METHOD OF APPLYING A FARADAYCAGE ONTO THE
RESONATOR OF A MICROWAVE LIGHT SOURCE
The present invention relates to a light source for a microwave-powered lamp.
It is known to excite a discharge in a capsule with a view to producing light.
Typical examples are sodium discharge lamps and fluorescent tube lamps. The latter
use mercury vapour, which produces ultraviolet radiation. In turn, this excites
fluorescent powder to produce light. Such lamps are more efficient in terms of
lumens of light emitted per watt of electricity consumed than tungsten filament lamps.
However, they still suffer the disadvantage of requiring electrodes within the capsule.
Since these carry the current required for the discharge, they degrade and ultimately
fail.
We have developed electrodeless bulb lamps, as shown in our patent
application Nos. PCT/GB2006/002018 for a lamp (our "'2018 lamp"),
PCT/GB2005/005080 for a bulb for the lamp and PCT/GB2007/001935 for a
matching circuit for a microwave-powered lamp. These all relate to lamps operating
electrodelessly by use of microwave energy to stimulate light emitting plasma in the
bulbs. Earlier proposals involving use of an airwave for coupling the microwave
energy into a bulb have been made for instance by Fusion Lighting Corporation as in
their US Patent No. 5,334,91 3. If an air wave guide is used, the lamp is bulky,
because the physical size of the wave guide is a fraction of the wave length of the
microwaves in air. This is not a problem for street lighting for instance but renders
this type of light unsuitable for many applications. For this reason, our '2018 lamp
uses a dielectric wave-guide, which substantially reduces the wave length at the
operating frequency of 2.4Ghz. This lamp is suitable for use in domestic appliances
such as rear projection television.
In our International Application No. PCT/GB2008/003829, now published
under No. WO 2009/063205, we provide a light source to be powered by microwave
energy, the source having:
• a solid plasma crucible of material which is transparent or translucent for exit
therefrom, the plasma crucible having a sealed void in the plasma crucible,
• a Faraday cage surrounding the plasma crucible, the cage being at least
partially light transmitting for light exit from the plasma crucible, whilst being
microwave enclosing,
• a fill in the void of material excitable by microwave energy to form a light
emitting plasma therein, and
• an antenna arranged within the plasma crucible for transmitting plasmainducing
microwave energy to the fill, the antenna having:
• a connection extending outside the plasma crucible for coupling to a source of
microwave energy;
the arrangement being such that light from a plasma in the void can pass through the
plasma crucible and radiate from it via the cage.
As used in that application and this specification:
"lucent" means that the material, of which the item described as lucent, is transparent
or translucent;
"plasma crucible" means a closed body enclosing a plasma, the latter being in the
void when the latter' s fill is excited by microwave energy from the antenna.
The object of the present invention is to provide an improved method of
applying a Faraday cage to a lucent crucible or other resonator of a light source to
powered by microwave energy.
According to the invention there is provided a method of applying a Faraday
cage to a lucent resonator, the resonator having a void containing microwaveexcitable
material and being adapted for microwave resonance in the resonator and
within the Faraday cage for driving a light emitting plasma in the void, the method
consisting in the steps of:
• deposition of a conductive material on to the lucent resonator;
• applying, patterning and developing a photoresist material over the conductive
material to leave the conductive material exposed where it is not required;
• removing the conductive material where not required and the photoresist
material from the required conductive material, leaving a reticular network of
conductive material providing a Faraday cage and
• depositing a layer of protective material over the cage of conductive material.
Normally the deposited conductive material will be at least twice the skin
depth of microwaves to be used in exciting the lucent resonator, and preferably more
than three times the skin depth.
Conveniently the conductive material and the protective material are vacuum
deposited either by sputtering or by electron-beam evaporation. The conductive
material is preferably highly conductive metal such as copper and the protective
material is preferably of the same material as the resonator, conveniently quartz, i.e.
silicon dioxide, or possibly silicon monoxide.
For fixing the lucent resonator, a ring of the conductive material - either in
continuous form or as part of the reticular network - is left uncovered by the
protective material and the fixture ring is soldered or brazed to the exposed
conductive material.
For directing light from the plasma forwards, a back face of the resonator
conveniently has deposited on it a reflective material, forming a continuous extension
of the Faraday cage. This can be of the same material as the reticular network, but is
preferably of a different material, albeit in conductive contact with it. Conveniently,
this reflective material is aluminium.
