Azlin M. Biaggi-Labiosa and Laura J. Evans

Glenn Research Center, Cleveland, Ohio

Gordon M. Berger

Universities Space Research Association, Cleveland, Ohio

Gary W. Hunter

Glenn Research Center, Cleveland, Ohio

Integration of Nanostructures Into Microsensor

Devices on Whole Wafers

NASA/TM—2015-218826

June 2015
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Azlin M. Biaggi-Labiosa and Laura J. Evans

Glenn Research Center, Cleveland, Ohio

Gordon M. Berger

Universities Space Research Association, Cleveland, Ohio

Gary W. Hunter

Glenn Research Center, Cleveland, Ohio

Integration of Nanostructures Into Microsensor

Devices on Whole Wafers

NASA/TM—2015-218826

June 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

We would like to extend our gratitude to Dr. Liangyu Chen for his efforts and assistance on the electrical contacts of the whole 

wafer alignment. We also say thank you to José M. González of Gilcrest and Michelle Mrdenovich of Sierra Lobo Inc. for their 

technical support. We acknowledge the contributions of Dr. Jennifer C. Xu, Dr. Carl W. Chang, Dr. Lawrence G. Matus, and

Dr. Mary V. Zeller for their helpful discussions and critical review of the manuscript. This research was supported by the Vehicle 

Systems Safety Technologies Project.

Available from

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This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
NASA/TM—2015-218826 1 

Integration of Nanostructures Into Microsensor 

Devices on Whole Wafers 

Azlin M. Biaggi-Labiosa and Laura J. Evans 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Gordon M. Berger 

Universities Space Research Association 

Cleveland, Ohio 44135 

Gary W. Hunter 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Chemical sensors are used in a wide variety of applications, such as environmental monitoring, 

fire detection, emission monitoring, and health monitoring. The fabrication of chemical sensors involving 

nanostructured materials holds the potential for the development of sensor systems with unique properties 

and improved performance. However, the fabrication and processing of nanostructures for sensor 

applications currently are limited in the ability to control their location on the sensor, which in turn 

hinders the progress for batch fabrication. This report discusses the advantages of using nanomaterials in 

sensor designs, some of the challenges encountered with the integration of nanostructures into 

microsensor/devices, and then briefly describes different methods attempted by other groups to address 

this issue. Finally, this report will describe how our approach for the controlled alignment of 

nanostructures onto a sensor platform was applied to demonstrate an approach for the mass production of 

sensors with nanostructures. 

Introduction 

A major objective in aerospace sensor development is to produce sensors that are small in size, 

low in cost, have low power consumption and easy to batch fabricate (Refs. 1 and 2). Chemical sensors 

involving nanostructured materials have the potential to provide these properties and more, however, the 

ability to control the location of the nanostructures on a microsensor platform is in its early stage of 

development. This report describes efforts to align nanostructures onto sensor platforms, including the 

demonstration of the fabrication of a whole wafer with aligned nanostructures. First, there is a brief 

summary of the qualities and properties of nanostructures that are used for chemical gas sensing, followed 

by a brief discussion of the various approaches that have been taken by other groups to integrate 

nanostructures into microsensor devices along with some of the challenges encountered. Lastly, this paper 

provides details on the demonstration of the controlled placement of nanostructures in a sensor structure 

having a sawtooth patterned electrode design and how this approach was extended and applied to a 

2-in. alumina wafer. The whole wafer was patterned using standard photolithography and thin film 

deposition techniques, combined with dielectrophoresis. This approach makes possible the alignment of 

nanostructures on more than 1000 sensors at once while also allowing for the fabrication of different 

sensor sizes. 
NASA/TM—2015-218826 2 

Benefits of Nanomaterials 

Limitations of traditional gas sensor technology for aerospace applications, such as low selectivity 

and sensitivity or the ability to operate at high temperatures, have prompted the study of new materials 

and the new properties that come about from using the nanostructured form of traditional sensor 

materials. One of the important characteristics of nanomaterials is their high surface area to volume ratio. 

This characteristic can be exploited to increase the selectivity and sensitivity of the sensor if the sensing 

mechanism is dependent upon the interaction, such as adsorption, of analyte gases on the sensor’s surface. 

