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

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

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 preferentially developed (Sakai et al., 2008).

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 cleaning unit should be established to accommodate a G2 fluid.

#### **3.1 Fluid degradation**

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 not a concern as an immersion fluid.

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

#### **3.1.1 Dissolved oxygen**

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

High-Index Immersion Lithography 403

Figure 6 shows dissolved oxygen using two types of oxygen removal functions. In the case with the nitrogen bubbling unit alone, the oxygen concentration was 4.5 ppm. It corresponds to the absorption of 1.83 %/mm, which can not be permitted. By using the nitrogen bubbling and the nitrogen injection units simultaneously, the oxygen concentration was reduced to 0.2 ppm. Further reduction was achieved increasing the nitrogen flow rate into the injection unit from 5 to 10 L/min. The oxygen concentration of 0.1 ppm is the permissible level because it corresponds to only 0.04 %/mm degradation. The nitrogen injection unit removes the dissolved oxygen from the oxygen-rich fluid just after recovery and prevents the oxygen-rich fluid from going back to the container. That is why the

> 0 20 40 60 80 100 Time (min)

Fig. 6. Experimental results of dissolved oxygen using oxygen removal functions. At first, the dissolved oxygen was not sufficiently low with the nitrogen bubbling unit alone. Then, the large reduction of dissolved oxygen was achieved with the simultaneous use of the

The absorbance of a G2 fluid increases with the photodecomposed materials of a G2 fluid induced by ArF laser irradiation. To achieve the target cost of \$1/layer, it is necessary to

Figure 7 shows the instruments used for a laser irradiation test. ArF excimer laser G41A3 (Gigaphoton), which can irradiate with the rep. rate of up to 3 kHz, is used for irradiation. The irradiation chamber has two beam lines and is purged with nitrogen gas to keep the oxygen concentration less than 1 ppm. The fluid circulation system with the oxygen removal functions can maintain dissolved oxygen at 0.1 ppm and below. The fluid absorbance monitor contains

The experimental conditions are shown in Table 3. Fluid suppliers have developed in-line purification units for their G2 fluid. Therefore, fluid degradation was evaluated with the combination of a fluid and a purification unit made by the same supplier, such as HIF-A and

The experimental results are shown as the induced absorbance against fluid dose. The fluid dose is defined as an incident dose modified by the volume dilution factor as shown in eq.

Dfluid=I·N·Virr/Vtotal , (3)

remove the photodecomposed materials using an in-line purification unit.

four flow-through cells which can evaluate a number of fluids simultaneously.

(3). It has been suggested as the appropriate dose metrics (Liberman et al., 2007).

Bubbling (5L/min) 4.5ppm (T=1.83%/mm) Bubbling + Injection (5L/min) 0.2ppm (T=0.08%/mm)

> Bubbling + Injection (10L/min) 0.1ppm (T=0.04%/mm)

injection unit is efficient for the reduction of dissolved oxygen.

0

nitrogen bubbling and the nitrogen injection units.

1

2

Dissolved oxygen (ppm)

**3.1.2 Laser irradiation** 

Unit-A.

3

4

5

first reason for this large degradation is the high oxygen solubility. It has been reported that the solubility of oxygen in G2 fluids are 55~70 ppm in air atmosphere (Furukawa et al., 2006). These values are 7~9 times higher than that in water. According to the research at Columbia University (Gejo et al., 2007), dissolved oxygen forms a charge-transfer complex with cycloalkane of a G2 fluid and the complex has strong absorption in the ultraviolet wavelength range. This is the second reason for the large degradation induced by dissolved oxygen.

The induced absorbance against dissolved oxygen in a G2 fluid has been experimentally obtained. Figure 4 shows the induced absorbance of HIL-203 (JSR) compared with water. It was confirmed that the oxygen-rich HIL-203 exhibits strong absorption. Therefore, a removal function of oxygen is necessary to keep the fluid absorbance low in a circulation system.

Fig. 4. Experimental results of induced absorbance with dissolved oxygen. Open circles show the induced absorbance of HIL-203 and filled circles show that of water.

An immersion nozzle supplies a G2 fluid under a final lens and sucks the fluid with the neighbouring gas. If the atmosphere around a wafer is air, the oxygen concentration in the recovered fluid increases with the sucked air. Under such a condition, two types of oxygen removal functions were evaluated using an experimental system as shown in Fig. 5. The fluid used in this experiment was HIL-203 (JSR) and the fluid flow rate was 400 mL/min.

Fig. 5. Schematic view of an experimental system to investigate two types of oxygen removal functions. One is a nitrogen bubbling unit in the fluid container, and the other is a nitrogen injection unit attached to the fluid line. Oxygen concentration in the fluid is measured at the position just before the immersion nozzle.

first reason for this large degradation is the high oxygen solubility. It has been reported that the solubility of oxygen in G2 fluids are 55~70 ppm in air atmosphere (Furukawa et al., 2006). These values are 7~9 times higher than that in water. According to the research at Columbia University (Gejo et al., 2007), dissolved oxygen forms a charge-transfer complex with cycloalkane of a G2 fluid and the complex has strong absorption in the ultraviolet wavelength range. This is the second reason for the large degradation induced by dissolved oxygen. The induced absorbance against dissolved oxygen in a G2 fluid has been experimentally obtained. Figure 4 shows the induced absorbance of HIL-203 (JSR) compared with water. It was confirmed that the oxygen-rich HIL-203 exhibits strong absorption. Therefore, a removal function of oxygen is necessary to keep the fluid absorbance low in a circulation system.

> 0 10 20 30 40 50 60 Dissolved oxygen (ppm)

> > Wafer

Nozzle

Nitrogen injection unit

Water

show the induced absorbance of HIL-203 and filled circles show that of water.

Container

G2 fluid

Pump

G2 fluid

Air Air

Fig. 5. Schematic view of an experimental system to investigate two types of oxygen removal functions. One is a nitrogen bubbling unit in the fluid container, and the other is a nitrogen injection unit attached to the fluid line. Oxygen concentration in the fluid is

Fig. 4. Experimental results of induced absorbance with dissolved oxygen. Open circles

An immersion nozzle supplies a G2 fluid under a final lens and sucks the fluid with the neighbouring gas. If the atmosphere around a wafer is air, the oxygen concentration in the recovered fluid increases with the sucked air. Under such a condition, two types of oxygen removal functions were evaluated using an experimental system as shown in Fig. 5. The fluid used in this experiment was HIL-203 (JSR) and the fluid flow rate was 400 mL/min.

