The International Technology Roadmap for Semiconductors requires improvements in resolution for each lithographic node. In order to meet the resolution requirements for the sub-65-nm nodes, image placement (IP) errors induced by chucking the mask during e-beam patterning, metrology, and exposure must be characterized and minimized. This study focused on a 200-mm electron projection lithography (EPL) stencil mask designed for high throughput. Finite element models were developed to simulate the response of the mask throughout a typical fabrication process flow, including the electrostatic chucking during e-beam patterning and EPL exposure. The results of this predictive study were used to identify the primary sources of IP error as a function of the system parameters.
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 liquid's 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.
For 157 nm lithography the pellicle material will be most probably a 800 μm thick inorganic (fluorine doped fused silica) plate instead of a standard thin (~ 1 μm) organic (polymer) film. The thickness of the pellicle makes it an additional optical element in the 157 nm exposure tool. This puts tight requirements on the optical properties of the pellicle. One of the largest challenges is to control the pellicle induced overlay errors that result from small variations in pellicle flatness. A local tilt of 12 μrad already introduces an image displacement of 1 nm. This paper deals with the theoretical understanding of the pellicle indued overlay errors. It shows the relation between offline pellicle flatness measurements and exposure tool overlay performance. Two potential solutions are presented to obtain the pellicle within the desired overlay specification. System overlay corrections in combination with a new mounting strategy based on 'correctable pellicle shapes' seem to make the desired overlay specification (≤ 1 nm) feasible. The proposed 'one-dimensional' pellicle shape seems to be very promising. Distortion data, as obtained from exposures on a 193 nm system with and without pellicle, indicate that the proposed solution for automatically and fully correcting for a non-flat pellicle is feasible.
The challenges in fabricating next-generation lithography (NGL) masks are distinct from those encountered in optical technology. The masks for electron proximity lithography, as well as those for ion and electron projection, use freestanding membranes incorporating layers that are different from the traditional chrome-on-glass photomask blanks. As a promising NGL technology, low-energy electron-beam proximity-projection lithography (LEEPL) will be subject to strict error budgets, requiring high pattern placement accuracy. Meeting these stringent conditions will necessitate an optimization of the design parameters involved in the mask fabrication process. Consequently, comprehensive simulations can be used to characterize the sources of the mechanical distortions induced in LEEPL masks during fabrication, pattern transfer, and mounting. For this purpose, finite element (FE) structural models have been developed to identify the response of the LEEPL mask during fabrication and chucking. Membrane prestress, which is used as input in the FE models, was measured on a 200-mm test mask and found to low in magnitude with excellent cross-mask uniformity. The numerical models were also validated both analytically and experimentally considering intrinsic and extrinsic loading of the mask. Finally, simulations were performed to predict the response of the LEEPL mask during electrostatic chucking. FE results indicate that the mask structure is sufficiently stiff to remain relatively flat under gravitational loadings. The results illustrate that mechanical modeling and simulation can facilitate the timely and cost-effective implementation of the LEEPL technology.
The International Technology Roadmap for Semiconductors requires improvements in resolution for each lithographic node. In essence, all sources of distortion in the chip fabrication process must be minimized to meet the stringent error budgets for the sub-90-nm nodes. These include the thermal distortions of the device wafer caused by energy deposition during exposure. Absorbed energy from the beam produces temperature increases and structural displacements in the wafer, which directly contribute to pattern placement errors and image blur. In this research, the thermomechanical response of the device wafer was investigated and compared for 193-nm lithography and EUV lithography. Thermal and structural finite element (FE) models were developed to numerically simulate the exposure process for both types of tools. The three-dimensional FE models include the full wafer and chuck to identify the time-dependent response. For verification purposes, the FE models were benchmarked against an analytical test case. Since the thermomechanical response is relatively sensitive to exposure energy and wafer chucking, parametric studies were performed to illustrate the effects of resist sensitivity, backside contact conductance, and effective boundary conditions. Results for both 193-nm lithography and EUV lithography are presented.
Extreme ultraviolet lithography (EUVL) is one of the leading technologies for Next-Generation Lithography. Continued progress in its development will be facilitated by characterizing all sources of distortion in the chip fabrication process. These include the thermal distortions of the wafer caused by deposited EUVL energy during scanning exposure. Absorbed energy from the beam produces temperature increases and structural displacements in the wafer, which directly contribute to pattern placement errors and image blur. Because of the vacuum conditions of EUVL systems, wafer chucking will be electrostatic, which has a number of advantages over mechanical clamping systems. The goals of this research are to predict the transient temperature increases and corresponding displacements (locally and globally) consistent with the thermomechanical boundary conditions of the wafer. Both thermal and structural finite element models were constructed to numerically simulate wafer exposure. The response of the wafer is relatively sensitive to the interface conditions between the substrate and electrostatic chuck. Thus, parametric studies of the response to changes in the contact conductance and the friction coefficient were performed and are presented in this paper.
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.