FABRICATION OF NANOSTRUCTURED SUBSTRATES COMPRISING A PLURALITY OF NANOSTRUCTURE GRADIENTS ON A SINGLE SUBSTRATE

Abstract

The invention relates to a method for producing a nanostructured substrate comprising an array of protruding nanostructures, which method comprises at least the following steps: a) providing a primary substrate; b) depositing at least one layer of a material capable to be removed by means of reactive ion etching (RIE) onto said primary substrate which layer comprises a predetermined gradient of its thickness; c) depositing a nanostructured etching mask onto the graded layer deposited in step b); d) generating protruding structures, in particular nanopillars, in the graded layer deposited in step b) by means of reactive ion etching (RIE), wherein simultaneously at least 2, preferably 3, predetermined continuous gradients of geometric parameters of the protruding structures are generated on the same substrateMore specifically, the geometric parameters are selected from the group comprising the height, diameter and spacingof the protruding nanostructures. A further aspect of the invention relates to a nanostructured substrate comprising an array of protruding nanostructures obtainable by the method as outlined above. In a preferred embodiment of said nanostructured substrate, each of the protruding nanostructures simultaneously represents an element of 3 continuous gradients of the height, diameter and spacing of said protruding nanostructures.

Claims

1. A method for producing a nanostructured substrate comprising an array of protruding nanostructures, which method comprises at least the following steps: a) providing a primary substrate; b) depositing at least one layer of a material capable to be removed by reactive ion etching (RIE) onto said primary substrate which at least one layer comprises a predetermined gradient of its thickness; c) depositing a nanostructured etching mask onto the at least one layer deposited in step b); d) generating the protruding structures, which are nanopillars, in the at least one layer deposited in step b), wherein simultaneously at least 2 predetermined continuous gradients of geometric parameters of the protruding structures are generated on the same substrate.

2. The method according to claim 1, wherein the layer deposited in step b) comprises a 2-dimensional or 3-dimensional gradient of its thickness.

3. The method according to claim 1, wherein the geometric parameters are selected from the group consisting a of height, a diameter and a spacing of the protruding structures.

4. The method according to claim 1, wherein the nanostructured etching mask comprises an array of nanoparticles which is provided by micellar diblock or multiblock copolymer nanolithography.

5. The method according to claim 1, wherein the etching comprises at least one treatment with an etchant which is selected from the group consisting of chlorine, gaseous chlorine compounds, fluorinated hydrocarbons, fluorocarbons, oxygen, argon, SF.sub.6 and mixtures thereof.

6. The method according to claim 5, wherein the etching comprises at least one treatment with a mixture of Ar/SF.sub.6/O.sub.2 or Ar/SF.sub.6 as etchant and at least one treatment with a mixture of Ar/CHF.sub.3 as etchant.

7. The method according to claim 1, wherein each etching treatment is carried out for a period in the range of 30 s to 60 min.

8. The method according to claim 1, which further comprises a mechanical treatment of the protruding structures generated.

9. The method according to claim 1, which comprises a further etching treatment by reactive ion beam etching (RIBE), chemically assisted ion beam etching (CAIBE), reactive ion etching (RIE) or inductive coupled plasma (RIE-ICP), wherein the protruding structures generated in the at least one layer deposited in step b) are used as an etching mask and gradients of the protruding structures corresponding to the protruding structures of the at least one layer above are generated in the primary substrate and the layer(s) above said primary substrate are removed in part or completely.

10. A nanostructured substrate comprising an array of protruding nanostructures, which are nanopillars, wherein the protruding nanostructures comprise at least 2 continuous gradients of geometric parameters of the protruding nanostructures on a single substrate.

11. The nanostructured substrate according to claim 10, wherein the protruding nanostructures have a mean height in the range from 800 nm to 1500 nm.

12. The nanostructured substrate according to claim 10, wherein the geometric parameters are members selected from the group consisting of a height, a diameter and a spacing of the protruding nanostructures.

13. The nanostructured substrate according to claim 12, wherein each of the protruding nanostructures simultaneously represents an element of 3 continuous gradients of the height, diameter and spacing of said protruding nanostructures.

