1. Field of the Invention
The present invention relates to a technology for realizing low damage sputtering regardless of materials (such as, an inorganic or organic material) in a surface analysis method, and relates to a technology for realizing improvement of sensitivity by improving secondary ion yield in a secondary ion mass spectroscopy method.
2. Description of the Related Art
An ion source for surface analysis that can perform sputtering without any damage to a target sample has not yet been developed. In the surface analysis, argon ion (Ar+) is the most common ion species for sputtering, but it is known that the occurrence of damage due to the sputtering is high.
In addition, in a secondary ion mass spectroscopy method (SIMS) as one of surface analysis methods, a primary ion beam that has been used so far is a noble gas ion or a metal ion (Cs+, Ar+, Ga+, Au+, or the like). Some of them can be reduced to a small beam in the order of several tens nanometer, but the occurrence of large damage to a sample is a common drawback.
In addition, if these ions are used as a primary ion source, secondary ion yield is very low, and secondary ion generation efficiency is low. Therefore, in order to overcome the drawback of the SIMS using them as the primary beam, a cluster ion SIMS has been developed. A beam source thereof is Au3++, Bi3++, or the like. By using the cluster ion (Au3++, Bi3++, or the like) consisting several atoms, desorption efficiency of the secondary ions is significantly increased in a non-linear manner. Such result is due to the generation of ablation.
On the other hand, because a target sample surface and its vicinity are significantly damaged, application of the conventional system to a biological material is difficult; and nondestructive observation of molecule ions is difficult; specifically, the sample receives large fragmentation, and a surface of the sample is decomposed and polymerized.
A cluster ion source of C60+ ion is commercialized; and hence, a low damage sputtering technology is realized though in a limited manner. Further, the desorption efficiency is further increased in the SIMS using the C60+ ion source as the primary ion source. However, the following phenomena are caused: (1) an inorganic material is contaminated with a carbon component derived from C60; (2) craters are generated in a surface of the material so that surface destruction occurs; (3) a biological sample or the like is significantly damaged; and (4) the secondary ion yield is low in the SIMS, and when the beam diameter is decreased, ionic strength is weakened so that utility value as the SIMS is deteriorated (particularly in an organic material). Refer to Japanese Patent Application Laid-open No. 2005-134170, Journal of Physical Chemistry B, 108, pp 7831-7838, and Applied Surface Science 231-232, pp 936-939, FIG. 4.
There is a surface analysis method utilizing a gas cluster ion beam (GCIB) that has been recently popular, in which noble gas (such as argon (Ar)) is ejected in vacuum to form a jet stream, gas temperature is decreased, and neutral clusters having an n value of Arn+ of a few thousands to a few tens of thousands are formed and ionized to generate Arn+, which is accelerated to impact the sample.
With this method, depth profile analysis with low-damage sputtering for an organic material (such as a polymer) is confirmed to be effective and is commercialized. However, for an inorganic material (such as a ceramic material) that is relatively hard, the sputtering speed is extremely slow so that it is not practical. Therefore, a range of the sample types to be analyzed is inevitably limited to mainly organic industrial materials.
In addition, when the GCIB is used as the primary ion source in the secondary ion mass spectroscopy method, it is known that the secondary ion yield thereof is low; and hence, it is not practical when used for improving sensitivity in the secondary ion mass spectroscopy method. Refer to Japanese Patent Application Laid-open No. Hei 04-354865, Japanese Patent Application Laid-open No. 2008-116363, and Analytical Chemistry, 2011, 83(10), pp 3793-3800, FIG. 7.
In addition, an ion beam technology using a charged droplet method has been developed. In this method, a capillary is disposed in the atmosphere, solvent is supplied through inside of the capillary, and an extraction electrode that is applied with a high voltage negative with respect to the capillary is disposed in front of the capillary so as to generate ions in the atmosphere.
A vacuum chamber is separated into several steps from low vacuum side to high vacuum side with small diameter orifices. The ions are made to pass through the orifices and are transported to vacuum atmosphere so as to be used as ion beam. In this case, the cluster ions generated in the atmosphere inevitably collide with gas molecules in the atmosphere so that many ions are scattered. Therefore, the amount of ions that are actually transported to the vacuum side and can be effectively used is small; and in addition, downsizing of the cluster ion (fission of the cluster) also occurs due to vaporization in the atmosphere side.
In addition, to use the ion beam, it is necessary to apply a high voltage, which is positive with respect to the ground potential, to the capillary as a source, and it is also necessary to apply a high voltage to parts for lens effect or the like in a low vacuum region during the ion transportation process. Therefore, discharge phenomenon tends to occur in various parts. Consequently, it becomes difficult to stably obtain the ion beam, and it is also difficult to decrease the beam size to be small.
