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J. Semicond. > 2013, Volume 34 > Issue 12 > 126001

SEMICONDUCTOR TECHNOLOGY

CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system

Chenwei Wang, Suohui Ma, Yuling Liu, Rui Chen and Yang Cao

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 Corresponding author: Wang Chenwei, cwtjy206@163.com

DOI: 10.1088/1674-4926/34/12/126001

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Abstract: CMP process optimization for bulk copper removal based on alkaline copper slurry was performed on a 300 mm Applied Materials Reflexion LK system. Under the DOE condition, we conclude that as the pressure increases, the removal rate increases and non-uniformity is improved. As the slurry flow rate increases, there is no significant improvement in the material removal rate, but it does slightly reduce the WIWNU and thus improve uniformity. The optimal variables are obtained at a reduced pressure of 1.5 psi and a slurry flow rate of 300 ml/min. Platen/carrier rotary speed is set at a constant value of 97/103 rpm. We obtain optimized CMP characteristics including a removal rate over 6452 Å/min and non-uniformity below 4% on blanket wafer and the step height is reduced by nearly 8000 Å/min in the center of the wafer on eight layers of copper patterned wafer, the surface roughness is reduced to 0.225 nm.

Key words: CMP processoptimizationalkaline copper slurrydesign of experiment

Copper is the material favored most for integrated circuit (IC) manufacture, owing to its excellent electro migration resistance and low electrical resistance. The integration of copper into an IC manufacturing process can be implemented by using the dual damascene technique, in which chemical mechanical planarization (CMP) has been widely used for planarizing the surface globally and locally[1-4]. Several factors have been considered in the literature, including the effects of numerous variables such as the choices of slurry and pad as well as system facility problems, post-CMP cleaning, and pad conditioning techniques which may affect the CMP process[5-9]. Among the above CMP components, slurry consumables and process variables controlled by equipment are very important parameters in determining removal rate and non-uniformity (within-wafer non-uniformity; WIWNU%). The optimization of CMP process variables by applying the design of experiment (DOE) technique is expected to improve the CMP process ability of new materials and to conserve expense and time, which trial and error techniques may otherwise cause. In this paper, CMP process optimization for bulk copper removal based on alkaline copper slurry was performed on a 300 mm Applied Materials Reflexion LK system. We obtained the optimum process variables by using the DOE method, which can apply the CMP process to the global planarization of multilevel interconnection structures. Optimum process variables have been studied from the viewpoint of removal rate and non-uniformity. Finally, through the above DOE results, we obtain the optimal CMP equipment parameters.

In this experiment, the CMP process was performed by using an Applied Materials Reflexion LK 300 mm tool with a five-zone polishing head. The polish pad was IC 1010 (DOW Chemical, Co.). The platen/carrier rotary speed was set at a constant value of 97/103 rpm in all main polishing steps based on Semiconductor Manufacturing International Corporation guidelines. The pressure and slurry flow rate were changed from 1.0 to 2.5 psi and from 250 to 400 mL/min for verifying the effect of pressure and slurry flow rate. Dummy wafers were polished for 1 min prior to the main 3 polishing steps. The alkaline copper slurry (namely FA/O copper slurry) applied in this experiment was obtained from the Hebei University of Technology. The slurry solutions included: 4 wt% colloidal silicon with a median particle diameter of 20 nm as the abrasive; 3 wt% polyhydroxy polyamino (FA/O) used as the copper complexing agent and obtained from Tianjin Jingling Mircroelectronics Material Co., LTD; 1 wt% hydrogen peroxide (H2O2), which added to the slurry to oxidize the copper and enhance its removal. The pH value of the slurry was 10.7.

