Radiant energy – Means to align or position an object relative to a source or...
Patent
1997-01-24
1998-07-28
Nguyen, Kiet T.
Radiant energy
Means to align or position an object relative to a source or...
25044211, 2504422, G01B 902
Patent
active
057866015
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates to an electron beam lithography machine and has particular reference to the mounting of components of a monitoring system in such a machine.
2. Description of the Prior Art.
In electron beam lithography a vertically directed electron beam is required to intersect a horizontal substrate, on which a pattern such as an integrated circuit is to be written, with a high degree of accuracy. The pattern is conventionally fractured into fields and the fields into subfields, with the latter written by controlled deflection of the beam and the former by movement of the substrate to position each field in turn in a zone swept by the deflected beam. In order to ensure precise positioning of the substrate, its movement in X and Y axial directions in a horizontal plane must be accurately monitored. Present standards of accuracy, achieved by laser interferometry measurement systems, are in the order of 50 nanometres. In such systems a divided laser beam is aimed at orthogonal mirrors associated with a movable stage carrying the substrate and measurements of the stage movement are derived from the interference of the outward and return beams. The measurement accuracy of this system can be influenced to a substantial degree by undesired movement of the interferometers relative to each other and to the neutral axis of the beam. Movements of this nature can arise due to, in particular, atmospheric pressure changes and temperature changes. The former occur because the electron beam, and thus also the mounting locations of the interferometers, must be situated in a region of high vacuum. The vacuum vessel required for this purpose will deflect under change in external atmospheric loading and will also expand or contract with temperature change. These movements are transmitted directly to the interferometers if mounted on the vessel.
Various methods have been adopted to ensure that the interferometer positions are as stable as possible. Vacuum vessels have been made with massive, reinforced constructions to minimise deflection due to pressure change and of materials which have a low coefficient of expansion so as to reduce susceptibility to thermally-induced deformation. The vessels themselves have been designed to be small, in order to keep mass within limits and to enhance rigidity, and the machines have been operated in temperature-controlled environments to eliminate change in ambient temperature. These solutions are all subject to compromise in one form or another. Construction of massive pressure vessels from relatively expensive low-expansion material adds to the overall cost of the machine and temperature control of the environment, which itself contributes to operating costs, has little effect on heat radiation from sources within the machine.
Another approach is construction of the vacuum vessel to be deformable under load and to mount the position-sensitive components of the machine, that is to say electron beam column, substrate stage and interferometer heads, on a single rigid body. A machine of that kind is disclosed in United Kingdom patent specification No 1 578 538. This machine is characterised by a massive ring which supports the column, inclusive of electron-optical components for beam focussing and correction, and from which depends a rigid structure carrying the substrate stage. The interferometer heads are mounted on the underside of the ring or on side members of the depending structure. The vacuum vessel encloses the body consisting of ring and depending structure and is connected to the body at a relatively small contact area at the top of the ring.
The arrangement described in No 1 578 538 has proved successful in uniting the column, stage and interferometers into a rigid sub-assembly largely resistant to relative displacements of these components due to atmospheric change. There still remains scope, however, for further reduction in the susceptibility to displacement of the critical position monitoring system compone
REFERENCES:
patent: 5644137 (1997-07-01), Waggener et al.
Dean Andrew
Sturrock John Monro
Leica Lithography Systems Ltd.
Nguyen Kiet T.
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