Investigation on inhibition of ruthenium corrosion by glycine in alkaline sodium hypochlorite based solution

Journal Pre-proofs Full Length Article Investigation on inhibition of ruthenium corrosion by glycine in alkaline sodium hypochlorite based solution Shuai Shao, Bingbing Wu, Peng Wang, Peng He, Xin-Ping Qu PII: DOI: Reference:

S0169-4332(19)33793-6 https://doi.org/10.1016/j.apsusc.2019.144976 APSUSC 144976

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Applied Surface Science

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22 August 2019 3 December 2019 5 December 2019

Please cite this article as: S. Shao, B. Wu, P. Wang, P. He, X-P. Qu, Investigation on inhibition of ruthenium corrosion by glycine in alkaline sodium hypochlorite based solution, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144976

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Investigation on inhibition of ruthenium corrosion by glycine in alkaline sodium hypochlorite based solution Shuai Shao, Bingbing Wu, Peng Wang, Peng He, and Xin-Ping Qu* State key lab of ASIC and systems, School of Microelectronics, Fudan University, Shanghai, 200433, China, [email protected] Abstract: Removal of Ru by etching in alkaline sodium hypochlorite (NaClO) based solution is carried out. The glycine on the corrosion of Ru in alkaline sodium hypochlorite (NaClO) based solution is systematically investigated. The results show that in the alkaline NaClO solution, glycine is a good corrosion inhibitor for Ru. Through electrochemical measurements, electrochemical quartz crystal microbalance (EQCM) and XPS measurements, it is found that the glycine adsorption isotherm on the Ru surface follows TEMKIN’s model. Glycine can not only adsorb on the Ru surface to inhibit the corrosion of Ru, but also promote the dissolution of the formed oxide layer through complexing action. Finally, we used glycine and NaClO for chemical mechanical polishing (CMP) of Cu/Ru interconnect structure and the removal rate selectivity of Cu and Ru can be adjusted to 1:1.

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1. Introduction Conventional Cu interconnect structure with Ta/TaN bi-layer as adhesion barrier has met challenges when technology node goes beyond 14 nm nodes, such as poor filling property of electroplated Cu on the Cu seed with Ta liner in the trench [1]. In order to solve the above challenges, Co [2], Mo [3], Ru [4], and other potential materials as alternative liners have been widely studied. And Co [5] and Ru [6] are also being studied as local interconnect conductor material. Ru is identified as liner for Cu with the greatest potential in the next generation interconnect if using Cu extension, because of good Cu reflow properties on Ru [7, 8]. However, removal of Ru is difficult. Chemical mechanical polishing (CMP) of Ru in acidic slurry will cause formation of toxic RuO4 [9], while CMP of Ru in conventional H2O2 based alkaline slurry results in very slow Ru removal rate (RR) and high Cu and Ta removal rate, leading to Cu corrosion or TaN barrier loss [10]. CMP of Ru in nonconventional slurry such us using NaIO4 [11], KIO4 [12, 13] and KMnO4 [14] based oxidants might cause corrosion of the polishing pad and the slurry material cost is big. Normally the liner thickness is around 1-3 nm, thus etching of Ru instead of CMP is also a possible solution. Recently, fully-self-aligned via (FAV) technology is proposed to control the alignment of the structure [15]. In the FAV technology, via recess will be formed, which needs either half-filling of the Cu, Ru or Co via or etching the formed via. Therefore, systematical investigation on corrosion of Ru and its application on liner removal or via etching is very important.

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NaClO is well known to be widely used in water disinfection and biological medicine domains. The cost of NaClO is relatively low. The etching and CMP properties of Ru by using NaClO have been studied [11, 16-18]. Cui et al. have reported that with 6 psi down force and oxidant concentration of 0.072 mol/L at pH 7, the polishing rate of Ru by using NaIO4 is much higher (~130 nm/min) than using NaClO (~75 nm/min). However, there were more pitting corrosion for the Ru in the NaIO4 solution than in the NaClO solution [11]. Recently, Yadav et al. have reported that Ru can be effectively polished by using NaClO as oxidant [18]. CMP properties for Ru with NaClO as oxidant and with silica or titania as abrasives at pH value from 3 to 11 have been studied. It is of great significance to improve the CMP process of Ru and apply Ru to interconnect materials. Using strong oxidants can improve the removal rate of Ru ,yet in the meantime, it would also cause spot corrosion of Ru and possible higher removal rate of Cu than that of Ru. Therefore, it is very important to find a suitable corrosion inhibitor to inhibit the Ru corrosion and balance the polishing rate of Ru/Cu. BTA was used as corrosion inhibitor for Ru [16-18]. We have demonstrated that glycine can be the corrosion inhibitor for Ru in the KIO4 solution, yet the inhibition mechanism was not systematically studied [12]. In this work, we systematically investigated etching properties of Ru in the NaClO solution with glycine as corrosion inhibitor. Glycine can reduce pitting on Ru surface. At the same time, it can inhibit ruthenium corrosion and promote copper corrosion, which is very suitable for balancing the polishing rate of Ru/Cu. We further studied the corrosion inhibition mechanism of glycine. 3

