US20140272280A1

US20140272280A1 - Anodized aluminum oxide nanoporous membrane integrated with micro-channel and method of formation thereof

Info

Publication number: US20140272280A1
Application number: US13/845,514
Authority: US
Inventors: Nitin Afzulpurkar; Ajab Khan Kasi
Original assignee: ASIAN INSTITUTE OF TECHNOLOGY
Current assignee: ASIAN INSTITUTE OF TECHNOLOGY; Asian Inst of Tech
Priority date: 2013-03-18
Filing date: 2013-03-18
Publication date: 2014-09-18

Abstract

The present invention relates to an Anodized Aluminum Oxide nanoporous membrane integrated with micro channel and method of formation thereof. The invention further relates to formation of AAO pillars that are integrated in the membrane to create micro-channels to enhance mechanical stability and substantially reduce membrane thickness to nanometer range. This intrinsic configuration results in obviating the use of any external added material or support. The integrated membrane comprises of a substrate, plurality of alumina micro pillars that form respective micro-channels wherein the said pillars are attached with the substrate, nanoporous structure integrated with the pillars wherein the micro channel is formed between two consecutive pillars bound by the nanoporous structure surface and the substrate surface.

Description

FIELD OF INVENTION

The present invention relates to an Anodized Aluminum Oxide nanoporous membrane integrated with micro channel and method of formation thereof. The invention further relates to formation of AAO pillars that are integrated in the membrane to create micro-channels to enhance mechanical stability and substantially reduce membrane thickness to nanometer range.

