WO2008138361A1

WO2008138361A1 - Mold for generating nanostructures, and mold holder unit

Info

Publication number: WO2008138361A1
Application number: PCT/EP2007/004109
Authority: WO
Inventors: Stefan Harrer; Sebastian Strobel; Sebastian Luber; Giuseppe Scarpa; Francesca Brunetti; Paolo Lugli; Marc Tornow; Gerhard Abstreiter
Original assignee: Technische Universität München
Priority date: 2007-05-09
Filing date: 2007-05-09
Publication date: 2008-11-20

Abstract

In a method for fabricating a mold (228, 230) for generating nanostructures a sequence (200) of layers comprising a first layer (202) and a second layer (204) is provided, a third layer (208) is provided on a first surface (206) formed by the first layer (202) and the second layer (204), wherein the material of the first layer (202) and the material of the second and third layers (204, 206) are selectively processable with respect to each other; and a second surface (210) formed by the first layer, the second layer (204) and the third layer (208) is processed to remove a portion of the material of the first layer (202) or a portion of the material of the second and third layers (204, 208), or to add material to the first layer (202) or to the second and third layers (204, 208) to generate a 3-D surface profile.

Description

MOLD FOR GENERATING NANOSTRUCTURES , AND MOLD HOLDER UNIT

Background of the Invention

Embodiments of the invention concern a mold or stamp for generating nanostructures, a method for fabricating the mold, a mold holder unit, a system for transferring a nanostructure to a substrate using the mold holder unit, a method for setting a position of a nanostructure mold held in a mold holder unit, and a method for transferring a nanostructure to a substrate

The constantly decreasing feature sizes of integrated circuit devices limit the applicability of optical litho- graphical to define patterns. To overcome the deficiencies of optical lithography other patterning techniques are used such as nanoimprinting or nanotransfer printing.

Summary of the Invention

Embodiments of the invention concern a method for fabricat- ing a mold for generating nanostructures, comprising providing a sequence of layers comprising a first layer and a second layer; providing a third layer on a first surface formed by the first layer and the second layer, wherein the material of the first layer and the material of the second and third layers are selectively processable with respect to each other; and processing a second surface formed by the first layer, the second layer and the third layer to remove a portion of the material of the first layer or a portion of the material of the second and/or third layers, or to add material to the first layer or to the second and/or third layers to generate a 3-D surface profile.

Further embodiments of the invention concern a mold for generating nanostructures, the mold comprising a sequence of layers comprising a first layer and a second layer, the second layer having a thickness in the nanometer region; and a third layer on a first surface formed by the first layer and the second layer, the third layer having a thickness in the nanometer region; wherein on a second surface formed by the first layer, the second layer and the third layer the first layer protrudes from the second and third layers or the second and third layers protrude from the first layer to define a 3-D surface profile.

Yet further embodiments of the invention concern a mold holder unit for holding a nanostructure mold, comprising a support; a mold receptacle for receiving a nanostructure mold, the mold receptacle supported by the support in a partial cardanic manner; and a fastening device securing the mold receptacle at a predetermined position with respect to the support when the fastening device is tightened, and allowing for the partial cardanic movement of the mold receptacle when the fastening device is not tightened.

Description of the Drawings

Embodiments of the invention will be described with reference to the attached drawings, in which

Fig. l(a) to (g) show an example of the thermal nanoim- print lithography;

Fig. 2 (a) to (d) show a mold fabrication scheme according to embodiments of the invention; Fig. 3 (a) to (g) show a further embodiment of a CEO-mold fabrication scheme;

Fig. 4 (a) to (g) show yet a further embodiment of a CEO- mold fabrication scheme;

Fig. 5 (a) to (e) show the principles of CEO-NIL;

Fig. 6 (a) to (d) show the principles of CEO-nTP;

Fig. 7 (a) to (b) show AFM cross-sections and top-views of 3-D surface profiles of both a 2D- lateral CEO-mold as well as a ID- lateral MBE-mold;

Fig. 8 shows an imprint result of a room- temperature nanoimprint step performed using the mold shown in Fig. 7b;

Fig.9 shows the lattice constant transitions for semiconductor compound materials;

Fig. 10 shows an apparatus or system for transferring a nanostructure to a substrate;

Fig. 11 shows an embodiment of a mold holder unit;

Fig. 12 illustrates the functionality of the mold holder unit; and

Fig. 13 shows a schematic view of the holder unit as shown in Fig. 11 with the mold attached Description of the Embodiments

The nanoimprint lithography addresses the task of imprint- ing features in the sub-10-nm region. Applications for the nanoimprint lithography (NIL) capable of imprinting features in the sub-10-nm region may be printing nanowires connecting molecules or metal pads that act as electrodes in molecular electronics, and imprinting magnetically ac- tive polymer layers in order to fabricate storage elements and quantum cellular automata.

The nanoimprint lithography transfers a pattern from a template into a substrate. The nanoimprint lithography uses a template or mold to stamp a pattern into a rubbery polymer layer on a substrate. Before removing the mold, the patterned polymer may be solidified by cooling. The nanoimprint lithography may also be performed using UV light to cure a liquid polymer precursor or without any cooling steps at all (room-temperature nanoimprint lithography) . After removing the template, several etching steps allow for transferring the printed pattern into the substrate.

Fig. 1 illustrate an example of the thermal nanoimprint Ii- thography. One thermal imprint cycle may comprise the following steps. In a first step shown in Fig. l(a) a layer of an imprint polymer 100 may be spin-cast onto the substrate 102 and raised to a temperature above its glass transition temperature T₉ resulting in a rubbery and deformable poly- mer. A stamp or a mold 104 comprising the structure 106 of the final desired pattern is provided.

In a second step shown in Fig. l(b) the imprint is performed by pressing the stamp 104 into the polymer layer 100 and deforming the polymer layer 100.

Before the stamp 104 is removed the imprint polymer layer 100 is cured in a third step shown in Fig. 1 (c) by cooling. The imprint polymer layer 100 is cooled down to a temperature below its glass transition temperature T_g.

In a fourth step shown in Fig. l(d) the stamp 104 is re- moved from the imprint polymer layer 100 leaving the patterned surface 108 comprising a plurality of protrusions 110 and grooves 112.

In a fifth step shown in Fig. l(e) a short etch is per- formed to remove the solid polymer 100 from the bottom of the patterned grooves 112 thereby exposing the surface of the substrate 102 in the grooves 112.

