Topography Planarization

FIGURE 6.6 Chemical and mechanical rate constants and rate ratio for the data in Fig. 6.4. 

Topography planarization differs in several respects from blanket wafer polishing. Topography height variations over the wafer surface induce variations in contact local pressure high areas have higher contact pressure, and low areas may see little or no contact pressure. At equilibrium, the integral of the contact pressure over the wafer surface must equal the applied load. 

Because of this, pressures over the wafer surface are coupled, complicating the problem of determining the pressure distribution over patterns. Pressure variations should also affect the heating of asperity summits since the summit temperature should decrease when the summit passes over a low or noncontacting area on the wafer. Finally, as layers are uncovered, the coefficient of friction will in general change, further affecting the temperature distribution on the wafer. 

We describe next what happens to trench topography contact mechanics for isolated trenches of different widths and depths in a severely mechanically limited process at constant COF in order to illustrate different behavioral regimes (Fig. 6.7). 

For very wide trenches (a large multiple of the pad thickness, depending on the hardness of the pad), the pad conforms easily to both the trench sides and the bottom, and except very close to the sides, the contact pressures and polishing rate are nearly equal. The planarization rate (the rate at which the step height is reduced) is therefore very low, and we could describe the feature as being larger than the pad planarization length. 

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Among numerous planarization technologies, CMP provides a global planarization of topography with a low post-planarization slope [97,98]. It can also dramatically reduce topographical variations to a degree not possible with any other planarizing process [97,99,100]. 

Sediments of Tertiary and Quaternary age, including volcanic ash and aeolian materials, make up the parent material of the soils. In the more arid parts of the Andean System (the coastal plain of Peru and Chile, and the Altiplano of Bolivia) the topography is level. The Altiplano is a very large closed basin with numerous salt flats. In northwestern Argentina, the planar topography is broken by mountains composed of Precambrian rocks and Quaternary sediments. 

Atomic force microscopy (AFM) is a commonly employed imaging technique for the characterization of the topography of material surfaces. In contrast to other microscopy techniques (e.g., scanning electron microscopy), AFM provides additional quantitative surface depth information and therefore yields a 3D profile of the material surface. AFM is routinely applied for the nanoscale surface characterization of materials and has been previously applied to determine surface heterogeneity of alkylsilane thin films prepared on planar surfaces [74,75,138]. 

Rothman (52) investigated the planarization of polyimide films over features tens of micrometers in size and separation. Bassous and Pepper (55) studied planarization of PMMA and AZ1350J over features pertinent to Si wafer processing. A mechanical stylus was used to determine the topography of the wafer and the corresponding surface variation of the resist thickness as shown in Figure 29 where a 1.7 - pm thick AZ1350J layer was spun on steps of different space and width combinations. 

Stylus profilometry is a very simple and powerful tool in CMP. Profilometry can be used to determine the surface planarity change before and after CMP. Basically, in this technique, a stylus scans across a pattern feature in contact with a wafer, while the Z motion (height) of the stylus is monitored. This Z motion signal reflects the surface topography scanned. 

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