Patent Publication Number: US-2018050959-A1

Title: Fabrication of nanostructures in and on organic and inorganic substrates using mediating layers

Description:
Several methods for generating nanostructures on various substrate surfaces are known in the art. Such nanostructures may be, e.g., used for immobilizing target entities such as biomolecules or for providing antireflective coatings on the respective substrates. 
     One approach for generating nanostructures or nanopatterns on substrate surfaces is based on electron beam lithography. Typically, however, this methodology is rather slow and costly. 
     Another commonly used approach involves various etching techniques, including reactive ion etching (RIE), reactive ion beam etching (RIBE), plasma processes etc. 
     In particular, an especially advantageous method for providing antireflective structures, so called moth-eye structures (MOES), on quartz glass by means of a combination of block copolymer micellar nanolithography (BCML) and RIE has been developed (see, e.g. WO 2008/116616 A1). 
     However, these methods are often limited with respect to suitable substrate materials and applications. For example, a number of highly refractive substrates such as SF10 glass, CaF 2 , Al 2 O 3 , are difficult or impossible to etch using said combination of BCML and RIE because the BCML mask and the substrates have similar etching rates and/or the etching parameters of the RIE treatment are unsuited for the desired substrate material. 
     In order to provide non-etchable primary substrates with such nanostructures, it is known to coat the primary substrate with an additional etchable material (such as silica) and to generate the desired nanostructures therein. Typically, however, the refractive index of these etchable layers differs from that of the refractive index of the primary substrate. This results in a considerable decrease of the antireflective properties of the nanostructured substrate. Further, the additional etchable layers tend to be mechanically instable and separate from the primary substrate under some conditions (mechanical or thermal stress). 
     It was therefore an object of the present invention to provide nanostructures, in particular antireflective nanostructures, in and on a variety of organic and inorganic substrates, including substrate surfaces which were not amenable to conventional etching techniques, and the corresponding nanostructured substrates which overcome the above drawbacks of the prior art. 
     This object is achieved according to the invention with the provision of the method according to Claim  1  and the composite substrate according to Claim  14 . Specific or preferred embodiments and aspects of the invention are the subject matter of the further claims. 
     DESCRIPTION OF THE INVENTION 
     The method according to the invention for creating nanostructures in and on organic or inorganic substrates according to Claim  1  comprises at least the following steps:
     a)providing a primary substrate having a predetermined refractive index;   b) coating the primary substrate with one or more (preferably a plurality of) mediating layers each having a predetermined refractive index different from that of the primary substrate, wherein the sequence of the layers is arranged so that a predetermined gradient of the refractive index is generated between the primary substrate and the uppermost layer of the one or more mediating layers;   c) optionally coating the uppermost layer of the one or more mediating layers with an additional top layer;   d) depositing a nanostructured etching mask onto the uppermost layer of the composite substrate obtained after steps a)-b) or a)-c);   e) generating protruding structures, in particular conical or pillar structures, or recessed structures, in particular holes, in at least the uppermost layer of the composite substrate by means of reactive ion etching.   

     In a specific embodiment of the method according to the present invention, the nanostructured etching mask comprises an ordered array of nanoparticles or statistically distributed nanoparticles in which the spatial frequencies of the statistical distribution shows only contributions which are larger than the inverse of the wavelength of light (typically from 30 nm to 300 nm). 
     In a preferred embodiment of the invention, an ordered array of nanoparticles is provided on the substrate surface by means of a micellar diblock copolymer nanolithography technology, as described e.g. in EP 1 027 157 B1 and DE 197 47 815 A1. In micellar nanolithography, a micellar solution of a block copolymer is deposited onto a substrate, e.g. by means of dip coating, and under suitable conditions forms an ordered film structure of chemically different polymer domains on the surface, which inter alia depends on the type, molecular weight and concentration of the block copolymer. The micelles in the solution can be loaded with inorganic salts which, following deposition with the polymer film, can be oxidized or reduced to inorganic nanoparticles. A further development of this technology, described in the patent application DE 10 2007 017 032 A1, enables to two-dimensionally set both the lateral separation length of the polymer domains mentioned and thus also of the resulting nanoparticles and the size of these nanoparticles by means of various measures so precisely that nanostructured surfaces with desired spacing and/or size gradients can be manufactured. Typically, nanoparticle arrangements manufactured with such a micellar nanolithography technology have a quasi-hexagonal pattern. 
