Customizing CMP
10/01/2004
The increasing number of CMP consumables suppliers has led to many possible combinations of pads, copper, and barrier slurries. When packaged together in one process, these components must be compatible; however, the burden for ensuring such performance is falling more and more on materials suppliers. While many fabs once managed the CMP process entirely in-house, this new level of intricacy and required expertise is expediting a shift among IC manufacturers to move away from these "homebrews" and look to materials suppliers for development of new slurry and pad technologies.
IC manufacturers are also looking to suppliers to develop solutions that can address universal issues such as defectivity, planarization, and selectivity, and also easily integrate with unique manufacturing processes as the industry moves to the 65nm node. In concert with this trend, some suppliers of CMP consumables are developing custom formulations and advanced abrasives.
Challenges for copper and barrier
To address the challenges posed by porous low-k materials, process trends are shifting to lower-pressure and lower-speed polishing, and barrier slurry formulation is becoming increasingly specialized. CMP slurries must perform effectively in this new regime; for example, slurries are being developed and optimized to achieve acceptable removal rate in a <1psi polishing range.
With the large variety of materials in development — including spin-on and CVD, capped and uncapped (SiN and SiON, SiC) — the introduction of sacrificial layers for topography correction, and the use of various barrier metals (Ta, TaN, TiN, and WN), the range of potential required slurry characteristics is broadening significantly.
Suppliers are also seeing a reduction in barrier-slurry abrasives to reduce defect levels. One approach is the development of tunable slurries. A barrier slurry can comprise multiple components — up to three or more — where each component is known to affect performance on Cu, low-k, cap, and barrier relatively independently. The end user mixes these in ratios that achieve the desired results for a given application.
Abrasive technologies
Many companies are moving toward low-abrasive and abrasive-free slurries, which cause the chemical composition to become more complex. Among other ingredients, these slurries may contain oxidizers, chelating agents, surfactants, and corrosion inhibitors. With this increase in chemical activity, careful formulation is needed to minimize copper corrosion and etching. In the case of abrasive-free slurries, the removal rates and planarization are more chemically driven but still require the pad to provide mechanical abrasion.
Other efforts are being made to enable gentler polishing and lower defectivity. One such abrasive, engineered by JSR, is composed of a compressible polymer core with a chemically coated inorganic shell of conventional abrasives. This shell can be composed of various inorganics, such as silica, ceria, or alumina, depending on the application. Scratches can be generated when standard abrasives are subjected to unusually high local pressure during the polish process. By implementing this type of abrasive, such as JSR Soft·brasives, the locally high pressure on the Cu film becomes distributed due to the compressible polymer core, resulting in lower defectivity.
The physical characteristics of these abrasives can be controlled and adjusted through changes in the materials used for the core and shell, ratio of core volume to shell, and the percent coverage of inorganic layer. By optimizing all these factors, a particle can be tuned to affect removal rate and defectivity.
 Figure 1. Composite abrasives. |
Other developments in unconventional abrasives are underway at Clarkson U., where much work has been done on mixed abrasive systems [1]. In such a system, blending two or more types of abrasive in a slurry has shown improvements in removal rate, selectivity, and stability (Fig. 1). In another composite abrasive approach, the inorganic abrasive is used at the core of the engineered particle and coated on the outside for soft polishing [2].
Pad technology: hard vs. soft
Efforts are also being made to improve pad performance. Ideal CMP pad performance will provide excellent defect-free planarization. Hard pads are historically better for planarization but can increase the number of defects. Soft pads can decrease defects but have poor planarization performance. To combine the positive characteristics of both types, JSR has developed filler pads with water-soluble particles (WSP) dispersed in the pad matrix (Fig. 2).
 Figure 2. a) Diagram and b) SEM photo of water-soluble particles. |
When exposed to a wet polishing environment, the particles on the pad surface dissolve, creating a thin, textured, compressible layer at the surface. In the pad bulk, the WSPs remain solid. The soft surface and firmness of the bulk material provide the characteristics of both a soft and hard pad. WSP size and distribution can be controlled, yielding consistent performance throughout the pad lifetime.
While CMP vendors are formulating solutions for leading-edge technology, most do not have the latest and greatest polishers or metrology tools. Although trends are moving toward low-downforce polishing for low-k, the current installed base of polishers generally is not designed for operating in a <1psi range.
The introduction of low-k materials also increases difficulty because hydrophobic low-k surfaces can leave watermarks, which translate into defects. As a solution, equipment vendors are developing alternative low-k cleaning methods.
Emphasis on defect reduction means that additional optimization is needed during the material development cycle. This often leads to the creation of custom slurry formulations, which can be capital-intensive for the materials supplier.
Acknowledgment
Soft·brasives is a trademark of JSR Microelectronics.
References
- A. Jindal, S. Hegde, S.V. Babu, "Mixed Abrasive Slurry: A Study on Metal CMP," Semiconductor FabTech, 16, p. 239, 2002.
- S.-H. Lee, Z. Lu, S.V. Babu, E. Matijevic', "Chemical-Mechanical Polishing of Thermal Oxide Films Using Silica Particles Coated with Ceria," J. Materials Res., 17, p. 2744, 2002.
Halbert Tam received his BS in chemical engineering from the U. of California at Berkeley and is an applications engineer at JSR Micro Inc., 1280 N. Mathilda Ave., Sunnyvale, CA 94089; ph 408/543-8906, e-mail [email protected].