HIL-203 (JSR)

0

O2

measured at the position just before the immersion nozzle.

Oxygen monitor

Nitrogen bubbling unit

0.2 0.4 0.6 0.8

Induced absorbance

(/cm, @193.4nm)

1

Figure 6 shows dissolved oxygen using two types of oxygen removal functions. In the case with the nitrogen bubbling unit alone, the oxygen concentration was 4.5 ppm. It corresponds to the absorption of 1.83 %/mm, which can not be permitted. By using the nitrogen bubbling and the nitrogen injection units simultaneously, the oxygen concentration was reduced to 0.2 ppm. Further reduction was achieved increasing the nitrogen flow rate into the injection unit from 5 to 10 L/min. The oxygen concentration of 0.1 ppm is the permissible level because it corresponds to only 0.04 %/mm degradation. The nitrogen injection unit removes the dissolved oxygen from the oxygen-rich fluid just after recovery and prevents the oxygen-rich fluid from going back to the container. That is why the injection unit is efficient for the reduction of dissolved oxygen.

Fig. 6. Experimental results of dissolved oxygen using oxygen removal functions. At first, the dissolved oxygen was not sufficiently low with the nitrogen bubbling unit alone. Then, the large reduction of dissolved oxygen was achieved with the simultaneous use of the nitrogen bubbling and the nitrogen injection units.

#### **3.1.2 Laser irradiation**

The absorbance of a G2 fluid increases with the photodecomposed materials of a G2 fluid induced by ArF laser irradiation. To achieve the target cost of \$1/layer, it is necessary to remove the photodecomposed materials using an in-line purification unit.

Figure 7 shows the instruments used for a laser irradiation test. ArF excimer laser G41A3 (Gigaphoton), which can irradiate with the rep. rate of up to 3 kHz, is used for irradiation. The irradiation chamber has two beam lines and is purged with nitrogen gas to keep the oxygen concentration less than 1 ppm. The fluid circulation system with the oxygen removal functions can maintain dissolved oxygen at 0.1 ppm and below. The fluid absorbance monitor contains four flow-through cells which can evaluate a number of fluids simultaneously.

The experimental conditions are shown in Table 3. Fluid suppliers have developed in-line purification units for their G2 fluid. Therefore, fluid degradation was evaluated with the combination of a fluid and a purification unit made by the same supplier, such as HIF-A and Unit-A.

The experimental results are shown as the induced absorbance against fluid dose. The fluid dose is defined as an incident dose modified by the volume dilution factor as shown in eq. (3). It has been suggested as the appropriate dose metrics (Liberman et al., 2007).

$$\mathbf{D}\_{\rm fluid} \equiv \mathbf{I} \cdot \mathbf{N} \cdot \mathbf{V}\_{\rm irr} / \mathbf{V}\_{\rm total} \tag{3}$$

High-Index Immersion Lithography 405

Figure 8 shows the induced absorbance of each G2 fluid with and without an in-line purification unit. For example, the degradation rate of HIF-C with Unit-C is one-seventhfold smaller than that without Unit-C. Using a purification unit, fluid lifetime will be up to 1 week before the fluid absorbance will reach an unacceptable level. Therefore, it can be

It has been reported that the exposed surface of a final lens is polluted by photodecomposed materials of a G2 fluid (French et al., 2007). This lens contamination diminishes the uniformity of exposure dose on a wafer and would be a source of particles. Thus, the suppression of lens contamination is a serious issue for an immersion system using a G2

The suppression of lens contamination was examined by three procedures. The first is to select an appropriate fluid, which has a lower deposition rate. The second is to use an in-line purification, which can remove photodecomposed materials. The third is water-addition

Figure 9 shows a schematic diagram of the experimental setup. The cell absorbance, which consists of the absorbance of windows and a fluid of 1 mm path length in the cell, was measured using two power meters. In addition, the fluid absorbance was measured by a fluid absorbance monitor. Then, the absorbance of windows was calculated by subtracting

 Awindow=Acell—Afluid , (4) where Awindow is the absorbance of windows, Acell is the cell absorbance, and Afluid is the

The laser fluence was 1.0 mJ/(cm2·pulse), which is almost the same as in an actual exposure tool. The dose estimation on an actual tool is approximately 60 kJ/(cm2·day). Although the effective cleaning method was proposed (Liberman et al., 2007), the necessity for lens

(Water addition unit)

A Fluid absorbance monitor

Fig. 9. Schematic diagram of the experimental setup. G2 fluid flows through the 1 mm gap space between the SiO2 windows of the irradiation cell. The particle counter can detect

Container

Pump

(In-line purification unit)

G2 fluid

concluded that recycling of a G2 fluid is feasible using an in-line purification unit.

into a G2 fluid, which is an original suppression method.

fluid absorbance through 1 mm path length.

Cell with SiO2 windows

particles larger than 60 nm in diameter.

P

ArF laser

Particle counter

the fluid absorbance from the cell absorbance as shown in eq. (4).

cleaning should be less than once a week for minimizing tool downtime.

Power meter

**3.2 Lens contamination** 

fluid.

where Dfluid is the fluid dose, I is the fluence per pulse, N is the total pulse count, and Virr and Vtotal are the irradiated volume and the total fluid reservoir volume, respectively.

Fig. 7. Instruments for a laser irradiation test. The fluid circulation system consists of a fluid container, a pump, and oxygen removal units.


Table 3. Experimental conditions for evaluation of in-line purification units.

Fig. 8. Induced absorbance of a G2 fluid against the fluid dose. The dashed line shows the induced absorbance without a purification unit. The solid line shows the result with a purification unit. The fluid dose is calculated using eq. (3) and the conditions as shown in Table 3.

where Dfluid is the fluid dose, I is the fluence per pulse, N is the total pulse count, and Virr and Vtotal are the irradiated volume and the total fluid reservoir volume, respectively.

with in-situ transmittance measurement

Fig. 7. Instruments for a laser irradiation test. The fluid circulation system consists of a fluid

Irradiation chamber

container, a pump, and oxygen removal units.

0 0.02 0.04 0.06 0.08 0.1 0.12

Induced absorbance (/cm)

Table 3.

