The invention relates to a method for the production of porous material, in particular of porous aluminium oxide with a periodic arrangement of pores using a stamp, and also a method for the manufacture of the stamp.
In the area of nano-technology the development tends towards increasingly fine structures with increasingly smaller and well defined dimensions which lie in the submicron range. A special area of nano-technology is concerned with regular arrangements of holes or tubes in a substrate with at least substantially identical hole dimensions lying in the submicron range. Such highly ordered two- or three-dimensional structures find applications, for example, in optical components, for example photonic crystals, high-density magnetic storage media, but also in structures which are required for the template synthesis of mono-disperse nano-rods or nano-tubes. These can be used in optical, electronic, chemical or biological fields. Further applications include the field of fine filters.
Customary structuring methods for the manufacture of submicron structures, for example highly ordered pore arrangements, are based on photolithography or ion beam lithography and plasma-chemical structuring.
Another possibility for the manufacture of highly ordered pore arrangements is based on the electro-chemical etching of aluminium. It has been known for a long time that porous aluminium oxide structures with hole diameters in the submicron range arise under certain conditions during the anodization of aluminium. In the year 1995, Hideki Masuda of the Tokyo University, Japan, observed that under certain conditions self-ordering pore structures can be achieved. Typical average pore spacings thereby amount to 50, 65, 110 or 500 nm.
So-called nano-embossing processes, also termed nano-imprint processes or nano-indentation processes, are also known which are used for the generation of submicron structures in polymer films and which are, for example, used in the photolithographic structure transfer to a substrate (U.S. Pat. No. 5,772,905).
Similar nano-imprint processes can also assist an intentional formation of porous aluminium oxide. The U.S. Pat. No. 6,139,713 describes for example the process of direct embossing of recesses into the surface of an aluminium substrate with the aid of a stamp and the subsequent formation of tube-like pores in the aluminium oxide by electrochemical etching of the aluminium substrate, with the pores having a spacing which, in accordance with the statement of the document, is identical to the spacing of the recesses produced by the stamp in the aluminium substrate.
In addition a series of further publications describes the manufacture of ordered porous aluminium oxide structures using the nano-imprint technique (H. Masuda et al., Appl. Phys. Let. 71, 2770 (1997); S. W. Pang et al., J. Vac. Sci. Techn. B16, 1145 (1998); H. Masuda et al., Jap. J. Appl. Phys. 38, L1403 (1999); H. Masuda et al., Jap. J. Appl. Phys. 39, L1039 (2000); H. Asoh et al., J. Vac. Sci. Technol. B19, 569 (2001)). Although hexagonally arranged pore structures in aluminium oxide are described in the majority of the publications other arrangement such as for example square arrangements or graphite lattice arrangements can be produced with the aid of the nano-imprint technique (H. Masuda et al., Adv. Mater. 13, 189 (2001)).
Furthermore, H. Masuda describes that by omitting individual imprints in the surface of the aluminium substrate, i.e. with a conscious generation of lattice defects pores nevertheless grow at the defects. Through this “self-repairing effect” lattice defects in a regular array, for example non-present or failed impressions of the stamp, are self-healed (H. Masuda et al., Appl. Phys. Lett. 78, 826 (2001)).
A method for the generation of a hexagonal nano-imprint in the surface of an aluminium substrate is also known in which a stamp is used having a stamp surface which is provided with elongate strip-like projections extending parallel to one another. The stamp is pressed for the first time onto the surface of the aluminium substrate and then, after a rotation through 60 degrees with respect to the aluminium substrate, is pressed onto the surface a second time. In this manner hexagonally arranged imprints are produced at positions where the linear imprints of the stamp intersect in the surface of the aluminium substrate and serve as the starting points for ordered pore growth in subsequently formed aluminium oxide. Through the double stamp process and the rotation of the stamp a hexagonally arranged pore structure can consequently be produced with a periodicity which is smaller than the periodicity of the linear grid structure on the stamp surface.
