Wafer bonding with intermediate polymer adhesives is an important fabrication technique for advanced microelectronic and microelectromechanical systems, such as three-dimensional integrated circuits, advanced packaging, and microfluidics. In adhesive wafer bonding, the polymer adhesive bears the forces involved to hold the surfaces together. The main advantages of adhesive wafer bonding include the insensitivity to surface topography, the low bonding temperatures, the compatibility with standard integrated circuit wafer processing, and the ability to join different types of wafers. Compared to alternative wafer bonding techniques, adhesive wafer bonding is simple, robust, and low cost. This article reviews the state-of-the-art polymer adhesive wafer bonding technologies, materials, and applications.

Adhesive wafer bonding

Thin copper (Cu) films of 80 nm thickness deposited on a diffusion barrier layered 8 in. silicon wafers were directly bonded at room temperature using the surface activated bonding method. A low energy Ar ion beam of 40–100 eV was used to activate the Cu surface prior to bonding. Contacting two surface-activated wafers enables successful Cu–Cu direct bonding. The bonding process was carried out under an ultrahigh vacuum condition. No thermal annealing was required to increase the bonding strength since the bonded interface was strong enough at room temperature. The chemical constitution of the Cu surface was examined by Auger electron spectroscope. It was observed that carbon-based contaminations and native oxides on copper surface were effectively removed by Ar ion beam irradiation for 60 s without any wet cleaning processes. An atomic force microscope study shows that the Ar ion beam process causes no surface roughness degradation. Tensile test results show that high bonding strength equivalent to bulk ...

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Room temperature Cu-Cu direct bonding using surface activated bonding method

We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick (20-mum) polysilicon encapsulation to package micromechanical resonators in a pressure 600 cycles of -50 to 80degC, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400degC have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400degC without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process

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Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for MEMS Resonators

Although MEMS technologies and device structures have made significant progress in the past three decades and have found widespread application in many areas, including Micro-Opto-Electro-Mechanical Systems (MOEMS), packaging and assembly techniques suitable for many of these emerging applications have not kept pace. Packaging is one of the most costly parts of microsystem manufacturing, and it is also often the first to fail or negatively influence the system response. This paper addresses the packaging and assembly challenges of microsystems and MEMS for different applications. Hermetic and vacuum micropackaging, wafer-level packaging and bonding, and miniature sealed interconnection and feedthrough technologies will be reviewed. Results from long-term accelerated testing, and from in-situ tests, especially in biological hosts, will also be discussed. Issues and challenges facing packaging of MOEMS will be discussed.

Micropackaging technologies for integrated microsystems: applications to MEMS and MOEMS

A new approach to vacuum packaging of micro-machined resonant, tunneling, and display devices is covered in this paper. A multi-layer, thin-film getter, called a NanoGetter, which is particle free and does not increase the chip size of the microsystem has been developed and integrated into conventional wafer-to-wafer bonding processes. Hermetic electrical feedthroughs are also provided as part of this total-solution technology. Experimental data taken with silicon resonators is presented in which Q values in excess of 21,000 have been obtained. Applications for this technology include gyroscopes, accelerometers, displays, flow sensors, density meters, infrared (IR) sensors, microvacuum tubes, radio frequency microelectromechanical systems (RF-MEMS) and pressure sensors.

Chip-level vacuum packaging of micromachines using NanoGetters

The fabrication and reliability of a solder wafer-to-wafer bonding process is discussed. Using a solder reflow process allows vacuum packaging to be accomplished with unplanarized complementary metal-oxide semiconductor (CMOS) surface topography. This capability enables standard CMOS processes, and integrated microelectromechanical systems devices to be packaged at the chip-level. Alloy variations give this process the ability to bond at lower temperatures than most alternatives. Factors affecting hermeticity, shorts, Q values, shifting cavity pressure, wafer saw cleanliness and corrosion resistance will be covered.

https://iopscience.iop.org/article/10.1088/0960-1317/11/6/303/pdf

L Jordan

Papers

Wafer-to-wafer bonding of nonplanarized MEMS surfaces using solder