nanoscale plasmonic lithography on silicon

7/25/2019 Nanoscale Plasmonic Lithography on Silicon

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Soft Lithography

Soft lithography is a technique for fabricating or replicating structures usingelastomeric stamps (most notably PDMS), molds, and conformable photomasks.

Elastomer is a polymer with viscoelasticity and very weak intermolecular forces,

generally having low Youngs modulus and high failure strain.

The advantages of soft lithography are:

Lower cost in mass production;

Well-suited for applications in biotechnology and plastic electronics;

Well-suited for applications involving large or non-planar surfaces;

More pattern transferring methods (more ink options);

Does not need a photo reactive surface to create nanostructures;

Smaller details than photolithography in laboratory settings. The resolution

depends on the mask used.

Soft Lithography Process

1. Inking a stamp.


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2. ODT has attached to the stamp.

3.

The PDMS stamp with the ODT is placed on the gold substrate. When the

stamp is removed, the ODT in contact with the gold stays stuck to the gold.

PDMS (Polydimethylsiloxane)

PDMS or polydimethylsiloxane belongs to a group of polymeric

organosilicon compounds.

It is the most-widely used silicon-based organic polymer and particularly

known for its unusual rheological properties.

It is optically clear, and in general, inert, non-toxic, and non-flammable.


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Its chemical formula is CH3[Si(CH3)2O]nSi(CH3)3.

PDMS Mechanical Properties

PDMS is viscoelastic, meaning that at long flow times (or high

temperatures), it acts like a viscous liquid, similar to honey. However, at

short flow times (or low temperatures), it acts like an elastic solid, similar to

rubber.

In other words, if some PDMS is left on a surface overnight (long flow time),

it will flow to cover the surface and mold to any surface imperfections.

However, if the same PDMS is rolled into a sphere and thrown onto the

same surface (short flow time), it will bounce like a rubber ball.

The shear modulus of PDMS varies with preparation conditions, but is

typically in the range of 100 kPa to 3 MPa. The loss tangent is very low

(tan 0.001).


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PDMS Chemical Compatibilites

After polymerization and cross-linking, solid PDMS samples will present an

external hydrophobic surface. This surface chemistry makes it difficult for

polar solvents (such as water) to wet the PDMS surface, and may lead to

adsorption of hydrophobic contaminants.

Plasma oxidation can be used to alter the surface chemistry,

adding silanol (SiOH) groups to the surface. Atmospheric air plasma &

argon plasma will work for this application.

This treatment renders the PDMS surface hydrophilic, allowing water to wet

it. This is frequently required for water-based microfluidics. The oxidized

surface resists adsorption of hydrophobic and negatively charged species.

PDMS Applications

Applications:

Surfactant and antifoaming agents

Hydraulic fluids and related applications

Medicine and cosmetics

Foods (cooking oils, processed foods)

Domestic and niche uses

PDMS Stamp Process Scheme

1. PDMS master is created by patterning silicon, pouring and curing the

PDMS, and peeling away from the substrate.


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2. Thiol is poured over the stamp and let dry. Conformal contact is made with

the substrate and pattern is left behind.


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PDMS Stamp Process

1.

Preparing the master

a) The master is typically created on silicon, but can be done on any solid

patterned surface.

b) Photoresist is applied to the surface and patterned by a photomask and

UV light.

c) The master is then baked, developed and cleaned before use.

In typical processes the photoresist is usually kept on the wafer to be used as a

topographic template for the stamp. However, the unprotected silicon regions can

be etched, and the photoresist stripped, which would leave behind a patterned

wafer for creating the stamp. This method is more complex but creates a more

stable template.

2. Creating and Inking the PDMS stamp

Creating the PDMS stamp:

After fabrication the master is placed in a walled container, typically a petri dish

and the stamp is poured over the master.

Inking the stamp:

Inking of the stamp occurs through the application of a thiol solution either by

immersion or coating the stamp with a Q-tip. The highly hydrophobic PDMS

material allows the ink to be diffused into the bulk of the stamp, which means the

thiols reside not only on the surface, but also in the bulk of the stamp material. This

diffusion into the bulk creates an ink reservoir for multiple prints. The stamp is let

dry until no liquid is visible and an ink reservoir is created.


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3. Applying the stamp to the substrate

Direct contact

The stamp is brought to physical contact with the substrate and the thiol solution is

transferred to the substrate.

Other techniques

Printing of the stamp onto the substrate can also take place with a rolling stamp

onto a planar substrate or a curved substrate with a planar stamp.

PDMS Stamp Technical Problems

Stamp deformation

Deformation of the stamp can occur during removal from the master and during the

substrate contacting process. When the aspect ratio of the stamp is high buckling of

the stamp can occur. When the aspect ratio is low roof collapse can occur.

