193nm Immersion Research Articles

Extreme ultraviolet lithography: overview and development status

Extreme ultraviolet (EUV) lithography has emerged as the most likely successor to 193-nm lithography. We provide a technical overview of EUV lithography and a discussion of the advantages of EUV lithography over alternative technologies. The key challenges in developing EUV exposure tools for high-volume production are discussed. A brief assessment is given of the cost of ownership of EUV lithography in comparison with 193-nm immersion lithography.

Study and Control of the Interfacial Mass Transfer of Resist Components in 193nm Immersion Lithography

Study and Control of the Interfacial Mass Transfer of Resist Components in 193nm Immersion Lithography

Synthesis of Fluorinated Materials for 193-nm Immersion Lithography and 157-nm Lithography

Various fluorinated polymers were synthesized for application in 193-nm immersion lithography with the goal of improving 157-nm photoresist performance. Their fundamental properties were characterized, such as transparency at 193-nm and 157-nm (wavelength) and solubility in water and a standard alkaline developer. High transparency, i.e., absorbance better than 0.2 μ-1 at 193-nm wavelength, was achieved. The dissolution behaviors of them were studied by using the Quartz Crystal Microbalance (QCM) method. We find that the dissolution rate of Poly(norbornene-2-fluoro-2-hexafluoroalchol) (PNB1FVIP) in 0.065N tetramethylammonium hydroxide (TMAH) was >100 times (nm/s) faster than that of the copolymer of tetrafluoroethylene (TFE) and norbornene-2-fluoro-2-hexafluoroalchol (TFE/NB1FVIP). A resist based on TFE/NB1FVIP was able to delineate 75 nm dense lines by exposure at 193-nm (wavelength) with an alternating phase shift mask using a 0.75 NA ArF scanner. The dissolution rates of the fluoropolymers in water and a 0.262N and 0.065 TMAH can be controlled by optimizing counter monomers containing hexafluoroisopropanol (HFA) unit, carboxylic acid unit and so on. In addition, we have collect water contact angle data. This data shows that fluoropolymers can be used as top-coats for 193-nm immersion lithography resists.

The Photopolymer Science and Technology Award

The Photopolymer Science and Technology Award No. 072100, the Best Paper Award 2007, was presented to Makiko Irie, Kotaro Endo, and Takeshi Iwai, all from Tokyo Ohka Kogyo Co., Ltd., for their outstanding contribution published in Journal of Photopolymer Science and Technology 19(4), 565-568 (2006), entitled "Surface Property Control for 193nm Immersion Resist".

Water immersion optical lithography at 193 nm

Historically, the application of immersion optics to microlitho-graphy has not been seriously pursued because of the alternative technologies available. As the challenges of shorter wavelength become increasingly difficult, immersion imaging becomes more feasible. We present results from research into 193-nm excimer laser immersion lithography at extreme propagation angles. This is being carried out in a fluid that is most compatible in a manufacturable process, namely water. By designing a system around the optical properties of water, we are able to image with wavelengths down to 193 nm. Measured absorption is below 0.50 cm−1 at 185 nm and below 0.05 cm−1 at 193 nm. Furthermore, through the development of oblique angle imaging, numerical apertures approaching 1.0 in air and 1.44 in water are feasible. The refractive index of water at 193 nm allows for exploration of the following: k1 values near 0.25 leading to half-pitch resolution approaching 35 nm at a 193-nm wavelength; polarization effects at oblique angles (extreme NA); immersion and photoresist interactions with polarization; immersion fluid composition, temperature, flow, and micro-bubble influence on optical properties (index, absorption, aberration, birefringence); mechanical requirements for imaging, scanning, and wafer transport in a water media; and synthesizing conventional projection imaging via interferometric imaging.

Advantage and feasibility of immersion lithography

Immersion lithography has an advantage in the numerical aperture of optics by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer. In case of 193-nm exposure tools, water (n = 1.44) has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193-nm) immersion lithography (NA = 1.05 to 1.23) has almost equivalent performance to F2 (157-nm) dry (NA = 0.85 to 0.93) lithography. Issues in the ArF immersion exposure tools are discussed with fluid-dynamic and thermal simulations results. In the fundamental issues, there seems to be no showstoppers so far, however, there exist several challenges to realize viable exposure tools.

