Jackson M.J. Micro and Nanomanufacturing

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Manufacturing High Aspect Ratio Microstructures 121

3.8 Micromolding Processes

There are four processes that are employed for micromolding of

thermoplastic polymers: Injection molding, reaction injection mold-

ing, hot embossing, and injection compressing molding.

3.8.1 Injection Molding

The well-known macroscopic injection molding process can be

adapted to the micro-scale by employing a variotherm process [2].

It comprises the following process steps: The mold cavity equipped

with a micro-structured tool (mold insert) is closed, evacuated, and

heated above the glass transition temperature of the polymer, an in-

jection unit heats the polymer and presses the viscous polymer into

the mold, and the polymer (and the tool) is cooled down below its

glass transition temperature and de-molded from the tool. This cycle

temperature control is called variotherm (variothermal).

(^)

Injoetion oHa.nnerl

Fig. 3.12. Process procedure for the micro-injection molding process: (a) the

mold is closed, evacuated, and heated to above the glass transition temperature;

(b) polymer material is injected into mold; and (c) mold and polymer product are

cooled down and the polymer de-molded

122

Micro-and Nanomanufacturing

Table

3.1.

Thermoplastic polymers used

for

micromolding

Acro-

nym

COC

PMMA

POM

PFA

PVC

PET

PEEK

PSU

PVDF

Full name

Cyclo-olefin

copolymer

Polymethyl-

methacrylate

Polycarbonate

Polystyrene

Polyoxymethyl-

ene

Perfluoralkoxy

copolymer

Polyvinylchlo-

ride

Polypropylene

Polyethylene

terephthalate

Polyethere-

therketone

Polyamide

Polysulfone

PolyvinyHdene-

fluoride

Temperature

stability

140

130

260

110

250

80-120

150

Properties

High

transparency

High

transparency

High

transparency

Transparent

Low

friction

High

chemi-

cal

resistivity

Low

cost

Mechanical

properities

Transparent,

low

friction

High

tem-

perature

resistivity

Chemical

and

temperature

resistivity

Chemical

and

temperature

resistivity

Chemically

inert,

piezo-electric

Structure

Amorphous

Semi

Crystalline

Semi

crystalline

Amorphous

Semi

crystalline

Amorphous/

Semi

crystalline

Semi

crystalline

Semi

crystalline

Amorphous

Semi

crystalline

Manufacturing High Aspect Ratio Microstructures 123

X 50.0 pm/div

Z 2.5 pm/div

(b)

Fig. 3.13. Injection molded continuous relief micro-lens showing AFM structure

of the micro-lens in three dimensions: (a) microlens; and (b) AFM image of the

surface of the microlens

124 Micro-and Nanomanufacturing

In general, cycle times are shortest (on the order of minutes)

when producing micro-parts from polymers by injection molding.

Figure 3.12 demonstrates the basic principle of micro-injection

molding. Figures 3.13 and 3.14 show an array of microscale prod-

ucts manufactured using the injection molding technique.

3.8.2 Reaction Injection Molding

Reaction injection molding is similar to injection molding, but in-

stead of one type of plastic, two components are injected into the

closed molding tool. This technique allows fabrication of parts from

polymers that are not thermoplastic, such as thermosetting materials

and elastomers. The manufacture of micro-parts by reaction injec-

tion molding was investigated in the mid-1980s [3, 4] but turned out

to be difficult to perform because a good mixture of the components

needs to be achieved on the micro-scale and a chemical reaction has

to take place in micro-structures of the molding tool, which requires

a comparatively long time and results in long cycle times. Now,

with the possibility of UV-curing instead of thermal initiation of the

polymerization stage, reaction injection molding has again appeared.

Fig. 3.14. Reaction injection molded sensor chips and optical modules

Manufacturing High Aspect Ratio Microstructures 125

3.8.3 Hot Embossing

The principal process steps of hot embossing are [5]: A thermoplas-

tic film is inserted into the molding machine, a micro-structured tool

(mold insert) in an evacuated chamber is pressed into the film,

which has been heated above its softening temperature, and the mold

insert is filled by the plastic material which replicates the micro-

structures in detail. Then the set-up is cooled and the mold insert is

withdrawn from the plastic.

