Patent Grant
6892142
1. Field of the Invention
The present invention relates to a method of analyzing particles suspended in a liquid, and a liquid-suspended particle analyzer for carrying out the method. More particularly, the present invention relates to a method capable of accurately measuring the particle size of particles having sizes on the order of nanometers (hereinafter, referred to as “nanoparticles”) suspended in a liquid and of determining the particle size distribution of the nanoparticles in a short time, and a liquid-suspended particle analyzer for carrying out the method.
2. Description of the Related Art
In a semiconductor device fabricating process for instance, a silicon wafer is cleaned with cleaning water, such as ultrapure water, to remove contaminants from the surfaces of the silicon wafer. If the cleaning water contains particles, the particles contained in the cleaning water remain and adhere to the surfaces of the silicon wafer after cleaning and drying operations. Such particles cause detrimental effects on the formation of an integrated circuit on the surface of the silicon wafer and reduce the yield of semiconductor devices.
In precision machine manufacturing processes, workpieces are cleaned with a volatile solvent to remove anticorrosive oil coating the workpieces and cutting fluids flowed over the workpiece in machining. If the volatile solvent contains hard particles, such as fine grains of sand and fine fragments of cutting tools, precision-machine parts are contaminated with those hard particles, and precision machines are constructed by assembling such precision-machine parts contaminated with hard particles. When a precision machine thus assembled is operated, the particles adhering to sliding surfaces cause abnormal abrasion and, consequently, the precision machine unable to function properly.
To avoid such problems and to improve the yield and the reliability of products, the cleanliness of the liquid, such as the cleaning water and the volatile solvent, must be monitored and proper measures must be taken to prevent contamination with particles.
Optical methods, such as light scattering methods and light transmission methods, and microscopic methods that analyzes an image formed by an electron beam microscope have been generally used for measuring the particle size and number of particles suspended in liquids, such as cleaning water and volatile solvents, to monitor the cleanliness of the liquids.
Optical methods, such as light scattering methods and light transmission methods, are subject to restrictions on the measurable particle size of particles. Even an optical particle size measuring instrument having the highest sensitivity is capable of measuring particle sizes on the order of submicrometers and incapable of measuring particle sizes below submicrometers. Microscopic methods that analyze an image formed by an electron beam microscope require advanced techniques and need a long time for image analysis.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a method of analyzing particles, such as nanoparticles, suspended in a liquid, and a comparatively inexpensive liquid-suspended particle analyzer for carrying out the method, capable of being easily operated, of accurately measuring the particle size of particles in a short time and determining a particle size distribution.
According to a first aspect of the present invention, a liquid-suspended particle analyzer for analyzing particles suspended in a liquid comprises: a liquid supply device that supplies a liquid to be analyzed; a fine liquid droplet producing device that produces fine liquid droplets suspended in a carrier gas by atomizing the liquid supplied by the liquid supply device; an evaporator that produces an aerosol of the carrier gas and particles suspended in the carrier gas by evaporating the liquid parts of the fine liquid droplets produced by the fine liquid droplet producing device; a classifier that classifies the particles of the aerosol produced by the evaporator into particle groups by particle size; and a particle analyzer that analyzes the particle groups of the particles classified by particle size by the classifier.
In the liquid-suspended particle analyzer according to the first aspect of the present invention, it is preferable that the fine liquid droplet producing device includes an electrospraying device adapted to convert the liquid supplied by the liquid supply device into charged fine liquid droplets, and an atomizer adapted to suspend the charged fine liquid droplets produced by the electrospraying device in the carrier gas. Preferably, the atomizer is provided with a radiation source capable of charging the charged fine liquid droplets produced by the electrospraying device in the Boltzmann equilibrium charge distribution. Preferably, an ammeter is connected to the atomizer to measure a quantity of charge discharged from the fine liquid droplets collided against an inner wall of the atomizer.
In the liquid-suspended particle analyzer according to the first aspect of the present invention, it is preferable that the classifier is a differential mobility classifier adapted to classify the particles of the aerosol produced by the evaporator according to mobility.
In the liquid-suspended particle analyzer according to the first aspect of the present invention, it is preferable that the particle analyzer is a particle counter adapted to count the respective numbers of the particles of the particle groups classified by the classifier. It is preferable that the particle counter is one selected from a group consisting of a Faraday cup electrometer, an ion counter and a nuclear condensation counter.
