JP4496355B2 - Droplet supply method and apparatus - Google Patents

Description

  The present invention relates to a method and apparatus for supplying droplets ejected from a nozzle.

  Droplets ejected from nozzles are widely used in inkjet and other various industrial fields, but the droplet technology is also useful as a target supply method for laser plasma light sources that use plasma generated by a pulsed laser as a light source. . An object of the present invention is to provide a droplet supply method and apparatus as means for supplying a laser plasma material continuously for a long period of time, including a material that exists as a solid at room temperature.

  High-temperature and high-density plasma generated by irradiating a short pulse laser is a high-intensity light source that generates radiation from the EUV region to the X-ray region. The spectrum to be radiated varies greatly depending on the laser irradiation conditions and the types of elements constituting the plasma. Therefore, it is necessary to select a material to be converted into plasma depending on the application. For long-time continuous operation of the light source, it is necessary to replenish the material lost by making it into plasma at high speed, but copper, iron, aluminum, tungsten, etc. can be made into a long tape or wire, Plasma can be generated continuously for a long time.

  Since a liquid can be ejected from a nozzle to form a continuous jet, a material that becomes a liquid near room temperature, such as water, alcohol, and Ga (Non-patent Document 1), can be continuously irradiated with high repetition. An experiment for generating plasma is also performed on tin, which has a melting point higher than room temperature but not so high, as a liquid jet (Non-patent Document 2).

  Even in the case of a material that can be liquefied, in order to suppress contamination in the container or an increase in pressure, it is desirable not to use a continuous jet but to form liquid droplets so that there is no liquid in unnecessary places. The continuous jet may be inconvenient in that it is sprayed at a distance from the nozzle, and it is desirable to further form droplets as a target for plasma generation. By applying vibration to the nozzle that blows out the liquid, the continuous jet can be changed into a droplet flow (Non-patent Document 3). In order to generate a stable droplet flow, it has been found that a certain relationship is required for the nozzle diameter, the liquid velocity, the vibration frequency, and the vibration amplitude (Non-patent Documents 4 and 5). .

  Two conditions are required for a target supply device for generating a plasma light source of high repetition continuously for a long time. One is continuous supply of various materials for a long time, and the other is a large supply rate.

(Continuous supply of target)
Targeting for a relatively long time is possible by forming a tape or wire, but materials that can be formed into a tape or wire are extremely limited. Even if it can be taped or wired, its length is limited.
Even an element that cannot be made into a tape or wire and cannot be made into a liquid can be continuously supplied by mixing fine particles containing the element into a liquid such as water and forming a liquid jet. Even a material that can be made into a tape or wire can be supplied in practically infinite time by making it into a liquid jet by making it into fine particles.

  A patent application has been filed for a method of continuously supplying a solid material by containing fine particles in a droplet (Patent Document 1). When the liquid is directly irradiated with a pulse laser for generating plasma, only about several μm of the solvent surface is turned into plasma, and the fine particles contained therein are not turned into plasma. This problem can be solved if the solvent of the droplets can be removed before the liquid is supplied in droplets and converted into plasma. However, no practical approach has been presented to date regarding droplet solvent removal.

(Drop velocity)
In the case of the plasma light source, if the droplets are too close to each other, the droplet for the next shot is blown away by the pressure reaching the GPa of the plasma created by the immediately preceding droplet. As the plasma expands, the pressure drops abruptly in inverse proportion to the fifth power of the radius, so this problem can be solved by increasing the interval between the droplets. In order to increase the repetition rate of the plasma light source and widen the interval between the droplets, the supply rate of the droplets must be increased.

Consider the case where all of the droplets become plasma. If the cross-sectional area of the droplet is S, the supply speed is V, the distance between the droplets is L, the expansion rate of the plasma is v, the collision of the next shot with the expanding plasma generated in the previous shot In order for the droplet not to be greatly decelerated, V / v >> (3 / 4π) S / L ² is required according to the theoretical consideration of the present inventor. Since the expansion speed of the plasma is about v = 5E4m / sec, if the droplet diameter is d = 100μm, if V = 10m / sec, L ² >> (3/4) d ² v / V = (6 mm) ² and the distance between clusters must be L >> 6 mm. Since the repetition rate is R = V / L, the upper limit is about 1 kHz.

  The present inventor has clarified that it is important to supply a target with a uniform density distribution over a wide area in order to achieve a large conversion efficiency with a plasma light source. In order to realize this, a technique is disclosed in which clusters composed of a large number of fine particles are supplied to a plasma generation region and the clusters are diffused immediately before plasma generation. In order to supply a target having a diameter of 500 μm and a density of 1 / 5,000 of the solid density, the diameter of the fine particle cluster is 30 μm. As a method for supplying fine particle clusters, a method is also disclosed in which a liquid containing fine particles is formed into droplets, the solvent is evaporated, and the fine particles are aggregated to form clusters. From the above formula, if the particle cluster has a diameter of 30 μm, the required interval is L >> 2 mm, and when the flight speed of the cluster is 10 m / sec, the repetition rate can be up to about 3 kHz, It is difficult to repeat 10kHz required for mass production of EUV lithography.

