Qin Y. Micromanufacturing Engineering and Technology

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residual layer decreases with packing time. Dur-

ing this molding process, the temperature remains

constant. This isothermal embossing under vac-

uum is required to fill the cavities of the tool

completely. Air inclusions or cooling during mold

filling may result in an incomplete molding of the

microstructures, in particular at high aspect

ratios. Upon the expiry of the packing time, cool-

ing of the tool and substrate starts, while the

embossing force is maintained. Cooling is contin-

ued until the temperature of the mol ded part

drops below the glass transition temperature or

melting point of the plastics. When the demolding

temperature of the polymer is reached, the molded

part is demolded from the tool by relative move-

ment between the tool and the substrate. Demold-

ing only works in connection with an increased

adhesion of the molded part to the substrate plate.

Due to this adhesion, the demolding movement is

transferred homogeneously and vertically to the

molded part. Demolding is the most critical pro-

cess step of hot embossing. Depending on the

selected process parameters and the quality of

the tool, demolding forces may vary by several

factors. In extreme cases, demolding is no longer

possible; the structures are destroyed during

demolding.

Apart from the one-sided molding described,

the process is also used for double- sided posi-

tioned embossing. The principle of the process

remains the same. Instead of the substrate, how-

ever, another tool is applied. To demold the

molded part from one of the two tool halves,

special demolding mechanisms, such as ejector

pins or pressurized-air demolding, are used. For

a better understanding, the schematic representa-

tion of embossing in Fig. 5-2 is limited to the

major process steps. Depending on the tool and

the polymer used, the process and process para-

meters have to be adapted to any changes.

The characteristics and merits of hot emboss-

ing may be summarized as follows:

The molding process is characterized by short

flow paths, only from the molten polymer film

into the micro-cavities.

Because of the moderate molding velocities in

the range of 1 mm/min only moderate shear

stress in the polymer will be generated. This

results in comparative low residual stress in

the molded parts.

If the molding temperature is set to the range

where the relaxation times of a polymer corre-

spond to the cycle times of molding, the stress

induced by molding can be decreased by relax-

ation processes. This opt ion requires knowl-

edge of the temperature dependent relaxation

behavior of the polymer.

The use of standardized mold inserts allows

for the quick change of a micro-structured

mold, which underlines the flexibility of the

technology.

Beside the quick change of the mold insert, the

polymer can also be changed quickly. Only a

new polymer foil has to be placed between the

mold insert and the substrate plate (Fig. 5.1).

This allows for replicating a mold insert into

several polymers in a short time.

The technology allows for a variety of process

variations, for example double-sided molding,

molding of through-holes, multi-layer molding

and also thermoforming of a high temperature

polymer foil by a low temperature polymer melt.

Compared to injection molding, the process

may be characterized by longer cycle time.

Although the development shows that with an

effective technology the cycle times could be

reduced, the process is economically well suited

for small and medium series production. Nev-

ertheless, because of the high flexibility this

process can be adapted to several requirements

and it, therefore, is of high potential for the

development of protot ypes within laboratories.

PROCESS VARIATIONS

As mentioned above, hot embossing is not limited

to the case of a single-sided molding cycle. The

concept of hot embossing allows for the process to

be modified according to the requirements of fur-

ther applications of the molded parts. Represen-

tative of a large number of modifications, the

concepts of double-sided molding, multi-layer

molding, the molding of through-holes, and roller

embossing, are described in this section.

CHAPTER 5 Hot Embossing 71

Double-sided Molding

Double-sided molding paves the way to a further

large family of molded parts. For example, dou-

ble-sided structured parts are part of micro-fluidic

systems where through-holes are esse ntial fea-

tures. Although fraction lines with thin residual

layers can be designed to separate molded parts , in

general, with doubl e-sided molding, the possibil-

ity of three-dimensional structuring could be

enabled. A precondition for such designs is the

correlation of two mold inserts which are posi-

tioned opposite each other and adjusted laterally

relative to each other. Use of markers or the ori-

entation on characteristic structures of the design

is recommended. After a coarse adjustment of

both mold inse rts, a first part should be molded.

In a second step the differences in the lat eral

dimension between the markers on the top side

and the bottom side of the molded part have to be

measured. Transparency of the polymer is helpful,

because it enables the determination of the mis-

alignment of the markers with an optical measure-

ment system, focusi ng on the top and bottom of a

thin molded part. The misalignment can be split

into a transversal (x,y) direction and a rotational

(in the perpendicular axis of the part) misalign-

ment. With this measured data the alignment of

both mold inserts can be corrected by an align-

ment system integrated in a hot embossing

tool. The alignment can be done separately in the

transversal and rotational directions (Fig. 5-3).

