WO2008148411A1

WO2008148411A1 - A method and an apparatus for producing microchips

Info

Publication number: WO2008148411A1
Application number: PCT/EP2007/005051
Authority: WO
Inventors: Shahab Jahromi; Leonardus Gerardus Bernardus Bremer
Original assignee: Dsm Ip Assets B.V.
Priority date: 2007-06-07
Filing date: 2007-06-07
Publication date: 2008-12-11
Also published as: TW200848942A

Abstract

The invention relates to a method for producing microchips by using immersion lithography, while using immersion fluids comprising a host fluid having at least about 40 wt% of at least one organic fluid with a refractive index of 1.5 or higher the immersion fluid further comprising particles with a refractive index substantially equal to that of the neat host fluid, the particles being present in about 10 wt% or more.

Description

A METHOD AND AN APPARATUS FOR PRODUCING MICROCHIPS

The invention relates to a method as well as an apparatus for producing microchips by using immersion lithography. Since the invention of integrated circuits in 1959, the computing power of microprocessors has been doubled every 18 months and every three years a new generation of microchips has been introduced, every time reducing the size of electronic devices. This phenomenon is known as Moore's law. The performance of the microchip is, to a large degree, governed by the size of the individual circuit elements, such as for example cupper and aluminium lines, in the microchip. A microchip in general comprises a complex three-dimensional structure of alternating, patterned layers of conductors, dielectrics, and semiconductor films. As a general rule, the smaller the circuit elements, the faster the microchip and the more operations it can perform per unit of time. This phenomenal rate of increase in the integration density of the microchips has been sustained in large by advances in optical lithography, which has been the method of choice for producing the microchips.

A higher degree of integration of the circuit requires a shorter wavelength of exposure light used in the method of producing microchips by optical lithography. Changing the exposure light to shorter wavelengths has indeed been the method of choice to increase the resolution. However, switching to shorter wavelengths is becoming increasingly a daunting task as new exposure tools and materials such as photo-resists must be designed. This is a difficult task and it often results in implementation issues and delays. Therefore chip manufacturers generally tend to postpone the introduction of a new exposure wavelength as long as possible and attempt to prolong the lifetime of an existing technology using alternative approaches. Already for a period of time immersion lithography is considered to be an effective method to improve the resolution limit of a given exposure wavelength. Here the air between the bottom lens of the apparatus for producing the microchips and the silicon wafer having a layer of photoresist on top, is replaced with an immersion fluid, leading essentially to a decrease in effective wave length, see for example: US Patent No.

4,480,910, US Patent 6,781 ,670 and US Patent No 6,788,477. Preferably the fluid has a high transparency at least at the wavelength of the exposure light, does not influence the chemistry of the photoresist on top of the silicon wafer used to produce the microchip and does not degrade the surface of the lens. Immersion lithography is for example possible for the wavelengths 248 nm, 193 nm and 157 nm. Because of its transparency at 193 nm water is the main candidate for immersion fluid at this wavelength. (See for example: J. H. Burnett, S. Kaplan, Proceedings of SPIE, Vol. 5040, P. 1742, 2003). Because of exceptional transparency of fluorinated and siloxane-based compounds at 157 nm, such fluids are being considered for 157 nm immersion lithography.

EP 1557721 describes immersion lithography fluids with additives. The reference describes mainly water based systems. Water with a refractive index of 1.44 at 193 nm establishes itself as suitable fluid for printing features up to 45 nm. The application of immersion technology for printing even smaller features can be feasable if fluids with higher index can be developed.

Depending on the exposure wavelength, various types of organic solvents could be used. It appears that different types of linear and cyclic alkanes have a very low absorption in the UV (below 400 nm) and Deep UV (below 250 nm) regions, and at the same time have a high refractive index. At an exposure wavelength of 193 nm, different organic liquids have been reported which indeed combine the high transparency, i.e. low absorption, with high refractive index. Although the organic fluids have a number of advantages, they also have several drawbacks. The most important disadvantage of such liquids is that the physical properties, such as refractive index, tend to show strong variation with changes in temperature. Such a variation of index with temperature (dn/dT) is not desirable because it can results in deterioration of printed images. Also organic fluids tend to decompose under exposure of high energy wavelengths resulting in loss of transparency and thus photo-darkening. Therefore, although a number of organic fluids exist with a higher refractive index, these are not yet in use because of a number of practicable problems. Aim of the invention is to provide a method for producing microchips by using immersion lithography showing further resolution enhancement using immersion fluids comprising a host fluid having at least about 40 wt% of an organic fluid with a refractive index of 1.47 or higher and having better properties.

