Photon source

The present invention relates to a photon source comprising a plasma source with at least one cathode and at least one anode, between which a system of one or more mutually separated cascade plates is placed, which cascade plates are provided with at least one passage opening, wherein corresponding passage openings of successive cascade plates lie at least substantially mutually in line, with at least one gas inlet for admitting at least during operation a gas to be excited at an inlet pressure and with an outlet opening close to the anode for allowing exit of photons emitted by a generated plasma, wherein an electric power source is connected at least during operation between the cathode and the anode.

Photon sources are used for many purposes, for instance as light source for infrared (IR), visible, ultraviolet (UV) to far-ultraviolet (FUV) light, and are known in diverse forms. A photon source of the type stated in the preamble is thus known from the International patent application WO 2004/048950. It is otherwise noted here that where the present patent application refers to light, unless expressly stated otherwise, this is understood to mean not only visible light, but also radiation not discernible to the human eye, such as for instance infrared and (far-)ultraviolet radiation. Particularly in the case of research, production and development of microscopic surface structures, the maximum brightness of the applied light source is often a limiting factor. This relates for instance to inspection and lithography of sub-micron and nanostructures, and spectroscopic and spectrometry applications, wherein use is preferably moreover made of a point source in order to obtain the best possible result. Conventional light sources have shortcomings in this respect, particularly in the sense that the maximum brightness which can be achieved therewith lies below a level which is desirable in practice.

The present invention has for its object, among others, to provide a photon source with which a high brightness can be achieved. It moreover has for its object to provide such a photon source which acts to an at least large extent as a point source.

In order to achieve the intended object, a photon source of the type stated in the preamble has the feature according to the invention that the power source is able and adapted to generate a modulated current and that, at least during operation, the gas inlet is coupled to gas supply means which are able and adapted to admit the gas to be excited at a modulated, sub-atmospheric to above-atmospheric inlet pressure. Use is made here as the basis of the photon source of a plasma which is generated in a gas to be excited when the current from the power source is conducted therethrough. Because the gas is admitted at a relatively high, i.e. sub- to above-atmospheric pressure, the plasma can be formed with a relatively high degree of ionization and electron density. Photons with an exceptionally high light intensity are continuously emitted mainly due to recombination and interaction of ions with free electrons which are created in the plasma between the cathode and the anode. This intensity will increase further by increasing the cathode current. Within the scope of the invention a sub- to above-atmospheric inlet pressure is here understood to mean for instance a pressure between roughly 50-85 kPa and 1000 kPa.

The inventors have realised that by not making use of a constant but a modulating current and gas pressure, a higher photon intensity can be achieved while heat generation in the source remains the same. The invention is herein based on the insight, among others, that the average heat dissipation in the source is roughly proportional to the average electric current density in the source, while the average photon intensity is roughly proportional to the average of the square of this electric current. By modulating the electric current instead of keeping it constant, a higher average photon intensity can thus be achieved at a heat dissipation which on average remains the same.

In order to enhance a stable and undisturbed operation of the photon source, a preferred embodiment thereof has the feature that the electric current is modulated to a sufficiently high base current, which is able to maintain a stable plasma. A particular embodiment of the photon source has in this respect the feature according to the

invention that the electric current alternately decreases during a time span of less than about 1 millisecond. The base current maintains a sufficiently high degree of ionization in the plasma and thus avoids the plasma collapsing prematurely, so that ignition phenomena are avoided and a uniform average yield is obtained. Because the plasma maintains an axisymmetrical charge distribution and thus field distribution, the distribution and the extra intensity of the light is the same at each modulation.

In order to also ensure a sufficiently high gas density at a temporarily increased electric current, with a view to the desired increased photon yield, a further preferred embodiment of the photon source according to the invention has the feature that the power source and gas supply means are adapted at least during operation such that the inlet pressure and the electric current vary relative to each other in at least practically synchronous manner. The electric current and gas pressure are thus mutually phase- modulated during operation so that the gas pressure is always adjusted to the momentary electric current.

In a particular preferred embodiment the photon source according to the invention has the feature that a system of a number of cascade plates is placed between the cathode and the anode, and that the passage openings of at least some of the cascade plates successively narrow and widen in a direction from the cathode to the anode. By thus successively narrowing and widening the passage openings the plasma beam will successively converge and diverge. The highest electron density occurs at or close to the narrowest part, where the highest light intensity will therefore prevail. At the outlet opening the photon source thus behaves at least substantially as a point source, which is an advantage in numerous applications. In addition, a combination of such a profiling of the plasma beam and a modulated control according to the invention provides further advantages. The narrowed plasma beam thus reduces the etendue of the light source, which can thereby be applied in an optical system with fewer adjustment losses. The narrowed plasma channel moreover results in a confinement of the plasma in the relevant part of the source within limited macroscopic dimensions, which can in particular be in the order of or smaller than an average free photon path

-A- length. A higher gas pressure can hereby be permitted, and self-absorption of the produced photon light will remain limited, which results in higher efficiency, and thereby in a higher light intensity.

