Development and applications of hard microstamping

DEVELOPMENT AND APPLICATIONS
OF HARD MICROSTAMPING

WU LEI
(B.Sci., Hebei University of Technology)

A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF SCIENCE
DEPARTMENT OF MATERIALS SCIENCE
NATIONAL UNIVERSITY OF SINGAPORE
2004


Acknowledgement
First of all, I would like to express my deepest appreciation to my supervisors,
Dr. Peter Moran, Dr. Mark Yeadon and Dr. Sean O’Shea, for their continuous
guidance and advice during the course of my research study.
It is also my pleasure to give my sincere thanks to all the staff and students in
IMRE. For their friendship, helps, and encouragement, my special hearty thanks are
due to Mr. Sunil Madhukar Bhangale, Dr. Li Bin, Dr. Li Zhongli, Dr. Zhang Jian, Dr.
Deng Jie, Dr. Y. Nikolai and Ms. Doreen.
In addition, I would acknowledge National University of Singapore (NUS) for
providing me an opportunity to pursue my Master degree, and Institute of Materials
Research and Engineering (IMRE) of Singapore for providing laboratory space and
the equipment, which have made this research possible.
I am indebted to my wife and my parents for their support, expectation and
encouragement, which are a significant part behind the work.

I

Table of Contents
Acknowledgement ......................................................................................................... I
Table of Contents..........................................................................................................II
Statement of Research Problems ................................................................................ IV
Summary ...................................................................................................................... V
List of Tables .............................................................................................................VII
List of Figures .......................................................................................................... VIII
Nomenclature........................................................................................................... XIII
List of Publications ....................................................................................................XV
Chapter 1 Introduction ................................................................................................ 1
Chapter 2 Literature Review....................................................................................... 4
2.1

Background ......................................................................................................... 4

2.2

Microcontact Printing (µCP) .............................................................................. 9

2.3

Nanoimprinting Lithography (NIL).................................................................. 12

2.4

Channel Stamping Technique ........................................................................... 14

2.5

Our Hard Microstamping Technique ................................................................ 16

Chapter 3 Experimental ............................................................................................ 23
3.1

Hard Stamping of Pd/PVP Nanoparticles ......................................................... 23

3.1.1 Materials ............................................................................................................ 23
3.1.2 Experimental Procedure..................................................................................... 24
3.2

Hard Stamping of Thin Metal Films................................................................. 30

3.2.1 Materials ............................................................................................................ 30
3.2.2 Experimental Procedure..................................................................................... 30
II


Chapter 4 Results and Discussion............................................................................. 40
4.1

Hard Stamping of Pd/PVP Nanoparticles ......................................................... 40

4.1.1 Effect of Adsorption Time ................................................................................. 41
4.1.2 Possible Adsorption Mechanisms of Pd/PVP Nanoparticles............................. 42
4.1.3 Transfer of Pd/PVP Nanoparticles..................................................................... 48
4.1.4 Effect of Stamping temperature on the Transfer of Pd/PVP Nanoparticles ...... 50
4.1.5 Engulfing of Pd/PVP Nanoparticles beneath the top layers of the polymer
substrate ...................................................................................................................... 52
4.1.6 Calculation of the Surface Energy of A Nanoparticle ....................................... 56
4.1.7 Effect of Stamping Temperature on Nanoparticle Engulfing ............................ 60

4.2

Hard Stamping of Thin Metal Films................................................................. 63

4.2.1 Mechanism of Hard Stamping of Thin Metal Films.......................................... 63
4.2.2 Effect of Stamping Temperature on Metal Film Transfer ................................. 64
4.2.3 Effect of Separating Temperature on Metal Film Transfer ............................... 65
4.2.4 Selection of Polymeric Substrate Materials....................................................... 66
Chapter 5 Applications ............................................................................................. 69
5.1

Microfabrication of Metal Patterns................................................................... 69

5.2

Stamped Metal Masks for Patterning Proteins.................................................. 78

5.2.1 Surface Modification and Characterization ....................................................... 79
5.2.2 Preliminary Study of the Cell Outgrowth .......................................................... 86
Chapter 6 Conclusions and Future Work.................................................................. 90
References................................................................................................................... 94

