Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 18
Текст из файла (страница 18)
These are WSi 2 , MoSi 2 ,TaSi 2 and TiSi 2 •The first problem occurs with the gate electrode. The solution that hasbeen developed has been to create a "polycide" structure. Here, a thin layer92Thermal CVD of Metallic Conductors93of phosphorus-doped poly is deposited and then a conducting layer of silicideis deposited on top of the first layer. The combination is a much better conductor than the doped poly. Additional details of this polycide film will be coveredlater. The properties of the silicide films have been reviewed by Murarka.
3 Wewill restrict our discussion to these films when they are deposited by CVD.Again, as VLSI requirements become more demanding, multilevel conductor circuits are being developed. The final metallization layer can be aluminum,since there are no additional processing steps that require temperatures above350°C. However, if we wish to use a second conductor level between the gateelectrode and the final metallization, then aluminum is no longer acceptable. Itmelts at about 660°C, should not be heated above 500°C, and there would beadditional processing steps well above these temperatures.
Some integratedcircuit developers have used two layers of polysilicon in this application, andothers have tried to develop low-temperature processing techniques for dielectric deposition to permit two aluminum levels. Both approaches have severeshortcomings, so CVD of refractory metals has some attraction. The resistivityof the refractory metals or silicides are not as good as aluminum, but for tungsten or molybdenum, it can be within a factor of two, a large improvementover doped poly. So, in addition to the refractory metal sil icides, there is muchinterest in refractory metals, and these will be discussed later.As a point of interest, approximate values of the thin film resistivities ofthese materials are tabulated in Table 1.Table 1: Resistivities of Representative Thin Film Metallic ConductorsMaterialWSi 2MoSi 2TaSi 2TiSi 2MoWAIDoped PolySiResistivity (pS1-cm)501005025894500An excellent presentation showing the future direction of gate/interconnect materials is shown in Figure 1.4 Since 1-MB DRAMs are now appearing onthe market, the pressure to move to the newer metal! ic conductors is strong.As a final point, we note that as device dimensions shrink, it becomes increasingly difficult to obtain good conformal coverage over steps and trencheswhen Iine-of-sight techniques are used (i.e., evaporation or sputtering).
In general, CVD offers excellent conformal coverage, so that has provided a furtherpush toward CVD metals. In fact, poor step coverage with the traditional sputtered aluminum has led to an interest in CVD aluminum. We will conclude ourchapter with a review of work in this area.94Chemical Vapor Deposition for MicroelectronicsSi-;1 00 - (1 010~E30 · em)JY1Q~_kc(1040·cm)W, Mo(1 0 -50 · em):>.rc~4~b ~i>J-=5~Kb 6~Kb0.1~-----L...---~0.7123I416Kb5(/-l m)Gate / InterconnectwidthFigure 1: Future generation MOS V LSI gate electrode and interconnect material choices. 4 Reprinted by permission of the publisher, The ElectrochemicalSociety, Inc.4.2 REFRACTORY METAL SILICIDESIn this section, we will restrict our attention to the silicides of tungsten,molybdenum, tantalum and titanium.
CVD WSi 2 is currently being used commercially, and the other three have either been considered seriously or used toa limited extent. We will start with WSi 2 •4.2.1 Tungsten SilicideUsing a cold-wall CVD reactor similar to the internally-heated barrel described in Figure 22 of Chapter 1, tungsten silicide was deposited from WF 6and SiH 4 ,s which is often described by the overall reactionExperimental evidence shows, however, that there is very little H F foundas a byproduct when WSi 2 (s) is deposited from WF6 and SiH 4 .
6 From thermodynamic considerations, we could anticipate reaction products such as Si F4 ,SiHF 3 , SiH 2 F2 , SiH 3 F, SiF2 , HF and H 2 as well as possibly others. Therefore, amore accurate representation would be0Depositions were done over a temperature range of 330 to 450°C and apressure range of 50 to 300 mTorr. A SiH 4 to WF6 ratio of 70:1 was used,which resulted in a Si:W ratio of 2.2 to 2.7.
In other words, they achieved filmswith the composition of WSi x where 2.2x2.7. Deposition rates varied withWF6 flow rate, as shown in Figure 2. On the other hand, they did not vary withpressure or deposition temperature. The stoichiometry of the as-deposited filmalso varied with the WF 6 flow rate, as shown in Figure 3.< <Thermal CVD of Metallic Conductors9560CEE.s5040G)asa:.r::30~30201050101520WF 6 Flow Rate (seem)Figure 2: Deposition rate of WSi x versus WF6 flow rate. 72.7 - - - - - - - - - - - - - - - ,2.62.52.42.32.2~__o~__5_ _ l __ ___"""'1015....~20WF e Flow Rate (seem)Figure 3: Film stoichiometry as a function of WF 6 flow rate.
7As mentioned earlier, the "polycide" structure can be used to replace thetraditional gate poly. A sketch of such a configuration is shown in Figure 4,when x2.0, the WSi x is stable on the poly. Otherwise, it cracks and/or peelsoff during high temperature processing.>A; WSi x25004000A POL YSI LICON (DOPED)_ _ _ _ _ _ _ _ _ _ _ _ _ _~ 1000)\~Si(SUB~:;:;~~Figure 4: Polycide structure.Si0THERMAL296Chemical Vapor Deposition for MicroelectronicsThe as-deposited WSi x exhibits high resistivity, which drops to acceptablevalues upon heat treatment.
