Thin Solid Films 437 (2003) 51–56
An oxidation barrier layer for metal–insulator–metal capacitors: ruthenium silicide Y. Matsuia,*, Y. Nakamurab, Y. Shimamotoa, M. Hiratania a
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan b Elpida Memory Inc., Sagamihara, Kanagawa 229-1198, Japan
Received 7 October 2002; received in revised form 27 February 2003; accepted 2 April 2003
Abstract Ruthenium silicide (Ru2 Si3 ) films were fabricated by an inter-layer reaction between ruthenium and silicon, and their oxidation resistance was investigated. These films remain conductive even after oxidation at 700 8C, which is more than 100 8C higher than that in the case of a TiN film. When a RuyRu2Si3 ySi structure is heat-treated at 700 8C, the ruthenium film reacts with the silicon that diffused through the Ru2 Si3 layer from the substrate, and forms Ru2 Si3 . On the other hand, if the Ru2Si3 layer is oxidized at 600 8C in advance and then the ruthenium film is deposited, further silicidation in the RuyRu2 Si3 ySi structure can be suppressed. This is because a 2 nm thick amorphous oxide layer formed at the RuyRu2 Si3 interface suppresses the diffusion of silicon. However, the oxide layer causes the contact resistance of the RuyRu2Si3 ySi structure to increase. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium; Silicides; Diffusion; Capacitors
1. Introduction Metal–insulator–metal (MIM) capacitors using high´ dielectrics must be used in order to shrink the area for storage capacitor in dynamic random access memories w1x. The most important component in MIM capacitors is the barrier metal that is inserted between the bottom electrode and the poly-silicon plug to prevent interdiffusion between them as well as oxidation of the poly-silicon plug. The barrier metal, therefore, plays the crucial role of keeping good electrical contact between the capacitor and a transistor. Titanium nitride (TiN) has been investigated as a barrier metal for MIM capacitors w2,3x, because it is known to be an excellent diffusion barrier against interdiffusion of aluminum and silicon w4x. However, TiN is intolerant of high-temperature oxidation above 600 8C. That is, the TiN barrier metal is oxidized during the post-heat treatment of the dielectrics, and a TiO2 layer with high resistivity is formed at the interface between the bottom electrode and the poly-silicon plug *Corresponding author. Tel.: q81-42-323-1111; fax: q81-42-3277773. E-mail address: [email protected]
w5x. Moreover, nitrogen gas generated according to the oxidation reaction, 2TiNq2O2™2TiO2qN2, destroys the capacitor structure w6x. We have directed our attention to silicides as a barrier metal, because they have high thermal stability and no gas is generated even if they are oxidized. In particular, ruthenium silicide, Ru2Si3, has higher oxidation resistance than other metal silicides, such as CoSi2, CrSi2, NiSi, NiSi2, MoSi2, WSi2, Ir3Si5, ReSi2 and Mn11Si19 w7x. Moreover, the oxidation rate of a Ru2Si3 film is as low as that of single-crystal silicon w7x. Therefore, Ru2Si3 is recognized as a promising material as a barrier metal with high oxidation resistance. In the present study, the Ru2Si3 films were prepared by an inter-layer reaction between ruthenium and silicon, and the oxidation resistance of the Ru2Si3 films and the thermal stability of the RuyRu2Si3 ySi structures were investigated. The results of this examination have verified the possibility of using Ru2Si3 films for the barrier metal in MIM capacitors. 2. Experimental details An 8-inch silicon wafer and a wafer with a 300 nm thick phosphorus-doped poly-silicon film on its surface
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00606-0
Y. Matsui et al. / Thin Solid Films 437 (2003) 51–56
Fig. 1. X-ray diffraction patterns of Ru(50 nm)ypoly-Si(300 nm)ySi structure: (a) the as-deposited film and (b) the structure after silicidation by inter-layer reaction at 700 8C for 60 s in N2. The indices marked with hkl and hkl* denote Ru and Ru2Si3, respectively.
