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Fundamentals of Microfabrication Technologies
Published in Ghenadii Korotcenkov, Handbook of Humidity Measurement, 2020
The effect of temperature on the unbonded area for different structures is plotted in Figure 15.20. Studies performed by Wei (2006) showed that for Si-to-glass anodic bonding, the unbonded area decreases markedly from 1.55% to 0.13% when the bonding temperature is increased from 200°C to 300°C. However, the bubble-free interface cannot be achieved. For a Si-to-glass bonding, the unbonded area is largely reduced. The unbonded area decreases from 0.4% to 0.13% when the bonding temperature is increased from 200°C to 225°C. When the bonding temperature is increased to higher than 250°C and when the voltage is increased beyond 600 V, the total wafer area becomes bonded together. For glass-to-glass bonding, a bubble-free interface can be achieved when the bonding temperature is higher than 275°C. The bond strength also increases with an increase in the bonding temperature (see Figure 15.20b). Therefore, in anodic bonding, the substrates are typically heated to a temperature between 350°C and 450°C when 500 V or more is applied across the structure. In the case of silicon-to-silicon bonding, low-temperature anodic bonding (Т~400°C) is only possible when using the intermediate layer bonding approach. Wei et al. (2006) established that with careful control of the cleaning and bonding processes, for direct silicon-to-silicon bonding at temperature of 400°C only bond efficiency of about 90% and bond strength of about 10 MPa can be achieved.
Fabrication Techniques for Capacitive Silicon Resonators
Published in Nguyen Van Toan, Takahito Ono, Capacitive Silicon Resonators, 2019
There are many different ways to bond silicon wafers with different materials. One of the popular ways to bond silicon wafers with other material is anodic bonding, which was discovered by Willis and Pomerantz [15]. The anodic bonding process has a variety of commercial applications including pressure sensors, photovoltaics, and microelectronic device packaging. It has been used to bond many different materials such as ceramics with metal, ceramics with semiconductors, ceramics with ceramics, and semiconductors with semiconductor pairs. In anodic bonding, the process relies on charge migration to produce bonded wafers. This usually works well for elements with high alkali metal content. Silicon wafer is one of the main materials used in the bonding process. Glass is another material that is high in alkali metal, therefore it is often used in the anodic bonding process to bond silicon to glass. Figure 3.15 is a schematic of the mechanism of anodic bonding. In this work, an anodic bonding process to bond silicon wafer and LTCC is performed with a temperature of approximately 400°C and an electric field of approximately 0.8 kV. The anodic bonding setup for LTCC and SOI is shown in Figure 3.16. The silicon and LTCC wafers are aligned and fixed on a sample holder, as shown in Figure 3.17.
Fundamentals of MEMS Fabrication
Published in Sergey Edward Lyshevski, Mems and Nems, 2018
Anodic (electrostatic) bonding is used to bond silicon to glass. The glass can be in the form of a plate or wafer, or as a thin film between two silicon wafers. Anodic bonding is performed at lower temperatures (450°C or less), bonding metallized microdevices. In anodic bonding, the silicon wafer is placed on a heated plate, the glass plate is placed on top of the silicon wafer, and a high negative voltage is applied to the glass. As the glass is heated, positive sodium ions become mobile and drift towards the negative electrode. A depletion region is formed in the glass at the silicon interface, resulting in a high electric field at the silicon-glass interface. This field forces the silicon and glass into intimate contact and bonds oxygen atoms from the glass with silicon in the silicon wafer leading to permanent hermetically sealed bonds. Anodic bonding of two silicon wafers can be formed by coating two wafer surfaces using sputtered glasses. Anodic bonding requires smooth bonding surfaces. This requirement is not so critical compared with the fusion bonding process because the high electrostatic forces pull small gaps into contact. It is important that glass must be selected with the thermal expansion coefficient that matches silicon. The difference in the thermal expansion coefficients of the glass and silicon will result in stress between the bonded pair after cooling to room temperature. Corning 1729 and Pyrex 7740 are widely used.