In a sixth step shown in Fig. l(f) the pattern 108 is transferred into the substrate 102 and in a seventh step shown in Fig. 1 (g) the residual imprint polymer 102 is stripped off the surface of the substrate 102.

Variants of the above described technique are known, e.g. dip-pen lithography (DPL) , microcontact and nanotransfer printing, step-and-flash UV nanoimprint, nanoindent lithography and room-temperature nanoimprint lithography. When using imprint polymers that can be imprinted at room- temperature by room-temperature nanoimprint lithography (RT-NIL) no heating or cooling steps may be necessary.

Recently the nanoimprint lithography has emerged to address some of the semiconductor-industry' s future requirements for manufacturing at the 10-nm length scale. Several ways of sub-10 nm nanoimprint lithography have been demonstrated. One conventional approach uses molds that have been fabricated by conventional molecular beam epitaxy followed by selective etching of the grown layers (MBE-NIL) . This approach only enables nanoimprinting of 1-D grating structures. Pitch and linewidth variations are the only degrees of freedom regarding the design of these grating structures. To create 2-D patterns multi-step RT-MBE-NIL may be performed onto the same area of an imprint polymer using the same grating mold and rotating this mold between subsequent imprint steps. However, overlay structures generated by NIL through subsequent imprint steps onto the same area of the imprint polymer layer result in deforma- tions that destroy and/or distort patterns that were created in previous imprint steps. In the 10-nm region these distortions become dominant with respect to the shape of the final desired pattern so that multi-step superposition RT-MBE-NIL may not be used to fabricate 2-D patterns in the sub-10-nm region in a controlled and predefined way. Further, conventional NIL in the sub-10-nm-region is not capable of imprinting 2-D patterns with the necessary flexibility and repeatability that is needed to successfully address the above applications.

Therefore, a need exists for an approach capable of generating repeatable and predefined nanometer pattern geometries.

Embodiments of the invention provide an approach for a nanoimprint lithography technique and a nanotransfer printing (nTP) technique that may both be capable of printing repeatable 2-D structures with feature sizes in the sub-10- nm region forming orthogonal pattern geometries having down to atomic dimensions into an imprint polymer layer, and onto a substrate or polymer material respectively.

Fig. 2 shows a mold fabrication scheme according to embodi- ments of the invention. In a first step shown in Fig. 2 (a) a sequence 200 of layers is provided. The sequence 200 of layers comprises a first layer 202 and a second layer 204. The second layer 204 has a thickness d in the nanometer region. The sequence 200 of layers may be formed by a first MBE step growing the layer 204 of e.g. AlGaAs with a thickness di of down to a few nm on top of a substrate 202 formed of e.g. GaAs. As can be seen, the layer 204 of Al- GaAs is grown on a surface of the substrate 202 (see the arrow in Fig. 2 (a)). In Fig. 2 (a) a first surface 206 is shown which is formed by the side surfaces (as seen in Fig. 2 (a)) of the first layer or substrate 202 and the second layer 204.

In a second step shown in Fig. 2 (b) a third layer 208 is grown on the first surface 206 (see Fig. 2 (a)). The third layer 208 has a thickness d₂ in the nanometer region. The third layer 208 may be grown of e.g. AlGaAs in a second MBE step with a thickness d₂ of down to a few run on the first surface 206 (see the arrow in Fig. 2 (b) ) . In Fig. 2 (b) a second surface 210 is shown which is formed by the upper surfaces (as seen in Fig. 2(b)) of the first layer or substrate 202, the second layer 204 and the third layer 208.

In a third step the second surface 210 (see Fig. 2(b)) is processed to remove a portion of the material of the first layer 202 (see Fig. 2 (c) ) or a portion of the material of the second layer 204 and the third layer 208 (see Fig. 2 (d) ) to generate a 3-D surface profile. The topographical 3-D landscape with 2-D lateral down to atomic resolution may be created by selectively etching the GaAs substrate 202 with respect to the AlGaAs layers 204 and 208 (see Fig. 2(c)) or vice versa (see Fig. 2 (d) ) . The 2-D lateral direc- tions are spanning a plane that may be perpendicular to both surfaces 204 and 206. The spacing between lines and grooves as well as the number of lines and grooves may be dependent on the character of the layer sequences grown in the first and second steps.

Fig. 2(c) and Fig. (2d) show embodiments of a mold for generating nanostructures. The mold comprises the sequence of layers of the first layer 202 and the second layer 204, and a third layer 208 on the first surface 206. The second layer 204 and the third layer 208 have a thickness in the nanometer region. On the second surface 210 the first layer 202 protrudes from the second and third layers 204 and 208 or the second and third layers 204 and 208 protrude from the first layer 202 to define a 3-D surface profile.

Further embodiments of the invention concern a mold that may be fabricated by a conventional multi-step molecular beam epitaxy (MBE) on a substrate material, in-situ cleaving of the sample, cleaved edge overgrowth (CEO) , a further ex-situ cleaving, and etching steps to selectively etch the grown layers and create a topographical landscape being the mold surface. A mold fabricated following this fabrication scheme will be referred to in the following as CEO-mold.

Several techniques may be used to fabricate (planar) 1-D and 2-D structures in the sub-10-nm region: (1) a combina- tion of MBE and electron beam lithography (EBL) may be used to create connected nanowires, CEO may be used to create 2- D and lower dimensional quantum wire structures by in situ cleaving the MBE treated sample and further growing of additional layers on the cleaved edge. Embodiments of the in- vention are an extension of these techniques. The layers grown by CEO may be selectively etched in order to generate a 3-D surface profile that may be used as the CEO-mold to perform ultrahigh resolution nanoimprint lithography (CEO- NIL) into a polymer layer by using conventional thermal NIL, room-temperature NIL (RTNIL) , ultraviolet-curing NIL

(UVNIL) , or a combination thereof. The CEO-mold may also be used for ultra-high resolution nanotransfer printing (CEO- nTP) . Here the CEO-mold may be used to perform classical

NTP processes leading to sub-10-nm resolution of orthogonal patterns printed onto polymer or other substrate materials. Embodiments of the CEO-mold fabrication technique provide 3-D rectangular mold patterns with down to atomic resolution in both 2-D lateral directions because the dimensions in both lateral directions are determined through the sub- sequent MBE growth steps combined in the CEO mold fabrication procedure. Hence conventional techniques may be ex^¬ tended from fabricating 3-D rectangular mold patterns with a longitudinal 1-D down to atomic resolution to fabricating 3-D rectangular mold patterns with a lateral 2-D down to atomic resolution. The complexity of the lateral 2-D mold geometry of the CEO-molds is exclusively determined by the CEO growth scheme since the selective etch characteristics of the CEO grown layer sequence is determined by the layer materials and thicknesses.