These alignment systems are optional components

of commercial hot embossing machines (for exam-

ple, Jenoptik Hex03) and are a precondition for

double-sided molding.

Multi-layer Molding

Another illustrated approach [12], e.g. for the

fabrication of through-holes, is the use of a poly-

mer composite, a stack of tw o different polymer

films (polymer 1 and polymer 2). The structured

part with through-holes will be molded onto a

layer of a second polymer (polymer 2) (Fig. 5-4).

After demolding of the molded composite the first

polymer can be separated from the second poly-

mer, in this case the carrier layer, by peeling. The

advantage of this method is the maintenance of

the adhesion on the substrate plate. This allows

for demolding the molded composite in the verti-

cal direction by the precise movement of the hot

embossing machine. The adhesion between the

first polymer and the second polymer has to be

high enough to withstand the tensile forces during

demolding. Addi tionally, a form closed connec-

tion, effected by the penetration of the mold

insert into the second polymer, will support the

connection of both polymers. This approach is

only suitable for selected material combinations,

FIGURE 5-3 Principle of alignment for double-sided molding.

72 CHAPTER 5 Hot Embossing

because of the required a dhesion and further sep-

aration.

Through-holes

In micro-system technology the need for molded

parts with through-holes is increasing; micro-flu-

idic components like micro-pumps, micro-valves

or fluidic components for lab-on-a-chip systems

are common examples. The fabrication of

through-holes is, therefore, an actual aspect in

fabrication of micro-system components. To

reduce the post-processing steps, through-holes

should be fabricated already during molding.

Because of the molding principle, this require-

ment is difficult to achieve. As mentioned abo ve,

hot embossing is characterize d by a squ eeze flow

of a polymer melt. Typical molded parts are there-

fore characterized by a residual layer. On the one

side, this residual layer is necessary to obtain the

pressure for filling micro-cavities; on the other

side, for the fabrication of parts with through-

holes, this residual layer should be completely

displaced. The methods for fabrication of these

holes can be split into the post-processing meth-

ods to remove thin residual layers and methods

that are characterized by the selected substrate

plates allowing for fabricating through-holes

already during molding. Finally, a new develop-

ment is discu ssed – a principle that allows

for fabricating through-holes with a two-step

process, a molding cycle and a cutting cycle which

gives the name of this process – hot punching.

To avoid any post-processing of molded parts,

through-holes should be fabricated already dur-

ing the embossing cycle. This can be achieved by

the use of a combination of modified mold inserts

and selected substr ates [13]. The principle refers

to a complete displacement of the residual layer in

selected areas, achieved by embossing free-stand-

ing structures of the mold inserts into modified

substrates. This enhanced molding principle

requires a sensitive set-up of the process para-

meters and a proper selection of the substrate

materials, because of the relatively high load on

the structures during molding. The risk of dam-

aging filigree structure s of the mold insert

increases, especially when the diameter of the

through-holes decreases.

Representative of the process variation of

molding through-holes, the approach of molding

onto a stac k of foils is described here. In this

approach, in addition to the conventional metal

substrate plate, a flexible layer is put on top. This

layer consists typically of a polymer laye r and a

metal film. By the selection of the materials and

film thickness, the flexibility of this combination

can be influenced. Figure 5-5 illustrates the prin-

ciple. Because of the flexibility of the layer, in

selected areas the polym er melt can be displaced

completely from the top side of the mold insert. By

using this technique it is possible to retain the

FIGURE 5-4 Principle of two-layer molding for molding through-holes.

CHAPTER 5 Hot Embossing 73

residual layer in large contact surfaces, e.g. at the

margin regions of the mold insert, and to

completely displace the residual layer in small

contact areas. During the cooling state the desired

dwell pressure will be generated by the flexible

layer and not by the residual layer.

Hot Punching

The principle of hot punching is a two-step

method for molding through-holes (Fig. 5-6). In

the first step a selected mold insert will be repli-

cated in an amorphous polymer with high glass

FIGURE 5-5 Schematic view of a configuration for the molding of through-holes with the use of a stack of foils.

FIGURE 5-6 Principle of hot punching for molding through-holes.

