US20060169592A1

US20060169592A1 - Periodic layered structures and methods therefor

Info

Publication number: US20060169592A1
Application number: US11/047,089
Authority: US
Inventors: Peter Mardilovich; Pavel Kornilovich; Sriram Ramamoorthi
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2005-01-31
Filing date: 2005-01-31
Publication date: 2006-08-03

Abstract

A periodic layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers is made by providing a substrate, depositing a quantity of non-porous electrochemically oxidizable material over the substrate, at least partially anodizing the non-porous electrochemically oxidizable material, and repeating similar steps until a layered structure having a desired periodicity and a desired total structure thickness is completed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to co-pending and commonly assigned application Ser. No. 10/817,729, filed Apr. 2, 2004 (attorney docket no. 200311571), the entire disclosure of which is incorporated herein by reference, and is related to co-pending and commonly assigned applications Ser. No. ______ (attorney docket no. 200401845) and Ser. No. ______ (attorney docket no. 200406118).

TECHNICAL FIELD

This invention relates generally to periodic layered structures and methods for fabricating and using such periodic layered structures.

BACKGROUND

Industrial interest in materials having structural and functional features with nanoscale dimensions has been growing rapidly. Nano-structures have been fabricated by semiconductor processing techniques including patterning techniques such as photolithography, electron-beam lithography, ion-beam lithography, X-ray lithography, and the like. Other nano-structures have also been fabricated utilizing structures formed by self-ordering processes.
Some devices for manipulating optical signals have incorporated nanostructures, often including periodic structures such as photonic crystals, for example. Fabrication of optical devices, including macroscopic, microscopic, and nanoscopic elements, has usually used glasses such as silica glasses, transparent crystals, or polymeric materials.
Nanostructures have been applied to display devices, magnetic recording media, quantum-well devices, molecular and gas sensors, optical devices, electroluminescent devices, and electrochromic devices, for example. Periodic layered structures such as superlattice structures may be used in many such applications. Efficient, reproducible, low-cost methods for making periodic layered structures are therefore needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawings, wherein:
FIG. 1A is a flowchart of a first embodiment of a method for fabricating a periodic layered structure.
FIG. 1B is a flowchart of a second embodiment of a method for fabricating a periodic layered structure.
FIGS. 2A-2F are cross-sectional side elevation views of an embodiment of a periodic layered structure at various stages of its fabrication.
FIGS. 3A-3F are cross-sectional side elevation views of another embodiment of a periodic layered structure at various stages of its fabrication.
FIG. 4 is an exemplary graph showing oxide thickness as a function of time as observed while practicing a step of an embodiment of a method for fabricating a periodic layered structure.
FIG. 5 is an exemplary graph showing oxide thickness as a function of anodization voltage as observed while practicing a step of an embodiment of a method for fabricating a periodic layered structure.
FIGS. 6A-6C are cross-sectional side elevation views of another embodiment of a periodic layered structure at various stages of fabrication for an embodiment of an imprint lithography stamp.
FIGS. 7 and 8 are cross-sectional side elevation views of two particular embodiments of periodic layered structures.
FIG. 9 is a top plan view of an embodiment such as the embodiments shown in FIGS. 7 and 8.

