Structural Characterization of a Manganese Oxide Barrier Layer

Nov 27, 2012 - Patrick Casey , Anthony P. McCoy , Justin Bogan , Conor Byrne , Lee Walsh , Robert O'Connor , and Greg Hughes. The Journal of Physical ...
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Structural Characterization of a Manganese Oxide Barrier Layer Formed by Chemical Vapor Deposition for Advanced Interconnects Application on SiOC Dielectric Substrates Nguyen Mai Phuong, Yuji Sutou, and Junichi Koike* Department of Material Science, Tohoku University, Sendai 980-8579, Japan ABSTRACT: We investigated the microstructure and growth behavior of a manganese oxide layer deposited on SiOC substrates for the purpose of providing a new method to form a thin diffusion barrier layer for advanced LSI interconnections. The Mn oxide layer was formed by chemical vapor deposition (CVD), using bis(ethylcyclopentadienyl)Mn as a precursor and H2 as a carrier gas at 300 °C for 30 min. The Mn oxide layer could be formed on plasma-treated SiOC, but not on as-received SiOC. By using thermal desorption spectroscopy (TDS), moisture absorption in SiOC was evidenced after plasma treatment, using various gases of O2, N2, and Ar. Two adsorbed moisture components, physisorbed (α) and chemisorbed (β, γ), were observed which were responsible for the formation of crystalline MnOx and amorphous MnSixOy, respectively. The position of the Mn oxide layer was also investigated by measuring the variation of the SiOC thickness. The Mn oxide layer was formed within the SiOC substrate. The result indicates that the Mn oxide barrier layer would have no influence on the line resistance value, but influences on the dielectric capacitance value.

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INTRODUCTION Decrease in the size of complementary metal oxide semiconductors is accompanied by increase in on-chip interconnect RC delay that is directly dependent on wiring metal resistivity and insulator dielectric constant (k value).1 To reduce the resistance of wiring metal, Al has been replaced with Cu because of its low resistivity, high melting point, and good electromigration resistance.2 However, Cu has poor adhesion to interlayer dielectric (ILD) materials and easily diffuses to a dielectric layer. Therefore, a diffusion barrier layer is necessary to prevent interdiffusion. Conventional barrier layers are formed by physical vapor deposition (PVD) to have a bilayer structure of Ta/TaN, W/TiN, Ru/TaN, or Mo/WN.3−5 The metallic layer is necessary to ensure a good adhesion with the overlying Cu layer. Since its polycrystalline structure suffers from fast diffusion along grain boundaries, the amorphous nitride layer is also formed, thus resulting in the bilayer structure. However, the poor step coverage of the PVD films leads to the barrier-layer thickness on the side wall of trenches and vias to be much thinner than that on the bottom, which may cause the difficulty in preventing Cu from diffusing into the dielectric materials. An additional problem of the PVD diffusion barrier layer is an increase in effective line resistivity because of the partial occupation of interconnection line volume with the barrier layer. Thus, the thickness of the diffusion barrier layer should be reduced to a minimum possible value. For example, the thickness of the barrier layer is required to be only 2.4 nm for the 32 nm technology node according to the International Technology Roadmap for Semiconductors.6 On the other hand, chemical vapor deposition (CVD) or atomic layer deposition (ALD) is known to have good step © 2012 American Chemical Society

