**High-Index Immersion Lithography**

Keita Sakai *Canon Inc. Japan* 

#### **1. Introduction**

396 Recent Advances in Nanofabrication Techniques and Applications

Wagner, C. & Harned, N. (2010). *EUV lithography: Lithography gets extreme*, Nature Photonics

Yoshioka, M.; Teramoto, Y.; Zink, P.; Schriever, G.; Niimi, G.; & Corthout, M. (2010). in Proceedings Vol. 7636, Extreme Ultraviolet (EUV) Lithography, 2010, 763610-1

4, 24 – 26 ISSN 1749-4885

The resolution capability of photolithography is given by Rayleigh's equation.

$$\mathbf{R} \mathbf{=} \mathbf{k}\_{\mathbf{l}'} \lambda / \mathbf{N} \mathbf{A} \tag{1}$$

where R is the half-pitch resolution of the image, k1 is a constant that depends on the resist process and exposure method, λ is the exposure wavelength, and NA is the numerical aperture of the projection optic. According to Rayleigh's equation, there are three ways to enhance the resolution. The first is to shorten the exposure wavelength such as extreme ultraviolet lithography (EUVL). The second is to improve the k1 value, for example, using the double-patterning technique. The third is to increase the numerical aperture (NA) as ArF immersion lithography. It has already realized the NA up to 1.35 and moreover can increase the NA using high-index materials. In this chapter, high-index immersion lithography with the NA over 1.45 is focused.

The NA is actually determined by the acceptance angle of the lens and the refractive index of the medium surrounding the lens and is given by eq. (2).

$$\mathbf{NA} \equiv \mathbf{n} \cdot \sin \theta \,, \tag{2}$$

where n is the refractive index of the medium surrounding the lens and θ is the acceptance angle of the lens. Therefore, the NA can be enlarged by replacing the air (n=1) with a fluid (n>1) as a medium. In this immersion lithography, the film stack consists of a lens, a fluid layer, and a resist layer. The value of n·sinθ is invariant through the film stack because it obeys Snell's Law. Since sinθ is smaller than 1, the maximum NA (=n·sinθ) is limited by the layer with the minimum refractive index. For example, the refractive index of water is 1.44 at 193.4 nm, thus the NA over 1.44 cannot be established as shown in Fig. 1 (a) because of the total reflection. To realize the NA over 1.44, the water must be replaced with a fluid which has a higher refractive index than water (Fig. 1 (b)). In Fig. 1 (b), fused silica has the smallest refractive index in the film stack and it limits the maximum NA. For further increasing the NA, a high-index lens material must be used as a lens material.

As described above, high-index lens materials and high-index immersion fluids are indispensable to realize high-index immersion lithography. One of the candidates of a highindex lens material is lutetium aluminium garnet (LuAG), which has a refractive index of 2.14. Second-generation (G2) and third-generation (G3) fluids are saturated hydrocarbon fluids whose refractive indices are approximately 1.64 and 1.80, respectively.

High-Index Immersion Lithography 399

confinement is also a challenge for a G2 fluid system because residual fluid is easy to remain on a wafer. Some issues arising with residual fluid, such as fluid darkening due to reentry of oxygen-rich residual fluid, were solved. By accepting residual fluid on a wafer, the scanning

Finally, the remaining challenges to realize high-index immersion lithography are

Key parameters for high-index lens materials are refractive index, absorbance, and intrinsic birefringence (IBR). High-index lens materials must have a refractive index to permit NA scaling sufficient to justify the development cost. The absorbance must be sufficiently low to avoid the image degradation by thermal aberrations. The intrinsic birefringence must be minimal to allow a correction to avoid introducing unacceptable aberrations in the final

The National Institute of Standards and Technology (NIST) has searched for high-index lens materials that meet the above requirements such as barium lithium fluoride (BaLiF3) and LuAG (Burnett et al., 2006). BaLiF3 developed by Tokuyama is available in various sizes with low absorbance (Nawata et al., 2007). However, the refractive index of BaLiF3 is not high enough to achieve a sufficient NA for the enhancement of the resolution. Only LuAG remains as a candidate for a high-index lens material. LuAG has the intrinsic birefringence over 30 nm/cm and still has a high absorbance caused by impurities. Thus, the status of LuAG should be paid attention to and an IBR correction method should be developed.

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.,

Year

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).

2005 2006 2007 2008 2009

Target

2010), the absorbance will be less than 0.005 /cm by further reduction of impurities.

speed and the throughput can be raised.

**2. Projection optics with LuAG** 

discussed.

aerial image.

**2.1 Status of LuAG** 

Absorbance (/cm, base 10)

0.001

0.01

0.1

1

10

Fig. 1. Light propagation through the film stack at 1.45 NA. (a) The light does not reach the resist layer because of the total reflection at the lens-water boundary. (b) A high-index fluid enables the light to propagate into the resist.

High-index immersion lithography can be classified into three types by combining a lens material and an immersion fluid: fused silica and a G2 fluid (type 1), LuAG and a G2 fluid (type 2), and LuAG and a G3 fluid (type 3). With these types, the maximum NAs are estimated 1.45, 1.55, and 1.70, respectively.

