graphy
· nanostructures
· resists
Ion projection lithography can be used to perform many different functions.
594
2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim
DOI: 10.1002/smll.200500050
small 2005, 1, No. 6, 594 608
reviews
A. A. Tseng I
on projection lithography (IPL) is an emerging technology and a major
candidate for the next-generation lithography (NGL) designed to comple-
ment and supplement current optical lithographic techniques for future chip
manufacturing. In this Review, the recent developments of IPL technology
are examined with an emphasis on its ability to fabricate a wide variety of
nanostructures for the semiconductor industry. Following an introduction of
the uniqueness and strength of the technology, the basics of ion-source
development and iontarget interactions with and without chemical
enhancement are presented. The developments in equipment systems, masks,
and resists are subsequently studied. The resolution of printed nanostructures
and the corresponding throughput of the current system are assessed for
NGL. Finally, concluding remarks are presented to summarize the strengths
and weaknesses of the current technology and to suggest the scope for future
improvement.
1. Introduction
In ion projection lithography (IPL), ions that are ex-
tracted from a source and collimated through a mask with
the imaging pattern are accelerated through a series of elec-
trostatic lenses that project the ions onto a wafer substrate,
where the ions penetrate and modify the substrate materials.
While passing through the lens system, the ions are acceler-
ated from between tens of keV to hundreds of keV and
thus, IPL can perform many different functions including
resist exposure, direct material sputtering, and the initiation
of chemical reactions for etching or deposition (as indicated
in the frontispiece). In performing its major task, resist ex-
posure, IPL is very similar to optical lithography (OL).
Both use reduction optics to project an image onto the
wafer, and stepping and repeating exposures are similarly
performed with the use of a precisely controlled laser inter-
ferometer stage.
Basically, IPL has the capability to realize printed fea-
tures at 50 nm resolution by using lightweight ions to
expose a resist. A wide range of projection energy and ion
species can be tailored to meet proper exposure or modifi-
cation conditions of the material while at the same time
causing no damage to the underlying materials or circui-
tries.
[1, 2]
IPL also has a large depth of focus (up to
Æ 500 mm) and very short exposure times (less than 0.5 s). A
variety of materials have been shown to work well with IPL
and no major effort is needed to develop new resist materi-
als. As a result, IPL has been selected as a major candidate
for next-generation lithography (NGL). NGL refers to the
post-optical lithography era and is designed to complement
and supplement OL for future semiconductor manufactur-
ing.
In addition to IPL, the candidates for NGL include ex-
treme ultraviolet (EUV) lithography, electron projection
lithography (EPL), direct-write e-beam lithography (EBL),
and X-ray lithography (XRL). Also, imprinting lithography
(IL) was added as an NGL candidate in the 2003 edition of
the International Technology Roadmap for Semiconductors
(ITRS) by SEMATECH.
[3]
Based on the current trend to
produce ever-shrinking device sizes and increased processor
speeds, it is expected that OL will become inadequate for
making some critical elements by the end of this decade.
This is when NGL technologies should take over production
of these critical elements for state-of-the-art semiconductor
devices. It is also expected that even after NGL becomes
mature, OL will still be around and perform noncritical fea-
tures. As such, NGL would coexist with OL for many more
years and should be seamlessly implemented into the manu-
facturing lines built for OL.
Since the worldwide market for semiconductor products
is huge (US$ 166 billion in 2003 according to the Semicon-
ductor Industry Association), NGL technologies are all
competing for a share of the next paradigm shift in lithogra-
phy techniques. All of these technologies also have their
own particular weaknesses. IPL certainly is not perfect, but
it offers several advantages for semiconductor manufactur-
ers. For example, XRL is too expensive and has a shorter
lifetime than IPL. The e-beam technology is too slow and
has far more potential for pitfalls than IPL. The optics for
EUV are still problematic and have a relatively shorter life-
time than IPL. It is inevitable that all these NGL technolo-
gies have to make room for an alternative and complemen-
tary technology such as IPL.