14. The nanostructured substrate according to claim 9, which is an optical filter, and wherein the protruding structures form an anti-reflective surface structure on the optical filter.

15. An optical device, comprising the nanostructured substrate according to claim 10.

16. A device comprising the nanostructured substrate according to claim 10, wherein the device is configured for use in a field selected from the group consisting of semiconductor technology, optics, sensor technology and photovoltaics.

17. The device according to claim 16, which is configured for use in optical devices, CCD sensors or solar cells.

18. The nanostructured substrate according to claim 10, wherein the protruding nanostructures comprise at least 3 continuous gradients of geometric parameters of the protruding nanostructures on a single substrate.

19. An optical device, comprising the nanostructured substrate according to claim 14.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0079] FIG. 1 shows the effect of different moth-eye structures as present in different areas on a single graded substrate of the invention. The transmission of the nanostructured substrates (broken lines) is significantly higher than the transmission of the plain substrate (continuous line). Different pillar geometries (A/B) result in different transmission maxima and reflection minima, respectively (arrows).

[0080] FIG. 2 shows examples of a 2D (top) or 3D (bottom) gradient sacrificial layer formed on top of a primary substrate.

[0081] FIG. 3 shows a comparison of different methods to affect the geometry of antireflective nanopillars.

[0082] FIG. 4 shows a surface covered with a graded sacrificial layer and a combination of different nanoparticle sizes and distances (top) and the resulting etched substrate structured with a complex pattern of pillars with different geometries (bottom).

[0083] FIG. 5 shows a single substrate covered by nanopillars with different height and diameter (left) and different height and spacing (right).

[0084] FIG. 6 shows the basic principle of an optical device wherein a beam of light passes through one aperture, a nanostructure gradient filter (NGF) and another aperture.

[0085] FIG. 7 shows the basic principle of an optical device wherein a rotating mirror is used to deflect a beam of light towards a pre-selected region of a NGF.

[0086] FIG. 8 shows the basic principle of an optical device wherein a movable light-source projects a beam of light into a prism with at least one side covered by a NGF.

[0087] The following examples are used for more in depth explanation of the present invention, without limiting the same thereto, however. It will be evident for the person skilled in the art that variations of these conditions in dependence of the specific materials used may be required and can be determined without difficulty by means of routine experiments.

Example 1

Creation of a Plurality of Nanostructure Gradients on the Same Single Substrate

1. Providing a Composite Substrate

[0088] A primary substrate, such as suprasil glass, is coated with at least one layer of an easily RIE etchable material such as SiO.sub.2 by means of sputter coating (UHV system; 99.995% SiO.sub.2 target, 3.00 inch diameter, 0.125 inch thickness, room temperature sputter at 150 W RF power with O.sub.2 and Ar at 2*10.sup.3 mbar, base pressure: 10.sup.6 mbar).

[0089] By slowly tilting the substrate during the deposition process, a gradient with varying thickness is deposited onto the substrate. By selecting the appropriate deposition methods and changing the tilting orientation during the deposition, it is possible to create simple 2D gradient or more complex 3D gradients (compare FIG. 2) with a linear or any other increase of layer thickness.

2. Providing an Array of Nanoparticles on the Substrate Surface

[0090] The surface of the graded sacrificial layer of the composite substrate was coated with gold nanoparticles in a defined arrangement by means of micellar nanolithography. In this step, one of the protocols described in EP 1 027 157 B1, DE 197 47 815 A1 or DE 10 2007 017 032 A1 can be followed.

[0091] The method involves the deposition of a micellar solution of a block copolymer (e.g. polystyrene(n)-b-poly(2-vinylpyridine(m)) in toluene) onto the substrate, e.g. by means of dip or spin coating, as a result of which an ordered film structure of polymer domains is formed on the surface. The micelles in the solution are loaded with a gold salt, preferably HAuCl.sub.4, which, following deposition with the polymer film, can be reduced to the gold nanoparticles.

