WO2005101924A1

WO2005101924A1 - Method and device for obtaining euv radiation from a gas-discharge plasma

Info

Publication number: WO2005101924A1
Application number: PCT/EP2005/003869
Authority: WO
Inventors: Vladimir Mikhailovich Borisov; Alexander Yurevich Vinokhodov; Alexander Sergeevich Ivanov; Yuriy Borisovich Kiryukhin; Valentin Alexandrovich Mishchenko; Alexander Vasilevich Prokofiev; Oleg Borisovich Khristoforov
Original assignee: Xtreme Technologies Gmbh
Priority date: 2004-04-14
Filing date: 2005-04-13
Publication date: 2005-10-27
Also published as: RU2004111488A; RU2278483C2

Abstract

The invention relates to a method and device for obtaining EUV radiation from a gas-discharge plasma by means of rotating electrodes (2, 3) inducing a discharge between heteropolar electrodes, and igniting a pinch-type discharge with the shaft (1) rotating at defined speed. The object of the invention to find a novel possibility for obtaining EUV radiation from a gas-discharge plasma by the use of rotating electrodes (2, 3) without limited average power and pulse repetition rate of the EUV source, is met according to the invention by pulse-forming a channel of working substance vapors in a peripheral region of the inter-electrode gap (d) located a defined distance from the rotational axis (R), and then igniting the pinch-type discharge, wherein the consumption of working substance is compensated by continuously supplying low-melting metal to the surface of at least one of the electrodes. In a preferred embodiment the vapor channel is generated by a pulse laser or electron beam focused on the layer (4) of low-melting metal.

Description

METHOD AND DEVICE FOR OBTAINING EUV RADIATION FROM A GAS-DISCHARGE PLASMA

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of Russian Application No. 2004 111 488 filed April 14, 2004 and relates to a co- pending Russian Application No. 2002 120 301 filed July 31, 2002 (not published yet) , the complete disclosure of both are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention relates to a device and a method for obtaining primarily extreme ultraviolet (EUV) radiation from a dense, hot plasma of pinch-type discharges. The field of application includes EUV lithography, particularly within a spectral band of 13.5 ± 0.135 nm, corresponding to the effective reflection range of mirror optics having a Mo/Si coating.

b) Description of the Related Art

Known in the art is a device and a method for obtaining EUV radiation form a cylindrical z-pinch plasma in xenon with a RF discharge ignition system described in US 5,504,795 A. In this disclosure, the auxiliary RF discharge formation system in tandem with the discharge chamber prevents the effective cooling of the electrodes, and the dielectric wall of the discharge chamber is exposed to powerful radiation fluxes, limiting the ability to increase the EUV radiation power.

These disadvantages are partially eliminated in an EUV radiation source according to US 6,677,600 B2, in which a rotating discoid insulator with multiple openings is arranged between two electrodes with coaxial openings. In the method for obtaining EUV radiation from gas-discharge plasma using the device specified above, a pinch-type discharge ignites at the moment when the axis of one rotating insulator opening overlaps the axis of the electrode openings. However, this device and method do not resolve the problem of effectively cooling the electrodes and increasing their service life while increasing the power of EUV radiation.

To a significant extent, these shortcomings are eliminated in a method for obtaining EUV radiation disclosed in WO 03 085 707 Al . EUV radiation is obtained from laser plasma in a high-frequency pulse tracking mode while focusing a pulsed laser beam on a sub-millimeter target containing a working substance, whose atomic elements having emission lines in the required EUV range. The device used to implement this method incorporates an injector of consecutive solid or liquid sub-millimeter targets containing a working substance such as xenon (Xe) , lithium (Li), tin (Sn) or tin dioxide (Sn0₂), and a laser system that forms a powerful laser beam with a high repetition frequency. One shortcoming of the EUV radiation source based on laser plasma involves its low efficiency by comparison to gas-discharge EUV sources. The closest technical solution of the prior art is an EUV source with rotating electrodes described in the Russian patent application RU 2002 120 301. Said rotating electrodes are rigidly secured to a shaft, which encompasses a discharge initiation system and a discharge power supply connected to the electrodes. The EUV source incorporates n identical pairs of electrodes with coaxial openings arranged at an identical distance from the rotational axis, wherein the discharge initiation system is designed as a device used to form an auxiliary slipping discharge on the surface of a cylindrical dielectric immovably mounted at the same distance from the shaft rotational axis as the n pairs of rotating electrodes .

