Patterning pattern density

The three-dimensional displacements inherent to NIL require resist materials that easily deform under an applied pressure and/or elevated temperature. These resists must have a low viscosity during imprinting, a Young s modulus less than that of the mold, and a low sheer modulus. It should also be mentioned that the resist material should have excellent adhesion to the substrate, provide high-quality, uniform film thickness through deposition via spin-coating, and have sufficient thermal and mechanical properties for subsequent processes. When determining the type of NIL resist to use, one should consider the critical dimensions of the pattern, pattern density, release properties from the mold, required imprint temperature and pressure, etch selectivity for subsequent pattern transfer, and route to eventual removal by dissolution or other processes. 

All of these approaches are intended to address global planarization issues. We close this section with a brief note about an alternative approach that focuses on achieving planarization at the die level. The distributed polish head [37], extends the concept of hydrostatic pressure by integrating polish blocks in the carrier design to minimize the locally the spatial variation in the polish. As shown in Fig. 9, each polish block makes contact with several die, each of which may contain regions of high and low pattern density. 

Fig. 9. Polish thickness evolution profiles illustrating the significance of layout pattern density.

Stine et al. proposed a simple analytic model for patterned feature removal incorporating an effective density determination step [54]. Figure 10 defines terms used in the model, which reformulates Preston s equation [Eq. (2)] as a function of blanket rate K and effective pattern density... 

Fig. 16. Local pattern density across a die evaluated in 40-/im cells.

FiQ. 17. Effective pattern density obtained with an elliptic weighting filter. 

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... 

A key benefit of accurate CMP models that needs emphasis is the capability to optimize layout design before polishing. Post-CMP ILD thickness variation is a serious concern from both functionality and reliability concerns. An effective method of minimizing this effect is the use of dummy fill patterns that lead to a more equitable pattern density distribution across the chip. Evaluation of such schemes before actual product implementation has become a major use of CMP modeling [53]. Dummy fill is also being investigated for front-end processes where shallow trench isolation CMP suffers from substantial pattern dependencies. 

A. Maury, D. Ouma, D. Boning, J. Chung, A Modification to Preston s Equation and Impact on Pattern Density Effect Modeling, Conf. for Advanced Metallization and Interconnect Systems for VLSI Applications, San Diego, CA, Oct. 1997. 

To understand the impact of a CMP process on a certain product with a unique integrated circuit pattern, it is desirable to measure areas with different feature sizes and shapes. Since CMP polish rate may be affected by pattern density, areas encompassing various features should be included in the measurement program. The within-die thickness nonuniformity will indicate the planarization capability of a CMP process. 

Many endpoint detection systems, based on mechanisms, such as those based on reflected optical light [9], spindle motor current [10], pad temperature [11,12], have been used to resolve this problem, with limited success. Some systems may work with blank wafers or wafers with relatively low pattern density (at the STI level, for example), but for the PMD or ILD levels no useful results have been reported. The presence of a pattern at the PMD or ILD levels adds a great deal of complexity to the signals. Currently, use of an endpoint detection system to control the final post-CMP thickness is still a fertile topic for research and development. 

For thick substrates, backscattered electrons from the substrate decrease contrast and the minimum dimension increases to about 20 nm. For thick resists, and samples thick compared to the primary electron penetration range, electron scattering in the resist (forward scattering) and backscattering of electrons from the substrate, become more important than the electron interaction range. In these cases, exposure dose is sometimes altered according to the local pattern density to compensate for variations in the backscattered... 

When dishing occurs in a patterned wafer, the ILD beside the dished area polishes faster than the ILD in isolated field regions. The ILD height difference between patterned and field areas, shown in Fig. 5.20, is called erosion. Experimentally, erosion depends on the pattern density. 

There are several pressures relevant to polishing of patterned wafers. Pm and PiLD are the pressures on the metal and ILD of a wafer with pattern density 9, for applied pressure P and with AP = Pild Pm- Because the metal is recessed relative to the ILD, the pad expands into the cavity and exerts less pressure on the metal than on the ILD. The total force P on a wafer of areaA-w is the sum of pressures on the metal and ILD areas, F = PA-w = Pm A-w + f iLoCl d)Aw It follows that Pm = P — (1 — 0)AP while Pild = P + dAP and... 

Equation 5.10 predicts an exponential decay to the final dishing depths, with the decay rate dependent on various parameters described above. This expression for dishing as a function of time, linewidth, and pattern density is compared to experimental data in Fig. 5.21. 

FIGURE 5.22 Erosion as a function of time and pattern density. Symbols are for data from (a) Figure 3.14 of Tugbawa [78] and (b) Figure 3.16 of Tugbawa [78]. The solid lines are fit to Equation 5.12. 

FIGURE 5.26 Erosion as a function of pattern density after CMP. FAP Polishing with fixed abrasive pad. Slurry Polishing with abrasive-containing slurry and conventional pad [80]. 

Choi J, Dornfeld DA. Modeling of pattern density dependent pressure nonuniformity at a die scale for ILD chemical mechanical planarization. Mater Res Soc Symp Proc 2004 816 K4.4.1-K4.4.6. 

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