 2-ethoxyethyl acetate:ethanol (2:5) for 5 seconds and 20 seconds, and respectively with a 10 second rinse in IPA rinse after both stepsfor 10 seconds.

Gold at a thickness of 100nm was evaporated on the sample to form the contact pads and was preceded by a 10 nm thick chromium underlayer to promote surface adhesion. The evaporator base pressure was 1x10 -6 torr and two resistive evaporation sources were used to avoid venting the chamber in between evaporationsThe base pressure for both evaporations was 1x10-6 torr. Lift-off of the excess metal was performed by placing the gold-coated sample in acetone for approximately 2 hours with periodic agitation to remove the EBR and unwanted metal. Lift off yielded four sets of patterned gold electrical contact pads with alignment marks atop a SWCNT mat.

The needles were patterned by applying spin casting another layer of bi-layer

EBR and exposing four 100 80 micron by 5 3 micron iron needle patterns using the same EBL process as above with an alignment step, such that. The four each iron needles were was centered between the four gold contact patternsgold electrodes. The iron needle pattern was exposed in the SEM and the sample was developed following the same process described above.

The iron needle evaporation consisted of a 30nm layer of chromiume, followed by a 300nm layer of iron, capped by another 30nm layer of chromiume. The chromiume layers wereare evaporated used to passivate the iron against oxidation. The evaporator base pressure was 1x10-6 torr and two thermal evaporation sources were used to avoid venting the chamber in between evaporations. Lift off was again performed as above and yielded chromium/four sets of iron/chromium needles centered between gold contact pads atop ensemble the mats of SWCNTs.

The trenches beneath the needles were patterned using photolithography. 1812 photoresist (PR) was applied to the sample and soft baked at 110˚C for 105 minutes. An optical microscope was then used to expose oval shaped patterns in the PR over each needle. To form each oval-shaped exposure, four overlapping circles were exposed for

45 seconds at 95% illumination intensity using the microscope’s smallest aperture at

1000x magnification. These exposures yielded an oval-shaped trench pattern of approximate dimensions 35 µm by 90 µm. The sample was then developed for 1 minute and then was hard baked at 110˚C for 90 minutes.

The trench was fabricated formed by etching the sample oxide layer in buffered oxide etch (BOE) for 20 minutes with slight agitation, where the patterned photoresist acts as an etch stop, followed by a 20 minute rinse in DI deionized water. BOE is known to isotropically etch through the silicon dioxide surface layer. With the patterned oxide as an etch stop, tThe sample was then etched for 2 hours in agitated KOH at 65˚C. KOH anisotropically etches through silicon and forms a pyramidal trench pattern in (100) oriented wafers. Notably, the chromium/iron/chromium metal was found to be chemically robust to this wet etch.

The etched sample was finally submerged in IPA in a critical point dryer (CPD).

The CPD dried the sample withoutmitigated the potentially damaging surface tension effects that could cause the needle to adhere to the trench bottom during normal evaporative drying. Scanning electron microscopy was used to examine the devices after processing (see Figures 2, 3).

Adhesion between the needle and SWCNT network was found to be a point of failure during agitated etching of the trench; as a result, the needles were often found to be partially removed after trench formation. For magnetic testing, the most pristine complete structure was prepared selected to measure the electric current across the

SWCNTs as a magnetic field was applied perpendicular to the SWCNT mat. The two gold contact pads were wire bonded to a chip carrier which andwas connected to a probe station. Current across the contacts (device current) was measured using a Stanford

Research Systems SR570 low-noise current pre-amplifier as a voltage of 30mV was applied across the contacts using a Sorensen XT15-4 power supply, resistively divided using an electrostatic discharge-mitigating electronic circuit.

The test magnetic field was produced generated by a GMW 5201 Projected Field

Electromagnet and the sample was oriented such that the magnetic field lines passed perpendicularly through the SWCNT mat, which would tend to rotate the iron needle out of the SWCNT plane. The magnetic field was incrementally applied by increasing the electromagnet current in 1 A steps up to a maximum of 15 A. This corresponds to a magnetic-field step of 0.0267, yielding a 0.4 Tesla maximum field, after which the field was stepwise decreased. Each step lasted 1 minute to allow eliminate for transient signalseffects to die out.

RESULTS

An array of four magnetometer test structures was successfully fabricated. SEM investigation revealed that the SWCNT ensembles, the gold contact pads and the iron needles survived were chemically robust to the two etching processes. Further, the needles were successfully suspended atop the SWCNT ensembles above trenches etched through the silicon. A successful test structure is shown in Figure 3.

Representative magnetic field measurements are shown in Figure 4. Magnetic field is increased for 500 seconds, and then decreased back to baseline. Initial increase in magnetic field causes an increase in device current. Device breakdown at around time

700 seconds prohibits further conclusions. An increase in device current means that the ensemble resistance of the device was decreasing with applied magnetic field. Other tests showed greater noise with lower signal and suggest refinements to the electrode and needle geometry and to the nanotube density, discussed below.