Substrate contamination

During the curing process some fragments can potentially be left uncured and

contaminate the process. When this occurs the quality of the printed SAM is

decreased. When the ink molecules contain certain polar groups the transfer of

these impurities is increased.

Shrinking/swelling of the stamp

During the curing process the stamp can potentially shrink in size leaving a

difference in desired dimensions of the substrate patterning. Swelling of the stamp

may also occur. Most organic solvents induce swelling of the PDMS stamp.

Ink mobility

Ink diffusion from the PDMS bulk to the surface occurs during the formation of the

patterned SAM on the substrate. This mobility of the ink can cause lateral

spreading to unwanted regions. Upon the transfer this spreading can influence the

desired pattern.


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PDMS Stamp Advantages

Advantages:

The simplicity and ease of creating patterns with micro-scale features;

Can be done in a traditional laboratory without the constant use of a

cleanroom (cleanroom is needed only to create the master);

Multiple stamps can be created from a single master;

Individual stamps can be used several times with minimal degradation of

performance;

A cheaper technique for fabrication that uses less energy than conventional

techniques.

Plasmonic Nanoparticles

Plasmonic nanoparticles are particles whose electron density

can couple with electromagnetic radiation of wavelengths that are far larger than

the particle due to the nature of the dielectric-metal interface between the medium

and the particles: unlike in a pure metal where there is a maximum limit on what

size wavelength can be effectively coupled based on the material size.

The nanoparticles can form clusters to form plasmonic molecules and interact with

each other to form cluster states. The symmetry of the nanoparticles and the

distribution of the electrons within them can affect a type of bonding or

antibonding character between the nanoparticles similarly to molecular orbitals.

Since light couples with the electrons, polarized light can be used to control thedistribution of the electrons and alter the mulliken term symbol for the irreducible

representation. Changing the geometry of the nanoparticles can be used to

manipulate the optical activity and properties of the system, but so can the

polarized light by lowering the symmetry of the conductive electrons inside the

particles and changing the dipole moment of the cluster. These clusters can be used

to manipulate light on the nano scale.


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Applications of Plasmonic Nanoparticles

Plasmonic solar cells

Due to their ability to scatter light back into the photovoltaic structure and

low absorption, plasmonic nanoparticles are under investigation as a method for

increasing solar cell efficiency. Forcing more light to be absorbed by the dielectric

increase efficiency.

Spectroscopy

Plasmonic nanoparticles have been explored as a method for high

resolution spectroscopy. One group utilized 40 nm gold nanoparticles that had

been functionalized such that they would bind specifically to epidermal growth

factor receptors to determine the density of those receptors on a cell.

Cancer treatment

Preliminary research indicates that the absorption of

gold nanorods functionalized with epidermal growth factor is enough to amplify

the effects of low power laser light such that it can be used for targeted radiation

treatments.

Surface Plasmon Resonance (SPR)

Surface plasmon resonance(SPR) is the resonant oscillation of conduction

electrons at the interface between a negative and

positive permittivity material stimulated by incident light.

The resonance condition is established when the frequency ofincident photonsmatches the natural frequency of surface electrons

oscillating against the restoring force of positive nuclei. SPR in

subwavelength scale nanostructures can be polaritonic or plasmonic in

nature.

SPR is the basis of many standard tools for measuring adsorption of

material onto planar metal (typically gold or silver) surfaces or onto the


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surface of metal nanoparticles. It is the fundamental principle behind many

color-based biosensor applications and different lab-on-a-chip sensors.

Localized Surface Plasmon

A Localized Surface Plasmon (LSP) is the result of the confinement of a surface

plasmon in a nanoparticle of size comparable to or smaller than the wavelength of

light used to excite the plasmon. The LSP has two important effects:

(1) Electric fields near the particles surface are greatly enhanced. This

enhancement falls off quickly with distance from the surface;

(2) The particles optical excitation has a maximum at theplasmon resonant

frequency. For noble metal nanoparticles, this occurs at visible wavelengths.

Hydrosilylation

Hydrosilylation, also called catalytic hydrosilation, describes the addition

of Si-H bonds across unsaturated bonds. Ordinarily the reaction isconducted catalytically and usually the substrates are unsaturated organic

compounds. Alkenes and alkynes give alkyl and vinyl

silanes; aldehydes and ketones give silyl ethers. The process was first

reported in academic literature in 1947.