DETAILED DESCRIPTION OF EMBODIMENTS

For clarity of the description, the drawings are not drawn to a uniform scale. In particular, vertical and horizontal scales may differ from each other and may vary from one drawing to another. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the drawing figure(s) being described. Because components of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting.
The term “anodization” is used in this specification and the appended claims to mean electrochemical oxidation of an oxidizable material (such as an oxidizable metal) by employing the oxidizable material as an anode in an electrolytic cell and by operating the electrolytic cell with voltage and current suitable to partially or fully oxidize the material of the anode. An “anodic oxide” is the oxide thus formed. An “anodizable material” is a material that can be oxidized in that manner. “Partial anodization” refers to oxidation of less than the entire thickness of a metal layer; i.e., some thickness of unoxidized metal remains after partial anodization, unless full anodization is explicitly specified. “Full anodization” refers to oxidation of the entire thickness of a metal layer. References herein to a layer of electrochemically oxidizable metal are intended to include semiconductor materials such as silicon which, with respect to their anodization, behave like the electrochemically oxidizable metals.
One aspect of the invention provides embodiments of a periodic layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers. The periodicity (pitch) of the layered structure may be characterized by a nanoscale dimension. Throughout this specification and the appended claims, the term “nanoscale” refers to a length of scale corresponding generally to the scale in the definition of U.S. Patent Class 977, generally less than about 100 nanometers (nm).
The term “superlattice” has been used to denote structures such as periodic heterojunctions having a periodicity longer than that of the characteristic crystal lattice of a base material, e.g., due to lattice mismatch. The term “superlattice” has also been used more generally to denote periodic structures constructed from other crystalline, polycrystalline, or non-crystalline materials. “Superlattice” is used in this more general sense in the present specification and claims.
An embodiment of a periodic layered structure may be made by providing a substrate, depositing a quantity of non-porous electrochemically oxidizable material such as a metal over the substrate, anodizing the non-porous electrochemically oxidizable material (partially or fully), and repeating similar steps until a layered structure having a desired periodicity and a desired total structure thickness is completed. The thickness of each layer and/or the periodicity of the periodic layered structure may be nanoscopic. Thus, another aspect of the invention provides methods for fabricating embodiments of periodic layered structures, including structures whose layers have nanoscale dimensions and periodic structures with nanoscale pitch.
One embodiment of a method for fabricating a periodic layered structure (having physical and chemical properties varying periodically at least along a direction perpendicular to its layers) employs steps of providing a substrate, depositing a quantity of non-porous electrochemically oxidizable material over the substrate to form an electrochemically oxidizable layer, anodizing the non-porous electrochemically oxidizable material until a layer of oxide is formed, and repeating alternately the depositing and anodizing steps until a periodic layered structure having a desired periodicity and a desired total thickness is completed. The periodic layered structure may be one of the types known as a superlattice. The non-porous electrochemically oxidizable material is anodized until a layer of oxide having a desired thickness is formed. In some cases, that anodization may be a partial anodization, i.e., less than the entire thickness of the electrochemically oxidizable material is oxidized.
Many electrochemically oxidizable materials are known, including the metals aluminum (Al), tantalum (Ta), niobium (Nb), tungsten (W), bismuth (Bi), antimony (Sb), silver (Ag), cadmium (Cd), iron (Fe), magnesium (Mg), tin (Sn), zinc (Zn), titanium (Ti), copper (Cu), molybdenum (Mo), hafnium (Hf), zirconium (Zr), titanium (Ti), vanadium (V), gold (Au), and chromium (Cr), along with their electrochemically oxidizable alloys, mixtures, and combinations, all of which are suitable for use in this method. Another suitable material is non-porous silicon (Si), although it is not classified as a metal, but as a semiconductor. Thus, references herein to a layer of non-porous electrochemically oxidizable material or metal are intended to include non-porous semiconductor materials such as non-porous silicon which, with respect to their anodization, behave like the non-porous electrochemically oxidizable metals. To simplify the description and drawings, embodiments using metals for a non-porous electrochemically oxidizable material will be described. Those skilled in the art will understand that any non-porous electrochemically oxidizable material may be substituted wherever “metal” is mentioned, except where the metal is explicitly described as not being electrochemically oxidizable.
While porous oxides have been formed by anodization, control of the thickness of oxides thus formed can be problematical. However, the thickness of dense oxide films (with densities comparable to theoretical oxide densities) formed by electrochemical oxidation of non-porous electrochemically oxidizable material is precisely controllable by controlling the anodization voltage, as described in more detail hereinbelow.
Suitable non-porous electrochemically oxidizable materials include the “valve metals,” defined in the review paper by M. M. Lohrengel, “Thin anodic oxide layers on aluminium and other valve metals: high field regime,” Materials Science and Engineering Vol. R11 (1993) pp. 243-294, for example. As described in the review paper by Lohrengel, valve metals may be defined to be in accordance with i=i₀*exp(β*E), where i is the oxide formation current, i₀and β are material-dependent constants, and E is the electric field strength in the oxide. Lohrengel goes on to list various properties typical of a valve metal: The surface of an (electro-) polished electrode is covered with 2-5 nanometers of oxide from air or electrolyte passivation. This corresponds to an open circuit potential of about zero V (vs. a hydrogen electrode in the same solution). The thickness of the oxide layer increases during anodization. In a galvanostatic experiment the potential increases almost linearly with time; in a potential sweep experiment with constant potential sweep the current is almost constant. These are equivalent and correspond to a constant charge and, therefore, a constant increase of thickness for a given potential change. The oxide layer is not reduced by (moderate) cathodic currents. Further oxide growth is only observed when the potential exceeds the previous formation potential. The ionic conductivity is small (steady state conditions or at potentials smaller than the formation potential). The electronic conductivity (of undoped oxides) and, hence, oxygen evolution are negligible. An addition of redox systems to the electrolyte causes no additional currents. Corrosion is small at moderate pH values. The oxide grows independently of the composition of the electrolyte. A (possible) incorporation of anions from the electrolyte, for example, causes no fundamental changes of the layer properties. The combination of the low oxide electrode potential (and, therefore, air passivation), negligible electronic conductivity, and the lack of oxygen evolution is not accidental, as the oxide electrode potential depends almost linearly on the band gap. Valve metals are usually covered by oxide films of the barrier type. An ideal barrier oxide “. . . is a nonporous, thin oxide layer possessing electronic and ionic conductivity at high electric field strength.”
Returning now to the description of a method embodiment for fabricating a periodic layered structure, the layer of non-porous electrochemically oxidizable metal (or, in the case of silicon, for example, non-porous electrochemically oxidizable semiconductor) may be deposited by any suitable conventional deposition method, such as evaporation, sputtering, plating, electroplating, atomic layer deposition (ALD), or chemical vapor deposition (CVD). The metal layer may have a thickness of about two nanometers (2 nm) or greater, for example, with essentially no theoretical upper limit, but limited only by practical considerations such as anodizing voltage, application requirements, etc.