Fig. 3 and 4 describe further embodiments of a CEO-mold fabrication scheme, and Fig. 5 and Fig. 6 demonstrate the principles of CEO-NIL and CEO-nTP.

In Fig. 3 elements already described in Fig. 2 are denoted with the same reference signs. In a first MBE step shown in Fig. 3 (a) a sequence 200 of layers of a AlGaAs layer 204 and a GaAs layer 212 comprising thicknesses di and d₂ of down to a few nm is grown on top of each other on a GaAs substrate 202 (see the arrow in Fig. 3 (a)). The thicknesses di and d₂ may be the same or different. In Fig. 3 (a) a first surface 206 is shown which is formed by the side sur- faces (as seen in Fig. 3 (a)) of the GaAs substrate 202, the AlGaAs layer 204 and the GaAs layer 212.

In a first cleaving step shown in Fig. 3(b) the structure or sample shown in Fig. 3 (a) is cleaved. As can be seen, the structure shown in Fig. 3 (a) is cleaved in a plane parallel to the surface 206 as is indicated by arrow 214. The cleaved part 216 comprising a part 202' of the substrate 202, a part 204' of the AlGaAs layer 204 and a part 212' of the GaAs layer 212 is removed from the structure 200 as in- dicated by arrow 218. The cleaving plane is perpendicular to the MBE growth plane, i.e. the surface of the substrate 202 on which the AlGaAs layer 204 and the GaAs layer 212 are grown. The cleaving step exposes a cleaving surface 206' as shown in Fig. 3(b).

In a second MBE step shown in Fig. 3(c) a sequence of layers of a GaAs layer 220, a AlGaAs layer 208, and a GaAs layer 222 is grown on top of each other on the cleaving surface 206' (see the arrow in Fig. 3(c)). The GaAs layer 220 and the AlGaAs layer 208 comprise thicknesses d₃ and d₄ of down to a few nm. The thicknesses d^ and d₄ may be the same or different. In Fig. 3(c) a second surface 210 is shown which is formed by the upper surfaces (as seen in Fig. 3(c)) of the GaAs substrate 202, the AlGaAs layer 204, the GaAs layer 212, the GaAs layer 220, the AlGaAs layer 208, and the GaAs layer 222.

In a second cleaving step shown in Fig. 3(d) the structure or sample generated as outlined above is cleaved. As can be seen, the structure is cleaved in a plane parallel to the surface 210 as is indicated by arrow 223. The cleaved part 224 comprising a part 202" of the substrate 202, a part 204" of the AlGaAs layer 204, a part 212" of the GaAs layer 212, a part 220" of the GaAs layer 220, a part 208" of the AlGaAs layer 208, and a part 222" of the GaAs layer 222 is removed from the structure as indicated by arrow 226, and exposes the cleaving surface 210' as shown in Fig. 3(d). The cleaving plane is perpendicular to the first MBE growth plane, i.e. the surface of the substrate 202 on which the AlGaAs layer 204 and the GaAs layer 212 are grown, and to the second MBE growth plane, i.e. the surface 206 of the substrate 202.

After the second cleaving step a topographical 3-D landscape with 2-D lateral down to atomic resolution is created by selectively etching the AlGaAs layers 204 and 208 with respect to the GaAs layers 202, 212, 220 and 222 or vice versa. These 2-D lateral directions are spanning a plane that is parallel to the surface 210 and perpendicular to both cleaving planes 204 and 206. The spacing between lines and grooves as well as the number of lines and grooves are dependent on the character of the layer sequences grown in the above described steps. Fig 3(e) shows the mold 230 after selectively etching the AlGaAs layers 204 and 208 with respect to the GaAs layers 202, 212, 220 and 222. As can be seen, by the selective etch a structure is created in which the AlGaAs layers 204 and 208 are recessed with respect to the GaAs layers 202, 212, 220 and 222 to define grooves 232 and 234.

The embodiment described with regard to Fig. 3(e) was fabricated from the structure as it is shown in Fig. 3 (d) in which the layers 202, 212, 220 and 222 were made from the same material. Likewise, the layers 204 and 208 were made of the same material, so that by etching the structure shown in Fig. 3(d), the grooves 232 and 234 in Fig. 3(e) are generated. Since the layers 204 and 208 were made of the same material, the processing, for example, the etching, of the material of the layers 204 and 208 resulted in the grooves shown in Fig. 3(e) having the same dimensions, e.g. the same depth. The depth may be determined by the duration of the etch process and/or the etching agent used.

Other embodiments of the application resulted in a structure as shown in Fig. 3(f) in which the layers 202, 212, 220, 222 are of different material, which can be selectively processed with respect to each other and/or also the layers 204 and 208 may be of different materials, which can be processed selectively with respect to each other. Fig. 3(f) shows the mold 228, which is obtained by processing the surface 210 of the structure shown in Fig. 3 (d) in which the layers 202, 212, 220 and 222 are of the same material and wherein layers 204 and 208 are of different material. The structure of Fig. 3(f) is obtained, for example, by etching the material of the layers 202, 212, 220 and 222 and by additionally selectively etching the material of the layer 208, so that the projections 204''' and 208''' have different heights.

Fig. 3(g) shows a further mold 231, also obtained by proc- essing the surface 210 of the structure shown in Fig. 3(d) in which the layers 202, 212 and 222 are formed of the same material and the layer 220 is formed of a different material, which can be selectively processed with regard to the material of the layers 202, 212 and 222 as well as with respect to the layers 204 and 208. The layers 204 and 208 were formed of the same material. By etching the material of layers 204, 212 and 222 as well as the material of the layers 204 and 208, the mold 231 as shown in Fig. 3(g) was obtained comprising the grooves 232 and 234 in a similar manner as in Fig. 3(e), however, in addition, the projection 220' ' ' was obtained due to the selective etching of the material of the layers 202, 212 and 222 and the mate- rial of the layer 220.

