The combinatorial effect of complexing agent and inhibitor on chemical–mechanical planarization of copper

Microelectronic Engineering 71 (2004) 90–97 www.elsevier.com/locate/mee

The combinatorial effect of complexing agent and inhibitor on chemical–mechanical planarization of copper Tianbao Du *, Ying Luo, Vimal Desai Advanced Materials Processing and Analysis Center (AMPAC) and Mechanical, Materials, Aerospace Engineering (MMAE), Eng 381, University of Central Florida, 12146 Mendal Dr., 4000 University Blvd., Orlando, FL 32816, USA Received 1 July 2003; received in revised form 5 August 2003; accepted 11 August 2003

Abstract Chemical–mechanical planarization (CMP) is a vital process for the fabrication of advanced copper multilevel interconnects schemes. The focus of this investigation was to understand the oxidation, dissolution and surface modification characteristics of Cu in slurries with varying pH. Hydrogen peroxide was used as the oxidizer, glycine as complexing agent and 3-amino-triazol (ATA) as inhibitor in the slurry. The electrochemical process involved in the oxidative dissolution of copper was investigated by potentiodynamic polarization studies. X-ray photoelectron spectroscopy was used to investigate the surface modification of copper and understand the interaction between Cu–H2 O2 – glycine–ATA during CMP. In the absence of glycine and ATA, copper removal rate was found to be high in the slurry with 5% H2 O2 at pH 2. The removal rate then decreased and reached the minimum at pH 6 and started to increase in alkaline conditions. With the addition of 0.01 M glycine, the removal rates of copper were lowered in acidic slurries, but increased significantly in alkaline slurries. The addition of ATA lowered copper removal rates, however, better surface planarity was achieved. The present investigation provides an insight to the mechanism of Cu removal in the presence of oxidizer, complexing agent and inhibitor. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Copper; Chemical–mechanical planarization; Glycine; Hydrogen peroxide

1. Introduction Copper is currently used as an interconnect material in integrated circuits (ICs) due to its low resistivity and high electromigration resistance [1,2]. Integration of copper into an IC manufac-

*

Corresponding author. Tel.: +1-407-823-4635; fax: +1-407882-1462. E-mail address: [email protected] (T. Du).

turing process is implemented by using the dual Damascene technique [3,4]. Here, copper is deposited by chemical vapor deposition (CVD) or electroplating into vias and trenches etched in the interlayer dielectric (ILD) over a diffusion barrier usually made from titanium, tantalum, or their nitrides. Chemical–mechanical polishing (CMP) technique is then applied to remove the overburden material and to planarize the wafer surface. The planarization capability is an important advantage of the CMP process. It prevents issues

0167-9317/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2003.08.008

T. Du et al. / Microelectronic Engineering 71 (2004) 90–97

associated with the increased number of interconnect layers, since the topography of device structures varies significantly as additional layers are stacked one on top of another. The Cu-CMP process is not only important but also a challenging task for developing interconnects. This is due to the high dissolution rates of copper at low pH and the unfavorable selectivity values between copper and its barrier layer due to the wide differences in their mechanical properties. Carpio et al. [5] have studied copper CMP in slurries including oxidizing agents, complexing agents, corrosion inhibitors and buffering agents. Several promising chemistries for copper CMP slurries have been reported. Steigerwald et al. [6–9] investigated the CMP of copper using ammonia as the copper etchant since ammonia can react with copper in the presence of oxidizer and form soluble copper–amine complexes. Such investigations revealed that the removal of copper could be achieved as result of mechanical abrasion, and the role of the chemical etchants was only to dissolve the abraded copper particles rather than to etch the material directly from the surface. The drawback of the alkaline copper CMP process is the low polish selectivity of copper to SiO2 , which results in the unwanted removal of oxide during polishing. Fayolle and Romagna [10] have reported that CMP slurry containing H2 O2 yielded better results than that containing Fe(NO3 )3 as oxidizer. Such results might be attributed to the slow rate of chemical reaction in case of H2 O2 than Fe(NO3 )3 . Luo et al. [11] studied copper CMP in acidic media using Fe(NO3 )3 as oxidizer and benzotriazole (BTA) as inhibitor. They concluded that the copper CMP process is primarily mechanical in nature and direct chemical etching contributes minimally to the copper polish rate. In addition to ammonia, inorganic complexing agents and several water-soluble organic complexing agents, such as ethylenediamine-tetra acetic acid (EDTA) [12] and glycine [13–15] have also been examined. Favorable polishing results were obtained using a mixture of glycine and H2 O2 as the main components of alkaline [13] and neutral [14] slurries for copper CMP. In either pH regime, copper dissolution and polish rates increased with increasing glycine concentration and decreased

