SOI by CVD: An Overview of Material Aspects and Implications of Device Properties
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SOI BY CVD: AN OVERVIEW OF MATERIAL ASPECTS AND IMPLICATIONS OF DEVICE PROPERTIES L. Jastrzebski and A. G. Xokkas RCA Laboratories, Princeton, NJ
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ABSTRACT Two CVD techniques producing monocrystalline SOI films, silicon-onsapphire (SOS) and Epitaxial Lateral Overgrowth (ELO), are described and the nature of the crystallographic defects in the films is discussed. The geometrical structure of SOI devices, device properties, dynamic characteristics, capacitance, and radiation hardness are then examined with emphasis on evaluating the potential of SOI technologies in future applications. INTRODUCTION During the last twenty years substantial interest has been generated in the growth of device quality silicon over insulators (SOI) and attempts have been made to achieve dielectrically isolated silicon islands suitable for the fabrication of a variety of integrated circuits. The most mature of the SOI technologies is silicon-on-sapphire (SOS) which is presently employed in the commerical manufacturing of dielectrically isolated thin films for CMOS applications. Numerous other approaches have also been proposed and are currently pursued for the fabrication of SOI films. They include silicon on Si0 2 recrystallized with the aid of lasers [I], lamps [2], and strip heaters [3], or achieved by graphoepitaxy [4], deep oxygen implantation [5], porous oxidation [6], and Epitaxial Lateral Overgrowth (ELO) [7]. In this paper two chemical vapor deposition (CVD) technologies providing SOI films will be described. The first is ELO [7-9] where a clean silicon surface, achieved by etching openings through an oxidized silicon substrate, acts as the seed for silicon growth within the openings and subsequently over the oxide. The second is SOS [10] where seeding and growth take place over a sapphire substrate. Both technologies take advantage of the nature and the well developed techniques and equipment of silicon CVD. We shall first outline the growth procedures and describe the defect structures of SOI films obtained by the above two methods. We shall then identify the device properties and characteristics of special interest in thin film technologies and attempt to critically evaluate them in terms of their advantages and disadvantages for future applications including radiation hardness. 1. ELO GROWTH PRINCIPLES 1.1 Suppression of Polysilicon Formation In ELO, the substrate consists of an oxidized wafer with oxide openings exposing bare silicon. Under correctly chosen deposition conditions, silicon will grow selectively only in these openings until its thickness reaches that of the Si0 2 and then will continue to grow vertically as well as laterally along the Si0 2 surface (Fig. 1). Growth is carried out until a continuous silicon film is formed over the Si0 2 surface. Mat. Res. Soc. Symp. Proc. Vol. 23 (1984) @Elsevier Science Publishing Co., Inc.
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The major problem encountered during the ELO deposition process has been formation of polysilicon nuclei on the Si0 2 surface. When incorporated into the overgrowing ELO layer,
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