EP1535307A2 - Discharge radiation source, in particular uv radiation - Google Patents

Abstract

The invention concerns a radiation source, comprising an anode (2), a cathode (3), an electric discharge gap (4) between the anode (2) and the cathode (3) and a gas input conduit (30) in the discharge gap (4). The gas input conduit (30) is electrically connected to the anode and the cathode. The invention is characterized in that the gas input conduit (30) is supplied with gas by a gas supply conduit (32), designed to form between its portion (42) connected to the gas input conduit (30) and another of its portions connected to a fixed potential, an electric impedance such that it counters the generation of electric discharges inside the gas input conduit (30).

Description

 Source of radiation, especially ultraviolet, with discharges

The invention relates to a radiation source, and in particular a radiation source in the ultraviolet or extreme ultraviolet spectral range.

One field of application of the invention relates to the production of ultraviolet radiation used in the manufacturing by lithography of integrated circuits.

The increase in the integration density of integrated circuits necessarily implies a reduction in the dimensions of their constituent parts.

Thus, it is desired to produce circuits having constituent parts of dimensions less than 70 nanometers by using sources of ultraviolet radiation, hence the need to decrease the wavelength of the ultraviolet radiation produced by these to lower values, for example at 13.5 nanometers.

There are several types of sources that can emit in the extreme ultraviolet range: synchrotron sources, sources of radiation produced by laser and sources produced by electrical discharges.

The document "Spectroscopic and energetic investigation of capillary discharges devoted to EUV production for new lithography generation" by Eric Robert, Branimir Blagojevic, Rémi Dussart, Smruti R. Mohanty, Moulay M. Idrissy, Dunpin Hong, Raymond Viladrosa, Jean-Michel Pouvesle, Claude Fleurier and Christophe Cachoncinlle, “Emerging Lithographie Technologies V, Proceedings of SPIE” Vol. 4343 (2001) describes a source of radiation in the extreme ultraviolet range, by electrical discharges.

The discharge occurs between an anode and a cathode in a space filled with gas. The discharge excites the gas molecules which emit radiation to de-excite.

One of the drawbacks of discharge radiation sources is their small amount of radiated energy.

The invention aims to obtain a source of radiation by discharge making it possible to increase the amount of energy radiated. To this end, the invention relates to a radiation source, comprising:

- an anode,

- a cathode, - an electrical discharge space between the anode and the cathode,

a gas introduction conduit into the discharge space, the gas introduction conduit being electrically connected to the anode or to the cathode,

- Means for creating in the gas present in the discharge space an electric discharge causing the emission of radiation to the outside, characterized in that the gas introduction pipe is supplied with gas by a supply pipe gas, arranged to form between its part connected to the gas introduction conduit and another of its parts, connected to a fixed potential, an electrical impedance such that it obstructs the creation of electric discharges inside the gas introduction pipe.

The inventors have discovered that in known sources the connection of the gas introduction conduit in the discharge space to the cathode was the cause of the appearance of a large number of discharges in the gas located in the conduit. introduction and not into the discharge space, thereby further decreasing the amount of energy radiated.

Thanks to the invention, the number of parasitic electrical discharges in the gas located in the introduction conduit is reduced or even eliminated, which increases the number of discharges taking place in the discharge space and consequently the quantity of radiated energy.

According to other features of the invention,

- the fixed potential and the anode are grounded and the gas introduction conduit is electrically connected to the cathode;

- The source further includes an anode cooling system;

- The cooling system is circulating cooling fluid in or on the anode;

- the cooling fluid comprises water;

- or the cooling fluid comprises air; - or the cooling fluid comprises oil;

- the electrical impedance formed by the gas supply pipe comprises an electrical inductance; - Between its part connected to the fixed potential and its part connected to the gas introduction conduit, the gas supply conduit comprises an electrically conductive material and is wound to form said inductance;

- The gas supply pipe is wound against and at a distance from an electrically insulating part for assembling the source;

the means for creating a discharge in the discharge space comprise at least one charge storage capacitor electrically connected by a first terminal to the cathode and by a second terminal to a first terminal of at least one switching capacitor connected electrically by its second terminal at the anode, electrical switching means being provided between the first and second terminals of the at least one switching capacitor and a load voltage source being provided between the first and second terminals of the at least one switching capacitor switching; the switching means comprise a switch controlled in single-shot mode;

- Or the switching means comprise a switch controlled in pulsed mode at a repetition frequency less than or equal to 10 kHz;