On the other hand, a differential pumping system for evacuating the separated vacuum chamber also becomes large in scale which causes difficulty when in use. Refer to Japanese Patent Application Laid-open No. 2011-141199.
Consequently, a practical ion source that can support various types in etching layer-by-layer without damaging a surface of the sample after irradiation has not been developed yet, and an ion source succeeding in dramatic improvement of sensitivity in the secondary ion mass spectroscopy method has also not yet been developed.
A charged droplet ion source of the related art is described below. In FIG. 5, a charged droplet ion source 701 includes a vacuum chamber 710.
The vacuum chamber 710 is connected to first and second vacuum evacuating devices 729a and 729b so that the inside of the vacuum chamber 710 can be evacuated.
An extracting electrode 721 is provided with a small hole (orifice) so that gas flows in the vacuum chamber 710 through the extracting electrode 721 when the inside of the vacuum chamber 710 is evacuated. First, the inside of the vacuum chamber 710 is evacuated by the first and second vacuum evacuating devices 729a and 729b. 
An emission tube (capillary) 703 is disposed outside the vacuum chamber 710.
The distal end of the emission tube 703 is directed towards the small hole of the extracting electrode 721; and a base part thereof on the opposite side is connected to a liquid supply pipe 743. The liquid supply pipe 743 is connected to an ionization liquid supply device 705.
The ionization liquid supply device 705 includes a liquid storing portion 732 and a liquid feeding pump 731. The ionization liquid stored in the liquid storing portion 732 is supplied to the base part of the emission tube 703 through the liquid supply pipe 743 by the liquid feeding pump 731, passes a thin tube in the emission tube 703, and is emitted to the outside of the emission tube 703 from an emission opening 735 at the distal end of the emission tube 703. The emission tube 703 is surrounded by an outer cylinder 707. When carrier gas (here, nitrogen gas) is supplied from a carrier gas source 708 to the inside of the outer cylinder 707, the gas is released from a distal end opening 736 of the outer cylinder 707.
The emission opening 735 is disposed between the distal end opening 736 of the outer cylinder and the small hole of the extracting electrode 721. Around the emission opening 735, there is formed a flow of the carrier gas from an upstream side as the base side of the emission tube 703 to a downstream side on which the extracting electrode 721 is located with the small hole.
An extraction power supply 728 is disposed outside the vacuum chamber 710.
In a state where the carrier gas supplied from the carrier gas source 708 is released from the distal end opening 736, the liquid feeding pump 731 supplies the ionization liquid to the emission opening 735, the extraction power supply 728 applies a voltage between the emission tube 703 (made of a metal here) and the extracting electrode 721 so that an electric field thereof extracts droplet cluster ions charged with a positive charge from the ionization liquid positioned in the emission opening 735. Then, the cluster ions pass through the small hole of the extracting electrode 721 and enter the inside of the vacuum chamber 710.
On the downstream side of the extracting electrode 721, there are disposed accelerating electrodes 722 and 723 with small holes and transport lens electrodes 724 and 725. When voltages are applied to the electrodes 722 to 725, the droplet cluster ions entering the inside of the vacuum chamber 710 pass through holes formed in the electrodes 722 to 725 so as to be a droplet cluster ion beam, and further propagates toward the downstream side.
A size of an initial droplet cluster ion generated in the atmosphere is approximately 100 nm in diameter. However, the droplet cluster ion generated in the atmosphere is downsized due to Rayleigh fission that occurs when Coulomb repulsion of itself exceeds surface tension of the droplet. Further, the droplet cluster ions inevitably collide with gas molecules in the atmosphere so that many ions are scattered. Therefore, only a small amount of the droplet cluster ions can enter the inside of the vacuum chamber 710, and the size of the droplet cluster ion is decreased to be smaller than that of initially generated one.
In addition, for use as the droplet cluster ion beam, it is necessary to apply a positive high voltage with respect to the ground potential to the emission tube 703 as the generation source. Further, it is also necessary to apply high voltages to the extracting electrode 721, the first accelerating electrode 722, and the transport lens electrode 724 disposed in the low vacuum environments in the vacuum chamber 710. Therefore, an arcing phenomenon is apt to occur in the vacuum chamber 710, and hence it is difficult to obtain the droplet cluster ion beam.
In addition, it is necessary to separate the atmosphere outside the vacuum chamber 710 from the inside space of the vacuum chamber 710, both of which are connected to each other through the small hole of the extracting electrode 721. Therefore, the first and second vacuum evacuating devices 729a and 729b for evacuating the inside space of the vacuum chamber 710 are required to be large ones; and hence, difficulty arises when they are used in that they occupy large areas and in terms of cost.
Consequently, in the ion source on the conventional technology, disposing the emission opening of the emission tube in the atmosphere so that the droplet cluster ion beam is generated in the atmosphere provides small amount of the droplet cluster ions that can be actually used. Hence, the conventional technology is of little practical use.