Process for fabricating an array of nanowires

• US 6,231,744 B1
• Filed: 04/22/1998
• Issued: 05/15/2001
• Est. Priority Date: 04/24/1997
• Status: Expired due to Fees

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Zhang, Lei, Ying, Jackie Y., Dresselhaus, Mildred S., Zhang, Zhibo
• Primary Examiner(s): Wong, Edna
• Application Number:
  US09/064,439
• Time in Patent Office: 1,119 Days
• Field of Search: 205/213, 205/221, 205/223, 205/131, 205/324, 205/328, 205/206, 427/294
• US Class Current: 205/324
• CPC Class Codes: B82B 1/00 Nanostructures formed by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
• CPC Class Codes: B82Y 30/00 Nanotechnology for materials or surface science, e.g. nanocomposites
• CPC Class Codes: C30B 11/00 Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method C30B13/00, C30B15/00, C30B17/00, C30B19/00 take precedence; under a protective fluid C30B27/00
• CPC Class Codes: C30B 29/02 Elements
• CPC Class Codes: C30B 29/605 Products containing multiple oriented crystallites, e.g. columnar crystallites
• CPC Class Codes: H01L 21/31111 by chemical means
• CPC Class Codes: H01L 21/76885 By forming conductive members before deposition of protective insulating material, e.g. pillars, studs
• CPC Class Codes: Y10S 977/762 Nanowire or quantum wire, i.e. axially elongated structure having two dimensions of 100 nm or less
• CPC Class Codes: Y10S 977/781 Possessing nonosized surface openings that extend partially into or completely through the host material
• CPC Class Codes: Y10S 977/888 Shaping or removal of materials, e.g. etching
• CPC Class Codes: Y10T 428/12431 Foil or filament smaller than 6 mils
• CPC Class Codes: Y10T 428/12486 Laterally noncoextensive components [e.g., embedded, etc.]
• CPC Class Codes: Y10T 428/1259 Oxide

Abstract

An array of nanowires having a relativley constant diameter and techniques and apparatus for fabrication thereof are described. In one embodiment, a technique for melting a material under vacuum and followed by pressure injection of the molten material into the pores of a porous substrate produces continuous nanowires. In another embodiment, a technique to systematically change the channel diameter and channel packing density of an anodic alumina substrate includes the steps of anodizing an aluminum substrate with an electrolyte to provide an anodic aluminum oxide film having a pore with a wall surface composition which is different than aluminum oxide and etching the pore wall surface with an acid to affect at least one of the surface properties of the pore wall and the pore wall composition.

Claims

1. A method for providing a substrate for use in the fabrication of a nanowire array comprising:

a) preparing an aluminum substrate;

b) anodizing the aluminum substrate using an acidic electrolyte solution to provide a porous aluminum oxide film on a surface of said aluminum substrate;

c) contacting said porous aluminum oxide film with an acid etchant solution for a period of time sufficient to enlarge the cell size of the pores of said aluminum oxide film, said acid selected from the group consisting of H₃PO₄ and H₂C₂O₄; and

d) contacting said etched film from step (c) with a solution of H₂SO₄.

2. The method of claim 1 wherein said aluminum substrate is polished prior to anodization.

3. The method of claim 1 wherein the aluminum substrate is anodized under constant voltage conditions.

4. The method of claim 1 wherein:

the acidic electrolyte solution has a weight of H₂SO₄ in the range of about 15 percent to about 20 percent; and

the anodizing voltage has a value in the range of about 5 volts to about 30 volts.

5. The method of claim 4 where:

the acidic electrolyte solution has a weight of H₂SO₄ in the range of about 15 percent to about 20 percent; and

the anodizing voltage has a value in the range of about 13 volts to about 25 volts.

6. The method of claim 1 wherein:

the acidic electrolyte solution has a weight of H₂C₂O₄ of about 1 to about 8 percent; and

the anodizing voltage has a value in the range of about 5 volts to about 120 volts.

7. The method of claim 6 wherein:

the acidic electrolyte solution has a weight of H₂C₂O₄ of about 4 percent; and

the anodizing voltage has a value in the range of about 30 volts to about 60 volts.

