Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 22
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J. Electrochem. Soc. 131 :1427 (1984).25. Gargini, P., Tungsten barrier eliminates VLSI circuit shorts. Ind. Res. &Dev., March 1983, p. 141.26. Green, M.L. and Levy, R.A., Structure of selective low pressure chemically vapor-deposited films of tungsten. J. Electrochem. Soc. 132:1243 (1985).27. Broadbent, E. K. and Stacy, W. T., Selective tungsten processing by lowpressure CVD. Solid State Technol., Dec.
1985, p. 51.28. Stacy, W.T., Broadbent, E.K. and Norcott, M.H., Interfacial structure oftungsten layers formed by selective low pressure chemical vapor deposition.J. Electrochem. Soc. 132:444 (1985).29. Itoh, H., Nakata, R. and Moriya, T., Creep-up phenomena in tungsten selective CVD and their application to VLSI technologies. IEDM Technical Digest, International Electron Devices Meeting, IEEE (1985).30.
Wilson, R.H., Stoll, R.W. and Calacone, M.A., Highly selective high rateW deposition for via filling. Proceedings of the Second InternationalIEEE VLSI Multilevel Interconnection Conference (1985), p. 343.31. Cooke, M.J., Heinecke, R.A., Stern, R.C. and Maes, J.N.C., LPCVD ofaluminum and AI-Si alloys for semiconductor application. Solid StateTechnology, Dec. 1982, p. 62.32. Green, M.L., Levy, R.A., Nuzzo, R.G.
and Coleman, E., Aluminum filmsprepared by metal-organic low pressure chemical vapor deposition.Thin Solid Films 114:362 (1984).5Plasma-Enhanced CVD5.1 INTRODUCTIONIn the previous two chapters, we examined the chemical vapor depositionprocess when well-defined reactant gases were supplied to the chamber thatheld the wafer. In all these cases, it was necessary to bring the wafer to a hightemperature (350° to 900°C) before the desired reaction could be achieved.When integrated circuit dimensions are reduced (i.e., VLSI), diffused regionsbecome quite thin (""2000 A), and high-temperature processing is a disadvantage.
Also, if we want to use aluminum in a multilevel metallization scheme,we have to keep temperatures below 500°C.In such cases, another CVD deposition technique has been sought. Thesolution for some applications has been the use of a glow discharge. If the reactant gases in the reactor chamber are kept at low pressure «2 Torr), then aglow discharge can be sustained, as discussed in Chapter 2. In a glow discharge,the entering gases are dissociated so that the reactive species that reach thewafer surface are atomic or molecular fractions.
For example, a glow dischargein SiH 4 will produce SiH, SiH 2 , or SiH 3 free radicals. All of these will reactwith unit probabil ity on a wafer surface. Thus, it is not necessary to have thewafer at a high temperature in order to achieve acceptable deposition rates.It may still be desirable to have a moderately high wafer temperature forother reasons. For example, at very low temperatures, film density may below, or temperature may play an important role in determining film structure.Nonetheless, it is possible to operate at lower wafer temperatures than wouldbe allowed by a strictly thermal process.In the present chapter, we will review the nature of plasma-enhanced CVD(PECVD) films for a variety of applications.
We will look at dielectrics (siliconnitride, silicon dioxide), semiconductors (polysilicon, epi silicon) and metals(refractory metals, refractory metal silicides, aluminum). There are many otherimportant films (i.e., amorphous silicon for solar cells and TiN for tool harden-119120Chemical Vapor Deposition for Microelectronicsing, among others) that have been put down by PECVD but they fall outsidethe scope of this text.5.2 SILICON NITRIDEPECVD of silicon nitride has been of commercial importance since 1976.
1The original motivation was to find a final passivation layer for an integratedcircuit that would replace the doped silicon dioxide films then in use. Thelatter were not reliable enough to permit packaging of integrated circuits inplastic. Silicon nitride was recognized as a better final passivation film, butthe only available technique for its deposition was the high-temperature thermalprocess. Since it had to cover an aluminum final metallization layer that wouldmelt at 600°C, this clearly could not work. The solution was to use PECVDat 350° to 400°C.The reactant gases that have been used for PECVD of silicon nitride havebeen either SiH 4 + NH 3 with a diluent gas such as N 2 , Ar or He, or SiH 4 + N 2 •Reactors using parallel-plate, capacitively-coupled R F have generally been employed (see Chapter 2).
The specific reactor geometry or condition (hot orcold wall) does not have a major influence on the film characteristics. It does,however, playa large role in determining film deposition rate and uniformity,which for batch systems determines the commercial viability of the reactor.After we evaluate the nature of the silicon nitride deposited by PECVD,we will return briefly to the question of "characterizing" a production reactor.Once the reactor configuration has been selected, some of the parametersthat will determine film quality are:(1) operating pressure(2) operating temperature(3) discharge frequency(4) reactant gas mixturePECVD silicon nitride films are amorphous at the temperature used forgrowth, so film structure (i.e., grain size) is not an issue. The film quality isdeterm ined by:(1 ) stoichiometry(2) H 2 content(3) impurities(4) density(5) stressThe first three items relate to the chemical nature of the film.
