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Open Access Nano Idea

A flexible nanobrush pad for the chemical mechanical planarization of Cu/ultra-low-к materials

Guiquan Han, Yuhong Liu, Xinchun Lu and Jianbin Luo*

Author Affiliations

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

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Nanoscale Research Letters 2012, 7:603  doi:10.1186/1556-276X-7-603


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/7/1/603


Received:3 September 2012
Accepted:10 October 2012
Published:30 October 2012

© 2012 Han et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

Keywords:
Flexible nanobrush pad; Polishing pad; Chemical mechanical planarization; Cu/ultra-low-к integration

Background

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 [1]. 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 [2]. 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 [3]. 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 [5]. However, the low down pressure leads to a low material removal rate (MRR) in the CMP process with conventional polishing slurries and pads [6]. 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. [19] reported a next generation pad with soft materials and smaller pore size (from 2 to 10 μm) to reduce scratch defects. Sung et al. [20] 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 [21] based on a compression compliance mechanism and the low-shear surface-engineered pad [22] 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 [39].

A schematic diagram of the FNP is shown in Figure  1. The FNP includes two layers, i.e., a nanobrush layer and a subpad. The nanobrush layer consists of a flexible nanofiber layer, a fixation layer, and a basal layer. Similar to the conventional porous polishing pad, the FNP can optionally have macro-textures such as grooves in the surface for the purpose of better slurry delivery. The basal layer, made of hard materials, provides a support for the flexible nanofiber layer and the fixation layer. It also maintains a high enough stiffness for the surface layer of the FNP, which is very essential to achieve the good planarization. The subpad made of soft materials enables the FNP to conform to wafer surface flatness variation and achieve good polishing uniformity. The top-hard and bottom-soft structures, inheriting from the conventional stacked pad, can achieve both good planarization and polishing uniformity [40]. The biggest difference between the FNP and the conventional pad is that there are a large number of flexible brush-like nanofibers (i.e., flexible nanobrush) in the surface layer of the FNP rather than micropores or microfibers. The functions of the flexible nanobrush will be discussed in particular below.

thumbnailFigure 1. Schematic diagram of flexible nanobrush pad. (a) Side view and (b) top view.

The most serious challenge under low down pressure for conventional CMP is its low MRR. How is MRR raised under the low down pressure using the FNP? In fact, it has been believed that the CMP system is mechanically limited at low down pressures [41-43]. Meanwhile, the mechanical removal rate depends on how many abrasives on the pad are pressed against the wafer, with the indentation depth of the abrasives being proportional to the applied pressure [44,45]. Therefore, the key to improve the MRR under the low down pressure is to increase the number of abrasives in contact with the wafer and the contact area between the pad and the wafer. From this point, the FNP has been designed. Material removal mechanism on mechanical aspects in CMP by the FNP is shown in Figure  2. By adjusting the types and properties of the nanofibers and slurry compositions, it is believed that nanoscale abrasives in slurry can be ‘actively’ carried by the nanofibers during polishing. The large number of nanofibers can absorb large quantities of abrasives. As a result, the contact frequency between the pad and the wafer can be greatly increased, as well as the contact area. Hence, the MRR can be enhanced to a great content. The abrasive-carrying capability of the FNP is independent of the down pressure, and meanwhile, only a small force is needed to keep the contact between the nanobrush and the wafer. Therefore, a high MRR can also be expected under the low down pressure.

thumbnailFigure 2. Material removal mechanism on mechanical aspects in CMP using flexible nanobrush pad.

