10.2417/spepro.005250

To optimize nanofabrication,consider unique traitsZhifu Yin, E Cheng, and Helin Zou

With optimized parameters, nano hot embossing can create high-

quality patterns again and again.

In recent years, the study of nanofluidic systems has been reported in a

wide spectrum of fields, including chemistry, biology, and medicine.1–3

Researchers use nanofluidic systems to study the behavior, manipula-

tion, and control of fluids confined to nanosized structures. Recent pub-

lications have highlighted applications such as protein analysis,1, 2, 4

DNA extension,5–7 and ion separation8, 9 based on nanofluidic devices.

One fabrication technique for nanofluidic devices, called hot emboss-

ing, entails squishing together a polymer material and a mold at

elevated temperature. This technique enables the creation of 2D or

3D structures by applying appropriate temperature, pressure, and time.

In contrast to conventional nanofabrication methods such as litho-

graphy and etching on semiconductor substrates, the hot embossing

method has demonstrated considerably better results in terms of cost,

resolution, productivity, and ease of operation. However, during hot

embossing the processing parameters must be optimized to fabricate

nanostructures with high replication accuracy.

We developed numerical simulations for the hot embossing process

using 2D polyethylene terephthalate (PET) nanochannel fabrication as

a model system. We used the generalized Maxwell model to optimize

the processing parameters, but calculated the constants with a newly

established fitting formula rather than by the conventional method.

Traditionally, the constants of the generalized Maxwell model are

based on the tensile stress relaxation test.10–12 However, hot emboss-

ing is a compressive process. In most cases the compressive behavior

of a given viscoelastic polymer and its tensile behavior follow very

different curves.13, 14 In addition, the cross-sectional area of the poly-

mer substrate is usually assumed to be constant during stress relaxation

tests. During a hot embossing process, however, the cross-sectional

area varies with time. Therefore, the material parameters calculated by

tensile tests may not be applicable to hot embossing.

Here, the constants of the generalized Maxwell model were calcu-

lated with a newly established fitting formula based on experimen-

tal compressive creep tests, and consider the change in the polymer’s

cross-sectional area during the embossing process.

Figure 1. Curves demonstrating correlations between material creep

in experiment (dots) versus curve-fitting data (lines), at 3MPa pressure

and temperatures between 60 and 90◦C, over 500 seconds.

The newly established fitting formula can be expressed as

" .t/ D

��0b1 C b1 �

F

V� A1

�C b2 �

F

V� A1 �

�1

�J

�e

�t�J � t

C

��0b2 � b1 �

F

V� A1

�e

�t�J ; (1)

where " is the compressive strain, �0 is the initial stress, F is the force

applied on the PET substrate during the creep experiments, V is the

volume of the polymer substrate, A1 is a constant in the displacement

formula, and b1, b2, and �J are the constants of the creep compli-

ance when the number of generated Maxwell units is one. The de-

tailed calculation process that created this fitting formula was published

previously.15 Figure 1 shows the experimental compressive strain data

at different temperatures and their fitting curves, as calculated by

Equation 1. The average correlation of curve fitting from 60 to 90◦C is

as high as R2 D 96.66%, suggesting that the experimental data match

the fitting data closely.

Continued on next page

10.2417/spepro.005250 Page 2/4

Figure 2. Orthogonal simulation results of polymer nanochannels during hot embossing in a series of side views of the nanomold (top) imprinting

into the polyethylene terephthalate (PET) substrate (bottom). Each image corresponds to an experiment under specific conditions of temperature,

pressure, and time. Samples in a–d were fabricated at 65◦C, e–h at 75◦C, i–l at 85◦C, and m–p at 95◦C. Pressures increase in increments from

0.5�106 to 2�106Pa along rows (increases as a!d) and times increase in increments from 60 to 240 seconds along columns (a!m). Dark blue

indicates lowest-stress areas in the nanomold and PET substrate (usually 0MPa), while warmer colors indicate greater stress. Note that the black

area from a to p, which designates an unfilled area of PET, decreases with increasing embossing temperature.

Table 1. Factor level table of orthogonal experiments used to create a matrix of experimental conditions.