An outstanding feature of PECVD silicon nitride films is that their stoichiometry canbe controlled, and that they can have as much as 30 atomic percent hydrogenin them. The last two items relate to the mechanical behavior of such films.If they are not dense enough, they will nc t be effective barriers to moisturePlasma-Enhanced CVD121and oxygen. Films with tensile stresses tend to crack, and ones with strongcompressive stresses tend to delam inate readily.For silicon nitride films made in a cold-wall, parallel-plate reactor operatingat 50 kHz, 200 mTorr, gas flows of SiH 4 /NH 3 /N 2 = 140/270/800 sccm and500 watts of power, we can compare chemical and physical properties withthermally-deposited silicon nitride. Such a comparison is shown in Table 1. 2Table 1: Physical and Chemical Properties of CVD and PECVDSilicon Nitride Films 2PropertyCompositionSi/N ratioSolution Etch Rate20°_25°CBuffered H F23°C49% HF155°C85% H 3 P0 4180°C85% H 3 P0 4Plasma Etch Rate92% CF 4 -8% O2 , 700 WNa + penetrationI R AbsorptionSi-N max.Si-H minorDensityRefracti ve IndexDielectric ConstantDi el ectric StrengthBul k ResistivitySurface ResistivityIntrinsic StressThermal ExpansionColor, TransmittedStep CoverageH 2 0 PermeabilityHigh Temp.
Nitride900°CSiN x0.8-1.0Si 3 N 40.7510-158015120Plasma Dep. Nitride300°CA/minA/minA/minA/min600 A/min<100 A---830 cm-12.8-3.1 g/cm 32.0-2.16-71 x 10 7 V/cm10 15 _10 17 n-cm13n/sq>10101.2-1.8 x 10dyn/cm 2Tensile4 x 10-6NoneGoodZerorC200-3001500-3000100-200600-1000A/minA/minA/minA/min1000 A/min<100 A---830 cm-12,200 cm- 12.5-2.8 g/cm 32.0-2.16-96 x 10 6 V/cm10 15 n-cm1 x 10 13 n/sq1-8 x 10 9 dyn/cm 2CompressiveYellowConformalLow-NoneIf we restrict our attention to one set of chemical precursors (SiH 4 , NH 3 ,N 2 ), we have available more detailed data describing the quality of PECVDsilicon nitride thin films as a function of the several operating parameters.
3Experiments were carried out in a parallel-plate reactor placed in a horizontalhot tube system where the wafer was placed on the grounded electrode.We can see the contribution of relative NH 3 concentration on film composition (N A at %), density (p g/cm 2 ) and deposition rate (G nm/min) for a300°C, 310-kHz and 65-Pa (487.5 mTorr) deposition in Figure 1. For thiscase, the SiH 4 flow was 100 seem and the N 2 + NH 3 flow was 1,400 seem.122Chemical Vapor Deposition for MicroelectronicsIIII300°C-310 kHz-65PaG-30~(nm/min)1c-061'0 __0----- 00-20>-11'-----0 ---10II2.9II)( .---.,. xx",P2.8(g/cm3 )ix<)(----x2.7soN-+- - - .
. -----+~+- ... - - ...NA 40(at.%)i,+00, 0---- 0----.-§ i o-0--- .------ ----- .30H20•10o0.250.500.751.00PNH 3 I Ptot ----.Figure 1: Silicon nitride film characteristics as a function of relative NH 3 concentration in reactant flow. 3 Reprinted by permission of the publisher, TheElectrochemical Society, Inc.We observe that as the partial pressure of NH 3 is increased, the amountof hydrogen incorporated into the film rises from 14 to 26 atomic %. The filmdensity is a maximum where the Si/N ratio is 0.75.Sim ilar measurements were made as temperature varied from 300° to 600°Cat 130 Pa (975 mTorr) and flows set at SiH 4 = 100 sccm/N 2 = 200 sccm/NH 3 ==Plasma-Enhanced CVD1231,200 sccm with a discharge frequency of 310 kHz.
For comparison, resultsat 700°C without a discharge were included and are shown in Figure 2. As thetemperature increases, the density rises to a limiting value and the hydrogencontent of the film drops rapidly to less than 10%. It is interesting to notethat IIthermalli silicon nitride still has several percent hydrogen in the film.IIIII310kHz-130PaG30~(nm/min)20t______0___ 0---°0 -c ---0 -°10IIII3.1P~x3.0(g/cm 3 )ix/2.9--x-xxx/'2.82.7N50+---+----+40--+--+-+-- +00.___0 Si- 0 -0oo~30201b"""'-'.".~.~.a300H----- •____.----.•400Figure 2: Silicon nitride film characteristics as a function of deposition tem3peratures.
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