Laser-Generated Ultrasound for Thermosonic Bonding

这个项目的目的是探索使用激光产生的超声波在热超声（TS）键合。TS键合是一种结合热、压力和超声波能量的连接技术，以促进形成牢固的金属-金属键合。它主要用于将键合线连接到封装内的硅芯片上，与其他连接方法相比，它具有许多优点。例如，它不需要额外的材料（例如焊料或粘合剂），它可以在比热压缩粘合更低的温度和压力下进行，比纯超声波焊接更低的超声波功率。TS键合的一个重要潜在应用是倒装芯片组装，这是一种用于先进电子制造的技术。倒装芯片允许未封装的集成电路以面朝下的方式附着在电路板或其他基板上，芯片上的触点垫和基板之间的电连接是通过传导“凸起”来提供的。与其他芯片连接方法相比，倒装芯片组装具有几个优点，例如更高的电气性能，更高的互连密度（每单位面积更多的电气连接），更小的占地面积和更低的高度。基于焊料连接的倒装芯片工艺已经建立了很多年。然而，随着小型化的不断发展，它们在互连密度方面正在接近极限。基于粘合剂粘合的替代方法可扩展到更精细的互连间距，但无法达到许多应用所需的性能或可靠性。TS键合可以构成高度可靠、超细间距倒装芯片技术的基础。然而，到目前为止，开发稳健的工艺已被证明是具有挑战性的，主要是因为它对共面误差和碰撞高度变化高度敏感，这可能导致结合强度不均匀甚至损坏芯片。随着芯片尺寸的增加，这些问题变得更加严重，因此TS倒装芯片被限制在涉及低互连数的小型设备的狭窄应用范围内。我们建议开发一种TS键合工艺，其中使用脉冲激光在特定键合部位局部产生超声波，使用夹在工件和透明键合头之间的牺牲载流子带的受限烧蚀。这种方法将使我们能够以灵活的方式传递超声波能量，允许补偿共面性和凹凸高度误差的可能性。利用该系统，还可以通过激光对界面进行局部预热，从而产生一个整体热负荷非常低的过程。如果成功，这项研究将最终导致下一代倒装芯片技术在电子制造领域的广泛应用。新工艺也应该在其他领域找到应用，如MEMS（微机电系统）和光电子学，在这些领域需要连接精密元件。

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.