WO2001039285A2

WO2001039285A2 - Nanostructure light emitters using plasmon-photon coupling

Info

Publication number: WO2001039285A2
Application number: PCT/US2000/042026
Authority: WO
Inventors: David Crouse; Yu-Hwa Lo; Shabbir A. Bashar; Zuhua Zhu
Original assignee: Nova Crystals, Inc.
Priority date: 1999-11-08
Filing date: 2000-11-08
Publication date: 2001-05-31
Also published as: WO2001039285A3

Abstract

A metal plasma light emitter includes a metal-insulator-metal (MIM) structure (40) on a substrate (20). The MIM structure (40) includes a top metal layer (41) and a bottom metal layer (43) sandwiching an insulator layer (42) in between. A plurality of nanoholes are in at least the top metal layer. A passivation and frequency-matching layer (50) is formed over the MIM structure to reduce a mismatch in plasma frequencies at an output interface. The surface plasma oscillation is excited by hot electrons injected through the thin insulator layer (42). The devices generates light that matches the frequencies of surface plasma in metal thin films. The emission covers the spectrum in the visible and ultra violet (UV) regimes. The nanostructure built in the metal thin film enhances the external optical efficiency from 10-5 to as high as 10-1, making the device an extremely attractive light source for illumination, signs, displays, and biomedical applications.

Description

ELECTRON EXCITED LIGHT EMITTERS USING PLASMON

PHOTON COUPLING ENHANCED BY NANOSTRUCTURES AND

METHOD FOR MAKING SAME

FIELD OF THE INVENTION

The invention pertains to the field of solid state light emitters. More particularly, the invention pertains to metal plasma light emitters and a method for making same.

BACKGROUND OF THE INVENTION High efficiency solid state light emitters in the visible regime have found important applications in signs, displays, traffic lights, etc. The most popular device structure today is a light emitting diode (LED) made of semiconductors. The state-of-the-art semiconductor LEDs of red, orange, and yellow colors are made of InAlGaP compounds, and the LEDs of green and blue colors are made of InGaN compounds. To achieve high performance, the semiconductor material has to have high quality, not only being crystalline but also containing a minimum number of defects. This is a nontrivial task and requires expensive epitaxial growth equipment such as organmetallic chemical vapor deposition (OMCVD) machines. For LEDs to enter the mainstream lighting industry to replace incandescent light sources, the device cost has to be reduced by at least one order of magnitude from today's price of LED lamps. Although the technology for semiconductor LEDs is advancing rapidly, it is optimistically projected that it will take at least 10 years to penetrate the general lighting industry. Researchers are therefore looking for other efficient light sources that are potentially much cheaper than semiconductor LEDs.

One alternative approach for light generation is metal plasma induced light emission. The physical principle of metal plasma induced light emission has been known for decades. However, this approach has never been found practical for light generation because the highest quantum efficiency demonstrated so far is only about 10^"5, which is 3 to 4 orders of magnitude lower than the efficiency of high brightness semiconductor LEDs. When the electrons in metal move collectively with respect to their positive ions, metal plasma is excited. When the excitation occurs near the surface of a metal/dielectric interface, the excitation frequencies are reduced from the bulk plasma frequencies due to the dielectric material which may be oxide, nitride, quartz, or simply air. The plasma modes caused by oscillations of electrons near the metal/dielectric interface are called surface plasma. In other words, in metals, free electrons vibrate collectively, thereby producing a compression wave referred to as a plasma wave. The compression wave occurring on the metal surface and having been quantized is referred to as the surface plasmon. The resonant frequencies of the surface plasma depend not only on the material next to the metal but also on surface characteristics such as roughness and morphology.

Since the surface plasma frequencies are often in the near IR, visible, and UV frequency range, surface plasma generates light if a coupling mechanism between photon and surface plasmon exists.

The cost advantage of using metal thin films instead of semiconductors for light generation is tremendous. The metal layer does not need to be crystalline and can be deposited using simple techniques such as sputtering and evaporation. In addition, the metal thin film can be formed on essentially any substrates such as silicon, unlike semiconductor LEDs that have to be formed on much more expensive and fragile III-V substrates. The challenges for metal plasma light emitters are how to electronically excite the surface plasma and how to convert surface plasma into light.