coverage and is a promising method to form a very thin barrier layer for the advanced technology node. Bystrova et al.7 reported the growth behavior of tungsten nitride films formed by ALD. A 7 nm-thick W1.5N layer showed an excellent diffusion barrier property at 400 °C for 30 min. Becker et al.8 reported the CVD formation of amorphous WN films which showed shiny surface appearance, good adhesion to a Cu film, and 100% step coverage for aspect ratios greater than 40:1. They also found that a 1.5 nmthick WN layer had a good barrier property up to 600 °C for 30 min. Kim et al.9 reported the CVD formation of TiN films, and found that the in situ plasma treatment of the TiN films with N2/ H2 gas could improve barrier properties at 650 °C because of a higher film density by the incorporation of oxygen in the film. Au et al. reported the CVD formation of a MnNx layer and its good diffusion barrier property.10 Meanwhile, manganese oxide has been known to be a good barrier layer. Koike et al.11,12 deposited a Cu−Mn alloy on SiO2 by sputtering. After annealing at various temperatures from 250 to 450 °C, a Mn oxide layer was formed to the thicknesses of 2 to 7 nm. After additional thermal annealing at 400 °C for 5 h, no interdiffusion was observed. Usui et al. also reported that the Mn oxide barrier was highly resistant against stress- and electromigration in a device level at 90 nm technology node.13 Recently, similar results have been reported by others to confirm the good reliability of the Mn oxide barrier layer.14 Received: April 5, 2012 Revised: October 17, 2012 Published: November 27, 2012 160

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The Mn oxide barrier layer can also be formed by CVD or ALD, so that their good step coverage could be utilized for conformal formation of the barrier layer in narrow trenches in the advanced technology node. Gordon et al.15 reported the CVD formation of a Mn oxide layer by using bis(N,N′diisopropylpentylamidinato)manganese(II) as a precursor gas. The presence of Mn oxide was found to improve adhesion at a Cu/dielectric interface. The good adhesion and step coverage of the Mn oxide layer were also reported earlier by Neishi et al.,16 Matsumoto et al.17 and Dixit et al.18 with use of bis(ethylcycropetadienyl)manganese. Excellent barrier properties of the Mn oxide layer were found after thermal annealing at 400 °C for 100 h and after bias temperature annealing under the applied electric field of 3 MV/cm at 277 °C for 100 min. In addition, the structure and thickness of the Mn oxide layer was found to be influenced by adsorbed moisture in SiO 2 substrates.19 A bilayer structure of a crystalline MnO upper layer and an amorphous MnSixOy lower layer was formed on asreceived SiO2 substrates, whereas a single amorphous MnSixOy layer was formed on preannealed SiO2 substrates. The observed difference in the Mn oxide structure was due to the difference in adsorbed moisture type (physisorbed or chemisorbed moisture) in the substrates. These results indicated that the adsorbed moisture had an important role in the formation of the Mn oxide barrier layer. So far, most reports on the CVD formation of the Mn oxide layer had used SiO2 substrates having a dielectric constant of about 4.0. However, lower dielectric constant (k) materials are used in the advanced technology node, and the CVD formation of the Mn oxide should be investigated by using low-k materials such as SiOC. SiOC is carbon-doped silica having a dielectric constant of 2.7 to 3.0. In SiOC, highly polarized bonds of Si−O are replaced with Si−CH3 groups with uniformly distributed micropores. The disruption of Si−O bond networks by methyl (CH3) groups does not only make the film lower in density and polarization but also makes the film hydrophobic. On the other hand, during the fabrication process of line trenches and vias, the SiOC films are subjected to plasma dry etching. It is well-known that the Si− CH3 bonds are broken and dangling bonds are formed by plasma treatment with various gases such as O2, CF4, N2, Ar, H2 or their mixed gases. Then the dangling bonds can react with environmental moisture to form Si−OH bonds or with hydrogen to form Si−H.20,21 Therefore, the plasma-treated SiOC becomes hydrophilic. As already mentioned in the case of the SiO2 substrates, the adsorbed moisture on the substrates had strong influences on the structure of the Mn oxide films. However, the effect of moisture has not been investigated at all for the SiOC substrates. So, the first purpose of the present work is to study the effects of plasma treatment on moisture adsorption on SiOC substrates and the influence of the adsorbed moisture on the formation of the Mn oxide layer. Meanwhile, the position of the Mn oxide layer is important in terms of RC delay. In the case of a conventional barrier layer of Ta/TaN or Ti/TiN, the barrier layer is formed on a dielectric layer and occupies the line volume of Cu. Thus, the conventional barrier does not influence the capacitance value of the dielectric layer, but influences the line resistance value. On the other hand, if the barrier layer is formed within a dielectric layer, it does not influence the line resistance value, but influences the dielectric capacitance value. Therefore, the second purpose of this work is to determine the position of the Mn oxide layer.

