Laser-Generated Ultrasound for Thermosonic Bonding

本计画的目的是探讨雷射超音波在热超音波接合中的应用。TS键合是一种结合使用热、压力和超声波能量以促进形成牢固的金属-金属键合的连接技术。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，它不涉及额外的材料（例如焊料或粘合剂），并且它可以在比热压结合更低的温度和压力下以及比纯超声焊接更低的超声功率下进行。 TS键合的一个重要的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配置附接到电路板或其它衬底，其中芯片上的接触垫与衬底之间的电连接由导电“凸块”提供。与其他芯片连接方法相比，倒装芯片组装具有许多优点，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连接）、更小的基底面和更低的高度。然而，随着对小型化的持续驱动，它们在互连密度方面接近其极限。基于粘合剂接合的替代方法可扩展到更精细的互连间距，但不能实现许多应用所需的性能或可靠性。TS键合可以形成高度可靠的超细间距倒装芯片技术的基础。然而，到目前为止，它已被证明是具有挑战性的开发强大的工艺，主要是因为它是高度敏感的共面误差和凸块高度变化，这可能会导致键合强度不均匀，甚至损坏芯片。这些问题变得更加严重的芯片尺寸的增加，因此TS倒装芯片已被限制到一个狭窄的范围内的应用，涉及小型设备与低互连count.We建议开发一个TS键合过程中，脉冲激光被用来产生超声波局部在特定的键合位点，使用有限的烧蚀的牺牲载带夹在工件和透明的键合头之间。这种方法将使我们能够以灵活的方式提供超声波能量，从而可以补偿共面性和凸块高度误差。利用所提出的系统，还可以通过激光局部预热界面，从而产生具有非常低的总体热负荷的工艺。如果成功，拟议的研究将最终导致下一代倒装芯片技术，在电子制造业中具有广泛的应用。这种新工艺还可以应用于其他领域，如MEMS（微机电系统）和光电子学，其中需要连接精密部件。

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.