A flexible nanobrush pad for the chemical mechanical planarization of Cu/ultra-low-к materials
© Han et al.; licensee Springer. 2012
Received: 3 September 2012
Accepted: 10 October 2012
Published: 30 October 2012
A new idea of polishing pad called flexible nanobrush pad (FNP) has been proposed for the low down pressure chemical mechanical planarization (CMP) process of Cu/ultra-low-к materials. The FNP was designed with a surface layer of flexible brush-like nanofibers which can ‘actively’ carry nanoscale abrasives in slurry independent of the down pressure. Better planarization performances including high material removal rate, good planarization, good polishing uniformity, and low defectivity are expected in the CMP process under the low down pressure with such kind of pad. The FNP can be made by template-assisted replication or template-based synthesis methods, which will be driven by the development of the preparation technologies for ordered nanostructure arrays. The present work would potentially provide a new solution for the Cu/ultra-low-к CMP process.
KeywordsFlexible nanobrush pad Polishing pad Chemical mechanical planarization Cu/ultra-low-к integration
With the development of semiconductor industry, the feature size of device is scaling down, and the density of integrated circuit (IC) is continuously increasing, as well as the wafer sizes. As a result, the fabrication techniques are facing new challenges. For instance, the conventional silica is replaced by ultra-low-к materials integrating with copper (Cu) for reduction of the dielectric permittivity. Chemical mechanical planarization (CMP), which is thought as the only one that can offer excellent local and global planarization at the same time, has become one of the most important fabrication technologies adopted by the semiconductor industry. However, due to low density, poor mechanical strength, and deficient adhesion properties, ultra-low-к dielectrics may be damaged by stresses applied during the conventional CMP. The pace of incorporating advanced ultra-low-к materials has been slowing down as compared to the original projections[1, 4]. One solution is to reduce down pressure in the CMP process. However, the low down pressure leads to a low material removal rate (MRR) in the CMP process with conventional polishing slurries and pads. Therefore, it is an urgent problem to be solved for the planarization of wafers by CMP under the low down pressure.
Numerous attempts have been made to meet the new Cu planarization requirements due to the use of fragile ultra-low-к materials in the near future. Most of them are focused on slurries[7–11] and the derivative technologies of CMP such as electrochemical mechanical planarization[12–15] and electrochemical mechanical deposition[16–18]. As we know, the polishing pad is one of the most important consumables and plays a critical role in CMP. However, up to now, very few researches have been done on the polishing pad for the low down pressure CMP process of Cu/ultra-low-к materials. Kasai et al. reported a next generation pad with soft materials and smaller pore size (from 2 to 10 μm) to reduce scratch defects. Sung et al. pointed out that ‘dry spots’ of polishing could be caused by this soft pad with smaller pores, and they designed a black pore-free pad with microscope graphite particles impregnated in a polyurethane matrix. Some new kinds of pads potentially used for the low down pressure CMP process have also been reported, such as the eSQ pad based on a compression compliance mechanism and the low-shear surface-engineered pad using ‘pad engineering’ technologies. Most of them are conceptual, confidential, and not fully developed.
Polishing pads[23–30] with free fibers on the surface have been widely studied due to their numerous advantages in the CMP process under normal down pressure, i.e., from 2 to 8 psi. However, the polishing pad with ordered nanofiber arrays on the surface has been seldom reported so far, and its CMP performances under the low down pressure (less than 1 psi or even 0.5 psi) are yet unknown. Previous simulation and experimental works in our group have already indicated that the interaction between abrasive particles and wafer surface has important effects on the CMP performances[31–38]. In the present work, a new idea of polishing pad called flexible nanobrush pad (FNP) has been proposed. A large number of flexible brush-like nanofibers which are supposed to be useful for the low down pressure CMP process of Cu/ultra-low-к materials will be made in the surface layer of the pad. The material removal and planarization mechanisms of the FNP, as well as the possible implementations, have been discussed.
Presentation of the hypothesis
Polishing pad is one of the most important components in CMP, while it is also one of the most poorly understood components. Pad structures and materials have changed little in the past few decades since the CMP technology was used in the semiconductor industry; nevertheless, the evolution is arrived at empirically for the most part.
Furthermore, because of the existence of flexible nanofiber layer, the contact between the pad and the wafer will become more uniform; thus, the potential contact ‘hot spots’ can be eliminated. Slurry can be sucked to the contact area along the nanofibers by capillary force, so ‘dry spots’ of polishing can also be avoided during the CMP process. Therefore, a better wafer surface with reduced scratches can be anticipated. In addition, the residual abrasives and polishing byproducts such as pad debris could be cleaned out by just using ultrasound, rinse, or other means. Hence, it becomes easier to maintain the FNP performance without diamond conditioning used by the conventional porous pad.
Testing the hypothesis
Implications of the hypothesis
As semiconductor technology develops and new materials are introduced for more advanced ICs, novel consumables of CMP must be developed to meet these new requirements. The work puts forward a flexible nanobrush technique for the polishing pad used in the low down pressure CMP process of Cu/ultra-low-к materials. Better polishing performances including high material removal rate, good planarization, good polishing uniformity, and low defectivity are expected to be achieved with such kind of pad. The FNP can be prototyped by template-assisted replication or template-based synthesis methods. It is expected that the work would potentially provide a new solution for the Cu/ultra-low-к CMP process. It is also expected that the work can be driven by the development of the preparation technologies for ordered nanostructure arrays.
GH is a Ph.D. candidate and engages in novel chemical mechanical planarization research. YL is a doctor, assistant professor, and an expert in the field of nanostructures and nanotechnology. XL is a doctor, professor, and an expert in equipment and technology of CMP. JL is a doctor, professor, an academician of Chinese Academy of Sciences (CAS), and director of State Key Laboratory of Tribology (SKLT) and specializes in tribology and nanomanufacturing.
chemical mechanical planarization
flexible nanobrush pad
material removal rate
The authors thank Dr. Guoxin Xie for revising the manuscript critically. The work was supported by the National Natural Science Foundation of China (grant no. 51021064) and the State Key Development Program for Basic Research of China (grant no. 2009CB724201).
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