The Materials Science of Thin Films 2011 Part 14 doc

628 Modification of Surfaces and Films 24. J. K. Hirvonen and C. R. Clayton, in Surface Modifcation and Alloying, eds. J. M. Poate, G. Foti, and D. Jacobson, Plenum Press, New York (1983). 25.* G. K. Wehner, J. Vac. Sci. Tech. A3, 1821 (1985). 26. P. Auciello and R. Kelly, Ion Bombardment Modification of Surfaces -Fundamentals and Applications, Elsevier, Amsterdam (1984). 27. J. L. Whitton, G. Carter, and M. J. Nobes, Radiation Effects 32, 129 (1977). 28. G. K. Celler, Solid State Technology 30(3), 69 (1987). 29. A. E. White and K. T. Short, Science 241(8), 930 (1988). hapter 74 3Esk- Emerging Thin-Film Materials and Applications In this final chapter an attempt is made to present a perspective of some emerging thin-film materials and applications that promise to have a significant impact on future technology. For this reason the discussion will be limited to the following topics: 14.1. Film-Patterning Techniques 14.2. Diamond Films 14.3. High T, Superconductor Films 14.4. Films for Magnetic Recording 14.5. Optical Recording 14.6. Integrated Optics 14.7. Superlattices 14.8. Band-Gap Engineering and Quantum Devices This potpourri of subjects encompasses covalent, metallic, and semiconduc- tor film materials deposited by an assortment of PVD and CVD methods. Represented are mechanical, electrical, magnetic, and optical properties, whose optimization hinges on both processing and the ability to characterize struc- ture-property relationships. Thus the spirit of materials science of thin films-the theme and title of this book-is preserved in microcosm within this chapter. For completeness however, it is necessary to start with Section 14.1, 629 630 Emerging Thin-Film Materials and Applications which is devoted to the topic of thin-film patterning techniques. This subject is crucial to the realization of the intricate lateral geometries and dimensions that films must assume in varied applications, particularly some of those in this chapter. 14.1. FILM-PATTERNING TECHNIQUES 14.1 .l. Lithography Until now the only film dimension considered has been the thickness, which is controlled by the growth or deposition process. However, irrespective of eventual application, thin films must also be geometrically defined laterally or patterned in the film plane. The complexity of patterning processes depends on the nature of the film, the feature dimensions, and the spatial tolerance of the feature dimensions. For example, consider an evaporated metai film that must $. JJJJ$+J ULTRAVIOLET RADIATION MASK " I POSITIVE RESIST ' NEGATIVE RESIST Figure 14-1. Schematic of the lithographic process for pattern transfer from mask to film. Both positive and negative resist behavior is illustrated. 14.1. Filmpatterning Techniques 631 possess features 1 mm in size with a tolerance of kO.05 mm. The desired pattern could possibly be machined into a thin sheet stencil or mechanical mask. Direct contact between this mask template and substrate ensures genera- tion of the desired pattern in uncovered regions exposed to the evaporation flux. This method is obviously too crude to permit the patterning of features 100 to loo0 times smaller in size that are employed in integrated circuits. Such demanding applications require lithographic techniques. The lithographic process shown schematically in Fig. 14-1 consists of four steps. 74.7.7.7. Generation of the Mask. The mask is essentially equivalent to the negative in photography. It possesses the desired film geometry patterned in Cr or FeO thin films predeposited on a glass or quartz plate. Masks for integrated circuit use are generated employing computer-driven electron beams to precisely define regions that are either opaque or transparent to light. Other processing steps to initially produce the patterned mask film parallel those used in subsequent pattern transfer to the involved film. 74.7.7.2. Printing. Printing of this negative mask requires the physical transfer of the pattern to the film surface in question. This is accomplished by first spin-coating the film-substrate with a thin photoresist layer (< 1 pm thick). As the name implies, photoresists are both sensitive to photons and resistant to chemical attack after exposure and development. Photoresists are complex photosensitive organic mixtures, usually consisting of a resin, photo- sensitizer, and