Process Monitoring of Copper Baths by a Novel Impedance Method Alan Zduneka, Delia Nieto Sanzb Air Liquide a Chicago Research Center, Countryside, IL, USA b Centre Recherche Claude Delorme, Les Loges en Josas, France
[email protected] Claude Gabrielli, Philippe Mocoteguy, Hubert Perrot CNRS, Laboratoire Interfaces et Systemes Electrochimiques, Université P. & M. Curie, Paris, France Christelle Mace, Anne Quennoy, Eric Chabal Altis Semiconductor Corbeil-Essones, France Abstract Electrochemical impedance spectroscopy (EIS) measurements were performed on copper interconnect deposition baths during copper metallization to characterize changes in the plating bath condition. Results from multiple process tool bath experiments indicate the shape of the impedance scan is sensitive to changes in the steady state of the bath chemistry and may be an indicator of overall bath quality. Development of this method as an online monitor to track overall bath quality will also be discussed. Introduction Copper baths for electroplating copper interconnects on integrated circuits requires special additives to achieve “bottom-up” or “superfilling” of the trenches and vias. The ability to perform such superconformal deposition strongly depends on the organic additives added to the copper sulfate/sulfuric acid-based electroplating bath. Monitoring or controlling the concentration of these additives as well as their by-products is therefore crucial to guaranteeing the properties of the copper deposit. State-of-the art, copper interconnect plating tools use online monitoring to quantify bath component concentrations to achieve tighter bath control. Recent progress to quantify bath ageing and measure byproducts has been reported [1-3], although implementation remains difficult, partly due to the proprietary nature of copper bath formulations. On the other hand, changes in the overall bath quality, which is dependent on manufacturing conditions, tool upsets, additive degradation, and byproduct accumulation, can profoundly affect the copper deposit. Clearly, a method is desirable that follows changes in copper bath and therefore, in copper film quality, so that out-of-spec bath conditions can be readily measured, identified and controlled. Electrochemical impedance spectroscopy has been postulated as a possible method for following bath quality during wafer processing [4-5]. Experiments in the laboratory during simulated bath ageing concluded that the impedance technique follows copper bath ageing during and after plating [4]. Furthermore, these results appeared to correlate with deposit morphology (deposit roughness) and thus may indicate overall quality of the bath during plating operation. In this paper, impedance measurements were made on actual
process bath solutions at a semiconductor-manufacturing site to validate the laboratory experiments. The results are presented below. Electrochemical Impedance Spectroscopy Method Electrochemical impedance spectroscopy, or EIS, is a common method used in electrochemical characterization, battery and fuel cell research, and corrosion measurements [610]. EIS is a steady-state technique capable of observing phenomena in electrochemical systems whose relaxation times vary over many orders in magnitude [8]. The EIS technique applies a small-amplitude sinusoidal voltage to a working electrode at a number of discrete frequencies, ω, ranging from 0.001 to 100,000 Hertz as shown in Figure 1. At each of these frequencies, the resulting current exhibits a sinusoidal response, I (ω) that is out-of-phase with the applied sinusoidal voltage signal. The electrochemical impedance, termed Z(ω), is the frequency-dependent proportionality factor between the voltage signal and current response [10]: Z(ω) = V(ω)/ I (ω) Z(ω) is a complex-valued vector quantity with real and imaginary components, whose values are frequencydependent: Z(ω) = Z’ (ω) + j Z”(ω), Where Z’ (ω) is the real component of the impedance and Z”(ω) is the imaginary component of the impedance. The real and imaginary impedance can be plotted against each other at each frequency to generate a “Nyquist” plot and the familiar semicircle shapes as shown in Figure 2. Figure 2 shows an ideal Nyquist plot for an electrochemical system consisting of a single charge transfer reaction (Me+ + e- ! Me) with a diffusion component. The ohmic, or electrolyte solution resistance is observed at the highest frequency, the charge transfer resistance which is related to the rate of the electrochemical reaction is observed in the mid frequencies, and the diffusion component occurs at the lower frequencies. In systems where additives are present and influence the deposition process, additional semicircles and inductive loops on the Nyquist plot can be observed and are related to ion-additive complexation, adsorption
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mechanisms or additive degradation processes. A calculated Nyquist plot derived from a model of copper deposition that accounts for organic additive adsorption is shown in Figure 3 [11]. Frequency generator ∆E.Sinωt, F = ω/2π
j (A/m2)
Iav + ∆I.Sin(ωt-Φ)
Potensiostat
Copper bath solutions were periodically obtained from the 6 process tools from a sampling port in each of the tools. The EIS equipment and setup consisted of a potentiostat and frequency generator/analyzer for imposing and acquiring the electric signal, as well as an electrochemical cell and ancillary components (rotating disc electrode (RDE) assembly, reference electrode, counter electrode) for conducting the experiments on the bath solutions. A small DC current was imposed on the system in addition to the AC current signal that was varied in frequency from 100,000 Hz to 0.01 Hz. 50
A
WE
Loop 2
2
∆E = Z .e jφ . ∆I Figure 1. AC voltage signal imposed on an electrochemical system and the resulting AC current and impedance.