L ² >> (3/4) d ² v / V => L >> (3 v / 4 V) ^1/2 d => V / L << V (4 V / 3 v) ^1/2 / d Therefore, since R = V / L << 2V ^3/2 / d (3v) ^1/2 , the repetition rate R can be increased by increasing the supply speed V. For example, 1E4 / sec x 3E-5m x 4E2 (m / sec) ^1/2 = 120 (m / sec) ^3/2 << 2V ^3/2 , so if V >> 16m / sec, the diameter d = 30μm fine particle clusters can be supplied at 10kHz repetition.

In order to increase the injection speed, the pressure at which the liquid is ejected from the nozzle may be increased. The pressure to push p, the density of the liquid [rho, putting the rate of jetted v, if the pressure difference is converted to all kinetic energy is p = ρv ^2/2. When water with a specific gravity of 1 is extruded at 10 atm (1 MPa), if the resistance at the inner wall of the nozzle can be ignored, it will be injected at 44 m / sec. Actually, there is resistance on the inner wall of the nozzle, which is slightly slower than this, but it is possible to push out at a high speed by applying a large pressure.

However, in order to produce stable droplets, the liquid flow must be laminar. Laminar flow occurs when the Reynolds number Re = UL / v determined from the fluid size L, viscosity v, and velocity U is 1,200 or less. The droplet diameter for transporting fine particle clusters with a diameter of 30 μm is 0.1 mm or more. Since the viscosity v of water is v = 1E-6 m ² / sec, when the diameter is 0.1 mm, laminar flow occurs at an ejection speed of 12 m / sec or less. At higher speeds, turbulence occurs and stable droplet generation is not possible. That is, the droplet ejection speed cannot be greater than about 10 m / sec at a nozzle diameter of 100 μm and greater than about 5 m / sec at 200 μm.

  Therefore, in order to increase the droplet velocity, it is necessary to accelerate the droplet after nozzle ejection. From the analogy of an ion accelerator, a means for charging particles and accelerating the electric field can be considered. For example, Non-Patent Document 3 describes means for selecting particles by applying an electric field. In this method, particles ejected from the nozzle are charged, an electric field is applied only to the particles to be selected to deflect the direction of movement, and the particles pass through a small hole about 80 mm from the nozzle. In this method, the deflection distance may be about 2 mm, and the horizontal speed given by applying the electric field is about 1/40 of the vertical speed. That is, for example, in order to select a water droplet (diameter of about 100 μm) having a mass of 1 μg flying in the vertical direction at 5 m / sec, the water droplet may be moved in the horizontal direction at 0.1 m / sec, and the applied kinetic energy is 10 nJ.

  However, the acceleration in the vertical direction that must be performed for supplying the plasma light source at a high repetition rate is as large as 20 m / sec. Compared with the acceleration to about 0.1 m / sec in the horizontal direction for particle selection, it is 400 times larger, and the kinetic energy must give 40,000 times. Therefore, it is very difficult to accelerate fine particles having a diameter of several tens of μm or more by electric field acceleration.

(evaporation)
As for the evaporation of the solvent, microwave heating can be considered. However, as described below, it is difficult to apply it to a droplet flying at high speed.
In microwave ovens that are popular in ordinary households, the electric dipole of water vibrates due to microwave irradiation at 2.45 GHz, and the frozen food is thawed by its dielectric loss. According to the literature, if 10 ml of water is heated in a 500 W microwave, the temperature rises by 60 degrees in 30 seconds. The heating time of 30 seconds is short enough for thawing food, but too long for droplet heating. As will be discussed in detail later, the droplet velocity needs to be 10 m / sec or more, so it will fly 300 m during heating, which is not a practical distance.

  2.45 GHz microwaves have a wavelength of 120 mm, and even if a mirror is used, they cannot be condensed to 100 mm or less. To heat droplets with a diameter of 1 mm or less at high density, increase the microwave power. It will be. However, as can be seen from the following discussion, in microwave heating, if the power is increased in order to quickly evaporate the droplets, bumping from the inside occurs instead of gentle evaporation from the surface.

Since the diameter of the droplet is 1 mm or less, heating with microwaves having a wavelength of 120 mm uniformly heats the entire droplet. On the other hand, the droplet surface is cooled by the heat of vaporization. The center is cooled by heat conduction to the cooled droplet surface. The power absorbed by the entire droplet and the heat of vaporization from the droplet surface are balanced. This state is the heat conduction equation
d ² T / dr ² + (2 / r) dT / dr + Q ₀ / K = 0 ---- (1)
It is described by. Here, Q ₀ is the absorption power per unit volume, and K is the thermal conductivity coefficient of the solvent.

