TECHNICAL PAPER

An intermediate-layer lithography method for generatingmultiple microstructures made of different conducting polymers

Anirban Chakraborty Æ Xinchuan Liu ÆGanga Parthasarathi Æ Cheng Luo

Received: 17 July 2006 / Accepted: 2 November 2006 / Published online: 1 December 2006� Springer-Verlag 2006

Abstract An intermediate-layer lithography (ILL)

method has been developed in this work to generate

multiple microstructures of different conducting poly-

mers on the same substrate. Previous and current ef-

forts in developing conducting polymer microsystems

mainly focus on generating a device of a single func-

tion. When multiple micropatterns of different con-

ducting polymers are produced on the same substrate,

many microsystems of multiple functions can be envi-

sioned. However, existing techniques present signifi-

cant technical challenges of degradation, low

throughput, low resolution, depth of field, and/or

residual layer in producing conducting polymer mi-

crostructures. To circumvent these challenges, the ILL

method has been explored to generate multiple micr-

opatterns of different conducting polymers in a parallel

manner. In this method, conducting polymer materials

and a non-conducting polymer intermediate layer are

first coated on a substrate, and are then patterned

through a mold insertion at a raised temperature. In

this work, the ILL has been used to successfully pattern

three types of commonly used conducting polymers on

the same substrate under a single mold insertion, and

simulation has been conducted to gain a good under-

standing of the molding process. Due to distinctive

advantages of simplicity, low cost and high throughput,

the ILL has promising applications in fabricating

micropatterns for polymer-based microsystems.

1 Introduction

The discovery of high conductivity in doped polyacet-

ylene in 1977 (garnering the 2000 Nobel Prize in

Chemistry for the three discovering scientists) has at-

tracted considerable interest in the application of

polymers as the semiconducting and conducting

materials due to their promising potential to replace

silicon and metals in building devices. The electronic

‘‘signature’’ of a pristine (undoped) conducting poly-

mer is an alternating SP2 hybridized carbon single and

double bond backbone (MacDiarmid Alan 2001) with

overlapping of pz orbital in the atomic structure. The

conductivity of a pristine conducting polymer (in the

range of 10–10 to 10–5 S cm–1) can be increased mani-

folds (to the range of 1–104 S cm–1) by a doping pro-

cess. Doping is achieved by exposing the insulating or

semiconducting polymers to specific chemical species,

which adds charge carriers to the polymer backbone.

Both electron donating and accepting kinds of dopants

have been used. De-doping the conducting polymers

can cause it to revert back to its previous semi- or non-

conducting state. There is no alteration of the polymer

backbone in the process. This mechanism allows pre-

cise control of conducting polymer properties.

A significant technical challenge in building con-

ducting polymer microsystems is the patterning of

conducting polymer microstructures. Unlike metals,

A. Chakraborty � G. ParthasarathiElectrical Engineering and Institute forMicromanufacturing, Louisiana Tech University, Ruston,LA 71270, USA

X. Liu � C. Luo (&)Biomedical Engineering and Institute ofMicromanufacturing, Louisiana Tech University, Ruston,LA 71270, USAe-mail: chengluo@latech.edu

123

Microsyst Technol (2007) 13:1175–1184

DOI 10.1007/s00542-006-0321-x

most polymers are sensitive to the environment, and

their material properties also tend to deteriorate over

time due to overoxidation (air), moisture, high tem-

perature and chemical alteration. The current fabrica-

tion techniques (i.e., lift-off, dry and wet etching

processes) used in photolithographic approach (i.e.,

ultra-violet lithography) involve gases (for instance,

SF6 and oxygen), DI water, and/or chemical solution

(such as photoresist and acetone), making them

improper to pattern conducting polymers. Non-pho-

tolithographic approaches like inkjet lithography

(Diepold et al. 1998; Smith et al. (1994), soft lithogra-

phy (Xia and Whitesides 1998; Xia et al. (1999), and

hot-embossing lithography (Hecklele et al. 1998;

Heckele and Schomburg (2004) are potential methods

to make polymer microstructures, because these

methods do not involve aggressive chemistry and thus

avoid those degrading factors. Inkjet lithography uses

an inkjet printer to ‘‘write’’ patterns on the substrate.

Due to serial nature of the writing, this approach has

low throughput in generating patterns. In contrast,

both hot-embossing and soft lithography methods are

suitable for massive fabrication of features. Soft

lithography employs a polydimethylsiloxane (PDMS)

master to transfer patterns from a mold to substrates.