Earlier work has shown that surface plasma can be excited by hot electron injection using a metal-insulator-metal (MIM) junction with top and bottom metal layers sandwiching an insulator between them. When a positive bias voltage is applied to the top metal layer, the electrons near the Fermi energy in the lower metal layer cross the thin insulator to reach the top metal. This electron transport is predominantly due to tunneling although transport due to thermionic emission is also possible. Once the electron reaches the top metal layer of the MIM structure, surface plasma is excited at the metal/insulator interface if the injected electrons have enough energy. The energy of injected electrons is typically within 1.5 eV of the applied bias voltage which roughly sets an upper limit for the electron energy. Different surface plasma frequencies equal to or lower than the electron energies can be excited. Using an Nu/alumina/Nl MIM junction as an example, the surface plasma energy excited by electrons falls in the range of 2 to 3.5 eV at a bias voltage of 3.5V. These energies happen to be in the visible and UV regimes.

The condition of phase matching, or in other words, conservation of momentum, has to be satisfied to achieve efficient plasmon-photon coupling. Since the wave vector of surface plasma is in the plane of the metal film (x-y plane), the phase matching condition only needs to be satisfied in the x- and y-directions. The magnitude of the in-plane k-vector for photons can not be over 2π/λ, which is on the order of 10^"3 N^"1. On the other hand, the magnitude of the in-plane k-vector for surface plasmons could be much larger as determined by the lateral momentum of the injected electrons that excite plasmons. Assuming that the electrons are accelerated by the bias voltage along the z-direction and the average energy of the electrons in the x-y plane is equal to the thermal energy, we calculate the magnitude of the in-plane k-vector for the electrons using Eq. (1). k_px * k_py * 2π (2 m₀ T)^,/2/h (1) where m₀, K, T, and h are the free electron mass, Boltzman constant, absolute temperature in kelvin, and the Plank constant, respectively, and k_px and k_py are the x- and y- components of the wave vector. Calculated from Eq. (1), k_px and k_py for surface plasmon are on the order of 0.1 N¹ at room temperature, which is two orders of magnitude greater than the wave vectors of photons. The large mismatch between the electron-excited surface plasmons and the photons makes it impossible for efficient plasmon-photon coupling without the introduction of artificial structures to make up the phase mismatch. Surface corrugations are one type of such artificial structure. The corrugations can be classified as gratings (periodic) or roughness (random) with the purpose of obtaining additional in-plane k-vectors to satisfy the phase matching condition. The plasmon/photon phase matching condition can then be represented as

where k_px and k_py are the x and y components of the wave vector for surface plasmon, k_oX and k_oy are the x and y components of the wave vector for photons, and K_x and K_y are the reciprocal lattice vectors for the surface grating or Fourier components of surface roughness. Because k_oX « k_px and k_oy « k_py, K_x and K_y have to be nearly equal to k_oX and k_oy to satisfy Eqs (2) and (3). K_px and k_py are on the order of 0.1 N^"1 which corresponds to a grating period of as small as 6 nm for effective plasmon-photon coupling. If the surface corrugation does not have an appreciable Fourier component at this short period, the plasmon-photon coupling will be extremely weak. This is one reason why the output efficiency for metal plasma MIM devices reported in the prior art has always been so low. SUMMARY OF THE INVENTION

Briefly stated, a metal plasma light emitter includes a metal-insulator-metal (MIM) structure on a substrate. The MIM structure includes a top metal layer and a bottom metal layer sandwiching an insulator layer in between. A plurality of nanoholes are in at least the top metal layer. A passivation and frequency-matching layer is formed over the MIM structure to reduce a mismatch in plasma frequencies at an output interface. The surface plasma oscillation is excited by hot electrons injected through the thin insulator layer. The devices generates light that matches the frequencies of surface plasma in metal thin films. The emission covers the spectrum in the visible and ultra violet (UV) regimes. The nanostructure built in the metal thin film enhances the external optical efficiency from 10-5 to as high as 10-1, making the device an extremely attractive light source for illumination, signs, displays, and biomedical applications. Since metal plasma light emitters are estimated to have less than one tenth of the cost of semiconductor LEDs, the invention opens up a real opportunity of using solid state devices to create energy saving, light weight, and long lived sources for general lighting. The new designs and implementation methods in this invention solve the low coupling efficiency problem, and as a result, are expected to raise the efficiency by at least 2 orders of magnitude to 10^"3 and finally to as high as 50%.