Article

EXPERIMENTAL METHODS

Substrates were 40-nm-thick SiOC on p-type Si wafers, provided by Tokyo Electron Ltd. Some substrates were used in an asreceived condition, and others were subjected to plasma treatment for 10 min with different gases of O2, N2, or Ar. Plasma treatment was performed at room temperature at a plasma power of 50 W and a frequency of 13.56 MHz, respectively. Since the adsorbed moisture on the substrates may influence the growth of the Mn oxide layer, thermal desorption spectroscopy (TDS) was carried out to determine the type and quantity of moisture desorbed from the substrates during heating. Ion current for the mass number of 18 for H2O was monitored during heating to 900 °C at a heating rate of 1 deg/ min. The same TDS analysis was performed for tetraethylorthosilicate (TEOS)-SiO2 substrates for comparison. Then, a Mn oxide layer was deposited on the SiOC substrates by CVD at 300 °C for 30 min with use of H2 as a carrier gas and bis(ethylcyclopentadienyl)manganese, (EtCp)2Mn, as a precursor. The vapor pressure of the precursor measured in the CVD chamber was 2.0 Torr. The detailed CVD procedure is described in ref 19. Next, we investigated the effects of adsorbed moisture on Mn oxide formation. Ar plasma-treated SiOC substrates were chosen for this experiment. The substrates were treated with Ar plasma at room temperature for 10 min and exposed to air for one day to let the substrate adsorb moisture. These substrates were preannealed at various temperatures from 150 to 400 °C in a vacuum of 1.0 × 10−5 Pa for 30 min with use of an infrared furnace. Then, a Mn oxide layer was deposited by CVD with the same deposition condition as mentioned above. The microstructure and thicknesses of the Mn oxide layer were investigated by transmission electron microscopy (TEM). In addition, deposition time was varied to investigate the thickness dependence of Mn oxide on deposition time. To determine the location of the Mn oxide layer, the possible thickness loss of the SiOC substrates was measured and compared with the growth thickness of the Mn oxide layer. If the SiOC thickness is decreased by the formation of the Mn oxide layer, the Mn oxide layer is considered to be formed within the SiOC substrate. If the SiOC thickness remains unchanged, the Mn oxide layer is considered to be formed on top of the SiOC substrate. Since the SiOC substrates may change the thickness just by keeping them at high temperature, the thickness of the SiOC substrates alone was measured before and after going through the same thermal history as for the CVD formation of the Mn oxide layer. Any change in thickness is due to the densification of the substrates. Additional change in thickness by CVD is due to the formation of the Mn oxide layer.

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RESULTS AND DISCUSSION The type of adsorbed moistures and their desorption temperature were investigated for SiOC. Figure 1 shows the TDS results of SiO2 and SiOC substrates before and after plasma treatment with different gases. The vertical axis represents the spectrometer ion current for molecular weight of M = 18 that is equivalent to H2O. As reported by Proost et al. for silica-based dielectrics,22 there are four types of water-related chemical groups which are attached to silica skeletons. They are composed of physisorbed moisture (type α) and chemisorbed moisture (type β, γ). The TDS curve of the SiO2 substrate shows the sequential desorption of all three types of moisture at about 130, 200, and 400 °C. In 161

