Patent Publication Number: US-9431229-B2

Title: Sputter neutral particle mass spectrometry apparatus with optical element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2014-055418, filed Mar. 18, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment of the present invention relates to a sputter neutral particle mass spectrometry apparatus. 
     BACKGROUND 
     In recent years, a sputter neutral particle mass spectrometry apparatus using a focused ion beam device and a light beam oscillation device has been developed. In light beam post-ionized neutral particle mass spectrometry using a focused ion beam, an ion beam is irradiated on a sample to generate neutral particles, and a light beam is made incident horizontally on the surface of the sample to perform post-ionization a single time. Here, a secondary ion mass spectrometry apparatus is equipped with a light beam for post-ionization to improve detection sensitivity. Therefore, the measuring system is simplified to facilitate optimization of the timing for irradiating the light beam and the timing for drawing in ions. 
     In such neutral particle mass spectrometry, since the output of the light beam is small, it has been necessary to make the light beams converge at a particular position on the sample surface in order to secure sufficient detection sensitivity. Therefore, to prevent background noise caused by direct contact of the light beams with the sample from occurring, the optical axis of the light beam has been required to be designed horizontal to, and at a fixed distance from the sample surface. As a result, the light beam irradiation timing needed to be delayed, and thus density per unit volume of a sputter neutral particle group decreased, causing a decline in yield. 
     In recent years, objects to be analyzed by the sputter neutral particle mass spectrometry apparatus have become microscopic. Therefore, there is a need for a sputter neutral particle mass spectrometry apparatus which is capable of improving detection sensitivity by increasing the yield of any element in the region of analysis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a sputter neutral particle mass spectrometry apparatus according to a first embodiment. 
         FIG. 2  is a diagram showing a mass spectrometry process of the sputter neutral particle mass spectrometry apparatus. 
         FIG. 3  is a diagram showing the mass spectrometry process observed from an ion beam irradiation direction. 
         FIG. 4A  is a diagram showing a density of a sputter neutral particle from above in a light beam irradiation direction (25°) of the sputter neutral particle mass spectrometry apparatus. 
         FIG. 4B  is a diagram showing the density of the sputter neutral particle from the side in the light beam irradiation direction (25°) of the sputter neutral particle mass spectrometry apparatus. 
         FIG. 5A  is a diagram showing the density of the sputter neutral particle from above in a light beam irradiation direction (55°) of the sputter neutral particle mass spectrometry apparatus. 
         FIG. 5B  is a diagram showing the density of the sputter neutral particle from above in the light beam irradiation direction (55°) of the sputter neutral particle mass spectrometry apparatus. 
         FIG. 6  is a diagram comparing an optical axis of a laser in a sputter neutral particle mass spectrometry of a comparative example and an optical axis in the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A sputter neutral particle mass spectrometry apparatus according to one embodiment includes a sample table holding a sample which is a mass spectrometry target, an ion beam which is irradiated on the sample held by the sample table to generate neutral particles in an adjacent region of the sample, a light beam irradiation device which irradiates a light beam on the neutral particles positioned in the adjacent region to obtain photoexcited ions, a draw-out electrode which draws out the photoexcited ions, a mass spectrometer which draws in the drawn out photoexcited ions to perform mass analysis, and an optical element which is provided in a light path after the light beam passes the adjacent region, and changes a traveling direction of the light beam so that the light beam passes the adjacent region again. 
       FIG. 1  is a schematic diagram showing a sputter neutral particle mass spectrometry apparatus  10  according to a first embodiment,  FIG. 2  is a diagram showing a mass spectrometry process of the sputter neutral particle mass spectrometry apparatus  10 , and  FIG. 3  is a diagram showing the mass spectrometry process observed from an ion beam irradiation direction. 
     The sputter neutral particle mass spectrometry apparatus  10  comprises: a sample table  20  which is accommodated in a vacuum chamber, etc. and holds a sample W which is to be analyzed; an ion beam irradiation device  30  which is arranged above the sample table  20  and irradiates an ion beam P on the sample W to generate neutral particles; a light beam irradiation device  40  which irradiates a light beam G on a proximity region Q directly above the sample table  20 ; a mass spectrometry apparatus  50  which is arranged near the proximity region Q and draws in the neutral particles to perform mass analysis; and a concave mirror (optical element)  60  placed on the sample W and which changes the traveling direction of the light beam G. 