When this is solved, the temperature distribution in the droplet shown in FIG. 1 is obtained. FIG. 1 shows a temperature distribution in a droplet when the droplet is heated uniformly by a microwave or the like and the droplet surface is cooled by heat of vaporization. It has a parabolic temperature distribution with a high temperature at the center, and the temperature difference ΔT between the droplet center and the droplet surface is ΔT = (Q ₀ / 6K) r ₀ ² . r ₀ is the droplet radius. Q ₀ is expressed using vaporization rate Zn and heat of vaporization Eevap. Since Q ₀ = 3Zn Eevap / r ₀ , ΔT = Zn Eevapr ₀ / 2K. The larger the droplet size, the more the vaporization rate. The larger the temperature, the higher the internal temperature.

In the case of water, since Eevap = 2.2 kJ / g and K = 0.7 W / mK, the vaporization rate for evaporating a droplet with a radius r ₀ of 0.1 mm in 10 msec, that is, when Zn = 0.1 mm / 10 msec, ΔT = 1600 degrees. Since the vapor pressure is 400 degrees Celsius and the atmospheric pressure is 200 atmospheres, the vapor pressure exceeds 1,000 atmospheres and the droplets explode. If the vaporization rate Zn is reduced to 1/100, the temperature rise will be 16 degrees, so bumping can be prevented. However, in that case, it takes 1 second for a droplet having a radius r ₀ = 0.1 mm to evaporate. If the speed is 10 m / sec, it will fly as much as 10 m, making it difficult to become a practical device.

Due to the above circumstances, in the case of uniform heating, it is necessary to reduce the size of the droplets in order to avoid bumping and complete heating at a distance of 1-2 m that can be said to be practically acceptable. There is. If the radius is reduced to r ₀ = 0.025 mm, the calculation can be completely evaporated at 1.25 m even when the speed is 20 m / sec.

  Simple uniform heating will greatly limit the radius of the droplets, which can be inconvenient for various applications. A technique that relaxes the restriction on the size of droplets is desired. Further, when the droplets fly for a long distance, the stability problem described below becomes serious.

(Stability in the direction of droplet movement)
Fluctuations are unavoidable in the direction of droplet ejection. It is not uncommon for fluctuations of about 1% to occur due to the shape of the nozzle, the vibration of the droplet generation container, and the like. Also, our experience shows that when the evaporation space is exhausted, the droplet path becomes unstable, and the magnitude of the instability is larger for the droplet jet than for the continuous jet. This can be interpreted as the droplets being affected by the airflow accompanying the exhaust. When there is a fluctuation of 1% in the traveling direction of the droplet, the place where it passes changes by 1 mm when flying 100 mm. If a heating distance exceeding 1 m is required for extremely slow evaporation, the position of the fine particles carried by the droplets will vary by 10 mm.

  Non-Patent Document 3 describes: It was reported that 60% of the droplets had a fluctuation of about 1/10 of the diameter (150-400 μm) at 80 mm from the nozzle, but the remaining 40% had a fluctuation of more than that. is doing. In addition, 2. 2. If the pressure in the nozzle space is lowered too much, the nozzle may be cooled by evaporation and clogged momentarily. To avoid this, make it 300-500 Pa. In order to obtain a better degree of vacuum, differential evacuation is performed through a micro-opening, but when the pressure difference between the nozzle space and the lower part of the micro-opening is 1E4 Pa or more, the droplets are shattered by the strong air flow 4. For this reason, the pressure in the nozzle space has been reduced to 600-1,000 Pa, but by using a cryopump that uses liquid nitrogen, the pressure in the space below the micro-opening could be reduced to 15 Pa. It is reported that there is a variation of 2.5 mm from 150 mm to 150 mm. Thus, the traveling direction of the droplet fluctuates due to various factors.

When droplets are used as a means for transporting fine particles, evaporation of the solvent is necessary, but the diameter of the fine particle clusters that transport the droplets is about 0.03 mm. Therefore, in order to generate a stable plasma, it is necessary to suppress the position variation of the droplets to about the droplet diameter. As discussed in droplet heating, evaporation over short flight distances is not easy and may require an evaporation space of over 1 m. Even in such a case, since the droplet position fluctuation is 30 μm or less, the angle stability of the droplet flight path needs to be accurate to about 3E-5. Such ultrastable droplet flight is considered impossible without external control.
JP 2004-288517 A A. Thoss et al, J. Opt. Soc. Am B20 (2003) 224: Kilohertz sources of hard x rays and fast ions with femtosecondlaser plasmas PAC Jansson, et al., Appl. Phys. Lett. 84, (2004) 2256; Liquid-tin-jetlaser-plasma extreme ultraviolet generation Schwenn and Sigel, J.Phys.E: Sci.Instrum.7 (1974) 715): Continuous droplet source for plasma production withpulse lasers Rayleigh Lord, Proc. R. Soc., 29 (1879) 81 Teruhiko Yoshida, Nobuki Nagai, Proceedings of the Japan Opportunity Association (Part B), Vol. 46, 171 (1980): Study on Generation of Uniformly Droplets by Vibration

As described above, both the accuracy of the droplet flight path and the droplet velocity required in the method of transporting fine particle clusters for generating plasma with droplets continuously for a long time are obtained by the conventional technology. I can't.
It is an object of the present invention to enable control stabilization of a flight path, acceleration of flight speed, or rapid removal of a solvent, which are required when droplets are used.

