Liquid loss occurs at the receding contact line that forms when a substrate is withdrawn from a liquid. This behavior, often called film pulling, is fundamental to coating and cleaning processes, as well as other systems. There has been substantial prior work relative to understanding the static and dynamic behavior of the receding contact line and film pulling, but this work has focused primarily on operating conditions where the interfacial and viscous forces dominate. In the current work, experimental investigations are presented that identify a second regime, where inertial forces are dominant. These results are used to develop a semiempirical model for predicting the velocity at which an arbitrary liquid is deposited onto an arbitrary smooth substrate from the receding meniscus. The model is verified for a range of fluid properties and is accurate to within 20% mean average error.
This paper is a revision of the authors' previous work entitled "Experimental characterization of the receding meniscus
under conditions associated with immersion lithography," presented in Optical Microlithography XIX, edited by Donis
G. Flagello, Proceedings of SPIE Vol. 6154 (SPIE, Bellingham, WA, 2006) 61540R.
Several engineering challenges accompany the insertion of the immersion fluid in a production tool, one of the most
important being the confinement of a relatively small amount of liquid to the under-lens region. The semiconductor
industry demands high throughput, leading to relatively large wafer scan velocities and accelerations. These result in
large viscous and inertial forces on the three-phase contact line between the liquid, air, and substrate. If the fluid
dynamic forces exceed the resisting surface tension force then residual liquid is deposited onto the substrate that has
passed beneath the lens. Liquid deposition is undesirable; as the droplets evaporate they will deposit impurities on the
substrate. In an immersion lithography tool, these impurities may be transmitted to the printed pattern as defects.
A substantial effort was undertaken relative to the experimental investigation of the static and dynamic contact angle
under conditions that are consistent with immersion lithography. A semi-empirical model is described here in order to
predict the velocity at which liquid loss occurs. This model is based on fluid physics and correlated to measurements of
the dynamic and static contact angles. The model describes two regimes, an inertial and a capillary regime, that are
characterized by two distinct liquid loss processes. The semi-empirical model provides the semiconductor industry with
a useful predictive tool for reducing defects associated with film pulling.
Immersion lithography seeks to extend the resolution of optical lithography by filling the gap between the final optical element and the wafer with a liquid characterized by a high index of refraction. Several engineering obstacles are associated with the insertion of the immersion fluid. One issue that has recently been identified is the deposition of the immersion liquid onto the wafer from the receding contact line during the scanning process; any residual liquid left on the wafer represents a potential source of defects. The process of residual liquid deposition is strongly dependent on the behavior of the receding three-phase contact line. This paper focuses on an experimental investigation of this behavior under conditions that are relevant to immersion lithography. Specifically, the static and dynamic contact angle and the critical velocity for liquid deposition are presented together with a semi-empirical correlation developed from these measurements. The correlation allows the film-pulling velocity to be predicted for a given resist-coated surface using only a measurement of the static receding contact angle and knowledge of the fluid properties. This correlation represents a useful tool that can serve to approximately guide the development of resists for immersion systems as well as to evaluate alternative immersion fluid candidates to minimize film pulling and defects while maximizing throughput.
193 nm immersion lithography is rapidly moving towards industrial application, and an increasing
number of tools are being installed worldwide, all of which will require immersion-capable
photoresists to be available. At the same time, existing 193 nm processes are being ramped up using
dry lithography. In this situation, it would be highly advantageous to have a single 193 nm resist that
can be used under both dry and wet conditions, at least in the initial stages of 45nm node process
development. It has been shown by a number of studies that the dominant (meth)acrylate platform of
193 nm dry lithography is in principle capable of being ported to immersion lithography, however, it
has been an open question whether a single resist formulation can be optimized for dry and wet
For such a dry/wet crossover resist to be successful, it will need to make very few
compromises in terms of performance. In particular, the resist should have similar LER/LWR,
acceptable process window and controlled defects under wet and dry exposure conditions.
Additionally, leaching should be at or below specifications, preferably without but at very least with
the use of a top protective coat. In this paper, we will present the performance of resists under wet
and dry conditions and report on the feasibility of such crossover resists. Available results so far
indicate that it is possible to design such resists at least for L/S applications. Detailed data on
lithographic performance under wet and dry conditions will be presented for a prototype dry/wet
crossover L/S resist.
The semiconductor industry has used optical lithography to create impressively small features. However, the resolution of optical lithography is approaching limits based on light wavelength and numerical aperture. Immersion lithography is a means to extend the resolution by inserting a liquid with a high index of refraction between the lens and wafer. This enables the use of higher numerical aperture optics. Several engineering obstacles must be overcome before immersion lithography can be used on an industry-wide scale. One such challenge is the deposition of the immersion liquid onto the wafer during the scanning process; any residual liquid left on the wafer is a potential defect mechanism. The residual liquid deposition is controlled by the details of the fluid management system, and is strongly dependent on the three-phase contact line. Therefore, this work concentrates on understanding the behavior of this contact line, specifically by measuring the dynamic contact angle and the critical velocity for liquid deposition. A contact angle measurement technique is developed and verified; the technique is subsequently applied to measure the dynamic advancing and receding contact angle for a series of resist-covered surfaces under conditions that are relevant to immersion lithography.
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.
Immersion lithography allows the semiconductor industry to create next-generation devices without requiring a large shift in infrastructure, making it an appealing extension to optical lithography. Improved resolution is enabled by placing an immersion fluid with a high refractive index between the final lens of the optical system and the resist-coated wafer. Several engineering challenges accompany the insertion of the immersion fluid in a production tool, one of the most important being the confinement of a relatively small amount of liquid to the under-lens region. The semiconductor industry demands high throughput, leading to relatively large wafer scan velocities and accelerations. These result in large viscous and inertial forces on the three-phase contact line between the liquid, air, and substrate. If the fluid dynamic forces exceed the resisting surface tension force then residual liquid is deposited onto the substrate. Liquid deposition is undesirable; as the droplets evaporate, they will deposit impurities on the substrate. In an immersion lithography tool, these impurities may result in defects. An experimental investigation was undertaken to study the static and dynamic contact angle under conditions that are consistent with immersion lithography. A semi-empirical model is described here to predict the velocity at which liquid loss occurs. This model is based on fluid physics and correlated to measurements of the dynamic and static contact angles. The model describes two regimes, an inertial and a capillary regime, characterized by two distinct liquid loss processes. The semi-empirical model provides the semiconductor industry with a useful predictive tool for reducing defects associated with film pulling.
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.
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.