Roland A. - PVD for microelectronics (779636), страница 41
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Brctt, and T. J. Stay, "The effects of collimation on intrinsic stress in sputter-deposited metallic thin films," Thin Solid Fihns 253:372 (1994).6.13. D. Liu, S. K. Dew, M. J. Brett, T. Janacek, T. Smy, and W. Tsai, "Experimental study and computer simulation of collimated sputtering of Ti thin films over topographical features," J. Appl.Phys. 74:1339 (1993).6.14. D.
Liu, S. K. Dew, M. J. Brett, T. Janacek, T. Smy, and W. Tsai, "Properties of Ti and AI thinfilms deposited by collimated sputtering," Thin Solid Films 236:267 (1993).6.15. S. Meikle, S. Kim, and T. Doan, "Semiconductor process considerations for collimated sourcesputtering of Ti films," Proc. VMIC, Santa Clara, CA, 1992, pp. 289-291 (unpublished).6.16. T. Hara, T. Nomura, and S.
C. Chen, "Properties of titanium layers deposited by collimationsputtering," Jpn. J. Appl. Phys. 31:LI746-L1749 (1992).6.17. J. G. Ryan, S. Brodsky, T. Katata, M. Honda, N. Shoda, and H. Aochi, "Collimated sputteringof Ti and TiN films," MRS Bulletin, 42-45 (November 1995).6.18. Varian Associates, Palo Alto, CA.6.19. "Bipolar 212TIN," from Sputtered Films, Inc, 320 Nopal St., Santa Barbara, CA 93103.6.20. E. Demeray (formerly of Varian Assoc.), 1989.6.21. S. Roehl, L. Camilletti, W. Cote, D. Cote, E. Eckstein, K. H.
Froehner, P. I. Lee, D. Restaino,G. Roeska, V. Vynorius, S. Wolff, and B. Volimer, "High density damascene wiring and borderless contacts for 64M DRAM," in Proc. VMIC, Santa Clara, CA, 1992, pp. 22-28 (unpublished).6.22. C. C. Fang, R. V. Joshi, V. Prasad and C.
Ouyang, "Modeling of intrinsic stresses of titaniumthin films deposited by collimated sputtering," Advanced Metallization and InterconnectDIRECTIONAL DEPOSITION213Systems for ULSI Applications in 1995, R. C. Ellwanger and S.-Q. Wang eds., MaterialsResearch Society, Pittsburgh PA, 1996, p. 423.6.23. D. S. Bang, J.
P. McVittie, M. M. Islamraja, K. C. Saraswat, Z. Krivokapic, S. Ramaswami, andR. Cheung, "Dynamic modeling of collimator clogging in physical vapor deposition systems,"Proc. VMIC, Santa Clara, CA, 1994, p. 554 (unpublished).This Page Intentionally Left BlankChapter 7 Planarized PVD: Use of ElevatedTemperature andfor High PressureThe diffusion rate of sputter-deposited atoms in a thin metal film, eitheralong the film surface or through the bulk, is highly sensitive to temperature.
Therefore, temperature provides the PVD user with a "processknob" that can be used to control metaI atom mobility, which in turn allows one to engineer the profile of a PVD fiIm for a given application.For example, the use of elevated temperature (== 350-550°C) either during or after PVD deposition has successfully been exploited for improvedstep coverage and even complete filling of PVD A1 alloy and Cu films inhigh aspect ratio structures. Most of the work to date has focused on A1metallurgy with the intention of replacing CVD W plugs with more conducting PVD A1 plugs in multilevel metallization schemes, allowing anall-aluminum solution with vertical A1 contacthia plugs and horizontalA1 interconnect wires.
In addition, since PVD is a blanket and not a selective process, A1 is deposited on both the field regions and in the viaholes. This opens the possibility of using a "hot Al" process to simultaneously fill the plugs and planarize the free A1 surface (Fig. 7.
I), with thebenefit of fewer overall process steps. As shown i n Fig. 7.la. since thethicknesses of the field and the via hole are usually comparable, the volume of the incident s\ug of material that enters the hole is not greatenough to both fill the hole and form a planarized surface. There is a"missing mass" that must be provided. Figure 7. lb shows a more realistic shape of the missing volume. Hot A1 processing induces mass to migrate into the hole from the much larger volume of the field and also re.net result is a simultaneouslydistribute within the hole (Fig.
