EA042061B1 - A PLASMA SOURCE OF EXTREME UV RADIATION AND A LITHOGRAPHIC SYSTEM WITH ITS USE - Google Patents

Description

Cross-reference to related patents and patent applications

This application is a continuation of patent application PCT/EP 2020/061562, filed 04/24/2020, which in turn is a continuation of patent application US 16/536,404 filed 08/08/2019, now US 10588210, which, in turn, is a continuation of RF application 2019113052 filed on April 26, 2019, now RF patent 2806713, and RF application 2020103063, filed on January 25, 2020, now RF patent 2826316, as well as a continuation of PCT application RU/2018/000520, filed on August 08, 2018, which, in its turn, is a continuation of the application for the patent of the Russian Federation 2017141042, filed on October 24, 2017, now the patent of the Russian Federation 2670283, which are included in this description by reference in its entirety.

Technical field

The invention relates to a high-brightness source of extreme ultraviolet (EUV) or soft X-ray radiation (MRI) based on laser plasma and to lithographic systems using it. Applications include the fabrication of semiconductor devices, in particular the mid- and low-volume production of nanoscale integrated circuits (ICs), as well as ancillary work in the large-scale production of next-generation ICs by EUV lithography.

Prior Art

EUV radiation (also sometimes referred to as soft X-rays) is usually defined as electromagnetic radiation with a wavelength in the range of about 5-100 nm. Modern EUV lithography mainly uses the wavelength range of 13.5±0.135 nm, which corresponds to the reflection range of Mo/Si multilayer mirrors, but is not limited to this option. Methods for producing EUV radiation include, but are not necessarily limited to, converting a target material into a plasma of a chemical element having emission lines of ions in the EUV range. For highly efficient generation of EUV radiation at 13.5 nm, it is known to use high-temperature plasma of a number of elements, in particular, xenon (Xe), lithium (Li), tin (Sn).

A lithographic system or lithograph is a machine that applies a desired pattern to a wafer, typically a target portion of the wafer in the manufacture of semiconductor devices (transistors, diodes, etc.), typically metal-oxide-semiconductor (MOS) devices.

The EUV lithography system, or EUV lithograph, is a key element in a production line or factory for the production of semiconductor devices.

The EUV lithograph includes an EUV radiation source and a mirror-based illumination system that generates a radiation beam incident on a patterning device. Usually, the pattern forming device is also called a mask or photomask. In the EUV lithograph, the mask (photomask) is made in the form of a reflective element. The EUV radiation beam formed by the projection optical system, which carries the image of the mask on a reduced scale, is used to create nanoscale structural elements on the plate.

The EUV lithograph can be used, in particular, in the manufacture of integrated circuits (ICs). In this case, the pattern forming device is used to create a circuit pattern to be formed on a separate section or layer of the IC. The pattern is usually transferred onto a layer of a radiation-sensitive material (photoresist) deposited on the plate. As a rule, one plate will contain many adjacent target parts, which will be sequentially structured in accordance with the topology of the mask image transferred to these parts.

Known lithographic systems include so-called steppers, in which each target area is irradiated by simultaneously exposing the entire pattern to a wafer area, and so-called scanners, in which each wafer area is irradiated by scanning the mask with a radiation beam in a predetermined direction (scanning direction) while simultaneously scanning the target parts of the plate parallel or anti-parallel to this scanning direction, as is known, for example, from patent application WO 2021121985 published on 06/24/2021.

A high-performance lithographic system requires a very advanced infrastructure to function. In particular, it includes the most powerful (~ 250 W/2%BW/2π cp) and highly efficient (with a conversion factor of CE13.5 ~ 5%) EUV radiation source using a tin laser plasma obtained by irradiating a target material in the form drops with two laser beams, as is known, for example, from WO 2014082811, publ. 06/05/2014. High-performance nanolithographs with such an EUV source make it possible to carry out large-scale (up to 160 plates with a diameter of 300 mm/h) production of ICs (memory chips) and processors of a new generation with a chip element size of only a few nanometers. With over 100,000 components, this EUV lithograph is one of the most complex machines ever built. It includes the most powerful gas CO ₂ laser system on the lower floor of the lithograph ever mass-produced. In total, such a nanolithograph weighs 180 tons and consumes more than 1 MW of electricity, and its cost exceeds 100 million USD.

The most complex and expensive elements of such a lithographic infrastructure are masks. A set of masks in the production of one IC can cost up to several million

- 1,042,061 dollars. Therefore, due to the high complexity and cost of equipment, high cost of masks, complex and expensive infrastructure, projection photolithography becomes competitive only in mass production. All this together makes this technology inaccessible or accessible only to individual global players.

Accordingly, there is a need to have relatively simple and compact EUV lithographs available for a relatively wide range of applications in relatively small batch production.

One of the approaches to solving this problem is the development of maskless lithographic systems that provide topological norms typical for projection lithography: in the region of 100–10 nm or less. The performance of a maskless nanolithograph can be one to two orders of magnitude lower than in the case of mass projection lithography, and the cost of the lithographic process should not depend strongly on the scale of production, which will ensure greater availability of lithographic production.

In one of these lithographic systems, the light beam is divided into two (or four) beams that interfere on the target part of the plate, as is known, for example, from application WO/2009/083229, publ. 07/09/2009. The interference causes the formation of lines or other pattern on the target part of the plate, Ekinci, Yasin et al. Evaluation of resist performance with EUV interference lithography for sub-22nm patterning, Advanced Lithography (2012).

In another maskless lithographic system, known, for example, from US Pat. In this technology, the image topology on the photoresist is encoded by the state of MOEMS pixels (micromirrors) that reflect EUV radiation.

Currently, the MOEMS pixel size is 16x16 µm with the number of pixels 1.05x106. Also known are MOEMS with a pixel size of 8x8 μm, and a pixel size of 4 μm can be implemented within the existing technology. Accordingly, when using a mirror lens with a reduction of 800x, the technological standards in lithographic production can be 20 nm, 10 nm or less, Salashchenko, N.N., Chkhalo, N.I. & Dyuzhev, N.A. Maskless X-Ray Lithography Based on Microoptical Electromechanical Systems and Microfocus X-Ray Tubes. J. Synch. Investig. 12, 944-952 (2018).

