Radiant energy – Inspection of solids or liquids by charged particles – Positive ion probe or microscope type
Reexamination Certificate
1999-07-26
2002-08-20
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Positive ion probe or microscope type
C250S310000
Reexamination Certificate
active
06437330
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an adjusting method and apparatus for a beam optical system in a scanning microscope that scans and irradiates a charged particle beam to a sample surface and detects emitted secondary charged particles to obtain an observational image. More particularly, the present invention relates to a method and apparatus suited for adjusting a focus correction and an astigmatism correction of a focused ion beam microscope.
The existence of scanning ion microscopes (SIM) and electron microscopes for use as high-magnification microscopes is widely known. Although similar in principle to scanning electron microscopes (SEM), scanning ion microscopes are significantly different in that ions, instead of electrons, are irradiated as a beam to a sample surface, and in that secondary charged particles given off from the sample surface are not limited to only electrons, but include ions as well. Because of the difference in the secondary charged particles given off in using SEM's and SIM's, the scanning images also differ in resolution depending on the different types of sample materials being scanned. Accordingly, it is a common practice to select an ion beam microscope when the SIM image is clearer than that of a SEM image for a particular type of sample being examined. Both SEM and SIM images may also be obtained to compare both of the scanned images to enable further detailed observation.
The focused ion beam apparatus has been in use in the semiconductor manufacturing field for about ten years. However, there has been some recent developments in the basic performance of the ion beam optical system. The high-brightness and high-resolution powered systems have become observational microscopes, while the high-accuracy systems have become processing apparatuses used to form and shape the samples.
The scanning microscope operates by using a charged particle beam irradiated onto a sample surface. Secondary charged particles are driven out of the sample surface in the irradiated area. By detecting the secondary charged particles, image information for that area can be obtained. Accordingly, if the beam is spread out and irradiated onto the sample surface, the image information obtained would cover a broader area. When the beam is spread out, the beam energy per unit area is decreased, and the secondary charged particles given off in that particular area are also reduced. Consequently, the image obtained is low in resolution and contrast (signal power/brightness). Also, if the beam is in an elliptical form at a certain direction, that is, oval as opposed to being circular, the area of irradiation will also be elliptical, and the scanned image will have a difference in resolution in that direction, resulting in an image that appears to “flow” in that direction with loss of resolution. Therefore, in order to obtain an image with even clarity in every direction, it is required to create a beam that is circular when projected onto the sample surface (that is, circular in sectional form) and which is well-focused onto the sample surface.
A focus adjustment for a scanning microscope of this kind using charged particles has been conventionally available. There is also disclosure in a publication, JP-B-4-40825, of an automatic focus adjusting method for an electron microscope, or the like. This publication teaches one how to perform focus adjustment by sequentially varying a current flowing to an objective lens, while taking scanning images in order to search for the highest point in the detection signal level of the secondary charged particles using a peak detector. When a beam is restricted and irradiated onto a sample surface, the number of secondary charged particles emitted would be high. Therefore, by varying the current flowing to an objective lens and detecting the number of secondary charged particles emitted, the peak level may be detected and automatically set.
Consistent focus adjustment can only be made if the strength of the irradiated beam and the correspondence of the secondary charged particles emitted were always the same. But, it is not possible to perform the desired automatic focus adjustment due to the “aging” of the sample. That is, when a focused ion beam is irradiated onto a semiconductor device with a passivation film, a phenomenon is exhibited where in an initial stage of irradiation, the amount of secondary charged particles is great and the image is bright. But, if the scanning is repeated several times, the image may become darker, and then become brighter again. The difference in the intensity of the images is due to the variable amount of secondary charged particles in the sample surface at any given time, even though the strength of the beam is constant. The variance in the number of secondary charged particles makes automatic focusing mechanisms difficult to operate. Moreover, correction of distortions in the beam sectional form, i.e., adjusting for astigmatism correction, requires an adjustment operation skill that simply cannot be performed by a novice operator.
The present invention aims to provide an adjusting method for a charged particle beam optical system that enables focus adjustment that is unaffected by the irradiation beam intensity and the variable nature of the number of secondary charged particles being emitted at any point during irradiation of a sample surface. The method of the present invention allows even a beginner to make focus adjustments, including astigmatism corrections, with less operator training required than in the prior methods.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for obtaining an observational image of a sample surface by scanning a charged particle beam to detect secondary charged particles given off from the sample surface. Charged particle beam focusing and astigmatism correction are performed by comparing scanning images: one image obtained from an initial adjusting value, and other images obtained from a ±&Dgr; of the initial adjusting value, wherein &Dgr; is a predetermined selected value. The clearest image of the images is selected, and the adjusting value of the clearest image is then set as the new initial adjusting value. The entire scanning, comparison, and adjusting process is repeated until an optimal satisfactory image is obtained.
In performing focus correction in the present invention, focusing a beam with a lens of an optical system occurs by varying the power provided to the lens in order to focus the beam onto a sample surface. If the charged particle beam is a focused ion beam, then the intensity of the electric field applied is varied by the power setting of an electrostatic lens (by varying the applied voltage to the lens). If the beam is an electron beam, then the lens power setting (of an electromagnetic lens) is varied by the current supplied to the coil to vary the intensity of the magnetic field. By varying the lens power settings, a focal point f may be changed by ±&Dgr;f, and the scanning images obtained at f, f+&Dgr;f, and f−&Dgr;f may be stored in an image memory, and these three images may be displayed for comparison. It is preferable that these three images are all displayed at the same time, so that direct comparison of these three images may be made by the operator.
If the clearest image of the three is not the image obtained at f, then a new focal point f is shifted by +&Dgr;f or &Dgr;f, corresponding to the adjustment distance of the clearest image selected. Then, the entire scanning and comparison process may be repeated, and a new &Dgr;f may be selected as well, preferably a smaller &Dgr;f value then the previous value selected (such as one-half of the &Dgr;f previously used) in order to perform a more precise (fine) adjustment. The process is repeated until an optimal image is obtained, and thus an optimal focal distance is also obtained.
The present invention also allows for astigmatism correction adjustment. Astigmatism in a scanned image occurs as a result
Nguyen Kiet T.
Pillsbury & Winthrop LLP
Seiko Instruments Inc.
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