Jackson M.J. Micro and Nanomanufacturing

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Micro- and Nanomanufacturing 643

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Fig. 12.7. Labcard product by ACLARA showing hot embossed microchannels

and reservoirs.

The development of fabricating three-dimensional microfluidic

devices has led to the development of orthogonal hot embossing

with soft tools for manufacturing on the mass scale. This technique

is shown in Figures 12.10 and 12.11 and is leading the way to the fu-

ture development of hot embossing three-dimensional microstruc-

tures.

644 Micro- and Nanomanufacturing

Photoresist

(AZ4620

SU-8)

Silicon

Force

PDMS tool

<^,

Elastom

m ixture

Silicon

ttt

Fig.

12.8. Hot embossing with soft PDMS tools

?3>9»{

i}D:37HR $:e8aee P'^eaBe

zi.n

»xu HD^oitn

s^m«9

tmi

90 jjiii thick

PDIVB

tool.

An array of

150

loiiiwide

features with

250

)jni center-to-center spacing

90 |jm deep PMMA channel. An array of 300 pni wide

features with 500 ^ni center-to-centCT spacing

J. Narasimhan and

Papautsky,

Mcromeck

Mcroeng.,

vol.

14, pp.

96-103,2004.

Fig.

12.9. Soft PDMS tool showing impression left after embossing PMMA

thermoplastic material [2]

Micro- and Nanomanufacturing 645

AZ4620 SU-8

Silicon

Force

PDMS Tool

T^mwiwc

^J^^

Elastomer

mixture

'""PIWySBJP'

Twwr

Fig. 12.10. Orthogonal hot embossing with soft PDMS tools

PDMS embossing tool illustrating the 20 |im deep

rectangular indents (250

jxm

x 500 |im) on the 110 ^m thick

structure

Embossed orthogonal 3-D PMMA channel

J. Narasimhan and I. Papautsky,

Micromech.

Microeng.,

vol.

14,

pp. 96-103, 2004.

Fig.

12.11.

PDMS embossing tools and orthogonal channel in PMMA material

[2]

646 Micro- and Nanomanufacturing

The critical issue for the future development of micromolding

techniques such as hot embossing with soft tools is concerned with

the faithful replication of the master pattern during hot embossing,

which requires the application of a uniform force for the appropriate

cycle time on the tool-polymer sandwich. This criterion in turn re-

quires the embossing tool to be uniform in thickness throughout,

which is achieved by casting on a highly flat and leveled hot plate.

The embossing parameters are different from those normally used

with Ni or Si tools, in part due to a much lower thermal conductivity

of PDMS (^0.2 W/mK) compared with Ni (--150 - 500 W/mK) or Si

(157 W/mK) and the soft nature of the tool iself The lifetime of soft

tools have also been determined to last approximately twenty mold-

ings with aspect ratios between 2 and 4, creating a minimum feature

size of about

// m. The use of soft tools and polymers looks set to

open up future opportunities for creating all polymer transistors, cir-

cuits,

and microprocessors.

12.2.4 Machining

The process of machining has always been a strong contender for

playing a role in the future of micromanufacturing. The ability to

machine any material guarantees that micromachining will continue

to be a micromanufacturing process of choice. In recent years, the

size of the cutting tool has become smaller and smaller, and as such

very small tools are now being fabricated by laser micromachining

and ion beam milling processes [4]. Figure 12.12 shows a collection

of cutting tools fabricated by ion beam milling. Laser micromachin-

ing shows promise in being able to remove very small areas of mate-

rial using extremely high fluences. This can be achieved using fem-

tosecond pulsed lasers and future applications of this type of laser

may include sharpening very small diamonds that can be attached to

tips such as AFM probes. The AFM may be developed as a

nanomachining tool that can cleave the bonds between atoms.

Micro- and Nanomanufacturing 647

\xm

••3*^"

1:2teM^^8fic:%-^v^

Fig. 12.12. Focused ion beam milled diamond cutting tools [4]

Diamond and titanium based cutting tools are required to ma-

chine a wide variety of engineering materials. Developments in

PVD and CVD coating technologies will also be fundamental to the

future development of micromachining cutting tools. The future de-

velopment of very stiff machine tools will also figure significantly in

648 Micro- and Nanomanufacturing

the future too. Developments in this area have already begun by

various research institutions in the United States and are described

as "meso-micro machine tools" (mMTs).

12.2.5 Future Developments in Micromanufacturing

The development of micromanufacturing processes that use silicon-

based materials are already highly developed. The next stage in the

development of micromanufacturing processes lies in the ability to

manufacture micro-products from engineering materials such as

metals and ceramics. Processes that show promise in promoting the

next stage of development include selective laser sintering of metals,

laser micromachining using pulsed femto- and attosecond lasers,

droplet-based manufacturing, and cold gas dynamic molding and

spraying of microparts. Combinations of these techniques and es-

tablished methods of manufacturing that produce hybrid manufactur-

ing processes will create the short term "stepping stones" required to

meet the demand generated to economically manufacture microscale

products.