The embodiments described above with regard to Fig. 3 generated the molds shown in Figs. 3(e) to (g) by etching the surface 210 of the structure shown in Fig. 3(d). Further embodiments of the invention may avoid the selective etching and rather apply a selective growth of material to the respective layers of the structure shown in Fig. 3 (d) to obtain similar molds as shown in Fig. 3(e) to (g) .

Fig. 4 shows a further embodiment of a CEO-mold fabrication scheme, which is similar to the one described with regard to Fig. 3, except that the sequence of layers, which is grown on the substrate 202 comprises further layers 236, 238, 240 and 242 as it is shown in Fig. 4 (a) to obtain the sequence 200' of layers. When compared to Fig. 3 (a), the structure is rotated by 90°. In addition, Fig. 4 (a) shows the surface 206, which is now formed by the side surfaces (in Fig. 4 the front surfaces) of the layers 202, 204, 212, 236, 238, 240 and 242.

In step 4 (b) , the structure shown in Fig. 4 (a) is cleaved in a similar manner as described in Fig. 3 (b) . In Fig. 4 (c) , in a similar manner as described with regard to Fig. 3 (c) , the layers 220, 208 and 222 are grown on the surface 206', the cleaved surface. Again, in a second cleaving step shown in Fig. 4 (d) , the above-described structure is cleaved in a plane parallel to the surface 210. Starting from the structure shown in Fig. 4 (d) , the surface 210 is processed and depending on the material selected for the respective layers, molds having different 3-D profiles may be generated.

Fig. 4(e) discloses a mold 244 in which the layers 202, 212, 236, 238, 242, 220 and 222 are of the same, first material and in which the layers 204, 208 and 238 are of the same, second material, which can be selectively processed with regard to the first material. By selectively growing additional material onto layers 204, 208, 236 or by selectively etching layer 202, 212, 220, 222, 236, 238, 242, the projections 204'", 208'" and 240'" may be obtained.

In Fig. 4 (f) , a similar structure is used as a starting point, except that the layers 204 and 240 are formed of the same, second material, whereas the layer 208 is formed of a different, third material, wherein the second and third material are selectively processable with respect to each other and also with respect to the first material. The structure shown in Fig. 4(f) forms a mold 246, which is obtained by etching the first material of layers 202, 212, 236, 238, 242, 220 and 222 and by further etching the layer 208, thereby generating the groove 234 and the projections 204"' and the projections 240'".

In Fig. 4 (g) , a further mold 248 is shown in which the layers 202, 212, 238, 242, 220 and 222 are of the same, first material, the layers 204, 208 and 240 are of the same, sec- ond material and the layer 236 is of a third material. The three materials are selectively processable with respect to each other so that, for example by etching the layers 202, 212, 238, 242, 220 and 222 and by further etching the layers 204 and 240 as well as the layer 208, the structure shown in Fig. 4 (g) having the grooves 232a and 232b as well as the groove 234 and the projection 236'" is obtained. In the following examples for the nanoimprint lithography technique and the nanotransfer printing technique using a mold in accordance with embodiments of the invention will be described. The nanoimprint lithography (NIL) technique is based on the conventional thermal NIL, room-temperature NIL (RTNIL) or ultraviolet-curing NIL (UVNIL) , or a combination thereof. The nanotransfer printing technique relies on traditional process steps for this technique.

Fig. 5 demonstrates the principles of CEO-NIL. The nanoimprint lithography process is analogue to the process described in Fig. 1. A substrate 300 is provided having applied to one surface thereof a polymer layer 302 as is shown in Fig. 5 (a). In this embodiment a mold 230' similar to the mold 230 shown in Fig. 3(e) is used for the CEO-NIL. When compared to Fig. 3(e) the mold 230 does not include the layer 220 - otherwise the mold 230' corresponds to mold 230 in Fig. 3(e). The mold 230' is oriented upside down when compared to the orientation shown in Fig. 3(e), i.e. the mold 230' is arranged such that the cleaved surface having the grooves 232 and 234 faces the polymer layer 302. The pattern is imprinted into the polymer layer 302 by pressing the mold 230' into the polymer layer 302 as is indicated by the arrow 304. When a transparent substrate ma- terial 300 is used as carrier for the imprint polymer layer 302 UV- and/or infrared-curing NIL may be performed by applying backside exposure of the sample. Optionally an anti- sticking layer may be evaporated onto the CEO-mold surface facing the polymer layer 302 to prevent imprint polymer ma- terial from being ripped of the imprint sample during stamp release.

In Fig. 5 (b) the mold 230' is removed from the polymer layer 302 as is indicated by the arrow 306 leaving an im- printed pattern 308 in the polymer layer 302. According to the grooves 232 and 234 in the mold 230' the imprinted pattern 308 comprises a first rib 314 having a thickness di, and a second rib 316 having a thickness d₄ and arranged perpendicular to the first rib 314.

The imprinted pattern may be transferred into the underly- ing substrate layer 300 in two different ways that yield a positive or negative pattern structure, respectively. When a negative final design pattern is needed in the substrate 300, a lift-off process may be performed as is shown in Fig. 5(c) to 5(e). After forming the structure in the poly- mer layer 302 as it is shown in Fig. 5(b) the polymer layer is removed in the non-structured portions 310, 312 and 313 so that the surface of the substrate 300 is exposed in these regions. A thin-film etch mask material 318, for example a metal, is evaporated onto the exposed surfaces 310, 312 and 313 of the substrate 300 and onto the polymer structure 314 and 316 as is shown in Fig. 5(c), thereby forming a mask layer 320 as is shown in Fig. 5 (d) . In a subsequent polymer etching step, the polymer components 314 and 316 are removed (also removing the etch mask material thereon) , as it is indicated by arrow 318 as is shown in Fig. 5(e). The residual etch mask material 322 serves as the etch mask for transferring the pattern into the substrate 300. After etching the substrate the residual etch mask material is removed completing the transfer of the structure into the substrate 300. In case a positive final pattern in the substrate layer is desired, the pattern transfer process as described in Fig. 1 of above may be carried out.