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The thickness of the Mn oxide layer is 5.0, 4.5, and 2.0 nm for the plasma gas of Ar, O2, and N2, respectively. Furthermore in Figure 2b, a Mn oxide layer on the Ar plasma-treated substrate is composed of two layers, an upper layer on the side of the Cu cap layer having a thickness of 3.5 nm and a lower layer on the side of the SiO2 substrate having a thickness of 1.5 nm. The periodic lattice fringes can be seen in the upper layer, which indicates a crystal structure. On the other hand, no lattice fringe can be seen in the lower layer, which indicates an amorphous structure. Similarly, in Figure 2c, the Mn oxide layer on the O2 plasmatreated substrate is composed of two layers. The variation of diffraction contrast is observed in the upper layer on the side of the Cu cap layer, which indicates a crystal structure. Meanwhile, the uniform contrast is observed in the lower layer on the side of the SiOC layer, which indicates an amorphous structure. In Figure 2d, the Mn oxide layer on N2 plasma-treated substrate is composed of a single layer with a thickness of about 1.8 nm. The uniform contrast in the layer indicates the formation of an amorphous structure. These results indicate that the Mn oxide layer can be formed on the plasma-treated SiOC substrates, but not on the pristine SiOC substrate. The thickness and structure of the Mn oxide layer is found to be dependent on the plasma gas. The dependence of the Mn oxide formation on the plasmatreated gases is exemplified in the structure and thickness variation of the Mn oxide. As shown in panels b and c of Figure 2, the thick bilayer of a crystal layer on the top surface and an amorphous layer on the bottom adjacent to the substrate is formed on the Ar and O2 plasma-treated SiOC substrate, whereas a thin single amorphous layer is formed on the N2 plasma-treated SiOC. It is well-known for O2 plasma treatment23,24 that a fraction of SiOC volume is converted to silicon dioxide-like composition. The surface of O2 plasma-treated SiOC tends to contain more O and less C than that without plasma treatment. In the case of the N2 plasma treatment, the SiOC surface tends to contain more N and less O and C.23,24 Also, silicon nitride bonds can be formed at the surface of SiOC. In the case of the Ar plasma treatment, the SiOC surface is more damaged than with O2 plasma treatment, showing more O and less C due to the increase in the ion density by a factor of 3.8 times and in the electron temperature (the ionization potential of O2 is 12.1 eV, whereas that of Ar is 15.75 eV). Thus, the OH bonds are adsorbed more easily on the substrate after plasma treatment with O2 and Ar gases than with N2 gas. Accordingly, the thick bilayer structure including the crystalline MnOx is obtained by Ar and O2 plasma treatment, but not by N2 plasma treatment. In this section, Ar plasma treatment was performed for 10 min on all SiOC substrates. After the exposure of the substrates in air for 1 day, the substrates were preannealed under vacuum at various temperatures for 30 min to investigate the influence of preannealing on the formation of Mn oxide. Panels a and b of Figure 3 are cross-sectional TEM images of a CVD Mn oxide layer deposited on Ar-plasma-treated substrates followed by preannealing at 150 and 300 °C for 30 min, respectively. As shown in Figure 1, physisorbed moisture desorbs at around 180 °C. On the basis of this result, physisorbed moisture is present in the preannealed substrate at 150 °C but is absent at 300 °C. Thus, this observation is designed to reveal the effects of physisorbed moisture on the formation of Mn oxide. The Mn oxide layer on the preannealed sample at 150 °C appears to be composed of two layers having a total thickness of 5.0 nm. The spatial variation of diffraction contrast is observed in the upper layer, which indicates the formation of a crystal structure. Meanwhile, a uniform contrast is observed in the lower layer,

Figure 1. TDS spectra of SiO2 substrates, pristine SiOC (p-SiOC), and plasma-treated SiOC in O2, N2, and Ar gases.

contrast, the pristine SiOC substrate without plasma treatment shows no desorption peak of the physisorbed α moisture, but shows the weak desorption peaks of the chemisorbed β and γ moistures. These results confirm a hydrophobic nature of the pristine SiOC substrate. On the other hand, desorption of all three types of moistures is observed in the plasma-treated SiOC substrates. These results indicate that the SiOC substrates are changed from hydrophobic to hydrophilic after plasma treatment. Note that the desorption temperature of the physisorbed α moisture is dependent on the plasma gas species. It is about 130, 150, and 180 °C for O2, N2, and Ar plasma treatment, respectively. On the other hand, the desorption temperatures of the chemisorbed β and γ moistures are about 300 and 500 °C, respectively, regardless of the plasma gas species. These desorption temperatures in the plasma-treated SiOC substrates are higher than those in the SiO2 substrate, which indicates the higher bonding energy of the chemisorbed moistures in SiOC. Next, plasma surface treatment was performed on SiOC surface and its influence on the Mn oxide formation behavior was investigated. Panels a−d of Figure 2 are cross-sectional TEM