12.3 Nanomanufacturing

12.3.1 Semiconductor IVIanufacturing

Semiconductor manufacturers have made great strides in the past

few years overcoming the diffraction limit of light waves to produce

nanoscale transistors. The scale of engineering the limit to favor the

semiconductor industry involves the use of extreme ultra violet light

sources and soft "x-rays". However, the use of photolithographic

processes to produce transistors at length scales of the order of tens

of nanometers will be reached within the next 5 years. Alternative

manufacturing methods are required to produce features sizes in the

range of tens of nanometers. A number of processes are emerging

from universities and research institutes that will form the basis of

future nanomanufacturing processes. The following descriptions of

Micro- and Nanomanufacturing 649

processes form the basis of the future direction of nanomanufactur-

ing.

12.3.2 Soft Lithographic IVIanufacturing

Soft lithographic processes are currently being developed for nano-

manufacturing of semiconductor and non-semiconductor applica-

tions and include techniques such as microcontact printing (mini-

mum feature size of 35 nm), replica molding (minimum feature size

of 30 nm), microtransfer molding (minimum feature size of 500

nm),

micromolding in capillaries (minimum feature size of 350 nm),

and solvent assisted micromolding (minimum feature size of 60 nm)

amongst others. Micro-contact printing is a non-photolithographic

method that routinely forms patterned self assembled monolayers.

An elastic stamp made from PDMS is used to transfer inks of mole-

cules or colloids of nanoparticles to the surface of the substrate be-

ing printed. The process can be made into a "manufacturing" proc-

ess by adding a roller to form a rolling stamp. Figure 12.13 shows

the principle of the process. Contact times of less than 30ms are re-

quired to deposit molecules to the surface of the substrate making

microcontact printing a relatively fast process. Deposition on the

surface of silver appears to be better than gold owing to silver's re-

activity to etchants and its high electrical conductivity. Multilayers

possess fewer defects than gold layers and edge distortions are less

because the deposited layers are 1 - 3 nm in thickness. PDMS

stamp require rigid backings so that the stamp does not lose form

during printing. The development of higher quality elastomers may

be necessary although reinforcing existing elastomers such as Syl-

gard 184 with metal particles may be the answer. The patterning of

non-planar surfaces is possible with this mechanical form of hthog-

raphy compared to the optical techniques. The development of mi-

crocontact printing as a nanomanufacturing process depends on the

ability to deposit a variety of materials. So far, organic molecules

and colloids of metals such as palladium have been successfully de-

posited. Future development of this process lies in the deposition of

materials to substrates other than silicon and glass.

650 Micro- and Nanomanufacturing

PGMS

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V"'"f V""/ '<•'

Fig.

12.13.

Schematic diagram of microcontact printing of self-assembled

monlayers (hexadecanethiol, HDT) to a gold surface: (a) printing a planar surface

with a planar stamp; (b) large area planar printing with a rolling stamp; and (c)

printing on a non-planar surface with a planar stamp [5]

Microcontact printing can also be performed to produce three-

dimensional structures such as microfluidic devices using the sand-

wich method of printing. A PDMS master stamp is coated with a

drop of PDMS pre-polymer which is stamped against the master pat-

tern of a silicon substrate. Masters are aligned with features just

touching where the PDMS pre-polymer cures under pressure. The

resulting membrane is peeled off the master and can be laid on top

of each other in order to build a three-dimensional device (Fig.

12.14).

Micro- and Nanomanufacturing 651

/j^f\

PDMS master

dropof PDMS

prepolyinor

Si master

Align masters face-to-facD

with features touching;

cure under pressure

Peel off membrane

and top master

150 nm

Seal bottom of membrane

flat piece of PDMS

Peel off top master

k 1/

Seal top of membrane to

flat piece of PDMS; trim

Inlet

Outlet

200 fim

Fig. 12.14. Membrane sandwich method for producing three-dimensional micro-

fluidic devices. The figure to the right shows flapper valves, a reciprocating pump,

and channel systems fabricated using the membrane sandwich method [6]

652 Micro- and Nanomanufacturing

12.3.3 Nanomanufacturing by Molding

Nanomanufacturing by use of micromolding techniques has re-

cently been demonstrated as a viable future process for nanomanu-

facturing. Micromolding applied to the nanoscale is comprised of

processes such as replica molding, microtransfer molding, micro-

molding in capillaries, and solvent assisted micromolding. Figure

12.15 shows the basic process procedures.

mold prepolymer

cure, peal oU

S)UTM

propotymer

romovo exc«ss

pre polymer

ptaco on tho support

crijre.

remove mold

rasiduai tiim

C) MIMIC

place a drop of

prepolymer

at one end

PDMS mold

support

D) SAMIM

- PDMS mold

waning .

liquid I pj^ce the mold on a support

fill clionneis by

capillary action

cure,

remove moid

residual film

Fig. 12.15. Nanomanufacturing procedures associated with molding techniques:

(a) repHca molding; (b) microtransfer molding; (c) micromolding in capillaries;

(d) solvent assisted micromolding [5]