Fluid absorbance monitor

Immersion fluid / Purification unit

Table 3. Experimental conditions for evaluation of in-line purification units.

ArF excimer laser

Fluid circulation system

Fluence 0.5 mJ/(cm2 pulse) Rep. rate 2000 Hz Irradiation area 8.0 mm Fluid gap 1.0 mm Fluid flow rate 100 ~ 350 mL/min

0 0.2 0.4 0.6 0.8 1 Fluid dose (J/cm2)

Fig. 8. Induced absorbance of a G2 fluid against the fluid dose. The dashed line shows the induced absorbance without a purification unit. The solid line shows the result with a purification unit. The fluid dose is calculated using eq. (3) and the conditions as shown in

w/o purification HIF-A HIF-B HIF-C

HIF-C with Unit-C

HIF-A with Unit-A

HIF-B with Unit-B

with oxygen removal unit

HIF-A / Unit-A HIF-B / Unit-B HIF-C / Unit-C

Figure 8 shows the induced absorbance of each G2 fluid with and without an in-line purification unit. For example, the degradation rate of HIF-C with Unit-C is one-seventhfold smaller than that without Unit-C. Using a purification unit, fluid lifetime will be up to 1 week before the fluid absorbance will reach an unacceptable level. Therefore, it can be concluded that recycling of a G2 fluid is feasible using an in-line purification unit.

#### **3.2 Lens contamination**

It has been reported that the exposed surface of a final lens is polluted by photodecomposed materials of a G2 fluid (French et al., 2007). This lens contamination diminishes the uniformity of exposure dose on a wafer and would be a source of particles. Thus, the suppression of lens contamination is a serious issue for an immersion system using a G2 fluid.

The suppression of lens contamination was examined by three procedures. The first is to select an appropriate fluid, which has a lower deposition rate. The second is to use an in-line purification, which can remove photodecomposed materials. The third is water-addition into a G2 fluid, which is an original suppression method.

Figure 9 shows a schematic diagram of the experimental setup. The cell absorbance, which consists of the absorbance of windows and a fluid of 1 mm path length in the cell, was measured using two power meters. In addition, the fluid absorbance was measured by a fluid absorbance monitor. Then, the absorbance of windows was calculated by subtracting the fluid absorbance from the cell absorbance as shown in eq. (4).

$$\mathbf{A}\_{\text{winddown}} = \mathbf{A}\_{\text{cell}} - \mathbf{A}\_{\text{fluid}} \,\,\, \,\tag{4}$$

where Awindow is the absorbance of windows, Acell is the cell absorbance, and Afluid is the fluid absorbance through 1 mm path length.

The laser fluence was 1.0 mJ/(cm2·pulse), which is almost the same as in an actual exposure tool. The dose estimation on an actual tool is approximately 60 kJ/(cm2·day). Although the effective cleaning method was proposed (Liberman et al., 2007), the necessity for lens cleaning should be less than once a week for minimizing tool downtime.

Fig. 9. Schematic diagram of the experimental setup. G2 fluid flows through the 1 mm gap space between the SiO2 windows of the irradiation cell. The particle counter can detect particles larger than 60 nm in diameter.

High-Index Immersion Lithography 407

It was found that lens contamination can be suppressed by addition of a small amount of water into a G2 fluid (Sakai et al., 2008). Since water in a G2 fluid is removed by an oxygen removal unit or an in-line purification unit, a water-addition unit is necessary for a fluid circulation system to keep water in a G2 fluid. Figure 12 shows a schematic diagram of a water-addition unit. Using the water-addition unit, a sufficient amount of water can be

Degasser

Fig. 12. Schematic diagram of a water-addition unit. The unit consists of a membrane contactor and a circulation unit of degassed water. Degassed water contacts a G2 fluid

Figure 13, 14, and 15 shows the experimental results using HIL-203 (JSR) as a G2 fluid. In the case without water-addition, the absorbance of windows increased gradually (Fig. 13) and many streak contaminations were observed on the window (Fig. 14 (a)). On the other hand, the window was not contaminated in the case with water-addition (Fig. 13, Fig. 14 (b)). A strong impact was also obtained in terms of particle. As shown in Fig. 15, particles increased with lens contamination in the case without water-addition. Some portions of lens contamination would be removed by laser irradiation and flow into the fluid as particles. Using the water-addition unit, there were no particles in the fluid made from lens contamination. It was also confirmed

G2 fluid out

Oxygen monitor

through the porous membrane with a large area.

that the water-addition unit does not generate any particles.

Induced absorbance

of windows

(2 surfaces)

0 0.05 0.1 0.15 0.2 0.25 0.3

contrast, the water-addition unit suppressed contamination (solid line).

O2

Pump

G2 fluid in

Water

Container

Membrane Contactor

Water in Water out

0 20 40 60 80 100 Dose (kJ/cm2)

Fig. 13. Induced absorbance of windows against exposure dose. The absorbance of windows increased with the exposure dose in the case without water-addition (dashed line). In

with water-addition

w/o water-addition

**3.2.3 Water-addition** 

solved into a G2 fluid.

#### **3.2.1 Appropriate fluid**

Induced absorbance of windows, which correspond to a final lens of a projection optic, was evaluated by using various G2 fluids. Figure 10 shows induced absorbance of windows against exposure dose. HIF-C exhibits a higher deposition rate than HIF-A and D. Therefore, HIF-A or D should be selected from the point of view of lens contamination. However, lens contamination occurs in a few hours on an actual exposure tool, even if using HIF-A or D. It is necessary to introduce some methods for suppressing contamination.

Fig. 10. Induced absorbance of windows using various G2 fluids. The deposition rates of HIF-A (solid line) and HIF-D (dashed line) are lower than that of HIF-C (dashed-dotted line).

#### **3.2.2 In-line purification**

It is expected that an in-line purification unit can suppress lens contamination because it can reduce photodecomposed materials. The experiments with several G2 fluids and in-line purification units were carried out using the experimental setup as shown in Fig. 9. Figure 11 shows experimental results of window absorbance with and without a purification unit. It was confirmed that each purification unit can improve lens contamination. But the performance of purification units are not enough and a final lens needs to be cleaned more than once a day under the practical use conditions. Therefore, the additional method is desired to extend the cleaning interval.