According to an embodiment of the invention, a metal plasma light emitter includes a substrate; a metal-insulator-metal (MIM) structure on the substrate, the MIM structure including a top metal layer and a bottom metal layer sandwiching an insulator layer in between; and a plurality of nanoholes in at least the top metal layer extending at least entirely through the top metal layer, wherein the plurality of nanoholes are either periodic or quasi-periodic with an average spacing of about 6 nm to about 200 nm. According to an embodiment of the invention, a method for making a metal plasma light emitter includes the steps of (a) providing a substrate; (b) depositing an aluminum layer on the substrate, wherein the aluminum layer is greater than about 0.5 μm thick; (c) anodizing the aluminum layer in an etching solution, wherein the aluminum layer is converted into a quasi-periodic self-ordered porous alumina layer having a plurality of holes in the alumina layer; (d) using the alumina layer as an etch mask, transferring a pattern of the plurality of holes to the substrate using a dry etching process, thus producing a plurality of holes in the substrate; (e) removing the alumina layer; (f) forming a bottom metal layer on the substrate and in each of the plurality of holes; (g) forming an insulator layer on the bottom metal layer; and (h) forming a top metal layer on the insulator layer.

According to an embodiment of the invention, a method for making a metal plasma light emitter includes the steps of (a) providing a substrate; (b) depositing a metal-insulator- metal (MIM) structure on the substrate, the MIM structure including a top metal layer and a bottom metal layer sandwiching an insulator layer in between; (c) depositing an aluminum layer on the top metal layer, wherein the aluminum layer is greater than about 0.5 μm thick; (d) anodizing the aluminum layer in an etching solution, wherein the aluminum layer is converted into a quasi-periodic self-ordered porous alumina layer having a plurality of holes in the alumina layer; (e) using the alumina layer as an etch mask, transferring a pattern of the plurality of holes to the top metal layer using a dry etching process, thus producing a plurality of holes in the top metal layer; and (f) removing the alumina layer.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A shows a part of a process flow and device structure for a first embodiment of the present invention.

Fig. IB shows a part of a process flow and device structure for the first embodiment of the present invention.

Fig. 1C shows a part of a process flow and device structure for the first embodiment of the present invention.

Fig. ID shows a part of a process flow and device structure for the first embodiment of the present invention.

Fig. IE shows a part of a process flow and device structure for the first embodiment of the present invention. Fig. 2 shows a SEM micrograph showing a top view of the device of the first embodiment.

Fig. 3 A shows a part of a process flow and device structure for a second embodiment of the present invention.

Fig. 3B shows a part of a process flow and device structure for the second embodiment of the present invention.

Fig. 3C shows a part of a process flow and device structure for the second embodiment of the present invention. Fig. 3D shows a part of a process flow and device structure for the second embodiment of the present invention.

Fig. 3E shows a part of a process flow and device structure for the second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invented device has a MIM structure for surface plasmon excitation and nanostructures for plasmon/photon coupling. Of the MIM structure, the metal that is positively biased receives electrons and is called a collector. Similarly, the metal that is negatively biased emits electrons and is called an emitter. Surface plasmons and photons are both generated in the collector. The collector is usually the top layer of the MIM structure although it is also possible to reverse the design and to put the collector physically at the bottom. For convenience, we define the "top layer" as the "collector" regardless of its physical position. Although the basic structure is similar to the previous design, this invention contains several unique features to be described next. It is these features that can increase the overall quantum efficiency by more than two orders of magnitude from 10^"5 to at least 10^" and eventually to as high as 50%.

Nanometer via holes in the top metal layer

The top metal thin film in which surface plasmon is excited is patterned with nanometer holes that penetrate through the top metal film. These nanoholes are periodic or quasi-periodic so an average spacing of the nanostructure can be defined. The average spacing should be no larger than 200 nm and the optimal average spacing is between 6 to 20 nm where an external quantum efficiency of greater than 1% can be achieved. If the period of the nanostructure is greater than 100 nm, the side wall of the structure needs to be as vertical as possible (greater than 70 degrees). It is believed that the device efficiency is primarily determined by two coupling processes, plasma coupling between two surfaces of the top metal layer and plasmon-photon coupling. The efficiency of both coupling processes is greatly enhanced by forming nanostructures satisfying the above requirements.

