Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 21
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When the tungsten film becomes too thick,an alternate path for WF 6 to find silicon is at the corner junction. Accordingly,when the WF6 flow is maintained long after the limiting thickness of W has beenachieved, lIencroachment" occurs. Such behavior is illustrated in Figure 19for a WF 6 + Ar deposition at 300°C for 40 minutes.OXIDETUNGSTENSILICON(A)TUNGSTEN(B)Figure 18: Selective tungsten deposition in a contact hate.Thermal CVD of Metallic Conductors111Figure 19: Encroachment of tungsten under Si0 2 .
28 Top figure is SEM; bottom one is TEM. Reprinted by permission of the publisher, The ElectrochemicalSociety. Inc.A phenomenon related to encroachment is the appearance of "tunnels."One example of such tunnels is shown in Figure 20. There appear to be manyindividual tunnels, each of constant diameter (200 to 400 Al, with a singletungsten-containing particle at the end. The tunnels are observed when the sameprocess is done on aluminum.
They also occur whether or not H2 is added tothe WF 6 _ Therefore, it appears that Si F4 or Si F2 (which are gaseous) must be theproduct of whatever reaction is causing the tunnel formation.112Chemical Vapor Deposition for MicroelectronicsWI*"tlckt\•O.2 ,l mFigure 20: TEM illustration of tunnel formation for selective deposition at300°C.
28 Reprinted by permission of the publisher, The Electrochemical Society,Inc.When the selective deposition is carried out at different temperatures, thedegree of encroachment varies. 29 This is illustrated in Figure 21 for a short 3minute deposition.1.53 min0.02 Torr0.2DEPO.
TIMEWF6PT~1.0E..=0.5o300400 500 600700DEPO. TEMP lOCIFigure 21: Encroachment length at different deposition temperatures. 29Thermal CVD of Metallic Conductors113It appears that if the deposition temperature is chosen precisely at 550°C,that there is no encroachment. In fact, if the deposition is continued beyondthe time when the limiting W thickness has been achieved, then "creep up"occurs.
This is shown in Figure 22. 29 Additional W is deposited, but it occursalong the outer surface of the adjacent oxide, rather than at the silicon/silicondioxide interface.Figure 22: Creep-up phenomena onto (a) field oxide and (b) the wall of contact hole. 29114Chemical Vapor Deposition for MicroelectronicsIn contrast to the many studies of selective tungsten deposition in a hot30wall reactor done to date, only one has been done in a cold-waif reactor.It was done in a laboratory-scale, horizontal tube with wafer heating done byradiation from high-intensity tungsten filament lamps.
Selective depositionsdone at temperatures as high as 520°C (p "-'300 to 800 mTorr) were done atrates in excess of 1000 A/min. Such high rates would be particularly valuablefor via hole filling, where on the order of 1 micron of material has to be deposited. It should be noted that although deposition on Mo, AI and Si arementioned, the test data reported were for depositions on Mo.
These authorsalso reported a more rapid growth of W up the side of their contact holes, whichmay be related to the creep-up phenomena mentioned earlier. 294.4 ALUMINUMEvaporated and sputtered aluminum films have been successfully used inintegrated circuit manufacture for some years. Problems due to spiking ofaluminum into the silicon it is contacting have been minimized by doping thefilms with 0.5 to 1.0% Si.
Again, aluminum is susceptible to electromigrationfailure due to aluminum transport at high current densities. That is, at highcurrent densities through a narrow conductor, the conductor material can betransported in the direction of current flow. In the extreme, this results inopens occurring in these conductors.
Such problems have also been minimizedby introducing 1 to 4% Cu into the films. The primary shortcoming of thesealuminum films is their lack of conformality when attempting to cover 1 micronsteps or trenches. This is due to the inherent line of sight nature of the evaporation and sputtering techniques.Since CVD processes offer the possibility of conformal coverage of a severe terrain, this method of depositing aluminum has been considered. Themost completely explored method employs an aluminum-organic compound,TIBAL,31,32 -which isand the overall reaction is thought to be(6)AI (C 4 H 9 )3-+AI+ 3/2 H 2 + 3C 4 H s.Depositions were carried out in a hot-waif tube reactor.
Since TI BALis a liquid with a vapor pressure of 1.5 Torr at 45°C, it was introduced into thereactor by heating in an evaporator. Special precautions were taken becauseTIBAL is pyrophoric.Prior to the aluminum deposition, it was found necessary to "activate"the wafer surface by exposing it to TiCl 4 vapor. Presumably, Ti was deposited,and the authors claim that this served to provide numerous nucleation sites forthe pyrolysis of the TIBAL. Deposition was carried out at temperatures in therange of 220° to 300°C and pressures were, typ~cally, 200 to 500 mTorr. Filmgrowth rates were about 200 to 800 A/min. Incorporation of Si into the filmsThermal CVD of Metallic Conductors115was achieved by exposing the wafers to SiH 4 + H 2 (10 Torr) in the same reactor.Apparently, no attempt was made to add Cu to the films.The films were adherent, and no appreciable quantities of C, H or otherimpurities were detected.