The invention is based on the object of providing a simple method for the production of porous material with a periodic pore arrangement with the periodicity being dissimilar to the imprints produced by means of a stamp in a material layer. Furthermore, it is the object of the invention to make available a method for the simple and cost-favourable manufacturing of the stamp.
In order to satisfy the first object a method with the features of claim 1 is provided in accordance with the invention.
This is a method for the generation of porous material with a periodic pore arrangement in which starting points are produced in the surface region of the material layer with the aid of a stamp, the stamp having a stamp surface which is provided at least regionally with periodically arranged projections with an average point spacing (DKeim) and the surface region of the material layer is exposed to an electro-chemical etching solution and an electrical potential in such a way that, in dependence on the arrangement of the starting points and on the selected potential, a self-organizing regular pore structure having an average pore spacing (DPor) forms which is different from the average point spacing (Dkeim).
The method of the invention is particularly well suited to producing a porous aluminium oxide layer on the aluminium substrate in which the regular pore structure, in particular a periodic arrangement of tube-like pores, is formed with a high aspect ratio. In accordance with the invention an average pore spacing can be achieved this which is either larger or smaller than the average starting point spacing.
In particular it is possible in this way to produce pore arrangements with an average pore spacing which is smaller than the average starting point spacing in a simple manner, i.e. smaller than the average spacing of the projections on the stamp surface of the stamp. Thus, finer pore arrangements can be produced than would be possible with a 1:1 transfer of the stamp structure to the material layer, the minimum periodicity of which is restricted by the lithographic process used for the manufacture of the stamp.
An average pore spacing (DPor′) is advantageously set by the potential which is smaller than the average starting point spacing (DKeim), with additional pores disposed between the starting points being formed by a self-organization process.
The potential is preferably set such that a pore forms at each starting point and in addition a pore is formed in each case in the centre of a triangle formed by three adjacent starting points.
The potential is preferably set such that the ratio DPor/DKeim amounts to approximately 0.6. With this potential setting, i.e. this ratio of the average pore spacing DPor to the average starting point spacing DKeim, the self-organizing effect proves to be particularly effective. Above all, it is possible in this manner to produce particularly good “interstitial” pores during the formation of porous aluminium oxide.
In order to satisfy the second object a method with the features of claim 11 is provided in accordance with the invention.
This is a method for the production of a stamp, for example for use in the above-explained method, in which a first three-dimensional structure is produced in a surface region of an auxiliary substrate, thereafter a hard material layer is applied at least to the surface region of the auxiliary substrate having the first structure in such a way that a second structure is formed at the surface of the hard material layer bordering on the auxiliary substrate which is inverse to the first structure, thereafter the surface of the hard material layer pointing away from the auxiliary substrate is connected to a carrier substrate and thereafter the auxiliary substrate is separated from the hard material layer.
The method of the invention enables a manufacture of large area stamps and is moreover fully VLSI-compatible, i.e. can be carried out with customary processes used in semiconductor technology.
By first producing a first three-dimensional structure in an auxiliary substrate and then transferring this as an inverse structure to the hard material layer, which is finally held by the carrier substrate, suitable materials for the auxiliary substrate, for the hard material layer and for the carrier substrate can be selected separately from one another depending on the particular application. Thus, a material can be selected for the auxiliary substrate, independently the hard material layer and the material layer from which the porous material is to be formed, with the material for the auxiliary substrate being optimized solely with respect to the generation of the first three-dimensional structure. For example, a mono-crystalline silicon wafer can be used as the auxiliary substrate in which inverted pyramids can be etched in a simple manner.
In contrast, the hard material layer, which preferably has a non-metallic material, can be adapted directly to the material layer in which the starting points for the periodic pore arrangement of the porous material are to be produced. If, for example, the production of porous aluminium oxide from an aluminium layer is desired, then a hard material layer of Si3N4, SiN, SiC, SiO2 or C with a hardness which is greater than that of the aluminium proves to be particularly advantageous in order to produce imprints in the aluminium layer.
As a result of the strength of the hard material layer the starting points can be produced by a direct impression of the stamp onto the material layer, so that it is possible to dispense with additional method steps in which, for example with the aid of the stamp, first of all a photo resist layer covering the material layer is perforated and the starting points are subsequently formed in the material layer through the holes of the photo resist layer, for example by means of ion beam etching.