When electrons are injected through the insulator layer, surface plasmon is excited near the metal/insulator interface. The remainder of this paragraph is theoretical. We believe that the short mean free path for the energetic electrons above the surface plasmon energies makes it difficult to excite surface plasmons at the metal surface to the outside world. The electric field of the surface plasmons decays rapidly in metal due to the screening effect, so it is very difficult to couple the field through metal from the metal/insulator interface to the other interface. If the nanostructures have via holes, the two metal interfaces of the top metal layer are essentially connected and the E- field coupling can be greatly enhanced. Although the surface plasmons are originally excited near the metal/insulator interface, they can be coupled through the via holes, not through the metal itself, to the other interface. The side profile of the nanometer via holes are critical to the device efficiency, particularly when the period or average spacing between the via holes is greater than lOOnm. Calculations from Eq. (1) show that, at room temperature, the wavelength of the surface plasmon is about 6 nm due to thermal energy. We believe that the plasmon/photon coupling strength depends on the square of the magnitude of the Fourier components of the corrugation. The strongest coupling occurs when the via hole spacing is equal to or slightly greater than 6 nm. If the spacing is much greater than 6 nm, the strength relies on the high order Fourier components of the surface structure for plasmon/photon coupling. These high order Fourier components are significant only if the side profile of the via holes are nearly vertical. A 90-degree vertical side wall is most desirable in this regard. To the contrary, if the side profile of the via hole approaches the shape of a sinusoidal wave, the high order

Fourier components vanish and the coupling efficiency is minimal.

Front surface dielectric coating for plasma frequency matching

To the first order approximation, the surface plasma frequency is equal to ω_s = ω_b / ( 1 + ε)^{1 2} (4) where ω_s and ω_b are the angular frequencies of surface plasma and bulk plasma, respectively, and ε is the dielectric constant of the material next to the metal. The surface plasma frequency is lower than the bulk plasma frequency and dependent on the dielectric constant of the neighboring material because the E-field of the surface plasma is present in the dielectric material. When the surface plasma is excited by the hot electrons of the MIM structure, the plasma frequency is determined by the dielectric constant of the insulator according to Eq. 4. The dielectric constant in Eq. 4 is normally frequency dependent, is known as the dispersion relation of the material, and is always greater than 1 for normal dielectric material. If the output interface is the metal/air interface (ε=l for air and vacuum), the plasma frequency at the output interface is significantly higher than the plasma frequency at the metal/insulator interface. We theorize that the mismatch in plasma frequencies at both interfaces reduces the plasma coupling efficiency, thus reducing the device efficiency. Experimentally, the peak emission frequency approximately matches the lower of the two plasma frequencies. The coupling of the non-equal surface plasma frequencies becomes possible because of the strong scattering mechanisms (electron scattering and photon scattering) that cause linewidth broadening of the plasma resonant frequency and relax the requirement of energy conservation. However, if the top metal surface is preferably but optionally coated with material of a similar dielectric constant and dispersion property as the insulator in the MIM structure, the surface plasma frequency mismatch problem is eliminated and the device efficiency is enhanced.

MIM on substrates with arrays of nanoholes

Referring to Figs. 1 A- IE, the device structure and process flow is shown. Referring to Fig. 1A, an Al layer 10 is deposited on a Si substrate 20. Although nearly any solid state material available in wafer form can be used as the substrate, Si substrate is preferred because of its low cost and high material quality. Layer 10 is typically 1 to 5 μm thick, although there is no upper limit for the Al thickness, but the lower limit of the Al layer is around 0.5 μm.

Referring to Fig. IB, Al layer 10 is then anodized in an etching solution. The peculiar etching process converts the Al thin film into porous alumina (aluminum oxide) with a self-ordered honeycomb structure. A plurality of hexagonal holes 11 are etched, thereby forming a plurality of alumina pores 12. Holes 11 and pores 12 together make up a self-ordered porous alumina 30. The hexagonal quasi-periodic structure has a period of 10 to 200 nm, linearly proportional to the bias voltage. For example, a 75 nm period was obtained at 30 V bias during etching and a 50 nm period was obtained at 20 V bias. Among the key advantages of using self-ordered nanostructure over lithographically defined nanostructure are the much lower cost and easier process. Because the self-ordered nanostructure is formed electrochemically as a result of the minimization of the surface energy, the process is easy to control and reproduced. Referring to Fig. 1C, after the nanostructure alumina pores are formed, the pattern is transferred from the alumina to Si substrate 20 using dry etching (e.g., reactive ion etching) with the porous alumina as the etch mask, with a plurality of holes 13 corresponding to hexagonal holes 11 and a plurality of Si mesas 14 corresponding to pores 12. Referring to Fig. ID, a MIM structure 40 is formed by direct metal evaporation on the nanostructure patterned substrate surface of now patterned substrate 20. A top metal layer (collector) 41 is on an insulator 42 which in turn is on a bottom metal layer (emitter) 43. An aluminum layer may form bottom metal layer 43 after which the surface of the aluminum is oxidized to form insulator 42. The top metal layer 41 is preferably formed by evaporating aluminum on the insulator layer to complete MIM structure 40. The typical thickness of the MIM structure 40 is about 1000 N or less. Where the insulator 42 is no thicker than 100 N, the plasma excited top metal layer 41 is no thicker than 500N, with the remainder being the bottom metal layer 43. Since the total thickness of the MIM structure is less than the depth of the nano-holes, top metal layer 41 has via holes important for two critical coupling processes: plasma coupling between both sides of top metal layer 41 and plasmon/photon coupling.