74 CHAPTER 5 Hot Embossing

transition or a semicrystalline polymer with high

melting temperature. The replication refers to a

single molding on a rough substrate plate. Instead

of peeling off the molded part from the substrate

plate, the replicated part remains on the residual

layer after demolding. The molded part will be

used as a second to the first mold aligned mold

insert, here the so-called mat rix. In the second

process step a thin film of another polym er with

a lower glass transition temperature than the

matrix is positioned between the mold insert

and the molded part. The mold insert and sub-

strate are heated to a temperature in the range of

the glass transition temperature of the thin poly-

mer film which should be significantly below the

glass transition temperature of the matrix. The

third step of this method is simply another

embossing cycle structuring the thin film of poly-

mer between the mold insert and the remaining

matrix. Because of the high accuracy of the align-

ment of the previously molded matrix compared

to the mold insert, the structures of the mold insert

will match the inverse structures of the matrix

properly. Differences of thermal expansions will

be compensated for by the relative flexibility of

the polymer matrix, compared to that of the mold

insert of metal. During the second molding step

the mold insert now cuts holes insi de the thin

polymer film. This principle can be seen as a

shearing process. The punched areas will remain

in the matrix, where the matrix has to be renewed

after several punching steps. Depending on the

design and the thickness of the polymer film , the

demolding of the punched polymer film from

the matrix can be difficult to achieve and should

be accomplished carefully, otherwise damage to

the polymer film will occur. To help the process a

release agent may be used [14].

The advantage of this method is the use of a

wide range of mold inserts without any modi-

fications. The material combinations are

manifold. Besides the differences in softening

temperatures, no adhesion between the used

polymers is a precondition. Because of the two-

step molding, this method is more time consum-

ing. For series production the method has to be

improved.

Thermoforming of High Temperature

Polymers by Hot Embossing

In accordance with the molding of through-holes

the concept of two-layer molding can also

be used for the thermoforming of thin films of

semicrystalline high temperature materials like

LCP or PEEK. Thermoforming refers to the

straining of a polym er, typically by air pressure.

The gas pressure is substituted with a polymer

melt. The starting point refers to a combination

of two polymer films, a thin film of a high tem-

perature polymer and a second film of a material

with lower softening temperature. A suitable

combination is, for example, PMMA as molding

material and PEEK as thermoforming material on

top. Similar to multi-layer molding, the combina-

tion of materials has to be selected carefully. The

molding temperature has to be set such that the

low temperature polymer is in the melting range

and the semicrystalline high temperature polymer

foil is in a temperature range above the transition

temperature but below the melting temperature.

In this temperature range the amorphous part is

softened which reduces the elastic modulus of the

high temperature semicrystalline polymer which

supports the straining of the polymer film. A char-

acteristic of this method is the combination of a

molding step (PMMA) and a simultaneous ther-

moforming step of the high temperature polymer

foil (PEEK).

The achievable thickness of the foils depends

on the process parameters and the necessary

molding force. Experiments show that good

results for this kind of thermoforming can be

achieved up to a film thickness of 100 mm. The

thickness of the polymer film has to be selected in

relation to the size of the cavities. To obtain a

three-dimensional shape the thickness of the poly-

mer foils has to be much low er than the lateral

dimensions of the mold cavity, otherwise the poly-

mer foil will not fill the cavity completely. Fur-

ther, the material combination has to be selected

in such away that only a moderate adhesion is

effective between both foils after molding,

because the thin foil has to be separated from

the molded low temperature polymer manually

CHAPTER 5 Hot Embossing 75

by peeling. With this method a three-dimensional

structuring is possible, the rear side of the thin

polymer foil showing the inverse structure of

the front side that is similar to the mold insert

(Fig. 5-7).

Roller Embossing

Roller embossing or roll-to-roll embossing is

a well-established modification of the hot

embossing process. Using rolls instead of plates,

a continuous molding can be achieved with

advantages regarding the molding times and dis-

advantages regarding the height and the aspect

ratio of the molded structures (Fig. 5-8). Tan

et al. [15] used this approach, so-called roller

nano-imprint lithography, to fabricate sub-

100 nm patterns. Two methods were investi-

gated. The cylinder mold method refers to a thin

structured metal film bent around a smooth

roller. In particular, a compact disc master with

a thickness of 100 mm was used. The second

FIGURE 5-8 Thermal roller hot embossing.

FIGURE 5-7 Principle of thermoforming of a thin polymer film by hot embossing.

76 CHAPTER 5 Hot Embossing

method, t he so-called flat mold method, refers to

a structured silicon wafer mold placed on a poly-

mer substrate. A smooth roller mold is rotated

over the mold and the deformation of the mold

under t he pressure of the roll imprints the struc-

tures into the polymer. In both methods the

roller temperature is set significantly above

the glass transition temperature. For PMMA

the roller temperature is set in a range between

170 and 200



C and the platform temperature in

a range between 50 and 70



C. Roller speeds

from 0.5 up to 1.5 cm/s were investigated, the

pressure being set in the range between 300 and

4800 psi.