2. Experimental The Ru (200 nm)/ TaN (20 nm) film, Cu (800 nm)/ TaN (20 nm) film, and Ta (100 nm)/ TaN (20 nm) were deposited on 12-inch silica (SiO2) or silicon (Si) substrate, respectively, by using magnetron sputtering (PVD) without breaking the vacuum. Then these samples were cut into 2 cm  2 cm coupons for further tests. The solutions were prepared with deionized (DI) water and analytical reagent grade chemicals. The pH value was adjusted to 10 by using diluted HNO3 and NaOH. The NaClO (SCR, China, 7.5% active chlorine) and glycine (Enox, China) were used as oxidant and additive, respectively. If not mentioned, the static etching and electrochemical tests were carried out without abrasives. The coupons in 2 cm  2 cm cut from 12’’ Ru and Cu wafers were first rinsed with DI water and then dried in nitrogen (N2) flow. Both the Ru and Cu coupons were immersed in the different NaClO solutions in a 500 mL glass beaker for 5 min. After etching, the samples were cleaned with DI water and then dried by N2 flow. The sheet resistance of Ru film before and after etching was measured by a 4-point probe to calculate the static etching rate (SER). Each result was the average of three individual tests. Some Ru and Cu coupons were also polished using a CP-4 bench-top polisher with an IC 1000-XY/ SUBA pad from Dow. Colloidal silica (from Dow chemical) particles with size of 60 nm were used as abrasives. The down force was 2 psi and the pad/ head rotating rates were100/ 100 rpm, respectively. The slurry flow rate was controlled to 100 mL/min. Each polishing test lasted for 5 min. The removal rate (RR) values were calculated by measuring 4

the change of sheet resistance by using four-point probe (FPP) using the equation of 𝑅𝑅/𝑆𝐸𝑅 =

ℎ−ℎ0 𝑡

=

(𝑅𝑠0 −𝑅𝑠 ) 𝑅𝑠 ×𝑡

× ℎ0 . Rs0 represents the sheet resistance of the as-deposited Ru

film measured by FPP, and h0 represents the the thickness of the as-deposited sample which was measured by SEM. The Rs and h represent the sheet resistance of the sample after etching (or polishing) for certain time t. Each result was the average of three individual tests. The potentiodynamic polarization, open circuit potential (OCP) and in-situ open circuit potential (in-situ OCP) measurements, and electrochemical impedance spectroscopy (EIS) method were carried out on a three-electrode cell PARSTAT 2273 electrochemical workstation (the working electrode and the counter electrode are platinum (Pt) and the reference electrode is standard calomel electrode (SCE)). For potentiodynamic polarization, OCP, and EIS experiments, the working electrodes were Ru and Cu coupons as mentioned above. For the in-situ OCP experiments the working electrodes were Ru and Cu cylinders (99.99% purity), sealed with epoxy resin, with 0.195 cm2 exposed. Prior to the tests, both the Ru and Cu cylindrical electrodes were polished with a set of waterproof emery papers until the electrode surface was mirror-like. Then both the film and cylindrical electrode were rinsed in DI water, and dried with N2. There were three steps in the in-situ OCP experiment: (1) polishing for 300 s, (2) static etching for 500 s, and (3) polishing for 300 s. In the polishing process, the down force was 2 psi and the electrode rotation speed was 1000 rpm. The EIS experiments were conducted in the NaClO based electrolytes. The applied voltage in each case was set equal to the value of Eoc (OCP) for the corresponding system. Before each scan, Eoc was measured until the working electrode reached the equilibrium state. The 5

amplitude of the applied alternating voltage was 5 mV and the scan frequency range was from 10 mHz to 10 kHz. Scanning electron microscopy (SEM) (Sigma HD, ZEISS), atomic Force Microscopy (AFM) (Bruker, Dimension FastScan) and X-ray photoelectron spectroscopy (XPS) (Kratos, United Kingdom) measurements were applied to analyze the surface morphology and chemical composition of the films after treatment in the different solutions. Electrochemical quartz crystal microbalance (EQCM) (Inificon, MAXTEK) measurements were applied to measure the mass change of the Ru films during the corrosion process. The crystal is AT-Cut 5 MHz covered with Au/Ti films. The 200 nm Ru film was deposited on the crystal surface by sputtering. Before each test, the crystal was washed with DI water and dried with N2 flow. Then the crystal was put into the corrosion solution and the frequency change of crystal oscillator was monitored.

