Plasma Assisted Nanofabrication Processes

1. Plasma Assisted Nanofabrication Processes Suiqiong Li ELEC 7730 Instructor Dr. Y. Tzeng

2. Outline • Nanotechnology • Plasma assisted nanofabrication • PECVD for carbon nanotube fabrication • Nanofabrication with scanning nanonozzle • Plasma etching for nanofabrication

3. Questions • What are the typical hydrocarbon sources used in plasma-based growth of CNTs? • How does the growth temperature affect the characteristics for PECVD carbon nanotubes?

4. Review of Nanotechnology It all started with a famous talk…. There's Plenty of Room at the Bottom An Invitation to Enter a New Field of Physics by Richard P. Feynman What I want to talk about is the problem of manipulating and controlling things on a small scale. this talk was given in 1959 http://courses.ece.cornell.edu/ece336/lec/nano-01.pdf

5. Nanoelectronics Sensors, NEMS Inorganic Organic Structural Applications Bio Materials Applications Review of Nanotechnology http://www.ipt.arc.nasa.gov/Graphics/new_talk.pdf

6. Review of Nanotechnology Past Shared computing thousands of people sharing a mainframe computer Present Personal computing Future Ubiquitous computing thousands of computers sharing each and everyone of us; computers embedded in walls, chairs, clothing, light switches, cars….; characterized by the connection of things in the world with computation. http://www.ipt.arc.nasa.gov/Graphics/new_talk.pdf

7. Carbon Nanotube • Carbon nanotube (CNT) is a new form of carbon, configurationally equivalent to two dimensional graphene sheet rolled into a tube. It is grown now by several techniques in the laboratory. The strongest and most flexible molecular material because of C-C covalent bonding and seamless hexagonal network architecture http://physicsweb.org/article/world/11/1/9

8. Carbon Nanotube Device • CNT can be metallic or semiconducting and offers amazing possibilities to create future nanoelectronics devices, circuits, and computers. Nanotube transistor CNT exhibits extraordinary mechanical properties. It is stiff as diamond. These properties are ideal for reinforced composites, nanoelectromechanical systems (NEMS). carbon nanotube gears http://www.ipt.arc.nasa.gov/carbonnano.html

9. PECVD for Nanofabrication • Plasma enhanced chemical vapor deposition (PECVD) has emerged as a key growth technique to produce vertically-aligned nanotubes. • A variety of plasma sources and widely varying results have been reported.

10. chemical vapor deposition (CVD) • Chemicals in the gas phase are introduced to the substrate and adsorb on surface • Chemical reaction occurs at substrate (assisted by heat, plasma, or other energy) • New material grows on substrate surface • Good for many thin films such as silicon oxide, silicon nitride, polysilicon http://mems1.inrf.uci.edu/ece119/lectures/lecture03.pdf

11. Plasma Enhanced Chemical Vapor Deposition(PECVD) • Plasma helps reaction • Low substrate temperature • Good step coverage • Chemical contamination http://www.me.utexas.edu/~lishi/L18.ppt

12. PECVD for Carbon Nanotube Growth • Typical hydrocarbon sources used in plasma-based growth of CNTs include methane (CH4), ethylene (C2H4) and acetylene(C2H2). • Since the plasma can dissociate the hydrocarbon creating a lot of reactive radicals, the use of pure hydrocarbon feedstock may lead to substantial amorphous carbon deposition. Therefore, it is desirable to dilute the hydrocarbon with argon, hydrogen or ammonia. http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

13. PECVD for Carbon Nanotube Growth • For CNT growth, PECVD reactors are typically operated at 1–20 Torr pressure levels with a hydrocarbon fraction of up to 20%. • Variable of plasma technology are used for CNT growth. • Dc • Hot-filament aided with dc • Inductively coupled plasma reactors • Microwave plasma

14. PECVD System CNTs Growth Schematic of a PECVD set-up The growth chamber itself is grounded. All plasma reactors are cold-wall systems with the substrate directly heated using some form of heat source from below the substrate-holder. http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

15. Growth Mechanisms for CNTs • Transition metal catalysts are needed for CNT growth by PECVD. • The catalyst on the substrate must be in the form of particles instead of smooth, continuous films. • The metals used to date as catalysts include Fe, Ni, Co, and Mo. • It is possible to apply these onto the substrate from solutions containing them or they can bedirectly deposited using some physical techniques.

16. - H2 CxHy CxHy MCy - H2 - H2 CxHy CxHy MCy M = Fe, Ni, Co, Pt, Rh, Pd and others Tip Growth Base Growth Occurs when the metal-surface interactions are strong Typically occurs when there are very weak metal-surface interactions Growth Mechanisms for CNTs • Adsorption and decomposition of feedstock on the surface of the catalyst particle • Diffusion of carbon atoms into the particle from the supersaturated surface • Carbon precipitates into a crystalline tubular form http://www.ipt.arc.nasa.gov/Graphics/new_talk.pdf

17. CNTs Growth in PECVD CNTs grown by PECVD are more vertically-aligned than CVD-grown structures. The electrostalic force F creates a uniform tensile stress across the entire particle/CNT interface. the stress at the interface keep vertical orientation maintained. Alignment mechanism as proposed by Merkulov et al http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