7 . 1 ~ )The(Fig.7.I d).plugged hole and planarized surfaceThis chapter discusses underlying physics and practical aspects of hotPVD processing with an emphasis on Al. The trend in hot PVD, as withMLM processing in general, is toward lower process temperatures - e.g..to reduce thermal stress voiding in metal lines and to ensure compatibilitywith future low-k dielectrics that are likely to be highly temperature-sensitive. Therefore, we also discuss several methods that have been developedto lower the temperature needed to reliably coat or fill a given aspect ratiostructure, including multistep processes such as the two-step or "cold-hot"Al process, the use of unconventional alloys such as Al-Si-Ge, and the application of ultrahigh-pressure annealing after PVD deposition to force theas-deposited metal to fill the feature by enhanced plastic flow - theForcefillTMprocess.216R.
POWELL AND S. M. ROSSNAGELFIG. 7.1 Hot PVD AI process allows simultaneous lilling o l ' a thrcc-din~cnsional plug and planarization of the two-dimensional interconnect surface over the hole.7.1 Physics of Hot PVDFigure 7.2 shows the general case of hot deposition of a metal film over asurface with nonplanar topography, although similar considerations applywhen the PVD film is annealed after deposition. Although gas-phase transport accounts for the transfer of sputtered atoms from the face of the targetto the surface of the growing film, it is solid-phase transport that accountsfor the operation of hot PVD. In particular, the fundamental idea behindhot PVD is to increase the self-diffusion rate of the metal (e.g., the diffusion rate of AI atoms in an A1 film) to rapidly build up the film in areas oflow coverage at the surface and/or fill up voids within the bulk.
In the firstcase, transport of material occurs by the mechanism of surface diffusion;in the second case, the mechanism is by bulk diffusion of vacancies withgrain boundary diffusion also possible in a real polycrystalline film.PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSUREAdatom Surtace Dmusion217Vacancy-AssistedBulk OmusionAreas d Low Surlace PotentlalFIG. 7.2 During hot PVD, surface diffusion redistributes depositing adatoms along the surface,while bulk diffusion transports activated atoms into the bulk.In either case, the driving force is the reduction of the surface potentialof the film, which is determined by the surface curvature of the film (orsubsurface curvature in the case of a buried void).
The surface potential isnegative for concave surfaces and positive for convex ones with the overall effect that matter is transported from the upper part of the film to thelower part of the film growing nearer to the substrate. This behavior isanalogous to that of a fluid in that regions of lowest curvature (concave)will be filled first. Assuming no film stress and an isotropic material, thesurface potential in two dimensions can then be written as p = pu yRIfl,where pOis the chemical potential of a flat surface, y is the surface tension,0 is the atomic volume, and R is the radius of curvature of the surface[7. I]. It is the gradient of this curvature that will cause the preferential diffusion of adatoms into areas where the surface is concave, such as the inside of a contact or via hole.For the materials and temperatures typically encountered with reflowprocessing (A1 alloys and Cu at 350-550°C), the metal vapor pressure isso low that transport of matter by the mechanism of gas evaporationcondensation need not be considered.
If the process temperature is muchgreater than one-half of the melting point of the metal in degrees Kelvin(i.e., >> 470°C for Al), then plastic flow may contribute to the bulk movement of the film - an effect that occurs more readily in films that areunder compressive stress. On the other hand, process temperatures arerarely so close to the melting point that viscous flow is significant.
In this+218R. POWELL AND S. M. ROSSNAGELcase, it is the more conventional mechanisms of surface and bulk diffusionthat dominate film transport during high-temperature deposition and/orpostdeposition annealing. Due to the relatively low activation energy fordiffusion at a free metal surface. surface diffusion rates are usually muchlarger than bulk diffusion rates. For example, representative activation energies for surface and bulk diffusion in a clean A1 film at moderate temperatures (450-550°C) are about EU = 0.40 eV and 1.45 eV, respectively.Also, diffusion theory shows that the l/e decay time for a sinusoidal surforface feature of frequency w goes as l / w 3 for bulk diffusion but l/04surface self-diffusion 17.21. Hence, one expects "high-frequency" submicron scale surface features to be smoothed out far more quickly by transport of material along the surface than through the bulk.The surface diffusivity D, i n cm2/sec, depends exponentialIy on temperature through an expression of the formwhere E,, is the activation energy for surface diffusion ( E , = 0.4 eV forAl), and the prefactor D,,is a strong function of material and depositionconditions (the bulk diffusiviiy of Al has the same form as Eq.
(7.1) but-= 1.7 cm2/sec and E,, = 1.45 eV). In modeling of hot A1 PVD prowith D,,cessing, it has been useful to further refine Eq. (7. I ) by defining a diffu?,T i q the meansion length along the surfdce given by L = ( D T ) ! ~wherelifetime for a mobile surface adatom 17.11. This lifetime is limited by suchfactors as the deposition rate - which determines the time i t takes to deposit a monolayer over the atom and effectively bury it - and by the presence of reactive gas species that can chemically bind with the atom and fixits position on the surface.
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