Dr. Marc J. Madou
roadmap

In this section, we describe the logic behind the organization of this book into ten chapters, describe the appendixes, explain how to use the insets and the glossary, and introduce the Internet site devoted to the book.

The ability to coax a wide variety of materials into ever smaller devices is based on progress in micromachining, nanomachining, nanochemistry (molecular engineering), artificial intelligence, biomimetics, smart materials, and other research areas.  Although the jargon associated with the field sometimes sounds like buzzwords competing for popular press and research dollars, work in these areas contributes to the science and engineering of miniaturization and holds real potential for ground-breaking discoveries and applications.  We characterize and define these major research areas and illustrate them with typical examples culled from the recent literature.

Fundamentals of Microfabrication explores the science of miniaturization, that is, the science of making very small things.  This science comprises an intimate understanding of the intended application, knowledge of the different manufacturing options, familiarity with all materials choices, and an understanding of scaling laws.  Miniaturization techniques surveyed are both top-down methods, in which one builds down from the large to the small, and bottom-up methods, in which one builds up from the small to the large.  Top-down miniaturization methods and materials surveyed include micromachining with single crystal and polycrystalline Si and other micromachining methods and materials based on lithography as well as nonlithographic miniaturization.  In dealing with micromachining techniques borrowed from the electronics industry, we emphasize those that differ most from standard processes and materials used in regular integrated circuit (IC) and hybrid manufacturing.  Although miniaturization and IC fabrication methods are intertwined, in miniaturization science, a much wider variety of processes, more and different materials, and other applications are considered.

We have named the bottom-up methods nanochemistry (or molecular engineering).  These methods are often inspired by natural processes (biomimetic) and include manufacturing options such as self-assembled monolayers (SAM), self-repair, individual particle manipulation tools, hydrogels, DNA-assisted micro assembly, replication, template chemistry, Langmuir-Blodgett films, and bottom-up artificial intelligence.  Bottom-up methods are often quite a stretch for the more traditional micromachinists, who tend to be mechanical and electrical engineers.  For chemists familiar with supramolecular chemistry and for biotechnologists already coaching nature into producing useful chemicals, this is an almost expected trend.  Hence, we have also tried to provide a good foundation for the understanding of bioengineering principles so the gap between “dry” and “wet” engineering might be narrowed somewhat.

New developments in lithography largely determine which direction the IC industry and derived fields like micromachining and nanomachining will take. Therefore, Chapter 1 introduces the book by discussing different lithography techniques.  The optimum future lithography might well be different for miniaturization science than for IC technology.  Whereas finer line-widths and standardized materials are the main quest in the IC industry, miniaturization seeks higher features, high aspect ratios, and the introduction of new materials.  Using the latest lithography tools, we access the nano domain and examine nanofabrication or nanomachining.  Even atom lithography is now becoming possible—for example, by using a proximal probe as a tool to add or erode atom by atom.  We classify the latter as a nanochemistry technique (Drexel calls it mechanosynthesis).  Resist patterns created by lithography on a substrate can be transferred to the substrate by subtractive (etching) or additive (deposition) techniques, the subjects of Chapters 2 and 3, respectively.

Dry etching, discussed in Chapter 2, is an important subtractive pattern transfer method in both IC and miniaturization science in general.  Recent progress in deep directional etching as well as environmental concerns about chemical wet etching have helped push dry etching even more to the forefront.

Chapter 3 covers additive pattern transfer techniques.  A limited description of thin film techniques and doping methods suffices, as these techniques are, in most cases, the same for both IC and the rest of miniaturization science.  On the other hand, thick film deposition technologies such as silk-screening and drop delivery methods are important in the manufacture of new chemical and biological sensors and sensor arrays.  More effort is dedicated to thick film materials and processes, as they are mostly foreign to IC production.

Chapter 4 describes wet bulk micromachining, a key process in miniaturization of sensor and sensor systems, often involving single crystal materials but used less in IC manufacturing.

Surface micromachining, a method involving thin film additive techniques as well as wet and dry etching and “sacrificial layers,” is presented in Chapter 5. The rapid commercial acceptance of surface micromachining for fabricating mechanical sensors is explained in terms of its compatibility with existing IC equipment and processes.

LIGA (a German acronym for lithography, electroplating, and molding), a versatile miniaturization tool for making primary molds, is based on deep x-ray lithography, electrodeposition, and micromolding; and is covered extensively in Chapter 6.  Since the advent of LIGA, electrodeposition and micromolding have become important tools in conjunction with alternative, less-expensive methods of making miniature primary molds, such as ultrafine mechanical machining, deep UV lithography, and electrodischarge machining (EDM).  Hence, both electroplating and micromolding methods are covered beyond their use in the context of LIGA.