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’ [21] can be eliminated. Slurry can be sucked to the contact area along the nanofibers by capillary force, so ‘dry spots’ [20] 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

The crucial difficulty in producing the FNP is how to fabricate large-area flexible ordered nanofiber arrays. The diameter of the conventional porous pad has been up to 1 m. However, it is very difficult to fabricate such a large nanofiber array with present existing techniques. Fortunately, there still have a few approaches to fabricating small-area ordered nanofiber arrays, such as template-based replication methods [46-49] and template-assisted synthesis [50-55], despite these technologies are yet immature. On the other hand, considering the grooves at the pad surface, we can use many small-area arrays (i.e., the small FNP unit as shown in Figure  1) to make up a large-area FNP. We can improve the existing polishing tool to test and optimize the performance of the FNP and develop other matching technologies such as slurries, process parameters, and pad conditioning technologies. As shown in Figure  3, two small area FNP units have already been prototyped by a simple template-assisted synthesis using anodic aluminum oxide membrane and thermoplastic polyurethane solution. In the FNP, as shown in Figure  3a, the nanofibers are clustered and tend to be perpendicular to the pad surface. In another FNP, as shown in Figure  3b, the up ends of the nanofibers are fallen, but the roots (as shown in the inset of Figure  3b) are perpendicular to the pad surface. The preparation, characterization, and evaluation of the prototypal FNP will be detailedly discussed in our future work.

thumbnailFigure 3. Scanning electron microscope images of two FNPs prototyped by a template-assisted synthesis. (a) The surface of one FNP; (b) the surface of another FNP and the cross-section of its nanofiber layer (inset).

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.

Abbreviations

CMP: chemical mechanical planarization; FNP: flexible nanobrush pad; MRR: material removal rate; IC: integrated circuit.

Competing interests

Patent concerning flexible nanobrush pad and manufacturing methods thereof is pending (China patent 201010217079.5, 2010).

Authors' contributions

GH, YL, and JL conceived of the study and participated in its design and coordination. GH drafted the manuscript. YL, JL, and XL were involved in revising the manuscript. All authors read and approved the final manuscript.

Authors' information

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.

Acknowledgments

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).

References

  1. International technology roadmap for semiconductors

    2011.

    http://www.itrs.net/Links/2011ITRS/Home2011.htm webcite

  2. Li YZ: Microelectronic Applications of Chemical Mechanical Planarization. John Wiley & Sons, Inc., Hoboken, New Jersey; 2007. OpenURL

  3. Wang DH, Chiao S, Afnan M, Yih P, Rehayem M: Stress-free polishing advances copper integration with ultralow-k dielectrics.

    Solid State Technol 2001, 44:101-106. OpenURL

  4. Tsujimura M: CMP for Cu processing. In Advanced Nanoscale ULSI Interconnects: Fundamentals and Applications. Edited by Shacham-Diamand Y, Datta M, Osaka T, Ohba T. Springer, New York; 2009:343-357. OpenURL

  5. Wada Y, Noji I, Kobata I, Kohama T, Fukunaga A, Tsujimura M: The enabling solution of Cu/low-k planarization technology. In IEEE 2005 International Interconnect Technology Conference: June 6–8 2005; Burlingame. IEEE, New York; 2005:126. OpenURL

  6. Wada Y, Tsujimura M, Kohama T, Noji I, Kobata I, Fukunaga A: The enabling solution of low k planarization based on the general principle of planarization technologies. In PacRim-CMP 2004: December 6 2004; Tokyo. Japan Planarization and CMP Technical Committee, JSPE, Tokyo; 2004:111. OpenURL

  7. Cheemalapati K, Chowdhury A, Duvvuru V, Lin Y, Tang K, Bian GM, Yao L, Li YZ: Novel pure organic particles for copper CMP at low down force. In 2004 MRS Spring Meeting: April 12–16 2004; San Francisco. Edited by Boning DS, Bartha JW, Philipossian A, Shinn G, Vos I. Materials Research Society, Warrendale; 2004:K1.7.1. OpenURL

  8. Surisetty CVVS, Goonetilleke PC, Roy D, Babu SV: Dissolution inhibition in Cu-CMP using dodecyl-benzene-sulfonic acid surfactant with oxalic acid and glycine as complexing agents.

