CN113179573A - Cluster beam generating method and equipment of extreme ultraviolet and soft X-ray source - Google Patents

Abstract

A method of generating extreme ultraviolet and soft X-ray radiation comprising the steps of: flowing a gas mixture comprising a cluster-forming gas, a diluent gas, and a tertiary gas into a pressurizable valve, an input channel in communication with the chamber defining a housing for the chamber through which the gas flows into the chamber, an output channel in communication with the chamber, and a temperature-controllable nozzle having a gas inlet end fixedly attached to the output channel and an opposite gas outlet end, the nozzle having an aperture disposed therein, the valve contained within a vacuum chamber; pressurizing the gas mixture contained in the valve; reducing the pressure in the vacuum chamber; adjusting the temperature of the temperature control nozzle to a temperature suitable for forming molecular clusters from the selected cluster forming gas.

Description

Cluster beam generating method and equipment of extreme ultraviolet and soft X-ray source

Technical Field

The present invention relates generally to the generation of electromagnetic radiation in the range of 0.5-100 nanometers by irradiating a target material with a high energy laser beam, and more particularly to the generation of target materials.

Background

The generation and use of Extreme Ultraviolet (EUV) radiation has wide applicability in the fields of material science, microlithography, and microscopy. Two common sources of such radiation are laser-generated plasma radiation and synchrotron radiation. With appropriate modifications, the brightness of the laser plasma source is comparable to that of a synchrotron, but is more suitable for small laboratory or commercial environments.

Although laser plasmas are effective EUV light sources, they suffer from the problem of unacceptably high destructive atomic and particulate debris due to the method of generating the plasma. The discharge of such debris limits the applicability of the plasma source. Laser plasma sources typically use planar solid-state metal targets, generally 10 deg.f¹¹-10¹³Tile/cm²Is irradiated with a pulsed laser. Under these conditions, the laser generates a plasma of 10-100eV above the target surface. The thermally expanding plasma may exert a pressure of greater than 100 kbar on the target, causing the target to melt and/or break up, and the rapid ejection of hot particulate matter. These jets range in size from atomic size to greater than 10 microns in diameter. By using a low pressure gas, atom-sized injectors can be effectively prevented from damaging nearby optical systems. On the contrary, it may be up to 10⁵Large particle jets traveling at cm/sec are difficult to clog or divert. These debris can adhere to and/or attack nearby solid surfaces and are therefore extremely harmful to optics and other instruments placed in the vicinity of the plasma. The severity of this problem can be explained by the fact that: as little as

A typical target material (e.g., gold deposited on a multilayer coated mirror) will reduce its reflectivity to 13% of the original value. Because "zero debris" laser plasma sources of EUV and soft X-ray radiation will greatly expand the applicability of these sources in commercial and scientific applications that cannot tolerate debris-induced damage, efforts are constantly being made to develop methods of mitigating debris and novel target geometries that reduce or eliminate this problem.

Disclosure of Invention

A method for solving the problems of the prior artThe method is to place the optical element at an angle relative to the laser plasma target surface to reduce exposure to debris. Another strategy to reduce high energy target fragments is to use "mass limited" targets. U.S. patent nos. 4,872,189, 4,837,793, and 5,151,928 disclose target geometries in which a ribbon or ceramic membrane supports a thin metal film, thereby limiting the volume of target material exposed to the laser beam, thereby reducing, but not eliminating, the total amount of debris ejected. However, these systems are difficult to construct and are prone to breakage, requiring the vacuum system to be opened and the operation of the system to be interrupted. U.S. patent No.4,866,517 discloses the use of a cryogenic solid target to avoid the formation of condensable solid fragments. These targets are made of a frozen inert or noble gas (e.g., k or xenon). Due to the high cost of using these gases, a sophisticated containment and recovery system is necessary to keep the cost per injection at 10^-6The following. Us patent 4,723,262 discloses the use of discrete droplets of a metal (e.g. mercury, gallium, indium, cesium or potassium) with a melting point below 100 ℃ as the target material. To minimize debris generation, the size of the droplets is set to correspond to the size of the laser plasma beam. This requires a sophisticated control device and means for heating metals with melting points above 25 c. Some target debris is inevitably generated. Furthermore, due to the low vapor pressure of these materials at room temperature, they will tend to condense on the optical components, resulting in a loss of optical performance. As described in us patent No.5,235,200, a fast mechanical shutter and a magnetic shutter are used. U.S. patent nos. 4,866,517 and 4,969,169 have been proposed to block or divert debris, but because of the speed of travel of the jets, they are only marginally effective. None of these approaches show the ability to reduce debris while maintaining sufficient source brightness or collecting solid angles.