Fig. 11. Induced absorbance of windows with and without a purification unit. The deposition rate with a purification unit (solid line) is lower than that without a purification unit (dashed line).

#### **3.2.3 Water-addition**

406 Recent Advances in Nanofabrication Techniques and Applications

Induced absorbance of windows, which correspond to a final lens of a projection optic, was evaluated by using various G2 fluids. Figure 10 shows induced absorbance of windows against exposure dose. HIF-C exhibits a higher deposition rate than HIF-A and D. Therefore, HIF-A or D should be selected from the point of view of lens contamination. However, lens contamination occurs in a few hours on an actual exposure tool, even if using HIF-A or D. It

0 20 40 60 80 100

HIF-D

Dose (kJ/cm2)

Fig. 10. Induced absorbance of windows using various G2 fluids. The deposition rates of HIF-A (solid line) and HIF-D (dashed line) are lower than that of HIF-C (dashed-dotted

It is expected that an in-line purification unit can suppress lens contamination because it can reduce photodecomposed materials. The experiments with several G2 fluids and in-line

Figure 11 shows experimental results of window absorbance with and without a purification unit. It was confirmed that each purification unit can improve lens contamination. But the performance of purification units are not enough and a final lens needs to be cleaned more than once a day under the practical use conditions. Therefore, the additional method is

0 20 40 60 80 100

Dose (kJ/cm2)

deposition rate with a purification unit (solid line) is lower than that without a purification

purification units were carried out using the experimental setup as shown in Fig. 9.

HIF-A w/o purification

HIF-D w/o purification

Fig. 11. Induced absorbance of windows with and without a purification unit. The

HIF-A

HIF-A

HIF-D

with purification

with purification

is necessary to introduce some methods for suppressing contamination.

HIF-C

0 0.02 0.04 0.06 0.08 0.1 0.12

Induced absorbance

of windows

(2 surfaces)

**3.2.1 Appropriate fluid** 

line).

**3.2.2 In-line purification** 

desired to extend the cleaning interval.

Induced absorbance

unit (dashed line).

of windows

(2 surfaces)

0 0.02 0.04 0.06 0.08 0.1 0.12 It was found that lens contamination can be suppressed by addition of a small amount of water into a G2 fluid (Sakai et al., 2008). Since water in a G2 fluid is removed by an oxygen removal unit or an in-line purification unit, a water-addition unit is necessary for a fluid circulation system to keep water in a G2 fluid. Figure 12 shows a schematic diagram of a water-addition unit. Using the water-addition unit, a sufficient amount of water can be solved into a G2 fluid.

Fig. 12. Schematic diagram of a water-addition unit. The unit consists of a membrane contactor and a circulation unit of degassed water. Degassed water contacts a G2 fluid through the porous membrane with a large area.

Figure 13, 14, and 15 shows the experimental results using HIL-203 (JSR) as a G2 fluid. In the case without water-addition, the absorbance of windows increased gradually (Fig. 13) and many streak contaminations were observed on the window (Fig. 14 (a)). On the other hand, the window was not contaminated in the case with water-addition (Fig. 13, Fig. 14 (b)). A strong impact was also obtained in terms of particle. As shown in Fig. 15, particles increased with lens contamination in the case without water-addition. Some portions of lens contamination would be removed by laser irradiation and flow into the fluid as particles. Using the water-addition unit, there were no particles in the fluid made from lens contamination. It was also confirmed that the water-addition unit does not generate any particles.

Fig. 13. Induced absorbance of windows against exposure dose. The absorbance of windows increased with the exposure dose in the case without water-addition (dashed line). In contrast, the water-addition unit suppressed contamination (solid line).

High-Index Immersion Lithography 409

that the simultaneous use of ARP and a water-addition unit has a sufficient performance. It

0 100 200 300 400 500

Dose (kJ/cm2)

Fig. 16. Induced absorbance of windows against exposure dose. Simultaneous use of ARP

(a) (b) Fig. 17. Micrographs of window surfaces; (a) with ARP after 130 kJ/cm2 exposure, (b) with

A bubble is one of the origins of immersion defect. A bubble is entrapped on an air-liquid boundary and is translated with a wafer motion. If the bubble lifetime is sufficiently short, the bubble will be eliminated before reaching an exposure area. Therefore, the reduction of

The gas in a bubble diffuses from the surface of the bubble into the fluid. This diffusion causes the bubble to shrink with time and to eventually vanish. The bubble lifetime is

> *S d DC C*

where τ is the bubble lifetime, ρ is the density of the gas inside the bubble, d is the diameter of the bubble, D is the diffusion coefficient, CS is the saturated concentration, and C∞ is the concentration of the dissolved gas at a position far from the bubble. Figure 18 shows a schematic diagram of the experimental setup for the bubble lifetime measurement. By

2

, 8 ( ― ) (5)

and a water-addition unit suppressed lens contamination until 420 kJ/cm2.

1mm 1mm

with ARP

w/o any suppression units

with ARP + Water-addition

is concluded that lens contamination is not a critical issue anymore.

0

ARP and a water-addition unit after 450 kJ/cm2 exposure.

approximated using eq. (5) (Honda et al., 2004).

**3.3 Bubble** 

bubble lifetime is important.

0.05

0.1

Induced absorbance

of windows

(2 surfaces)

0.15

0.2

As described above, water-addition is the superior method for suppressing lens contamination. In addition, it seems to be a method without any disadvantages. For example, the refractive index of a G2 fluid does not change substantially with wateraddition because the solubility of water into a G2 fluid is very low (several tens of ppm).

Fig. 14. Micrographs of window surfaces; (a) without water-addition after 100 kJ/cm2 exposure, (b) with water-addition after 100 kJ/cm2 exposure.

Fig. 15. Particles in a fluid against exposure dose. The number of particles increased with the exposure dose without water-addition (dashed line). In contrast, the water-addition unit suppressed particles induced by lens contamination (solid line).

#### **3.2.4 Long-term test**

A long-term test for lens contamination was done with an in-line purification unit and a water-addition unit simultaneously. IF132 (DuPont) and ARP (DuPont) were used as a G2 fluid and an in-line purification unit, respectively.