To help understanding of the invention, a specific embodiment thereof will
now be described by way of example and with reference to the accompanying
drawings, in which:
Figure 1 is a perspective view of a lucent crucible with a Faraday cage
applied in accordance with the invention;
Figure 2 is a scrap cross-sectional v iew of a back corner of the crucible
showing a fixture ring;
Figure 3 is a scrap cross-sectional view of the cage showing a protection layer
sputtered over the cage; and
Figure 4 is a diagrammatic view of a crucible holder for use during sputtering
of the front face and sidewall of the crucible.
Referring first to Figures 1 to 3 of the accompanying drawings, a lucent
crucible 1 is of quartz, being circular and having a diameter of 49mm and a length of
20mm. Centrally, it has a void 2, which is 20mm long and 6mm in diameter. The
diameter could be decreased to as little as 3mm. A 5mm long by 10mm diameter cap
3 closes the void at a front face 4 of the crucible. A metal halide and noble gas charge
is contained in the void. An antenna bore 5 is provided from the back face 6 of the
crucible and extends into it adjacent the central void.
The crucible has a Faraday cage formed of a hexagonal network 11 of copper
lines - 50micron wide by 2 micron thick in the radial direction - covering its circular
face 7. The network extends onto the front face 4 and indeed onto the cap 3. A plain
line 2 of copper extends around the corner edge between the front face 4 and the
circular face 7; and a band 1 of copper extends around the circular cylindrical sidewall
adjacent the back face 6. A brass fixture ring 14 is silver soldered to the band 13.
The back face is covered in an aluminium layer 15, in electrical contact with the band
13 and the rest of the Faraday cage. Inside the aluminium is a reflective layer 31
enhancing the reflectivity of the aluminium layer. A protective layer 15 of quartz
material covers the copper network 11.
Application of the Faraday cage to filled plasma crucible will now be
described. It should be noted that in practice a plurality of crucibles would be
processed together in a batch. For simplicity of explanation, a single crucible only is
referred to below:
1. The crucible is cleaned with standard glass cleaning practices to prepare it for
metal deposition.
2. The crucible is heated up in a clean furnace to 450°C to eliminate any surface
water vapour.
3. The crucible is immediately loaded into a Sputtering Vacuum Chamber,
preferably while still hot. For coating of the rear face of the crucible, it is
fixedly mounted with the rear face directed towards an aluminium sputtering
target. For coating the front face and the circular cylindrical sidewall, it is
mounted obliquely on holder 20 such as shown in Figure 4. This has a
stationary member 21, having 45° angled bores 22 in which are journalled
individual holders 23. These have chucks 24 able to grip the crucible via a
vestigial sealing tube 25. Mounted with the chucks is a bevel gear 26 in mesh
with a complementary gear 27 mounted on a shaft 28, sealingly extending
through the member 2 1. Rotation of the shaft turns the crucible so that its
front face and the sidewall are evenly exposed to sputtering as described
below.
Before sputtering, RF energy @ 13.56MHz is first applied to an isolated
holder retaining the crucibles. This is for the order of 10 seconds, and will
clean the crucibles by sputtering off an atomic layer. It will also eliminate any
foreign particulate matter or water vapour from the crucible surface.
With the crucible set up with its back face exposed, a preliminary optical
multi-layer coating 31 is applied to the back face of the crucible for high
reflectivity from 400 nm to 800 nm.
The crucible is manipulated and mounted at 45° facing a copper sputter
electrode. The RF is applied and the deposition process will begin. The
deposition rate is on the order of 1 micron per minute, so for a three micron
layer the deposition would run for three minutes. Copper 32 is deposited
where the mesh is desired, i.e. on the front face and sidewall. Enough of it 33
migrates onto the back face around the edge for electrical contact.
The crucible is manipulated again and the RF is then applied to an aluminium
sputter electrode and the aluminium coating 1 is applied to the back face,
including to the copper rim 33, making electrical contact with it. It should be
noted that the aluminium coating has two further functions: (i.) completion of
the Faraday cage and (ii.) reflection infra red forwards out of the crucible, to
reduce heat transmission towards a source of microwaves exciting the
crucible.
The crucible is removed after the final deposition and the photoresist is
applied. The output front face of the crucible will have the photoresist applied
by a spin coater. A blob of photoresist is poured in the centre and then the
crucible is spun at high speed. This leaves a very thin and uniform layer over
the face. No residual photoresist should drip over the edge and the circular
cylindrical side-wall and the back face are still uncoated with resist. The
crucible is then put in a special holder and dipped into a container of
photoresist just to the top edge, being careful not to let any run over the top
onto the thin layer that was applied by the spin on technique. This is not
difficult because the photoresist has a very high surface tension and it doesn't
run over easily. Once the crucible is lowered into position in the cup so that
the resist is at the edge it is then slowly removed at a constant rate. The rate of
removal determines the thickness of the photoresist. It is important that the
thickness of the resist be uniform or the UV laser source may not expose the
resist for the full depth, causing defects.