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with increasing peroxide concentration. Hariharaputhiran et al. [16] reported that the presence of H2 O2 along with amino acid catalytically produced hydroxyl radicals, which attributed to the very high copper-polishing rate. Zhang and Subramanian [17] have shown that the removal rates of copper increased with the increase of glycine concentration in the presence of 5 wt% of H2 O2 . The effect of benzotriazole on the corrosion inhibition of copper during CMP was investigated by Luo et al. [11]. Although benzotriazole is an efficient inhibitor reducing copper dissolution rate significantly, it affects the stability of the alumina particle in the CMP slurry. Also, it is not an environment friendly inhibitor. 3-Amino-triazol (ATA), which shows superior inhibition property than BTA, is evaluated as inhibitor for copper CMP in this study. Future optimization of CMP for copper will require a deeper understanding of the material removal mechanism during polishing. In order to reach this understanding, the specific role of each chemical constituent in the slurry should be elucidated at a fundamental level. Although a significant amount of work has been done on CuCMP using H2 O2 , glycine and ATA, the Cu– H2 O2 –glycine–ATA interaction during CMP is still not fully understood. Cu surface oxidation and dissolution play a critical role in CMP removal. Therefore, the present investigation was aimed at studying the oxidation, dissolution and modification of Cu surface by using H2 O2 as oxidizer, glycine as complexing agent and ATA as inhibitor. Electrochemical methods and static and dynamic removal rate measurements were used to study polishing characteristics. Surface modification of copper was investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) to understand the interaction between Cu– H2 O2 –glycine–ATA during Cu-CMP.

2. Experimental 2.1. Chemical mechanical polishing MSW 2000 alumina polishing slurry was used for the polishing experiments. The abrasive

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concentration in the slurries was maintained at 4.6 wt%. The pH was adjusted using buffer solutions (Fisher Scientific). Hydrogen peroxide, glycine and ATA were supplied by Alfa Aesar. All the experiments were done using 99.995% pure copper disk with 1-in. diameter and 1/4-in. thick, which were obtained from Target Materials. The polishing experiments were carried out using a BuehlerMinimet 1000 Polisher with perforated, non-woven Buehler Polimet pads. After about 15 min of initial pad breakout time, the pad was found to give stable and reproducible results without the need for conditioning. The applied downward pressure was 7.63 psi. The polishing time for each run was 5 min. Removal rates reported in this study are an average of five polishing runs. Dissolution experiments were carried out in a 1000-ml glass beaker with 400 ml of etchant solution in order to find out the static etch rates. The removal rates and etch rates were calculated by measuring the weight loss of the disk using Sartorius A210 P balance. 2.2. Electrochemical measurements Static potentiodynamic polarization tests were performed using EG&G Princeton Applied Research model 273 potentiostat/galvanostat. A standard three-electrode corrosion flat cell was used. A platinum foil was used as the counter electrode, and saturated calomel electrode was used as the reference electrode. The reference electrode was inserted into the corrosion cell through a Luggin bridge whose tip was at a distance of 2 mm from the working electrode. The voltage scan rate was used at 1 mV s
1 . 2.3. Surface morphology and chemistry studies The surface modification of copper specimens in various solutions of pH, glycine, ATA and H2 O2 was characterized using an XPS system (5400 PHI ESCA). XPS studies were conducted on cleaned Cu samples after dipping them into the requisite solution for 10 min. Then, these samples were taken out, rinsed with distilled water and dried in inert atmosphere. The sample was transferred to the vacuum chamber quickly with care to

avoid further oxidation. The base pressure during analysis was 10
9 Torr and Mg Ka X-radiation (1253.6 eV) at a power of 350 W was used. Both the survey and the high-resolution narrow spectra were recorded with electron pass energy of 44.75 and 35.75 eV, respectively, to achieve the maximum spectral resolution. Any charging shift produced by the samples was carefully removed by using a B.E. scale referred to C(1s) B.E. of the hydrocarbon part of the adventitious carbon line at 284.6 eV. Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peak shape after the background removal. 2.4. Surface planarity studies AFM experiments were performed on a Digital Instruments Dimension Series 3100 AFM using Tapping ModeTM to determine the nanoscale morphology of the Cu surface after CMP. A Ônano-roughnessÕ, or secondary roughness, was subsequently calculated for these high areas to compare the surface planarity for CMP of Cu in different conditions.

