Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-05-22
2003-06-24
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06583426
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an ion beam machining apparatus for directly machining a very small portion of an electronic part, such as a semiconductor or the like, and, more particularly, to an ion beam machining apparatus which is capable of high speed machining by using a projection ion beam.
BACKGROUND OF THE INVENTION
In this technical field, there is a direct machining technology utilizing sputtering produced by irradiating a Focused Ion Beam (FIB) on a sample, and there is a method of observing a section of a semiconductor device or modifying wirings by using the machining technology (Japanese Unexamined Patent Publication No. 02-295040).
As is well known, a FIB is formed by focusing an ion beam emitted from a liquid metal ion source onto a sample using an electrostatic lens system to thereby produce an image of the ion source. The image of the ion source is very small and, therefore, there is provided a beam diameter determined mainly by aberration of the electrostatic lens system, and the size of which practically falls in a range of several 10 nm through several &mgr;um. Further, in the application thereof to direct machining, generally the acceleration voltage is set to 20 through 60 kV and the current falls in a range of several pA through 10 nA. The FIB can be irradiated to an arbitrary point and scanned in a region of about 1 mm at maximum on a sample by using an electrostatic deflector. In this case, in the case of using a FIB, due to the aberration of the electrostatic lens, when the current is increased, the beam diameter is enlarged. When the beam current reaches several nA, blur is rapidly enhanced due to the spherical aberration of the electrostatic lens and the current density is reduced. Therefore, the beam diameter is switched by changing the aperture or the focused state in accordance with the machining area and machining accuracy such that the machining speed is reduced as little as possible. Particularly, in forming a cross section of a sample for observation by a scanning electron microscope (SEM), as shown in FIG.
7
(
a
), which is most generally carried out by machining using a FIB, a hole having a rectangular region of about 10 &mgr;m is dug to a similar degree of depth and machining is carried out by switching the beam to a slender beam successively twice through three times in order to finish only a face for observing the cross section. Further, a similar operation is carried out in forming a wall of a sample for observation by a transmission electron microscope (TEM), as shown in FIG.
7
(
b
).
Further, there is known projection ion beam technology capable of forming a pattern with high accuracy. According to Japanese Unexamined Patent Publication No. 2-65117, the beam is used in lithography and an area having a size of several 10 mm is exposed by an accuracy of sub &mgr;m. It is regarded that, although the projection ion beam is suitable for irradiating a large area with a high accuracy, the beam is not suitable for an application which needs a high current density to carry out direct sputtering machining. In this case, a technology for carrying out high accuracy machining by applying an FIB apparatus to a projection ion beam apparatus is disclosed in Japanese Unexamined Patent Publication No. 8-213350. In accordance with this technology, the beam current is 10 nA at most, similar to that in a FIB, since the constitution of the FIB apparatus is used and there is no description with regard to a technology enabling high speed machining which can replace the FIB.
As described above, there is no known ion beam forming technology capable of realizing a machining speed exceeding that of a FIB and which is capable of forming a region of several 10 &mgr;m or smaller with an accuracy of sub &mgr;m.
It is an object of the present invention to provide a projection ion beam machining apparatus which is capable of machining a region having a size of several 10 &mgr;m or smaller, at high speed, and of processing an edge of the region with high accuracy by using an ion beam projecting a pattern of a member having an opening portion (stencil mask).
SUMMARY OF THE INVENTION
The present invention is based on optimum conditions in the design of an electrostatic lens system and indispensable matters in constituting an apparatus which we have found for constituting a projection ion beam apparatus capable of machining at high speed and with high accuracy in comparison with a FIB. The optimum conditions in the design of an electrostatic lens system referred to here are mainly optimum ranges of a distance between an ion source and a lens proximate to the ion source, a distance between a sample and a lens proximate to the sample and a distance between these two lenses. To satisfy such design conditions, specific combinations with regard to the constitution of the apparatus are needed.
An explanation will be given of conditions in the design of an electrostatic lens system. First, for simplicity an investigation has been made of a case in which two electrostatic lenses, that is, a lens
1
proximate to an ion source and a lens
2
proximate to a sample are used, as shown in FIG.
8
(
a
) and FIG.
8
(
b
). FIG.
8
(
a
) shows a case of forming a FIB in which the intensities (inverse number of focal length) of the two lenses are adjusted such that an image of the ion source is formed on the sample by the ion beam. FIG.
8
(
b
) shows a case of forming a projection ion beam in which the strength of the lens
2
is adjusted such that an image of a stencil mask is formed on the sample by the ion beam. In this case, the lens
1
is an illumination lens for adjusting the amount of irradiating ions which impinge onto the stencil mask, and the lens
1
converges the ion beam to a center of the lens
2
. Further, the lens
2
is a projection lens for projecting the image of the mask onto the sample, and the lens
2
converges the ion beam radiated from respective points of the stencil mask onto the sample along trajectories shown as dashed lines in FIG.
8
(
b
).
The following has been found by investigating characteristics of the two ion optical systems by calculation. That is, when the distance between the ion source and the center of the lens
1
is designated by notation Lo, the distance between the sample and the center of the lens
2
is designated by notation Li and the distance between the centers of the lens
1
and the lens
2
is designated by notation L, in the case of FIG.
8
(
a
), the current density of the FIB on the sample is proportional to 1/(Lo×Li). In the meantime, in the case of FIG.
8
(
b
), the current density of the projection ion beam on the sample is proportional to the square of L/(Lo×Li). That is, when a comparison is made with the same lens arrangement, a ratio of the current density of the projection ion beam as compared with the current density of the FIB is proportional to (L/Lo)(L/Li). Further, in the case of the projection ion beam, when the current density is increased, distortion is increased in proportion to the ninth power of Lo and the third power of Li even in a pattern having the same size and the same current density.
It has been found from the foregoing results that in the electrostatic lens system of the projection ion beam apparatus according to the present invention, it is necessary to increase L and reduce Lo and Li, and more particularly to reduce Lo to minimize the distortion to a degree which is not conceivable in the case of a FIB apparatus. For such purpose, it is indispensable to arrange all of the elements of the ion optical system, other than the electrostatic lenses, between the electrostatic lenses. In the meantime, when L is increased, the accuracy of the setting voltage of a lens power source necessary for adjusting the strength (inverse number of focal length) of the electrostatic lens proximate to the ion source becomes more and more severe and, accordingly, it has been found that there is an upper limit for L. However, it has been also found that this restriction can be alleviated
Kawanami Yoshimi
Madokoro Yuuichi
Tomimatsu Satoshi
Umemura Kaoru
Lee John R.
Smith II Johnnie L
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