Radiant energy – With charged particle beam deflection or focussing – With target means
Reexamination Certificate
1999-04-15
2003-09-02
Anderson, Bruce (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
With target means
C250S3960ML, C250S3960ML
Reexamination Certificate
active
06614026
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a charged particle beam column for the examination of specimen. In particular, this invention relates to a beam column where the beam may land on the specimen surface under an oblique landing angle.
BACKGROUND OF THE INVENTION
In charged particle beam devices, such as a scanning electron microscope (SEM) the typical aperture angle as well as the typical landing angle of the charged particle beam is of the order of several millirads. For many applications it is desirable that the charged particle beam lands on the sample surface under a much larger angle of typically 5° to 10°, corresponding to 90 to 180 millirads. Some uses require tilt angles in excess of 15° or even 20°.
One application which requires large landing angles is the stereoscopic visualization of a specimen surface. Stereographic techniques using a SEM date back to the early developmental period of scanning electron microscopy. Since electrons can be collected from practically all parts of a relatively rough sample, a SEM image has a rather “real” appearance. The main reason for this real appearance is that the secondary electron signal produced at the point of beam impact varies with the local slope of the surface in the same way as the perceived brightness of the surface of a diffusely illuminated macroscopic object. Furthermore, variations in the efficiency with which this signal is collected by the weak electric field from the detector modifies the signal as a function of position such that it appears as if the sample surface contained shadows. While the images have thus all the visual cues of a conventional black and white photograph, these cues are in many situations deceptive. It is therefore essential that a method which provides authentic perspective information is available. Stereoscopic visualization is such a method. It is useful and sometimes indispensable for detecting and resolving situations where other coding mechanisms yield ambiguous results.
In another application, topographical information about the specimen surface may be extracted, for example, from the parallax between stereo pairs of images obtained with a tilted beam. A further application, three-dimensional imaging of a specimen, requires also a beam tilted by several degrees, see, e.g., U.S. Pat. No. 5,734,164.
In all these applications, the beam tilting mechanism plays a key role. In early solutions, a stereo effect was achieved by mechanically tilting the specimen to provide two perspectives. However, due to mechanical imperfections, a lateral movement of the specimen is inevitable, which often results in misregistrations between the elements of a stereo image pair. This problem is especially pertinent for highly regular structures such as an array of memory cells in an integrated circuit.
When beam tilting is carried out electrically, the fact that the specimen can remain horizontally is a significant advantage as far as the lateral coordinate registration is concerned. Electrical tilting is also much faster than its mechanical counterpart. The electrical method, however, has also certain drawbacks. In one method, the beam is deflected above the objective lens (pre-lens deflection) in such a way that each ray seems to emerge from a point coincident with the apparent position of the electron source (see FIG.
3
). This way, each ray is focussed on the same area of the sample as long as the sample surface is in focus. However, as a consequence, the beam traverses the field of the objective lens considerably off-axis with its attendant degradations due to lens aberrations. Especially chromatic aberrations limit the attainable resolution to several tens of nanometers. Many applications require a much higher resolution of about 5 nm.
If, as in another method, the deflection coils are arranged below the objective lens (post-lens deflection), the beam passes through the lens on the optical axis (FIG.
3
). However, the physical dimensions of the coils below the final lens imposes a limit on the minimum attainable working distance, i.e., on the minimum attainable distance between the final lens and the specimen to be examined. An acceptable resolution is then not achieved due to the degraded instrument resolution arising from the enlarged working distance.
SUMMARY OF THE INVENTION
The present invention intends to overcome the above-mentioned drawbacks and disadvantages of the prior art. Specifically, the invention intends to provide an improved charged particle beam column allowing specimen to be examined with an obique beam landing angle while maintaining a high resolution of the charged particle image. According to one aspect of the present invention, to achieve this, there is provided a column as specified in claim
1
and a method as specified in claim
13
.
Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach to define the invention in general terms.
According to one aspect, the invention provides a column for directing a beam of charged particles with a finite energy spread onto a specimen surface under an oblique beam landing angle, the column comprising: a particle source for providing the beam of charged particles propagating along an optical axis; an objective lens for focussing the beam of charged particles onto the specimen surface; a deflection unit for deflecting the beam of charged particles away from the optical axis such that the beam of charged particles traverses the objective lens off-axis, thereby causing a chromatic aberration, a compensation unit adapted to disperse the beam of charged particles, thereby substantially compensating said chromatic aberration in the plane of the specimen surface, whereby the combined action of the objective lens and the deflection unit directs the beam of charged particles to hit the specimen surface under said oblique beam landing angle.
As discussed hereinbefore, the deflection leads to an off-axis path of the beam through the objective lens which gives rise to large chromatic aberrations due to the finite energy spread of the beam. It has surprisingly been found by the present inventors that this first chromatic aberration caused by the deflection can be compensated in the plane of the specimen surface by adding an element which introduces a second chromatic aberration of substantially the same kind and magnitude as the first chromatic aberration but which is substantially in the opposite direction. Such a second chromatic aberration may be introduced by dispersing the beam of charged particles.
In a preferred embodiment the compensating element comprises means for generating crossed electrostatic and magnetic deflection fields. Preferably, the crossed electrostatic and magnetic fields are created substantially perpendicular to the optical axis and form a so-called Wien filter. The compensation unit is advantageously in the form of an electrostatic and magnetic multipole (2n-pole, with n=1, 2, 3 . . . ), preferably selected from the group consisting of electrostatic and magnetic dipole (2-pole), quadrupole (4-pole), hexapole (6-pole) and octupole (8-pole).
In a further preferred embodiment, the electrostatic and magnetic 2n-pole comprises 2n pole pieces and 2n electrodes which are distinct from said pole pieces. The pole pieces and the electrodes are arranged in a plane perpendicular to the optical axis. In a still further preferred embodiments, the electrostatic and magnetic 2n-pole comprises 2n pole pieces, wherein each of the 2n pole pieces is adapted to be used at the same time as an electrode. The pole pieces are arranged in a plane perpendicular to the optical axis.
Without being bound to a particular theory, the compensating effect of a Wien filter in the column is presently understood as follows:
For a certain beam landing angle, for example 5°, the necessary deflection causes the center of the beam to pass the objective lens at a certain distance f
Anderson Bruce
Applied Materials Inc.
Sughrue Mion LLP.
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