were used as substrates. The wafers were etched in 1% HF solution for 2 min to eliminate the native oxide on their surface, rinsed with de-ionized water for 5 min, dried in flowing nitrogen gas, and immediately transferred to a deposition chamber under a pressure of approximately 2=10y6 Pa. A ruthenium film was deposited by DC magnetron sputtering from a 12 inch metal target in argon discharge gas. The deposition conditions for the ruthenium film were an argon gas flow rate of 100 sccm, a total pressure of 0.53 Pa, an incident power density of 1 kW and a deposition temperature of 300 8C. After it was deposited, the ruthenium film was reacted with the silicon underlayer at 700 8C for 60 s in nitrogen gas to form a silicide layer of Ru2Si3 by inter-layer diffusion. To evaluate its oxidation resistance, a Ru2Si3 film was oxidized in oxygen gas at temperatures from 400 to 700 8C for 5 min. For comparison, a TiN film was also oxidized under the same conditions. The TiN film was deposited on a silicon wafer by reactive DC magnetron sputtering from a titanium metal target in AryN2 discharge gas. The deposition conditions for the TiN films were an AryN2 gas flow ratio of 3y34 sccm, a total pressure of 0.09 Pa, an incident power of 12 kW, a deposition temperature of 500 8C, and a film thickness of 50 nm. A ruthenium film was then deposited on the Ru2Si3 film as a bottom electrode under the same conditions as described above. To evaluate the mutual reaction between Ru and Ru2Si3, the RuyRu2Si3 ySi structure was heat treated in nitrogen gas at 700 8C for 30 s. The crystallographic nature was analyzed by X-ray diffraction (XRD). A Cu Ka line was used as the Xray source. The electrical resistivity was measured by a DC four-probe method. The cross-sectional morphology was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The film
Fig. 2. Cross-sectional SEM views of Ru(50 nm)ypoly-Si(300 nm)ySi structure (the same sample as in Fig. 1): (a) the as-deposited film and (b) the structure after silicidation by inter-layer reaction at 700 8C for 60 s in N2.
composition was determined by energy-dispersive X-ray spectrometry (EDX) of field-emission TEM. The probe diameter was 3 nm. To measure the contact resistance of the RuyRu2Si3 ySi structure, the Ru and Ru2Si3 layers were patterned by sputter etching. The area of Ruy Ru2Si3 electrode is 9–100=10y6 cm2. 3. Results and discussion 3.1. Growth of ruthenium silicide Fig. 1 shows an X-ray diffraction pattern of a Ru(50 nm)ypoly-Si(300 nm)ySi structure and the pattern after the structure was reacted at 700 8C for 60 s in a nitrogen atmosphere. In the as-deposited structure (Fig. 1a), all diffraction lines can be assigned to ruthenium metal (indexed with hkl), except for the lines from silicon. After the reaction (Fig. 1b), all diffraction lines from the ruthenium metal disappeared completely, and alternative lines corresponding to Ru2Si3 (indexed with hkl*) appeared. The Ru2Si3 compound is typically crystallized ˚ and c of in a tetragonal symmetry with a of 11.075 A ˚ w8x. However, an orthorhombic polymorph with 8.954 A ˚ b of 8.937 A ˚ and c of 5.525 A ˚ has been a of 11.052 A, reported w9,10x. The two variants give quiet similar diffraction angles and intensities, since the orthorhombic unit cell is defined as half of the tetragonal cell. In Fig. 1b, the peaks were assigned as tetragonal Ru2Si3, because peak splitting inherent to the orthorhombic cell was not observed. The change in the X-ray diffraction patterns reveals that the 50 nm thick ruthenium films were completely transformed to Ru2Si3 after the reaction at 700 8C.
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perature above 500 8C, and then the TiN film lost its electrical conductivity at 600 8C. This result indicates that an oxide layer with high resistivity was formed according to a chemical reaction of TiNqO2™TiO2q 1y2N2. On the other hand, the resistivity of the Ru2Si3 film did not increase up to 700 8C, although the asdeposited Ru2Si3 film had higher resistivity than the asdeposited TiN film. It is, therefore, concluded that the Ru2Si3 film maintains its resistivity up to 700 8C (i.e. more than 100 8C higher than the TiN film), and that it remains conductive even after oxidation at 700 8C. 3.3. Thermal stability of RuyRu2Si3 ySi structure Fig. 3. Dependence of the thickness of Ru2Si3 films formed by the inter-layer reaction at 700 8C for 60 s in N2 on the original Ru film thickness (processed value). The theoretical value is calculated from the density of Ru and Ru2Si3.
