**2.1 Status of LuAG**

Lithography-grade LuAG has been aggressively developed by Schott Lithotec since 2005. The absorbance, which is the biggest issue for LuAG, was largely improved down to 0.035 /cm by purifying the raw material and optimising the crystal growing process as shown in Fig. 2. Since it was found that the intrinsic absorbance of LuAG is 0.00118 /cm (Letz et al., 2010), the absorbance will be less than 0.005 /cm by further reduction of impurities.

Fig. 2. History of LuAG development by Schott Lithotec. The absorbance of LuAG has been improved down to 0.035 /cm but it has not reached the target (0.005 /cm).

High-Index Immersion Lithography 401

Figure 3 shows wave front aberration maps for a projection optic of 1.55 NA. These show the difference between the radial and the tangential polarized wave fronts. The wave front difference without the correction has a large rms value of 217 mλ and a clear 4θ component. On the other hand, the original correction method reduces the rms difference to 4.2 mλ. This

In the case of 1.70 NA, the wave front without the correction has a large rms difference of 680 mλ and the rms difference after the correction is 21.2 mλ. Although this value is still large compared with the case of 1.55 NA, it is close to the practical level. Therefore, it can be

High-index immersion fluids are categorized as second-generation (G2) or third-generation (G3) fluids. G2 fluids have refractive indices of approximately 1.64 and enable an NA to increase up to 1.55 with LuAG as a final lens material. G3 fluids are targeted to have

DuPont, JSR, and Mitsui Chemicals have reported both G2 and G3 fluids (French et al, 2007, Furukawa et al., 2007, Kagayama et al., 2007). Their G2 fluids are saturated hydrocarbon and have sufficient performance as an immersion fluid. They also developed G3 fluids as some extension of organic G2 fluid materials. This type of G3 fluids has not yet met the requirements of refractive index or absorbance. The other types of G3 fluids, which include nanoparticles, are being developed, but they are still within the research phase (Zimmerman et al., 2008). In the above situation, an immersion system using a G2 fluid has been

To achieve the target cost of \$1/layer for a G2 fluid, the recycling of a G2 fluid is required. G2 fluids are easily degraded by laser irradiation and the dissolution of oxygen from the atmosphere. Therefore, a fluid circulation system is desired to keep the fluid absorbance sufficiently low using purification and oxygen removal functions. In addition, lens contamination, bubble, and residual fluid on a wafer are also important issues. An immersion system composed of a fluid circulation system, an immersion nozzle and a

The absorbance of water does not change largely with dissolved oxygen, and immersion water is not used repeatedly as a G2 fluid. These are the reasons the degradation of water is

On the other hand, the absorbance of a G2 fluid is easily increased by dissolved oxygen or laser irradiation. Moreover, the dn/dT of a G2 fluid is 5~6 times larger than that of water (French et al., 2006, Santillan et al., 2006, Furukawa et al., 2007). This means that a temperature change causes a larger change in the refractive index and results in larger thermal aberration (Sekine et al., 2007). To suppress a temperature rise with the absorption

Although the absorbance of a G2 fluid is lower than that of water (French et al., 2005, Wang et al., 2006), the degradation of a G2 fluid with dissolved oxygen is larger than that of water. The

value is within the practical level.

**3. Immersion system using a G2 fluid** 

preferentially developed (Sakai et al., 2008).

**3.1 Fluid degradation** 

**3.1.1 Dissolved oxygen** 

not a concern as an immersion fluid.

concluded that the IBR correction is feasible even at 1.70 NA.

refractive indices over 1.80 for achieving an NA over 1.65 with LuAG.

cleaning unit should be established to accommodate a G2 fluid.

of exposure light, the fluid absorbance has to remain sufficiently low.

Table 2 shows the target and the status of each requirement. The stress birefringence (SBR) and the homogeneity seem to have gotten closer to the targets. However, these were achieved with a small crystal of yttrium aluminium garnet (YAG). Therefore, it is necessary to confirm the SBR and the homogeneity of large-sized LuAG.