For example, the sensing mechanism for n-type metal oxide semiconductors relies on perturbations of the 

surface conductivity induced by chemical reactions between target gases and oxygen species adsorbed 

onto the surface. The adsorption of oxygen creates surface-acceptor sites that immobilize conduction band 

electrons near the surface region, creating a depletion layer (Ref. 3). Typically, under ambient conditions, 

that is, in the presence of air, the near-surface region of each grain or crystallite is depleted of electrons 

forming a depletion layer, L, relative to the interior parts of the grain. In the depletion layer the 

conductance is lower as compared to the absence of oxygen gas. Thus, the change in conductance of the 

sensing material occurs at the exterior regions of the grains. Beyond the depletion layer, the interior parts 

of the grains do not contribute to the gas response. As a consequence, the grain size (D) affects the sensor 

performance. For large grains, D >> 2L, the conductance of the film is limited by Schottky barriers at 

grain boundaries (GB) (Fig. 1(a)). In this case, the sensitivity is practically independent of D. When grain 

size is comparable to 2L (D ≈ 2L) every conducting channel in the necks between grains becomes small 

enough to influence the total conductivity (Fig. 1(b)). Since the number of necks is much larger than the 

Figure 1.—Three mechanisms of conductance in metal-oxide gas 

sensitive materials. The shaded part shows the core region (high 

conductivity) and the unshaded part shows the depletion layer (low 

conductivity) and GB = grain boundary. (a) D >> 2L, the conductance, 

which is higher in the non-depleted core region, is controlled by grain 

boundaries. (b) D ≈ 2L, where necks between coalesced primary grains 

control the conductance. (c) D < 2L, when the grains are small enough 

to be fully depleted the conductance is grain controlled (Ref. 4). 
NASA/TM—2015-218826 3 

grain contacts, the necks govern the conductivity of the material and define the size-dependence of gas 

sensitivity. If D < 2L, every grain is fully involved in the space charge layer, and the electron transport is 

affected by the charge on the particles' surfaces (Fig. 1(c)). Thus, by reducing the particle size, the 

conduction of the sample may be controlled by the grain boundaries, necks, or grains. The latter case is 

the most desirable, since it allows achieving the highest resistance change (Ref. 4). Therefore, for the 

same chemical composition of the material, the smaller the nanomaterial grain size, the more sensitive the 

sensor can be in principle. Because of these potential advantages over the same sensor built with bulk 

materials, sensors fabricated with nanostructures (nanoparticles, nanotubes, nanorods, etc.) have been 

investigated for the detection of a wide variety of gases (Refs. 5 to 8). 

From the operative point of view, gas sensors that utilize nanostructures as the sensing material need 

to be compact and manufacturing costs need to be low. Nanostructured materials (nanomaterials) can be 

used to reduce operating temperatures, which reduces power consumption and, at the same time, makes it 

safer to operate. What is more, because they are in such wide demand, devices need to be developed that 

are not only robust but also have high sensitivity, selectivity and stability in relevant temperature 

conditions. The stability of a sensor is the ability to provide reproducible results for a certain period of 

time. This includes retaining the sensitivity, selectivity, response and recovery time. 

While, in theory, the increase in active surface area and consequently gas diffusion should translate into 

a proportional increase in sensitivity, there are other factors involved in the transduction of the analyte gas–

surface reactions into an electrical conductance change. These are (1) electrode–nanomaterial contact, (2) 

nanocrystal organization within the nanomaterial and (3) electrical and surface properties of the electrodes. 

No matter how sensitive the nanomaterial, if you cannot make good electrical contact with the sensor 

platform, then the device will be ineffective. This, in turn, introduces a notable challenge in the use of 

nanostructures for gas sensor applications: how to integrate nanostructured materials into a micro/macro 

structure in a time-efficient and cost-effective manner. If this can be overcome, then the ability to massproduce sensors with nanostructured materials in a reproducible and controllable way is more achievable. 

Previous Approaches to Incorporate Nanomaterials 

The standard method of deposition of nanostructures onto a sensor platform is by dispersing the 

nanostructures in a suspension and depositing the suspension on a substrate (Refs. 9 to 11). Although this is 

a very simple method, as can be seen in Figure 2, there is limited or no control of the density of 

nanomaterial being deposited, and the quality of the electrical connection is poor because the alignment of 

the nanomaterial between the electrical contacts is random and uncontrolled rather than reproducible and 

uniform. These factors make it difficult to mass-produce sensors in a controllable way using this approach. 