As shown in Fig. 16, the absorbance of windows started to increase at 20 kJ/cm2 without any suppression units. The dose estimation on an actual exposure tool is approximately 60 kJ/(cm2·day). Therefore, lens cleaning must be done three times per day in this case. Even if with ARP, the absorbance started to rise at 60 kJ/cm2 and a lens needs to be cleaned once a day. The cleaning interval is not allowed in the case with ARP alone. On the other hand, lens contamination was suppressed until 420 kJ/cm2 using ARP and a water-addition unit simultaneously. The necessity for lens cleaning is once a week and it is practical. Figure 17 shows micrographs of window surfaces after the experiments. The micrographs also exhibit

As described above, water-addition is the superior method for suppressing lens contamination. In addition, it seems to be a method without any disadvantages. For example, the refractive index of a G2 fluid does not change substantially with wateraddition because the solubility of water into a G2 fluid is very low (several tens of ppm).

1mm 1mm

exposure, (b) with water-addition after 100 kJ/cm2 exposure.

suppressed particles induced by lens contamination (solid line).

fluid and an in-line purification unit, respectively.

Particle (counts/ml)

**3.2.4 Long-term test** 

(a) (b)

Particle (>60nm), 30min average

w/o water-addition

0 20 40 60 80 100 Dose (kJ/cm2)

Fig. 15. Particles in a fluid against exposure dose. The number of particles increased with the exposure dose without water-addition (dashed line). In contrast, the water-addition unit

A long-term test for lens contamination was done with an in-line purification unit and a water-addition unit simultaneously. IF132 (DuPont) and ARP (DuPont) were used as a G2

As shown in Fig. 16, the absorbance of windows started to increase at 20 kJ/cm2 without any suppression units. The dose estimation on an actual exposure tool is approximately 60 kJ/(cm2·day). Therefore, lens cleaning must be done three times per day in this case. Even if with ARP, the absorbance started to rise at 60 kJ/cm2 and a lens needs to be cleaned once a day. The cleaning interval is not allowed in the case with ARP alone. On the other hand, lens contamination was suppressed until 420 kJ/cm2 using ARP and a water-addition unit simultaneously. The necessity for lens cleaning is once a week and it is practical. Figure 17 shows micrographs of window surfaces after the experiments. The micrographs also exhibit

with water-addition

Fig. 14. Micrographs of window surfaces; (a) without water-addition after 100 kJ/cm2

that the simultaneous use of ARP and a water-addition unit has a sufficient performance. It is concluded that lens contamination is not a critical issue anymore.

Fig. 16. Induced absorbance of windows against exposure dose. Simultaneous use of ARP and a water-addition unit suppressed lens contamination until 420 kJ/cm2.

Fig. 17. Micrographs of window surfaces; (a) with ARP after 130 kJ/cm2 exposure, (b) with ARP and a water-addition unit after 450 kJ/cm2 exposure.

#### **3.3 Bubble**

A bubble is one of the origins of immersion defect. A bubble is entrapped on an air-liquid boundary and is translated with a wafer motion. If the bubble lifetime is sufficiently short, the bubble will be eliminated before reaching an exposure area. Therefore, the reduction of bubble lifetime is important.

The gas in a bubble diffuses from the surface of the bubble into the fluid. This diffusion causes the bubble to shrink with time and to eventually vanish. The bubble lifetime is approximated using eq. (5) (Honda et al., 2004).

$$
\tau = \frac{\rho d^2}{8D(\mathbb{C}\_s - \mathbb{C}\_v)} \tag{5}
$$

where τ is the bubble lifetime, ρ is the density of the gas inside the bubble, d is the diameter of the bubble, D is the diffusion coefficient, CS is the saturated concentration, and C∞ is the concentration of the dissolved gas at a position far from the bubble. Figure 18 shows a schematic diagram of the experimental setup for the bubble lifetime measurement. By

High-Index Immersion Lithography 411

The surface tension and viscosity of G2 fluids are approximately 30 mN/m and 2~4 mPa·s, respectively. The film pulling velocity (υfp), which is the minimum scanning velocity to leave fluid behind the nozzle, is proportional to the surface tension (γ) and the inverse of the viscosity (μ) as shown in eq. (6) (Shedd et al., 2006). If a wafer has the same static receding contact angle (SRCA, θs,r) for water and a G2 fluid, the film pulling velocity for a G2 fluid is approximately one-sixth-fold lower than that for water. Although the higher SRCA can increase the film pulling velocity, the target SRCA over 120° is too high to develop an appropriate topcoat material (Sanders et al., 2008). Therefore, it is difficult to achieve a sufficient scanning speed without residual fluid in an immersion system using a G2 fluid.

> 3 , , *fp C s r*

(6)

Wafer

G2 fluid Nozzle

Figure 20 shows the results of the fundamental scanning test using a prototype topcoat for G2 fluids. HIL-203 (JSR) was used as a G2 fluid for the experiment. Even with the scanning speed of 30 mm/sec, the dynamic receding contact angle became 0° and fluid was left behind the nozzle. As a result, it was confirmed that residual fluid should be allowed to realize a sufficient scanning speed. Since the contact time of a G2 fluid and a resist becomes longer in such a system, the interaction between a G2 fluid and a resist should be discussed.

10 mm/sec 20 mm/sec 30 mm/sec

Fig. 20. Side views of the fundamental scanning test. The dynamic receding contact angle on

To investigate the interaction between a G2 fluid and a resist, the defect inspection of soaked wafers is effective. Figure 21 shows the procedure of a soaking test. Between the exposure and post-exposure bake (PEB), a wafer was soaked in a G2 fluid for 60 seconds and the fluid was removed by spin drying. The defect inspection was carried out using KLA2371 (KLA-Tencor). The defects induced by post-exposure soaking were evaluated with the above

Figure 22 shows the results of the defect inspection. A large number of defects were found on the resist after soaking in HIF-E. In a microscopic review, there were many stains as shown in Fig. 22. As a result, it was confirmed that it is necessary to use an inert resist for a G2 fluid. On the other hand, it is not apparent that the wafer soaked in HIF-A or HIF-D had more defects than the reference wafer. There were some variations in the number of defects because the experiment was carried out in off-line process. Defect studies should be done in

Dynamic receding contact angle

52<sup>o</sup> 42o 0o

a wafer became 0° with the scanning speed of 30 mm/sec.

**3.4.1 Scanning test** 

where C is the empirical constant.