The copper removal rate was measured with a 300 mm copper blanket wafer with 1.2 μm of copper film. The eight layer copper pattern wafers used in this experiment had an initial step height of 12.7 kÅ and the starting thickness of copper film was 16 kÅ. The planarization test pad was 50 × 70 μm2. The material removal rate was determined by calculating the film thickness before and after polishing for 1 min. A RESMAP 463 FOUP (Cornell Dubilier Electronics, Inc.) resistivity measurement device was used to measure the copper film thickness and removal rate profile. WIWNU was calculated according to the results. The measured copper was an 81-point diameter scan with 3 mm of edge exclusion. MRR was determined by taking the averaged result. A Dimension AFP (atomic force profiler, Veeco) was performed to measure the step height of the patterns.

The DOE technique used for the optimized CMP process is summarized in Table 1.

Table  1.  Parameters of CMP equipment for DOE technique.
DownLoad: CSV  | Show Table

Figure 1 shows the MRR and WIWNU after CMP as a function of various pressures. The polishing conditions were a head/platen speed of 103/97 rpm and a slurry flow rate of 300 ml/min. As shown in Fig. 1, the MRR is increased continuously with the increase of pressure. The average MRR variation between 4640 to 9425 Å/min is observed at applied pressures in the range of 1.0-2.5 psi. Copper removal rates of 5000 Å/min (2.0 psi) or greater are desirable in IC manufacturing where process time and wafer throughput are a major concern. The polishing results obtained from Fig. 1 show that FA/O copper slurry has a higher removal rate of 8553 Å/min at 2.0 psi. WIWNU also increased as the pressure increased, as shown in Fig. 1. The profile of MRR with various pressures indicates that the increases of WIWNU were caused by the low removal rate near the wafer edge and a slightly reduced removal rate around the wafer center. We conclude that as the pressure increases, the removal rate increases and non-uniformity is improved.

Figure  1.  Material removal rate and WIWNU after CMP as a function of various pressures.

The effects of various slurry flow rates on the MRR and WIWNU of a copper blanket wafer are present in Fig. 2. The polishing process was conducted at a pressure of 1.5 psi and a rotational velocity of 103 rpm (polish head) and 97 rpm (platen). As the slurry flow rate is increased, there is no significant improvement in the material removal rate, but there is a slight reduction of the WIWNU and thus an improvement in uniformity. As the flow rate is increased, the MRR profiles presented in Fig. 2 are not changed substantially, and most profiles maintain a shape having a higher removal rate around the center than its edge. The Applied Materials Reflexion LK CMP system applied in this experiment was equipped with 5-zone ContourTM heads, and the lower removal rates near the edge of the wafer are caused by the lower pressures of zones 1 and 2 on the polishing head. There is a strong relationship between uniformity and the pressure of zones 1 to 5, and thus if good uniformity is obtained, the pressure should be optimized.

Figure  2.  Material removal rate and WIWNU after CMP as a function of different slurry flow rates.

Because of the ongoing development of interconnects toward the nanoscale, new interconnect structures are designed to use low-dielectric constant (low-k) materials. Many of these dielectrics are mechanically fragile and require low down-pressure ( 2 psi) polishing. Under the DOE conditions, we obtained optimized CMP characteristics including a removal rate over 6452 Å/min and non-uniformity below 4% at a reduced down pressure of 1.5 psi and a slurry flow rate of 300 ml/min. Thus, the CMP studies on eight-layer patterned wafers (M8) were performed under this pressure and slurry flow rate. The difference in the step height of the patterns before and after polishing was reported as the step height reduction (SHR). A planar surface or a high step height reduction results from protruding areas on the wafer surface being polished at a relatively higher rate than the polish rate for recessed areas. Figure 3 shows the step height reduction of eight-layer copper patterned wafers at different positions as a function of polishing time. It can be seen that the SHR increases continuously with an increase in polishing time. When polished for 60 s, the step height reduced by nearly 8000 Å in the center of the wafer and step height reduction increased to 12040 Å when polished for 100 s. The results presented in Fig. 3 demonstrate that the slurry can provide good planarization performance and a high SHR efficiency under the DOE conditions.

Figure  3.  Step height reduction of eight-layer copper pattern wafers at different positions as a function of polishing time.

