Chemical mechanical planarization (CMP) is one of the key enabling
technologies in the semiconductor manufacturing industry today for
the fabrication of extremely smooth and flat surfaces on a variety of
semiconductor substrate materials, and processs details are discussed
in Chapter 10. In order to meet the requirements of current lithography
tools which require extremely stringent tolerances for flatness
and planarity, CMP is capable of planarizing a 300 mm (current industry
standard) diameter wafer achieving surface roughness on the
order of 1–2 nm Ra and global planarity well below 0.5 μm. However,
CMP has also become one of the key bottleneck or roadblock
issues in semiconductor manufacturing today188. The decreasing line
widths of semiconductor devices require new materials, such as
copper and the so-called low-k dielectrics, which further challenge
the process. Preferential polishing rates of adjacent materials, or surface
features resulting from previous manufacturing steps, often lead
to defects such as dishing which frustrate efforts to obtain planarity.
The abrasive slurry can also induce defects such as surface contamination,
scratches, slurry residue, etc., hence predicating the need for
a reliable means of monitoring the CMP process.
Sources of AE at the pad asperity/surface interface are believed
to dominate the measured AE signal, and are diagrammed
schematically in Figure 8.21. AE generation due to plastic deformation
induced by abrasive particle interaction with the wafer surface
is believed to be a primary component of the total AE signal, with
AE generation via elastic contact (stick-slip mechanisms, etc) between
the wafer and pad asperities also contributing. At the macroscale,
friction and rubbing between two surfaces (such as at the wafer/
pad/retaining ring level) are potential sources; surface asperities
come into contact and are elastically and/or plastically deformed,
and possibly even welded together. As the surfaces slide over one
another, these asperities are deformed further and possibly even
fractured. A schematic of the AE generation mechanism at the
atomic scale is shown in Figure 8.22, where individual atoms are