Microfabricated particle thin film filter and method of making it

• US 5,948,255 A
• Filed: 05/02/1997
• Issued: 09/07/1999
• Est. Priority Date: 03/07/1994
• Status: Expired due to Term

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Ferrari, Mauro, Keller, Christopher G.
• Primary Examiner(s): Drodge, Joseph W.
• Application Number:
  US08/848,981
• Time in Patent Office: 858 Days
• Field of Search: 210/321.6, 210/321.84, 210/500.22, 210/490, 210/491, 210/500.25, 210/500.26, 210/243, 210/506, 216/2, 216/56, 427/2.1, 427/2.3, 427/245, 422/101, 428/314.2, 428/315.5, 428/315.9
• US Class Current: 210/321.84
• CPC Class Codes: B01D 2325/021 Pore shapes
• CPC Class Codes: B01D 2325/26 Electrical properties
• CPC Class Codes: B01D 2325/38 Hydrophobic membranes
• CPC Class Codes: B01D 39/1692 Other shaped material, e.g. perforated or porous sheets
• CPC Class Codes: B01D 39/20 of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires porous ceramic material C04B38/00 ; sintering metals C22C1/04; making porous sintered metal bodies B22F3/10, honeycomb filters B01D46/2418, materials used for filtering exhaust gases of an internal combustion engine F01N3/022, ceramic honeycomb structures C04B38/0006
• CPC Class Codes: B01D 67/003 by selective elimination of components, e.g. by leaching
• CPC Class Codes: B01D 67/0062 by micromachining techniques, e.g. using masking and etching steps, photolithography
• CPC Class Codes: B01D 69/02 characterised by their properties
• CPC Class Codes: B01D 69/1216 Three or more layers
• CPC Class Codes: B01D 71/0215 Silicon carbide; Silicon nitride; Silicon oxycarbide
• CPC Class Codes: B01D 71/022 Metals
• CPC Class Codes: C23C 16/01 on temporary substrates, e.g. substrates subsequently removed by etching
• CPC Class Codes: G03F 7/00 Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor using photoresist structures for special production processes, see the relevant places, e.g. B44C, H01L, e.g. H01L21/00, H05K
• CPC Class Codes: G03F 7/0015 Production of aperture devices, microporous systems or stamps

Abstract

A thin film filter fabricated using surface micromachining. The width of the filter pores is determined by the thickness of a sacrificial thin-film layer. This dimension can be precisely controlled, and may be as small as about 50 angstroms. The pore length may also be determined by the thickness of thin film layers and can therefore be smaller than the limit of resolution obtainable with photolithography. The filters are suitable for use at high temperatures and with many harsh solvents.

Claims

1. A filter, comprising:

a first thin film structure having openings therethrough; and

a second thin film structure having openings therethrough, said second thin film structure positioned relative to said first thin film structure such that said openings of said first thin film structure are partially blocked by said second thin film structure and said openings of said second thin film structure are partially blocked by said first thin film structure to produce pores of a substantially uniform predetermined width and length, said pores being spaces between said first thin film structure and said second thin film structure and said pores being formed as a result of removing a sacrificial layer from between said first thin film structure and said second thin film structure.

2. The filter of claim 1, wherein said length is in the range of about 1 micron to about 10000 microns.

3. The filter of claim 1 further including at least one reinforcing rib.

4. The filter of claim 3 wherein said rib has a height of between about 0.01 mm and about 0.25 mm.

5. The filter of claim 1 wherein said width is at most about 3000 angstroms.

6. The filter of claim 5 wherein said width is at most about 2000 angstroms.

7. The filter of claim 6 wherein said width is at most about 1000 angstroms.

8. The filter of claim 1 wherein said first and second thin film structures include silicon.

9. The filter of claim 8 wherein walls of said pores are doped with boron.

10. The filter of claim 9 wherein said pores have a length between about 1000 angstroms and about 5000 angstroms.

11. The filter of claim 9 wherein said length is in the range of about 2 microns to about 4 microns.

12. The filter of claim 1 wherein said second thin film structure forms a rivet-like shape underneath said first thin film structure.