The remaining challenges, such as the absorbance, will be solved if the development is accelerated by strong supports from the industry.


Table 2. Target and status of LuAG.

#### **2.2 Intrinsic birefringence correction**

Since the absorbance of LuAG is higher than that of fused silica, it is difficult to use many LuAG lenses in a projection optic. From the viewpoint of optical design, applying LuAG to the final lens is most effective. In the final lens through which light converging at an image plane passes, the range of incident angles to the optical axis is wide. Assuming that the bottom surface of the final lens, which touches an immersion fluid, is flat, the maximum ray angles in LuAG are 46.4 and 52.6° for the NAs of 1.55 and 1.70, respectively.

The IBR distribution depends on the direction of a crystal. The distribution of a (111) oriented crystal has three-fold rotational symmetry. When the (111)-oriented crystal is used, the angle at the maximum IBR is 35.3° from the (111) axis. On the other hand, a (100) oriented crystal has four-fold rotational symmetry, and the angle at the maximum IBR is 45° from the (100) axis. In short, the maximum ray angle exceeds both angles of the maximum IBR even in the case of NA 1.55.

Fig. 3. Difference between radial and tangential polarized wave front for a projection optic of 1.55 NA; (a) without IBR correction, (b) with an original correction method.

Table 2 shows the target and the status of each requirement. The stress birefringence (SBR) and the homogeneity seem to have gotten closer to the targets. However, these were achieved with a small crystal of yttrium aluminium garnet (YAG). Therefore, it is necessary

The remaining challenges, such as the absorbance, will be solved if the development is

(/cm, base 10) < 0.005 0.035

(RMS, nm/cm) < 0.5 0.73

(-Z36, ppm) < 0.05 0.03

Since the absorbance of LuAG is higher than that of fused silica, it is difficult to use many LuAG lenses in a projection optic. From the viewpoint of optical design, applying LuAG to the final lens is most effective. In the final lens through which light converging at an image plane passes, the range of incident angles to the optical axis is wide. Assuming that the bottom surface of the final lens, which touches an immersion fluid, is flat, the maximum ray

The IBR distribution depends on the direction of a crystal. The distribution of a (111) oriented crystal has three-fold rotational symmetry. When the (111)-oriented crystal is used, the angle at the maximum IBR is 35.3° from the (111) axis. On the other hand, a (100) oriented crystal has four-fold rotational symmetry, and the angle at the maximum IBR is 45° from the (100) axis. In short, the maximum ray angle exceeds both angles of the maximum

> (a) (b) -0.5

of 1.55 NA; (a) without IBR correction, (b) with an original correction method.

Fig. 3. Difference between radial and tangential polarized wave front for a projection optic


(λ) (λ)




x10-3


0

0.00














0.0 0.0

angles in LuAG are 46.4 and 52.6° for the NAs of 1.55 and 1.70, respectively.

Target Status

(40 mm, YAG)

(45 mm, YAG)

to confirm the SBR and the homogeneity of large-sized LuAG.

Absorbance

Stress birefringence

Index homogeneity

Table 2. Target and status of LuAG.

IBR even in the case of NA 1.55.

**2.2 Intrinsic birefringence correction** 

accelerated by strong supports from the industry.

Figure 3 shows wave front aberration maps for a projection optic of 1.55 NA. These show the difference between the radial and the tangential polarized wave fronts. The wave front difference without the correction has a large rms value of 217 mλ and a clear 4θ component. On the other hand, the original correction method reduces the rms difference to 4.2 mλ. This value is within the practical level.

In the case of 1.70 NA, the wave front without the correction has a large rms difference of 680 mλ and the rms difference after the correction is 21.2 mλ. Although this value is still large compared with the case of 1.55 NA, it is close to the practical level. Therefore, it can be concluded that the IBR correction is feasible even at 1.70 NA.