3. Results and analysis 3.1 Effect of NaClO concentration on surface corrosion of Ru The effect of NaClO concentration on static etching of the Ru film is first investigated. An alkaline enviroment is chosen (at pH = 10) becasue Ru can react with oxidants to form toxic gaseous RuO4 in the acidic solution. Fig. 1 shows the SER of the Ru film in the NaClO solution at pH 10 as a function of NaClO concentration. With 0.1 wt.% NaClO, the SER of Ru is only 0.1 nm/ min, indicating a weak chemical corrosion. With the increase of NaClO 6

concentration, the SER of Ru increases markedly to 2.2 and 7.0 nm/ min at 0.5 and 0.9 wt.% NaClO, respectively. NaClO will oxidize Ru to form oxide layer and then the formed oxide will dissovle in the alkaline solution. The higher concentration of oxidizer leads to increase of SER in our work. 3.2 Effect of glycine on corrosion of Ru Corrosion of Ru often results in pitting problem. So, either during CMP of Ru or static etching of Ru, a corrosion inhibitor is needed. Glycine is added as corrosion inhibitor in this work. Fig. 2 shows the SER of the Ru film in the 0.5 wt.% NaClO solution at pH 10 as a function of glycine concentration. It can be seen that without glycine, the SER of Ru is 2.2 nm/min, while with increase of glycine concentration from 0.5 to 2.0 mM, the SERs of Ru decrease from 1.9 to 0.7 nm/min. This result shows that glycine can inhibit the Ru etching. The SER results measured above were obtained from the sheet resistance variation. Since the surface oxide RuO2 is also conductive, it might cause some uncertainties of resistance calculation. We further checked the result by using thickness change obtained from SEM. Fig. 3 shows the cross-sectional SEM images of the as-deposited Ru film and the Ru films after treatment in the 0.5 wt.% NaClO solutions with different glycine concentration. The total thickness of the as-deposited Ru-TaN layer stack (shown in Fig. 3(a)) and the one treated in the solution with 0.5 wt.% NaClO for 10 min (Fig. 3(b)) is 216.7 nm and 193.1 nm, respectively. Therefore, the etch rate of the Ru film in the 0.5 wt.% NaClO condition is 2.4 nm/min, similar as the one obtained from FPP. After adding 4 mM GLY in the NaClO

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solution for 10 min, the total thickness of Ru-TaN layer is 209.5 nm in Fig. 3(c), so the calculated Ru etch rate is about 0.7 nm/min. Fig. 4 shows the surface morphologies of the Ru films after different treatments and the inset images are the corresponding optical images. Seen from the Fig. 4(a), the as-deposited sample has metallic shining color with nano-grain of around 20 nm. For the Ru film soaked in a solution of 0.5 wt. % NaClO at pH 10 for 10 min, Fig. 4(b) shows that the grain structure is not clear and the Ru film underwent severe pitting in the NaClO solutions. It is reported that this pitting was accompanied by the formation of surface RuO3/RuO2•H2O layer [11]. The pitting corrosion can also be observed from the cross-sectional SEM image shown in the Fig. 3(b). Fig. 4(c) - (f) are surface morphologies of the Ru films etched with 0.5 wt.% NaClO and glycine with concentration of 1 - 4 mM at pH 10 for 10 min, respectively. With the increase of glycine concentration, the color of the Ru surface in the treated area gradually becomes lighter and lighter as shown in the insets, indicting decreased oxidation content of the Ru film. Also the pitting corrosion decreases and the shapes of grains are more and more close to those of the as-deposited samples. 3.3 Electrochemical study - Inhibition mechanism of Ru corrosion by glycine Fig. 5 shows the potentiodynamic polarization curves of the Ru coupons in the solutions containing 0.5 wt.% NaClO with and without 10 mM glycine at pH 10. The corrosion potential (Ecorr) and corrosion current density (icorr) were shown in the inset table. As Ru is always used as liner between Cu and TaN, we also show the curves for the Cu and Ta coupons in the same solutions. In the 0.5 wt.% NaClO solution at pH 10, the corrosion 8

current density of Ru is 121.30 μA/cm2, while those for Cu and Ta are 5.53 μA/cm2 and 0.30 μA/cm2, respectively. When 10 mM glycine is added in the solution, the corrosion current densities of Cu and Ta increase to 20.83 and 2.60 μA/cm2, respectively, while that of Ru decreases significantly to 0.41 μA/cm2. The cathodic reaction can be written as: ClO- + 2e- + H2 O → Cl- + 2OH(1) O2 + 2H2 O + 4e- → 4OH(2) The anodic reaction can be written as [19]: Ru + 4OH- → RuO2 •2H2 O + 4e(3) Ru + 6OH- → RuO3 + 3H2 O + 6e(4) In the alkaline solution, the RuO2•2H2O and RuO3 can form soluble RuO42-. RuO2 •2H2 O + ClO- + 2OH- → RuO24 + H2 O + Cl