     The BCML etching mask may be deposited on the primary substrate by any suitable method known in the art such as, e.g., dip coating or spin coating. 
     Principally, the material of the nanoparticles is not particularly limited and can comprise any material known in the prior art for such nanoparticles. Typically, this is a metal or metal oxide. A broad spectrum of suitable materials is mentioned in DE 10 2007 014 538 A1. Preferably, the material of the metal or the metal component of the nanoparticles is selected from the group made up of Au, Pt, Pd, Ag, In, Fe, Zr, Al, Co, Ni, Ga, Sn, Zn, Ti, Si and Ge, mixtures and composites thereof. Specific examples for a preferred metal oxide are titanium oxide, iron oxide and cobalt oxide. Preferred examples for a metal are gold, palladium and platinum and gold is particularly preferred. 
     The term “particle” as used here also comprises a “cluster”, particularly as described and defined in DE 10 2007 014 538 A1 and DE 197 47 815 A1 and both terms can be used here interchangeably. 
     Advantageously, the material of the primary substrate is not especially limited and may be selected from a wide range of organic and inorganic substrates. 
     In one specific embodiment, the material of the primary substrate is selected from glasses, in particular comprising one of the following base systems of inorganic glasses with their main components: 1) B 2 O 3 —La 2 O 3 —M m O n  (m being an integer from 1 to 2 and n being an integer from 2 to 5; M m O n  preferably selected from ZrO 2 , Ta 2 O 5 , Nb 2 O 5 , Gd 2 O 3 , Y 2 O 3 , TiO 2 , WO 3 ) ; 2) (B 2 O 3 , SiO 2 )—La 2 O 3 —MO, MO being a metal oxide typically selected from MgO, CaO, SrO, BaO, ZnO; 3) SiO 2 —PbO—M 2 O with (for example) M 2 O selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O; the PbO content in glasses of the system SiO 2 —PbO—M 2 O can be replaced partially or completely by TiO 2 ; 4) SiO 2 —B 2 O 3 —BaO; 5) (SiO 2 , B 2 O 3 )—BaO—PbO; 6) SiO 2 —M 2 O—TiO 2  (preferably the glass lattice/matrix comprises additional molecules, atoms, or ions of fluorine (e.g. F 2 ) and/or oxygen) with M 2 O being a metal oxide typically selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O; 7) P 2 O 5 —Al 2 O 3 —MO—B 2 O 3  with (for example) MO selected from: MgO, CaO, SrO, BaO, ZnO; 8) SiO 2 —BaO—M 2 O with (for example) M 2 O selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O. 
     In more specific embodiments, the material of the primary substrate comprises or consists of glasses and quartz glasses, in particular SF10, N-LASF 9 (Schott, refractive index 1.85), N-LASF 45 Schott, refractive index 1.80), N-SF6 (Schott, refractive index 1.81, N-SF57 (Schott, refractive index 1.85), S-LAM 66 (Ohara, refractive index 1.80), S-TIH14 (Ohara, refractive index 1.76), S-BAH28 (Ohara, refractive index 1.72), NBF1 (Hoya, refractive index 1.74), NBFD3 (Hoya, refractive index 1.81), or E-FD8 (Hoya, refractive index 1.69) glass, SiO 2 , CaF 2 , GaAs, Al 2 O 3 . 
     In preferred embodiments, the material of the primary substrate is selected from quartz glasses, in particular high quality quarz glasses, such as suprasil glass. 