In practice, the metal layer may be made thinner on a smooth, substantially planar substrate than on a substrate which is not smooth and planar. The substrate may be prepared by polishing to a smooth planar surface before depositing the metal layer. Also, the metal layer may be planarized after its deposition, e.g., by mechanical polishing, chemical polishing, electrochemical polishing, chemical mechanical polishing (CMP), or other planarization technique.
The desired periodicity of structures that are made by such a method depends on the intended application of the periodic layered structure. For example, a periodic layered structure to be used as a photonic crystal may require a periodicity determined by the wavelength of the electromagnetic radiation that is to be processed by the photonic crystal. Method embodiments performed in accordance with the present invention may make periodic layered structures having periodicities (pitch) of about five nanometers (nm) or greater.
FIGS. 1A and 1B are flowcharts illustrating embodiments of methods for fabricating periodic layered structures. Steps of the methods are denoted by reference numerals S10, S20, . . . , S160. FIGS. 2A-2F and 3A-3F are cross-sectional side elevation views of embodiments of periodic layered structures at various stages of their fabrication.
FIG. 1A is a flowchart of a first embodiment of a method for fabricating a periodic layered structure. As shown in FIG. 1A, a suitable substrate is provided (step S10). For many applications, a suitable substrate is a smooth planar silicon wafer as is commonly used in semiconductor manufacturing. For some applications, a layer of insulating material such as silicon oxide or silicon nitride may be formed on the silicon wafer so that the top surface of the substrate is an insulator. In step S20, a layer 20 of a first metal is deposited (FIG. 2A). The first metal is a non-porous electrochemically oxidizable material. The thickness of this layer of first metal is chosen to provide a suitable amount of metal such that anodization in the next step (step S30) will contribute suitably to the desired periodicity (pitch) of the periodic layered structure.
When the metal layer is anodized, the total thickness typically increases. The volume ratio of oxide to consumed metal is typically greater than one. For example, a five-nanometer-thick aluminum layer may be converted by anodization to about six and a half nanometers of aluminum oxide if the full thickness of aluminum is anodized, and a partially anodized layer (the entire film, aluminum and alumina together) has a thickness intermediate between five and about 6.5 nanometers. Similarly, partial anodization of a five nanometer film of tantalum results in a tantalum oxide film having a thickness intermediate between zero (or none) and about 11.5 nanometers (the entire film having thickness intermediate between 5 and 11.5 nanometers).
FIG. 2B shows the oxide layer 30 formed by partial oxidation of layer 20, leaving a thinner remaining metal layer 21. In step S40, a layer 40 of a second metal is deposited (FIG. 2C). The second metal is also a non-porous electrochemically oxidizable material. In step S50, the second metal is anodized, forming the oxide layer 50 shown in FIG. 2D by partial oxidation of layer 40, leaving a thinner remaining metal layer 41. If the second metal deposited and anodized in steps S40 and S50 is different from the metal as deposited and anodized in steps S20 and S30, the completion of step S50 serves to provide one composite layer, i.e., one period, of the periodic layered structure. However, the second metal deposited and anodized in steps S40 and S50 may be the same as the metal which was deposited and anodized in steps S20 and S30. In that case, if the corresponding thicknesses are also made equal, the completion of step S50 serves to provide two composite periodic layers, i.e., two periods, of the periodic layered structure. If the structure is complete, this ends the process. The structure is determined to be complete if the total structure thickness has reached the desired thickness and/or if the total number of layers provided is the desired total number. If the periodic layered structure is not complete, suitable steps S20, S30, S40, and S50 are repeated until the desired periodic layered structure is complete, as determined in decision step S60. FIGS. 2E and 2F show deposition of layer 60 and partial anodization to form layer 70, for example, leaving a thinner remaining metal layer 61. As shown by the dashed line in FIG. 1A, steps S40 and S50 may be repeated separately (for example, if the second metal deposited and anodized in steps S40 and S50 is the same as the metal deposited and anodized in steps S20 and S30 and if the corresponding thicknesses are equal). Those skilled in the art will recognize that, if the first and second metals deposited in steps S20 and S40 are different, the periodicity may be different than if they are the same metal, especially if the deposition thicknesses and corresponding anodization thicknesses are the same. Also, for example, the volume ratio (oxide/consumed metal) and/or expansion coefficients can be different for different metals. FIG. 2F shows a completed periodic layered structure 80.
Thus, another aspect of the invention provides embodiments of a method for fabricating a periodic layered structure by employing the steps of providing a substrate, depositing a quantity of a first metal, anodizing the first metal until a desired thickness of first oxide is formed, depositing a quantity of a second metal over the first oxide, anodizing the second metal until a desired thickness of second oxide is formed, and repeating the preceding four steps a number of times until a layered structure having a desired periodicity and a desired total structure thickness is completed.
The first and second metals may be distinct and different metals, or they may be the same metal. As in embodiments described above, each of the metals may be a material selected from among Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and their alloys, mixtures, and combinations. Some embodiments worth mentioning specifically are those in which the first metal comprises aluminum or tantalum, and those in which the second metal comprises aluminum or tantalum. Examples of such embodiments are described in more detail hereinbelow.
Those skilled in the art will recognize that the number of distinct metals and their corresponding oxides may also be more than two, as illustrated in FIG. 1B. FIG. 1B is a flowchart of a second embodiment of a method for fabricating a periodic layered structure. As shown again in FIG. 1B, a suitable substrate is provided (step S10). In step S100, a layer 20 of a first metal is deposited (FIG. 3A). The first metal may be a non-porous electrochemically oxidizable material, but for some applications this first metal may be a metal, such as platinum, that is not readily oxidizable electrochemically. In step S110, a layer 35 of a second metal is deposited (FIG. 3B). The second metal is a non-porous electrochemically oxidizable material. The total thickness of these two layers of first and second metals is such that anodization of the second metal in the next step (step S120) will contribute suitably to the desired periodicity of the periodic layered structure. FIG. 3C shows second-metal-oxide layer 45 formed by anodic oxidation of second-metal layer 35. In step S130, another layer 55 of the first metal is deposited (FIG. 3D). In step S140, a layer 65 of a third metal is deposited
(FIG. 3E). The third metal is also a non-porous electrochemically oxidizable material. In step S150, the third metal is anodized. FIG. 3F shows third-metal-oxide layer 75 formed by anodization of third-metal layer 65.
If the second metal and third metals deposited in steps S120 and S140 are different from each other and different from the first metal as deposited in steps S100 and S130, the completion of anodization step S150 serves to provide one composite layer 95, i.e., one period, of the periodic layered structure. For some applications the series of steps S100-S150 is then repeated until the desired periodic layered structure is complete, as determined in decision step S160.
Thus, when the structure is complete after a suitable number of repetitions, completion of anodization step S150 ends the process. The structure is determined to be complete if the total structure thickness has reached the desired thickness and/or if the total number of layers provided is the desired total number. If the periodic layered structure is not complete, suitable steps S100, S110, S120, S130, S140, and S150 are repeated until the desired periodic layered structure is complete. If the first, second, and third metals shown in FIG. 1B comprise three different materials, for example, the suitable set of steps to be repeated may be the full set of steps S100-S150.
As shown by the dashed line in FIG. 1B, steps S130, S140 and S150 may be repeated separately (for example, if the third metal deposited in step S140 and anodized in step S150 is the same as the second metal deposited in step S110 and anodized in step S120 and if the corresponding thicknesses are equal). In that case, if the corresponding thicknesses are made equal, the completion of step S150 serves to provide two composite periodic layers, i.e., two periods, of the periodic layered structure.
