NANOIMPRINT LITHOGRAPHY PROCESS AND PATTERNED SUBSTRATE OBTAINABLE THEREFROM

Abstract

The present invention pertains to the field of nanoimprint lithography (NIL) processes and more specifically to a soft NIL process used for providing a sol-gel patterned layer on a substrate. Specifically, this process comprises a step of adjusting the solvent uptake of the sol-gel film to 10 to 50% vol., preferably between 15 and 40% vol., by varying the relative pressure of the solvent while a soft mould is applied onto the substrate coated with the sol-gel film.

Claims

1. A nanoimprint lithography (NIL) process, comprising the successive steps of: a) preparing a solution of metal or metalloid oxide precursor(s), b) applying said solution onto a substrate to form a film, c) equilibrating the film with the atmosphere under adjusted solvent vapor pressure, d) applying a soft mould onto said film to provide an assembly such that said film fills up the cavities of the mould, and maintaining said assembly under solvent vapor, e) thermally treating said assembly so as to rigidify the gel film thus obtained, f) removing the mould to obtain a substrate coated with a patterned gel, and g) curing said patterned gel so as to obtain a patterned metal or metalloid oxide material on said substrate, wherein the solvent uptake of the film is adjusted to 10 to 50% vol. by varying the vapor pressure in a volatile solvent at the vicinity of the gel during steps c) and d).

2. The process of claim 1, wherein the solvent is selected from the group consisting of water, ethanol, isopropanol, acetone, THF, hexane, toluene and their mixtures.

3. The process according to claim 1, wherein the metal or metalloid oxide precursors are selected from the group consisting of inorganic salts, organics salts or alkoxides of at least one metal or metalloid or of a combination of at least one metal with at least one metalloid.

4. The process according to claim 3, wherein that the metal or metalloid oxides obtained from the metal or metalloid oxide precursors are selected from the group consisting of TiO.sub.2, ZnO:TiO.sub.2, AlO(OH), V.sub.2O.sub.5, VO.sub.2, any silicon oxides and YZrO.sub.2.

5. The process according to claim 1, wherein the mould is manufactured from silicone elastomers, fluorinated polymers, polyvinylpyrrolidone (PVP), polylactic acid (PLA) or polyetherimide (PEI).

6. The process according to claim 1, wherein the thermal treatment is performed at a temperature of from 25 to 200 C.

7. The process according to claim 1, further comprising a step (a) of converting the metal or the metalloid oxide precursors into the metal or the metalloid oxide suspension after step (a) and before step (b), and does or does not comprise step (g).

8. The process according to claim 1 wherein the curing step is performed at a temperature of from 200 to 800 C.

9. A patterned substrate obtainable by the process according to claim 1.

10. A device for optical, photonic, electrical or biological applications comprising a patterned substrate according to claim 9.

11. The process of claim 1, wherein the solvent uptake of the film is adjusted between 15 and 40% vol.

12. The process of claim 2, wherein the solvent is water.

13. The process of claim 5, wherein the mould is manufactured from silicone elastomers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1 represents the thickness evolution of a TiO.sub.2 film next to a pattern, when a mould is applied onto the film, when varying the relative humidity.

[0052] FIG. 2 shows the replicas obtained from the same TiO.sub.2 film under various relative humidities.

[0053] FIG. 3 illustrates the variation in the aspect ratio and surface roughness of the same TiO.sub.2 film when varying the relative humidity.

[0054] FIG. 4 illustrates the thickness variation of a TiO.sub.2 film as a function of the relative humidity during replication.

[0055] FIG. 5 illustrates the thickness variation of an aluminum oxide film as a function of the relative humidity during replication.

[0056] FIG. 6 illustrates the thickness variation of a ZnO:TiO.sub.2 film as a function of the relative humidity during replication.

[0057] FIG. 7 shows the variation in the swelling of a SiO.sub.2 film when varying the relative pressure two different solvents.