3. Results and discussion The CMP removal rates and static etch rates of copper as a function of pH in alumina slurries with 5% H2 O2 , without and with 0.01 M glycine and 0.1 wt% ATA, are shown in Fig. 1. In the absence of glycine, the copper removal rate was found to decrease with the increase of pH and reached the minimum at pH 6, with the further increase of pH to alkaline value, the removal rate started to rise again. In the presence of 0.01 M glycine, the removal rate was observed to decrease at pH 2 and 4, but increased at pH higher than 6, and this increase was more significant in alkaline conditions. Further study indicated that Cu removal rate was enhanced with the addition of 0.1 M glycine in acidic conditions. This could imply that, at lower concentrations, glycine acts as an inhibitor in acidic slurries. This is confirmed by the results reported by Szocs et al. [18], where glycine was found to be a very effective inhibitor in preventing corrosion. The decreased removal rate of copper in

T. Du et al. / Microelectronic Engineering 71 (2004) 90–97 Removal Rate (Ang/ min)

1200

93

2

(a) 1000 800

1.5

(c)

400 200

(d) 0 0

2

4

6

8

10

12

pH Fig. 1. Dynamic and static removal rate of Cu in alumina slurry with varying pH values. (a) Dynamic with 5% H2 O2 . (b) Dynamic with 5% H2 O2 and 0.01 M glycine. (c) Dynamic with 5% H2 O2 , 0.01 M glycine and 0.1 wt% ATA and (d) Static with 5% H2 O2 .

acidic slurry might be due to the inhibition effect of glycine. With the addition of ATA, the copper removal rates were further suppressed. The static etch rates were significantly lower than that of dynamic polishing, which can be attributed to the influence of abrasives in the slurry. Potentiodynamic polarization studies were carried out to measure the corrosion current density at various pH with and without glycine. Polarization plots for copper as a function of pH without and with 0.01 M glycine solution are presented in Figs. 2 and 3, respectively. It can be observed 1.5

Potential vs SCE / V

1

0.5

(b) (c)

(a) (d)

0

-0.5

-1 -7

-6

-5 -4 -3 Log (Current density/A.cm-2)

-2

-1

Fig. 2. Potentiodynamic polarization plots of copper without glycine with 5% H2 O2 at pH. (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 10.

Potential vs SCE / V

(b)

600

1

0.5

(b) (c) (a)

(d)

0

-0.5 -7

-6

-5 -4 -3 log (Current density/A.cm-2)

-2

-1

Fig. 3. Potentiodynamic polarization plots of copper with 5% H2 O2 and 0.01 M glycine at pH of: (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 10.

from Fig. 2 that all the polarization curves show active dissolution behavior with no obvious evidence of passivation. Initially, the potential was scanned in the positive direction from a potential – 500 mV below open circuit potential (OCP). It was observed that the cathodic reduction of oxygen or hydrogen peroxide predominated over the anodic dissolution of copper. As the electrode potential was scanned further in the positive direction to 200 mV above OCP, the net anodic current density increased by almost three orders of magnitude from corrosion current density. In contrast to the active anodic behavior at pH 2, 4 and 10, the polarization curves at pH 6 revealed some kind of passivation consistent with the potential-pH diagram at these pH values. The Tafel slope for the initial anodic portion of the polarization curves shows an increased value, followed by a transition, which could be due to the transition from cuprous oxide to cupric oxide. In the presence of 0.01 M glycine, the polarization curves at the four different pH values show active copper dissolution, with no evidence of passivation. This is consistent with the observation that, there is an absence of any solid oxides on copper surface at these pH values. Aksu and Doyle [19] have studied the electrochemical behavior of glycine in aqueous solutions extensively. They reported that glycine could exist

