Extreme ultra violet light source apparatus

In an LPP type EUV light source apparatus, the intensity of radiated EUV light is stabilized by improving the positional stability of droplets. The extreme ultra violet light source apparatus includes: a chamber in which extreme ultra violet light is generated; a target supply division including a target tank for storing a target material therein and an injection nozzle for injecting the target material in a jet form, for supplying the target material into the chamber; a charging electrode applied with a direct-current voltage between the target tank and itself, for charging droplets when the target material in the jet form injected from the injection nozzle is broken up into the droplets; a laser for applying a laser beam to the droplets of the target material to generate plasma; and a collector mirror for collecting extreme ultra violet light radiated from the plasma to output the extreme ultra violet light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.

2. Description of a Related Art

Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, the development of exposure equipment is expected by combining an EUV light source generating EUV light having a wavelength of about 13 nm and a reduced projection reflective optics.

As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP type EUV light source apparatus has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.

Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be explained. By applying a laser beam to a target material supplied into a vacuum chamber, the target material is excited and plasmarized. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit. For the purpose, a multilayer film in which thin films of molybdenum (Mo) and thin films of silicon (Si) are alternately stacked (Mo/Si multilayer film), for example, is formed on the reflecting surface of the EUV collector mirror.

FIG. 7shows a droplet target generating device and a part around the device in a conventional EUV light source apparatus. As a target material, for example, tin (Sn) melted into the liquid state, lithium (Li) melted into the liquid state, or a material formed by dissolving colloidal tin oxide fine particles in water or a volatile solvent such as methanol is used.

The target material introduced into a target tank101is pressurized with a pure argon gas or the like, for example, and a jet of the target material is ejected from an injection nozzle102attached to the leading end of the target tank101and having an inner diameter of several tens of micrometers. When regular disturbance is provided to the jet by using a vibrator (not shown) attached to the injection nozzle102or near the injection nozzle102, a jet part1aof the target material immediately breaks up into droplets1bhaving homogeneous diameters, shapes, and intervals. The method of generating the homogeneous droplets in this manner is called a continuous jet method.

The generated homogeneous droplets1bmove within a vacuum chamber100according to the inertia when the jet is ejected from the injection nozzle102, and a laser beam radiated from a CO2laser or YAG laser, for example, is applied thereto at a laser application point. Thereby, the target material is plasmarized and EUV light is radiated from the plasma. The droplets that have not irradiated with laser are collected by a target collecting unit106provided at the opposite side to the injection nozzle102with the laser application point in between.

However, in the conventional technology, the stability of the positions of droplets are gradually lost and the positions become unstable before the droplets reach the laser application point, and variations in positions are increased especially in the traveling direction of the droplets. As a result, the laser beam is no longer applied to the droplets constantly in the same condition, and there is a problem that the intensity of the radiated EUV light varies and, in the worst case, the laser beam is not applied to the droplets and no EUV light is generated. The trouble due to instability in positions of droplets becomes significant as the inner diameter of the injection nozzle102becomes smaller and the diameters of the droplets and intervals between the droplets become smaller.

FIG. 8is a photograph of droplets generated by the droplet target generating device shown inFIG. 7. As the target material, melted tin is used. As shown inFIG. 8, the turbulence occurs in the positional stability of droplets at the laser application point, and the intervals between droplets are inhomogeneous and plural droplets are combined in some locations.

As one method of solving the problem, it is conceivable to apply laser beam to the droplets in a point where the positional stability of droplets is in a relatively good condition, that is, a point at a flying distance from the injection nozzle102is short (e.g., a point at a distance of about 50 mm from the injection nozzle102). However, since the laser poser to be used in the EUV light source is 10 kW or more, the heat input to the injection nozzle102or the part around the nozzle is greater, the stably droplet generation is not maintained, and consequently, the performance of the EUV light source is deteriorated.