solvent. During exposure, light (usually UV) passes through the mask and is imaged on the resist surface by appropriate exposure tools or printers. Either full-scale or reduced latent images can be produced in the photoresist layer. There are two types of photoresists and their behaviors are distinguished in Fig. 14-1. The positive photoresist faithfully reproduces the (opaque) mask film pattern; in this case light exposure causes scission of polymerized chains rendering the resist soluble in the developer. Alternatively, negative resists reproduce the transparent portion of the mask pattern because photon-induced polymerization leaves a chemically inert resist layer behind. For yet greater feature resolution X-ray and electron-beam lithography tech- niques are practiced. 14.1.1.3. Etching. After resist exposure and development, the underlying film is etched. Wet etching in appropriate solutions dissolves away the exposed 632 Emerging Thin-Film Materials and Applications film, leaving intact the film protected by resist. Equal rates of lateral and vertical material removal (isotropic etching) however, lead to loss of resolution due to undercutting of film features. This presents a problem in VLSI processing where 1 pm (or so) features must be defined. For this reason dry etching is practiced. Material is removed in this case through exposure to reactive plasmas that interact with film atoms to produce volatile by-products that are pumped away. For example, typical dry etchants for Si, SiO, and Al are SF, + Cl,, CF, + H,, and BCl, + C1, gas mixtures, respectively (Ref. 1). Alternatively, inert-gas plasmas are also employed to erode the film surface in a process that resembles the inverse of sputtering deposition. In both cases, positive ion bombardment normal to the surface leads to greater vertical than horizontal etching, i.e., anisotropic etching. Steep sidewall topography and high aspect ratio features such as shown in Fig. 14-2 are the result of anisotropic material removal. An important issue in dry etching is the etchant selectivity or ability to preferentially react with one film species relative to others that are present. Simply changing the plasma gas composition can significantly alter etching selectivity. For example, the SiO, etch rate exceeds that of poly-Si by only 25% in a pure CF, plasma. In an equimolar mixture of H, + CF, , however, Figure 14-2. SEM micrograph of reactive plasma-etched pattern in photoresist re- vealing development of submicron features. (Courtesy of L. F. Thompson, AT&T Bell Laboratories). 14.1. Film-Patterning Techniques 633 the etch rate of poly-Si drops almost to zero; the selectivity or ratio of etch rate of SiO, relative to poly-Si exceeds 45 (Ref. 1). 74.7.1.4. Resist Removal. The final step requires removal of the resist. Special resist stripper solutions or plasmas (e.g., 0, rich) are utilized for this purpose. What remains is a high fidelity thin-film copy of the mask geometry. Only the briefest summary of the basic steps comprising the very important technology of lithography has been presented. For more detailed accounts of mask production (Ref. 2), photoresists (Ref. 3), printing (Ref. 4), and etching (Ref. 1) the reader is referred to the indicated references. 14.1.2. Silicon Micromachining Silicon micromachining can be defined as a high-precision shaping technique that uses photolithographic and etching methods to form miniature three-di- mensional shapes in Si (and SO,) such as holes, wells, pyramids, grooves, hemispheres, needles, etc. In the same way that Si has revolutionized electron- ics, this versatile material has altered conventional perceptions of miniature mechanical components, devices and systems. Though small, micromachined features are generally large compared to VLSI dimensions. Examples include the microcantilever thin film beams discussed on p. 412, tiny gears, valves, springs and tweezers, X-ray Fresnel lenses, pressure and strain transducers, ink jet nozzle arrays, electrochemical sensors, multisocket electrical connec- tors, and force and acceleration transducers (Refs. 5, 6). Among the recent developments are the fabrication of a triode vacuum microelectronic device (Ref. 7) and an optical microassembly. The former shown in Fig. 14-3a is impervious to radiation damage, insensitive to heat with the potential for very VACUUM SPACE METAL ANODE INSULATING LAYER METAL GATE OR GRID *-DIELECTRIC (SUCH AS Si021 METAL EMITTER +-SILICON SUBSTRATE Figure 14-3a. Schematic structure of Si triode vacuum microelectronic device. (From Ref. 7). 