10 Hz
10
No mass transfer limited system. Mass transfer limited system.
1,0 2
F = f(Rtc, Cdl)
1 Hz 10 mHz
-10 -20 1 mHz
-40 F→0
100 mHz
100 Hz
0
-30
1,5
Loop 3
1000 Hz
20
Eav + ∆E.Sinωt
For each frequency : Z(F) =
-Im(∆E/∆I)/Ω.cm
Loop 1
30
E (V)
Ref
-Im( ∆ E/∆ I)/Ω cm
CE
40
-50
Loop 4
0
20
40
60
Re( ∆ E/ ∆ I)/Ω cm
0,5 45° F→∞
0,0
Rtc (Charge transfer resistance)
0,5
1,0 1,5 2,0 2 Re(∆E/∆I)/Ω.cm
10 0
Figure 3. Calculated impedance from the model.
F→0
Ohmic resistance
-0,5 0,0
80 2
2,5
3,0
Figure 2. Ideal Nyquist plot for a single charge transfer reaction with a mass transfer component. Experimental Electrochemical impedance spectroscopy measurements were performed on plating bath solutions from six copper plating tools during copper deposition manufacturing. The tools were Novellus Sabre electrodeposition tools. Two different bath chemistries were studied. Bath Solution 1 contained a two additive system (B and C) with an inorganic base composition of copper sulfate, sulfuric acid and Cl-. Tool 2 and tool 3 were operated using Bath solution 1. Bath Solution 2 was a three additive system (A,B,C) with an inorganic base of different composition. Tool 1, and Tools 4,5,6 were operated using Bath Solution 2. The process tools were followed using EIS measurements during a ten-day campaign to determine the steady state stability of the bath solution as a function of processing time.
Results Bath Solution 1. Figure 4 compares the impedance spectra of Bath Solution 1 obtained from process tool 2 and tool 3. As shown in Figure 4, the impedance spectra from the two process tools are very similar and the baths appear to be at the same steady state. The tool bath impedance spectra were compared with those obtained in the laboratory for fresh Bath Solution 1 (no ageing or processing). It can be seen that the spectra obtained from the tool baths are close to the fresh solution but the 3rd loop is slighter larger in diameter. In addition, two spectra previously obtained in the laboratory on partially aged Bath Solution 1 were also added to Figure 4. The shape of the spectra is similar to those obtained in the process tool baths, however the third capacitive loop diameter is half the one obtained for the process tool baths. A possible explanation is that the steady state condition of the bath in tool 2 and tool 3 baths are close to the fresh solution, or in other words, the process tool bleed and feed operation is compensating the bath to the point that the “aged” bath is nearly like a fresh bath. This suggests that bath compensation could be optimized to reduce bath bleeding without effect deposit quality.
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1
perturbation during manufacturing. These results indicate that the impedance measurements made over time are able to detect changes in the steady state of the bath. However, these changes may or may not be significant to the overall quality of the bath but may represent the normal fluctuations associated with the dynamic plating process. Further measurements will be made to confirm this behavior and correlate with bath composition and deposit quality.
