sublayer interface after SF6 based reactive ion etching

Applied Surface Science 70/71 (1993) 613-618 North-Holland

applied surface science

In situ XPS analysis of the tungsten/sublayer interface after SF 6 based reactive ion etching N. Couchman a,b C. Pacifico a,c G. Turban a and B. Grolleau a a Laboratoire des Plasmas et des Couches Minces, Institut des Matdriaux de Nantes, UMR 110, CNRS-UniversitE de Nantes, 2 Rue de la Houssini&e, 44072 Nantes CEdex 03, France b M A T R A MHS, Route de Gachet, Case Postale 3008, 44087 Nantes Cddex 03, France c Centro di Studio per la Chimica dei Plasmi, CNR c / o Dipartimento di Chimica, Universit?t di Bari, Tray 200 Via Re David 4, 70126 Bari, Italy Received 20 August 1992; accepted for publication 20 November 1992

The characteristics of the interface between CVD tungsten and its underlayer have been investigated after successive short plasma etching steps of the metallization bilayer. An X-ray photoelectron spectroscopy (XPS) analysis has been performed in situ at each etching stage to identify the species formed on the etched surface depending on the sublayer and the etching method. The tungsten/sublayer structures had first been investigated "as-deposited" by the XPS profile as a reference. Three different materials were tested for the sublayer: titanium, titanium nitride and titanium-tungsten alloy TixWy and two reactive ion etching processes were compared: pure SF6 and SF6/O 2 mixture. It has been observed that titanium is preferentially fluorinated than tungsten when the two materials are present at the interface. Both titanium and titanium nitride are covered with a quite thick fluorinated layer of etching residues after total etching of the tungsten film whereas only a few percent of titanium present in the tungsten-titanium alloy is fluorinated. Tungsten-titanium is then far less polluted after an SF6 based ion etching of the tungsten and seems therefore an interesting candidate between the materials studied as a barrier layer for tungsten.

I. Introduction CVD tungsten is now currently used for ULSI multilevel interconnects applications to achieve the first metal layer. It has proved to be the best alternative to sputtered aluminium for filling contacts and vias of the VLSI and ULSI circuits or even for "plug technologies". Attractive features of tungsten deposited by chemical vapor deposition (CVD W) include its low resistivity, high electromigration resistance, good step coverage and its ability to withstand high process temperatures [1]. There are two basic ways of depositing tungsten: one is a blanket deposition process and the other a selective deposition process [2]. We have been interested here in the first technology which consists of three deposition steps. First an adhesion layer is deposited, usually a titanium based material. This intermediate de-

posit has to be used because of the poor adhesion of CVD W on SiO 2. Next, the tungsten film is deposited by reduction of tungsten hexafluoride (WF6). This is followed by an etchback process of tungsten in a fluorinated chemistry. In this study, we have investigated the mechanism of the etching at the interface between tungsten and the sublayer considering that the surface of the sublayer after etching of the tungsten layer may be contaminated and that it would be a problem for the following steps of the process [3]. Different processings may be envisaged after tungsten has been removed. The sublayer may also be etched in a chlorinated plasma [4] and then its surface has to be freed of plasma residues pollution to avoid micromasking effects. Otherwise, contacts may be directly deposited on the sublayer which has then to be "clean" enough for a good electrical contact to be established.

0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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N. Couchman et al. / XPS analysis of tungsten/sublayer interface after SF6 etching

Anyway it is of prime importance to know precisely the surface chemistry.

