JP2012180536A - Anodized alumina using electron beam drawing method, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly regular anodized alumina film formed in an optional shape within an optional area by use of an electron beam drawing method, and a method for producing the same.SOLUTION: The method for producing the anodized alumina film includes: preliminarily aligning points of origin of pores on an SiOprotective layer surface on an Al thin film in a predetermined shape in a high regular manner by means of electron beam drawing, thereby controlling the regularity of pore diameters, pore intervals, and vertical lateral and depth directions. The anodized alumina film produced by the method is also provided. The resulting anodized alumina film can be used as a material for manufacturing a functional device and an insulator on a semiconductor device.

Description

The present invention relates to an anodized alumina film using an electron beam drawing method and a method for producing the same. More specifically, the present invention proposes a highly ordered anodized alumina film formed in an arbitrary shape in an arbitrary region using an electron beam drawing method and a method for manufacturing the same.

The anodized alumina film is a porous oxide film obtained by anodizing aluminum (Al) in an acidic, alkaline or neutral electrolyte solution, and the pores are perpendicular to the film surface. It has a hole array structure. For this reason, anodic alumina films are attracting interest as various functional devices and as a starting structure for producing various functional devices, and as high dielectric insulating films of semiconductor devices.

  Typical examples of functional application fields of anodized alumina films include magnetic recording media filled with magnetic material in pores, quantum elements filled with semiconductors and metals, optical elements filled with metals, and optics utilizing pore diameters. And a sonic element. In addition, it is also possible to manufacture a resistance change type memory element by filling pores with other functional oxides (zinc oxide, titanium oxide) or the like. Furthermore, in recent years, the anodized alumina film itself can also be used as a functional device material typified by a resistance change type memory element.

  In these functional applications, the regularity of the pore arrangement is extremely important in addition to the uniformity of the pore diameter. As an example, taking a magnetic recording medium as an example, it has been reported that the regularity of the pore arrangement contributes to the medium performance. In addition, disorder of the pore arrangement in the anodized alumina film causes distortion of the pore shape and nonuniformity of the pore diameter. Therefore, the regularity of the pore arrangement is an element that affects the formation of the anodized alumina itself. is there.

  It is known that the regularity of the pore arrangement of the anodized alumina film depends on the production conditions. References (Japanese Patent Laid-Open No. 2002-285382, Japanese Patent Laid-Open No. 2009-299188, two-step anodizing method) show that an anodized alumina film in which pores are regularly arranged can be obtained under appropriate anodizing conditions. Has been. However, it is known that the pitch and pore diameter of the regular arrangement obtained by this method depend on the anodizing conditions, particularly the anodizing voltage, and are defined by a pitch of 2.5 nm / V and a pore diameter of 1.0 nm / V. ing. Further, the regular arrangement region by this method is limited to a maximum of about several μm from the aluminum particles (and domains).

  In this two-step anodizing method, after anodizing for a long time from electropolishing of the substrate material, it is dissolved and removed with a mixed solution of phosphoric acid and chromic acid or alkali, and the resulting structure is used as a seed layer. As a result, an ordered structure is obtained by the second anodic oxidation. Therefore, the first stage of anodic oxidation requires long-term anodic oxidation, and the aluminum material used as the substrate also requires a thickness of 500 nm or more. Furthermore, when a noble metal is used as the substrate material, a large current flows at the moment when the hole bottom reaches the substrate material during anodization, and there is a restriction in use such that a large amount of gas is generated and the film structure is destroyed. .

In recent years, an imprint method has been devised as a method for obtaining an ordered structure by only one-step anodization. However, in this method, in order to produce a regular structure, a mold having a regular structure formed in advance using a material harder than aluminum such as silicon carbide (SiC), silicon (Si), and chromium (Cr). There is a need. An electron beam drawing method is usually used for the production of this mold (electron beam imprint method). It is a method of transferring the regular structure onto aluminum by using the mold as a starting point for anodization by pressing and transferring the resin onto a resin or SiO ₂ or aluminum, and the structure that can be produced is limited by the mold. . Furthermore, there is a possibility that regularity is deteriorated and mass productivity is lowered due to breakage of the mold due to mold pressing, and the degree of freedom is low.

