Sputter Deposition Processes
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THIN FILM PROCESSES II
11-4Sputter Deposition ProcessesROBERT PARSONSDepartment of Physics The University of British Columbia Vancouver, British Columbia Canada
I. IntroductionA. General B. Features of a Sputter Coater C. Considerations of Film Properties Sputter Sources A. Nonmagnetron Sources (Diode and Triode) B. Magnetron Sputter Sources C. Ion Beam Sputter Sources Sputter Deposition of Conducting Films A. General B. Specific Applications Sputter Deposition of Dielectric Films A. Reactive Sputtering Sputter Coating Systems A. General B. Technical Considerations Emerging Technologies Concluding Remarks References
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The intent of this chapter is to present a comprehensive treatment of sputter deposition of thin films, with the main emphasis on the practical, engineering aspects of the sputter technique. For a detailed discussion of177Copyright 0 1991 by Academic Press, Inc All rights of reproduction i any form reserved. n
the basic underlying physics of sputtering, the reader is referred to Chapter 11.1 and the excellent review articles on cathode sputtering [l],discharge sputtering , magnetron sputtering [3, 41, and reactive sputtering [ 5 , 61. Sputtering involves many interrelated physical and chemical processes. Although our basic understanding of this complex subject is incomplete, sputtering is a very mature technology, as evidenced by the many applications of the process. Examples include magneto-optical storage media, compact disks, planarized coatings for multilayer circuits, optical multilayer coatings for mirrors and filters, solar control and low emissivity window coatings, conductors and barrier layers for very large scale integrated circuits, solar cells, diamondlike coatings, transparent conducting electrodes, amorphous optical films for integrated optics devices, luminescent films, microcircuit photolithographic mask blanks, wear-resistant coatings for cutting tools, and decorative coatings. One of the main reasons for this development has been the apparent ease of extending results obtained empirically on a small, research-size sputter coater to a highly reliable, production process. Other reasons for using sputtering include: Excellent film uniformity, particularly over large areas; Surface smoothness and thickness control; Deposition of films with nearly bulklike properties, which are predictable and stable; Versatility; the sputter process is essentially a kinetic process involving momentum exchange rather than a chemical and/or thermal process and, therefore, virtually any material can be introduced into a gas discharge or sputtered from the solid; Good adhesion; Either conformal or planarized coatings; and High rates, which are comparable to evaporation.
Sputter sources for film deposition can be categorized in two ways: glow discharge (diode, triode, and magnetron) and ion beam. In all cases the particles are ejected by the same basic mechanism of momentum exchange between energetic particles and surface atoms. However, as discussed in Chapter 11.1, the various source configurations cover quite different process parameters and all have their particular advantages. Nonmagnetron sources, especially ion beam sources and the rf planar diode, are widely used; however, the planar magnetron with its high efficiency and convenient geometry for scale-up is well established as the sputter source of choice. Ion beam sputtering can be subdivided into deposition and etching applications. Ion beams are used for film deposition in two basic configurations; primary ion beam deposition and ion beam sputtering (also called
11-4. SPUTTER DEPOSITION PROCESSESLEAK V A L V E INERT REACTIVE
SPUTTERED ATOM SUBSTRATE
HIGH VACUUM PUMP
Fig. 1. General features of a sputter coater.
secondary ion beam deposition) [7, 81. The latter technique is discussed in this chapter. Sputter etch applications are treated in Chapter V.2.
