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I have imposed upon myself, as a law, never to advance but from what is known to what is unknown, never to form any conclusion which is not an immediate con-sequence necessarily flowing from observation and experiment; and always to arrange the facts, and the conclusions which are drawn from them, in such an order as to render it most easy for beginners in the study of chemistry thoroughly to understand them.

Antoine Lavoisier, Traite´ E´ le´mentaire de Chimie

It is hard to think of an invention that has had a greater influence on the mass production of devices that have enabled humankind to communicate information with printed matter or with photons or with electrons or with ions or even with atoms than lithography. The information highways and pathways of our present age—the information age—are literally paved with chips of crystalline silicon made by lithography. Nearly every book, magazine, newspaper, brochure, flyer, catalog, and other print piece that has been produced during the last three hundred years has been printed with offset lithography. Nearly every integrated circuit (IC) in the chips that run the computers and telecommunication systems that power the information highway, as well as medical devices, electronics, home and industrial appliances, automobiles, and airplanes—to mention but a few—is made by semiconductor lithography. Without such integrated circuits, we would have no powerful computers, no large-scale automation, no communi-cation satellites, or even space exploration. There would certainly be no electronic calculators or digital watches, no transistor radios, portable tape recorders, personal digital assistants, Internet, cell phones, etc. Many diagnostic procedures in medi-cine and dentistry rely on integrated circuits, as does the heart pacemaker and the modern hearing aid. The impact of all these things on our lives is tremendous.

For instance, we can watch events on our television sets or mobile phones or on the Internet as they are happening thousands of miles away. We can withdraw our money from automatic teller machines almost everywhere in the world, without the aid of a bank clerk, whose function has been transformed from that of an acco-unts keeper to one of an intermediary between the customer and the computer.

Many products are now manufactured, assembled, and shipped today by automatic machines that rely on integrated circuits for their operations. Airplanes are guided by computer-controlled systems and even airline seats are reserved worldwide by an instantaneous computer booking system. The list is almost endless and is growing every day. A world without lithography would be unrecognizable to any one of us today, although we may not always be cognizant of its pervasiveness and reach in our daily lives.

How did we get to where we are today? It all started with the invention of lithography in 1798, of photolithography and photography simultaneously in 1826, and subsequent developments in lithography ever since, all of which were made possible by antecedent developments in chemistry and optical physics over a period spanning more than 30 centuries, since records began to be kept.

Through this unique marriage of chemistry and optics, the science and technology of lithography have evolved and made possible the mass production of printed materials, starting from the late eighteenth century during the industrial revolution and culminating in the microelectronics revolution that ushered in the mass pro-duction of microelectronic equipment, starting from the middle of the twentieth century with the invention of the transistor in 1947 and the invention of the integrated circuit 12 years later. Innovations in lithography, new materials, and scaling to ever-smaller dimensions have led to many orders of magnitude of improvement in the capability of transistors to carry out computation, thus paving the way for the information age in which we currently live. All of these developments have radically influenced the course and trajectory of human civili-zation and development.

This book therefore deals with how chemistry mediates lithography, a topic that has not been previously discussed at length. In particular, it traces the arc of developments in lithography from a chemical perspective, starting from its invention and reaching back in an unmistakably continuous line to a period much earlier still. Like an arc, it has a beginning, a bow, and a tip. The beginning of this arc is the invention of lithography in 1798, the bow of the arc is the invention of photolithography in 1826, and the tip of the arc coincides with the develop-ment of semiconductor lithography in the 1950s and culminates in the state of the art in the field today.

The objective of this book is not to lay out 30 centuries of the history of science, particularly that of physics and chemistry, like a long piece of wallpaper, and divide it into so many superficial categories after the manner of the encyclopedist and the abridger. Instead, we will focus on the lines of strategic change and care-fully examine those moments in the history of physics and chemistry that seem con-sequential and uncover the intellectual knots that had to be untied, which directly or indirectly aided the development of lithography. The treatment therefore needs not follow in chronological order and linear fashion, but rather must be organized around similar coherent themes.

It is very useful to learn from the mistakes of early scientists, to examine parti-cular intellectual hurdles associated with given periods, as well as the course of scientific developments that ran into blind alleys, but that nonetheless affected the progress of science in general and lithography in particular.

Since its invention, lithography has witnessed tremendous evolution. Many of its variants are now practiced, ranging from stone plate lithography used in fine art printing, to offset lithography used in the printing of newspapers and the like, and to semiconductor lithography, which utilizes a variety of exposure radiations to print integrated circuits. While all of these variants of lithography are covered in this book, our emphasis will be on semiconductor lithography, since it is the most scien-tifically and technically advanced form of lithography. And within semiconductor lithography, optical lithography is the most dominant technique used in fabrication of integrated circuits. Most importantly, relative to other lithographic techniques, semiconductor lithography best exemplifies the marriage of chemistry and optics—a theme that we explore in depth in this book.