[0092] The reduction can take place chemically, e.g. with hydrazine, or by means of energy-rich radiation, such as electron radiation or light. Optionally, after or at the same time as the reduction, the polymer film can be removed (e.g. by means of plasma etching with Ar-, H- or O-ions). Thereafter, the substrate surface is covered with an arrangement of gold nanoparticles.

[0093] By using the BCML technology in combination with dip-coating techniques it is possible to create gold-dots with different spacing and diameter on the SiO.sub.2 sacrificial layer (compare FIG. 3). By selecting the dipping-orientation of the substrate, the BCML gradient can either be parallel or in any orientation relative to the SiO.sub.2 gradient on the substrate.

3. First Etching

[0094] Subsequently, the etching of the substrate surface covered with gold nanoparticles took place in a desired depth. A reactive ion etcher from Oxford Plasma, device: PlasmaLab 80 plus was used to this end. Other devices known in the prior art are likewise fundamentally suitable, however. The etching consisted of two treatment steps with various etchants which were carried out several times one after the other.

[0095] The following protocol was used to create conical nanostructures:

Step 1:

[0096] A mixture of Ar/SF.sub.6/O.sub.2 in the ratio 10:40:8 (sccm) was used as etchant (process gas).

Pressure: 50 mTorr
RF power: 120 W
ICP power: 0 W

Time: 60 s

Step 2:

Etchant: Ar/CHF.SUB.3.:40:40

[0097] Pressure: 50 mTorr
RF power: 120 W
ICP power: 20 W

Time: 20 s

[0098] These 2 steps were carried out alternately 8 times.

[0099] Alternatively, the following protocol was used to create pillar-shaped nanostructures:

Step 1:

[0100] A mixture of Ar/SF.sub.6 in the ratio 40:40 (sccm) was used as etchant (process gas).

Pressure: 50 mTorr
RF power: 120 W
ICP power: 0 W

Time: 60 s

Step 2:

Etchant: Ar/CHF.SUB.3.:40:40

[0101] Pressure: 50 mTorr
RF power: 120 W
ICP power: 20 W

Time: 20 s

[0102] These 2 steps were carried out alternately 8 times.

[0103] The total duration of the etching treatment varied depending on the desired depth of the etching within about 1-15 minutes. As a result, column-like or conical nanostructures were obtained, which still can show gold nanoparticles on their upper side.

4. Second Etching

[0104] The nanostructures created on the graded layer as outlined above can further be used as an etching mask for transferring said nanostructures into the primary substrate layer by means of RIE or reactive ion beam etching (RIBE). Compared to the RIE process described above, the RIBE process is less selective and can etch substrates, which cannot be etched using RIE.

[0105] Reactive ion beam etching (RIBE) uses an energetic, broad beam collimated and highly directional ion source to physically mill material from a substrate mounted on a rotating fixture with adjustable tilt angle. In contrast to ion beaming (IBE), in the RISE process reactive ions are incorporated in whole or in part in the etching ion beam.

[0106] The ion sources used are gridded ion sources, e.g. of the Kaufman type or microwave electron cyclotron resonance (ECR). The etching process involves the control of the ion incident angle and a separate control of the ion flux and ion energy. Typical reactive and inert gases used for RIBE are Ar, N.sub.2, O.sub.2, CHF.sub.3 CF.sub.4 and SF.sub.6.

[0107] The RIE or RIBE process directly transferred the nanostructure of the mediating sacrifical layer into the primary substrate.

Example 2

Characterisation of Nanostructured Composite or Primary Substrates

[0108] Suprasil samples with a gradient SiO.sub.2 layer and electroless treated BCML patterns were etched on one side using a RIE process as described above. Wavelength-dependent transmittance was then measured using a spectrometer set-up with a beam size of 1 mm. After the measurements the samples were cleaved to take SEM pictures of the pillar cross sections. FIG. 1A shows the geometry of the pillar structure on one side of the sample (42 nm diameter, 350 nm height, FIG. 1B on the opposite side of the same sample (78 nm diameter, 420 nm height). FIG. 10 shows the corresponding improved overall transmittance (compared to a plain suprasil substrate) and the shift of the transmission maximum.