DE102005024682A1 - Optical material e.g. glasses, for UV microlithography, has earth alkali metal fluoride doped with bivalent metal ions that possess ion radius, which is similar to that of alkali ions, so that bivalent ions are integrated in crystal lattice - Google Patents

Abstract

The material has earth alkali metal fluoride, which is doped with bivalent metal ions. The bivalent metal ions are selected in such a manner that the ions possess an ion radius, which is so similar to ion radius of earth alkali metal ions, in such a manner that the bivalent metal ions can be integrated in the earth alkali metal fluoride crystal lattice. The earth alkali metal fluoride is calcium fluoride and/or barium fluoride. Independent claims are also included for the following: (1) an optical unit in a lens system for the UV microlithography (2) a lens system for the UV microlithography (3) a method for production of an optical material.

Description

The Invention relates to an optical material with high refractive indices, which are suitable for the production of optical elements for microlithography are a method of making such materials, optics, containing such materials and the use of such optics for the production of electronic components.

In An electronic computer or control unit will be very many Circuits arranged in a confined space. These are also called integrated circuits (IC) or referred to as a chip. There are through sophisticated arrangements circuits possible that many millions of switching functions can. By the permanent increase the computing or switching functions of such circuits will be their Arrangement in a confined space, d. H. their packing density getting bigger. So should be at regular reduction such chips according to the so-called. Moore's law the computing power or the computing speed such circuits every 18 to 24 months. To this, too reach, it is necessary Transistors for Such arithmetic or Circuits to shrink ever smaller and ever closer to and on top of each other to arrange. Only in this way is the miniaturization possible promote the development of heat and thus further improve the computing speed.

such Miniaturized circuits are made by so-called microlithography made in which a light-sensitive on a wafer Lacquer, a so-called photoresist, illuminated by means of a complex optic with light becomes. In this way, images of previously designed Create tracks and circuits on the photoresist and through a further treatment from this figure an entire network of manufacture integrated circuits.

The Industry is gradually moving towards ever smaller structures. The individual stages along this path are referred to as "nodes." So are currently integrated Circuits produced whose interconnects distances of 180, 150 and 130 nm exhibit. Currently, the 90nm "Node" is implemented while already at the end of the year 2005, the 65 nm "Node" is targeted and Planning already plans the 45 nm "Node".

Around to produce such small structures by microlithography, that must be Lithography system a possible have high resolution.

The resolution of a projection optic is calculated according to the formula d = λ / 2A, where d is the distance between two neighboring points which the optics can barely distinguish from each other, λ is the wavelength of the light used and A is the numerical aperture. From this equation it follows that in order to improve the resolving power of the optics (ie to reduce the value of d), one uses light with the shortest possible wavelength and / or increases the numerical aperture.

For this reason, exposure wavelengths in the UV range are used in microlithography, in particular in the deep UV range (DUV). For this purpose, coherent light, ie laser light, namely excimer laser is used, such as KrF laser with a wavelength of 248 nm, ArF laser with a wavelength of 193 nm, and F ₂ laser with a wavelength of 157 nm Glass in the UV range has a poor transmission, must be used for the projection optics UV-transparent glasses. Preference is given to using high-purity, crystallized fluorite (fluorspar, CaF ₂ ) whose transmission is sufficiently high deep into the UV range.

The numerical aperture is calculated according to the equation A = n · sin σ, where n is the refractive index of the optical system and σ is half the aperture angle of the projection optics. In this case, an optical system usually has different refractive indices, since the light as it passes through the system different materials (usually different glasses and air) happens. The refractive index of the overall system always depends on the material with the lowest refractive index. This is usually air, which has a refractive index of 1. Since, for practical reasons, half the opening angle of the projection optics can assume values of a maximum of about 72 °, the numerical aperture achievable in practice according to the above equation is a maximum of 0.95.

With the currently used in microlithography ArF lasers with a wavelength of 193 nm can therefore be chip structures with a width of minimal (193 nm / 2) x 0.95 = 101.6 nm produce.

Out Light microscopy is known as a medium between the front lens and projection object instead of air to use a liquid that a higher one Refractive index as air, and so the numerical aperture to increase.

So has e.g. deionized water has a refractive index of n = 1.44 on, while special oils Have refractive indices of up to n = 1.70. With this as immersion microscopy known technique can be Following the above equation, the resolving power of light microscopes almost double that.

Also In microlithography, immersion technology is used as a means of Improvement of resolving power meanwhile known. This is a liquid placed between the projection optics and the wafer to be exposed. There are already immersion intentions for commercial available lithography equipment developed (e.g., the Twinscan system of ASML, The Netherlands).