     In another specific embodiment, the material of the primary substrate is selected from organic materials such as polymethylmethacrylate (PMMA), polycarbonate (PC), polycarbonate-comprising copolymers (e.g. PC-HT), styrene-methylmethacrylate-copolymer (SMMA), methacryl-acrylnitrile-butadien-styrene-copolymer (MABS), polystyrene (PS), styrene-acrylnitrile-copolymer (SAN), polymethacrylmethylimide (PMMI), cycloolefin-based polymers (COP), cycloolefin-based copolymers (COC), polyethersulfones (PES), polyetherimides (PEI), polmethylenepentene (TPX), polyamide 12 (PA 12), allyldiglycol-carbonate. 
     The material of each of the one or more mediating layers, preferably of a plurality of mediating layers, is selected such that a desired gradient of the refractive index is generated between the primary substrate and the uppermost layer of the intermediate layers. 
     In a specific embodiment of the invention, the material of the one or more mediating layers is selected from the group comprising glass and quartz glass, in particular SiOx (with 1&lt;x&lt;2) and SiOxNy (with 1&lt;x&lt;2 and y/x+y in the range from 0 to 0.5 and with N/(N+O) from 0% to 50%). 
     The thickness of the one or more mediating layer(s) is not especially limited and may be adjusted as appropriate for the respective substrate and coating materials and applications. 
     For optical applications, such as antireflective lenses, mediating layers having a total thickness in the range from 200 to 500 nm may be typically used. 
     The mediating layers typically are deposited by reactive pulse sputtering using a magnetron system from a silicon target. Argon can be used as inert gas and a mixture of oxygen and nitrogen as reactive gas. The mediating layers are formed by continuously adapting the ratio of the mixture of oxygen and nitrogen. The deposition of the layers typically starts from the refractive index of the substrate glass and is decreased down to that of the top layer, typically SiO 2 . 
     Typically, the nanostructures created by one or more etching treatments are conical or pillar-shaped structures. The height of the structures typically lies in a range of 50 nm to 400 nm, preferably of 150 nm to 300, and typically they have a diameter in the range of 5 nm to 50 nm, preferably of 10 nm to 30 nm, (measured in half height of the structures). 
     In a typical embodiment, the refractive index of the primary substrate is in the range from 1.46 to 2.01, more typically in the range from 1.6 to 1.9, and the refractive index of the uppermost layer of the composite substrate obtained after steps a)-b) or a)-c) above is in the range from 1.3 to 1.6., typically in the range of SiO 2 . 
     The material of the optional additional top layer may be identical with or different from the material of the mediating layer(s). In the first case, the material of the additional top layer may be selected from the group comprising quartz glass, such as SiO 2 and SiOxNy (with x and y as defined above), with N/(N+O) from 0% to 50%. 
     In any case, the additional top layer does not contribute to the gradient of the refractive index generated by the layers below, thus its refractive index will ideally be identical with the uppermost layer of the intermediate layers. 
     The thickness of the top layer may in the range from 50 to 1200 nm. 
     Suitable methods for plasma etching or reactive ion etching are principally known in the art (see, e.g. DE 10 2007 014 538 A1 and Lohmüller et al. (NANO LETTERS 2008, Vol. 8, No. 5, 1429-1433). 
     The etching step e) can comprise one or several treatments with the same etching agent and/or with different etching agents. The etchant can basically be any etchant known in the prior art and suitable for the respective substrate surface. Preferably, the etchant is selected from the group of chlorine gases, e.g. Cl 2 , BCl 3  and other gaseous chlorine compounds, fluorinated hydrocarbons, e.g. CHF 3 , CH 2 F 2 , CH 3 F, fluorocarbons, e.g. CF 4 , C 2 F 8 , oxygen, argon, SF 6  and mixtures thereof. 