- the load voltage source and the switching means are such that the at least one charge storage capacitor is charged by the load voltage source shortly before the switching means are switched;

- a plurality of charge storage capacitors are provided, the cathode has an annular part connected to a central part connected to the discharge space, and the charge storage capacitors are distributed around the central part by being connected by their first terminal to the annular part and by their second terminal to a conductive ring electrically connected to the first terminal of the at least one switching capacitor; - the anode has a frustoconical hole for passage of the radiation emitted in the discharge space, the hole being connected by its small base to the discharge space and by its large base towards the outside for the passage towards the outside radiation emitted into the discharge space; - Or the anode has a central cylindrical hole for passage of the radiation emitted in the discharge space, the hole being connected to the discharge space for the passage to the outside of the radiation emitted in the discharge space; - The cathode has a central frustoconical hole for the passage of gas, the small base of which is connected to the discharge space and the large base of which is connected to the gas introduction conduit;

- Or the cathode has a central cylindrical hole for the passage of gas, connected on the one hand to the discharge space and on the other hand to the gas introduction conduit.

The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example with reference to the appended drawings, in which:

- Figure 1 shows schematically in axial section a radiation source according to the invention;

- Figure 2 shows schematically the source according to Figure 1 seen from the rear; and

- Figure 3 shows schematically in side view the source according to Figures 1 and 2. In the figures, the source 1 of radiation according to the invention comprises an anode 2 and a cathode 3, both conductive of electricity and separated l 'from one another by an electric discharge space 4.

The anode 2 is for example formed by a part of generally circular metallic circular shape with a longitudinal axis 5, while the cathode 3 is for example formed by a metal part in the form of a cap, comprising a central part 6 in the form of a pot. circular cylindrical with axis 5, the bottom 7 of which faces the discharge space 4 and the open part of which is connected to an annular part 8 with axis 5. The anode and / or the cathode are made of conductive materials having a high melting point, for example thoriated tungsten, for example 3%.

The discharge space 4 is for example formed by a capillary directed along the axis 5 and delimited transversely to the axis 5 by an electrically insulating disc 9 mounted between the bottom 7 of the part 6 and the anode 2. The anode 2 has in its center a frustoconical hole 10, for example with a half-angle at the top equal to 25 °, whose small base is connected to the discharge space 4 and whose large base 11, distant from the discharge space 4 and the disc 9, opens outward to allow the radiation emitted by the discharge to pass through space 4. The bottom 7 of the cathode 3 also has a frustoconical hole 12, for example with a half-angle at the top equal to 25 °, the small base of which is connected to the discharge space 4 and the large base of which is directed towards the annular part 8. The holes 10 and 12 can also be cylindrical with an axis 5. The disc 9 is for example made of ceramic and has a longitudinal thickness of 0.1 to 40 mm and for example equal to 10 mm. An electrically conductive ring and axis 5 is fixed around the central part 6, at a distance from the annular part 8, electrically insulating from the part 6. A plurality of charge storage capacitors 14 are arranged around the central part 6 while being connected by their first electrical terminal 15 to the annular part 8 and by their second electrical terminal 16 to the ring 13. In addition, the ring 13 is connected, on a part of its remote side on the side of connection to the terminals 16, to a conductor 17 for connection to a first electrical terminal 18 of one or more switching capacitors 19 connected by its or their second electrical terminal 20 to a conductive ring 21, for example of steel, at the center of which the anode 2 is mounted, the anode 2 being in electrical contact with the inner face 22 of the ring 21 by means of a contact piece 23.

The electrical circuit for creating discharges in space 4 is for example of the Blumlein type, as is known.

Between the terminals 18 and 20 of the switching capacity 19 are provided electrical switching means (not shown) for connecting them, these comprising for example a thyratron or any other suitable switch and being for example controlled by an external generator delivering current pulses at repetition frequencies which can range from 1 Hz to 10 kHz, in particular from 1 Hz to 5 kHz, and for example equal to 1 kHz. Between the terminals 18 and 20 of the switching capacitor 19 is provided, in parallel with the switching means, a voltage source, for example continuous of a value up to 30 kV or pulsed at a frequency between 0, 1 and 1 kHz and for example equal to 1 kHz. In Figure 2, six charge storage capacitors 14 are shown, each with a value of 4 nF capable of supporting 20 kV. However, any number of capacitors 14 can be provided for an overall capacitance of between a few nF and a few tens of nF, and for example a single capacitor 14.