(5) RuO3 + 2OH- → RuO24 + H2 O (6) 9

To further investigate the inhibition mechanism of glycine on Ru, the potentiodynamic polarization curves of the Ru films in the 0.5% NaClO solution with addition of 0 ~ 4 mM glycine were measured and are shown in Fig. 6. With increase of glycine concentration, the corrosion current decreases. This proves that glycine can inhibit the chemical interaction between NaClO solution and Ru. Both the anode and cathode branches of the curve move to the left, indicating glycine can inhibit the reaction between the anode and the cathode at the same time. Using corrosion currents, we can calculate the inhibition efficiency (IE%) and surface coverage (𝜃) according to the following formula [20]: IE% =

icorr0 -icorr icorr0

× 100 (7)

θ=

icorr0 -icorr icorr0

(8) In which icorr0 is the corrosion current density without adding the corrosion inhibitor. To analyze the adsorption isotherm, we fit the surface coverage (𝜃) as a function of inhibitor concentration (𝐶) . The best-fitted straight line of 𝜃 ~ 𝑙𝑜𝑔 (𝐶) is obtained shown in Fig. 7 indicating that the adsorption of the additives onto Ru can be approximated using TEMKIN adsorption isotherm [21-23], given as: exp(-2aθ) = kads C (9) 10

In which, kads is the equilibrium constant of the adsorption process, and ‘a’ is molecular interaction parameter. In this work, we get: 𝑎 = −1.0022 and 𝑘𝑎𝑑𝑠 = 1485.44. In our solution system, glycine exists as a polar molecule and adsorbs on the surface of the sample. This indicates that there is some interaction inside the adsorption layer, so Temkin isotherm can better describe this real physical phenomenon. A negative ‘a’ in the equation (9) indicates repulsion in the adsorption layer, possibly due to polar glycine molecules, while kads represents the adsorption strength between the adsorbent and the sample and the higher kads means the better the inhibition effect. In our case, the higher kads indicates glycine can effectively adsorb on the surface of the sample to form a protective layer, inhibiting both cathode and anode reaction. Fig. 8(a) shows the in-situ OCP curves of Ru in the solutions containing 0 mM and 2 mM glycine, 7.5 vol.% colloidal silica, and 0.5 wt.% NaClO at pH 10. In the absence of glycine, the OCP gradually increases after the transition from the polishing to the static state. This indicates that after the removal of mechanical force, the exposed metal further reacts with NaClO to form passivation layer on the Ru surface. The gap of the potential value between the highest of in the polishing (I) and static (II) stage is 29.4 mV. After adding 2 mM glycine, the potential difference reduces to 1.9 mV, indicating that the addition of glycine makes the oxide layer thinner. In the static zone, the OCP slightly decreases, indicating dissolution of the passivation layer in the presence of glycine. Fig. 8(b) shows the OCP curves of Ru in the solutions containing 0.5 wt.% NaClO with different concentrations of glycine at pH 10. With the increase of glycine concentration, the OCP continues to decrease 11

from 620 mV to 590 mV. OCP value can characterize the thickness of oxide layer on the surface of metal films, so the addition of glycine can effectively reduce the oxide layer thickness. The EIS measurements were performed to further study the interface information and electrochemical process of the Ru film in solutions with different NaClO and glycine concentrations. Fig. 9 (a) - (c) show the Nyquist and Bode plots of the Ru film treated with different NaClO concentrations, and Fig. 9 (d) - (f) show the Nyquist and Bode plots of the Ru film treated with different glycine concentrations, and the equivalent circuit in Fig. 9 (g) was employed to fit the EIS data. As seen in Fig. 9, the Nyquist plots only exhibit one capacitance loop and the Bode plots also show one peak from high frequency to low frequency. Meanwhile, there is a near 45° linear-like tail at low frequency in the Nyquist plots. Therefore, the equivalent circuit with one time-constant and a Warburg impedance was used to fit EIS data [24]. In view of the equivalent circuit, Rs represents the solution resistance, and Rct stands for the charge transfer resistance, respectively. The constant phase element (CPE) ,CPEdl, corresponds to the double layer capacitance. 𝐶𝑃𝐸 = [𝑄(𝑗𝜔)𝑛 ]−1 (10) In CPE, n is a mathematical coefficient (−1 ≤ n ≤ 1). When n = −1 CPE means inductance, n = 0 means resistance, n = 0.5 means Warburg impedance, and n = 1 means capacitance. The n value can also give indication of the roughness of the film surface: smaller n value indicates a rough surface of the electrode [13, 25].