0
1,5 4
Figure 4. Impedance spectra comparison between process tool, fresh solution and lab-aged solution: Bath solution 1. Results Bath Solution 2. Figure 5 compares the impedance spectra obtained on the 4 process tools using Bath Solution 2. Differences between the inductive loop diameters were observed between the spectra obtained between the different tools. The 4 process tools appear to be at slightly different steady states from an impedance spectra standpoint. A synthetic solution of Bath Solution 2 was made and EIS performed. The impedance spectra obtained from this fresh solution is close to those obtained from the 4 process baths. Again, it can be tentatively concluded that the steady state of the new chemistry bath in the process tool is close to the fresh solution. Comparison Between the Two Bath Solutions Comparison between Figures 4 and 5 shows a strong difference between the spectra of each bath solution. The impedance method is able to clearly distinguish between two types of bath compositions. Interestingly, Bath Solution 2, which is designed for better superfilling of narrow vias, does not exhibit a third capacitive loop diameter, but rather a large inductive loop. These characteristics were found to be the same as those driving the smoothest deposits in the ageing studies performed with Bath Solution 1 in reference 4. In that paper, the third capacitive loop had to be minimized to obtain the smoothest deposits. Bath Stability with Time Figures 6 and Figure 7 show the evolution with time of the Bath Solution 2 impedance spectra for Tool 1, and Tool 6. The impedance spectra do not exhibit a third capacitive loop, but rather a large inductive loop. In addition, the impedance spectra over 10 days of measurements are similar for Tool 1 (Figure 6), indicating a fairly good stability of the plating bath. A change in the diameter of the inductive loop was seen in Figure 7 for three spectra (30/06/2003 to 01/07/2003). This may indicate a change in the steady state of the bath on these days. Also, in Figure 7, a change in the impedance spectra associated the formation of a small third capacitive loop was observed (Curve corresponding to the bath sampled on july 2nd at 14h55). This change was correlated with a
1,0 2
2 3 2 Re(∆E/∆I)/Ω.cm
Re(∆E/∆I)/Ω.cm
1
Fresh solution : VMS + 8 mL/L of S + 5 mL/L of A + 1.5 mL/L of L Sabre 1, 27/06/2003, 11h00 (t = t0 + 0h30). Sabre 4, 25/06/2003, 14h35 (t = t0 + 0h50). Sabre 5, 25/06/2003, 14h40 (t = t0 + 0h15). Sabre 6, 26/06/2003, 16h45 (t = t0 + 0h50). Sabre 6, 30/06/2003, 15h00 (t = t0 + 0h20).
0,5
0,0
-0,5
-1,0 0,0
0,5
1,0
1,5
2
2,0
2,5
Re(∆E/∆I)/Ω.cm
Figure 5. Impedance spectra comparison between process tool, fresh solution and lab-aged solution: Bath solution 2.
1,5 1,0
25/06/2003, 14h41 (t = t0 + 1h10). 26/06/2003, 11h00 (t = t0 + 0h50). 27/06/2003, 11h00 (t = t0 + 0h30). 01/07/2003, 8h45 (t = t0 + 1h10). 02/07/2003, 8h33 (t = t0 + 5h00). 03/07/2003, 9h00 (t = t0 + 1h50). 04/07/2003, 8h30 (t = t0 + 2h20).
2
0
-Im(∆E/∆I) (Ω.cm )
-Im(∆E/∆I)/Ω.cm
2
2
Sabre 2, 03/07/2003, 15h32 (t = t0 + 0h50). Sabre 3, 26/06/2003, 10h52 (t = t0 + 0h50). Fresh VMS + 11 mL/L of B + 1.8 mL/L of C. (Exp. 1) Fresh VMS + 11 mL/L of B + 1.8 mL/L of C. (Exp. 2) Aged industrial solution (Vieil 63 : 1.76 A.h/L < I*t/v < 2.21 A.h/L). Aged industrial solution (Vieil 53 : 2.40 A.h/L < I*t/v < 2.81 A.h/L).
0,5 0,0 -0,5 -1,0 0,0
0,5
1,0 1,5 2,0 2,5 2 Re(∆E/∆I) (Ω.cm ) Figure 6. Evolution of time of the impedance spectra for Tool 1, Bath Solution 2.