III


Statement of Research Problems
We have developed two low-cost, versatile micropattering methods for
fabricating micron and deep sub-micron conformal metal patterns on planar and nonplanar polymeric substrates. We refer to our methods collectively as “hard
microstamping” since both of them use a pre-patterned rigid silicon stamp to emboss
a polymeric substrate while selectively transferring substances, such as nanopaticles

or thin metal films, to a substrate.
We investigate their ability to generate to high-quality micron and deep submicron metal patterns in systems presenting problems in materials, topology, surface
functionality that cannot easily be solved by photolithography. Scientific issues
unique to the use of rigid stamps arise. Our work also involves optimizing the
techniques, examining and explaining the principles, processes, and limitations.
Our “hard microstamping” techniques have potential applications in fields as
diverse as semiconductor packaging and bioengineering. One such application is
briefly examined.

IV


Summary
Our broad aim is to develop alternative micro- and nanopatterning techniques
to complement established methods such as photolithography. These techniques
would ideally circumvent the diffraction limits of photolithography (i.e. be applicable
well into the nanoscale), be able to fabricate to two-dimensional and threedimensional structures, tolerate a wide range of materials and surface chemistries, be
inexpensive, and experimentally simple. In this research, we have focused on
fabricating micro- and nanoscale metal patterns on polymeric substrates and their
applications.
We have developed two cost-effective, versatile micropatterning techniques,
collectively called “hard microstamping”, for fabricating micro- and nanoscale
conformal metal patterns on planar and non-planar polymeric substrates in a parallel
process. We refer to these techniques as “hard microstamping” since both methods
use a pre-patterned rigid stamp, normally made of silicon or metal, to emboss a
polymeric substrate while selectively transferring substances to a substrate.
We investigate their ability to provide routes to high-quality patterns and
structures with lateral dimensions of micron and sub-micron scale in systems
presenting problems in materials, topology, surface functionality that cannot (or at
least not easily) be solved by photolithography. Scientific issues unique to the use of

rigid stamp arise. Our work involves developing and optimizing the techniques,
examining and explaining the principles, processes, and limitations. The ability of our
method to easily and accurately fabricate metal patterns on the micro- and sub-micron

V


over large regions opens new possibilities in fields as diverse as microelectronics and
bioengineering.

VI


List of Tables
Table 2.1

The recent past, present, and future of semiconductor technology.
These represent the smallest features that can be economically
mass produced.

Table 2.2

Non-photolithographic methods for micro- and nanofabrication.

Table 3.1

Physical vapor deposition parameters of gold.

Table 4.1

Different treatments of 5 Si<100> samples (with the native oxide
layers).

Table 4.2

Pd intensity on the stamp measured before and after hard
stamping which occurred at 100oC, with varying adsorption
periods*.

Table 4.3

PS samples stamped at different temperatures.

Table 4.4

PS samples treated in different ways for ToF-SIMS measurement.

Table 4.5

Parameters and their meanings in the calculation of surface
energy.

Table 4.6

PS samples with stamped Pd/PVP nanoparticles prepared for
ToF-SIMS measurements.

VII

List of Figures
Figure 2.1

Schematic illustration of the procedure used to fabricate a PDMS
stamp from a master having relief structures in photoresist on its
surface.

Figure 2.2

Schematic illustration of procedures for µCP of hexadecanethiol
(HDT) on a gold surface: a) printing on a planar surface with a
PDMS stamp; b) etching through the printed SAM as mask; c)
Depositing other materials through the printed SAM as mask.
After the “ink” was applied to the PDMS stamp with a cotton
swab, the stamp was dried in a stream of N2 and then brought into
contact with the gold surface.

Figure 2.3

First stage of hot embossing lithography: imprint replication in
polymer followed by window opening.

Figure 2.4

Schematic of the process of hard microstamping bi-polymer
features. (Figure 2.4 adapted from P. M. Moran and C. Robert[34])

Figure 2.5

Illustration of possible deformations and distortions of

microstructures in the surfaces of elastomers such as PDMS. a)
Pairing, b) sagging, c) shrinking. (Figure 2.5 adapted from Y. Xia
and G. M. Whitesides[3])

Figure 2.6

The process of hard microstamping: (a) A cleaned Si stamp (b)
Inking of nanoparticles or deposition of metal (c) A silicon stamp
coated with nanoparticles or metal is pressed into a heated
polymer (d) After separating, nanoparticles or metal are
selectively transferred from stamp to the polymeric substrate.

