Thermal laser separation for wafer dicing

Thermal laser separation (TLS) is used to separate brittle materials using well known basic principles. The range of applications vary from cutting display glass (including laminated glass), cutting the edges in float glass production lines, and the scribing of Al2O3 ceramics. Additionally, TLS wafer dicing is an interesting alternative to the established mechanical dicing saws and has many advantages, compared to other laser technologies. In this paper, the application of the TLS-technology for the separation of semiconductor wafers will be described.

The thermal laser separation (TLS) process uses very simple techniques to separate brittle materials; it is a cleaving process that uses thermal-induced mechanical stress. The process can be explained in two steps. To initiate the process, a very small initial scribe (defect) is required. This defect could be the result of former process steps, e.g., grinding or wire sawing, or - in most cases - it must be made by a tool like a diamond tool tip or a special scribing laser (step 1). This initial scribe gives the cleaving process a well-defined starting point; otherwise, the cleaving process might start at the nearest defect site.

MiddleIn the cleaving step, the material is heated up by a laser with a well-defined quantum of energy (step 2). The heated material expands and radial pressure forces occur in the heated zone. Around this heated zone, tangential tensile stress (TS I) is induced.

The laser focus is directly followed by cooling; as the material cools, it shrinks. Within the cooling zone, additional tensile forces (TF II) are induced (step 3). Within the overlaying zone of both tensile forces (TF I and TF II) - and only at this point in the process - a crack can be opened. So the cleaving process can be done, starting from the initial defect. The result is excellent edge quality for the separated dies

For crystalline materials such as silicon, the separation is complete (step 4). The local maximum of tensile stress is located on the axis between focus of heat and cooling, so the system of forces and strength is a self-centering one. If the crack tends to run faster than the combination of heat and cooling, it runs into the zone of pressure forces and is delayed.