RU2278483C2 - Extreme ultraviolet source with rotary electrodes and method for producing extreme ultraviolet radiation from gas-discharge plasma - Google Patents

Abstract

FIELD: technology for producing extreme ultraviolet radiation from gas-discharge plasma.

SUBSTANCE: as electrodes, two disk-shaped elements are utilized, central symmetry axes of which are combined with axis of shaft, peripheral portion of surface of at least one of electrodes is covered by later of low melting-point metal, and also provided is a system for feeding low-melting point metal onto surface of at least one of aforementioned electrodes, and as system for initiation of discharge device for forming steam channel in peripheral area of inter-electrode gap is utilized.

EFFECT: increased frequency of movement of pulses, increased average power of extreme ultraviolet emission of gas-discharge plasma while providing for small dimensions thereof, increased lifetime and increased efficiency of extreme ultraviolet source, high stability of emission from pulse to pulse.

2 cl, 2 dwg

Description

The invention relates to a device and method for producing predominantly extreme UV radiation, as well as X-ray and soft X-ray radiation from a dense hot plasma of pinch-type discharges with rotating electrodes. The field of application includes EUV lithography, in particular, in the spectral band of 13.5 ± 0.135 nm, which corresponds to the effective reflection range of mirror optics with a Mo / Si coating.

A known device and method for producing EUV radiation from a plasma of a cylindrical z-pinch in xenon with a rf discharge initiation system [1]. For the data of the device and method, the auxiliary RF discharge generation system combined with the discharge chamber prevents efficient cooling of the electrodes, and the dielectric wall of the discharge chamber is exposed to powerful radiation fluxes, which limits the possibility of increasing the power of EUV radiation.

Partially these disadvantages are deprived of the EUV radiation source, in which a rotating disk-shaped insulator with many holes is placed between two electrodes having coaxial holes [2]. In the method for producing EUV radiation from a gas-discharge plasma by means of said device, a pinch-type discharge is ignited at the moment of alignment of the axis of one of the holes of the rotating insulator with the axis of the holes of the electrodes. However, in this device and method, the problem of effective cooling of the electrodes and increasing their life time with increasing power of EUV radiation has not been solved.

To a large extent, these drawbacks are devoid of the method of obtaining EUV radiation from a laser plasma obtained in the regime with a high pulse repetition rate when focusing a pulsed laser beam on a submillimeter target containing a working substance whose emission lines of atomic elements are in the desired region of the EUV range [3]. A device [3] that implements this method includes an injector of successive solid or liquid submillimeter targets containing a working substance, such as xenon (Xe), lithium (Li), tin (Sn) or tin oxide (SnO ₂ ) , and a laser system that forms a powerful laser beam with a high repetition rate. One of the drawbacks of the source of EUV radiation based on laser plasma is due to its low efficiency compared to gas-discharge EUV sources.

The closest technical solution, selected as a prototype, is an EUV source with rotating electrodes that are rigidly fixed to the shaft, which includes a pulsed discharge initiation system and a discharge power source connected to the electrodes [4]. The EUV source contains n identical pairs of electrodes with coaxial holes located at the same distance from the axis of rotation, and the discharge initiation system is made in the form of an auxiliary sliding discharge forming device on the surface of a cylindrical dielectric mounted motionless at the same distance from the axis of rotation of the shaft as n pairs of rotating electrodes.

A method of obtaining EUV radiation from a gas-discharge plasma by means of the indicated device consists in initiating a discharge between bipolar electrodes and igniting a pinch-type discharge during rotation with a shaft frequency ν [4]. In the indicated method, a discharge is initiated at the moment of alignment of the axis of one of the rotating electrode pairs with the axis of the stationary auxiliary discharge formation system by gas preionization in the discharge zone, and a pinch-type discharge is ignited in a gas or vapor-gas mixture with a repetition frequency f = n · ν sequentially in each of n electrode pairs.

The prototype allows for a long electrode life at a high average power of EUV radiation.

The disadvantage of the prototype is the limited ability to increase the pulse repetition rate, which complicates the increase in the average power of EUV radiation to a level that meets the requirements of industrial applications, the most important of which is EUV lithography at λ = 13.5 nm. This disadvantage is due to the fact that in the prototype a pinch-type discharge is ignited in the discharge zone between electrodes having axial symmetry. Such a discharge geometry determines a sufficiently large (> 1 cm) distance between the axes of adjacent electrode pairs, which at pulse repetition frequencies (≥7 kHz) required for EUV lithography requires high (> 70 m / s), linear electrode velocities that are difficult to realize. In addition, in the prototype, the use of metal vapors as a working substance may not be optimal, since the possibility of automatic generation of such vapors is provided for at the high-current phase of the discharge, and not at the stage of its initiation.

