Gate electrodes for millimeter-wave operation and methods of fabrication

• US 8,741,715 B2
• Filed: 04/29/2009
• Issued: 06/03/2014
• Est. Priority Date: 04/29/2009
• Status: Active Grant

• Current Assignee: MACOM Technology Solutions Holdings, Inc.
• Original Assignee: CREE Incorporated (Wolfspeed, Inc.)
• Inventors: Moore, Marcia, Heikman, Sten
• Primary Examiner(s): Sefer, A.
• Application Number:
  US12/432,478
• Publication Number:
  US 20100276698A1
• Time in Patent Office: 1,861 Days
• Field of Search: 438/172, 438/270
• US Class Current: 438/270
• CPC Class Codes: H01L 21/0272 for lift-off processes
• CPC Class Codes: H01L 21/28587 characterised by the sectional shape, e.g. T, inverted T
• CPC Class Codes: H01L 29/66462 with a heterojunction interface channel or gate, e.g. HFET, HIGFET, SISFET, HJFET, HEMT

Abstract

A transistor device having a tiered gate electrode fabricated with methods using a triple layer resist structure. The triple layer resist stack is deposited on a semiconductor structure. An exposure pattern is written onto the resist stack using an e-beam writer, for example. The exposure dose is non-uniform across the device. Portions of the three resist layers are removed with a sequential development process, resulting in tiered resist structure. A conductive material is deposited to form the gate electrode. The resulting “Air-T” gate also has a three-tiered structure. The fabrication process is well-suited for the production of gates small enough for use in millimeter wave devices.

Claims

1. A method of fabricating a gate electrode on a semiconductor structure, comprising:

depositing a resist layer that comprises an inner layer, a middle layer, and an outer layer on said semiconductor structure, said inner layer being the closest of any resist layers to said semiconductor structure and comprising a resist material different from the material of said middle layer, and said outer layer comprising a resist material different from the material of said middle layer;

removing selected portions of said resist layer in sequence starting with said outer layer, wherein a portion of said inner layer is removed such that a remaining portion of said inner layer defines sidewalls generally curving inward as they approach said semiconductor structure; and

depositing a conductive material in a space left after said removal of selected portions.

2. The method of claim 1, said inner and outer layers comprising electron beam resist materials.

3. The method of claim 1, said middle layer material associated with a developer that does not develop said outer and inner layers.

4. The method of claim 1, further comprising, after depositing said resist layer, exposing a pattern on said resist layer.

5. The method of claim 4, wherein an electron beam is used to expose said pattern.

6. The method of claim 5, wherein the dose of exposure from said electron beam is non-uniform over said resist layer.

7. The method of claim 4, further comprising developing at least a portion of said outer layer such that at least a portion of said middle layer is exposed.

8. The method of claim 7, further comprising developing at least a portion of said middle layer such that a portion of said inner layer is exposed.

9. The method of claim 8, wherein a portion of said middle layer is removed that is laterally beyond the edge of the remaining portion of said outer layer, forming an undercut feature.

10. The method of claim 9, further comprising developing at least a portion of said inner layer such that at least a portion of said semiconductor structure is exposed, such that the remaining portion of said inner layer defines said generally curved sidewalls.

11. The method of claim 5, wherein during said exposure, said electron beam has a highest exposure dosage in the center of said resist layer and an exposure dosage that decreases with distance moving away from the center on both sides.

12. The method of claim 1, wherein said conductive material is deposited using an evaporation process and the orientation of said resist layer is varied during deposition.

13. The method of claim 1, wherein said inner and outer layers comprise ZEP-520A and said middle layer comprises polymethylglutarimide (PMGI).

14. The method of claim 1, wherein said inner layer comprises polymethylmethacrylate-950k (PMMA-950k), said middle layer comprises PMGI, and said outer layer comprises co-PMMA.

15. The method of claim 1, wherein said inner layer is deposited to be approximately 200-800 nm thick, said middle layer is deposited to be approximately 500-600 nm thick, and said outer layer is deposited to be approximately 200 nm thick.

16. The method of claim 1, wherein said sidewalls are generally convex and angle away from the center of said semiconductor structure.

17. A method of fabricating a gate electrode on a semiconductor structure, comprising:

depositing a resist layer that comprises an inner layer, a middle layer, and an outer layer on said semiconductor structure, said inner layer being the closest of any resist layers to said semiconductor structure and comprising a resist material different from the material of said middle layer, and said outer layer comprising a resist material different from the material of said middle layer;

removing selected portions of said resist layer in sequence starting with said outer layer, wherein a portion of said inner layer is removed such that a remaining portion of said inner layer defines sidewalls generally curving inward as they approach said semiconductor structure;

depositing a conductive material in a space left after said removal of selected portions;

after depositing said resist layer, exposing a pattern on said resist layer;

developing at least a portion of said outer layer such that at least a portion of said middle layer is exposed;

developing at least a portion of said middle layer such that a portion of said inner layer is exposed;

wherein a portion of said middle layer is removed that is laterally beyond the edge of the remaining portion of said outer layer, forming an undercut feature; and

developing at least a portion of said inner layer such that at least a portion of said semiconductor structure is exposed, such that the remaining portion of said inner layer defines said generally curved sidewalls;

wherein said developing of said outer layer is done with a mix of methyl ethyl ketone and methyl isobutyl ketone (MEK;

MIBK), said developing of said middle layer is done with a tetramethylammonium hydroxide (TMAH) based developer, and said developing of said inner layer is done with amyl acetate.