Figure 3.1

Schematic of the process of inking and hard microstamping of
catalytic nanoparticles. (a) The silicon stamp is thoroughly
cleaned. (b) The stamp is immersed into a PVP-stabilized Pd
nanoparticle solution. (c) The inked stamp is heated and pressed
against the surface of a PS substrate that had been heated above it
Tg. (d) After cooling, the stamp and substrate were separated.
Nanoparticles were transferred to the areas where the polymer
was in contact with the stamp.

Figure 3.2

Shematic of hard microstamping of catalytic nanoparticles for
three dimensional polymer features with high aspect ratio (a) A Si
stamp was inked with Pd/PVP nanoparticles. (b) The
nanoparticles on the raised regions of the stamp were removed.
(c) The stamp was pressed against a PS substrate that had been

VIII


heated above its Tg and the pressure was applied to ensure that
the polymer flow filled up the cavities of the stamp entirely.
Figure 3.3

Schematic of the process of hard micro-/nanostamping of metals.
(a) The Si stamp is thoroughly cleaned. (b) The cleaned stamp is
deposited with a thin film (50-200 nm) of metal. (c) The metalcoated stamp is pressed into a polymer substrate that has been
heated above its Tg. (d) After cooling, the stamp and substrate are
separated. The metal film is transferred to the areas where the
polymer was in contact with the stamp.

Figure 3.4

PEAA surface for protein conjugation. Proteins, such as laminin,
can be readily conjugated with surface –COOH groups.

Figure 3.5

Schematic of surface modification of PS (a) PS surface (b) Ar
plasma treament and O2 oxidization (c) acrylic acid grafting
under UV irradiation. Proteins, such as laminin, can be readily
conjugated with surface –COOH groups.

Figure 4.1

ToF-SIMS measurements of Pd coverage of SiO2 surface as a
function of the adsorption time. The intensity of the Pd signal is

normalized by the signal from the Ga source.

Figure 4.2

Illustration of the adsorption of PVP-stabilized Pd nanoparticles
on the SiO2 surface.

Figure 4.3

Schematic representation of the adsorption mechanism for weakly
and strongly absorbing polymers on Pd particles (a) Weakly
adsorbing polymer (PVA); (b) strongly adsorbing polymer (PVP).
(Figure 4.3 adapted from W. Hoogsteen and L. G. J. Fokkink[39])

Figure 4.4

Illustration of possible configurations of PVP-stabilized Pd
nanoparticles adsorbed on the SiO2 surface (a) before and (b) after
heating (Not to scale)

Figure 4.5

XPS scans of Si<100> samples showing two peaks: Pd 3d3/2
(~342 eV) and Pd 3d5/2 (~336 eV). The lack of Pd peaks in
samples Si1_3, Si1_4, and Si1_5 show that the vast majority of
the nanoparticles have been removed from the silicon surfaces.
(see table 4 for preparation details of each sample 1-5) The
intensity of Pd signal is normalized by Si intensity used as the
reference.

Figure 4.6

ToF-SIMS measurements of the transfer percentage of the
Pd/PVP particles at different stamping temperatures. This is
simply a graphical representation of Table 4.3.

IX


Figure 4.7

Schematic of PVP-stabilized Pd nanoparticles engulfed beneath
of a few topmost layers of the polymer substrate.

Figure 4.8

ToF-SIMS measurements of the distribution of Pd particles on PS
surfaces. The sputtering time is indicative of the depth below the
surface. The Pd intensity in each sample is normalized against the
Ga intensity.

Figure 4.9

Illustration of nanoparticle on and embedded below the surface
(a) stage 1: Nanoparticle on the surface; (b) stage 2: Nanoparticle
partially embedded below the surface.

Figure 4.10

The relationship between θ and the change of the total surface

energy ∆Σ.

Figure 4.11

ToF-SIMS measurements of Pd/PVP nanoparticles on PS surfaces
stamped at different temperatures.

Figure 4.12

Optical micrograph of 100 µ m x 100 µ m, square PS features,
separated by Au regions, stamping at 80oC. Defects are due to the
low stamping temperature.

Figure 4.13

Optical micrograph of Au lines, roughly 10 µm in width, on
PEAA, stamping at its Tg, but the stamp and substrate were
separated almost immediately.