634 Emerging Thin-Film Materials and Applications Figure 14-3b. SEM micrograph of optical microassembly. (Courtesy of K. L. Tai, AT&T Bell Laboratories). high frequency operation. The latter shown in Fig. 14-3b has been employed to provide low-loss coupling between optical fibers and optoelectronic devices in optical communications systems. Here the laser (or detector) rests beneath the apex of the etched pyramid in which the optical fiber is precisely positioned. This microassembly package provides for low-loss electrical interconnection between optoelectronic and other electronic devices on a common Si substrate. Precise knowledge of etch rate anisotropies and selectivities for Si and SiO, is required for designing successful micromachining etching treatments. In a recent study (Ref. 8), utilizing KOH/H,O etchants, the following etch rates (R) were measured as a function of temperature: 0.61 eV kT RSi(100) = 6.19 x 108exp - - (Ccm/min) 9 (14-1) 0.77 eV kT Rsi(lll) = 3.19 x l0’exp - - (Ccm/min) 9 (14-2) 1.07 eV kT RSiO2 = 5.49 x 10”exp - - (~cm/min), (14-3) 14.2. Diamond Films 635 d n100 = T (X + A@ sin 54.7" Rlll = t Figure 14-4. Etching geometry of Si-SiO, structure. The ratio Rsi(lOO)/Rsi(lll) defines the etch rate anisotropy ( Aloo,,l,) and the ratio Rsi( 100)/Rsio2 represents the selectivity. As an example in the use of these etch rates consider a (100) Si wafer containing a 2 pm thermally grown SiO, film so patterned to open windows to the Si surface (Fig. 14-4). After etching at 100 "C for 15 min, how much does the SiO, etch mask overhang the slanted Si wall? During etching, both the (100) and (11 1) planes recede along their direction normals. The angle between the [ 1001 and [l 1 13 directions is 54.7". Therefore geometric considerations indicate that the net overhang length x at any time t is given by x = (~~~(lll)/sin54.7 - ~,~~,)t, 04-41 where the isotropic etching of SiO, is accounted for. Direct substitution of RSi(lll) = 0.126 pm/min, Rsi020.0191 pm/min, t = 15 min, and sin54.7 = 0.816, yields x = 2.03 pm. Depending on the width of the SiO, mask window, V-shaped pits or flat-bottomed troughs can be etched into Si. 14.2. DIAMOND FILMS 14.2.1. Introduction Derived from the Greek 01Bap01~ (adamas), which means unconquerable, diamond is indeed an invincible material. In addition to being the most costly 636 Emerging Thin-Film Materials and Applications on a unit weight basis, and capable of unmatched beauty when polished, diamond has a number of other remarkable properties. It is the hardest substance known (H, > 8OOO kg/mm2), and has a higher modulus of elasticity (E = 1050 GPa) than any other material. When free of impurities, it has one of the highest resistivities (p > 1OI6 Q-cm). It also combines a very high thermal conductivity ( K = 1100 W/m-K) that exceeds that of Cu and Ag, with a low thermal expansion coefficient (a = 1.2 x lop6 K-' ) to yield high resistance to thermal shock. Lastly, diamond is very resistant to chemical attack. These facts, the first three, in particular, have spurred one of the most exciting and competitive quests in the history of materials science-the synthe- sis of diamond. Success was achieved in 1954 with the General Electric Corp. process for producing bulk diamond utilizing extremely high pressures and temperatures. Interestingly, however, attempts to produce diamond from low- pressure vapors date back at least to 1911 (Ref. 9). P. D. Bridgeman, in a 1955 Scientific American article, speculated that diamond powders and films should be attainable by vapor deposition at low pressures (Ref. 10). By the mid- 1970s the Russian investigators Derjaguin and Fedeseev had apparently grown epitaxial diamond films and whiskers during the pyrolysis of various hydrocarbon-hydrogen gas mixtures (Ref. 11). After a decade of relative quiet, an explosive worldwide interest in the synthesis of diamond films and in their properties erupted, which persists unabated to the present day. Isolated C atoms have distinct 2s and 2p atomic orbitals. When these atoms condense to form diamond, electronic admixtures occur, resulting in four equal hybridized sp3 molecular orbitals. Each C atom