Furthermore, an electron beam imprint method using electron beam drawing has been devised as a one-step anodic oxidation method. However, in the conventional electron beam imprinting method, the anodic oxidation pore formation starting point is formed by indentation on the resin on the aluminum film or on the aluminum film itself, and thus damage to aluminum cannot be avoided. For this reason, aluminum other than the part for producing the pore structure is also exposed to the anodizing process, and thus flatness cannot be avoided due to the formation of the pores.

In any of the above methods, in the formation of fine pores for a short time (thin), formation in the direction perpendicular to the film surface may be hindered by the influence of aluminum crystal grains (domains, grains, etc.).

JP 2009-299188 A JP 2006-213992 A JP 2002-285382 A JP 2010-229506 A

Q. Huang et al., APL 88 (2006) 233112, Observation of isolated nonporous formed by patterned anodic oxidation thin films S. Chen et al., JJAP 49 (2010) 015201, Nanoimprinting Pre-patterned effects on anodic aluminum oxide. H. Shirakiet al., Appl. Surf. Sci. 237 (2004) 369, Investigation of formation processes of an anodic porous alumina film on a silicon substrate.

The present invention has been made to solve the above-described problems in the prior art. In a wide range of several tens of nm to 600 μm (maximum exposure area in electron beam drawing), not only hexagonal but also an arbitrary shape such as a lattice shape is used. An object of the present invention is to provide an anodized alumina film formed with any possible size (pore diameter and pore spacing of 10 nm or more and 500 nm or less) and a method for producing the same.

  Here, the anodized alumina film in which pores are regularly arranged means that when a 0.3 mol / L oxalic acid aqueous solution is used, pores having a diameter of 40 nm are several hundred μm at a pitch of 100 nm at an anodic oxidation voltage of 40 V. A film having hexagonal and lattice-like arrangement without defects is produced in the following range.

  Another object of the present invention is to provide a functional device, particularly a resistance change memory, an insulating film for a semiconductor device, a filter, and a method for manufacturing the same, which can be manufactured using the anodized alumina film.

The invention of this application is primarily to solve the above problems,
A method for producing an anodized film comprising:
(1) On the Si substrate after degreasing and cleaning coated with SiO _2, an adhesion layer made of a 3d transition metal or a metal nitride is formed, and then an Al layer is formed.
(2) A SiO ₂ layer is formed on the Al layer,
(3) An electron beam positive resist is coated on the SiO ₂ layer,
(4) Draw a desired pattern on the electron beam resist coat by electron beam lithography,
(5) selectively dry-etching the SiO ₂ layer having a pattern formed on the surface;
(6) Anodizing the Al layer using the SiO ₂ layer as a mask,
(7) Provided is a method for producing an anodized alumina film, wherein a highly ordered anodized alumina film is formed in a step of completely removing the remaining resist coat.

  Second, the present invention provides a method for producing an anodic oxide film characterized in that the 3d transition metal is Ti or Cr and the metal nitride is TiN or AlN as the constituent material of the adhesion layer.

Third, the pattern formed by electron beam drawing is one of a lattice arrangement, a triangle arrangement, a pentagon arrangement, an octagon arrangement, or an arrangement in which the five kinds of arrangements are mixed. A method for manufacturing an oxide film is provided.

Fourth, production of an anodic oxide film characterized by using sulfuric acid, oxalic acid, phosphoric acid or chromic acid, and having an anodizing voltage of 200 V or less, a pore diameter of 200 nm or less and a pitch of 500 nm or less. Provide a method.

