Plasma source

Details Plasma source Plasma source

2001-04-09

2004-03-23

Wong, Don (Department: 2821)

Electric lamp and discharge devices

Fluent material supply or flow directing means

Plasma

C315S111210, C315S111810, C118S7230ER

Reexamination Certificate

active

06710524

ABSTRACT:

FIELD
This invention relates to a plasma source. The invention also relates to a method of generating plasma, and to an apparatus for coating or cleaning substrates. More particularly, this invention relates to a plasma source in which radiofrequency energy is inductively coupled to both a thermionic-field emitter, thereby generating electrons with broad energy distribution for plasma generation and neutralisation, and a discharge process generating a plasma having ions and electrons.
BACKGROUND
Such a plasma source can be effectively used in the vacuum processing of thin film coatings during electron beam or thermal deposition. The energy imparted by the source to the growing film is capable of modifying the microstructure producing dense, near stoichiometric films that are impervious to temperature and humidity variations. The refractive index achieved is near that of the bulk materials, thus extending the possibilities for multilayer thin film design.
Substrate heating is superfluous with assisted deposition processes. Low temperature coating is a major process advantage offering low-cost fixturing, time/cost and compatibility with low-temperature materials such as plastics.
Plasma sources are also exceptional for in situ substrate cleaning. In particular, argon cleaning provides physical sputter removal of adsorbed water and residual cleaning solvents. Oxygen cleaning can stimulate chemical removal of hydrocarbons through the formation of volatile species.
A primary application for such sources includes precision optical coatings of oxide and fluoride based deposition materials. Examples include anti-reflection coatings for ophthalmic lenses, high tolerance multilayer dielectric optical coatings for telecommunications and high laser damage coatings.
Currently available plasma or ion sources for assisted vacuum deposition processes are have been described in the prior art, such as, for example, U.S. Pat. No. 4,862,032, EP-A-0463230, WO96/30928, FR-2557415 and in S. Pongratz and A. Zoller, J. Vac. Sci. Technol. A 10(4), p 1897, Jul./Aug. 1992.
Certain commercially available plasma sources have a length such that they require a well in the base plate to ensure that positioning within the vacuum chamber does not mask deposition sources due to excessive source height. Consequently such systems require a specialised vacuum chamber for operation and are not readily retrofittable to other vacuum systems.
In plasma deposition, the term neutralisation refers to a state in which there is a balance of ions and electrons. In the absence of neutralisation (which usually involves a surplus of ions) three deleterious effects can occur:
1. Electrons can be drawn to the beam in short-duration arcs that can eject small particles from the arc location. These arcs can cause damage to a sensitive substrate surface and also introduce contamination into the growing film.
2. The occurrence of the arcs as described in 1 also leads to temporal variation in beam-plasma voltage which causes process variation.
3. Space charge effect which spreads the plasma spatial distribution and also introduces edge effects for dielectric substrates mounted in metal holders.
This effect manifests itself as a film thickness variation.
Plasma/ion sources which rely on only thermionic emission have a very narrow electron energy emission characteristic with minimal lower energy electrons as shown in FIG.
1
. This problem is overcome in ion sources through use of a separate supply of electrons injected into the plasma to provide neutralization. Current plasma sources rely upon the thermionic electron emission to provide sufficient electrons with necessary energy to provide neutralisation. This method does not provide adequate control over neutralisation and as such effects 1, 2 and 3 above are encountered.
Other ion source systems employing inductively coupled RF energy have been described (see, for example, U.S. Pat. No. 4,104,875). Such systems are susceptible to conductive deposits on the non-conductive window isolating the inductor from the plasma region. Capacitively coupled RF discharge processes have also been used in ion and plasma sources (see, for example, EP-A-0474584).
All of the plasma/ion sources described above have fixed spatial distribution of the ion/plasma flux at the substrate plane, engineered for ion/plasma overlap over the full deposition area or positioned to provide the best overlap with the evaporant fluxes. Such sources compromise achieving the full benefits of assisted deposition as each application requires a specific match of ion/plasma spatial distribution depending upon coating type, required substrate loading over deposition area, deposition material(s), evaporant source flux and film parameter(s) to be optimised via ion/plasma bombardment.
Moreover, sputtering of the thermionic emitter material causes changes in the emitter spatial profile which varies the spatial distribution of emitted electrons with source operating time and hence the plasma distribution.
A general object of the present invention is to provide a radiofrequency energy driven plasma source with enhanced plasma generation, control and neutralisation. Another object of the present invention is to provide a plasma source which avoids the disadvantages and deleterious features of such plasma sources as described above. Broadly we achieve this by using induction to help generate the ions from an electron emitter of a plasma source.
SUMMARY
According to a first aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, an RF induction coil at least partially surrounding the electron emitter, and an anode.
Preferably, the plasma source further comprises a cylindrical former of electrical insulator material capable of withstanding high temperature, and the emitter is disposed within the former. In an embodiment, the emitter is in the form of multiple cylinders of thermionic-field emitting material arranged on the circumference of a circle lying within the insulating former. We also prefer that plasma source includes a removable base, said base including at least part of the gas inlet. The base may also include apertures through which electrical wires for the emitter, the anode and the induction coil may extend.
The emitter emits thermionic electrons for the generation of the plasma, when held at a negative potential and subjected to heating. In effect, the emitter acts as a cathode.
In a preferred embodiment, at least part of the electron emitter is dome-shaped. More preferably, the emitter is in the form of a cylinder having a domed top.
In an embodiment, the emitter is in the form of a cylinder of varied width and height with a flat top, thereby allowing the spatial distribution of emitted electrons to be changed.
The anode, which is desirably cylindrical, is preferably concentric with the emitter and axially displaced therefrom, generating a potential difference between anode and emitter. The potential difference between anode and ground and axial magnetic fields causes the plasma to be extracted from the source. More preferably, the axial displacement of the anode from the emitter is adjustable. We prefer that a cap is disposed between the anode and cathode, and we also prefer that the cap has an aperture of variable size.
Preferably, the electron emitter is supported by a conductive support column, by means of which the emitter can be held at a negative potential. The emitter is desirably disposed substantially concentrically within the induction coil and the former, the former being disposed within the induction coil. Preferably the induction coil is water cooled.
The induction coil can be operated to perform a number of advantageous functions. In order to generate the plasma it is necessary to heat the emitter, and this can be achieved by means of the induction coil which can be operated to deliver energy to heat the emitter. There are important benefits to induction heating, as

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