Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 20
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Unless some steps are taken beforehand,the H2 reduction of WF6 on Si0 2 will not produce an adherent film. The solution to this problem was to undertake the selective process initially.18 It isthen pointed out that WF6 will decompose on Si, but it will slowly etch Si0 2 •Once the Si0 2 has been etched by WF6 , then the W deposited from the H2reduction process adheres very well.The two-step process just described was operated at 700°C and atn10sphericpressure. Films up to 0.5 micron were reported to be specular with no indication of cracking or loss of adhesion to the oxide.
A resistivity of 6 J1n-cm wasachieved.More recently, cold-wall blanket W depositions have been done at low pressures. 19 ;20 In this case, the problem of poor adhesion to the oxide surface wassolved by depositing an intermediate layer of WSi 2 • The silicide adheres verywell to oxide, and if properly treated, the W will adhere to the silicide. Onesuggested treatment is a plasma etch of the freshly deposited WSi 2 with NF3 •All depositions were carried out at 500° to 600°C and pressures of 200 to 500mTorr in the same barrel reactor used for tungsten silicide studies. 8 The silicide layer was typically 10% of the thickness of the final layer.
Accordingly,the silicon content was approximately 5% and the film resistivity was 8 to 10pn-cm for 1-micron thick films. Good conformality of these films was obtained, as can be seen in Figure 13. 19The hydrogen reduction of the hexafluoride process has also been developedin a hot wall process. 21 Experiments were carried out in a traditional diffusionfurnace tube with deposition rates approximately 100 A/min. Resistivities of14 pn-cm were found for 2000 A thick pure tungsten films. Adhesion of thesefilms on silicon was reported to be good, but not good on silicon dioxide.Tungsten hexachloride has not been studied as extensively as the fluoridefor tungsten deposition, since the chloride is a solid at room temperature andmust be heated to 170°C to achieve a reasonable vapor pressure.
Of course, allThermal CVD of Metallic Conductors105Figure 13: CVD tungsten on sil icon. 19lines leading from the sublimator to the reactor must also be heated to 170°Cto prevent condensation. In one experiment,22 WCI 6 was reduced by H 2 in anatmospheric pressure reactor similar to the horizontal cold-wall reactor usedby Shaw and Amick. 18 Depositions were carried out at 600°C and depositionrates of 100 A/min were achieved. No information on the quality of the tungsten film was reported, however.Finally, blanket tungsten can be deposited from tungsten carbonyl, W(COkSome data has been published and such depositions appear to result in films withappreciable carbon and oxygen. 23106Chemical Vapor Deposition for Microelectronics4.3.2 Selective TungstenAlthough there are a number of reactions from which the selective deposition of tungsten could be implemented, the only ones that have been investigated are the reduction of WF6 by either Si or H2 [see Equations (5) and (6)] .The first of these will obviously be selective.
The second can be selective whenthe H2 reduction is favored on W surfaces, as compared to oxide surfaces.The most comprehensive experiments have been performed in low-pressure CVD hot tube reactors. 24 When WF6 is reduced on clean, flat silicon surfaces, the deposition rate is very rapid (>1000 A/min) and self limiting. Gen0erally, a tungsten film of less than 200 A (grown at 300 to 425°C and 500mTorr) is sufficient to completely block this reaction, as shown in Figure 14.200~< 150~z:::.:::():r: 100....• 30QoCA 375°C• 425°CAenenw~••~50III•I-•A..• •••• •• •...• I•A.•••••••• • -I-IIIt0.11.010100DEPOSIT TIME (min)Figure 14: Silicon reduction of WF 6 -thickness of tungsten versus depositiontime. 24 Reprinted by permission of the publisher, The Electrochemical Society,Inc.If a selective tungsten layer thicker than 200 A is required, selectively deposited, then the two-step process originally suggested by Shaw l8 can be used.In this procedure, shortly after the limiting tungsten layer has been deposited,H2 is added to the reactant gases and tungsten deposition continues.