An electrically insulating centering ring 24 is mounted between the ring 21 and the disc 9 around the anode 2. In front of and in the center of the ring 21 is mounted a cylindrical part 25 of axis 5, having an external opening 26 letting the radiation from the hole 10 pass. A conduit 30 for introducing discharge gas into the discharge space 4 is mounted on the cathode 3, inside its central part 6. The conduit 30 for introducing gas has a longitudinal wall 31 defining a longitudinal circular cylindrical space 32 for gas passage, which ends on the one hand in a gas-tight manner at the hole 12 and which is closed on the other hand in a gas-tight manner by a fixed plug 33 gas tight, for example screwed, inside the wall 31.

An electrically insulating ring 34 is mounted against the annular part 8, and is crossed in its center by the gas introduction conduit 30. The rings 21 and 34 are fixed to each other by means of rods 35 arranged around the elements mentioned above and electrically insulating, for example made of PVC. Each rod 35 is fixed to the ring 34 and the ring 21 by screws 36, respectively 37 of longitudinal compression. The rings 21 and 34 are compressed longitudinally towards one another by the rods 35 so that the connections between the gas introduction conduit 30 and the cathode 3, the cathode 3 and the discharge space 4, the discharge space 4 and anode 2 are made gas tight.

The wall 31 of the gas introduction conduit 30 has a transverse through hole 41, which opens on the one hand into the gas passage space 32 and into which is inserted on the other hand a gas-tight connection 42 of a conduit 43 for discharging gas.

The gas supply conduit 43 has a part 44, remote from the connector 42 and connected to a fixed electrical potential.

The gas supply pipe 43 is connected, upstream of or in its part 44 to a source 50 of discharge gas, to send the gas through the pipe 43, the fitting 42, space 32 for passage and hole 12 in space 4 for discharge. The connector 42 is electrically conductive and metallic. The prior charge of the storage capacitors 14 and then the adequate control of the switching means causes the appearance of an electric discharge in the gas present in the discharge space 4 and the emission of radiation therein towards the hole 10 and opening 26. The gas is for example xenon, oxygen, nitrogen, argon or krypton. The gas is for example chosen to produce by discharges extreme ultraviolet radiation having a wavelength between 10 and 50 nm and for example 13.5 nm. The gas supply conduit 43 is such that it defines, between its part 44 connected to the fixed potential and its part 42 connected to the gas introduction conduit 30, an electrical impedance obstructing the creation of electric discharges at the inside the gas introduction conduit 30.

For example, this fixed potential is grounded as is the anode, while the cathode is at a potential below that of ground, which advantageously makes it possible to provide a cooling system by circulation of any cooling fluid in the body of the anode 2 or on the body of the latter, whether the cooling fluid is electrically conductive or not. In FIG. 1, the cooling system of the anode 2 comprises a recess 45 in the body of the latter, in which the cooling fluid circulates. The invention thus offers the possibility of using a good coolant, such as water, which conducts electricity and which is inexpensive. The coolant can be or include water, air or oil.

The impedance formed by the conduit 43 for supplying gas between its part 42 and its part 44 comprises for example an electrical inductance, formed by the winding of the conduit 43 partially or entirely of electrically conductive material between its parts 42 and 44. This winding is for example formed by a little more than one turn of the conduit 43 from the part 42 towards the part 44 around the conduit 30. The conduit 43 is located at a distance from the ring 34 and the others parts of the source 1, and is surrounded transversely and at a distance by the rods 35 and the screws 36. The arrangement of the conduit 43 for supplying gas at a distance and against the ring 34 makes it possible to reduce its bulk.

The winding of the gas supply duct 43 can be in a spiral or in a coil or, as shown in FIG. 3, have its internal part 45 and radially close to the gas introduction pipe 30 closer longitudinally to the ring 34 than its radially outer part 46 and away from the pipe 30.

Thus the electrical inductance formed by the conduit 43 for supplying gas has a value making it possible to annihilate the electric discharges in the conduit 30 for introducing gas. The gas supply conduit 43 makes it possible both to prevent electrical discharges from appearing in the gas introduction conduit 30 and to supply the latter with gas, by saving additional electrical components. Thus, the electrical energy supplying the storage capacitors 14 for charging them is no longer wasted in parasitic electrical discharges in the gas introduction conduit 30, thereby increasing the energy efficiency of emission of radiation from the source, with equal power supply.

In addition, this efficiency is further increased by charging the storage capacitors 14 only immediately or shortly before each switching of the switching means. This switching can be carried out in single-shot mode or in pulsed mode up to 10 kHz.