Hydrosilylation involves the insertion of an unsaturated bond, in this case

alkene, into a silicon-hydride group on the surface, which results in

attachment of the alkyl to the silicon via a SiC bond. This reaction is of

broad interest because of its utility for covalent interfacing of a broad

range of organic molecules to silicon surfaces. The reaction pathways

proposed for low energy light-induced hydrosilylation reactions on the

silicon surface typically involve electronhole pairs, since visible light is

incapable of homolytic SiH bond cleavage or electron photoemission.


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Proposed Mechanism for the Plasmonic Stamp-Assisted Hydrosilylation

Scheme of Hydrosilylation Reaction on Silicon Surface

a. Hydrosilylation of an alkene on Si(111)H, resulting in SiC bond formation and

covalent attachment of the molecule to the surface.


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b. Plasmonic stamp-assisted photohydrosilylation on a Si(111)H surface. A

continuous wave bandpass filter, where it shines upon the optically transparent,

PDMS-based plasmonic stamp with embedded gold nanoparticle patterns.

Hydrosilylation occurs in a spatially defined fashion on the Si(111)H surface to

produce a hydrosilylated surface pattern that mirrors that of the parent gold

nanoparticle pattern in the plasmonic stamp.

Fabrication of Plasmonic Stamps

a.

Conversion of a gold nanopattern on silicon, prepared via block copolymerself-assembly, into a plasmonic stamp. A gold nanopattern is produced using a

self-assembled block copolymer (BCP) template and then lifted off the Si/SiOx

surface embedded within the cured PDMS to yield the plasmonic stamp.

b. Atomic force microscopy (AFM) height map and section analysis of a gold

nanopattern on silicon surface, produced via self-assembly of the BCP PS-b-

P2VP (molecular weight of 125 k-b-58.5 k). The scale bar is 100 nm. (c) AFM

height map and section analysis of the plasmonic stamp surface. The scale bar

is 100 nm. (d) Schematic image and optical photograph of a plasmonic stamp

(on a reflective surface). The scale bar is 2 mm. (e) UV_vis spectrum of a PDMS

plasmonic stamp showing the absorption at 530 nm due to the embedded gold

nanoparticles.


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Block-copolymer Self Assembly

Self-assembly is a process in which a disordered system of pre-existing

components forms an organized structure or pattern as a consequence of

specific, local interactions among the components themselves, without

external direction. When the constitutive components are molecules, the

process is termed molecular self-assembly.

Block copolymers comprise two or more homopolymer subunits linked by

covalent bonds. The union of the homopolymer subunits may require an

intermediate non-repeating subunit, known as a junction block. Block

copolymers with two or three distinct blocks are called diblock

copolymers and triblock copolymers, respectively.

BCP-based films are compatible with existing silicon-based lithography due

to the central role played by organic photoresists that are handled in much

the same manner.


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Pattern Modification/Visualization using 10 nm Gold Nanoparticles

(a-c) Schematic outline of pattern visualization via multiple surface modification

reactions. The BCP used for generating the gold nanoparticle pattern within the

plasmonic stamp was PS-b-P2VP. Hydrosilylation with 1,7-octadiene resulted in

alkene-terminated patterns; a UV-mediated thiolene addition of 1,6-hexanedithiol

resulted in thiolterminated patches. Exposure of this patterned surface to gold

nanoparticles resulted in binding only to the thiolterminated areas on the silicon

surface. (d) Scanning electron microscopy (SEM) micrograph of the patterned

silicon surface before exposure to gold nanoparticles. (e) SEM micrograph of

patterned silicon surface after exposure to 10 nm Au nanoparticles. (SEM scale

bar: 100 nm)


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AFM height maps of silicon surfaces with 1-dodecene and plasmonic stamps

(a-c) Effect of time on illumination of a Si(111)H surface with 1-dodecene, through

a plasmonic stamp, from 0 (a), 30 (b), and 60 min (c).

(d-f) Control over pattern spacing using plasmonic stamps prepared with BCPs of 3

different molecular weights: center-to-center spacings of PS-b-P2VP and

illumination time of 60 min was 70 nm (d), PS-b-P2VP (91.5 k-b-105 k) was 120

nm (e), and PS-b-P2VP was 160 nm (f). All scale bars correspond to 200 nm.