In order not to break under a suitable contact pressure, the half material layer is held by a carrier substrate. The carrier substrate can be selected in this connection solely with respect to its stability characteristics in order to achieve a uniform pressure distribution on the hard material layer and thus on the material layer to be imprinted.
The carrier substrate, which is preferably formed of a crystal material, in particular of silicon, is preferably connected by means of adhesive or bonding to the hard material layer.
An intermediate layer, in particular a layer of spin-on-glass, is advantageously arranged between the hard material layer and the carrier substrate. The layer of spin-on-glass can easily be applied to the hard material layer where, as a buffer layer, it evens the surface of the hard material layer facing the carrier substrate and forms a particularly good adhesive undercoat for the connection to the carrier substrate.
A further subject of the invention is a stamp for use or when used in the above explained method for the production of porous material with a periodic pore arrangement and manufactured with the aid of the above explained manufacturing method, the stamp having at least one carrier substrate on which at least one hard material layer, in particular a non-metallic hard material layer, is arranged which has projections, at least regionally, at its surface facing away from the substrate.
The projections are advantageously periodically arranged, with the periodicity preferably lying in the submicron range and in particular in the range of a few 10 nm to a few 100 nm. With such an arrangement of projections, particularly highly ordered regular pore structures can be produced.
The projections are preferably pyramids. Such pyramidic projections can be manufactured in a particularly simple manner in that inverted pyramids are produced as a first three-dimensional structure in the auxiliary substrate during the manufacture of the stamp and can easily be etched for example in a mono-crystalline silicon wafer.
Through the pointed form of the projections the contact pressure of the stamp required to produce the starting points in the material layer can be reduced by a factor of 50 in comparison to customary stamps (H. Masuda et al., Jpn. J. Appl. Phys. 38, L140 (1999); S. Pang et al., J. Vac. Sci. Technol. B16, 1145 (1998)), which leads to reduced requirements for the pressing device and, on the other hand, to reduced danger of breakage, i.e. to an increased working life of the stamp.
In the following the invention will be described purely by way of example and with reference to the accompanying drawing. There are shown:
a-c a schematic representation of different steps of the method of the invention for the production of porous material with a periodic pore arrangement;
a-j a schematic representation of various steps of the method of the invention for the manufacture of a stamp to produce the imprints shown in
a, b raster electron microscope recordings of the stamp surface of a stamp in accordance with the invention in a perspective view (a) and a plan view (b);
First of all, the method of the invention for the production of porous material with a periodic pore arrangement will be explained with reference to
In a first step of the method of the invention which can be seen in
In the embodiment described here the projections 18 are hexagonally arranged, they can however also form a square grid or a graphite grid. Furthermore the projections 18 in the example described here are formed as pointed pyramids, although variants of the invention can also be formed as pyramids with rounded tips, as truncated pyramids, as cylinders, cones, cones with rounded cone tips or made spherical. The hexagonally arranged pyramidic projections 18 have an average spacing which is termed here the starting point spacing DKeim, for a reason which come clear further below and which typically lies in the submicron range, preferably in the range of a few 10 nm to a few 1000 nm. The height of the projections 18 lies in a similar range.
By pressing the stamp 10 against the surface 14 of the aluminium layer 16 the three-dimensional structure of the stamp surface 12 is transferred inversely to the surface 14 of the aluminium layer as shown in
Subsequently the structured surface 14 of the aluminium layer 16 is exposed to an electrochemical etching solution, for example a sulphuric acid, oxalic acid or phosphoric acid solution and a potential U and is anodized, whereby, as can be seen in
During the production of the porous aluminium oxide 22 the recesses 20 in the surface 14 of the aluminium layer 16 act as starting points for the tubular pores 24 which form perpendicular to the surface 14. Insofar as the applied potential U is selected such that the value of the potential corresponds, in accordance with the above named proportionality, directly to the starting point spacing DKeim, i.e. amounts to the average spacing of the recesses 20 divided by 2.5 nm/V the tubular pores 24 arise at every lattice location of the hexagonal lattice, i.e. always there where a recess 20 is located.