18. CNTs Grown by Hot filament DC PECVD The well-aligned carbon nanotubes are grown on the nickel-coated display glass by plasma-enhanced PE-HF-CVD. Parameters: Gas: C2H2/NH3/N2 : 80/160/0 Filament current (A): 8.5 Plasma intensity : 0.2/700/150 (A/V/W) Growht time : 25 (min) • SEM micrograph of carbon nanotubes. • Enlarged view of (A) showing the diameters and their distributions. A site density of about 107 tubes/mm2 was estimated. Ren Z F, Huang Z P, Xu J W, Wang J H, Bush P, Siegal M P and Provencio P N 1998 Science 282 1105

19. PECVD CNTs -- effect of growth temperature The average diameter of CNTs increases with temperature. Effect of temperature on growth characteristics for MWCNFs grown in an ICP reactor at 3 Torr, 20/80: C2H2/H2 for a total flow of 100 sccm, 100W inductive power, 200W capacitive power and a 10 nm thick iron catalyst. http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

20. PECVD CNTs -- Effect of catalyst layer thickness Thicker catalysts lead to shorter and fatter MWCNFs Effect of catalyst layer thickness. DC plasma of C2H2/NH3 http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

21. PECVD CNTs -- Effect of Substrate bias Increasing the bias changes the CNT growth morphology The CNTs were grown from patterned 20 nm thick nickel catalyst film on a 100 nm thick chromium underlayer. Process gas mixture consisted of 80 sccm NH3 and 30 sccm C2H2 at 4 Torr. Panel A shows the exclusive growth of well-aligned MWCNFs at a bias voltage of .550V (360W, 670 mA). Panel B, grown at .575V bias (400W, 710 mA) shows a mixture of MWCNFs and MWCNTs. Panel C, further increasing the bias to .600V (470W, 780 mA) gives exclusive growth of MWCNTs http://ej.iop.org/links/q90/DuX,3fwZcIaZarb2fF3IIw/ps3212.pdf

22. Nanofabrication with scanning nanonozzle ‘Nanojet’ scanning micro/ nanonozzle: atomic force microscopy combined with nanoscale localised chemical etching by gaseous radicals. Reactive species created in a plasma source are pumped through a capillary which is pulled to form a nanonozzle. The electrically neutral radicals are forced in direction of the substrate just by a pressure gradient along the tube. J. Voigt , F. Shi , K. Edinger , P. Guthner , I.W. Rangelow, Microelectronic Engineering 57–58 (2001) 1035–1042

23. Nanofabrication with scanning nanonozzle ‘Nanojet’ (Cont.) Due to the high directionality of the particle beam emerging the high aspect ratio nanonozzle, a strongly localized etching or deposition is induced. By scanning the substrate a small distance under the nozzle, a pattern generation with nearly the same resolution as the nozzle diameter is performed. J. Voigt , F. Shi , K. Edinger , P. Guthner , I.W. Rangelow, Microelectronic Engineering 57–58 (2001) 1035–1042

24. Nanofabrication with scanning nanonozzle ‘Nanojet’ (Cont.) Process gas was SF6/O2 with the plasma pressure of 2.2 mbar. A nozzle with aperture diameter of 250 nm was scanned in close proximity over the substrate, with a scanning speed of approximately 150 nm/min. Length of the nanostructured line is about 2.7 mm, full width half maximum (FWHM) and thus the resolution of the process is 270 nm. J. Voigt , F. Shi , K. Edinger , P. Guthner , I.W. Rangelow, Microelectronic Engineering 57–58 (2001) 1035–1042

25. Plasma Etching Radicals produced in the plasma will drift to the surface Ions accelerated across the sheath deliver energy, driving the chemical reaction(s) between the radicals and the surface material The resulting molecules leave in gaseous form http://www.utdallas.edu/~goeckner/plasma_sci_class/Plasma%20Process%203%20Types.pdf

26. Fabrication of diamond nano-whiskers • High-density diamond whiskers can be applied to electronic devices, such as heat sinks and secondary electron sources as well as field emitters. • High-density aligned diamond whiskers, whose morphology looked similar to that of the aligned carbon nanotube could be formed by directly etching a polycrystalline diamond film. E.-S. Baika, Y.-J. Baika, S.W. Leeb, D. Jeonb, . Thin Solid Films 377-378 (2000) 295-298

27. Fabrication of diamond nano-whiskers (Cont.) • Polycrystalline diamond films were grown on a boron-doped Si(100) wafer using hot-filament CVD. • Mo was deposited on the diamond substrate by sputter deposition. This can be used as a randomly distributed micro-mask. • Diamond substrates were etched by radio frequency air-plasma. Dried room air was introduced into the chamber through a leak valve to maintain the pressure at 30 mtorr. The plasma power was 300 W, and the substrate was biased at -470 V. E.-S. Baika, Y.-J. Baika, S.W. Leeb, D. Jeonb, . Thin Solid Films 377-378 (2000) 295-298

28. Fabrication of diamond nano-whiskers (Cont.) SEM micrographs of the aligned diamond whiskers grown by etching a polycrystalline diamond substrate. (a) The original diamond substrate; (b) side; and (c) top view of the whiskers. The whiskers shown were formed from a substrate deposited with Mo for 5 s and plasma-etched for 50 min. E.-S. Baika, Y.-J. Baika, S.W. Leeb, D. Jeonb, . Thin Solid Films 377-378 (2000) 295-298

29. Conclusion • Plasma technology combining with other techniques would be a powerful tool for nanofabrication.

30. Answers • Typical hydrocarbon sources used in plasma-based growth of CNTs include methane (CH4), ethylene (C2H4) and acetylene(C2H2). • The average diameter of CNTs increases with temperature.