BACKGROUND OF THE INVENTION

Anodized Aluminum Oxide (AAO) ultrathin membrane (having thickness in nanometer range) find the application in sensors, filters, separation process etc. When filtration is conducted very thin membrane is desirable. However, AAO is not mechanically stable because of the substantial distance between the supports of the membrane. To overcome this problem the thickness of the membrane needs to be increased. This poses problem in filtration and separation because substantial pressure differential is required to pass the medium through the thick membrane. The thin membrane has problem of withstanding the pressure necessary for mass transfer. There is need in the market place to reduce the thickness and provide mechanical stability at the same to sustain the pressure.
It is to be noted that in the conventional system and process for making substantially straight channels is very expensive due to the use of complex process and equipment such as ion milling that takes enormous time resulting in very high process cost. There is limitation of using chemical etching that results in irregular formation of the channels.
Patent application number 20120267337 discloses ultrathin porous nanoscale membranes, methods of making, and uses thereof. A process for forming a porous nanoscale membrane is described. The process involves applying a nanoscale film to one side of a substrate, where the nanoscale film includes a semiconductor material; masking an opposite side of the substrate; etching the substrate, beginning from the masked opposite side of the substrate and continuing until a passage is formed through the substrate, thereby exposing the film on both sides thereof to form a membrane; and then simultaneously forming a plurality of randomly spaced pores in the membrane. The resulting porous nanoscale membranes, characterized by substantially smooth surfaces, high pore densities, and high aspect ratio dimensions, can be used in filtration devices, microfluidic devices, fuel cell membranes, and as electron microscopy substrates.
United States Patent Application 20100219079 discloses methods for making membranes based on anodic aluminum oxide structures. It comprises of membranes including anodic aluminum oxide structures that are adapted for separation, purification, filtration, analysis, reaction and sensing. The membranes can include a porous anodic aluminum oxide (AAO) structure having pore channels extending through the AAO structure. The membrane may also include an active layer, such as one including an active layer material and/or active layer pore channels.
United States Patent Application 20110210259 discloses micro-channel plate detector. An anodized aluminum oxide membrane is provided and includes a plurality of nanopores which have an Al coating and a thin layer of an emissive oxide material responsive to incident radiation, thereby providing a plurality of radiation sensitive channels for the micro-channel plate detector.
WIPO Patent Application WO/2008/124062 discloses composite structures with porous anodic oxide layers and methods of fabrication. It includes a composite gas separation module having a porous metal substrate; a porous anodic aluminum oxide layer, wherein the porous anodic aluminum oxidelayer overlies the porous metal substrate; and a dense gas-selective membrane, wherein the dense gas-selective membrane overlies the porous anodic aluminumoxide layer.
Mahadi Hasan et al have reported “Anodic Aluminum Oxide (AAO) to AAO Bonding and Their Application for Fabrication of 3D Microchannel”. (Mahadi Hasan, Ajab Khan Kasi, Jafar Khan Kasi, and Nitin Afzulpurkar, “Anodic Aluminum Oxide (AAO) to AAO Bonding and Their Application for Fabrication of 3D Microchannel”, Nanoscience and Nanotechnology Letters, Vol. 4, 569-573, 2012.)
Dong Sung Kim et. al. report fabrication of micro channel containing nanopillar arrays using micromachined AAO mold. A fabrication method of a microchannel containing nanopillar arrays based on the soft lithographical replication of PDMS (polydimethylsiloxane) using a micromachined AAO (anodic aluminum oxide) master mold without any photolithographic processes is disclosed. (Dong Sung Kim, Han UI Lee, Dong Sung Kim, Kun-Hong Lee, Dong-Woo Cho, Fabrication of microchannel containing nanopillar arrays using micromachined AAO (anodic aluminum oxide) mold, Journal Microelectronic Engineering, Volume 84 Issue 5-8, May, 2007, Pages 1532-1535).
Seon Woo Lee et. al report development of AAO based micro-channel plate for large area photo detector (Seon Woo Lee, Qing Peng2, Anil U. Mane, Jeffrey W. Elam2, Karen Byruml, Henry Frisch and Hau Wang, The Development of Anodic Aluminum Oxide Based Micro-channel Plate for Large-area Photo-detector, 2010 MRS Fall Meeting)
In the prior art the attempt is made to prepare an ultrathin membrane with the use of micro-channel, however they suffer from the limitation of substantially high cost because of use of expensive process, material and machines that uses micro fabrication tools, clean room requirement, use of silicon. This semi-conductor technology is complex process steps and expensive. The scaling and use of such membranes for filtration is a challenge due to cost and scaling limitations.
It is required to provide intrinsic means to enhance the mechanical stability of the membrane obviating the use of any external added material or support. It is necessary to integrate the process of membrane preparation and the support for mechanical strength from a single substrate in a single process without separation of the membrane from the support. It is necessary that the support is integrated with the membrane itself.

SUMMARY OF THE INVENTION

The main object of the invention is to provide micro channel integrated Anodized Aluminum Oxide membrane and method of preparation thereof. Further object of the invention is to substantially reduce the membrane thickness in nanometer range.
Another object of the invention is to form micro-channels in the membrane substrate as an integrated structure.
Yet another object of the invention is to the membrane and the pillars are formed from the same substrate.
Another object of the invention is to enable formation of the supporting pillar structures from membrane.
Yet another object of the invention is to create cavity for the micro-channel from the said structure using selective etching of the membrane.
Yet another object of the invention is to form micro-channel using external support to provide mechanical stability to the pillars and in turn the nano-porous structure.
Yet another object of the invention is to enable attachment of the said pillars to the external surface that could be used as external support as per the end use.
Another object of the invention is to enable creation of the micro-cavity in the substrate that is bound by the integrated ultrathin membrane at one end. Further object of the invention is to use such a structure for sensing purpose.
Another object of the invention is to provide the micro-channel integrated membrane structure as a template for creating a desired structure on another substrate. Further object of the invention is to tailor the micro-channel dimensions as per the requirement of the configuration of the template to create desired structure on another substrate.
Thus in accordance with the invention the micro-channel integrated nanoporous membrane comprises of
plurality of alumina micro pillars to form respective micro-channels wherein the said pillars are attached with a surface/substrate with the help of adhesive layer; the nanoporous structure is supported by the said pillars and is integrated with the said pillars wherein the said nanoporous structure functions as a membrane
wherein the method comprises steps of:

- electro-polishing of the said substrate comprising steps of:
  - placing the said substrate in the mixture of perchloric acid and ethanol respectively wherein the ratio in the range of 1:3 to 1:5 by volume wherein purity of ethanol is in the range of 99%-99.9% and that of Perchloric acid is in the range of 69-72%;
  - Applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V;
  - Applying potential for 3 to 10 min depending on the surface roughness;
- first step anodization comprising steps of:
  - selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;
  - sing oxalic acid as electrolyte in the range of 0.2M to 0.3M;
  - applying a potential in the range of 35 to 45V wherein process time is in the range from 1 h to 6 h;
- chemical etching of the anodized aluminum oxide comprising steps of:
- etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%;
- second step anodization comprising steps of:
  - repeating the process in the first step anodization wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer wherein process time depends on the membrane thickness, which makes height of pillars, it can range from 1 h to 48 h depending on pillars height.
- selective etching of AAO comprising steps of
  - spin coating of positive photo resist at spinner speed 1000 to 3000 rpm for 10 to 20 seconds;
  - soft backing at 50 to 80° C. for 20 to 30 seconds;
  - placing mask of the desired texture;
  - UV exposure of 200 to 1000 W lamp for 2 to 10 seconds;
  - hard backing at 80 to 100° C. for 20 to 60 seconds;
  - development of photo resist for 5 to 10 seconds in developer;
  - selective chemical etching of aluminum oxide in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%, wherein etching time from 1 to 5 minutes;
  - chemical etching of photoresist in the specific etcher;
- anodization for making ultrathin nanoporous structure/membrane comprising steps of
  - selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;
  - using oxalic acid as electrolyte in the range of 0.2M to 0.3M;
  - applying a potential in the range of 35 to 45V wherein process time is in the range from 10 to 60 minutes.
- spin coating of adhesive on glass substrate at spinner speed 300 to 3000 rpm depending on adhesive viscosity
- attachment of glass with alumina pillars
- etching of Al comprising steps of
- chemical etching of Al substrate using CuCl2 and HCI
- wherein the concentration of CuCl2 is in the range of 0.2 to 0.25M and the concentration of HCI is in the range of 6 to 6.1M HCI, the temperature is in the range of 40 to 45° C. wherein etching time depends on Al thickness;
- Barrier layer (BL) removal by placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings. The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.

FIG. 1 illustrates schematic of the configuration of the AAO ultrathin membrane system

FIG. 2 illustrates schematic of the result of second step of anodization process

FIG. 3 illustrates schematic of the result of the selective etching process.

FIG. 4 illustrates schematic of the result of the anodization process that follows the selective etching process.

FIG. 5 illustrates schematic of the micro pillar attachment to the substrate

FIG. 6 illustrates schematic of the barrier layer formation

FIG. 7 illustrates schematic of the membrane after barrier layer removal

FIG. 8 illustrates schematic of the embodiment of the system

DEFINITION OF THE TERM USED

The term ultrathin nanoporous member used in the context of this invention refers to the membranes having thickness in the nanometer range.