Fig. 6 demonstrates the principles of CEO-nTP for an ultrahigh resolution nano-transfer printing. A CEO-mold 230' as shown in Fig. 5 (a) is used. Onto the cleaved surface gold 340 is applied (see the arrow 342 in Fig. 6 (a)) in such a manner that primarily the cleaved surface portions are cov- ered, but not the grooves. Fig. 6(b) shows the mold 230' comprising a gold layer 344. The nano-transfer printing is carried out according to conventional nano-transfer printing processes, i.e. the pattern is imprinted onto a sub- strate 346 by pressing the mold 230' onto the substrate 346 as it is indicated by arrow 348 (see Fig 6(b)). Then the mold 230' is removed from the substrate 346 leaving the pattern 350 comprising the parts 350a, 350b and 350c as shown in Fig. 6(c). The pattern 350 in the described embodiment is formed of the gold layer 344. The rectangular part 350a of the pattern 350 is separated from the rectangular part 350b by the distance d₄ and from the elongated part 350c by the distance d_x. The rectangular part 350b is separated from the elongated part 350c by the distance d4. The embodiment described in Fig. 6 is a CEO-nTP procedure that allows for achieving 2-D down to atomic resolution and simultaneously a significantly higher flexibility of the printed patterns. These patterns may be used to build a mo- lecular three terminal device 352 (see Fig. 6(d)), such as for example a molecule-based transistor. While the spacing between contact pads 350a to 350c reaches down to atomic dimensions the overall dimensions of the pattern components may be larger allowing for contacting these components by classical optical or e-beam lithography forming the contacts 352a to 352c.

Embodiments of the invention may use other materials to be transferred than gold. Also additional layers, like a tita- nium layer may be applied. The layer 344 may be applied to the mold 230' by a self assembly process, or by using the so-called dip pen lithography approach in combination with the above described CEO-mold. When using the dip pen lithography approach, the mold may be immersed into a mate- rial to be transferred repeatedly, so that a pattern may be printed repeatedly.

Fig. 7a and Fig. 7b show AFM cross-sections and top-views of 3-D surface profiles of both a 2D-lateral CEO-mold as well as a ID-lateral MBE-mold, while Fig. 8 provides data on first imprint results. Fig. 7a shows an AFM image of the 3D surface profile of a

2D-CE0-mold after selective etching. The top view (top) shows a 25-nm-wide AlGaAs line (horizontal) being spaced

30nm from a larger AlGaAs layer (vertical) . The topographi- cal height legend (bottom right) and the cross-sections

(bottom middle/left) are corresponding to AFM scan-lines that are indicated by dashed lines in the top-view image: one cross-section images the spacing region (bottom left) while the other cross-section (bottom middle) shows the width of the AlGaAs line.

Fig. 7b shows a vertical SEM cross-section image of the 3D surface profile of a lD-MBE-mold after selective etching. The white area on the far right side of the image is the surface area of the GaAs layer, i.e. surface 212 in Fig. 3 (a). The MBE-mold was fabricated according to the following process: (1) A 270nm thick AlGaAs layer was grown on top of a GaAs sample in a first MBE step, (2) a second GaAs layer was grown on top of the AlGaAs layer in a second MBE step, and (3) the GaAs-AlGaAs-GaAs sandwich structure was selectively etched yielding a 270 nm wide and ~400nm deep trench where AlGaAs was etched away. This negative pattern can be used as a MBE-mold to imprint a positive 270 nm wide line into an imprint polymer layer.

Fig. 8 shows an imprint result of a room-temperature nano- imprint step performed using the mold shown in Fig. 7b. Polystyrene with a molecular mass of 60 kg/mol was used as imprint polymer. Fig. 8 provides a top-view image of the imprinted PS layer taken by means of an optical microscope at 10Ox magnification. The 270 nm-wide line was continuously printed over the complete length of the mold. The second, upper line was generated by imprint polymer that was pushed to the side at the edge of the mold during the imprint step. Fig. 8 shows an imprint experiment for determining imprint parameters for CEO-molds and MBE-molds. Fig. 8 shows an optical microscope top-view image of a 600-nm- thick polystyrene layer that was successfully imprinted by using a custom-built nanoimprint machine (see below) and the MBE-mold shown in Fig. 7b. The MBE-mold was pressed into the imprint polymer layer according to the NIL-scheme described in Fig. 5. The imprint pressure was between 1 MPa and 100 MPa, preferably around 10 MPa. The imprint polymer was imprinted at room-temperature for 10s. No anti-sticking layer was applied to the mold surface before imprinting.

Fig. 9 depicts the lattice constant transitions for semi- conductor compound materials. The GaAs/AlGaAs system that was used for the demonstrated MBE-NIL experiments (see Fig. 7b and Fig. 8) is highlighted in a circle in the diagram. The line connecting the dots labeled "GaAs" and "AlAs" represents the transition from pure GaAs (Alx-lGaxAs with X=I) via compounds comprising all three materials (AIx- lGaxAs with l>x>0) to pure AlAs (Alx-lGaxAs with x=0) . The general rule of thumb for which material combinations may be suitable for MBE-and CEO-growth and hence MBE-and CEO- mold fabrication is as follows: (a) the more the lattice constants of substrate and grown materials match the better the quality of the MBE grown sandwich structure, and (b) the better substrate and grown materials can be etched selectively with respect to each other the better the side- wall characteristics of the fabricated mold structures. As can be deduced from the diagram possible other substrate/growth material systems would for example be HgCdTe (HgTe to CdTe), AlGaSb (GaSb to AlSb), or InAs/AlGaSb. The InAs/AlGaSb system is not directly connected in the diagram but all compounds can still be used for MBE- and CEO-growth since in good approximation all involved lattice constants match. The diagram also provides energetic band-gaps and corresponding wavelengths for all shown semiconductor compounds. Also combinations of four or more materials which can be grown homogeneously on each other may be used.

In the above description embodiments of the invention were described but the invention is not limited to these embodiments. Besides the mentioned materials for the layers for creating the mold also other materials for the three or more layers may be used. The materials may be selected to be selectively removable or etchable. Embodiments of the invention use materials, which can be grown on each other so that a homogeneous single crystalline structure is obtained and such structures yield a good cleaving edge. Such materials may be selected from the III V group comprising Al, Ga, En and P, As, Sb, N. Also, such materials may, for example, be selected from the II VI group comprising, for example, Zn, Cd, Hg and Se, S and Te.

At least the thickness of the second and third layers defining the pattern may be from about a monolayer to about 1000 nm, or from about 1 nm to about 10 nm.

In addition the invention is not limited to the sequence of layers described above. Rather, a plurality of layers may be grown on the first surface and/or on the second surface to define a plurality of pattern elements on the mold.

