Plasma Etching

1. Plasma Etching Josh Gentle Matthew Grund ELEC 6570

2. Overview • What is etching? • Two types of etching • Application of Plasma in etching • Process • Mechanisms • Reactive Ion Etching (RIE) System • Problems

3. Questions • What are the three types of grass like cones and ways to stop the forming of cones? • What are two benefits of the laser inferometer has compared to the optical emission spectrometer?

4. Introduction • What is etching? • the removal of material from the surface by physical and or chemical process • Two classes of etching processes • Wet etching is where the material is dissolved when immersed in a chemical solution • Dry etching is where the material is sputtered or dissolved using reactive ions or a vapor phase etchant

5. Types of Etching • Anisotropic Etching • means different etch rates in different directions in the material. • Isotropic (wet) Etching • means uniform etch rates in all directions in the material. Figure 1: Difference between anisotropic and isotropic wet etching. http://www.memsnet.org/mems/beginner/etch.html

6. Comparison • Review of wet etching characteristics • Wet etching is limited to pattern sizes of 3 microns or less. • Wet etching is isotropic (equally in all directions), can result in sloped walls. • Wet etching requires rinse and dry steps. • Wet etching has an intrinsic contamination potential. • Wet etching can undercut if the resist-wafer bond fails. • Review of dry etching characteristics • Dry etching has lower cost of chemicals • Dry etching has lower impact on the environment • Dry etching has higher cost of equipment • Dry etching has variable selectivity http://planet.tvi.cc/fabian/newpage7.htm

7. Application of Plasma in Etching • Four step process • Formation of active gas species • Transport of the active species to the surface • Reaction at the surface • Pump out the reaction products http://www.gs68.de/tutorials/plasma/node12.html

8. Mechanisms of Plasma Etching Sputter Etching • Ions created by the plasma and propelled by the sheath potentials to knock loose particles • Pressure has to be low http://web.ask.com/redir?bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dplasma%2betching%26o%3d0%26page%3d1&q=plasma+etching&u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d244ddcdb544ddcdb5%26sid%3d344ddcdb544ddcdb5%26qid%3d12D092709F75EE45A75DE585C39D7B48%26io%3d0%26sv%3dza5cb0ddf%26o%3d0%26ask%3dplasma%2betching%26uip%3d44ddcdb5%26en%3dte%26eo%3d-100%26pt%3dPlasma%2bEatching%2b-%

9. Mechanisms of Plasma Etching Chemical Etching • Plasma generates neutral species • Neutral species spontaneously react with substrate material http://web.ask.com/redir?bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dplasma%2betching%26o%3d0%26page%3d1&q=plasma+etching&u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d244ddcdb544ddcdb5%26sid%3d344ddcdb544ddcdb5%26qid%3d12D092709F75EE45A75DE585C39D7B48%26io%3d0%26sv%3dza5cb0ddf%26o%3d0%26ask%3dplasma%2betching%26uip%3d44ddcdb5%26en%3dte%26eo%3d-100%26pt%3dPlasma%2bEatching%2b-%

10. Mechanisms of Plasma Etching Accelerated Ion-assisted Etching • Ion are created by the plasma and accelerated by the sheath potentials which “damage” the surface • Neutral etchant are now able to react more easily with surface http://web.ask.com/redir?bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dplasma%2betching%26o%3d0%26page%3d1&q=plasma+etching&u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d244ddcdb544ddcdb5%26sid%3d344ddcdb544ddcdb5%26qid%3d12D092709F75EE45A75DE585C39D7B48%26io%3d0%26sv%3dza5cb0ddf%26o%3d0%26ask%3dplasma%2betching%26uip%3d44ddcdb5%26en%3dte%26eo%3d-100%26pt%3dPlasma%2bEatching%2b-%

11. Application of Plasma in Etching • Reactive Ion Etching (RIE) Typical parallel-plate reactive ion etching system http://www.memsnet.org/mems/beginner/etch.html