The method for obtaining EUV radiation from a gas-discharge plasma using the approach of RU 2002 120 301 involves the steps of inducing a discharge between heteropolar electrodes, and igniting a pinch-type discharge with the shaft rotating at speed v. In this method, the discharge is initiated at the moment the axis of one of the rotating electrode pairs overlaps the axis of the stationary auxiliary discharge formation system, pre-ionizing the gas in the discharge zone, and the pinch-type discharge is consecutively ignited in the gas or vapor-gas mixture at a repetition rate of f=n'v in each of the n electrode pairs.

The approach of RU 2002 120 301 makes it possible to ensure a long electrode service life at a high average power for the EUV radiation. But there is also a disadvantage in the limited capability of increasing the pulse repetition rate, making it more difficult to increase the average power of EUV radiation to a level that satisfies the requirements of industrial applications, the most important of which is EUV lithography at λ=13.5 n . This shortcoming arises from the fact that the pinch-type discharge is ignited in the discharge zone between axially symmetrical electrodes . This discharge geometry defines a rather large (> 1 cm) distance between the axes of adjacent electrode pairs, which requires high (> 70 m/sec) , difficultly realized linear rotational speeds for the electrodes at the pulse repetition rates required for EUV lithography (>7 kHz) . In addition, the use of metal vapors as a working substance in the prior art device might not be optimal, since the capability of automatically generating such vapors is envisaged for the high-current discharge phase, not for its initiation phase.

OBJECT AND SUMMARY OF THE INVENTION

The object of the invention is to increase the pulse repetition rate and average power of gas-discharge plasma EUV radiation while ensuring its small dimensions, lengthen the service life and increase the efficiency of the EUV source while maintaining a high radiation stability from pulse to pulse.

This object can be achieved by improving the EUV source with rotating electrodes rigidly secured to the shaft, which incorporates a pulse discharge initiation system and a discharge power supply connected to the electrodes.

The device was improved by having the electrodes be comprised of two disk-shaped elements, whose central axes of symmetry overlap the shaft axis, covering the peripheral part of the surface of at least one of the electrodes with a layer of low-melting metal, and introducing a system for supplying the low-melting metal on the surface of at least one of the electrodes, while a device for forming a vapor channel in the peripheral region of the inter-electrode gap is used as the discharge initiation system.

The device used to form a vapor channel is a laser or electron beam with a system for focusing the beam on a layer of low-melting metal, comprised of a working substance, primarily tin (Sn) , the plasma of which has radiation lines in the EUV spectral range.

In one embodiment for realizing the device, the system for supplying the low-melting metal on the electrode surface consists of two baths filled with liquid tin, wherein the electrodes are mounted so that they can be partially loaded in the baths during rotation.

The baths can be electrically insulated from each other, and the supply source is connected to the rotating electrodes by means of liquid tin filling the baths .

A heat exchanger for dissipating heat from the electrodes can be situated in each bath with liquid tin.

In addition, the electrodes can be fitted with blades to ensure that the liquid tin circulates through the heat exchanger .

The method for obtaining EUV radiation from gas-discharge plasma using an EUV radiation source with rotating electrodes involves initiating a discharge between heteropolar electrodes and igniting a pinch-type discharge with the shaft rotating at speed v.

