Pushing the limits of CMP mechanical control
03/01/2007
Any chemical-mechanical planarization (CMP) process can be conceptually separated into chemical and mechanical components. While process models have become very complex-including variables such as the contact area of a pad asperity, the slurry film thickness, and stress levels within the wafer and any covering films-the average material removal rate in a CMP process can still be described by the classic Preston equation:
where ξ is the thickness of the layer removed (m), t is the polishing time (s), p is the nominal pressure (N/m2), υR is the relative velocity (m/s), and kp is the Preston constant (m2/N).
In standard CMP tools, a rotating head holds the wafer and forces it against a rotating porous polyurethane pad through which slurry flows. The head is now a complex pneumatic device with backside pressure often applied to multiple zones across the wafer’s backside to achieve optimal uniformity. The engineered polymer materials used in the retaining ring must also meet high uniformity specs.
The polyurethane pad rides on a platen, and the uniformity of the rotating platen must also be tightly controlled. All tools have a natural harmonic frequency that is typically below 30 Hz, according to Tom Tucker, principle with Laredo Technologies and renowned expert on CMP technology. Higher order harmonic frequencies can induce vibrations at process rotation rates, so the mass of the platen must be as uniformly distributed as possible to minimize deformation.
HP Labs’ Quantum Science Research group is developing an ultra-uniform CMP process for the platinum lines used in its novel cross-bar circuit architecture (see Ed’s Threads 070116; http://www.pennwellblogs.com/sst/eds_threads). This CMP process creates the vertical spacing between cross-bars, and thus the switching characteristics of the molecules within each junction, so process control is extraordinarily demanding.
HP Labs had some of their physicists model, measure, and modify the rotating platen to minimize vibration. They ultimately stuck weights around the platen to “balance” it like a wheel on a car. The ultimate smoothness possible with this finely tuned tool has not been disclosed, but we may assume that it is better than the 0.2nm Ra surface roughness best seen using standard commercial CMP tools.
For good post-CMP wafer surface uniformity, excellent mechanical control is needed in the consumables set, including the porous polyurethane pads and the nanoparticles within the slurry. Slurry particles may be easily filtered to eliminate large excursions.
One solution to particle uniformity is to use composite posts in a fixed-abrasive (FA) pad, as developed by 3M Corp. and distributed by Rohm&Haas. Fixing the abrasive allows for processing with higher relative velocity and lower pressure, providing reduced dishing for certain structures. While the FA pad may be highly uniform, wafer surface topographies can induce pattern-specific variations in local removal rates, which limit overall within-wafer uniformity. Thus, FA CMP processes seem to be limited to working with HDP deposited oxides in regular STI applications.
Most CMP pads used in IC fabrication are now slices from a cylindrical solid “cake” that is cured into solid form with entrapped pores. “Actually when you form a cake, the pores can be very nonuniform,” explained Tucker. With very large pores, agglomerated slurry particles can get trapped and then cause scratches. Maintaining a tight distribution of small pores also lowers the slurry flow requirements.
Applied Materials has worked with Praxair on a more uniform “DuraPad,” which is formed by continuous sheet cross-linking and curing; the basic manufacturing process was originally developed by Madison CMP Group before that company’s acquisition by Praxair in 2002. The new pad also incorporates Praxair’s patented inert-gas foaming process, which optimizes the pad pore structure to limit large excursions (see figure).
Liquid polyurethane is mixed with uniform nitrogen bubbles, and then sprayed onto a belt and cured into a solid sheet with open-celled pores. Round disks for the pads are then simply stamped from the continuous sheet. Venkata R. Balagani, global product manager of CMP consumables for Applied Global Services commented, “There are 4 to 6 key parameters that are controlled to >1.5 Cpk.” Compared to standard CMP pads, the company claims 30% improved lifetime, PM downtown, and thus CoO for this new pad.
“The current state of the art in CMP goes far beyond what was anticipated in the early days in IBM’s labs, when 50% within-wafer uniformity for oxide planarization over aluminum lines was considered good,” explained Michael Fury, who helped develop CMP at IBM in the 1980s and is now managing director of InterCrossIP Management LLC. “So much of our work in those days was empirical, with limited understanding of the removal mechanisms. Now after 20 years, the industry’s grasp on CMP process fundamentals is finally reaching the levels necessary to ensure sustainable technology growth.”