The orientation of the two surfaces on which the layers are grown was described as being perpendicular. The invention is not limited to such an arrangement and the surfaces may be arranged to each other under an angle different from 90°, which may be defined by the crystal structure.

The embodiments of the invention described the provision of a defined surface by cleaving, It is noted that the invention is not limited to such embodiments. Instead of cleav- ing, a substrate may also be cut and polished, for example, by CMP or, upon manufacturing the substrate the generation of defined surfaces having a predefined property may be controlled so that no cleaving or cutting is necessary at all.

The above embodiments were described as using an MBE process for growing the respective layers, however, the invention is not limited to such approaches. The materials may be grown, for example, by vapor deposition, with chemical deposition, self assembly deposition and sputtering deposition.

Embodiments of the invention were described in which the processing of the second surface concerns the removal of a portion of the material of the first layer or a portion of the material of the second and third layers to generate the 3-D surface profiles. However, instead of removing mate- rial, also additional material may be applied to the first layer or additional materials may be applied to the second and third layers. Further, the embodiments described above describe the processing of the second layer as comprising the processing of the second and third layers in the same manner to obtain projections or recesses having the same dimensions. When using the same materials for the second and third layers, such similar dimensions may be obtained. However, the invention is not limited to such an approach and, as mentioned above, also different materials for the second and third layers may be used, as long as both can be processed selectively with regard to the material of the first layer. When using second and third layers of different materials, also the processing may be different, so that for example, different heights or depths of the re- cesses may be obtained by selectively processing not only the first layer and the second and third layers, but also by selectively processing the second and third layer with respect to each other.

Further embodiments of the present invention concern a stamp or mold holder unit for holding the nanostructure mold described above.

Fig. 10 shows an apparatus or system for transferring a nanostructure to a substrate. The system 400 comprises a wafer-chuck 402, which is supported by three wafer-chuck holders 404a, 404b, and 404c. The system 400 further comprises a system bottom plate 406, to which the wafer-chuck holders 404a to 404c are mounted. The wafer-chuck 402 is for holding a substrate or a wafer, to which a nanostruc- ture is transferred using the above-described nanostructure mold or stamp. To allow for this transfer, the apparatus 400 comprises a hydraulic subsystem 407 comprising a piston 408, which is moveable in a vertical direction, as it is indicated by arrow 410. The piston 408 is actuated by the hydraulic device 412 shown schematically in Fig. 10. The piston 408 engages a mold holder unit 450, which is for holding the above-described nanostructure stamp or mold. As can be seen, a slit 452 is shown in the mold holder unit 450 which is for holding the mold in a manner as described in the following.

Fig. 11 shows an embodiment of a mold holder unit 450 as it may be used with the system or apparatus shown in Fig. 10. The mold holder unit 450 comprises a support 454, which is L-shaped to define a bottom surface 456 and a wall surface 460, which extends from the bottom surface 456. As can be seen, the wall surface 460 comprises a stepped portion 462 defining a support surface 464, which is substantially parallel to the bottom surface 456. On the wall surface 460, two projections 466a and 466b are formed, which extend from the wall surface 460 by a distance, which corresponds to the distance by which the stepped portion 462 extends from the wall surface 460.

The mold holder unit 450 further comprises a mold or stamp receptacle 468, a resilient member 470, like a spring or a foamed material, and a first and a second moveable plate 472a and 472b.

The mold holder unit 450 further comprises a fastening device 474 having a surface 476 facing the surface 460 of the support 454. The surface 476 of the fastening device 474 comprises two recessed portions 478a and 478b for receiving the moveable plate 472a and 472b when the mold holder unit 450 is assembled. In addition, the fastening device com- prises a plurality of openings 480a to 48Od and 482a to 482d for receiving respective fastening elements 484a to 484d and 486a to 486d.

The mold holder unit 450 has an assembled state, in which the resilient member 470 is provided on the surface 464 of stepped portion 462, and the mold receptacle 468 is supported by the resilient member 470. The resilient member 470 and a lower portion 468b of the mold receptacle 468 have a width, which corresponds substantially to the width of the support 454, as can be seen from Fig. 11 so that the lower portion 468b of the receptacle extends between the surface 464 of the stepped portion 462 and the projections 466a and 466b formed on the surface 460. The mold recepta- cle 468 further comprises an upper portion 468a, the upper surface of which receives the mold and which extends from the lower portion 468b so that in the assembled state the upper portion 468a would extend into the space between the two projections 466a and 466b. The mold receptacle 468 is arranged such that without screws tightened (see below) a movement of the receptacle is possible when the support 454 and the fastening device 474 are assembled, i.e. the fastening device 474 is fixed to the support 454.

The fastening device 474 is fixed to the support 454 by means of the screws 486a to 486d, which will pass through the respective holes 482a to 482d and which will be received in respective screw holes 488a to 488d formed in the body of the support 454. The moveable plates 472a and 472b will be received in the recesses 478a and 478b, wherein the moveable plates 472a and 472b can be pressed against the mold when held by the receptacle 468 and against the lower portion 468b of the receptacle 468 by tightening the screws 484a to 484d, which extend through the holes 480a to 480d and which engage the moveable plate 472a and 472b as it is shown by the dashed lines thereby urging same towards the surface 460 and securing the mold and the receptacle 468 at a predetermined position, when the screws 484a to 484d are tightened. The receptacle 468 is moveable in a partial car- danic manner when the screws 484a to 484d are not tightened, i.e. when the moveable plates 472a, 472b do not press against the mold and the receptacle 468. This partial car- danic movement is allowed, due to the resilient member 470, which might be a spring or a foam material, so that a receptacle 468 may be tilted in a direction parallel to the surface 460, while the receptacle 468 is guided by the surfaces 460 and 476, which may be considered guide surfaces.

The functionality of the device shown in Fig. 11 is such that for transferring a nanostructure to a substrate using a system as it is shown in Fig. 10, a mold, which was manufactured as described above, is inserted in the mold holder 450 such that it rests on an upper surface of the upper portion 468a of the receptacle 468. The receptacle 468 and the mold are dimensioned in such a manner that the mold extends above an upper surface 490 of the support 454. In this situation, the screws 484a to 484d are not tightened, i.e. the receptacle 468 can be tilted as described above.