     Preferably, the etching comprises at least one treatment with a mixture of Ar/SF 6 /O 2  or Ar/SF 6  as etchant and at least one treatment with a mixture of Ar/CHF 3  as etchant. 
     For example, a combination of a first etching step with a mixture of Ar/SF 6  as etchant and a second etching step with a mixture of Ar/SF 6 /O 2  may be used to produce pillar-shaped nanostructures from a SiOx substrate. A combination of a first etching step with a mixture of Ar/SF 6 /O 2  and a second etching step with a mixture of Ar/CHF 3  may be used to produce conical nanostructures from a SiOx substrate. 
     Typically, each etching step is carried out for a period in the range of 5 s or 10 s to 10 min, preferably in the range from 10 s to 60 s. 
     The duration of the entire etching treatment typically lies in the range of 10 s to 60 minutes, preferably 1 to 15 minutes. 
     Typically, the obtained nanostructures have a diameter in the range of 10-100 nm, preferably 10-30 nm, and a height of 10-800 nm, preferably 250-500 nm. In the case of conical structures, the diameter data refer to the thickness at half height. The average spacings of the nanostructures are preferably in a range from 15 to 250 nm. 
     For some applications it is preferred that the nanoparticles used as an etching mask have a predetermined two-dimensional geometric arrangement on the substrate surface. Such arrangement has predetermined minimum or average particle spacings as a characteristic, wherein these predetermined particle spacings can be the same in all regions of the substrate surface or various regions can have different predetermined particle spacings. A geometric arrangement of this type can fundamentally be realized with any suitable method of the prior art, micellar nanolithography in particular, as explained in more detail above. 
     Some embodiments of the method according to the present invention involve at least one further processing step of a mechanical treatment, such as sonication, of the protruding structures generated in the course of the etching step. 
     In a specific embodiment of the present invention, the structures generated in the top layer and/or the mediating layer(s) of the composite substrate are used as an etching mask and protruding structures corresponding to the protruding structures of the layer (s) above are generated in the primary substrate and the layer (s) above the primary substrate are removed in part or completely. 
     This further etching treatment may be accomplished by means of reactive ion beam etching (RIBE), chemically assisted ion beam etching (CAIBE), reactive ion etching (RIE) or inductive coupled plasma (RIE-ICP) as appropriate for the respective substrate layer. 
     Using the above multi-step-etching process enables to achieve nanostructures on or in the primary substrate which were not obtainable by a direct etching treatment of the primary substrate. 
     A nanostructured primary substrate which is free from any coating layers (mediating layers and optionally a top layer) is mechanically more stable, since no coating layers are potentially detachable under conditions of mechanical or thermal stress. Also, if the coating layers are completely removed in the final etching step, it is possible to use toxic or nonbiocompatible intermediate layers and still obtain a biocompatible final product. This allows more flexibility in conducting the method of the invention. 
     In a preferred embodiment of the invention, the composite substrate obtained by the method as outlined above is an optical element and the structures generated form an anti-reflective surface structure on the optical element. 
     A further aspect of the present invention relates to the nanostructured composite substrate obtainable by the method as outlined above. 
     Typically, the composite substrate with a nanostructured surface according to the present invention comprises a primary substrate having a defined refractive index, preferably in the range from 1.3 to 2.1; one or more (preferably a plurality of) mediating layers having a predetermined refractive index different from that of the primary substrate wherein the sequence of the layers is arranged so that a defined gradient of the refractive index is provided between the primary substrate and the uppermost layer of the one or more mediating layer; optionally an additional top layer; and nanostructures, in particular conical or pillar structures, on the surface of the composite substrate, which structures are composed of the material of the additional top layer of the composite substrate (optionally capped with nanoparticles) and/or the material of the one or more mediating layers. 
     In a more specific embodiment of said composite substrate, the protruding structures further comprise of material of the primary substrate. 
     Preferably, the nanostructured composite substrate or primary substrate is an optical element and the protruding structures form an anti-reflective surface structure on the optical element. 
