Since the 1980's, immersion exposure has been proposed several times. At the end of 1990's, however, these concepts were almost forgotten because other technologies, such as electron beam projection, EUVL, and 157 nm were believed to be more promising than immersion exposures. The current work in immersion lithography started in 2001 with the report of Switkes and Rothschild. Although their first proposal was at 157 nm wavelength, their report in the following year on 193 nm immersion with purified water turned out to be the turning point for the introduction of water-based 193 nm immersion lithography. In February, 2003, positive feasibility study results of 193 nm immersion were presented at the SPIE microlithography conference. Since then, the development of 193 nm immersion exposure tools accelerated. Currently (year 2008), multiple hyper NA (NA>1.0) scanners are generating mass production 45 nm half pitch devices in semiconductor manufacturing factories. As a future extension, high index immersion was studied over the past few years, but material development lagged more than expected, which resulted in the cancellation of high index immersion plans at scanner makers. Instead, double patterning, double dipole exposure, and customized illuminations techniques are expected as techniques to extend immersion for the 32 nm node and beyond.
High index immersion lithography is one of the candidates for next generation lithography technology following water
immersion lithography. This technology may be most attractive for the industry since it is effective in raising resolution
without seriously changing the chip making processes. This motivates us to continue to study further NA expansion
although there are many challenges with respect to either high index fluid development or high index lens material
development. In this paper, the current status of high index lithography development compared with the industry's
requirements is discussed while considering design feasibility.
In this paper we will present the progress that has been made in the area of tool development for ArF Immersion. The
local fill nozzle design adopted by Nikon has been implemented in the world's first production Immersion tools, the
S609B and S610C, to produce bubble free and low defect imaging. Defect, imaging and overlay results from the S609B
are presented showing manufacturing level results. First imaging results from the 1.30 NA S610C are also reported
showing the tools capability to image at the 45nm node and beyond. Beyond 1.30 NA it is likely that high index
materials will be required. We examine the prospects for taking immersion to lens NA's of around 1.55 with second
generation fluids and even 1.70 NA with third generation fluids. However, it cannot be forgotten that this also requires
new glass materials for lenses; the status of these will also be discussed. It is likely that high index immersion, if
implemented, will not be in time for most customers' roadmaps, in the interim it is likely that Double Patterning (DP)
will be used with potential cost penalites. The potential applications of this technique will be briefly discussed.
High index immersion lithography (HIL) is one candidate for the next generation lithography technology following
water immersion lithography. This technology may require only moderate changes of chip making processes and result in
lower cost of ownership (CoO) compared with other technologies such as double processing, extreme ultra violet
lithography (EUVL), and nano-imprinting, and other technologies. In this paper, the current status of high index lens
material and immersion fluid development compared with our requirements is discussed considering microlithographic
lens design feasibility and attainable NA.
Immersion lithography is rapidly approaching the manufacturing phase. A production-quality exposure tool system with NA=1.07 (Nikon NSR-S609B) was constructed to target the start of immersion lithography for IC manufacturing in 2006. Its projection optics have very small wavefront aberration and lowest local flare levels. The overlay issue has been analyzed, and its cause was found to be evaporation cooling. With the tandem stage and local fill nozzle implemented in the S609B, we have successfully avoided the evaporation cooling so that the good wet-to-dry mix-and-match overlay data have been obtained. The major part of immersion specific defects is caused by dried water-droplets, i.e. water-marks. The local fill nozzle has eliminated this defectivity by avoiding air flow in the nozzle. In the future, water immersion with NA=1.30 optics will be used for half-pitch 45nm manufacturing. Finer pattern imaging down to 32nm seems to need high-index material immersion or nonlinear double patterning, but these have several issues and concerns to be solved.
Immersion lithography is becoming a realistic method of high resolution pattern generation for semiconductor manufacturing. Nikon has a roadmap of full-field immersion exposure tools starting with an Engineering Evaluation Tool (EET, NA=0.85), succeeded with production models of S609B (NA=1.07) and S6xx (NA=1.30). EET was constructed in 2004, and is being used for evaluation of immersion technology and process development. With EET, focus, stepping, overlay and across-wafer CD uniformity data are demonstrated to be better or equivalent to dry tools, while the depth of focus (DOF) is significantly improved as expected. A remarkable point is the defectivity result with EET. We have detected no bubbles and a negligible level of “immersion specific” defects even with hydrophobic top coat. A production model S609B will have the NA=1.07 optics, which will be the highest NA of “all refractive optics”, and will be shipped at 2005/4Q. S6xx, with planned shipment timing is 2006/2H, will have NA=1.30 catadioptric optics, whose NA will be the highest NA of “water-immersion”. Both S609B and S6xx will be equipped with loss-less polarized illuminators, which will enable 50nm L/S with S609B and 42nm L/S with S6xx. Resist and top coat are studied from the viewpoints of chemical contamination and scanning properties. Tentative specifications are proposed for leaching of PAG and amines against chemical contamination. As for scanning properties, static contact angle was found to be not a good parameter; instead, sliding angle is proposed.
Feasibility of ArF (193nm) immersion lithography is reported based on our recent experimental and theoretical studies. Local fill method of water, edge shot, high NA projection optics, focus sensing, water supply, polarization effect, polarized illumination and resist are investigated. Although we recognize there are some remaining engineering risks, we have judged that ArF immersion lithography is basically feasible and is a very promising method that can reach the half pitch required for the 45nm node. On this basis we have planned our development schedule of immersion exposure tools.
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.
Present status of development of F2 (157nm) exposure tool in Nikon is described. Key points of F2 exposure tool are reported; low aberration projection optics, CaF2 quality, coating durability and gas purging of the pellicle space. We also report the measurement of refractive index inhomogeneity inside CaF2 crystals, which is suspected as the cause of local flare. Characteristics of high NA optics over 0.9 are investigated by imaging simulations for both 193nm and 157nm wavelengths, which are compared NA=0.85 imaging.
Imaging performance and issues of immersion lithography are discussed with the results of the recent feasibility studies. Immersion lithography has 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 193nm exposure, water (n = 1.44) has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193nm) immersion lithography (NA=1.05 to 1.23) has equivalent performance to F2 (157nm) dry (NA=0.85 to 0.93) lithography. Six fundamental issues in the ArF immersion lithography are investigated and studied. Results of the study indicate that there are no “show stoppers” that prevent going into the next phase of feasibility study.
Purging and reduction of out-gassing are very important issues that need to be treated in order to realize F2 laser lithography system. Several methods of purging are tried and out-gases from metals, O-rings, lubricants, and an adhesive are analyzed. Metal surfaces mainly release oxygen and water independent of surface roughness, Ni plating, or elements. Other substances are not detected by API-MS or GC-MS. Since O-rings are indispensable to make gas-tight structures, several kinds of O-rings made of fluoro-compounds are tested. Black fluoro-rubber o-ring, O-ring F, is recommended from the view of organic out-gassing but Teflon-based fluoro-elastomer, O-ring A, is a good candidate in terms of the water out-gassing. Greases emit a large amount of out-gases even when the samples are not irradiated by 157 nm laser. As an adhesive, Adhesive A is recommended because of the fact that it does not release as much organic and inorganic compounds which may absorb 157 nm laser light. Finally preliminary demonstration using a model exposure system is performed to obtain purging time for several cases.