In transferring the nanostructures defined by the mold to a substrate it may be important to provide for a correct alignment of the surface of the mold or the surface of the nanostructure and the surface of the substrate to which the nanostructure is to be transferred. To allow for such a correct alignment between the respective surfaces, in a first step the mold is inserted into the mold holder unit 450. Without the screws 484a to 484d tightened, the recep- tacle 468, and thereby the mold may tilt. Fig. 12 illustrates the functionality of the mold holder unit in further detail. As can be seen in Fig. 12, a substrate 494 is shown and by the vertical movement indicated by the arrow, the mold 500 is brought into contact with the substrate 494. In Fig. 12 the resilient member 470 is formed as a spring and arrow 503 illustrates the tilting of the receptacle 468 so that by the tilting, for example, a non-planar back surface 502 of the mold 500 may be compensated for. For determining the correct position of the receptacle 468 to insure for the correct alignment of the surfaces as described above, the hydraulic system is engaged (see Fig. 10) so that the mold holder unit 450 is raised until the mold comes into contact with the substrate. The forces, which are generated by bringing the mold into contact with the substrate will be sufficient to deform the resilient member 470 such that the receptacle 468 and the mold take up a position in which the surface of the mold and the surface of the substrate are aligned with respect to each other. Once this position is obtained, the screws 484a to 484d are tightened to secure the receptacle 468 at the determined position and then the hydraulic system is further engaged for pressing the mold against the substrate to transfer the nanostructure to the substrate.

Embodiments of the invention may use a modified approach in that after finding the aligned position, the screws 484a to 484d are only slightly tightened to secure a receptacle 468 at the found position. Then, the mold holder unit is retracted for truly tightening the screws for the following pressing step. This approach may be desirable in case fastening of the screws would result in a movement of the mold holder unit, which is already in contact with the substrate and which might deteriorate the surface of the substrate, to which the nanostructure is to be transferred.

Fig. 13 shows a schematic view of the holder unit 450 as shown in Fig. 11 with the mold 500 attached, but without the clamping or fastening device 474 shown. In Fig. 13, the support 454 is shown, and Fig. 13 is provided to illustrate the manner in which the mold 500 is provided on the upper portion 468a of the receptacle 468. As can be seen at 492 the upper end of the mold 500 extends above the upper sur- face 490 of the support 454. Also the moveable plates 472a and 472b are shown, wherein the position where same engage with respective portions of the receptacle 468 are indicated by the arrows. Embodiments of the invention concern CEO-NIL and CEO-nTP which may be lithography techniques capable of generating 2-D orthogonal pattern geometries in the sub-10-nm region in a repeatable and predefined way comprising more degrees of freedom regarding the design of the pattern geometry than any other existing lithography technique. This may be a core need of future semiconductor fabrication industry. CEO-NIL/nTP may be the next step towards significantly pushing its fabrication limits towards the 1-nm barrier. On top of that multiple areas for applications of CEO-NIL/nTP may be identified:

• fabrication of 3-D nano-electronic and/or nano- optoelectronic circuits comprising down to 2-D lateral atomic dimensions and components serving as sensors, (photo) detectors, measuring setups enabling physical characterization of molecules, and data storage units

• fabrication of molds for other NIL techniques

• CEO-NIL/nTP may also provide classical advantages of NIL over e-beam lithography (EBL) and other competing radiation based lithography techniques: (i) no exposure of material to be patterned to potentially damaging radiation, (ii) no shot-noise, (iii) no proximity effect, and (iv) NIL is a very fast and cheap lithography technique.

Claims

1. A method for fabricating a mold (228, 230) for gener- ating nanostructures, comprising:

providing a sequence (200) of layers comprising a first layer (202) and a second layer (204);

providing a third layer (208) on a first surface (206) formed by the first layer (202) and the second layer (204), wherein the material of the first layer (202) and the material of the second and third layers (204, 206) are selectively processable with respect to each other; and

processing a second surface (210) formed by the first layer, the second layer (204) and the third layer (208) to remove a portion of the material of the first layer (202) or a portion of the material of the second and/or third layers (204, 208), or to add material to the first layer (202) or to the second and/or third layers (204, 208) to generate a 3-D surface profile.

2. The method of claim 1, wherein the second layer (204) and the third layer (208) have a thickness in the nanometer region.

3. The method of claim 1 or 2, wherein

providing the sequence (200) of layers comprises:

growing the first layer (202) and the second layer (204) on each other in a first growth plane, and

cleaving the structure in a first cleaving plane which is perpendicular to the first growth plane; providing a third layer comprises:

growing the third layer (208) on the first cleaving plane in a second growth plane, and

cleaving the structure in a second cleaving plane which is perpendicular to the first and second growth planes; and

processing comprises:

etching the material of the first layer (202) or the material of the second and third layers (204, 208) of the second surface (210) exposed by cleav- ing the structure in the second cleaving plane, or growing material to the first layer (202) or to the second and third layers (204, 208) of the second surface (210) exposed by cleaving the structure in the second cleaving plane.

4. The method of claim 3, wherein growing the layers (202, 204, 208) or material on the layers (202, 204, 208) comprises molecular beam epitaxy, vapor deposition, wet chemical deposition, self assembly deposi- tion, or sputtering.

5. The method of one of claims 1 to 4, wherein the material of the first layer (202) and the material of the second and third layers (204, 206) are selectively etchable or growable with respect to each other.

6. The method of one of claims 1 to 5, wherein the second and third layers (204, 208) are of the same material.

7. The method of one of claims 1 to 5, wherein the second and third layers (204, 208) are of different materials, and wherein processing the second surface com- prises selectively processing the second and third layers (204, 208).

8. The method of one claim 7, wherein the second and third layers (204, 208) are processed to obtain projections (204"', 208"') on the second surface (210) or grooves (232, 234) in the second surface (210) with different dimensions.

9. The method of one of claims 1 to 8, wherein the material of the first, second and third layers (202, 204, 208) is selected from the III-V group, the II-VI group or the II group.

10. The method of claim 9, wherein the material of the first, second and third layers (202, 204, 208) is selected from the group comprising Al, Ga, In, P, As, Sb, Zn, Cd, Se, S, N and combinations thereof.

11. The method of one of claims 1 to 10, wherein the second and third layers (204, 208) have a thickness from about a monolayer to about 1000 nm.

12. The method of claim 11, wherein the second and third layers (204, 208) have a thickness from about 1 nm to about 10 nm.

