Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2002-09-19
2004-11-30
Wells, Nikita (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S310000, C250S3960ML
Reexamination Certificate
active
06825475
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of inspection techniques of the kind utilizing irradiation of a sample by a focused beam of electrically charged particles, such as electrons, positrons, or ions, and relates to a deflection method and system, and a focusing/deflecting assembly utilizing the same, for use in a charged particle beam column.
BACKGROUND OF THE INVENTION
Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor devices, being utilized in CD metrology tools, the so-called CD-SEM (critical dimension scanning electron microscope) and defect review SEM. In an SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
SEM typically includes such main constructional parts as an electron beam source (cathode having a small tip called “electron gun), an electron beam column, and a detection unit. The electron beam column comprises inter alia a beam aligning means (the so-called “alignment coils”), a beam shaping means (stigmator), and a focusing/deflecting assembly including a lens arrangement and a deflection system for directing a primary electron beam onto a sample and directing secondary electrons towards one or more detection unit. The deflection of the primary beam provides for scanning the beam within a scan area on the sample, and also for adjusting incidence of the primary beam onto the sample (an angle of incidence and/or beam shift).
Some systems of the kind specified utilize an objective lens arrangement in the form of a combination of a magnetic objective lens and an electrostatic lens, the so-called “compound magnetic-electrostatic lens” (e.g., WO 01/45136 and EP 1045425 both assigned to the assignee of the present application, and WO 01/56056). The electrostatic part of the compound magnetic-electrostatic lens is an electrostatic retarding lens (with respect to the primary charged particle beam), and has two electrodes held at different potentials, one of the two electrodes being formed by a cylindrical anode tube which is arranged within a magnetic objective lens along its optical axis, and the other electrode being a metallic cup provided below the magnetic objective lens. A need for a retarding field is associated with the following. In an SEM, in order to reduce the “spot” size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optic elements are more effective (i.e., produce smaller aberrations) when the primary electrons are accelerated to high kinetic energy. Generally, the landing energy of the primary electron beam is defined by the potential difference between the cathode (a source of primary electrons formed with a small tip called “an electron gun) and the sample. To achieve the desired acceleration of electrons, an appropriate potential difference between the cathode and anode (which is typically in the form of a tube defining a primary beam drift space for the primary beam propagation to the sample) should be provided. For example, the cathode voltage V
c
can be about (−1) kV and the anode voltage V
a
can be about (+8) kV. Hence, the electrons are accelerated on their way towards the magnetic objective lens having the velocities of 9 keV However, it has been observed that such a highly energized electron beam causes damage to resist structures and integrated circuits, and, in the case of dialectical samples, causes the undesirable charging of the sample. To avoid these effects, a retarding field is created in the vicinity of the sample. The electric field created by the electrostatic lens also facilitates the extraction of secondary charged particles from the sample.
The above-indicated publication WO 01/56056 also discloses the use of a magnetic deflector integrated into a magnetic objective lens, which has an excitation coil and upper and lower pole pieces. The magnetic deflector comprises excitation coils located on the lower pole piece of the magnetic lens, and the lower pole piece is divided into four pole piece segments, each segment having its corresponding additional excitation coil of the deflector. The additional excitation coils are wrapped around the pole piece segments of the magnetic lens, so that by exciting one the additional excitation coils, a magnetic field is generated in the corresponding segment of the lower pole piece. The magnetic field is basically perpendicular to the path of the electron beam (to the optical axis). Accordingly, a magnetic field across the path of the electron beam is generated which leads to a deflection of the electron beam. Due to the segments of the lower pole piece of the magnetic lens, the magnetic field is guided to an area close above the sample and generates the required strong deflection field. The segments of the lower pole piece of the magnetic lens at the same time also guide the magnetic field generated by the excitation coil of the magnetic lens.
SUMMARY OF THE INVENTION
There is a need in the art to improve the control of charged particle beam propagation through a lens arrangement in a charged particle beam column towards a sample under inspection, by providing a novel deflection method and system, and a lens arrangement utilizing the same.
The present invention is aimed at increasing the deflecting magnetic field at the optical axis of the lens arrangement in the vicinity of the sample's plane at a given electric current through the excitation coils of the magnetic deflector, or obtaining a high magnetic field with a lower electric current through the excitation coils of the deflector. This allows for obtaining a desirably high deflecting magnetic field within the closest vicinity of the sample at the optical axis of the lens arrangement, without increasing a working distance, also in cases where the electrode of an electrostatic retarding lens is located between the magnetic objective lens and the sample.
The term “working distance” is typically referred to as a distance between the electrode of the lens arrangement closest to the sample's plane and the sample's plane. This distance should be as small as possible, and the minimal possible working distance is typically defined by an arcing problem. The present invention provides for concentrating the magnetic deflecting field at the optical axis of the lens arrangement in the vicinity of the sample's plane without affecting (increasing) the working distance, by providing a pole piece assembly at least partly located within the magnetic field of a magnetic deflector.
The problem solved by the present invention is associated with the following: To enable effective control of the magnetic field intensity in the vicinity of a sample (either grounded or not), an electrode closest to the sample should be formed with an opening as small as possible (e.g., of about 2 mm). Making the external pole piece of the magnetic objective lens with such a small opening will result in non-homogeneity of the magnetic and electric fields (due to the gaps between the pole piece segments of the magnetic objective lens or pole pieces of a magnetic deflector, as the case may be), and accordingly, in a distorted (blurred) image of the irradiated area of the sample. Using a larger inner diameter of the pole pieces is ineffective for both controlling the field intensity and deflecti
Adamec Pavel
Krivts Igor
Petrov Igor
Rosenberg Zvika
Applied Materials Israel, Ltd.
Blakely & Sokoloff, Taylor & Zafman
Kalivoda Christopher M.
Wells Nikita
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