Also, both the second metal deposited and anodized in steps S110 and S120 and the third metal deposited and anodized in steps S140 and S150 may be the same as the first metal as deposited in steps S100 and S130. In that case, if the corresponding thicknesses are also made equal, the completion of step S150 serves to provide three composite periodic layers, i.e., three periods of the periodic layered structure. Again, if the structure is complete as determined in decision step S160, this ends the process. Otherwise, suitable steps are repeated until the structure is complete. The structure is determined to be complete if the total structure thickness has reached the desired thickness and/or if the total number of layers provided is the desired total number.
In another example, an embodiment may be made that is related to the embodiments illustrated by FIG. 1B. In that embodiment, adjacent layers of two electrochemically oxidizable materials may be anodized together. For example, as previously described, the second and third metals in steps S110 and S140 respectively are non-porous electrochemically oxidizable materials. If the first metal deposited in steps S100 and S130 is also a suitable non-porous electrochemically oxidizable material, it can be anodized when the second metal is anodized in step S120 and/or when the third metal is anodized in step S150. Those skilled in the art will recognize that the anodization time, voltage, current density, composition and concentration of the electrolyte, temperature and/or other parameter may be suitably adjusted during this combined anodization step for a second layer to be anodized, e.g., when anodization of a first material to be anodized is complete.
Those skilled in the art will recognize that various suitable electrolytes and various suitable conditions of anodization may be used for different non-porous electrochemically oxidizable materials. For example, dense anodic oxide films on Al and Ta may be prepared in electrolytes based on citric acid, boric acid, ammonium tartrate, ammonium borate, and many others. Tungsten may be oxidized in sulfuric-acid-based electrolyte, for example, and zinc may be oxidized in NaOH and K₂Cr₂O₇, etc. In general, electrolytes may also include other surfactants and/or buffer materials.
Additional embodiments related to the embodiments illustrated by FIGS. 1A or FIG. 1B include embodiments in which a layer of a non-porous electrochemically oxidizable material is electrochemically oxidized substantially completely, i.e., substantially its entire thickness is converted to oxide. For example, in FIG. 1A, the layer of a first metal may be electrochemically oxidized completely in step S30 to form a layer of first-metal oxide, whereby the first metal is substantially replaced by the first oxide. Then, if the second metal were not completely electrochemically oxidized in step S50, the periodic layered structure would comprise repeated instances of a stack consisting of three sublayers: the oxide of the first metal, the second metal, and the oxide of the second metal. On the other hand, if the second metal were completely electrochemically oxidized in step S50, whereby the second metal is substantially replaced by the second oxide, then the periodic layered structure would comprise repeated instances of a stack consisting of two sublayers: the oxide of the first metal and the oxide of the second metal. Periodic layered structure embodiments employing many variations of such sequences may be readily constructed by those skilled in the art.
Thus, another aspect of the invention provides a method for fabricating a periodic layered structure including the steps of providing a substrate, depositing a quantity of a first metal over the substrate to form a first metal layer, depositing a quantity of a second metal over the first metal layer to form a second metal layer, anodizing both the first and second metal layers until a composite layer of first and second oxides is formed, and repeating the three steps (two depositions and one combined anodization) a number of times until a layered structure having a desired periodicity and a desired structure thickness is completed.
In all the embodiments illustrated in FIGS. 1A and 1B and the variations described above, the method may include one or more steps of planarization or polishing. The substrate may be planarized or polished if it is not already sufficiently planar or smooth. As mentioned hereinabove, the first metal layer may be planarized after its deposition, e.g., by mechanical polishing, chemical polishing, electrochemical polishing, chemical mechanical polishing (CMP), or other planarization technique. Similarly, the second and/or third metal layers may be planarized after their depositions if necessary.
The method embodiments illustrated by FIG. 1B may be generalized from the three metals enumerated in FIG. 1B to a more general method employing a number n of metals. Thus, another aspect of the invention provides embodiments of a method for fabricating a periodic layered structure, comprising the steps of providing a substrate, depositing a quantity of a first metal of the n metals over the substrate to form a first layer, anodizing the first metal until a desired thickness of first oxide is formed, and repeating the depositing and anodizing steps a number of times respectively for n metals until a layered structure having a desired periodicity and a desired structure thickness is completed. Again, each of the n metals may be a non-porous electrochemically oxidizable material, such as Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, Cr, or their alloys, mixtures, and combinations. Such periodic layered structures may be made in which the n metals all comprise the same material, the n metals comprise at least two different materials, or, more generally, the n metals comprise a number m of different materials, where m is less than or equal to n.
All of the periodic layered structure embodiments made by the methods described herein may be one of the types of layered structure known as a superlattice. The periodicities (pitch) of the periodic layered structures may be comparable to the thickness of native oxides commonly formed on metals that are electrochemically oxidizable, e.g., about 5 nanometers or greater.
It was mentioned above that a first metal in an embodiment of a periodic layered structure may comprise a metal, such as platinum, that is not readily oxidizable electrochemically. More generally, one or more of the layers of the periodic layered structure may comprise such a metal, e.g., platinum, palladium, or rhodium, while one or more other layers comprise a material that is readily oxidizable electrochemically.
Of the known electrochemically oxidizable metals, the anodization processes of tantalum and aluminum, especially, have been extensively studied. Thus, specific embodiments in which a first metal comprises aluminum or tantalum, and those in which a second metal comprises aluminum or tantalum (and the corresponding oxides formed are aluminum oxide and tantalum oxide respectively) provide good examples.
At least some of the method embodiments described herein are believed to operate in accordance with a common regime for electrochemical oxidation of metals such as tantalum (Ta), aluminum (Al), and other metals to produce dense oxides, including two major stages: galvanostatic and potentiostatic. During the galvanostatic stage, characterized by constant current density, steady state oxidation of metal occurs. During the potentiostatic stage, characterized by constant cell voltage, generally there is no more metal consumption, but the oxide layer thickness is still increasing due to diffusion of oxygen ions into the oxide matrix. However, the invention should not be construed as being limited to the consequences of any particular theory of operation.
FIGS. 4 and 5 illustrate quantitative details of the anodization process for tantalum. Anodization for the measurements shown in FIGS. 4 and 5 was performed at 20° C., using 0.1% citric acid in water as the electrolyte with current density of 0.5 milliamperes/square centimeter (mA/cm²), and using a platinum (Pt) cathode.
FIG. 4 is an exemplary graph showing resulting oxide thickness as a function of time as observed in performing the second anodization step on a tantalum metal layer while practicing an embodiment of a method for fabricating a periodic layered structure. Curve 210 shows how tantalum oxide thickness in nanometers (nm) shown on the vertical axis varied with time in minutes, shown on the horizontal axis. The final anodization voltage was fixed at 70 volts.