DESCRIPTION OF THE INVENTION

In the following description, various embodiments will be disclosed. However, it will be apparent to those skilled in the art that the embodiments may be practiced with some or shall disclosed subject matter. For purposes of explanation, specific numbers, materials, and/or configuration are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without one or more of the specific details, or with other approaches, materials, components etc. In other instances, well-known structures, materials, and/or operations are not shown and/or described in detail to avoid obscuring the embodiments. Accordingly, in some instances, features are omitted and/or simplified in order to not obscure the disclosed embodiments. Furthermore, it is understood that the embodiments shown in the Figures are illustrative representation and are not necessarily drawn to scale.
The nanoporous alumina structure/membrane module is illustrated schematically in FIG. 1. It comprises of substrate 1, plurality of alumina micro pillars 2 to form respective micro-channels 5. The said pillars are attached with substrate with the help of adhesive layer 3. The ultrathin porous structure 4 is supported by the said pillars and is integrated with the said pillars. The said porous structure 4 functions as a membrane. In one of the embodiments, the substrate is made up of glass. In one of the preferred embodiments, the nanoporous structure/membrane is ultrathin having thickness is in the nanometer range.
The method of preparation comprises steps of:

- electro-polishing of the said substrate comprising steps of:
  - placing the said substrate in the mixture of perchloric acid and ethanol respectively wherein the ratio in the range of 1:3 to 1:5 by volume wherein purity of ethanol is in the range of 99%-99.9% and that of Perchloric acid is in the range of 69-72%;
  - applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V;
  - applying potential for 3 to 10 min depending on the surface roughness;
- first step anodization comprising steps of:
  - selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;
  - using oxalic acid as electrolyte in the range of 0.2M to 0.3M;
  - applying a potential in the range of 35 to 45V wherein process time is in the range from 1 h to 6 h;
- chemical etching of the anodized aluminum oxide comprising steps of:
- etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%;
- second step anodization comprising steps of:
  - repeating the process in the first step anodization wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer wherein process time depends on the membrane thickness, which results in the height of the micro pillars, it can range from 1 h to 48 h depending on pillars height.
- The result of second step of anodization is depicted schematically in FIG. 2. Plurality of micro pillars (indicated by numeral 20) of AAO from the substrate 21 are formed.
- selective etching of AAO comprising steps of
  - spin coating of positive photo resist at spinner speed 1000 to 3000 rpm for 10 to 20 seconds;
  - soft backing at 50 to 80° C. for 20 to 30 seconds;
  - placing mask of the desired texture;
  - UV exposure of 200 to 1000 W lamp for 2 to 10 seconds;
  - hard backing at 80 to 100° C. for 20 to 60 seconds;
  - development of photo resist for 5 to 10 seconds in developer;
  - selective chemical etching of aluminum oxide in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%, wherein etching time from 1 to 5 minutes;
  - chemical etching of photoresist in the specific etcher.

The result of this selective etching process is depicted schematically in FIG. 3. The cavity 32 is formed as a result of the selective etching in the plurality of micro pillars (indicated by numeral 30) of AAO.

- Anodization for making ultrathin membrane comprising steps of
  - selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;
  - using oxalic acid as electrolyte in the range of 0.2M to 0.3M;
  - applying a potential in the range of 35 to 45V wherein process time is in the range from 10 to 60 minutes.

The result of this anodization process is depicted schematically in FIG. 4. Plurality of micro pillars (indicated by the numeral 40) of AAO from the substrate 41 are formed in the said cavity. The surrounding plurality of AAO micro pillars are indicated by numeral 42.
The adhesive is applied on the on the glass using spin coating. The said micro pillars of alumina are attached to the glass substrate. In another embodiment the substrate is selected depending on the end use and application. Person skilled in art can contemplate such a variety of substrates. FIG. 5 illustrates the schematic of the system wherein the said micro pillars 50 (formed from the Al substrate 53) are attached to the substrate 51 with the aid of the adhesive 52.
To form the membrane, it is required to etch Al and remove barrier layer. It comprises steps of chemical etching of Al substrate using CuCl2 and HCI wherein the concentration of CuCl2 is in the range of 0.2 to 0.25M and the concentration of HCI is in the range of 6 to 6.1M HCI, the temperature is in the range of 40 to 45° C. wherein etching time depends on Al thickness. FIG. 6 illustrates the schematic indicating the formation of the barrier layer 60 as a result of the etching process.
The barrier layer (BL) is removed by placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. FIG. 7 illustrates the schematic after removal of the barrier layer to form the membrane.
One of the embodiments of the system is depicted in FIG. 8. It comprises of micro pillars (indicated by the numeral 80) surrounding the cavity 81. The membrane 82 is at the end of the cavity (as seen from top). In one of the variants of this embodiment, this system configuration is used in sensor application.

