Photolithographic LED fabrication using phase-shift mask

• US 8,796,053 B2
• Filed: 12/21/2010
• Issued: 08/05/2014
• Est. Priority Date: 12/21/2010
• Status: Expired due to Fees

• Current Assignee: Ultratech Incorporated (Veeco Instruments Incorporated)
• Original Assignee: Ultratech Incorporated (Veeco Instruments Incorporated)
• Inventors: Hawryluk, Andrew M., Hsieh, Robert L., Flack, Warren W.
• Primary Examiner(s): Such, Matthew W
• Assistant Examiner(s): SPALLA, DAVID C
• Application Number:
  US12/928,862
• Publication Number:
  US 20120153323A1
• Time in Patent Office: 1,323 Days
• Field of Search: 257/79, 257/98, 257/103
• US Class Current: 438/29
• CPC Class Codes: G03F 1/26 Phase shift masks [PSM]; PSM blanks; Preparation thereof
• CPC Class Codes: G03F 7/70283 Mask effects on the imaging process
• CPC Class Codes: G03F 7/703 Non-planar pattern areas or non-planar masks, e.g. curved masks or substrates
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• CPC Class Codes: H01L 2933/0083 Periodic patterns for optical field-shaping in or on the semiconductor body or semiconductor body package, e.g. photonic bandgap structures
• CPC Class Codes: H01L 33/007 comprising nitride compounds
• CPC Class Codes: H01L 33/22 Roughened surfaces, e.g. at the interface between epitaxial layers

Abstract

Photolithographic methods of forming a roughened surface for an LED to improve LED light emission efficiency are disclosed. The methods include photolithographically imaging a phase-shift mask pattern onto a photoresist layer of a substrate to form therein a periodic array of photoresist features. The roughened substrate surface is created by processing the exposed photoresist layer to form a periodic array of substrate posts in the substrate surface. A p-n junction multilayer structure is then formed atop the roughened substrate surface to form the LED. The periodic array of substrate posts serve as scatter sites that improve the LED light emission efficiency as compared to the LED having no roughened substrate surface. The use of the phase-shift mask enables the use of affordable photolithographic imaging at a depth of focus suitable for non-flat LED substrates while also providing the needed resolution to form the substrate posts.

Claims

1. A photolithographic method of forming a roughened surface for a light-emitting diode (LED) to improve LED light emission efficiency, comprising:

providing a semiconductor substrate having a surface covered with photoresist;

photolithographically imaging a phase-shift mask pattern onto the photoresist using a single exposure, the phase-shift mask pattern comprising a two-dimensional periodic array of first and second phase-shift regions that are separated by an opaque background section and that have a sub-resolution dimension and a sub-resolution spatial frequency, said photolithographic imaging forming in the photoresist a periodic array of sub-resolution photoresist features having substantially the sub-resolution spatial frequency and substantially the sub-resolution dimension of the first and second phase-shift regions;

defining the roughened substrate surface by processing the photoresist and the photoresist features therein to form a corresponding periodic array of substrate posts in the substrate surface; and

forming a p-n junction multilayer structure atop the roughened substrate surface to form the LED, with the periodic array of substrate posts serving as scatter sites that improve the LED light emission efficiency as compared to the LED having no roughened substrate surface.

2. The method of claim 1, further comprising the substrate being made of one of:

sapphire, SiC, GaN and Si.

3. The method of claim 1, wherein the photolithographic imaging is performed at an imaging wavelength, and wherein first and second phase-shift regions are configured to provide respective phase shifts of 0°

and 180°

at the imaging wavelength.

4. The method of claim 1, further comprising the imaging wavelength being 365 nm and the photolithographic imaging being performed at unit magnification.

5. The method of claim 1, wherein the substrate posts have a diameter, a spacing and a height, the method further comprising performing the photolithographic imaging at a numerical aperture that provides a maximum depth of focus for the diameter, the spacing and the height.

6. The method of claim 1, wherein the first and second phase-shift regions have at least one of a circular shape, an oval shape and a polygonal shape.

7. The method of claim 1, further comprising forming the substrate posts to have a width of 2 microns or less.

8. The method of claim 7, further comprising forming the substrate posts to have an edge-to-edge spacing of between 0.5 microns and 3 microns.

9. The method of claim 8, further comprising forming the substrate posts to have a height of up to 2 microns.

10. The method of claim 1, further comprising the periodic array of substrate posts having a substantially 1:

1 pitch.

11. The method of claim 1, wherein the photolithographic imaging has a depth of focus of greater than 30 microns.

12. A method of forming a light-emitting diode (LED), comprising:

performing a single photolithographic exposure to photolithographically expose photoresist supported by a semiconductor substrate to form therein an array of photoresist posts having a second spatial frequency, the single photolithographic exposure including passing illumination light once through a phase-shift mask having a two-dimensional pattern comprising first and second phase-shift regions that are spaced apart by an opaque background section and that have a sub-resolution dimension and a sub-resolution spatial frequency, wherein the single photolithographic exposure has a numerical aperture of 0.5 or less and an imaging wavelength defined by a mercury line;

processing the photoresist to form an array of substrate posts that defines a roughened substrate surface and that have substantially the sub-resolution dimension and substantially the sub-resolution spatial frequency of the first and second phase-shift regions; and

forming a p-n multilayer structure atop the roughened substrate surface to form the LED, wherein the roughened substrate surface acts to scatter light generated by the p-n multilayer structure to increase an amount of light emitted by the LED as compared to the LED having an unroughened substrate surface.

13. The method of claim 12, further comprising:

the substrate posts having at least one dimension of 0.5 microns; and

the single photolithographic exposure having a numerical aperture of 0.5 or less and an imaging wavelength of about 365 nm.

14. The method of claim 12, further comprising the substrate being a sapphire substrate.

15. The method of claim 12, wherein the opaque background section comprises a coating of either aluminum or chrome.

16. The method of claim 12, wherein the first and second phase-shift regions have at least one of a circular shape, an oval shape and a polygonal shape.

17. The method of claim 12, wherein the photolithographic exposure is performed at an imaging wavelength, and wherein first and second phase-shift regions are configured to provide respective phase shifts of 0°

and 180°

at the imaging wavelength.

18. The method of claim 12, further comprising the photolithographic exposure being performed at unit magnification.

19. The method of claim 12, further comprising the substrate posts have a dimension of one micron or less and further comprising performing said photolithographic exposure at a numerical aperture of 0.5 or less.