8. The method of claim 1 wherein:

the acidic electrolyte solution has a weight of H₃PO₄ in the range of about 4 percent to about 8 percent; and

the anodizing voltage has a value in the range of about 5 volts to about 200 volts.

9. The method of claim 8 wherein:

the acidic electrolyte solution has a weight of H₃PO₄ in the range of about 4 percent to about 8 percent; and

the anodizing voltage has a value in thie range of about 15 volts to about 120 volts.

10. The method of claim 1 where the step of anodizing includes the step of providing a current having an amplitude between about 1 and about 200 mA/cm² of substrate surface.

11. The method of claim 10 where the step of anodizing includes the step of providing a current having an amplitude between about 1 and about 80 mA/cm² of substrate surface.

12. The method of claim 11 where the step of anodizing includes the step of providing a current having an amplitude between about 5 and about 40 mA/cm² of substrate surface.

13. The method of claim 1 wherein the anodizing electrolyte temperature is in the range of about −

5°

C. to about 50°

C.

14. The method of claim 13 wherein the anodizing electrolyte temperature is in the range of about 0°

C. to about 10°

C.

15. The method of claim 1 wherein the acidic electrolyte solution comprises H₂C₂O₄ at a weight in the range of about 1 percent to about 8 percent.

16. The method of claim 1 wherein the acidic electrolyte solution comprises H₃PO₄ at a weight in the range of about 1 percent to about 40 percent.

17. The method of claim 16 wherein the acidic electrolyte solution has a weight of H₃PO₄ in the range of about 4 percent to about 8 percent.

18. The method of claim 1 wherein said step of preparing an aluminum substrate comprises electrochemical polishing of said substrate.

19. The method of claim 18 wherein the step of electrochemical polishing includes the steps of:

calcining the substrate in air at a predetermined temperature for a predetermined period of time to provide the substrate having a uniform surface oxide layer;

placing the aluminum substrate in an electrolyte provided from a polishing acid solution;

providing a cathode; and

applying a voltage.

20. The method of claim 19 wherein the composition of the polishing acid solution is 95 vol % phosphoric acid solution (85 wt %)+5 vol % sulfuric acid solution (97 wt %)+20 g/l chromium oxide (CrO₃) and whereirn the temperature of the electrolyte is about 85°

C.

21. The method of claim 19 wherein said voltage is in the range of about 20 to about 24 V.

22. The method of claim 1 wherein the cell size of said porous aluminum oxide film prepared in step (b) is determined in accordance with the formula C=[M]mV, wherein C is the cell size in nanometers, V is the anodizing voltage and [M]m is a constant in the range of 2.0 to 2.5.

23. The method of claim 1 further including the step of:

e) injecting molten material into the pores of said aluminum oxide film to form a plurality of nanowires.

24. The method of claim 23 wherein:

the molten material is provided from a material having a melting temperature lower than a temperature at which the substrate undergoes a structural change.

25. The method of claim 24 wherein the molten material is provided from a metal, a semiconductor, an alloy or a polymer gel.

26. The method of claim 23 further comprising the step of removing the aluminum oxide film from the aluminum substrate.

27. The method of claim 23 wherein said anodic aluminum oxide film is removed from said substrate after step (e).

28. The method of claim 23 wherein said nanowires have a diameter in the range of about 1 to 500 nm.

29. The method of claim 28 wherein said nanowires have a diameter in the range of about 8 to 200 nm.

30. The method of claim 23 wherein the step of injecting molten material comprises melting a material in a vacuum to provide the molten material and injecting the molten material with a pressure in the range of 0 bar to about 350 bar.

31. The method of claim 30 wherein said molten material is a metal.

32. The method of claim 31 further comprising the step of cooling the molten metal injected into said plurality of pores at a predetermined rate such that said nanowires form a single crystal structure.

33. The method of claim 31 wherein said metal comprises bismuth and said pressure is in the range of 0 bar to about 115 bar.

34. The method of claim 1 wherein the acidic electroyte solution comprises H₂SO₄ at a weight of H₂SO₄ in the range of about 5 percent to about 40 percent.

35. The method of claim 34 wherein the electrolyte solution has a weight of H₂SO₄ in the range of about 15 percent to about 20 percent.

36. The method of claim 35 wherein the electrolyte solution has a weight of H₂SO₄ in the range of about 1 percent to about 20 percent.