Figure 4.14

(A1) Optical micrograph of Si stamp with 2 µm x 2 µm square
microwells; (A2) Optical micrograph of raised PMMA regions, 2
µm x 2 µm in cross section, separated by gold regions stamped
from the Si stamp shown in (A1); (B1) Optical micrograph of Si
stamp with 20 µm x 20 µm square microwells; (B2) Optical
micrograph of raised LDPE regions, 20 µm x 20 µm in cross
section, separated by gold regions stamped from the Si stamp
shown in (B1); (C1) SEM micrograph of 350 nm wide gold lines
(bright) on PEAA substrate, separated by PEAA regions (dark)

(C2) SEM micrograph of raised PS regions, 200 nm x 200nm in
cross section, separated by gold regions.

Figure 4.15

Optical micrograph of raised cured epoxy resin regions, (a) 2 µm
x 2 µm in cross section, (b) 10 µm x 10 µm in cross section,
separated by stamped gold regions.

Figure 5.1

Schematic of electroless Ni plating on micropatterned polymer
surfaces fabricated by hard stmaping methods. (Strategy I) Pd
particles are transferred from the raised portions of the stamp and
polymer does not fill the cavities entirely; (Strategy II) Pd

X


particles are first removed from the raised areas of the stamp and
polymer is conformally embossed against the stamp. The raised
regions of the polymer are coated with Pd nanoparticles only
where the subsequent Ni plating occurs.
Figure 5.2

Optical micrographs of polystyrene surfaces metallized
selectively by our hard nanoparticle microstamping method. The
light gray regions are where nickel has been deposited. The dark
areas are polystyrene regions free of nickel. The insets show cross
sections (not to scale) of the metallized substrates. (a) Nickel

lines, roughly 40 µm wide, separated by 10 µm wide bare
polystyrene regions. (b) 1 µm wide nickel lines forming a grid
pattern.

Figure 5.3

Micrographs of three-dimensional raised PS microstructures
fabricated by our hard stamping technique. The insets show cross
section schematics (not to scale) (a) Optical micrograph of raised
PS columns (b) SEM micrograph of raised grid patterns (c) SEM
micrograph of raised PS columns. All raised features in (a), (b)
and (c) are coated with nickel by electroless plating and separated
by sunken PS regions.

Figure 5.4

(a) Optical micrograph of the surface of the stainless-steel scissors
used as a stamp for hard stamping. (b) Optical micrograph of a
selectively metallized polymer surface fabricated using the
scissors as the stamp. (c) SEM micrograph of the nickel film
plated on the polystyrene surface. After plating, the surface was
scratched with a sharp metal object (~50 µm wide running from
the top to the bottom of the micrograph) to demonstrate that the
plating has covered the whole surface. Bright areas within the
scratched region are exposed PS surfaces that are “charging” due
to the electron beam.

Figure 5.5

Surface roughness of the PS specimen plated with Ni.

Figure 5.6

Schematic of neuron attachment on protein patterns (a) Protein
patterning (b) neuron attached only on the protein regions.

Figure 5.7

Fluorescence images of micro-stamped gold/PEAA pattern after
conjugation of Avidin-FITC (a) Square PS regions with avidinFITC (green), roughly 2 µm x 2 µm, separated by Au regions
(dark); (b) 10 µm wide PS lines with avidin-FITC (green),
separated by roughly 5 µm wide Au regions (dark).

Figure 5.8

XPS Spectra on gold regions of a PEAA/gold patterned sample at
various steps of the modification reactions.

XI


Figure 5.9

XPS Spectra on polymer regions of a PEAA/gold patterned
sample at various steps of the modification reactions.

Figure 5.10

Mass resolved images of an area of gold-patterned PEAA that has
been treated with laminin without any prior PEG treatment (a) Au

and (b) NH secondary ions. Both (a) and (b) are images exactly
the same area of the substrate. Scan area is 200 µm × 200 µm.

Figure 5.11

Mass resolved images of an area of gold-patterned PEAA that has
been treated with mercapto-terminated PEG and thereafter was
treated with laminin (a) Au and (b) NH secondary ions. Both (a)
and (b) are images of exactly the same area of the substrate. Scan
area is 200 µm × 200 µm.

Figure 5.12

PC12 cultured on micropatterned polymer substrates with laminin
conjugation, separated by gold regions (a) 24h culture on PEAA
surface containing 2 µm x 2 µm square-like feaures (b) 24 h
culture on PS surface containing 10 µm wide lines. The cell has
differentiated and is growing on (a) but cells in (b) are confined to
the protein regions.

