Dry Etching of Copper Using Plasma

1. Dry Etching of Copper Using Plasma Kejun Xia Semiconductor TCAD Lab. Auburn University, AL Oct 19, 2003

2. Outline • Background Introduction > Why Copper & low k for IC Interconnection > Some methods of getting fine copper lines • Halogens Plasma Etching > Chlorine Plasma Etching > Basic problems and solutions > Chlorine-Argon Plasma Etching > Using Iodine-containing substance

3. Questions • What’s the function of Argon in Chlorine-Argon plasma etching? • Why is the process gas preferable in some case to detect the starting and ending point other than CuCl when using Optical Emission Spectroscopy Process monitoring ?

4. Introduction • Interconnection, The veins of ICs > line RC delay prevail over gates delay • For today’s technology node and beyond, Aluminum is no longer suitable > relatively high resistance (2.66um-Ohm) > Electromigration

5. Electromigration Hillock formation Whisker bridging Accumulation & depletion http://www.mse.berkeley.edu/groups/doyle/serdar/RESEARCH/copper.pdf

6. How about Copper/low k INTERCONNECT OF NEW MILLENNIUM: COPPER BY SERDAR AKSU, FALL 1999

7. SEM pictures of Cu interconnection http://www.mse.berkeley.edu/groups/doyle/serdar/RESEARCH/copper.pdf

8. Ways to get fine Cu lines • Subtractive etch Dry etching using Chlorine Plasma Main problem is low rate, improved with several methods • Damascene (CMP) Currently used Unavailable when feature size down to 0.1um • Thermally Produce the CuO, then etch it away with an organic acid (hfacH) High etching rate of CuO, 1um/min at 423K Low rate of oxidation and the incompatibility with plasma Chlorine Plasma’Copper Reaction in a New Copper Dry Etching Process Journal of The Electrochemical Society, 148 ~9! G524-G529 ~2001

9. Chemical Mechanical Polish Single/dual Damascene Presently used Difficult when feature size down to 0.1um http://www.mse.berkeley.edu/groups/doyle/serdar/RESEARCH/copper.pdf

10. Dry Etching system http://www.ifm.liu.se/~petas/mikrosystem/Lectures/Lectures_files/L2Dryetching.pdf

11. Main Difficulty in Etching Cu with Chlorine plasma CuClx, the main reaction product, has a low volatility.

12. Morphology of CuClx as a function of Chlorine Plasma process parameters Journal of The Electrochemical Society, 148 ~9! G524-G529 ~200112-20

13. Time 1 min 2 min at 25 0C, 20mTorr, 600W, Cl2 20sccm. Grain size and porosity increases with time because of ion bombardment ppt:13-21Journal of The Electrochemical Society, 148 ~9! G524-G529 ~2001

14. Time CuClx remain increasing by enhancement of Cl

15. Pressure 20mTorr 100mTorr Ion bombardment play an important role

16. Pressure The thickness of CuClx decreases becoz of ion bombardment, but Why should Cathode Self-Biased Voltage drop with increasing pressure? The ratio of cathode area to anode area increases.

17. Cathode self-biased voltage:ion acceleration energy

18. Power 300W 600W Higher etching rate

19. Power Vsc=(VRF)pp/2-Vp

20. Substrate temperature 25oC 250oC

21. Substrate temperature Why has a peak for CuClx ? Below 150oC, Cl transport rate and reaction rate increase; Above 150oC, Evaporation of CuClx dominates.

22. How to remove CuClx ? • High substrate temperature 473-523K required in order to adequately remove this product. Thus it seems this will not be used for the sake of thermal budget in ICs fabrication. • High density plasma source, Adding Argon • Ultraviolet, Infrared or laser

23. Chlorine-Argon plasma for Cu etching Comparison of etching rate between with and without Argon Russian Microelectronics, Vol. 31, No. 3, 2002, pp. 179–192. Translated from Mikroelektronika, Vol. 31, No. 3, 2002, pp. 211–226.

24. Role of Argon • Accelerate bulk generation of atomic chlorine by electron-impact dissociation Cl2+Ar(3P0,3P1,3P2)-->Cl+Cl+Ar • Accelerate removal of reaction products from surface active centers under the action of incident ions and UV photons

25. Process monitoring by Optical Emission Spectroscopy (OES) 435.8nm CuCl band at different temperatures

26. Ultraviolet Light Irradiationassisted Low temperature Cu etching http://mirine.kaist.ac.kr/research/cu_1.html

27. Actions under UV irradiation

28. Desorbed gas species in different CuClx Not all of the component desorbed

29. SEM photographs after etching

30. Schematic of ICP with a collimated UV lamp (300-400nm) http://mirine.kaist.ac.kr/research/cu_1.html

31. Etching rate with Power Etching rate is approximately five times higher than that without UV Journal of The Electrochemical Society, 146 (8) 3119-3123 (1999)

32. Effects from Polycrystalline characteristic of copper during plasma etching 200nm width copper lines with a granulated surface http://www.infotech.tu-chemnitz.de/~zfm/eng/research/fokum.html

33. (1) Higher reaction rate at the grain boundary Result in unsmooth profile

34. (2) Ion Channeling Effect in plasma etching Etching rate 2 Larger than Etching rate 1, consequently Etching deviates from the vertical direction.

35. SEM pictures of Ion channeling Dark area is <110>plane which has a least atomic density. In the case of multilayer interconnection, this is greatly harmful Decrease uniformity Decrease etching selectivity Dry Etching Considerations for Copper Metallizations V.V.Makarov, et dl, NPTest Inc., 150 Baytech Dr. San Jose, CA D.P.Griffis, P.E.Russell, North Carolina State University, 1010 Main Campus Dr. Raleigh, NC

36. One Solution • preliminary ion bombardmentation beam incidence different from normal, so that grains showing strongest channeling and therefore most resistive for etching under normal ion bombardment are effectively amorphized/etched. (short time) Without ion preliminary bombardment With ion preliminary bombardment

37. Using Iodione-containing substances for copper etching • CH3I+HI using RF or DC discharge. • CuI has a high volatility • High etching rate. 60nm/min at 353K. Hampden-Smitch, M.J. and Kodas, T.T., Copper Dry Etching: New Chemical Approaches, Mater.Res.Bull,1996,no.6,p.39

38. Conclusion • Cu/low k interconnection has its great advantage • CuClx has a low volatility. Ion bombardment and UV photon can assist to remove CuClx at room temperature. • Chlorine-Argon plasma has a higher etching rate than Chlorine plasma itself.

39. Answers • What’s the function of Argon in Chlorine-Argon plasma etching? Do favor to increase the concentration of atomic Chlorine; Accelerate removal of products With bombardment.

40. Answers Why is the process gas preferable in some case to detect the starting and ending point other than CuCl? CuCl has a low saturated pressure at room temperature, thus can not be detected accurately by OES.