Chapter 7 compares top-down and bottom-up techniques.  The arsenal of miniaturization tools has increased dramatically over the last twenty years, and since different applications require different fabrication means, a thorough appreciation of all the possible options is essential.  It is one of the objectives of this book to broaden the perspective of the reader on the different options available in the manufacture of small things.  Applying miniaturization tools more correctly to the problem at hand might generate more commercial successes.  In Chapter 7, we speculate on the future of miniaturization.  We believe that for the IC world, top-down nanofabrication—heir to microfabrication, using the same subtractive and additive processes to build devices on a finer and finer scale—will continue to be the path of progress for perhaps two more decades.  In research on quantum devices, biological applications, and sensors, nature will be the guide, and bottom-up nanochemistry (molecular engineering) will be the path of most progress.  The synergy between nanofabrication and nanochemistry may prove the most fruitful research domain for the next two decades.  In Chapter 7, we also introduce some of the molecular biology concepts of life, as progress in biotechnology (disposable diagnostics, PCR, the human genome project, high-throughput drug screening, proteomics, genomics, etc.) is proving a greater force towards miniaturization than have mechanical applications such as pressure sensors, accelerometers, and gyros.

All currently available miniaturization tools explored, we turn our attention in Chapter 8 to the design of new miniaturized devices, their packaging, and how to install a brain into these devices (if needed).  Because no standard design rules permeate miniaturization and because of the very difficult partitioning decisions of the different functions in a miniaturized system (e.g., sensing, electronics, power, actuators, etc.), early attention to the design and packaging of miniature systems is even more important than in the IC industry.  We cannot stress enough the importance of starting a design from a good understanding of its application and from the application-specific package and real world interface.  Only then should the preferred miniature system be applied.  We will see that new miniaturization techniques themselves provide many excellent solutions for future device packaging issues, including batch packaging of sensors and a few early attempts at self-assembly of macrocomponents.  Merging of IC design software with micromechanical design code is helping a slow but certain introduction of miniaturization methods in even the most conservative companies.  With respect to artificial intelligence (AI) built into microsystems, we note that the newer bottom-up of AI (neural networks) has so far been more successful than the traditional, analytical top-down AI approach (based on logical principles and complex algorithms).  It will be interesting if bottom-up manufacturing also eventually becomes the more successful approach.

After fabrication, design, packaging, and brains have been explored, we turn in Chapter 9 to scaling, actuating, and powering of miniaturized systems.  The chapter also covers the importance of miniaturization in general, with an emphasis on the most difficult devices to miniaturize, actuators and power sources.  A good understanding of scaling law can help the reader develop “micro intuition” and assist in making decisions about the optimum miniaturization approach and design.  From a fundamental perspective, we see the emergence of the most exciting miniaturization opportunities in those areas where the macrocontinuum models break down in the micro domain.

In Chapter 10, we present current and potential applications and review market opportunities.

Appendixes A through G provide useful, complementary information for aspiring miniaturization scientists, with topics ranging from information on metrology tools (Appendix A) to the glossary (Appendix G).  Words that may be unfamiliar for someone new to the field can be found in the glossary.

Text/figure combinations in the text, referred to as insets, are self-explanatory additions to the subject matter, less integrated in the text than the regular figures.  Because of the rapidly changing nature of the science of miniaturization, http://www.biomems.net, dedicated to Fundamentals of Microfabrication, was set up to transform this book into a living, hyperlinked document with frequent updates of relevant Web sites, educational materials such as tutorials and problem sets, a glossary, and companies with the latest breakthrough MEMS products.  A compact disc with links both internal (e.g., from text to reference or from text to glossary) and external (e.g., from the word lithography to the IBM Almaden site dedicated to that topic) will be available shortly. 

MICROMACHINING (MEMS)

Micromachining, also microfabrication, micromanufacturing, or micro electromechanical systems (MEMS) refers to the fabrication of devices with at least some of their dimensions in the micrometer range.  Commercial MEMS products are on the market and a large part of the book is dedicated to this type of miniaturization; it is the main topic of Chapters 1 through 6.

In the early years, this discipline was almost exclusively based on thin and thick film processes and materials borrowed from IC fabrication labs. Emphasis was on UV lithography (Chapter 1), single crystal Si (Chapter 4), and polycrystalline Si (Chapter 5) for mechanical applications such as pressure sensors, accelerometers, and gyros.  In the nineties, as the applications of micromachining broadened, emphasis shifted to a more all-inclusive view of micromanufacturing methods.  Besides the IC methods, techniques such as micromolding, drop delivery systems, wire electrodischarge machining (WEDM), laser machining, ion- and electron- beam machining, and computer numerically controlled (CNC) ultra fine diamond milling have been reevaluated for their merits in miniaturization.  A plethora of “exotic” materials needs to be “adapted” to make these new micro device applications possible.  In the case of MEMS applied to medical and biochemical problems (i.e., BIOMEMS), these materials include hydrogels, gas-permeable membranes, biological cells, enzymes, antigens and antibodies, and membranes doped with ionophores.

Although the term MEMS is still very popular, a much more apt description today for microfabrication and the other techniques described below is miniaturization engineering, a discipline based on a thorough knowledge and understanding of intended applications, different micromanufacturing options, the behavior of materials, and scaling laws.  The latter describes the laws that express how structures scale when all their dimensions are isomorphically reduced (Chapter 9).

The figure below illustrates an example of a micromachined biomedical device.  It is an analytical lab on a CD.  The lab may be used, for example, to analyze blood gases and blood electrolytes.  Machining options for fluidic channels with diameters greater than 80 µ