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26/06/2003, 9h25 (t = t0 + 6h10) 26/06/2003, 16h45 (t = t0 + 0h50) 27/06/2003, 14h45 (t = t0 + 2h30) 30/06/2003, 15h00 (t = t0 + 0h20) 01/07/2003, 11h00 (t = t0 + 0h50) 01/07/2003, 17h25 (t = t0 + 0h10) 02/07/2003, 8h23 (t = t0 + 3h20) 02/07/2003, 14h55 (t = t0 + 0h20) 03/07/2003, 9h00 (t = t0 + 1h30) 03/07/2003, 15h23 (t = t0 + 0h30)
2,0 1,5
Conclusions Electrochemical impedance spectroscopy (EIS) measurements were performed on copper interconnect deposition baths during copper metallization to characterize changes in the plating bath condition. A comparison between the two bath solutions indicates that the impedance method is able to clearly distinguish between different bath compositions. In addition, the results show that the impedance spectra can detect changes in the bath over time, making it a possible candidate for an online monitor of bath quality. Further measurements will be undertaken to confirm this behavior.
2
-Im(∆E/∆I) (Ω.cm )
1,0 Increase in inductive loop
0,5 0,0 -0,5
Perturbation
-1,0 0,0
0,5
1,0 1,5 2,0 2 Re(∆E/∆I) (Ω.cm )
2,5
3,0
Figure 7. Evolution of time of the impedance spectra for Tool 6, Bath Solution 2.
References 1. M. Pavlov, E. Shalyt, P. Bratin, D.M. Tench, “Detection of Accelerator Breakdown Products in Plating Baths”, Electrochem. Soc. Proceedings Volume PV 2003-10, pp. 53-62, The Electrochemical Society, Inc. Pennington, NJ, 2003. 2. A. Jaworski, K. Wikiel, “The Effect and Detection of Short Chain PEG’s in Copper Damascene Electroplating Process”, ECS 203rd Meeting, April 2003, Paris, France, Paper 678. 3. P. Bratin, G. Chalyt, A. Kogan, M. Pavlov, M.J. Perpich, D.M. Tench, ‘Detection of Suppressor Breakdown Contaminants in Copper Plating Baths”, ECS 203rd Meeting, April 2003, Paris, France, Paper 676. 4. C. Gabrielli, P. Mocoteguy, H. Perrot, A. Zdunek, D. Nieto Sanz, M.Clech, “Investigation of Copper Bath Ageing in the Damascene Process by Electrochemical Impedance Spectroscopy”, Electrochem. Soc. Proceedings Volume PV 2003-10, pp. 129-136., The Electrochemical Society, Inc. Pennington, NJ, 2003. 5. C. Gabrielli, J. Kittel, P. Mocoteguy, H. Perrot, A. Zdunek, P. Bouard, M. Haddix, L. Doyen, M.Clech, Forum Impedances Proceedings 15th Electrochimiques, p. 159, December, 2002. 6. C. Gabrielli, “Identification of Electrochemical Processes by Frequency Response Analysis”, Schlumberger Technologies Technical Report Number 004/83, August, 1984. 7. A.J. Bard, “Electroanalytical Chemistry”, Vol. 4, M. Dekker, NY, 1970, pp-3-121. 8. R.Varma and J.R. Selman “Techniques for Characterization of Electrodes and Electrochemical Processes”, Chapter 11, pp. 515-647, John Wiley & Sons, 1991. 9. J.R. Scully, D.C. Silverman, M.W. Kendig, “Electrochemical Impedance: Analysis and Interpretation”, ASTM STP 1188, ASTM Philadephia, PA, 1993. 10. R. Baboian, ed. “Corrosion Test and Standards: Application and Intrepretation”, ASTM Manual
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Series: MNL 20, ASTM Philadelphia, PA, 1995, pp.80-81. 11. C. Gabrielli, J.Kittel, P. Mocoteguy, H. Perrot, A. Zdunek, M. Haddix, P. Bouard, L. Doyen, “A Model of Copper Deposition for the Damascene Process”, ECS 203rd Meeting, April 2003, Paris, France, Paper 404.
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