is covalently attached to four other atoms in tetragonal bonds 1.54 A long creating the well-known diamond cubic structure (Fig. 1-2c). Graphite, on the other hand, has a layered structure. The C atoms are arranged hexagonally with strong trigonal bonds (sp2) and have an interatomic spacing of 1.42 in the basal plane. A fourth electron in the outer shell forms weak van der Waals bonds between planes that account for such properties as good electrical conductivity, lubricity, lower density, a grayish-black color and softness. In addition, C exists in a variety of metastable and amorphous forms that have been characterized as degenerate or imperfect graphitic structures. In these, the layer planes are disoriented with respect to the common axis and overlap each other irregularly. Beyond the short-range graphitic structure, the matrix consists of amorphous C. A complex picture now emerges of the manifestations of C ranging from amorphous to crystalline forms in a contin- uum of structural admixtures. Similarly, the proportions of sp2-sp3 (and even sp') bonding is variable causing the different forms to have dramatically different properties. Not surprisingly, this broad spectrum of metastable car- 0 14.2. Diamond Films 637 bons have been realized in thin-film deposits. What now complicates matters further is that the many techniques to produce carbon films use precursor hydrocarbon gases. Hydrogen is, therefore, inevitably incorporated, and this adds to the complexity of the deposit structure, morphology, and properties. Given the structural and chemical diversity of carbon films, an understand- able confusion has arisen with regard to the description of these materials. Labels such as hard carbon, amorphous carbon (a-C), hydrogenated amor- phous carbon (a-C:H), ion-beam-processed carbon (i-C), diamondlike carbon (DLC), as well as diamond have all been used in the recent literature. The ensuing discussion will treat the deposition processes and properties of these films with the hope of clarifying some of their distinguishing features. 14.2.2. Film Deposition Processes At the outset it is important to realize that synthesis of bulk diamond occurs in the diamond stable region of the P- T phase diagram (Fig. 1-1 1). Thin “diamond” films, on the other hand, clearly involve metastable synthesis in the low-pressure graphite region of the phase diagram. The possibility of synthesizing diamond in this region is based on the small free-energy differ- ence (500 cal/mole) between diamond and graphite under ambient conditions (Ref. 12). Therefore, a finite probability exists that both phases can nucleate and grow simultaneously, especially under conditions where kinetic factors dominate, such as high energy or supersaturation. In particular, the key is to prevent graphite from forming or to remove it preferentially, leaving diamond behind. The way this is done practically is to generate a supersaturation or superequilibrium of atomic H. The latter can be produced utilizing 0.2-2% CH,-H, mixtures in microwave plasmas or in CVD reactors containing hot filaments. Under these conditions, atomic H is generated and, in turn, fosters diamond growth either by inhibiting graphite formation, dissolving it if it does form, stabilizing sp3 bonding, or by promoting some combination of these factors. In general, hydrocarbon, e.g., CH,, C,H, , decomposition at sub- strate temperatures of 800-900 “C in the presence of atomic H is conducive to diamond growth on nondiamond substrates. Paradoxically the copious amounts of atomic H result in very little hydrogen incorporation in the deposit. The modem era of CVD synthesis is coincident with the beautiful SEM images of diamond crystallites produced in the manner described. These have captured the imagination of the world and examples of the small faceted “jewels,” grown at high temperatures on nondiamond substrates, are shown in Fig. 14-5. The a-C :H materials are formed when hydrocarbons impact relatively low-temperature substrates with energies in the range of a few hundred eV. [...]... properties, i.e., soft magnetic materials for the recording and playback head components and hard magnetic materials for the storage media The magnetic properties of some of these materials are listed in Table 10-4 In 14. 4 Films for Magnetlc Recordlng 647 the next two sections we further explore their use in magnetic recording applications 14. 