Roland A. - PVD for microelectronics (779636), страница 70
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S. M. Rossnagel, C. A. Nichols, S. Hamaguchi, D. Ruzic, and R. Turkot, "Thin, high atomicweight refractory film deposition for diffusion barrier, adhesion layer and seed layer applications," J. Vac. Sci. & Tech. B14:1819 (1996).10.23. K.-H. Mueller, "'Stress and microstructure of sputter deposited thin films: Molecular dynamics investigations," J. Appl. Phys.
62:1796-1799 (1987).10.24. C.-C. Fang, V. Prasad, R. V. Joshi, F. Jones, and J. J. Hsieh, "A Process Model for SputterDeposition of Thin Films Using Molecular Dynamics," in Modeling ~?["Fihn Depositionfor Microelectronic Applications, S. M. Rossnagel, Ed., Academic Press, San Diego,! 996.10.25. D. Liu, S. K. Dew, M. J. Brett, T. Smy, and W. Tsai, "Compositional variations in Ti-W filmssputtered over topographical features." J. Appl.
Phys. 75:8114 (1994).10.26. T. S. Cale and V. Mahadev, "Feature Scale Transport and Reaction During Low PressureDeposition Processes," in Modeling of Film Deposition for Microelectronic Applications, S.M. Rossnagel Ed., Academic Press, San Diego, 1996.This Page Intentionally Left BlankChapter 11 Sputtering TargetsSimply stated, PVD is the controlled erosion and transfer of material froma target to a substrate by means of the sputtering process. The sputteringsource initiates the process and provides the needed control to turn bulktargets into thin films suitable for microelectronic applications.
Such filmsmust be deposited economically (i.e., low cost-of-ownership) and withtight tolerances on film uniformity, chemical purity, microstructure, andin-film particles. Each of these film attributes is in turn strongly influencedby the sputtering target itself ~ often in an interrelated way. For example,to deposit uniform films on the wafer, a planar magnetron source is oftenengineered to give relatively high erosion at the edge of the target, whichcompensates for its finite geometric size. This in turn impacts target utilization and PVD cost-of-ownership since the entire target must be replaced when the preferentially eroded region at the edge reaches the targetbacking plate.
As another example, A1203 inclusions can affect the purityof an AI target, and these same insulating inclusions can give rise to electrical arcing at the target surface with resulting particle generation. As a result, targets are no longer regarded as passive elements in a PVD system,and increasing attention is being placed by both target manufacturers andPVD users on target purity, target metallurgy (e.g., grain size and crystallographic orientation), and the design of targets tailored for both a givenPVD cathode design and process application.The focus of this chapter will be predominantly on metallic sputteringtargets. These can be used either for deposition of metals when sputteredwith an inert gas species or for the reactive deposition of nitrides or oxides(see Chapter 3 for a discussion of reactive PVD).
Compound targets, suchas nitrides or oxides, are rarely if ever used for the deposition of mostsemiconductor materials such as TiN, TaN, WN, SiO 2, TiO 2, etc., for several reasons. For example, these targets are generally insulating, which requires the use of RF power. This adds significant complexity to the sputtertool in the form of RF matchboxes, tuning circuits, additional electromagnetic shielding, filtering of other circuits, etc. Another primary constraintis that oxide targets, and to some extent nitride targets, are difficult to bondand handle. They are brittle and have poor thermal conductivity, whichmakes them susceptible to cracking and structural failure ~ particularlywhen used under high-power, high-deposition rate conditions.The primary exceptions to the metals-only approach to PVD are piezoelectric materials and superconducting thin films ~ both of which arerarely, if ever, extended to production manufacturing systems ~ as wellas a class of complex oxides with very high dielectric constant that are375376R.
POWELL AND S. M. ROSSKAGELbeginning to be applied to IC applications. Members of this latter classinclude barium strontium titanate (BST = Ba,Sr, -xTiO,), strontium bismuth tantalate (SBT = SrxBil- rTaO,) and lead zirconium titanate (PZT =PbrZrl-rTiO,), which can have dielectric constant value k of several hundred compared to k .= 4 for SiO, and k = 1 for vacuum. The primary interest in these high-k materials is their use as the dielectric of a small, planar capacitor that could then replace more complex designs such as thedeep trench capacitors or multilevel capacitors currently used to storeelectrical charge (i.e., to store digital information) on advanced memorychips such as a 1 Gb DRAM.
