Extreme Ultraviolet Lithography (EUVL) is one of the leading candidates for Next-Generation Lithography in the sub-45-nm regime. One of the key components in the development of EUVL is understanding and characterizing the response of the mask when it is electrostatically chucked in the exposure tool. In this study, finite element (FE) models have been developed to simulate the reticle / chuck system under typical exposure conditions. FE simulations are used to illustrate (a) the effects of the nonflatness of the reticle and chuck, (b) the image placement errors induced by back-side particulates, (c) the influence of the coefficient of friction between the reticle and chuck during exposure scanning, and (d) the effects of contact conductance on the thermomechanical response of the reticle. The focus of this paper is to illustrate that mechanical modeling and simulation has now become a fundamental tool in the design of electrostatic pin chucks for the EUVL technology.
Electron Projection Lithography (EPL) has been identified as a viable candidate of the next-generation lithography technologies for the sub-65-nm nodes. The development of a low-distortion mask is essential for meeting the stringent requirements at these lower nodes. This research focused on predicting the influence of mask fabrication and pattern transfer on the image placement (IP) accuracy of a 200-mm EPL mask. In order to quantify the in-plane distortions of the freestanding membranes, three-dimensional finite element (FE) models (full mask and submodels) have been developed.
A typical process flow including thin-film deposition, pattern transfer, and tool chucking was simulated with the FE models. Full mask models were used to characterize the global response of the mask, whereas submodels of the individual membranes provided details of the localized distortions on a subfield-by-subfield basis. In addition, local (subfield) correction schemes were replicated in the FE simulations. A parametric study was conducted to identify critical variables in the mask fabrication process. Pattern transfer was modeled using appropriate equivalent modeling techniques. IP errors of membranes with patterned areas of 4 mm × 4 mm and 1 mm × 1 mm were compared in the current study, illustrating the advantages / disadvantages of the two formats. The numerical models developed here have been used to investigate the proposed EPL mask formats, as well as the materials, fabrication processes, and general system parameters required to achieve the necessary pattern placement accuracy.
The development of a low-distortion mask is of prime importance to Extreme Ultraviolet (EUV) Lithography. The mask consists of a standard ultra low expansion (ULE®) substrate measuring 152.4 mm x 152.4 mm x 6.35 mm, with a 280 nm thick reflective multilayer deposited on the top surface. Nonflatness of the mask patterned surface will manifest itself as image placement errors on the device wafer. Bottom surface nonflatness can interfere with securely holding the mask in the patterning and exposure tools as well as exacerbating patterned surface nonflatness. Of great concern is the effect of the mounting technique employed in the patterning and exposure tools on mask flatness. One such design, the electrostatic pin chuck, consists of a 'bed of pins' on the top surface of the chuck that will support the EUV mask during patterning and exposure. The pin design has been proposed to minimize the likelihood of particulates becoming lodged between the mask and chuck that would adversely distort the mask. To ensure that a chuck of this design will minimize image placement errors while still securely holding the mask, three-dimensional finite element (FE) models have been created to predict the influence of the electrostatic pin chuck on mask flatness. Legendre polynomials were used as input to the models to represent experimentally-measured substrate bottom surface shapes. The FE results illustrate that mechanical modeling provides an invaluable tool for quantifying the influence of mounting techniques on mask flatness, and, ultimately optimizing system parameters to successfully meet the stringent requirements at the 45-nm node (and below).
Nanoimprint lithography (NIL) was placed on the 2004 ITRS Roadmap, thus signifying its growing potential as a viable next-generation lithography technique. A particularly promising NIL technology is Step-and-Flash Imprint Lithography in which the pattern from a quartz template is transferred into a UV-curable silicon-rich monomer. The process of squeezing the monomer film during the imprint process produces significant flow-related pressures on the template which result in out-of-plane distortions (OPD). These OPD inherently produce in-plane distortions which compromise the quality of the resulting features. A single droplet imprint process, wherein a single puddle of monomer is used to cover the entire active area, suffers from throughput limitations due to the low imprint velocities that are required to control the flow-related pressures exerted on the template. In response to these limitations, recent research has focused on a multiple droplet imprint process wherein many droplets are dispensed and coalesce during the imprint process, resulting in lower flow-related pressures. In this paper, a numerical model is described that is capable of predicting both the pressures and the template distortions during a multiple droplet imprint process. The model consists of a finite element structural model of the template interfaced to a fluid-dynamic model of the flow through the gap; the distortion of the template affects the pressure applied on the template and vice versa, therefore a coupled, fluid-structure model is required. The pressure distribution during the imprint process is described by an analytical solution to the Reynolds equation that is modified to account for the coalescing process as well as the affects of absorption and surface tension. The modified solution is developed and verified through the use of computational fluid dynamic simulations. Results are described for a nominal set of conditions and a parametric study of the effect of droplet density is presented.