A further particular embodiment of the photon source has the feature according to the invention that on a side of the system of cascade plates remote from the outlet opening a reflective surface of a mirror, in particular a reflective surface of a spherical mirror, is incorporated in a light path of the photons. The reflective surface herein provides for the interception and reflection of photons which would otherwise have been lost. Owing to this internal reflection and possible resonance, the source will produce a greater light output A spherical mirror is recommended here in the case of a relatively simple alignment of the optical system.

Although the photon source is suitable as an open and as a closed system, a further particular embodiment has the feature that the outlet opening is sealed at least substantially gas-tightly with a window which is at least substantially transparent to the photons. By thus sealing the source, the source can be applied generally in both a vacuum environment and in an atmospheric environment. Furthermore, less of the gas to be excited will thus be consumed, whereby the photon source can be operated in economically more efficient manner. Owing to the at least substantially transparent window the generated light can nevertheless exit.

hi order to be able to control an exposure or irradiation with the photon source relatively quickly, a further particular embodiment thereof according to the invention has the feature that a controllable shutter is arranged in a photon path close to the outlet opening. The electric current and gas supply in the photon source can thus remain unchanged, while the photon outflow at the outlet opening can nevertheless be controlled as wished. The shutter can in particular be synchronized with the modulation according to the invention of the electric current and/or the inlet gas pressure of the source.

The invention will be further elucidated hereinbelow on the basis of an exemplary embodiment and an associated drawing. In the drawing figure 1 shows a schematic view of a structure of an exemplary embodiment of a photon source according to the invention; figure 2 shows an inlet pressure of a gas flow in the photon source of figure 1 as measured in time during operation of the device; figure 3 shows an electric current from a power source of the photon source of figure 1 as measured in time during operation of the device; figure 4 shows a schematic cross-section of a second exemplary embodiment of a photon source according to the invention; and figure S shows an enlarged view of the cascade package of the device of figure

4.

The figures are otherwise purely schematic and not drawn to scale. Some dimensions in particular may be exaggerated to greater or lesser extent for the sake of clarity. Corresponding parts are designated as far as possible in the figures with the same reference numeral.

An exemplary embodiment of a photon source according to the invention is shown in figure 1. The photon source comprises a plasma source 10 for generating a thermal plasma. The plasma source comprises one or more cathodes 11 and an anode 12, between which a system of one or more, in this example seven, mutually separated cascade plates 13 is placed. The cascade plates are manufactured from copper and are provided internally with cooling channels through which a cooling agent such as (demineralized) water flows during operation. Plates 13 have a thickness of about S millimetres and are separated from each other by separating plates 14 of a thickness of about 1 millimetre of PVC (polyvinyl chloride) or another electrical insulator for the purpose of electrically insulating plates 13 from each other.

Situated centrally in the plates is a bore 15 with a cross-section between 1.0 and 2.7 millimetres, which forms a passage opening for a plasma arc which is drawn between cathode 11 and anode 12 during operation when a sufficiently high current is applied

therebetween. The passage openings of successive plates lie mutually in line and thus form a plasma channel between cathode 11 and anode 12. Sealing rings 16 between cascade plates 13 here ensure a vacuum-tight sealing of this plasma channel 15. Sealing rings 16 are herein shielded from the plasma by rings 17 of boron nitride in order to protect them from premature degradation. Plasma channel 1 S thus has an overall length of about 42 millimetres.

The plasma source comprises a gas inlet 21 which is coupled to gas supply means 20 in order to admit a gas for exciting at a controllable, sub- to above-atmospheric inlet pressure into the plasma channel. As in this example, use is preferably made here of an inert gas such as argon or xenon, which, owing to its chemical inertness, will not produce any undesirable by-products, or hardly so, which could otherwise adversely affect the operation and stability of the source. The gas supply means comprise a pump 25 which is coupled to an argon cylinder 26 with an outgoing line 24 leading to the gas inlet. During operation a small gas flow is maintained by the system in order to drive possible contaminants out of source 20. The used gas is vented into the open air from an outlet 22 via a narrow and long capillary 27 in order to stabilize the gas flow. The capillary typically has a diameter in the order of several tenths of millimetres and a length in the order of 10 metres. Situated between gas inlet 21 and gas outlet 22 is a shunt line 23 which herein ensures a uniform pressure build-up in the system. Situated in lines 21..24 are the control valves (not shown) necessary during operation for automatic regulation and modulation of the inlet pressure of the system in accordance with the invention.