Fig. 2 shows cross-sectional SEM views of the same samples as in Fig. 1. The 50 nm thick layer in the asdeposited structure is ruthenium metal (Fig. 2a), while the 120 nm thick layer after the reaction is Ru2Si3 (Fig. 2b). The poly-silicon layer was consumed in the silicidation and the thickness decreased from 300 to 200 nm. Consequently, the 120 nm thick Ru2Si3 film was obtained by an inter-layer reaction between the 50 nm thick ruthenium metal and the 100 nm thick polysilicon. As a result, the silicidation decreased the total thickness by approximately 10%. Fig. 3 shows the thickness of the Ru2Si3 film formed by the inter-layer reaction plotted against the original thickness of the ruthenium film. The thickness of the Ru2Si3 film increased in proportional to that of the ruthenium film. The ruthenium metal is crystallized in a hexagonal unit ˚ and c 4.2819 A, ˚ which includes cell with a 2.7058 A two chemical formulae for ruthenium with a density of 7.35=1022 ruthenium atomsycm3. On the other hand, Ru2Si3 is crystallized in a tetragonal unit cell with a ˚ and c 8.954 A, ˚ which includes sixteen chem11.075 A ical formulae for Ru2Si3 with a density of 2.92=1022 ruthenium atomsycm3. Accordingly, the silicidation due to the inter-layer reaction increases the original thickness of ruthenium metal by approximately 2.5 times. This theoretical calculation agrees well with the processed thickness, as shown in Fig. 3, and indicates that the Ru2Si3 thickness can be controlled by the original ruthenium thickness.
In the fabrication process of a capacitor, Ta2O5 dielectric is post-annealed at approximately 700 8C to crystallize the film. If the bottom electrode is deformed during the post-annealing, the capacitor properties may be degraded. Accordingly, the thermal stability of the RuyRu2Si3 ySi structure, in which a ruthenium film was deposited as a bottom electrode on the Ru2Si3 barrier layer, was investigated by heat treatment of the Ruy Ru2Si3 ySi structure at 700 8C. Figs. 5 and 6 show the changes in cross-sectional SEM views and X-ray diffraction patterns during the heat treatment, respectively. Fig. 5a and Fig. 6a represent the as-deposited ruthenium film (with a thickness of 50 nm) on the silicon substrate. The inter-layer reaction at 700 8C for 30 s provided the 120 nm thick Ru2Si3 film (Fig. 5b) that produced the X-ray diffraction peaks (Fig. 6b). A 50 nm thick ruthenium film was further deposited on the Ru2Si3 layer without intentional heating to form a RuyRu2Si3 bilayer, as seen in the SEM view (Fig. 5c) and the XRD pattern (Fig. 6c). After this RuyRu2Si3 bilayer was heat-treated at 700 8C for 30 s, the Ru2Si3 film grew to a thickness of 250 nm (Fig. 5d), and in the X-ray diffraction pattern, the peaks for ruthenium disappeared and only those for Ru2Si3 remained (Fig.
3.2. Oxidation resistance The oxidation resistance of Ru2Si3 was compared with that of TiN. Fig. 4 shows the dependence of resistivity on oxidation temperature. The resistivity of the TiN film increased with increasing oxidation tem-
Fig. 4. Resistivity change of TiN and Ru2Si3 films with oxidation.
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Fig. 5. Change in cross-sectional SEM views accompanied by silicidation, oxidation and post-annealing: (a) an original RuySi bi-layer structure; (b) a Ru2Si3ySi structure after silicidation at 700 8C for 60 s in N2; (c) a RuyRu2Si3ySi structure where a Ru film was further deposited on Ru2Si3; (d) a thick Ru2Si3 layer after post-annealing at 700 8C for 60 s in N2 ; (e) a RuyRu2 Si3 ySi structure where the Ru2 Si3 surface was oxidized at 600 8C for 10 s in O2 and then a Ru film was further deposited and (f) a stable bi-layer structure after post-annealing at 700 8C for 60 s in N 2.