This schemeis effective as long as there is no simultaneous reduction on adjacent dielectricsurfaces. Alternately, the H2 can be added to the reactant gases at the start,and the same process will occur 24 since Si is a much more effective reducingagent for WF6 than H2 . This phenomenon is shown clearly in Figure 15.Thermal CVD of Metallic Conductors107o 270°C1:1300 °Co 325°C1000• 350°C800.~CJ)CJ)wz 600::t:~:::t:t-~400200H2 DISSOCIATIONON WWF6· Si REACTION, LIMITING THICKNESS246810DEPOSIT TIME (min)Figure 15: Tungsten deposit thickness as a function of time for several temperatures. 24 Reprinted by permission of the publisher, The Electrochemical Society,Inc.Since the mechanism whereby silicon reduces WF 6 involves the consumption of silicon (i.e., production of Si F4 , Si F2 , etc.), there is concern that thereshould not be too much encroachment of the tungsten into the silicon. Assuming the silicon is consumed uniformly, then two atoms of silicon will bereleased for each tungsten aton1 deposited.
As tungsten is a denser material thanSi, this translates into a thickness of Si consumed which is twice the thicknessof tungsten deposited.The Si consumption can be beneficial in that a more intimate contactbetween Wand Si is promoted. 25 It can, however, lead to encroachment problems that will be discussed later. As in the blanket tungsten deposition case,film resistivities are higher than bulk. For selective deposition, thinner films areuseful, so it is important to recognize that resistivity varies with film thickness,as shown in Figure 16. 26108Chemical Vapor Deposition for Microelectronics100-~~---------------------'9080..7060402010•oFigure 16: Tungsten film resistivity versus. film ~hickness.26Reprinted bypermission of the publisher, The Electrochemical Society, Inc.It is important to recognize that substrate pretreatment is important forselective tungsten depositions.
The character of the silicon surface, and its oxidecoating, playa decisive role in tungsten growth behavior. When starting with aclean Si substrate and only a thin native oxide or no oxide on it, silicon reduction yields good quality films that self limit at approximately 200 A. If thesubstrate is treated with a glow discharge (CF 4 /0 2 ), a rough 200 A oxide layeris formed.
In this case, the limiting tungsten layer can be thicker, but it is nota good quality surface. 27 This phenomenon is illustrated in Figure 17.Thermal CVD of Metallic Conductors109wI..10~",••.-'.J(a){Il(b)w0.3 m(a)(b)(II )Figure 17: Photographs of two limiting tungsten films: (I) Tungsten depositedwith dilute HF preclean; (II) Tungsten deposited with CF4 /0 z glow dischargeclean; (a) SEM; (b) TEM.
z7Depending on deposition conditions, tungsten may nucleate and depositon dielectric films. In order for a selective process to be successful, such deposition must be prevented. Qualitatively, higher deposition temperatures andhigher concentrations of WF6 in the reactant flow promote nucleation anddeposition.
However, even if low temperatures «500°C) and a dilute reactantgas stream (H zIWF6 = 220) are used, selectivity can be lost under certain conditions. It has been observed z7 that where there are large areas of exposedsilicon on a wafer, that selectivity is lost on oxide adjacent to these areas.Therefore, it has been suggested that reaction by products from the selectivedeposition of tungsten (Si F4 , H F) may cause th is loss of selectivity. Backsidecoating of wafers with oxide, for example, seems to be helpful in maintainingselectivity.
For depositions up to about 3000 A, it appears that good selectivity is achieved. To date, experimental results have been inconsistent for thickerfilms.Finally, the type of dielectric also plays a role in selectivity. The percentage monolayer coverage achieved in a WF6 deposition at 300°C for 40 minutesis shown in Table 2 for several different films. z4110Chemical Vapor Deposition for MicroelectronicsTable 2: Monolayer Coverage of Selective Tungsten on Different DielectricsPECVD S;3N4LPCVD Si 3N4Thermal S;02 (p-doped)2712%5.21.3Atmos. PSG (7% P)0.86Thermal S;02 (undoped)0.40The remaining important phenomena that occur with selective tungstendeposition are "encroachmentll and "tunneling." 28 When the silicon in a contact hole reduces WF6 , it is covered by a layer of tungsten. When the layer becomes thick enough so that WF6 can no longer diffuse through the tungstenlayer, the reaction stops.
However, consider an idealized contact hole as shownin Figure 18. The corner where Si, Wand Si0 2 meet is illustrated in expandedfashion in (b) of Figure 17. Observe that the reaction proceeds by WF6 diffusing down through the tungsten, reacting at the silicon surface and then Si F4 diffuses back out through the tungsten.
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