FIG. 5 is an exemplary graph showing resulting oxide thickness as a function of final anodization voltage as observed in performing the anodization step on a tantalum metal layer. Line 230 shows how tantalum oxide thickness in nanometers (nm) shown on the vertical axis varied with final anodization voltage in volts (V), shown on the horizontal axis. The final anodization voltage (V) is the constant voltage used in a potentiostatic step. The oxide thickness can depend on duration of the potentiostatic step if that duration is less than about 15 minutes, but that variation is reproducible and predictable, so that the thickness of the oxide can be controlled precisely. The anodization time at final voltage V for the anodization illustrated in FIG. 5 was 30 minutes. Line 230 shows that tantalum oxide thickness varied linearly, directly proportional to final anodization voltage. The anodization coefficient was 1.9 nanometer/volt (nm/V). Similar results for aluminum had an anodization coefficient of 1.3 nanometer/volt (nm/V).
As illustrated by FIGS. 6A-6C, another aspect of the present invention provides embodiments of a method for fabricating an imprinting stamp for lithography (e.g., a nano-imprinting stamp for nano-lithography). In such an embodiment, a suitable substrate 10 is provided, a quantity of a first metal is deposited over the substrate to form a first metal layer having a first-metal edge, and the metal is anodized to convert all of the first metal to its oxide, forming a first-metal oxide layer 45 having a first-metal oxide edge.
A second metal layer is deposited over the first-metal oxide, forming a second-metal layer 20 having a second-metal edge. Similarly to the other embodiments described hereinabove, the first-metal depositing, anodizing, and second-metal depositing steps are repeated alternately until a periodic layered structure having a desired periodicity and a desired structure thickness is completed (as shown in FIG. 6A). For simplicity of illustration, only a few layers of the periodic layered structure are shown in FIGS. 6A-6C. While FIG. 6A shows only two and a half periods of the periodic layered structure, those skilled in the art will recognize that any number of periods may be made. In FIG. 6A, layer 75 is the first-metal-oxide layer of the second period, layer 40 is the second metal of the second period, and layer 85 is the first-metal-oxide layer of a third (incomplete) period. Then the second-metal edge of each second-metal layer is selectively anodized, forming oxide edges 100 (as shown in FIG. 6B). The oxide edges 100 are selectively etched back (as shown in FIG. 6C), forming recesses 105 and leaving salient portions at the edges of the first-metal-oxide layers. The salient portions and intervening recesses form an imprinting stamp having the desired periodicity and dimensions.
Thus, another aspect of the invention provides embodiments of a method for fabricating an imprinting stamp for lithography (such as nano-lithography), comprising steps of providing a substrate, depositing a quantity of a first metal over the substrate to form a first metal layer having a first-layer thickness, anodizing the first metal until a desired thickness of first oxide is formed, depositing a quantity of a second metal over the first oxide, whereby a second metal layer having a second-metal layer edge is formed, repeating the previous three steps a number of times until a layered structure having a desired periodicity and a desired structure thickness is completed, selectively anodizing each second-metal layer edge until a desired thickness of second oxide is formed, and selectively etching back one of the two different oxides (the first oxide and second oxide), thus forming an imprinting stamp having the desired periodicity. The imprinting stamp has salient portions of one of the two oxides, separated by recesses where the other oxide has been etched back.
For this method embodiment, the first and second metals should comprise different metals, at least one of which can be electrochemically oxidized, such as Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, or their alloys, mixtures, or combinations.
For example, in FIGS. 6A-6C, the first-metal- oxide layers 45, 75, and 85 may be tantalum oxide formed by anodization of tantalum layers, and second- metal layers 20 and 40 may be aluminum. Together, the alternating tantalum oxide and aluminum layers form a periodic layered structure. Edges suitable for the next steps may be made, for example, by cutting, sawing, cleaving, and/or polishing the periodic layered structure. After the edges of aluminum layers 20 and 40 are anodized, aluminum oxide edges 100 are formed. These aluminum oxide edges 100 may be etched back with a selective etchant comprising a mixture of phosphoric acid (H₃PO₄), chromium oxide (CrO₃), and water.
A suitable etchant, for example, has about 5-40 wt. %,of H₃PO₄, about 2-15 wt. %, of CrO₃, and etching temperature of 80-100° C. This etchant does not appreciably etch the aluminum or tantalum oxide. When the aluminum oxide has been etched out, remaining recesses 105 have reproducible profiles and dimensions suitable for use as an imprinting stamp. The salient portions composed of tantalum oxide also have suitable reproducible profiles and dimensions.
Another embodiment of an imprinting stamp also has alternate layers of metal and oxide. To fabricate such an embodiment, a suitable substrate is provided, a quantity of metal is deposited over the substrate to form a metal layer having a metal edge, and the metal is anodized until a second-layer thickness of oxide is formed. As in other embodiments described hereinabove, the depositing and anodizing steps are repeated alternately until a periodic layered structure having a desired periodicity and a desired structure thickness is completed. Then the metal edge of each metal layer is selectively etched back to form a recess, whereby an imprinting stamp having the desired periodicity is formed. Alternatively, in principle, the edges of the oxide layers could be selectively etched using a suitably selective etchant, leaving salient portions of the metal layers. This etching may be performed for a predetermined time.
The method embodiments described above for fabricating an imprinting stamp for lithography are specific examples of a more general method for fabricating an imprinting stamp. The more general method includes steps of providing a periodic layered structure comprising layers of at least two materials differing in etch rate, and of selectively etching back the edge of at least one of the at least two materials to form recesses separated by salient portions, thereby making an imprinting stamp having the periodicity of the periodic layered structure. In the imprinting stamp embodiments disclosed herein, the periodic layered structure is provided by methods using electrochemical oxidation as described above.
FIGS. 7 and 8 are cross-sectional side elevation views of two particular embodiments of periodic layered structures. FIG. 9 shows a top plan view of a test pattern embodiment corresponding to the embodiments shown in FIGS. 7 and 8. Thus, FIGS. 7 and 8 are both cross-sectional side elevation views of the structure illustrated by FIG. 9.
The periodic layered structures of FIGS. 7 and 8 are formed on non-planar substrates, resulting in layers 20 and 30 having edges which are not simply straight lines. The non-planar substrates of FIGS. 7 and 8 are formed by deposition and patterning of a dielectric material feature 240 on a base substrate 10 (which itself may be planar as shown in FIGS. 7 and 8). The dielectric feature 240 in FIG. 7 has sloped sides, while the dielectric feature 240 in FIG. 8 has vertical sides. In both cases, periodic layered structures are formed over the dielectric features 240. Layers of the metals for the periodic layered structures are deposited over the dielectric features 240, using conventional deposition methods (such as atomic layer deposition (ALD), or chemical vapor deposition (CVD)), including deposition on side walls of the dielectric features 240. The layers may be deposited conformally over the dielectric features 240, using conventional conformal deposition methods. The metals are anodized as described hereinabove. While, for clarity of illustration, FIGS. 7 and 8 show only a few layers 20 and 30 in the periodic layered structures, many more layers may be used. In each case, when the periodic layered structure is complete, a dielectric material 250 may be deposited over the periodic layered structure. For many applications, material 250 may be the same material as dielectric feature 240. The resulting surface may be planarized, as shown in FIGS. 7 and 8. (FIGS. 7 and 8 show the structures after this planarization step.) If the periodic layered structures of FIGS. 7 and 8 are formed in accordance with the embodiments described above for imprinting stamps for lithography, they may be suitably etched to form the imprinting stamps, with imprinting features that are not simply straight lines, but form angles, corners, and curves, as shown in FIGS. 7-9.