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Table 1 shows the fitted results for Ru film in the NaClO solutions with concentration. When the NaClO concentration increases from 0.3 wt. % to 0.7 wt. %, the Rct distinctively decreases from 66.20 to 34.18 Ω·cm2, indicating a higher corrosion rate of Ru in the 0.7 wt. % NaClO solution in comparison with 0.3 wt. % NaClO soluition. This is consistent with the SER test. Meanwhile, the Warburg impedance exhibits an increasing trend, suggesting that with the increase of NaClO concentration, the oxidation layer gradually thickens and the influence of diffusion on the chemical reaction is enhanced [26, 27]. The fitted results of Ru with different glycine concentrations are listed in Table 2. When the concentration of glycine increases from 1 mM to 4 mM, the Rct distinctively increases from 81.66 to 413.40 Ω·cm2, the Warburg impedence decreases and ndl drecreases from 1 to 0.9279. This reveals that with the increase of glycine concentration, the corrosion rate of Ru obviously decreases, the whole chemical reaction becomes a less diffusion-controlled mass transfer process due to reduction of the surface Ru oxide layer. The decrease of ndl value may be due to the adsorption of glycine, which makes electrode surface become more rough. It is also worth noting that with the increase of glycine concentration, the slope of the linear-like tail is more or less deviated from 45° and the fitting error of Warburg impedance gradually increases. This could be ascribed to the following two reasons: the electrode surface is rough; or there exists another state-variable [13, 25]. We then try to characterize how glycine can inhibit the formation of oxide layer. Ru was sputter-deposited onto the EQCM. We first dip the Ru covered EQCMs into the aqueous solution with 1 mM glycine at pH 10 for 10 min to remove the natural oxide layer on the Ru 13

surface and then let the glycine have enough time to adsorb on the Ru surface. Then one EQCM was cleaned by DI water for 1 min while the other one wasn’t cleaned. Then both samples were dip in the 0.5 wt.% NaClO at pH 10 for 30 min. The mass change plots are shown in Fig. 10. It can be seen that the curve (a) can be divided into two regions. Region I is in the initial 3.13 min and the corrosion speed is 930 ng/ min. During this initial region, Ru surface was in direct contact with NaClO solution and the surface oxide layer rapidly formed and quickly dissolved in the alkaline environment. At this time, the passivation layer was thin, so it had weak effect to block the chemical reaction. After 3.13 min, the formed passivation layer was thicker, and the corrosion rate decreased to 427 ng/ min, shown in Region II. The corrosion rate tended to be stable and the formation rate reached a balance with the dissolution rate. For the one without cleaning, the curve can also be divided into two regions. In the initial 12.48 min (Region III), the corrosion rate was only 169 ng/ min. Compared with Fig. 10(a) region I, there is an obvious plateau, indicating an inhibited Ru corrosion which is because of glycine adsorption on these surfaces. Water cleaning do wash the surface adsorbed glycine away, which causes the difference of these two curves. In the region IV, with the progress of chemical reaction, the glycine adsorption layer on Ru surface gradually reacted with the surface layer consumed, so the dissolution rate increased to 473 ng/ min that is similar with region II.

3.4 Mechanisms for glycine inhibition of Ru corrosion In order to further explore the roles of glycine on the Ru oxidation, the XPS measurements 14

were carried out for the Ru film with treatment by different solutions. We designed four experiments. The first three experiments are to check the effect of glycine concentration. The Ru films were immersed in the solution of 0.5 wt.% NaClO without glycine, with 1 mM and 2 mM glycine at pH 10 for 5 min, respectively, and then all were immersed in DI water at pH 10 for 3 min. The fourth reference experiment was to first immerse Ru film in the solution of 0.5 wt.% NaClO at pH 10 for 5 min and then immersed in the aqueous solution of 2 mM glycine at pH 10 for 3 min. Fig. 11 shows the Ru 3d spectra of the Ru films after above treatments. The fitted XPS results are shown in Table 3. By comparing Fig. 11(a) - (c), it is found that Ru4+ and Ru6+ are the main oxidation state of Ru after treatment in the alkaline NaClO solution. Although it is said amorphous hydroxide Ru(OH)3 can form on the Ru surface, yet we didn’t observe this III valence oxide [19]. With the increase of glycine concentration, the Ru (0) concentration increases from 10.29% to 29.47%, the tetravalent metal content decreases from 40.62% to 27.44%, and the hexavalent metal content decreases from 49.09% to 43.09%, respectively. Combining the SER, OCP and EIS data, we can conclude that glycine effectively reduces the surface oxide content. From the reference experiment, shown in Fig. 11(d), it can be seen that, in comparison to the sample treated without glycine (Fig. 11(a)), the Ru (0) state increases from 10.29% to 21.70%, meanwhile, Ru4+ and Ru6+ decrease from 40.62% to 32.38% and 49.09% to 45.92%, respectively. These results clearly demonstrate that glycine can promote the dissolution of the formed oxide layer. From these results, we can also assume that glycine has a stronger complexing ability of Ru (IV) than with Ru(VI) since in all cases Ru(VI) changes a little while Ru(IV) content changes a lot. The total reaction of Ru in the NaClO solution can be written as: 15