Surprisingly this aim is achieved with an immersion fluid that comprises particles with a refractive index substantially equal to that of the neat host fluid, the particles being present in about 10 wt% or more.

Surprisingly we have found that organic fluids can be stabilized by dispersion of certain inorganic particles resulting in much less variation of index with temperature, and of reduced photo-decomposition. We have found that this can be ideally achieved by dispersing particles with an index close or identical to the index of host liquid. Matching the index of particles with liquid results in significant relaxation of stringent requirements in particle size as is the case in the immersion fluids described in WO 2005/050324. Also we have observed that the dispersion of particles with identical index as the host liquid is much easier than in the case where there is large difference between two indices.

Preferably the difference of the refractive index of the neat host fluid and the particles is at most about 0.03 or less, more preferably at most about 0.02 or less, still more preferably at most about 0.01 or less.

In another embodiment, the difference in refractive index of the neat host fluid is less than 1 %, preferably about 0.9% or less.

Examples of organic fluids are various types of fluids that may have been suggested as immersion fluids per-se, such as for example alkanes, cyclic or multicyclic alkanes, alkane-nitrile, siloxane based fluids or cyclic sulfone and sultone compounds, as well as fluorinated variants from any of the alkane-comprising compounds.

Suitable alkanes used as diluent in the immersion fluid of the present invention preferably comprise 6 carbon atoms or more. The alkanes may comprise 30 carbon atoms or less, preferably about 20 carbon atoms or less, and even more preferred 15 carbon atoms or less. The alkanes preferably comprise one or more ring structures, such as for example decaline, cyclo-octanes, cyclo-dodecanes and the like.

Suitable nitrile group comprising compounds used as diluents in the immersion fluid of the present invention are mono, di or tri-nitrile compounds. Preferred nitrile-group comprising compounds are alkyl-dinitrile compounds having 5 to 16 carbon atoms or branched alkanes with nitrile end-groups. Suitable examples of nitrile compounds include di-nitrile butane, dinitrile-hexane, di-nitrile-octane, di-nitrile-decane and the like.

We have found for example that at 193 nm, decalin is exceptionally transparent and has high refractive index. Cis-decalin has a higher refractive index than trans-decalin. Depending on concentration of these two isomers, the refractive index of decalin at 193 nm can vary between 1.63 and 1.65.

The host fluid preferably is present in an amount of 35 vol% or more. Preferably, the organic fluid constitutes about 70wt% or more of the host fluid, more preferably about 80wt% or more, and even more preferred, the organic fluid constitutes about 95% or more of the host fluid. Host fluid means the part of the immersion fluid that - if measured without the particles - is liquid at the processing - A -

conditions. The organic fluid may contain other components like for example larger molecules of organic solids or liquids like for example polycyclic alkanes. These molecules increase n and decrease dn/dT to some extent. The organic fluid may contain smaller molecules like acetonitril, sulphuric acid, methanole and other alcohols or water. The smaller molecules can stabilize the particles and also have some effect on n and dn/dT. It may be preferred that the organic fluid can be free of oxygen and other molecules that lead to oxidation of (cyclic) alkanes. This can be achieved by for example bubbling an inert gas like He through the fluid. Generally, lithography is performed at a well conditioned temperature, e.g. at 20 ⁰C. Other temperatures are possible, such as at about 0 ⁰C or higher, or at about 80 ⁰C or lower

In one embodiment of the invention, the organic fluid comprises more that one organic compound, each having a refractive index of about 1.5, at 248, 193 or 157 nm preferably each having a refractive index of about 1.55 or higher at said wavelengths, and most preferred, at least one of the organic compounds has a refractive index of about 1.6 or higher at said wavelengths.

