Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 23
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Reprinted by permission of the publisher, The ElectrochemicalSociety, Inc.124Chemical Vapor Deposition for MicroelectronicsVariations with pressure for a 300°C, 310-kHz deposition with SiH 4 =100 sccm/N 2 = 300 sccm/NH 3 = 1,100 sccm are shown in Figure 3. Most notably here, the hydrogen content rises and the film density drops sharply aspressure is increased above 150 Pa (1.125 Torr).,,300°-310kHz30_____ 0Gc-___ 0 - -_0(nm/min~Oi_____ c---10II3.0pJ(g/cm )i-_ x--- x -- x2.8--x2.62.450-+-NA 40.0---(at.%)iN+-+ - - - - +-.......-......Si o--o __ ~0--30-e_e20He __e~e1050100150200P (Pa)---..Figure 3: Silicon nitride film characteristics as a function of pressure.
3 Reprinted by permission of the publisher, The Electrochemical Society, Inc.A fairly dramatic change in film characteristics is seen as frequency isvaried. For a 300°C, 130 Pa (975 mTorr) discharge with SiH 4 = 100 sccm/N 2 =700 sccm/NH 3 = 700 sccn1, the variation with frequency is shown in Figure4. At frequencies of 4 MHz and above, the film density drops sharply and thehydrogen content rises sharpl y.Plasma-Enhanced CVD125300°C-130 Po2.8xx-----xP 2.6(g/cm3 )i2.450NA40(at.o/o)r-0-+0N+3020-.•o~SiH0 - - - ' 00 -+--.........+ --++-.J.-..-100.1to10- - - .
freq. (MHz)Figure 4: Silicon nitride film characteristics as a function of frequency.3 Reprinted by permission of the publisher, The Electrochemical Society, Inc.Finally, we can see how the film stress behaves with pressure, temperature and frequency in Figure 5. For each variable, the flow conditions are thesame as those specified in the corresponding prior figure. Clearly, depositionsat lower frequencies and temperatures tend to favor the desirable compressivestress condition.As noted in Chapter 2, when discharge frequency is low, there is a greatertendency toward ion bombardment.
If we argue that ion bombardment compresses the film as it is being deposited, we would expect to see compressivestresses and higher density films, as shown in Figures 4 and 5. Attempts to minimize the H2 content of the films is best done by operating with less NH 3and at higher temperatures.126Chemical Vapor Deposition for Microelectronics-12....o "'" 0-9~o~-6~0"-.....-3P(Pa)~50-91000_I150200coo6 L..------L.-------0.1-6 _ei-3oc~mpressive:tensile300freq(MHz)-----'-=--_...l-1------l._ _----""10-~_"-~-----T(OCl_·400500~e.;600700Figure 5: Stress in silicon nitride films as functions of pressure, frequencyand temperature. 3 Reprinted by permission of the publisher, The Electrochemical Society, Inc.As would be expected, there are some differences in the films formed whendifferent diluent gases are used (i.e., Ar or H 2 rather than N 2 ).
Recent experiments have been carried out in a parallel-plate, cold-wall PECVD reactor 4operating at 300°C, p := 0.33 mbar (330 mTorr), an RF power (50 kHz) of200 W, and a total gas flow of 1.2 slm. The ratio of NH 3 to N 2 , Ar or H2 wasvaried while maintaining the sum of the NH 3 and the diluent constant. Inplasma silicon nitride films, the composition can vary widely depending ondeposition conditions.
When we speak of film composition, we are talkingabout the Si/N ratio, which would ideally be 0.75, and the H 2 content. Theseauthors demonstrate that the film refractive index is directly related to theSi/N ratio, as shown in Figure 6. Then, when the fraction of NH 3 is variedduring deposition, we can see in Figure 7 that the stoichiometry varies differently with NH 3 concentration, depending on the diluent used .. For example,to achieve a Si/N ratio of 0.85, Figure 6 says we need a refractive index of2.0.
For the higher SiH 4 flow in Figure 7, we need only 35% NH 3 in the reactant gases when we use N 2 as compared to 53% if we use argon as the diluent.Plasma-Enhanced CVDI2.62.5o• SiH4 -NH 3 -N z+ SiH 4 -NH 3 -Hz/SiH 4 -NH 3 -Aro1272.4n12.32.2o2.12.01.9t1.•.o ...~+~0. 0~ +.~0.7 0.8 0.9 1.0 1.11.2 1.3 1.4 1.5 1.6 1.7~Si/NFigure 6: Refractive index of silicon nitride films as a function of stoichiometry (Si/N).4 Reprinted by permission of the publisher, The ElectrochemicalSociety, Inc.Si HI.
