CAMP CMP, Day 3: CMP fangs, dual-purpose slurry injector, “true CMP” HB-LED wafer polishing; karaoke time!

by Michael A. Fury, Techcet Group

August 16, 2010 – The 3rd and final half-day of the 15th Annual Clarkson CAMP CMP Symposium progressed with a sustaining attendance that belies the significance of the content value of this meeting. It was gratifying to see a quite respectable audience size even for the final talk.

Click to EnlargeLieve Teugels of IMEC woke us up with horrifying pictures of "fangs" — those nasty copper chemical mechanical planarization (CMP) galvanic corrosion artifacts at the copper-barrier interface. Electrochemical studies were conducted with BTA and another commercial surfactant in development, and Ta barrier with other barrier metals to come.

Srini Raghavan of the U. of Arizona showed how to use sonoluminescence to study the role of dissolved gases in megasonic cleaning, with an eye toward optimizing PCMP and other wafer cleaning. Dissolved CO2 may function as a free radical scavenger that extends the lifetime of ozone in water.

Len Borucki described a joint effort between Araca joint with Entrepix: a slurry injector that rests on the pad, serving a dual purpose of distributing fresh slurry immediately ahead of the wafer carrier and forming a bow wave to divert the used slurry coming around on the platen to the platen edge. Slurry flow was reduced 36% while maintaining removal rate. Under optimized conditions, there is a minimal slurry bow wave at the wafer carrier as slurry flows in a thin film, reducing slurry waste.

Deepak Mahulikar of Planar Solutions talked about the requirements for ultrapure colloidal silica for Cu slurry. Chemical contaminants in the silica have been implicated in corrosion, scratching & fangs in Cu CMP. Planar is producing a colloidal silica using fumed silica as a source material.

James Kelly of IBM Research discussed the correlation of Cu plating levelers with CMP planarization results. Rather than optimizing the leveler to reduce the load on CMP, the motivation for the work was to simplify to copper plating bath in order to rely more heavily on CMP for excess Cu removal. Even though the post-CMP topography showed <10nm difference between leveler and no leveler, the difference in electrical serpentine shorts was almost 100×. Yield results could be made comparable by adding a 20sec touch-up polish to the no-leveler wafers. The conclusion, however, is that if you want to maintain a low Cu plating overburden, the leveler should not be eliminated.

SEE ALSO:
Day 2: Porosity on demand, CMP whack-a-mole, end-of-line acronym soup
Day 1: CMP’s FEOL future, "dark art" defect work, mysterious Cu dendrites

Takeshi Nogami, representing IBM’s Interconnect Group at Albany Nanotech, presented work on CVD Co as a Cu barrier and the resulting CMP implications. Co is an effective barrier for Cu, but not for O2 and H2O. To eliminate Cu oxidation, a PVD TaN/CVD Co/PVD Cu seed scheme was evaluated. Corrosion fangs post-CMP were worse with TaN/Co than with TaN/Ta. Co is not ruled out for 22nm, but additional work is required.

David Merricks from Ferro showed work on sapphire polishing with alumina abrasive for HB-LED applications. Samsung, TSMC, UMC, Micron, and others are entering the LED foundry market to utilize 150mm and 200mm excess capacity. Sapphire single-crystal wafers can be grown and processed like Si up to 200mm. The Ferro alumina slurry replaces a diamond slurry, and delivers ~8μm/hr removal and a final 0.2nm Ra. The alumina process is true chemical-mechanical, whereas diamond is all-mechanical.

The use of combinatorial methods for developing PCMP cleaning chemistries was presented by Steve Bilodeau, reporting on joint work involving ATMI, Intermolecular and IBM (Albany). Complex chemical processes are often highly non-linear, making multivariant techniques not only more effective but essential. Key variables included pH, solvating agent, passivator, corrosion inhibitor, and surfactant at two levels each. Measured responses included data scatter, passivation layer thickness, Cu loss and slurry cleaning. Minimum corrosion inhibitor provided the best cleaning, while pH could be modulated to minimize the passivation layer thickness.

In addition to the many fine papers presented at the Clarkson CAMP CMP Symposium, there were 14 poster papers presented by the students and faculty at Clarkson. The poster titles & professor are provided here; for further information, contact the professor at Clarkson directly.

  1. Effect of polishing pad on tunable removal rates of polysilicon, SiO2 and SiNx films (Babu).
  2. CMP of black diamond films (Babu).
  3. Role of ionic strength, hydroxyl ions & radicals, & abrasives on Ge CMP in H2O2-based slurries (Babu).
  4. A high removal rate Cu slurry for TSV applications (Li, with BASF).
  5. Particle agglomeration in the CMP process (Li).
  6. Development of an amine-free Cu PCMP cleaner (Li).
  7. Electrochemical impedance studies of surface reactions for chemically enhanced CMP of Ta using oxalic acid as a complexing agent (Roy & Babu).
  8. Chemical mechanisms of CMP of Cu & Ta in tartaric acid-based slurries investigated with electrochemical techniques (Roy & Babu).
  9. Study of mechanical properties of polymer pads with AFM & nanoindentation techniques (Sokolov).
  10. Studies on preparation of polymer core — ceria shell particles for CMP evaluation (Partch).
  11. Effects of electrostatic & capillary forces on particle detachment with surface deformation in turbulent flows (Ahmadi).
  12. Particle adhesion & removal mechanisms by hydrodynamic force in CMP cleaning processes (Ahmadi & Ferro).
  13. Electrochemical methods for testing Ta CMP slurries (Suni & Li).
  14. Fundamental studies of Cu ECMP electrolytes (Suni & Li, with Air Liquide).

My attendance at this meeting was motivated by the fact that it was the 15th anniversary of this symposium. Prof. Babu thought it would be nice for me to give a dinner talk on Tuesday evening about the early days of CMP, as I am the individual held most responsible for instigating the CMP programs at Clarkson. I was honored to do so — and I express my thanks to Prof. Babu and his colleagues at Clarkson for developing and sustaining a program that has grown far beyond my expectations.

I also decided to have a little fun with my time on the podium in the relaxed environment at Lake Placid, NY. The lyrics to my ode to chip development can be found online, but interested performers will have to supply their own karaoke music and stage support.


Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail [email protected]

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