As a related technology, U.S. Patent Application Publication US 2006/0192154 A1 discloses EUV plasma formation target delivery system and method. The target delivery system includes: a target droplet formation mechanism comprising a magneto-restrictive or electro-restrictive material, a liquid plasma source material passageway terminating in an output orifice; a charging mechanism for applying electric charge to a droplet forming jet stream or to individual droplets exiting the passageway along a selected path; a droplet deflector positioned between the output orifice and a plasma initiation site, for periodically deflecting droplets from the selected path, a liquid target material delivery mechanism comprising a liquid target material delivery passage having an input opening and an output orifice; an electromotive disturbing force generating mechanism for generating a disturbing force within the liquid target material, a liquid target delivery droplet formation mechanism having an output orifice; and/or a wetting barrier around the periphery of the output orifice. However, US 2006/0192154 A1 does not particularly disclose improvements in positional stability.

Further, HEINZL et al., “Ink-Jet Printing”, ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS, U.S., Academic Press, 1985, Vol. 65, pp. 91-171 describes an explanation about the continuous jet method. According to HEINZL et al., the transformation from laminar to turbulent-like jet flow depends on the aspect ratio L/d of the nozzle, where “L” is the length and “d” is the diameter. Further, laminar-flow jets break up into a train of drops at some point due to surface tension. This is due to the fact that the surface energy of a liquid sphere is smaller than that of a cylinder having the same volume. Therefore, a jet of fluid column having, for example, a cylindrical shape is inherently unstable and will eventually transform itself into drops having a spherical shape (page 132).

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the above-mentioned problems. A purpose of the present invention is to stabilize the intensity of radiated EUV light by improving the positional stability of droplets in an LPP type EUV light source apparatus.

In order to accomplish the above purpose, an extreme ultra violet light source apparatus according to one aspect of the present invention is an extreme ultra violet light source apparatus for generating extreme ultra violet light by applying a laser beam to a target material to turn the target material into a plasma state, and the apparatus includes: a chamber in which extreme ultra violet light is generated; a target supply division including a target tank for storing a target material therein and an injection nozzle for injecting the target material in a jet form, for supplying the target material into the chamber; a charging electrode applied with a direct-current voltage between the target tank and itself, for charging droplets when the target material in the jet form injected from the injection nozzle is broken up into droplets; a laser for applying a laser beam to the droplets of the target material to generate plasma; and a collector mirror for collecting extreme ultra violet light radiated from the plasma to output the extreme ultra violet light.

According to the present invention, since there is provided the charging electrode for charging the droplets when the target material in the jet form injected from the injection nozzle is broken up into the droplets, the intensity of the radiated EUV light can be stabilized by homogenizing the intervals between the droplets with the repulsive force between the droplets due to the electric charge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be explained in detail by referring to the drawings. The same reference characters are assigned to the same component elements and the description thereof will be omitted.

FIG. 1is a schematic diagram showing an extreme ultra violet (EUV) light source apparatus according to the first embodiment of the present invention. The EUV light source apparatus adopts an LPP (laser produced plasma) type and used as a light source of exposure equipment.

As shown inFIG. 1, the EUV light source apparatus according to the embodiment includes a vacuum chamber (EUV light generation chamber)9in which EUV light is generated, a target supply unit10that supplies a target material, a target tank11for storing the target material therein, an injection nozzle (nozzle unit)12for injecting the target material in a jet form, a laser oscillator13, a laser beam focusing optics14, an EUV collector mirror15, a target collecting unit16, a vibration mechanism19, a charging electrode20, a power supply and control unit21. Here, the target supply unit10to the injection nozzle12form a target supply division for supplying the target material into the vacuum chamber9.

The vacuum chamber9is provided with a lead-in window17that leads in a laser beam (excitation laser beam)3for exciting a target1and generating plasma2into the vacuum chamber9and a lead-out window18that leads out EUV light4radiated from the plasma2to an exposure unit. The exposure unit as an output destination of the EUV light4is also provided in vacuum (or under reduced pressure) like the interior of the vacuum chamber9.

The target supply unit10introduces the target material in the liquid state into the target tank11, and supplies the target material stored in the target tank11to the injection nozzle12at a predetermined flow rate by pressurizing it with a pure argon gas or the like, for example.

As the target material, tin (Sn) melted into the liquid state, lithium (Li) melted into the liquid state, or a material formed by dissolving colloidal tin oxide fine particles in water or a volatile solvent such as methanol, or the like is used. For example, when tin is used as the target material, liquefied tin formed by heating solid tin, an aqueous solution containing tin oxide fine particles, or the like is supplied to the injection nozzle12.

