- Open Access
Fracture analysis of anodically bonded silicon substrates during the CMP process
© The Author(s) 2018
- Received: 7 October 2018
- Accepted: 4 December 2018
- Published: 7 December 2018
In this paper, a stress and fracture study, occurring during the chemical mechanical polishing (CMP) of anodically bonded substrates is presented. The samples contain glass pillars, used to form the glass cavities and a silicon substrate sealing the glass structure, the samples are fabricated using the anodic bonding process. The mechanical stresses of the bonded silicon substrate are simulated using the COMSOL software. The fracture strength after post-processing is investigated based on the criterion value, which is the ratio of the anodically bonded area over the cavity area. It is found that the bonded area and the distribution of pillars are related to the mechanical stability of the bonded substrate during the CMP process. Studies on the stability of subsequent processes, like CMP after anodic bonding, plays an important role in improving the fabrication yield of anodic bonded devices.
- Anodic bonding
- Chemical mechanical polishing
- von Mises stress
Wafer bonding is used in microelectromechanical systems (MEMS) to protect and package sensitive internal structures from environmental influences, such as temperature, humidity and pressure. Wafer bonding is classified into direct bonding, anodic bonding and bonding performed by introducing an intermediate layer . The anodic bonding process is among the wafer bonding techniques widely used for MEMS packaging, since it provides strong bonding strength, hermetic encapsulation, high temperature resistance and permanent bonding; frequently used to package devices such as accelerometers and pressure sensors [2, 3].
Anodic bonding should be done under an electric field at high temperatures. When temperature and voltage are applied after the silicon and glass substrates are in contact, they are bonded by covalent bonds, formed at the bonding interface between the silicon and glass. In the process, both substrates have typical requirements for a successful bond, involving low surface roughness (< 10 nm). A temperature lower than the glass transition temperature is applied to increase the glass ion mobility. The applied temperature and voltage induce glass chemical bond dissociation, the cations (Na+) in the dissociated ions drift toward the backside of the glass, where the cathode is applied. As the cations move, the anions (O2−) in the dissociated ions remain at the bonding interface. Then, a depletion region is formed by the applied voltage, resulting in an irreversible bond. In other words, the remaining anions (O2−) react with the silicon surface to form silicon oxide, resulting in two anodically bonded substrates.
After the anodic bonding process, the bonded substrates usually become the final electronic device, through performing subsequent processes such as chemical mechanical polishing (CMP), thin film deposition, etch and dicing [4, 5]. Subsequent processes are indispensable for the complete fabrication of a specific MEMS device; however fracture is frequently observed on the anodically bonded substrates, caused during the subsequent processes [5–7]. Since the two materials (glass and silicon) have slightly different thermal expansion coefficients, the substrates bonded at high temperatures will have a residual stress at room temperature, resulting in fractures on the bonded substrate, obtained during the subsequent processes. In other words, the anodically bonded substrate involving residual stress may be fractured or deformed, due to mechanical and chemical influences present during the subsequent processes. Therefore, stress analysis is important to improve subsequent process stability and device production yield.
In this paper, the stability and fabrication yield regarding the subsequent processes can be improved by studying the fractures occurring on anodically bonded substrates, with respect to the area and shape of the bonding interface. A stress and fracture model of Microinfinity Co., Ltd., company that manufactures and commercializes MEMS devices, was simulated using the COMSOL software. The stresses present on the anodically bonded substrates were analyzed based on the ratio of bonded area to cavity area. The results allow to predict the deformation and fracture possibility during the CMP process, after anodic bonding.
The glass substrate cavity area is 6.37 × 6.37 mm2, and the cavity depth is 30 μm. The anodically bonded area of sample 1 is 2.8 mm2, and the areas of samples 2, 3, and 4 are 3.1 mm2, 3.5 mm2, and 8.6 mm2, respectively. All samples have an internal structure for the MEMS sensor in the cavity (see Fig. 2). Sample 1 with a BAR of 7.0% has only the internal structures for the MEMS sensor in the cavity, these are also used to support the sealed cavity. Glass pillars with a diameter of 20 μm are arranged in the cavity at 320 μm-intervals in sample 2, with a BAR of 7.6%. A glass pillar array with a pitch of about 1.2 mm is arranged to support the cavity space in sample 3, with a BAR of 8.5%. For sample 4, glass pillars with a radius of 15 μm are arranged at intervals of 60 μm, with a BAR of 21.2%.
CMP process conditions of anodically bonded substrate
Silicon thickness (μm)
Glass cavity depth (μm)
Glass thickness (μm)
Comparison of bonding properties for the cavity patterns
Area of glass pillar (mm2)
Bonding area rate (%)
Maximum von Mises stress (MPa)
Occurrence of fractures
Anodically bonded substrates having a large cavity are fabricated for a silicon MEMS device. Anodic bonded substrates with four different bonding areas are simulated for the analysis of stresses, occurring during the CMP process. In order to prevent anodically bonded substrate fracture, the stress intensity of four samples is analyzed according to the bonded area and pattern of the glass pillar arrays inside the cavity. As the ratio of bonded area to cavity area increased from 7.0 to 21.2%, the von Mises stress occurring during the CMP process was relieved from 89.5 to 0.3 MPa. As a result, the anodically bonded area and the distribution of the glass pillars inside the cavity are important factors for a stable subsequent CMP process, impacting the productivity, reliability and fabrication yield of the MEMS device.
All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
This work was financially supported by the National Research Foundation of Korea (NRF-2017R1A2B4005687). Part of this work has been supported by the Generalitat de Catalunya under Grant 2017 SGR 891.
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