  The present invention provides laser or particle beam irradiation or other means required to control flight path stabilization, increase flight speed, or quickly remove solvent required when using droplets. Thus, a means for removing a part of the droplets once, intermittently many times or continuously is provided.

(Suppression of bumping)
In the case of microwave heating, the problem of sudden boiling due to the extremely high temperature at the center of the droplet when rapid evaporation is performed can be solved by removing the solvent with an energy source having a large absorption coefficient. The heat conduction equation when the droplet surface is uniformly heated with an energy source having a large absorption coefficient α is expressed by equation (2).
d ² T / dr ² + (2 / r) dT / dr + Q ₀ exp (-α (r ₀ -r)) / K = 0 ---- (2)
This solution is T = (Q ₀ / α ² K) {(1-2 / αr ₀ ) − (1-2 / αr) exp (−α (r ₀ −r)} + T ₀ , FIG. 2 shows the temperature distribution in the droplet when the surface of the droplet is uniformly heated by a heat source having a large absorption coefficient.The internal temperature can be increased by increasing the absorption coefficient. As shown in FIGS.4 and 7, the temperature rise inside can be further reduced by arranging the liquid droplets so as not to be irradiated.
The internal temperature rise ΔT = Q ₀ / (α ² K) = Eevap Zn / αK can be reduced by increasing the absorption coefficient α.

The absorption coefficient α needs to be α = 1,000 cm ⁻¹ or more. However, a particle beam such as an electron beam or an ion beam can be used because it is absorbed at the surface of nanometer depth. However, since particle beams, particularly electron beams, are absorbed and scattered by a very small amount of gas, it is important to exhaust evaporated droplet molecules.

(Local heating)
Substituting vaporization rate Zn = 0.1 mm / 10 msec and absorption coefficient α = 1,000 cm ⁻¹ into the result of equation (2), the internal temperature rise ΔT = 330, and increasing the absorption coefficient increases the temperature rise. It is considerably reduced. This is a case where the droplet surface is heated uniformly. However, when locally heating for controlling or accelerating the flight path, the internal heating power is reduced, so the amount of increase in the internal temperature can be further reduced. .

The size of the droplet is about 0.1 mm, and in order to perform local heating, the absorption coefficient α at the wavelength of the removing laser needs to be about 1E3 cm ⁻¹ or more. For this reason, the present invention provides a means using a CO ₂ laser. At a CO ₂ laser wavelength of 10 μm, the absorption coefficient of water is as large as 1 / (10 μm), and local heating is possible. Even if the absorption coefficient of the solvent itself is small, the absorption of the removal laser can be increased by dissolving a dye having a large absorption coefficient.

In the case where the linear absorption coefficient due to the solvent is small, the present invention provides a means for irradiating an ultrashort pulse laser. When the irradiation intensity is increased to 1E10 W / cm ² or more, the laser beam is absorbed by nonlinear absorption and the droplet surface is ablated. If the impact due to ablation is too great, the droplets will break up, so ablation is performed in multiple steps. To reduce the impact on the entire droplet due to ablation, the pulse energy is decreased. On the other hand, high peak power is required to increase nonlinear absorption, and the pulse width is shortened. Although nanosecond pulses can be used in some cases, sub-picoseconds or less are usually desirable.

(Position control and acceleration of droplets)
When the absorption coefficient is large, local evaporation by local heating is possible, and the position control and acceleration of the flight path of the droplet using this can be performed.

  Speaking of a case where a droplet having a mass of a fine particle cluster of m and a mass of a droplet solvent containing the fine particle cluster is used as a target supply means, in the case of M >> m, as shown in FIG. By evaporating a small part of the mass from one place on the side surface of the droplet, the movement direction of the droplet is changed by the reaction of evaporation. As shown in the figure, the movement direction of the droplet 2 can be changed by heating the surface of the droplet 2 nonuniformly with a laser beam 3 or the like to evaporate nonuniformly. If the velocity at which the droplet is ejected is v and the velocity at which the solvent molecules leave the droplet is V, V is about 300 m / sec and v is about 10 m / sec. The direction can be changed by 1/100 and the position of the droplet can be controlled.

As shown in FIG. 4, if the droplet is irradiated with a continuous oscillation CO ₂ laser from obliquely upward in four directions, the amount of evaporation from the lower portion of the droplet is small, so that the flight speed is accelerated. By increasing the evaporation from above the evaporation amount from below, the reaction speed of evaporation increases the speed of the remaining droplets downward. In the case of droplets containing fine particle clusters, it is possible to accelerate the speed of the fine particle clusters remaining after evaporation of the solvent to several hundred m / sec.