The injection nozzle12injects the supplied target material into the vacuum chamber9. The target material is broken up and changes from the jet state into droplet state. In order to generate droplets at a predetermined frequency, the vibration mechanism (e.g., a PZT vibrator)19is provided for vibrating the injection nozzle12at the predetermined frequency. Further, a position adjustment mechanism for adjustment of the position of the injection nozzle12may be provided such that the target1as droplets may pass through the application position of the excitation laser beam3.

The laser oscillator13outputs the excitation laser beam3to be applied to the target1by laser oscillation. The laser beam focusing optics14collects the laser beam3outputted from the laser oscillator13to apply it to the target1via the lead-in window17.

The EUV collector mirror15has a concave reflecting surface and reflects a predetermined wavelength component (e.g., EUV light having a wavelength of 13.5 nm±0.135 nm) of the light emitted from the plasma and collects it toward the exposure unit. For the purpose, a multilayer film (e.g., an Mo/Si multilayer film) for selectively reflecting the wavelength component is formed on the reflecting surface of the EUV collector mirror15. The number of layers of the multilayer film is typically about several tens to several hundreds. An opening for passing the laser3is formed in the EUV collector mirror15.

The target collecting unit16recovers the target material that has been injected from the injection nozzle12but not irradiated with the laser beam3and has not contributed to plasma generation. Thereby, reduction in the degree of vacuum (rise in pressure) within the vacuum chamber9and contamination of the EUV collector mirror15, the lead-in window17, and so on are prevented.

In such an EUV light source apparatus, the target1in a droplet form is formed and the laser beam3is applied to the target1by laser oscillation, and thereby, the plasma2is generated. The light radiated from the plasma2contains various wavelength components at various energy levels. A predetermined wavelength component (EUV light) of them is reflected and collected toward the exposure unit by the EUV collector mirror15. Thus generated EUV light is used as exposure light in the exposure unit.

FIG. 2shows a droplet target generating device and a part around the device in the EUV light source apparatus according to the first embodiment of the present invention. As shown inFIG. 2, the target material is in the jet state (a jet part1a) immediately after injected from the injection nozzle12, and changes into the droplet state (droplets1b) at a predetermined distance from the injection nozzle12.

The charging electrode20for charging the droplets is provided under the injection nozzle12. In order to apply an electric field between the charging electrode20and the jet part1ainjected from the injection nozzle12, the power supply and control unit21is provided for applying a constant direct-current voltage between the target tank11and the charging electrode20. In the embodiment, the potential of the target tank11is set to the ground potential (0V) and a positive direct-current voltage is applied to the charging electrode20.

Thereby, the jet part1afunctions as one electrode of a pair of electrodes. In this regard, electric charge depending on the voltage between the electrodes emerges at the leading end of the jet part1a, and thus, while the jet part1ais being broken up into homogeneous droplets1b, the amount of electric charge accumulated in the respective droplets1bbecomes extremely homogeneous. Therefore, the respective droplets1bwill have the same mass and electric charge, and therefore, intervals between them are kept equal to each other by the repulsive force due to charge.

FIG. 3is a photograph of droplets generated by the droplet target generating device shown inFIG. 2. As the target material, melted tin is used. This photograph is taken at the same observation location as the observation location of the conventional technology inFIG. 8, and it is known that intervals between the droplets are kept equal to each other due to charge of the droplets.

The charging electrode20shown inFIG. 2may take a cylindrical shape, a parallel plate shape, a ring shape, or the like, and the cylindrical charging electrode20is used in the embodiment. Further, the droplet generation position where the jet part1achanges to droplets1bis desirably within the charging electrode20. In this case, effective charging of the droplets1bbecomes possible according to the theoretical equations explained as below, and the positional stability of the droplets1bcan be improved by a simple configuration and a small voltage.