Figure 5.13

SEM micrograph of gold stamped onto PEAA. The amount of
exposed PEAA forms a gradient in the horizontal direction. The
holes in the gold mask are 200 nm in diameter.

XII


Nomenclature

Notation
Mw

Molecular weight

Tg

Glass transition temperature

Abbreviation
AA

Acrylic acid

DUV

Deep ultraviolet

EDAC

1-ethyl-3-(3-dimethylamino)propyl carbodimide

EUV

Extreme ultraviolet

IPA

Isopropyl alcohol

µCP

Microcontact printing

MIMIC

Micromolding in capillaries

µTM

Microtransfer molding

NHS

N-hydroxysuccinimide

NIL

Nanoimprinting lithography

PBS

Phosphate buffer solution

PDMS

Poly(dimethyl siloxane)

PEAA

Poly(ethylene-co-acrylic acid)

PEG

Poly(ethylene glycol)

PMMA

Poly(methyl methacrylate)

PS

Polystyrene

XIII


PVD

Physical vapor deposition

PVP

Poly(vinylpyrrolidone)

QSE

Quantum size effect

REM

Replica molding

RIE

Reactive ion etching

SAM

Self-assembled monolayer

SEM

Scanning electron microscopy

SET

Single electron tunneling

ToF-SIMS

Time-of-flight secondary ion mass spectrometry

XPS

X-ray photoelectron spectroscopy

XIV

List of Publications
1. W. K. Ng, L. Wu, P. M. Moran, Appl. Phys. Letts. 81, 3097 (2002).
2. L. Wu, P. M. Moran, to be submitted to Appl. Phys. Letts.
3. S. M. Bhangale, L. Wu, P. M. Moran, to be submitted to Adv. Mater.

XV


Chapter 1 Introduction

Chapter 1 Introduction
Microfabrication has long been the basis for microprocessors, memories, and
other microelectronic devices for information technology. Miniaturization and
integration of a range of devices have resulted in portability; reductions in time, cost,
sample size, and power consumption; improvements in detection limits; and new
types of functions.
New technical challenges arise with the continued shrinking of feature sizes
towards and below 100 nm. Further miniaturization will require major technological
breakthroughs

in

the

processes

underlying

microfabrication,

especially

photolithography, the heart of microfabrication. The breakthroughs must not only
allow further reductions in the size of the smallest features, but also must be
economically feasible to implement within the manufacturing process. Below 100 nm,
however, it is generally accepted that current strategies for photolithography may be
blocked by optical diffraction and by the opacity of the materials used for making
lenses and photomask supports. Furthermore, even for fabrication on the micrometer
scale, photolithography may not be the only and/or best method for all tasks.
We aim to develop a non-photolithographic, cost-effective microfabrication
method that is able to produce micro- and nanoscale conformal metal patterns on
planar and non-planar polymeric substrates in a parallel process. We have developed
two strategies and refer to these methods collectively as “hard microstamping” since
both use a pre-patterned rigid silicon stamp to mold a polymeric substrate while
selectively transferring materials to a substrate.

1


Chapter 1 Introduction
Our work involves developing and optimizing the stamping process. This
includes studying the transfer of materials, such as nanoparticles or thin metal films
from a rigid stamp, understanding and examining the principles, materials, and
limitations of the techniques, and demonstrating their ability to generate patterns and
structures with features that range from nanometers to micrometers in size. We have
also studied some scientific issues and problems related to material science, unique to
the use of hard stamps.
This thesis was organized into six chapters. In the first, we review
micropatterning techniques that have been developed in the past ten years. In the
second, we give a brief overview of our methods and compare them to other

micropatterning techniques. In the third section, we introduce the development of our
hard microstamping techniques. The emphasis in this section is on how to fabricate
conformal metal micro- and deep sub-micron patterns on planar and non-planar
polymeric substrates. One strategy is hard microstamping of catalytic Pd/PVP
nanopaticles. This work involves transferring catalytic nanoparticles selectively to
polymer surfaces. Subsequent electroless plating allows the formation of microscale
metal patterns. The other technique involves hard microstamping of thin metal films
directly on polymeric substrates. Both methods allow us to generate conformal metal
micro- and sub-micron patterns on common polymeric substrates.
In the fourth section, we demonstrate the experimental results and discuss
some scientific issues and problems related to materials science, unique to the use of
hard stamps. For the process of hard stamping of Pd/PVP nanoparticles, this involves
explaining the mechanisms of adsorption of nanoparticles on the rigid stamp, and the