FIG. 4 is a diagram in which water droplets containing fine particles are irradiated with a CO ₂ laser from four diagonally upper directions, the left diagram from the front and the right diagram from the top. By reducing the laser irradiation intensity on the flight axis, the flight path of the droplet can be stabilized. In this arrangement, the droplet flight path is stabilized and simultaneously accelerated downward. As shown in the right figure of FIG. 4, if the strength on the flight axis is lowered, the amount of evaporation from the deviated direction increases when the liquid drops off the flight axis. The return force works and the flight path is stabilized.

(Linear focused irradiation)
I want to focus the laser beam on a droplet with a diameter of about 0.1 mm for effective use of energy. On the other hand, the droplets fly at a high speed of 10 m / sec or more, and the evaporation time needs to be several milliseconds or more to avoid bumping, so they fly a distance of several tens of centimeters during heating. . Therefore, the present invention provides means for performing linear light collection.

(Reuse of transmitted beam)
As described above, the distance between droplets needs to be finally L >> 6 mm due to acceleration. Since the diameter of the droplet is about 0.1 mm, only 1/100 or less of the linear focused beam is used. The droplet interval immediately after ejection from the nozzle is 4 to 8 times the droplet diameter, and the utilization rate of the linear focused beam is about 10-20%.

  In order to increase the utilization rate of the irradiation laser, the present invention provides a means for utilizing a linear focused laser beam that has passed through the droplet stream. Most of the linear focused laser beam that irradiates the droplet stream is transmitted. The transmitted beam is reflected and the droplet stream is again condensed into a linear shape. Most of the reflected and re-irradiated beam is also transmitted. This transmitted beam is reflected again and reused. By this means, the utilization factor can be increased and the power of the heating laser can be lowered.

(Differential exhaust)
As described in [Background Art] (Stability in the traveling direction of a droplet), the degree of vacuum in the space for ejecting the droplet is only several hundred Pa. In many cases, it is desired to further increase the degree of vacuum in the plasma generation region. Although there is a description in Non-Patent Document 3, a space partitioned by a partition with a minute opening through which a droplet passes is provided, and the droplet is guided to a higher vacuum space by performing differential evacuation. I can do it. However, an airflow through the micro-opening flows from the low vacuum space to the differential exhaust space, thereby disturbing the movement of the droplet. According to the report of Non-Patent Document 3, it is 80
Although the fluctuation was about 1/10 or less of the diameter (150-400μm), that is, about 0.03 mm, the fluctuation after passing through the micro-hole was as large as 2.5 mm at 150 mm. .

  For this reason, the present invention provides means for removing droplets even in one or a plurality of differential evacuation spaces provided with one or a plurality of differential evacuation spaces and partitioned by a partition wall from the droplet ejection space. I will provide a. After a certain amount of the volume of the droplet is removed in the droplet ejection space, the droplet is introduced into the differential exhaust space through a minute opening in the partition wall. Even in the differential exhaust space, droplet removal with reduced strength on the flight axis is performed. When the droplet deviates from the flight axis, the amount of evaporation from the deviated direction is increased, and the reaction returns a force to return the droplet to the flight axis. This makes it possible to control the instability of the droplet trajectory caused by the airflow passing through the minute openings in the partition walls.

  The pressure in the differential exhaust space can be increased by two orders of magnitude by setting the diameter of the micro-openings in the partition wall to 1 mm and performing differential exhaust with a vacuum pump with an exhaust capacity of 10 l / sec. If the pressure in the droplet ejection space is 1,000 Pa, the pressure in the differential exhaust space is 10 Pa. If a vacuum level of 1E-3 Pa or higher is required, add two stages of differential exhaust. The pressure in the first stage differential exhaust space is sufficiently low, and even if the droplet passes through the additional differential exhaust micro-opening, the trajectory is hardly affected. Therefore, it is not necessary to perform a droplet removing operation in the additional differential exhaust space. However, if necessary, the droplet removal operation for stabilizing the orbit is performed also in the added differential exhaust space.

According to the present invention, when a material that is a solid at room temperature is used as a target of a plasma light source, high repetition and long-time supply are possible.
In addition, according to the present invention, it is possible to stabilize the flight path of a droplet including or not including a solid material.
Furthermore, according to the present invention, since the droplet can be accelerated, the interval between the droplets is increased, and the multi-kHz light source limited by the plasma pressure can be realized.

