Drying and gas or vapor contact with solids – Process – Gas or vapor pressure varies during treatment
Reexamination Certificate
2000-08-18
2001-07-03
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor pressure varies during treatment
C034S428000, C034S446000, C034S448000, C034S493000, C034S494000, C034S508000, C156S345420, C118S715000, C118S722000, C118S7230AN, C118S724000
Reexamination Certificate
active
06253464
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains generally to method and apparatus for preventing surface contamination by deposition of particulate matter and more particularly to preventing deposition of particulate matter onto lithographic components such as reticles (masks) and wafers during lithographic use, fabrication, inspection, repair, handling and storage.
The ability to produce high quality microelectronic devices and reduce yield losses is strongly dependent upon maintaining the surfaces substantially defect-free. This is particularly true as design rules drive integrated circuits to finer feature size. Generally, surface defects can be related to particulate matter being deposited onto surfaces of reticles (masks) and wafer substrates during the various operations required to produce integrated circuits. The need to maintain these surfaces substantially free of particulate matter has long been recognized in the microelectronics industry and various schemes to do so have been proposed, such as those set forth in U.S. Pat. Nos. 5,373,806 and 5,472,550. The former discloses the use of thermal energy, such as the use of radiant energy, RF, or resistance heating, to substantially eliminate electrostatic attraction as a mechanism for particle transport and deposition during gas phase processing while the latter describes the use of the photophoretic effect to capture particles by projecting a laser beam inside the processing chamber along a trajectory that does not contact the substrate surface.
The concern about printable defects caused by particle deposition onto surfaces is of particular importance for the next generation of lithographies, including proximity x-ray lithography, direct-write and projection electron-beam lithography (SCALPEL), direct-write and projection ion-beam lithography, and extreme ultraviolet (radiation having a wavelength in the region of 3.5-15 nm) lithography (EUVL) which must provide for exclusion of particles with diameters greater than 0.01 &mgr;m. The situation is exacerbated by the fact that for a beam of high energy radiation (photons, electrons, ions, or atoms), such as used for the aforementioned advanced lithographies, a pellicle which is customarily employed to protect lithographic reticles (masks) from particle deposition cannot be used. The protective benefit provided by a protective membrane such as a pellicle is negated by its deleterious effect on the beam of high energy incident radiation. By way of example, a half micron thick Si film will reduce the light intensity at 13 nm by 60%, which is an intolerable reduction for most lithographic applications. Coupled with this is the difficulty of forming a durable pellicle consisting of a ½ &mgr;m Si film. In the case of electron lithography, the pellicle will absorb some of the electron current and, by inelastic scattering, introduce undesirable chromatic aberration into the electron beam and intolerable deviations in beam angle. While it is possible to produce organic polymeric materials in the proper thickness to form pellicles, they suffer from the disadvantage that they will decompose under the influence of high energy radiation, releasing volatile degradation products which, in turn, will coat optical surfaces and reduce their efficiency. Moreover, many of the advanced lithographic concepts must operate in a vacuum to reduce degradation of high energy radiation used for finer design rules consequently, the pellicle surface will be subjected to large changes in pressure (from 760 Torr to 5×10
−4
Torr) over a surface area that may be as large as 100 cm
2
and thus, forces larger than a thin organic membrane pellicle can withstand will be generated.
Because of the importance of protecting lithographic surfaces, such as reticles, from deposition of particulate matter for next generation lithographies alternative protection schemes such as clean encapsulation of the exposure chamber, protective gas blankets, and in-situ cleaning of mask surfaces are being investigated. However, each of these alternative schemes has disadvantages and none have been developed to the point of application.
What is needed is a means to protect lithographic surfaces, such as those of the reticle and wafer, from particle deposition without comprising lithographic performance or contaminating lithographic optical elements. Moreover, in order to be useful in advanced lithographic applications it is necessary that the protecting means operate effectively in a sub-atmospheric pressure environment.
SUMMARY OF THE INVENTION
The present invention generally employs a physical phenomenon known as thermophoresis to protect lithographic surfaces from particle deposition and is particularly designed to operate in an environment where the pressure is substantially constant and can be sub-atmospheric. Protection from particle deposition is afforded during lithographic use, fabrication, repair, handling, and storage without compromising lithographic performance or contaminating other lithographic components.
Thermophoresis can be a useful tool to overcome particle deposition onto surfaces because it is capable of overwhelming those mechanisms that lead to particle deposition such as: 1) electrostatic forces, 2) inertia, 3) Brownian motion, and 4) gravity. Thermophoretic forces operate to cause particles to be driven from regions of higher gas temperature to regions of lower gas temperature. However, it is known that the thermophoretic effect begins to become less effective as gas pressure is lowered, generally precluding its use where the surface to be protected is held at pressures below about 5 m Torr, as can be the case in many lithographic operations and, particularly for advanced lithographic concepts where operation in a vacuum is necessary to reduce attenuation of the radiation. The present invention discloses a novel system that reduces particle deposition onto a surface by the use of thermophoresis, directed gas flow to isolate the surface from particles in the environment, orientation of the surface to eliminate gravitational deposition, and elimination of electric fields to protect the surface from electrostatic deposition. This invention is designed to provide particle protection in situations where the atmosphere is at substantially constant but sub-atmospheric pressure. However, because of the novel features of this invention, which have been disclosed above, the present invention can also be used in those applications where the surrounding atmosphere is at atmospheric pressure or above.
Because the system of the present invention functions in a manner similar to that of a conventional pellicle and for ease of description the system disclosed herein will be referred to hereinafter as a thermophoretic pellicle. While intended principally to provide protection to reticles (hereinafter the terms reticle and mask will be used interchangeably and synonymously) from particle deposition during operation of the lithographic process, it is contemplated that the thermophoretic pellicle can also provide protection for a reticle during fabrication, inspection and repair as well as storage, manual and robotic handling. Furthermore, the protection provided by the thermophoretic pellicle can extend to other critical lithographic components such as wafers, wafer chucks, filters, lenses, mirrors and reticle stages.
The thermophoretic pellicle, which is generally deployed in a chamber operating at a sub-atmospheric pressure, comprises an enclosure that surrounds a lithographic component having a surface needing protection from particle deposition, means for introducing a flow of gas into the enclosure, and at least one aperture that provides access to the surface being protected. Here, access is defined as permitting the entry and/or exit of a beam of radiation as well as the exit and control of the gas flow from the interior of the thermophoretic pellicle into the environment of the chamber containing the thermophoretic pellicle as well as admitting entry of mechanical devices, such as the probe of
Klebanoff Leonard E.
Rader Daniel J.
EUV LLC
Nissen Donald A.
Wilson Pamela
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