Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2000-05-26
2003-08-19
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S306000, C250S307000, C430S296000
Reexamination Certificate
active
06608308
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to electrostatic lenses and electrostatic optical systems as used in charged-particle-beam (CPB) 2-D mapping microscopes (e.g., mapping electron microscopes) and related systems for observing a surface of a specimen in two dimensions.
BACKGROUND OF THE INVENTION
A scanning electron microscope (SEM) generally is used for examining the surface of a specimen, such as the product of a step in a process for manufacturing semiconductor integrated circuits, especially to ascertain the presence of surficial defects. In view of the fact that an electron beam is an exemplary charged particle beam, work has been done directed to the use of other charged particle beams (such as a focused ion beam) for similar applications.
Since principles generally applicable to an electron beam are applicable to an ion beam, the discussion below is made in the context of an electron-beam system. However, in view of the above, it will be understood that the invention is not limited to electron-beam systems.
In an SEM, as is generally known, an electron beam is irradiated onto a point on the surface of the specimen being observed. Impingement of the electron beam on the specimen surface causes the surface to emit secondary electrons. The secondary electrons are accelerated away from the surface, collected, and quantified by a suitable detector. To image a region on the sample, the electron beam simply is scanned in two dimensions in a raster manner. Secondary electrons generated at each irradiation point in the scan are collected and quantified. The data collected by the detector are processed to form an image that is displayed on a screen (CRT) or the like.
A main disadvantage of conventional SEMs is the long period of time required for obtaining an image of the surface being observed. The time is related to the need to two-dimensionally scan a point-focused electron beam over the observed surface. As a result, “mapping electron microscopes” are being investigated for use, as a possible alternative to SEMs, in examining semiconductor wafers and chips and in other applications in which high speed is required. This is because a mapping electron microscope offers prospects of simultaneously viewing an entire region of the target surface in two dimensions. To such end, a mapping electron microscope utilizes an electron-optical system (i.e., a system comprising multiple electron lenses) to direct the electron beam onto an area of the sample surface that is larger than a point. Unfortunately, various technical problems remain unresolved with mapping electron microscopes.
One problem concerns the substantial aberration that is encountered whenever a wide visual field is imposed on an electron-optical system. A conventional electron-optical system for use in a mapping electron microscope utilizes multiple electrostatic lenses to achieve a suitably high magnification of the image of the target surface. In such systems, simple Einzel (unipotential) lenses typically are used for all lenses except for the initial (cathode) lens. However, with such lenses, suitably large fields cannot be obtained because large fields produce serious aberrations.
With simple Einzel lenses, image-degrading aberrations can be reduced somewhat by forming the image using two lenses. However, such a configuration cannot produce the desired high image magnification.
More specifically, whenever high-magnification enlargement and projection are performed using electrostatic lenses, a simple short-focal-length (f) lens may be situated an axial distance (f+dz), wherein dz<<f, to the rear of the object plane or of an intermediate imaging plane. However, if the voltage applied to the electrostatic lens is increased, the field intensity within the lens increases and f shortens. If the impressed voltage is excessive, a potential barrier is formed that exceeds the kinetic energy of the electrons in the beam. In such a condition, the electrons are repelled by the lens and the desired lensing action is not obtained.
At a given impressed voltage to an electrostatic lens, the focal length f can be shortened simply by making the lens smaller. However, this approach is problematic in that there are practical limitations on the spacing between adjacent electrodes of the lens. These limitations mainly concern, for example, breakdown voltages between the electrodes. Also, off-axis aberrations tend to be excessive whenever small electrostatic lenses are used that have narrow on-axis fields. Therefore, it has been difficult to construct short-f electrostatic lenses that exhibit acceptably reduced aberrations.
Conventional approaches that achieve increased magnification by multi-stage imaging, especially in systems comprising multiple axially aligned simple Einzel lenses, have other problems. For example, whenever an electron beam passes through a lens located remotely downstream, portions of the beam pass through extreme off-axis regions of the lens. In such situations, even though aberrations could be suppressed adequately by making the remote lens extremely large, this approach is impractical. Alternatively, the electron-optical system is made extremely long in the axial direction so as to achieve high magnification with adequate suppression of aberrations. The great length of such a system is a serious disadvantage.
In other types of conventional electron-beam mapping-projection apparatus, as shown in
FIG. 9
, an “E cross B” (“ExB”; sometimes also termed a Wien filter)
42
is used to achieve perpendicular irradiation of the target surface
44
with an electron beam passing through an “irradiation column”
41
. Secondary electrons emitted from the target surface
44
are routed through a “projection system” PL having an optical axis AX that is perpendicular to the target surface
44
. More specifically, the “irradiation beam” (having a predetermined transverse area) approaches the ExB
42
along an axis Al that is angled relative to an optical axis AX. The irradiation beam is directed to the optical center of the ExB
42
. Upon passing through the ExB
42
, the irradiation beam propagates along the optical axis AX through a front lens
43
to the target surface
44
to “down-illuminate” (irradiate) the target surface at a zero angle of incidence. Secondary electrons emitted by the target surface
44
return along the optical axis AX through the front lens
43
and pass straight through the ExB
42
without being deflected. Upon passing through the front lens
43
, the secondary electrons form a first intermediate image at a first intermediate-imaging plane Ml situated at the optical center of the ExB
42
. An aperture
45
is provided to decrease aberrations in the first intermediate image.
From the ExB
42
, the beam of secondary electrons enters the projection system PL. The projection system PL comprises first and second projection lenses
46
,
47
, respectively. An image of the first intermediate image is formed, as a second intermediate image, at a second intermediate-imaging plane M
2
by the first projection lens
46
. An image of the second intermediate image is formed on a detector surface (imaging surface)
48
by the second projection lens
47
. The overall magnification of the image on the detector surface
48
can be varied in a continuous manner (i.e., “zoomed”) by varying the electrical energy supplied to the first projection lens
46
, which varies the position of the second intermediate-imaging plane M
2
.
The first intermediate-imaging plane M
1
is located at the optical center of the ExB
42
to eliminate, in a substantial manner, chromatic aberrations of the ExB
42
arising from its function as a conventional Wien filter, and to eliminate, in a substantial manner, the astigmatism that is characteristic of ExBs.
Unfortunately, a secondary-electron mapping-projection system such as that shown in
FIG. 9
has key disadvantages. First, the axis distance between the ExB
42
and any lenses downstream of it must be very large. As noted above, aberrations caused by the ExB
42
can
Nishimura Hiroshi
Takagi Toru
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
Souw Bernard E.
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