2


Chapter 1 Introduction
subsequent transfer to the polymeric substrate, analyzing the effects of the various
factors, including adsorption time and stamping temperature, on the quality of the
microstructures fabricated. For hard microstamping of thin metal films, the principle
and process of the metal film transfer were examined and the effects of stamping
temperature and separating temperature on the quality of micropatterns were
investigated.
In the fifth section, we describe some applications of the resulting
micropatterned surfaces. We chose to demonstrate that our micro- and deep submicron metal patterns can be used as masks to pattern proteins. Patterned surfaces
with protein concentration gradients have been fabricated in order to study the
directional outgrowth of nerve cells. In the last section, we give an overall conclusion
of the work.

3


Chapter 2 Literature Review

Chapter 2 Literature Review
2.1 Background
Microfabrication is key to much of modern science and technology. A number
of opportunities exist if new microstructures can be fabricated or existing structures
can be downsized.[1] The most obvious examples are in microelectronics, where
“smaller” has meant better ⎯ lower cost, more components per chip, faster operation,
higher performance, and lower power consumption.
Ever since its adoption into integrated circuit manufacturing, photolithography
has thrived thanks to the evolution of optics and other peripheral technology
innovations such as photoresist development, advanced resist processing, and mask
making. Photolithography is the most successful micropatterning technology.
Photolithographic methods currently used for manufacturing microelectronic
structures are based on a projection printing system in which the image of a reticle is
reduced and projected through a high numerical aperture lens system onto a thin film
of photoresist that has been spin-coated on a wafer. The resolution “R” of the stepper
is subject to the limitations of optical diffraction according to the Rayleigh Equation
(1) [2],
R = k1λ/NA

(eq. 1)

where λ is the wavelength of the illuminating light, NA is the numerical aperture of
the lens system, and k1 is a constant that depends on the photoresist. Although the
theoretical resolution limit of optical diffraction is usually about λ/2, the minimum
feature size that can be obtained is approximately the wavelength of the light used. As

4


Chapter 2 Literature Review
a result, illuminating sources with shorter wavelengths are progressively introduced
into photolithography to generate structures with smaller feature sizes (Table 2.1).[3]
As structures become increasingly small, they also become increasingly difficult and
expensive to produce.

Table 2.1 The recent past, present, and future of semiconductor technology.
These represent the smallest features that can be economically mass produced.

Year

Lithographic method

Resolution (nm)[a]

Bits (DRAM)[b]

Photolithography (λ [nm])
1992

UV(436), g line of Hg lamp

500

16M

1995

UV(365), i line of Hg lamp

350

64M

1998

DUV(248), KrF excimer laser

250

256M

2001

DUV(193), ArF excimer laser

180

1G

2004

DUV(157), F2 excimer laser

120

4G

100

16G

<100[c]

>16G

2007
2010

DUV(126), dimmer discharge
from an argon laser
Advanced lithography
Extreme UV (EUV, 13 nm)
Soft X-ray (6-40 nm)
Focused ion beam (FIB)
Electron-beam writing
Proximal-probe methods

[a] The size of the smallest feature that can be manufactured. [b] The size of the dynamic random
access memory (DRAM). [c] These techniques are still in early stages of development, and the
smallest features that they can produce economically have not yet been defined. (Table 2.1 adapted
from Y. Xia and G. M. Whitesides[3])

5


Chapter 2 Literature Review

The continued shrinking of feature sizes towards and below 100 nm poses
new technical challenges for photolithography. It might be extended to feature size
down to 100 nm by employing advanced mask/resist technologies and deep
ultraviolet (DUV) radiation. Below this size, however, it is generally accepted that
current strategies for photolithography may be ineffective due to optical diffraction
and the opacity of lens and photomask materials. Furthermore, it may not be the best
method for all tasks even on the microscale. For example, it is high-cost; it cannot be
easily adopted for patterning nonplanar surfaces;[4] and it is directly applicable to only
a limited set of materials used as photoresists.[5]
New approaches must be developed to extend patterning capability into the
range below 100 nm. Advanced lithographic techniques currently being explored for
this regime include extreme UV (EUV) lithography, electron-beam writing, X-ray
lithography, focused ion beam writing, and proximal-probe lithography.[6] These
techniques can define sub-100 nm features, but their commercial applications still
require great ingenuity due to high cost and low throughput. These limitations suggest
the need for alternative microfabrication techniques. The development of practical
methods capable of generating structures smaller than 100 nm for a range of materials
with low cost and high throughput represents a major task, and is one of the greatest
technical challenges now facing microfabrication.
A number of non-photolithographic techniques have been demonstrated for
fabricating high-quality microstructures and nanostructures (Table 2.2).[7-25] Among
these, there is a family of micropatterning techniques collectively termed “soft

