Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2002-04-30
2004-04-06
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
Reexamination Certificate
active
06717143
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to providing improved resolution for a surface imaging technique which uses a beam of charged particles and, in particular, for controlling the electron beam of a scanning electron microscope (“SEM”) to improve beam alignment in the lower column.
BACKGROUND OF THE INVENTION
Various instruments are known which rely on emission of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are electron microscopes (e.g., SEMs), focused ion beam microscopes, and mass spectrometers which utilize various well known means to analyze charged particles emitted from the sample.
For facilitating the description of the present invention, it will be explained in connection with an SEM. However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments that steer a beam of charged particles through more than one lens, such as a magnetic lens and an electrostatic lens.
An SEM operates by generating a primary, or incident, electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and have respective trajectories backward along the original beam direction and at angles diverging therefrom. Emitted electrons are collected by a detector, which is arranged above the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is typically processed to create an image of the surface, which is then displayed on a video screen.
An SEM has its main components set up in a part of the apparatus commonly referred to as a column. The column, as the name implies, is usually a vertical arrangement starting at the top with an electron source, or gun, and ending at the bottom with the sample. Positioned between the gun and sample are various well known components that constitute the upper, middle and lower portions of the column and which are used to, for example, correct the shape of the beam, align the beam, and provide scanning by deflecting the beam along an x-direction and a y-direction (see
FIG. 3
) in a plane perpendicular to the incident beam. SEMs can contain more than one of any of such components, as well as other components that are not discussed herein. Also, it should be understood that the position of the various components need not be as shown in the drawings and/or as described herein, such position being presented for illustrative purposes rather than accuracy.
FIG. 1
shows electron beam
1
passing through a conventional lower column
3
shown in cross section. A typical lower column includes beam deflector
5
, scan deflector
7
, and electromagnetic lens
11
. Deflector
5
is used for aligning the beam within the column. Scan deflector
7
causes beam
1
to controllably depart from its path for a minute range of scanning motion along the x-direction and y-direction (the scanning limit in the x-direction is shown in broken lines, with the deflection angles to points
23
and
25
being exaggerated) in order to scan the surface of sample
9
.
Electromagnetic lens
11
is provided for focusing beam
1
to a very small spot on sample
9
to enable high resolution imaging. Such a lens is commonly called an objective lens or a final lens. Its physics and its operation are well known. In the illustrative representation of objective lens
11
in
FIG. 1
, it includes a toroidal, channel-shaped magnetic polepiece
13
with a lens inner pole
15
and a lens outer pole
17
, and a winding
19
inside the channel.
It is well known that a beam directed along the axis of a cylindrically symmetric lens will remain on the axis, i.e., it will not be deflected, as the focus of the lens is changed. If a thin lens model of such a lens were to be applied to simplify the physics involved, and thus facilitate understanding of the invention, even if the beam is not directed along the lens axis but it passes through the lens center it will not be deflected as the focus of the lens is changed. However, there is a degradation of resolution for the case where the beam is not traveling along the optical axis. Similarly, for any optical system, there is a trajectory along which a beam is minimally deflected due to changes in focus. This trajectory, or system axis, will generally not be a simple straight line starting from the electron source and ending perpendicular to the target. The trajectory will have several elements. Although each individual lens might have a well defined axis, these will generally not fall on a single straight line. For charged particle optics systems where the final lens is a compound lens (i.e. comprised of electrostatic and electromagnetic elements) the two lenses have centers that are not generally coincident but, under optimal physical construction, it is possible for the two axes to fall on exactly the same line. In this case it is best to align the beam to this common axis. More generally, as a result of unavoidable imperfections in manufacture and mechanical placement of elements, the two lenses will not have a common optical axis. In this case the optimal alignment (to achieve minimal deflection due to changes in focus) occurs when the beam is directed along the line formed by the two lens centers. When such an alignment is achieved, the image will not shift when the focus of either lens is changed—a critical condition to achieve success with an automated metrology system. Additionally, aligning to the axis of the electrostatic lens is optimal to faithfully reproduce the sample characteristics in an image (especially for achieving image symmetry) while concurrently aligning to the axis of the magnetic lens is optimal for resolution.
Objective lens
11
has an axis identified on the drawing with the unit vector
29
in association with center E. It is necessary to align the path of beam
1
to travel substantially along axis
29
in order to achieve satisfactory performance of the SEM. For example, although it is necessary to vary the focus of objective lens
11
during normal operation of the SEM, unless beam
1
travels essentially along axis
29
, such variations in focus induce adverse effects with respect to resolution, distortion, magnification and/or motion of features in the derived image of the sample surface. Thus, deflector
5
is used to align beam
1
with axis
29
. Beam
1
remains within the confines of axis
29
even during scanning because the range of motion created by scan deflector
7
is too small and because the alignment of beam
1
with deflector
5
is accomplished by taking into account the scanning motion provided by the scan deflector.
Sample
9
is maintained by voltage source
33
at a predetermined voltage relative to polepiece
13
. For example, polepiece
13
can be grounded. The biased sample
9
and the grounded magnetic poles of objective lens
11
form an electrostatic “lens”. The primary function of this electrostatic lens is to provide a deceleration field for controlling the landing energy of the particles in the beam as they impact on the sample surface. The deceleration field is controlled by adjusting the voltage of source
33
. The combination of this electrostatic lens with the above described objective lens constitutes the effective final (compound) lens.
The electrostatic lens has an axis identified on the drawing with the unit vector
31
in association with the center F. Due to the inevitable mechanical misalignments, the electromagnetic axis
29
and the electrostatic axis
31
are typically not coincident nor are they necessarily on a line perpendicular to the sample surface. (The spacing between centers E, F and the difference in orientation between axes
29
,
31
are exaggerated for illustrative purposes.) Consequently, merely directing the beam perpendicularly to the sample surface is not necessarily the optimum choice. Thus, if beam
Duval Paul J.
Sullivan Neal T.
Tyurina Anastasia Y.
Cohen & Pontani, Lieberman & Pavane
Johnston Phillip
Schlumberger Technologies Inc.
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