A second edition of a text has a number of advantages over the first edition: in
addition to representing the accumulated wisdom of the readers who have
suggested ideas for improving the previous edition, it also incorporates
developments in the field that have taken place since the publication of the
first edition. From their familiarity with the previous edition and in keeping
with the general aim to make the text accessible, readers will be aware that I
have provided not a slight tinkering with the text in this edition, but a
wholesale rewriting of some chapters, while introducing entirely many new
ones.
This present edition is a series of three volumes, namely, The Chemical
History of Lithography (volume 1), Chemistry in Lithography (volume 2), and
The Practice of Lithography (volume 3). Each volume is a unit in itself and can
stand alone, which is fortunate in view of their different subject matters.
My motivation for splitting the second edition of Chemistry and
Lithography into the above three volumes is because the subject matter of
these three books is very broad and lends itself nicely to multiple angles of
analysis and interpretation. Lithographically fabricated structures appear in
an increasing wide range of scientific and technical fields, beyond their
traditional niches in fine arts, paper printing, and electronics. In particular,
the last ten years since the publication of the first edition of Chemistry and
Lithography have witnessed a phenomenal pace of development in advanced
lithography, perhaps the fastest pace of development in any decade over the
222-year history of the field. The transistor count of the leading-edge
integrated circuit when the first edition was published was around 2.5 billion,
and a short five years later that count rose to 10 billion. By the time the
present edition is published, the transistor count of an equivalent device may
well be over 20 billion, representing a nearly ten-fold increase within a decade
in the ability to process, communicate, and store electronic information, made
possible in no small part by advances in lithography. Such is the pace of the
development in this field that the analysis I make in these three volumes is a
sufficiently different and significantly expanded version of the treatment of the
subject I rendered in the first edition. As such, there are a considerable
number of changes within the text of the present edition.
The first edition covered some brief historical material in Part I, and in
volume 1 of the present edition these historical aspects are extended. It seems
reasonable to me that with the rapid advances being made in advanced
lithography it is very important to realize that some of the concepts that we
now take for granted in chemistry and physics that enervate lithography are
by no means self-evident and often were developed only after much struggle
and controversy. Improvements are still being made in all of these fields and
can all the more be clearly understood by those who appreciate how our
present understanding of the fields were attained.
In developing volume 1 of the present edition, I drew considerably from
Part I of the first edition. Relative to Part I of the first edition, volume 1 of the
present edition comprises five significantly expanded chapters: Chapters 1 and
2 of first edition with slight tinkering; an expanded version of Chapter 4 of the
first edition; and two new chapters derived from expanding and splitting up
Chapter 3 of the first edition. The object of volume 1 of the present edition is
to weave together threads of a narrative on the history of optical and
molecular physics, optical technology, chemistry, and lithography, with a
view to creating a rich tapestry that gives the reader new insights into an
aspect of the relationships between these fields that are often not fully
appreciated: how the marriage between chemistry and optics led to the
development and evolution of lithography. I show how major developments
in the chemistry, physics, and technology of light influenced the invention and
development of lithography, well beyond what its inventor envisioned. I also
show how developments in lithography have not only influenced the
development of optics and chemistry, but also played a critical role in the
large-scale manufacture of integrated circuits that run the computers and
machineries on which our modern electronic and information age depend.
Part of the analysis in volume 1 of the present edition is necessarily skewed
toward the underlying science and technologies of advanced lithographic
patterning techniques, in terms of materials, processes, and imaging, along
with their unique features, strengths, and limitations. This book also provides
an analysis of the emerging trends in lithographic patterning, as well as the
current and potential applications of the resulting patterned structures and
surfaces.
The object of volume 2 of the present edition is to deconstruct lithography
into its essential chemical principles and to situate its various aspects in
specific fields of chemistry. It comprises 16 chapters developed around a
rewriting of Chapters 5 through 8 of Part II of the first edition and parts of
Chapters 13 and 14 of Part III of the first edition. It also includes eight entirely
new chapters that explore in a fundamental manner the role of chemistry in
mediating specific aspects of the lithographic process.
Volume 2 of the present edition is in fact an outgrowth of the SPIE
Advanced Lithography Short course Chemistry and Lithography (SC1099)
that I have taught at the SPIE Advanced Lithography Symposium over the
past six years. In this volume, I develop a chemistry and lithography
interaction matrix, and use it as a device to illustrate how various aspects and
practices of advanced lithography derive from established principles and
phenomena of chemistry. For instance, lithographic unit operations involving
principally the resist fall within the realm of process chemistry. Photochemistry
is involved in the generation of photons from the exposure sources of
optical, extreme ultraviolet, and x-ray lithographic exposure tools; the
interaction of O2, H2O, NH3, SOx, and hydrocarbons with photons within
the optical lithographic tool exposure chamber, as well as their roles in the
oxidation, carbon deposition and growth, and formation of inorganic salt
crystals (also called haze crystals) on masks and optical elements; cleaning of
contaminated optics and masks with UV photons; clear defect repair of masks
via photo-induced decomposition of organometallic precursors and deposition
of metals on defective areas of the mask; and UV curing and photooxidative
degradations of resists during exposure. Photochemistry is also the
basis of the exposure action of photoacids and photobases in resists, as well as
resist poisoning by airborne molecular bases. It is also the basis of laserproduced
and discharge produced plasmas—the radiation sources for
extreme-ultraviolet (EUV) lithographic patterning. Finally, photochemistry
is the basis of the plasma (dry) stripping of resists and other hydrocarbon
contaminants from wafers and masks.