When using an ArF laser, an immersion optics of CaF ₂ and deionized water as immersion liquid can thus at least theoretically a resolution of (193 nm / 2) x 1.44 = 67 nm be achieved. However, the maximum numerical aperture is also limited by the refractive index of the lens material, if smaller than that of the immersion liquid used. While quartz glass has a refractive index (n ₁₉₃ ) of 1.56 for 193 nm wavelength light, CaF ₂ , which is preferred because of its favorable transmission properties, has a refractive index of n ₁₉₃ = 1.50 and BaF _{2 has} a refractive index of n ₁₉₃ = 1.58 , In contrast, immersion liquids with refractive indices of up to 1.70 are available.

Around To be able to produce circuits with even finer conductor tracks therefore glasses needed the with higher transmission in the DUV range higher refractive indices exhibit.

From the prior art glasses with high refractive indices are known, which consist of MgO / CaO, Al ₂ O ₃ or MgAl ₂ O ₄ . However, these glasses have high spatial dispersion caused te birefringence values (<50 nm / cm) and are therefore not suitable for optical purposes. Corresponding data can be found in the script for the lecture "High-Index Materials for 193 nm and 157 nm Immersion Lithography" by John H. Burnett, which was presented at the SPIE conference Microlithography 2005 in San José.

Of the Invention has for its object to provide materials the with very sufficient transmission in the DUV range very high refractive indices have and, moreover for optical Purposes are suitable. The invention also has the goal of optical elements or lens systems to provide, at the wavelengths mentioned a high resolution or high image sharpness enable and in particular for the immersion microlithography are suitable. These tasks will be by the characterizing features of the present claims 1, 14 and 18 solved.

Accordingly, it is envisaged to provide an optical material, in particular for UV microlithography, which consists of crystallized alkaline earth metal fluoride. The material is characterized in that the alkaline earth metal fluoride is doped with divalent metal ions, wherein the divalent metal ions so ge are selected to have a radius that is so similar to the radius of the alkaline earth metal ion that the divalent metal ions can be incorporated into the alkaline earth metal fluoride crystal lattice.

The Applicant has surprisingly found that this doping contributes to an increase in the refractive index wavelength leads, the below the wavelength the additional Absorption lie. It is believed that this doping to a direct electron transfer below the band gap of alkaline earth metal fluoride, the one additional Absorption causes.

simultaneously Show that way doped materials birefringence of less than 10 nm / cm and are therefore for the proposed uses are suitable.

In Another variant of the invention is provided, an optical Material, especially for UV microlithography, which also consists of crystallized Alkaline earth metal fluoride exists. This variant is characterized that the alkaline earth metal fluoride with monovalent and trivalent Ions in a stoichiometric relationship of 1: 1, wherein the monovalent and trivalent ions so chosen are that the sum of the square of the radius of the monovalent Ions and the square of the radius of the trivalent ion of the sum the squares of the radii of two alkaline-earth metal ions are so similar that pairs of monovalent and trivalent ions in the alkaline earth metal fluoride crystal lattice can be installed.

This condition can be expressed by the formula (r 1+ ) 3 + (r 3+ ) 3 ~ 2 (r 2+ ) 3 where r ^{1+ is} the radius of the monovalent metal ion, r ^{3+ is} the radius of the trivalent metal ion, and r ^{2+ is} the radius of the alkaline earth metal ion. After r ²⁺ dissolved the formula is:

These Variant is connected to the first variant in a way that both realize a single general inventive idea, which is to dope an alkaline earth metal fluoride crystal with metal ions, to increase its refractive index, where the selection of the ions of their radius in relation to Radius of the alkaline earth metal ion depends. So it's about two closely related alternative solutions for a specific task.

In a preferred embodiment of the invention, it is provided that the alkaline earth metal fluoride is CaF ₂ . CaF ₂ crystals are widely used in microfotolithography because of their high transmission in the DUV range.

In another preferred embodiment of the invention, it is provided that the alkaline earth metal fluoride is BaF ₂ . BaF ₂ has a higher refractive index compared to CaF ₂ even without doping (n ₁₉₃ = 1.58 vs. n ₁₉₃ = 1.50), which can be further increased by the doping according to the invention.