**3.4.2 Soaking test** 

more clean circumstances.

procedure.

increasing the pressure of the fluid from 1 to 2 atm, the bubble vanishes at a certain lifetime. It corresponds to the lifetime in the half-degassed fluid. Figure 19 shows the bubble lifetimes in water and two types of G2 fluids. The experimental results in water correlated with the theory fairly well. The bubble lifetime in the G2 fluid was shorter than that in water. Hence, the concept of the bubble elimination method for a water immersion system, i.e., using degassed fluid, is also feasible in an immersion system using a G2 fluid.

Fig. 18. Schematic diagram of the experiment setup for the bubble lifetime measurement. The size of a bubble is observed using the microscope.

Fig. 19. Bubble lifetimes in various fluids. The lifetimes in G2 fluids (triangles) are shorter than that in water (circles).

#### **3.4 Residual fluid on a wafer**

The surface tension and viscosity of water are 72 mN/m and 1 mPa·s, respectively. This extremely high surface tension and low viscosity enable the immersion nozzle to keep water under a final lens (Kubo et al, 2007). On the other hand, it has been reported that a G2 fluid is easy to remain on a wafer (Sewell et al, 2007). After the verification that residual fluid is hard to avoid, some issues arising with residual fluid are discussed. The experimental results for the issues, such as the interaction between a G2 fluid and a resist, metal contamination in a G2 fluid, and fluid darkening due to oxygen-rich residual fluid, are explained.

#### **3.4.1 Scanning test**

410 Recent Advances in Nanofabrication Techniques and Applications

increasing the pressure of the fluid from 1 to 2 atm, the bubble vanishes at a certain lifetime. It corresponds to the lifetime in the half-degassed fluid. Figure 19 shows the bubble lifetimes in water and two types of G2 fluids. The experimental results in water correlated with the theory fairly well. The bubble lifetime in the G2 fluid was shorter than that in water. Hence, the concept of the bubble elimination method for a water immersion system, i.e., using

Valve

0 50 100 150 Diameter of a bubble (μm)

Fig. 19. Bubble lifetimes in various fluids. The lifetimes in G2 fluids (triangles) are shorter

The surface tension and viscosity of water are 72 mN/m and 1 mPa·s, respectively. This extremely high surface tension and low viscosity enable the immersion nozzle to keep water under a final lens (Kubo et al, 2007). On the other hand, it has been reported that a G2 fluid is easy to remain on a wafer (Sewell et al, 2007). After the verification that residual fluid is hard to avoid, some issues arising with residual fluid are discussed. The experimental results for the issues, such as the interaction between a G2 fluid and a resist, metal contamination in a G2 fluid, and fluid darkening due to oxygen-rich residual fluid, are

Fig. 18. Schematic diagram of the experiment setup for the bubble lifetime measurement.

dots: experiment line: theory

Microscope

Fluid

HIF-E

HIF-D

Pressure gauge

degassed fluid, is also feasible in an immersion system using a G2 fluid.

Chamber Bubble

Water

The size of a bubble is observed using the microscope.

Lifetime (sec)

than that in water (circles).

explained.

**3.4 Residual fluid on a wafer** 

The surface tension and viscosity of G2 fluids are approximately 30 mN/m and 2~4 mPa·s, respectively. The film pulling velocity (υfp), which is the minimum scanning velocity to leave fluid behind the nozzle, is proportional to the surface tension (γ) and the inverse of the viscosity (μ) as shown in eq. (6) (Shedd et al., 2006). If a wafer has the same static receding contact angle (SRCA, θs,r) for water and a G2 fluid, the film pulling velocity for a G2 fluid is approximately one-sixth-fold lower than that for water. Although the higher SRCA can increase the film pulling velocity, the target SRCA over 120° is too high to develop an appropriate topcoat material (Sanders et al., 2008). Therefore, it is difficult to achieve a sufficient scanning speed without residual fluid in an immersion system using a G2 fluid.

$$
\omega\_{\circ} = \mathbb{C} \frac{\mathcal{Y}}{\mu} \theta\_{\*,r}^{\flat} \tag{6}
$$

where C is the empirical constant.

Figure 20 shows the results of the fundamental scanning test using a prototype topcoat for G2 fluids. HIL-203 (JSR) was used as a G2 fluid for the experiment. Even with the scanning speed of 30 mm/sec, the dynamic receding contact angle became 0° and fluid was left behind the nozzle. As a result, it was confirmed that residual fluid should be allowed to realize a sufficient scanning speed. Since the contact time of a G2 fluid and a resist becomes longer in such a system, the interaction between a G2 fluid and a resist should be discussed.

Fig. 20. Side views of the fundamental scanning test. The dynamic receding contact angle on a wafer became 0° with the scanning speed of 30 mm/sec.

#### **3.4.2 Soaking test**

To investigate the interaction between a G2 fluid and a resist, the defect inspection of soaked wafers is effective. Figure 21 shows the procedure of a soaking test. Between the exposure and post-exposure bake (PEB), a wafer was soaked in a G2 fluid for 60 seconds and the fluid was removed by spin drying. The defect inspection was carried out using KLA2371 (KLA-Tencor). The defects induced by post-exposure soaking were evaluated with the above procedure.

Figure 22 shows the results of the defect inspection. A large number of defects were found on the resist after soaking in HIF-E. In a microscopic review, there were many stains as shown in Fig. 22. As a result, it was confirmed that it is necessary to use an inert resist for a G2 fluid. On the other hand, it is not apparent that the wafer soaked in HIF-A or HIF-D had more defects than the reference wafer. There were some variations in the number of defects because the experiment was carried out in off-line process. Defect studies should be done in more clean circumstances.

High-Index Immersion Lithography 413

Table 4. Experimental conditions for metal contamination on a wafer dipped into a G2 fluid.