According to the method for producing an anodized film of the present invention, the following effects can be obtained.
(1) For the lower layer of Al, the same effect can be achieved even if Ti is replaced with a 3d transition metal such as Cr, or a nitride such as TiN and AlN in addition to Ti.
(2) The electron beam drawing pattern can be changed depending on the type of anodizing solution (for example, phosphoric acid, sulfuric acid, etc.).
(3) The anodized metal can be replaced with an anodizable metal (Si, Ti, etc.) other than Al.
(4) The controllability of the Al layer thickness is high by using a thin film.
(5) The substrate uses a Si substrate with thermally oxidized SiO ₂ in this manufacturing method, but higher electrical properties such as insulating (SrTiO ₃ , Al ₂ O ₃ , polyimide, etc.) and semiconducting (ZnO) substrates. It can be replaced with a substrate exhibiting resistance.
(6) By applying the present invention to a conventional semiconductor process, for example, it is possible to manufacture a high dielectric constant insulating film on a semiconductor device such as a MOS gate insulating film.
(7) By applying the present invention to a semiconductor process, it is possible to manufacture a specific wavelength filter with higher regularity, flatness, and film thickness controllability.
(8) Application as a mask material when forming nanostructures such as nanodots and nanorods on an arbitrary substrate on a substrate is also possible.

Schematic of an anodized alumina film. The SEM image (50,000 times) which shows the porous alumina structure on an aluminum substrate. Process flow diagram of electron beam imprint method. In the SEM image of the SiO ₂ protective layer after electron beam imprinting, an example in which the arrangement is a lattice and a hexagonal type is shown (100,000 times). Surface SEM image after anodic oxidation (50,000 times, anodic oxidation time 1 min). FIB-SEM cross-sectional image after anodization (150,000 times). In the cross-sectional image, a cross section is cut out in the direction perpendicular to the film surface, and observation is performed from a direction inclined 45 degrees from the vertical. Surface SEM image after anodization (50,000 times, anodization time 3 min). FIB-SEM cross-sectional image after anodization (150,000 times). The cross-sectional observation method is the same as in FIG. SEM (50,000 times) and FIB-SEM cross-sectional image (without SiO ₂ protective layer, 150,000 times) after electron beam imprint anodization of an Al thin film. The cross-sectional observation method is the same as in FIG. SEM image (50,000 times) and FIB-SEM cross-sectional image (150,000 times) of an electron beam imprint anodization (without Ti, without SiO ₂ film) of an Al thin film. The cross-sectional observation method is the same as in FIG. SEM image after anodization of an Al thin film (no electron beam imprint) (100,000 times).

The present invention has the features as described above, and an embodiment thereof will be described below.

First, a regular structure of the anodic oxidation was produced by direct drawing imprinting by an electron beam drawing method. As the method, an adhesion layer (Ti, Cr, etc.) and an Al thin film are formed on a Si substrate with thermally oxidized SiO ₂ by vacuum deposition or sputtering. Thereafter, a SiO ₂ layer serving as a protective layer is formed by sputtering. A regular structure is directly drawn on the SiO ₂ by electron beam lithography. SiO ₂ can selectively form a pattern by dry etching (RIE, F-based gas) without damaging the underlying Al. Thereafter, anodization is performed using SiO ₂ as a mask, and an Al layer is anodized. As a result, only the Al portion exposed to the anodizing solution is anodized, and a highly ordered anodized alumina having a pattern equivalent to electron beam drawing can be formed.

FIG. 3 is a process flow of a method for forming an anodized film using an electron beam imprint method. Details are described below.

First, the organic contaminant layer on the surface of the Si substrate with thermally oxidized SiO ₂ is degreased and cleaned using an organic solvent (acetone, ethanol). Thereafter, the organic solvent is removed with pure water or distilled water, and the organic substances and metal contamination layer adhering to the surface are removed by a semiconductor cleaning solution (Semico Clean 23: manufactured by Furuuchi Chemical Co., Ltd.), RCA cleaning method, and piranha cleaning method. To do.

