# Patent

## Title
Using isotopically specified fluids as optical elements

## Abstract

Fluidic optical elements and systems use isotopically specified fluids for processing light passing therethrough. The isotopic composition of the fluid may be adjusted to vary the optical properties. The properties of the isotopically specified fluid may be monitored and adjusted to obtain the desired optical characteristics of the fluidic optical element. In one embodiment, a method of optically processing light includes directing light through an optical element that includes an isotopically specified fluid disposed in a confined space. The isotopically specified fluid is selected to provide a preset desired effect on the light directed therethrough for optically processing the light.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation of International Application No. PCT/US2004/021159 filed Jun. 30, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/484,276 filed Jul. 1, 2003. The disclosures of these applications are incorporated herein by reference in their entireties.


BACKGROUND
The invention relates generally to optical systems and elements and, more particularly, to the use of isotopically specified fluids as optical elements including applications in immersion lithography.
An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly (sometimes referred to as a projection lens), a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultraviolet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left.
Immersion lithography is a technique that can enhance the resolution of projection lithography by permitting exposures with numerical aperture (NA) greater than one, which is the theoretical maximum for conventional “dry” systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer) with an immersion fluid, immersion lithography permits exposure with light that could otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where only the final optical element of the optical assembly and its housing and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography.
The immersion fluid in an immersion lithography system serves as a fluidic optical element. Fluids can also be used to form optical elements such as those inside the optical assembly of an exposure apparatus. The use of a fluid as an optical element raises certain issues and challenges but also provides new opportunities.
SUMMARY
Embodiments of the invention are directed to fluidic optical elements and systems using isotopically specified fluids for processing light passing therethrough. The isotopic composition of the fluid may be adjusted to vary the optical properties. The properties of the isotopically specified fluid may be monitored and adjusted to obtain desired optical characteristics of the fluidic optical element. Examples of such properties include the isotopic composition, index of refraction, temperature, and pressure. Water has optical properties that are suitable as the immersion fluid in an immersion lithography system. Water further has mechanical properties such as low viscosity and surface tension characteristics that render it particularly desirable for use in immersion lithography. The index of refraction of an immersion fluid plays a role in the imaging during lithography. Generally, an immersion fluid having a higher index of refraction is more desirable. The heavier isotopes of water can provide a desired higher index of refraction.
In accordance with an aspect of the invention, a method of optically processing light comprises directing light through an optical element that includes an isotopically specified fluid disposed in a confined space. The isotopically specified fluid is selected to provide a preset desired effect on the light directed therethrough for optically processing the light.
In some embodiments, the isotopically specified fluid comprises one or more isotopes of a fluid. The isotopically specified fluid may comprise a plurality of isotopes of a fluid, and an isotopic composition of the fluid is monitored. The method further comprises adjusting amounts of the plurality of isotopes of the fluid to change the isotopic composition based on the monitored isotopic composition of the fluid. The method may also comprise adjusting amounts of the plurality of isotopes of the fluid to form a variable isotopic composition of the fluid to provide variable optical characteristics of the optical element.
In specific embodiments, the isotopically specified fluid comprises one or more isotopes of water. The method further comprises positioning the optical element in an immersion lithography apparatus at a location between an optical assembly and a substrate to be processed by lithography, wherein the light is directed through the optical assembly and the optical element to the substrate. The isotopically specified fluid of the optical element has a first index of refraction and the confined space is provided in a container comprising a material having a second index of refraction that is different from the first index of refraction. The method further comprises recirculating the isotopically specified fluid between the confined space of the optical element and a recirculation reservoir. The method may also comprise controlling at least one of a temperature, a pressure, and an isotopic composition of the isotopically specified fluid in the optical element. The confined space is provided in a container which comprises fused silica. The light has a wavelength of about 193 nm or greater.
In accordance with another aspect of the invention, an optical element comprises a container and an isotopically specified fluid disposed in the container. The isotopically specified fluid is selected to provide a preset desired effect on a light directed therethrough for optically processing the light.
In accordance with another aspect of the present invention, an optical system for processing a substrate comprises an optical assembly spaced from a substrate by a space and an optical element disposed in the space between the optical assembly and the substrate. The optical element includes an isotopically specified fluid selected to provide a preset desired effect on a light directed therethrough for optically processing the light.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in conjunction with the accompanying drawings of exemplary embodiments in which like reference numerals designate like elements and in which:
FIG. 1 is a simplified schematic view of an optical assembly including an optical element having an isotopically specified fluid according to an embodiment of the invention;
FIG. 2 is an elevational view of an optical element having an isotopically specified fluid according to another embodiment of the invention;
FIG. 3 is a simplified schematic view of a fluidic system for an optical element illustrating a recirculation arrangement according to another embodiment of the invention;
FIG. 4 is a simplified schematic view of a fluidic system for an optical element illustrating an open system arrangement according to another embodiment of the invention;
FIG. 5 is a simplified schematic view of an immersion lithography system employing an isotopically specified fluid according to another embodiment of the invention; and
FIG. 6 is a plot of refractive index versus D2O concentration in water based on calculations.



DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 shows an optical assembly 10 including two solid optical elements 12, 14, and a fluidic optical element 16 disposed therebetween. In the specific embodiment shown, the solid optical elements 12, 14 are converging lenses and the fluidic optical element 16 is a diverging lens. The solid optical elements 12, 14 may be made of glass or the like. The fluidic optical element 16 includes an isotopically specified fluid 20 disposed in a confined space defined by the surfaces of the two solid optical elements 12, 14 and a side wail 18. The side wall 18 is sealed to the solid optical elements 12, 14. The side wall 18 may be made of a transparent material such as glass.
As used herein, the term “isotopically specified fluid” refers to a fluid of defined chemical composition, where the molecules making up the fluid are distinguished by the specific isotopes of their elemental constituents, the corresponding molecular weight, and their concentration in the fluid. For example, for the case of water, the basic molecular constituent is H2O and the fluid would be characterized by information similar to the following:










TABLE 1





Molecular
Molecular




constituents
weight
Concentration
Comments







H2O16
18
0.99985
H = most abundant isotope





of hydrogen; O16 = most





abundant isotope of oxygen


HDO16
19
<10−5
D = deuterium, heavy





isotope of hydrogen


D2O16
20
0.00015


H2O17
19
<10−5


HDO17
20
<10−5


D2O17
21
<10−5


H2O18
20
<10−5


HDO18
21
<10−5


D2O18
22
<10−5









Only stable isotopes are listed in Table 1. Radioactive isotopes, for the case of water, are short lived enough that they are not likely to be present in a high enough concentration to be significant. In other fluids however, radioactive isotopes may have to be considered as well. Table 1 is representative of “normal water” which contains approximately 0.015% heavy water. The concentrations labeled “<10−5” represent levels below which no significant change in refractive index occurs. In the case of water, a change of approximately 10−5 may change the refractive index by about 1 ppm which is assumed to be insignificant for the present application. For other fluids and applications the level of significant concentration would have to be determined.
FIG. 2 shows another fluidic optical element 30 including an isotopically specified fluid 32 disposed in a container 34. The container 34 may be made of glass, fused silica, or the like. In some cases, the isotopically specified fluid 32 has a first index of refraction and the material of the container 34 has a second index of refraction that is different from the first index of refraction. The isotopically specified fluid 32 may include a plurality of isotopes of a fluid. Different isotopes have different optical properties such as the index of refraction. The amounts of the plurality of isotopes of the fluid may be adjustable to form a variable isotopic composition of the fluid to provide variable optical characteristics of the optical element. In addition to the isotopically specified fluid, the shape of the container 34, the optical properties of the container material, and the configuration of the cavity for the isotopically specified fluid 32 in the container 34 can be selected to produce the desired characteristics of the fluidic optical element 30.
The fluidic optical element 30 has two materials, namely, the material of the isotopically specified fluid 32 and the material of the container 34. This makes it possible to correct for chromatic aberrations or spectral dispersions. Some prior optical elements rely on the introduction of calcium fluoride into fused silica or a similar glass material to form a solid optical element. While calcium fluoride provides suitable optical properties, it is difficult and expensive to produce due to its crystalline nature and it introduces certain undesirable material properties such as intrinsic birefringence, as well as additional birefringence arising from mechanical stress in the optical element. The isotopically specified fluid 32 in the fluidic optical element 30 may replace the use of calcium fluoride in some prior optical elements.
FIG. 3 shows a system 40 for providing a fluidic optical element 42 to process light passing therethrough. In this closed system, the isotopically specified fluid is recirculated between the optical element 42 and a reservoir 44. One or more sensors 48 are used to monitor the properties of the isotopically specified fluid, such as the isotopic composition, index of refraction, temperature, pressure, and the like. In FIG. 3, the sensors 48 are coupled to the reservoir 44, but may be coupled to the optical element 42 or another part of the recirculation system to monitor the fluid. The sensor information is provided from the sensors 48 to a controller 50, which adjusts the properties of the isotopically specified fluid based on the sensor information, such as controlling any of the temperature, pressure, index of refraction, and isotopic composition of the fluid.