FIG. 4is a diagram for explanation of changing of the target material by the charging electrode. As shown inFIG. 4, when the charging electrode20has a cylindrical shape, the amount of charge “Q” of one droplet is expressed by the equation (1) of the charge emerging at the electrode of a coaxial capacitor.
Q=2πε·V·vJ/{f·log(b/a)}  (1)
where “ε” is permittivity of vacuum, “V” is a voltage applied to the charging electrode20, “vJ” is an injection velocity of the target, “f” is a generation frequency of the droplets1b(vibration frequency of the vibrator), “a” is a diameter of the jet part1a, and “b” is a diameter (inner diameter) of the cylindrical charging electrode20.

On the other hand, the repulsive force “F” between the droplets is expressed by the following equation (2).
F=k·Q2/L2
where “k” is a proportional constant, “L” is an interval between droplets. When the amounts of charge “Q” of the droplets becomes excessive, the repulsive force “F” between the droplets becomes too strong, the droplets are displaced in a direction perpendicular to the traveling direction, and the row of the droplets becomes out of line.

For example, in experiments using water, it is known that intervals are maintained with the row of the droplets in line under a condition that the acceleration by the repulsive force between charged droplets is equal to or less than 500 m/s2regardless of the size and interval of the droplets. On the other hand, the row of the droplets inevitably becomes out of line under a condition that the acceleration by the repulsive force between charged droplets is equal to or less than 2000 m/s2Therefore, the amount of charge of the droplets should be an amount of charge enough to make the intervals between droplets homogeneous and make the acceleration with which adjacent droplets do not repulsively act (according to the experimental result, about 500 m/s2or less).

FIG. 5shows relationships between the voltage of the charging electrode and the change in the acceleration generated in the droplets. InFIG. 5, the horizontal axis indicates the voltage (kV) of the charging electrode, and the vertical axis indicates the acceleration (m/s2) by the repulsive force between the droplets. Further, the droplet generation frequency (kHz) is taken as a parameter.

In the acceleration shown inFIG. 5, range (A) is a stable range in which initial variations in position of droplets do not affect the subsequent positional relationship, range (B) is an intermediate range in which there is a possibility that the row of droplets may be out of line due to initial variations in position of droplets, and range (C) is an unstable range in which the row of droplets is inevitably out of line due to initial variations in position of droplets. According toFIG. 5, it is known that, if the voltage of the charging electrode is set to 1 kV or less, when the droplet generation frequency is at least 50 kHz to 120 kHz, the acceleration by the repulsive force between the droplets falls in the stable range (A).

Next, the second embodiment of the present invention will be explained.

FIG. 6shows a droplet target generating device and a part around the device in an EUV light source apparatus according to the second embodiment of the present invention. In the second embodiment, at least a part of an injection nozzle12ahas an electric insulation property. The rest is the same as that of the first embodiment.

Generally, the actual length of the jet part ejected from the injection nozzle is extremely short. For example, the length of the jet part is 1 mm or less in most cases when vibration is applied by a vibrator to the jet ejected from an injection nozzle having an inner diameter of 15 um at a velocity of 20 m/s and droplets are formed. Therefore, from a practical point of view, in order to allow the droplet generation position to exist within the injection nozzle in an ideal condition as explained in the first embodiment, it is necessary to place the charging electrode as close to the injection nozzle as possible.

However, since the voltage applied to the charging electrode is of the order of kV, when the injection nozzle has conductivity, a very large electric field is generated between the charging electrode and the injection nozzle. Accordingly, in the second embodiment, at least the part of the injection nozzle (nozzle unit)12a, especially, the part close to the charging electrode20is formed of an insulating material such as ceramics, and thereby, the charging electrode20can be placed closer to the injection nozzle12a.

More preferably, if the charging electrode20is directly attached to the insulating part of the injection nozzle12a, even when a voltage of several kilovolts is applied to the charging electrode20, breakdown do not occur between them and the relative positions of them are stable. In this case, even when the jet part1aof the target is short, the droplet generation position can be allowed to exist within the charging electrode20, and thereby, the droplets1bcan be charged in the ideal condition as explained in the first embodiment.

In fact, even in the case where the droplet generation position does not exist within the charging electrode20and the target becomes droplets at the upstream of the charging electrode20, the droplets are charged by the charging electrode20. According to the embodiment, since the charging electrode20can be placed close to the injection nozzle12a, the droplets can be efficiently charged in that case.