The technical result of the invention is to increase the pulse repetition rate and the average power of the EUV radiation of a gas-discharge plasma while ensuring its small size, increase the lifetime and increase the efficiency of the EUV source with high stability of the pulse-to-pulse radiation.

This task can be carried out by improving the EUV source with rotating electrodes, rigidly mounted on a shaft, which includes a pulse system for initiating a discharge and a discharge power source connected to the electrodes.

An improvement of the device consists in the fact that two disk-shaped elements are used as electrodes, the central axis of symmetry of which are aligned with the axis of the shaft, the peripheral part of the surface of at least one of the electrodes is covered with a layer of low-melting metal, and a system for supplying low-melting metal to the surface is introduced, according to at least one of the electrodes, and a device for generating a vapor channel in the peripheral region of the interelectrode gap was used as a system for initiating a discharge.

As a device for forming a vapor channel, a laser or an electron gun with a system for focusing the beam onto a layer of low-melting metal, which uses a working substance, mainly tin (Sn), the plasma of which has emission lines in the EUV spectral range, is used.

In one embodiment of the device, the system for supplying low-melting metal to the surface of the electrodes is made in the form of two bathtubs filled with liquid tin, while the electrodes are installed with the possibility of partial immersion in the baths during rotation.

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

A heat exchanger can be placed in each liquid tin bath to remove heat from the electrodes.

In addition, the electrodes can be equipped with blades to circulate liquid tin through a heat exchanger.

A method for producing EUV radiation from a gas discharge plasma by means of an EUV radiation source with rotating electrodes consists in initiating a discharge between bipolar electrodes and igniting a pinch-type discharge during rotation with a shaft frequency ν.

An improvement of the method consists in the fact that in a peripheral region of the interelectrode gap that is remote from the axis of rotation, a channel is formed from the vapor of the working substance to close the interelectrode gap of d value, and then a pinch type discharge is ignited, and discharge cycles with a repetition rate are performed f≥2πνR / d and compensate for the flow of the working substance by continuously supplying low-melting metal to the surface of at least one of the electrodes.

The improvement of the method also consists in the fact that the formation of the vapor channel is carried out by a pulsed laser or electron beam focused on a layer of low-melting metal.

The invention is illustrated by the accompanying drawings.

Figure 1 schematically shows a device EUV source with rotating electrodes. Figure 2 - device EUV source, in which the coating of the electrodes with fusible metal is carried out by partial immersion in baths with liquid tin.

The EUV source contains electrodes 2, 3, rigidly fixed on the shaft 1, made in the form of disk-shaped elements, the central axis of symmetry of which are aligned with the axis of the shaft. The peripheral part of the surface of at least one electrode 3 is covered with a layer 4 of fusible metal. As a system for initiating an EUV discharge, the source contains a vapor channel forming device 5, which includes a laser or an electron gun with a system for focusing the beam onto layer 4 of low-melting metal, covering the peripheral part of the surface of electrode 3. As a low-melting metal, a working substance, mainly tin, is used whose elements have emission lines in the EUV range of the spectrum. The switching power supply 6 is connected to the electrodes 2, 3 through the sliding contacts 7, 8. The sliding contacts can be made brush, liquid metal, or plasma. A switching power supply may include elements such as a storage capacitor with a switch, a pulse transformer, a pulse capacitor, and a magnetic key. In order to reduce the inductance of the discharge circuit, part of the elements of the switching power supply, for example, a switching capacitor and a magnetic key, can be mounted directly on the rotating electrodes. The electrodes 1, 2 are located in the discharge chamber 9. Near the discharge zone, the position of which is determined by the focusing point of the beam of the vapor channel forming device 5, a trap 10 is installed that prevents the propagation of vapor of the working substance and the discharge plasma ions into the output region of the EUV radiation. A collector chamber 11 is connected to the discharge chamber 9, into which the EUV radiation is output. The discharge chamber 9 is equipped with a vacuum pumping system 12 to provide a low vapor and gas pressure therein, and a pair of rotational motion is sealed to seal the shaft 1. In another embodiment of the device, a magnetic coupling can be used to seal the shaft 1. The EUV source also contains a system 13 for supplying fusible metal to the surface of at least one of the electrodes. As a fusible metal, a working substance is used, mainly tin, the plasma of which most efficiently emits in the EUV spectral region. The system 13 serves to supply the fusible metal to the surface of the electrodes 2, 3 with one of the following methods: tinning (wetting), spraying, hydraulic filling of porous structures made in the material of the electrodes. In order to remove heat, the electrodes 2, 3 are equipped with channels for the circulation of the cooling fluid 14 (Fig.1).