According to a second aspect of the present invention, a liquid-suspended particle analyzing method of analyzing particles suspended in a liquid comprises the steps of: producing fine liquid droplets suspended in a carrier gas by atomizing a liquid to be analyzed; producing an aerosol of the carrier gas and particles suspended in the carrier gas by evaporating the liquid parts of the fine liquid droplets; classifying the particles of the aerosol into particle groups by particle size; and analyzing the particle groups of the particles classified by particle size.
In the liquid-suspended particle analyzing method according to the second aspect of the present invention, it is preferable that, in the step of analyzing the particles of the particle groups, respective numbers of the particles of the particle groups classified by particle size are counted so that a particle size distribution of the particles suspended in the liquid is determined on the basis of the counted numbers of the particles.
According to the present invention, the liquid suspending the particles is atomized into fine liquid droplets suspended in the carrier gas, and the liquid parts of the fine liquid droplets is evaporated to produce the aerosol of the carrier gas and the particles suspended in the carrier gas. Therefore, the particles of optional particle sizes including nanoparticles and suspended in the liquid can be suspended in the carrier gas such that the particles do not aggregate. The particles thus suspended in the carrier gas can be classified by particle size by the classifier and the groups of the particles can be analyzed. Thus, the particle size distribution and such of the particles, such as nanoparticles, can be accurately measured in a short time.
The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which:
Referring to
The fine liquid droplet producing device 3, the evaporator 4, the differential mobility classifier 5 and the Faraday cup electrometer 6 of the liquid-suspended particle analyzer shown in
Referring to
The atomizer 13 has a cylindrical vessel 14, an entrance end wall 15 attached to one end of the cylindrical vessel 14, and an exit end wall 16 attached to the other end of the cylindrical vessel 14. The entrance end wall 15 is formed of an insulating material. The front end part of the nozzle 10 of the electrospraying device 8 is inserted through a central part of the entrance end wall 15 in the atomizer 13. A carrier gas inlet 14a is formed in the cylindrical vessel 14, and a carrier gas supply device 17 is connected to the carrier gas inlet 14a by a pipe 7′ to supply a carrier gas, such as nitrogen gas, into the atomizer 13. Thus, charged fine liquid droplets produced in the atomizer 13 are suspended in the carrier gas. An outlet 16a is formed in the exit end wall 16 to discharge the carrier gas suspending the charged fine liquid droplets into the evaporator 4. Preferably, the temperature of the atmosphere in the atomizer 13 is on the order of a room temperature, and the pressure of the same is on the order of the atmospheric pressure. Preferably, the carrier gas is supplied into the atomizer 13 at a flow rate in the range of about 0.5 to about 5 l/min. The cylindrical vessel 14 and the exit end wall 16 are formed of a conductive material.
A radiation source 18, such as a radioactive isotope of americium, is placed in the atomizer 13 to charge the charged fine liquid droplets produced by the electrospraying device 8 provided with the nozzle 10 in the Boltzmann equilibrium charge distribution. A conductor 19a connects the cylindrical vessel 14 of the atomizer 13 to the ground. An ammeter 19 is inserted in the conductor 19a.
Referring to
Referring to
Parameters, including the flow rate of the sheath gas, the voltage to be applied to the center rod 27, and dimensions of the center rod 27 and the case 28, defining the functions of the differential mobility classifier 5 are determined properly according to the particle size of the particles to be classified, which is mentioned in JP-A Nos. 288609/1998, 264790/1999 and 46720/2000.
Referring to
The operation of the liquid-suspended particle analyzer thus constructed will be described. Referring to
Referring to
The aerosol of the carrier gas and the charged particles is classified by particle size by the differential mobility classifier 5 shown in FIG. 5. The aerosol discharged from the evaporator 4 flows through the aerosol inlet 28a of the case 28 into a space extending between the center rod 27 and the case 28. Since the sheath gas supplied through the sheath gas inlet 28b of the case 28 and straightened by the straightening mesh 31 flows down in a laminar flow, only charged particles of a specific particle size among those included in the aerosol supplied through the aerosol inlet 28a are drawn through the annular slit 27a of the center rod 27 into the particle discharge pipe 32 and are discharged toward the Faraday cup electrometer 6. The particle size of the charged particles to be thus collected is dependent mainly on the flow rate of the sheath gas and the voltage applied to the center rod 27.