In the above work, it is proposed to use a relatively simple EUV radiation source based on xenon laser plasma with a high (theoretically up to 4%) expected efficiency of laser radiation conversion into plasma radiation at a wavelength of 11.2 nm. Also, a shorter wavelength compared to the 13.5 nm wavelength, which has already become traditional for EUV lithography, will improve the technological standards for manufacturing ICs.

However, the low absorption efficiency of laser pump radiation in Xe and the strong self-absorption of EUV radiation by xenon severely limit the possibility of obtaining high efficiency EUV radiation sources based on Xe plasma. You can also note the high cost of xenon, which requires a complex system of its recirculation. All this explains the reasons why EUV radiation sources based on laser Xe plasma have not gained popularity. In addition, operation with a radiation wavelength of 11.2 nm is associated with the use of Mo / Be mirrors containing beryllium, which is not always acceptable due to its toxicity.

Another spectral interval is associated with the use of La/B4C and C/B4C mirrors, which have a very high (theoretically up to 80%) reflectance in the spectral region of 6.6-6.7 nm, which makes it possible to significantly improve the technological standards of the IC manufacturing process or other lithographic process for by reducing the wavelength of the EUV radiation. To obtain radiation at 6.7 nm, the most effective (CE6.7~1.8%) is the use of refractory (~1300°C) gadolinium (Gd) or terbium (Tb) as a plasma-forming material. The material may contain a compound of the eutectic alloy Gd or Tb, which reduces the melting point of the target up to 650°C, as is known from the application WO 2009116867, publ. 09/24/2009.

However, the use of a refractory plasma-forming material in EUV lithographic sources is still an unresolved technical problem.

This disadvantage is overcome by using Li as a plasma-forming material with its relatively low melting point, 160.54°C, known, for example, from WO 2017217882, publ. 12/21/2017. Unlike other materials, the use of Li makes it possible to obtain practically monochromatic radiation from a single resonant transition of hydrogen-like Li ²⁺ at a wavelength of 13.5 nm. The width of the spectral emission line at half-height of the intensity does not exceed 0.02-0.03 nm, as is known, for example, from the publication of Schriever G, et al. Laser-produced lithium plasma as a narrow-band extended ultraviolet radiation source for photoelectron spectroscopy. Apple Opt. 1998 Mar 1; 37(7):1243-8.

Along with the emission of EUV radiation as an undesirable by-product, a stream of polluting particles (debris) is generated from the region of the emitting plasma, which include microdroplets, and

- 2 042061 also charged and neutral particles polluting the optical system integrated with the EUV radiation source. However, the proposed evaporative cleaning of optical elements when using lithium as a plasma-forming material does not completely solve the problem of their protection from contamination.

To a large extent, these disadvantages are eliminated in clean compact EUV radiation sources of a relatively simple design, known from the patent application PCT/RU 2018/000520, published on May 31, 2019 under the number WO 2019103648, PCT/EP 2020/061562, published on October 29, 2020 under the number WO 2020216950, and patent RU2743572, published February 20, 2021. These sources use a low-melting liquid metal laser target rotating at a high linear speed of about 100 m/s. This, firstly, ensures high stability of the laser target surface and does not require a complex system for synchronizing the laser beam and the target. Secondly, the high speed of the target makes it possible to suppress the flux of contaminant particles generated in the plasma along with the radiation and, above all, its most problematic droplet fraction, by redirecting it away from the spatial angle of the EUV radiation output and from the entrance window of the vacuum chamber, intended for laser beam input. .

This method of suppression of impurities is highly effective when using a target material with a sufficiently high density p, for example, tin with p=7.31 g/cm ³ . However, the use of Li as a target material is characterized by a too high speed of the droplet fraction of impurities due to the low density of the target material (p = 0.534 g/cm ³ ), fragments of which, as a result of exposure to explosive shock pulses in the interaction zone, acquire velocities reaching 1000 m /s, which sharply reduces the effectiveness of protection against the droplet fraction Li of pollution. In addition, the use of only a liquid metal target limits the choice of target materials, such as Gd or Tb, when it is necessary to obtain highly efficient generation of EUV radiation at a shorter wavelength.

Accordingly, there is a need to eliminate at least some of the disadvantages mentioned above. In particular, there is a need for lithographic systems of a relatively simple design using EUV sources, which would have sufficiently high output parameters of EUV radiation (average power, brightness) with almost complete attenuation of the flow of pollutant particles emerging from the plasma, would be compact, and capable of using various target materials: Sn, Li, Gd or Tb, - to expand the functionality of these lithographic systems.

Brief summary of the invention

The technical task and technical result of the invention is the improvement of EUV radiation sources based on laser plasma with a rapidly rotating target and the expansion of the arsenal of lithographic systems, mainly intended for small- and medium-scale production of semiconductor products, through the use of new improved EUV radiation sources of the specified type.

The goals set are satisfied by the features of the independent claims. The dependent claims describe embodiments of the invention.

According to an aspect of the invention, an EUV radiation source is proposed, containing a vacuum chamber with a rotating target assembly that feeds the target into the interaction zone with a pulsed laser beam focused on the target, which is a layer of target material formed under the action of centrifugal force on the surface of the annular groove facing the axis of rotation, made in the rotating target assembly, an EUV radiation beam (may also be referred to as a plasma radiation beam) emerging from the interaction zone to the optical collector, and pollution suppression means.

The difference between the source of EUV radiation is that the target material has fluidity under the action of centrifugal force, while the target has a centrifugal acceleration of at least 3000 g, where g is the acceleration of free fall, and a linear velocity of at least 100 m/s.

In a preferred embodiment of the invention, the target surface is circular with an axis of symmetry coinciding with the axis of rotation.

In a preferred embodiment of the invention, the target material belongs to the group consisting of a liquid, such as a molten metal or alloy; powder, such as metal powder, or mixtures thereof.

In a preferred embodiment of the invention, the target material is tin (Sn) or its alloy with indium (In).

In an embodiment, the target material contains a composition of lithium (Li) with at least one additional element selected from the group consisting of silver (Ag), gold (Au), bismuth (Bi), barium (Ba), strontium (Sr ).

The target material may be a eutectic alloy in which the atomic fraction of Li is in the range of 60 to 90%.

- 3 042061

In a preferred embodiment of the invention, the EUV radiation source has a maximum spectral brightness at a wavelength of 13.5 nm.

