Roland A. - PVD for microelectronics (779636), страница 7
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11-77, John L. Vossen and Werner Kern, Eds. Academic Press, NewYork, 1991.1.12. Robert Parsons, "Sputter Deposition Processes," in Thin Fihn Processes 1I, pp. 177-208, JohnL. Vossen and Werner Kern, Eds., Academic Press, New York, 1991.1.13. Handbook of Thin Film Process Technology, David A. Glocker and S. Ismat Shah, Eds.,Institute of Physics (IOP) Publishing, Bristol, UK, 1995.1.14.
VLS! Electronics: Microstructure Science, Norman G. Einspruch, Series Ed. Academic Press,New York, 1990.INTRODUCTION211.15. Handbook of Multilevel Metallization for Integrated Circuits, Syd R. Wilson, Clarence J. Tracy,and John L. Freeman Jr., Eds., Noyes Publications, Park Ridge, N J, 1993.1.16. Donald L. Smith, Thin Film Deposition: Principles & Practice, McGraw-Hill, New York, 1995.1.17. Mitsuharu Konuma, Film Deposition by Plasma Techniques, vol.
10 in the Springer SeriesAtoms + Plasmas, G. Ecker, P. Lambropoulos, I. Sobelman, and H. Walther, Series Eds.,Springer-Verlag, New York, 1992.1.18. Michael A. Lieberman and Allan J. Lichtenberg, Principles of Plasma Discharges andMaterials Processing, Wiley, New York, 1994.This Page Intentionally Left BlankChapter 2 Physics of SputteringThin film, vacuum-based deposition technologies fall into two basiccatagories: physical vapor deposition (PVD) and chemical vapor deposition(CVD). PVD techniques include physical sputtering, which is the underlying topic of this volume, thermal evaporation [2.1, 2.1 ], and arc-based deposition [2.3, 2.4]. These techniques are generally atomic in nature, in thatthe films are deposited from single atoms or small clusters and any reactions that occur (such as oxidation or nitridization) occur at the film surfaceindependently of the source process.
This differs from CVD techniques, inwhich molecular species in the gas phase chemically react at a film surface,resulting in the formation of a condensed film as well as the emission ofvolatile by-products. CVD techniques will not be discussed in this volume.Sputtering is a relatively simple process in which an energetic particlebombards a target surface with sufficient energy to result in the ejection ofone or more atoms from the target. The sputter yield, Y, is just the ratio ofthe number of emitted particles to the number of bombarding ones:y = (number of ejected particles)(number of incident particles)(2.1)Physical sputtering can result from bombardment with a variety of incidentspecies.
The most commonly used species is an inert gas ion (e.g., Ar §Kr+), but sputtering can also result from the bombardment of other energetic ions, neutrals, electrons, and even photons. In general, the physicaleffects caused by bombardment with a neutral or an ion of the same speciesand energy will be identical.
The ion is usually neutralized by pulling anelectron from the near-surface region just prior to impact, and so it impactsthe surface as a neutral. However, since the electrical current to the targetdue to ion bombardment is easily measured and it is quite easy to generatelarge fluxes of ions at controlled energies, virtually all applications of sputtering use ions as the bombarding particles. Because of the vast variety ofpossible effects that can occur, we will confine the discussion primarily toinert gas positive ion bombardment, with occasional divergences to neutrals or appropriate negative ions.2.1 SputteringThe sputtering process is one of relatively violent, kinetic collisionsfirst between the incident energetic particle and one or two substrateatoms, and then subsequent collisions between multiple atoms as theR.
POWELL AND S. M. ROSSNAGEL24incident kinetic energy and momentum are distributed among many atoms(Fig. 2.1). Depending on the kinetic energy, E, of the incident ion, four different physical results can occur:1. Low Energy (0 < E < 2 0 - 5 0 eV). This regime is known classically(and somewhat inaccurately) as the subthreshold region. In this regime, itwas thought that the incident ion had too little energy to dislodge and ejecta target atom and that the resultant yield was zero. For many years, it wasobserved that sputtering seemed to have a threshold of about 40 eV formost materials, below which sputtering did not occur (Fig. 2.2).
