Chemical-mechanical Planarization - Working Principles

Working Principles

Typical CMP tools, such as the ones seen on the right, consist of a rotating and extremely flat platen which is covered by a pad. The wafer that is being polished is mounted upside-down in a carrier/spindle on a backing film. The retaining ring (Figure 1) keeps the wafer in the correct horizontal position. During the process of loading and unloading the wafer onto the tool, the wafer is held by vacuum by the carrier to prevent unwanted particles from building up on the wafer surface. A slurry introduction mechanism deposits the slurry on the pad, this is represented by the slurry supply in Figure 1. Both the platen and the carrier are then rotated and the carrier is kept oscillating as well; this can be better seen in the top view of Figure 2. A downward pressure/down force is applied to the carrier, pushing it against the pad; typically the down force is an average force, but local pressure is needed for the removal mechanisms. Down force depends on the contact area which, in turn, is dependent on the structures of both the wafer and the pad. Typically the pads have a roughness of 50µm; contact is made by asperities (which typically are the high points on the wafer) and, as a result, the contact area is only a fraction of the wafer area. In CMP, the mechanical properties of the wafer itself must be considered too. If the wafer has a slightly bowed structure, the pressure will be greater on the edges than it would on the center, which causes non-uniform polishing. In order to compensate for the wafer bow, pressure can be applied to the wafer’s backside which, in turn, will equalize the centre-edge differences. The pads used in the CMP tool should be rigid in order to uniformly polish the wafer surface. However, these rigid pads must be kept in alignment with the wafer at all times. Therefore, real pads are often just stacks of soft and hard materials that conform to wafer topography to some extent. Generally, these pads are made from porous polymeric materials with a pore size between 30-50µm, are consumed in the process, and must be regularly reconditioned. In most cases the pads are very much proprietary, and are usually referred to by their trademark names rather than their chemical or other properties.