FIG. 4 shows an open system 60 for providing a fluidic optical element 62 to process light passing therethrough. The isotopically specified fluid is supplied to the optical element 62 from a source 64 and exits the optical element 62 to a drain 66. One or more sensors 68 are coupled to the optical element 62 or another part of the flow path upstream or downstream of the optical element 62, and are used to monitor the properties of the isotopically specified fluid. The sensor information is provided from the sensors 68 to a controller 70 which adjusts the properties of the isotopically specified fluid based on the sensor information.
The closed and open flow systems of FIGS. 3 and 4 maintain a constant flow of the isotopically specified fluid through the fluidic optical elements. The flow can ensure uniformity of the isotopically specified fluid and properties of the optical elements, and can eliminate the formation of voids or the like. It can also facilitate monitoring and control of the isotopically specified fluid for the optical elements.
FIG. 5 shows an immersion lithography system 110 including a reticle stage 112, a projection lens or optical assembly 114, and a wafer or substrate 116 supported on a wafer stage 118. An immersion apparatus 120 is disposed between the final optical element 122 of the projection lens 114 and the wafer 116 to provide an immersion fluid therebetween. In the present embodiment, the immersion fluid is an isotopically specified fluid, and forms a fluidic optical element in the path of the light between the projection lens 114 and the wafer 116. The isotopically specified fluid is typically provided as a continuous flow through the space between the projection lens 114 and the wafer 116 to form the fluidic optical element.
The system for providing the isotopically specified fluid to the immersion lithography system 110 is preferably a closed immersion fluid control system, where the fluid is recovered and recycled after exposure of a wafer. This is highly desirable because it is important to maintain the isotopic composition and thus the fluid's optical properties, and because at least some of the isotopes are likely to be rare and expensive. Even the vapor from the fluid should be recovered. A system with many of the desired characteristics is described in PCT Application No. PCT/US04/10055, entitled Environmental System Including Vacuum Scavenge For An Immersion Lithography Apparatus, filed Mar. 29, 2004. Additional components may be used to ensure the purity and/or optical properties of the fluid. Some change in isotopic composition may occur from small differences in chemical reactivities or physical properties among the isotopes. Also some impurities may be introduced into the fluid. The chemical and isotopic composition can be monitored, for example, using either optical or mass spectroscopy. Alternatively, the optical properties such as the refractive index can be monitored using a refractometer, and appropriate isotopic molecular constituents added to maintain the property at a constant value. Since impurities may affect the refractive index, this method may maintain a constant index even though the basic chemical composition of the fluid is changing.
The isotopically specified fluid is selected to provide a preset desired effect on a light directed therethrough for optically processing the light, for example, to perform a lithography procedure. The isotopically specified fluid may include a plurality of isotopes of a fluid. The fluid should not react with surrounding components such as the container of the fluidic optical element, the neighboring optical components, and substrates with which the fluid has contact. Water is particularly suitable for certain applications in lithography. Water is generally stable and nonreactive with other optical components and substrates. If water forms part of the optics in a system such as a lithography system, the optical properties of water will need to be known to almost ppm accuracy. The optical properties of water are being measured.
Water may play an important role in immersion lithography. Water has very low optical absorption at wavelengths as short as 193 nm which is the wavelength of an ArF beam. Water has a relatively low viscosity. Thus wafer stage motion and vibrations may not couple through the water to the projection lens to an unacceptable extent. It is important to monitor the isotopic purity or composition of water, because the different isotopes can have significantly different optical properties. First, different sources of water may have slightly different isotopic compositions, leading to possibly significant differences in optical properties. Second, the index of refraction of water can be varied significantly by varying the isotopic composition, which may allow some amount of tuning of the projection optics design. Third, water tuned to an appropriate index of refraction may be introduced elsewhere in the projection optics, to tune the optics further. Because the optical dispersion of water differs from that of fused silica, some amount of chromatic correction is possible.
The refractive indices of the water isotopes are not well determined at the short wavelengths used in optical lithography. The refractive indices for the various isotopes of water are estimated here from the Lorentz-Lorenz model of molecular polarizability.