INDUSTRIAL APPLICABILITY

Methods performed in accordance with the invention are useful for fabricating imprinting stamps for lithography. Structures made in accordance with the invention may also be used for photonic-crystal applications and, more generally, for many other applications requiring superlattices or other periodic layered structures.
Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims. For example, the steps of the various method embodiments may be performed in the order recited, or the order of steps may be varied somewhat. Functionally equivalent materials may be substituted for materials described in this specification and the claims. It is not intended that the methods and the resulting structures described should exclude from the periodic layered structures incorporation of layers that are not anodized. Thus, for example, an insulating layer that is not formed by electrochemical oxidation may be included in the stack of layers of the periodic layered structure. For specific examples of this, a metal or semiconductor layer may be thermally oxidized, or a layer of silicon oxide, silicon nitride, or diamond may be periodically deposited as one or more sublayers of the stack if desired for a particular application.

Claims

1. A method for fabricating a layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers, the method comprising the steps of:

a) providing a substrate,

b) depositing a quantity of non-porous electrochemically oxidizable material over the substrate to form a first layer having a first-layer thickness,

c) anodizing the electrochemically oxidizable material until a second-layer thickness of oxide is formed, and

d) repeating alternately the steps b) of depositing and c) of anodizing until a layered structure having a desired periodicity and a desired structure thickness is completed.

2. The method of claim 1, wherein the electrochemically oxidizable material comprises a non-porous material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

3. The method of claim 1, wherein the electrochemically oxidizable material comprises non-porous silicon (Si).

4. The method of claim 1, wherein the periodic layered structure comprises a superlattice.

5. The method of claim 1, wherein the first-layer thickness of the first layer is greater than or about equal to two nanometers (2 nm).

