Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 25
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Dep.-0.5 (C)APCVD PSG(Low Temp.)+ (1-4) (T)2.9(0.7) to -1C = CompressiveT = TensileFinally, we will consider PECVD silicon oxynitrides, and their unique characteristics. When oxygen is added to a PECVD nitride film, there are indications that it may improve its crack resistance as a final passivation layer. 13Also, there may be advantages in terms of its electrical characteristics as an interlayer dielectric. Therefore, the nature of films grown when N 2 0 is addedto a SiH 4 , NH 3 and He gas mixture in a high frequency (13.56 MHz), coldwall, parallel-plate reactor have been studied.Measurements were made showing that the film composition could bevaried uniformly from nitride to oxide, with varying composition oxynitridesin between, simply by changing the ratios of reactive gases appropriately.
Therefractive index of the films, accordingly, varied from 1.93 (nitride) to 1.45(oxide). Annealing did not alter the composition, except to reduce the hydrogen content. Increasing the oxygen content of the films had the effect of decreasing the plasma etch rate of the film.The ability to deposit oxynitride films at low temperatures by PECVDis just another example of the unique ability of this technique to tailor thechemical composition of a film to match desired film characteristics.5.4 POLYSILICONUp to this point, we have been considering PECVD films that are beingused commercially in the manufacture of integrated circuits. We will nowconsider PECVD of other films that may find practical application in the future.At the present time, they are still considered experimental.The major advantage to the use of PECVD for deposition of polysil iconfilms is the possibility of depositing them at low temperatures.
The conventional LPCVD process requires T ~650°C for times on the order of a half hour.PECVD of polycrystalline films can be carried out with a wide variety of gases0(SiH 4 , Ar, He, H 2 and doped or undoped) at temperatures ranging from 200Plasma-Enhanced CVD137to 400°C. 14 The morphology of the films created is an important issue. Insome cases, polycrystalline films are formed, while in others a mixture of microcrystals in an amorphous matrix is found.Aside from the reactant gases that can be used, another factor influencingthe nature of the films is ion bombardment.
If ion bombardment is increased ata given deposition temperature, it is possible to convert an amorphous film toone which is microcrystalline. As ion bombardment continues to increase, thevolume of crystall ites display lower resistivity, as would be expected. Theas-deposited resistivity of PECVD polysilicon has been shown to have a valueof 0.78 x 106 n-cm, as compared to similar film done by LPCVD which is13.8 x 10 6 n-cm.Many questions remain. It will have to be shown that the doped resistivitycan be as good as conventional doped LPCVD films.
Also, the role of speciesother than sil icon has to be clarified (j .e., hydrogen, noble gases).At the same time, the fully doped polysilicon resistivity (LPCVD or PECVD)is proving inadequate for future generation IC devices, and interest is shiftingto refractory metal silicides and/or refractory metals as replacements. Therefore, it is not clear how significant the development of such a new processwill be for integrated circuit manufacture. There are other applications, however, where it may be better suited.5.5 EPITAXIAL SILICONReferring to our earlier discussion of the thermal CVD of epitaxial silicon,we recall that a very high temperature initial etch (HCI at 1150° to 1250°C)was necessary to prepare the silicon substrate for epitaxial deposition. Suchvery high temperatures can, and do, lead to difficulties with slip defects.
Thenit was noted that commercial practice was to deposit from SiCI 4 or Si HCI 3rather than SiH 2 CI 2 or SiH 4 . The latter two reactants permit high depositionrates at lower temperatures, but in some cases, do not produce good qualityfilms. Therefore, deposition temperatures range up to 1000° to 1050°C, andat these temperatures, autodoping becomes a serious problem, especially forthe thinner epi films of current interest.Accordingly, there has been considerable interest in carrying out this process at lower temperatures. Son1e research has been done using a PECVD approach to deposit epitaxial silicon films from SiH 4 at temperatures as low as750°C without the high-temperature cleaning step.IS Unfortunately, similarresearch has not been done using the reactants traditionally used.
In fact, referring to Figure 17 of Chapter 4, we recognize that epi silicon from SiH 4has always been possible at relatively low temperatures. The issue has been,that the film quality has been inadequate. It remains to be seen whether or notthe PECVD approach can produce better qual ity films than can be producedthermally.The research at M IT has been done in the cold-wall vertical tube reactorshown in Figure 14. The wafer is aligned almost parallel to the flow on a vertical silicon carbide-coated susceptor. The wafer is heated by optical radiationfrom high-intensity lamps to a temperature of 775°C. Silane was introduced138Chemical Vapor Deposition for Microelectronicsat a pressure of 15 mTorr for the deposition.