The photon source is operated by admitting the gas via inlet 21 into an inlet chamber 31 of the system at a pulsed inlet pressure in the order of between about 100 and 500 kPa, see figure 3, and the gas is then allowed to flow through plasma channel 15 with a relatively low flow. A power source 40 is connected between cathode 11 and anode 12, with which an electric current is transmitted through plasma channel 15. The power source can supply a power in the order of several kilowatts at a potential difference in the order of 100 Volt between the anode and cathode, so that a current of

several tens of amperes is drawn. In this example use is made of a power source with a rated output of 2.2 kilowatts at a potential difference of about 95 Volt, so that the nominal rating will amount to about 23 ampere. The free electrons collide in plasma channel 1 S with gas atoms and ions, which ionize and/or become excited under the influence thereof, whereby the gas eventually changes to a plasma phase.

Three mechanisms in the plasma are responsible for the generation of optionally visible light. These are free-bound emission caused by recombination of electrons with ions, free-free emission as a result of electron-ion and electron-gas atom interactions, and line emission as a result of excited atoms falling back autonomously to a lower state. The proportion of line emission and electron-atom interaction is relatively small compared to broadband continuum emission, so that these two mechanisms can be left out of consideration. The photon yield of the source is caused mainly by free-free and free-bound emissions in electron-ion interactions. Due to the relatively high, sub- to above-atmospheric inlet pressure, roughly between 50-85 kPa and 1000 kPa, which is applied in the source according to the invention, and is here in the order of about 340 kPa, a relatively high degree of ionization in the order of 1-15% is achieved, which results in a relatively high light intensity. The electron temperature typically lies around 1 eV (11600 K) and the electron density in the plasma is typically in the order Of I ⁽ P m- ³ .

The plasma generation can be monitored via an inspection window 33. The light exits plasma channel 15 to a plasma chamber 32 via an outlet opening 18 formed in or close to anode 12. This light can leave plasma chamber 32 via a transparent window 35. An electronic shutter 34 which transmits or blocks the exiting light as required is placed in front of the window. Window 35 is vacuum-tight so that the system can be applied in an atmospheric environment. The material of the window is herein adapted to the nature of the emitted light. If the photon source is intended for application in a vacuum environment, window 35 can also be omitted if desired, so that light which would otherwise be absorbed thereby, such as in particular far and extreme UV light, can also be used effectively.

In accordance with the invention the power source does not supply a constant output current, but a modulated one. This is shown schematically in figure 2. The electric current I is modulated to an average base current I ₅ of about 13 ampere, which is sufficiently high to maintain a stable plasma. Below a certain level I _1111n , which is indicated schematically in the figure with a broken line, there is a danger that the plasma will lose stability or even collapse completely. From the base level I ₅ of about 13 ampere the amperage is increased temporarily to a higher level I _1112x in the order of 100-500 ampere, in this example about 500 ampere, for short intervals of only a few tens of microseconds to several tenths of a millisecond. Use is made in this example of a relatively short pulse duration of about 30 microseconds. The time span between these pulses is relatively short and amounts to for instance a maximum of 1 millisecond in order to ensure the stability of the plasma. The average current level I _g0n thus amounts to about 28 ampere. The potential difference between the cathode and anode is held constant at all times. This results in a temporarily increased flow of free electrons through the plasma channel for successive intervals of about 30 microseconds, which are about a millisecond apart.

Because both the free-bound emission and the free-free emission are roughly proportional to the square of the electron density as a result of interactions between free electrons and ionized gas atoms, the light emission during these time intervals will increase quadratically. The heat dissipation in the source, caused by collisions of the electrons with the wall, is however roughly proportional to the electron density, so that it will only increase linearly during said intervals. A significantly greater light output can thus be obtained in the case of a relatively small increase in heat dissipation or, conversely, a greater light output can be achieved when the heat dissipation remains roughly the same, as compared to a plasma source powered with a constant current.

Synchronously with the electric current I through the source, the inlet pressure p is also varied in time t, this being shown schematically in figure 3. A minimum pressure p _mm of about 85 kPa also applies here in order to ensure a stable plasma generation,

above which the gas is admitted at a base pressure p, of about 100-300 kPa and increased to a higher pressure P^ _x of about 500 kPa during successive intervals of several tens of microseconds, synchronously with the intervals in which the electric current is increased. A greater gas density is thus always available in the case of a temporarily increased electric current.