It is particularly preferred that the material be present as a single crystal, as can be prepared by conventional alkaline earth metal fluoride fusion crystal growth methods. Such crystals have particularly favorable imaging properties and are therefore very well suited for microlithography. The growing of single crystals from the melt is known per se. In textbooks for crystal growing, such as the 1088 pages comprehensive work "Crystal Growth" by K.-Th. Wilke and J.Bohm, the most diverse methods for growing crystals are described. Further breeding methods are for example in the DE 100 10 484.3 described.

In a preferred embodiment of the first variant of the invention it is provided that the divalent metal ions have a radius, the 80-120 % of the radius of the Erdalkalimetallions. Ions within this size range can be installed relatively easily in the alkaline earth metal fluoride crystal lattice, while larger or minor ions to interference in the crystal lattice and are therefore unsuitable.

Particularly preferably, the divalent metal ions to be incorporated into a CaF ₂ crystal lattice have a radius between 80 and 120 μm. This may be, for example Cd ^2+, Sr ^2+, Hg ^2+, Sn ^2+, Zn ²⁺ and / or Pb ²⁺ trade. All of these ions have radii so similar to Ca ²⁺ that they can be incorporated into the CaF ₂ crystal lattice. While Ca ^{2+ has} a radius of 100 pm, Cd ^{2+ has} a radius of 95 pm, Sr ²⁺ a radius of 118 pm, Hg ²⁺ a radius of 102 pm, Sn ²⁺ a radius of 118 pm, and Pb ^{2 +} a radius of 119 pm. Particular preference is given to using Cd ²⁺ , Hg ²⁺ , Sn ²⁺ and / or Pb ²⁺ .

The same applies to material from BaF ₂ . Since Ba ^{2+ has} an ionic radius of 143 pm, it is possible to use divalent metal ions for doping whose radius is between 110 and 170 pm, which is similar to that of Ba ^2+, so that the ions integrate into the BaF ₂ crystal lattice to let.

In a preferred embodiment of the second variant of the invention, it is provided that the monovalent metal ion is Na ⁺ and the trivalent metal ion is La ³⁺ , Bi ³⁺ , Y ³⁺ , Tm ³⁺ , Yb ³⁺ , Lu ^{3 +} and / or Tl ³⁺ acts. Likewise, the monovalent metal ion may be Ag ⁺ and the trivalent metal ion may be Y ³⁺ , Ir ³⁺ , In ³⁺ , Sb ³⁺ and / or Tl ³⁺ . Furthermore, the monovalent metal ion may be K ⁺ and / or Au ⁺ and the trivalent metal ion may be Al ³⁺ .

The ionic radii of said ions are shown in the following table: TABLE 1

folgende Werte: Tabelle 2

With the mentioned combinations arise for the term

the following values: Table 2

As can be seen from the table, values of 95.51 pm-110.80 pm result for the combinations mentioned. By contrast, Ca ²⁺ has a radius of 100 μm. Basically, all the combinations of monovalent and trivalent metal ions are suitable for the doping in which, according to the above equation, values result that correspond to 80-120% of the ionic radius of Ca ²⁺ , ie values of 80-120 μm. The mentioned combinations of monovalent and trivalent ions therefore fulfill the condition that the sum of the square of the radius of the monovalent ion and the square of the radius of the trivalent ion is so similar to the sum of the squares of the radii of two calcium ions that pairs of these ions can be installed in the calcium fluoride crystal lattice.

The same applies to the doping of BaF ₂ . Since Ba ^{2+ has} an ionic radius of 143 μm, here combinations of monovalent and trivalent metal ions are suitable for the doping in which, according to the above equation, values correspond to 80-120 of the ionic radius of Ba ²⁺ , ie values of 110 -170 pm.

In a further embodiment according to the invention, it is provided that the optical material according to the invention has a refractive index of at least n ₁₉₃ > 1.5. It preferably has a refractive index of n ₁₉₃ > 1.6, particularly preferably a refractive index of n ₁₉₃ > 1.7.

Of Another invention is an optical Element in a lens system for provided the UV microlithography, the material of the invention containing one of the properties described above.

Prefers is this optical element to a lens, a Prism, a light guide rod, an optical window, an optical Component for DUV photolithography and / or a front lens.

Especially Preferably, the optical element is a front lens in an immersion optics for microlithography. In this case, the use of the optical according to the invention High refractive index material particularly preferred because instead from air a medium with a likewise high refractive index (such as oil or water) used and increased the numerical aperture of the system and so on the resolution of the Optics can be improved.

As well is the use according to the invention of the optical material having any of the properties described above intended for the production of optical elements for lens systems.

