Review on Micro- and Nanolithography Techniques
and their Applications
Alongkorn Pimpin
*
and Werayut Srituravanich
**
Department of Mechanical Engineering, Faculty of Engineering, Chulalongkorn University,
Pathumwan, Bangkok 10330, Thailand
E-mail: alongkorn.p@chula.ac.th
*
, werayut.s@chula.ac.th
**
Abstract. This article reviews major micro- and nanolithography techniques and their
applications from commercial micro devices to emerging applications in nanoscale
science and engineering. Micro- and nanolithography has been the key technology in
manufacturing of integrated circuits and microchips in the semiconductor industry.
Such a technology is also sparking revolutionizing advancements in nanotechnology.
The lithography techniques including photolithography, electron beam lithography,
focused ion beam lithography, soft lithography, nanoimprint lithography and scanning
probe lithography are discussed. Furthermore, their applications are summarized into
four major areas: electronics and microsystems, medical and biotech, optics and
photonics, and environment and energy harvesting.
Keywords: Nanolithography, photolithography, electron beam lithography, focused
ion beam lithography, soft lithography, nanoimprint lithography, scanning probe
lithography, dip-pen lithography, microsystems, MEMS, nanoscience, nanotechnology,
nano-engineering.
ENGINEERING JOURNAL Volume 16 Issue 1
Received 18 August 2011
Accepted 8 November
Published 1 January 2012
Online at http://www.engj.org
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ENGINEERING JOURNAL Volume 16 Issue 1, ISSN 0125-8281 (http://www.engj.org)
1. Introduction
For decades, micro- and nanolithography technology has been contributed to the manufacturing of
integrated circuits (ICs) and microchips. This advance in the semiconductor and IC industry has led to a
new paradigm of the information revolution via computers and the internet. Micro- and nanolithography
is the technology that is used to create patterns with a feature size ranging from a few nanometers up to
tens of millimeters. By combining lithography with other fabrication processes such as deposition and
etching, a high-resolution topography can be produced while this cycle may be repeated several times to
form complex micro/nanoscale structures. Lithography techniques are divided into two types by the use
of masks or templates: masked lithography and maskless lithography. Masked lithography makes use of
masks or molds to transfer patterns over a large area simultaneously, thus, enabling a high-throughput
fabrication up to several tens of wafers/hr. The forms of masked lithography include photolithography
[1-10], soft lithography [11-13], and nanoimprint lithography [14-21]. On the other hand, maskless
lithography, such as electron beam lithography [22-29], focused ion beam lithography [30-33], and
scanning probe lithography [34-44], fabricates arbitrary patterns by a serial writing without the use of
masks. These techniques create patterns in a serial manner which allows an ultrahigh-resolution
patterning of arbitrary shapes with a minimum feature size as small as a few nanometers. However, the
throughput of this type is limited by its slow serial nature which makes it inappropriate for mass
production.
Not only micro- and nanolithography has been the main driving technology in the semiconductor
and IC industry, it also plays an increasingly important role in manufacturing of commercial
microelectromechanical system (MEMS) devices [45-50] as well as prototype fabrication in emerging
nanoscale science and engineering [51-56]. These applications are expected to significantly improve our
quality of lives in many ways from electronic gadgets to healthcare and medical devices. Some
examples of commercial MEMS products include MEMS accelerometers employed in automobiles and
consumer electronic devices [45, 46], digital micromirror devices (DMD) for display applications in
projectors and televisions [45, 47, 48], and MEMS pressure sensors for detecting pressures in car tires
and blood vessels [49, 50]. Furthermore, nanoscience and engineering has increasingly contributed to
conventional technologies by opening up alternative routes to overcome current technical barriers, to
name a few of them, nanoelectronics for denser and faster computing, nanomedicine for diagnosis and
treatment of many diseases including cancers [51-54], heart disease and Alzheimer's disease [55, 56],
nanoelectromechanical systems for high-sensitivity and high-resolution sensing and manipulating, and
nanobiosensors for ultra-low concentration and single molecular detection. Table 1 summarizes the
specifications (i.e. minimum feature size and throughput) and applications of the major lithography
techniques.
2. Micro- and Nanolithography Techniques
2.1. Photolithography
Photolithography has been the main workhorse in the semiconductor and IC industry [1-10]. It has been
employed for pattern generation in manufacturing of ICs, microchips and commercial MEMS devices.