The method is improved by pulse-forming a channel of working substance vapors in the peripheral region of the inter-electrode gap located at a distance R from the rotational axis in order to close the inter-electrode gap with value d, and then igniting the pinch-type discharge, wherein discharge cycles are carried out at a repetition rate of f > 2πvR/d, and the consumption of working substance is compensated by continuously supplying low- melting metal to the surface of at least one of the electrodes .

The method is also improved in that the vapor channel is formed by a pulse laser or electronic laser focused on the layer of low-melting metal.

The essence of the invention and the preferred embodiments are shown in the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic representation of the EUV source device with rotating electrodes.

Fig. 2 shows a EUV source device in which the electrodes are covered with a low-melting metal by partially loading them into baths with liquid tin. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The EUV source contains electrodes 2, 3 that are rigidly secured to the shaft 1 and designed as disk-shaped elements whose central axes of symmetry overlap the shaft axis. The peripheral part of the surface of at least one electrode 3 is covered by a layer 4 of low-melting metal. As the discharge initiation system, the EUV source contains a device 5 for forming a vapor channel, which comprises a laser or electron beam with a system to focus the beam on the layer 4 of low-melting metal covering the peripheral part of the surface of the electrode 3. The working substance used as the low-melting metal is primarily tin, the atomic elements of which have emission lines in the EUV spectral range. The pulse supply source 6 is connected to the electrodes 2, 3 by slipping contacts 7, 8. The slipping contacts can be in the form of a brush, liquid metal or plasma. The pulse power supply can contain elements like a accumulating capacitor with switch, pulse transformer, pulse capacitor and magnetic key. In order to reduce the inductivity of the discharge circuit, some of the elements in the pulse power supply, e.g., the pulse capacitor and magnetic key, can be mounted directly onto the rotating electrodes. Electrodes 1, 2 are accommodated in the discharge chamber 9. In proximity to the discharge zone, whose location is determined by the focus of the beam from the vapor channel formation device 5, a blade 10 is mounted, which prevents working substance vapors and discharge plasma ions from spreading into the EUV radiation discharge area. The discharge chamber 9 is connected to a collection chamber 11, into which the EUV radiation is emitted. The discharge chamber 9 is equipped with a vacuum purging system 12 to ensure a low vapor and gas pressure therein, and with a rotational motion gasket to seal the shaft 1.

In another embodiment of the device, the shaft 1 can be sealed with a magnetic coupling for its drive. The EUV source also encompasses a system 13 for supplying low- melting metal on the surface of at least one of the electrodes. The low-melting metal is a working substance, primarily tin, the plasma of which is most effectively radiating in the EUV spectral range. The system 13 is used for supplying low-melting metal on the surface of electrodes 2, 3 based on one of the following methods: tinning (wetting) , metallizing, hydraulic filling of porous structures incorporated in the electrode material. For purposes of heat dissipation, electrodes 2, 3 are equipped with channels to circulate coolant 14 (Fig. 1) .

In the embodiment of the device shown in Fig. 2, both rotating electrodes 2, 3 are covered by a tin layer 4. To this end, the area not optically connected with the discharge zone accommodates two baths 15, 16 electrically insulated from each other, while the liquid metal 17 filling the bath simultaneously serves as liquid-metal slipping contacts, which connect the power supply source 6 to the rotating electrodes 2, 3. In addition, the baths 15, 16 with liquid tin incorporate heat exchangers to dissipate heat, inside of which the coolant circulates. In this case, each rotating electrode is equipped with blades 19 to ensure that the liquid tin circulates through the heat exchangers 18.