Construction of a spherically bent crystal analyzer through anodic bonding for a non-resonant inelastic X-ray scattering spectrometer
Published in Instrumentation Science & Technology, 2021
The anodic bonding method was discovered by Wallis and Pomerantz[16] in 1969. It has been widely used both in the microassembly and the encapsulation processes of microelectromechanical systems (MEMs)[17] such as accelerometers,[18] pressure sensors,[19] micropumps,[20] tactile sensors,[21] and flow sensors.[22] The protection of microdevices from environmental influences such as moisture and pollution is of great importance in terms of their durability and performance. Anodic bonding is a hermetic sealing between a two-material interface through an electrostatic field at elevated temperature, and occurs between glass-glass, silicon-glass and silicon-quartz interfaces. The method is mostly used to bond alkali ion-rich glass (borosilicate or pyrex glass) and a semiconductor or metal.[23] A sodium-containing glass substrate and a silicon wafer are widely used in the process. Borosilicate is preferred, because its thermal expansion coefficient is very close to that of silicon, and it has the appropriate electrical conductivity at the bonding temperature.[24] The optical transparency of the glass also provides an advantage of observing the microstructures behind it.
Thermoplastic-based microfluidic chip bonding with PES hot melt adhesive film
Published in The Journal of Adhesion, 2023
Yaohua Wang, Fan Xu, Yiqiang Fan
For silicon or glass-based microfluidics, anodic bonding or thermal fusion bonding are the most commonly used bonding methods, which usually require highly sophisticated instruments working under high temperature.[5] For polymer-based microfluidics, the bonding methods can be divided into two categories, bonding methods for PDMS (polydimethylsiloxane), and bonding methods for thermoplastics. The microstructures cast on PDMS were usually sealed with a piece of glass slide or another layer of PDMS after oxygen plasma (or corona) treatment.[4,6]
Progress in wafer bonding technology towards MEMS, high-power electronics, optoelectronics, and optofluidics
Published in International Journal of Optomechatronics, 2020
Jikai Xu, Yu Du, Yanhong Tian, Chenxi Wang
Compared with the room-temperature bonding, more kinds of direct bonding methods have been investigated at low temperatures, such as multistep chemical treatment,[49–51] single plasma activation, sequential plasma activation,[52,53] ultraviolet or vacuum ultraviolet/ozone (UV or VUV/O3) activation,[54–58] and anodic bonding. Most of them have the advantages of low cost and batch production. The temperatures of some bonding methods are lower than the requirements of the COMS process, therefore they are popular in practical production. Among many low-temperature bonding methods, anodic bonding is the most widely used in MEMS device packaging. This bonding method has a low requirement in surface roughness but can obtain ultrahigh bonding strength. The schematic diagram for the anodic bonding is shown in Figure 2(g). The bonding temperature and applied voltage are usually performed in the range of 200–500 °C and 500–1000 V, respectively. During the bonding process, Na+ in the glass substrate will be drifted to the negative electrode. A depletion layer with a thickness of hundreds of nanometers adjacent to the Si will be formed, as confirmed by the TEM observation in Figure 2(h). To increase the bonding area, a pre-cleaning process on surfaces is important, as shown in Figure 2(i). By observing current changes, the bonding process can be clearly shown. When the voltage is just applied, there is a larger current pulse, then the current decreases. Finally, the current is almost zero, indicating that the bonding process has been finished. Xin Guo et al. from Southeast University has ever used the anodic bonding for their device packaging.[41,59] In a novel stereoscopic symmetrical quadruple hair gyroscope (SQHG), excellent performances of an angular rate sensitivity of 16.03 mV/deg/s and a bias instability of 16.26°/h at room temperature have been achieved.[60] The schematic and physical diagrams are presented in Figure 2(j). Besides, wafer-level hermit vapor cell, as shown in Figure 2(k), and capacitive MEMS sensors, as shown in Figure 2(l), are also successfully fabricated via the anodic bonding method.[34,35] A variety of commodities has proven that anodic bonding is a commercially mature MEMS packaging technique.