    J Electrochem Soc 2008, 155:H971-H980. Publisher Full Text OpenURL

  9. Pandija S, Roy D, Babu SV: Achievement of high planarization efficiency in CMP of copper at a reduced down pressure.

    Microelectron Eng 2009, 86:367-373. Publisher Full Text OpenURL

  10. Zhang W, Lu XC, Liu YH, Pan GS, Luo JB: Inhibitors for organic phosphonic acid system abrasive free polishing of Cu.

    Appl Surf Sci 2009, 255:4114-4118. Publisher Full Text OpenURL

  11. Liu XY, Liu YL, Liang Y, Liu HX, Zhao ZW, Gao BH: Effect of slurry components on chemical mechanical polishing of copper at low down pressure and a chemical kinetics model.

    Thin Solid Films 2011, 520:400-403. Publisher Full Text OpenURL

  12. Jeong S, Lee S, Jeong H: Effect of polishing pad with holes in electro-chemical mechanical planarization.

    Microelectron Eng 2008, 85:2236-2242. Publisher Full Text OpenURL

  13. Kondo S, Tominaga S, Namiki A, Yamada K, Abe D, Fukaya K, Shimada M, Kobayashi N: Novel electro-chemical mechanical planarization using carbon polishing pad to achieve robust ultra low-k/Cu integration. In IEEE 2005 International Interconnect Technology Conference: June 6–8 2005; Burlingame. IEEE, New York; 2005:203. OpenURL

  14. Suni II, Du B: Cu planarization for ULSI processing by electrochemical methods: a review.

    IEEE Trans Semiconduct Manuf 2005, 18:341-349. Publisher Full Text OpenURL

  15. Sato S, Yasuda Z, Ishihara M, Komai N, Ohtorii H, Yoshio A, Segawa Y, Horikoshi H, Ohoka Y, Tai K, Takahashi S, Nogami T: Newly developed electro-chemical polishing process of copper as replacement of CMP suitable for damascene copper inlaid in fragile low-k dielectrics. In IEDM 2001: December 2–5 2001; Washington. IEEE, Piscataway; 2001:4.4.1. OpenURL

  16. BaŞol BM, Uzoh CE, Talieh H, Wang T, Guo G, Erdemli S, Cornejo M, Bogart J, Basol EC: Planar copper plating and electropolishing techniques.

    Chem Eng Commun 2006, 193:903-915. Publisher Full Text OpenURL

  17. Stcikney B, Nguyen B, Basol B, Uzoh C, Talieh H: Topography reduction for copper damascene interconnects.

    Solid State Technol 2003, 46:49-54. OpenURL

  18. BaŞol BM, Uzoh CE, Talieh H, Young D, Lindquist P, Wang T, Cornejo M: Electrochemical mechanical deposition (ECMD) technique for semiconductor interconnect applications.

    Microelectron Eng 2002, 64:43-51. Publisher Full Text OpenURL

  19. Kasai T, Nam CW, Li S, Kasthurirangan J, Fortino W, Homma Y, Tanaka S, Prasad A, Gaudet G, Sun F, Naman A: Next generation polish pad tunability on CMP performance. In ICPT 2009: November 19–21 2009; Fukuoka. Japan Planarization and CMP Technical Committee, JSPE, Tokyo; 2009:91. OpenURL

  20. Sung JC, Tso P, Tsai M, Chen Y, Chen P, Hu S, Sung M: ICPT 2009: November 19–21 2009; Fukuoka. Japan Planarization and CMP Technical Committee, JSPE, Tokyo; 2009:426. OpenURL

  21. Carpio R, Pham J, Tolic F, Hymes S, Bajaj R: CMP pad design for ultra-low k compatible Cu CMP process. In VMIC 2006: September 26–28 2006; Fremont. IMIC, Tampa; 2006:438. OpenURL

  22. Deopura M, Hwang E, Misra S, Roy PK: Stress characterization of post-CMP copper films planarized using novel low-shear and surface-engineered pads. In 2005 MRS Spring Meeting: March 28-April 1 2005; San Francisco. Edited by Kumar A, Lee JA, Obeng YS, Vos I, Johns EC. Materials Research Society, Warrendale; 2005:W2.7.1. OpenURL