Claims

1. Radiation source, comprising: - an anode (2), - a cathode (3, 8), - a space (4) for electrical discharge between the anode (2) and the cathode (3), - a conduit (30) for introducing gas into the discharge space (4), the conduit (30) for introducing gas being electrically connected to the anode (2) or to the cathode (3, 8) , - means (13 to 23) for creating in the gas present in the discharge space (4) an electric discharge causing the emission of radiation to the outside, characterized in that the conduit (30) for introduction gas is supplied with gas by a gas supply conduit (43), arranged to form between its part (42) connected to the gas introduction conduit (30) and another (44) of its parts, connected to a fixed potential, an electrical impedance such that it prevents the creation of electrical discharges inside the conduit (30) for introducing gas. 2. A radiation source according to claim 1, characterized in that the fixed potential and the anode (2) are grounded and the gas introduction conduit (30) is electrically connected to the cathode (3, 8) . 3. A radiation source according to any one of the preceding claims, characterized in that it further comprises a system (45) for cooling the anode (2). 4. A radiation source according to claims 2 and 3, characterized in that the cooling system (45) is circulating cooling fluid in or on the anode (2). 5. A radiation source according to claim 4, characterized in that the cooling fluid comprises water. 6. A radiation source according to claim 4, characterized in that the cooling fluid comprises air. 7. A radiation source according to claim 4, characterized in that the cooling fluid comprises oil. 8. A radiation source according to any one of the preceding claims, characterized in that the electrical impedance formed by the gas supply conduit (43) comprises an electrical inductance. 9. A radiation source according to claim 8, characterized in that between its part (44) connected to the fixed potential and its part (42) connected to the conduit (30) for introducing gas, the conduit (43) of gas supply comprises an electrically conductive material and is wound to form said inductance. 10. A radiation source according to claim 9, characterized in that the conduit (43) for supplying gas is wound against and at a distance from an electrically insulating part (34) for assembling the source. 11. A radiation source according to any one of the preceding claims, characterized in that the means (13 to 23) for creating a discharge in the discharge space (4) comprise at least one capacitor (14) for storing charge electrically connected by a first terminal (15) to the cathode (3, 8) and by a second terminal (16) to a first terminal (17) of at least one switching capacitor (19) electrically connected by its second terminal ( 20) at the anode (2), electrical switching means being provided between the first and second terminals (18, 20) of the at least one switching capacitor (19) and a load voltage source being provided between the first and second terminals (18, 20) of the at least one switching capacitor (19). 12. A radiation source according to claim 11, characterized in that the switching means comprise a switch controlled in single-shot mode. 13. A radiation source according to claim 11, characterized in that the switching means comprise a switch controlled in pulsed mode at a repetition frequency less than or equal to 10 kHz. 14. A radiation source according to any one of claims 11 to 13, characterized in that the charge voltage source and the switching means are such that the at least one charge storage capacitor (14) is charged by the load voltage source shortly before the switching means are switched. 15. A radiation source according to any one of claims 11 to 14, characterized in that a plurality of capacitors (14) charge storage are provided, the cathode (3, 8) has an annular part (8) connected to a central part (6) connected to the discharge space (4), and the charge storage capacitors (14) are distributed around the central part (6) by being connected by their first terminal (15) to the annular part (8) and by their second terminal (16) to a conductive ring (13) electrically connected to the first terminal (18 ) of at least one switching capacitor (19). 16. A radiation source according to any one of the preceding claims, characterized in that the anode (2) has a frustoconical hole (10) for passage of the radiation emitted in the discharge space (4), the hole being connected by its small base in the space (4) of discharge and by its large base towards the outside for the passage towards the outside of the radiation emitted in the space (4) of discharge. 17. A radiation source according to any one of claims 1 to 15, characterized in that the anode (2) has a cylindrical central hole for passage of the radiation emitted in the discharge space (4), the hole being connected to the discharge space (4) for the passage towards the outside of the radiation emitted in the discharge space (4). 18. A radiation source according to any one of the preceding claims, characterized in that the cathode (3, 8) has a central frustoconical hole (12) for gas passage, the small base of which is connected to the space (4 ) of discharge and whose large base is connected to the conduit (30) for introducing gas. 19. A radiation source according to any one of claims 1 to 17, characterized in that the cathode (3, 8) has a central cylindrical hole for the passage of gas, connected on the one hand to the space (4) of discharge and secondly to the gas introduction conduit (30).