Apart from achieving high planarization efficiency, reducing the surface roughness of the processed surface also is important for Cu CMP. The surface roughness of copper films has been investigated in the proposed optimum CMP process. The results are presented in Fig. 4, along with the root mean square (RMS) surface roughness and peak/valley (P/V) distances. As shown in Fig. 4, the surface parameters of the copper surface before and after CMP are RMS = 5.84 nm, P/V = 47 nm and RMS = 0.225 nm, P/V = 16.4 nm. The RMS and P/V values obtained from Fig. 4 indicate that the copper films polished by using the FA/O copper slurry yielded a good surface quality and considerably low P/V values under the optimized situation.

Figure  4.  3D AFM image and surface roughness (a) before and (b) after CMP.

The process variables of the CMP equipment were apparently dependent on removal rate and non-uniformity. Under the DOE condition, we conclude that as the pressure increases, the removal rate increases and non-uniformity is improved. As the slurry flow rate is increased, there is no significant improvement in the material removal rate, apart from a slight reduction of the WIWNU and thus an improvement of uniformity. We obtained optimized CMP characteristics including a removal rate of over 6452 Å/min and non-uniformity below 4% on a blanket wafer and the step height was reduced by nearly 8000 Å/min in the center of the wafer. On an eight-layer copper patterned wafer, the surface roughness is reduced to 0.225 nm.



[1]
Tsai T H, Yen S C. Localized corrosion effects and modific-ations of acidic and alkaline slurries on copper chemical mechanical polishing. Appl Surf Sci, 2003, 210:190 doi: 10.1016/S0169-4332(02)01224-2
[2]
Liu X Y, Liu Y L, Liang Y, et al. Effect of slurry components on chemical mechanical polishing of copper at low down pressure and a chemical kinetics model. Thin Solid Films, 2011, 520:400 doi: 10.1016/j.tsf.2011.06.050
[3]
Hernandez J, Wrschka P, Oehrlein G S. Surface chemistry studies of copper chemical mechanical planarization. J Electrochem Soc, 2001, 148(7):G389 http://cat.inist.fr/?aModele=afficheN&cpsidt=1095187
[4]
Lee H, Park B, Jeong H. Influence of slurry components on uniformity in copper chemical mechanical planarization. Microelectron Eng, 2008, 85:689 doi: 10.1016/j.mee.2007.12.044
[5]
Pandija S, Roy D, Babu S V. Achievement of high planarization efficiency in CMP of copper at a reduced down pressure. Microelectron Eng, 2009, 86:367 doi: 10.1016/j.mee.2008.11.047
[6]
Nagar M, Vaes J, Ein-Eli Y. Potassium sorbate as an inhibitor in copper chemical mechanical planarization slurries. Part Ⅱ:effects of sorbate on chemical mechanical planarization performance. Electrochim Acta, 2010, 55:2810 doi: 10.1016/j.electacta.2009.10.086
[7]
Du T, Luo Y, Desai V. The combinatorial effect of complexing agent and inhibitor on chemical-mechanical planarization of copper. Microelectron Eng, 2004, 71:90 doi: 10.1016/j.mee.2003.08.008
[8]
Nagendra Prasad Y, Ramanathan S. Chemical mechanical planarization of copper in alkaline slurry with uric acid as inhibitor. Electrochim Acta, 2007, 52:6353 doi: 10.1016/j.electacta.2007.04.044
[9]
Luo Q, Campbell D R, Babu S V. Chemical-mechanical polishing of copper in alkaline media. Thin Solid Films, 1997, 311:177 doi: 10.1016/S0040-6090(97)00454-9
Fig. 1.  Material removal rate and WIWNU after CMP as a function of various pressures.

Fig. 2.  Material removal rate and WIWNU after CMP as a function of different slurry flow rates.

Fig. 3.  Step height reduction of eight-layer copper pattern wafers at different positions as a function of polishing time.

Fig. 4.  3D AFM image and surface roughness (a) before and (b) after CMP.

Table 1.   Parameters of CMP equipment for DOE technique.