6


Chapter 2 Literature Review
lithography”, since all methods use a soft elastomeric stamp or mold to transfer the
pattern to the substrate.

Table 2.2 Non-photolithographic methods for micro- and nanofabrication.
Resolution[a]

Reference

Injection molding

10 nm

[7]

Imprinting (embossing)

10 nm

[8]

Cast molding

50 nm

[9]

Laser ablation

70 nm

[10]

Micromachining with a sharp stylus

100 nm

[11]

Laser-induced deposition

1 µm

[12]

Electrochemical micromachining

1 µm

[13]

Silver halide photography

5 µm

[14]

Pad printing

20 µm

[15]

Screen printing

20 µm

[16]

Ink-jet printing

50 µm

[17]

electrophotography (xerography)

50 µm

[18]

Stereolithography

100 µm

[19]

Method

[20]

Soft lithography
Microcontact printing(µCP)

35 nm

[21]

Replica molding(REM)

30 nm

[22]

Microtransfer molding(µTM)

1 µm

[23]

Micromolding in capillaries(MIMIC)

1 µm

[24]

Solvent-assisted micromolding(SAMIM)

60 nm

[25]

[a] The lateral dimension of the smallest feature that has been generated. These numbers do not
necessarily represent ultimate limits. (Table 1.2 adapted from Y. Xia and G. M. Whitesides[3])

7


Chapter 2 Literature Review
Soft lithography generates micropatterns of self-assembled monolayers
(SAMs)[26] by contact printing, and also forms microstructures in materials, such as
plastics and glasses, by embossing[8] and replica molding[9]. It has expanded the range
of materials that can be used and has suggested routes to previously inaccessible
three-dimensional structures. Figure 2.1 shows the general procedure for producing
an elastomeric “master” and stamp for soft lithography.[27] The strength of soft
lithography is in replicating rather than fabricating the master, but rapid prototyping
and the ability to deform the elastomeric stamp or mold give it unique capabilities
even in fabricating master patterns. Soft lithographic techniques require remarkably
little capital investment and are procedurally simple. They can often be carried out in
an ambient laboratory environment. They are not subject to the limitations set by
optical diffraction, and they provide alternative routes to structures that are smaller
than 100 nm. The only advanced lithographic techniques needed are for making the
master. Since this master can then be reproduced many times, it may be fabricated
with slow and expensive techniques.
Substantial effort has been put into developing new techniques for fabricating
nanostructures inexpensively and in very large numbers. During the 1990's two
significant breakthroughs in unconventional lithographic methods were made:
"microcontact printing" (µCP)[21] developed by George Whitesides and coworkers
and "nanoimprinting lithography" (NIL)[8] by Stephen Chou and coworkers. In
general µCP is based on the use of a soft, poly(dimethyl siloxane) (PDMS) stamp to
ink a solid substrate with a self-assembled monolayer(SAM). NIL involves using a

8

Chapter 2 Literature Review
rigid mold to emboss a heated polymer layer coated on a substrate. Both µCP and
NIL have been extensively studied and appear close to commercial application.

Si wafer

(a)

Spin coat photoresist
Photoresist

(b)

Expose to UV light through a mask and then
expose to a solution of developer
“master”

(c)
Cast PDMS

(d)
Remove PDMS from master

(e)

PDMS stamp

Figure 2.1 Schematic illustration of the procedure used to fabricate a PDMS
stamp from a master having relief structures in photoresist on its surface.

2.2 Microcontact Printing (µCP)
Microcontact printing (µCP), developed by George Whitesides and coworkers
at Harvard University, is a flexible, non-photolithographic method that routinely
forms patterned SAMs containing regions terminated by different chemical
functionalities with micron and sub-micron scale lateral dimensions. The procedure is
schematically represented in Fig. 2.2(a). An elastomeric PDMS replica is produced
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