Jackson M.J. Micro and Nanomanufacturing

Подождите немного. Документ загружается.

Micro- and Nanomanufacturing 653

Replica molding is used to make topographically complex struc-

tures using a soft PDMS stamp to replicate non-conformal surfaces

on substrates. The process has been used to produce diffraction grat-

ings,

chirped, blazed diffraction gratings on curved surfaces, pat-

terned microfeatures on hemispherical domes, and arrays of rhombic

microlenses. Figure 12.16 shows the process procedure for making

a variety of structures.

PDMS

silicon

• photoresist

I peel off PDMS

PDMS

1) compress

2) mold PU

I PDMS \[.

force

1) cure PU

2) peel off

finite-elemeni analysis

(30%

strain)

PDMS

|ens__p^a 50 |irn

1-2

PU J cure

PU,

oeel oft PDMS

-^ glass

tube

deform PDMS

, 2) mold PU

n-i

I cjre

PU,

peel off

Fig. 12.16. Schematic illustration of replica molding against an elastomeric

PDMS mold under: (a) mechanical compression; (b) bending; and (c) stretching

[5]

654 Micro- and Nanomanufacturing

Microtransfer molding is a process that produces very thin layers

of patterned surfaces that can be used to build three-dimensional

products with nanoscale features. A selection of products made us-

ing this technique is shown in Fig. 12.17.

Fig. 12.17. Images of structures produced using microtransfer molding [5]

Micro- and Nanomanufacturing 655

Another future method of nanomanufacturing is micromolding in

capillaries (MIMIC). The process allows non-planar surfaces to be

patterned using a network of channels that are filled with pre-

polymer which is cured and forms a solid network of small-scale

features. This process has been used with materials such as ceram-

ics,

biological materials, functional and structural polymers, inor-

ganic salts, polymer beads, colloids, and sol-gel materials. Func-

tional devices have been created using MIMIC and are described in

depth by Xia and Whitesides [5]. The devices made include: Schot-

tky diodes, GaAs/AlGaAs FETs, and silicon MOSFETs.

SAMIM is a process that uses solvents to form small channels in

micro-and nanoscale products that is also a contender for develop-

ment into a nanomanufacturing process [6]. The process can be

used for making MEMS and NEMS devices such as micro-and nan-

ofluidic devices as shown in Fig. 12.18.

Nanomanufacturing by molding will become more prevalent in

the future because of the replication and reproducibility of feature

size achievable using mechanical methods of lithography, and due to

the fact that non-planar, convex and concave surfaces can be pat-

terned with ease compared to optical lithographic methods. The

molding of polymeric materials will experience the largest growth

due to the construction of all polymer transistor circuits and micro-

processors

[7,8].

12.3.4 Nanoimprint Lithography

Nanoimprint lithography is a very promising low-cost nano-

manufacturing process that involves two steps. The first step is "im-

printing" and this relies on deforming a thin resist film using a mold

with nanostructures imprinted on it. In the "pattern transfer" step, an

etching process such as reactive ion etching (RIE) is used to remove

residual resist material. The resist can be a thermoplastic, thermal

curable polymer, or other deformable material. The process is a me-

chanical deformation process as opposed to soft lithography that im-

parts ink with colloidal fluid or self-assembled molecules to the sub-

strate. The resolution is dependent on the mechanical strength of the

mold but resolution as low as 5 nm are achievable.

656 Micro- and Nanomanufacturing

soft Uthogrnphic

techniques

MIMiC -

HTM

HCP

—

SAMIM -

REM™-

•100-

k- 10-

t-0.1-

-0,01-

applications

1 -MEMS

! '•

sensors

—. microreaclors

J combinatorial arrays

j mfcroanalyticai systems

L biosurfaces and materials

— microelectronics

- optical systenns

• nanoeleclronic devices

-"' 2 (Am

oa.

&0 rim

Fig. 12.18. Images of nanostructures created using MIMIC and a comparison of

the micromolding techniques with their respective application to the micro-and

nanoscales [5]

Micro- and Nanomanufacturing 657

-HH-jOnm

-^N-10nm

"Hk-10nm

-- • • • • •

Fig. 12.19. Silicon mold with 10 nm diameter pillars that have imprinted holes

into pieces of PMMA that are 60 nm deep and 40 nm apart from each other [9]

In general small holes are easier to imprint than pillars because

they tear off the mold quite easily. Figure 12.19 shows images of

molds and imprinted materials.

Nanoimprint lithography can be performed in one operation as a

nanofabrication process but its future lies in its development as a

nanomanufacturing process. This has recently been achieved by de-

veloping the roller nanoimprint lithography technique that uses a

rolling mill to create nanofeatures.

Such features have been created in the form of microwave tran-

sistors T-gates and PMMA gratings. Figure 12.20 shows the roller

nanoimprint process and transistor T-gates manufactured using the

process. An interesting review of nanoimprint lithography has re-

cently been published by Guo [10] who advances the ideas created

by Chou [9] by commenting on the development of step-and-flash

nanoimprint lithography, and combining processes such as photo-

lithography and nanoimprint lithography to form hybrid processes

such as reverse nanoimprint lithography.

