Cascaded self-induced holography

• US 5,760,960 A
• Filed: 08/07/1996
• Issued: 06/02/1998
• Est. Priority Date: 05/19/1995
• Status: Expired due to Term

• Current Assignee: Cornell Research Foundation Incorporated (Cornell University)
• Original Assignee: Cornell Research Foundation Incorporated (Cornell University)
• Inventors: Lin, Chih-Hsien Jason, Lo, Yu-Hwa
• Primary Examiner(s): Henry, Jon W.
• Application Number:
  US08/694,477
• Time in Patent Office: 664 Days
• Field of Search: 359/566, 359/569, 359/571, 359/573, 359/576, 359/577, 359/1, 359/34, 359/35, 430/1, 430/2, 430/321, 385/37
• US Class Current: 359/569
• CPC Class Codes: G02B 5/1857 using exposure or etching means, e.g. holography, photolithography, exposure to electron or ion beams
• CPC Class Codes: G02B 6/02138 based on illuminating a phase mask
• CPC Class Codes: G02B 6/124 Geodesic lenses or integrated gratings

Abstract

Cascaded diffraction grated pairs are employed to form submicron gratings and other patterns in various types of devices using photolithographic, UV exposure and other techniques. Cascaded grating pairs are formed in masks which are positioned between a source of radiation and the device or substrate in which a grating is to be formed. The interaction of the diffracted beams generated by each diffraction grating in a cascade grating pair results in the device grating having a period which is substantially smaller than that of either of the gratings in the cascaded grating pair. As a result, gratings with substantially smaller periods than were previously obtainable can now be achieved using conventional fabrication techniques. To eliminate sensitivity to background exposure in photolithographic fabrication applications, a mask is employed in which first and second cascaded grating pairs are separated from one another by an opaque region that acts as a spatial filter. The mask is positioned above the target device or substrate at a distance which causes the diffracted radiation beams from each of the cascaded grating pairs to interfere with one another on the device in the area where the diffraction grating is to be formed.

Claims

1. Apparatus for forming diffraction gratings in a material comprising:

a) a radiation source for projecting radiation onto a radiation sensitive portion of a material to form a pattern therein;

b) a first cascaded diffraction grating pair for receiving radiation from said radiation source and projecting a first diffraction grating pattern on a material, said first cascaded diffraction grating pair comprising;

1) a first diffraction grating for forming a first diffracted radiation beam, said first diffraction grating having a first grating period; and

2) a second diffraction grating for diffracting said first diffracted radiation beam and forming a second diffracted radiation beam, said second diffraction grating having a second grating period, and said first diffraction grating pattern having a grating period which is smaller than said first and second grating periods;

c) a second cascaded diffraction grating pair positioned in a plane parallel to said first cascaded diffraction grating pair, and comprising;

1) a third diffraction grating disposed in a plane parallel to said first diffraction grating for receiving radiation from said radiation source and forming a third diffracted radiation beam, said third diffraction grating having a third grating period that is substantially equal to said first grating period; and

2) a fourth diffraction grating disposed in a plane parallel to said second diffraction grating for diffracting said third diffracted radiation beam and forming a fourth diffracted radiation beam which interferes with said second diffracted radiation beam, thereby forming said first diffraction grating pattern, said fourth diffraction grating having a fourth grating period that is substantially equal to said second grating period; and

d) spatial filtering means to be positioned between said radiation source and a material in which a diffraction grating is to be formed.

2. The apparatus of claim 1, wherein said first and third diffraction gratings are formed on a top portion of a transparent substrate at first and second ends thereof, respectively;

said second and fourth diffraction gratings are disposed on a bottom portion of said substrate at said first and second ends, respectively; and

said spatial filtering means comprises an opaque section of said substrate disposed between said first and second ends.

3. The apparatus of claim 1, further comprising:

e) third and fourth cascaded grating pairs positioned in a plane parallel to one another for receiving radiation from said radiation source and projecting a second diffraction grating pattern on a material, said third and fourth cascaded grating pairs comprising;

1) a fifth diffraction grating for receiving radiation from said radiation source and forming a fifth diffracted radiation beam, said fifth diffraction grating having a fifth grating period;

2) a sixth diffraction grating for diffracting said fifth diffracted radiation beam and forming a sixth diffracted radiation beam, said sixth diffraction grating having a sixth grating period;

3) a seventh diffraction grating positioned in a plane parallel to said fifth diffraction grating for receiving radiation from said radiation source and forming a seventh diffracted radiation beam, said seventh diffraction grating having a seventh grating period that is substantially equal to said fifth grating period; and

4) an eighth diffraction grating positioned in a plane parallel to said sixth diffraction grating for diffracting said seventh diffracted radiation beam and forming an eighth diffracted radiation beam which interferes with said sixth diffracted radiation beam to form a second diffraction pattern, said eighth diffraction grating having a grating period that is substantially equal to said sixth grating period;

wherein, said fifth and seventh grating periods differ from any of said first through fourth grating periods by a preselected amount, and said sixth and eighth grating periods differ from any of said first through fourth grating periods by an amount equal, but opposite, to said preselected amount;

whereby, said first and second diffraction grating patterns having grating periods which differ from one another.