(n2−1)/(n2+2)=Aρ,

    
    
        where n is the index of refraction, ρ is the density, and A is an empirical constant. If one assumes that A is the empirical constant for ordinary water, the isotope effect arises basically from changes in the density. Ordinary water is presumed to be H216O. Normal “heavy” water D2O is taken to be D216O, where D is deuterium (2H), the heavy isotope of hydrogen. Of course, there are other isotopes including HDO and heavier isotopes of oxygen, 17O and 18O, that will add to the density effect. At a temperature of 30° C. for a wavelength of 193 nm, the constant A for ordinary water is about 0.262936. Table 2 shows the calculated results of the index of refraction for different isotopes of water. The index of refraction of an immersion fluid plays a role in the imaging during lithography. Generally, an immersion fluid having a higher index of refraction is more desirable. The heavier isotopes of water may provide the desired higher index of refraction.
    
    


The above model cannot represent a complete description of the optical properties of the water isotopes, because it neglects differences in the molecular energy levels among the isotopes. Therefore Table 2 represents an approximation. Better measurements of the refractive index of the water isotopes are desirable to confirm or adjust these numbers. The differences in the indices of refraction among the isotopes also cause some difference in the optical absorption of the different isotopes.











TABLE 2







Isotope
Density ρ (gram/cc)
Index of Refraction




















H2O
0.995678
1.43664



D2O
1.10315
1.49188



D217O
1.158308
1.52112



D218O
1.213465
1.55103










The natural abundance of heavy water is about 0.015%. This causes a change of about 8 ppm in the refractive index from that of pure H2O, possibly a significant difference. FIG. 6 shows a plot of the variation in refractive index with different concentrations of heavy water based on calculations. Pure D2O has a refractive index of about 1.492, which is close to the refractive index of calcium fluoride (Ca2F) of about 1.50 at the wavelength of 193 nm. Therefore, heavy water may be able to replace some Ca2F for chromatic corrections in an optical system containing optical elements formed of fused silica or the like. Table 3 further lists the index of refraction and dispersion at 193 nm for fused silica, calcium fluoride, and different isotopes of water based on calculations. The dispersion of the water isotopes has not been measured at 193 nm. At longer wavelengths, the dispersion of D2O is lower than that of H2O.









TABLE 3





Material
Index of Refraction (n)
Dispersion (dn/dλ)

















Fused Silica
1.5607
−0.00158


Ca2F
1.502
−0.00099


H2O
1.4355
−0.00199


D2O
1.49188
not measured at 193 nm


D217O
1.52112
not measured at 193 nm


D218O
1.55103
not measured at 193 nm









It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reviewing the above description. There are many alternative ways of implementing the methods and apparatus of the invention.

## Claims

1. A method of performing immersion lithography in which a substrate is exposed to an exposure light, the method comprising:
exposing the substrate by directing the exposure light through an optical element onto the substrate,
wherein the optical element includes an isotopically specified liquid disposed in a space, the isotopically specified liquid having an isotopic composition that has been adjusted to provide a preset desired effect on the light directed therethrough for optically processing the light.