The pattern transfer to the substrates is analogous to

postal printing of seals on envelopes with the PDMS

master, the polymer, and the substrates interpreted as

the postal stamp, the ink, and the envelopes, respec-

tively. However, unlike the postal stamp, PDMS is soft

because of its low Young’s modulus. The Young’s

modulus of the PDMS is 750 KPa (Lotters et al. 1997),

which is 1200,000

of that of silicon (Spiering et al. 1993).

Due to residual stress, the free-standing PDMS may

have undesirable deformations, such as pairing, sag-

ging, and shrinkage (Xia and Whitesides 1998; Xia

et al. (1999), that lead to failure of the patterns (Xia

et al. 1999; Delamarche et al. (1997) or misalignment

(Luo et al. 2002) problems.

In the hot-embossing method, a polymer film

coated on a solid substrate is patterned through the

insertion of a rigid mold at a temperature above the

glass transition temperature of the polymer. A char-

acteristic residual layer is generated between neigh-

boring patterns in the polymer material after the hot-

embossing process, connecting these patterns and

making them not electrically isolated. Meanwhile, the

height variations, which exist in the mold structures

due to non-uniformity in processing, may make some

short structures lose contact with the substrate and

not be transferred to the conducting polymer layer.

Also, unwanted conducting polymer structures may

be generated on the substrate during the pattern

transfer in soft lithography when the aspect ratios of

voids in a PDMS master are too low. For example,

voids of low aspect ratio (<0.2) are susceptible to

sagging deformations (Delamarche et al. 1997), i.e.,

the surfaces at the bottom of the concave PDMS

features may have large deflections, making those

surfaces come into contact with the substrate and

generating unwanted patterns on the substrate. To

circumvent those obstacles of degradation, low

throughput, low resolution, depth of field, and/or

residual layer present in the existing lithographic ap-

proaches, a new lithographic method is needed to

fabricate conducting polymer microstructures in a

proper manner.

Previous and current efforts in developing micro-

systems mainly focus on generating device of a single

function. For example, chemical sensors have been

traditionally made based on a ‘‘lock-and-key’’ design,

wherein a specific receptor is synthesized in order to

bind strongly and highly selectively to the analyte of

interest. When multiple micropatterns of different

conducting polymers are produced on the same sub-

strate, many microsystems of multiple functions can be

envisioned. For example, analogous to the mammalian

olfactory system which includes over 1,000 receptor

genes in detecting various odors (Stuart 2001), a sensor

consisting of multiple distinct conducting polymer

sensing elements will be capable of detecting a number

of analytes simultaneously. Another example is an

intelligent display employing a multiple conducting

polymer matrix (Heuschen Mark et al. 2005). There-

fore, a lithographic method was developed in this

work, which can not only be used to fabricate con-

ducting polymer microstructures but also applied to

generate multiple microstructures of different con-

ducting polymers on a substrate in a parallel fashion.

Three commonly used conducting polymers, polyy-

pyrrole (PPy) (Komilla et al. 2002), sulphonated

polyaniline (SPANI) (Barbero et al. 1997) and

poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulph-

onate) (PEDOT-PSS) (Daoud Walid et al. 2005) were

tested and patterned.

2 Experimental procedure in the intermediate-layer

lithography (ILL) method

The ILL has been developed to generate multiple

micropatterns of different polymers on a substrate

simultaneously. Figure 1 shows an example of such

patterns in the form of microwires. The multiple mi-

crostructures of different conducting polymers are

1176 Microsyst Technol (2007) 13:1175–1184

123

generated using the ILL as follows: (1) a Si substrate,

coated with a layer of a non-conducting thermoplastic

material and a layer of multiple conducting polymer

coatings, is heated up to the printing temperature,

which is above the glass transition temperature (Tg;

softening temperature) of the non-conducting polymer

and below the Tg of each conducting polymer (Fig. 2a);

(2) a microstructure-formed Si mold and the substrate

are brought into physical contact by pressure, followed

by subsequent cooling (Fig. 2b); and (3) they are sep-

arated when their temperatures are below the Tg of the

non-conducting polymer, completing the pattern

transfer from the mold to the conducting polymer layer

(Fig. 2c). In the first step, the layer of multiple con-

ducting polymer coatings is generated by first spin-

coating conducting polymer I on the substrate with

other areas covered by tape and then spin-coating

conducting polymer II with other areas covered by

tape. Using a similar procedure, additional conducting

polymer coatings can also be generated on this layer.