13. The method of one of claims 1 to 12, wherein the sequence (200) of layers comprises a plurality of first and second layers arranged alternately on each other.

14. The method of one of claims 1 to 13, wherein the sequence (200) of layers comprises a fourth layer (212) on the second layer (204), and wherein the processing of the second surface (210) removes a portion of the material of the first layer (202) and the further layer (212) or a portion of the material of the second and third layers (204, 208).

15. The method of one of claims 1 to 14, comprising

providing a fifth layer (220) between the first sur- face (206) and the third layer (208) to form a further sequence of layers,

wherein the processing of the second surface removes a portion of the material of the first layer (202) or a portion of the material of the second, third and fifth layers.

16. The method of claim 15, wherein the further sequence of layers comprises a plurality of third and fifth layers arranged alternately on each other.

17. The method of one of claims 1 to 16, comprising

providing a sixth layer (222) on the third layer (208),

wherein the processing of the second surface (210) removes a portion of the material of the first layer (202) or a portion of the material of the second, third and sixth layers (208, 220, 222) .

18. The method one claim 17, wherein the fourth to sixth layers (212, 220, 222) have a thickness in the nanometer region.

19. A mold for generating nanostructures, comprising:

a sequence (200) of layers comprising a first layer (202) and a second layer (204), the second layer (204) having a thickness in the nanometer region; and

a third layer (208) on a first surface (206) formed by the first layer (202) and the second layer (204), the third layer (208) having a thickness in the nanometer region;

wherein on a second surface (210) formed by the first layer (202), the second layer (204) and the third layer (208) the first layer (202) protrudes from the second and third layers (204, 208) or the second and third layers (204, 208) protrude from the first layer (202) to define a 3-D surface profile.

20. The mold of claim 19, wherein the material of the first layer (202) and the material of the second and third layers (204, 208) are selectively etchable with respect to each other.

21. The mold of claim 19 or 20, wherein the second and third layers (204, 208) are of the same material.

22. The mold of one of claims 19 to 21, wherein the mate- rial of the first, second and third layers (202, 204,

208) is selected from the III-V group, the II-VI group or the IV group.

23. The mold of one claim 22, wherein the material of the first, second and third layers (202, 204, 208) is selected from the group comprising Al, Ga, In, P, As, Sb, Zn, Cd, Se, S and combinations thereof.

24. The mold of one of claims 19 to 23, wherein the second and third layers (204, 208) have a thickness from about a monolayer to about 1000 nm.

25. The mold of claim 24, wherein the second and third layers (204, 208) have a thickness from about 1 nm to about 10 nm.

26. The mold of one of claims 19 to 25, wherein the sequence (200) of layers comprises a plurality of first and second layers arranged alternately on each other.

27. The mold of one of claims 19 to 26, wherein the sequence (200) of layers comprises a fourth layer (212) on the second layer (204), and wherein the first and further layers (202, 212) protrude from the second and third layers (204, 208), or the second and third lay- ers (204, 208) protrude from the first and or the second and third layers (202, 204, 208) protrude from the first and further layers (202, 212) .

28. The mold of one of claims 19 to 27, comprising

a fifth layer (220) between the first surface (206) and the third layer (208) to form a further sequence of layers,

wherein the first layer (202) protrude from the second, third and fifth layers (204, 208, 220), or the second, third and fifth layers (204, 208, 220) protrude from the first layer (202) .

29. The mold of claim 28, wherein the further sequence of layers comprises a plurality of third and fifth layers arranged alternately on each other.

30. The mold of one of claims 19 to 29, comprising

a sixth layer (222) on the third layer (208),

wherein the first layer (202) protrude from the second, third and sixth layers (204, 208, 222), or the second, third and sixth layers (204, 208, 222) protrude from the first layer (202) .

31. The mold method one claim 30, wherein the fourth to sixth layers (212, 220, 222) have a thickness in the nanometer region.

32. A method for fabricating a nanostructure, comprising:

providing a mold (228, 230) of one of claims 19 to 31 having a desired 3-D surface profile;

performing a nanoimprint lithography or a nanotransfer printing using the mold (228, 230) to define the nanostructure in a layer of a desired material.

33. A mold holder unit for holding a nanostructure mold, comprising:

a support (454) /

a mold receptacle (468) for receiving a nanostructure mold (500), the mold receptacle (468) supported by the support (454) in a partial cardanic manner; and

a fastening device (474) securing the mold receptacle (468) at a predetermined position with respect to the support (454) when the fastening device (474) is tightened, and allowing for the partial cardanic movement of the mold receptacle (468) when the fastening device (474) is not tightened.

34. The mold holder unit according to claim 33, comprising a resilient member (470) arranged between the support (454) and the mold receptacle (468).

35. The mold holder unit according to claim 34, wherein the resilient member (470) is selected from the group comprising a spring, a foamed material, or a rubber material.

36. The mold holder unit according to claim 34 or 35, wherein

the support (454) comprises a support bottom (464) and a first guide surface (460) extending from the support bottom (464);

the fastening device (474) comprises a second guide surface (474) parallel to the first guide surface (460) and a fastening element (472a, 472b, 486a-d) ; and

the mold receptacle (468) is moveable between the guide surfaces (460, 474), when the fastening element (472a, 472b, 486a-d) is not tightened.

37. The mold holder unit of one claims 33 to 36, comprising a mold (500), according to one of claims 19 to 31 held in the mold receptacle (468) .

38. A system for transferring a nanostructure to a substrate, the system comprising:

a substrate holder (402, 404a-c) ;

a mold holder unit (450) according to one of claims 33 to 37; and

a hydraulic device (406, 408, 410) adapted for moving the mold holder (450) .

39. A method for setting a position of a nanostructure mold held in a mold holder unit (450) according to one of claims 33 to 37 and to be aligned with a substrate to which a nanostructure is to be transferred, the method comprising: moving the mold holder unit (450) , including the mold

(500) into contact with the substrate surface with the fastening device (474) not tightened so that the mold surface aligns with the substrate surface and takes up the predetermined position; and

tightening the fastening device (474) after taking up the predetermined position.

40. A method for transferring a nanostructure to a substrate, the method comprising:

setting the position of a nanostructure mold according to claim 39;

pressing the nanostructure mold (500) against the surface of the substrate; and

withdrawing the nanostructure mold from the surface of the substrate.