-NH 3 -ArSi HI. -NH 3 - H2• Si HI. -NH J -N 2o2.6+ni2.52.42.32.22.12.01.90.10.20.30.40.50.6Figure 7: Refractive index of silicon nitride films versus ammonia concentra4tion for different diluents. Reprinted by permission of the publisher, TheElectrochemical Society, Inc.128Chemical Vapor Deposition for MicroelectronicsAs diluents are changed, there does not appear to be any change in hydrogen content of the films. However, the film density does show some dependence on diluent used, as shown in Figure 8. Evidently, the densest films aredeposited when argon is used as the diluent.2.9\o2.8• SiH,-NH3 -N 2e,+ SiH 4 -NH 3 -H z(g/cm3) 2.7SiH4 -NH3 -ArP2.62.52.42.32.22.1•,2.0 ~--"-""""'--------"---'-"_-""-'-'"0.6 0.8 1.0 1.2 1.4 1.6 1.8--. Si/NFigure 8: Silicon nitride film density as a function of Si/N ratio for different4diluents.Reprinted by permission of the publisher, The ElectrochemicalSociety, Inc.When NH 3 is omitted from the reactant gases and the N in Si 3 N4 is provided by N 2 , substantial changes occur in the films deposited.
The % H incorporated in such films is much less than when NH 3 is used, for the samedeposition temperature as shown in Figure 9. 5It is also interesting to note that the % H in silicon nitride films varieswidely depending on the type of reactor used (15.4 to 30.7 at. %).5 It is notclear that the hydrogen content of these films is undesirable in a plasma silicon nitride film. However, if for some reason the decision is made to reduce HPlasma-Enhanced CVD12930::r:~2010O~----"-100"&--_ _~200300- " - -_ _- - I o400"'"500TEMPERATURE (OC)Figure 9: Hydrogen % versus deposition temperature for films deposited withSiH 4 + NH 3 , SiH 4 + N2 and SiH 4 + NH 3 + N2 (one point).5in the film, it would be best to work with a SiH 4 + N2 process at as high a temperature as possible.
As noted earlier in Chapter 3, however, even thermallydeposited, low-pressure CVD films retain some hydrogen in the film.Earlier, we reviewed silicon dioxide (thermal) films deposited with addedphosphorus to serve as a getter for mobile ion impurities, as a final passivationfilm. Plasma-enhanced silicon nitride can also be doped with phosphorus. 6Some of the film characteristics have been reviewed, and it was found thatthe fil ms with 2 to 3% P had the best el ectrical qual ity. No measurements ofstress or H2 content were reported, so it is not clear that these would be useable films.It is also interesting to note that the H bound into the plasma silicon nitridefilm can be driven off, to some extent, by annealing at low temperatures. Datafor SiH 4 + N2 + He and SiH 4 + N2 + Ar depositions are shown in Figure 10: Observe that for a film deposited with He at 300°C and then annealed for 10hours at 300°C, the hydrogen content drops from 19 to 12%.
The curves markedNOT are unannealed. Depositions with Ar did not display this behavior.After the quality of the plasma silicon nitride films and their dependenceon the several system parameters has been evaluated, there still remains thequestion of whether or not a given process can be commercially viable. Herethe issue is the deposition rate and the uniformity of deposition on a waferand over all wafers in the reactor. The ideal solution is to deposit at a highrate uniformly over many wafers at one time.
We cannot simply stack manywafers close together and run a low-pressure process, as in thermal LPCVD,because we have to be sure the plasma discharge is uniform as well.130Chemical Vapor Deposition for MicroelectronicsAT%H302010H100----__-0-_--.---...00O-+----+-----+----+--~o100200(0)300 T ( DC )AT%H302010o-+---+--~~--+---..-a100200(b)300 T(oc)Figure 10: Effect of annealing on hydrogen % in plasma silicon nitride filmsversus deposition temperature: (a) He diluent; (b) Ar diluent. 7 Reprinted bypermission of the publisher, The Electrochemical Society, Inc.Plasma-Enhanced CVD131In fact, there are many factors that make a reactor "commercial" or production ready, and they will be discussed along with many examples in Chapter 6.
In the present chapter, we will just touch on the typical process modifications necessary to develop a "commercial" system.The process of "characterizing" a reactor can be illustrated for a parallelplate cold-wall reactor operated at 50 kHz. 8 System power was kept at 500 W,pressure at 200 mTorr, and wafer temperature of 240°C. Wafers are placed ona circular electrode which is rotated to promote uniformity of deposition.Therefore, we are only interested in the radial variation of deposition rate.Reactive gases enter at the center and flow out at the periphery.Typically, deposition uniformity depends on the reactive gas mixtures,as is illustrated in Figure 11.
1 Clearly, much trial and error is required to establish reactor operating conditions that will yield wafers with uniform deposits.It is interesting to observe that, as shown in Figure 11 (a), as the SiH 4flow is decreased, the deposition rate at the outer portions of the platen riserapidly. Running with N2 and no NH 3 , as in Figure 11 (d) makes it impossibleto obtain uniform depositions.There is no convenient, obvious, explanation for these diverse results.Basically, as we change the gas composition, we are modifying the glow discharge behavior even though power and pressure have remained unchanged.Such changes in the discharge are the cause of unexpectedly nonuniform deposition behavior.
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