The introduction of the intermediate layer between

the conducting polymer film and the substrate is the

critical point of the developed lithography approach,

and thus this approach is called ILL method. When the

substrate has only the layer of the polymer to be

printed, which is the case of hot-embossing approach

(Hecklele et al. 1998; Heckele and Schomburg (2004),

the patterning faces two obstacles (Fig. 3a). First, the

convex mold structures may have height variations

induced by surface roughness of the mold and/or fab-

rication errors. When these height differences are lar-

ger than the thickness of the conducting polymer layer,

some convex mold structures lose contact with the

conducting polymer and cannot transfer their patterns

to the substrate. Meanwhile, the small height differ-

ences will be transferred to the substrate due to the

replica nature of the molding process, making the

conducting polymer patterns have different thickness-

es. Second, the part of the conducting polymer right

underneath the convex mold structures is just com-

pressed by the mold insertion, but not separated from

the neighboring conducting polymer, causing the

shorting problem in the electrical applications of the

generated patterns. The introduction of the interme-

diate layer overcomes these two obstacles. At the

printing temperature, which is above Tg of the inter-

mediate layer material, this material is softened such

that the mold penetration occurs in both conducting

polymer and intermediate polymer layers, enabling the

intimate contact of the convex mold structures with the

conducting polymer layer and the creation of a thick-

ness contrast in this intermediate layer to assist the

cutting and separation of the conducting polymer, as

illustrated in Fig. 3b. Furthermore, the height differ-

ences among the convex mold structures are trans-

ferred to the intermediate layer, making the generated

conducting polymer patterns have a uniform thickness.

Naturally, this intermediate layer should be at least

thicker than the potential height variations of the

convex mold structures for solving the two obstacles.

In the ILL, the conducting polymer right underneath

a convex mold structure is first cut-off from the

neighboring conducting polymer at the beginning of

the mold insertion and then pushed down to the bot-

tom of the intermediate layer (Fig. 3b). The cutting is

due to high shearing forces at the two edges of each

convex mold structure induced by the mold penetra-

tion. The conducting polymer is heated up to reduce its

break stress for easily cutting it off. Meanwhile, the

printing temperature is below its Tg to avoid the

alteration of its material properties.

This technique of patterning conducting polymers is

more of a physical process than a chemical one. This

patterning approach is parallel and can be used to

massively pattern conducting polymers. Unlike con-

ventional lithography, ILL does not use any energetic

beams or any chemicals to pattern the conducting

Microwires of Conducting Polymer I

Microwires of Conducting Polymer II

Microwires of Conducting Polymer III

Fig. 1 Schematic of three arrays of microwires made of differentconducting polymers

(a)(b)

(c)

Si

Si mold Conducting polymer coating I Intermediate polymer layer

Conducting polymer coating II

Fig. 2 The three-step procedures to fabricate polymeric patternsusing the proposed ILL method: a heating of the substrate, binsertion of the mold into the two polymer layers, and cseparation of the mold and the substrate

Microsyst Technol (2007) 13:1175–1184 1177

123

polymer, avoiding degradation of the conducting

polymers.

3 Experimental method

Non-conducting polymethylmethacrylate (PMMA)

sheets were adopted as the intermediate layers. The

sheets were 500-lm thick, 170-mm wide and 170-mm

long. PMMA was chosen as the intermediate-layer

material, because it was a good hot-embossing material

(Chou Stephen et al. 1996). The PMMA has small

thermal expansion coefficient of ~5.0 · 10–5 �C–1 and a

small pressure shrinkage coefficient of ~3.8 · 10–7 per

psi (Chou Stephen et al. 1996). Its Tg is around 105�C.

PPy (Sigma Aldrich Co.), SPANI (Sigma Aldrich Co.)

and PEDOT-PSS (Baytron Co.) were considered in

this work. They were used as received from the man-

ufacturers. Since they were water-soluble, they were

dissolved in water. Their thin layers were generated by

spin-coating the corresponding solutions on the

PMMA sheets. Before coating the conducting poly-

mers over the PMMA, all three polymer solutions were

kept in an ultrasonic bath for 1 h to remove any

aggregate formation in solution from prolonged stor-

age. The PMMA surface is hydrophobic under normal

conditions due to its low surface energy. In order to

make the top surface of PMMA hydrophilic such that

the liquid conducting polymers could be effectively

spin-coated over it, the PMMA surface was exposed to

oxygen plasma in a commercial machine (Techniques

MicroRIE series 800). The plasma power was 300 W

and the duration of exposure was 45 s. Once high en-

ergy species bombarded the PMMA surface, the

adhesion between the PMMA and conducting poly-

mers was increased due to the increase of surface en-

ergy.