Since this application involves a very thin,planar film (< 1000 A) with only moderate step coverage, PVD is a suitable deposition method. On the other hand, CVD methods continue to bedeveloped for high-k film deposition and may offer a more cost-effectivesolution as well as extendability to very advanced devices (> 4 GbDRAM) with more stringent step coverage needs.11.1 Target FabricationA sputtering target for ~nicroelectronicapplication consists of an appropriately shaped and dimensioned slab of target material. which is then attached (typically bonded) to a simple backing plate or to a more complexmechanical assembly for use with a given supplier's sputterin2 cathode.Figure I1. I shows a variety of targets and assemblies used in advanced ICproduction. The target might be formed or machined into a large-diametercircular disk to be used with a company-specific planar magnetron cathode~ ~ , RMX1".
or Varian Quantum"h'.such as the AMAT D u r a ~ o u r c e ' MRCAlternatively, the target could consist of a single piece with complextopography like the ring source of a conical magnetron (e.g., Varian ConMag'".) or separate pieces intended for a multiple-cathode source such asSputtered Films' dual-cathode S - G U ~ ' " ~ ' .The lateral dimension of the target is primarily set by the need to uniformly coat a stationary large-diameter wafer, which favors large targets,while at the same time minimizing deposition of expensive target materialon anything other than the wafer, which favors small targets.
The thicknessof the target is primarily set by the desire to maximize the time for any partof the target to erode to the backing plate without needlessly wasting material since, regardless of starting thickness, the eroded front surface of thetarget will eventually become sufficiently nonuniform that film uniformityis out of spec. These considerations applied to planar magnetron coating of6- and 8-inch wafers have led to the use of circular targets with diameterSPUTTERING TARGETS377FIG.
11.1 A variety of PVD target s h a p e s - planar, conical, and r i n g - s h a p e d - are used in advanced microelectronic manufacturing and depend on cathode design (courtesy of Tosoh, SMD,Grove City, OH).12 inches and thickness up to ~ 0.5 inch, excluding a possible backingplate that might add another 0.25 inch of thickness to the assembly.Regarding target diameter in the future, one supplier (C.
Wickersham ofTosoh SMD) has plotted the historical increase in target diameter ~bt~rgetwith time as wafer diameter has increased in production from d~w~fer -4 inch to 200 mm. He finds that these two diameters are strongly correlatedand track each other through an empirical relationshipof the form 4)target =91.46 ~bw~fe~ + 35 mm. Using this formula, we would expect the diameter oftargets needed to coat next-generation 300-mm wafers to be ~ 475 mm( 18-19 inch).PVD targets are machined from a solid block of material" how one fabricates this starting block depends on the specific metallurgy in question.For example, AI alloys (A1-Si, AI-Cu, A1-Si-Cu) are made by first castingthe metal and then working it by rolling or forging with various annealingtreatments.
Ti targets are manufactured in much the same fashion as the AIalloys" however, Ti-W targets utilize powder metallurgy. In this case, powders of W and Ti or Till 2 are mixed and then compacted under high pressure and high temperature (ranging from ~ 750-1500~The result is alump of metal at near 100% of bulk density that is then machined to thefinal target shape and surface finish. For example, hot isostatic pressing(HIP) at 800~ and 400 MPa can be used to produce W-10 wt % Ti targets378R. POWELL AND S.
M. ROSSNAGELwith greater than 99% of bulk density [11.1]. Depending on process conditions, the Ti-W target may contain a single phase or two phases, with thesingle phase being very brittle and difficult to work.11.2 Target CoolingDC magnetrons consume a great deal of energy relative to what they deliver to the wafer and have been described somewhat cynically as an expensive wafer heating device.
For example, planar magnetron depositionof AI at 1 /zm/min from a 700-cm 2 sputter target might have a specific deposition rate (i.e., film deposition rate normalized to magnetron power) of15 ~/sec-kW, requiring a cathode power of 12 kW (e.g., cathode currentof Idc -- 25 A and discharge voltage of Vdc - - 500 V).
As calculated inSection 5.5.4, the flux of A1 atoms to the wafer represents ~ 0.2 W/cm 2 or60 W over a 200-mm wafer. Even assuming that several times thispower is delivered to nonwafer surfaces such as the PVD shields, the totalrepresents only a small fraction of the 12 kW applied to the PVD source.This is not to say that the power delivered into the process chamber isunimportant. For example, it can lead to large temperature rises of thewafer and other exposed chamber parts. However, from the standpoint ofenergy use, almost all of the magnetron discharge power (the product of Idcand Vdc) is consumed by the cathode, w h i c h - for 12-kW and conventional diameter t a r g e t s - represents > 10 W/cm 2.
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