HIF-A 1 0.15 0.30 0.09 1.30 0.28 0.28 0.71 0.29 0.09 1.57 0.87 0.05 0.13 <0.03 2 0.11 0.92 0.05 0.85 0.97 0.36 0.45 0.13 0.19 0.82 0.40 0.04 0.10 <0.03

HIF-B 1 <0.03 0.45 0.05 0.23 0.13 0.38 0.48 <0.03 <0.03 0.10 <0.03 0.04 0.20 <0.03 2 <0.03 0.48 0.07 0.29 0.14 0.47 0.58 <0.03 <0.03 0.06 <0.03 0.03 0.40 <0.03

HIF-D 1 <0.03 0.36 0.12 0.57 0.10 0.21 0.51 <0.03 <0.03 0.09 <0.03 0.07 0.06 <0.03 2 <0.03 0.26 0.09 0.47 0.13 0.17 0.66 <0.03 <0.03 0.10 <0.03 0.04 0.03 <0.03

Oxygen in the atmosphere diffuses into the residual fluid on a wafer. When the oxygen-rich residual fluid goes back under the lens, it decreases the transparency of the fluid under the lens. That is the reason why dissolved oxygen is one of the issues arising with a residual fluid. Oxygen concentration in the fluid under the lens was evaluated while a wafer was moving along the sequence as shown in Fig. 23. HIL-203 (JSR) was used as a G2 fluid. A prototype immersion nozzle designed for a G2 fluid was used for the experiment. The processing time for one wafer was 30 seconds and it consisted of a scanning time and a waiting time at the edge of the stage. The fluid under the lens was sucked through the hole opened in the lens and the oxygen concentration in the sucked fluid was measured. The absorbance was

Figure 24 shows fluid absorbance induced by dissolved oxygen against the number of scanned wafers. The absorbance rises with the scanning speed but it is low enough even at

Waiting position

Shot sequence

Scan direction

Table top

Wafer

Fluid Metal concentration (×1010atoms/cm2)

Table 5. Metal concentrations on a wafer evaluated by VPD/ICP-MS.

calculated by using the relation as shown in Fig. 4.

Fig. 23. Sequence of a scanning test on an 8 inches wafer.

Immersion fluid HIF-A, HIF-B, HIF-D Volume of fluid 200 mL Dipping time 5 minutes

Wafer Bare Si (200 mm)

Method VPD/ICP-MS Evaluation area 180 mm Detection limit 0.03×1010 atoms/cm2

Li Na Mg Al K Ca Ti Cr Mn Fe Ni Cu Zn Pb

1 <0.03 0.03 <0.03 0.03 <0.03 <0.03 0.43 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 2 0.06 <0.03 <0.03 0.05 <0.03 <0.03 0.32 <0.03 <0.03 <0.03 <0.03 0.03 <0.03 <0.03

Dipping

Analysis

Reference

**3.4.4 Dissolved oxygen** 

Fig. 22. Results of defect inspection. The number of defects after soaking in HIF-A, B, D, and E are 66, 1794, 8, and 69023, respectively.

#### **3.4.3 Metal contamination**

If the fluid contains a lot of metals, a wafer would be polluted by metals in the residual fluid. Thus, the objective of the following experiment is to confirm that G2 fluids are sufficiently clean. The requirement is less than 1010 atoms/cm2 on a silicon wafer.

The conditions for the experiment are shown in Table 4. At first, a silicon wafer was dipped into a fluid for 5 minutes. Then, the wafer was pulled up and was dried by air blowing. Then, the metal concentrations on the wafer were evaluated by vapor phase decomposition inductively coupled plasma mass spectrometry (VPD/ICP-MS). The detection limit of the spectrometry is 0.03×1010 atoms/cm2. The experiment was done twice for each fluid.

Table 5 shows the experimental results. The metal concentrations of the reference mean those on a wafer without dipping into any fluids. In the case of HIF-A, the concentrations of some metals are more than 1010 atoms/cm2 (black-out cells in Table 5). On the other hand, HIF-B and D are sufficiently clean. As a result, the residual fluids of HIF-B and D are acceptable from the viewpoint of metal contamination.

Exposure Soak

Fig. 21. Experimental procedure of a post-exposure soaking test to investigate the interaction

HIF-A HIF-B HIF-D HIF-E Reference (dry)

Fig. 22. Results of defect inspection. The number of defects after soaking in HIF-A, B, D, and

If the fluid contains a lot of metals, a wafer would be polluted by metals in the residual fluid. Thus, the objective of the following experiment is to confirm that G2 fluids are

The conditions for the experiment are shown in Table 4. At first, a silicon wafer was dipped into a fluid for 5 minutes. Then, the wafer was pulled up and was dried by air blowing. Then, the metal concentrations on the wafer were evaluated by vapor phase decomposition inductively coupled plasma mass spectrometry (VPD/ICP-MS). The detection limit of the

Table 5 shows the experimental results. The metal concentrations of the reference mean those on a wafer without dipping into any fluids. In the case of HIF-A, the concentrations of some metals are more than 1010 atoms/cm2 (black-out cells in Table 5). On the other hand, HIF-B and D are sufficiently clean. As a result, the residual fluids of HIF-B and D are

sufficiently clean. The requirement is less than 1010 atoms/cm2 on a silicon wafer.

spectrometry is 0.03×1010 atoms/cm2. The experiment was done twice for each fluid.

500μm

Spin dry

Soak

Spin drying

• Liquid: HIF-A, HIF-B HIF-D, HIF-E • Liquid volume: 100mL • Soaking time: 60sec

• Spin speed: 2000rpm • Spin time: 30sec

69023 44

Coat

Coat

Exposure • FPA-6000 AS4 • Pattern: 90nm LS

Inspection

between a G2 fluid and a resist.

• Resist: TARF-P6228ME

• KLA2371 (KLA tencor) PS 0.25um

PEB/Development • ACT12 (TEL)

E are 66, 1794, 8, and 69023, respectively.

acceptable from the viewpoint of metal contamination.

**3.4.3 Metal contamination** 

Threshold 150 Inspection

66 1794 8

Microscopic review

Post-apply bake

PEB

Development


Table 4. Experimental conditions for metal contamination on a wafer dipped into a G2 fluid.


Table 5. Metal concentrations on a wafer evaluated by VPD/ICP-MS.

#### **3.4.4 Dissolved oxygen**

Oxygen in the atmosphere diffuses into the residual fluid on a wafer. When the oxygen-rich residual fluid goes back under the lens, it decreases the transparency of the fluid under the lens. That is the reason why dissolved oxygen is one of the issues arising with a residual fluid.

Oxygen concentration in the fluid under the lens was evaluated while a wafer was moving along the sequence as shown in Fig. 23. HIL-203 (JSR) was used as a G2 fluid. A prototype immersion nozzle designed for a G2 fluid was used for the experiment. The processing time for one wafer was 30 seconds and it consisted of a scanning time and a waiting time at the edge of the stage. The fluid under the lens was sucked through the hole opened in the lens and the oxygen concentration in the sucked fluid was measured. The absorbance was calculated by using the relation as shown in Fig. 4.