Si molds of depths 17, 100 and 120 lm were used in

the ILL. The molds were fabricated by a combination

of ultra-violet lithography and deep reactive ion etch

(DRIE) as below: (1) spin-coat a photoresist on a SiO2-

coated Si wafer and create the desired patterns in the

photoresist using ultra-violet lithography; (2) pattern

the 2-lm-thick oxide coating on the Si wafer via wet

etch using the photoresist as a masking layer; (3) create

mold structures in the Si wafer using DRIE (the pho-

toresist and SiO2 serve as the masking layers during the

DRIE); and (4) remove the photoresist, completing the

fabrication of the desired Si mold. The DRIE is a dry-

etching technique commonly used in micro/nanofabri-

cation to etch deep, high-aspect-ratio structures in sil-

icon by alternating an SF6 plasma etch with a C4F8

protective deposition (Madou 1995).

The imprinting process was done in vacuum using a

commercial hot-embossing system HEX 01/LT (JE-

NOPTIK Mikrotechnik Co.), which had a precise

control over the critical process parameters. Emboss-

ing temperatures could be raised to 200�C with an

accuracy of 0.1�C, embossing forces could be raised to

20 KN with an accuracy of 0.01 N, and positions could

be controlled with an accuracy of 1 lm. Both the

substrate and mold were heated up to the same tem-

perature for embossing. If the mold was not heated as

much as the substrate, then the mold would cool down

the PMMA when the mold was placed on the sub-

strate. This might increase the viscosity of the PMMA,

and the polymer would not flow effectively. Demolding

temperature was below the Tg of the PMMA. Tem-

perature control during the demolding was important.

For example, cooling the sample in a fast manner

would make the PMMA solidify quickly inside the

silicon mold, causing a stiction problem. Conducting

polymer undergoes conformational changes at its Tg,

which might affect the polymer matrix and conductiv-

ity. Therefore, the Tg of the conducting polymer should

be higher than that of the intermediate-layer polymer.

In our case, the intermediate-layer polymer was

PMMA, and its Tg was lower than those of the three

conducting polymers. The adopted printing tempera-

ture was 120�C, which was above the Tg of the PMMA

and below that of a targeted conducting polymer.

The silicon mold and substrate were aligned in a hot-

embossing machine. The top and bottom plates of the

hot-embossing machine were pre-aligned and centered.

Conducting polymer No contact between

the structure and the substrate

(a) (b)

Residual layer connecting neighboring conducting polymer patterns

Conducting polymer layer

Intermediate polymer layer

Fig 3 a The ‘‘height variations’’ and ‘‘residual layer’’ obstaclesthat both hot-embossing and nanoimprint lithography ap-proaches face in patterning a conducting polymer, and b these

two obstacles are overcome in the ILL method due tointroduction of the intermediate layer of a nonconductingpolymer

1178 Microsyst Technol (2007) 13:1175–1184

123

The silicon mold was attached and centered to the top

plate of the embossing machine. A 170 mm · 170 mm

PMMA sheet was centered on the bottom plate of the

embossing machine and the mold coverage area was

marked. This area was equally divided into multiple

blocks and each block was coated with one conducting

polymer while masking the other blocks. After spin-

coating of the conducting polymers on all blocks, the

PMMA sheet was again centered on the bottom plate of

the embossing machine and imprinting was carried out.

In case three conducting polymers were considered,

three sets of patterns were generated in the mold. The

mold coverage area on the PMMA sheet was divided

into three blocks for the three conducting polymers,

respectively. The only alignment requirement was that

the three sets of molding patterns should fall into the

three blocks on the substrate, respectively, during the

embossing.

Tape masking was used in this work. In the long run,

a better masking approach should be used to precisely

define the area of each block on the substrate due to

two concerns. First, commercially available tapes were

adopted here. Their available sizes and shapes are

limited, which limits the block sizes and shapes that

could be produced. Second, along their edges, there

was a size variation of tens of microns, which makes

the block boundary not uniform. A silicon stencil may

be a potential tool to replace a tape in this masking

effort, because the patterns in the silicon stencil could

be well defined using ultra-violet lithography. The

masking using a silicon stencil is currently being

investigated in our research group.

