Laser-Generated Ultrasound for Thermosonic Bonding

点击翻译按钮获取中文摘要

The aim of this project is to explore the use of laser-generated ultrasound in thermosonic (TS) bonding. TS bonding is a joining technique which uses a combination of heat, pressure and ultrasonic energy to facilitate the formation of strong metal-metal bonds. It is used mainly for attaching bond wires to silicon chips inside their packages, where it offers a number of advantages over other joining methods. For example, it involves no additional materials (e.g. solders or adhesives), and it can be carried out at lower temperature and pressure than thermo-compression bonding and lower ultrasonic power than pure ultrasonic welding. An important potential application for TS bonding is flip chip assembly, a technique used in advanced electronics manufacturing. Flip chip allows unpackaged integrated circuits to be attached to a circuit board or other substrate in a face-down configuration, with electrical connections between the contact pads on the chip and the substrate being provided by conducting "bumps". Flip chip assembly offers several advantages over other chip attachment methods, such as higher electrical performance, higher interconnect density (more electrical connections per unit area), smaller footprint and lower height.Flip chip processes based on solder attachment have been established for many years. However, with the continual drive for miniaturization they are approaching their limits in terms of interconnect density. Alternative approaches based on adhesive bonding are scalable to finer interconnect pitches, but do not achieve the performance or reliability required for many applications. TS bonding could form the basis of a highly reliable, ultra-fine-pitch flip chip technology. However, up to now it has proved challenging to develop robust processes, mainly because it is highly sensitive to co-planarity errors and bump height variations which can lead to bond strength non-uniformity and even damage to the chip. These issues become more severe as the chip size increases, and consequently TS flip chip has been limited to a narrow range of applications involving small devices with low interconnect count.We propose to develop a TS bonding process in which pulsed laser light is used to generate ultrasound locally at specific bonding sites, using confined ablation of a sacrificial carrier tape sandwiched between the workpiece and a transparent bond head. This approach will enable us to deliver the ultrasonic energy in a flexible manner, allowing for the possibility of compensating for co-planarity and bump height errors. With the proposed system it will also be possible to pre-heat the interface locally by laser, yielding a process with very low overall thermal loading. If successful, the proposed research will ultimately lead to a next generation flip chip technology with wide ranging applications in electronics manufacturing. The new process should also find applications in other fields such as MEMS (microelectromechanical systems) and optoelectronics where joining of delicate components is required.