The method for obtaining EUV radiation from gas-discharge plasma is realized as follows. Disk-shaped electrodes 2, 3 rigidly secured to the shaft 1 are rotated uniformly around its axis, wherein the peripheral part of at least one of these electrodes is covered by a layer 4 of low-melting metal. During activation of the pulse device 5 with a laser or electron beam focused on a layer 4 of low-melting metal in the peripheral area of the inter-electrode gap, an optimally small portion of the low-melting metal evaporates. This results in a vapor channel that closes the discharge interstice between the rotating electrodes 2, 3 in a fixed area of space defined by the beam focus point. The pulse supply source 6 is activated, and the pinch-type discharge is ignited in the medium of metal vapor and circulating gas. One characteristic of a pinch-type discharge can be a high-current vacuum spark. The discharge current flows along circuits that incorporate the pulse power supply 6, slipping contacts 7, 8, and rotating electrodes 2, 3. As the discharge current passes, the low-melting metal is subjected to additional evaporation, increasing the density of the gas-discharge plasma of the charge. Using a working substance as the low-melting metal, in particular tin, in whose vapors the pinch-type discharge takes place, ensures a highly effective discharge of EUV radiation from the discharge plasma. The used portion of radiation is passed out of the discharge chamber 9 through a semi-translucent trap 10 and into a collection chamber 11. The trap 10 prevents vapors, electrode erosion products and plasma streams from getting into the collection chamber 11. To ensure more efficient operation, the trap 10 is provided with a buffer gas, e.g., Argon.

After the electrodes 2, 3 have rotated by an angle sufficient for taking a sample of hot vapor and gas from the discharge zone, which includes the laser or electron beam focusing region of the vapor channel formation system 5, and the work cycle is repeated. Since the transverse dimensions of the plasma pinch do not exceed measure d for the inter-electrode gap, the peripheral part of the electrodes only has to be shifted by a distance less than the value d to spatially stabilize the pinch. This makes it possible to perform discharge cycles with a frequency f > 2πvR/d, where R is the distance from the axis of shaft 1 to the discharge zone. At R = 300 mm, d = 3 mm, and v = 650 RPM, the allowable pulse repetition rate f exceeds the value 7 kHz. A low vapor and gas pressure in the discharge chamber 10 is achieved when working with a vacuum purging system 12, which lowers EUV radiation absorption. Tin (Sn) is also continuously supplied to the surface of the electrode 3 when working with system 13. This compensates for its losses resulting from discharge pulses. The electrodes 2, 3 are cooled y circulating coolant 14 in them.

In the embodiment of the device shown in Fig. 2, the pinch- discharge type current flows along circuits that include the electrodes 2, 3, pulse power supply 6 and slipping contacts . Low-melting metal is supplied to the surface of both electrodes by wetting them while partially loading the electrodes into baths 15, 16 with a liquid metal (tin) 17. This restores the expended material at both electrodes, increasing the operating life of the EUV source. During operation, heat is dissipated from the electrodes by way of a liquid metal 17 that fills the baths 15, 16 and routed to the heat exchangers 18. The blades 19 on the disk-shaped electrodes 2, 3 ensure that the liquid metal circulates between the electrodes and heat exchangers 18, leading to improved electrode cooling. Having the EUV radiation source incorporate rotating electrodes in the manner described makes it possible to execute a pinch-type discharge in a peripheral region of the disk-shaped electrodes having any azimuth coordinate. As opposed to the prior art containing a discrete quantity of rotating discharge chambers, discharge ignition system activation, according to the invention, need not be carefully synchronized with the moment at which the rotating electrode system passes a specific coordinate. This greatly simplifies the operation of the EUV radiation source with rotating electrodes .

In addition, the magnitude of electrode system rotation between consecutive pulses measures several degrees in the prior art, while the EUV source according to the invention requires a rotation of less than tenths of a degree. This makes it possible to significantly increase the discharge pulse repetition rate and average power of the EUV radiation while reducing the rotational velocity. As a result, it becomes possible to obtain the average radiation power, in particular at λ = 13.5 nm, required for industrial applications in EUV lithography at a resolution of < 50 nm.

