Laser-induced particle formation in a methane discharge

Eva Stoffels, Winfred Stoffels,

Eindhoven University of Technology,

PO Box 513, 5600 MB Eindhoven.

stoffels@discharge.phys.tue.nl

Giacomo Ceccone, Francois Rossi,

European Commission, Joint Research Center,

Ispra (VA), Italy.

• CH4 radio-frequency plasma for diamond deposition

• pressure 0.2 - 1 Torr

• 3rd harm. Nd:YAG laser ( = 355 nm), 8 ns pulse, 10 Hz repetition frequency

• detection at 90o by photo-camera

Experimental setup

0

50

100

150

200

250

250 450 650 850

power (mW)

time

(s)

Appearance of particles as a function of laser power

•Laser photon energy (ca. 3.5 eV) matches the bond energy of CH4.

•High radical densities are created locally by photodissociation.

• Nanoparticles (<10 nm) are nucleated.

•Nanoparticles are detected by the same laser, due to their fluorescence upon laser irradiation

Time needed to nucleate carbon nanoparticles during laser irradiation, as a function of laser power

500 nm

SEM micrograph of particles

After nucleation, nanoparticles grow in the plasma until they reach a size of 100 nm.

Coulomb crystal formation

Formation of vertical strings

First organised structures

100 nm particles have a permanent negative charge.

Coulomb interactions lead to formation of organised structures.

0.2 mm

SEM x2000 Optical microscope x7510 m

Vertical strings

•In the plasma-sheath transition electric fields are present

•Charged particles gain kinetic energy of about 1 eV, enough to overcome Coulomb repulsion

•Particle coalescence takes place

V-shape structures floating in plasma

1 cm

Forces on a single particle and on V-shaped conglomerate

Charge on a single particle of size a:

Ze = 40 a Vp, mass M = 4/3a

Charge on a linear string of N particles with size a:

Ze = 40 a Vp N/ln(4N), mass M = 4/3a

1 cm

Floating network above wafer

0.1 mmCoulomb repulsion

Large structures in the plasma

Deposited conglomerates

Both charge and mass of a string increase with the string size N. Coulomb and gravitation forces remain balanced. Large strings are still floating.

SEM x50 Light microscope x50

0.5 mm 0.5 mm

Structure of floating network

Growth on electrode

5 mmEventually, floating network collapses and deposits on the electrode.

Overview of all phases