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LOW-TEMPERATURE PLASMAS 42 PLASMA PROCESSING OF MATERIALS Plasmas used for the processing of materials affect several of the largest manufacturing industries, including national defense, automobiles, biomedicine, computers, waste management, paper, textiles, aerospace, and telecommunications. The importance of plasma processing to the electronics industry is illustrated in Figure 1.1. An NRC study reviewed plasma processing of materials in detail in 1991.2 This section briefly summarizes its findings and recommendations. Applications of plasma-based systems used to process materials are diverse because of the broad range of plasma conditions, geometries, and excitation methods that may be used. This technology is multidisciplinary and, ideally, the researchers should have a basic knowledge of several scientific disciplines, including elements of electrodynamics, atomic science, surface science, computer science, and industrial process control. The impact ofâand urgent need forâplasma-based materials processing is overwhelming for the electronics industry. In its report, the NRC study panel made the following statements:3 ⢠"In recent years, the number of applications requiring plasmas in the processing of materials has increased dramatically. Plasma processing [such as that illustrated in Plate 1] is now indispensable to the fabrication of electronic components and is widely used in the aerospace and other industries. However, the United States is seeing a serious decline in plasma reactor development that is critical to plasma processing steps in the manufacture of VLSI [very large scale integrated] microelectronic circuits. In the interest of the U.S. economy and national defense, renewed support for low-energy plasma science is imperative." (p. 2) ⢠"The demand for technology development is outstripping scientific understanding of many low-energy plasma processes. The central scientific problem underlying plasma processing concerns the interaction of low-energy collisional plasmas with solid surfaces. Understanding this problem requires knowledge and expertise drawn from plasma physics, atomic physics, condensed matter physics, chemistry, chemical engineering, electrical engineering, materials science, computer science, and computer engineering. In the absence of a coordinated approach, the diversity of the applications and of the science tends to diffuse the focus of both." (p. 2) ⢠"Currently, computer-based modeling and plasma simulation are inadequate for developing plasma reactors. As a result, the detailed descriptions required to guide the transfer of processes from one reactor to another or to scale 2 See footnote 1, p. 36. 3 See footnote 1, p. 36.
LOW-TEMPERATURE PLASMAS 43 processes from a small to a large reactor are not available. Until we understand how geometry, electromagnetic design, and plasma-surface interactions affect material properties, the choice of plasma reactor for a given process will not be obvious, and costly trial-and-error methods will continue to be used. Yet there is no fundamental obstacle to improved modeling and simulation nor to the eventual creation of computer-aided design (CAD) tools for designing plasma reactors. The key missing ingredients are the following: (1) A reliable and extensive plasma data base against which the accuracy of simulations of plasmas can be compared.⦠(2) A reliable and extensive input data base for calculating plasma generation, transport, and surface interactions. ⦠(3) Efficient numerical algorithms and supercomputers for simulating magnetized plasmas in three dimensions." (p. 3) FIGURE 1.1 The world electronics "food chain." Although revenues for plasma technology are a small portion of the world electronics market, plasma technology is a critical component upon which the industry rests. Note that the plasma reactor business is expected to quadruple in this decade. (Courtesy of R.A. Gottscho. Adapted from the National Advisory Committee on Semiconductors report, Preserving the Vital Base, Arlington, Va., July 1990, and from data in "Semiconductor Equipment Manufacturing and Materials Worldwide," Dataquest, Inc., 1994.) ⢠"In the coming decade, custom-designed and custom-manufactured chips, i.e., application-specific integrated circuits (ASICs), will gain an increasing fraction of the world market in microelectronic components. This market, in turn, will belong to the flexible manufacturer who uses a common set of processes and
LOW-TEMPERATURE PLASMAS 44 equipment to fabricate many different circuit designs. Such flexibility in processing will result only from real understanding of processes and reactors. On the other hand, plasma processes in use today have been developed using a combination of intuition, empiricism, and statistical optimization. Although it is unlikely that detailed, quantitative, first- principles-based simulation tools will be available for process design in the near future, design aids such as expert systems, which can be used to guide engineers in selecting initial conditions from which the final process is derived, could be developed if gaps in our fundamental understanding of plasma chemistry were filled." (p. 4) ⢠"Three areas were recognized by the PLSC [Plasma Science Committee] panel as needing concerted, coordinated experimental and theoretical research: surface processes, plasma generation and transport, and plasma- surface interactions. For surface processes, studies using well-controlled reactive beams impinging on well-characterized surfaces are essential for enhancing our understanding and developing mechanistic models. For plasma generation and transport, chemical kinetic data and diagnostic data are needed to augment the basic plasma reactor CAD tool. For studying plasma-surface interactions, there is an urgent need for in situ analytical tools that provide information on surface composition, electronic structure, and material properties." (p. 4) ⢠"Breakthroughs in understanding the science will be paced by development of tools for the characterization of the systems. To meet the coming demands for flexible device manufacturing, plasma processes will have to be actively and precisely controlled. But today no diagnostic techniques exist that can be used unambiguously to determine material properties related to device yield. Moreover, the parametric models needed to relate diagnostic data to process variable are also lacking." (p. 4) ⢠"The most serious need in undergraduate education is adequate, modern teaching laboratories. Due to the largely empirical nature of many aspects of plasma processing, proper training in the traditional scientific method, as provided in laboratory classes, is a necessary component of undergraduate education. The Instrumentation and Laboratory Improvement Program sponsored by the National Science Foundation has been partly successful in fulfilling these needs, but it is not sufficient.'' (p. 5) ⢠"Research experiences for undergraduates made available through industrial cooperative programs or internships are essential for high- quality technical education. But teachers and professors themselves must first be educated in low-energy plasma science and plasma processing before they can be expected to educate students. Industrial-university links can also help to impart a much needed, longer-term view to industrial research efforts." (p. 5) Plasma processing of materials is a technology that is of vital importance to several of the largest manufacturing industries in the world. Foremost among these industries is the electronics industry, in which plasma-based processes are