In another embodiment of the invention, the EUV radiation source has a maximum spectral brightness in the wavelength range of 6.6-6.8 nm, and the target material contains gadolinium (Gd) or terbium (Tb) or their compounds.

The target material may be an alloy of Gd and/or Tb with a metal selected from the group consisting of Ag, Cu, Ni, Fe, Co, Mn, Al, Ga, Cd, Ru and Rh.

In an embodiment of the invention, the optical collector comprises at least one multilayer mirror.

In an embodiment of the invention, the optical collector contains two blocks of ellipsoidal grazing-incidence mirrors arranged in tandem along the path of the EUV radiation beam, transmitting an image of the emitting region of the plasma without distortion, on a scale determined by the design of these blocks, to the second focal point of the second ellipsoidal mirror block, and the second focal point the point of the first ellipsoidal mirror block is located at the first focal point of the second ellipsoidal mirror block.

In a preferred embodiment of the invention, the paths of the pulsed laser beam and the EUV radiation beam are located in spatial regions with a minimum pollutant particle ejection velocity, which is at least 104 times less, preferably 106 times less than the maximum pollutant particle ejection velocity.

In a preferred embodiment of the invention, the linear velocity vector of the target in the interaction zone is directed on one side of the plane passing through the interaction zone and the axis of rotation, and the focused laser beam and the output beam of EUV radiation are located on the other side of the specified plane.

In a preferred embodiment of the invention, between the interaction zone and the optical collector, there is one or more means of protection against contamination, including a screen located outside the area of passage of laser and EUV radiation beams, a shielding gas flow, a magnetic field, a foil trap, a membrane with a transparency of more than 60% for EUV radiation from a material belonging to the group including carbon nanotubes (CNTs), Ti, Al, Si, Zr, Si, BN.

In a preferred embodiment of the invention, the pulsed laser beam is generated by a Q-switched, diode-pumped solid-state laser.

In another aspect, the invention relates to a lithographic system comprising an EUV radiation source and an illumination system for illuminating a patterning device.

The lithographic system is characterized in that the EUV radiation source is made in accordance with the present invention.

In a preferred embodiment of the invention, the patterning device refers to a group of devices including a photomask or lithographic mask; scanned photomask; micro-optical electromechanical system (MOEMS) of micromirrors; translucent quasi-template designed to form interfering beams of EUV radiation.

In a preferred embodiment of the invention, the membrane, which is part of the protection against pollution, also serves as a gas seal separating volumes with different pressures.

In a preferred embodiment of the invention, the volume of the vacuum chamber of the EUV radiation source contains argon and hydrogen gases or a mixture thereof, and the volume with the lighting system contains hydrogen gas.

Between the set of essential features of the present invention and the achieved technical result, there are the following cause-and-effect relationships.

The use of the proposed source of EUV radiation, characterized by simplicity of design, deep suppression of pollution, high brightness and stability, in general, provides a simplification of the design of the EUV lithograph, reduces its weight, dimensions, improves its reliability, technological and commercial availability, reduces operating costs, increases ease of use. .

The present invention has the advantage of using a wide range of target materials. When using Sn- or Li-containing liquid metal targets, the operation of the EUV lithograph is carried out at a wavelength of 13.5 nm, which has become traditional for EUV lithography. In the case of using Li, the radiation source is essentially monochromatic, which in some cases makes it possible to improve the quality of the lithographic process. At the same time, the use of a heavy Li alloy (instead of pure low-density lithium) provides highly effective suppression of contaminant particles generated in the plasma along with radiation.

The use of not only liquid-metal, but also any liquid and/or bulk materials of targets makes it possible to generate and use laser plasma radiation in various spectral ranges. In a particular case of implementation, the invention provides for the operation of a lithographic system with radiation at a wavelength of 6.7 nm due to the use of refractory materials as a target material, for example, Gd or Tb powder, allowing a significant reduction in technological standards

- 4 042061 lithographic production.

For a wide range of fluid target materials, high centrifugal acceleration (more than 3000 g) makes it possible to form a highly stable target under the action of centrifugal force, the surface of which, by the next pulse, assumes a constant round-cylindrical shape in the interaction zone, which ensures highly stable operation of the radiation source and the EUV lithographic system generally.

As is known, the drop fraction of polluting particles, which flies out of the interaction zone at a relatively low speed, plays a decisive role in the contamination of an optical collector. In accordance with the invention, it is effectively suppressed due to the fast rotation speed of the target (hundreds of Hz at a linear velocity of more than 100 m/s), which redirects the overwhelming majority of the droplet fraction of polluting particles away from the optical collector and from the input window designed to introduce the laser beam into the chamber with a source of EUV radiation. Further ensuring the purity of the source of EUV radiation is achieved by a whole range of pollution suppression means. Among them, it is worth noting a replaceable membrane, for example, from CNTs, which is practically transparent to EUV radiation, which also plays the role of a gas seal that separates the volumes of a lithographic system with different pressures, which contain an EUV radiation source and an illumination system.

The first of these volumes contains argon and hydrogen gases or a mixture thereof, and the second contains hydrogen gas to protect the surfaces of optical elements from contamination. The CNT membrane serves as a cardinal means of suppressing relatively small (less than 300 µm) droplets, as well as ionic and vapor fractions of contaminants, while for large droplets (more than 300 µm), the most effective method of protection is the rapid rotation of the target, redirecting them away from the optical collector. Together with the use of other means of protection against contamination, namely protective screens, shielding gas flows, magnetic fields, foil traps, this ensures the highest purity of the radiation source and a very long period of operation of the membrane before it is changed due to a decrease in transparency.

In the present invention, in contrast to the known use of drop targets, the need to synchronize the target with the laser beam and its preliminary formation is eliminated. In addition, the use according to the invention of a compact high-performance solid-state laser, in contrast to the use of a complex and cumbersome system of CO2 gas lasers, greatly simplifies the source of EUV radiation.

On the whole, all this increases the reliability of the EUV lithographic system, simplifies its design, and increases the ease of use.

The invention provides for the possibility of using various types of optical collector: based on multilayer normal-incidence mirrors and/or grazing-incidence mirrors, which makes it possible to optimize the pollution protection system and the parameters of the lighting system of the lithographic system.

