Tribology and Lubricants

Korean Tribology Society (한국트라이볼로지학회)
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Effect of Citric Acid in Cu Chemical Mechanical Planarization Slurry on Frictional Characteristics and Step Height Reduction of Cu Pattern

  • Lee, Hyunseop (School of Mechanical Engineering, Tongmyong University)
  • Received : 2018.09.14
  • Accepted : 2018.11.11
  • Published : 2018.12.31
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Abstract

Copper chemical mechanical planarization (CMP) has become a key process in integrated circuit (IC) technology. The results of copper CMP depend not only on the mechanical abrasion, but also on the slurry chemistry. The slurry used for Cu CMP is known to have greater chemical reactivity than mechanical material removal. The Cu CMP slurry is composed of abrasive particles, an oxidizing agent, a complexing agent, and a corrosion inhibitor. Citric acid can be used as the complexing agent in Cu CMP slurries, and is widely used for post-CMP cleaning. Although many studies have investigated the effect of citric acid on Cu CMP, no studies have yet been conducted on the interfacial friction characteristics and step height reduction in CMP patterns. In this study, the effect of citric acid on the friction characteristics and step height reduction in a copper wafer with varying pattern densities during CMP are investigated. The prepared slurry consists of citric acid ($C_6H_8O_7$), hydrogen peroxide ($H_2O_2$), and colloidal silica. The friction force is found to depend on the concentration of citric acid in the copper CMP slurry. The step heights of the patterns decrease rapidly with decreasing citric acid concentration in the copper CMP slurry. The step height of the copper pattern decreases more slowly in high-density regions than in low-density regions.

Keywords

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Fig. 1. Static etch rate as a function of citric acid concentration in Cu CMP slurry.

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Fig. 1. Static etch rate as a function of citric acid concentration in Cu CMP slurry.

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Fig. 2. Top view SEM images of wafer surface static etched for 3 minutes; (a) as received, (b) 0.005M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%, (c) 0.01M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%, (d) 0.05M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%.

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Fig. 2. Top view SEM images of wafer surface static etched for 3 minutes; (a) as received, (b) 0.005M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%, (c) 0.01M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%, (d) 0.05M citric acid + DIW + H2O2 3vol% + colloidal silica 3wt%.

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Fig. 3. XPS analysis of copper surface immersed into slurries which contain various concentration of citric acid.

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Fig. 3. XPS analysis of copper surface immersed into slurries which contain various concentration of citric acid.

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Fig. 4. Material removal rate as a function of citric acid concentration.

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Fig. 4. Material removal rate as a function of citric acid concentration.

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Fig. 5. Friction signals during CMP with slurries which contain various concentration of citric acid.

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Fig. 5. Friction signals during CMP with slurries which contain various concentration of citric acid.

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Fig. 6. Temperature signals during CMP with slurries which contain various concentration of citric acid.

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Fig. 6. Temperature signals during CMP with slurries which contain various concentration of citric acid.

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Fig. 7. Step height of patterns as received and polished for 80 seconds with copper CMP slurry containing 0.005 M, 0.01 M, and 0.05 M citric acid; (a) 10 μm/90 μm (100 μm pitch, 10% density), (b) 50 μm/50 μm (100 μm pitch, 50% density), (c) 90 μm/10 μm (100 μm pitch, 90% density).

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Fig. 7. Step height of patterns as received and polished for 80 seconds with copper CMP slurry containing 0.005 M, 0.01 M, and 0.05 M citric acid; (a) 10 μm/90 μm (100 μm pitch, 10% density), (b) 50 μm/50 μm (100 μm pitch, 50% density), (c) 90 μm/10 μm (100 μm pitch, 90% density).

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Fig. 8. Schematic diagram of step height reduction; (a) low citric acid concentration and (b) high concentration citric acid.

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Fig. 8. Schematic diagram of step height reduction; (a) low citric acid concentration and (b) high concentration citric acid.

Table 1. Experimental conditions

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Table 1. Experimental conditions

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Table 2. Hardness of copper and copper oxide [18]

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Table 2. Hardness of copper and copper oxide [18]

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