[4]
A shift by the semiconductor industry to any NGL tech-
nology would require the introduction of a new infrastruc-
ture of tools, materials, and processing techniques, the re-
[*] A. A. Tseng
Department of Mechanical and Aerospace Engineering
Arizona State University, Tempe, AZ 85287-6106 (USA)
Fax: (+ 1) 480-965-1384
E-mail: ampere.tseng@asu.edu
small 2005, 1, No. 6, 594 608
DOI: 10.1002/smll.200500050
2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim
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Ion Projection Lithography search and development costs of which would be enormous.
A review article on IPL should provide the necessary infor-
mation for making a good judgment in the selection of a
NGL technique. As a result, the purpose of this Review is
to assess the technical capabilities of IPL through an over-
view of its recent technical advances, especially the strength
and weakness of the lithographic equipment already devel-
oped, as well as the resolution of the nanostructures made
by the equipment. The ability to make a high-resolution
structure is the most important and basic criteria in judging
the suitability of NGL. At the present stage, to be competi-
tive, any vital NGL candidate should have the ability to
make nanoscale structures. Here, a nanoscale structure or
nanostructure can act as a component, device, or system,
having a feature size in the range from 0.1 to 100 nm.
In this Review, the current developments in the litho-
graphic capabilities of IPL, including ion sources, equip-
ment, masks, and resists are first assessed. To illustrate the
versatility and advancement of these lithographic capabili-
ties, a wide variety of nanostructures made by different ex-
posures and trial conditions are subsequently examined with
an emphasis on the resulted resolution and equipment
throughput. Finally, a summary of the current progress and
the scope recommended for future developments are pro-
vided to conclude the present study.
2. Ions and Interactions
Ions are particles with net electrical charges, which usu-
ally are atoms lacking one or more orbiting electrons.
Therefore, they can be steered by electric or magnetic
fields. In IPL, ions are collimated into a beam that passes
through a stencil mask and is projected onto the substrate
using electromagnetic lens systems. In this section, the char-
acteristics and sources of ions as well as their interactions
with other materials and chemicals are discussed and ana-
lyzed.
2.1. Characteristics of Ions
One of the most important features of IPL is that its
ions have extremely small particle wavelengths (for in-
stance, the de Broglie wavelength of 100 keV He
+
ions is
just 5 10
À5
nm), whereas photon-based OL or EUV lithog-
raphy is operated at the diffraction-limited resolution at
which the shortest wavelength currently considered is on
the order of 10 nm in the EUV region. Certainly, charged-
particle- (including ion-) based optical resolution is limited
by lens aberrations. In general, for particle-based optics, one
requires that the diffraction-limited resolution should be
one tenth of the minimum feature size to be printed.
Moreover, ions possess advantages over other high-
energy particles used in nanofabrication. For example, when
compared to electrons, ions are much heavier and can strike
with greater energy at relatively shorter wavelengths to di-
rectly transfer patterns onto hard materials (such as semi-
conductors, metals, or ceramics) without major forward- and
back-scattering. Thus the feature size of the patterns is
largely dictated by the beam size and the interaction of the
beam with the target material. On the other hand, electrons
or photons can mainly be applied for writing on soft materi-
als (such as polymers or resists) and the corresponding fea-
ture sizes are determined by the proximity of the back-scat-
tered electrons or wave diffraction limits. Moreover, the lat-
eral exposure in an ion beam is very low, thereby exposing
only the correct areas and writing very narrow lines in the
substrate, which makes it more capable to directly fabricate
nanostructures.
[57]
2.2. Ion Sources and Beam Quality
The ion source is important because its properties affect
many parameters involved in forming an ion beam as well
as the interaction between the beam and substrates in fabri-
cation. Two major types of ion sources, point and volume-
plasma sources, have been developed to produce nanome-
ter-resolution patterns. Normally, point sources are used to
form a focused ion beam (FIB), in which a sharp dot image
is focused directly on the substrate for direct writing. On
the contrary, volume-plasma sources are used for IPL, in
which a parallel ion beam is printed onto a substrate or
resist through a mask with or without demagnification.
In general, the axial energy spread of the ion beam
when coupled with th