6d). This is because, the ruthenium film that was deposited on the Ru2Si3 layer reacted with the silicon that diffused through the Ru2Si3 layer from the substrate. On the other hand, when the Ru2Si3 layer was oxidized at 600 8C for 10 s in advance and then the 50 nm thick ruthenium film was deposited (Fig. 5e and Fig. 6e), the RuyRu2Si3 bilayer remained intact with a distinct interface (Fig. 5f), and X-ray diffraction lines assignable to both ruthenium and Ru2Si3 were observed (Fig. 6f), even after the heat treatment at 700 8C for 30 s. Fig. 7 shows cross-sectional TEM views of the sample in which the ruthenium film was deposited after oxidation at 600 8C for 10 s (the same sample as shown in Fig. 5e). Fig. 7a shows the entire image of the Ruy Ru2Si3 ySi structure, and Fig. 7b and c show magnified lattice images of the RuyRu2Si3 and Ru2Si3 ySi interfaces, respectively. The Ru2Si3 film has columnar grains, which seemed to affect the grain growth of the ruthenium layer (Fig. 7a). A very thin amorphous layer of approximately 2 nm in thickness is observed at the Ruy Ru2Si3 interface (Fig. 7b), while there is no additional layer at the Ru2Si3 ySi interface (Fig. 7c). The film composition was analyzed at points marked with *1 to *9 by EDX (Fig. 7b and c). The ruthenium (at *1 and *2) and silicon (at *9) layers away from the interfaces consisted solely of ruthenium and silicon, respectively. Both ruthenium and silicon were detected throughout the Ru2Si3 layer (*5, *6, *7 and *8). Within the layer at the RuyRu2Si3 interface (*3 and *4), not only ruthenium and silicon, but also oxygen was detected. Accordingly, the 2 nm thick amorphous oxide layer
made it possible to suppress the diffusion of silicon and thus suppress further silicidation. Fig. 8 shows the dependence of the resistance of the RuyRu2Si3 ySi structures on the contact area. When the ruthenium film was deposited on Ru2Si3 without oxidation, the contact resistance of the RuyRu2Si3 ySi structure was on the order of 102 V (s). On the other hand, when the Ru2Si3 layer was oxidized at 600 8C for 100 s (h) or at 650 8C for 10 s (n) in advance and then the ruthenium film was deposited on the Ru2Si3 layer, the contact resistance was increased to approximately 104 V. It is clearly understood that the increase of the contact resistance results from an oxide layer formed at the RuyRu2Si3 interface by oxidation, as shown in Fig. 7b.
Fig. 6. Change in X-ray diffraction patterns of the same structures as in Fig. 5.
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Although the resistance increased a little with decreasing contact area, it is not possible to extrapolate the contact resistance in the case of an actual device structure with a fine size (to approximately 10y10 cm2). In any case, the contact resistance that is increased due to the ultrathin interfacial oxide does not meet the specification necessary for device operation. The Ru2Si3 film itself is highly oxidation resistant, however, its surface must be slightly oxidized to stabilize the Ruy Ru2Si3 structure and, thus, to enable a storage node to be fabricated. This oxidation results in increased contact resistance. Consequently, it is concluded that Ru2Si3 is difficult to use as a barrier metal of MIM capacitors as
Fig. 8. Dependence of the contact resistance on the area of RuyRu2Si3ySi structures. The Ru film was deposited on the Ru2Si3 layer without oxidation (s), and after oxidation at 600 8C for 100 s (h) and 650 8C for 10 s (n).
long as the high oxidation resistance is not compatible with the high thermal stability in the RuyRu2Si3 ySi structure. 4. Conclusions The growth process and oxidation resistance of the Ru2Si3 films, prepared by inter-layer reaction between ruthenium and silicon and the thermal stability of the RuyRu2Si3 ySi structure were examined. When a RuySi structure is heat treated at 700 8C, the ruthenium film is transformed into Ru2Si3 whose thickness is 2.5 times larger than the original Ru film. The oxidation resistance of the Ru2Si3 film is maintained up to 100 8C higher than that of the TiN film, and the resistivity of the Ru2Si3 film does not increase even after the oxidation at 700 8C. After the heat treatment of the RuyRu2Si3 ySi structure, the ruthenium layer is transformed into Ru2Si3 by reaction with the silicon diffused through the Ru2Si3 layer from the substrate. Once the ruthenium film is deposited on the Ru2Si3 layer via oxidation at 600 8C, the RuyRu2Si3 ySi structure can be maintained. This is because that the very thin oxide layer (approx. 2 nm) formed by the oxidation process suppresses the silicon diffusion. However, the thin oxide layer increases the contact resistance: therefore, it is difficult to use Ru2Si3 films as the barrier metal in MIM capacitors. References Fig. 7. Cross-sectional TEM views of a RuyRu2Si3ySi structure where the Ru2Si3 surface was oxidized at 600 8C for 10 s in O2 in advance and then a Ru film was further deposited: (a) an entire image of the cross-section; (b) a magnified lattice image of RuyRu2Si3 interface and (c) a magnified lattice image of Ru2Si3ySi interface. The points marked as *1 to *9 denote the positions at which composition was analyzed by EDX.
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