Prior to deposition, an argonplasma was struck and the wafer biased 300 V negatively. The argon ion bombardment effectively removed the approximately 20 A native oxide from thewafer and permitted epi film growth even with the plasma off ("-340 A/min)at 775°C. Using the plasma during epi growth increased the deposition rate to450 A/min. If the plasma preclean is not used, or less than 300 V bias is applied, then films deposited are polycrystalline.GASPYROMETERD-~\.......... _--. ..LAMPsDiRFo:L--..-..-.ro- SAMPL E"DC~~TURBOPUMPFigure 14: M IT system for PECVD of epi silicon. ISThere is no doubt that the deposition rate for silicon deposition can beenhanced by plasma excitation of the reactive gases. The modest increase described above (340 ~ 450 A/min) is probably more a function of the reactorconfiguration than anything basic in the process.
Whether the epi films deposited at these higher rates are of good enough quality for integrated circuitfabrication remains to be seen. It would also appear that if the traditionalhigh-temperature Hel and/or H 2 predeposition etch can be avoided by a plasmaPlasma-Enhanced CVD139process, then process temperatures can probably be realistically reduced by50° to 100°C. This would reduce slip defects and problems from autodoping.5.6 REFRACTORY METALS AND SILICIDES5.6.1 TungstenIn the previous chapter, it was shown that it is possible to deposit qualitytungsten films by eVD for either selective or blanket applications at moderate temperatures.
As-deposited resistivities are low, so post-deposition hightemperature anneals are not essential.As research on thermal CVD of tungsten proceeded, parallel studies werebeing done on plasma-enhanced depositions. 16 At the present, commercialapplication of thermal CVD tungsten has begun, but all the problems have notbeen solved.
Therefore, it will be useful to review some of the PECVD of tungsten studies as well as similar research for other refractory metals or their silicides.Tungsten films have been deposited in a parallel-plate, cold-wall reactoroperating at 4.5 MHz. Initial depositions were attempted at temperatures from40°C on up with WF 6 as the reactant gas. It was discovered that depositionscould not be accomplished at temperatures above 90°C. Apparently, the glowdischarge creates many free fluorine atoms, and their etching of tungsten overtakes any deposition from WF 6 decomposition on the surface at this temperature. The solution to higher temperature depositions was the addition of H2to the WF6 to promote the formation of HFin the glow discharge.
The reaction is sim ilar to the surface reaction we discussed for blanket tungsten deposition in the last chapter, only now W is released in the gas phase.(1 )In effect, the hydrogen serves to scavenge any free fluorine in the plasma, thereby promoting the deposition reaction.Films deposited at low temperatures were porous and exhibited high resistivity. Therefore, depositions were carried out from 250° to 400°C and produced deposition rates that increased rapidly and then leveled off, as shown inFigure 15. Deposition rate increased Iinearly with pressure and power level.The as-deposited resistivity of the PECVD tungsten was found to be quitehigh, as is shown in Figure 16.
X-ray diffraction analysis 17 showed that theas-deposited film was largely a metastable J3-phase of tungsten. Pure J3-phasetungsten has a resistivity over 100 pn-cm, whereas the material we have beenconsidering all along has a low resistivity (""5 pn-cm) and is referred to as thea-phase. Heat treatment of the as-deposited films, as shown in Figure 17, bringsthe tungsten resistivity back into the expected range. Analysis of the chemical composition of the PECVD tungsten films showed less than 1% fluorineincorporated.If Si H4 is added to the reactive gas mixture, then tungsten silicide canbe deposited. 1s In this case, He was used as a diluent, along with the WF6and Si H4 , and deposition was carried out in a parallel-plate, cold-wall plasma140Chemical Vapor Deposition for Microelectronics6.0 ---~------,.----,""",,:,c5.0E"E.5 4.0Q)0a::c3.00<n0a.2.0Q)01.00200400300500Substrate Temperature (OC)Figure 15: Tungsten deposition rates versus substrate temperature for p = 200mTorr, H2 /WF 6 = 3 and 30 watts.
162208.0200160Euc:7.01606.0:l:.->140.~&nQJ50 a::~IJ.c4.0100eu0"0&It08030Ia.eu0(/)<tV)eu41)0~~.->41)Ul'u; 120eua::"00......c:I60(/)2.0<t401.020°1002003004005000Temperature (Oe)Figure 16: As-deposited resistivity and sheet resistance of PECVD tungstenversus deposition temperature. 16Plasma-Enhanced CVD14150Annealing Temp (Oe).•040E•u•500600650900c:=l30~...>\I).(/)<U0:2010•OL----....-.----..-----......--...o153045Annealing Time (min)Figure 17: Resistivity of PECVD tungsten versus annealing time and temperature in a 10% H2 /90% N2 ambient. 16reactor.
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