Another approach has been to generate laser plasma targets. Free jet expansion of gases, a thermally dense plasma is generated by the interaction of a high power laser with a small gas cloud or cluster by pulsing a gas through a nozzle (free jet expansion) into a vacuum chamberFormed, from Nd3 +: the results of experimental studies of YAG laser irradiation free jet expanding gas targets produce X-rays indicating that this technique can be used to produce intense soft X-ray emissions (laser plasma X-ray source with an insufflation target, H, Fiedorowicz, a. bartnik, R Parys and z. patron, journal of physics, No. 130, chapter 7, page 515, IOP publishing ltd, 1993 and SPIE 1994). In the article by McFransen et al (Applied Phys.B 57, p337,1993, Phys.Rev.Lett.72, p 1810, 1994), the mechanism of X-ray generation was studied using clusters, where the extremely high laser intensity was 5X10¹⁶-8x10¹⁸ watts/cm²A short pulse (300 femtoseconds) laser source was used to generate cluster enhanced X-ray emission. It has been demonstrated that clusters do provide benefits in the emission of direct multi-photon induced specific X-ray transitions. However, there is no report that the use of molecular clusters can minimize plasma debris while serving as an effective target for EUV or soft X-ray generation, whereas conventional laser plasmas can serve as an effective target and use a heating scheme. We disclose a new concept for producing an ultra-low fragmentation laser plasma source based on medium intensity (10)¹¹-10¹²watts/cm²) Wherein the target is generated by supersonic expansion of a gas composed of large molecular clusters through a nozzle and held together by van der waals forces. A particular advantage inherent in the invention, apart from the fact that in operation debris is produced many orders of magnitude less than in conventional laser plasma sources, is the long life of the uninterrupted operation due to the periodic replacement of the laser head. Used targets, such as metal tape or drum targets, may use inexpensive targets, supply targets almost continuously, and focus the laser away from the nozzle orifice further reducing debris or eliminating the need to clean and/or replace optical components.

It is well known to those skilled in the art that under isentropic conditions, supersonic expansion of a gas through a nozzle from a high pressure region to one of the lower pressures results in a drop in the temperature of the gas. As the temperature of the gas decreases, the relative intermolecular velocity of the gas decreases, and the weak attractive van der waals forces present between the molecules cause the expanding gas to condense, subsequently forming molecular clusters, such as dimeric polymers, eventually forming droplets. The formation of molecular clusters is a key element for efficient absorption of laser light, followed by laser heating and EUV production. From the perspective of laser plasma generation, these clusters, aggregates of atoms or molecules will respond locally like microscopic solid particles. The electron density of each cluster is well above the critical density required for efficient absorption of laser energy. Without these clusters, the gas jet density at 10-30mm from the orifice was so low that the laser energy was not absorbed and plasma was not formed. The gas pressure in the laser focal volume required to reach the critical density of the target necessary to completely absorb the laser in, for example, water vapor, will be close to 100 atmospheres.

To increase the total pressure and number of clusters before expansion, a diluent gas such as Ne or Ar may be added to the gas mixture. A ternary (inert) gas (e.g. Ne, Ar or Kr) may be added to the mixture to optimize the collision energy transfer to produce large molecular clusters such that the electron density of each cluster is at least as great as the critical density required to complete the process, allowing absorption of the laser radiation.