After cleaning, the Si substrate is a Ti adhesion layer (growth rate 1 to 10 nm / min, film thickness 20 to 50 nm), Al thin film (growth rate) by vacuum deposition or sputtering in a high vacuum chamber of 10 ⁻⁴ Pa or higher. 1-10 nm / min, film thickness 100-500 nm).

As the adhesion layer, it is desirable that the physical bond or lattice constant is intermediate between the substrate material and the anodized material. In the 3d transition metal, it is known that the 2p electrons of O in SiO ₂ and the d electrons in the 3d transition metal are likely to cause orbital hybridization and have a strong bonding force. Further, it is known that the AlN and TiN nitride adhesion layers have very close lattice constants between the substrate material Si and the anodized material Al. Of course, a metal having a close number of lattices and a nitride having a strong bonding force for both the substrate and the anodized material are also selected as the adhesion layer.

Thereafter, when the film is formed by the RF sputtering method, the state is left as it is. When the film is formed by the vacuum evaporation method, in order to remove the natural oxidation layer of Al, in the sputtering chamber, an RF output of 100 W is reversed for 2 minutes. Sputtering is performed, and then a SiO ₂ protective layer having a thickness of 20 to 100 nm, more preferably 50 to 80 nm is formed. As a result, the Al surface can be anodized without being oxidized. All the steps so far may be performed at temperatures from 100 ° C. or lower to room temperature so as to be compatible with various substrate materials.

After film formation, an electron beam resist (ZEP520A: manufactured by Nippon Zeon Co., Ltd., positive type electron beam resist) is coated by spin coating to about 200 to 500 nm (depending on the pore diameter), and is required for an electron beam drawing apparatus. An ordered structure of the anodic oxide film is drawn.

After the drawing, the film is developed with an electron beam resist developer, and the resist in the pore forming portion is removed.

The developed sample is subjected to selective etching of only the SiO ₂ layer with an F-based gas such as CF ₄ or CHF ₃ in a RIE (Reactive Ion Etching) apparatus.

In this step, there is a high possibility that the electron beam resist is peeled off by the plasma of the etching apparatus. In the case where the resist remains on the surface, the resist is stripped by an electron beam resist stripping solution and O ₂ plasma treatment. If the resist remains in this process, it becomes a contaminant in the subsequent process. FIG. 4 is an image of the surface of the anodized film by SEM observation after all the steps are completed.

The regular pattern drawn on the electron beam resist by electron beam lithography is either a grid array, a triangle array, a pentagon array, an octagon array, or an array in which the five types of arrays are mixed. You can choose freely from home.

Moreover, the pitch and the hole diameter in the regular pattern drawn on the electron beam resist are limited by the anodizing solution used. As an anodizing solution, sulfuric acid, oxalic acid, phosphoric acid, and chromic acid are known. Table 1 shows the relationship between the voltage, hole diameter, and pitch at which a pattern with high regularity is naturally obtained in sulfuric acid, oxalic acid, and phosphoric acid that are often used. This is an appropriate condition for enhancing regularity by anodizing with a bulk material for a long time, but this condition is not necessarily followed when an artificial pattern is formed on a thin film and anodizing for a short time as in the present invention. There is no need.

<Example 1>
In a high vacuum deposition apparatus of 10 ⁻⁵ Pa or less, a 50 nm Ti adhesion layer is formed at a growth rate of 5.7 nm / min by RF sputtering on a Si substrate coated with a 200 nm SiO ₂ layer by thermal oxidation. Filmed. Further, an Al layer having a thickness of 500 nm was formed on the Ti layer at a growth rate of 8.2 nm / min. Thereafter, a SiO ₂ protective layer having a thickness of 60 nm was formed on the Al layer in a high vacuum state of 10 ⁻⁴ Pa or less by RF sputtering.