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in aqueous solutions in three different forms, namely, þ H3 NCH2 COOH (cation), þ H3 NCH2 COO
(Zwitterion) and H2 NCH2 COO
(anion). The cation form is predominant at pH values below 2.35, while anions form predominantly above pH of 9.78. The zwitterion predominates at intermediate pH values. Glycine forms soluble complexes with both cupric and cuprous ions. The principal copper (II) glycinate complexes are Cu(H3 NCH2 COO)2þ , Cu(H2 NCH2 COO)þ and Cu(H2 NCH2 COO)2 , while the principal Cu(I) species is Cu(H2 NCH2 COO)
. As can be seen, glycine exists as a cation at pH 2, which will inhibit the dissolution of copper, and therefore we observe decreased removal rates in the presence of glycine. At pH 4, the predominant form is zwitterion, but as the equilibrium indicates, some cations still exist; hence lower decrease in removal rate was observed. However, at pH 6 and 10, more anions are present, therefore increased removal rates were observed. It is well known that the corrosion current density is a parameter, which represents the corrosion rate of a material in the presence of an electrolyte. Fig. 4 represents a plot of Icorr at static conditions as a function of pH without and with 0.01 M glycine. It can be observed from Fig. 4 that the corrosion current density decreases as pH increases and reaches a minimum at pH 6, it then increases again at pH 10 in the absence of glycine. Such a decrease in Icorr indicates the formation of a more protective passive film. The direct contribu-

tion of electrochemical dissolution in the removal rates can be calculated from the Icorr measurements. At acidic pH, the removal due to electrochemical dissolution constitutes a small portion of the actual polish rates. Therefore, the Cu-CMP process is primarily mechanical in nature. However, at near neutral and alkaline pHs, the calculated electrochemical removal rate is comparable to the measured polishing rate. This might be due to the harder protective film formed at these conditions [20]. The Icorr value in the presence of glycine remains almost the same at acidic pH and increases significantly at pH 10. The significant increase of Icorr at pH 10 indicates the strong reaction between copper and glycine and the formation of highly soluble copper complexes. Perhaps the formation of such Cu–glycine complexes has contributed to an increased removal rate of copper. XPS has been widely used to study the changes in surface chemistry of Cu after exposure to varying slurry chemistries [21,22]. In this work, XPS studies were carried out to understand the interaction of glycine and H2 O2 on copper surface at varying pH in static conditions. Fig. 5 shows the Cu 2p3=2 XPS spectra for copper in pH 2, 4, 6 and 10 solutions after 10 min of immersion. The characteristic binding energy (BE) value of Cu2 O at 932.2 eV was observed for samples at all pH

Cu/Cu2O CuO

500

Icorr/uA.cm-2

(a)

Counts/arb.units

Without glycine

400

with 0.01M glycine

300 200

Cu(OH)2

CuO satellite

(b) (c) (d)

100 0

948

0

2

4

6

8

10

12

943

938

933

928

Binding Energy/eV

pH

Fig. 4. Variation of Icorr values of copper as a function of pH without and with 0.01 M glycine.

Fig. 5. Cu (2p3=2 ) XPS of copper oxide films formed in solutions with 5% H2 O2 at different pH after 10 min of immersion. (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 10.

T. Du et al. / Microelectronic Engineering 71 (2004) 90–97

values. The relatively high photoelectron signal intensity for pH 2 and 10 samples when compared to that for pH 4 and 6 samples indicates that the signals possibly responded not only from Cu2 O on the surface, but also from the substrate copper metal. The deconvolution of the XPS spectra reveals two additional peaks presented at 933.5 and 934.5 eV for samples at pH 2 and 6. The binding energies for copper in CuO and Cu(OH)2 states are about 933.4  0.2 and 934.5  0.2 eV, respectively [23,24]. The signals of CuO could be identified clearly for the sample at pH 6. It was characteristic of CuO to have the shakeup satellite features at BE 10 eV higher than the Cu 2p3=2 peaks at 933.5 eV. Fig. 6 shows the Cu 2p3=2 XPS spectra for copper in pH 2, 4, 6 and 10 solutions after 10 min of immersion in the presence of glycine. The XPS core level Cu(2p) peak of the sample immersed in pH 2 solution clearly indicated the presence of cupric oxide. The Cu-oxide satellite peak was also observed on the copper surface. For copper immersed in pH 4 solution, the XPS core level Cu(2p) peak appeared at 932.5 eV, corresponding to metallic copper or cuprous oxide. However, the deconvolution of this peak revealed the presence of cupric oxide. The decreased removal rates of copper in these acidic slurries in the presence of H2 O2 and glycine might be due to the enhanced formation of