Initiating the discharge by forming the vapor channel in the peripheral region of the inter-electrode gap ensures a high positional stability of the pinch-type discharge between the rotating electrodes and, as a consequence, the EUV radiating unit itself. Forming the vapor channel with a pulse laser or electron beam focused on a layer of low- melting metal on the electrode surface makes it possible to form the vapor channel over less time in a smaller volume, yielding small dimensions for the radiating surface necessary for primary applications, along with a high spatial stability for the EUV radiating unit.

Using a working substance, particularly tin, as the low- melting metal ensures that a high discharge plasma efficiency is obtained in the EUV spectral range, in particular close to λ = 13.5 nm. In this case, introducing a system for the continuous supply of low-melting metal on the surface of at least one of the electrodes compensates for the consumption of working substance during operation, and ensures a high stability for the EUV radiation energy from pulse to pulse.

Covering the electrodes with the low-melting metal by partially loading them in baths with liquid tin compensates for the material consumption of both electrodes that takes place as the result of discharge pulses. This increases the service life of the electrode system and EUV source as a whole. Connecting the supply source to the rotating electrodes via the liquid tin filling the baths ensures that energy is supplied to the discharge through liquid- metal contacts having a long service life. Arranging a heat exchanger in each bath with liquid tin and equipping the electrodes with blades for circulating the liquid tin through the heat exchanger ensures a high electrode-cooling rate by means of the liquid metal heat carrier.

Therefore, the proposed device and method makes it possible to increase the efficiency and average power of the EUV radiation from a hot plasma discharge while reducing its dimensions, extend the life of the EUV source, and increase its spatial and energy stability.

Claims

1. An EUV radiation source with rotating electrodes rigidly secured to the shaft, which includes a pulse discharge initiation system and a discharge supply source connected to the electrodes, characterized in that the electrodes consist of two disk-shaped elements, whose central axes of symmetry overlap the shaft axis, the peripheral part of the surface of at least one of the electrodes is covered by a layer of low-melting metal, and a system is incorporated for supplying the low-melting metal to the surface of at least one of the electrodes, while a device for forming a vapor channel in the peripheral region of the inter- electrode gap is used as a discharge initiation system.

2. The EUV radiation source with rotating electrodes according to claim 1, characterized in that the device used to form a vapor channel is a laser or electron beam with a system for focusing the beam on a layer of low-melting metal, comprised of a working substance, primarily tin (Sn), the plasma of which has radiation lines in the EUV spectral range.

3. The EUV radiation source with rotating electrodes according to claim 1, characterized in that the system for supplying the low-melting metal on the electrode surface consists of two baths filled with liquid tin, wherein the electrodes are mounted so that they can be partially loaded in the baths during rotation.

4. The EUV radiation source with rotating electrodes according to claim 3, characterized in that the baths are electrically insulated from each other, and the supply source is connected to the rotating electrodes by means of liquid tin filling the baths .

5. The EUV radiation source with rotating electrodes according to claim 3, characterized in that a heat exchanger for dissipating heat from the electrodes is situated in each bath with liquid tin.

6. The EUV radiation source with rotating electrodes according to claim 5, characterized in that the electrodes are fitted with blades to ensure that the liquid tin circulates through the heat exchanger.

7. A method for obtaining EUV radiation from a gaseous discharge plasma by means of an EUV radiation source with rotating electrodes, in which a discharge between hetero-polar electrodes is induced and a pinch-type discharge is ignited while the shaft rotates at speed v, characterized in that a channel of working substance vapors is pulse-formed in the peripheral region of the inter-electrode gap located a distance R from the rotational axis in order to close the inter-electrode gap with value d, after which the pinch-type discharge is ignited, wherein discharge cycles are carried out at a repetition rate of f > 2πvR/d, and the consumption of working substance is compensated by continuously supplying low-melting metal to the surface of at least one of the electrodes.

8. The method for obtaining EUV radiation from a gaseous discharge plasma according to claim 7, characterized in that the vapor channel is formed with a pulse laser or electron beam focused on the layer of low-melting metal.