An electron beam resist ZEP520A was coated to 300 nm by spin coating on the substrate after film formation, and a regular hole structure having a pore diameter of 40 nm and a pitch of 100 nm was drawn with an electron beam drawing apparatus. This ordered structure is a structure corresponding to the pore structure of the alumina film by a two-step anodic oxidation method using a 0.3 mol / L oxalic acid aqueous solution. The sample after drawing and development was etched using a RIE apparatus for 70 s under conditions of a CF ₄ flow rate of 54 cm ³ / min, an O ₂ flow rate of 17 cm ³ / min, and an RF output of 80 W.

After the etching, anodization is performed. A Pt substrate was used for the cathode, and a 0.3 mol / L oxalic acid aqueous solution was used as the electrolyte in accordance with the regular structural period. The anodization time is 60 s. FIG. 5 shows a SEM image of the completed anodic oxide film, and FIG. 6 shows a cross-sectional image of the pores perpendicular to the film surface based on the FIB-SEM image.

<Example 2.>
In a high vacuum deposition apparatus at 10 ⁻⁵ Pa or less, a 20 nm Ti adhesion layer was formed at a growth rate of 6 nm / min by an electron beam deposition method on a Si substrate coated with a SiO ₂ layer by thermal oxidation treatment. Further, an Al layer of 500 nm was formed on the Ti layer at a growth rate of 6 nm / min. Thereafter, a 50 nm thick SiO ₂ protective layer was formed on the Al layer in a high vacuum state of 10 ⁻⁴ Pa or less by RF sputtering.

The substrate after film formation was coated with 200 nm of electron beam resist ZEP520A by spin coating, and a regular hole structure having a pore diameter of 40 nm and a pitch of 100 nm was drawn with an electron beam drawing apparatus. This ordered structure is a structure corresponding to the pore structure of the alumina film by a two-step anodic oxidation method using a 0.3 mol / L oxalic acid aqueous solution. The sample after drawing and development was etched using a RIE apparatus for 300 s under the conditions of CHF ₃ flow rate 2.0 cm ³ / min bias output 10 W and RF output 100 W.

After the etching, anodization is performed. A Pt substrate was used for the cathode, and a 0.3 mol / L oxalic acid aqueous solution was used as the electrolyte in accordance with the regular structural period. The anodization time is 180 s. FIG. 7 shows an SEM image of the completed anodic oxide film, and FIG. 8 shows a cross-sectional image perpendicular to the film surface of the pores by FIB-SEM image. From FIG. 8, it can be confirmed that the regular structure is proportional to the time and proceeds in the direction perpendicular to the film surface in the aluminum film. The etching rate (hole depth rate) at that time is 100 nm / min.

<Comparative Example 1>
Anodized alumina was produced in the same steps and conditions as in Example 1 without forming a SiO ₂ protective layer. FIG. 9 is a FIB-SEM image of the electron beam imprinted anodic oxide film in Comparative Example 1. Although the regular structure is formed in the same manner, regarding the surface flatness, crystal grains in the Al thin film and surface roughness due to anodic oxidation are observed. In addition, regarding the vertical direction of the film surface, in the hole traveling direction, a disorder of the regular arrangement in the vertical direction of the film surface is observed due to irregularities in the film in the current direction due to aluminum crystal grains and anodization.

<Comparative example 2>
Anodized alumina was produced in the same steps and conditions as in Example 1 without forming the Ti adhesion layer and the SiO ₂ protective layer. 10 is a surface SEM and cross-sectional FIB-SEM image of an electron beam imprinted anodic oxide film in Comparative Example 2. FIG. The hole structure and surface state depending on the Al crystal grains and the current direction are observed as in Comparative Example 1. Moreover, the progress of anodic oxidation to the Al exposed part other than the pattern part is observed. The conventional electron beam imprint method corresponds to the comparative example 2.

