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

Abstract

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 capable of resolving the flow characteristics over 100 nm scale features without the need to simulate the entire macroscopic flow through the lens-wafer gap. Results of these models show that bubble formation occurs only for extreme geometries and flow conditions. In actual production the velocities, contact angles, and feature profiles are well out of these extreme ranges, and will not cause bubble formation.

Advances in Patterning Materials for 193 nm Immersion Lithography

“…Macroscale air entrainment in the advancing meniscus is calculated to occur only when the substrate velocity is greater than 17 m/s, which is more that 30 times larger than relevant wafer scan rates . Similarly, air entrainment due to meniscus flow over microscale topographical features during filling or wafer scanning was not found to pose a problem for immersion lithography. , Other possible sources of surface-bound air bubbles include the spontaneous formation of nanobubbles on water-immersed hydrophobic surfaces ,, and outgassing of volatile resist components. While transient bubbles were formed during exposure of a 248 nm photoresist (APEX-E, which employs a t -butoxycarbonyl protecting group) under immersion conditions, no such bubble formation was observed with typical 193 nm resists. , …”

“…However, another source of scattering, air bubbles, can arise from air entrainment during fluid filling, fluid flow, or wafer scanning, evolution of dissolved gases, and resist outgassing . Macroscale air entrainment in the advancing meniscus is calculated to occur only when the substrate velocity is greater than 17 m/s, which is more that 30 times larger than relevant wafer scan rates . Similarly, air entrainment due to meniscus flow over microscale topographical features during filling or wafer scanning was not found to pose a problem for immersion lithography. , Other possible sources of surface-bound air bubbles include the spontaneous formation of nanobubbles on water-immersed hydrophobic surfaces ,, and outgassing of volatile resist components.…”

Modeling and experimental investigation of bubble entrapment for flow over topography during immersion lithography

“…The progression of a flow over sharp-edged, vertical-sidewall features with 0.5 μm depth was investigated using CFD modeling via the technique described by Wei et al 6 As in the experiments detailed above, a contact line velocity of 1 m/s was chosen, since this seems to be an upper bound for contact line motion in immersion lithography systems. As noted above, the advancing dynamic contact angle increases with velocity, so performing the simulation at the upper bound of 1 m/s presents conservative results.…”

“…These include macroscopic mechanisms such as meniscus break-up, filmpulling, and air entrainment, as well as microscopic mechanisms such as flow over wafer topography related to printed features. 2,3 In this paper, the solid-liquid-gas interface is examined; this region is referred to as the three-phase contact line, or the triple line. The angle between the liquid-gas interface and the solid surface in the absence of any relative motion between the two is called the static contact angle, θ S , illustrated in Fig.…”

“…Equation 1contains an important non-dimensional grouping known as the capillary number, Ca, σ μU = Ca ( 2 ) This is essentially a ratio of the viscous forces acting on the fluid near the contact line and the surface tension forces acting at the contact line. Since the surface tension is assumed to be constant in this study, Ca can be thought of as a non-dimensional velocity, scaled according to the forces that are significant to the phenomena of interest.…”