By properly adjusting the conditions under which the clusters are formed, the clusters will maintain their integrity at a significant distance from the orifice. Experiments have shown that the laser focus can be moved 10-30mm away from the nozzle aperture and still allow the laser beam to effectively absorb the plasma generated around each cluster. The benefit of moving the laser focal point away from the nozzle orifice is reduced debris generation and better differential pumping to reduce X-ray attenuated gaseous products produced by the jet.

By appropriate selection of the operating conditions, an infrared laser can be used to efficiently produce EUV radiation. The use of an infrared laser has particular advantages because the gas density required to achieve good X-ray generation is reduced since the critical plasma density for effective laser coupling decreases inversely with the square of the laser wavelength.

It is therefore an object of the present invention to eliminate the generation of target fragments by using molecular clusters. Another object is to form these targets by supersonic expansion of a gas through a nozzle. Another object is to use a gas which is an efficient radiator in the spectral range of 0.5-100 nm and which forms macromolecular clusters. Another object is to efficiently generate the desired EUV or soft X-ray radiation within a desired bandwidth using an infrared laser. Another object is to form clusters using materials with high vapor pressure at room temperature to eliminate condensation on critical optical components and the resulting loss of optical performance. Another object is to keep the laser focus away from the orifice of the nozzle used to generate the molecular clusters to reduce debris formation and provide better differential pumping to reduce X-ray attenuated gaseous products.

These and other objects of the invention are achieved by supersonic expansion of a gas through an orifice to form molecular clusters which are subsequently irradiated by a laser source to produce EUV or soft X-ray radiation.

The technical scheme of the invention is that the method for generating extreme ultraviolet and soft X-ray radiation comprises the following steps:

flowing a gas mixture comprising a cluster-forming gas, a diluent gas and a tertiary gas (which may be the same) into a (pressurizable) valve, an input channel defining a housing of a chamber in communication with the chamber through which the gas flows into the chamber, an output channel in communication with the chamber, and a temperature-controllable nozzle having a gas inlet end fixedly attached to the output channel and an opposite gas outlet end, the nozzle having an aperture disposed therein, the valve contained within a vacuum chamber; pressurizing the gas mixture contained in the valve; reducing the pressure in the vacuum chamber; adjusting the temperature of the temperature control nozzle to a temperature suitable for forming molecular clusters from the selected cluster-forming gas;

forming a molecular gas-cluster by expanding the high-pressure gas contained in the valve into the vacuum chamber through the orifice; and irradiating said molecular clusters with a laser beam focused on said molecular clusters to produce radiation of a selected wavelength.

The cluster-forming gas comprises a gas that is effective radiation of spectral energy in the region of about 0.5 to about 100 nanometers in wavelength.

Wherein the cluster forming gas comprises a gas that is an effective radiator of spectral energy in a range of about 11nm to about 18 nm.

The cluster forming gas is selected from the group consisting of water, carbon tetrafluoride, hydrogen chloride, fluorine, hydrogen sulfide, diborane, oxygen, argon, krypton, and xenon.

Wherein various cluster forming gases selected from the group consisting of water, carbon tetrafluoride, hydrogen chloride, fluorine, hydrogen sulfide, diborane, oxygen, argon, krypton, and xenon are combined to produce a cluster forming gas mixture.

The diluent gas is selected from the group consisting of hydrogen, helium and neon.

The ternary gas is selected from neon, argon or krypton.

The pressure in the valve is at least up to two atmospheres.

The pressure in the vacuum chamber is at least as low as 1 torr.

The laser light is at least 10%¹¹Tile/cm²Is irradiated to the molecular cluster.

Has the advantages that: the objectives of the invention, as well as its advantages over the prior art forms, which will become apparent to those skilled in the art from the detailed disclosure of the invention set forth below, are achieved by the improvements described and claimed below.

Drawings

FIG. 1 is a schematic diagram illustrating the basic concept of target formation of molecular clusters, which is a preferred embodiment of the device.

Fig. 2 shows an embodiment of a pulse valve for generating gas clusters.