Table 1 shows k1 values for typical half-pitch and NA. The k1 is calculated using Rayleigh's equation and needs to be at least 0.25 to resolve the patterns of the half-pitch for the theoretical limit. The resolution of 36 nm is achieved with 1.45 NA optic, which can be realized using only a G2 fluid. Those of 1.55 NA and 1.65 NA can achieve the resolutions of 34 and 32 nm, respectively.


Table 1. k1 values for typical half-pitch and NA.

Although an exposure tool of 1.45 NA does not need new materials except for a G2 fluid, customers have little interest in the tool because of its modest gain in resolution. On the other hand, a tool of over 1.65 NA seems attractive for resolution enhancement. However, it would be difficult to realize G3 fluids immediately because no materials meet the requirements. In such a situation, it is a realistic way to develop a tool of 1.55 NA using LuAG and a G2 fluid.

In the next subchapter, projection optics with LuAG are explained. Development of LuAG is a hard work because the specifications of lithography-grade lens materials are extremely stringent. According to the history of LuAG development by Schott Lithotec (Parthier et al., 2008), great progress was achieved in the absorbance but it does not reach the target. The key issue for the optical design with LuAG is a correction of intrinsic birefringence (IBR). An effective method has been developed for IBR correction and it reduces the wave-front aberration to a practical level.

In the third subchapter, an immersion system using a G2 fluid is described. It was demonstrated that fluid absorbance can be kept low enough through an in-line purification unit and an oxygen removal unit. Although lens contamination is an important issue in a G2 fluid system, it was found that contamination can be suppressed by addition of a small amount of water into a G2 fluid. With this water-addition and in-line purification, the necessity for lens cleaning decreases from three times per day to once a week. Fluid

High-index fluid (n2=1.64)

Fig. 1. Light propagation through the film stack at 1.45 NA. (a) The light does not reach the resist layer because of the total reflection at the lens-water boundary. (b) A high-index fluid

High-index immersion lithography can be classified into three types by combining a lens material and an immersion fluid: fused silica and a G2 fluid (type 1), LuAG and a G2 fluid (type 2), and LuAG and a G3 fluid (type 3). With these types, the maximum NAs are

Table 1 shows k1 values for typical half-pitch and NA. The k1 is calculated using Rayleigh's equation and needs to be at least 0.25 to resolve the patterns of the half-pitch for the theoretical limit. The resolution of 36 nm is achieved with 1.45 NA optic, which can be realized using only a G2 fluid. Those of 1.55 NA and 1.65 NA can achieve the resolutions of

Half-pitch 1.35 NA 1.45 NA 1.55 NA 1.65 NA 1.70 NA 39 nm 0.272 0.292 0.313 0.333 0.343 36 nm 0.251 0.270 0.289 0.307 0.316 34 nm 0.237 0.255 0.272 0.290 0.299 32 nm 0.223 0.240 0.257 0.273 0.281

Although an exposure tool of 1.45 NA does not need new materials except for a G2 fluid, customers have little interest in the tool because of its modest gain in resolution. On the other hand, a tool of over 1.65 NA seems attractive for resolution enhancement. However, it would be difficult to realize G3 fluids immediately because no materials meet the requirements. In such a situation, it is a realistic way to develop a tool of 1.55 NA using

In the next subchapter, projection optics with LuAG are explained. Development of LuAG is a hard work because the specifications of lithography-grade lens materials are extremely stringent. According to the history of LuAG development by Schott Lithotec (Parthier et al., 2008), great progress was achieved in the absorbance but it does not reach the target. The key issue for the optical design with LuAG is a correction of intrinsic birefringence (IBR). An effective method has been developed for IBR correction and it reduces the wave-front

In the third subchapter, an immersion system using a G2 fluid is described. It was demonstrated that fluid absorbance can be kept low enough through an in-line purification unit and an oxygen removal unit. Although lens contamination is an important issue in a G2 fluid system, it was found that contamination can be suppressed by addition of a small amount of water into a G2 fluid. With this water-addition and in-line purification, the necessity for lens cleaning decreases from three times per day to once a week. Fluid

θ2

θ2

θ3

NA = 1.45 = n1·sinθ<sup>1</sup> = n2·sinθ<sup>2</sup> = n3·sinθ<sup>3</sup>

θ<sup>1</sup> θ<sup>1</sup>

SiO2 (n1=1.56)

Resist (n3=1.70) (a) (b)

SiO2 (n1=1.56) Water (n2=1.44) Resist (n3=1.70)

enables the light to propagate into the resist.

estimated 1.45, 1.55, and 1.70, respectively.

Table 1. k1 values for typical half-pitch and NA.

34 and 32 nm, respectively.

LuAG and a G2 fluid.

aberration to a practical level.

confinement is also a challenge for a G2 fluid system because residual fluid is easy to remain on a wafer. Some issues arising with residual fluid, such as fluid darkening due to reentry of oxygen-rich residual fluid, were solved. By accepting residual fluid on a wafer, the scanning speed and the throughput can be raised.

Finally, the remaining challenges to realize high-index immersion lithography are discussed.