In accordance with the invention provision is however made to achieve a pore spacing DPor which is not the same as the starting point spacing DKeim but rather larger or smaller than the starting spacing DKeim. This is achieved in that a potential U is selected which does not correspond to DKeim/x nm/V with x=2.5 but rather to a value x different from 2.5 and with the self-organization characteristic of the pore grid being exploited.
If the voltage U is set such that x=5, then, as shown in
The self-organization effect of the periodic pore grid can be particularly well exploited when the potential is selected such that the equation
applies for the ratio of the pore spacing to the starting point spacing. In this case, an additional interstitial pore 26 is produced in each case at the centre of a triangle formed by three adjacent pores 24 which have grown at the starting points 20.
By exploiting the self-organization effect it is, however, not only possible to produce pore spacings DPor which are smaller than the average starting point spacing DKeim but rather also pore spacings DPor which are larger than the average starting point spacing DKeim. In this case a potential must be set which is correspondingly larger than the potential which would lead to the pore spacing DPor=DKeim. Excess starting points, i.e. recesses 20 of which too many have been produced are reduced or healed by the self-organization of the pore arrangement. A particularly suitable ratio of average pore spacing to average starting point spacing is in this respect DPor/DKeim=1.66.
In the determination of the average pore spacings by the setting of a suitable anodization potential approximately the following applies, that the potential U at DPor=DKeim to the potential U at DPor≠DKeim is the same as the ratio of the average pore spacing at DPor=DKeim to the average pore spacing at DPor≠DKeim, i.e.
U(DPor=DKeim)/U(DPor≠DKeim)=DPor(DPor32 DKeim)/DPor(DPor≠DKeim),
wherein a deviation from this ratio up to ±8% is possible.
In
As can be seen from
The result is thus a hexagonal arrangement of tubular pores 24, 26, the periodicity of which is considerably smaller than that of the structure transferred by the stamp 10. The average pore spacing DPor amounts to approximately three fifths of the average point spacing DKeim (DPor≈0.6 DKeim).
Through the removal of a surface near region of the porous aluminium oxide layer 22 and the aluminium layer 16 an aluminium oxide layer 22 can be produced which has a highly ordered periodically pore structure which includes at least substantially identical tubes 24, 26.
In
After a cleaning treatment, the Si auxiliary substrate 28 is first provided with a silicon dioxide layer 30, for example by means of a thermal oxidation process (
Thereafter inverted pyramids 34 are anisotropically etched into the Si auxiliary substrate 28, for example in KOH through the open photo resist layer 32 and the oxide layer 30. In this connection the use of the (100)-silicon substrate 28 proves to be particularly advantageous because a preferential etching along the specific crystal directions can be achieved by a suitable concentration of the KOH solution and it can thus be anisotropically etched. In this manner very regular inverted pyramidic structures can be produced (
After the formation of the inverted pyramids 34 the photo resist layer 32 is dissolved in acetone and the SiO2-layer 30 is removed from the Si auxiliary substrate 28 in hydrofluoric acid and the silicon wafer 28 is cleaned (
Thereafter a non-metallic hard material layer 36 can be applied on the structured surface of the silicon wafer 28 (
A layer of spin-on-glass (SOG) is applied (
Thereafter the Si auxiliary substrate 28 is removed from the hard material layer 36. This can take place both in a mechanical manner, for example by grinding and/or polishing, by wet chemical etching, by plasma assisted etching or by any desired combination of these removal methods (
The result is a stamp 10 in accordance with the invention consisting of a carrier substrate 40, an SOG layer 38 and a hard material layer 36 provided with a periodical array of pyramidic projections 18 which has been manufactured exclusively by VLSI-compatible method steps. As a result of the hard material layer 36 formed from Si3N4 the three-dimensional structure of the stamp 10 can be non-destructively transferred onto a comparatively soft aluminium layer 16 so that the stamp 10 can be multiply used, i.e. can be used for numerous stamp processes.