Similarly, radiation chemistry is involved in the generation of electrons
and ions from the exposure sources of electron-beam and ion-beam
lithographic exposure tools, respectively; cleaning of contaminated optics
and masks, and repair of defective masks with electrons and ions; the electronand
ion-mediated exposure process as well as electron-beam and ion-beam
curing, and crosslinking of resists. The manufacture of lithographic exposure
sources, optical elements, masks, and resist materials involves materials
chemistry. Polymer chemistry is the basis of the synthesis of polymeric resist
resins used in optical, charged particle, and imprint lithography, as well as
block copolymers used in directed block copolymer self-assembly lithography.
Surface chemistry is the basis of priming of relevant mask and wafer
substrates, and their subsequent coating with resists and associated layers used
in the fabrication of masks and semiconductor device wafers. Colloid
chemistry is the basis of the cleaning of masks and wafers, and development
process of exposed resists. It also explains the basis for the stability of resist,
developer, and wet cleaning solutions. Electrochemistry is the basis of the
corrosion and electromigration processes of Cr and Mo mask or reticle
absorber features, as well as electrostatic damage of the same objects. It is also
the basis of electrochemical imprint lithography. Organometallic chemistry is the basis for the precursor materials used in clear defect repair of masks, as
well as EUV metal-oxide resists.
The Practice of Lithography (volume 3) comprises 12 chapters, made up of
Chapters 9 through 17 of the first edition, in addition to three new chapters
covering full treatment of EUV lithography, as well as imprint lithography,
directed block copolymer self-assembly lithography, and proximal probe
lithography. The object of this volume is to present how the more important
lithographic patterning techniques are used to print images on appropriately
prepared flat substrate surfaces using radiations as varied as photons,
electrons, and ions, as well as mechanical force, thermodynamically driven
directed self-assembly of block copolymers, and even electron tunneling
phenomena. In particular, I cover photolithography (or optical lithography),
electron-beam lithography, ion-beam lithography, EUV lithography, imprint
lithography, directed block copolymer self-assembly lithography, and
proximal probe lithography.
Immersion ArF laser optical lithography is currently used in high-volume
manufacture of integrated circuits at 22-, 15-, and 10-nm nodes, using doublepatterning
techniques to decrease feature pitch, where appropriate. EUV
lithography is now entering high-volume production at the 7-nm node in some
of the leading-edge semiconductor companies. Imprint lithography, especially
in its roll-to-roll format, is increasingly being used in the fabrication of flexible
and wearable electronic devices, diffractive optical elements, and large-area
electronics. Directed block copolymer self-assembly is increasingly being used in
the fabrication of functional nanostructures used in applications ranging from
photonics to biomimetics, and from electrochemical energy storage to patterned
electronic media. Now is an auspicious moment to provide an in-depth look
into the chemistry that underpins these most advanced of lithographies.
Along with the introduction of EUV lithography into device manufacturing
at single-digit-nanometer technology nodes, we enter a regime where the
resist suffers from increased stochastic variation and the attendant effects of
shot noise—a consequence of the discrete nature of photons, which, at very
low number per exposure pixel, show increased variability in the response of
the resist relative to its mean. Examples of resist response that may experience
shot noise effects under very low-photon-count-per-pixel conditions within
small exposure volumes, such as in EUV lithographic patterning, include
photon absorption by the resist, and chemical conversion of light sensitive
components in the resist, as well as the chemical changes that make a resist
molecule soluble in the developer. We will examine in this volume of the
present edition the role of stochastics in EUV lithography in far greater detail
than we did in the first edition.
As in the first edition, I have made an attempt throughout the three books
of the present edition to provide examples illustrating the diversity of chemical
phenomena in lithography across the breadth of scientific spectrum, from fundamental research to technological applications. The format of this book is
not necessarily chronological, but it is such that related aspects of lithography
are thematically organized and presented with a view to conveying a unified
view of the developments in the field over time, spanning many centuries,
from the very first recorded reflections on the nature of matter to the latest
developments currently at the frontiers of lithography science and technology.
The emphasis is mostly placed on applications that have relevance to the
semiconductor industry.
A great many of the pioneers of chemistry and lithography are not
represented at all in the three books of the present edition. I can only record
my immense debt to them and all who have contributed to the development of
the two fields to the state in which I have reported it.
I am most grateful for suggestions from a number of experts, particularly
the following: Andreas Erdman of Fraunhofer IISB, Manuel Thesen of micro
resist technology GmbH, and Folarin Latinwo of Synopsys. Special thanks go
to SPIE Senior Editor, Dara Burrows, for her editorial assistance in producing
this book, which is much improved because of her efforts.
I am also grateful to my colleagues in the Department of Polymer Science
and Engineering of University of Massachusetts at Amherst and, in
particular, Prof. Jim Watkins, for lively scientific and technical discussions
on polymers and flexible electronics. The opportunity to work in this
department has not only helped me to broaden and deepen my scientific
research interests, but also, in a direct way, has made it possible for me to
write this book.
Lastly, I acknowledge the informal assistance I have received from my
family members, in particular, from my wife Anett and daughter Sophie, who
created a conducive atmosphere to work on this book at home.
Uzodinma Okoroanyanwu
Florence Village, Northampton, Massachusetts
January 2020
