Wenhua Yang

Low temperature Cu-Cu direct bonding using formic acid vapor pretreatment

This paper describes a novel low temperature Cu-Cu direct bonding method using formic acid vapor pretreatment. The in situ dry process of reduction using formic acid vapor is developed to realize Cu-Cu direct bonding at low temperature and in low vacuum. In order to analyze the Cu film surface reduction by formic acid vapor, the CMP Cu film surface is investigated by X-ray photoelectron spectroscopy (XPS) before treatment and after treatment. It shows that the Cu peak becomes strong and O, C peaks become weak after formic acid vapor treatment at 200°C. It is successful to bond Cu film chips when formic acid vapor treatment conditions are 150°C for 10min, 175°C for 5min and 200°C for 1min, respectively. The bonding strength is about 9.8MPa when sample is treated by formic acid vapor at 200°C for 20min. The bonding interface is observed by transmission electron microscopy (TEM).

Effect of Formic Acid Vapor In Situ Treatment Process on Cu Low-Temperature Bonding

Low-temperature Cu/Cu direct bonding technology using formic acid vapor in situ treatment was developed. Effect of formic acid vapor treatment conditions on Cu surface and bonding was studied. Cu surface oxide was reduced using formic acid vapor in situ treatment at 150 and 200°C, respectively. With higher temperature and longer treatment time, surface reduction is more effective. Grain boundary etching was found on chemical mechanical polished Cu film after initial treatment by formic acid. However, Cu surface roughness is minimally influenced by long-time formic acid vapor treatment. Cu film/Cu film direct bonding was realized in N2 atmosphere with formic acid vapor in situ treatment under temperature below 200°C. For lower treatment temperature, longer treatment time is required to achieve good bonding. The bonding strength is about 9.0 MPa when Cu surface is treated at 200°C for 10 min.

Formic acid vapor treated Cu-Cu direct bonding at low temperature

Formic acid vapor treated low temperature Cu-Cu direct bonding method was developed. The in situ dry process of reduction using formic acid vapor was applied to realize Cu-Cu direct bonding at low temperature and in low vacuum. In order to evaluate the effect of formic acid vapor on Cu film surface, the CMP-Cu film surface is investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM) before treatment and after treatment. It shows that the Cu peak becomes strong and O, C peaks become weak after formic acid vapor treatment at 200 °C. Grain boundary was found on the surface after treatment due to surface etching by formic acid. Surface roughness only increases a little with formic acid vapor treatment. CMP-Cu/EB-Cu film direct bonding and CMP-Cu film/Cu bumpless electrodes direct bonding were conducted in N2 atmosphere with formic acid vapor treatment at 150°C–200°C, respectively. The bonding strength of CMP-Cu/EB-Cu film direct bonding is about 9.8MPa when samples are treated by formic acid vapor at 200°C for 20min. CMP-Cu film/Cu bumpless electrodes bonding also was done, and the bonding strength is about 24.1MPa. Finally, the bonding interface is observed by transmission electron microscope (TEM) and scanning electron microscope (SEM).

Formic acid treatment with Pt catalyst for Cu direct bonding at low temperature

The paper describes a new process of Cu-Cu direct bonding at a temperature lower than 200°C in atmospheric pressure. The method consists of reduction process of Cu oxide using formic acid treatment and subsequent generation of Cu nano-particles by means of Pt catalyst. The main idea of the proposed method is that due to Pt catalytic effect, combined to the conventional formic acid treatment, a part of the formic acid is decomposed to generate hydrogen radicals to reduce the oxide of the Cu surface, CuO and produce Cu formats very effectively. As the result, nano-particles of Cu are precipitated from the formats on the Cu surface and a tight bonded interface can be formed at a temperature below a 200°C. Cu surface was heated in water vapor to enhance the oxidation process and compared regarding to the precipitation of the Cu nano-particles.

Formic acid treatment with Pt catalyst for Cu direct and hybrid bonding at low temperature

A new process of Cu-Cu direct bonding at lower than 200°C in atmospheric atmosphere was developed. The method is composed of formic acid treatment with Pt catalyst. To demonstrate the feasibility of the method, Cu electrodes samples were designed for characterizing the bonding strength and the contact resistance for the Cu-Cu direct bonding. The bonding strength is about 40MPa, and the contact resistance at the bonding interface is about 0.17mΩ. Both of them are better than those without Pt catalyst. The effects of the Pt catalyst on bonding were investigated and it is concluded that the surface oxide of Cu is reduced effectively by the hydrogen radical generated by formic acid decomposition with Pt catalyst and the Cu formats resulted by the formic acid treatment is decomposed to precipitate very fine Cu grains at the bonded interface.

Low temperature Cu/Cu direct bonding using formic gas in-situ treatment

A low temperature Cu/Cu direct bonding technology using formic acid vapor treatment was developed. Using this technology, 6mm×6mm Cu film chips were bonded at low temperature in N₂ atmosphere. XPS surface analysis shows that Cu surface oxide was reduced after formic gas in-situ treatment. The bonding was conducted in N₂ at 200°C after Cu surface was treated at 200°C for 10min.

Formic acid with Pt catalyst combined treatment process for Cu low temperature bonding

In this study, formic acid combined Pt catalyst in situ pretreatment process was developed for low temperature Cu/Cu direct bonding. Cu film surface was treated by formic acid without/with Pt as catalyst at 200°C for 10min to reduce Cu surface oxide. Through XPS analysis, it is found that, with Pt as catalyst, formic acid reduction effect on Cu film surface is better than that of formic acid treatment without Pt as catalyst. Cu film and Cu micro-electrodes was bonded together at 200°C in N2 atmosphere after formic acid treatment without/with Pt catalyst. The bonding strength is evaluated using die shear tester. When sample was treated by formic acid with Pt catalyst, the bonding is stronger compared with the bonding treated by formic acid without Pt catalyst. Bonding strength also increases with keeping the contact load for long time during the bonding. When sample was treatedat200°C for 10min with Pt catalyst, and the contact load as 1000N was kept for 60min during bonding, the bonding strength is about 28.5MPa. Finally, the cross-section image of bonding interface is obtained through SEM observation.