Plasma sputtering process for producing particles
A high production rate plasma sputtering process for producing particles having a size of 10 μm or less is disclosed. The process causes ionization of at least a part of the sputtered target atoms and is performed at such parameters that the pick-up probability of ionized sputtered target atoms on the surface of grains is high.
1. A plasma sputtering process for producing particles having a size of about 1 nm to about 10 μm by growth to said size of the particles through pick-up of sputtered target atoms on the surface of grains, the grains constituting previously formed particles or particle nuclei, the process comprising
providing a target in a plasma generating apparatus, said apparatus having a characteristic length L c wherein L c 3 equals the volume of the plasma generated in said apparatus,
introducing a process gas into said plasma generating apparatus and controlling the pressure inside the plasma generating apparatus in order to obtain a process gas number density n G ,
creating a plasma inside said plasma generating apparatus and controlling the energy provided to said plasma in order to obtain a plasma electron number density n e and a plasma electron temperature T e ,
sputtering atoms from said target by means of said plasma,
allowing the sputtered atoms to be picked-up on the surface of the grains, the grains being present in the plasma, thus producing the particles having a size of about 1 nm to about 10 μm, and
collecting the produced particles,
wherein the plasma electron number density n e and plasma electron temperature T e are sufficient to cause ionization of at least a part of the sputtered target atoms thus resulting in a pick-up flux of ionized sputtered target atoms on the surface of the grains;
wherein the following criterion is fulfilled L ITA /L c ≦0.5
where L ITA is the mean free path of the ionized sputtered target atoms inside said plasma,
wherein the plasma generating apparatus is operated by a pulsed electric power supply with a frequency of at least 100 Hz and with a duration of the pulses of at least 5 microseconds, and
wherein the energy provided to the plasma is sufficient to obtain ionization of at least 20 % of the sputtered atoms in said plasma.
2. The process of claim 1 wherein Γ ITA /(Γ ITA +Γ NTA ) 0.5, wherein Γ ITA is the pick-up flux of ionized target atoms on the surface of grains and Γ NTA is the pick-up flux of neutral target atoms on the surface of grains.
3. The process of claim 1 wherein the energy provided to the plasma is sufficient to obtain ionization of at least 30% of the sputtered atoms in said plasma.
4. The process of claim 1 wherein the plasma generating apparatus is a hollow cathode apparatus.
5. The process of claim 1 wherein the plasma generating apparatus is a magnetron sputtering apparatus.
6. The process of claim 1 wherein (1+3L c /L ITA )(1+4W e /W ITA )>10, wherein W ITA is the average kinetic energy of ionized sputtered atoms in said plasma and W e is the average electron kinetic energy inside said plasma related to T e through W e =(3/2)k B T e .
7. The process of claim 1 wherein the electric power is supplied essentially in the form of square pulses.
8. The process of claim 1 wherein the pulsed electric power supply is applied with a frequency of 200 -2000 Hz and with a duration of the pulses of 10-100 microseconds.
9. The process of claim 1 wherein the average power applied to the cathode is at least 30,000 W/m 2 .
10. The process of claim 1 wherein the process gas number density n G and the electron number density n e are such that
W
ITA
W
e
≤
0.5
wherein W ITA is the average kinetic energy of ionized sputtered atoms in said plasma and W e is the average electron kinetic energy inside said plasma related to T e through W e =(3/2)k B T e .
11. The process of claim 1 wherein
L
ITA
L
c
≤
0.4
.
12. The process of claim 11 wherein
W
ITA
W
e
≤
0.4
.
13. The process of claim 1 wherein the process gas is an inert gas, a reactive gas, a gas mixture of inert gases, or a gas mixture comprising at least a reactive gas.
14. The process of claim 1 wherein said plasma is created at least partly of said process gas.
15. The process of claim 1 wherein the process gas is an inert gas, and the process further comprises introducing a reactive gas or a gas mixture comprising a reactive gas, into said plasma generating apparatus.
16. The process of claim 1 wherein said grains are introduced into said plasma generating apparatus.
17. The process of claim 1 wherein said grains are formed in-situ in said plasma generating apparatus.
18. The process of claim 1 wherein the plasma electron number density n e is at least 10 17 per m 3 .
19. The process of claim 1 wherein the process produces particles having a size ranging from about 5 nm to about 500 nm.
20. The process of claim 2 wherein Γ ITA /(Γ ITA +Γ NTA ) ≧0.66.
21. The process of claim 20 wherein Γ ITA /(Γ ITA +Γ NTA ) ≧0.70.
22. The process of claim 11 wherein
L
ITA
L
c
≤
0.3
.
23. The process of claim 12 wherein
W
ITA
W
e
≤
0.3
.
24. The process according to claim 1 , wherein collecting the produced particles comprises cooling the produced particles and thereafter collecting the particles on a substrate.
25. The process according to claim 1 , wherein collecting the produced particles is performed by a system wherein the produced particles are suspended in a carrier gas.