Claims

We claim:

1. An anodized aluminum oxide nanoporous membrane integrated with micro channel comprising of a substrate, plurality of alumina micro pillars that form respective micro-channels wherein the said pillars are attached with the substrate, nanoporous structure integrated with the pillars

wherein the micro channel is formed between two consecutive pillars bound by the nanoporous structure surface and the substrate surface.

2. An anodized aluminum oxide nanoporous membrane integrated with micro channel prepared in steps of

electro polishing of Al substrate

first step anodization;

chemical etching;

second step anodization;

selective etching of anodized aluminum oxide to form one or plurality of cavity/cavities;

anodization for forming anodized aluminum oxide structure in the said cavities;

attachment of the pillars to the substrate;

etching Al for separation of alumina and barrier layer removal or voltage pulse detachment for barrier layer removal and detachment of membrane from Al substrate.

3. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the process of electro polishing of Al sheet comprises steps of:

placing the said substrate in the mixture of perchloric acid and ethanol respectively wherein the ratio in the range of 1:3 to 1:5 by volume wherein purity of ethanol is in the range of 99%-99.9% and that of Perchloric acid is in the range of 69-72%;

applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V;

applying potential for 3 to 10 min depending on the surface roughness.

4. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the process of first step anodization comprising steps of:

selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;

using oxalic acid as electrolyte in the range of 0.2M to 0.3M;

applying a potential in the range of 35 to 45V wherein process time is in the range from 1 h to 6 h.

5. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the process of chemical etching of the anodized aluminum oxide comprising steps of:

etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of chromic acid is 99% and purity of phosphoric acid is 85%.

6. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the second step anodization comprises steps of:

using oxalic acid as electrolyte in the range of 0.2M to 0.3M;

applying a potential in the range of 35 to 45V wherein process time is in the range from 1 h to 6 h

wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer that results in the formation of the pillars.

7. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the selective etching of anodized aluminum oxide to form cavity/cavities comprises steps of:

spin coating of positive photo resist at spinner speed 1000 to 3000 rpm for 10 to 20 seconds;

soft backing at 50 to 80° C. for 20 to 30 seconds;

placing mask of the desired texture;

UV exposure of 200 to 1000 W lamp for 2 to 10 seconds;

hard backing at 80 to 100° C. for 20 to 60 seconds;

development of photo resist for 5 to 10 seconds in developer;

selective chemical etching of aluminum oxide in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt %, wherein etching time from 1 to 5 minutes;

chemical etching of photoresist in the specific etcher.

8. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the process of anodization for the formation of the anodized aluminum oxide structure in the cavities comprises steps of:

using oxalic acid as electrolyte in the range of 0.2M to 0.3M;

applying a potential in the range of 35 to 45V wherein process time is in the range from 10 to 60 minutes.

9. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein the process of attachment of the pillars to the substrate comprises steps of applying adhesive on the glass substrate surface using spin coating and attaching the pillars.

10. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 2 wherein barrier layer removal process comprises steps of chemical etching of Al substrate using CuCl2 and HCI wherein the concentration of CuCl2 is in the range of 0.2 to 0.25M and the concentration of HCI is in the range of 6 to 6.1 M HCI, the temperature is in the range of 40 to 45° C.

11. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 1 comprising of micro pillars surrounding a cavity wherein there is a membrane at the end of the cavity.

12. An anodized aluminum oxide nanoporous membrane integrated with micro channel as claimed in claim 1 wherein the substrate is selected from glass, metal, alloy, nonmetal, polymer based or any surface on which the membrane is supported.