Figure 2. TEM images of a Mn oxide layer deposited on (a) as-received, (b) Ar plasma-treated, (c) O2 plasma-treated, and (d) N2 plasma-treated substrate.

images of a CVD Mn oxide layer deposited on a pristine or a plasma-treated SiOC with Ar, O2, or N2 gas, respectively. A Cu cap layer is formed to protect the Mn oxide layer from possible change during exposure in air. As shown in Figure 2a, a Mn oxide layer is not noticeable on the pristine SiOC. In contrast, the Mn oxide layer is observed in the plasma-treated SiOC substrates. 162

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Figure 4. Thickness variation of the Mn oxide layer and the SiOC substrate with preannealing temperatures.

α moisture is completely desorbed and the chemisorbed β moisture begins to be gradually desorbed. Therefore, the remaining moisture type and its amount are not much different, which led to the similar thickness of the Mn oxide layer at the preannealing temperature of 300 to 400 °C. In Figure 4, two types of SiOC thickness are also plotted, and they are denoted as “SiOC w/MnOx” and “SiOC w/o MnOx”. The former is the thickness of SiOC after pre-annealing at the indicated temperatures for 30 min followed by the deposition of a Mn oxide layer at 300 °C for 30 min. The latter is the thickness of SiOC alone after being subjected to the same thermal history as the SiOC w/MnOx. To find whether the Mn oxide layer is formed on top of SiOC or within SiOC, the thickness gain of Mn oxide is compared with the thickness loss of SiOC. It is noted that the thickness loss of SiOC can occur not only by the Mn oxide deposition but also by densification during annealing. The densification loss can be quantitatively estimated by annealing SiOC substrates alone. The two red datum points of SiOC w/o MnOx are the SiOC thickness due to densification by annealing. Comparison of these values with the blue datum points at the same pre-annealing temperature gives the thickness loss of SiOC by Mn oxide deposition. At the pre-annealing temperature of room temperature, meaning without pre-annealing, the Mn oxide thickness of 5.0 nm is gained at the expense of the SiOC thickness loss of 4.0 nm. At the pre-annealing temperature of 300 °C, the Mn oxide thickness of 2.0 nm is gained at the expense of the SiOC loss of 3.3 nm. The gain and the loss are well balanced within experimental error. The results indicate that the Mn oxide layer is formed so as to embed itself within the SiOC layer. Similar results were obtained in a self-forming barrier layer of Mn oxide formed in sputter-deposited Cu−Mn alloy on TEOSSiO2.14 These results are in contrast to a conventional barrier of Ta/TaN that is formed on an insulating layer and occupies Cu interconnect volume. In the case of the embedded Mn oxide, no negative influence is expected on the effective resistivity of the Cu interconnect. Rather, possible influence on effective capacitance should be considered. In this regard, our preliminary results suggest negligible influences on the effective capacitance. Further work is required to better understand the effects of the embedded Mn oxide barrier layer on the electrical properties of multilayer interconnect structure. Finally, the growth behavior of the Mn oxide layer was investigated as a function of deposition time up to 30 min. Panels a and b of Figure 5 are the TEM images of the Mn oxide layer deposited for 2 and 30 min at 300 °C. The thickness of Mn oxide deposited for 2 min is about 3.5 nm, confirming that the Mn oxide layer can be formed just after the reaction time of 2 min. A

Figure 3. TEM images of a Mn oxide layer deposited on preannealed substrates at (a) 150 and (b) 300 °C for 30 min.