B. Features of a Sputter Coater
Figure 1 shows the standard parts of a sputter coater:A stainless steel or mild steel chamber, which has been certified vacuum-tight with a helium leak detector;
Pumping capacity that is capable of reducing the chamber pressure to about 1 x lop6 torr (or lower in the case of ultrahigh-vacuum applications); Pressure gauges; In the case of the glow discharge sources, a means to raise the chamber pressure to about 5 X torr for sputter operation; for example, with a combination of mass flow controllers, and a variable orifice valve (throttle) to reduce the pumping speed; Sputter source and power supply; and Substrate holder. In addition, the coater can have the following hardware: Substrate heater; Power supply to apply a voltage to the substrate (for bias sputtering); Separate ion source for bombardment of the growing film; Multiple sputter sources for co-sputtering; Residual gas analyzer and/or optical emission monitor to measure partial pressures and sputtered flux; and Automation control system.C. Considerations of Film Properties
Before we consider specific sputter processes, it is useful to review the dependence of film microstructure on the growth parameters. By control of film microstructure, many film properties of practical importance, such as intrinsic stress, refractive index, surface roughness, and electrical resistivity, can be promoted. Microstructure is determined primarily by the surface and near-surface environment during film growth-more specifically, the adatom mobility. The main parameters for the control of adatom mobility are substrate temperature and particle bombardment [9-131. The microstructure of sputtered films is usually classified in terms of four zones [14, 151. Zone 1 structure, consisting of tapered columns and significant voids between columns, is prevalent which the ratio T/T, is less than about 0.3, where T i s the growth temperature and T, is the melting point of the deposited material. In this range of growth temperatures, adatom diffusion is negligible and, as a result of shadowing effects, most of the sputtered flux is deposited on high points on the film, with little material reaching the valleys. Factors that increase shadowing, such as increased angle of incidence of the coating flux, promote the growth of Zone 1. These factors should be considered in the case of substrate movement, and are particularly important for wide-angle geometries such as the
cylindrical-post magnetron (Section 11,B) . Zone 1 structure is also promoted by the presence of impurities such as oxygen  that, presumably, reduce the movement of adatoms. Zone 1 structures tend to be associated with rough surfaces, poor stability, and properties that are far from those of the bulk material . Zone 2 is usually found when 0.3 < TIT,,, < 0.5, which is associated with significant adatom diffusion on grain surfaces. Zone 2 structures are characterized by columnar, platelet, or whisker grains separated by dense intercrystalline boundaries. In the case of many compound semiconductors of interest, an optimum growth situation occurs in the structure-sensitive properties, such as surface smoothness, when the growth temperature is within a few percent of one-third of the boiling point of the compound . Zone 3 occurs at high relative temperatures, TIT,,, > 0.5, when diffusion within the grains is a significant mechanism of film growth. Zone 3 is associated with equiaxied grains and epitaxial growth on the substrate. By elevating the substrate temperature during film growth, semiconductor materials such as GaAs and Si have been epitaxially grown . The fourth zone, Zone T (transition), is the result of bombardmentinduced surface mobility. Films that would have been expected to be Zone 1 structures on the basis of TIT,,, can be grown with a very smooth surface and high density by bombarding the growing film with energetic particles during film growth 115, 20-251. The main bombarding species of importance in the case of magnetron sources are ions and energetic neutrals. The latter species originate from the target either as positive ions neutralized and reflected from the target surface , or as sputtered negative ions that are accelerated in the dark space and then neutralized in the gas . Film material is moved into the spaces between grains by forward sputtering [14, 281 and by energy deposited locally by the bombarding particles (i.e., thermal spikes). This movement of material leads to tightly packed fibrous grains. Surface mobility and, consequently, Zone T structures, can be promoted by control of positive ion bombardment. Since the plasma is always the most positive part of the glow discharge, the self-bias voltage on a floating substrate is approximately equal to the energy of the bombarding, positive ions (usually 10-30 eV). The ion energies can be increased by applying a negative bias to the substrate. The upper limit to the ion energy is usually set by resputtering of the film-e.g., about 200 eV. Insulating substrates can be biased with the use of a rf matching network to the substrate carousel, or indirectly with the use of a positively biased electrode separate from the substrate. Alternatively, bombardment can be accomplished with use of an ion source [22, 23, 291 or an unbalanced magnetron (Section 11,B).
Bombardment of the growing film is a well-known method [9,30-371 to make film properties more closely matching the bulk values. Zone