US20070195302A1 - Using isotopically specified fluids as optical elements - Google Patents

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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This is a divisional of U.S. patent application Ser. No. 11/311,247 filed Dec. 20, 2005, which in turn 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 would 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 anoptical assembly10 including two solidoptical elements12,14, and a fluidicoptical element16 disposed therebetween. In the specific embodiment shown, the solidoptical elements12,14 are converging lenses and the fluidicoptical element16 is a diverging lens. The solidoptical elements12,14 may be made of glass or the like. The fluidicoptical element16 includes an isotopically specifiedfluid20 disposed in a confined space defined by the surfaces of the two solidoptical elements12,14 and aside wall18. Theside wall18 is sealed to the solidoptical elements12,14. Theside wall18 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 10⁻⁵may 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 fluidicoptical element30 including an isotopically specifiedfluid32 disposed in acontainer34. Thecontainer34 may be made of glass, fused silica, or the like. In some cases, the isotopically specifiedfluid32 has a first index of refraction and the material of thecontainer34 has a second index of refraction that is different from the first index of refraction. The isotopically specifiedfluid32 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 thecontainer34, the optical properties of the container material, and the configuration of the cavity for the isotopically specifiedfluid32 in thecontainer34 can be selected to produce the desired characteristics of the fluidicoptical element30.
• The fluidicoptical element30 has two materials, namely, the material of the isotopically specifiedfluid32 and the material of thecontainer34. 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 specifiedfluid32 in the fluidicoptical element30 may replace the use of calcium fluoride in some prior optical elements.
• FIG. 3 shows asystem40 for providing a fluidicoptical element42 to process light passing therethrough. In this closed system, the isotopically specified fluid is recirculated between theoptical element42 and areservoir44. One ormore sensors48 are used to monitor the properties of the isotopically specified fluid, such as the isotopic composition, index of refraction, temperature, pressure, and the like. InFIG. 3, thesensors48 are coupled to thereservoir44, but may be coupled to theoptical element42 or another part of the recirculation system to monitor the fluid. The sensor information is provided from thesensors48 to acontroller50, 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 anopen system60 for providing a fluidicoptical element62 to process light passing therethrough. The isotopically specified fluid is supplied to theoptical element62 from asource64 and exits theoptical element62 to adrain66. One ormore sensors68 are coupled to theoptical element62 or another part of the flow path upstream or downstream of theoptical element62, and are used to monitor the properties of the isotopically specified fluid. The sensor information is provided from thesensors68 to acontroller70 which adjusts the properties of the isotopically specified fluid based on the sensor information.
• The closed and open flow systems ofFIGS. 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 animmersion lithography system110 including areticle stage112, a projection lens oroptical assembly114, and a wafer orsubstrate116 supported on awafer stage118. Animmersion apparatus120 is disposed between the finaloptical element122 of theprojection lens114 and thewafer116 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 theprojection lens114 and thewafer116. The isotopically specified fluid is typically provided as a continuous flow through the space between theprojection lens114 and thewafer116 to form the fluidic optical element.
• The system for providing the isotopically specified fluid to theimmersion lithography system110 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 and¹⁸O, 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 (14)

1. A liquid used in a liquid immersion lithography apparatus, the liquid comprising one or more isotopes of a liquid, wherein an isotopic composition of the liquid has been adjusted to provide desired optical characteristics.

2. The liquid according toclaim 1, wherein the isotopic composition has been adjusted by adjusting amounts of a plurality of isotopes.

3. The liquid according toclaim 1, wherein the optical characteristics include a refractive index.

4. The liquid according toclaim 3, wherein the refractive index of the liquid is greater than about 1.45.

5. The liquid according toclaim 1, wherein the liquid comprises one or more isotopes of water.

6. The liquid according toclaim 1, wherein the isotopes include one or more heavy isotopes.

7. The liquid according toclaim 6, wherein the one or more heavy isotopes include deuterium.

8. The liquid according toclaim 1, wherein the liquid is enriched with heavy isotopes.

9. The liquid according toclaim 8, wherein a proportion of at least one heavy isotope is at least doubled in comparison with the natural isotope distribution of the liquid.

10. The liquid according toclaim 8, wherein the liquid is enriched with deuterium.

11. The liquid according toclaim 10, wherein more than 1% of the molecules contained in the liquid contain deuterium.

12. The liquid according toclaim 11, wherein more than 80% of the molecules contained in the liquid contain deuterium.

13. The liquid according toclaim 13, wherein more than 99% of the molecules contained in the liquid contain deuterium.

14. The liquid according toclaim 13, wherein more than 99 molar % of the liquid consists of heavy water.

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