Ru + ClO- + 2OH- → RuO24 + H2 O + Cl

(13) Glycine will complex with surface oxide and form soluble product. The real reaction might be complex and we just write the final reaction. (Gly means H2NCH2COO-) Ru(OH)4 + 4HGly → Ru(Gly)4 + 4H2 O

(14)

2RuO3 + 6HGly + 2OH- → Ru(Gly)6 + 4H2 O + RuO24

(15)

Where Ru-Gly is a soluble complex of Ru. From above results it can be seen that the addition of glycine can inhibit the formation of oxide layer and glycine can reduce the oxide thickness in two ways: one is inhibiting the formation of oxide layer and another is promotion the dissolution of oxide layer. According to the XPS results, we can build a model for the passivation/corrosion of Ru surface in the NaClO solution with glycine. As can be seen from Table 3, for the sample A and D, when using 2 mM Gly to remove the surface oxide, the declining trend of Ru (IV) oxide is more obvious. Normally the oxide film growth obeys a layer-by-layer sequence, with higher valence oxide (Ru(VI) oxide in the outmost), so we would expect a higher Ru(VI) removal. However; in our case, Ru (IV) oxide would dissolve much more than Ru (VI) oxide. Therefore, we propose that in our system, the formed passivation layer structure is a RuRu4+/Ru6+ oxide mixed structure. Therefore, NaClO will enter the inner part of passivation layer along some pores and further oxidize the low-valence oxide (Ru (IV)) into the highvalence oxide (Ru (VI)). Some high valences oxide will dissolve in the alkaline solution and 16

corrosion pits will form. Finally, the surface passivation layer is a Ru-Ru4+/Ru6+ oxide mixed structure, as shown in Fig. 12(a). At pH 10, glycine in solution is mainly in the form of +H3NCH2COO− and H2NCH2COO- with a concentration ratio of 1.66/1 [3]. The isoelectric point of ruthenium surface oxides in solution is 4 ~ 6 [28]. Therefore, at pH 10, the surface of the sample is negatively charged. The positive charges end of the +H3NCH2COO− species will be attracted to the negatively charged surface. The schematic illustration of the inhibition model of glycine is shown in Fig. 12(b). Glycine can adsorb on the surface, inhibiting the further contact between oxidant and metal, thus inhibiting the reaction between oxidant and metal. At the same time, glycine will complex the formed Ru (Ⅳ) and Ru (Ⅵ) oxide, forming the soluble complex, which further accelerates the dissolution of the oxide layer.

3.5 Application of glycine and sodium hypochlorite in Chemical-Mechanical Planarization Then we carried out polishing experiments to investigate CMP properties of Ru and Cu in the NaClO slurries. Fig. 13(a) shows the RR of Ru and Cu in the NaClO slurry as a function of NaClO concentration at pH10. The abrasives content is 7.5 wt.%. When using 0.1 wt.% NaClO, the RR of Ru is about only 0.2 nm/ min. As the NaClO concentration increases, the Ru removal rate increases to 7.0 nm/min at 0.3 wt.% NaClO and 13.2 nm/min at 0.9 wt.% NaClO. As mentioned in the previous discussion, the formed Ru (IV) and Ru (VI) oxides can been removed by mechanical action, so the Ru removal rate is significantly improved. 17

Using 0.1 wt.% NaClO, the RR of Cu is about 2.3 nm/ min. Increase of NaClO concentration even decreases the RR of Cu to about 0.8-1.1 nm/min due to formation of thick passivation layer. In the conventional H2O2 solution, increase of Ru RR due to increase of H2O2 concentration will also increase the RR of Cu and Ta, which would result in barrier loss and dishing of Cu [29]. Fig. 13(b) shows the RR of Ru and Cu in the 0.5 wt.% NaClO slurry with 0 to 2 mM glycine at pH 10. The abrasive content is 7.5 wt.%. It can be seen that RR variation has the same trend as SER variation. When adding 1 mM glycine, the selectivity between Cu and Ru is about 1.09.

