Elsevier

Applied Surface Science

Volume 453, 30 September 2018, Pages 416-422
Applied Surface Science

Full Length Article
Direct bonding of silicon and quartz glass using VUV/O3 activation and a multistep low-temperature annealing process

https://doi.org/10.1016/j.apsusc.2018.05.109Get rights and content

Highlights

  • Direct heterogeneous bonding of silicon and quartz glass was realized in air.

  • The high bonding strength was achieved and the bonding interface was defect-free.

  • A model was proposed to elucidate the bonding mechanism at a low temperature.

Abstract

Low-temperature direct bonding is a promising method to integrate two or more dissimilar materials into one composite without large thermal stresses. In this paper, we describe a bonding process for silicon and quartz glass via vacuum ultraviolet/ozone (VUV/O3) activation and a multistep, low-temperature annealing process. A strong bonding strength and a bonding interface without any microcracks were obtained after annealing at 200 °C. The surfaces and bonding interface were characterized. After the organic contaminants were removed by VUV/O3, the treated surfaces were very hydrophilic. In addition, the VUV/O3-induced surface oxidation increased, resulting in oxide asperities on the substrates. These newly generated surface asperities might possess a strong deformability based on the water stress corrosion effect, leading to gap closure after low-temperature annealing. Moreover, a model for the mechanism of the VUV/O3-activated bonding was proposed.

Graphical abstract

Direct bonding of silicon and quartz glass is realized via VUV/O3 activation. Based on the surface and interface analysis, the bonding mechanism attributed to the gap closure has been proposed.

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Introduction

Today, silicon is the most widely used semiconductor material in microelectronics products, and it has been used as a substrate material for large-scale integrated circuits, large-scale integration and other microelectronics devices. Quartz glass is an amorphous material with a compact network structure composed of silica tetrahedron units, and it is often used for image sensors, displays, solar cells and optical waveguides due to its excellent thermal and chemical stability and extremely high light-transmission performance [1]. Therefore, the bonding of silicon- or glass-based materials is important for improving functional device performance or hermetic packages [2], [3]. The coefficient of thermal expansion (CTE) for silicon and quartz is 2.6 × 10−6/K and 0.56 × 10−6/K, respectively, at 25 °C [4], and as the temperature increases, the difference in the CTE values gradually increases. The large difference in the CTE values results in difficulties in bonding these two materials together.

Vacuum ultraviolet (VUV) radiation is a short wavelength (100–200 nm) UV light [5], [6] that can clean surfaces more effectively due to its high energy. Additionally, VUV irradiation has been considered an environmental-friendly dry cleaning method for the removal of surface contaminants without toxic solutions and can be conducted at room temperature and atmospheric pressure without cooling water and a warm-up procedure. The VUV activation method is also an emerging technique for direct wafer bonding [7]. Due to the lack of ion bombardment, VUV activation can clean the surfaces of substrates while causing less damage than plasma [8], [9]. In addition to achieving atomic-level surface cleaning, a hydrophilic surface with an ultrathin hydration layer, beneficial for direct bonding, can also be formed [10]. In this paper, we used VUV/O3-activated direct bonding for the preparation of a heterogeneous combination of silicon and quartz glass substrates. A low-temperature, multistep annealing process is proposed to enhance the bonding strength between the Si and quartz glass, which have a large difference in their CTE values. To determine the conditions for a strong bonding strength, the surfaces and bonding interfaces were characterized to gain insight into the bonding mechanism.

Section snippets

Experimental

P-type, (1 0 0)-oriented, double-polished silicon chips with a size of 10 mm × 10 mm × 400 μm were used in the experiments. The quartz glass chips were 10 × 10 mm2, polished on two sides, and 500-μm thick. Both types of samples were provided by Daheng Optics and Fine Mechanics Co., Ltd, Shanghai, China.

We employed a one-step VUV treatment for the silicon to quartz glass prebonding process. A Xe excimer lamp source (SUS 713, Ushio), which can emit a wavelength of 172 nm with a full width at half

Optimization of the bonding parameters

To obtain the optimum prebonding parameters, the changes in the bonding area ratio as a function of the humidity and irradiation time were examined, and the results are shown in Fig. 1. The bonding area ratio initially increased and then decreased with increasing irradiation time and reached the highest value after 15 min of VUV irradiation. Meanwhile, the largest bonding area was obtained at a humidity of 30 ± 5% after 15 min of irradiation. Therefore, we maintained the humidity and the VUV

Conclusions

We developed a VUV/O3-activated direct bonding method for silicon and quartz glass at a low temperature. This method does not require an ultrahigh vacuum, cleanroom or applied external force during the bonding procedure. After 15 min of VUV irradiation, the silicon and quartz glass surfaces become highly hydrophilic. In addition, some silicon oxide asperities grow on the VUV-irradiated surface. However, the overall surface roughnesses of the Si and quartz glass were very small. In addition,

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51505106) and the China Postdoctoral Science Foundation (Grant No. 2017M610207). The authors also acknowledge support from the Heilongjiang Postdoctoral Foundation (No. LBH-Z16074).

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