The generation of patterns is affected by four major

parameters: insertion speed, printing pressure, printing

temperature, and printing time. The insertion speed

should be constant and should be as small as possible,

aimed at reducing the dynamic effects on the materials

to be printed. The mold insertion depth ranged from 17

to 120 lm. The embossing pressures used in our

experiments were 2, 5 and 60 MPa, and the mold

insertion speeds were 0.5 and 1 mm min–1.

4 Numerical modeling

In order to have a good understanding about the

molding process, we conducted simulation using a

commercial finite-element package ANSYS 8.0. Since

the conducting polymer layer is much thinner than the

intermediate layer in the ILL method, the conducting

polymers and PMMA patterns should be generated

mainly due to the flow of the PMMA at the printing

temperature. Consequently, the PMMA should have

dominant effects on the finally generated patterns.

Therefore, for simplicity, during the simulation, we just

considered the case that only PMMA was coated on

the substrate.

The deformations of solid polymers are usually

characterized using the nonlinear Mooney–Rivlin

stress–strain relationship, instead of traditional linearly

elastic stress–strain function, since this relationship is

uniquely suited for rubber-like elastic deformations

(Mooney 1940; Yoshihiko et al. (2001). This relation-

ship has also been used to describe the deformation

behavior of PMMA in the nanoimprint lithography

method, in which a nanostructured Si mold is adopted

to pattern the PMMA through a hot-embossing process

(Madou 1995). According to Mooney–Rivlin model,

the stress is defined as (Mooney 1940; Yoshihiko et al.

(2001):

ri ¼ ki@W

@ki; ð1Þ

where k is the expansion rate and W is a strain density

function. W is expressed as:

W ¼ C10 I1 � 3ð Þ þ C01 I2 � 3ð Þ þ 1

dJ � 1ð Þ2; ð2Þ

where I1 ¼ k21 þ k2

2 þ k23; I2 ¼ k2

1k22 þ k2

2k23 þ k2

3k; I1 is

the first deviatoric strain invariant, I2 the second

deviatoric strain invariant, C10 and C01 material

constants characterizing the deviatoric deformation

of the material, and d the material incompressibility

parameter. The initial shear modulus is defined as:

l = 2(C01+C10) and the initial bulk modulus is

defined as: k ¼ 2d : C10 and C10 are the 1st and the

2nd strain energy constrains, respectively. They are

derived from the following approximated relations

(Mooney 1940):

C01 ¼ 0:25C10; 6ðC10 þ C01Þ � E; ð3Þ

where E is the Young’s modulus of the polymer. By

Eq. (3), we have

C01 ¼ 0:034E; C10 ¼ 0:134E: ð4Þ

Due to the nonlinear nature of Eq. (2), it is difficult

to find an analytical solution. Therefore, numerical

simulation is needed to find the molding deformation.

Since ANSYS 8.0 allows the simulation of materials

using the Mooney–Rivlin stress–strain relationship, it

was chosen in this work to establish the simulation

model. Rectangular hyper-elastic elements were used

to simulate the embossing of 5- and 50-lm microheater

Microsyst Technol (2007) 13:1175–1184 1179

123

patterns in PMMA (Figs. 4, 5). Each microheater

consisted of a serpentine-shaped line and two contact

pads. The PMMA layer was simulated as a rubber-

elastic material during the molding process. The

parameters used for simulation are listed in Table 1. In

case the mold had a depth of 100 lm, when the heights

of the 200 · 200 and 500 · 500 lm2 contact pads were

63 and 100 lm, respectively, the corresponding heights

of the 5- and 50-lm-wide microheater lines were 3 and

31 lm, separately (Figs. 4, 5). As the mold had a depth

of 120 lm, when the heights of the 200 · 200 lm2 and

500 · 500 lm2 contact pads were 75 and 120 lm,

respectively, the corresponding heights of the 5 and 50-

lm-wide microheater lines were 4 and 38 lm, sepa-

rately. These simulation results showed that PMMA

did not flow into the 5 and 50 lm silicon channels as

efficiently as that into the 200 · 200 lm2 and

500 · 500 lm2 contact pad cavities. Hence, height

differences were generated between them. In order to

increase the heights of small patterns, it is necessary to

decrease insertion speed but increase printing time,

such that polymers have more time to fill small mold

cavities.

