EUV Lithography—The Successor to Optical Lithography? 1
EUV Lithography—The Successor to Optical Lithography?
John E. Bjorkholm
Advanced Lithography Department, Technology and Manufacturing Group, Santa Clara, CA.
Intel Corporation
Index words: EUV lithography, lithography, microlithography
Abstract
This paper discusses the basic concepts and current state
of development of EUV lithography (EUVL), a relatively
new form of lithography that uses extreme ultraviolet
(EUV) radiation with a wavelength in the range of 10 to
14 nanometer (nm) to carry out projection imaging.
Currently, and for the last several decades, optical
projection lithography has been the lithographic
technique used in the high-volume manufacture of
integrated circuits. It is widely anticipated that
improvements in this technology will allow it to remain
the semiconductor industry’s workhorse through the 100
nm generation of devices. However, some time around
the year 2005, so-called Next-Generation Lithographies
will be required. EUVL is one such technology vying to
become the successor to optical lithography. This paper
provides an overview of the capabilities of EUVL, and
explains how EUVL might be implemented. The
challenges that must be overcome in order for EUVL to
qualify for high-volume manufacture are also discussed.
Introduction
Optical projection lithography is the technology used to
print the intricate patterns that define integrated circuits
onto semiconductor wafers. Typically, a pattern on a
mask is imaged, with a reduction of 4:1, by a highly
accurate camera onto a silicon wafer coated with
photoresist. Continued improvements in optical
projection lithography have enabled the printing of ever
finer features, the smallest feature size decreasing by
about 30% every two years. This, in turn, has allowed
the integrated circuit industry to produce ever more
powerful and cost-effective semiconductor devices. On
average, the number of transistors in a state-of-the-art
integrated circuit has doubled every 18 months.
Currently, the most advanced lithographic tools used in
high-volume manufacture employ deep-ultraviolet (DUV)
radiation with a wavelength of 248 nm to print features
that have line widths as small as 200 nm. It is believed
that new DUV tools, presently in advanced development,
that employ radiation that has a wavelength of 193 nm,
will enable optical lithography to print features as small
as 100 nm, but only with very great difficulty for high-
volume manufacture. Over the next several years it will
be necessary for the semiconductor industry to identify a
new lithographic technology that will carry it into the
future, eventually enabling the printing of lines as small
as 30 nm. Potential successors to optical projection
lithography are being aggressively developed. These are
known as “Next-Generation Lithographies” (NGL’s).
EUV lithography (EUVL) is one of the leading NGL
technologies; others include X-Ray lithography, ion-
beam projection lithography, and electron-beam
projection lithography. [1]
In many respects, EUVL may be viewed as a natural
extension of optical projection lithography since it uses
short wavelength radiation (light) to carry out projection
imaging. In spite of this similarity, there are major
differences between the two technologies. Most of these
differences occur because the properties of materials in
the EUV portion of the electromagnetic spectrum are
very different from those in the visible and UV
wavelength ranges. The purpose of this paper is to
explain what EUVL is and why it is of interest, to
describe the current status of its development, and to
provide the reader with an understanding of the
challenges that must be overcome if EUVL is to fulfill its
promise in high-volume manufacture.
Why EUVL?
In order to keep pace with the demand for the printing of
ever smaller features, lithography tool manufacturers
have found it necessary to gradually reduce the
wavelength of the light used for imaging and to design
imaging systems with ever larger numerical apertures.
The reasons for these changes can be understood from
the following equations that describe two of the most
fundamental characteristics of an imaging system: its