[0058] FIG. 8 shows the variation in water uptake of a YZrO.sub.2 film and a TiO.sub.2 film as a function of the relative humidity during replication.

[0059] FIG. 9 shows the variation in water uptake of a hybrid SiO.sub.2 film and a TiO.sub.2 film as a function of the relative humidity during replication.

[0060] FIG. 10 shows the variation in the swelling of a V.sub.2O.sub.5 film and a TiO.sub.2 film as a function of the relative humidity.

[0061] FIG. 11 illustrates the thickness and refractive index variations of a TiO.sub.2 film as a function of the relative humidity during replication.

EXAMPLES

[0062] This invention will be better understood in light of the following examples which are given for illustrative purposes only and do not intend to limit the scope of the invention, which is defined by the attached claims.

Example 1: Soft-NIL Method Applied to a TiO.SUB.2 .Film Under Relative Humidity

[0063] A master was manufactured by Focused Ion Beam lithography of a silicon wafer. Two types of patterns were used: [0064] pyramid arrays which were obtained by engraving parallel lines spaced by 1 m, then lines perpendicular thereto, on an area of 20 m20 m. The depth of the engraving could not be measured (destructive method) [0065] plots of 1.8 m diameter surrounded by an engraved area of 4 m diameter and 700 nm depth, which were obtained by engraving rings in the master.

[0066] A negative of this master was then prepared by moulding from a silicone elastomer (PDMS) material. For this purpose, the master was first immersed in an ethanoic solution of 0.05 M of SiCl.sub.2(CH.sub.3).sub.2 during 30 min, then withdrawn from the solution and abundantly rinsed with ethanol. The PDMS precursors (90% by weight Rhodorsil RTV141A, 10% RTV141B) were mixed then poured onto the master. After degassing, the PDMS was annealed at 120 C. for 1 h then demoulded. The mould was degassed for 10 min just before the replication step.

[0067] A sol-gel solution consisting of 2.5 g of TiC.sub.4, 42 g ethanol, 1.3 g H.sub.2O and 0.1 g of Pluronic F127 (PPG-PEG copolymer) was placed onto a defatted glass slide (VWR) by dip-coating in controlled chamber (v=2 mm/s, T=25 C., RH=20%). Shortly after dip-coating, the glass slide was placed into a chamber with controlled atmosphere, onto a cold heating plate. The gas relative pressure was precisely controlled and maintained for 1 min, then the mould was applied onto the film and kept for 5 min under controlled humidity. This assembly was then heated to 120 C. for 5 min (including 2 min of heating time to 120 C.). The substrate was finally demoulded and annealed at 500 C. in order to crystallize TiO.sub.2 into its anatase form.

1Influence of Humidity on Film Mobility

[0068] A preliminary experiment is first conducted by measuring with an Atomic Force Microscope the change in film thickness close to one of the patterns present on the mould. For this purpose, a precise control of water relative pressure (i.e. relative humidity, RH) is performed. When the mould is applied to the surface of the film, the capillary forces between the film and the PDMS surface induce a local deformation and are responsible for the displacement of the material around the patterns. This experiment showed that the film has no mobility at RH<40%. The film mobility increases gradually when the process is conducted at RH=50% then at RH=70%, as shown on FIG. 1. The sol-gel fil thus displays some mobility which may be controlled by adjusting relative humidity during the replication step of the soft-NIL process.

2Optimization of Relative Humidity in the Case of a TiO2 Film

[0069] The effect of relative humidity on the replication of structures has been studied. The patterns engraved on the master were replicated on the mould then on sol-gel samples and the samples were characterized by dark field optical microscopy and by atomic force microscopy in order to assess the accuracy of pattern replication between the master and the replica. In addition to accurate replication, an aspect ratio as close as possible to that of the master is desired, as shown on FIG. 2 where Z designates the aspect ratio, which corresponds to the height (H or h) of the pillars to their diameter (L or l, respectively). It is preferred that h/l be as close as possible to H/L.

