Ion-Assisted Surface Processing of Electronic Materials
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fabrication of large-area substrates. In what follows, we will first explore some of the basic physical and chemical interactions of low-energy ions with surfaces. Then we discuss ion-stimulated processes used for etching, cleaning, and depositing electronic materials. In each area, we first highlight processes already in application and then present novel processes still in the research stage. We emphasize the fundamental mechanisms rather than the process technology, but we will discuss both low-energy direct ion beams and plasma sources for processing to illustrate their relative roles in research and application.
Physical and Chemical Processes Introduction Why are low-energy ions relevant to the surface processing of electronic materials? The answer lies in the overriding trend of miniaturization in microelectronics. The achievement of these feats in ultrasmall architecture has required surface processing capabilities that allow layer addition and removal with incredible precision. The resulting benefits of greater capacity and speed at a plummeting cost per function are near legendary. The ability of low-energy ions to enhance the precision of surface etching, cleaning, and deposition/growth processes (Figure 1) provides one basis for the interest in ion-assisted processes. Low-energy ions are used, for example, to enhance the sharpness of side walls in plasma etching and to improve step coverage by metal layers in sputter deposition. Emerging optoelectronic applications such as forming ridges for wave-guides and ultrasmooth vertical surfaces for lasers further extend piesent requirements, and low-energy ions again provide one tool to help in this area of ultraprecise materials control. Trends associated with the decreased feature size include the movement from wet chemical processing to dry processing, the continuing need for reductions in defect densities, and the drive toward reduced temperatures and times in process steps. How do the above trends focus interest on studies of low-energy ion-assisted processes? In current applications, these trends are driving the need for increased atomiclevel understanding of the ion-enhancement mechanisms, for example, in reactive ion etching to minimize defect production and enhance surface chemical reactions. In research on new processes, trends toward miniaturization have prompted studies of the influence of low-energy ions on morphology control (e.g., surface smoothness, side wall sharpness, and layer continuity over steps), on the uniform nucleation of 52
thin layers, on incorporation of electrical dopants within regions approaching one monolayer, and on the ability to stimulate processes at lower temperatures. To study the effects of ion bombardment, beams of low-energy ions can be formed and directed at a sample surface, giving good control over the ion type, energy, and flux. Alternatively, in a plasma processing environment, low-energy ions continually bombard the surface by virtue of the electric fields built up between the plasma and the surfaces in contact
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