4. Conclusions Ru can be significantly etched in the alkaline NaClO solution. It is found that glycine can effectively inhibit the Ru corrosion. The inhibition behavior and mechanism are systematically investigated by using potentiodynamic, OCP, EIS, EQCM, SEM and XPS. It is found that the glycine adsorption on Ru follows the Temkin’s model. Glycine has two roles on inhibiting Ru corrosion: (1) Glycine will adsorb on the surface of Ru, limiting the further reaction between oxidant and Ru, thus reducing the formation rate of oxides; (2) Glycine will promote the dissolution of oxide layer through complexing action. From the XPS measurement it is found that that glycine seems complex more with the Ru(IV) oxide. And we propose the corrosion inhibition model of glycine on Ru. Finally, the NaClO based slurries with glycine is used to CMP of Cu and Ru. Through, addition of glycine into the NaClO solution, the selectivity of Cu and Ru RR can be adjusted to 1:1. Our results 18

demonstrate that the NaClO based slurry with glycine can be used for static etching of Ru with good surface control and may also apply on CMP of Ru with good selectivity between Cu and Ru, low corrosion of Ta barrier as well as good surface in the interconnect structure.

Acknowledgements This work is supported by National Natural Science Foundation of China (NSFC61574047). The authors would like to thank Hebei University of Technology for their support of wafers. The authors would also like to thank Prof. Jin Li's research group at Department of Materials Science, Fudan University for their help of electrochemical analysis.

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[4] X.P. Qu, J.J. Tan, M. Zhou, T. Chen, P.R. Guo, B.Z. Li, Improved barrier properties of ultrathin Ru film with TaN interlayer for copper metallization, Applied Physics Letters, 88 (2006) G154. [5] F. Griggio, J. Palmer, F. Pan, N. Toledo, A. Schmitz, I. Tsameret, R. Kasim, G. Leatherman, J. Hicks, A. Madhavan, Reliability of dual-damascene local interconnects featuring cobalt on 10 nm logic technology, in: 2018 IEEE International Reliability Physics Symposium (IRPS), IEEE, 2018, pp. 6E. 3-1-6E. 3-5. [6] X. Zhang, H. Huang, R. Patlolla, W. Wang, F.W. Mont, J. Li, C.-K. Hu, E.G. Liniger, P.S. McLaughlin, C. Labelle, Ruthenium interconnect resistivity and reliability at 48 nm pitch, in: 2016 IEEE International Interconnect Technology Conference/Advanced Metallization Conference (IITC/AMC), IEEE, 2016, pp. 31-33. [7] C.C. Yang, F.R. McFeely, B. Li, R. Rosenberg, D. Edelstein, Low-Temperature Reflow Anneals of Cu on Ru, Ieee Electron Device Letters, 32 (2011) 806-808. [8] K. Yu, T.H.M. Oie, F. Amano, S. Consiglio, C. Wajda, K. Maekawa, G. Leusink, Integration of ALD barrier and CVD Ru liner for void free PVD Cu reflow process on sub-10nm node technologies, in: 2014 IEEE International Interconnect Technology Conference, IEEE, 2014, pp. 117-120. [9] R. Kötz, S. Stucki, D. Scherson, D. Kolb, In-situ identification of RuO4 as the corrosion product during oxygen evolution on ruthenium in acid media, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 172 (1984) 211-219. [10] B.C. Peethala, D. Roy, S.V. Babu, Controlling the Galvanic Corrosion of Copper during Chemical Mechanical Planarization of Ruthenium Barrier Films, Electrochem. Solid State 20

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23

Table 1 0.3 wt.%

0.5 wt.%

0.7 wt.%

Fitted

Err%

Fitted

Err%

Fitted

Err%

Rs (Ω cm2)

82.31

0.2994%

62.09

0.3357%

49.20

0.309%

Qdl×105 (Ω-1 sn cm-2)

1323.0

2.988%

1206.0

4.152%

562.6

4.609%

ndl

0.9169

2.123%

0.9333

2.444%

0.8689

1.958%

Rct (Ω cm2)

66.20

3.158%

41.23

3.155%

34.18

1.985%

W×105 (Ω-1 s0.5 cm-2)

21950

13.46%

25060

10.47%

26040

6.247%

24

Table 2 1mM

2mM

3mM

4mM

Fitted

Err%

Fitted

Err%

Fitted

Err%

Fitted

Err%

65.06

0.6363%

67.59

0.3451%

58.11

1.194%

52.52

2.591%

83.29

6.495%

83.36

2.787%

67.75

6.255%

59.21

10.15%

ndl

1

1.941%

0.9490

0.8978%

0.9418

2.056%

0.9279

3.415%

Rct (Ω cm2)

81.66

2.008%

122.70

0.985%

231.30

2.801%

413.40

5.967%

11480

8.739%

9112

4.93%

6014

17.41%

2866

30.7%

Rs (Ω cm2) Qdl×105 (Ω-1 sn cm-2)

W×105 (Ω-1 s0.5 cm-2)

25

Table 3. The XPS fitting results of Ru film after treatment in 0.5 wt.% NaClO concentration with different glycine. Ru (%)

Ru4+ (%)

Ru6+ (%)

A: 0.5 wt.% NaClO at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min.

