A Comparative Study: Void Formation in Silicon Wafer Direct Bonding by Oxygen Plasma Activation with and without Fluorine

Abstract

A quantitative evaluation of void formation in plasma activated bonding of 200-mm silicon wafers is demonstrated. As a comparison, the wafers were treated with oxygen plasma or fluorine containing oxygen plasma prior to bonding, respectively. The bonding was performed at the room temperature (∼25 • C), and all the bonded pairs were annealed from 200 to 800 • C. Results shows adding a small amount of fluorine into the oxygen plasma could decrease the hydrophilicity of the silicon surfaces, and effectively mitigate void formation at the bonding interfaces during the subsequent annealing process. Moreover, long plasma treatments (e.g. >60 s) may produce more porous surfaces resulting in more annealing voids. A short fluorine containing plasma treatment (e.g. ∼10 s) is able to remove organic contaminants efficiently and causes restricted hydrophilic surfaces with less subsurface damages. It may lead to much fewer voids at bonding interfaces during the annealing. Void formation models are proposed to gain insight the mechanism of the plasma activated bonding without and with fluorine, respectively.

Progress in wafer bonding technology towards MEMS, high-power electronics, optoelectronics, and optofluidics

“…The fluorine-contained plasma activated wafer direct bonding method is firstly investigated by Chenxi Wang from Harbin Institute of Technology. [46][47][48] By adding an ultrasmall amount of CF 4 into the traditional O 2 plasma, the surface energy can reach to 2.5 J/m 2 equal to the fracture energy of Si substrate Figure 2. Wafer-level bonding for MEMS devices.…”

Effect of Combined Hydrophilic Activation on Interface Characteristics of Si/Si Wafer Direct Bonding

“…Nitrogen (N 2 ), oxygen (O 2 ) and argon (Ar) are the most commonly used activated gases [19]. A mixed gas of O 2 /CF 4 also has been proposed to enhance bonding strength and restrain the formation of interfacial voids [20,21]. However, to develop compatible activation processes for different wafers and different activation systems, there is still a challenge of plasma activated wafer direct bonding.…”

“…Its influence on the formation of SiO x layer and electrical properties of bonding interface are also analyzed [24]. Although the better hydrophily, the higher surface energy, the excess H 2 O will prevent Si-OH bonds converting into Si-O bonds [21], resulting in the decrease of bonding strength. Therefore, there is a balance between Si-OH bond and Si-O-Si bond during bonding, an optimal amount of H 2 O molecules on wafer surface is needed for excellent bonding.…”

Investigation of Plasma Activated Si-Si Bonded Interface by Infrared Image Based on Combination of Spatial Domain and Morphology

“…These voids not only influence the bonding yield, but also decrease the bonding strength. Many researchers have studied the morphology of interfacial voids with different process parameters and attempted to suppress the void formation [19,20,21]. The generally used void detection methods are infrared imaging method [22], ultrasonic method [23], X-ray image method [24], and mirror image method [25].…”