which indicates the formation of an amorphous structure. On the other hand, as shown in the Figure 3b, the Mn oxide layer on the preannealed substrate at 300 °C appears to be composed of a single layer of 2.0 nm thickness. A uniform contrast of the layer indicates the formation of an amorphous layer. It is noted that a single amorphous layer of Mn oxide is also observed on other preannealed substrates at 250, 350, and 400 °C for 30 min. The TEM results in Figure 3 show that the preannealed sample below 180 °C is composed of the two layers of the crystal layer on the top surface and the amorphous layer on the bottom. Meanwhile, the preannealed sample above 180 °C is composed of a single amorphous layer. It was reported19 that the reaction of (EtCp)2Mn precursor with physisorbed moisture is responsible for the formation of a crystalline MnOx, whereas reaction with chemisorbed moisture is responsible for the formation of an amorphous MnSixOy. In addition, based on the TDS results in Figure 1, the physisorbed (α) and chemisorbed (β, γ) moistures are desorbed at 180, 300, and 500 °C, respectively. Therefore, below 180 °C of preannealing temperature, the remaining moisture in the SiOC substrate is composed of all three types of moisture (α, β, γ), which leads to the formation of the two layers of crystalline-MnOx/amorphous-MnSixOy. On the other hand, above 180 °C of preannealing temperature, the remaining moisture is only chemisorbed (β, γ), which leads to the formation of the single amorphous layer of MnSixOy. Figure 4 shows the thickness variation of the Mn oxide layer on the Ar plasma-treated SiOC substrates as a function of preannealing temperature. All values are obtained from crosssectional TEM images. It is found that the Mn oxide thickness is 5.0 nm at the preannealing temperature below 150 °C, and rapidly decreases with increasing preannealing temperature above 250 °C. This variation can be understood by considering the influence of physisorbed moisture. As already shown in Figure 1 and discussed in Figure 3, physisorbed moisture is present on the SiOC substrates below 180 °C of preannealing temperature, but is absent above 180 °C. The presence of the physisorbed moisture induces the formation of a MnOx layer in addition to an amorphous MnSixOy layer and makes the total Mn oxide layer rather thick at room temperature and 150 °C, but not at 250 °C and above. At 300 and 400 °C of preannealing temperature, TDS results in Figure 1 show that the physisorbed 163

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AUTHOR INFORMATION

Corresponding Author

*TEL/FAX: +81-22-795-7360. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank S. Hosaka, K. Maekawa, and K. Matsumoto at Tokyo Electron Ltd. for their fruitful discussion. We appreciate T. Miyazaki at Tohoku University for technical assistance with TEM analysis. This work was supported by JSPS Grant-in-Aid for Scientific Research (S) No. 22226012, Japan Science and Technology Agency (CREST), and Tokyo Electron Ltd.

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Figure 5. TEM images of a Mn oxide layer deposited for (a) 2 and (b) 30 min. (c) Thickness variation of the Mn oxide layer with deposition times.

similar result was observed in the case of Mn oxide formed on SiO2 substrate for 1 min.16 The thicker Mn oxide layer is obtained for the longer deposition time. The deposition-time evolution of the Mn oxide thickness is shown in Figure 5c. With increasing deposition time, the thickness of MnOx is increased up to 10 min. Thereafter, the Mn oxide thicknesses do not change much with time, showing the self-limit growth behavior of Mn oxide formation.

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REFERENCES

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CONCLUSION

A Mn oxide layer formed by chemical vapor deposition on low-k dielectric SiOC substrates was investigated in various plasma treatment conditions and at various preannealing temperatures of the substrates. The Mn oxide layer could be deposited on plasma-treated SiOC substrates, but not on an as-received substrate. After plasma treatment with various gases of O2, N2, and Ar, moisture was adsorbed on the substrates that acted as a catalyst for reaction between a precursor gas and the substrate to form a Mn oxide layer. Below 180 °C of preannealing temperature, a bilayer of crystalline MnOx and amorphous MnSixOy is formed; whereas above 180 °C, a single layer of amorphous MnSixOy is formed. In addition, the barrier layer is formed within the SiOC substrate, suggesting that the Mn oxide barrier layer has influence on the effective capacitance of the dielectric layer, but not on the effective resistance of the Cu lines and vias. 164

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