Ultrasonic Bonding of Ag Ribbon on Si Wafers With Various Backside Metallization

Introduction

Semiconductor chips are connected electrically to other components or systems in order to function. Such electrical connections are critical to electronic packaging and are accomplished by utilizing various forms of conductors to build up the signal or power connection pathways within a single package. Therefore, the bonding should provide a decent electrical interconnection that will not decay with time, while providing excellent performance of the semiconductor chips. Wire bonding is one of the standard interconnection techniques for electrically connecting microchips to the terminal of a chip package or directly to a substrate (Harman, 2010). Generally, the wire bonding technology can either be categorized by the wire bonding method (ball–wedge or wedge–wedge) or the bonding mechanism that creates the metallic interconnection between wire and substrate (thermo-compression, ultrasonic or thermosonic) (Jung et al., 2019).

Each bonding method entails a range of advantages and drawbacks; therefore, the decision to use one of them has to be made with consideration of the application. A recent industry survey has shown that about 90% of all electronics packages and assemblies are made of ball bonds, while the rest are produced with wedge bonds. The most established material for bond wire is gold alloy (> 90% Au), usually doped with elements such as beryllium to increase the strength and ductility so as to improve properties such as wire loop height, elongation at break, temperature strength, breaking load or tensile strength (Simons et al., 2000). Meanwhile, different types of bond wires have been designed to adapt to technological niches such as high-current applications, assemblies restricted to low processing temperatures, or bonds with enhanced mechanical strength. Some common bond wires are aluminum wire, palladium coated copper wire, and silver-alloy wires. The wire sizes are typically 0.6 to 2 mils; however, for power devices, larger diameter wires ranging from 5 to 20 mils in diameter are used for a higher current carrying capacity (Zhou et al., 2023). Thermosonic bonding enhances the reliability of previous mechanisms by preheating the bonding wire in advance of the ultrasonic cycle to reduce cracking issues in Si chips. As an alternative to thermocompression bonding, thermosonic bonding has been broadly used for Au gold wire bonding of ICs in both packages and multichip modules. In power electronic packaging, however, most of the wire diameters are around 300–500 μm due to the high voltage and current applied (Liu, 2012). The formation of the ball bond in the thermosonic bonding method becomes rather tricky when the wire diameter increases, and this challenge has given rise to efforts to achieve bonding through ultrasonic bonding (Maeda & Takahashi, 2013). Ultrasonic bonding can be used to join a wide variety of metals (Neppiras, 1965), rendering it a popular bonding technique today. However, the operation requires careful handling to prevent mechanical damage, and the acoustical properties of the joining materials can change causing variations in strength.

As for wire materials, metals with high electrical conductivity and a high diffusion rate, such as Cu (Lim et al., 2014), Au (Wulff et al., 2003), Ag (Chen et al., 2022), and Al (Schneider-Ramelow & Ehrhardt, 2016), are commonly used for wire bonding. Among these metals, gold wire is a very attractive choice because this metal is fully utilized in ultrasonic, mechanical force, and heat processes. Moreover, Au wire is profoundly electrically conductive, almost a significant degree more so than other metals. Au has much higher oxidation resistance than other metals and is relatively soft, which is important for delicate surfaces.