The immersion fluid further comprises about 10 wt% particles or more, which particles are insoluble in the host fluid and therefore are dispersed as particles.

Examples of particles include organic, inorganic or metallic nano particles.

Examples of inorganic particles include: Phosphates, polyphosphates, sulphates, perchlorates, fluorides, chlorides, borates, silicates and aluminates from compounds as defined hereafter, wherein X, Y and Z denote the following: X = H, Li, Na, Ka, Rb, Cs

Y = Be, Mg, Ca, Sr, Ba Z = B, Al

Examples of phosphates and polyphosphates include: X₃PO₄, X₂HPO₄, XH₂PO₄, H₃PO₄, P₂O₅ reacted with 0 - 4 equivalent water, P₂O₅ glass compositions obtained by reactions with X₂O, YO or Z₂O₃ ,YXPO₄, YHPO_4, (XPO₃)₆ and other polyphosphates

Examples of sulphates include: X₂SO₄ and YSO4. Examples of perchlorates include: XCIO₄, Y(CIO₄)_2, and Z(CIO₄)₃ Examples of fluorides and chlorides include: XF, XCI; YF₂, YCI_2, YXF_3, and ZF₃ Examples of borates and aluminates include: X₃ZO_3, YX ZO_3, and Z₂O₃ Examples of silicates include: SiO₂, quartz and X₂SiO₄

Further examples include various mixed crystals and glasses from these groups like the following example from group 4 and 6, AI₂(SiO₄F₂)

Further examples include materials obtained after hydration like AI(OH)₃ (Gibbsite), NaHSO₄-H₂O

Further examples include materials mainly based on fluorides but with rare earth metals, Ytrium, Sc and Zr. Example Ytrium₂Ca₅F₁₆

Preferred the particles include fused silica, quartz, oxides (such as MgO, CaO₁ AI₂O₃), spinal-related materials (such as MgAI₂O₄), garnets (such as Lu₃AI₁₅O₁₂), various fluorinated materials, such as barium fluoride and KY₃F₁₀, BaLiF₃.

The transparent particles may consist of a material having a transmission of least 40% (as measured over a theoretical light path of 1mm), at the desired lithography wavelength of 248, 193 or 157 nm. Preferably this transmission is at least 60%, more preferably at least 80%, still more preferably at least 90 %, most preferably at least 95%. Examples of suitable transparent particles are particles of transparent crystals, for example SiO₂, AI₂O_3, MgO and HfO₂. Preferably amorphous SiO₂ particles, sapphire particles or MgO particles are used.

More preferably particles of fused amorphous SiO₂ are used, having a purity of at least 99 wt.%, more preferably at least 99.5 wt.%, still more preferably at least 99.9 wt.%. In this way a fluid having still further improved transparency is obtained.

Examples of particles of fused amorphous SiO₂ suitable for use in the immersion fluid are of the Lithosil™ series preferably Lithosil™Q0/1-E193 and Lithosil™Q0/1-E248 (produced by Schott Lithotec), and fused amorphous SiO₂ of the HPFS series with the Corning code 7980 (produced by Corning) as used for the production of lenses for apparatus for the production of chips. Such fused amorphous SiO₂ is very pure and therefore may have a transparency of more than 99%. A method of producing such particles is by flame hydrolysis, a method known to the person skilled in the art. In order to increase the refractive index of the particles of fused amorphous SiO₂ it possible to dope the particles with small amounts of suitable doping elements, for example Germanium.

Very good results are obtained by using particles of a material, which material is highly transparent for radiation at the exposure wave length, for example at a wave length of 248, 193 or 157 nm, for example the material having a transmission of at least 50%, as measured over a theoretical light path of 1mm.