The ability to control the orientation and alignment of nanostructures on microdevices is still in an 

early stage of development although various groups have made significant progress for a range of 

materials as can be seen in references (Refs. 13 to 21). Some of the techniques used are summarized 

below along with their advantages and disadvantages. These brief descriptions are meant to provide the 

reader with examples, and not an exhaustive survey. 

Figure 2.—Reproducibility problems encountered when 

depositing a nanostructure suspension onto a substrate. 

(a) Nanorods contacted with the substrate via a silver epoxy 

(b) Nanorods precipitated onto substrate between a pair of 

interdigitated electrodes (Ref. 12). 
NASA/TM—2015-218826 4 

Laser or optical tweezers use a highly focused laser beam to provide an attractive or repulsive force 

(typically on the order of piconewtons), to physically hold and move microscopic dielectric objects. They 

are very sensitive instruments and are capable of the manipulation and detection of sub-nanometer 

displacements for sub-micron dielectric particles. For this reason, they are often used to manipulate and 

study single molecules. When applied to the alignment of nanostructures onto a sensor platform, they are 

not viable for mass production as it is only possible to align one nanostructure at a time. If you want many 

nanostructures aligned, it becomes a very labor intensive process (Ref. 13). 

Electron beam (e-beam) lithography consists of emitting a narrow, concentrated beam of electrons 

onto a resist-coated substrate. Electrons can induce the deposition of substances onto a surface (additive), 

or etch away at the surface (subtractive). E-beam lithography is particularly important in microelectronics, which require extremely precise placement of micro sized circuit elements. When integrating 

nanostructures to form microsensors, this method is mainly used to deposit electrical contacts on top of 

the nanostructure. This provides precision and a good quality electrical contact. However, with such 

precision, components can only be made very slowly and only one at a time, greatly increasing the time 

and cost and prohibiting mass commercial acceptance (Refs. 14 and 15). 

In the Langmuir-Blodgett (LB) method, nanostructures are suspended at an air-liquid interface and 

assembled into dense films through compression. The nanostructures adopt a close-packed organization to 

minimize the energy of the system during compression. Dip coating is used to transfer the nanostructures 

from the air-liquid interface onto a solid substrate. The nanostructures can adhere to this substrate through 

van der Waals, hydrophobic-hydrophilic, or electrostatic interactions as it is drawn in a vertical direction 

through the layer of compressed nanostructures. A shear force is created along the vertical direction as the 

solvent withdraws from the surface of the substrate, leading to alignment of the nanostructures along the 

same direction. The density of the deposited nanostructures is controlled by the speed of withdrawing the 

substrate and the pressure applied to the assembly. Limitations of this technique include reorganization of 

the nanowires during dip coating that leads to overlapping features and gaps within the dense arrays of 

nanowires (Refs. 16 to 18). 

In the Superlattice Nanowire Pattern Transfer (SNAP) technique, a superlattice consisting of 

alternating layers of material, typically a metal, is grown. Then, one of the layers is selectively etched 

away and a metal is evaporated onto its end in a tilted position. The metal-coated superlattice is placed 

face down onto an epoxy film on top of a silicon wafer and cured, thus “gluing” the wires to the substrate. 

A final etching step frees the wires from the superlattice. SNAP can be harnessed to produce large arrays 

of virtually any type of nanowire, the only limitation is that the material from which the nanowires (NW)s 

are made must begin as a thin film. However, assembling of the individual nanowires for device/circuit 

function is complicated. A major challenge associated with constructing electronic devices is the many 

fabrication and processing steps. A good device performance from these nanowires is yet to be 

demonstrated (Refs. 19 and 20). 

Dielectrophoresis (DEP) is the motion of particles toward or away from regions of high electric-field 

intensity. This motion is produced by the action of an electric field on dipole moments induced in the 

particle and the suspending fluid. In an electric field, a dielectric particle behaves as an effective dipole 

with induced dipole moment p

 , proportional to the electric field E

 (Ref. 22) that is, 

p E

   (1) 

The constant of proportionality depends on the geometry of the dielectric particle. The force, F



, on a 

dipole, in the presence of an electric field, is given by 

F p E

   

  (2) 

where 



 is the vector differential operator, Del. 
NASA/TM—2015-218826 5 

Figure 3.—Schematic representation of dielectrophoretic alignment of nanowires between 

two parallel plate electrodes.	