The resistivity was measured to be only 10% higherthan bulk aluminum. As shown in Figure 23, the films appeared to be quiteconformal.Figure 23: CVD aluminum over vertical oxide step with overhang. 31The primary difference between these CVD films and those obtained byevaporation or sputtering was their roughness. The films produced were notspecular. They had a peak-to-peak surface roughness of 1000 to 1500 A. SEMpictures of a film deposited at 270°C and 0.4 Torr (see Figure 24) show thisroughness.In summary, CVD aluminum films produced by low-pressure pyrolysisof TIBAL have been shown to achieve the improved conformality desired.Unfortunately, they are rougher than standard films, and no feasible way ofintroducing Cu into the film has been found to prevent electromigration.116Chemical Vapor Deposition for Microelectronicsa4fLm0·--~·~···..
··~1Figure 24: SEM photos of CVD aluminum film. 32Thermal CVD of Metal! ic Conductors117REFERENCES1. Kern, W., Chemical Methods of Film Deposition, in Thin Film Processes,eds. J. L. Vossen and W. Kern, Academic Press, NY (1978)2. Saraswat, K.C. and Mohammadi, F., Effect of interconnection scalingon time delay of VLSI circuits, IEEE Trans. Electron Devices. ED-29:645 (1982).3. Murarka, S.P., Silicides for VLSI Applications. Academic Press, NY (1983).4. Yamamoto, N., Kume, H., Iwata, S., Yagi, K., Kobayashi, N., Mori, N.and Miyasaki, H., Fabrication of highly reliable tungsten gate MOSVLSI's.J. Electrochem. Soc.
133:401 (1986).5. Saraswat, K.C., Brors, D.L., Fair, J.A., Monnig, K.A. and Beyers, R., Properties of low-pressure CVD tungsten silicide for MOS VLSI interconnections.IEEE Trans. on Electron Dev. ED-30:1497 (1983).6. Private communication, Gaczi, P., and Reynolds, G.7. Brors, D.L., Fair, J.A., Monnig, K.A. and Saraswat, K.C., Deposition parameters and properties of low pressure chemical vapor deposited tungstensilicide for integrated circuits manufacture, in Proceedings of the NinthInternational Conference on Chemical Vapor Deposition.
eds. Robinson,McD., van den Brekel, C.H.J. Electrochem. Soc., Pennington, NJ (1984),p. 283. These figures were originally presented at the Spring 1984 Meeting of The Electrochemical Society, Inc. held in Cincinnati, Ohio.8. Brors, D. L., Fair, J.A., Monnig, K.A., and Saraswat, K.C., Properties oflow pressure CVD tungsten silicide as related to IC process requirements.Solid State Technology, April 1983, p. 183.9. Inoue, S., Toyokura, N., Nakamura, T., Maeda, M., and Takaji, M., Properties of molybdenum silicide film deposited by chemical vapor deposition.J.
Electrochem. Soc. 130:1603 (1983).10. Gaczi, P. To be published.11. Lehrer, W.I., Pierce, J.M., Good, E., and Justi, S., Low temperature LPCVDdeposition of tantalum silicide, in VLSI Science and Technology/1982,eds. C.J. Dell'Oca and W.M. Bullis (The Electrochemical Society Pennington, NJ, Vol. 82-7), p. 258.12. Wieczorek, C., Chemical vapor deposition of tantalum disilicide.
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Blocker, Jr., Electrochem. Soc.,Pennington, NJ (1973).118Chemical Vapor Deposition for Microelectronics18. Shaw, J.M. and Amick, J.A., Vapor-deposited tungsten as a metallizationand interconnection material for silicon devices. RCA Review 31 :306(1970).19. Brors, D. L., Monnig, K.A., Fair, J.A., Coney, W., and Saraswat, K.C., CVDtu ngsten-A sol ution for the poor step coverage and high contact resistance of aluminum, Solid State Technology, April 1984, p. 313.20. Smith, G.C., CVD tungsten contact plugs by in-situ deposition and etchback, Proceedings of the Second International IEEE VLSI Multi-LevelInterconnection Conference (1985).21.
Miller, N.E. and Beinglass, I., Hot-wall CVD tungsten for VLSI. Solid StateTechnology, December 1980.22. Lehrer, W.f. and Pierce, J.M., Low temperature CVD growth of tungstendisilicide, in Semiconductor Silicon, 1981, ed. H.R. Huff, R.J. Krieglerand Y. Takeishi, Electrochem. Soc., Pennington, NJ (1981 ).23. Vogt, G.J., Low-temperature chemical vapor deposition of tungsten fromtungsten hexacarbonyl. J. Vac. Sci. TechnoI. 20: 1336 (1982).24. Broadbent, E.K. and Ramiller, C.L., Selective low pressure chemical vapordeposition of tungsten.
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