658 Micro-and Nanomanufacturing

imprinting using a cylinder mold

i^H^S^13^-!3'#%^-tf4

Substrate

Imprinting using a flat mold

Fig. 12.20. Schematic diagram of roller nanoimprint lithography and two images

of microwave transistor T-gates with footprints of 40 nm and 90 nm, respectively

[9]

When combined with polymer inking techniques, a process is

created that has common features associated with soft lithography

and nanoinprint lithography. The simplicity associated with nano-

imprint lithography has made it a strong candidate for a future

nanomanufacturing process that can imprint nanoscale features and

deposit a wide range of colloidal particles and self-assembled

monolayers.

12.3.5 Lithographically Induced Self-Assembly

A variation of the nanoimprint lithography process is the litho-

graphically induced self-assembly process, LISA (Fig. 12.21). This

process allows pillars to "grow" from the surface of a viscous liquid

between substrate and a mask. The self-formation of pillars forms a

pillar array that can be the basis of creating three-dimensional

Micro- and Nanomanufacturing 659

nanostructures. A variety of patterns have been observed depending

on the template shape and size as well as the polymer used in the

experiments. Processing conditions are also of vital importance

when creating nanostructures. When processing conditions change,

concentric rings are formed under certain conditions. Dynamic

growth of the pillars has been observed using video techniques that

show that pillars form one-by-one at the comer of the mask then at

the edges propagating to the center. Figure 12.22 shows the protrud-

ing triangular pattern on the mask along with an AFM image of the

surface of the features produced.

Spin polymer

HOMOPOLYMER

Si SUBSTRATE

Place mask with spacers

SPACER

Si SUBSTRATE

3. Heat, Induce self-assembly

^^^-WIASK_^^

^HHMHI

Si SUBSTRATE |

Fig.

12.21.

Lithographically induced self-assembly process [9]

660 Micro- and Nanomanufacturing

Fig. 12.22. LISA process showing: (a) protruding triangular pattern; (b) PMMA

pillar array formed under the triangular pattern; (c) AFM image of pillar array;

and (d) Shape and size of pillar arrays, which are 310 nm height, 5 // m diameter,

and 9 // m spacing [9]

Applications of LISA include semiconductors, metal nanostruc-

tures,

and biological arrays. Periodic arrays created by LISA are

useful in memory devices, photonic materials, and biological struc-

turing of hard and soft tissue. The future of both nanoimprint lithog-

raphy and LISA lies in their ability to be adapted to manufacturing

large-scale nanostructures using rollers and stamps.

Micro-and Nanomanufacturing 661

12.3.6 Dip Pen Nanomanufacturing

Dip pen nanolithography can be adapted to deposit self-assembled

monolayers in a manufacturing context by simply adding additional

dip pen probes to the cantilever beam and adding reservoirs in order

to deposit a multitude of nanoscale particles such as colloidal metals,

molecules, and carbon nanotubes etc. This type of arrangement can

be used to form nanoscale features very quickly by modifying an

atomic force microscope. Figure 12.23 shows the construction of an

array of "nanopens" that can directly write "ink" to the surface. The

deposition of molecules by the nanoplotter is shown in Fig. 12.24.

The process involves depositing the ink to a surface then selectively

etching around the molecules to create trenches and removing the

residue left behind. The resulting structure is viewed using the

atomic force microscope to check topographic features of

the

nanos-

tructure that has been created [11].

Another promising way of creating nanofeatures using a deposi-

tion process is ink jet printing, which was first used for depositing

microstructures to substrates by Sachs at MIT.

Laser

Detector

V"^"^"

X—^"

Flexible

^''^^^^'^^^^

cantilever array

Pen1 Pen 2 Pen 3 Pen 4 Pen5 Pen 6 Pen 7 ?enB< Nano-Pen

Till Stage

:::::£.::.

Lslefal translation stage

9§

Inkwells

Fig. 12.23. Dip pen nanomanufacturing using the nanoplotter that dispenses a va-

riety of

inks,

or one ink deposited multiple times [11]

662 Micro- and Nanomanufacturing

Molecules written

<><>>> >>S> yyyy

'" ^y

nanoplotter

^^...*,rf..»,.._.„*..A.*.^ .#,*.M„rf. —.lO-nm Au layer

'-"R C~ 5-nm

layi

.J ^"SiO, layer

layer

E iii5 iii5 iii5

ra iiii i^ ,iiii

}r'iy77rt-|^ yrr-rr^i^-A^

^rrr-Trr,\^

rSi(110^ tSi(IOO)

Fig.

\1,1A.

Deposition of molecules written by the nanoplotter on to

gold layer

on silicon. The AFM topographic image shows the nanometer resolution afforded

by the process [11]

The ink jet printing of polymer transistors has resulted

in a

re-

markable level of resolution and is

non-contact printing technique

that is environmentally friendly and can be adapted for nanomanu-

facturing processing by adding more nozzles

the ink jet printer.

Inkjet printing allows the precise control of liquid droplets forming

on the surface of the substrate. Figures 12.25 and 12.26 show the

resulting structures created by Inkjet printing [12].