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.
A new fabrication process flow is being developed for X-ray lithography masks to simplify the wafer bonding procedure while allowing for the use of a standard, non-distortive mount in the e-beam tool. A conventional flow includes a support ring that is anodically-bonded to the mask wafer prior to writing the pattern in the e-beam tool. The new flow includes a support ring that is bonded to the mask wafer at a “single point” after the pattern is written. Because mask membrane distortions due to fabrication, pattern transfer, and mounting give rise to image placement errors on the device wafer, this research focused on the impact the new process flow has on mask membrane distortions in comparison to those that result from a conventional process flow. The resulting simulations showed that distortions that lead to image placement errors decrease when employing the new fabrication process. The results also illustrate that mechanical modeling provides an invaluable tool for quantifying image placement errors, and, ultimately, optimizing the system parameters to successfully meet the stringent error budgets at the 45-nm node (and below).
Step-and-Flash Imprint Lithography (S-FILTM) is a principal candidate for the next-generation lithography at the 45-nm node (and below). In imprint lithography, a monomer solution is dispensed onto the wafer. The monomer fills small features in a template that is lowered onto the wafer. The monomer is cured, causing it to solidify so that a three-dimensional replica of the template features is produced and remains on the wafer after the template is removed. Because this is a one-to-one process, any distortions of the template during the squeezing process will be manifested directly as errors in the features that are imprinted on the substrate. A finite element (FE) structural model of the S-FIL template has been created to predict the distortions due to mounting, gravity, and the fluid pressure distribution that arises from the viscous flow of the polymer liquid during the imprint process. Distortions take the form of both in-plane and out-of-plane displacements. An axisymmetric, finite difference (FD) model is used to predict the pressure distribution over the template due to viscous flow and surface tension effects. The FE and FD models are coupled using an iterative process in which the pressure distribution and template distortions are calculated at progressing time intervals until the final, desired gap height is achieved, nominally 200 nm. The coupled models are capable of characterizing the fluid-structure interaction that occurs during the imprint process. The results of the model will facilitate the design of system components that are capable of meeting the stringent error budgets associated with the sub-45-nm nodes.
Electron-beam projection lithography is a prime candidate for producing sub-100 nm linewidths. Critical to its success is the development of a low-distortion membrane mask. Membrane distortions are a result of fabrication and exposure and manifest themselves as pattern placement errors; thus, the sources of distortion must be identified, controlled, and minimized. Mechanical modeling via finite element (FE) methods provides an invaluable tool for accomplishing this task. Consequently, the FE method was used in conjunction with a series of designed experiments to efficiently identify and control the most influential parameters involved in the development of the Scattering with Angular Limitation Projection Electron-beam Lithography (SCALPEL) mask.
A virtual mask laboratory has been developed at the UW Computational Mechanics Center to aid in the design and optimization of the SCALPEL mask. Finite element models have been generated to simulate the thermomechanical response of the mask during fabrication, pattern transfer, mounting and exposure. Results on the mask-related distortions can be used to assess image placement accuracy and mask stability; examples of accurate procedures to vectorially sum in-plane distortion maps from the various sources are presented. In addition, experimental methods to provide material properties and stress characterization data are outlined, along with techniques to verify and benchmark the mechanical models.
Considering semiconductor industry projections, sub-0.10 micrometers technology will most likely require a new advanced lithography. Scattering with Angular Limitation Projection Electron-beam Lithography (SCALPEL) is one such lithography being developed to meet this need. As with all lithographies, successful implementation of the SCALPEL technique is dependent upon the development of a low- distortion mask; distortions lead to pattern placement errors on the integrated circuit. Therefore, finite element (FE) models have been developed in order to quantify and minimize mask membrane distortions. The support grillage, i.e., the struts, in the pattern area on a SCALPEL mask require a large number of elements to determine mechanical displacements of the mask membrane. The element density becomes computationally expensive and may exceed the computer hardware limitations. Therefore, an equivalent modeling technique has been developed to reduce the number of elements required to simulate the behavior of the mask, thereby reducing computation time and remaining within the computer hardware limitations.