2. Experimental aspects

2.1. Deposition All the metallization structures ( W / s u b l a y e r ) studied here were deposited on a 1000 A thick layer of SiO 2 grown on a 125 m m diameter p-type Si(100) wafer. Three different materials were c o m p a r e d for the adhesion layer: titanium (Ti), titanium nitride (TIN) and titanium-tungsten alloy (Ti-W). They were all deposited in an Electrotech Plasmafab MS6200 magnetron sputtering system. Titanium and the titanium-tungsten alloy were 800 ,& thick sputtered layers while titanium nitride was a 400 A thick reactively sputtered layer deposited on a 400 ~, thick titanium layer. The titanium tungsten target consists of 10wt% Ti (atomic concentration of 30%) and the actual film composition is around 5% Ti as analysed by means of the XPS profile. The tungsten films were chemical vapor deposited in a six wafer, cold-wall, Genus 8710 CVD reactor. We have compared two chemistries using tungsten hexafluoride (WF 6) as the source gas: (i) The "silane reduced" WF 6 chemistry: W F 6 + Sill 4 ~ W + 2 H F + Sill 4 + H 2 at low pressure (200 mTorr) and at a t e m p e r a t u r e of 410°C with W F 6 at 200 sccm and Sill 4 at 180 sccm. (ii) The "hydrogen reduced" process: W F 6 + 3H2 ~ W + 6 H F at high pressure (30 Torr) and at a t e m p e r a t u r e of 390°C with W F 6 at 400 sccm and H 2 at 7500 sccm. In both cases, the carrier gas is argon. The main attraction of "hydrogen reduced tungsten" is that it offers an excellent step coverage whereas "silane reduced chemistries" offer poor step coverage properties but higher deposition rates. Several other differences exist between the two processes including the silicon consumption.

2.2. Etching The etching was performed in a 40 cm diameter, 29 cm high, stainless steel reactor, including two 15 cm electrodes 47 m m apart from each other. The lower silicon electrode is coupled to a 13.56 M H z radiofrequency generator through a matching network. The samples placed on it are submitted to the plasma in a R I E m o d e and a water cooling system avoids their heat degradation. The standard etching conditions we use are 30 sccm gas flow rate, 100 m T o r r total pressure and 50 Watt applied power. An SF 6 plasma and an S F 6 / O 2 (80/20) plasma, which both give good etching rates and therefore are both commonly used in the industry, were compared.

2.3. Analyses The XPS analyses have been performed in a L e y b o l d - L H S 1 2 surface analysis system where a vacuum in the range of 10 -9 Torr (1.33 × 10 -7 Pa) is obtained. The spectrometer uses a M g K a (1253.6 eV) X-ray source combined with the analyser whose resolution is approximately 0.5 eV using a pass energy of 50 eV. This gives a total resolution of 1 eV. This source is set up in an analysis chamber which is connected to the etch reactor. The sample carriage is executed in less than 5 minutes at a pressure lower than 10 -6 Torr. This avoided the ex situ pollution problems.

3. Results and discussion The same species, described below, have been detected throughout the interface with titanium, titanium nitride and the titanium-tungsten alloy as an adhesion layer. Very few differencies have been observed between the bilayers etched in SF 6 and S F 6 / O 2 plasmas, except for the oxygen percentage on the analysed surfaces that slightly increases in S F 6 / O 2 plasma conditions. It can be noticed that the tungsten film is etched more rapidly in an SF 6 plasma than in an S F 6 / O 2 plasma.

N. Couchman et al. / XPS analysis of tungsten/sublayer interface after SF6 etching 3.1. The etched reactive layer

The W 4f distribution shows the W 4f (W metal) contibution at 31.3 eV at every step of the etching and a slight shoulder always subsides on the high energy side which corresponds to WOxFy species (figs. l a and 2a). The F ls distribution in tungsten decomposes in two components (figs. lb and 2b): the one at 684.7 eV corresponds to covalent F - W bonds and the other, at 687.7 eV, is attributed to WF 6 or WOxFy species trapped on the surface. As soon as the titanium of the sublayer is reached, the fluorine distribution is modified. The WOxFy species (687.7 eV) are still present but the covalent F - W bond contribution (684.7 eV) disappears and the T i - F bond of TiOxFy (y = 3 or 4) located at 685.3 eV is detected as the most important contribution of the fluorine distribution. The Ti2p distribution shows five different doublet contributions (figs. lc and 2c). The Ti 2p (Ti metal) contribution (Ti2P3/2 at 454.3 eV) is detected from under a layer of titanium which has reacted both with fluorine that gives the TiFx and the TiOxFy species (Ti2P3/2 at 460.4 and 462.1 eV), and with oxygen resulting in the formation of several oxides TiO~ (1 < x < 2) (Ti 2P3/2 at 456.6 and 458.8 eV). The O ls distribution displays the O - W bond (531 eV) on the spectra acquired in tungsten. The O ls contribution clearly increases in the titanium whose likeness for the oxygen is well-known and the T i - O bond (530.5 eV) is detected. 3.2. The etching mechanism 3.2.1. The titanium sublayer As noticed above, as soon as titanium of the sublayer appears under the etched tungsten, a fluorine-titanium bond is created: between 67% and 83% of fluorine is readily bonded to titanium at the interface (figs. lb and c). It seems then that titanium is preferentially fluorinated than tungsten. The bond strengths are therefore quite similar: 136 k c a l / m o l for the F - T i bond and 131 kcal for the F - W bond. Nevertheless the fact that WOxFy is still visible means that the etching of