Furthermore, a lens system for UV microlithography is provided according to the invention, the min contains at least one of the optical elements described above.

there It is preferably provided that the optical element is a front lens is. This is in immersion lenses in contact with the immersion liquid. Therefore, just with this lens, a refractive index is required, which is at least as high as that of the immersion liquid. In a particularly preferred embodiment, it is The lens system to an immersion optics for wavelengths below 200 nm. It should the lens system is preferred for wavelength to 193 nm and 157 nm, respectively. Such a lens system allows due the principles set out in the introduction provide optimal resolution of the Microlithography optics and thus the production of very small semiconductor structures.

Farther is the use according to the invention the described optical element and / or described Lensensystems for the production of steppers, lasers, in particular of excimer lasers, computer chips, as well as integrated circuits and electronic devices that such circuits and chips included provided.

Claims (20)

Optical material, in particular for UV microlithography, consisting of crystallized alkaline earth metal fluoride, characterized in that the alkaline earth metal fluoride is doped with divalent metal ions, the divalent metal ions being chosen to have an ionic radius which is so similar to the ionic radius of the alkaline earth metal ion, that the divalent metal ions can be incorporated into the alkaline earth metal fluoride crystal lattice. Optical material, in particular for UV microlithography, consisting of crystallized alkaline earth metal fluoride, characterized that the alkaline earth metal fluoride with monovalent and trivalent Ions in a stoichiometric relationship of 1: 1, wherein the monovalent and trivalent ions so are chosen that the sum of the cube of the ionic radius of the monovalent Ions and the cube of the ionic radius of the trivalent ion the sum of the third powers of the ionic radii of two alkaline earth metal ions so similar is that pairs of monovalent and trivalent ions incorporate into the alkaline earth metal fluoride crystal lattice to let. An optical material according to claim 1 or 2, characterized in that the alkaline earth metal fluoride is CaF ₂ and / or BaF ₂ . Optical material according to one of the preceding claims, characterized characterized in that the material is a single crystal. Optical material according to one of claims 1, 3, and 4, characterized in that the divalent metal ions have an ionic radius that is 80-120% of the radius of the alkaline earth metal ion is. Optical material according to one of claims 1, 3, 4 or 5, characterized in that the divalent metal ions have an ionic radius between 80 and 120 pm. An optical material according to any one of claims 1, 3 or 4-6, characterized in that the divalent metal ion Cd ^2+, Sr ^2+, Hg ^2+, Zn ^2+, Sn ²⁺ and / or Pb ^2+. An optical material according to any one of claims 2-4, characterized in that the monovalent metal ion Na ⁺ and the trivalent metal ion is La ^3+, Tm ^3+, Yb ^3+, Lu ^3+, Bi ^3+, Y ³⁺ and / or Tl ³⁺ is. An optical material according to any one of claims 2-4, characterized in that the monovalent metal ion is Ag ⁺ and the trivalent metal ion is Y ³⁺ , Ir ³⁺ , In ³⁺ , Sb ³⁺ and / or Tl ³⁺ . An optical material according to any of claims 2-4, characterized in that the monovalent metal ion is K ⁺ and / or Au ⁺ and the trivalent metal ion is Al ³⁺ . Optical material for optical purposes according to one of the preceding claims, characterized in that it has a refractive index of n ₁₉₃ > 1.5. Optical element in a lens system for UV microlithography, characterized in that the element is a glass according to one of claims 1-11 contains. Optical element according to claim 12, characterized in that that the optical element is a lens, prism, light guide rod, optical Window, an optical component for DUV photolithography and / or a Front lens is. An optical element according to claim 12 or 13, characterized in that the optical element is a front lens in one Immersion optics for microlithography is. Use of a glass according to any one of claims 1-11 for Production of optical elements for lens systems. Lens system for the UV microlithography, characterized in that the lens system at least one optical element according to any one of claims 12-14. Lens system according to claim 16, characterized that the optical element is a front lens. Lens system according to claim 16 or 17, characterized that there is an immersion optics for wavelengths below 200 nm. Use of the optical element according to one of claims 12-14 and / or a lens system according to any one of claims 16 to 18 for producing steppers, lasers, in particular excimer lasers, computer chips, as well as integrated circuits and electronic devices that contain such circuits and chips. Method for producing an optical material according to one of the claims 1-11, comprising the following steps: Heating a crystal raw material, of an alkaline earth metal fluoride, the divalent metal ions according to claim 1 and / or mono- and trivalent metal ions in stoichiometric relationship of 1: 1 according to claim 2 contains on a temperature above the melting point until a crystal melt forms, and slowly lowering the temperature to at least the crystallization temperature of the crystal raw material, under education a single crystal.