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cupric oxide. However, for samples immersed in pH 6 and pH 10 solution in the presence of glycine, the main Cu(2p) peak corresponds to metallic copper which indicates that, in the presence of glycine, the oxide film dissolves into the solution after forming a soluble complex. XPS Cu(2p) spectra from surface of copper treated in a solution containing 5% H2 O2 , 0.01 M glycine and 0.1 wt% ATA for 10 min are shown in Fig. 7. Surprisingly, no evidence of copper oxide formation was found in these XPS spectra. This clearly indicates that with the addition of ATA inhibitor, oxide formation was highly suppressed. To further confirm the ATA adsorption and the Cu–glycine complexation, XPS spectra of N(1s) obtained from the copper surface after 10 min treatment in pH 10 solution containing 5% H2 O2 + 0.01 M glycine without and with 0.1 wt% ATA were analyzed (Fig. 8). The XPS N(1s) peak appearing at 397.1 eV on the copper surface treated with 5% H2 O2 and 0.01 M glycine indicates the presence of Cu–glycine complex on the surface. The relatively low intensity of this N(1s) peak clearly indicates that this Cu–glycine complex is highly water soluble. XPS N(1s) peak appeared at around 400 eV for the sample treated in solution containing ATA inhibitor. The deconvolution of this N(1s) peak revealed two separate peaks. The

Cu(2p3/2)

Counts/arb.unit

Counts/arb.units

Cu/Cu2O

CuO

(b) (a) (d)

CuO satellite

(c) (d) (a)

(c)

(b)

948

948

943

938

933

928

943

938

933

928

Binding Energy/eV

Binding Energy/eV

Fig. 6. Cu (2p3=2 ) XPS of copper oxide films formed in solutions with 5% H2 O2 and 0.01 M glycine at different pH after 10 min of immersion. (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 10.

Fig. 7. Cu (2p3=2 ) XPS of copper oxide films formed in solutions with 5% H2 O2 , 0.01 M glycine and 0.1 wt% ATA at different pH after 10 min of immersion. (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 10.

T. Du et al. / Microelectronic Engineering 71 (2004) 90–97

Intensity

96

390

395

(a)

400

405

Intensity

Binding Energy (eV)

2

1

395

(b)

397

399

401

403

405

Binding Energy (eV)

Fig. 8. XPS N(1s) spectra taken from the surface of Cu after treating with: (a) 0.01 M glycine and (b) 0.01 M glycine and 0.1 wt% ATA for 10 min in a solution at pH 10 containing 5% H2 O2 .

one appearing at 398.9 eV corresponds to the amino group on the ATA molecule; while the peak appearing at 400.1 eV is due to the N on the triazol ring. This clearly indicates that ATA molecule can be preferentially adsorbed on copper surface and inhibit the oxide formation. Similar results were observed for all ranges of pH. AFM images of copper surface after CMP polishing in solutions at pH 10 containing 5% H2 O2 , 5% H2 O2 + 0.01 M glycine and 5% H2 O2 + 0.01 M glycine + 0.1 wt% ATA are depicted in Fig. 9. AFM images indicate that the surface roughness value of the copper surface polished in 5% H2 O2 is 18.5 nm. With the addition of glycine, the surface roughness drops to 6.7 nm. This is due to the strong dissolution of copper by forming Cu–glycine soluble complex. With further addition of 0.1 wt% ATA, the surface roughness goes further down to 1.6 nm. This clearly indicates that with the addition of ATA inhibitor, the best surface planarity is obtained.

Fig. 9. AFM images from the surface of Cu after CMP polishing at pH 10 in solutions containing (a) 5% H2 O2 , (b) 5% H2 O2 and 0.01 M glycine and (c) 5% H2 O2 , 0.01 M glycine and 0.1 wt% ATA.

4. Conclusions Cu–glycine–H2 O2 –ATA interaction in Cu-CMP has been studied successfully using electrochemistry and XPS. The CMP removal rates and etch rates of copper in slurries with 5% H2 O2 were found to vary with pH. The removal rate first decreases

T. Du et al. / Microelectronic Engineering 71 (2004) 90–97

with the increase of pH and reaches the minimum at pH 6, it then increases with further increase of pH. Corrosion products formed on copper surface were found to be responsible for this CMP removal rate. Addition of 0.01 M glycine in the hydrogen peroxide containing solution enhances the dynamic removal rate of copper at alkaline pH due to the formation of a highly soluble copper–glycine complex. The addition of ATA as inhibitor can improve the surface planarity by inhibiting the high dissolution of copper through preferential adsorption on the copper surface. Acknowledgements Lucent Technologies and the Florida High Tech Corridor supported this research.

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