Lithography in its very essence is a series of chemical transformations—a fact recognized by its inventor who called it chemical printing. Like all chemical transformations, its currency of transaction is the electron—outer-shell electrons of the atoms, molecules, and compounds of the material (also called resist) on which the image of the mask or the object to be printed is recorded, to be precise. These electrons orchestrate distinct bond-breakage and bond-formation events in all of the process steps in lithography that ultimately lead to the contrast between the clear (exposed) area and the dark (unexposed) area of the image-recording medium. Preparation of the substrate, coating of the resist, the actual exposure, and subsequently the postexposure thermal and related processing are all characterized by distinct chemical processes that taken together are ultimately about chemical bond breakage and formation.

In the substrate preparation step involving priming, the surface chemistry of the substrate is modified to promote the adhesion between the substrate and the resist material during coating. In the exposure process proper, exquisite radiation chemistry takes place inside the radiation sources in order to generate the exposure radiation, but also because of the interaction of these radiations—be they photons, ions, electrons, x rays—with the outer-shell electrons of the radiation-sensitive compounds and molecules in the resist, leading to bond breakage and/or bond formation.

In the postexposure thermal processing steps, thermally driven diffusion and reaction of the active species that catalyze deprotection, bond scission, or cross-linking reactions in the resist are engendered. In the development step, appropriate areas of the resist film are dissolved away either through physical dissolution (involving no chemical reaction) or through acid-base neutralization reactions between the exposed areas of the resist and the developing solvent (depending on polarity). This is the basis of the contrast between the exposed and unexposed areas of the resist film.

The main attribute of optical lithography that made it the manufacturing tech-nology of choice for ICs since the beginning of the IC era is the tremendous throughput advantages it offers through its ability to reproduce an entire IC layout from a master (or reticle) in a single exposure, in contrast to other technol-ogies that address a field point by point. In addition to the compelling throughput advantages, there were resolution and cost advantages as well. The infrastructure for light sources, lenses, reticles, photosensitive polymers, and other optical materials developed for other optical and photographic applications were

appropriated and applied to IC lithography, allowing development resources to be shared.

In 1965, Gordon Moore¹postulated that the exponential growth in the number of transistors in an IC led to certain technical and economic advantages. Smaller transistors switch faster, allowing more operations per second. And more transis-tors with more interconnections enable computations of much greater complexity to be achieved. This postulate has since been codified as Moore’s law,²which states that the complexity of ICs as measured by the number of transistors approximately doubles every two years (see Fig. 1.1). This law has led to unprecedented growth in the computer industry. Technologies that were once available only in supercompu-ters are now commonly available in children’s toys. Satellite communications net-works that were once the domain of the military now help drivers find their way to their locations.

A good metric for measuring progress in IC lithography is resolution, the ability to resolve and distinguish two neighboring features on the chip. Two main approaches for improving resolution include decreasing the wavelength of

0 5 10 15 20 25 30 35

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year

Log_base_2 of transistor count

Intel AMD Linear (Intel) Linear (AMD)

Figure 1.1 Plot of the number of transistors versus year for microprocessors manufac-tured by Advanced Micro Devices, Inc. and Intel Corporation microprocessors.³

1G.E. Moore, “Cramming more components onto integrated circuits,” Electronics 38(8), 114 117 (1965).

2G.E. Moore, “Progress in digital electronics,” IEEE Meeting 1975, Tech. Digest 11 13 (1975);

G.E. Moore, “Lithography and the future of Moore’s law,” Proc. SPIE 2440, 2 17 (1995).

3For information on transistor count number versus year of manufacture for AMD’s microprocessors, please see http://www.amd.com; for Intel Corporation’s microprocessors, please see http://

www.intel.com/museum/archives/history docs/Moore.htm/Intel Microprocessor Transistor Count Chart.

the optical source and increasing the numerical aperture of the optical system, as derived from Rayleigh’s resolution criterion⁴and shown in Eq. (1.1).

w ¼ k1

l

NA, (1:1)

where w is the half pitch of the feature being printed, k1 is a process-dependent parameter, l is the exposure wavelength, and NA is the numerical aperture of the optical system, which is defined in terms of the maximum cone angle of rays (umax) subtended by the maximum pupil diameter at the image plane as:

NA n sin u_max(where n is the refractive index in image space). Equation (1.1) unites chemistry with optics in lithography, for it is the interaction of the exposure photons, electrons, ions, or x rays transmitted through the numerical aperture of the optics, with the electrons of the high-contrast recording medium—photosensitive materials (resists)—that mediates chemical phenomena on which lithographic pat-terning is based. The exposed part of the resist is altered relative to the unexposed part, leading to contrast between the two regions during development.