Referring to Fig. IE, for the purposes of device passivation and surface plasma frequency matching, the MIM structure is covered with a passivation and plasma frequency matching layer 50 deposited as the top dielectric layer of a finished device 100. Layer 50 is preferably of a material of similar dielectric properties as the insulator in MIM structure

40. Aluminum oxide is one of the best choices in this case.

Referring to Fig. 2, a top view is shown of a MIM device deposited on a Si substrate with self-ordered nanostructure holes. Besides the nanostructure that should satisfy the previously stated requirements, the MIM structure can have many varieties. Au/AlO/Al, Ag/AlO/Al, and many others are among the most popular choices. The insulator thickness of the MIM structure typically falls between 20 and 100 N to maintain appropriate operating voltage and current density for light emitters.

Nanoholes etched through the MIM structure Referring to Figs. 3A-3E, another way to form the device is to etch the nanometer holes through the MIM structure as summarized in the device process shown. Referring to Fig. 3A, a MIM structure 60 is first formed on a substrate 20 such as silicon. MIM structure 60 includes a top metal (collector) layer 63 on an insulator 62 which in turn is on a bottom metal (emitter) layer 61.

Referring to Fig. 3B, a thick layer 70 of aluminum (typically 1 to 5 μm, although there is no upper limit for the Al thickness, but the lower limit of the Al layer is around 0.5 μm) is deposited on MIM structure 60 after which the aluminum is anodized in an etching solution.

Referring to Fig. 3C, similar to the method of the previous embodiment, a self- ordered hexagonal porous alumina layer 80 is formed of a plurality of hexagonal holes 81 and a plurality of alumina 82, with a period of the structure being proportional to an applied voltage during etching. A period of 10 to 200 nm is achieved with a proper choice of etch solution and voltage.

Referring to Fig. 3D, the pattern is transferred from self-ordered porous alumina layer 80 to top metal layer 63, producing a plurality of vias 65 in a collector 64, after which alumina 82 are removed. If top metal layer 63 of MIM structure 60 is etched by the same solution, the nanostructure can be directly transferred to the MIM layers. Otherwise, the nanostructure is formed in the alumina and the alumina is used as etch mask for subsequent pattern transfer by dry etching.

Referring to Fig. 3E, a top dielectric layer 90, which is a passivation and plasma frequency match layer, is deposited, resulting in a finished device 200. After the nanoholes are formed in the MIM structure, one can either remove the alumina layer and coat the surface with a new insulating layer for passivation and plasma frequency match or fill the alumina pores with newly deposited alumina.

Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments are not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.

What is claimed is:

Claims

1. A metal plasma light emitter, comprising: a) a substrate; b) a metal-insulator-metal (MIM) structure on said substrate, said MIM structure including a top metal layer and a bottom metal layer sandwiching an insulator layer in between; and c) a plurality of nanoholes in at least said top metal layer extending at least entirely through said top metal layer, wherein said plurality of nanoholes are one of periodic and quasi-periodic with an average spacing being between about 6 nm to about 200 nm.

2. A metal plasma light emitter according to claim 1, further comprising a passivation and frequency-matching layer over said MIM structure.

3. A metal plasma light emitter according to claim 2, wherein said passivation and frequency-matching layer and said insulator layer have similar dielectric properties.

4. A metal plasma light emitter according to claim 1, wherein said plurality of nanoholes have an average spacing between about 6 nm to about 20 nm.

5. A metal plasma light emitter according to claim 1, wherein when an average spacing between said plurality of nanoholes is between about 100 nm to about 200 nm, a verticality of a sidewall of each nanohole is greater than about 70 degrees.

6. A metal plasma light emitter according to claim 1 , wherein: said plurality of nanoholes extend into said substrate; and each of said nanoholes includes a second MIM structure within it.