4.2 Thin- Film Head Materials (Ref 17) The phenomena of magnetic... Waveguides The simplest planar waveguide has the sandwich structure shown in Fig 14- 13 In the center is a thin film whose thickness is of the order of the wavelength of the light propagating through it, i.e., 1 pm The index of refraction of this film (nf)exceeds that of either the substrate (n,) or cover layer (nJ,which serve as the cladding Consider the case where nf > n, > n, Only Snell’s law of refraction,... of the written domains, but now with N in the direction of the original film magnetization 14. 5.3 Magneto-Optical Film Materials Before addressing their actual properties and compositions the issue of why films are used deserves brief mention The primary reasons are the great speed of heating and cooling that is possible in films of low thermal mass, and the high-storage-density continuous films (rather... to the thin- film guides Miniature prisms, thin- film diffraction gratings and tapered film couplers are some of the components used to achieve these ends 14. 6.4 Film Materials and Processing A sizeable number of different materials have been employed over the years to fabricate thin- film waveguides and other integrated optics components Some of these are listed in Table 14- 3 together with values for their... computer The analyzer is shown in Fig 14- 15 Light from a laser diode is coupled into a planar waveguide lens, and then into a Bragg cell, the heart of the system The rf signals to be analyzed are converted into the operational bandwidth of the Bragg cell and then fed into the SAW transducer The latter consists of an interlaced, or interdigitated, comblike pair of electrodes Acoustic waves of the same... toward or away from the surface along the track, There are no demagnetizing fields at the points of magnetic reversal, thus sharpening the transition and increasing the recording density The discovery that CoCr alloy films (15 -20 at % Cr) exhibit an easy axis of magnetization normal to the fl im has made the concept of high-density perpendicular recording a reality In these materials the tendency toward... essential for the functioning of most of the components and devices employed in integrated optics Common configurations of raised and embedded channel waveguides are shown in Fig 14- 14together with a pair of components that incorporate them At the outset it is important to distinguish between passive and active waveguide components The channels of Fig 14- 14a, b are passive components because their optical... refractive index of the intervening medium by electro-optical methods In Fig 14- 14d the electro-optic waveguide phase modulator is depicted The waveguide is embedded in an electro-optical crystal substrate When the modulation voltage signal is applied to parallel electrodes, the refractive index of either the guide or substrate is altered, influencing the phase of the guided wave A different group of devices... Rotation of the plane of polarization of a linearly polarized light beam after reflection from a vertically magnetized magnetic material is the basis of the effect The sense of rotation depends on the magnetization direction in the recording film layer Compared with the writing process, the laser beam intensity for reading is much lower Finally the recorded information can be erased by laser irradiation of. .. index of refraction Transparency to IR Heteroepitaxial films required High thermal conductivity Large energy band gap High hardness Commercially available 14. 3 High Tc Superconductor Fllms 641 properties that come with better control of deposition processes, the expanded use of these films can certainly be anticipated 14. 3 HIGHT, SUPERCONDUCTOR FILMS THIN 14. 3.1 lntroduction The unexpected discovery of . and the ability to characterize struc- ture-property relationships. Thus the spirit of materials science of thin films -the theme and title of this book-is preserved in microcosm within. chemical attack. These facts, the first three, in particular, have spurred one of the most exciting and competitive quests in the history of materials science -the synthe- sis of diamond. Success. in the recent literature. The ensuing discussion will treat the deposition processes and properties of these films with the hope of clarifying some of their distinguishing features. 14. 2.2.