Preliminary analysis of laser-pulse-induced pressure variation for immersion lithography

We describe an assessment of the pressure rise that may be induced by the short-duration but high-power pulses associated with the immersion lithography exposure process. A conservative model provides an upper bound on the pressure rise related to the expansion of the fluid near the wafer due to rapid heating. This rapid heating process is simulated as a constant heat flux from the substrate. The resulting temperature rise causes a change in pressure that propagates into the fluid at the speed of sound. The net change in the mass of the fluid contained within the pressure wavefront must be zero. This continuity requirement allows an estimate of the pressure rise and its penetration depth into the gap. For the nominal conditions associated with 193-nm immersion lithography, the model predicts that the pressure near the wafer surface may rise by as much as 7.3 kPa during the laser pulse. At the end of the laser pulse, this pressure rise will extend nominally 75 μm into the gap. Following the laser pulse, this pressure rise will rapidly decay as the pressure wavefront continues to propagate across the gap and eventually out of the under-lens region.

Simulation of optical projection with polarization- dependent stray light to explore the difference between dry and immersion lithography

When imaging at high incident angles to the resist, both the reflected light from the resist surface and the image forming process of the transmitted light into the resist are polarization dependent. The transmitted TM component and the reflected TE and TM components tend to induce stray light as a function of the incident angle. In this work, these components are analyzed and evaluated quantitatively. The polarization-dependent stray light (PDS) and the system stray light found in regular optical imaging systems are incorporated into a simulation program for diffracted intensity distribution to construct exposure-defocus (E-D) trees and windows to evaluate the exposure latitude and depth of focus (DOF) of typical circuit elements. The DOF of line-space pairs and contact holes with and without PDS are compared in the cases of two- and three-beam interference, first- and second-order beams, binary intensity masks, and phase shifting masks. Even though the 193-nm immersion system is shown to be better than 157- and 193-nm dry systems, using polarized illumination can improve DOF, particularly in low k1 situations, and in other situations when the stray-light-free image contrast is low.

Image characterization of bubbles in water for 193-nm immersion lithography—far-field approach

The scattering of bubbles in water by 193-nm light is characterized analytically with Mie scattering theory. The angular-resolved spectra with bubble sizes 100 nm, 1 μm, and 10 μm are calculated. For large bubbles, the forward scattering becomes very strong and therefore introduces a pattern-dependent flare. The normalized cross section with variant bubble size is also calculated. For bubble sizes smaller than the incident wavelength, the cross section decreases steeply and is explained by Rayleigh scattering. A contour plot of the normalized cross section versus the bubble size and the variance of refractive index is also calculated. This plot is explained with the Born approximation and is used to characterize the temperature control of the water during exposure. Finally, a statistical model is suggested to predict the image degradation caused by the phase information loss after the scattering. A bubble density 0.03/μm3 with 1-μm bubble size causes a loss of exposure latitude from 28 to 22%.

Simulation of the 45-nm half-pitch node with 193-nm immersion lithography—imaging interferometric lithography and dipole illumination

With immersion in water (n = 1.44), the highest spatial frequency available with ArF-based (193-nm) lithography tools with a numerical aperture (NA) = n×sin θ of 1.3 (1.44×0.9) corresponds to a half-pitch of 37 nm. This suggests that the 45-nm half-pitch node should be accessible. A detailed vector simulation study is reported for two approaches to printing for this node. Both imaging interferometric lithography (IIL, with a single mask and multiple exposures incorporating pupil plane filters) and dipole illumination (with two masks separating the x and y oriented small features) are shown to be capable of printing arbitrary structures under these conditions. There is a substantial loss of contrast for TM polarization at this NA that demands that different polarizations be used to capture the high spatial frequencies in the x and y directions. Both dipole and IIL schemes offer this capability; IIL provides more robust imaging results.

Benefits and limitations of immersion lithography

Liquid immersion has been used for more than 100 years to increase the numeric aperture (NA) and resolution in optical microscopy. We explore the benefits and limitations of immersion technology in lithography. Immersion optical lithography has the potential to extend the resolution below 40 nm. The theory of immersion is decribed. Simulations show that a 193-nm immersion system at NA = 0.95 can double the depth of focus as compared to a dry system. Also, an immersion 193-nm system at NA = 1.05 has slightly more depth of focus than a 157-nm dry system at NA = 0.85. However, the exposure latitude at 193 nm is decreased due to the impact of polarization in imaging. Design schemes are presented to realize an immersion step and scan system. Two configuration approaches are proposed and explored. A localized shower type solution may be preferred over a bath type solution, because the impact on the step and scan platform design is significantly less. However, scanning over the wafer edge becomes the main design challenge with a shower solution. Studies are presented that look at the interaction of immersion fluids with the lens and the photoresist. Water seems to be a likely candidate, as it does not impact productivity of the step and scan system; however, focus and aberration levels need to be carefully controlled. For 157 nm, per-fluor-polyether (PFPE) materials are currently being studied, but their characteristics may limit the productivity of the exposure system. Further research on fluid candidates for 157-nm immersion is required.