The average size of the particles is preferably about the size of the wavelength or lower, preferably about 2 times smaller or less, more preferable about 4 times smaller or less, still more preferably about 8 times smaller or less and even still more preferably 20 times smaller or less than the corresponding exposure wavelength, the wave length of the exposure light used in the method according to the invention. The average size of the nano particles may be about 1000 nm or less, preferably about 100 nanometer (nm) or less, and is preferably about 50 nm or less, more preferably about 30 nm or less, still more preferably about 20 nm or less, and, if a very short wavelength (193 nm or 157 nm) is used, it may be preferred to have a size of about 10 nm or less. This results in a high transparency of the immersion fluid, especially at the wavelength of the exposure light. The smaller sizes of about 20 nm or lower are in particular useful in embodiments where there is a difference in refractive index between the host liquid and the particles.

The minimum size of the particles is not critical, and is dependent on the manufacturing method, smaller particles being more difficult to make. The particles may have a minimum size of 0.1 nm, and are preferably of a minimum size of 1 nm or more, more preferable, about 2 nm or more. For measuring the dimensions of the nano-particles the particles are in a very dilute mixture applied on a surface in a thin layer, so that at a microscopic (for example FE-SEM (Field Emission Scanning Electron Microscopy) or AFM (atomic force microscopy)) photographic image of the layer, the single nano-particles are observable. Than from 100 nanoparticles, ad random selected, the dimensions are determined and the average value is taken. In case of particles having an aspect ratio above 1 , like platelets, rods or worm-shaped nano-particles, as the size the distance from one end to the most remote other end is taken.

The volume percentage of the nano particles in the fluid is about 10 % or more, preferably about 20% or more, still even more preferably about 30% or more, even still more preferably about 40% or more. Most preferably the volume percentage is about 50% or more, as this results in a fluid having very stable properties, like low dn/dT, a high transparency and low amount of scattering of the incident light. Generally, the volume percentage is about 80% or lower, more preferably about 70% or lower. It is known to the skilled person how to make nano particles and stable dispersions of the nano particles in immersion fluids.

For the preparation of nano particles both wet and solid state techniques may be used. Wet methods include sol-gel techniques, hydrothermal processing, synthesis in supercritical fluids, precipitation techniques and micro emulsion technology. Solid state techniques include gas phase methods like flame / plasma techniques and mechano-chemical processing. In particular good results are obtained with wet methods such as sol-gel techniques. The sol-gel reaction can be carried out in aqueous media in which case the particles are charged stabilised. The counter ions are chosen in such a way to ensure high optical transmission at corresponding wavelengths. Preferably phosphorous containing counter ions such as phosphoric acid are used. Alternatively the sol-gel reaction may be carried out in nonaqueous media for example alkanes like decane or cyclic alkanes like decaline. In this case, the nano-particles are stabilised by addition of suitable dispersing agents. In this way high concentration, so high refractive index, and low viscosity are obtained. To ensure low absorption at deep-UV wavelengths, preferably fluorinated dispersing agents are used. After the sol-gel synthesis at ambient pressures, the fluid containing nanoparticles may be heated under pressure to increase the density and also change the crystalline structure of particles. In this way, particles with superior optical properties such as high refractive index can be produced. Also a combination of the flame hydrolysis and a wet method may be used in which the particles, produced at elevated temperatures, are directly deposited in the fluids such as water or alkanes such as for example decane or cyclic alkanes such as for example decaline. This method has the advantage that aggregation and agglomeration of highly pure nano-particles is prevented. It is also possible to use an immersion fluid in the process according to the invention, comprising a mixture of more than one organic fluid and/or more than one type of particle.

In a further preferred embodiment a fluid is used comprising transparent particles having a refractive index substantially equal to the refractive index of a mixture of fluid materials.

In a further preferred embodiment a fluid is used comprising transparent particles which are functionalised on their surface in such a manner that they become readily dispersible in the immersion fluid. This is for example possible by grafting the particles with a surfactant, preferably a polymeric surfactant. It is also possible for purpose of dispersing the transparent particles to add a surfactant to the immersion fluid comprising the transparent particles. In another preferred embodiment, the particles are functionalised using a compound having an alkoxy-siloxane group and an aliphatic chain, chloor atom or other suitable group. The alkoxysiloxane group may be for example a tris-methoxy or bis- or tris-ethoxy group. The aliphatic chain may be for example stearyl, isodecyl, cyclohexyl or n-butyl. Such a compound can be reacted with the particles through the siloxane group. The hydrophobic chain at the other side of the molecule makes the particle easily dispersable in for example aliphatic organic liquids.

