Amr Y. Abdo

Theoretical analysis of 157-nm hard pellicle system purification via a cyclic purge'fill process

Optical lithography with 157-nm light is expected to bridge the gap between 193-nm technology and next-generation lithography. One important practical difficulty facing the implementation of 157-nm tech- nology is gas absorption of 157-nm light. The exposure process for 193-nm technology is carried out in an air environment, but oxygen gas and water vapor severely attenuate 157-nm radiation. However, 157-nm exposure can be carried out in a nitrogen environment, which can be achieved by purging. A challenging aspect of the nitrogen purging pro- cess is the evacuation of the volume delineated by the pellicle frame, and the 800-mm-thick hard pellicle plate, which can fracture when sub- jected to an excessive pressure difference. A technique for pellicle puri- fication via a cyclic purging and filling process is investigated. A theoret- ical analysis of the gas flow and pressure variation in the system is presented. The maximum stress induced in the hard pellicle during the process is predicted using finite element modeling. The minimum time for purification without causing excessive stress in the pellicle plate is estimated for a nominal set of conditions. Finally, a parametric analysis of important geometric variables including the size and number of purg- ing holes as well as the filter resistance is presented.

Simulating fluid flow characteristics during the scanning process for immersion lithography

Immersion lithography has been proposed as a method for improving optical lithography resolution to 50 nm. The premise behind the concept is to increase the index of refraction in the space between the lens and wafer by insertion of a high refractive index liquid in place of the low refractive index air that currently fills the gap. Because the liquid will act as a lens component during the lithographic process, it must maintain high uniform optical quality. One source of optical degradation may be due to changes in the liquid’s index of refraction caused by a change in temperature. During the exposure process, energy is deposited onto the wafer, causing a rise in temperature. Consequently, any liquid in direct contact with elevated temperature portions of the wafer will also experience an increase in temperature. Two-dimensional computational fluid dynamics models were created to assess the thermal and fluid effects of the exposure process on the liquid temperature. This article presents the results of t...

Microfluidic simulations for immersion lithography

The premise behind immersion lithography is to improve resolution by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer by inserting a high-index liquid in place of the low-index air that currently fills the gap. We present a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer. The objectives of this feasibility study are to identify liquid candidates that meet the fluid mechanical requirements and to verify modeling tools for immersion lithography. The filling process was analyzed to simplify the problem and identify important fluid properties and system parameters. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer are developed and used to investigate a passive technique for filling this gap, in which a liquid is dispensed onto the wafer as a puddle, and then the wafer and liquid move under the lens. Numerical simulations include a parametric study of the key dimensionless groups influencing the filling process, and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements.

Preliminary microfluidic simulations for immersion lithography

The premise behind immersion lithography is to improve the resolution for optical lithography technology by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer. This is accomplished through the insertion of a high index liquid in place of the low index air that currently fills the gap. The fluid management system must reliably fill the lens-wafer gap with liquid, maintain the fill under the lens throughout the entire wafer exposure process, and ensure that no bubbles are entrained during filling or scanning. This paper presents a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer in immersion lithography. The objective of this feasibility study was to identify liquid candidates that meet both optical and specific fluid mechanical requirements. The mechanics of the filling process was analyzed to simplify the problem and identify those fluid properties and system parameters that affect the process. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer were developed for simulating the process. The CFD simulations were used to investigate two methods of liquid deposition. In the first, a liquid is dispensed onto the wafer as a “puddle” and then the wafer and liquid move under the lens. This is referred to as passive filling. The second method involves the use of liquid jets in close proximity to the edge of the lens and is referred to as active filling. Numerical simulations of passive filling included a parametric study of the key dimensionless group influencing the filling process and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements. For active filling, preliminary simulation results characterized the influence of the jets on fluid flow.

Thermomechanical distortions of advanced optical reticles during exposure

If optical lithography is to be extended into the 157 nm regime, controlling mask-related distortions will be a necessity. Thermomechanical distortions during exposure could be a major source of pattern placement error, especially if alternative materials such as CaF2 or MgF2 are employed. Full 3D finite element heat transfer and structural models have been developed to simulate the response of the reticle during both full-field and scanning exposure systems. Transient and periodic steady-state temperature distributions have been determined for typical exposure duty cycles. Corresponding in-plane and out-of- plane thermal distortions have been identified for both fused silica and calcium fluoride substrates. Under equivalent exposure conditions, the distortions in the CaF2 are significantly higher.