Hereinafter, the present invention will be described based on examples. In the following description, a droplet supply method as a target supply for a laser plasma light source is described. However, the method and apparatus provided by the present invention can be applied to a wide variety of other applications.
FIG. 5 shows a first embodiment. FIG. 5 is a diagram showing a continuous linear condensing optical system of a droplet flow using two cylindrical mirrors. In the figure, 1 is a droplet generating nozzle, 2 is a droplet containing fine particles, 3 is a CO ₂ laser for solvent evaporation, 4 is a linear focusing lens, 5 is a linearly focused laser beam, and 6 is A cylindrical mirror pair for converging multiple linear lines is shown. By repeatedly reflecting with the two cylindrical mirrors 6, the incident linear condensed beam can be efficiently absorbed into the droplet flow. A water droplet 2 having a diameter of 0.1 mm is generated from the droplet generating nozzle 1 at a velocity of 10 m / sec, a repetition rate of 10 kHz, and a droplet interval of 1 mm. A continuous wave CO ₂ laser 3 having an output of 100 W is linearly focused on the droplet stream with a length of 10 mm and a focusing width of 0.15 mm.

When transporting microparticles in droplets, the CO ₂ laser is selectively absorbed by the solvent, and even if the microparticles in the droplets absorb the laser energy and rise in temperature, the temperature of the solvent is increased by heat conduction. Rises and is cooled by vaporization of the solvent, so that fine particles having a melting temperature of about 1000 degrees do not dissolve.
Since the distance between the droplets is 1 mm and the droplet diameter is 0.1 mm, 10% of the linear focused beam is absorbed and the remaining 90% passes. This transmitted beam is reflected by a cylindrical mirror placed on the opposite side, and again, the droplet stream is subjected to linear focused irradiation. Since most of this beam is also transmitted, it is reflected again by a cylindrical mirror installed on the side where the beam is incident, and is again subjected to linear focused irradiation.

  FIG. 6 shows linear light collection viewed from above. By using concentric cylindrical mirrors, all the beams reflected by the two cylindrical mirrors are collected near the flight axis of the droplet. By making the centers of curvature of the two cylindrical mirrors concentric, all of the beams repeatedly reflected by the two cylindrical mirrors are collected almost at the center. Thus, the energy of the incident laser can be used for heating the droplets with high efficiency.

FIG. 7 shows an enlarged view of the vicinity of the droplet. An enlarged view of the path near the laser beam droplet is shown. The strength of the central part is lowered to stabilize the droplet flight path. The linear focused beam has a low intensity on the flight axis, and when the droplet is off the flight axis, the amount of evaporation from the off direction increases, and the reaction returns the force to return the droplet to the flight axis. Works to stabilize the flight path.
As shown in FIG. 5, since the amount of evaporation from the top of the droplet is large when irradiated from obliquely above, the flight speed of the droplet is also accelerated.
Of a linear beam with a power of 100 W, a length of 10 mm, and a width of 0.15 mm, about 1/200, that is, about 0.5 W is absorbed by one droplet. The time for a droplet of 10 m / sec to travel a distance of 10 mm is 1 msec, but the energy absorbed during this time is 0.5 mJ, which is consumed by the heat of vaporization and about 230 ng is evaporated. That is, when converted to the case where the surface evaporates uniformly, a thickness of 7 μm evaporates. It corresponds to a vapor pressure of 7,000 Pa and the surface temperature is about 40 ° C.

In FIG. 5, the laser beam is incident only from the left side. However, by entering the laser beam from the opposite side, the evaporative property can be improved.
Moreover, the stability of the flight path of the droplets can be improved by installing the cylindrical mirror pair in a direction orthogonal to the illustrated direction. By increasing the number of irradiation directions such as 6 or 8 equal angles instead of 4 equal, flight path stability can be further enhanced.

In the second embodiment, for removing the solvent of water droplets generated at a diameter of 0.1 mm, a speed of 10 m / sec, a repetition rate of 10 kHz, and a droplet interval of 1 mm, a wavelength of 800 nm at a repetition rate of 40 kHz with an output of 40 W is used. A femtosecond Ti: S laser is used. When the laser pulse width is 1 ps, the length is 1 mm, the condensing width is 0.15 mm, and when condensing linearly into the droplet stream, 1 mJ / (1 ps x 0.1 cm x 1.5E-2 cm) = 6.6E11 W / cm ² , and laser light is absorbed on the droplet surface by nonlinear absorption, and the droplet surface is ablated.

Of a linear focused beam (1 mJ / (10 mm x 0.15 mm = 60 mJ / cm ² ) with a pulse energy of 1 mJ, length of 10 mm, and width of 0.15 mm, about 1/20, that is, about 5 μJ is absorbed by a single droplet. This is consumed by the heat of vaporization, and about 2 ng is evaporated, that is, if the surface is evaporated uniformly,
20E-12 / π 1E-4 = 6E-8cm = 60 nm
A thickness of 60 nm evaporates. It is calculated that this pulse is irradiated for 1000 shots, that is, 25 msec, all droplets having a diameter of 0.1 mm are removed.