In the embodiment of the device shown in FIG. 2, both rotating electrodes 2, 3 are coated with a tin layer 4. For this, in the area which is not optically connected with the discharge zone, two baths 15, 16 with liquid tin 17 are placed in which the electrodes are partially immersed. The baths 15, 16 are electrically isolated from each other, while the liquid metal filling the baths 17 simultaneously serves as liquid-metal sliding contacts, through which the power source 6 is connected to the rotating electrodes 2, 3. In addition, to remove heat in the baths 15, 16 with liquid tin heat exchangers 18 are placed inside which coolant circulates. Moreover, each rotating electrode is equipped with blades 19 to ensure the circulation of liquid tin through heat exchangers 18.

A method of obtaining EUV radiation from a gas discharge plasma is implemented as follows.

Around the axis of the shaft 1 with a frequency ν, a uniform rotation of the disk-shaped electrodes 2, 3, rigidly fixed on it, is performed, the peripheral part of at least one of which is covered with a layer 4 of low-melting metal. When the pulse device 5 is turned on by a laser or electron beam focused on the low-melting metal layer 4 in the peripheral region of the interelectrode gap, an optimally small portion of the low-melting metal is evaporated. As a result, a vapor channel is formed that closes the discharge gap between the rotating electrodes 2, 3 in a fixed region of space determined by the beam focusing site. Turn on the switching power supply 6 and produce ignition of the discharge type pinch in the environment of metal vapor and ambient gas. One of the varieties of the pinch-type discharge can be a high-current vacuum spark. The discharge current flows through a circuit including a switching power supply 6, sliding contacts 7, 8, rotating electrodes 2, 3. In the course of the discharge current, additional evaporation of the fusible metal occurs, which increases the density of the gas-discharge plasma. Due to the use of a working substance as a low-melting metal, in particular tin, in the vapor of which a pinch-type discharge is carried out, highly efficient emission of EUV radiation from the discharge plasma is ensured. The used part of the radiation is removed from the discharge chamber 9 through a translucent trap 10 into the collector chamber 11. The trap 10 prevents vapor, erosion products of the electrodes and plasma flows from entering the collector chamber 11. For a more efficient operation of the trap 10, a buffer gas, for example Ar, is introduced into it. After the electrodes 2, 3 are turned by an angle sufficient to remove the hot steam and gas plugs from the discharge zone, which includes the focus area of the laser or electron beam of the vapor channel 5 formation system, the operation cycle is repeated. Since the transverse dimensions of the plasma pinch do not exceed the size d of the interelectrode gap, for spatial stabilization of the pinch it is sufficient to shift the peripheral part of the electrodes by a distance smaller than d. This allows for discharge cycles with a frequency f> 2πνR / d, where R is the distance from the axis of the shaft 1 to the discharge zone. At R = 300 mm, d = 3 mm, ν = 650 rpm, the permissible pulse repetition rate f exceeds 7 kHz. In the process, using the vacuum pumping system 12 provide a low vapor and gas pressure in the discharge chamber 10, which reduces the absorption of EUV radiation. Also in the process, using the system 13 is continuously fed to the surface of the electrode 3 tin (Sn). This compensates for its decrease as a result of discharge pulses. The cooling of the electrodes 2, 3 is performed by the coolant circulating in them 14.

In the embodiment of the device shown in FIG. 2, a pinch-type discharge current flows through a circuit including electrodes 2, 3, a switching power supply 6 and sliding contacts. Due to the partial immersion of the electrodes in the baths 15, 16 with liquid metal (tin) 17, fusible metal is supplied to the surface of both electrodes by wetting. This ensures the restoration of the consumable material at both electrodes, which increases the resource of the EUV source. In the process, heat is removed from the electrodes through the molten metal 17 filling the baths 15, 16 to the heat exchangers 18. The blades 19, which are equipped with disk-shaped electrodes 2, 3, circulate the liquid metal between the electrodes and the heat exchangers 18, which improves the cooling of the electrodes.

The implementation of the source of EUV radiation with rotating electrodes in the specified form allows you to discharge the pinch type in the peripheral region of the disk-shaped electrodes having any azimuthal coordinate. Moreover, unlike the prototype, which contains a discrete number of rotating discharge chambers, careful synchronization of the inclusion of the discharge initiation system with the moment of passage of a certain coordinate by the rotating electrode system is not required. This greatly simplifies the operation of the EUV radiation source with rotating electrodes. In addition, in the prototype the magnitude of the rotation of the electrode system between successive pulses is several degrees, while in the proposed EUV source, a rotation of less than tenths of a degree is necessary. This allows you to significantly increase the repetition rate of discharge pulses and the average power of the EUV radiation while reducing the rotation speed. Due to this, it is possible to obtain an average radiation power, in particular at λ = 13.5 nm, which is necessary for industrial use in EUV lithography with a resolution of ≤ 50 nm.