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tungsten has not stopped but has only slowed down in presence of titanium. Only 20% of titanium detected on the XPS spectra at the interface corresponds to metal titanium whereas 70% is bonded to fluorine in a way or another. As the thickness of the area analysed by XPS is approximately 50 ,~ the fluorinated layer is quite thick and it will be difficult to eliminate. 3.2.2. The titanium nitride sublayer The titanium nitride studied here reacts exactly like pure titanium. After etching of the tungsten layer the titanium nitride surface is covered with a non-negligible polluted layer of titanium fluorides. 3.2.3. The titanium-tungsten sublayer The titanium fluorination phenomenon is observed both on this material and on titanium and titanium nitride but what is remarkable is that 75% of titanium detected at the interface corresponds to metal titanium whereas the proportion was only around 20% on the titanium sublayer. We observe in that case a more important percentage of metal titanium on the surface of the sublayer after etching than on the pure titanium sublayer: only a small proportion of titanium has been fluorinated (fig. 2c). TiW is in fact a "pseudo-alloy"; titanium is most likely in solid solution with some distributed at the grain boundaries [5]. It is then quite easy to imagine that 5% Ti distributed in the tungsten lattice is not that easily reached by fluorine of the plasma. The percentage of fluorinated titanium is therefore far less important than in the pure titanium sublayer. The expression of "fluorinated layer" cannot be used here because only 5% of the sublayer surface consists of titanium and only a small proportion of it is binded to fluorine. The surface after etching has to be envisaged as a tungsten layer containing titanium clusters and sprayed with titanium fluorides islets. The tungsten-titanium alloy surface is therefore far less modified and polluted by the etching of the tungsten film than the titanium and the titanium nitride surfaces. The interest of this sublayer is then that it can be used as is for the following step of the integrated circuit fabrication process

N. Couchman et al. / X P S analysis of tungsten/sublayer interface after SF6 etching

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N. Couchman et al. / XPS analysis of tungsten/sublayer interface after SF6 etching intenelty [cpe] lin I0000

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Fig. 2. (a) W 4f distribution at the tungsten/tungsten-titanium interface for 10, 18 and 33 s etching times. (b) F ls distribution at the tungsten/tungsten-titanium interface for 10, 18 and 33 s etching times. (c) Ti 2p distribution at the tungsten/tungsten-titanium interface for 15, 18 and 33 s etching times.

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N. Couchrnan et al. / XPS analysis of tungsten/sublayer interface after SF6 etching

whereas the other barrier layers have to be cleared of the fluorinated residues layer either by strong heating or by sputtering to avoid any contact problem or etching masking phenomenon.

4. Conclusion

From an application point of view, it is clear that tungsten titanium is the most appropriate material to be used as a sublayer for CVD tungsten among the ones studied. Both titanium and titanium nitride are covered, after etching of tungsten, by a quite thick layer of titanium fluorides including trapped tungsten fluorides. In an industrial process, this pollution layer has to be removed, either to prevent masking of the titanium etching in the chlorinated plasma or to

make possible the direct contact on the sublayer. The tungsten-titanium surface seems to be far less polluted since titanium is diluted in the tungsten matrix and then difficult for fluorine to reach. The type of plasma - SF6 or SF6/O 2 used for the tungsten etching does not seem to influence these results.

References [1] T.I. Kamins, D.R. Bradbury, T.R. Cass, S.S. Laderman and G.A. Reid, J. Electrochem. Soc. 133 (1986) 2555. [2] P.H. Singer, Semicond. Int. 8 (1990) 36. [3] R. D'Agostino, F. Fracassi, C. Pacifico and P. Capezzuto, J. Appl. Phys. 71 (1992) 1. [4] R.L. Torrisi, P. Vasquez, O. Viscuso and C. Magro, J. Electrochem. Soc. 138 (1991) 1171. [5] P. Dipankar, J. Vivek, Solid State Technol. 5 (1991) 97.