MATERIALS FOR HOT EMBOSSING

Most applicable mat erials for hot embossing are

thermoplastic polymers. During a molding cycle a

thermoplastic polymer undergoes several temper-

ature dependent aggregate states. Switching to

another aggregate state changes the material

properties significantly. Based on the molecular

structure of amorphous and semicrystalline poly-

mers different aggregate states and different tran-

sition ranges can be determined. The thermal

behavior of thermoplastic polymers may be

described with the time dependent shear modulus

over temperature.

Based on the shear modulus temperature dia-

gram, typical molding windows for embossing

amorphous and semicrystalline polymers can

be specified. Amorphous polymers show, theo-

retically, a wide temperature range for hot

embossing, beginning at the glass transition tem-

perature and ending before the decomposition

range. In practice the molding window depends

on several other parameters and has to be set

dependant of the design, the molding area and

the technique used in the embossing machine.

Due to t hese factors, the actual molding window

is smaller than the theoretical window and can

be set approximately in the range of 20 up to

100 K above the transition temperature. In con-

trast to the amorphous polymers, semicrystalline

polymers show only a small gap of temperature

suitable for hot embossing. As described above,

the decrease in the shear modulus occurs in a

small temperature gap in the melting range. At

the beginning of this range the stiffness of the

polymer is too high for molding. The risk of

damage to a micro-structured mold is high. At

the end of the range, marked by a low shear

modulus, semicrystalline polymers show typi-

cally a behaviour like a fluid with low viscosity,

which i s not suitable for hot embossing because

the pressure neede d for embossing with f reefl ow

fronts cannot be achieved. Therefore, the mold-

ing window can be found approximately in the

middle of the small gap of the melting range. In

practice, an exact temperature has to be set dur-

ing molding. The temperature range of amor-

phous polymers is considerably larger than that

for semicrystalli ne polymers, which in practice

makes it easier to mold amorphous polymers in

hot embossing (Fig. 5-9).

HOT EMBOSSING TECHNIQUES

Components

The technology components for micro-hot

embossing mainly include four groups:

The embos sing machine – this is responsible for

delivering the press force and the accurate

molding velocity. The challenge of the technol-

ogy is to obtain a high stiffness at high force and

also a precise relative moti on between the mold

and the substrate plate. These requirements

result in a stiff frame design consisting of two

crossbars and mostly four massive guiding pil-

lars. To obtain a press force, one of the cross-

bars is fixed . Another crossbar is moved by a

precise motion system, like a spindle drive or a

hydraulic drive.

The tool – this is mounted between the two

crossbars and is responsible for the heating

and cooling of the mold, substrate plate and

polymer sheet. A typical tool consists of two

halves, the top half fixed onto the top crossbar

and the bottom half fixed at the lower cross-

bar, each with a single heating and cooling

system. With an approximation of both

halves, an integrated vacuum chamber will

CHAPTER 5 Hot Embossing 77

be closed and isolates the micro-structured

mold insert and the substrate plate against

the ambient pressure. Optionally, an align-

ment system is integrated that allows the

adjustment, for example, of two mold inserts

against each other or to mold on pre-struc-

tured substrates.

The micro-structured mold insert – this is

reversibly fixed onto the tool. Opposite the

mold insert another mold insert or a substrate

plate will be positioned. A wide range of micro-

structured mold inserts can be used, for exam-

ple inserts produced by mechanical machining

or mold inserts fabricated by lithographic tech-

niques.

A precise controlling system – the precise con-

trol of the press force, the m otion o f cross bars

and the temperature is one of the t echnologi-

cal challenges. Besides the c ontrol of the pro-

cess parameters, the measurement of the press

force, temperatures inside the mold inserts

and the distances between the mold insert

and the substrate plate are t asks for the con-

trol unit. As an interface to the user, it is also a

task to prepare and visualize all data, allowing

the user to set up and control the process in an

effective way.

Hot embos sing machines have been devel-

oped to differe nt levels, beginning with simple

manually controlled hot embossing machines,

for example for general laboratory use, up to

high-level machines with a high grade in automa-

tion, used in industry and scientific research.

FIGURE 5-10 Schematic view of the components of a hot

embossing machine.

FIGURE 5-9 Thermal molding windows for hot embossing.