[0070] As shown on FIG. 3, the aspect ratio increases with relative humidity. Mobility thus increases together with the amount of water absorbed by the film. Below 45% RH, mobility is not high enough and no replication occurs. Moreover, for patterns obtained at RH>70%, a high surface roughness is observed, as well as cracks in the sol-gel layer. During the consolidation process of the sol-gel layer, the temperature increase (from 25 to 120 C.) indeed results in the evaporation of volatile species (such as water and ethanol) through the PDMS mould. Since the latter is permeable to gases, it is likely that the excess of absorbed water (related to RH) is such that the volatile species cannot escape fast enough, which accounts for surface roughness and cracks on the edge of the pattern.

[0071] Optimal replication appears to be obtained between 50 and 70% RH, which corresponds to a swelling of 15-40% (increase of film volume), as shown on FIG. 4. At this relative humidity, the patterns have the highest aspect ratios while keeping a good homogeneity.

[0072] Optimizing relative humidity during sol-gel imprinting thus allows obtaining structures with high aspects ratios (>1) and structures higher than 460 nm from an initial layer which is 90 nm high. This result is difficult to achieve with other techniques using soft conditions. Moreover, this process results in low thermal shrinkage of the structures (30% in each direction, 65% in volume) for TiO.sub.2.

Example 2: Soft-NIL method applied to a ZnO/TiO.SUB.2 .film and to a AlOOH film

[0073] Similarly to Example 1, a soft-NIL method was conducted on a ZnO/TiO.sub.2 film. ZnO was mixed with TiO.sub.2 to improve its processability and its chemical and mechanical properties and to reduce thermal shrinkage.

[0074] Three sol-gel solutions were prepared, respectively consisting of: [0075] 1.5 g TiCl.sub.4, 1.75 g Zn(OAc).sub.2,2H.sub.2O, 20.5 g ethanol, 1.62 g H.sub.2O and 0.1 g of Pluronic F127 (PPG-PEG copolymer), to provide a 50:50 ZnO:TiO.sub.2 film, [0076] 0.76 g TiCl.sub.4, 2.63 g Zn(OAc).sub.2,2H.sub.2O, 20.5 g ethanol, 1.62 g H.sub.2O and 0.1 g of Pluronic F127 (PPG-PEG copolymer), to provide a 25:75 ZnO:TiO.sub.2 film, and [0077] 5.1 g Al(OiPr).sub.3, 33.4 g ethanol, 3.42 g HCl (37%), 0.04 g CTAB (cetyl trimethylammonium bromide).

[0078] Each of these solutions was deposited onto a defatted glass slide (VWR) by dip-coating in a controlled chamber (v=2 mm/s, T=25 C., RH=20%). Shortly after dip-coating, the glass slide was placed into a chamber with controlled atmosphere, onto a cold heating plate. The gas pressure was precisely controlled and maintained for 1 min, then the mould was applied onto the film and kept for 2 min under controlled humidity. This assembly was then heated to 120 C. for 5 min (including 2 min of heating time to 120 C.). The substrate was finally demoulded. In the case of TiO.sub.2, it was further annealed at 350 C.

[0079] High quality replicas were obtained from the ZnO:TiO.sub.2 films under 50% RH, which corresponds to a swelling rate of 20-30% vol., as shown on FIG. 6. In the case of the AlOOH film, replication was only possible between 20 and 50% RH, which corresponds to a swelling of 15-40% vol., as apparent on FIG. 5. Below 20% RH, no replication was observed, whereas the AlOOH film adhered to the mould during demoulding above 50% RH.

Example 3: Soft-NIL Process Under Controlled Atmosphere of Various Gases

[0080] A sol-gel solution consisting of TEOS (Aldrich):EtOH(absolute):H.sub.2O(milliQ):Pluronic F127 in a 1:20:5:0.0002 molar ratio was prepared and deposited onto a glass slide.