In contrast to injection molding, during hot embossing the poly-

mer flows a very short way from the foil into the micro-structure

only. As a result, very little stress is produced in the polymer and

the molded parts are well suited as optical components, such as

wave-guides and lenses. Hot embossing is particularly suited for

forming plane plates or foils, as a small amount of plastic has to be

molded. Figure 3.15 shows an outline of the process of hot emboss-

ing and a sample microscale product. Figure 3.16 shows a variety of

applications and commercial equipment available for hot embossing.

Originally designed for the LiGA standard format of 26 x 66

mm area, the embossing area has been extended considerably in the

past few years, and molding machines for samples with a diameter

of up to 200 mm are being introduced onto the market. With in-

creasing area, shrinkage of the plastic component is significant. A

sophisticated execution of the process is required to prevent defor-

mation, such as overdrawn edges of the components or even damage

to the mold insert. Shrinkage of hot embossed samples is smaller,

and therefore, it is especially suited for a further extension of the

format

[6,7].

A hybrid process developed for increasing productivity in the

manufacture of microscale products is laser-assisted hot embossing,

or infra-red assisted hot embossing. Figure 3.17 shows the general

outline of the process. Here, two variations are shown, transparent

mold embossing in which the embossing process can be viewed by

an observer to ensure that mold filling takes place, and transparent

substrate embossing. This method is currently in development and

demonstrates the advances currently being made in the manufacture

and fabrication of polymeric micro-and nanostructures.

126 Micro-and Nanomanufacturing

(a)

io)

;;.:•.•• ^ ..•• • •.••••• •' !

\-': • •• • •'••: .• ;• :* -:i

;.:.^......

A...-

]

ermof^astic foil

^ '

Mold inserts

"El. IJ

' .;.,,:. '• : •',:,'",••

__„rL

'•"

•}....

a..,_

iin

Residual layer

(a) (b)

(c)

Fig. 3.15. Process steps for hot embossing: (a) thermoplastic film is inserted into

the mold; (b) mold is evacuated and heated above the glass transition temperature;

and (c) the polymer is cooled down and de-molded to reveal a typical microscale

product such as an electrostatic comb drive, micro-lens, and a Fresnel zone plate

Manufacturing High Aspect Ratio Microstructures 127

• PI

\1 n

- i

F7l

mi^rm^Avm

Fig. 3.16. Commercial hot embossing system manufactured by JENOPTIK Mik-

rotechnik GmbH (Germany). Spec: press force: < 200kN, temperature inside

chamber: < 500^C, max substrate size: f==150mm, with vacuum and cooling sys-

tem. Samples of microformed structures include reflective surfaces and surfaces

with needles standing proud of the surface

_. —»-To laser generator

.--Fiber bundle

Laser radiation

-Mask

Transparent mold

Absorbing substrate

•-^To laser generator

Fiber bundle

Laser radiation

Mask

Transparent substrate

Absorbing mold

Transparent mold embossing Transparent substrate embossing

Fig. 3.17. Laser/infra-red assisted embossing

128 Micro-and Nanomanufacturing

3.8.4 Injection Compression IVIolding

Injection compression molding is a combination of injection mold-

ing and embossing to overcome the problem of heating the polymer

by the tool. The polymer is injected from a screw into the semi-

closed molding tool and then pressed into the microstructure by

closing the tool. In this way, the problem of injection through a

small gap is avoided when producing a microstructure on a thin car-

rier layer. Injection compression molding is widely used to produce

CDs and DVDs. CDs possess critical dimensions of less than 1 mi-

crometer, but the aspect ratio is small and, therefore, de-molding is

not a problem.