In the following a special embodiment of a stamp 10 in accordance with the invention and also of the periodic pore arrangement produced with it in porous aluminium oxide will be described. As explained above, a (100)-oriented 4″ silicon wafer is structured with a two-dimensional hexagonal grid with a grid constant of 500 nm and a hole diameter of 300 nm by means of low UV lithography (248 nm).
In accordance with the above described process, hexagonally arranged pyramidic projections with a height of 260 nm and a lattice constant of 500 nm are produced in a 300-500 nm thick Si3N4 layer by means of the inverted pyramids produced in the silicon substrate.
For the preparation for the formation of porous aluminium oxide layer the surface of an aluminium layer is mechanically polished in order to achieve a particularly smooth surface with a roughness of Rq<100 nm prior to the stamp process.
Using a pressure of 5 kN/cm2, the hexagonal structure of the stamp is then transferred onto the surface of the aluminium layer, with the rectangular recesses being produced with a depth of proximately 40 nm in the surface of the aluminium layer, which serve as starting points for the formation of the aluminium oxide. Through the use of a stamp surface with pyramidic projections it is possible to select a stamp which is approximately 50 times smaller than is the case with similar known stamp processes (H. Masuda et al., Jpn. J. Appl. Phys. 38, L140 (1999); S. Pang et al., J. Vac. Sci. Technol. B16, 1145 (1998)).
When using a stamp surface with 260 nm high pyramidic projections it is basically possible to produce approximately 260 nm deep imprints in the aluminium layer. In this connection, in the case of pyramidic projections, not only does the depth of the grid imprint increase on increasing the stamp pressure but rather also the lateral dimension of the recesses increases.
The imprinted surface of the aluminium layer is subsequently anodized in oxalic or phosphoric acid. In order to achieve a match of the pore spacing and the starting point spacing the proportionality dependency DPor=2.5 nm/V·U must be fixed at a grid constant of 500 nm at the voltage U of approximately 200 Volts in order to achieve an average pore spacing 500 nm. However, in order to achieve an average pore spacing of 250 nm, the anodization is carried at 100 V (see
With an anodization potential U of 120 V one succeeds, starting from the starting point grid with the grid constant of 500 nm in 1.7% phosphoric acid, in producing a perfect, highly ordered pore structure with an average pore spacing of 300 nm. From a depth of 3 μm measured from the surface of the aluminium oxide all the pores have the same diameter of 85 nm over a pore length of 80 μm. The pores disposed at the grid sides, i.e. at the starting points, cannot be distinguished from interstitial pores which are each disposed between 3 adjacent “regular” pores.
Consequently it is possible, through the self-organization effect, to produce highly ordered pore structures the grid constants of which are smaller than those of the structure on the stamp surface. This, for example, enables the production of pore structures with a grid constant of 100 nm by means of a stamp with a grid constant of 180 nm or indeed of 40 nm pore structures by means of a stamp with a 60 nm grid constant and indeed by pressing just once with the stamp.
Moreover, it has been found that the cross-section of the tubular pores of the surface of the aluminium oxides depends on the structure shape at the stamp surface and on the stamp pressure. A rectangular cross-section of the pores at the surface of the aluminium oxide can be achieved when the structure transfer takes place by rectangular projections and with a high stamp pressure, whereas one obtains circular pore cross-sections at low stamp pressure.
In contrast thereto the pore cross-section at the base of the pores, i.e. at the boundary surface between the aluminium oxide and the aluminium layer, is principally influenced by the current flow and the electrolyte and not by the shape of the impression in the surface of the aluminium layer. If the rectangular structure is consequently transferred at a high stamp pressure, then one can observe a change of the pore cross-section in the longitudinal direction of the pores starting from a rectangular cross-section at the surface of the aluminium oxide and up to a round cross-section at the base of the tubular pores.
The raster electron microscope recordings of the stamp surface 12 of the stamp 10 with the projections 18 (
Number | Date | Country | Kind |
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102 07 952.8 | Feb 2002 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/01138 | 2/5/2003 | WO | 5/23/2005 |