2. The method of claim 1, wherein the isotopically specified liquid comprises one or more isotopes of a liquid.

3. The method of claim 2, wherein the isotopically specified liquid comprises a plurality of isotopes of a liquid, and further comprising monitoring an isotopic composition of the liquid.

4. The method of claim 3, further comprising adjusting amounts of the plurality of isotopes of the liquid to change the isotopic composition based on the monitored isotopic composition of the liquid.

5. The method of claim 2, further comprising adjusting amounts of the plurality of isotopes of the liquid to form a variable isotopic composition of the liquid to provide variable optical characteristics of the optical element.

6. The method of claim 1, wherein the isotopically specified liquid comprises one or more isotopes of water.

7. The method of claim 1, wherein the optical element is disposed at a location between an optical assembly and a substrate, and wherein the light is directed through the optical assembly and the optical element to the substrate.

8. The method of claim 1, further comprising recirculating the isotopically specified liquid.

9. The method of claim 1, wherein the space is provided in a container.

10. The method of claim 1, wherein the light has a wavelength of about 193 nm or greater.

11. The method of claim 1, wherein the isotopically specified liquid has an optical property which depends on an isotopic condition of the liquid.

12. The method of claim 11, wherein the optical property includes the refractive index for the light.

13. The method of claim 2, wherein the isotopically specified liquid is defined by an isotopic composition thereof.

14. The method of claim 13, wherein the isotopic composition of the isotopically specified liquid is determined to tune an optical property of the liquid.

15. The method of claim 14, wherein the optical property includes the refractive index for the light.

16. The method of claim 2, wherein the isotopically specified liquid is defined by a concentration of the isotope therein.

17. The method of claim 16, wherein the concentration of the isotope in the isotopically specified liquid is determined to tune an optical property of the liquid.

18. The method of claim 17, wherein the optical property includes the refractive index for the light.

19. The method of claim 2, wherein the isotopically specified liquid includes one or more heavy isotopes.

20. The method of claim 19, wherein the heavy isotope includes deuterium.

21. An immersion lithography apparatus that exposes a substrate to an exposure light, the immersion lithography apparatus comprising:
a holding member that holds the substrate; and
an optical element through which the exposure light is transmitted to the substrate held by the holding member, the optical element comprising:
a container; and
an isotopically specified liquid disposed in the container,
wherein the isotopically specified liquid has an isotopic composition that has been adjusted to provide a preset desired effect on a light directed therethrough for optically processing the light.

22. The apparatus of claim 21, wherein the isotopically specified liquid comprises one or more isotopes of a liquid.

23. The apparatus of claim 22, wherein the isotopically specified liquid comprises a plurality of isotopes of a liquid, and wherein amounts of the plurality of isotopes of the liquid are adjustable to form a variable isotopic composition of the liquid to provide variable optical characteristics of the optical element.

24. The apparatus of claim 21, wherein the isotopically specified liquid comprises one or more isotopes of water.

25. An immersion lithography apparatus that exposes a substrate to an exposure light, the immersion lithography apparatus comprising:
a holding member that holds the substrate; and
an optical system through which the exposure light is transmitted to the substrate held by the holding member, the optical system comprising:
an optical assembly spaced from the substrate by a space; and
an optical element disposed in the space between the optical assembly and the substrate, the optical element including an isotopically specified liquid having an isotopic composition that has been adjusted to provide a preset desired effect on a light directed therethrough for optically processing the light.

26. The apparatus of claim 25, wherein the isotopically specified liquid comprises one or more isotopes of a liquid.

27. The apparatus of claim 26, wherein the isotopically specified liquid comprises a plurality of isotopes of a liquid, and wherein amounts of the plurality of isotopes of the liquid are adjustable to form a variable isotopic composition of the liquid to provide variable optical characteristics of the optical element.

28. The apparatus of claim 25, wherein the isotopically specified liquid comprises one or more isotopes of water.

29. The system apparatus of claim 25, wherein a continuous flow of the isotopically specified liquid is provided through the space between the optical assembly and the substrate to form the optical element.

30. The apparatus of claim 25, wherein the refractive index of the isotopically specified liquid is greater than about 1.45.