  In the second embodiment, firstly, the material selectivity in nonlinear absorption is small, and secondly, ablation occurs instantaneously within several μs, but the thermal diffusion distance between them is sub μm. Therefore, dissolution of fine particles may also occur. In order to avoid fusion of fine particles in the droplet, it is necessary to lower the irradiation energy density.

Since the thermal conductivity of the particles is about two orders of magnitude greater than that of water, in the case of fine particles having a diameter of about 0.1 μm, the temperature of the whole fine particles becomes almost uniform instantaneously in 1 ns or less. In the case of tin with a melting point of 232 degrees Celsius, the specific heat is about 6.5 cal / (mole K), so 200 x 6.5 cal / (mole K) x 4.2 J / cal x (1/2) x (1E-5cm) 3 x7 / 119 = 0.16 pJ energy absorption, tin fine particles with a diameter of 0.1μm rise in temperature from room temperature to melting point. That is, if the unit area is 0.16 pJ / (π / 4) (1E-5 cm) 2 = 2 mJ / cm ² or less, the melting point does not reach the melting point, so that the fine particles do not fuse.

Even if the irradiation energy density is 2 mJ / cm ² or more, fusion does not occur easily. When the fine particles are exposed on the droplet surface, the fine particles are heated. Although the thermal conductivity is low, 0.1 μm of water around the fine particles is heated in a time of several tens of ns. At 200 degrees Celsius, the vapor pressure of water is about 3 MPa and the vaporization rate is 3.3 m / sec. Therefore, 0.1 μm of water around the fine particles will be vaporized in 30 ns. As a result, the heat of vaporization of 3.4E-6J is removed from the fine particles.
(π / 2) (1E-5) ² x 1E-5 x 2.2kJ = 3.4E-12 J
This is greater than the energy required to raise the temperature of 0.1 μm diameter tin microparticles to the melting point, which is greater than 0.16 pJ. That is, the temperature of the tin fine particles having a diameter of 0.1 μm is determined by the balance with the heat of vaporization of the water surrounding the tin fine particles. The tin fine particles do not reach the melting point even with irradiation of 20 times the previous meter value, that is, 40 mJ / cm ² .

Even when the cooling effect due to the heat of vaporization of surrounding water is not taken into account, if the diameter of the fine particles is 1 μm, the allowable irradiation energy density is increased by an order of magnitude. In the case of tin oxide fine particles having a melting point exceeding 1000 degrees, irradiation with an energy density that is an order of magnitude higher is allowed. That is, if tin oxide fine particles are used and the fusion is allowed to 1 μm, irradiation of 0.2 J / cm ² is allowed. Thereby, the time until the removal of the solvent is completed can be shortened.

Until the evaporation of the solvent progresses and the density of the microparticles in the droplets increases, the probability that the microparticles are heated directly out of the droplet surface is low, and the probability that the microparticles are in contact with other microparticles is even greater. Since it is low, the probability of fusion between the fine particles is small even when irradiation is performed at a higher energy density. Therefore, in practice, there is no major problem even if irradiation of the order of J / cm ² is performed.

  Also in the second embodiment, the irradiation arrangement shown in FIGS. 5 to 7 shown in the first embodiment is used to effectively use laser energy, stabilize the droplet flight path, and accelerate the flight speed. .

In the second embodiment, since there is no substance selectivity, the first method may be used in combination after the evaporation of the solvent has progressed and the density of the fine particles in the droplets has increased. In particular, when the density increases to such an extent that the fine particles aggregate, a large acceleration is generated due to instantaneous thermal expansion, and the fine particle clusters are thereby decomposed. The decomposition of this cluster becomes remarkable at about 20 mJ / cm ² or more. The power of the CO ₂ laser to be irradiated when most of the solvent is evaporated may be very small. For example, it may be several W. In addition, the CO ₂ laser irradiation does not necessarily have to be linearly condensed, and it is not necessary to repeatedly reflect the light by two cylindrical mirrors.

A third embodiment is shown in FIG. The space 10 in which the droplet 2 ejected from the nozzle 1 is removed by the CO ₂ laser 3 and the space 10 partitioned by the partition wall 8 are exhausted by a vacuum pump (not shown). The droplet 2 is supplied to the differential exhaust space 10 through the minute opening 9 provided in the partition wall 8. The diameter of the minute opening 9 is 1
When it is about mm, the degree of vacuum of the space 10 can be improved by two orders of magnitude over the space 7 when the exhaust speed of a vacuum pump (not shown) that exhausts the space 10 is 10 l / sec. When a higher degree of vacuum is required, a plurality of differential exhaust spaces 10-2.10-3 partitioned by partition walls 8-2, 8-3 are provided as illustrated in FIG.

An air flow is generated from the droplet ejection space 7 to the differential exhaust space 10 through the minute opening 9, and thereby the trajectory of the droplet 2 after passing through the minute opening 9 fluctuates. In order to stabilize the trajectory of the droplet 2, the droplet is also removed in the differential exhaust space 10 by irradiating the CO ₂ laser 3 with reduced intensity at the center of the flight axis that guides the droplet.