As a result of the pulsed operation of the device a light output can be achieved which is an average of about ten times higher relative to a continuous operation at the same average amperage of about 28 ampere. The effect on the result of a measurement performed with the device, in particular the sensitivity thereof, is even greater as a result of the pulsed character (so-called phase-sensitive detection).

Instead of being straight, plasma channel IS is profiled in the present example in that the passage openings of at least a number of the cascade plates 13 successively narrow and widen in a direction from cathode 11 toward anode 12. The cascade plates are successively designated A..G in the figure. Passage opening 15 of a first 13 A of cascade plates 13 has a diameter of about 2.7 millimetres, while the passage openings in subsequent cascade plates 13B..13G are successively 2.0, 1.7, 1.0, 1.5, 2.0 and 2.5 millimetres wide. By thus slightly reducing the diameter of the plasma channel in the centre, the etendue of the generated light beam is reduced. Adjustment losses in an optical system in which the light source is applied can hereby be limited. Furthermore, the current density in the centre, and thereby thus the electron density, will thus increase quadratically. Because the light output is proportional to the square thereof, this will result in a considerably greater light intensity. In addition, the generated light beam will thus exit at a solid angle α imposed by the cascade plates, whereby it apparently exits from a point and the light source is apparently a point source which can accordingly be sharply depicted.

A second embodiment of a photon source according to the invention is shown in figures 4 and 5. The photon source largely corresponds with that of figure 1 , with the proviso that use is made in this example of a system of copper cascade plates 13 which

are mutually separated by insulating ceramic intermediate discs 19. Cascade package 13,19 is soldered together to one whole in a vacuum oven at a temperature of about 900 ⁰ C. Owing to the excellent heat-resistance of ceramic material compared to plastics as insulator, the device in this example demands considerably less of the cooling and the device can be loaded to considerably higher amperages.

According to the invention use is nevertheless also made in this example of a pulsed operation, in respect of both the amperage carried by the source and the inlet pressure which is applied here in order to thus achieve a higher efficiency and output. A carrier gas is admitted via an inlet 21 and pumped out at an outlet 22 in order to maintain a small flow during operation, while a plasma is developed between a cathode 11 and an anode 12. The light/photons emitted by the plasma can leave the source via an outlet opening 18 and a transparent window 35.

A reflective surface 36 of a spherical mirror 37 is in this example incorporated in photon path IS on a side of the system 13,19 of cascade plates remote from outlet opening 18 in order to reflect the photons exiting on this side to outlet opening 18. A higher light output is thus also realized.

Figure 5 shows in detail the profiling of plasma channel 15 applied in this exemplary embodiment. The thicknesses of cascade plates 13 and local diameters of plasma channel 15 shown herein are indicated in millimetres. On either side the generated light exits at a solid angle α of respectively about 5.4 and 4.0 degrees. By thus narrowing and widening the channel the plasma beam will converge and diverge. The highest electron density occurs in or close to the narrowest part so that the highest light intensity will prevail there. At outlet opening 18 the photon source thus behaves at least substantially as a point source.

A combination of such a profiling of the plasma beam and the modulated control according to the invention also provides further advantages. The narrowed plasma beam thus reduces the etendue of the light source, which can therefore be applied with

fewer adjustment losses in an optical system. The narrowed plasma channel moreover results in a confinement of the plasma in the relevant part of the source within limited macroscopic dimensions, which can in particular be in the order of or smaller than an average free path length of the photons. A higher gas pressure can hereby be permitted, which results in a higher efficiency, and thereby a higher light intensity, without resulting in self-absorption of the generated light

Although the invention has been further elucidated above on the basis of only a single exemplary embodiment, it will be apparent that the invention is by no means limited thereto. On the contrary, many other variations and embodiments are possible for the person with ordinary skill in the art without departing from the scope of the invention. Instead of argon or xenon, other optionally inert gases can for instance be applied for the creation of a plasma therewith. Each gas will result here in its own emission spectrum which can thus be adapted to a specific requirement.

The stated current pulse and pressure diagrams have only been provided for the purpose of elucidation and can be further optimized as desired in respect of level, amplitude and regularity. Instead of a single cathode, a number of cathodes can be applied in a source in order to limit the individual load at a determined electrical current. A number of sources can also be connected simultaneously in order to obtain a greater output. Finally, the invention also relates to a method for generating photons, wherein a photon source of the type described in the preamble is operated with the indicated modulating inlet pressure and current.