13. The filter of claim 1 wherein said pores have a length between about 500 angstroms and about 5000 angstroms.

14. The filter of claim 1 further comprising a polymer matrix holding said first and second thin film structures.

15. The filter of claim 1 wherein said first and second thin film structures are conductive and are not in electrical contact.

16. The filter of claim 15 wherein at least one of said first and second thin film structures includes a metal film.

17. The filter of claim 16 having metal films on opposite sides of said at least one film structure.

18. The filter of claim 1 wherein said first and second thin film structures have hydrophobic surfaces.

19. The filter of claim 18 wherein said width is less than about 3000 angstroms.

20. The filter of claim 1 wherein said first and second thin film structures have surfaces with monoclonal antibodies bonded thereto.

21. The filter of claim 1 wherein said first and second thin film structures have surfaces with a chemical coating covalently bonded thereto.

22. The filter of claim 21 wherein said chemical coating can effectively crystallize below a freezing point to stop flow through said pores.

23. The filter of claim 21 wherein said chemical coating comprises highly polarizable chain molecules capable of being polarized by an electric field.

28. The filter of claim 1 or 26 wherein said pores have ends, said ends not being in line of sight of each other.

24. A method for fabricating a filter, the method comprising:

providing a first thin film structure having openings therethrough;

forming a first sacrificial layer over at least part of said first thin film structure;

forming a second thin film structure over said first thin film structure and said first sacrificial layer, said first sacrificial layer and said second thin film structure blocking said openings of said first thin film structure, and said second thin film structure having openings therethrough, said opening exposing a portion of said first sacrificial layer; and

etching said first sacrificial layer to produce pores of a substantially uniform predetermined width and length.

25. The method of claim 24, wherein said step of providing a first thin film structure includes the substeps of providing a wafer, growing a second sacrificial layer on said wafer, growing said first thin film structure on said second sacrificial layer, and patterning said first thin film structure;

and further comprising etching said second sacrficial layer after said second thin film structure is formed whereby the wafer may be reused.

26. A filter produced by the method of claim 24.

27. The filter of claim 26 wherein said pores are straight-through pores.

29. The method of claim 24 wherein said first sacrificial layer is formed by a chemical reaction at an exposed surface of said first thin film structure.

30. The method of claim 29 wherein said first thin film structure includes silicon and said chemical reaction is an oxidation.

31. The method of claim 29 wherein before said step of forming said first sacrificial layer, a portion of an initially exposed surface of said first thin film structure is covered with an anchor layer.

32. The method of claim 31 wherein said first and second thin film structures are conductive and said anchor layer is insulating.

33. The method of claim 32 wherein said first and second thin film structures include silicon and said chemical reaction is an oxidation.

34. The method of claim 33 wherein said anchor layer includes a material from the group consisting of alumina and silicon nitride.

35. The method of claim 24 wherein said first thin film structure is comprised of a sidewall structure.

36. The method of claim 24 wherein said step of forming said second thin film structure includes patterning openings over unetched portions of said first thin film structure.

37. The method of claim 24 wherein said step of forming said first sacrificial layer includes etching anchoring holes in said first sacrificial layer to expose said first thin film structure.

38. A method of fabricating a filter, the method comprising:

providing a first thin film structure having openings therethrough;

forming a first sacrificial layer over at least part of said first thin film structure;

forming a second thin film structure over said first thin film structure and said first sacrificial layer, said first sacrificial layer and said second thin film structure blocking said openings of said first thin film structure, and said second thin film structure having openings therethrough, said openings exposing a portion of said first sacrificial layer;

etching said first sacrificial layer to form pores; and

after etching said first sacraficial layer, growing a thin film on walls of the pores.

39. The method of claim 38 further including:

etching said thin film.