The invention also provides for the possibility of using various types of pattern forming devices, which include:

lithographic mask, including scanned;

mask or pseudo-pattern for interference lithography;

MOEMS with mirrors having a reflection maximum at a wavelength of 13.5 or 6.7 nm.

In accordance with this, the invention provides an extension of the range of EUV lithographs made in accordance with the present invention, realizes the possibility of creating relatively simple universal EUV lithographic systems for creating semiconductor products for various purposes, mainly according to technological standards of 22 nm or less.

The above and other objects, advantages and features of the present invention will become more apparent from the following non-limiting description of embodiments, given by way of example with reference to the accompanying drawings.

Brief description of the drawings

The essence of the invention is illustrated by the drawings, in which Fig. 1 is a diagram of an EUV radiation source in accordance with an embodiment of the present invention, FIG. 2 is a simplified diagram of the EUV radiation source, FIG. 3 - the results of calculating the spatial distribution of the emission of polluting particles from the interaction zone, illustrating the choice of directions for the propagation of beams of laser and EUV radiation in spatial regions with minimal levels of emission of pollution, fig. 4 - emission spectra of laser plasma using Sn as target material and eutectic alloy Sn/In=52/48, fig. 5A and 5B show the results of calculating the spatial distribution of the emission of polluting particles from the interaction zone for Li target materials and compositions with atomic fractions of Li(80%) and Ag(20%), FIG. 6A and 6B are experimental results of comparison of pollutant emission for mi materials.

- 5 042061 sheni from Li and composition 80% Li + 20% Ag, fig. 7A and 7B show the spectra of the EUV source before and after reflection from the Mo/Si mirror for the target material of the composition 80%Li+20%Ag, FIG. 8 shows the emission spectrum of the laser plasma of the Gd target, FIG. 9A and 9B are cross-sectional fragments of a rotating target assembly with a powdered target material and a centrifuged high-density powder/melt mixture.

fig. 10 is a diagram of an EUV radiation source with an optical collector based on two blocks of ellipsoidal grazing-incidence mirrors, FIG. 11 is a diagram of a EUV plasma source lithography system in accordance with an embodiment of the present invention, FIG. 12 is a diagram of a maskless lithography system in accordance with an embodiment of the present invention, FIG. 13 is a diagram of a patterning device with a quasi-pattern for generating EUV interfering beams in an embodiment of a lithographic system.

In the drawings, matching device elements have the same reference numerals.

These drawings do not cover and, moreover, do not limit the entire scope of options for implementing this technical solution, but are only illustrative material of particular cases of its implementation.

Detailed description of the preferred embodiments of the invention

According to the embodiment of the invention shown in FIG. 1, a high-brightness EUV radiation source 1 intended for use in a lithographic system contains a vacuum chamber 2 with a rotating target assembly 3. A rotating target assembly 3 delivers a target 4, the material of which is fluid under the action of centrifugal force, into the interaction zone 5 with a focused laser beam 6. A part of the rotating target assembly 3 is made in the form of a disk fixed on the rotation shaft. The disk has a peripheral part in the form of an annular barrier with an annular groove facing the axis of rotation 7. The target 4 is a layer of target material formed by centrifugal force on the surface of an annular groove made in the rotating target assembly 3. The configuration of the annular groove prevents the target material from being ejected in the radial direction and in both directions along the axis of rotation 7 if the volume of the target material does not exceed the volume of said annular groove.

In a preferred embodiment of the invention, the target material can be a molten low-melting metal, melted and maintained in a given optimal temperature range using an induction or radiation heating system 8.

In another embodiment, the target material can be in the form of a powder, for example, of refractory Gd or Tb to produce EUV radiation with a maximum spectral brightness close to 6.7 nm.

To ensure high stability of the target surface and, accordingly, the output parameters of the EUV radiation source, a high rotation speed is used with a centrifugal acceleration of at least 3000g. The action of centrifugal force makes the surface of the target 4 parallel to the axis of rotation 7. i.e. the target surface is essentially a circular cylindrical surface, the axis of which coincides with the axis of rotation 7.

In the interaction zone 5, under the influence of a focused laser beam 6, a pulsed high-temperature plasma of the target material 4 is generated. The plasma generates radiation in the EUV range, which leaves the interaction zone 5 in the form of a divergent EUV radiation beam 9. The EUV radiation beam may also contain unused radiation other spectral ranges.

The EUV radiation beam in the spatial angle of radiation collection, limited by the casing 13, falls on the optical collector 10. The optical collector in a particular case of the invention is made in the form of a Mo/Si mirror with a reflection maximum at a wavelength of 13.5 nm. In other versions, an optical collector based on grazing incidence mirrors or optical elements in the form of Fresnel zone plates can be used.

After reflection from the mirror of the optical collector 10, the EUV radiation beam can be directed to the optical element 11, for example, in the form of a grazing incidence mirror, or in the form of a diffraction grating - to perform spectral filtering of the radiation by changing the tilt angle of the adjustable, as shown by the arrows in Fig. 1.

In the path of the focused laser beam 6 and the EUV radiation beam 9 are pollution suppression means, including all or some of the following means:

housings 12, 13 surrounding said beams of laser and EUV radiation;

protective gas flows directed through the gas inlets 14 into the housings 12, 13 to suppress the vapor and ion fractions of contaminants in the path of the laser and EUV radiation beams;

screen 15, separated from the rotating target assembly by 3 slit gaps, having only

- 6 042061 two small openings (for the input of the focused laser beam and for the output of the EUV radiation beam) through which contaminants can leave the target assembly;

a foil trap (not shown) with high transparency for radiation, which is a system of foils located radially with respect to the interaction zone 5, on the developed surface of which contaminants are deposited;

the magnetic field created by the permanent magnets 16, which deflect charged particles from hitting the optical collector;

membrane 17 is substantially transparent to EUV radiation, impermeable to contaminants and gas, preferably replaceable (quick change membrane assembly not shown).

The membrane installed in the path of the EUV radiation beam is preferably made of a material belonging to the group including: carbon nanotubes (CNT), Ti, Al, Si, ZrSi, BN.