Figure 3 illustrates the working principles of dielectrophoretic alignment of nanowires between two 

electrodes. If the induced dipole moment of the nanowire is greater than that of the fluid, the nanowire 

exhibits “positive dielectrophoresis,” experiencing a force toward regions of high electric field intensity. 

“Negative dielectrophoresis” occurs when the fluid is more polarizable than the suspended nanowire, and 

the nanowires are displaced from high-field regions by the fluid. Dielectrophoresis has captured much 

interest recently because it is an effective way to trap, manipulate, and separate particles ranging from 

large DNA strands to blood cells and larger particles in microfabricated devices. The attributes of the 

nanowire, such as diameter and length, and the electric field frequency influence the alignment of the 

nanowire (Refs. 23 to 28). 

Next is a discussion on the equations that govern dielectrophoresis. These theories and equations are 

well documented in various articles and some of them are referenced below. The frequency dependence 

of dielectrophoretic alignment follows from the equation for dielectrophoretic force (Eq. (3)) and the 

equation for electroosmotic fluid velocity (Eq. (9)). The need for the dielectric constant of the nanowires 

to be higher than the dielectric constant of the surrounding medium is expressed in Equation (3) for 

spherical particles (Ref. 29). 

2

* *

* *

2

F 2 pv Re E

p m

p m

DEP m

    

















  

     (3) 

where v is the volume of the particle, m the permittivity of the suspending medium, E is the external 

electric field, 















  

  

* *

* *

2

Re

p m

p m is the real part of the Clausius-Mossotti factor 















  

  

* *

* *

p 2 m

p m , and *

m *

p  are 

the complex permittivities of the medium and particle, respectively. Homogeneous dielectric particles 

experience a Maxwell-Wagner interfacial polarization at a frequency determined by the relationship 
NASA/TM—2015-218826 6 

between the complex permittivities of the particle and the surrounding medium, which is expressed by the 

real part of the Clausius-Mossotti factor, 

The complex permittivity constants, *

m and *

p  , can be calculated by 





    k

k k * j (4) 

where k is the real part of the permittivity in which k can be either p or m for particles or solvent medium, 

respectively, j is the square root of –1, k is the electrical conductivity of the nanowire material or solvent, 

and  is the angular frequency of the applied electric field. This shows that the real part of the 

Clausius-Mossotti factor is frequency dependent and determines both the magnitude and the sign of the 

dielectrophoretic force. The value of the factor ranges from −0.5 to +1.0, and can be calculated from the 

properties of the medium and the particle. However, for nanorods and nanowires, a shape-dependent 

factor Aa is introduced into the Clausius-Mossotti factor (Ref. 30), 

  



















    

    * * *

* *

p m a m

p m

a

A

K (5) 

Where Ka is the shape-dependent Clausius-Mossotti factor, in which the index a can either refer to L for 

the long axis of the nanowire or to S for its short axis, and where Aa is the depolarization factor along the 

long or short axis of a prolate ellipsoid. 





 



  



 







   e

e

e

e

e AL 2

1

1 ln

2

1

3

2

 (6) 

and 

 

2

1 L

S

A A   (7) 

Where ݁ is defined as the eccentricity: 

2

2

2 1

a

b

e   (8) 

Here a and b are the half length of the long axis and the radius of the short axis, respectively (Ref. 30). 

Equation (9) gives the fluid velocity v near the electrode surface (Ref. 29), 

 2 2

2 2

0

1 8

1

  

  

x

V

v m (9) 

Where V0 is the peak-to-peak potential,  is the dimensionless frequency, which is defined by 

Equation (10),  is the viscosity of the solvent, and x is the distance from the center of the electrode gap 

to one of the electrodes. 
NASA/TM—2015-218826 7 







   

m 2

m x (10) 

Where  is the inverse of the Debye-Hϋckel length (Ref. 29), which can be calculated by 

kT

q n z

m

i i





 

2 0 2

 (11) 

Where q is the elementary charge 1.610–19 C, 0 ni and 0

iz are the number concentration and ionic charge 

of the ionic species type ݅ present in the solution, respectively, k is Boltzmann’s constant 

(1.38110–23 J/K), and T is the temperature. 

An effect that AC electric fields have on polarizable objects is to orient them with respect to an 

electric field. The induced dipole moment p

 of the object interacts with the electric field to produce a 

torque T



, given by 

T p E

  

  (12) 

Neutral particles are subject to this effect, and therefore DEP is more versatile in its applications for 

manipulating live cells, macromolecules, nanostructures, etc. 

