In an immersion lithography tool, a high refractive index liquid is introduced into the space between the last projection
lens of the system and the wafer. The additional liquid increases the system's numerical aperture, thereby decreasing the
theoretical limit of resolution. In order to achieve the levels of throughput that are demanded by the semiconductor
industry, the wafer will be subjected to high velocities and accelerations which present challenges to the fluid
management system. As the wafer velocity increases, the dynamic receding contact angle is reduced. At high velocities
inertial forces can overcome surface tension forces that hold the fluid. If this occurs, the contact angle approaches zero
and a very thin film of liquid is "pulled" from the receding meniscus, which is not desirable. A two-dimensional (2-D) computational fluid dynamics model has been developed to investigate the behavior of the
receding meniscus under different operating conditions. The receding dynamic contact angle and film pulling velocity
predicted by the model are compared with the same quantities measured experimentally. It is shown that a 2-D model
provides predictions that are qualitatively accurate and therefore useful in the evaluation of alternative fluid management
techniques. A parametric study of the effect of static receding contact angle and external pressurization on the film
pulling velocity is described, as these quantities represent two design parameters that are currently being considered for
immersion tool fluid management.
In immersion lithography at 193 nm, water is inserted between a resist-coated wafer and the final lens element to improve resolution and depth of focus. Experiments have shown that some chemicals in the resist, particularly the photoacid generators, are soluble and therefore will leach out of the resist layer when exposed to water. Diffusion of this contamination across the lens-wafer gap may, over time, build up on the lens and therefore degrade the performance of the tool. We present models that describe the transport of contaminants in the under-lens region of an immersion tool. The mass flux of contaminants onto the lens is quantified for a reasonable range of parameters under various 2-D steady-state and transient flow conditions. A critical mass flux is estimated to provide a context for interpreting these results; the critical mass flux is defined as the level of mass flux that might, over a period of one year, result in a layer of contamination that is sufficiently thick so as to affect the optical transmission of the system.
In immersion lithography, the air gap that currently exists between the last lens element of the exposure system and the wafer is filled with a liquid that more closely matches the refractive index of the lens. There is a possibility that air bubbles, which represent a refractive index discontinuity, may be present in the liquid within the active exposure region and cause errors in imaging. One potential source of bubble generation is related to the flow of liquid over previously patterned features, or topography, during scanning or filling. This microscale entrainment mechanism is investigated experimentally and analyzed using computational fluid dynamics (CFD) modeling. The contact angle is a critical parameter that governs the behavior of the contact line and therefore the entrainment of air due to topography; the same topography on a hydrophobic surface is more likely to trap air than on a hydrophilic one. The contact angle can be a strong function of the flow velocity; a hydrophilic surface can exhibit hydrophobic behavior when the velocity of the free surface becomes large. Therefore, the contact angle was experimentally measured under static and dynamic conditions for a number of different surfaces, including resist-coated wafers. Finally, the flow of liquid across 500-nm-deep, straight-sidewall spaces of varying width was examined using both experimental visualization and CFD modeling. No air entrainment was observed or predicted over the velocity and contact angle conditions that are relevant to immersion lithography. The sharp-edged features studied here represent an extreme topography relative to the smoother features that are expected on a planarized wafer; therefore, it is not likely that the microscale entrainment of bubbles due to flow over wafer-level topography will be a serious problem in immersion lithography systems.
In immersion lithography, the air gap that currently exists between the last lens element of the exposure system and the wafer is filled with a liquid that more closely matches the refractive index of the lens. There is a possibility that air bubbles, which represent a refractive index discontinuity, may be present in the liquid within the active exposure region and cause errors in imaging. One potential source of bubble generation is related to the flow of liquid over previously patterned features, or topography, during scanning or filling. This microscale entrainment mechanism is investigated experimentally and analyzed using computational fluid dynamics (CFD) modeling. The contact angle is a critical parameter that governs the behavior of the contact line and therefore the entrainment of air due to topography; the same topography on a hydrophobic surface is more likely to trap air than on a hydrophilic one. The contact angle can be a strong function of the flow velocity; a hydrophilic surface can exhibit hydrophobic behavior when the velocity of the free surface becomes large. Therefore, the contact angle was experimentally measured under static and dynamic conditions for a number of different surfaces, including resist-coated wafers. Finally, the flow of liquid across 500-nm deep, straight-sidewall spaces of varying width was examined using both experimental visualization and CFD modeling. No air entrainment was observed or predicted over the velocity and contact angle conditions that are relevant to immersion lithography. The sharp-edged features studies here represent an extreme topography relative to the smoother features that are expected on a planarized wafer; therefore, it is not likely that the microscale entrainment of bubbles due to flow over wafer-level topography will be a serious problem in immersion lithography systems.
KEYWORDS: Ion beams, 3D modeling, Systems modeling, Process modeling, Photomask technology, Photomasks, Gases, Computational fluid dynamics, Computing systems, Statistical modeling
Focused Ion Beam (FIB) systems can be used to repair photomasks by accurately depositing and/or removing absorber material at the nanometer-scale. These repairs are enabled or enhanced by process gases delivered to the area of ion beam impact on the sample. To optimize gas delivery, three-dimensional computational fluid dynamics (CFD) models of selected gas delivery systems for FIB tools have been developed.
The models were verified through an experiment in which water vapor was dispensed onto a cryogenically-cooled substrate. Water vapor hitting the sample surface immediately freezes. The height of the deposited ice on the sample surface is proportional to the product of the local gas flux and the exposure time. The gas flux predicted by the CFD model was found to be in good agreement with the experimental measurement.
The CFD models were used to predict the mass flux of process gas and the pressure distribution at the sample surface for various gas delivery system designs. The mass flux and pressure relate directly to the amount of reactants that are available for the FIB repair processes. Parametric studies of key gas dispense system geometric parameters are presented and used to optimize the gas dispense system geometry.
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