The explanation is that the as-deposited film hasvery small grains and they grow to quite large grains as a result of the heat treatment. This phenomena is illustrated in Figure 5, where 1000 A thick films weredeposited on bare (100) silicon wafers and annealed for 1 hour in argon at different temperatures. The as-deposited film has a microcrystalline structure with30 A grains. When annealed at 1000°C, the film became polycrystalline, withcrystals 750 A across.-1500AI-oLPCVD WSi 2 : 5 (a) as-deposited, 400°C. or annealed,(b) 500°C, (c) 600°C, (d) 800°C, and (e) 1000°C.Figure 5: TEMs of 1000AThermal CVD of Metallic Conductors97The same effect is seen when the polycide structure of Figure 4 is annealed.In Figure 6 we see the effect of furnace annealing; Figure 7 shows similar effectsfor rapid thermal annealing.040a 9OO·CCo 1000·0CI)A0c 30tUC;;11oo·C22004000(IiA Sfficlde onA Undoped PolyCI)a:Q)Q).cCJ)10510152030253540Time (min)Figure 6: Resistivity of WSi x films on polysilicon with furnace anneals.
740oa---,302200 A Silicideon Undoped PolyCDoc:ftSiii 20"enQ)a:a;Q)t510102030405060708090100 110 120Time (sec)Figure 7: Resistivity of WSi x films with rapid anneals. 8Another feature of the annealed films is that their final resistivity is dependent on their unanneated stoichiometry. This effect is illustrated in Figure 8,where we see that the lowest resistivity film is obtained with a slightly siliconrich film (WSi 2 •2 ). Apparently during the annealing process, some of the Si inthe WSi 2 •2 migrates down to the poly underlayer, leaving this poly slightlythicker (about 150 A) and the silicide closer to WSi 2 , as desired.98Chemical Vapor Deposition for Microelectronics55,...----------------,---()..---,50- 45E~~Cl.40o302L.2---2.J...3---...!2.4----'2L..5-----::2:':.6:---~2.7x (WSixlFigure 8: Annealed resistivity versus WSi x stoichiometry.?The tungsten silicide films, when deposited by CVD as contrasted to sputtering, are very conformal.
This behavior is shown in Figure 9, where a 5000 Apolycide film (2500 A WSi z + 2500 A poly) is deposited over a small step. Onsteps with vertical walls, the thickness of the polycide on the vertical wall wasat least 75% of its thickness on the horizontal surface.Figure 9: SEM of step coverage of CVD WSi z·sThermal CVD of Metallic Conductors99One of the primary advantages of the polycide concept is that the silicidetop layer oxidizes readily to form a dense adherent Si0 2 overlayer, and the polycide structure underneath remains intact. Also, the oxide film forms within areasonable time.
Oxide thicknesses formed by dry O 2 oxidation are shown inFigure 10, as a function of time and temperature.WSi 2 OXIDATION IN DRY 020<..........(/)(f)w10950°C3z~900°Cu:r:....w0-)(0•.........----------------..j-------"'--------...J210 21010OXIDATION TIME (min)Figure 10: Oxide thicknesses for dry oxidation of WSi 2 . 5The mechanism whereby the silicide acquires its Si0 2 overlayer is importantto the understanding of the value of this structure. Basically, Si diffuses from thepoly layer up to the surface of the WSi 2 layer where oxidation begins. As theoxide grows, O2 diffuses from the surface of the oxide down to the interfacebetween the oxide and WSi 2 . At the same time, Si continues to diffuse up tothis same interface from the poly layer.
In fact, if a very thick oxide layer isgrown, and the underlying poly were fairly thin, it is possible to completelyconsume the polysilicon layer. Obviously, it would neither be necessary ordesirable to carry the process this far. The fact that the WSi 2 layer remainsintact during this process allows the integrated circuit designer to continueto use the usual process steps while obtaining the benefits of the lower resistivity polycide layer.100Chemical Vapor Deposition for Microelectronics4.2.2 Molybdenum SilicideCompared to the detailed studies of WSi 2 , relatively less has been reportedon LPCVD of the other refractory metals. One study in which MoSi 2 has beendeposited from MoCl s, Si H 4 and H 2 has been described recently.9 In th is work,a hot tube furnace was used with a MoCl s sublimator using an H 2 carrier gasattached to it.
The sublimator was operated at 160°C and all lines leading tothe furnace were heat traced to prevent MoCl s condensation.A typical deposition was carried out with MoCl s in H 2 gas at 190 sccm,Si H4 at 210 sccm, and N 2 as a diluent gas. Pressures were in the range of 600mTorr to 2.0 Torr. At a temperature of 670°C, a deposition rate of 180 A/minwas achieved. Although several molybdenum silicide phases are known (M0 3 Si,MosSi 3 , and MoSi 2 ), only MoSi 2 was found in these experiments.The as-deposited films had resistivities of 1000 J.lrl-cm. After annealing at1000°C for 20 minutes, this reduced to 120 J.lrl-cm, which compared to theWSi 2 described earlier is more than twice the value. One advantage of this filmis that some small amount of chlorine remains after deposition, and this chlorinecan act as a getter for any mobile impurities such as Na.Another recent study has examined the MoSi 2 deposition using MoF 6and Si H 4 rather than the chloride.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
