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

Dongling Li,Xiaohan Cui,Fenfen Lan,Ying Zhou,Mao Du

doi:10.3390/pr9091599

Dongling Li, Xiaohan Cui + Show 3 more

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https://doi.org/10.3390/pr9091599

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Abstract

Wafer direct bonding is an attractive approach to manufacture future micro-electro-mechanical system (MEMS) and microelectronic and optoelectronic devices. In this paper, a combined hydrophilic activated Si/Si wafer direct bonding process based on wet chemical activation and O2 plasma activation is explored. Additionally, the effect on bonding interface characteristics is comprehensively investigated. The mechanism is proposed to better understand the nature of hydrophilic bonding. The water molecule management is controlled by O2 plasma activation process. According to the contact angle measurement and FTIR spectrum analysis, it can be concluded that water molecules play an important role in the type and density of chemical bonds at the bonding interface, which influence both bonding strength and voids’ characteristics. When annealed at 350 °C, a high bonding strength of more than 18.58 MPa is obtained by tensile pulling test. Cross sectional SEM and TEM images show a defect-free and tightly bonded interface with an amorphous SiOx layer of 3.58 nm. This amorphous SiOx layer will induce an additional energy state, resulting in a lager resistance. These results can facilitate a better understanding of low-temperature hydrophilicity wafer direct bonding and provide possible guidance for achieving good performance of homogenous and heterogenous wafer direct bonding.

Highlights

Wafer direct bonding can combine two mirror-polished wafers together by van derWaals forces, and form strong chemical bonds without any intermediate material between surfaces
The contact angle measurement shows that O2 plasma activation can render the wafer surface highly hydrophilic
Fourier transform infrared spectroscopy (FTIR) spectrum analysis shows that H2 O molecules play an important role in Si/Si hydrophilic wafer direct bonding, which affects the formation of interfacial voids, and bonding strength

Summary

Introduction

Wafer direct bonding can combine two mirror-polished wafers together by van derWaals forces, and form strong chemical bonds without any intermediate material between surfaces. One of the main targets of wafer direct bonding is to obtain high bonding strength and void free bonding interface after low temperature post annealing. Different surface treatment methods have been developed to realize high bonding strength and good interface characteristics, such as wet chemical activation [10,11], reactive ion etching (RIE). Plasma activated bonding (PAB) is one of the promising candidates for low temperature wafer direct bonding, it can be achieved in any equipment producing plasma. To develop compatible activation processes for different wafers and different activation systems, there is still a challenge of plasma activated wafer direct bonding. Another important factor in plasma activation wafer bonding is the hydrophily of the wafer surface. There is a balance between Si-OH bond and Si-O-Si bond during bonding, an optimal amount of

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