  23. Budinger WD, Jensen EW: Pad material for grinding, lapping and polishing. US patent 4927432; May 22, 1990. OpenURL

  24. Tolles RD: Substrate polishing article. US patent 6688957; 10 February 2004. OpenURL

  25. Chen ST, Rodbell KP, Chi Hsu OK, Vangsness J, Gilbride DS, Billings SC, Davis K: Polishing pads with polymer filled fibrous web, and methods for fabricating and using same. US patent 2004/0162013 A1; 19 August 2004. OpenURL

  26. Suzuki M, Nakagawa H, Yoshida M, Nishiyama M, Shimamura Y: Polishing pad, process for producing the same and method of polishing therewith. US patent 2006/0199473 A1; 7 September 2006. OpenURL

  27. Nishiyama M, Habiro M, Iwatsuki Y, Hiraoka H: Polishing pad for CMP, method for polishing substrate using it and method for producing polishing pad for CMP. US patent 7374474; 20 May 2008. OpenURL

  28. Feng CC, Yao IP, Hung YC, Tsai KC, Lin CY: Polishing pad having abrasive grains and method for making the same. US patent 2010/0099343 A1; 22 April 2010. OpenURL

  29. Kim WJ, Hwang YN: Polishing pad and method of manufacturing the same. US patent 2010/0173573 A1; 8 July 2010. OpenURL

  30. Hsu OK, Lefevre P: Fabric containing non-crimped fibers and methods of manufacture. US patent 2011/0171831 A1; 14 July 2011. OpenURL

  31. Chen RL, Liang M, Luo JB, Lei H, Guo D, Hu X: Comparison of surface damage under the dry and wet impact: molecular dynamics simulation.

    Appl Surf Sci 2011, 258:1756-1761. Publisher Full Text OpenURL

  32. Huang YT, Guo D, Lu XC, Luo JB: A lubrication model between the soft porous brush and rigid flat substrate for post-CMP cleaning.

    Microelectron Eng 2011, 88:2862-2870. Publisher Full Text OpenURL

  33. Xu XF, Luo JB, Guo D: Nanoparticle–wall collision in a laminar cylindrical liquid jet.

    J Colloid Interf Sci 2011, 359:334-338. Publisher Full Text OpenURL

  34. Si LN, Guo D, Luo JB, Lu XC, Xie GX: Abrasive rolling effects on material removal and surface finish in chemical mechanical polishing analyzed by molecular dynamics simulation.

    J Appl Phys 2011, 109:84335-84338. Publisher Full Text OpenURL

  35. Luo JB: Variation of surface layer during chemical mechanical polish.

    Indian J Pure Ap Phy 2007, 45:403-405. OpenURL

  36. Xu J, Luo JB, Lu XC, Wang LL, Pan GS, Wen SZ: Atomic scale deformation in the solid surface induced by nanoparticle impacts.

    Nanotechnology 2005, 16:859-864. Publisher Full Text OpenURL

  37. Xu J, Luo JB, Lu XC, Zhang CH, Pan GS: Progress in material removal mechanisms of surface polishing with ultra precision.

    Chinese Sci Bull 2004, 49:1687-1693. OpenURL

  38. Lei H, Luo JB: CMP of hard disk substrate using a colloidal SiO2 slurry: preliminary experimental investigation.

    Wear 2004, 257:461-470. Publisher Full Text OpenURL

  39. Steigerwald JM, Murarka SP, Gutmann RJ: Chemical Mechanical Planarization of Microelectronic Materials. Wiley, New York; 1997. OpenURL

  40. Stavreva Z, Zeidler D, Plötner M, Drescher K: Characteristics in chemical–mechanical polishing of copper: comparison of polishing pads.

