James Osborne Plumpton

Monitoring of microfloor vibrations in a new research building

This paper describes efforts at monitoring microfloor vibrations in a newly-constructed research building. This building is intended to house a variety of delicate precision scientific instruments with performances that are deleteriously affected by even small floor vibrations. The building is five stories with welded steel construction. Upon completion of the construction, the initial occupants anecdotally complained of excessive floor vibrations. This resulted in an effort to measure the vibrations and to reduce them at their sources, including the mechanical systems for the building. The measurements are compared with industry standards and with measurements taken at nearby reinforced concrete buildings. The success of efforts at reducing the vibrations due to the mechanical systems of the building are also assessed.

Active membrane masks for improved overlay performance in proximity lithography

Membrane masks are thin (2 micron x 35 mm x 35 mm) structures that carry the master exposure patterns in proximity (X-ray) lithography. With the continuous drive to the printing of ever-finer features in microelectronics, the reduction of mask-wafer overlay positioning errors by passive rigid body positioning and passive stress control in the mask becomes impractical due to nano and sub-micron scale elastic deformations in the membrane mask. This paper describes the design, mechanics and performance of a system for actively stretching a membrane mask in-plane to control overlay distortion. The method uses thermoelectric heating/cooling elements placed on the mask perimeter. The thermoelectric elements cause controlled thermoelastic deformations in the supporting wafer, which in turn corrects distortions in the membrane mask. Silicon carbide masks are the focus of this study, but the method is believed to be applicable to other mask materials, such as diamond. Experimental and numerical results will be presented, as well as a discussion of the design issues and related design decisions.

Membrane mask aeroelastic and thermoelastic control

This paper describes the mechanics and control of mechanical distortions imposed on membrane masks during proximity (X-ray) lithography. Two sources of mechanical distortions are examined. The first is aeroelastic distortion caused by the coupling of aerodynamic fluid forces in the gap between the membrane and the wafer with the elastic mechanics of the membrane. Aerodynamic loadings on the membrane arise when the gap between mask and wafer is adjusted and during lateral stepping maneuvers. Results of stepping and gap closing experiments are presented. The results are correlated with numerical calculations based on Reynolds lubrication equation. Possible methods for reducing these aeroelastic distortions are examined. The second set of mechanical distortions contains those that give rise to some of the in-plane overlay errors. A thermoelastic technique for controlling in-plane errors using thermoelectric devices placed on the mask perimeter is described. Numerical and experimental results are presented.