Application of Standard Microfabrication Technology 

A common feature of the techniques described above is that they do not use standard silicon 

processing techniques that have been applied to microfabrication of Micro Electro-Mechanical Systems 

(MEMS) based sensors. Microfabrication techniques include the use of photolithography to produce 

miniaturized, repeatable structures. Recently, our team at the NASA Glenn Research Center developed a 

method for the alignment of nanostructures onto a sensor platform with standard microfabrication 

techniques (Ref. 21). This has been achieved by fabricating the sensors using standard photolithographic 

techniques combined with the alignment of the nanostructures using dielectrophoresis (DEP). Figure 4 

shows the six separate fabrication steps taken in this procedure. Briefly, step 1 begins by patterning the 

alumina substrates with a sawtooth electrode pattern with contact pads through photolithography, 

followed by deposition of 2500 Å of platinum (Pt) by sputter deposition. In the second step, the 

nanostructures are added to a photoresist solution, which is coated onto the alumina substrate containing 

the patterned electrodes. Dielectrophoresis is used in step 3 by applying an alternating current (AC) 

through the contact pads on the substrate to align the nanostructures (step 4) between the sawtooth 

patterned electrodes. Because of the geometry of our electrodes, the electric field is greater at the tips of 

the sawtooth patterned electrodes, which accounts for the preferential alignment of the nanostructures 

between the tips of opposing sawtooth electrodes (Refs. 31 and 32). 

After nanostructure alignment, a top platinum electrode is patterned through photolithography (step 5) 

and sputter deposited (step 6) on top of the bottom platinum electrode. This way, the ends of the 

nanostructures are buried between the bottom and top electrodes allowing for improved electrical contact. 

This last step can either reproduce and overlap the bottom sawtooth electrode structure, as shown in the 

SEM image of Figure 5(a), or be a rectangular pattern blanketing the bottom electrode layer as seen in the 

dashed lines in step 6 of Figure 4(a) and in the SEM image of Figure 5(b). For further details of this 

microfabrication process, see Reference 21. 
NASA/TM—2015-218826 8 

Figure 4.—Fabrication steps of the alignment and deposition process (Ref. 21). (a) Top view and (b) side view. 

Figure 5.—Top platinum electrode patterned through photolithography and sputter deposited on top of the bottom 

platinum electrode. (a) The top electrode pattern can either reproduce and overlap the bottom sawtooth electrode 

or (b) be a rectangular pattern blanketing the bottom electrode layer. (c) SEM image of a single set of opposing 

sawteeth with multiple oxide nanowires (Ref. 21). 

There are three key advantages to this processing approach. First, is the use of sawtooth electrodes, 

which provides flexibility in the choice of DEP solvents. Typically the solvent used to suspend the 

nanostructures is either dimethylformamide (DMF) or a light alcohol. However, in this new approach, 

conventional photoresist was used in order to assist with subsequent processing steps. Use of the sawtooth 

electrode configuration facilitates alignment and spatial localization of the nanostructures using DEP. 

Because of the geometry of our electrodes, the electric field is greater at the tips of the sawtooth patterned 

electrodes, which accounts for the preferential alignment of the nanostructures between the tips of 

opposing electrodes. Alignment between sawteeth occurs due to the differential hydrodynamic drag force 

dictated by the nanorod’s length/diameter ratio, as well as the torque induced by the AC electric field as 

the nanorod rotates out of line to the electric field. An AC field is used to prevent electrochemical 

reactions at electrode surfaces (Ref. 34). Through control of photoresist density and nanostructure 

concentration/dispersion, differing densities of aligned nanostructures can be obtained; single (Fig. 5(a) 

and (b)) or multiple nanostructures can be deposited across the electrode gaps in a given sensor 
NASA/TM—2015-218826 9 

(Fig. 5(c)). In addition, the length of the nanostructures deposited across the electrode gap can be affected 

by the AC frequency used to align the nanostructures. For instance, at low frequencies (~20 Hz) the 

majority of nanorods aligned were limited to lengths greater than 10 μm. When the frequency is 

increased, shorter nanorods are captured between the sawteeth. Therein, this approach allows for the 

possibility of reproducible manipulation of nanomaterials at a single point or over an array of devices. 