6. The method of claim 1, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

7. The method of claim 1, wherein the substrate is substantially planar.

8. The method of claim 1, wherein the steps are performed in the order recited.

9. The method of claim 1, further comprising the step of planarizing the first layer.

10. The periodic layered structure made by the method of claim 1.

11. A method for fabricating a layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers, the method comprising the steps of:

a) providing a substrate,

b) depositing a quantity of a first non-porous electrochemically oxidizable material over the substrate to form a first layer having a first-layer thickness,

c) anodizing the first layer until a desired thickness of first oxide is formed,

d) depositing a quantity of a second non-porous electrochemically oxidizable material over the first oxide,

e) anodizing the second non-porous electrochemically oxidizable material until a desired thickness of second oxide is formed, and

f) repeating the previous four steps b), c), d), and e) a number of times until a layered structure having a desired periodicity and a desired structure thickness is completed.

12. The method of claim 11, wherein the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprise different materials.

13. The method of claim 11, wherein each of the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprises a material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

14. The method of claim 11, wherein the first non-porous electrochemically oxidizable material comprises aluminum.

15. The method of claim 11, wherein the first non-porous electrochemically oxidizable material comprises tantalum.

16. The method of claim 11, wherein the second non-porous electrochemically oxidizable material comprises aluminum.

17. The method of claim 11, wherein the second non-porous electrochemically oxidizable material comprises tantalum.

18. The method of claim 11, wherein the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprise the same material.

19. The method of claim 18, wherein both the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprise the same material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

20. The method of claim 11, wherein the periodic layered structure comprises a superlattice.

21. The method of claim 11, wherein the first-layer thickness of the first layer is greater than or about equal to two nanometers (2 nm).

22. The method of claim 11, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

23. The method of claim 11, wherein the substrate is substantially planar.

24. The method of claim 11, wherein the steps are performed in the order recited.

25. The method of claim 11, further comprising the step of planarizing the first layer.

26. The method of claim 11, further comprising the step of planarizing the second layer.

27. The periodic layered structure made by the method of claim 11.

28. A method for fabricating a layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers, the method comprising the steps of:

a) providing a substrate,

c) anodizing the first non-porous electrochemically oxidizable material until a desired thickness of first oxide is formed, and

d) repeating steps b) and c) a number of times respectively for a number n of non-porous electrochemically oxidizable materials until a layered structure having a desired periodicity and a desired structure thickness is completed.

29. The method of claim 28, wherein each of the non-porous electrochemically oxidizable materials of the number n of non-porous electrochemically oxidizable materials comprises a material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

30. The method of claim 28 wherein the non-porous electrochemically oxidizable materials of the number n of non-porous electrochemically oxidizable materials all comprise the same material.

31. The method of claim 28 wherein the non-porous electrochemically oxidizable materials of the number n of non-porous electrochemically oxidizable materials comprise at least two different materials.

32. The method of claim 28 wherein the non-porous electrochemically oxidizable materials of the number n of non-porous electrochemically oxidizable materials comprise a number m of different materials, wherein m is less than n.

33. The method of claim 28 wherein the non-porous electrochemically oxidizable materials of the number n of non-porous electrochemically oxidizable materials comprise n different materials.

34. The method of claim 28, wherein the periodic layered structure comprises a superlattice.

35. The method of claim 28, wherein the first-layer thickness of the first layer is greater than or about equal to two nanometers (2 nm).

36. The method of claim 28, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

37. The method of claim 28, wherein the substrate is substantially planar.

38. The method of claim 28, wherein the steps are performed in the order recited.

39. The method of claim 28, further comprising the step of planarizing the first layer.

40. The method of claim 28, further comprising the step of planarizing the m^thlayer, wherein m is less than or equal to n.

41. The periodic layered structure made by the method of claim 28.

42. A method for fabricating a layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers, the method comprising the steps of:

a) providing a substrate,

b) depositing a quantity of a first non-porous electrochemically oxidizable material over the substrate to form a first electrochemically oxidizable layer having a first-layer thickness,

c) anodizing the first electrochemically oxidizable layer fully until a homogeneous layer of first oxide is formed, thereby substantially replacing the first electrochemically oxidizable layer with the first oxide,

d) depositing a quantity of a second material over the first oxide, and

e) repeating the previous three steps b), c) and d) a number of times until a layered structure having a desired periodicity and a desired structure thickness is completed.

43. The method of claim 42 wherein each of the first non-porous electrochemically oxidizable material and second material comprises a material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

44. The method of claim 42, wherein the first non-porous electrochemically oxidizable material and the second material comprise different materials.

45. The method of claim 42, wherein the second material comprises a material that is not electrochemically oxidizable.

46. The method of claim 42, wherein the second material comprises a metal selected from the list consisting of platinum, palladium, and rhodium.

47. The method of claim 42, wherein the first non-porous electrochemically oxidizable material comprises aluminum.

48. The method of claim 42, wherein the first non-porous electrochemically oxidizable material comprises tantalum.

49. The method of claim 42, wherein the second material comprises aluminum.

50. The method of claim 42, wherein the second material comprises tantalum.

51. The method of claim 42, wherein the first non-porous electrochemically oxidizable material and the second material comprise the same material.

52. The method of claim 42, further comprising the step of oxidizing the second material.

53. The method of claim 52, further comprising the step of forming an edge on the second material before oxidizing the second material.

54. The method of claim 53, wherein the step of oxidizing the second material is performed by anodizing its edge.

55. The method of claim 42, wherein the periodic layered structure comprises a superlattice.

56. The method of claim 42, wherein the first-layer thickness of the first electrochemically oxidizable layer is greater than or about equal to two nanometers (2 nm).