FIG. 3 shows the Extreme Ultraviolet (EUV) radiation yield obtained in the 70-108eV spectral band for a Xe jet compared to that obtained for a solid gold target.

Figure 4 shows the EUV radiation yield obtained for a Xe jet at 13mm from the nozzle orifice, plotted as a function of nozzle temperature, in the spectral band 70-108 eV.

Figure 5 shows the EUV radiation yield obtained as a function of the nozzle temperature as a function of the separation distance between nozzle and laser focus.

Detailed Description

The present invention replaces the traditional EUV or soft X-ray generation method, i.e. irradiation of a gas macroscopic target by a pulsed laser source, by supersonic expansion of the gas into vacuum-generating van der waals clusters which serve as zero-energy renewable source fragment targets. Figure 1 illustrates an apparatus and method for generating van der waals cluster targets. Here, a gas mixture consisting of cluster-forming gas, dilution gas and ternary gas is conducted at a pressure of 10-100 atm to the valve 10, which may be continuously opened or alternatively opened and closed.

The background pressure of the expanding gas in the vacuum chamber 1 through which the X-rays have to pass has to be kept to a minimum. To reduce the pumping speed requirements of the vacuum pump 30 and achieve the lowest background gas pressure, a pulsing valve may be employed. The valve may be, but is not limited to, a solenoid, a piezoelectric diaphragm, or a repulsive current loop design. An embodiment of a solenoid valve 10 is schematically illustrated in fig. 1. In this particular embodiment, a standard solenoid valve is fitted with a nozzle 20, the nozzle 20 being designed to achieve efficient cluster production. The gas in the nozzle 20 is cooled to a temperature 10-20% above its sublimation or boiling point by means of a yoke 100 mounted on the valve 10 to achieve efficient cluster production. In a preferred embodiment, a long conical nozzle is used, as this shape is known to maximize the size of the resulting clusters. To further increase the throughput of large clusters, the orifice 60 in the nozzle 20 has a conical shape approximately 25mm long with a full opening angle of-10 degrees. The entrance of the cone on the high pressure side 110 is-1 mm and the exit 120 is-5.4 mm. The inner wall of the conical nozzle should be as smooth as possible to avoid flow disruption and the deleterious effects of expanding gas flow diffusion. The length of the gas pulse applied by the valve should be ≧ 30 μ s to allow sufficient time for stable flow and good cluster formation. In a preferred embodiment, the pulse valve 10 functions in synchronism with a pulse laser 40 having an open period of 1-2000 Hz. The expanded flow of the gas cluster 50 is ejected from the pulse valve orifice 60 by the pressure difference existing between the pulse valve 10 and the vacuum chamber 1. When the gas clusters 50 exit the valve orifice 60, they are irradiated by the pulsed laser 40. Its light 80 enters the vacuum and is focused by lens 70 near nozzle exit 120 to produce a plasma that emits EUV and soft X-rays. As previously mentioned, the radiation generated by the interaction of the pulsed laser with the molecular clusters formed at the nozzle exit is focused on the wafer or other material of interest by optical methods well known to those skilled in the art.

The disclosed embodiments of the invention are shown in figures 1 and 2 to enhance the generation of EUV radiation. The EUV production of Xe molecular gas clusters in fig. 3-5 is compared to solid gold targets, which are one of the most efficient laser plasma source targets, but produce unacceptable levels of debris. The radiation yield obtained was 50% of the gold target value. Figure 4 shows the effect of reducing the temperature of the pulse valve nozzle on the generation of molecular gas clusters. Here, Nd3 +: the YAG laser beam is focused 13mm from the nozzle orifice and onto the Xe gas cluster flow. The EUV radiation yield is measured as a function of the nozzle temperature. As the nozzle temperature decreases, the EUV radiation yield increases abruptly to 227K, about 25% above the Xe boiling temperature (167K), indicating the formation of molecular gas clusters.

