Dusty Plasmas…………

1. Dusty Plasmas………… Presented by- Md Monirul Islam For: ELEC 7730 Department of Electrical & Computer Engineering Auburn University, AL

2. Introduction Forces acting on dust particles Sources and growth of particles Diagnostics of dusty plasma Electron microscopy Optical characterization of particle Technology impacts of dusty particle Conclusion Presentation Outline:

3. Questions: • What are the forces acting on dust particles in a plasma processing system? • Why electric force is dominant?

4. Introduction: • RF plasma processing is conducted inside a sealed chamber which is reduced to a medium vacuum before selected gases are introduced. • The gas or mixture of gases is then energized into a plasma by an AC electrical field which is generated between an array of electrodes. • The plasma then reacts at the surface of any material placed inside the chamber, modifying the surface and removing contamination which is turned into volatile molecules http://www.hybrid-tech.co.uk/hybrid2.htm

5. Intro… • Particulate contamination formed in plasmas for industrial applications is considered nowadays to be the limiting factor for production yield and device reliability. • They can cause local perturbations in the plasma characteristics which adversely effect processing. H. M. Anderson and S. B. radovanov “Dusty Plasma Studies in the Gaseous Electronics Conference Reference Cell” J. Res. Natl. Inst. Stand. Technol. 100, 449 (1995)

6. Forces acting on dust particles • Dust particles are subject to various forces which confine them in the plasma or drag them to the walls or to the pump ports. • Various forces are- • Electrostatic force • The force that dust particle experiences in a plasma in the presence of a non-zero macroscopic field ( for example- in the pre-sheath field of a glow discharge). Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

7. Forces acting on dust particles • Ion drag • The ion drag force is due to momentum exchange between positive ions and a dust particle • Important when ion flux is large • Two components • One due to the momentum transfer to the dust particle when as ion is collected • The other one due to momentum exchange during the electrostatic Coulomb interaction between a dust particle and a positive ion deflected by the potential around it. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

8. Forces acting on dust particles • Neutral drag • When moving through a neutral gas experiences a drag force due to momentum transfer during collisions with atoms or molecules. • Thermophoresis • For non-uniform neutral gas temperature, the momentum transfer rate during collisions between the gas molecules and a dust particle is larger on the hot side of the dust particle. • There will be a net momentum transfer from the gas to the dust particles • The resulting force is called Thermophoresis • Proportional to the temperature gradient Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

9. Forces acting on dust particles • Gravity • The gravitational force is generally small with respect to other forces in plasma processing for dust particles in the micrometer range or smaller • However, becomes important after discharge extinction since the electrostatic and ion drag forces quickly drop to zero. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

10. Forces acting on dust particles • Electrostatic • Confining force • All other • Accelerating the dust particles towards outside the plasma Fig: Forces acting on dust particles in a plasma processing tool H. M. Anderson and S. B. radovanov “Dusty Plasma Studies in the Gaseous Electronics Conference Reference Cell” J. Res. Natl. Inst. Stand. Technol. 100, 449 (1995)

11. Forces acting on dust particles • Order of magnitude of the different forces • The electric force is proportional to the particle radius (a) • The ion drag, the neutral drag and thermophoretic forces are proportional to the square of the particle radius • The gravitational force is proportional to the cube of the radius • It is therefore clear that the electric force will be dominant for small particle. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

12. Forces acting on dust particles • For capacitively coupled RF plasma • 100 mtorr of Ar • 5 × 109 cm-3 – plasma density • 500 K - ion and neutral temperature • 3 eV – electron temperature • 30 V/cm – local electric field • 0.05 mA/cm2 – ion flux to the electrode • 30 cm/s – velocity of gas flow • 10 K – gas temperature velocity over 1cm Assume that the ion velocity is mobility limited Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

13. Forces acting on dust particles • A rough estimate of the different forces for 100nm radius dust particle of mass density 2g/cm3 is the following: • Electrostatic – Fes ~ 2 × 10-13 N • ion drag - F0+d ~ 5 × 10-14 N • Neutral drag - FNd ~ 10-15 N • Thermophoretic - Fth ~ 10-15 N • Gravitational - Fg ~ 10-16 N Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

14. Sources and growth of particles • A four-step scenario of particle growth • Formation of primary clusters • Nucleation and cluster growth • Coagulation or agglomeration • Particle growth • During the cluster growth and nucleation stage the small particle containing at most 105 atoms are either neutral or singly charged anion clusters. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