In FIG. 2 shows a simplified diagram of the source of EUV radiation and the location of beams 6, 9 of laser and EUV radiation with respect to the rotating target assembly 3. In accordance with the present invention, the linear velocity vector of the target in the interaction zone and beams 9, 6 of EUV and laser radiation are located on opposite sides from the plane 18 passing through the interaction zone 5 and the axis of rotation of the target assembly 7, which ensures the purity of the EUV source in the spatial regions of the passage of laser and EUV radiation beams. This is illustrated by the results of numerical simulation of the spatial distribution of the emission of pollutants from the interaction zone 5, performed using the RZLINE program. The program is intended for work in the field of radiative hydrodynamics of dense hot plasma, it contains mathematical models based on many years of experimental and theoretical work, as is known, for example, from the publication of K. Koshelev, V. Ivanov, V. Medvedev, et al Return-to -zero line code modeling of distributed tin targets for laser-produced plasma sources of extreme ultraviolet radiation, Journal of Micro/Nanolithography, MEMS, and MOEMS Vol. 14, Issue 4, 4015.

In FIG. Figure 3 shows a map of the spatial distribution of the specific velocities of the emission of polluting particles (particles of all fractions of all velocities are taken into account) in experimental coordinates, in which φ is the angle to the target velocity vector in the rotation plane (azimuth angle), θ is the angle to the rotation axis (polar angle). The characteristic directions in the interaction zone are as follows:

I - parallel to the axis of rotation: θ = 0, φ - any;

II - along the target velocity: θ = 90°, φ = 0°;

III - along the normal to the target surface: θ = 90°, φ = 90°;

IV - against the target speed: θ = 90°, φ = 180°.

Spatial distribution of specific pollutant emission rate, fig. 3, was calculated in nm/(month-W) as the specific growth rate of the film thickness of the target material deposited on the surface of the exposed sample, located at a distance of 40 cm from the interaction zone, per unit of laser power with a 24-hour source operation 7 days a week. This distribution was obtained for typical parameters of the EUV radiation source: target material - Sn, laser radiation wavelength 1-2 μm, laser pulse energy - several units of mJ (1-5 mJ), laser pulse duration - several ns (1-5 ns) , the diameter of the focal spot is several tens of microns, the linear velocity of the target is 200 m/s. Apart from the rapid rotation of the target, no other methods of contamination suppression were taken into account.

As shown in FIG. 3, the mass of contaminants is mainly concentrated in a sector along the direction of the target velocity, limited by azimuth angles φ from 0° to 80° and polar angles θ from 0° to 90°. The ovals indicate the spatial regions selected in accordance with the invention with minimal levels of pollution output, in which the cones of the laser and EUV radiation beams are located. As can be seen from the data in Fig. 3, the maximum rate of emission of polluting particles along the direction of rotation of the target is about 107 nm/month-W, and in the places where laser and EUV beams 6, 9 pass - less than 2 nm/month-W, i.e. more than 106 times better.

In FIG. Figure 4 shows the emission spectra of laser plasma using Sn as the target material and Sn/In=52/48 eutectic alloy obtained under the same excitation conditions. It can be seen that the same high spectral brightness in the EUV range is achieved when using these materials. The use of tin or a tin alloy is preferable to achieve high brightness at a working length of EUV lithography of 13.5 nm with a high, about 3%, efficiency of conversion of CE13.5 laser radiation energy into EUV radiation in the 13.5+/-0.135 nm spectral band into spatial angle 2π sr. In this case, the use of the eutectic Sn/In alloy may be preferable, since its melting point of 125°C is significantly lower than the melting point of 232°C of pure tin, which simplifies the design of the EUV source and improves its operational convenience.

In another embodiment of the invention, in order to obtain essentially monochromatic EUV radiation at a wavelength of 13.5 nm, the target material contains Li. To reduce the yield of polluting particles, the target material, in addition to Li, contains at least one additional element, selected

- 7 042061 from the group consisting of silver Ag, Au, Bi, Ba, Sr. Preferably the target material is a eutectic alloy in which the atomic fraction of Li is in the range of 60 to 90%.

The results of calculating the release of pollutant particles from the plasma for target materials from Li, Fig. 5A and compositions of 80% Li and 20% Ag (in atomic fractions), FIG. 5B illustrate the significant benefits achieved in accordance with the present invention. An increase in the density of the target material (by a factor of 4.8 compared to the density of lithium ρ = 0.512 g/cm3) makes it possible to sharply (by more than an order of magnitude) reduce the rate of emission of pollutant particles in the regions where laser and EUV radiation beams pass.

The achievement of this positive effect of the invention is confirmed experimentally using a source of EUV radiation, fig. 1, in which the test sample was placed instead of the membrane 17.

In FIG. 6 shows Scanning Electron Microscope (SEM) photographs of test samples for Li target materials, FIG. 6A and compositions of 80% Li and 20% Ag, FIG. 6B. Frame size - 125 microns. The tests were carried out under the following conditions:

target linear velocity - 150 m/s;

acceleration - 23000g;

the energy of the laser beam on the target is 3.3 mJ in the focusing spot of 120 μm according to the intensity level (1/e2);

pulse repetition frequency - 25 kHz;

the exposure time of the samples was 2.5 h (Fig. 6A) and 10 h (Fig. 6B).

It can be seen that the increase in the density of the target material in accordance with the invention leads to a sharp suppression of contaminants.

Based on the tests performed, it was shown that relatively large droplets with a size of more than 300 nm, which can penetrate through the CNT membrane, are completely suppressed. Thus, replacing the test sample with a replaceable CNT membrane allows for ultra-high purity of the high-brightness 13.5 nm monochromatic radiation source made in accordance with the present invention.

In FIG. 7A and 7B show the spectra of the EUV radiation source for the target material from the composition 80%Li+20%Ag. The spectrum shown in Fig. 7A measured for radiation emerging directly from the plasma. The presence of Ag ions in the emitting plasma causes an emission band with a peak at a wavelength of 17 nm, which does not overlap with the Li emission line at 13.5 nm and can be easily filtered out, for example, using a Mo/Si mirror, as can be seen from Fig. 7B. In this case, the intensity of the Li ²⁺ emission line at 13.5 nm practically does not change when the target material from Li is replaced by a composition of 80% Li + 20% Ag.

The monochromaticity of the EUV radiation source achieved in accordance with the invention makes it possible to use with it a wide range of optical elements, which include multilayer Mo/Si mirrors installed at various angles, grazing incidence mirrors, Fresnel zone plates, and expands the range of applications essentially point (with a characteristic size ~ 100 microns and less) radiation sources disclosed in the present invention.