4. The apparatus of claim 3, wherein said first through fourth grating periods are substantially equal to one another.

5. Apparatus for forming diffraction gratings in a material comprising:

a) a radiation source for projecting radiation onto a radiation sensitive portion of a material to form a pattern therein;

b) a first cascaded diffraction grating pair for receiving radiation from said radiation source and projecting a first diffraction grating pattern on a material, said first cascaded diffraction grating pair comprising;

1) a first diffraction grating for forming a first diffracted radiation beam, said first diffraction grating having a first grating period; and

2) a second diffraction grating for diffracting said first diffracted radiation beam and forming a second diffracted radiation beam which generates said first diffraction grating pattern, said second diffraction grating having a second grating period, and said first diffraction grating pattern having a grating period which is smaller than said first and second grating periods; and

c) a second cascaded diffraction grating pair for receiving radiation from said radiation source and projecting a second diffraction grating pattern on a material, said second cascaded diffraction grating pair comprising;

1) a third diffraction grating for forming a third diffracted radiation beam, said third diffraction grating having a third grating period; and

2) a fourth diffraction grating for diffracting said third diffracted radiation beam and forming a fourth diffracted radiation beam which generates said second diffraction grating pattern, said fourth diffraction grating having a fourth grating period;

wherein, one of said third or fourth grating periods is larger than said first or second grating periods by a preselected amount, and the other of said third or fourth grating periods is smaller than said first or second grating periods by said preselected amount;

whereby, said first and second diffraction grating patterns having grating periods which differ from one another.

6. The apparatus of claim 5, wherein said material comprises a first optical fiber positioned adjacent said first cascaded diffraction grating pair and having a core for reception of said first diffraction grating pattern, and a second optical fiber positioned adjacent said second cascaded diffraction grating pair, and having a core for reception of said second diffraction grating pattern.

7. The apparatus of claim 5, wherein said material comprises an optical fiber positioned adjacent said first and second cascaded diffraction grating pairs, said optical fiber including a core for receiving said first and second diffraction grating patterns.

8. The apparatus of claim 5, wherein said first and second grating periods are substantially equal to one another.

9. Apparatus for forming diffraction gratings in a material comprising:

a) a radiation source for projecting radiation onto a radiation sensitive portion of a material to form a pattern therein; and

b) a first cascaded diffraction grating pair for receiving radiation from said radiation source and projecting a first diffraction grating pattern on a material, said first cascaded diffraction grating pair comprising;

1) a first diffraction grating for forming a first diffracted radiation beam, said first diffraction grating having a first grating period; and

2) a second diffraction grating for diffracting said first diffracted radiation beam and forming a second diffracted radiation beam which generates said first diffraction grating pattern, said second diffraction grating having a second grating period which is substantially equal to said first grating period, and said first diffraction grating pattern having a grating period which is smaller than said first and second grating periods.

10. The apparatus of claim 9, further comprising a material in which a diffraction grating is to be formed, said material being positioned adjacent said cascaded diffraction grating to receive said first diffraction grating pattern.

11. The apparatus of claim 10, wherein said material further comprises an optical fiber positioned adjacent said cascaded diffraction grating pair, said optical fiber including a core for receiving said first diffraction grating pattern.

12. Apparatus for forming diffraction gratings in a material comprising:

a) a radiation source for projecting radiation onto a radiation sensitive portion of a material to form a pattern therein;

b) a first cascaded diffraction grating pair for receiving radiation from said radiation source and projecting a first diffraction grating pattern on a material, said first cascaded diffraction grating pair comprising;

1) a first diffraction grating for forming a first diffracted radiation beam, said first diffraction grating having a first grating period; and

2) a second diffraction grating for diffracting said first diffracted radiation beam and forming a second diffracted radiation beam which generates said first diffraction grating pattern, said second diffraction grating having a second grating period, and said first diffraction grating pattern having a grating period which is smaller than said first and second grating periods; and

c) a material in which a diffraction grating is to be formed, said material being positioned to receive said first diffraction grating pattern.