(b) (c)

100µm

63µm

(a)

3µm

Fig. 4 ANSYS simulation ingenerating a 5-lmmicroheater pattern: a beforeand b after mold insertion,and c a close-up view

100µm

100µm 31µm

(b)

(c)

(a)

Fig. 5 ANSYS simulation ingenerating a 50-lmmicroheater pattern: a beforeand b after mold insertion,and c a close-up view

Table 1 Material properties of PMMA and silicon

Properties PMMA SiliconHyper elastic solid Rigid solid

Young’s modulus (GPa) 2.5–0.5 170C10 0.3325–0.0665 –C01 0.0825–0.0165 –Poisson ratio 0.45 0.23Density (kg m–3) 1.2 · 103 2.5 · 103

1180 Microsyst Technol (2007) 13:1175–1184

123

These simulation results provide a good under-

standing about the polymer deformations during the

molding process, while they could not be used to

quantitatively describe the deformation behaviors

due to the following concerns. The material proper-

ties of PMMA change with temperatures during the

heating-up and cooling-down processes in the ILL,

while the Mooney–Rivlin relationship does not de-

scribe the corresponding thermal behavior since this

relationship does not involve thermal effect. Also,

the cross-section profile produced during the molding

cannot be fully recovered, which is the characteristic

of a plastic deformation. Therefore, the PMMA should

be better modeled as a thermoplastic material during

the molding case, which is also our case. Unfortu-

nately, to our knowledge, such a thermoplastic model

has not been developed yet.

5 Experimental results

Microwire and microheater patterns were fabricated,

respectively, in the three different conducting polymers

(Fig. 6). The substrates were PMMA sheets. The gen-

eration of the microwires was used to examine whether

the three conducting polymers would be cut as illus-

trated in Fig. 2, while the production of the micro-

heater patterns was further employed to examine

whether functional devices, such as the microheaters

(consist of large contacts and small lines), could be

directly fabricated using the ILL.

The microwires had identical sizes. Each microwire

was 300-lm wide, 80-lm deep, and 5-mm long. The

microheater lines had different widths. They were 5-,

10- and 50-lm wide, separately (Table 2). The mi-

croheater patterns were smaller than the microwire

patterns. The PPy, PEDOT-PSS, and SPANI coatings

on those patterns were about 500 nm, 5 lm and

200 nm thick, respectively. The pattern density was

increased from 8 microwires per conducting polymer

to 49 microheaters per conducting polymer. The mi-

crowires were embossed at 120�C with an imprinting

pressure of 60 MPa. The mold insertion time was

120 s, and the demolding temperature was 70�C.

According to the simulation, the PMMA should flow

more effectively into large Si cativities. In order to

SPANI PEDOT-PSS PPy PEDOT-PSS

(b)

5 mm

SPANI PEDOT-PSS PPy

(a)

5mm

Fig. 6 Three different conducting polymers SPANI, PEDOTand PPy micropatterns on PMMA substrates: a microwires, bmicroheaters

Table 2 The structural dimensions of the silicon mold used for embossing, and the experimental results

Silicon microheater mold Embossing results

Physical dimensions Height of PMMA

Channel width (lm) Contact pad area (lm2) Mold depth (lm) Channel (lm) Contact pad (lm)

5 200 · 200 100 5 10010 500 · 500 120 5 12010 500 · 500 17 15.5 1750 500 · 500 120 30 10050 500 · 500 120 17 17

Microsyst Technol (2007) 13:1175–1184 1181

123

make the PMMA fill the smaller microheater

patterns, the embossing time was increased to 200 s.

The demolding temperature was raised to 80�C, while

the demolding velocity was reduced to 0.5 mm min–1.

The embossing pressure was 2 MPa. The examination

of microwire patterns generated in each conducting

polymer indicated that these conducting polymers

have been properly patterned (Fig. 7). The patterns of

the three conducting polymers had identical sizes, and

were generated under identical molding conditions.

Two differences between these patterns were: (1)

PPy, PEDOT-PSS and SPANI coatings on these

patterns had thicknesses of about 500 nm, 5 lm and

200 nm, respectively, and (2) these coatings had dif-

ferent morphologies since they were different mate-

rials. When other thicknesses and embossing

conditions are used, these patterns may show more

differences, which we have not tested yet.