In another embodiment, the invention makes it possible to generate radiation around the center wavelength of 6.7 nm, as illustrated by the emission spectrum of the Gd laser plasma of the target shown in FIG. 8.

In FIG. 9 schematically shows a fragment of a section of a rotating target unit 3 with a target 4 containing the main plasma-forming material related to Gd or Tb. To ensure fluidity, the refractory target material may be a powder 20 on the surface of a groove 19 made in the rotating target assembly 3, FIG. 9A. In yet another embodiment, the target material may be a mixture of a denser melt or liquid 21 and powder 20, FIG. 9B.

The high brightness plasma light source operates as described below and shown in FIG. 1.

The vacuum chamber 2 is evacuated using an oil-free vacuum pump system to a pressure below 10-5...10 ^-13 mbar. This removes gas components, such as nitrogen, oxygen, carbon, etc., that can interact with the target material and contaminate the mirror of the optical collector 10.

The rotating target assembly 3 is driven by a rotating drive, such as a magnetic coupling motor, which ensures that the vacuum chamber 2 is kept clean. Under the action of centrifugal force, the target 4 is formed as a layer of target material on the surface of the annular groove facing the rotation axis 7. With centrifugal acceleration of at least 3000g, the target surface is essentially parallel to the axis of rotation.

The target 4 is exposed to a focused laser beam 6 with a high pulse repetition rate, which can be in the range from 10 kHz to 1 MHz. Shortwave radiation is generated by a focused laser beam 6, which heats the target material to the plasma temperature. Laser plasma emits light in the shortwave range. Depending on the power density of laser radiation in the focal spot and the target material, short-wavelength radiation is generated in the range from 2.5 to 120 nm, in particular, in the spectral ranges near the lengths

- 8 042061 waves 6.7 nm or 13.5 nm.

Heat is removed from the target mainly by radiative cooling and through a narrow gap between the rotating target assembly 3 and a stationary water-cooled heat exchanger (not shown), through which gas is blown under a pressure of ~1 mbar.

The dense high-temperature plasma formed in the interaction zone 5 generates short-wave radiation. The EUV radiation beam 9 exits the interaction zone 5 through the pollution suppression means 13-17 onto the mirror of the optical collector 10. Due to the rapid rotation of the target 4, the droplet fraction of polluting particles emitted from the interaction zone acquires a significant tangential velocity component comparable to the linear velocity of the target. Accordingly, the resulting droplet velocity vector will be largely redirected from the cones of the laser beam 6 and the EUV radiation beam 9, which are preferably located on opposite sides of the plane 18 passing through the interaction zone 5 and the rotation axis 7.

Preferably, but not limited to, molten low-melting metal can be selected as the target material. In other embodiments, the target material is a powder, such as a metal powder or a mixture of powder and melt.

In yet another embodiment of the invention, shown schematically in FIG. 10, the collection of EUV radiation is carried out by an optical collector 10 containing two blocks 22, 23 of ellipsoidal grazing-incidence mirrors. Blocks 22, 23 of ellipsoidal mirrors are located on one common optical axis 24, preferably in tandem along the propagation path of the EUV radiation beam 9. The interaction zone 5 is located in the first focus of the first block 22, and the second focus of the first block 22 coincides with the first focus of the second block of ellipsoidal mirrors 23 Each block of ellipsoidal mirrors 22, 23 may consist of several nested coaxial elliptical mirrors so that their foci coincide.

The first block of ellipsoidal mirrors 22 transmits the image of the emitting region of the plasma located in the first focus of the first ellipsoidal mirror to the second focus. Due to the different gains of different sections of the ellipsoidal mirror, the resulting image of the emitting plasma is strongly distorted in the intermediate focus between the two blocks of mirrors 22, 23. In this case, the second block of ellipsoidal mirrors 23, the first focus of which coincides with the second focus of the first block of ellipsoidal mirrors 22, eliminates specified image distortion. Thus, in the second focus of the second ellipsoidal mirror block, the plasma image is projected without distortion with a scale factor determined by the design of the first and second blocks of ellipsoidal mirrors 22, 23.

In an embodiment of the invention, the optical collector 10 has a magnification close to or equal to 1, ranging from 0.8 to 1.2. In the preferred embodiment, the second block of ellipsoidal mirrors 23 is several times, from 2 to 15 times, smaller than the first block of ellipsoidal mirrors 22.

Depending on the operating wavelength range and the collection angle, one of the following materials with a relatively high grazing incidence reflectance can be selected as the reflective material of ellipsoidal grazing-incidence mirrors: Mo, Ru, Rh, Pd, U, Ni, W, Fe, Nb , Al, Si, Co and BN, providing a reflection coefficient of at least 20% at angles of incidence of at least 5 degrees in the wavelength range from 2.5 to 120 nm.

On the path of the EUV radiation beam 9 along the optical axis 24 of the optical collector 10 there are pollution suppression means, some of which are marked with reference numbers 13, 14, 15, 17, 26 in FIG. 10.

In accordance with the present invention, important advantages are achieved in comparison with light sources using an optical collector based on a normal incidence mirror. In the latter case, the area of placement of pollution protection means is very limited, since a significant part of the area between the interaction zone and the optical collector is occupied by the propagation zone of the reflected EUV radiation beam. According to the embodiment of the invention shown in FIG. 10, almost all the way between the interaction zone 5 and the optical collector 10 are placed pollution suppression means 13, 15. These means are partially located inside and outside the casings 12, 13 surrounding the beams 6, 9 of laser and EUV radiation.

In the zone between the first and second blocks 22, 23 of ellipsoidal mirrors, there is preferably a second section of anti-pollution means 26 with a gas inlet 14, through which a protective gas, preferably argon, is supplied through a series of nozzles installed around the optical axis 24. Thus, a zone of increased pressure is created in front of the membrane 17, which serves as an additional barrier to the spread of contaminants into the second block of ellipsoidal mirrors and the equipment integrated with the source. This gas is pumped out through an annular pipe, also located in this zone (not shown). Membrane 17, on the one hand, limits the flow of polluting particles, and on the other hand, separates zones with different protective gas pressures. All this provides highly effective protection against contaminants.