     In a more specific embodiment of said composite substrate or optical element, the protruding structures have a predetermined two-dimensional geometric arrangement, in particular a hexagonal arrangement, or are statistically distributed such that the spatial frequencies of the statistical distribution shows only contributions which are larger than the inverse of the wavelength of light (typically in a range from 30 nm to 300 nm). 
     In one specific embodiment, the material of the primary substrate is selected from glasses, in particular comprising one of the following base systems of inorganic glasses with their main components: 1) B 2 O 3 —La 2 O 3 —M m O n  (m being an integer from 1 to 2 and n being an integer from 2 to 5; M m O n  preferably selected from ZrO 2 , Ta 2 O 5 , Nb 2 O 5 , Gd 2 O 3 , Y 2 O 3 , TiO 2 , WO 3 ) ; 2) (B 2 O 3 , SiO 2 )—La 2 O 3 —MO, MO being a metal oxide typically selected from MgO, CaO, SrO, BaO, ZnO; 3) SiO 2 —PbO—M 2 O with (for example) M 2 O selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O; the PbO content in glasses of the system SiO 2 —PbO—M 2 O can be replaced partially or completely by TiO 2 ; 4) SiO 2 —B 2 O 3 —BaO; 5) (SiO 2 , B 2 O 3 )—BaO—PbO; 6) SiO 2 —M 2 O—TiO 2  (preferably the glass lattice/matrix comprises or is doped with additional molecules, atoms, or ions of fluorine and/or oxygen) with M 2 O being a metal oxide typically selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O; 7) P 2 O 5 —Al 2 O 3 —MO—B 2 O 3  with (for example) MO selected from: MgO, CaO, SrO, BaO, ZnO; 8) SiO 2 —BaO—M 2 O with (for example) M 2 O selected from Li 2 O, Na 2 O, K 2 O, Ca 2 O. 
     In more specific embodiments, the material of the primary substrate comprises or consists of glasses and quartz glasses, in particular SF10, N-LASF 9 (Schott, refractive index 1.85), N-LASF 45 Schott, refractive index 1.80), N-SF6 (Schott, refractive index 1.81, N-SF57 (Schott, refractive index 1.85), S-LAM 66 (Ohara, refractive index 1.80), S-TIH14 (Ohara, refractive index 1.76), S-BAH28 (Ohara, refractive index 1.72), NBF1 (Hoya, refractive index 1.74), NBFD3 (Hoya, refractive index 1.81), or E-FD8 (Hoya, refractive index 1.69) glass, SiO 2 , CaF 2 , GaAs, Al 2 O 3 . 
     In preferred embodiments, the material of the primary substrate is selected from quartz glasses, in particular high quality quarz glasses, such as suprasil glass. 
     In another specific embodiment, the material of the primary substrate is selected from organic materials such as polymethylmethacrylate (PMMA), polycarbonate (PC), polycarbonate-comprising copolymers (e.g. PC-HT), styrene-methylmethacrylate-copolymer (SMMA), methacryl-acrylnitrile-butadien-styrene-copolymer (MABS), polystyrene (PS), styrene-acrylnitrile-copolymer (SAN), polymethacrylmethylimide (PMMI), cycloolefin-based polymers (COP), cycloolefin-based copolymers (COC), polyethersulfones (PES), polyetherimides (PEI), polmethylenepentene (TPX), polyamide 12 (PA 12), allyldiglycol-carbonate. 
     Preferably, the material of the one or more mediating layers of the composite material is selected from the group comprising glass, in particular SiOx and SiOxNy, SiOx (with 1&lt;x&lt;2) and SiOxNy (with y/x+y in the range from 0 to 0.5 and N/(N+O) from 0% to 50%). 
     Also, the material of the additional top layer, if any, is preferably selected from the group comprising quartz glass, such as SiO 2  and SiOxNy, SiOx (with 1&lt;x&lt;2) and SiOxNy, with x and y as defined above and N/(N+O) from 0% to 50%. 