Optimizing the fluid dispensing process for immersion lithography

The concept behind immersion lithography is the insertion of a high refractive index liquid in the space between the final projection lens of an exposure system and the device wafer to improve the overall resolution of the exposure process. Computational fluid dynamics (CFD) simulations were performed in order to investigate the process of initially filling the lens-wafer gap with immersion fluid. The CFD models were used to investigate the effects of dispense velocity, gap height, and fluid dispense angle on the fill process; specifically on the possibility of air entrainment. The simulations revealed that there is an optimal region in the parameter space of gap height and dispense velocity for which the gap fills completely. Outside of this region, either excessive inertial or surface tension forces cause an undesirable, incomplete filling process. The optimal region was found to shift somewhat based on the fluid dispense angle. Finally, experiments were performed to verify the CFD models. The CFD simul...

Predicting air entrainment due to topography during the filling and scanning process for immersion lithography

Current optical lithography methods are nearing theoretical limits that prevent their use in the production of circuits for future nodes. A proposed solution is to increase the index of refraction of the transmission medium between the final lens of the exposure system and the wafer. When a liquid is used in this lens–wafer gap, the process is known as immersion lithography. A major concern is air bubbles in the liquid, since they are sources of index discontinuities. This article investigates the potential for trapping air as the free surface of the fluid front moves over features associated with wafer topography during the filling and scanning process. Optical simulations have shown that even very small bubbles located near or on the resist can significantly impact the imaging process. Therefore, the ability to predict the characteristics of the flow, liquid, and features that lead to air entrainment during filling is important. Modeling techniques were developed in order to create models that were capa...

Simulation of the coupled thermal optical effects for liquid immersion micro-/nano-lithography

Immersion lithography has been proposed as a method for improving optical microlithography resolution to 45 nm and below via the insertion of a high refractive index liquid between the final lens surface and the wafer. Because the liquid will act as a lens component during the imaging process, it must maintain a high, uniform optical quality. One potential source of optical degradation involves changes in the liquid’s index of refraction caused by changing temperatures during the exposure process. Two-dimensional computational fluid dynamics models from previous studies have investigated the thermal and fluid effects of the exposure process on the liquid temperature associated with a single die exposure. Here, the global heating of the wafer from multiple die exposures has been included to better represent the “worst case” liquid heating that will occur as an entire wafer is processed. The temperature distributions predicted by these simulations were used as the basis for rigorous optical models to predict effects on imaging. This paper presents the results for the fluid flow, thermal distribution, and imaging simulations. Both aligned and opposing flow directions were investigated for a range of inlet pressures that are consistent with either passive systems or active systems using filling jets.

Predicting microfluidic response during immersion lithography scanning

Immersion lithography has been proposed as a method of improving optical lithography resolution to 50 nm and below. The premise behind the concept is to increase the index of refraction in the space between the lens and wafer through the insertion of a high refractive index liquid in place of the low refractive index air that currently fills the gap. This paper presents three studies related to potential problem areas for immersion lithography. The first study investigates the entrainment of air as liquid flows over features in the wafer topology. Bubbles are undesirable because they introduce changes in the index of refraction in the optical path that can lead to imaging errors. The second investigation examines liquid heating due to the absorption of the incident energy by the fluid as well as heat transferred from the exposed wafer and viscous heating. This temperature elevation can lead to changes in the liquids index of refraction which may lead to optical degradation of the fluid. The final investigation examines the potentially significant normal and shear stresses induced on both the lens and wafer surface due to the increased viscosity and density of the liquid as compared with air. These mechanical loads may cause the lens to distort or shift in its mounting. This paper presents the results of the numerical thermal, flow, and structural simulations used to analyze these various critical issues.

Mask-related distortions of modified fused silica reticles for 157-nm lithography

Extending 157-nm lithography to the 70 nm node will be a difficult challenge due to the stringent requirements on image placement accuracy. At the University of Wisconsin Computational Mechanics Center, numerical and experimental studies are being conducted to investigate materials, fabrication processing, and system parameters necessary to achieve the required overlay error budget. This paper provides our latest results for 157-nm reticles, including the photomask / pellicle system. Mask blank fabrication and pattern transfer effects were simulated utilizing three-dimensional finite element (FE) structural models. The pattern-specific in-plane distortions (IPD) induced by each fabrication process step have been determined using the IBM Nighteagle / Falcon layout. To complete the static structural analysis, the effects of bonding a pellicle were also identified. The thermomechanical response of reticles during e-beam patterning and exposure were evaluated utilizing FE heat transfer models. Results from e-beam writing simulations indicate that transient thermal distortions from patterning the Nighteagle / Falcon design are not critical. However, under high throughput conditions, the IPD induced during scanning exposure can become relatively large. The simulation results provide an indication of the total overlay error budget to be expected, and demonstrate the importance of using predictive models to optimize mask system performance in a cost-effective manner.