Thus, the present invention provides EUV radiation sources with high average power, high EUV luminance, long service life and ease of operation in relation to the use of lithographic systems.

- 9 042061

In FIG. 11 schematically shows a lithographic system 27, which contains a plasma source of EUV radiation 1, an illumination system 28 designed to illuminate the pattern forming device 35 and a projection system 37, which are placed in separate (vacuum) housings and are located sequentially in the path of the EUV radiation beam 9 outgoing from the interaction zone 5 of the EUV radiation source 1. The EUV radiation source 1, the lighting system 28 and the projection system 37 are located in a vacuum housing (not shown) surrounding these systems. The EUV plasma source 1 is made in accordance with the present invention.

In FIG. 11 shows a lithographic system with a variation of the EUV source shown in FIG. 10 and described above, so a detailed description thereof will be omitted. An optical collector 10 collects the plasma radiation into an EUV radiation beam 29 and focuses it at a point 30 to form an image of an emitting plasma region that acts as a virtual radiation source for the illumination system 28. The point 30 where the radiation beam is focused may be referred to as an intermediate focus.

In accordance with the embodiment of the present invention shown in FIG. 11, the membrane 17, preferably a CNT membrane, which is an anti-contamination means and which can be installed between the mirrors 22, 23 of the optical collector, also serves as a gas seal separating the volumes of the EUV radiation source 1 and the illumination system 28 with different pressures. The higher pressure EUV source chamber 2 contains argon gases, the flows of which in the path of the laser and EUV beams 6, 9 are used to protect against contamination, as described above. Hydrogen, due to its reactivity, is used to clean optical surfaces.

Intermediate focus 30 may be located in aperture 31 for spatial filtering of the EUV radiation beam.

Additionally, a spectral filter can be installed along the path of the EUV radiation beam, for example in the form of a metal foil, but not limited to this option.

The lighting system 28, which is designed to form a beam of EUV radiation, may include a device 32 with a composite (faceted) field mirror and a device 33 with a composite pupillary mirror. The compound field mirror device 32 and the compound pupillary mirror device 33 together provide a radiation beam 34 with the desired cross-sectional shape and desired angular distribution. The radiation beam 34 extends from the lighting system 28 and onto the pattern generator 35 held by the support structure 36. The pattern generator 35 reflects and shapes the radiation beam 38 The lighting system 28 may include other mirrors or devices in addition to or instead of the devices 32, 33 with composite field and pupillary mirrors.

After reflecting off the pattern forming device 35, which may be a lithographic mask, the beam of directed radiation 38 enters the projection system 37. The projection system 37 includes a plurality of mirrors that are configured to project the radiation beam 38 onto a plate 39 held by a plate table 40. The projection system 37, characterized by a reduction ratio, forms an image with elements that are smaller than the corresponding elements on the pattern forming device 35. Although the projection system 37, simplified in FIG. 11 has two mirrors, it can include any number of mirrors (for example, six, seven, eight, nine or ten mirrors).

EUV radiation source 1, lighting system 28 and projection system 37 can be designed and located so that they can be isolated from the external environment. The EUV radiation source 1 may be provided with a gas at a pressure below atmospheric pressure (eg hydrogen and argon). The lighting system 28 and/or the projection system 37 may be provided with a vacuum. A small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure can be provided in the lighting system 28 and/or the projection system 37.

The projection system 37 focuses the radiation beam 38 on the target area of the plate 39. The target area can be called the exposed field. The wafer table 40 can be precisely moved, for example in such a way as to position different target parts of the wafer 39 in the path of the radiation beam 38 . Plate table 40 may be positioned by one or more devices (not shown). To improve positioning accuracy of the wafer table 40, one or more position sensors (not shown) can be used to measure the position of the wafer table 40 relative to the bundle 38. Measurements made by the sensors at one or more positions can be fed back to one or more positioners.

The pattern forming device 35 in the imaging embodiment is used in a scanning mode in which the mask table (support structure) 36 and the plate table 40 are scanned synchronously while the pattern structuring the EUV radiation beam is projected onto the plate 39 (dynamic exposure) . The speed and direction of the plate table 40 relative to the mask table 36 may be determined by the reduction and reversal characteristics of the projection system 37. The directional beam incident on the plate 39 may be in the form of a band of radiation that

- 10 042061 paradise can be called an exposure slot. During a scanning exposure, the movement of the plate table 40 and the support structure 36 may be such that the exposure slit moves across the exposure field of the plate 39.

The projection system 37 of the conventional lithographic system 27 may apply a reduction of about 4 times in both the x direction and the y direction. However, it may be advantageous to increase the reduction of the projection system 37 in at least one of the x and/or y directions.

After irradiation on the plate 39 of the photoresist layer, it is developed with the possibility of further etching the plate in accordance with this pattern.

At present, the EUV radiation sources according to the present invention provide about 10 W at the intermediate focus in the 13.5 ± 0.135 nm spectral band, and this value can be scaled up to at least 50 W. In principle, this makes it possible to use the proposed lithographic systems for mass production of semiconductor devices. At the same time, due to the high complexity and cost of equipment, high cost of masks, complex and expensive infrastructure, projection photolithography is competitive only in mass production. One of the approaches to solving this problem is the use of maskless lithographic systems that provide topological norms that are typical for projection lithography. The performance of a maskless nanolithographic system can be one to two orders of magnitude lower than in the case of mass projection lithography, but the cost of the lithographic process does not depend on the scale of production, which will ensure greater availability of lithographic production.

In FIG. 12 schematically shows an embodiment of the invention with a lithographic system 27 in which the patterning device 35 is a micro-optical electromechanical system (MOEMS) of micromirrors. The EUV plasma source 1 shown in FIG. 12 is described in detail above (FIG. 1) and its description is omitted. To simplify the lighting and projection systems can be placed in one common vacuum chamber 41. The lighting system can include a device 32 with a composite mirror for uniform illumination of the pattern forming device 35 in the form of MOEMS micromirrors. The radiation beam 34 passes from the compound mirror and onto the pattern forming device 35. The pattern forming device 35 reflects and forms the radiation beam 38, which, using the mirrors 42, 43, 44 of the projection lens, focuses the radiation beam 38 on the target area of the photoresist plate 39 moved by the table 40 plates (2D scanner). Lithographic system 27 also contains a gateway for loading and unloading exposed plates (not shown).