This technique utilizes an exposure of a light-sensitive polymer (photo-resist) to ultraviolet (UV) light
to define a desired pattern. Initially, UV light with wavelengths in the range of 193-436 nm is
illuminated through a photomask that consists of opaque features on a transparent substrate (e.g., quartz,
glass) to make an exposure on a photo-resist that is coated on a substrate [5, 6, 22]. In the exposed area,
the polymer chains of photo-resist break down resulting in more soluble in a chemical solution called
developer. Subsequently, the exposed photo-resist is removed in a developer to form the desired photo-
resist pattern. Figure 1 depicts the schematic illustration of the main steps in photolithography. This
patterned photo-resist can be used as a protective layer in subsequent etching or deposition processes to
build topography on the substrate.
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Table 1. Specifications and applications of the major lithography techniques.
Lithography Technique
Minimum
Feature Size
Throughput
Applications
Photolithography
(contact & proximity
printings)
2-3 μm
[22]
very high
typical patterning in
laboratory level and
production of various
MEMS devices
Photolithography
(projection printing)
a few tens of
nanometers
(37 nm)
[2]
high - very high
(60-80 wafers/hr)
[1]
commercial products and
advanced electronics
including advanced ICs
[1]
,
CPU chips
Electron beam lithography
< 5 nm
[23]
very low
[1, 3]
(8 hrs to write a
chip pattern)
[1]
masks
[3]
and ICs
production, patterning in
R&D including photonic
crystals, channels for
nanofluidics
[23]
Focused ion beam
lithography
20 nm with a
minimal lateral
dimension of 5
nm
[2]
very low
[3]
patterning in R&D
including hole arrays
[125,
134]
, bull’s-eye structure
[132]
, plasmonic lens
[137]
Soft lithography
a few tens of
nanometers to
micrometers
[2, 13]
(30 nm)
[2]
high
LOCs for various
applications
[13, 96]
Nanoimprint lithography
6-40 nm
[14, 15, 18]
high
(> 5 wafers/hr)
[1]
bio-sensors
[17]
, bio-
electronics
[18]
, LOCs: nano
channels, nano wires
[97, 102,
104]
Dip-pen lithography
a few tens of
nanometers
[39, 40,
43]
very low – low,
possibly medium
[39]
bio-electronics
[43]
, bio-
sensors
[40]
, gas sensors
[42]
There are three forms of photolithography: contact printing, proximity printing and projection
printing as schematically illustrated in Fig. 2 [1, 22]. Contact and proximity printings place the
photomask in contact with or in a close proximity to the photo-resist. Generally, contact and proximity
printings are capable of making patterns as small as a few micrometers. Therefore, they are typically
used in the fabrication of moderate-resolution patterns especially in laboratories and small to medium-
sized companies. It should be noted that photolithography in most of research works normally refers to
contact or proximity printings. In contrast, a projection printing system (so-called ‘stepper’) utilizes an
optical lens system to project a deep-UV pattern from an excimer laser (wavelength of 193 or 248 nm)
on the photo-resist enabling pattern-size reduction by 2-10 times. It is capable of fabricating high-
resolution patterns as small as a few tens of nanometers (37 nm) [2] at a high throughput (60-80
wafers/hr) [1]. However, it requires a sophisticated optical-lens system and precise control systems of
temperature and position resulting in a very expensive setup. Thus, it is employed in manufacturing of
advanced ICs and CPU chips. In recent years, immersion lithography [8], resolution enhancement
technology [9] and extreme-UV lithography [10] have been developed to improve the lithography
resolution of projection printing.
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photomask
UV light
(a) Exposure
(b) Development
photo-resist
substrate substrate
photo-resist
Fig. 1. Schematic illustration of the main steps in photolithography. (a) exposure step: photo-resist
coated on the substrate is exposed to UV light, (b) development step: the exposed photo-resist
is removed by immerging into a developer.
(b) Proximity printing(a) Contact printing
(c) Projection printing
substrate substrate
substrate
photo-
resist
lens
photo-
resist
photo-
resist
photomask
photomask
photomask
UV light
UV light
UV light
Fig. 2. Schematic illustration of three forms of photolithography: (a) contact printing; (b) proximity
printing; and (c) projection printing.