According to the work of Tombler and others1-5, a decrease in nanotube current with an increase in magnetic field is expected. As the magnetic field torques the iron needle, the nanotubes are strained and the conductivity of the nanotube mat should decrease. In contrast to this the current through the nanotube network increased when in a magnetic field. Carbon nanotubes themselves should not display any magnetically induced conductivity changes. Further, previous work suggests that a magnetic field does not change the conductivity of a SWCNT mat grown from an iron catalyst 14. Hence, any change in the conductivity of the nanotubes is likely due to the magnetic induced torque on the iron needle.

The data obtained through magnetic testing suggest several mechanisms to explain these results and possibilities for refinements to the fabrication process. possibilities for refinements to the fabrication process. It is possible that the noisy signal is due to the fact that, while the iron needle torques out-of-plane and strains the outer

SWCNTs, the center SWCNTs are not significantly strained and thus provide a low- resistance current path. This could be avoided by separately contacting the outer nanotubes using a device geometry such as Figure 5. There may also be an insufficient magnetic moment in the iron needle to torque the incredibly stiff nanotube network. By decreasing the nanotube density, the device sensitivity should increase. This will allow a more precise measurement of resistance versus magnetic field. Additionally, explanations include changing contact resistance or modulation of the number of conducting channels cannot be ruled out. IWithin the data available, it is difficult to rule out otherrefine the mechanism responsible for the unexpected increase in device conductances due to the limited amount of data collected before device failure..

Additional devices should be fabricated to confirm the reported effects. Nevertheless, the response shown in Figure 4 is suggestive of magnetic field response, and further investigations may reveal the its operation. scientifically and technologically important

reduce film stress by replacing cr with dielectric to improve yield CONCLUSIONS

A fabrication process for magnetometer test structures has been demonstrated.

An array of prototype magnetometer test structures has been successfully fabricated. The test structures consist of a high aspect-ratio iron needle suspended above a trench by a mat of SWCNT. Gold pads electrically contact the SWCNT mat. This fabrication process is now available and being used in various field sensing applications.

Initial magnetic testing indicates that there is some correlation between magnetic field and measured resistance in the device. Further device fabrication and testing is necessary to establish the extent of this relationship.

ACKNOWLEDGEMENTS

We gratefully acknowledge financial support from NASA GSFC Director’s

Discretionary Fund , the and NASA Internal Research and Development Fund

to S. A. Getty.

, the Rocky Mountain Space Grant Consortium Student/Faculty Internship Program for the summers of 2006 and 2007, and the Student/Faculty Internship Program for the summers of 2006 and 2007 which provided support for J. A. Brame, J. E. Goodsell, and

David. D. Allred. The authors also acknowledge technical assistance from D. Dove, D.

Stewart, and B. Munoz and numerous helpful discussions with B. Hicks, R. B. Davis, R.

Vanfleet, and D. Rowland. REFERENCES

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on , vol.2, no., pp. 465-468, (2006). Figure 1. Cross-section view of device schematic showing the edge of the gold needle (needle pointing perpendicular to page) suspended on the nanotubes, which are contacted by the gold pads on either side of the V-shaped trench. The dark line beneath the gold layer and holding the needle represents the nanotube mat.

Figure 2. Scanning Electron Microscope image of entire device. The large trapezoidal structures at the top and bottom are the gold contact pads, with the trench and needle in the middle. The trench is the dark region surrounding the needle. The nanotubes are not visible but are contacted by the C-shaped pad above and below the needle. The four small patterns near the outside of the C-shapes are alignment marks.

Figure 3. The test structure consisted of two gold pads making electrical contact with a SWCNT mat (the spider web looking structure). An iron needle was suspended on the mat above a trench etched through the SiO2 layer and into the silicon layer. Portions of the nanotube mat on either side of the device have delaminated from the SiO2 layer leaving a substrate bare of nanotubes on either side of the photo. A portion on the mat of the left side has rolled up around the alignment mark and continues across the trench and is on top of the lower left alignment mark.

Figure 4. Device resistance decreases as magnetic field is increased, labeled a on the graph. 500 seconds into the experiment the field is ramped down (b on graph). At some point contact with one of the pad becomes intermittent leading to the noisy signal in region c on the graph. The x-axis (time) also represents the magnetic field which increased in one minute intervals up to a maximum at time 500 seconds, then decreased in minute intervals back to zero.

Figure 5. The design for the next iteration of gold-pad geometry features three separate sets of pads (labeled a, b & c) to contact specific sections of nanotubes. In this figure the black Xs represents the nanotube mat and the horizontal line is the needle. Dividing the pads into three sets makes it possible to uses the outer set of pads (labeled a & c) to measure the strain across the outer portions of the nanotubes. These should be the most- strained. Figure 1

Needle Gold Si02

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Figure 3

SWCNT Mat Au Contact Pad Trench

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SWNT ST5 Fluxgate Magnetometer Max Op 450°C 100°C Temp Sensor 1cm x 1cm x 4cm x 4cm x 6cm Dimensions 100nm Sensor 10-5 g 75 g Mass Sensor Op 10-3 - 10-2 mW 50 mW Power