Factor no. Factor Level 1 Level 2 Level 3 Level 4

A Temperature/◦C 65 75 85 95

B Pressure/Pa 5:0 � 105 1:0 � 106 1:5 � 106 2:0 � 106

C Time/s 60 120 180 240

We employed the orthogonal experimental method to study the rela-

tionship between processing parameters and replication accuracy. The

orthogonal experimental method employs orthogonal arrays to orga-

nize parameters affecting the process and the levels at which they vary,

testing in pairs of combinations. Table 1 shows the factor-level table

for embossing temperature, pressure, and time. The simulation result

and its range analysis are shown in Figure 2. Range analysis is a simple

visual method to analyze the orthogonal experimental results. It allows

us to systematically explore which factors (temperature, pressure, and

time) have the most significant influence on nanochannel pattern trans-

fer quality.

Continued on next page

10.2417/spepro.005250 Page 3/4

Figure 3. Scanning electron microscopy (SEM) top-down view of the

fabricated 2D PET nanochannel array (top) and an inserted view of

the 2D nanochannel cross section (bottom).

During the hot embossing process, the stress in the nanomold and

PET substrate can be estimated from the colors shown in Figure 2. The

dark blue color indicates the smallest stress areas in the nanomold and

PET substrate (usually 0MPa), while warmer colors indicate greater

stress areas. Black areas indicate vacant space between the PET sheet

on the bottom half of each image and the mold pressing down from the

top. We can see that the higher the temperature applied to the nanomold

and PET sheet, the better the pattern transfer quality will be. At the

highest temperature, stresses are reduced to almost zero and the PET

almost completely fills the mold. The embossing temperature clearly

has the most significant influence on pattern transfer quality.

According to the simulation results, the best temperature, pressure,

and length of time for hot embossing are 95◦C, 2 � 106Pa, and 240s,

respectively. Consequently, we fabricated 2D PET nanochannels under

optimized parameters on the hot embossing apparatus.16, 17 The 2D

silicon nanomold was fabricated with a new nanomold fabricating

method based on a sidewall transfer technique. The scanning elec-

tron microscopy image of embossed 2D PET nanochannels is shown in

Figure 3. The fabricated 2D PET nanochannels measure approximately

80nm wide, 250nm deep, and 4mm long. High-aspect-ratio struc-

tures such as these have broad application potential in MEMS (micro-

electromechanical systems). Such nanostructures are usually difficult

to fabricate by hot embossing because the conditions must be exactly

controlled. These nanochannels with aspect ratio 3:1 were precision-

imprinted into a PET sheet with optimized hot embossing parameters.

The experimental results show that the proposed simulation method and

the measured material parameters have great potential for the process

analysis of hot embossing using viscoelastic materials.

Here we report our investigation of PET deformation by the finite el-

ement method (FEM). The PET material parameters required by FEM

software were calculated from experimental compressive creep curves

using a newly established fitting formula. Under optimized hot em-

bossing parameters, 2D PET nanochannels, approximately 80nm wide,

250nm deep, and 4mm long, were precision-imprinted into a PET

sheet. The simulation method based on our newly established fitting

formula is an accurate and time-saving method of optimizing the hot

embossing parameters. Hot embossing micro- or nanostructures with

all kinds of thermoplastic materials can be analyzed by this simulation

method to increase pattern transfer quality.

In the future we will bond the embossed 2D PET nanochannels with

another PET sheet with microchannels to form a nanofluidic chip. The

deformation of the nanochannels during thermal bonding will be ana-

lyzed by both experiments and simulations. The influence of bonding

parameters on the bonding strength and bonding rate (the ratio of the

effective bonding area over the whole area of the chip) will be investi-

gated as well. Based on this well-bonded nanofluidic chip, we plan to

perform DNA extension and ion separation to test the properties of the

device.

Author Information

Zhifu Yin, E Cheng, and Helin Zou

Key Laboratory for Micro/Nano Technology and Systems

of Liaoning Province

Dalian University of Technology

Dalian, China

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c 2014 Society of Plastics Engineers (SPE)