78 CHAPTER 5 Hot Embossing

Commercial Available Machines

The commercially available hot embossing

machines market is comprehensive. The first hot

embossing machine suitable for hot embossing of

high aspect ratio was developed in Karlsruhe at

FZK in cooperation with Jenoptik Mikrotechnik.

Jenoptik Mikrotechnik. Jenoptik Mikrote ch-

nik [9] was one of the first companies providing

a complete family of hot embossing machines.

Each member of the hot embossing machine fam-

ily is suited for different kinds of embossing

tasks. The machine HEX01 is the smallest

machine, compact and with a maximum force

of 100 kN already suitable for most molding

tasks, especially up to a molding area of four

inches. If larger areas are to be molded the

machine HEX02 fulfills the requirements rega rd-

ing molding forces up to 200 kN and molding

temperatures up to 300



C. If double-sided mold-

ing is required, t he machine HEX03 is equipped

with an addition precise alignment system and an

integrated microscope that allows the alignment

of the tool to be accomplished very easily . The

late st machine, HEX04, is especially des igned for

large area replication und er high molding forces

of up to 600 kN. All these machines are charac-

terized by an electrical heating unit, a convective

cooling system, a spindle drive and flexible con-

trolling by a macro-language. The fam ily of ho t

embossing machines is shown in Fig. 5-11.

Wickert Press. Another company that manufac-

tures hot embossing machines is Wickert Maschi-

nenbau [10]. In the past this company built the

first machine ‘MS1’ used in industry for hot

embossing of micro-spectrometers. In 2003 a

new generation of hot embossing machines was

developed. The machine WMP1000 (Fig. 5-12 )is

characterized by a hydraulic drive, a molding area

larger than 8 inches and a maximum force of

1000 kN. The machi ne was developed particu-

larly for industrial use, therefore an automatic

handling system was integrated with the machine.

In parallel an optimized molding tool was devel-

oped to reduce the heating and cooling times,

which finally resulted in a significant reduction

of cycle times. Further, a user-friendly control

panel was integrated, allowing the control of the

machine in an effective way. Nevertheless, the

realized concept requires more infrastructure for

the operation of the machine. For example, a sep-

arate room for the hydraulic pumps and a solid

foundation for the heavy machine which also has

a large overall height will be required.

EVGroup. The company EVGroup [16] offers

two hot embossing machines with different levels

of automation [17]. The hot embossing machine

EVG520HE is characterized by a semi-automatic

molding process, while the hot embossing machine

EVG750 is, in contrast, fully automated (Fig. 5-13).

This high level of automation is suited for large

serial production. The machines are compatible

with standard semiconductor manufacturing tech-

nologies and allow molding on substrates up to

200 mm. The hot embossing system EVG520HE

includes a vacuum chamber, a drive unit with a

high press force up to 600 kN, and a heating system

which allows the molding of a wide range of poly-

mers. Both machines are equipped with an align-

ment system.

HOT EMBOSSING TOOLS

Apart from the molding press and the micro-

structured mold insert, hot embossing tools are

essential components for any hot embossing

system. The hot embossing tool may be defined

as an interface between the molding press that

is responsible for applying the molding force

and molding velocity and the micro-structured

mold insert to be replicated in polymers. Com-

pared to macroscopic molding tools, such as

tools for injection molding, where the struc-

tures are part of the tool, tools for micro-

replication are characterized by a reversible

integration of a micro-s tructuredmoldinsert.

This concept results from the different and

incompatible fabrication processes of the mac-

roscopic tool and microscopic structures. On

the one hand, the tasks of a hot embossing tool

are similar to tasks known from m acroscopic

molding tools. On the other hand, the emboss-

ing tool has to fulfill tasks that are specific to

the molding of micro-structures, such as the

CHAPTER 5 Hot Embossing 79

generation of a vacuum. In detail, a hot emboss-

ing tool has to fulfill the following tasks.

Heating and cooling of the polymer film by heat

conduction of the mold insert and substrate plate;

Fixation of different kinds and sizes of mold

inserts;

Hermetic sealing of the mold inse rt, polymer,

and substrate plate against ambient pressure;

Generation of a vacuum to fill the micro-

cavities completely;

A demolding unit which allows the demolding

of the embossed parts in the vertical direction at

a controlled demolding velocity;

Optional alignment of both mold halves, if

double-sided mol ding or positioned molding

is desired.

From these tasks, the requirements to be met by

a molding tool can be deduced, including mini-

mum requirements for molding and optional

requirements for specific tasks.

FIGURE 5-11 The hot embossing family of Jenoptik.

80 CHAPTER 5 Hot Embossing