The progress of optical lithography is partly, then, the result of decreasing the exposure wavelength. From the initial broadband sources, the IC industry made a migration first to the mercury g-line (436 nm), then to the i-line (365 nm), and then switched to exciplex⁵laser sources—first KrF (248 nm) and now ArF (193 nm).

A migration to an F2excimer laser source was contemplated by the IC industry, but did not materialize, even after significant investments, primarily because of issues associated with the availability of IC industry-grade calcium fluoride (CaF2), used in the lens elements. The migration toward shorter wavelengths

4Lord Rayleigh, “Investigations in optics, with special reference to the spectroscope,” London, Edinburgh, Dublin Phil. Mag. J. Sci., Series 1 6, l8(49), Pt. XXXI, 261 274, Pt. XLVI, 403, 411, and Pt. LVI, 477 486 (1879); Lord Rayleigh, “Investigations in optics, with special reference to the spectroscope,” London, Edinburgh, Dublin Phil. Mag. J. Sci., Series 7, 9(53), Pt. V, 40 55 (1879); Lord Rayleigh, “On the theory of optical images, with special reference to the microscope,”

London, Edinburgh, Dublin Phil. Mag. J. Sci. 42(255), Pt. XV, 167 195 (1896).

5The term “exciplex” refers to a combination AB^of two different atoms; it exists only in an electro nically excited state and dissociates as soon as the excitation is lost. It differs from an “excimer,” an excited state dimer of two similar atoms AA^. The exciplex lasers that have found widespread appli cations in lithography are based on KrF^and ArF^formed in electrical discharge in a mixture contain ing krypton and fluorine in KrF lasers and argon and fluorine in ArF lasers, respectively. The only true excimer laser that has found application in lithography is based on excited state F2dimers (lasing at 157 nm). The KrF^and ArF^exciplex and F₂excimer survive for a few nanoseconds, long enough to participate in laser action. As soon as the excitation is gone, the atoms separate because the potential energy curve of their ground state is repulsive. Unfortunately, the widely used misnomer “excimer laser” appears in the literature to describe exciplex lasers XeCl^(lasing at 308 nm), KrF^(lasing at 248 nm), and ArF^ (lasing at 193 nm) when “exciplex laser” is appropriate. In this book, we will use the appropriate terms. [For the photochemistry of excimers and exciplexes, please see, for example, P.W. Atkins, Physical Chemistry, 5th ed., p. 609, W.H. Feeman, New York (1994);

P. Suppan, Chemistry and Light, pp. 104 110, Royal Society of Chemistry, Cambridge, England (1994)].

naturally limits the pool of available photosensitive materials that could be employed in resist formulations.

Because the NA of the optical system limits the spatial frequencies that can be transmitted to expose the resist, the NA of lens designs has migrated from 0.2 to 0.42 to 0.63 to 0.75 to 0.95. With a fundamental limit of NA 1.0 for a conven-tional optical system, the introduction of immersion ArF lithography has enabled the migration to hyper-NA (.1.0) optical systems. It is noteworthy that the drive toward high NA is at a cost of decreased depth of focus and increased difficulty in fabricating a lens with adequate field size.⁶

Today, the leading-edge microelectronic devices are being made with photo-lithography at 193 nm, which is inevitably a continuation of the progression from longer-wavelength lithographies and is dictated by the requirements for higher resolution and the drive in the IC industry toward greater packing density and higher speeds, as noted earlier.

The resolution that will be necessary for the manufacture of future generations of ICs with feature sizes below 22 nm is beyond the limits of 365-, 248-, and 193-nm UV lithographies. According to the International Technology Roadmap for Semiconductors (ITRS),⁷ extreme ultraviolet (EUV) lithography at 13.5 nm is a promising candidate for achieving such high resolution. With EUV lithography, sub-22-nm devices can be fabricated with conventional masks using reflective optics of 0.25 – 0.45 NA.

As stated above, this book attempts to systematically reappraise the main developments in chemistry and optics that have ultimately led to lithography as practiced today, especially in semiconductor lithography—the most advanced form of lithography.⁸The task is no doubt an onerous one, but one that must be done in order to unearth the hidden connections between the various streams of thoughts that materialized as lithography and subsequently as semiconductor lithography.

6M.J. Bowden, “The lithographic process: the physics,” Chapter 2 in Introduction to Microlitho graphy, L.F. Thompson, C.G. Willson, and M.J. Bowden, Eds., pp. 19 138, American Chemical Society, Washington, DC (1994).

7http://www.itrs.org

8Although the emphasis of this book is on semiconductor lithography, attempts will be made where necessary to highlight relevant aspects of the low technology variants of lithography as practiced in offset lithography and fine art lithography.

Chapter 2

Invention of Lithography