The initiation of the discharge by forming a vapor channel in the peripheral region of the interelectrode gap provides high positional stability of the pin-type discharge between the rotating electrodes and, as a result, of the EUV emitter itself. The formation of a vapor channel by a pulsed laser or electron beam focused on a layer of low-melting metal on the electrode surface allows one to form a vapor channel in small volumes for short times, which, along with high spatial stability of the EUV emitter, allows one to obtain small dimensions of the emitting region necessary for basic applications.

The use of a working substance, primarily tin, as a low-melting metal, provides high-efficiency discharge plasma radiation in the EUV spectral range, in particular, near λ = 13.5 nm. In this case, the introduction of a system for continuously supplying fusible metal to the surface of at least one of the electrodes compensates for the consumption of the working substance during operation and ensures high stability of the EUV radiation energy from pulse to pulse.

The coating of the electrodes with fusible metal, carried out by partially immersing them in bathtubs with liquid tin, provides compensation for the material flow of both electrodes resulting from discharge pulses. This increases the lifetime of the electrode system and the EUV source as a whole. The connection of the power source to the rotating electrodes by means of liquid tin filling the baths ensures the supply of energy to the discharge through liquid metal contacts with a high resource. Placing a heat exchanger in each bath with liquid tin and supplying electrodes with blades for circulating liquid tin through the heat exchanger provides a high cooling rate of the electrodes using a liquid metal coolant.

Thus, the proposed device and method can increase the efficiency and average power of the EUV radiation of a hot discharge plasma while decreasing its size, increase the lifetime of the EUV source, and increase its spatial and energy stability.

Information sources

1. US patent 5504795; class US 378/119; Declared 6.02.96.

2. US patent 6677600; class U.S. 250 / 504R; claimed 03/27/2003.

3. Patent WO 03085707; MKI6 H 01 L 21/027, H 05 N 1/24; claimed 04.04.2003.

4. Application for a patent of the Russian Federation No. 20022120301 dated 07/31/2002.

Claims (8)

1. A source of EUV radiation with rotating electrodes rigidly fixed to the shaft, including a pulsed discharge initiation system and a discharge power source connected to the electrodes, characterized in that two disk-shaped elements are used as electrodes, the central axis of symmetry of which are aligned with the axis of the shaft , the peripheral part of the surface of at least one of the electrodes is covered with a layer of fusible metal, and a system for supplying fusible metal to the surface of at least one of the electric ktrodov, and as the discharge initiation system applied to the device forming the vapor channel in the peripheral region of the interelectrode gap. 2. The source of EUV radiation with rotating electrodes according to claim 1, characterized in that a laser or an electron gun with a beam focusing system on a layer of low-melting metal, which is used as a working substance, mainly tin (Sn), is used as a device for forming a vapor channel whose plasma has emission lines in the EUV range of the spectrum. 3. The source of EUV radiation with rotating electrodes according to claim 1, characterized in that the system for supplying low-melting metal to the surface of the electrodes is made in the form of two bathtubs filled with liquid tin, while the electrodes are installed with the possibility of partial immersion in the baths during rotation. 4. A source of EUV radiation with rotating electrodes according to claim 3, characterized in that the baths are electrically isolated from each other and the power source is connected to the rotating electrodes by means of liquid tin filling the baths. 5. A source of EUV radiation with rotating electrodes according to claim 3, characterized in that a heat exchanger is placed in each bath with liquid tin to remove heat from the electrodes. 6. A source of EUV radiation with rotating electrodes according to claim 5, characterized in that the electrodes are equipped with blades for circulating liquid tin through a heat exchanger. 7. A method of obtaining EUV radiation from a gas discharge plasma by means of an EUV radiation source with rotating electrodes, which consists in initiating a discharge between bipolar electrodes and igniting a pinch type discharge during rotation with a shaft frequency ν, characterized in that in the peripheral one remote at a distance R from the axis rotation of the interelectrode gap region pulse form a channel of working substance vapors to close the interelectrode gap of d value, and then the pinch type discharge is ignited, while produce discharge cycles with a repetition rate f≥2πνR / d and compensate for the flow of the working substance by continuously supplying fusible metal to the surface of at least one of the electrodes. 8. The method of obtaining EUV radiation from a gas discharge plasma according to claim 7, characterized in that the formation of the vapor channel is carried out by a pulsed laser or electron beam focused on a layer of fusible metal.

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