PEDOT layer boundary

PEDOT layer

PMMA

(b)PPy layer

PMMA

PPy layer boundary

(a)

(c) SPANI layer

PMMA

SPANI layer boundary

Fig. 7 Sidewall (SEM) viewsof a PPy, b PEDOT-PSS andc SPANI microwires

5µm heater pattern

Contact pads

(a) (b)Fig. 8 PPy microheaterpattern on PMMA: a top viewand b side view. The heaterlinewidth was 5 lm and thecontact pads were200 · 200 lm2

(a) (b)50 µm heater patteren

Contact pads

Fig. 9 PPy microheaterpattern on PMMA: a top viewand b side view. The heaterlinewidth is 50 lm and thecontact pads are500 · 500 lm2

1182 Microsyst Technol (2007) 13:1175–1184

123

There existed significant height difference between

the contact pads and the heater lines in the 5 and

10 lm microheater patterns. The heights of the em-

bossed 5-lm-wide heater lines and 200 · 200 lm2

contact pads were 6 and 100 lm, respectively (Fig. 8).

For the 50-lm-wide heater lines, the height difference

was significantly reduced (Fig. 9). The heights of the

heater lines and the 500 · 500 lm2 contact pads were

30 and 100 lm, respectively. The possibility of reduc-

ing the height difference between the microheater lines

and the contact pads via the change of embossing

conditions was also explored. For this purpose, mi-

croheater patterns of 10- and 50-lm-wide lines were

particularly studied. Imprinting was carried out with

reduced depth of the silicon mold. The depth of the

silicon mold was 17 lm, the embossing pressure was

5 MPa, and the embossing time was 100 s. From SEM

images (Fig. 10), the height difference between contact

pads and 10-lm-wide lines was 1.5 lm, and no height

difference was visually observed between contact pads

and 50-lm-wide lines. The contact pads in these mi-

croheater patterns had a height of 17 lm. These results

demonstrate that, when the Si molds are not deep,

conducting polymers and PMMA will have a good

filling of both large and small cavities in the molds, thus

reducing the height difference between large and small

patterns generated.

During the ILL process, the temperature of the

conducting polymer coated PMMA substrate was first

increased from 25�C (room temperature) to 120�C (i.e.,

the printing temperature) and afterwards decreased

from 120� to 25�C. To investigate the effect of heating

on the conductivity of the conducting polymers during

the ILL, the following experiments were conducted.

Additional silver epoxy contacts were made on a PPy

microheater pattern and connected to the two probes

of a Keithley probe station. The I–V characteristic was

measured after the PPy microheater pattern had been

heated to a particular temperature and cooled back to

room temperature. Such a procedure was repeated for

temperatures of 25, 30, 50, 70, 90, 110, and 125�C. The

device current was measured for a sweep of the bias

voltage from 0 to 10 V at each temperature reading.

The value of the current at 10 V was used to plot

Fig. 11 since the I–V curve obtained by the sweeping

bias voltage was linear. Inherent structures of a con-

ducting polymer may be affected by temperatures after

the conducting polymer is heated up to an embossing

temperature and cooled down to room temperature.

Consequently, it is possible that conducting polymer

devices of identical configurations may have different

resistances (or say, different currents when the applied

voltages are the same) at their operation temperature

(i.e., room temperature), depending on the embossing

temperatures adopted to generate these devices. This

current-temperature test was performed to find how

much potential embossing temperatures might affect

electrical properties of the corresponding devices.

The data in Fig. 11 shows the variation in the cur-

rent as a PPy microheater pattern was exposed to the

gradually increasing temperatures. The currents varied

between 5.5 · 10–9 and 9 · 10–9 A. This small variation

should not affect the applications of conducting poly-

Fig. 10 PPy microheaterpatterns of a 50-lm-wide andb 10-lm-wide lines. The Simold used has a depth of17 lm

0.00E+00

1.00E-09

2.00E-09

3.00E-09

4.00E-09

5.00E-09

6.00E-09

7.00E-09

8.00E-09

9.00E-09

25 45 65 85 105 125

Temp(C)

Cu

rren

t(A

)

Fig. 11 The drift in the current of a 50-lm-wide PPy microheat-er pattern after successive heating and cooling cycles withincreasing temperature

Microsyst Technol (2007) 13:1175–1184 1183

123

mers. For example, in a sensing application, the current

changes of PPy micropatterns were in the order of 10–

7 A (Chakraborty et al. 2006). Therefore, it is reason-

able to conclude that the temperature increase and

decrease during the ILL have a minimal effect on

electrical properties of conducting polymers.

6 Conclusion

In this work, an ILL method has been used to simul-

taneously generate microwires and microheaters of

PPy, SPANI, and PEDOT-PSS on PMMA sheets.

