Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-09-09
2001-12-04
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S3960ML
Reexamination Certificate
active
06326629
ABSTRACT:
BACKGROUND OF THE INVENTION
Projection lithography device utilizing charged particles.
The invention relates to a lithography device for carrying out projection lithography by means of a beam of charged particles, which device includes an imaging particle-optical system for imaging a lithographic object structure on a lithographic imaging surface by means of said beam, said particle-optical system including a substantially telescopic system with a first and a second round particle lens which are arranged to produce a first and a second rotationally symmetrical lens field, respectively.
A device of this kind is known from an article in Proceedings SPIE “Electron-Beam Sources and Charged-Particle Optics”, Jul. 10-14, 1995, by W. K. Waskiewicz et al., entitled “Electron-Optical Design for the SCALPEL Proof-of-Concept Tools”, published in SPIE, Vol. 2522, 1995.
SUMMARY OF THE INVENTION
Particle-optical imaging, notably electron-optical imaging, can be used for the lithographic manufacture of very small structures, such as integrated electronic circuits or masks for such circuits, with a resolution which is less than the wavelength of light.
The imaging of a lithographic object structure on a lithographic imaging surface by means of electrons can in principle be carried out in two ways: sequentially and non-sequentially. In the case of sequential imaging, the emissive surface of an electron source, or a part thereof, is imaged, at a strongly reduced scale, on the lithographic imaging surface on which the lithographic structure to be formed is to be provided. This image of the electron source (the “spot”) is displaced across the object by means of, for example deflection coils, the electron beam being blanked or not during said displacement. The pixels of the pattern to be imaged are thus sequentially written onto the lithographic imaging surface. As the dimensions of the lithographic structure are larger, significantly more time will be required for the scanning writing of this structure, i.e. an increase in time in proportion to the surface area of the structure. Because nowadays in the integrated circuit technique there is a strong tendency to image increasingly larger structure, the throughput during the reproduction of integrated circuits decreases strongly, so that this method of imaging is becoming increasingly more objectionable.
In the case of non-sequential imaging, the lithographic object structure to be imaged is uniformly irradiated by means of the electron beam and a focusing lens system is used to form an image, reduced or not, of the lithographic object structure on the lithographic imaging surface. The pixels of the pattern to be imaged are thus simultaneously, i.e. not sequentially, projected onto the lithographic image surface. Therefore, this method of lithography is also called projection lithography.
The cited article describes a projection lithography method in which a lithographic object structure is imaged on a lithographic imaging surface by means of a system of rotationally symmetrical electron lenses. Such a lithographic object structure can be formed by a (comparatively large) rendition of a lithographic mask which is to be imaged on a lithographic imaging surface in order to derive the actual (much smaller) lithographic mask therefrom. The lithographic object structure to be imaged may also be formed by the actual mask, which is then imaged on the lithographic imaging surface (in that case being a wafer) in order to form integrated circuits therefrom. This known lithographic method is called SCALPEL® (“Scattering with Angular Limitation Projection Electron-beam Lithography”. The imaging system of electron lenses therein is formed by two electron lenses, having a rotationally symmetrical lens field, which together constitute a telescopic system.
In the context of the present invention a telescopic system is to be understood to mean a system of lenses which converts an incident parallel beam into a parallel outgoing beam. The simplest form of such a system consists of two lenses having a common optical axis, the rear focus of one lens being coincident with the front focus of the other lens. Projection lithography requires a telescopic system, because a comparatively large lithographic object structure (having a diameter of the order of magnitude of 1 mm) must be completely imaged on the lithographic imaging surface. The edges of the structure should in principle be just as sharp as its center, which means that the imaging defects at the edges of the structure to be imaged may hardly be greater than those at the central parts. This condition can be optimally satisfied only if the imaging system is a telescopic system, so that for the present invention it is of essential importance to perform the imaging by means of such a system.
During the production of integrated circuits by means of projection lithography the throughput is determined by the magnitude of the current in the electron beam whereby the lithographic object structure to be imaged (so the mask to be imaged in the case of IC manufacture) is irradiated. A limit is imposed as regards the current in the electron beam because the electrons in the beam repel one another (the so-called Coulomb interaction), causing an energy spread of the electrons in the beam and distortion of the beam. Both effects are greater as the current in the electron beam is larger, and cause imaging defects by the imaging system. The imaging defects may not exceed a specified value, so that an upper limit is also imposed as regards the current in the beam, and hence also as regards the throughput of the integrated circuits to be produced.
The described repulsion effect is strongest in the part of the electron beam where the spacing of the electrons in the beam is small, i.e. at the area of a cross-over in the electron beam. Such a cross-over occurs between said two round lenses which together constitute the telescopic system, that is to say at the area of the coincident focal points of the two lenses. Even though it may occur that cross-overs are also formed in the electron beam ahead of the telescopic system, such cross-overs do not have an effect on the (geometrical) imaging defects, because they are situated in the irradiating part of the beam and do not occur in the imaging beam path between the object (the lithographic object structure) and the image (the lithographic imaging surface).
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to make said limitation of the current in the electron beam less severe, and hence increase the throughput during the production of integrated circuits.
To this end, the lithographic device according to the invention is characterized in that the particle-optical system includes first quadrupole means and second quadrupole means which are arranged to produce a first and a second quadrupole field, respectively, the converging effects of said quadrupole fields on the beam intersecting one another at substantially right angles, said first and second quadrupole fields being substantially coincident with the first and the second rotationally symmetrical lens field, respectively, the strength of the quadrupole fields being such that the lithographic object structure to be imaged is stigmatically imaged on the lithographic imaging surface.
As is known from particle optics, a quadrupole field has a purely converging effect on a beam of charged particles in a first plane containing the optical axis whereas it has a purely diverging effect in a plane which extends perpendicularly thereto and contains the optical axis. When two successive quadrupoles are rotated 90° relative to one another, the converging effect of the first quadrupole will be directed perpendicularly to the converging effect of the next quadrupole. However, it may be that a rotation of the beam about the optical axis takes place between the two quadrupoles (as it occurs upon passage through a rotationally symmetrical magnet field), so that the next quadrupole would have to be rotated through the same angle in ord
Biren Steven R.
Nguyen Kiet T.
U.S. Philips Corporation
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