The temperature distribution in a droplet when the droplet is heated uniformly by microwaves and the droplet surface is cooled by heat of vaporization. Temperature distribution inside the droplet when the surface of the droplet is heated uniformly with a heat source with a large absorption coefficient. By increasing the absorption coefficient, the internal temperature rise can be reduced. As shown in FIGS. 4 and 7, the temperature rise inside can be further reduced by disposing the central portion of the droplet not to be irradiated. The movement direction of the droplet 2 can be changed by heating the surface of the droplet 2 non-uniformly with the laser beam 3 or the like to evaporate non-uniformly. Irradiation arrangement example in the case where only the flight path of the droplet is stabilized without accelerating downward. The figure on the right is seen from above. Continuous linear condensing optical system of droplet flow using two cylindrical mirrors. By repeatedly reflecting with two cylindrical mirrors, the incident linear condensed beam can be efficiently absorbed into the droplet flow. Linear condensing seen from above. By using concentric cylindrical mirrors, all the beams reflected by the two cylindrical mirrors are collected near the flight axis of the droplet. The enlarged view of the path | route near the droplet of a laser beam. The strength of the central part is lowered to stabilize the droplet flight path. A plurality of differential evacuation spaces are provided to guide the droplets to the high vacuum space. Since the droplets fluctuate when passing through the minute opening provided in the partition wall, the droplets are removed and the flight trajectory is stabilized even in the differential exhaust space.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Droplet generation nozzle 2 Droplet containing fine particles 3 Solvent evaporation CO ₂ laser 4 Linear condensing lens 5 Linear condensing laser beam 6 Cylindrical mirror pair 7 for condensing multiple linear condensate Ejection space 8 Differential exhaust partition 9 Droplet passage minute opening 10 Differential exhaust space

Claims (16)

In a method for supplying droplets ejected from a nozzle,
Removing a part of a droplet once, intermittently or continuously several times by laser or particle beam irradiation means; and
Reducing the removal amount at the center of the flight axis leading the droplet, and increasing the removal amount when it is off the flight axis,
A method for supplying droplets. The droplet supply method according to claim 1, wherein a removal amount from the backward direction is larger than that from the forward direction of the progression of the droplets. Provide one or more spaces partitioned by a partition with a micro-opening through which a solution containing fine particles passes to perform differential exhaust, and one or more of the differential exhaust space partitioned by the partition from the droplet ejection space and performing work also droplets removed at the space, a droplet supplying process according to claim 1 or 2. It used a CO ₂ laser as removing laser solvent contained in the droplets, or in a solvent solution, from claim 1, characterized in that subjected to means for increasing the absorption of laser solvent removed 3 A droplet supply method according to any one of the above. The droplet supply method according to any one of claims 1 to 4 , wherein the droplet surface is removed by laser-induced ablation or impact by ultrashort pulse laser irradiation. Droplet supplying method of claimed in any of 5 to a solution containing fine particles, characterized in that the droplet. In a supply device of liquid droplets ejected from a nozzle,
A droplet generator;
A laser or particle beam irradiation means for removing a part of the liquid droplets intermittently or continuously many times;
A function that can reduce the intensity at the center of the flight axis where the droplet flow is to be guided by shielding a part of the laser or particle beam or combining multiple lasers or particle beams from multiple directions,
A droplet supply device comprising: 8. The droplet supply device according to claim 7 , wherein the irradiation intensity of the rear portion is made stronger than the irradiation intensity of the portion in the traveling direction of the droplet. 9. The droplet supply apparatus according to claim 7 , wherein the laser or particle beam is linearly condensed in a narrow shape in a direction that is long in the traveling direction of the droplet and orthogonal thereto. The linearly focused laser beam that has passed through the droplet stream is condensed again using a cylindrical mirror , or the laser beam is reflected many times using the reflection of multiple cylindrical mirrors. The droplet supply device according to claim 7 , wherein the droplet is condensed. The droplet supply apparatus according to claim 7 , wherein a plurality of lasers or particle beams capable of tracking and irradiating droplets have a function of tracking and irradiating one or a plurality of droplets. Provide one or more spaces partitioned by a partition with a micro-opening through which a solution containing fine particles passes, and perform differential evacuation of each space, in the droplet ejection space and the space partitioned by the partition. 12. The droplet supply device according to claim 7 , wherein one or a plurality of spaces have a droplet removal function. The droplet supply device according to any one of claims 7 to 12 , wherein a CO ₂ laser is used for removing the solvent contained in the droplet. 14. The droplet supply device according to claim 7 , wherein a subpicosecond ultrashort pulse laser is used for removing the solvent contained in the droplet. 15. The droplet supply device according to claim 14 , wherein the repetition rate of the ultrashort pulse laser is 1 kHz or more. 16. The droplet supply device according to claim 7, wherein the solution containing fine particles is a droplet.