Discussion

From the above results, it appears that the local hydrosilylation reaction is driven

by the plasmon resonance of the Au nanoparticles. As such, energy is being

transferred from the Au nanoparticles/plasmons to the 1-dodoecene/Si(111)-H

interface. Upon excitation, the plasmon can decay radiatively through resonant

photon scattering or nonradiatively via the generation of an electron_hole pair

(EHP), typically referred to as hot carriers. These hot carriers can be directly


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injected into the surrounding medium, or they can recombine, which results in local

heating of the nanoparticle. Lastly, EHPs can be directly generated in the Si as a

result of the high intensity electromagnetic field of the LSPR, which is localized to

the neighborhood of the gold nanoparticle. Given that all four of these mechanisms

(resonant photon scattering, nanoparticle plasmonic heating, hot carrier injection

and near-field electromagnetic EHP generation) could be driving the local

hydrosilylation reaction, their relative contributions are discussed below. Efficiency

enhancement due to the resonant photon scattering or forward light scattering of

metal nanoparticles has been suggested in the field of light scattering-enhanced

solar cells. When metal nanoparticles are located on a solar cell surface, thesecondary radiation of incident electromagnetic energy scattered into the active

layers of solar cells, referred to as the forward light scattering, can increase the

effective optical length by light trapping in the active layers which could lead to

an increased rate of EHP generation at the silicon surfaces. However, the size of

the Au nanoparticles in the plasmonic stamp is 1015 nm, which is in the range in

which LSPR absorption would be dominant; the forward scattering intensity would

be less than 0.1% of incident light, according to previously reported simulations.

The next mechanism to be considered is in situ plasmonic heating, which could result

in a local increase of temperature of the gold nanoparticles due to the strong light

absorption at the plasmon resonance, which is dissipated as heat. For thermal

heating to play a role in hydrosilylation, the silicon surface would need to reach

temperatures in excess of 100C, within the time scale of experiments performed

in this work. The temperature increase, T, of a single isolated gold nanoparticle

can be estimated using the conventional model:

where is the particle absorption cross section, I is the incident light intensity, k is

the thermal conductivity of the surrounding medium, and D is the nanoparticle

diameter. Assuming a scattering cross section of 500 nm2, a thermal conductivity of

0.15 W/m.K (corresponding to PDMS), a nanoparticle diameter of 15 nm, and an

intensity of 50 mW/cm2, the predicted temperature increase would be 1.8x10-5

C. It is worth noting that steam generation has been observed for dilute solutions


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of light absorbing nanoparticle using moderate light intensities of 10 mW/cm2but

this phenomenon has been shown to be a result of enhanced light absorption due

to efficient volume scattering between nanoparticles in solution. A similar light

trapping effect is unlikely to play a role here since the nanoparticles are isolated

on a single plane. Moreover, in order for patterned surface hydrosilylation to

occur, it would be necessary for the temperature increase to be localized directly

underneath the nanoparticles such that the surrounding silicon does not react.

The third mechanism to consider is hot carrier injection from the gold nanoparticles

into the surrounding medium. In the event of nonradiative decay of the plasmon

(which is the dominant plasmon decay mode for nanoparticles much smaller than

the plasmon wavelength), an energetic EHP is formed, typically referred to as hot

carrier generation. These hot carriers have been extensively studied in catalytic

water-splitting, where it has been shown that the hot carriers are directly injected

into the semiconductor surface, facilitating the water-splitting reaction. In the case

of hydrosilylation, if a hole is injected into the silicon surface from the gold

nanoparticle, the hydrosilylation reaction can proceed via nucleophilic attack by an

alkene on a positively charged surface silicon site, as has been repeatedly

postulated.

Lastly, the effect of the highly concentrated electromagnetic field produced by the

LSPR is considered. It has been shown that the rate of EHP generation in a

semiconductor is proportional to the local intensity of the electric field. Given that

the electric field enhancement is on the order of ~103 at the edges of the

nanoparticle, the relative concentration of EHPs at the silicon surface (within the

neighborhood of the gold nanoparticle) would also be expected to ~103 times

higher. As such, the kinetic rate of the hydrosilylation reaction would proceed much

faster in these areas of increased EHP concentration.


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CONCLUSION

Soft lithography is a technique for fabricating or replicating structures using

elastomeric stamps (most notably PDMS), molds, and conformable photomasks.

The gold nanopatterns which produced via block copolymer self-assembly can be

incorporated into an optically transparent flexible PDMS stamp, termed a

plasmonic stamp, and used to directly functionalize silicon surfaces on a sub-100

nm scale.

Localized surface plasmons in a gold nanopattern, embedded within an optically

transparent plasmonic stamp, can be used to drive hydrosilylation of alkenes on

silicon surfaces, upon illumination with low intensity light with wavelengths that

corresponded to the absorption profile of the gold nanoparticles. The gold

nanopatterns were created via block copolymer self-assembly, but could

presumably be produced through other lithographic nanopatterning approaches.


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SOURCE

ACS Nano, 2015, 9(2), pp 21842193

Chem. Mater., 2014, 26(1), pp 763772

Micromachines2012, 3, 21-27

http://en.wikipedia.org/

nanoscale plasmonic lithography on silicon

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