Reference grid for use in charged particle beam spectroscopes

Abstract: A reference grid for use in charged particle beam spectroscopes in analyzing an impurity contained in a target sample, which includes a grid composed of a material free of the impurity.

Description: FIELD OF THE INVENTION The present invention relates to a test element including a reference pattern, for example, reference grid used in adjusting beam for charged particle beam spectroscopes such as secondary ion mass spectroscopes, focused ion beam apparatus and the like. RELATED ART A typical reference grid of this type is exemplified in FIG. 2 in which a grid 22 made of copper is bonded to an aluminum substrate 21 by means of an adhesive. Aluminum impurity contained in a high-purity silicon substrate is analyzed with a secondary ion mass spectroscope having the above reference grid in accordance with the following procedure: prior to the analysis, a charged particle beam (ion beam) is focused as small as possible onto the reference grid and adjusted to obtain a clear image of the grid; after the adjustment, a silicon substrate as a target sample to be analyzed is set on the spectroscope and irradiated with charged particle beam; secondary ions generated by the irradiation are mass-analyzed by applying a predetermined magnetic field and electric field to measure the mass number of secondary ions of the target elements, or specifically, to measure the total number of secondary ions for both aluminum and silicon; and the content of aluminum in the silicon substrate is found based on the ratio of the total number of secondary ions of aluminum to that of silicon. The conventional reference grid is, however, pointed out as having the following drawback in conducting analysis by charged particle beam spectroscopes; that is, when the reference grid is irradiated with charged particle beam for adjustment, metal elements sputtered from the reference grid are suspended in or attached to a sample chamber and optical or analytical system of the spectroscope and remain thereat even after the adjustment of the beam, degrading the analytical power of the charged particle beam spectroscope. The analytical power as termed herein is meant by a detection limit, or the lowest concentration possible to be detected or a resolving power with respect to signals. FIG. 3 is a graph showing a variation with time in the number of aluminum secondary ions detected for analyzing the aluminum impurity contained in a high purity silicon substrate with use of a secondary ion mass spectroscope having the conventional reference grid after the adjustment of beam. In this graph the number of aluminum secondary ions is represented in terms of the vertical axis, while the analyzing time (time for which the silicon substrate is irradiated with charged particle beam to sputter silicon) is represented in terms of the horizontal axis. Note that the vertical axis is in linear scale and so it appears that aluminum ions cannot be detected any more in about 4000 seconds, but if the vertical axis is converted in logarithmic scale, detection of the aluminum secondary ions would take several days. In the case of analyzing aluminum as the impurity, which is the same element constituting the reference grid, the presence of aluminum ions remaining for a prolonged time causes the resolving power of the secondary ion mass spectroscope to degrade. In addition, constituents of the adhesive used to bonding the reference grid 22 to the substrate 21 are also suspended in or attached to the sample chamber and the optical or analytical system, hindering high precision analysis. These drawbacks are not ascribable to the secondary ion mass spectroscopes only but are common to charge particle beam spectroscopes of this type. Journal of Vacuum Science and Technology: Part A, vol. 8, no. 3, June 1990, New York, US, pages 2209-2212 relates to chemical imaging using ion microscopy and digital imaging, and discloses the use of a tuning sample made of a Cu grid bonded to an Al disc. Journal of Vacuum Science and Technology: Part B, vol. 7, no. 2, April 1989, New York, US, pages 181-187 is concerned with secondary ion mass spectrometry as a diagnostic tool for micromachining applications which utilize a liquid Ga ion source. Patent Abstracts of Japan, vol. 5, no. 74 (E-57) (746), 16 May 1981; & JP-A-56 024 933 relates to a method of detecting the position of a reference mark on a substrate. US 4,163,900 discloses a composite electron microscope grid for support of specimens made of a hydrocarbon polymer and elemental carbon. SUMMARY OF THE INVENTION The present invention has been accomplished in view of the foregoing situation and the desire thereof is to provide a reference grid with which a charged particle beam spectroscope exerts its inherent analyzing power satisfactorily. Thus, the present invention provides a test element as set out in claim 1. The invention also provides a method of performing spectroscopic analysis as set out in claim 4. When a reference grid is made of a material free of an impurity contained in the target sample to be analyzed. Hence, such an impurity originating from the reference grid is no longer suspended in or attached to a sample chamber and an optical- or analytical system of the charged particle beam spectroscope in adjusting a charged particle beam prior to the analysis on the target sample. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a reference grid embodied in accordance with the invention; FIG. 2 is a perspective view showing a conventional reference grid; and FIG. 3 is a graph showing a change with time in the number of aluminum ions detected to analyze the aluminum impurity contained in a high purity silicon substrate with a secondary ion mass spectroscope in which a charged particle beam is adjusted in advance with use of the conventional reference grid. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In embodiments of the present invention, the reference grid may have any configuration such that a groove in a grid pattern is engraved in a substrate, or a grid member is bonded to a substrate by means of welding or an adhesive, provided that the reference grid does not contain substances which are contained as impurity in a target sample to be analyzed. The grid member is composed of a high purity silicon material while the substrate to be bonded or engraved may also be composed of a high purity silicon material. The adhesive as used in an embodiment of the invention may be made of any material that can bond the grid member to the substrate but does not affect analysis. The target sample may contain as impurity Al, Fe, Ni, Cr, Cu, B, P, As, Na, K or the like. The present invention is now described in detail by way of the preferred embodiments with reference to the drawings. Here, a reference grid as the embodiment of the invention is used in a secondary ion mass spectroscope in adjusting a charged particle beam (ion beam) incident on the target sample prior to analyzing aluminum impurity contained in high purity silicon substrate (target sample). Referring to FIG. 1, a reference grid 1 comprises a substrate 2 made of high purity silicon (as a material other than aluminum) in which a grid pattern groove 11 is formed. In this embodiment the groove 11 is cut with use of a slicer (first method) or by photolithography (second method). In the case of the first method, the grid pattern groove 11 is cut into the silicon substrate 2 with use of a slicer (dicing machine or the like) having a peripheral cutting edge with 10 µm edge width. Since chipping is prone to occur at a top edge portion of the groove, the number of revolutions of the cutting edge needs to be extremely large, while the cutting speed needs to be low. In this embodiment the number of revolutions of the cutting edge is set to 30000 r.p.m. and the cutting speed to 1 mm/sec or less. In such a manner the groove 11 is formed with depth T: 10 µm, width L: 5 µm, and pitch: 15 µm. For convenience, the reference grid 1 thus formed is herein referred to as A-type reference grid. In the case where the width and pitch of the groove 11 are desired to be smaller than described above, it is advantageous to utilize known semiconductor wafer process technology, particularly photolithography. Specifically, the surface of the silicon substrate 2 is covered with a photoresist which is then patterned to have a grid pattern, and subjected to anisotropic plasma etching to cut a groove in the substrate 2. Thereafter, the photoresist is removed from the substrate 2 by being immersed in sulfuric acid heated to 150 °C. In such a manner the groove 11 is formed with depth T: 0.4 µm, width L: 0.5 µm and pitch: 0.5 µm. For convenience, the reference grid 1 thus formed is herein referred to as B-type reference grid. Using the reference grid 1 including A and B types in a secondary ion mass spectroscope, a charged particle beam (ion beam) incident on the target is adjusted to be optimized. In this adjustment the reference grid 1 is continuously irradiated with charged particle beam, and accordingly sputtered constituents of the reference grid 1 are suspended in and attached to the inside of the spectroscope, as with the conventional reference grid. However, unlike the case of using the conventional reference grid, the sputtered constituents of the reference grid 1 do not include aluminum because the reference grid 1 is made of high purity silicon, and accordingly they do not greatly affect the analysis on aluminum impurity contained in the target sample (silicon substrate in this case). The reference grid 1 is made of high purity silicon in this embodiment because a simple substance material free of impurities is not so available. As a result of the use of the reference grid 1, it becomes possible to quantitatively analyze the aluminum content of the target sample in an extremely low concentration region (10 -17 atms/cm 3 or below). In other words, degradation in the resolving power inherent to the secondary ion mass spectroscope due to beam adjustment can be avoided by virtue of the reference grid of the invention. Further, unlike the conventional reference grid, nonuse of an adhesive in the reference grid 1 makes it possible to prevent the secondary ion mass spectroscope from being polluted by constituents of the adhesive, assuring precise analysis. It was found that the enlarged image of the B-type reference grid on the display of the secondary ion mass spectroscope was superior to that of the A-type reference grid in contrast. This is conceivable because when the depth L of the groove is too large relative to the width T thereof, secondary ions reflected at the bottom face of the groove may be irregularly reflected at the side face of the groove, with the result that an error in secondary ion level takes place. Experiments reveal that if the relationship between the depth T and the width L is set to satisfy (T/L) < 1, not so great a difference in secondary ion level is cased between the groove 11 and other portions, though there is a great difference between the edge portion of the groove 11 and other portions. As a result, the reference grid 1 according to the embodiment of the invention as claimed assures good contrast in its image even highly magnified. Further, it was found that decreasing the width and pitch of the groove 11 enhanced precision in beam adjustment. That is, use of a reference grid having a pitch of 15 µm in adjusting beam limits magnification of a secondary ion image to about 1000-2000, whereas use of a reference grid having a pitch of 1 µm can raise the magnification up to about 20000-30000. In addition, the reference grid 1 contributes to improvement in planar resolving power needed for analysis on the fabrication process of large scale integrated circuits. It should be understood that the test element according to the present invention is applicable to not only secondary ion mass spectroscopes but also other charged particle beam spectroscopes such as SEMs (Scanning Electron Microscopes), FIB (Focused Ion Beam) apparatus and RBS (Rutherford Back Scattering) spectrometers. While only certain presently preferred embodiments have been described in detail, as will be apparent with those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

Claims: A test element including a reference pattern for use in charged particle beam spectroscopes in analyzing an impurity contained in a target sample, wherein the test element is made of high purity silicon. A test element according to claim 1, wherein said reference pattern is a reference grid (1) and comprises a groove pattern (11) in the surface of a substrate. A test element according to claim 2, wherein the depth (T) of the grooves of said groove pattern (11) is smaller than the width (L) of the grooves of said groove pattern. A method of performing spectroscopic analysis of an impurity in a silicon substrate using a charged particle beam spectroscope, the method including a beam pre-adjustment step in which the charged particle beam is directed onto a test element (1) including a reference pattern (11), wherein said test element is made of high purity silicon.

Inventors: NAKANO AKIHIKO, NAKANO, AKIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H01J49
IPC codes: H01J37, H01J49, G01Q40