A second advantage of this approach is the possibility of improved electrical contacts. Each 

nanostructure “sandwiched” between two electrical contacts that are isolated upon deposition and 

separated by an alumina insulator. This is particularly important for minimizing the variability of 

necessary electrical interface connections. Through burying the nanostructures in a sandwich of metallic 

contacts, reproducible and reliable contacts to the nanostructures can be produced. The robustness of 

these contacts provides a clear advantage when operating at higher temperatures. However, the third and 

most notable advantage of this approach is that it permits incorporation of nanomaterials into standard 

photolithographic processing procedures for sensor fabrication. The use of nanostructure/photoresist 

solution is compatible with other mechanical fabrication methods, including drop coating, dip coating, 

and jet printing. In effect, the combination of including nanostructures in a processing solution, and then 

using the solution in standard processing with the addition of electromechanical nanostructure alignment, 

has a range of processing implications. 

This approach was extended for use with other nanomaterials with very promising results (Ref. 35). 

As an example, porous SnO2 nanorods were synthesized via an unique template approach, developed by 

our group, and incorporated onto a sensor platform using the photoresist-based microfabrication approach 

described above. From this, a room temperature methane (CH4) microsensor based on porous SnO2

nanorods was developed and operated, demonstrating the detection of 0.25 percent methane in air as can 

be seen in Figure 6(a), which is, to our knowledge, the first system of this type utilizing SnO2

nanostructures without the use of dopants or catalysts. Furthermore, this porous SnO2 nanorod sensor has 

a wide operating temperature range (25 to 500 °C) for CH4 detection (2500 to 125 ppm), in contrast with 

other material systems. Figure 6(b) is a current vs time curve of this sensor at 300 °C for the different 

concentrations, which illustrates the typical behaviour of the sensor when the methane concentration is 

lowered. The inset of Figure 6(b) shows a transmission electron microscope (TEM) image of the porous 

SnO2 nanorod used in this work (Ref. 35). 

Figure 6.—(a) Current versus Time curve for 0.25 percent CH4 in air at 25 C and 1.0 V and (b) Current versus Time 

curve for the different concentrations of CH4 tested at 300 C for the porous SnO2 nanorod sensor. The inset is a 

TEM image of a porous SnO2 nanorod (Ref. 35). 
NASA/TM—2015-218826 10 

Alignment of MWCNTs on a 2-in. Alumina Wafer 

Uniform, multiple sensor processing across a wafer (batch processing) is an objective if 

microfabrication techniques are to be fully applied to the processing of sensors based on nanostructures 

and to allow such sensors to be mass produced and thus applied in broader applications. This present 

work describes efforts to move from the fabrication of a limited number of sensors at a time to the mass 

production of sensors with uniformly controlled properties. As a demonstration of the proof-of-concept of 

this whole wafer approach, multi-walled carbon nanotubes (MWCNTs) were used from NanoLab Inc. 

(Ref. 36). The diameters of the MWCNTs typically range from 10 to 20 nm with lengths typically ranging 

from 5 to 20 microns (μm). 

The photomask used for this whole wafer approach was designed in such a way that the arrays of 

paired patterns on an alumina wafer were electrically connected, so that an electrical field applied across 

one set of sawteeth electrodes was simultaneously applied to the full array of multiple sawtooth paired 

contact patterns on the wafer, as shown in Figure 7(a). There are 16 patterns, each with 146 opposing 

sawtooth electrodes for a total of 2336 electrodes on the whole wafer. As can be seen in Figure 7(b), 

probe contacts made on two outside electrodes establishes electrical connection to all the individual 

sawtooth patterned electrodes. Furthermore, the uniqueness of this design is that it allows for the selective 

dicing of different sized individual sensors (red squares on Fig. 7(b)). Figure 7(c) magnifies one of the 

sawtooth electrode patterns to show further detail of the electrode design. The numbers on the bottom are 

the number of sawtooth on each electrode. This further demonstrates the versatility of this design where 

individual sensors can be diced into different sizes. More sensors (sawteeth) translate into larger signals, 

which translate to lower detection levels. Hence, the size of the sensor chosen may be determined by the 

application in which they are going to be used. 

The fabrication steps for this whole wafer array were similar to the one-sensor approach described 

previously. The major change was the design of the new photomask to accommodate as many sensors as 

possible on a 2-in. alumina wafer and to electrically connect them. Platinum (Pt) at a thickness of 2450 Å 

was used as the bottom electrodes due to its stability and compatibility to the other materials in the

Figure 7.—(a) Whole wafer photomask design. (b) Photograph of 2-in. alumina wafer with the Pt 

electrodes showing where electrical contact is established with the probes on the two outside 

electrodes. Red squares illustrate the different ways that individual sensors can be diced. 