As previously mentioned, positioning the laser focus near the nozzle orifice due to the pressure wave exerted by the thermally expanding plasma will result in increased debris formation, as described above. Moving the laser focus sufficiently far from the nozzle aperture will significantly reduce this effect while achieving better differential pumping. The ability to move the laser focus away from the nozzle depends on the target gas being of sufficient quality to form a plasma at a distance removed from the nozzle orifice. This capability is provided by the molecular gas clusters formed by the methods disclosed herein, shown in fig. 5. Here, the EUV radiation dose of xenon is a function of the distance from the nozzle orifice at different nozzle temperature values. When the nozzle temperature drops below the critical value of Xe (227K as shown in FIG. 4), the EUV yield remains 60% of its maximum value at a laser focus distance of at most 20mm from the nozzle aperture.

To generate wavelengths useful for different applications, various aggregating agents may be used, e.g., H₂O，CF₄，CO₂，HCl，F₂，H₂S，B₂H₆，O₂Ar and Xe. The wavelength generated by such sources can be extended to 0.5 to 100 nanometers, depending on the source gas selected. Furthermore, a mixture consisting of cluster gases selected from the above examples may be used to obtain an optimal combination of EUV and soft X-ray wavelengths.

A preferred embodiment for EUV production using a cluster gas is to use H2O vapor. He ions generated by irradiating a large volume of H2O with a laser intensity of 1012 watts/cm 2 are strongly irradiated in the spectral region of 11.4-18 nm. In this embodiment, a diluted mixture of H2O vapor (20-1000Torr) in He at 5-10atm and Ar at 10 vol% is expanded into a chamber through a 0.1mm nozzle maintained at a temperature of 375K. Resulting in the formation of a [ H2O ] n cluster, where n is 5000. Then, the mixture is mixed with 1.5J 5-10 ns of Nd: the YG or KrF excimer pulse laser irradiates the cluster, focused to a length of 1mm in diameter of about 630pm in diameter, thereby producing an attraction of 3x10-5cm 3. O5+ plasma radiation at 100eV 13 nm.

Claims (10)

1. A method of generating extreme ultraviolet and soft X-ray radiation comprising the steps of: flowing a gas mixture comprising a cluster-forming gas, a diluent gas, and a tertiary gas into a pressurizable valve, an input channel in communication with the chamber defining a housing for the chamber through which the gas flows into the chamber, an output channel in communication with the chamber, and a temperature-controllable nozzle having a gas inlet end fixedly attached to the output channel and an opposite gas outlet end, the nozzle having an aperture disposed therein, the valve contained within a vacuum chamber; pressurizing the gas mixture contained in the valve; reducing the pressure in the vacuum chamber; adjusting the temperature of the temperature control nozzle to a temperature suitable for forming molecular clusters from the selected cluster-forming gas;

forming a molecular gas-cluster by expanding the high-pressure gas contained in the valve into the vacuum chamber through the orifice; and irradiating said molecular clusters with a laser beam focused on said molecular clusters to produce radiation of a selected wavelength;

the cluster-forming gas comprises a gas that is effective radiation of spectral energy in a region of about 0.5 to about 100 nanometers in wavelength;

wherein the cluster forming gas comprises a gas that is an effective radiator of spectral energy in a range of about 11nm to about 18 nm;

the cluster forming gas is selected from the group consisting of water, carbon tetrafluoride, hydrogen chloride, fluorine, hydrogen sulfide, diborane, oxygen, argon, krypton, and xenon;

wherein various cluster forming gases selected from the group consisting of water, carbon tetrafluoride, hydrogen chloride, fluorine, hydrogen sulfide, diborane, oxygen, argon, krypton, and xenon are combined to produce a cluster forming gas mixture;

the diluent gas is selected from hydrogen, helium and neon;

the ternary gas is selected from neon, argon or krypton;

the pressure in the valve is at least up to two atmospheres;

the pressure in the vacuum chamber is at least as low as 1 torr;

the laser light is at least 10%¹¹Tile/cm²Irradiating the molecular cluster at an intensity of;

the laser is a pulsed laser.

2. The method of claim 1, wherein the pulsed laser is selected from the group consisting of an infrared laser and an ultraviolet laser; the pulsed laser light is at least 10%¹¹Tile/cm²Is irradiated for about 10 nanoseconds.