15. Sources and growth of particles • After coagulation the macroscopic particles bear a multiple negative charge and one enters the dusty plasma physics regime • Then macroscopic particle keep growing independently up to μm sizes by condensation of neutral or ion monomers. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

16. Schematic illustration of the four steps particle growth Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

17. Source of particle formation • Thermal plasmas: Arcs and Flames • The source material is injected in power form as a liquid spray • Glow discharge CVD and Plasma Polymerization: • Low-pressure glow discharges of molecular gases, plasma polymerization of organic molecules have been considered as trouble-making by-product Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

18. Source of particle formation • Sputtering and Reactive Ion Etching: • Atoms sputtered or etched from the electrode materials are the primary source of nucleation, even if the reactive gas may participate in the particle growth at a large stage. • Other sources of particle: • Polymeric residues from the walls • Due to shock induced ejection (so-called ‘tool-tapping test’) of dust • Weakly bond residues • Flake on the walls • Stress-induced spontaneous delamination of a deposited film Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

19. Diagnostics of a dusty plasma • The most relevant issues for investigation: • The presence of Particle • Density • Size and location in the discharge • Both in-situ and ex-situ methods can be used for detection technique. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

20. Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

21. Electron Microscopy • Extensively used technique to obtain information on particle size and morphology. • 3mm in dia. coated with a 5nm thick carbon layer as electron microscopy grids Fig: Experimental set-up with the multigrid holder . 8 TEM grids prepared with a thin (5nm) carbon layer Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

22. TEM micrograph of particles grown in an argon-silane plasma during 20s: total pressure 100 mtorr, PRF=10 W Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

23. Optical characterization of particle • Laser light scattering (LLS) is the primary means of detecting particles • A laser is directed into the plasma parallel to the electrode at the plasma/sheath interface. • For particle > 0.5μm and for visible light, elastic scattering (Mie) occurs • For particle < 0.5μm, isotropic manner (Rayleigh scattering) take place • The forward scattered light is collected and detected with photomultiplier or CCD video camera H. M. Anderson and S. B. radovanov “Dusty Plasma Studies in the Gaseous Electronics Conference Reference Cell” J. Res. Natl. Inst. Stand. Technol. 100, 449 (1995)

24. Laser light scattering Gary S Selwyn Fig: The rastered laser light scattering experimental set-up, showing the position of the laser, video detection system and the plasma tool. Gary S Selwyn “Optical characterization of particle traps” Plasma Sources Sa. Technol. 3 (1994) 340-347.

25. A photograph of the rastered laser light scattering image showing trapped particle clouds over three closely packed Si wafers on a graphite electrode Gary S Selwyn “Optical characterization of particle traps” Plasma Sources Sa. Technol. 3 (1994) 340-347.

26. Technological impacts of dusty plasma • Applications of low-pressure dusty plasma • Dusty plasmas are able to enhance, in a significant way, power deposition in the volume of low-pressure discharge • Inclusion of dust particles in nanostructured thin films, which could be attractive for optical, electrical and mechanical properties • Plasma chemistry can be modified in successive steps with the help of dust particles (because of efficient trap). Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

27. Technological impacts of dusty plasma • Applications to composite films: • Elasticity is reinforced by embedded small crystallites that can avoid propagation of dislocations • For example: inclusions of TiN nanocrystals in Si3N4 films increase hardness and elastic modulus to their maximum values. • Synthesis of films with small magnetic or metallic particles (data storage, sensors, etc.) Andre Bouchoule “Dusty Plasmas physics, chemistry and technological impacts in plasma processing” John Wiley & Sons , New York, 1999

28. Conclusion: • It is evident that particle “dust” in plasma processing is a threat to device yield. • Still now • the formation of particulate nuclei in plasma, • their growth from few nanometers to several microns, • the charging of particles in a plasma, • the transport of these suspended particles and • the consequences of particulates at the end ---------------are not well understood. • Although dust particles shows promising feature for several application, such as composite materials

29. Answer to the question • Dusty particles are subject to various forces which includes electrostatic force, ion drag, gas flow, neutral drag, thermophoresis, and gravitational force. • The electric force is proportional to the radius of particle. While the ion drag, the neutral frag and thermophoretic forces are proportional to the square of the particle radius. The gravitational force is proportional to the cube of the radius.

30. Thank you