     The concave mirror  60  is arranged at a position where the ion beam P is not irradiated directly, with a reflective surface  61  faced upwards. The highest periphery  62  of the concave mirror  60  is formed at a height H lower than where a diameter D of an adjacent region L explained later on is positioned. 
     The sputter neutral particle mass spectrometry apparatus  10  configured in the above manner performs mass analysis in the following manner. In other words, the ion beam irradiation device  30  generates the ion beam P and makes it collide against the surface of the sample W. This collision causes the neutral particles to discharge from the surface of the sample W and float in the adjacent region L which is directly above the sample table  20 . The adjacent region L in which neutral particles with high density float is almost in the shape of a spindle, with a largest portion at the diameter D somewhat on the upper side of the height direction. 
     Meanwhile, the light beam G generated from the light beam irradiation device  40  is irradiated on the neutral particles floating within the adjacent region L. The neutral particles are ionized near the focal point of the light beam G to become photoexcited ions. Since light beam G is condensed (condensing spot S 1 ) inside the adjacent region L, photon density is increased, which allows ionization of various elements simultaneously. The photoexcited ions are drawn out into the mass spectrometer  50 , separated, and electrically pulsed for composition analysis of the sample W. 
     At this point, the light beam irradiation direction is positioned in a normal direction of the sample W, that is, so that the light beam is incident on the concave mirror  60  from a direction angled less than 90° with respect to the incident direction of the ion beam P, in a manner that the light path of the light beam G and the light path of the ion beam P intersect. This is to increase the detection sensitivity in a manner mentioned later on. 
     The light beam G is reflected by the reflective surface  61  of the concave mirror  60  and changes direction, to be re-irradiated toward the adjacent region L as light beam GX. Here as well, the non-ionized neutral particles are irradiated, and the composition analysis is conducted by the mass spectrometry apparatus  50  in the same manner. By using the concave mirror  60 , the light beam G spread by passing the condensing spot can be converged (condensing spot S 2 ) again within the adjacent region L. In this manner, the photon density is increased and various elements can be ionized simultaneously. 
     In the sputter neutral particle mass spectrometry apparatus  10  according to the present embodiment configured in the above manner, by passing the light beam G and the light beam GX through the adjacent region L, the opportunity of ionizing the neutral particles can be doubled. Therefore, even if the output of the light beam is small, sufficient detection sensitivity can be secured. 
     Since there is no need to irradiate the light beam in parallel with the sample, background noise caused by direct contact can be prevented from occurring. 
       FIG. 4A  is a diagram showing a density of a sputter neutral particle from above in a light beam irradiation direction (25°) of the sputter neutral particle mass spectrometry apparatus  10 ,  FIG. 4B  is a diagram showing a density of a sputter neutral particle from the side in a light beam irradiation direction (25°) of the sputter neutral particle mass spectrometry apparatus  10 ,  FIG. 5A  is a diagram showing a density of a sputter neutral particle from above in a light beam irradiation direction (55°) of the sputter neutral particle mass spectrometry apparatus  10 , and  FIG. 5B  is a diagram showing a density of a sputter neutral particle from the side in a light beam irradiation direction (55°) of the sputter neutral particle mass spectrometry apparatus  10 . This shows distribution after 100 nsec of initiating irradiation of a SRIM2013 30 keV, Ga beam. 
     As mentioned above, the sputter particle distribution obtained in the case where the ion beam irradiation is incident from directions angled 25° and 55° from the normal direction of the sample W surface is shown. As could be understood from the particle distributions, even if the incident direction of the ion beams is changed, the trend of the sputter neutral particle distribution remains almost unchanged. 
       FIG. 6  is a diagram comparing the optical axis of a laser in a sputter neutral particle mass spectrometry of a comparative example and an optical axis in the present embodiment. It may be understood from the drawing that the number of particles included in a laser irradiation region according to the present embodiment has been increased from the number of sputter particles included in a laser irradiation region of the comparative example. The portion indicated as M in the present embodiment in  FIG. 6  is a calculation range. In fact, when considering the reflected lasers as well, this will septuplicate, and increase even more if the laser is made to be reflected over a plurality of times. However, since this will be a trade off with mass resolution, the number of reflections should be controlled along with its purpose. 
     In the above-mentioned embodiment, the traveling direction is changed once, and the light beam passes the proximity region twice. However, it is also fine to change the traveling direction of the light beam at least twice to increase the effect of the neutral particles and the light beam to at least three times. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.