3.9 Micromolding Tools

Molding tools used in micro-engineering generally consist of a mi-

cro-structured mold insert and the tool. The requirements, which

need to be fulfilled by the mold insert, are very different. The mold

insert has to provide for the primary microstructure and, therefore, is

manufactured with techniques appropriate for this. The microstruc-

ture should exhibit smooth sidewalls to avoid friction during de-

molding and a small inclination angle, if this can be tolerated by the

application of the microstructures to be molded.

While the tool and the ejection systems are manufactured by

classical metal processing, various methods are applied for micro-

structuring the mold insert. There are methods of direct structuring,

including mechanical micro-machining, laser structuring, and elec-

tric discharge machining (EDM), and indirect processes such as

lithographic processes that use x-rays or UV radiation combined

with electroplating.

Electroplating allows fabrication of

metal mold insert from mi-

crostructures made of plastics, silicon, and other materials not suit-

able for use as a mold insert

[8-10],

It may be advantageous to

transform a metal microstructure worked from a solid soft metal into

a hard alloy (e.g., Fe-Ni or Co-Ni) or hard metal by electroplating.

Another reason for employing electroplating is that some critical mi-

Manufacturing High Aspect Ratio Microstructures 129

crostructures such as narrow grooves or sharp concave comers can-

not be milled while their inverted forms (narrow rib or sharp convex

comer) can easily be manufactured by mechanical means. Even

stmctures in the nanometer range can be replicated by electroplating

and, therefore, this is an important technique for tool manufacturing.

For very high aspect ratio microstmctures, mold inserts need a sub-

stantial backing in order to provide the necessary mechanical stabil-

ity.

Mechanical micro-machining is closest to traditional tool tech-

nology. Techniques such as tuming, drilling, or milling are em-

ployed for fabricating mold inserts. Mold inserts fabricated by me-

chanical micro-machining with CNC machines are offered by many

companies and are available from research institutes. The smoothest

sidewalls of microstmctures are obtained with diamond tools but

these are not suitable for work in steel, which is a favored material

for mold inserts. Moreover, the smallest diameter of diamond tools

is approximately 200 |Lim. When narrower grooves need to be fabri-

cated on a mold insert or the mold insert needs to be made of tool

steel, milling and drilling tools made of hard metals can be used,

with the requirements regarding the smoothness of sidewalls being

reduced. Compared to lithographic processes it is easy to fabricate

mold inserts with three-dimensional micro-stmctures even with

curved surfaces by mechanical micro-machining, and sloped side-

walls of the micro-stmctures can be achieved by simply using mill-

ing tools with the required profile.

Another technique initially developed for macroscopic use is

electric discharge machining (EDM). Today, it also allows micro-

stmctured mold inserts to be produced. This is achieved by using

wires as thin as 30 |Lim for wire erosion or by sinking erosion with

electrodes produced by electroplating. In this way, microstmctures

made from nickel or other metals can be transformed into mold in-

serts made of steel. A disadvantage of this technique is that the

sidewalls of microstmctures fabricated in this way are rough com-

pared to mechanical micromilling processes.

Lithographic processes are particularly suited for minute stmc-

tures.

A resist is patterned with a microstmcture that is electroplated

to build up a mold insert. LiGA technology is characterized by pos-

sessing extremely high stmctural tolerances, small lateral dimen-

130 Micro-and Nanomanufacturing

sions,

and sidewalls with a roughness of less than 50 nm [11,12].

UV lithography is less complex and less expensive alternative to x-

ray technology, which is able to meet less demanding specifications.

Both methods are based on the lithographic generation of non-

conductive plastic microstructures that are filled by electroplating.

Mold inserts made by both methods are available from companies

and research institutes [13].

Another possibility for structuring metal is to use laser technol-

ogy [11]. This development is still at its beginning, but promises to

have enormous potential in terms of aspect ratios and minimum

structural dimensions. This technology is of particular interest, as it

allows processing of materials such as stainless steel or tungsten

carbide. Figure 3.18 shows an array of micro-products using molds

manufactured via a number of processes.

Fig. 3.18. Molds manufactured via different processes: (a) deep x-ray lithography

and electroplating; (b) photolithography with SU8 resin; and (c) electro-discharge

machining