<Comparative Example 3>
Anodized alumina was produced in the same steps and conditions as in Example 1 without using electron beam imprinting. FIG. 11 is a surface SEM image of the anodized film in Comparative Example 3. There is no regularity, and the influence of Al crystal grains is observed. A conventional anodic oxidation method using an aluminum thin film corresponds to Comparative Example 3.

Of course, the present invention is not limited to the above examples, and it goes without saying that various aspects are possible in detail.

The present invention can solve the problem of liberalization of the regular structure in the conventional imprinting method, and can produce anodized alumina having a regular arrangement structure and a regular arrangement with different pore diameters and pore intervals, compared to the prior art. There is an advantage. Specific application examples include a resistance change type memory element using a regular structure, a quantum device filled with metal in the pore, a recording medium filled with a magnetic recording layer in the pore, and a rule in the pore. There are a resistance change type memory element filled with a conductive oxide, a sound wave and optical filter element using a pore diameter and an interval, and the like.

Further, the present invention can solve the problem of using an aluminum substrate in the conventional two-step anodic oxidation method, and can be formed even in an aluminum thin film having a thickness of 50 nm to 500 nm or less using an aluminum film formed on an arbitrary substrate. In addition, there is an advantage over the prior art in that an anodic oxide film can be produced even if it is converted to an anodizable metal other than aluminum. Specific examples of applications include resistance change memory elements using regular structures, quantum devices filled with metal in the pores, recording media filled with magnetic recording layers in the pores, pore diameter and spacing Acoustic wave and optical filter elements, insulating films on semiconductor elements, and the like.

In addition, the present invention can solve the problem of the rough surface of the aluminum in the conventional electron beam imprint anodizing method and the formation of an anodized film other than the necessary part, and can produce an anodized film without losing the flatness of the film at an arbitrary place. From the point that it can be done, there is an advantage that the affinity with the device formation process after this step is higher than that of the prior art. Specific application examples include a resistance change type memory element using a regular structure, a sound wave and optical filter element using a pore diameter and an interval, and an insulating film on a semiconductor element.

Claims (10)

A method for producing an anodized film comprising:
(1) On the insulating substrate after degreasing and cleaning coated with SiO ₂ , an Al layer is formed after forming an adhesion layer made of metal or metal nitride,
(2) A SiO ₂ layer is formed on the Al layer,
(3) An electron beam positive resist is coated on the SiO ₂ layer,
(4) Draw a desired pattern on the electron beam resist coat by electron beam lithography,
(5) The SiO ₂ layer having a pattern formed on the surface is selectively etched dry or wet,
(6) Anodizing the Al layer using the SiO ₂ layer as a mask,
(7) A method for producing an anodized film, wherein a highly ordered anodized alumina film is formed in a step of completely removing the remaining resist coat. A process according to claim 1, wherein the method of anodic oxide film, wherein the layer thickness of the SiO ₂ layer is in the range of 20 to 100 nm. 2. The method according to claim 1, wherein the thickness of the electron beam positive resist coat is in the range of 100 to 300 nm. 2. The method for producing an anodized film according to claim 1, wherein the Al layer is formed by CVD or sputtering. 2. The method for producing an anodic oxide film according to claim 1, wherein the SiO ₂ layer is formed by a PVD method. 2. The method for producing an anodized film according to claim 1, wherein the metal is a 3d transition metal. The method for producing an anodized film, wherein the 3d transition metal according to claim 6 is Ti or Cr. The method for producing an anodic oxide film, wherein the metal nitride according to claim 1 is TiN or AlN.
2. The manufacturing method according to claim 1, wherein the pattern formed by electron beam drawing is a grid array, a triangle array, a pentagon array, an octagon array, or an array in which the five types of arrays are mixed. A method for producing an anodic oxide film, comprising: 2. The production method according to claim 1, wherein sulfuric acid, oxalic acid, phosphoric acid or chromic acid is used, and when the anodic oxidation voltage is 200 V or less, the pore diameter is 200 nm or less and the pitch is 500 nm or less. A method for producing an anodic oxide film.