During the fabrication of 5, 10 and 50 lm microheater

patterns, non-uniform fluid flows were found when the

silicon mold structures had different sizes, causing

height differences between features of smaller and

larger dimensions. However, when the mold structures

were not deep, these height differences could be sig-

nificantly reduced. Simulation was also conducted to

make a good understanding about the molding process.

In conclusion, the ILL technique is capable of gen-

erating well-resolved conducting polymer micropat-

terns with good repeatability and high throughput, and

thus has the potential to become an important ap-

proach in patterning conducting polymers. In particu-

lar, this technique gives substantial flexibility to

generate various micropatterns on different conducting

polymers simultaneously.

Acknowledgments This work was supported in part throughNSF–DMI-0508454 and NSF/LEQSF(2006)-Pfund-53 grants.The authors would also like to thank two anonymous reviewersfor their very constructive comments.

References

Barbero C, Miras MC, Kotz R, Haas O (1997) Sulphonatedpolyaniline (SPAN) films as catoin insertion electrodes forbattery applications Part II: exchange of mobile species inaqueous and non-aqueous solutions. J Electroanal Chem437:191–198

Chakraborty A, Parthasarathi G, Zhao W, Luo C (2006)Electronic nose for detecting multiple targets. In: SPIEproceedings on micro (MEMS) and nanotechnologies forspace applications, Orlando, FL, 04/2006

Chou Stephen Y, Krauss Peter R, Renstorm Preston J (1996)Nanoimprint lithography. J Vac Sci Technol B 14:4129–4133

Daoud Walid A, Xin John H, Szeto Yau S (2005) Polyethylen-edioxythiophene coatings for humidity, temperature andstrain sensing polyamide fibers. Sensor Actuat B 109:329–333

Delamarche E, Schmid H, Michel B, Biebuyck H (1997) Stabilityof molded polydimethylsiloxane microstructures. Adv Ma-ter 9:741–746

Diepold T, Obermeier E, Berchtold P (1998) A micromachinedcontinuous ink jet print head for high-resolution printing. JMicromech Microeng 8:144–147

Heckele M, Schomburg WK (2004) Review on micro molding ofthermoplastic polymers. J Micromech Microeng 14:R1–R14

Hecklele M, Bacher W, Muller KD (1998) Hot embossing-themolding technique for plastic microstructures. MicrosystTechnol 4:122–124

Heuschen Mark R, Moon Ronald L, Sheats James R (2005)Method of manufacture of active matric addressed polymerLED display. Agilent Technologies, Utility Patent Grant:US6965361

Komilla S, Annapoorni S, Sarkar AK, Tandon RP (2002) Gasand humidity sensors based on iron oxide–polypyrrolenanocomposites. Sensor Actuat B 81:277–282

Lotters JC, Olthuis W, Veltink PH, Bergveld P (1997) Themechanical properties of the rubber elastic polymer poly-dimethylsiloxane for sensor applications. J MicromechMicroeng 7:145–147

Luo C, Garra J, Schneider T, White R, Currie J, Paranjape M(2002) Determining local residual strains of polydimethylsilox-ane using ink dots, and stiffening. polydimethylsiloxane usingSU-8 particles. J Micromech Microeng 12:1–5

MacDiarmid Alan G (2001) Synthetic metals: a novel role fororganic polymers (Nobel Lecture). Angewandte Chemie -International Edition 40:2581–2590

Madou M (1995) Fundamentals of microfabrication. CRC Press,Boca Raton

Mooney M (1940) A theory of large elastic deformation. J ApplPhys 11:582–597

Smith L, Soderbarg A, Bjorkengren U (1994) Continuous ink-jetprint head utilizing silicon micromachined nozzles. SensorActuat A 43:311–316

Spiering VL, Bouwstra S, Spiering R (1993) On chip decouplingzone for package-stress reduction. Sensor Actuat A 39:149–156

Stuart F (2001) How the olfactory system makes sense of scents.Nature 413:211–218

Xia YN, Whitesides GM (1998) Soft lithography. Annu RevMater Sci 28:153–184

Xia YN, Rogers JN, Paul K, Whitesides GM (1999) Unconven-tional methods for fabricating and patterning nanostruc-tures. Chem Rev 99:1823–1848

Yoshihiko H, Masaki F, Takahiro O, Yoshio T (2001) Study ofresist deformation in nanoimprint lithography. J Vac SciTechnol B 19:2811–2815

1184 Microsyst Technol (2007) 13:1175–1184

123