Step height reduction

In the linear regime, an incompressible pad model gives a step height h as a function of time 

In the exponential regime, a compressible pad model results in a step height dependence as 

Duane S. Boning and Okumu Ouma Real pad with deformations 

The model by Grillaert et al. addresses step height dependencies and includes a density dependence. Smith et al. [48] integrated the effective pattern density model described earlier with the time and step-height dependent model of Grillaert et al. to accurately predict both up and down area polishing. The resulting analytic expression for the up area amount removed (AR) is 

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The model is empirical and can be used to explain aspects of measured data. Characterization or extraction of parameters would include D°, which is obtained as the average rate over a raised area divided by the blanket rate. Equation (9) is then used to trace the actual thickness evolution in conjunction with step height reduction in Eq. (10). These equations define the feature polish characteristics for relatively large features. For a given array of patterned features, the up polish rates follows the following relationship ... 

Fig. 19. Schematic of local steps and step height reduction as planarization proceeds.

J. Grillaert, M. Meuris, N. Heylen, K. Devriendt, E. Vrancken, and M. Heyns, Modelling Step Height Reduction and Local Removal Rates Based on Pad-Substrate Interactions, Proc. of CMP-MIC, pp. 79-86, Feb. 1998. 

The chemical-mechanical polishing or planarization (CMP) process is a complex interplay between the wafer and the consumables involved. The consumables include slurry, pad, conditioner, and so on. During polishing, the pad carries the slurry and delivers it to the wafer surface. It also transmits the normal and shear forces from the polisher to the wafer. Therefore, polishing pad plays a critical role in the CMP process and influences the outcomes such as material removal rate (MRR), within-wafer nonuniformity (WIWNU), wafer-to-wafer nonuniformity (WTWNU), step height reduction efficiency (SHRE), and defect counts. 

FIGURE 5.6 Comparison of patterned wafer profile and step height reduction efficiency (SHRE) for polishing on ICIOOO and NCP pads (a) Wafer profile before polishing, (b) wafer profile after polishing, (c) SHRE on ICIOOO pad, (d) SHRE on NCP pad. 

FIGURE 5.7 Comparison of normalized patterned wafer step height reduction efficiency (SHRE) for polishing on ICIOOO and various NCP pads. 

Guo Y, Chandra A, Bastawros A. Anal5dical dishing and step height reduction model for CMP with a viscoelastic pad. Electrochem Soc 2004 151(9) G583-G589. 

Fu G, Chandra A. An analytical dishing and step height reduction model for chemical mechanical planarization (CMP). IEEE Trans Semicond Manuf 2003 16(3) 477-485. 

Step Height Reduction Efficiency and Overpolishing Window... 

FIGURE 11.24 Dependency of step height reduction efficiency on electrical conductivity of the liquid used for ECP-DI polishing (from Ref. 32). 

Lim DS, Ahn JW, Park HS, Shin JH. The effect of Ce02 abrasive size on dishing and step height reduction of silicon oxide film in STI-CMP. Surf Coat Technol 2005 5-6 1751-1754. 

Note that the metrics of Figure 2.4 do not give the degree to which the maximum variation in topography (or the step height) is reduced by the planarization process. The step height reduction (SHR) is given by... 

A planarization monitor has been applied to the copper system to investigate pattern dependencies during copper overburden planarization. Conventional profilometry and a noncontact, acousto-optic measurement tool, the Insite 300, are utilized to quantify the planarization performance in terms of the defined step-height-reduction-ratio (SHRR). Illustrative results as a function of slurry, pad type and process conditions are presented. For a typical stiff-pad copper CMP process, we determined the planarization distance to be approximately 2mm. 

In this paper, we report the evolution and quantification of the step-height-reduction-ratio, and subsequently extract the planarization distance for copper CMP for the first time. Conventional profilometry and the InSiteSOO photo-acoustic measurement tool were employed to quantify copper film thickness and topography. The Insite 300 operates upon the transient gradient technique and allows for noncontact, nondestructive metal film thickness measurement [5]. The utility of this metal thickness tool bypasses a number of issues which arise with conventional profilometry. The ability to accurately delineate metal feature edge positions and circumvent stress-induced curvature present in long profilometry scans is of principle importance. 

Profilometry was performed on a wafer which was polished in 40 second time increments under baseline conditions using an experimental grade copper slurry at 4 psi on a perforated IClOOO/Suba rv pad on an IPEC 372MU. Figure 3 displays the extracted step-height-reduction as a function of trench width. Several important conclusions are immediately evident. 

The change of surface profile during chemical-mechanical planarization (CMP) is monitored continuously in this study. The influences fiom pattern dependency and substrate effects are discussed. Step height reduction rate is a function of pattern density and down force. The rate decreases with time until planarization is achieved. As the polish approaches the patterns underneath, the interaction between substrate effects and pattern dependency results in the resurgence of step height. The implication of this newly found phenomenon is discussed. 

In this work, we perform a series of CMP experiments on wafers with different pattern densities and width. The evolution of step height reduction is monitored continuously. The interaction between pattern dependency and substrate effects is observed, and the impacts of these effects on CMP process control are discussed. 

Initially, the step height reduction rate is very high due to the high local pressure on top of these up features. As time goes by, the up features are reduced rapidly and the pad starts to touch the down features. As a consequence, the pressure is partitioned between the up and down features, causing a fall in the up removal rate and a corresponding rise in the down removal rate. Finally, planarization is achieved and the removal rate reduced to the blanket removal rate. 

Careful examination over the measured data over different patterns reveals that the step height reduction rate is higher for smaller patterns and larger spacings, both of which indicate that local removal rate is higher for lower pattern densities over the range investigated. The results above are similar to previous studies. 

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