This wasdue to the dramatic fall-off in the yield as the ion energy decreased.FIG. 2.1Schematic of physical sputtering process.PHYSICS OF SPUTTERINGFIG. 2.225Sputter yield for Cr sputtered with Ar and Hg as a function of ion energy at low energies[2.5].Various models were developed that predicted thresholds of about 4 timesthe binding energy of the target material, which corresponded with energies in the 30 eV range.Experimentally, though, more evidence has become available that suggests that sputtering can occur at energies below 4 times the binding energy. In high-density plasmas, such as those formed using ECR techniques,sputtering and film deposition at effective ion energies of below 15 eV areroutinely observed.
The required yields are in the 10 -6 range, which is 2 to3 orders of magnitude below the earlier measurements that suggested athreshold at higher energy. However, since the effective ion currents in anECR tool may be many tens of amperes, even these very tiny yields can bequite significant.26R. POWELL AND S. M. ROSSNAGELEXAMPLE: With an ECR tool operated at 1 kW, the total ion fluxwithin the source is on the order of 20 Amperes (at 50 eV/ion production rate). Most of this ion current lands on the chamber walls.With a sputter yield of 10 -5 at perhaps 10-15 V of plasma potential, this leads to an erosion rate in a typical source (800-1000 cm 2)of about 0.005 atomic layers per second.
While this seems small,since the material sputtered is randomly redeposited and can landon the dielectric window through which the microwave power enters the source, an electrically opaque film (approx. 10 nm) will bedeposited in a little more than 1 hour of plasma run time. This filmthen reflects additional microwave power from entering the source.Obviously, even an extremely low level of sputtering can becomecrucial in these very high current tools.There has been relatively little theoretical work on very low energy sputtering, perhaps because there are few applications.
However, the previousconcept of a true sputter threshold is really not that accurate. Under theright conditions, even an incident particle with very low energy ( < 1 eV)might be able to dislodge an adsorbed surface atom.2. Moderate Energy (50 eV < E < 1000 eV). This range, sometimesknown as the knock-on sputtering regime, covers most of the practicalrange of energies used for PVD technologies. In this range, the incident ionimpacts a target atom, which recoils and strikes one or more atoms, whicheach then recoil, and the process continues much like in Fig. 2.1.
However,this is a difficult process to predict and measure because it is keenly dependent on the exact collision point of the incident ion. The sequence ofcollisions will be completely different for each bombarding particle because each particle will hit in a different place with regard to the locationof the surrounding atoms, and only a small fraction of the target atoms nearthe impact point will actually be dislodged as part of the collision chain.This process must be evaluated practically by simply looking at the average of a large number of impacting particles.
Various computer codeshave been developed that follow the collision chains for many impactingions. The most widely used program is called TRIM, and there are manyvariants [2.6-2.8] (see Section 10.1).The sputter yield depends strongly on the incident particle's mass andkinetic energy as well as the substrate's mass and orientation. For manyyears it was thought that the substrate's temperature was important also.However, in the early 1980s a group in Julich, Germany, clearly showedthat unless the temperature was very close to the melting point, it was notPHYSICS OF SPUTTERING27relevant to the sputtering process [2.9].
Conceptually, also, it would notmake sense that energies on the thermal scale (0.1 eV) present in a warmsubstrate would have that much influence on sputtering events, which contain energies in the hundreds of eV range.The yields for several materials of relevance to semiconductor applications are shown in Figs. 2.3 and 2.4. Sputter yields for many common materials used in semiconductor applications for several ion energies andinert gas species are given in Table 2.1 (adapted from [2.10]).3. High Energy (1 keV < E < 50 keV).
This region, which is not relevant to semiconductor processing, is nevertheless a more well understoodregion. At these energies, the incident ion causes a dense cascade of secondary particles (target atoms) after the initial impact. Within this cascadeRange of MagnetronOperation10::|1i:|I,ri,, ,,I-......Zn1 ..........Cuj|AI"1:31_ ..._1 ~el'iSi||__/*Ti.>-c~.or)s0.1j_,___21_ _/II/Il/I//tIII_/II I/0.01101 O010001 O, 000100,000Ion Energy (eV)FIG.
2.3 Sputter yields as a function of ion energy for Ar + b o m b a r d m e n t of c o m m o n materials forion energies up to 100 keV [2.10].R. POWELL AND S. M. ROSSNAGEL283.5t-.o3.0-Eo2.5--o-~9>..c_o.AgCuPb2.0Ni1.51.00.0FIG. 2.4Argon~0~ff ~Co~AIEr//~'-~',,C200400600Ion Energy (eV)Sputter yields as a function of ion energy for low energy: up to 600 eV (2.5).volume, all of the bonds between atoms are broken and the region can betreated with a statistical mechanics-like approach.
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