     The products of the method according to the invention offer a wide range of application options in the fields of semiconductor technology, optics, sensor technology and photovoltaics. 
     A few non-limiting examples for this are the use in optical devices, particularly optical elements such as lenses, diffraction gratings and other refracting or diffractive structures, sensors, particularly CCD sensors and solar cells. 
     A particularly preferred application relates to the use in optical elements, particularly for minimizing reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  schematically shows the main steps of the method according to the invention for preparing a nanostructured composite substrate or primary substrate. 
         FIG. 2  schematically shows the gradient of the refractive index formed by the primary substrate and the overlaid mediating layers. 
         FIG. 3  shows the data of transmission measurements performed on a plain SF10 substrate, a GRIN-coated SF10 substrate and a SF10 substrate coated with GRIN and an additional layer of MOES. The optical characterisation of plain, GRIN and MOES+GRIN substrates shows that the transmission of MOES+GRIN structured substrates is superior compared to the transmission of GRIN-structured surfaces alone. 
     
    
    
     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 Nanostructures on a Composite Substrate 
     1. Providing a Composite Substrate 
     A primary substrate of SF10 glass is coated with several intermediate layers of amorphous Si x O y N z  forming a gradient of refractive indices (GRIN). The indices x, y and z of each intermediate layer are selected to provide a desired refractive index difference to the underlying layer. The GRIN layers are deposited by reactive pulse sputtering using a magnetron system from a silicon target. Argon was used as inert gas and a mixture of oxygen and nitrogen as reactive gas. 
     The layered structure of the GRIN-layer is formed by continuously adapting the ratio of the mixture of oxygen and nitrogen. The deposition of the GRIN profile is starting from the refractive index of the base substrate (SF10) and is decreased down to that of the covering SiO 2  layer, forming the composite substrate consisting of SF10 (base), GRIN layer and a top SiO 2  layer. 
     2. Providing an Array of Nanoparticles on the Substrate Surface 
     The surface of the uppermost 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. 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 4 , which, following deposition with the polymer film, can be reduced to the gold nanoparticles. 
     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. 
     3. First Etching 
     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. 
     The following protocol was used to create conical nanostructures: 
     Step 1; 
     A mixture of Ar/SF 6 /O 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 3 :40:40 
         Pressure: 50 mTorr 
         RF power: 120 W 
         ICP power: 20 W 
         Time: 20 s 
       
    
     These 2 steps were carried out alternately 8 times. 
     Alternatively, the following protocol was used to create pillar-shaped nanostructures: 
     Step 1: 
     
         
         A mixture of Ar/SF 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 3 :40:40 
         Pressure: 50 mTorr 
         RF power: 120 W 
         ICP power: 20 W 
         Time: 20 s 
       
    
     These 2 steps were carried out alternately 8 times. 
     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 
     The nanostructures created in the mediating layers according to step 3 above can further be used as an etching mask for transferring said nanostructures into the primary substrate layer by means of reactive ion beam etching (RIBE). Compared to the previous RIE process, the RIBE process is less selective and can etch substrates, which cannot be etched using RIE. 
     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 RIBE process reactive ions are incorporated in whole or in part in the etching ion beam. 
     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 2 , O 2 , CHF 3  CF 4  and SF 6 . 
     The RIBE process directly transferred the nanostructure of the mediating layer into the base substrate. 
     EXAMPLE 2 
     Characterization of Nanostructured Composite or Primary Substrates 
     To illustrate the superior optical properties of the MOES+GRIN substrates, a plain SF10 surface, a GRIN coated SF10 surface and a single-sided MOES+GRIN coated surface were optically characterized using a spectrometer. 
     Compared to the plain SF10 substrate, the MOES+GRIN substrate shows a notably improved transmission, which covers a wide rage of wavelengths, as typical for MOES structures (see  FIG. 3 ).