In a preferred embodiment the method according to the invention further comprises the steps of: a) measuring the refractive index of the immersion fluid directly or indirectly, b) adding any one of organic fluid or adding any one of the particles to the immersion fluid as to adjust the refractive index of the immersion fluid at a predetermined value. In this way fluctuations in the refractive index due to variations in temperature and concentration of the organic fluid or particles are compensated for.

The refractive index may be measured as such directly. It is also possible to measure one or more other parameters, being a measure for the refractive index. As the immersion fluid comprises the particles with a refractive index equal to the refractive index of the fluid, it is possible to determine the light scattering of the particles and to add a certain other fluid or additive to reduce the light scattering. The addition of such a fluid may suitably be carried out by mixing additional fluid with the immersion fluid. The addition of extra particles may suitably be carried out by mixing a concentrated dispersion of the particles in the pure fluid with the immersion fluid. In another embodiment of the invention, the temperature of the immersion fluid is varied to adjust the refractive index of the host fluid with respect to the particles. In this way, it is easy to correct for small differences between the host fluid and the particles. The dn/dT of the host fluid generally will be substantially larger than the dn/dT of the particles. Thus, by lowering the temperature, the refractive index of the host liquid will decrease more than the refractive index of the particles. The dn/dT of the host fluid will generally be about -400 to -700 ppm^"1, whereas the particles will generally have a dn/dT of +70 to -30 ppm^'1. A ppm is the change in refractive index per degree temperature change: A dn/dT of ^400 ⁰C^"1 means that the refractive index decreases with 400 x 10^"6 (0.0004) with a temperature increase of 1 ⁰C. A still further preferred embodiment of the method according to the invention comprises the steps of a) transporting the immersion fluid after being used in the production of a microchip to a cleaning unit, b) cleaning the immersion fluid c) recycling the cleaned immersion fluid into the process for producing the chips.

Due to the extraction of components from the photoresist layer on top of the wafer, possible chemical changes in the fluid components during the exposure step and further reasons, the immersion fluid will tend to be contaminated. This means that after a certain period of using the fluid in the process of the present invention, the fluid has to be refreshed. However this increases fluid consumption and negatively influences the process economics. Surprisingly it is possible to clean the fluid and recycle the cleaned fluid into the process of the present invention.

Cleaning of the fluid is suitably carried out by cross flow filtration or dead end flow filtration using for example membranes for microfiltration, ultrafiltration, nanofiltration or reverse osmoses. Good results are obtained if a stirred pressure cell is used. An example of a stirred pressure cell is described in WO 2005/050324.

Another cleaning technique that can be used, is washing with strong acids like sulphuric acid or alkaline solutions, subsequent washing with water and then drying by passing over a column with CaCI₂ or P₂O₅. The acid, the alkaline or water layer can be separated from the immersion fluid and contains most of the contamination.

Further cleaning can be achieved by absorption on a column with activated alumina. These techniques can be used instead of filtration if the particles resist strong acids and water. Alternatively only the host liquid is cleaned this way.

Preferably the immersion fluid has a transmission at one or more wavelength out of the group of 248, 193 and 157 nm of at least 10% through a path- length of 1cm, more preferably at least 30%, still more preferably at least 40%, even still more preferably at least 50%, most preferably at least 60%. BaLiF₃ is very much transparent at 193 nm and has an index of 1.64 at 193 nm. We have found that it is possible to disperse BaLiF₃ particles in decalin and match the refractive index by changing the concentration of cis and trans isomers. The resulting dispersion shows almost no scattering and much less variation of index with temperature. Also the photo- decomposition of decalin has been suppressed dramatically. The invention also relates to an apparatus for immersion lithography for the production of microchips, comprising the immersion fluid.