13. The apparatus of claim 12, wherein said material is selected from the group comprising a substrate, a thin film and an optical fiber.

14. A method for forming diffraction gratings in a material comprising the steps of:

a) providing a material in which a diffraction grating is to be formed, said material including a radiation sensitive portion to permit formation of a pattern therein;

b) providing a radiation source for projecting a beam of radiation on said material;

c) positioning at least a first cascaded diffraction grating pair between said radiation source and said material, said cascaded diffraction grating pair comprising first and second diffraction gratings having first and second grating periods, respectfully;

d) directing a beam of radiation from said radiation source through said cascaded diffraction grating pair to form a first diffracted radiation beam which projects a first diffraction grating pattern on said material having a grating period which is smaller than said first or second grating periods; and

e) forming a diffraction grating in said material corresponding to said diffraction grating pattern.

15. The method of claim 14, wherein said first and second grating periods are selected to be substantially equal to one another.

16. The method of claim 14, wherein the step of positioning a first cascaded grating pair further comprises the steps of:

1) positioning at least a second cascaded diffraction grating pair between said radiation source and said material in a plane parallel to said first cascaded diffraction grating pair, said second cascaded diffraction grating pair comprising third and fourth diffraction gratings having third and fourth grating periods, respectfully; and

2) positioning a spatial filtering means between said radiation source and said material; and

wherein, the step of directing further comprises directing a beam of radiation from said radiation source through said first and second cascaded diffraction grating pairs, thereby forming said first and a second diffracted radiation beams, and projecting said first and second diffracted radiation beams in an interfering manner onto said material, thereby forming said first diffraction pattern thereon, said diffraction pattern having a grating period smaller than any of said first, second, third or fourth grating periods.

17. The method of claim 16, wherein the step of positioning said first and second cascaded grating pairs, and said spatial filtering means further comprises:

positioning third and fourth cascaded diffraction grating pairs between said radiation source and said material in a plane parallel to one another, said third cascaded diffraction grating pair comprising fifth and sixth diffraction gratings having fifth and sixth grating periods, respectfully, and said fourth cascaded diffraction grating pair comprising seventh and eighth diffraction gratings having seventh and eighth grating periods, respectfully, wherein said fifth and seventh grating periods are selected to be substantially equal to one another and differ by a preselected amount from any of said first through fourth grating periods, and said sixth and eighth grating periods are selected to be substantially equal to one another and differ from any of said first through fourth grating periods by an amount substantially equal, but opposite, to said preselected amount; and

wherein said step of directing further comprises directing a beam of radiation through said first through fourth cascaded diffraction grating pairs to form said first and second diffracted radiation beams which interfere with one another to form said first diffraction grating pattern in said material, and third and fourth diffracted radiation beams which interfere with one another to form a second diffraction grating pattern in said material, said first and second diffraction grating patterns having grating periods which differ from one another.

18. The method of claim 14, wherein said step of providing at least a first cascaded diffraction grating pair further comprises providing a second cascaded diffraction grating pair including third and fourth diffraction gratings having third and fourth grating periods, respectfully, said third grating period differing from either of said first or second grating periods by a preselected amount, and said fourth grating period differing from either of said first or second grating periods by an amount equal, but opposite, to said preselected amount;

and

wherein said step of directing further comprises directing a radiation beam through said first and second cascaded diffraction grating pairs to form said first and a second diffraction grating patterns in said material, said first and second diffraction grating patterns having grating periods which differ from one another and are smaller than any of said first through fourth grating periods.

19. The method of claim 18, wherein said step of providing a material further comprises providing an optical fiber including a core, and positioning said optical fiber adjacent said first and second cascaded diffraction grating pairs for reception by said core of said first and second diffraction grating patterns.

20. The method of claim 18, wherein said step of providing a material further comprises providing first and second optical fibers, said first optical fiber having a first core and said second optical fiber having a second core, and positioning said first optical fiber adjacent said first cascaded diffraction grating pair, and said second optical fiber adjacent said second cascaded diffraction grating pair for reception of said first diffraction grating pattern by said first core and said second diffraction grating pattern by said second core.

21. The method of claim 14, wherein the step of providing a material further comprises providing a material selected from the group comprising a substrate, a thin film and an optical fiber.

22. The method of claim 14, wherein the step of providing a material further comprises providing a material having a photoresist layer thereon, and wherein the step of forming further comprises:

1) developing said photoresist layer after said diffraction grating pattern is projected thereon; and

2) etching said material to form said diffraction grating therein from said diffraction grating pattern.