Fig. 23. Sequence of a scanning test on an 8 inches wafer.

Figure 24 shows fluid absorbance induced by dissolved oxygen against the number of scanned wafers. The absorbance rises with the scanning speed but it is low enough even at

High-Index Immersion Lithography 415

industry are indispensable to overcoming the remaining challenges such as LuAG. It is just

The author would like to acknowledge DuPont, JSR, and Mitsui Chemicals for supplying G2

Parthier, L.; Wehrhan, G.; Seifert, F.; Ansorg, M.; Aichele, T. & Seitz, C. (2008). High-index

Parthier, L.; Wehrhan, G.; Seifert, F.; Ansorg, M.; Aichele, T.; Seitz, C. & Letz, M. (2008).

Burnett, J. H.; Kaplan, S. G.; Shirley, E. L.; Horowitz, D.; Josell, D.; Clauss, W.; Grenville, A.

Nawata, T.; Inui, Y.; Masada, I.; Nishijima, E.; Mabuchi, T.; Mochizuki, N.; Satoh, H. &

Letz, M.; Gottwald, A.; Richter, M.; Liberman, V. & Parthier, L. (2010). Temperature-

French, R. H.; Liberman, V.; Tran, H. V.; Feldman, J.; Adelman, D. J.; Wheland, R. C.; Qiu,

Furukawa, T.; Kishida, T.; Miyamatsu, T.; Kawaguchi, K.; Yamada, K.; Tominaga, T.; Slezak,

Kagayama, A.; Wachi, H.; Namai, Y. & Fukuda, S. (2007). High-refractive index fluids for

*Lithography*, 6519-66, San Jose, California, USA, February 25-March 2, 2007 Zimmerman, P. A.; Byers, J.; Rice, B.; Ober, C. K.; Giannelis, E. P.; Rodriquez, R.; Wang, D.;

immersion generation-three fluid candidates, *Proc. SPIE* Vol. 6923, 69230A Sakai, K.; Iwasaki, Y.; Mori, S.; Yamada, A.; Ogusu, M.; Yamashita, K.; Nishikawara, T.;

index materials, *Jpn. J. Appl. Phys.* Vol. 47, No. 6, pp. 4853-4861

lens material LuAG : development status and progress, *presented at SEMATECH*

Development update of high index lens material LuAG for ArF hyper NA immersion systems, *presented at 5th Int. Symp. on Immersion Lithography*, HI-01, The

& Peski, C. V. (2006). High index materials for 193 nm immersion lithography, *presented at 3rd Int. Symp. on Immersion Lithography*, OO-21, Kyoto, Japan, October 2-

Fukuda, T. (2007). High-index fluoride materials for 193 nm immersion

dependent Urbach tail measurements of lutetium aluminum garnet single crystals,

W.; McLain, S. J.; Nagao, O.; Kaku, M.; Mocella, M.; Yang, M. K.; Lemon, M. F.; Brubaker, L.; Shoe, A. L.; Fones, B.; Fischel, B. E.; Krohn, K.; Hardy, D. & Chen, C. Y. (2007). High-index immersion lithography with second-generation immersion fluids to enable numerical aperatures of 1.55 for cost effective 32-nm half pitches,

M. & Hieda, K. (2007). High-refractive index materials design for ArF immersion

second-generation 193-nm immersion lithography, *persented at SPIE Advanced* 

O'Connor, N.; Lei, X.; Turro, N. J.; Liberman, V.; Palmacci, S.; Rothschild, M.; Lafferty, N. & Smith, B. W. (2008). Development and evaluation of a 193nm

Hara, S. & Watanabe, Y. (2008). Feasibility study on immersion system using high-

fluids and purification units. The author also thanks colleagues for their cooperation.

*Litho Forum*, Bolton Landing, New York, USA, May 12-14, 2008

Hague, Netherlands, September 22-25, 2008

lithography, *Proc. SPIE* Vol. 6520, 65201P

lithography, *Proc. SPIE* Vol. 6519, 65190B

*Phys. Rev. B* 81, 155109

*Proc. SPIE* Vol. 6520, 65201O

said, "No market, no tool."

**6. Acknowledgment** 

5, 2006

**7. References** 

the scanning speed of 800 mm/sec. This result exhibits that the nozzle can reduce the amount of residual fluid sufficiently and can suppress the fluid darkening. In conclusion, the issue of dissolved oxygen was solved by the immersion nozzle designed for a G2 fluid.

Fig. 24. Fluid absorbance between a lens and a wafer during the scan. The absorbance induced by dissolved oxygen is low enough even at the scanning speed of 800 mm/sec.

#### **4. Remaining challenges**

There are some remaining challenges to realize high-index immersion lithography. As described in the second subchapter, the quality of LuAG material does not reach the target specification. Especially, the absorbance of LuAG would be a critical issue. While the intrinsic absorbance of LuAG is lower than the target absorbance, further reduction of impurities is required.

On the other hand, there still remain a few issues to be studied in an immersion system using a G2 fluid. Although fluid absorbance can be kept low enough, dose homogeneity through a fluid layer should be confirmed. Defect study should be done with various resists. It is preferable to examine them with a preproduction tool using a G2 fluid.

For the extendability of high-index immersion lithography, the research activity on new materials such as G3 fluids and high-index resists is needed.

## **5. Conclusion**

It has been discussed the feasibility on a high-index immersion system of 1.55 NA using LuAG and a G2 fluid. Although the IBR correction of LuAG is feasible, the quality of LuAG is not enough and the acceleration of its development is desired. The immersion system using a G2 fluid is being developed without serious issues. It was demonstrated that fluid absorbance can be kept low enough through an in-line purification unit and an oxygen removal unit. Lens contamination can be suppressed by addition of a small amount of water into a G2 fluid. 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.

EUVL is the orthodox candidate of the next generation lithography but still needs real verifications in various items. If EUVL is delayed, high-index immersion lithography will be at the leading edge lithography. It is a common knowledge that strong supports from the industry are indispensable to overcoming the remaining challenges such as LuAG. It is just said, "No market, no tool."