57. The method of claim 42, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

58. The method of claim 42, wherein the substrate is substantially planar.

59. The method of claim 42, wherein the steps are performed in the order recited.

60. The method of claim 42, further comprising the step of planarizing the first electrochemically oxidizable layer.

61. The method of claim 42, further comprising the step of planarizing the first oxide.

62. The method of claim 42, further comprising the step of planarizing the second material.

63. The periodic layered structure made by the method of claim 42.

64. A method for fabricating a layered structure having physical and chemical properties varying periodically at least along a direction perpendicular to its layers, the method comprising the steps of:

a) providing a substrate,

c) depositing a quantity of a second non-porous electrochemically oxidizable material over the first layer to form a second layer having a second-layer thickness,

d) anodizing both the first and second layers until a composite layer of first and second oxides is formed, and

e) repeating the previous three steps b), c) and d) a number of times until a periodic layered structure having a desired periodicity and a desired structure thickness is completed.

65. The method of claim 64 wherein each of the first and second non-porous electrochemically oxidizable materials comprises a material selected from the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

66. The method of claim 64, wherein the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprise different materials.

67. The method of claim 64, wherein the first non-porous electrochemically oxidizable material comprises aluminum.

68. The method of claim 64, wherein the non-porous electrochemically oxidizable material comprises tantalum.

69. The method of claim 64, wherein the second non-porous electrochemically oxidizable material comprises aluminum.

70. The method of claim 64, wherein the second non-porous electrochemically oxidizable material comprises tantalum.

71. The method of claim 64, wherein the first non-porous electrochemically oxidizable material and the second non-porous electrochemically oxidizable material comprise the same material.

72. The method of claim 64, wherein step d) is performed by partially anodizing at least the second layer.

73. The method of claim 64, wherein the periodic layered structure comprises a superlattice.

74. The method of claim 64, wherein the first-layer thickness is greater than or about equal to two nanometers (2 nm).

75. The method of claim 64, wherein the second-layer thickness is greater than or about equal to two nanometers (2 nm).

76. The method of claim 64, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

77. The method of claim 64, wherein the substrate is substantially planar.

78. The method of claim 64, wherein the steps are performed in the order recited.

79. The method of claim 64, further comprising the step of planarizing the first layer.

80. The method of claim 64, further comprising the step of planarizing the second layer.

81. The periodic layered structure made by the method of claim 64.

82. A method for fabricating an imprinting stamp for lithography, the method comprising the steps of:

a) providing a periodic layered structure having a periodicity, the periodic layered structure comprising layers of at least two materials differing in etch rate to a predetermined etchant, at least one of the layers being formed by electrochemical oxidation, the layers having edges of the at least two materials, and

b) selectively etching back the edge of at least one of the at least two materials with the predetermined etchant to form recesses separated by salient portions, thereby forming an imprinting stamp having the periodicity of the periodic layered structure.

83. The imprinting stamp made by the method of claim 82.

84. A method for fabricating an imprinting stamp for lithography, the method comprising the steps of:

a) providing a substrate,

b) depositing a quantity of non-porous electrochemically oxidizable material over the substrate to form a first layer having a first-layer thickness and a first-layer edge,

c) anodizing the non-porous electrochemically oxidizable material until a second-layer thickness of oxide is formed,

d) repeating alternately the steps b) of depositing and c) of anodizing until a periodic layered structure having a desired periodicity and a desired structure thickness is completed, and

e) selectively etching back one of the first-layer edge or the second-layer oxide of each layer to form a recess, whereby an imprinting stamp having the desired periodicity is formed.

85. The imprinting stamp made by the method of claim 84.

86. A method for fabricating an imprinting stamp for lithography, the method comprising the steps of:

a) providing a substrate,

d) depositing a quantity of a second material over the first oxide, whereby a second layer having a second-layer thickness is formed,

e) repeating the previous three steps b), c), and d) a number of times until a layered structure having a desired periodicity and a desired structure thickness is completed,

f) forming a second-layer edge of at least each second layer,

g) anodizing each second-layer edge until a desired thickness of second oxide is formed, and

h) selectively etching back one of the first oxide and second oxide, whereby an imprinting stamp having the desired periodicity is formed.

87. The method of claim 86, wherein the first non-porous electrochemically oxidizable material and the second material comprise different materials.

88. The method of claim 86, wherein the first non-porous electrochemically oxidizable material and the second material comprise different materials selected form the list consisting of Al, Ta, Nb, W, Bi, Sb, Ag, Cd, Fe, Mg, Si, Sn, Zn, Ti, Cu, Mo, Hf, Zr, Ti, V, Au, and Cr, and alloys, mixtures, and combinations thereof.

89. The method of claim 86, wherein the first non-porous electrochemically oxidizable material is aluminum and the first oxide is aluminum oxide.

90. The method of claim 86, wherein the second material is tantalum and the second oxide is tantalum oxide.

91. The method of claim 86, wherein the periodic layered structure comprises a superlattice.

92. The method of claim 86, wherein the first-layer thickness is greater than or about equal to two nanometers (2 nm).

93. The method of claim 86, wherein the second-layer thickness is greater than or about equal to two nanometers (2 nm).

94. The method of claim 86, wherein the periodicity is greater than or about equal to five nanometers (5 nm).

95. The method of claim 86, wherein the substrate is substantially planar.

96. The method of claim 86, wherein the steps are performed in the order recited.

97. The method of claim 86 further comprising the step of planarizing the first layer.

98. The method of claim 86 further comprising the step of planarizing the first oxide.

99. The method of claim 86, further comprising the step of planarizing the second layer.

100. The imprinting stamp made by the method of claim 86.