The particle concentrations, i.e., the numbers of particles per unit volume, of the charged particles respectively included in groups of the charged particles respectively having different particle sizes classified by the differential mobility classifier 5 are measured by the Faraday cup electrometer 6 shown in FIG. 6. More concretely, the charged particles of each group discharged from the differential mobility classifier 5 and supplied through the particle supply pipe 37 into the outer vessel 34 deposit on the conductive filter 38 held in the inner container 36. The charged particles deposited on the conductive filter 38 discharge and produce a weak current. The weak current produced in the conductive filter 38 flows through the conductive rod 39 and the receiving part 41 into the preamplifier 42. The preamplifier 42 converts the weak current into a corresponding voltage and amplifies the voltage. The voltage thus amplified by the preamplifier 42 is applied through the double-shielded wire 43 to the electrometer 44. The electrometer 44 indicates the current, i.e., the amount of charge of the charged particles. The relation between the current i indicated by the electrometer 44, and the particle concentration Ng, i.e., the number of the charged particles contained in unit volume of the carrier gas is expressed by Expression (1):
Ng=i/(n·η·e·q), (1)
where n is the amount of charge of the charged particles, η is charging efficiency, e is elementary electric charge (1.6×10−19 C) and q is the flow rate of the carrier gas.
The concentration N1 of particles suspended in a liquid to be analyzed can be obtained by multiplying the concentration Ng calculated by using Expression (1) by the ratio of the flow rate Qg of the carrier gas to the flow rate Q1 of the liquid suspending the particles. Namely, the concentration N1 is calculated by Expression (2):
N1=Ng·(Qg/Q1). (2)
Thus, the concentrations (the number of particles per unit volume) of the groups of the particles of specific particle sizes classified by the differential mobility classifier 5 in the liquid to be analyzed can be determined. The particle size of each group of the particles to be classified by the differential mobility classifier 5 can be determined by properly determining the voltage to be applied to the center rod 27. The particle sizes of the charged particles to be classified by the differential mobility classifier 5 are changed successively and the particle sizes of the groups of the charged particles are determined successively, so that a particle size distribution (the relation between the particle size and the number of charged particles per unit volume) of the charged particles suspended in the liquid to be analyzed can be determined.
If the charged fine liquid droplets collide against the inner surface of the cylindrical vessel 14 of the atomizer 13 included in the fine liquid droplet producing device 3, the fine liquid droplets are discharged, which causes an error in the number of the charged particles measured by the Faraday cup electrometer 6. Therefore, it is preferable to measure the amount of charge discharged from the fine liquid droplets collided against the inner surface of the cylindrical vessel 14, i.e., a current, by the ammeter 19 connected to the cylindrical vessel 14 of the atomizer 13 and to correct the measurement provided by the Faraday cup electrometer 6 by using the measured charge.
The fine liquid droplet producing device 3 atomizes the liquid to be analyzed, i.e., the liquid suspending particles, into the fine liquid droplets suspended in the carrier gas, the evaporator 4 evaporates the liquid parts of the fine liquid droplets to produce the aerosol of the carrier gas and the charged particles suspended in the carrier gas. Therefore, charged particles including nanoparticles can be suspended in the carrier gas without causing the aggregation of the charged particles. Therefore, the charged particles suspended in the carrier gas can be classified by particle size by the differential mobility classifier 5, and the respective concentrations of the classified groups of the charged particles can be measured by the Faraday cup electrometer 6. Thus, the particle size distribution of the particles including nanoparticles and suspended in the liquid can be accurately determined in a short time.
Although the liquid-suspended particle analyzer in the preferred embodiment of the present invention described above employs the liquid supply device including the sample container 1 and the fixed-displacement pump 2, any suitable liquid supply device, such as a syringe, may be used instead of the foregoing liquid supply device.
An optional suitable measuring instrument, such as an ion counter or a nuclear condensation counter, may be used instead of the Faraday cup electrometer 6 employed in the foregoing embodiment as a particle counter for counting the charged particles classified by particle size by the differential mobility classifier 5.
Although the foregoing embodiment analyzes the particles by counting the number of charged particles classified by particle size by the differential mobility classifier 5 by using the particle counter, i.e., the Faraday cup electrometer 6, a mass analyzer or a particle collector may be used instead of the particle counter; and the composition of the particles may be determined or the source of the particles may be identified by analyzing the compositions of the groups of the particles classified by particle size.
Although the invention has been described in its preferred embodiments with a certain degree of particularity, many changes and variations are obviously possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001-349667 | Nov 2001 | JP | national |
This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2001-349667 filed in JAPAN on Nov. 15, 2001, which is herein incorporated by reference.
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