    Appl Surf Sci 1997, 108:39-44. Publisher Full Text OpenURL

  41. Paul E: A model of chemical mechanical polishing.

    J Electrochem Soc 2001, 148:G355-G358. Publisher Full Text OpenURL

  42. Paul E: A model of chemical mechanical polishing II. Polishing pressure and speed.

    J Electrochem Soc 2002, 149:G305-G308. Publisher Full Text OpenURL

  43. Paul E: CMP or CMP: the balance in chemical mechanical polishing.

    Electrochem Solid St 2007, 10:H213-H216. Publisher Full Text OpenURL

  44. Shi FG, Zhao B: Modeling of chemical–mechanical polishing with soft pads.

    Appl Phys A-Mater 1998, 67:249-252. Publisher Full Text OpenURL

  45. Hernandez J, Wrschka P, Hsu Y, Kuan TS, Oehrlein GS, Sun HJ, Hansen DA, King J, Fury MA: Chemical mechanical polishing of Al and SiO2 thin films: the role of consumables.

    J Electrochem Soc 1999, 146:4647-4653. Publisher Full Text OpenURL

  46. Kim DS, Lee HS, Lee J, Kim S, Lee K, Moon W, Kwon TH: Replication of high-aspect-ratio nanopillar array for biomimetic gecko foot-hair prototype by UV nano embossing with anodic aluminum oxide mold.

    Microsyst Technol 2007, 13:601-606. Publisher Full Text OpenURL

  47. Yanagishita T, Kondo T, Nishio K, Masuda H: Optimization of antireflection structures of polymer based on nanoimprinting using anodic porous alumina.

    J Vac Sci Technol B 2008, 26:1856-1859. Publisher Full Text OpenURL

  48. Grimm S, Giesa R, Sklarek K, Langner A, Gösele U, Schmidt H, Steinhart M: Nondestructive replication of self-ordered nanoporous alumina membranes via cross-linked polyacrylate nanofiber arrays.

    Nano Lett 2008, 8:1954-1959. PubMed Abstract | Publisher Full Text OpenURL

  49. Ho AYY, Yeo LP, Lam YC, Rodríguez I: Fabrication and analysis of gecko-inspired hierarchical polymer nanosetae.

    ACS Nano 2011, 5:1897-1906. PubMed Abstract | Publisher Full Text OpenURL

  50. Lee W, Jin M, Yoo W, Lee J: Nanostructuring of a polymeric substrate with well-defined nanometer-scale topography and tailored surface wettability.

    Langmuir 2004, 20:7665-7669. PubMed Abstract | Publisher Full Text OpenURL

  51. Chen GF, Soper SA, McCarley RL: Free-standing, erect ultrahigh-aspect-ratio polymer nanopillar and nanotube ensembles.

    Langmuir 2007, 23:11777-11781. PubMed Abstract | Publisher Full Text OpenURL

  52. Wu H, Wang W, Yang HX, Su ZH: Crystallization and orientation of syndiotactic polystyrene in nanorods.

    Macromolecules 2007, 40:4244-4249. Publisher Full Text OpenURL

  53. Martín J, Mijangos C: Tailored polymer-based nanofibers and nanotubes by means of different infiltration methods into alumina nanopores.

    Langmuir 2009, 25:1181-1187. PubMed Abstract | Publisher Full Text OpenURL

  54. Palacios R, Formentín P, Santos A, Trifonov T, Pallarés J, Alcubilla R, Marsal LF: Synthesis of ordered polymer micro and nanostructures via porous templates. In 7th Spanish Conference on Electron Devices: February 11–13 2009; Santiago de Compostela. Edited by Loureiro AG, Iglesias NS, Rodriguez MAA, Figueroa EC. IEEE, New York; 2009:424. OpenURL

  55. Del Campo A, Arzt E: Fabrication approaches for generating complex micro- and nanopatterns on polymeric surfaces.

    Chem Rev 2008, 108:911-945. PubMed Abstract | Publisher Full Text OpenURL