12. Aluminium and Tungsten Metal Etching • Most popular materials used for interconnnection • Chlorine is used for etching and bromine is being investigated • Tungsten has excellent deposition conformability http://web.ask.com/redir?bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dplasma%2betching%26o%3d0%26page%3d1&q=plasma+etching&u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d244ddcdb544ddcdb5%26sid%3d344ddcdb544ddcdb5%26qid%3d12D092709F75EE45A75DE585C39D7B48%26io%3d0%26sv%3dza5cb0ddf%26o%3d0%26ask%3dplasma%2betching%26uip%3d44ddcdb5%26en%3dte%26eo%3d-100%26pt%3dPlasma%2bEatching%2b-%

13. Polymer Etching • Cheaper Fabrication • Lower Processing Temperatures • Good vertical profile mask selectivity is important • Hydrogen generally used for etching • UHF-ECR etchers http://www.oxinst.com/pdf/PolymerEtchArticle.pdf Katasuya Watanabe, Kenetsu Yokogawa, Yutaka Omoto, Hiroyuki Makino, “UHF-ECR Plasma Etching System for Dielectric Films of Next-generation Semiconductor Devices,” Hitachi Review 166-170, Vol.52 (2003).

14. Ashing • Requirements • Ash rate 1-2um/min • Remove resist without contamination • High Temperature Oxygen Process • Uses pure • C • Infinite Selectivity to Silicon • Low Temperature Process with Fluorine • Carried out at room temperature • Inorganic materials can be removed • Low k material ashing http://www.gs68.de/tutorials/plasma/node19.html

15. Selectivity • Is the ability to etch one material and refrain from etching another • Crucial for avoiding line width loss • depends on both chemistry and charged particle bombardment • most demanding in etching gates for MOSFETs Commission on Physical Sciences, Mathematics, and Applications, ”Plasma Processing of Materials: Scientific Opportunities and Technological Challenges,” 13-36, (1991)

16. End Point Detection • Laser interferometer • In-situ etch rate monitoring • Endpoint does not require etch stop layer • Endpoint can be chosen anywhere within the layer (onnce etch rate has been established) • Optical Emission • Monitoring of reactive species or etch by-products provides endpoint signal. • Endpoint relies on etch stop layer. • Scanned monochromator allows full spectrum analysis. http://www.oxfordplasma.de/technols/rie.htm

17. Uniformity • Uniformity is the preservation of both the profile and critical dimension (CD) of the etch across the entire wafer • Reaction by-products are a cause of Irregularities in the etch • To achieve uniformity • Uniform plasma and etching gas distribution in the etching chamber • uniform distribution of etching by-products on the wafer. • Little or no charge damage must occur Shinji Kawamura, Naoshi Itabashi, Akitaka Makino, Masamichi Sakaguchi., “UHF-ECR Plasma Etching System for Gate Electrode Processing,” Hitachi Review Vol. 51 (2002), No. 4

18. Deposits (a) Trapezoidally shaped deposits (b) Horns (c) Bowed Walls (d) Rounded Bottoms http://users.ece.gatech.edu/~alan/Lecture11-Etching%20especially%20Plasma%20Etching.pdf

19. Grass • Formation of Cones • Several Types • Aluminum • Sputtered • Polymer • Solutions • Gold Coating • Eliminating O2 • Reducing Gas Pressure Aluminum Grass Sputtered Grass Polymer Grass http://www.triontech.com/techPapers/RIEGrass.html

20. Trenching • Etch attack at the edge of the gate • Usually observed at low gas pressure • Solved by increasing etch selectivity http://web.ask.com/redir?bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dplasma%2betching%26o%3d0%26page%3d1&q=plasma+etching&u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3da%26uid%3d244ddcdb544ddcdb5%26sid%3d344ddcdb544ddcdb5%26qid%3d12D092709F75EE45A75DE585C39D7B48%26io%3d0%26sv%3dza5cb0ddf%26o%3d0%26ask%3dplasma%2betching%26uip%3d44ddcdb5%26en%3dte%26eo%3d-100%26pt%3dPlasma%2bEatching%2b-%

21. Answers to Questions • Three types of grass and solutions. • Aluminum- eliminating O2 • Sputtered- gold coating • Polymer- Reducing gas pressure • Two benefits Laser interferometer has compared to the Optical emission spectrometer. • Does not require etch stop layer • Endpoint can be chosen anywhere within the layer