(c) Enlargement of one of the patterns to show further detail of the electrode design. The numbers 

on the bottom are the number of opposing sawtooth on each electrode. 
NASA/TM—2015-218826 11 

structure, as well as its capability to enhance the reaction of the nanotubes through its catalytic properties. 

The Pt was sputter deposited (32W DC, 4 millitorr, Ar/O2 (80%/20%)) on a polished alumina wafer and 

patterned using conventional lift-off techniques. The spacing between sawteeth was approximately 1 to 

2 μm: a distance small enough to capture the shorter nanorods as well as generate a stronger electric field. 

Shipley S1805 photoresist (Ref. 37) was used as the dispersing medium for the MWCNTs because of its 

low viscosity. A concentration of 0.03 g of MWCNTs in 15 mL of S1805 was used for this proof-ofconcept demonstration. The suspension was sonicated to thoroughly mix the nanotubes within the 

solution and to break up any clumps of MWCNTs. After the solution was prepared, it was deposited onto 

the existing electrode pattern using a pipette. Tungsten probes were placed on the outside electrodes using 

a standard probe station, and an AC field (20 MHz, 10 Vp-p) was generated for 20 min using a function 

generator to perform the dielectrophoresis technique and align the MWCNTs. A sinusoidal signal was 

used because it was convenient and maintained the linear relationship between the effective dipole 

moment and the electric field (Ref. 27). Following the alignment, the probes were removed and the wafer 

underwent typical photoresist processing steps to complete the deposition of the top layer electrodes as 

shown in step 6 of Figure 4 (Refs. 21 and 33). 

Under scanning electron microscopy (SEM) examination, it was observed that MWCNTs 

successfully bridged 2,313 of the 2,336 electrodes for a 99 percent yield, using this fabrication approach. 

This can be seen in Figure 8 where different SEM images show alignment of the MWCNTs between 

opposing sawtooth electrodes. An important aspect shown here is that the MWCNTs are only observed to 

be between the electrodes and not anywhere else on the substrate. 

Figure 8.—(a) Low magnification SEM image to show alignment 

on more than one sawtooth electrode. (b) Alignment of a few 

nanotubes between opposing electrodes. (c) Alignment of 

more nanotubes between opposing electrodes. (d) Image of 

alumina substrate showing that there are no nanotubes (or a 

little amount) outside of the electrode area demonstrating that 

the nanotubes are predominately found between opposing 

electrodes. 
NASA/TM—2015-218826 12 

Conclusions and Future Work 

In summary, we have demonstrated a standardized approach to chemical sensor processing using 

nanostructures and extended it to the mass production of sensors. This has been achieved by the 

integration and alignment of nanostructures with microfabrication methods. A photomask was designed to 

maximize the incorporation of sensors on a 2-in. alumina wafer and to electrically connect them. The 

design also made possible selective dicing of individual sensors into different sizes. The approach was 

successful in obtaining a 99 percent yield of alignment across the electrodes with MWCNTs. The 

combination of including nanostructures in a processing solution, and then using the electromechanical 

nanostructure alignment, has a range of processing implications. The low cost of the simple batch 

fabrication processes and its compact microsize has the potential for a highly affordable sensor that could 

be installed in a wide range of locations. This approach is intended to address the significant barriers of 

deposition control, contact robustness, and simplified processing to realize the potential of 

nanotechnology as applied to sensors. The resulting sensors can be used in applications where 

microsensors are presently used, but with the addition of nanostructured materials, sensor system 

capabilities can be improved. 

Significant work remains towards improving the process and realizing the potential of nanostructures 

for applications such as chemical sensing. Further refinement of the DEP technique and photoresist 

suspension of nanostructures is planned to increase and to better control the number of nanowires 

bridging each paired contact pattern. It has being determined that a minimum frequency of ~1 MHz is 

needed for alignment of nanostructures and that the peak-to-peak voltage used can be chosen arbitrarily as 

it only defines the strength of the generated dielectrophoretic force; the dielectrophoretic force depends 

more on the frequency as shown in Equations (3) and (4). Current work is also ongoing in applying this 

mass production approach with other types of nanostructures, such as metal and metal oxides. 

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