The photoresist covered crucible is then baked in a dark clean oven at 80°C for
10 minutes.
The photoresist is then ready for exposure. A laser galvanometer system is
used to write the mesh pattern onto the crucible. The crucible is mounted onto
a rotating vacuum fixture and held by the rear aluminium coated surface. The
laser galvanometer system then writes the mesh pattern onto the circular
cylindrical side-wall, by writing a section and then rotating a set amount and
then writing the next section. It takes six such rotations at present. This can
be improved with an upgrade to the system, whereby the laser galvanometer
moves in the Z axis and the rotation covers the theta rotation for pattern
writing. This would be much quicker. While the circular cylindrical side-wall
is being written, an additional galvanometer system writes the front face
pattern. The side-wall and front face patterns are calibrated so that the lines
meet at the edge for continuity. A thin line can be drawn around where the
side-wall and front faces meet for additional insurance of continuity from sidewall
to the front face.
Once exposed the photoresist is immediately developed in KTFR developer
solution. This takes two minutes. The unexposed photoresist is washed off in
high pressure deionised water. The crucible must immediately be rinsed in
alcohol and blown dry with dry nitrogen. The photoresist is no longer light
sensitive.
The photoresist is now be baked for 20 minutes at 100°C in a clean oven.
After baking the photoresist is ready to etch the pattern where there is no
photoresist. The copper quickly etches off in normal copper etchant, such as
Ferric Chloride. Some agitation is advantages for uniform etching. This
process will take of the order of 30 seconds. It is to be remembered that all of
these processes are batch processes, and many crucibles are being processed at
once. After etching the crucible is rinsed in flowing de-ionised water.
The crucibles are then blown off with dry nitrogen and immersed in
photoresist remover for two minutes. Once again, agitation is helpful. After
removing from the remover, the crucibles are rinsed in hot soapy water and
then de-ionised water. Finally ultrasonically rinsed in isopropyl alcohol and
dried with clean dry nitrogen.
Once clean, the crucibles are baked at 120°C and then reloaded into the
sputtering chamber. Once again, reverse sputtering is used to remove any
residual photoresist and to ensure that the crucibles are free of water and
particulate material. A three micron thick layer of Si02 is then sputtered onto
the crucible, covering the copper mesh and the aluminium rear reflector. The
chamber crucible holder masks a small ring 13 around the rear edge of the rear
reflector, leaving a small strip of copper exposed.
A mounting ring 14 is then soldered or brazed to this exposed ring and is used
for mounting and electrical connection to the crucible. A quarter wave
antireflection layer of MgF could be evaporated over the Si02 to gain an
addition 2-3% output.
CLAIMS:
1. A method of applying a Faraday cage to a lucent resonator, the resonator having a
void containing microwave-excitable material and being adapted for microwave
resonance in the resonator and within the Faraday cage for driving a light emitting
plasma in the void, the method consisting in the steps of:
• deposition of a conductive material onto the lucent resonator;
• applying, patterning and developing a photoresist material over the conductive
material to leave the conductive material exposed where it is not required;
• removing the conductive material where not required and the photoresist
material from the required conductive material, leaving a reticular network of
conductive material providing a Faraday cage and
• depositing a layer of protective material over the cage of conductive material.
2. A method as claimed in claim 1, wherein the conductive material is deposited to a
thickness at least twice the skin depth of microwaves to be used in exciting the
excitable material and preferably more than three times the skin depth.
3. A method as claimed in claim 1 or claim 2, wherein the conductive material is
vacuum deposited either by sputtering or by electron-beam evaporation.
4. A method as claimed in claim 1 or claim 2, wherein the protective material is
vacuum deposited either by sputtering or by electron-beam evaporation.
5. A method as claimed in any preceding claim, wherein the conductive material is
highly conductive metal, preferably copper.
6. A method as claimed in any preceding claim, wherein the protective material is of
the same material as the resonator, preferably quartz, i.e. silicon dioxide, or silicon
monoxide.
7. A method as claimed in any preceding claim, wherein a ring of the conductive
material, either in continuous form or as part of the reticular network, is left
uncovered by the protective material and a fixture ring is soldered or brazed to the
exposed conductive material.
8. A method as claimed in any preceding claim, wherein reflective material,
preferably forming a continuous extension of the Faraday cage, is deposited onto a
back face of the resonator.
9. A method as claimed in any preceding claim, wherein the reflective material
deposited onto the back face is of the same material as the reticular network.
10. A method as claimed in any one of claims 1 to 8, wherein the reflective material
deposited onto the back face is of a different material, preferably aluminium.