[0081] Various experiments were performed by varying the ethanol partial pressure in the chamber. As shown on FIG. 7, the swelling of the SiO.sub.2 layer was higher in the case of ethanol, compared to the same partial pressure of water.

Example 4: Soft-NIL Method Applied to a YZrO.SUB.2 .Films and Hybrid-Silica Films

[0082] Similarly to Example 1, a soft-NIL method was conducted on YZrO.sub.2 films, hybrid silica films and V.sub.2O.sub.5 films. Hybrid silica refers to a gel containing standard silica precursor (here tetraethoxysilane) and a modified silica precursor (here methyltriethoxysilane). The addition of yttria in zirconia films allows to stabilize the cubic phase having better properties.

[0083] Sol-gel solution were prepared, consisting of: [0084] 0.94 Zr(Cl).sub.4; 0.06 Y(NO.sub.3).sub.3,2H.sub.2O; 41 EtOH; 12 H.sub.2O; 2.10.sup.4 F127 molar ratios for Yttria-stabilised zirconia precursor solution [0085] 0.6 TEOS; 0.4 MTEOS; 40 EtOH; 10 H.sub.2O; 0.01 HCl; 0.01 CTAB molar ratios for hybrid silica. [0086] 1 V(Cl).sub.3; 60 EtOH: 10 H.sub.2O; 2.10.sup.4 F127 molar ratios for V.sub.2O.sub.5.

[0087] Each of these solutions was deposited onto a defatted glass slide (VWR) by dip-coating in a controlled chamber (v=2 mm/s, T=25 C., RH=20%). Shortly after dip-coating, the glass slide was placed into a chamber with controlled atmosphere. The gas pressure was precisely controlled and maintained for 1 min, then the mould was applied onto the film and kept for 2 min under controlled humidity. This assembly was then heated to 70 C. for 1 min then 120 C. for 1 min. The substrate was finally demoulded and annealed at 400 C. for 10 min.

[0088] High quality replicas were obtained from the YZrO.sub.2 films under 50% RH, which corresponds to a water uptake of about 30% vol., as shown on FIG. 8.

[0089] In the case of hybrid silica, replication yields high quality replicas at 70% RH which corresponds to a swelling rate of 20-30% vol as shown on FIG. 9.

[0090] High quality replicas were obtained from the V.sub.2O.sub.5 films under 20% RH, which corresponds to a water uptake of 20-30% vol, as shown on FIG. 10. V.sub.2O.sub.5 can subsequently be reduced to VO.sub.2 at 500 C. for 4 hours under an atmosphere composed of 5% of H.sub.2 in N.sub.2.

Example 5: Soft-NIL Method Applied to TiO.SUB.2 .Nanoparticle Films

[0091] Similarly to Example 1, a soft-NIL method was conducted on TiO.sub.2 nanoparticle films.

[0092] To do so, a sol-gel solution was prepared from:

[0093] 0.04 mol/L of titanium isopropoxide in a 1:2 ethanol:water solvent. pH is adjusted at 0.7 by addition of concentrated HNO.sub.3. The solution is stirred for 4 h at 240 C. (hydrothermal conditions) to yield 15 nm diameter TiO.sub.2 nanoparticles.

[0094] The ageing of the solution allows to deposit nanoparticles instead of small clusters as in Example 1.

[0095] The solution is deposited onto a defatted glass slide (VWR) by dip-coating in a controlled chamber (v=2 mm/s, T=25 C., RH=20%). Shortly after dip-coating, the glass slide was placed into a chamber with controlled atmosphere. The gas pressure was precisely controlled and maintained for 1 min, then the mould was applied onto the film and kept for 2 min under controlled humidity. This assembly was then heated to 70 C. for 1 min then 120 C. for 1 min. The substrate was finally demoulded.

[0096] High quality replicas were obtained from the TiO.sub.2 nanoparticle films under 95% RH, which corresponds to a water uptake of about 30% vol., as shown on FIG. 11.