[1]
Tsai T H, Yen S C. Localized corrosion effects and modific-ations of acidic and alkaline slurries on copper chemical mechanical polishing. Appl Surf Sci, 2003, 210:190 doi: 10.1016/S0169-4332(02)01224-2
[2]
Liu X Y, Liu Y L, Liang Y, et al. Effect of slurry components on chemical mechanical polishing of copper at low down pressure and a chemical kinetics model. Thin Solid Films, 2011, 520:400 doi: 10.1016/j.tsf.2011.06.050
[3]
Hernandez J, Wrschka P, Oehrlein G S. Surface chemistry studies of copper chemical mechanical planarization. J Electrochem Soc, 2001, 148(7):G389 http://cat.inist.fr/?aModele=afficheN&cpsidt=1095187
[4]
Lee H, Park B, Jeong H. Influence of slurry components on uniformity in copper chemical mechanical planarization. Microelectron Eng, 2008, 85:689 doi: 10.1016/j.mee.2007.12.044
[5]
Pandija S, Roy D, Babu S V. Achievement of high planarization efficiency in CMP of copper at a reduced down pressure. Microelectron Eng, 2009, 86:367 doi: 10.1016/j.mee.2008.11.047
[6]
Nagar M, Vaes J, Ein-Eli Y. Potassium sorbate as an inhibitor in copper chemical mechanical planarization slurries. Part Ⅱ:effects of sorbate on chemical mechanical planarization performance. Electrochim Acta, 2010, 55:2810 doi: 10.1016/j.electacta.2009.10.086
[7]
Du T, Luo Y, Desai V. The combinatorial effect of complexing agent and inhibitor on chemical-mechanical planarization of copper. Microelectron Eng, 2004, 71:90 doi: 10.1016/j.mee.2003.08.008
[8]
Nagendra Prasad Y, Ramanathan S. Chemical mechanical planarization of copper in alkaline slurry with uric acid as inhibitor. Electrochim Acta, 2007, 52:6353 doi: 10.1016/j.electacta.2007.04.044
[9]
Luo Q, Campbell D R, Babu S V. Chemical-mechanical polishing of copper in alkaline media. Thin Solid Films, 1997, 311:177 doi: 10.1016/S0040-6090(97)00454-9
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    Chenwei Wang, Suohui Ma, Yuling Liu, Rui Chen, Yang Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. Journal of Semiconductors, 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001
    C W Wang, S H Ma, Y L Liu, R Chen, Y Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. J. Semicond., 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001.
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    Received: 15 April 2013 Revised: 13 June 2013 Online: Published: 01 December 2013

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      Chenwei Wang, Suohui Ma, Yuling Liu, Rui Chen, Yang Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. Journal of Semiconductors, 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001 ****C W Wang, S H Ma, Y L Liu, R Chen, Y Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. J. Semicond., 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001.
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      Chenwei Wang, Suohui Ma, Yuling Liu, Rui Chen, Yang Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. Journal of Semiconductors, 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001 ****
      C W Wang, S H Ma, Y L Liu, R Chen, Y Cao. CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system[J]. J. Semicond., 2013, 34(12): 126001. doi: 10.1088/1674-4926/34/12/126001.

      CMP process optimization using alkaline bulk copper slurry on a 300 mm Applied Materials Reflexion LK system

      DOI: 10.1088/1674-4926/34/12/126001
      Funds:

      Project supported by the Major National Science and Technology Special Projects (No. 2009ZX02308), the Tianjin Natural Science Foundation of China (No. 10JCZDJC15500), the National Natural Science Foundation of China (No. 10676008), and the Fund Project of the Hebei Provincial Department of Education (No. 2011128)

      the National Natural Science Foundation of China 10676008

      the Tianjin Natural Science Foundation of China 10JCZDJC15500

      the Fund Project of the Hebei Provincial Department of Education 2011128

      the Major National Science and Technology Special Projects 2009ZX02308

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      • Corresponding author: Wang Chenwei, cwtjy206@163.com
      • Received Date: 2013-04-15
      • Revised Date: 2013-06-13
      • Published Date: 2013-12-01

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