Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-04-15
2002-09-17
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S3960ML
Reexamination Certificate
active
06452175
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a charged particle beam device for the examination of specimens. In particular, this invention relates to a beam column where the beam may land on the specimen surface at 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 at a much larger angle of about 5° to 10°, corresponding to 90 to 180 millirads.
One application which requires such 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 horizontal 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, also has 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.
2
). 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 attendant degradations due to lens aberrations. In particular, 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 specimens to be examined with a large beam landing angle while maintaining a high resolution of the charged particle image.
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 onto a specimen surface under a large 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 pre-lens deflection unit arranged between the particle source and the objective lens; the pre-lens deflection unit being adapted to deflect the beam of charged particles away from the optical axis on such a path that the combined action of the objective lens and the pre-lens deflection unit directs the beam of charged particles towards the optical axis to hit the specimen surface from a first direction; an in-lens deflection unit arranged in the vicinity of the objective lens such that the fields of the in-lens deflection unit and the objective lens overlap; the in-lens deflection unit being adapted to redirect the deflected beam of charged particles on such a path that the combined action of the objective lens and the in-lens deflection unit redirects the beam of charged particles towards the optical axis to hit the specimen surface under said large beam landing angle from a second direction substantially opposite to said first direction.
Preferably, the fields of the pre-lens deflection unit and the objective lens have substantially no overlap. It is further advantageous if the in-lens deflection unit and the objective lens have appreciable overlap.
As discussed above, pre-lens deflection leads to an off-axis path of the beam through the objective lens which gives rise to large chromatic aberrations. These chromatic aberrations have been found to be independent of the position of the deflecting system as long as the field of the deflector and the field of the objective lens do not overlap. When the deflection system is placed inside the field of the lens, the chromatic aberrations are reduced. The reduction can amount to 50% or more, if the deflection system is placed deep inside the field of the lens or even partly below the lens. However, the chromatic aberration of such an in-lens deflection system is still in the order of tens of nanometers and thus not acceptable for many applications.
It has surprisingly been found by the present inventors that the chromatic aberrations caused by pre-lens deflection can be compensated by an in-lens deflection in the opposite direction. The combined action of pre-lens deflection and in-lens deflection causes the charged particle beam to hit the sample surface from a direction substantially opposite to the direction from which the beam hits the sample when no in-lens deflection is carried out.
Without being bound to a particular theory, this effect is presently understood as follows: For example, the pre-lens deflection system alone may cause a chromatic aberration of 100 nm for a beam landing angle of 5°, and the in-lens deflection system alone may cause a chromatic aberration of 50 nm for a beam landing angle of 5°, i.e. one which is reduced by 50%. Exciting the pre-lens deflection system to tilt the beam by 5°, and the in-lens deflection system to tilt the beam by 10° in the opposite direction
Applied Materials Inc.
Nguyen Kiet T.
Sughrue Mion Zinn MacPeak & Seas LLP.
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