7. A metal plasma light emitter according to claim 1, wherein said top metal layer is Al, said insulator layer is A1O, and said bottom metal layer is Al.

8. A metal plasma light emitter according to claim 1, wherein said top metal layer is Au, said insulator layer is A1O, and said bottom metal layer is Al.

9. A metal plasma light emitter according to claim 1, wherein said top metal layer is Ag, said insulator layer is A1O, and said bottom metal layer is Al.

10. A metal plasma light emitter according to claim 1, wherein a thickness of said MIM structure is less than about 1000 N.

11. A metal plasma light emitter according to claim 10, wherein a thickness of said insulator layer is less than about 100 A and a thickness of said top metal layer is less than about 500 N.

12. A method for making a metal plasma light emitter, comprising the steps of: a) providing a substrate; b) depositing an aluminum layer on said substrate, wherein said aluminum layer is greater than about 0.5 μm thick; c) anodizing said aluminum layer in an etching solution, wherein said aluminum layer is converted into a quasi-periodic self-ordered porous alumina layer having a plurality of holes in said alumina layer; d) using said alumina layer as an etch mask, transferring a pattern of said plurality of holes to said substrate using a dry etching process, thus producing a plurality of holes in said substrate; e) removing said alumina layer; f) forming a bottom metal layer on said substrate and in each of said plurality of holes; g) forming an insulator layer on said bottom metal layer; and h) forming a top metal layer on said insulator layer.

13. A method according to claim 12, further comprising the step of forming a passivation and frequency-matching layer over said top metal layer.

14. A method according to claim 13, wherein said passivation and frequency-matching layer and said insulator layer have similar dielectric properties.

15. A method according to claim 12, wherein said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 6 nm to about 200 nm.

16. A method according to claim 12, wherein said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 6 nm to about 20 nm.

17. A method according to claim 12, wherein: said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 100 nm to about 200 nm; and a verticality of a sidewall of each hole is greater than about 70 degrees.

18. A method according to claim 12, wherein said top metal layer is Al, said insulator layer is A1O, and said bottom metal layer is Al.

19. A method according to claim 12, wherein said top metal layer is Au, said insulator layer is A1O, and said bottom metal layer is Al.

20. A method according to claim 12, wherein said top metal layer is Ag, said insulator layer is A1O, and said bottom metal layer is Al.

21. A method according to claim 12, wherein a thickness of said insulator layer is less than about 100 N and a thickness of said top metal layer is less than about 500 N.

22. A method for making a metal plasma light emitter, comprising the steps of: a) providing a substrate; b) depositing a metal-insulator-metal (MIM) structure on said substrate, said

MIM structure including a top metal layer and a bottom metal layer sandwiching an insulator layer in between; c) depositing an aluminum layer on said top metal layer, wherein said aluminum layer is greater than about 0.5 μm thick; d) anodizing said aluminum layer in an etching solution, wherein said aluminum layer is converted into a quasi-periodic self-ordered porous alumina layer having a plurality of holes in said alumina layer; e) using said alumina layer as an etch mask, transferring a pattern of said plurality of holes to said top metal layer using a dry etching process, thus producing a plurality of holes in said top metal layer; and f) removing said alumina layer

23. A method according to claim 22, further comprising the step of forming a passivation and frequency-matching layer over said top metal layer.

24. A method according to claim 23, wherein said passivation and frequency-matching layer and said insulator layer have similar dielectric properties.

25. A method according to claim 22, wherein said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 6 nm to about 200 nm.

26. A method according to claim 22, wherein said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 6 nm to about 20 nm.

27. A method according to claim 22, wherein: said plurality of holes are one of periodic and quasi-periodic with an average spacing being between about 100 nm to about 200 nm; and a verticality of a sidewall of each hole is greater than about 70 degrees.

28. A method according to claim 22, wherein said top metal layer is Al, said insulator layer is A1O, and said bottom metal layer is Al.

29. A method according to claim 22, wherein said top metal layer is Au, said insulator layer is A1O, and said bottom metal layer is Al.

30. A method according to claim 22, wherein said top metal layer is Ag, said insulator layer is A1O, and said bottom metal layer is Al.

31. A method according to claim 22, wherein a thickness of said MIM structure is less than about 1000 A.

32. A method according to claim 31 , wherein a thickness of said insulator layer is less than about 100 A and a thickness of said top metal layer is less than about 500 A.