Examples

Dispersions of nano particles of quartz nanocrystals can be produced by a hydrothermal synthesis as described in Nano Lett. Vol. 3, no 5 (2003) pp 655-659. Refractive indices are measured at 193 nm and 248 nm using ellipsometer VUV-VASE produced by J.A. Woollam Co., lnc (US). The results are shown in table 1.

Table 1. Refractive indices (Rl) of various nanoparticles and decaline host liquids measured at 193 nm and 248 nm.

As is clear from the table, immersion fluids can be made which - at appropriate temperature level - have a substantial or precise match of refractive index between host fluid and particles. Trans-decaline can be combined with quartz glass for use at 248 nm, at 30, but preferably at 50, and even more preferred at 70 °C. Cis- decaline can be combined with crystalline quartz at 193 nm, at 30, but preferably at 10 ⁰C. The immersion fluids can be used in an apparatus for producing microchips, based on immersion technology at wavelength of 248 or 193 nm. It is also possible to make particles with a refractive index between that of e.g. cis- and trans-decaline, and adjust refractive index differences with the organic fluids used in the host liquid.

Claims

1. Method for producing microchips by using immersion lithography, using immersion fluids comprising a host fluid having at least about 40 wt% of at least one organic fluid with a refractive index of about 1.5 or higher at 193 nm and comprising particles with a refractive index substantially equal to that of the neat host fluid, the particles being present in about 10 wt% or more.

2. Method using a fluid composition according to claim 1 where the difference of index of particles and index of host fluid is about 0.03 or less at 193 nm.

3. Method using a fluid composition according to claim 2 where the difference of index of particles and index of host fluid is about 0.02 or less at 193 nm.

4. Method using a fluid composition according to claim 3 where the difference of index of particles and index of host fluid is about 0.01 or less at 193 nm.

5. Method using a fluid composition according to any one of claims 1-4 where the host fluid comprises at least about 40 wt% of at least one organic fluid with a refractive index of about 1.6 or higher

6. Method using a fluid composition according to any one of claims 1-5 where the host liquid comprises (cyclic) alkanes as the organic fluid.

7. Method using a fluid composition according to any one of claims 1-5 where the host liquid comprises (partially) fluorinated (cyclic) alkanes.

8. Method using a fluid composition according to claim 6 where the host liquid is cis-decalin, trans decalin or a mixture of both.

9. Method using a fluid composition according to any one of claims 1-8 where the particles are crystalline or amorphous inorganic materials.

10. Method using a fluid composition according to claim 9 where the particles can be fused silica, quartz, oxides (such as MgO, CaO, AI₂O₃), spinal-related materials (such as MgAI₂O₄), garnets (such as Lu₃AI₁₅O₁₂), various fluorinated materials, such as barium fluoride and KY₃F₁₀, BaLiF₃.

11. Method using a fluid composition according to claim 10 where the particle is preferably BaLiF₃ and host fluid preferably decaline.

12. Method according to any one of claims 1-11 , wherein the immersion fluid comprises about 35 vol% or more of the host fluid which host fluid comprises about 90wt% or more organic fluids.

13. Method according to any one of claims 1-12, wherein the particles are present in an amount of about 30 wt% or more.

14. Method according to any one of claims 1-13, wherein the immersion fluid comprises nano particles with a size of about 2 times smaller than the wavelength of the exposure light or less.

15. Method according to claim 14, wherein the nano particles have an average size of about 1000 nm or less.

16. Method according to any one of claims 1-15, wherein the nano particles have an average size of about 100 nm or less.

17. Method according to any of claims 1-16, wherein the immersion fluid comprises at least 40 volume % of the nano particles.

18. Method according to any of claims 1-17, wherein the particles are of a material that has a transmission of at least 50%, as measured over a theoretical light path of 1 mm.

19. Method according to any of claims 1 -18, characterised in that the method comprises the steps of a) transporting the immersion fluid after being used in the production of a microchip to a cleaning unit, b) cleaning the immersion fluid c) recycling the cleaned immersion fluid into the process for producing the chips.

20. Apparatus for producing microchips, based on the technology of immersion lithography, wherein the apparatus comprises the immersion fluid as used in the process of any one of claims 1-19.