2.2. Electron beam lithography and focused ion beam lithography
Electron beam and focused ion beam lithographies have been the main techniques for fabricating
nanoscale patterns. Electron beam lithography [22, 23] utilizes an accelerated electron beam focusing
on an electron-sensitive resist [24, 25] to make an exposure. Subsequently, this electron-beam spot with
a diameter as small as a couple of nanometers is scanned on the surface of resist in a dot-by-dot fashion
to generate patterns in sequence (Fig. 3). Similarly, focused-ion beam lithography utilizes an
accelerated ion beam (typically gallium ion) instead of the electron beam to directly punch a metallic
film on the substrate [30-33]. This is possible due to the heavy mass of ions as compared to that of
electrons. Furthermore, focused ion beam systems are also employed for depositing materials such as
tungsten, platinum, and carbon via ion beam induced deposition. When a precursor gas such as tungsten
hexacarbonyl (W(CO
6
)) is added into the chamber, the precursor gas is hit by the focused-ion beam
leading to gas decomposition which leaves a non-volatile component (tungsten) on the surface [33]. In
terms of specifications, the resolution of electron beam and focused ion beam lithography techniques
are of the order of 5 - 20 nm [2, 23] due to ultra-short wavelengths of electron/ion beams in the order of
a few nanometers. However, the lack of throughput limits their applications within research and mask
fabrication. Therefore, these two techniques are normally used for fabricating prototypes of nanoscale
structures and devices. To increase the system throughput, multiaxis electron beam lithography [26, 27]
has been proposed. So far, the deployment of this technique in manufacturing process is still limited due
to the difficulty in developing practical electron beam sources [1]. In the past, electron beam
lithography was very expensive thus limiting the access. Recently, scanning electron microscopes were
able to be equipped with pattern generator modules, thus enabling the scanning of electron beam spot
within desired areas to generate nanoscale patterns as electron beam lithography systems [28, 29]. As a
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result, this technique has become widely used which greatly contributed to the progress in nanoscience
and engineering.
electron beam
substrate
resist
(b) Top view(a) Side view
exposed pattern
electron beam
source
beam blanker
final condense lens
2
nd
condense lens
deflector
1
st
condense lens
aperture
Fig. 3. Schematic illustration of electron beam lithography. Electron beam is focused on a resist film
to create a pattern by exposing dot by dot: (a) side view of the lithography setup; (b) top view
of the exposed pattern by a serial writing.
2.3. Soft lithography and nanoimprint lithography
Soft lithography arose from the innovation of using a relatively soft polymer stamp to imprint a solution
of molecules (ink) onto a substrate for pattern transferring. This technique requires inexpensive
materials and employs non-specialized equipment. It was first introduced by Bain and Whitesides in
1989 [11]. Their pioneered work greatly contributed to the development of this technique as
summarized in Ref 12 and 13. This process can be separated into two main steps: the fabrication of a
patterned polymer stamp, and the use of this stamp to transfer molecules in geometries defined by the
element’s relief structure. Figure 4 depicts the schematic illustration of soft lithography. The uniqueness
of this technique is on the utilization of a soft stamp for pattern transferring, thus it allows a conformal
contact between the stamp and the substrate resulting in a patterning capability on flexible or curved
substrates.
Similar to soft lithography, nanoimprint lithography utilizes a hard mold to imprint into a polymer
film for nanoscale patterning. Nanoimprint lithography has emerged as a candidate for next-generation
manufacturing methods as it has great potential to circumvent the issues in photolithography, thereby,
promising a high-throughput and high-resolution method with a relatively low cost [1, 3]. The details of
a cost comparison can be found in Ref 1. Nanoimprint lithography was first introduced by S.Y. Chou as
“hot embossing technique” enabling the definition of features with lateral sizes down to sub-10 nm [14,
15]. The procedure of imprinting lithography is shown in Fig. 5 (left). The technique heats
thermoplastic polymer above its glass transition temperature enables material flow, filling the structures
of a mold. After that, its temperature is lowered and the replicated patterns are solidified in place, after
which the mold is removed. The most commonly used materials for mold have been quartz and silicon
that are kinds of hard material. Typically, mold features are patterned by using conventional lithography
techniques such as photolithography and electron beam lithography. A hard material offers a number of
advantages for nanofabrication. The rigidity retains nanoscale features with minimal local deformation.
Moreover, a hard mold is thermally stable at high temperature. Although nanoimprint lithography has
made a great progress in a relatively short time, there are few issues to be resolved. One of them is life