10.29

40.62

49.09

B: 0.5 wt.% NaClO and 1m Mglycine at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min.

21.66

32.59

45.75

C: 0.5 wt.% NaClO and 2 mM glycine at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min.

29.47

27.44

43.09

D: 0.5 wt.% NaClO at pH 10 for 5 min, and then immersed in the aqueous solution of 2 mM glycine at pH 10 for 3 min.

21.70

32.38

45.92

26

Figure Caption Fig. 1. The SER of Ru and Cu films in the NaClO solutions with different concentrations at pH 10. Fig. 2. The SER of Ru films in the 0.5 wt.% NaClO solutions with different concentrations of glycine at pH 10. Fig. 3. The cross-sectional SEM images of Ru in the solutions at pH 10 with (a) pure water, (b) 0.5 wt.% NaClO, and (c) 4 mM GLY and 0.5 wt.% NaClO. Fig. 4. The surface SEM images of Ru film (a) without treatment and the ones immersed in the 0.5 wt.% NaClO at pH 10 after treatment for 10 min with different concentrations of glycine: (b) 0 mM, (c) 1 mM, (d) 2 mM, (e) 3 mM, and (f) 4 mM. The insets in each SEM figure show the corresponding optical images of the sample after each treatment. Fig. 5. Potentiodyamic polarization curves of Ru, Cu, and Ta in the solutions containing 0.5 wt.% NaClO with different concentrations of glycine at pH 10. Fig. 6. Potentiodyamic polarization plots of Ru in the solutions containing 0.5 wt.% NaClO with different concentrations of glycine at pH 10. Fig. 7. Temkin adsorption plot as surface coverage θ as a function of glycine concentrations. Fig. 8. (a) In-situ OCP of Ru in the solutions containing 7.5 vol.% colloidal silica, and 0.5 wt.% NaClO without glycine and with 2 mM glycine at pH 10, and (b) OCP of Ru in the solutions containing 0.5 wt.% NaClO with different concentrations of glycine at pH 10. Fig. 9. (a) The Nyquist plots and (b) and (c) the Bode plots of Ru in the NaClO solutions with 27

different concentration. (c) The Nyquist plots and (e) and (f) Bode plots of Ru in the 0.5wt.% NaClO solutions with different glycine concentration. (g) Equivalent circuit of the impedance spectra with Warburg impedance. Fig. 10. The mass change of two Ru covered EQCMs with different treatment. (a) from the sample first dipped in the aqueous solution with 1 mM glycine at pH 10 for 10 min, followed by DI water cleaning for 10 min and then dipped in the 0.5 wt.% NaClO at pH 10 for 30 min; (b) from the sample dipped into the aqueous solution with 1 mM glycine at pH 10 for 10 min, then the sample is dipped in the 0.5 w.t.% NaClO at pH 10 for 30 min. Fig. 11. The Ru 3d XPS spectra of the Ru films after treatments in different solutions at pH 10: (a) Ru is first immersed in the solution of 0.5 wt.% NaClO at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min; (b) Ru is first immersed in the solution of 0.5 wt.% NaClO with 1 mM glycine at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min; (c) Ru is first immersed in the solution of 0.5 wt.% NaClO and 2 mM glycine at pH 10 for 5 min, and then immersed in DI water at pH 10 for 3 min. (d) Ru is first immersed in the solution of0.5 wt.% NaClO at pH 10 for 5 min, and then immersed in the aqueous solution of 2 mM glycine at pH 10 for 3 min. Fig. 12. The schematic illustrations of (a) the formed Ru-Ru (Ⅳ)/Ru (Ⅳ) mixed passivation layer structure and (b) Glycine inhibition and complexation model. Fig. 13. (a) The RR of Ru and Cu in the solutions at pH 10 with different concentrations of NaClO. (b) The RR of Ru and Cu in the solutions containing 0.5 wt.% NaClO at pH 10 with different concentrations of glycine. 28

29

30

31

32

33

34

35

36

37

Author Contributions Section The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Highlights 1. In the alkaline NaClO solution, glycine is a good corrosion inhibitor for Ru. 2. The glycine adsorption isotherm on Ru surface in the alkaline NaClO solution follows TEMKIN’s model. 3. Glycine can not only adsorb on the Ru surface, but also promote the dissolution of the oxide layer. 4. Glycine can adjust the selectivity of Cu and Ru RR to 1:1.

38

The results show that in the alkaline NaClO solution, glycine is a good corrosion inhibitor for Ru. It is found that the glycine adsorption isotherm on the Ru surface in the alkaline NaClO solution follows TEMKIN’s model. Glycine can not only adsorb on the Ru surface to inhibit the corrosion of Ru, but also promote the dissolution of the formed oxide layer through complexing action.

39