Operation of the lithographic system, Fig. 12 proceeds as follows. The EUV radiation beam 29 reflected by the collector mirror 10 is redirected by the mirror 11 to the compound mirror 32 (homogenizer). Composite mirror 32 provides uniform illumination of the dynamic mask 35, which is a MOEMS of mirrors. The radiation reflected from the MOEMS mirrors falls on the first mirror 42 of the projection lens, and then, with the help of mirrors 43 and 44, the MOEMS image is transferred to the plate 39 with a photoresist installed in the image plane. A feature of the projection scheme is that the rays reflected from tilted micromirrors (this state corresponds to the non-reflecting part of the classical photomask) do not fall into the input aperture of the mirrors 42, 43, 44 of the projection lens. Thus, the state of the micromirror forms two image pixels: 0 is the deflected micromirror and 1 is the initial state of the micromirror.

The required topology on the plate 39 is formed due to the coordinated movement of the table 40 of the plate (scanner) and the MOEMS micromirrors, synchronized with the pulses of the EUV radiation source 1. The movement of the plate 39 in the exposed area preferably occurs at a constant speed.

The microchip layer alignment system provides accurate determination of the plate coordinates by markers, control and correction of coordinates during scanning with the required accuracy. The autofocus system controls and corrects the distance between the lens and the photoresist plate with an accuracy better than the depth of focus of the lens. Laying of plates with photoresist and excavation of exposed plates is carried out through a gateway.

In an embodiment of the invention, the lens reduction is 800x, the productivity is 3 plates with a diameter of 400 mm/h, the MOEMS size is 8x33 mm, the pixel is 16x16 μm, the number of pixels is 1.05 MP, the resolution is 20 nm.

In addition to the purely binary exposure method, it is possible to expose in grayscale mode. This is achieved by repeatedly, with a small shift (less than a pixel size), fixing the image. This is one of the ways to improve resolution. In general, due to the interconnection of the three systems: EUV radiation source-MOEMS-scanner, additional possibilities for optimizing the photoresist illumination profile are realized, thereby achieving high resolution and low line edge roughness.

In a particular case of implementation, the invention provides for the operation of a lithographic system with radiation at a wavelength of 6.7 nm due to the use of refractory materials as the target material.

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Claims (13)

catching, for example, Gd or Tb powder, making it possible to reduce the technological norms of lithographic production. In another embodiment of the lithographic system, the patterning device shown schematically in FIG. 13 is a quasi-pattern 46 illuminated by the EUV beam 45. Spatial structuring of the radiation on the exposed plate 39 is carried out using an optical system that includes a quasi-pattern in which, for example, two mutually perpendicular pairs of diffraction gratings 47 and 48 with periods px and py, respectively, are located. beams of the first diffraction order, diffracted on these gratings, leads to the formation in the photoresist covering the plate of a standing electromagnetic field (interference pattern), periodic in the x and y coordinates. The period of the interference pattern 49 (standing field) dx is uniquely determined by the period px of a given pair of diffraction gratings dx = px/2. The restriction on the period of the interference pattern is that the period of the diffraction gratings cannot be less than half the wavelength of the EUV radiation. Thus, this drives the development of simple and affordable EUV high-resolution lithography systems. Industrial Application The proposed devices are intended for applications involving lithography using an EUV plasma source. CLAIM 1. A source of extreme ultraviolet (EUV) radiation containing a vacuum chamber with a rotating target assembly (2) that feeds the target (4) into the interaction zone (5) with a pulsed laser beam (6) focused on the target, which is formed under the action of centrifugal forces a layer of target material on the surface of the annular groove facing the axis of rotation, made in the rotating target assembly, the EUV radiation beam emerging from the interaction zone to the optical collector, and pollution suppression means, characterized in that the target material has fluidity under the action of centrifugal force, when In this case, the target has a centrifugal acceleration of at least 3000 g, where g is the free fall acceleration, and a linear velocity of at least 100 m/s. 2. The source according to claim 1, in which the target surface (3) is round-cylindrical with an axis of symmetry coinciding with the axis of rotation. 3. The source according to claim 1, in which the target material belongs to the group including liquid, such as molten metal or alloy; powder, such as metal powder, or mixtures thereof. 4. The source according to claim 1, in which the target material is tin (Sn) or its low temperature alloy with other metals, including indium (In). 5. The source according to claim 1, in which the target material contains a composition of lithium (Li) with at least one additional element selected from the group including silver (Ag), gold (Au), bismuth (Bi), barium ( Ba), strontium (Sr). 6. The source according to claim 1, wherein the target material is a eutectic alloy in which the atomic fraction of Li is in the range of 60 to 90%. 7. The source according to claim 1, in which the EUV radiation source has a maximum spectral brightness at a wavelength of 13.5 nm. 8. The source according to claim 1, in which the EUV radiation source has a maximum spectral brightness in the wavelength range of 6.6-6.8 nm, and the target material contains gadolinium (Gd) or terbium (Tb) or their compounds. 9. The source according to claim 8, wherein the target material is an alloy of Gd and/or Tb with a metal selected from the group consisting of Ag, Cu, Ni, Fe, Co, Mn, Al, Ga, Cd, Ru, and Rh . 10. Source according to claim 1, in which the optical collector (8) contains at least one multilayer mirror. 11. The source according to claim 1, in which the optical collector (8) contains two blocks (9, 10) of ellipsoidal grazing-incidence mirrors arranged in tandem along the path of the EUV radiation beam (7), transmitting an image of the emitting region of the plasma without distortion, on a scale, determined by the design of these blocks, to the second focal point (17) of the second ellipsoidal mirror block (10), and the second focal point of the first ellipsoidal mirror block (9) is located at the first focal point of the block of the second ellipsoidal mirror (10). 12. The source according to claim 1, in which the paths of the pulsed laser beam (5) and the EUV radiation beam (7) are located in spatial regions with a minimum pollutant emission velocity, which is at least 10 times less, preferably 10 times less than the maximum rate of emission of pollutants. 13. The source according to claim 1, in which the linear velocity vector of the target in the interaction zone (4) is directed on one side of the plane (18) passing through the interaction zone (4) and the rotation axis (6), and the focused laser beam (5 ) and the EUV radiation beam (7) are located on the other side -