1. Field of the Art
The present invention relates to an apparatus adapted for observing an image composed of secondary electrons generated in a recessed portion when the charged particle beam is focused on a specimen having recessed portions, such as through holes formed in the processes carried out in an LSI production line, and a method for observing a surface by a charged particle beam.
2. Background of the Prior Art
Recently, there has arisen the problem of disconnection in wiring due to the residue of a photoresist on the bottom of a specimen having through holes, capacitive grooves, recesses and so on (all of which are referred to simply as "through holes" hereinafter in this specification) so that there has been a strong demand for an apparatus capable of observing the bottoms of through holes by a charged particle beam in the LSI production process.
FIG. 1 shows a column of an apparatus for observing a surface by utilizing a charged particle observation reported by Y. Furuya, T. Ohtaka, S. Yamada, H. Mori, M. Yamada, T. Watanabe and K. Ishikawa in "Model S-6000 Field Emission CD Measurement SEM", the 93rd study reference disclosed at the 132nd committee held on Japan Society for the Promotion of Science (November 8-9, 1985), p. 1.
Electrons 1 (to be referred to as "the primary electrons" hereinafter in this specification) emitted from a cathode 2 are accelerated by a first anode 3 and a second anode 4 and are focused on the upper surface of a specimen 8 through a condenser lens 5 and an objective lens 6. The focusing point on the surface of the specimen 8 at which the primary electrons 1 are focused is controlled by a deflection coil 7. Secondary electrons 9 produced in response to the impingement of the primary electrons 1 on the surface of the specimen 8 pass through the objective lens 6 and are detected by a secondary electron detector 10. The secondary electron image over the surface of the specimen 8 can be observed with the secondary electron detector 10 by detecting the secondary electrons 9 while the primary electrons 1 are caused to scan the surface of the specimen by using the deflection coil 7.
However, in order to observe the through holes with the above-mentioned conventional apparatus utilizing charged particle beam observation, almost all the secondary electrons emitted from the bottoms of through holes will not come out therefrom because the secondary electrons impinge the side walls of through holes. For instance, the trajectory of the secondary electrons emitted from a cylindrical through a hole 1 .mu.m in depth and 0.5 .mu.m in diameter is shown in FIG. 2. An insulating layer 12 is formed over the surface of a substrate 11 and the specimen 8 shown in FIG. 1 is composed of the substrate 11 and the insulating layer 12. The insulating layer 12 is formed with a through hole 0.5 .mu.m in diameter (D) and 1 .mu.m in depth (T). Corresponding to the following reference numerals: 13 represents the central axis of the through hole; 14 represents the trajectory of the secondary electron emitted at an angle of 5.degree. with respect to the central axis 13 from the center of the through hole; 15 represents the trajectory of the secondary electron emitted at an angle of 10.degree. with respect to the central axis 13 from the center of the through hole; and 16 represents the trajectory of the secondary electron emitted from the center of the through hole at an angle between 15.degree. and 85.degree. with respect to the central axis 13. The energy of the secondary electrons 14-16 is 5 eV. In the through hole, there exists magnetic flux of 1.times.10.sup.4 AT/m corresponding to the leakage flux of the objective lens 6 in parallel with the axis 13. It is seen that the secondary electrons emitted at angles in excess of about more than 15.degree. with respect to the axis 13 impinge on the side wall of the through hole and cannot come out therefrom. As described above, in the case of the conventional apparatus utilizing charged particle beam, almost all the secondary electrons cannot reach the secondary electron detector so that there is the problem that the secondary electron image at the bottom of the through hole cannot be observed.
The same inventors proposed to observe the secondary electrons by applying a strong magnetic field perpendicular to the bottom of the through hole and causing the secondary electrons to come out of the through hole by winding the secondary electrons around lines of the magnetic force as disclosed in "Low-energy-electron ray tracing and its application" in the 97th study reference (November 14-15, 1986) of the 132nd committee held on Japan Society for the Promotion of Science, pp. 118-123. In order to observe the secondary electron image at the bottom of the through hole formed during the LSI production process, it is estimated that the magnetic field of about higher than 10.sup.6 AT/m must be applied perpendicularly to the surface of the specimen.
In the case of the apparatus as shown in FIG. 1, the magnetic field applied perpendicularly to the surface of the specimen 8 is of the order of about 1.times.10.sup.4 AT/m. In order to observe the bottom of the through hole by the above-mentioned method for applying a strong magnetic field to draw the secondary electrons, the magnetic field applied to the specimen 8 must be increased by about 100 times so that the excitation current of the objective lens 7 must be increased also of the order of about 100 times.
However, in the above-mentioned surface observation apparatus utilizing a charged particle beam, when the excitation current for the objective lens 6 is increased, the focusing point of the primary electrons which has been located on the surface of the specimen 8 is moved upwardly so that the secondary electron image of the specimen becomes out of focus as shown in FIG. 3. In FIG. 3, corresponding to the following reference numerals 1 represents the trajectory of the primary electrons prior to the increase of the excitation current for the objective lens 6; 17 represents a point at which the primary electrons 1 converge; 18 represents a trajectory of the primary electrons after the excitation current for the objective lens 6 is increased; and 19 represents the point of convergence or focusing point of the primary electrons when the excitation current is increased. Before the excitation current for the objective lens 6 is increased, the focusing point 17 is at the upper surface of the specimen 8, but when the excitation current for the objective lens 6 is increased, the focusing point 19 of the primary electrons shifts upwardly of the specimen 8. As a result, the diameter of the primary electron beam impinged on the surface of the specimen 8 is increased so that there is the problem that the secondary electron image to be observed becomes out of focus.