Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Patent
1996-09-03
1997-12-02
Berman, Jack I.
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
25049223, 250251, H01L 21266, H01L 21263, H01J 3730, H01J 37317
Patent
active
056939507
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a projection system for charged particles, in particular ions, having a mask disposed in the beam path of charged particles which mask comprises transparent portions, more specifically holes, which are disposed in a particular asymmetrical manner with respect to the optical axis and which are imaged onto a wafer using lenses disposed in the beam path of the charged particles, wherein the beam of the charged particles comprises between the mask and the wafer at least one cross-over (crosses the optical axis).
BACKGROUND INFORMATION
In the case of the projection systems of this type, problems occur owing to the fact that in the case of identical current density at the mask provided with the transparent portions, depending upon the mask and arrangement and also the size of the transparent portions, different ionic currents and current density distributions occur behind the mask. The so-called space-charge effect, which occurs owing to the mutual influencing of the charged particles once they have passed through the transparent portions of the mask, causes the charged particles to be deflected differently, thus causing the charged particles to impact on the wafer at impact locations (actual impact locations) which differ from the desired impact location which can produce scale changes and image distortions.
These problems are particularly serious if the transparent portions in the mask are disposed in an extreme asymmetric manner with respect to the optical axis. In order to solve the ensuing problems it has already been proposed (image Projection Ion-Beam Lithography by Paul A. Miller in SPIE Vol, 1089 Electron-Beam, X-ray, and Ion-Beam Technology Submicrometer Lithographies VII I (1989), pages 26 and following) to attach compensating holes (or rather transparent regions) in the mask in order to achieve an arrangement of transparent regions (holes) which create an arrangement of transparent regions (holes) which are symmetrical with respect to the ionic-optical axis.
A disadvantage of these aforementioned features resides in the fact that the total current increases and thus the effect of the so-called stochastic space charge, which is based on the direct Coulombian interaction of the randomly distributed ions, is increased, resulting in the maximum achievable resolution being impaired. The stochastic space charge is according to A. Weidenhausen, R. Spehr, H. Rose, Optics 69, Page 126-134, 1985 proportional to the quadratic root of the total current.
Apart from this disadvantage, it is also difficult to calculate the size and arrangement of the transparent regions which are arranged on the mask and assist in the compensating process.
SUMMARY OF THE INVENTION
In order to provide a solution to the problem mentioned in the introduction and to avoid problems which occur in the arrangement of the transparent regions which assist in the compensating process, the invention proposes in the case of a projection system for charged particles to provide between the mask and the wafer a dispenser of particles which are oppositely charged with respect to the particles being imaged, e.g. a thermionic cathode, for projection systems which operate with positively charged ions, for the purpose of directing oppositely charged particles into the beam path of the charged particles. Apart from positive ions, the projection system embodying the invention can also be used to project electrons or to project negative ions, more specifically negative hydrogen ions. These negative ions render possible a more convenient mass separation.
The electrically charged particles which are to be directed between the mask and the wafer are oppositely charged with respect to the particles of the main beam. If the imaging particles are positively charged ions, then electrons are used as oppositely charged particles. If the imaging particles are negatively charged ions, more specifically negative H-ions, then the oppositely charged particles are positively charged ions. If electrons are used as t
REFERENCES:
patent: 4804837 (1989-02-01), Farley
patent: 4894549 (1990-01-01), Stengl
patent: 5378899 (1995-01-01), Kimber
Weidenhausen et al., Stochastic Ray Deflections In Focused Charged Particle Beams, Optik 69, No. 3 (1985) 126-134.
A. Yanof, "Electron-Beam, X-Ray, and Ion-Beam Technology: Submicrometer Lithographies VIII", SPIE, vol. 1089 (1989).
Chalupka Alfred
Stengl Gerhard
Vonach Herbert
Berman Jack I.
IMS-Ionen Mikrofabrikations Systeme GmbH
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