Pad-wafer contact

Fig. 20. Two-regime model for pad-wafer contact. In the linear regime the pad is modeled as an incompressible pad which only contacts raised areas. In the exponentitil regime, the pad is assumed to be compressible and able to also contact down areas [14].

Pad-Wafer Contact and Slurry Transport Dual Emission Laser Induced Fluorescence... 

Muldowney GP, James DB. Characterization of CMP pad surface texture and pad-wafer contact. Mater Res Soc S5unp Proc 2004 816 K5.2.1-K5.2.12. 

A wafer scale CMP process will fall into one of the three categories listed above. In the first case, the load is supported almost entirely via pad wafer contact. In the second case the load is supported partially by pad-wafer contact and partially by hydrodynamic pressure on the slurry between the wafer and pad. In the final case, the load is supported entirely by a continuous fluid layer of slurry between the wafer and pad. As discussed by Preston (Section 4.1), polish rate is proportional to pressure. Because each of these modes is likely to distribute pressure differently, the ability of a CMP process to remove material and to planarize will be affected by which mode a given CMP process operates within. 

Slurry fluid layer thickness is important because it will determine if the load is supported by a combination of pad-wafer contact and fluid layer or entirely by the fluid layer. If the fluid... 

Whether CMP occurs as Hertzian indentation or fluid-based wear is not clear and has been the subject of some debate. The difference between the two wear modes is in the slurry fluid layer between the pad and wafer. As discussed in Section 4.2, if the fluid layer is not continuous, then pad-wafer contact occurs. Note, however, that the pad does not contact the wafer surface directly, but rather the pad presses abrasive particles against the surface. In such instances, the pad will drag the abrasives across the surface, resulting in Hertzian indentation. 

For the case of a noncontinuous fluid layer, i.e., partial pad-wafer contact, contact between the pad and the wafer will occur at the pad asperities (Figure 4.25). As the pressure increases, more asperities will come into contact with die surface. The contact properties of a given asperity are derived from Hertz s equations. The contact area, a, and contact load, /, are given by ... 

Figure 4.25 From the case of partial pad-wafer contact, contact between the...

Pad—wafer contact scale ( 1 mm). Pohshing pad surface texture affects local pressure on the wafer surface in this range. Pad asperity height distrihutions are of interest. 

Yeruva, S.B., et al., 2009. Impact of pad-wafer contact area in chemical mechanical polishing. J. Electrochem. Soc. 156 (10), D408—D412. 

Elmufdi, C.L., Muldowney, G.P., 2006. A novel optical technique to measure pad-wafer contact area in chemical mechanical planarization. Mater. Res. Soc. Symp. Proc. 914, E12. http // dx.doi.org/10.1557/PROC-0914-F12-06. 

Sun, T., Zhuang, Y., Borucki, L., Philipossian, A., 2010. Characterization of pad-wafer contact and surface topography in chemical mechanical planarization using laser confocal microscopy. Jpn. J. Appl. Phys. 49, 066501. http //dx.doi.org/10.1143/JJAP.49.066501. 

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