3. The method of claim 1, wherein the valve is a pulse valve selected from a solenoid valve, or a piezoelectric diaphragm and a repulsion loop design; the pulse valve has an open period of at least 30 μ s;

the pulse valve has an open period synchronized with the firing rate of the pulse laser.

4. The method of claim 1, wherein the temperature of the temperature control nozzle is about 10% to 20% higher than the boiling or sublimation temperature of the cluster forming gas.

5. The method of claim 1, wherein the orifice has a conical shape with the axis of the cone parallel to the flow of the gas mixture through the nozzle, and wherein the apex of the cone is located proximal to the gas inlet; (ii) a The conical orifice is disposed within the nozzle having a length of about 25mm, and the apex of the cone has a diameter of about 1mm and the base has a diameter of about 5 mm.

6. The method of claim 1, wherein the focal point of the laser beam is at least 10mm from the exit of the nozzle.

7. A method of generating 13nm radiation, comprising the steps of:

(H) produced by supersonic expansion of a mixture of steam, helium and argon₂O)_nA molecular cluster of the form wherein n is at least equal to 100 wherein the pressure of water vapor is from about 20 to about 1000torr, the concentration of Ar is from about 10 vol%, and the pressure of He is from about 5 to 10 atm; entering a chamber having a pressure of less than about 1Torr through an orifice having a diameter of about 0.1 mm; and

using an Nd: YAG or KrF excimer pulsed laser is irradiated on the clusters, wherein the laser is operated at a power of about 1J for about 10 nanoseconds.

8. An apparatus for generating a laser plasma source by irradiating a target material consisting of a macromolecular gas cluster with laser light to generate ultra-low fragments of extreme ultraviolet and soft X-ray radiation, said gas cluster being generated by a laser beam; supersonic expansion of a cluster of gases through an orifice, comprising: a valve, the valve comprising:

an input channel defining a housing of a chamber in communication with the chamber, the cluster forming gas flowing into the chamber through the input channel, an output channel in communication with the chamber, and a temperature controllable nozzle having a gas inlet end fixedly attached to the output channel and an opposing gas outlet end, the nozzle having an orifice disposed therein configured to maximize a size of a gas cluster; generated by releasing the gas from the chamber through the orifice; means for releasing said gas from said valve, said valve contained within a vacuum chamber, said vacuum chamber held in a lower position;

a pressure higher than the plenum; means for pressurizing said gas in said valve;

a laser for irradiating the clusters; and means for focusing the laser light onto the molecular gas cluster;

the pressure in the pressurizable valve is at least up to two atmospheres;

the pressure in the vacuum chamber is at least as low as 1 torr;

the laser light is at least 10%¹¹Tile/cm²Irradiating the molecular cluster at an intensity of;

the laser is a pulsed laser.

9. The apparatus of claim 8, wherein,

the focal point of the laser beam is at least 10mm from the outlet of the nozzle;

the pulse laser is selected from an infrared laser and an ultraviolet laser; the pulsed laser light is at least 10%¹¹Tile/cm²Irradiating the molecular cluster at an intensity of; wherein the pulsed laser irradiates the molecular cluster at a power of about 1J for about 10 nanoseconds; the temperature of the temperature control nozzle is about 10% to 20% higher than the boiling or sublimation temperature of the cluster forming gas; the orifice has a conical shapeA shape with the axis of the cone parallel to the flow of the gas mixture through the nozzle, and wherein the apex of the cone is located proximal to the gas inlet.

10. The apparatus of claim 8, wherein the valve is a pulse valve selected from the group consisting of a solenoid, a piezoelectric diaphragm, and a repulsion loop design; the pulse valve has an open period of at least 30 μ s;

the pulse valve has an open period synchronized with the firing rate of the pulse laser; the conical orifice is disposed within the nozzle having a length of about 25mm, and wherein the apex of the cone has a diameter of about 1mm and the base has a diameter of about 5 mm.

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